POWER OUTPUT APPARATUS, METHOD OF CONTROLLING POWER OUTPUT APPARATUS, AND DRIVING SYSTEM WITH POWER OUTPUT APPARATUS INCORPORATED THEREIN

Abstract: A power output apparatus (20) of the present invention includes a clutch motor (30), an assist motor (40), and a controller (80). The clutch motor (30) and the assist motor (40) are controlled by the controller (80), so as to enable the power output from an engine (50) to a crankshaft (56) and expressed as the product of its revolving speed and torque to be converted to the power expressed as the product of a revolving speed and a torque of a drive shaft (22) and to be output to the drive shaft (22). The engine (50) can be driven at an arbitrary driving point defined by a revolving speed and a torque, as long as the energy or power output to the crankshaft (56) is identical. A desired driving point that attains the highest possible efficiency with respect to each amount of output energy is determined in advance. In order to allow the engine (50) to be driven at the desired driving point, the controller (80) controls the clutch motor (30) and the assist motor (40) as well as the fuel injection and the throttle valve position. Such control procedures of the power output apparatus (20) enhance the energy efficiency of the whole power output apparatus (20).

Description: DESCRIPTION POWER OUTPUT APPARATUS, METHOD OF CONTROLLING POWER OUTPUT APPARATUS, AND DRIVING SYSTEM WITH POWER OUTPUT APPARATUS INCORPORATED THEREIN Technical Field The present invention relates to a power output apparatus, a method of controlling a power output apparatus, and a driving system withapoweroutputapparatus incorporatedtherein. Moreconcretely, the present invention pertains to a power output apparatus for outputting power generated by an engine to a drive shaft at a high efficiency and a method ofcontrolling such a power outputapparatus, as well as a driving system with such a power output apparatus incorporated therein. Background Art In known poweroutput apparatuses mounted on avehicle, anoutput shaft of an engine is electromagnetically linked with a drive shaft, which connects with arotor of a motor, by means of anelectromagnetic coupling, so that power of the engine is transmitted to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). The electromagnetic coupling of the power output apparatus transmits part of the power output from the engine as a torque to the drive shaft via electromagnetic connection, and supplies electric power regenerated by sliding motions of the electromagnetic coupling to the motor and secondary cells, which are connected in parallel with the electromagnetic coupling. When the torque transmitted to the drive shaft by means of the electromagnetic coupling is insufficient, the motor applies an additional torque to the drive shaft with the electric power regenerated by the electromagnetic coupling or the electric power released from the secondary cells. The motor works as a generator when a braking force is applied to the drive shaft, so as to regenerate the energy of rotational motion of the drive shaft as electrical energy and store the regenerated electrical energy in the secondary cells. In the conventional power output apparatuses, however, problems, such as an extremely low energy efficiency of the whole apparatus or an extremely poor emission, arise in some cases. The power output apparatus can implement torque conversion of all the power output from the engine with the electromagnetic coupling and the motor and output the converted power to the drive shaft. The electromagnetic coupling and the motor carry out energy converεion of the power or energy expressed as the product of the torque and the revolving speed of the engine into the power or energy expressed as the product of the torque and the revolving speed of the drive shaft under the condition of constant energy. On the assumption that the conversion efficiency is an ideal state (that is, the value ' 1'), the object of torque conversion is to make the power output from the engineequal to the power output to the drive shaft. The engine may accordingly be driven at any driving point (defined by the revolving speed and the torque) that can output energy identical with the power. Without the active control of the driving point of the engine, this often causes the engine to be driven at driving points of low energy efficiency or poor emission. Disclosure of invention One object of the present invention is thus to provide a power output apparatus and a method of controlling the same that enhance the energy efficiency of the whole power output apparatus. Another object of the present invention is to provide a power output apparatus and a method of controlling the same that improve the emission. Still another object is to provide a power output apparatus and a method of controllingthe same that enable the engine to be smoothly shifted to a new driving point with a variation in power to be output to the drive shaft. Further object is to provide a driving system that prevents the driving system itself or a power output apparatus or any other equipment incorporated in the driving system from resonating due to an operation of the engine in the power output apparatus. At least part of the above objects is realized by a first power output apparatus for outputting power to a drive shaft. The first power output apparatus includes: an engine having an output shaft; energy adjustmentmeans having a first shaft connectedwiththeoutput shaft of the engine and a second shaftconnected with the drive shaft, the energy adjustment means adjusting a difference in energy between power input into or output from the first shaft and power input into or output from the second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between the drive motor and the drive shaft; targetpower setting means for setting a target power output to the drive shaft; driving state setting means for setting a target driving state of the engine that outputs energy corresponding to the target power set by the target power settingmeans, basedon apredeterminedcondition; and control means for controlling the engine, so as to enable the engine to be driven in the target driving state set by the driving state setting means, and for controlling the energy adjustment means and the drive motor, so as to enable power output from the engine to be subjected to torque conversion and output as the target power to the drive shaft. The first power output apparatus of the invention enables the engine to be driven in the target driving state that has been set based on the predetermined condition, among the available driving states which can output energy corresponding to the target power. In the first power output apparatus, the predetermined condition used for setting the target driving state may be a condition for enhancing an energy efficiency of the engine that outputs energy corresponding to the target power. This condition enhances the energy efficiency of the engine. The predetermined condition may also be a condition for enhancing a comprehensive efficiency, which is calculated by multiplying an energy efficiency of the engine that outputs energy corresponding to the target power by a transmission efficiency of the energy adjustment means and the drive motor when the power output from the engine is subjected to torque conversion and output to the drive shaft. This condition enhances the efficiency of the whole power output apparatus. The predetermined condition may otherwise be a condition for continuously varying a driving state of the engine with a variation in target power. This condition allows the engine to smoothly shift its driving state with a variation in target power. In accordance with one aspect of the present invention, the energy adjustment means of the first power output apparatus may be constructed as a twin-rotor motor comprising a first rotor connected with the first shaft and a second rotor connected with the second shaft, the second rotor being rotatable relative to the first rotor, whereby power is transmitted between the first shaft and the second shaft via an electromagnetic coupling of the first rotor with the second rotor, the twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of the first rotor with the second rotor and a difference in revolving speed between the first rotor and the second rotor. In the first power output apparatus including the twin-rotor motor as the energy adjustment means, the drive motor may include the second rotor included in the twin-rotor motor and a stator for rotating the second rotor. This effectively reduces the size of the whole power output apparatus. In accordance with another aspect of the present invention, the energy adjustment means of the first power output apparatus may be constructed as: three-shaft-type power input and output means connected with the first shaft, the second shaft, and a third shaft, the three-shaft-type power input and output means for, when powers input into or output from any two shafts among the three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with the third shaft, wherein power is transmitted between the third shaft and the shaft motor. In accordance with another aspect of the present invention, the first power outputapparatus further includes driving state detecting means for detecting a driving state of the engine. The control means further has means for controlling the energy adjustment means, so as to enable the engine to be driven in the target driving state, based on the driving state of the engine detected by the driving state detecting means. This structure ensures the operation of the engine in the target driving state. In accordance with another aspect of the present invention, the first power output apparatus further includes driving state detecting means for detecting a driving state of the engine. The control means further has tentative target driving state setting means for, when a state deviation of the driving state detected by the driving state detecting means from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the predetermined condition and setting the selected driving state as a tentative target driving state. The tentative target driving state set by the tentative target driving state setting means is used in place of the target driving state for operation control of the engine and control of the energy adjustment means and the drive motor, until the state deviation enters the predetermined range. Even when the newly set target driving state has a large state deviation, this structure enables the engine to stably approach to and eventually reach the target driving state. This effectivelyprevents the engine from stalling or stopping the revolutions of its output shaft due to the large state deviation. In the first power output apparatus wherein the control means has thetentative target drivingstate settingmeans, it is preferable that the tentative target driving state setting means further includes predetermined range setting means for setting the predetermined rangebasedonthedriving state detected bythe driving state detecting means. This structure enables the requirement or non-requirement for setting the tentative target driving state to be determined according to the driving state of the engine. In the first power output apparatus wherein the control means has the tentative target driving state setting means, the control means may further include means forcontrolling theenergy adjustment means, so as to enable the engine to be driven in the target driving state, based on the driving state detected by the driving state detecting means, when the state deviation is within thepredetermined range. This structure further ensures the operation of the engine in the target driving state when the state deviation is within the predetermined range. In accordance withone aspect of the present invention, the first power output apparatus, wherein the control means has the tentative target driving state setting means, further includes storage battery means being charged with electrical energy taken out of the energy adjustment means, being charged with electrical energy taken out of the drive motor, being discharged to release electrical energy used in the energy adjustment means, and being discharged to release electrical energy used in the drive motor. The control means further has means for, when the tentative target driving state is used in place of the target driving state for the operation control of the engine and the control of the energy adjustment means and the drive motor, utilizing the electrical energy stored into or released from the storage battery means and controlling the drive motor, so as to enable the drive motor to input or output a specific power into or from the drive shaft, the specific power corresponding to an energy difference between the target power and the power output from the engine that is driven in the tentative target driving state. This structure enables the target power to be output to the drive shaft even, when there is a large difference between the actual driving state of the engine and the target driving state. In the first power output apparatus, it is also preferable that the control means further includes: driving state estimating means for estimating a driving state of the engine when the target power setting means sets a different target power; and estimated- condition control means for controlling the energy adjustment means and the drive motorbasedonthe estimateddriving state of theengine. This structure enables the engine to smoothly shift to the target driving state. With a variation in target power, the first power output apparatus constructed as above can effectively implement torque conversion of the power output from the engine and output the converted power to the drive shaft with a high efficiency. In the first power output apparatus wherein the control means has the driving state estimating means and the estimated-condition control means, it is further preferable that the driving state estimating means includes means for estimating the driving state of the engine based on a revolving speed of the output shaft of the engine and a state of the energy adjustment means. In the first power output apparatus wherein the control means has the driving state estimating means and the estimated-condition control means, it is also preferable that the estimated-condition control means further has means forcontrolling theenergy adjustment means and the drive motor, so as to enable an estimated power output from the engine corresponding to the driving state of the engine estimated by the driving state estimating means to be subjected to torque conversion and output as the target power to the drive shaft. This structure enables the power output from the engine to be subjected to torque conversion and to be output to the drive shaft, even in the transient period of varied target power. In accordance with another aspect of the present invention, the first power output apparatus, wherein the control means has the driving state estimating means and the estimated-condition control means, further includes storage battery means being charged with electrical energy taken out of the energy adjustment means, being charged with electrical energy taken out of the drive motor, being discharged to release electrical energy used in theenergyadjustment means, and being discharged to release electrical energy used in the drive motor. The estimated-condition control means has means for utilizing the electrical energy stored into or released from the storage batterymeans andcontrolling thedrive motor, so as toenable the drive motor to input or output a specific power into or from the drive shaft, the specificpower corresponding to anenergydifference between the target power and the estimated power output from the engine corresponding to the driving state of the engine estimated by the driving state estimating means. Even when the engine is not driven in the target driving state in the transient period of varied target power, the target power can be thereby output to the drive shaft. The present invention is also directed to a second power output apparatus for outputting power to a drive shaft. The second power output apparatus includes: an engine having an output shaft; energy adjustment means having a first shaft connected with the output shaft of the engine and a second shaft connected with the drive shaft, the energy adjustment means adjusting a difference in energy between power input into or output from the first shaft and power input into or output from the second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between the drive motor and the output shaft of the engine; target power setting means for setting a target power output to the drive shaft; driving state setting means for setting a target driving state of the engine that outputs energy corresponding to the target power set by the target power setting means, based on a predetermined condition; andcontrol means forcontrollingtheengine, so as to enable the engine to be driven in the target driving state set bythe driving state settingmeans, and for controllingtheenergy adjustment means and the drive motor, so as to enable power output from the engine to be subjected to torque conversion and output as the target power to the drive shaft. The second power output apparatus of the invention enables the engine to be driven in the target driving state that has been set based on the predetermined condition, among the available driving states which can output energy corresponding to the target power. In the second power output apparatus, the predetermined condition used for settingthe target driving statemay beacondition for enhancing an energy efficiency of the engine that outputs energy corresponding to the target power. This condition enhances the energy efficiency of the engine. The predetermined condition may also be a condition for enhancing a comprehensive efficiency, which is calculated by multiplying an energy efficiency of the engine that outputs energy corresponding to the target power by a transmission efficiency of the energy adjustment means and the drive motor when the power output from the engine is subjected to torque conversion and output to the drive shaft. This condition enhances the efficiency of the whole power output apparatus. The predetermined condition may otherwise be a condition for continuously varying a driving state of the engine with a variation in target power. This condition allows the engine to smoothly shift its driving state with a variation in target power. In accordance with one aspect of the present invention, the energy adjustment means of the second power output apparatus may be constructed as a twin-rotor motor comprising a first rotor connected with the first shaft and a second rotor connected with the second shaft, the second rotor being rotatable relative to the first rotor, whereby power is transmitted between the first shaft and the second shaft via an electromagnetic coupling of the first rotor with the second rotor, the twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of the first ll rotor with the second rotor and a difference in revolving speed between the first rotor and the second rotor. In the second power output apparatus including the twin-rotor motor as the energy adjustment means, the drive motor may include the first rotor included in the twin-rotor motor and a stator for rotating the first rotor. This effectively reduces the size of the whole power output apparatus. In accordance with another aspect of the present invention, the energy adjustment means of the second power output apparatus may be constructed as: three-shaft-type power input and output means connected with the first shaft, the second shaft, and a third shaft, the three-shaft-type power input and output means for, when powers input into or output from any two shafts among the three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with the third shaft, wherein power is transmitted between the third shaft and the shaft motor. In accordance with another aspect of the present invention, the second power output apparatus further includes driving state detecting means for detecting a driving state of the engine. The control means further has tentative target driving state setting means for, when a state deviation of the driving state detected by the driving state detecting means from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the predetermined condition and setting the selected driving state as a tentativetargetdriving state. The tentativetargetdriving state set by the tentative target driving state setting means is used in place of the target driving state for operation control of the engine and control of the energy adjustment means and the drive motor, until the state deviation enters the predetermined range. Even when the newly set target driving state has a large state deviation, this structure enables the engine to stably approach to and eventually reach the target driving state. This effectivelyprevents the engine from stalling or stopping the revolutions of its output shaft due to the large state deviation. In the second power output apparatus , it is also preferable that the control means further includes: driving state estimating means for estimating a driving state of the engine when the target power setting means sets a different target power; and estimated- condition control means for controlling the energy adjustment means and the drive motorbasedonthe estimated driving state of theengine. This structure enables the engine to smoothly shift to the target driving state. With a variation in target power, the first power output apparatus constructed as above can effectively implement torque conversion of the power output from the engine and output the converted power to the drive shaft with a high efficiency. In the second power output apparatus wherein the control means has the driving state estimating means and the estimated-condition control means, it is also preferable that the estimated-condition control means further has means forcontrolling theenergyadjustment means and the drive motor, so as to enable an estimated power output from the engine corresponding to the driving state of the engine estimated by the driving state estimating means to be subjected to torque conversion and output as the target power to the drive shaft. This structure enables the power output from the engine to be subjected to torque conversion and to be output to the drive shaft, even in the transient period of varied target power. In accordance with another aspect of the present invention, the second power output apparatus, wherein the control means has the driving state estimating means and the estimated-condition control means, further includes storage battery means being charged with electrical energy taken out of the energy adjustment means, being charged with electrical energy taken out of the drive motor, being discharged torelease electrical energy used in theenergy adjustment means, and being discharged to release electrical energy used in the drive motor. The estimated-condition control means has means for utilizing the electrical energy stored into or released from the storage batterymeans andcontrolling thedrive motor, so as toenable the drive motor to input or output a specific power into or from the output shaft of the engine, the specific power corresponding to an energy difference between the target power and the estimated power output from the engine corresponding to the driving state of the engine estimated by the driving state estimating means. Even when the engine is not driven in the target driving state in the transient period of varied target power, the target power can be thereby output to the drive shaft. The present invention is further directed to a first driving system, which includes: an engine having an output shaft; energy adjustment means having a first shaft connected with the output shaft of the engine and a second shaft connected with a drive shaft of the driving system, the energy adjustment means adjusting a difference in energy between power input into or output from the first shaft and power input into or output from the second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between the drive motor and the drive shaft; target power setting means for setting a target power output to the drive shaft; driving state setting means for setting a target driving state of the engine that outputs energy corresponding to the target power set by the target power setting means, based on a first condition for enhancing an energy efficiency of the engine that outputs energy corresponding to the target power and a second condition for making a vibration due to an operation of the engine out of a range of resonance frequency of the driving system; and control means for controlling the engine, so as to enable the engine to be driven in the target driving state set by the driving state setting means, and for controlling the energy adjustment means and the drive motor, so as to enable power output from the engine to be subjected to torque conversion and output as the target power to the drive shaft. Typical examples of the first driving system of the present invention includevehicles, ships, airplanes, andvarious industrial machines. The first driving system can effectively prevent the driving system itself or a power output apparatus or any other equipment incorporated in the driving system from resonating due to an operation of the engine. No undesirable stress due to the resonance is thereby applied tothe driving systemor otherequipment incorporated in the driving system, so that the driving system and the other equipment have better durability and the engine can be driven at driving points of the highest possible efficiency. This improves the energy efficiency of the whole system. In accordance with one aspect of the present invention, the energy adjustmentmeans of the first driving systemmaybeconstructed as a twin-rotor motor comprising a first rotor connected with the first shaft and a second rotor connected with the second shaft, the second rotor being rotatable relative to the first rotor, whereby power is transmitted between the first shaft and the second shaft via an electromagnetic coupling of the first rotor with the second rotor, the twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of the first rotorwith the second rotor and a difference in revolvingspeed betweenthe first rotor and the second rotor. In the first driving system including the twin-rotor motor as the energy adjustment means, the drive motor may include the second rotor included in the twin-rotor motor and a stator for rotating the second rotor. This effectively reduces the size of the whole power output apparatus. In accordance with another aspect of the present invention, the energy adjustmentmeans of the firstdriving systemmaybeconstructed as: three-shaft-type power input and output means connected with the first shaft, the second shaft, and athird shaft, thethree-shaft-type power input and output means for, when powers input into or output from any two shafts among the three different shafts are determined, automatically setting a power input into or output from a residual shaft based on thepowers thus determined; anda shaftmotor connected with the third shaft, wherein power is transmitted between the third shaft and the shaft motor. The present invention is further directed to a second driving system, which includes: an engine having an output shaft; energy adjustment means having a first shaft connected with the output shaft of the engine and a second shaft connected with a drive shaft of the driving system, the energy adjustment means adjusting a difference in energy between power input into or output from the first shaft and power input into or output from the second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between the drive motor and the output shaft of the engine; target power setting means for setting a target power output to the drive shaft; driving state setting means for setting a target driving state of the engine that outputs energy corresponding to the target power set by the target power setting means, based on a first condition for enhancing an energy efficiency of the engine that outputs energy corresponding to the target power and a second condition for making a vibration due to an operation of the engine out of a range of resonance frequency of the driving system; and control means for controlling the engine, so as to enable the engine to be driven in the target driving state set by the driving state settingmeans, and for controlling the energy adjustment means and the drive motor, so as to enable power output from the engine to be subjected to torque conversion and output as the target power to the drive shaft. Typical examples of the second driving system of the present invention include vehicles, ships, airplanes, andvarious industrial machines. The second driving system can effectively prevent the driving system itself or a power output apparatus or any other equipment incorporated in the driving system from resonating due to an operation of the engine. No undesirable stress due to the resonance is thereby applied tothe driving system or other equipment incorporated in the driving system, so that the driving system and the other equipment have better durability and the engine can be driven at driving points of the highest possible efficiency. This improves the energy efficiency of the whole system. In accordance with one aspect of the present invention, the energy adjustment means of the second driving system may be constructed as a twin-rotor motor comprising a first rotor connected with the first shaft and a second rotor connected with the second shaft, the second rotor being rotatable relative to the first rotor, whereby power is transmitted between the first shaft and the second shaft via an electromagnetic coupling of the first rotor with the second rotor, the twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of the first rotor with the second rotor and a difference in revolving speed between the first rotor and the second rotor. In the second driving system including the twin-rotor motor as the energy adjustment means, the drive motor may include the first rotor included in the twin-rotor motor and a stator for rotating the first rotor. This effectively reduces the size of the whole power output apparatus. In accordance with another aspect of the present invention, the energy adjustment means of the second driving system may be constructed as: three-shaft-type power input and output means connected with the first shaft, the second shaft, and a third shaft, the three-shaft-type power input and output means for, when powers input into or output from any two shafts among the three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with the third shaft, wherein power is transmitted between the third shaft and the shaft motor. At least part of the above objects is also realized by a first method of controlling a power output apparatus for outputting power to a drive shaft. The first method includes the steps of: (a) providing an engine having an output shaft; energy adjustment means having a first shaft connected with the output shaft of the engine and a second shaft connected with the drive shaft, the energy adjustment means adjusting adifference inenergy betweenpower input into or output from the first shaft and power input into or output from the second shaft byregulating input and output of corresponding electrical energy; and a drive motor, wherein power is transmitted between the drive motor and the drive shaft; (b) setting a target power output to the drive shaft; (c) setting a target driving state of the engine that outputs energy corresponding to the target power set in the step (b), based on a specific condition of selecting a specific driving point that attains a highest possible efficiency among a plurality of available driving points of the engine that outputs energycorresponding tothe target power; and (d) controlling the engine, so as to enable the engine to be driven in the target driving state set in the step (c), and for controlling the energy adjustment means and the drive motor, so as to enable power output from the engine to be subjected to torque conversion and output as the target power to the drive shaft. The first method of the present invention enables the engine to be driven in a specific driving point that attains the highest possible efficiency among a plurality of available driving points of the engine that outputs energy corresponding to the target power. This further enhances the energy efficiency of the power output apparatus. In the first method of the present invention, the step (d) may further include the steps of: (e) detecting a driving state of the engine; (f) when a state deviation of the driving state of the engine detected in the step (e) from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the specific condition and settingthe selecteddriving state as a tentative target driving state; and (g) using the tentative target driving state set in the step (f) in place of the target driving state, in order to control the engine, the energy adjustment means, and the drivemotor, until the state deviation enters the predetermined range. Even when the newly set target driving state has a large state deviation, this structure enables the engine to stably approach to and eventually reach thetarget driving state. This effectivelyprevents the engine from stalling or stopping the revolutions of its output shaft due to the large state deviation. In the firstmethod of the present invention, it is also desirable that the step (d) further includes the steps of: (h) when a different target power is set, estimating a driving state of the engine based on a revolving speed of the output shaft of the engine and a state of the energy adjustment means; and (i) controlling the energy adjustment means and the drive motor, so as to enable power output from the engine to be subjected to torque conversion and output to the drive shaft, based on the estimated driving state of the engine. With a variation in target power, this structure enables the power output from the engine to be subjected to torque conversion and to be output to the drive shaft with a high efficiency. The present invention is further directed to a second method of controlling a power output apparatus for outputting power to a drive shaft. The second method includes the steps of: (a) providing an engine having an output shaft; energy adjustment means having a first shaft connected with the output shaft of theengine and a second shaft connected with the drive shaft, theenergy adjustmentmeans adjusting a difference in energy between power input into or output from the first shaft and power input into or output from the second shaft by regulating input and output of corresponding electrical energy; and a drive motor, wherein power is transmitted between the drive motor and the drive shaft; (b) setting a target power output to the drive shaft; (c) setting a target driving state of the engine that outputs energy corresponding to the target power set in the step (b), based on a specific condition of selecting a specific driving point that attains ahighest possible comprehensiveefficiencyamongaplurality of available driving points of the engine that outputs energy correspondingtothetargetpower, thecomprehensiveefficiencybeing calculated by multiplying an energy efficiency of the engine by a transmission efficiency of the energy adjustment means and the drive motor when the power output from the engine is subjected to torque conversion and output to the drive shaft; and (d) controlling the engine, so as to enable the engine to be driven in the target driving state set in the step (c) , and for controlling the energy adjustment means andthe drive motor, soas toenable poweroutput fromthe engine to be subjected to torque conversion and output as the target power to the drive shaft. The second method of the present invention enables the engine to be driven in a specific driving point that attains the highest possible comprehensive efficiency of the whole apparatus, among a plurality of available driving points of the engine that outputs energy corresponding to the target power. This further enhances the energy efficiency of the power output apparatus. In the second method of the present invention, the step (d) may further include the steps of: (e) detecting a driving state of the engine; (f) when a state deviation of the driving state of the engine detected in the step (e) from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the specific condition and settingthe selecteddriving state as atentativetarget driving state; and (g) using the tentative target driving state set in the step (f) in place of the target driving state, in order to control the engine, the energyadjustment means, and the drivemotor, until the state deviation enters the predetermined range. Even when the newly set target driving state has a large state deviation, this structure enables the engine to stably approach to and eventually reach thetarget driving state. This effectivelyprevents theengine from stalling or stopping the revolutions of its output shaft due to the large state deviation. In the second method of the present invention, it is also desirable that the step (d) further includes the steps of: (h) when a different target power is set, estimating a driving state of the engine based on a revolving speed of the output shaft of the engine and a state of the energy adjustment means; and (i) controlling the energy adjustment means and the drive motor, so as to enable power output fromthe engineto be subjected totorque conversionandoutput to the drive shaft, basedonthe estimateddriving state of the engine. With a variation in target power, this structure enables the power output from the engine to be subjected to torque conversion and to be output to the drive shaft with a high efficiency. These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings. Brief Description of Drawings Fig. 1 is a schematic view illustrating structure of a power output apparatus 20 as a first embodiment according to the present invention; Fig.2 is across sectionalviewillustratingdetailed structures of a clutch motor 30 and an assist motor 40 included in the power output apparatus 20 of Fig. 1; Fig. 3 is a schematic view illustrating general structure of a vehicle with the power output apparatus 20 of Fig. 1 incorporated therein; Fig. 4 is a graph showing the operation principle of the power output apparatus 20; Fig. 5 is a flowchart showing a torque control routine executed in the first embodiment by the control CPU 90 of the controller 80; Fig. 6 is a graph illustrating driving points of the engine 50 defined bythe targetengine torque Te* and the target revolvingspeed Ne*; Fig. 7 is a graph showing the efficiency of the engine 50 along the constant-output energy curves against the revolving speed Ne of the engine 50; Fig. 8 is a flowchart showing a fundamental procedure of controlling the clutch motor 30 executed by the controller 80; Fig. 9 is a graph showing the relationship between the torque command value Tc* of the clutch motor 30 and the target torque Te* and the target revolving speed Ne* of the engine 50; Figs. 10 and 11 are flowcharts showing a fundamental procedure of controlling the assist motor 40 executed by the controller 80; Fig. 12 is a flowchart showing a throttle valve position control routine executed by the EFIECU 70; Fig. 13 is a flowchart showing a fuel injection control routine executed by the EFIECU 70; Fig. 14 is a graph showing driving points of the engine 50 by taking into account aresonant revolving speed rangeNF of thevehicle or equipment mounted on the vehicle; Fig. 15 is an enlarged view showing the resonant revolving speed range NF of Fig. 14 and its vicinity; Fig. 16 is a flowchart showing a torque control routine executed in a second embodiment of the present invention by the control CPU 90 of the controller 80; Fig.17 shows variations in accelerator pedal positionAP, output energy Pe, and torque Tc when the driver steps on the accelerator pedal 64; Fig. 18 is a flowchart showing a control procedure of the assist motor 40 executed bythe controller 80 in a modification of the second embodiment; Figs. 19 and 20 are flowcharts showing a control procedure of the clutch motor 30 executed in a third embodiment of the present invention by the controller 80; Fig.21 shows a process of leading theestimated target revolving speed Nef* from the estimated torque Tef; Fig. 22 shows variations in accelerator pedal position AP, throttle valve position BP, and torque Te and revolving speed Ne of the engine 50 when the driver steps on the accelerator pedal 64; Fig. 23 shows structure of another power output apparatus 20C as a modification of the first through the third embodiments; Fig. 24 shows structure of still another power output apparatus 20D as a fourth embodiment of the present invention; Fig. 25 shows structure of another power output apparatus 20E as a modification of the fourth embodiment; Fig. 26 shows structure of still another power output apparatus 2OF as another modification of the fourth embodiment; Fig. 27 shows structure of another power output apparatus 110 as a fifth embodiment according to the present invention; Fig. 28 is an enlarged view illustrating an essential part of the power output apparatus 110 of the fifth embodiment; Fig. 29 is a schematic view illustrating general structure of a vehicle with the power output apparatus 110 of the fifth embodiment incorporated therein; Fig. 30 is a nomogram showing the relationship between the revolving speed and the torque of the three different shafts linked with the planetary gear 120; Fig. 31 is a nomogram showing the relationship between the revolving speed and the torque of the three different shafts linked with the planetary gear 120; Fig. 32 is a flowchart showing a torque control routine executed in the fifth embodiment by the control CPU 190 of the controller 180; Figs. 33 and 34 a re flowcharts showing a control procedure of the firstmotor MG1 executed inthe fifthembodiment bythe controller 180; Fig. 35 is a flowchart showing a control procedure of the second motor MG2 executed in the fifth embodiment by the controller 180; Fig. 36 shows structure of another power output apparatus 110A as a modification of the fifth embodiment; Fig. 37 shows structure of still another power output apparatus HOB as another modification of the fifth embodiment; Fig. 38 shows structure of another power output apparatus HOC as a sixth embodiment of the present invention; Fig. 39 is a nomogram showing the relationship between the revolving speed and the torque of the three different shafts linked with the planetary gear 120 in the power output apparatus HOC of the sixth embodiment; Fig. 40 is a nomogram showing the relationship between the revolving speed and the torque of the three different shafts linked with the planetary gear 120 in the power output apparatus HOC of the sixth embodiment; Fig. 41 shows structure of another power output apparatus HOD as a modification of the sixth embodiment; Fig. 42 shows structure of still another power output apparatus HOE as another modification of the sixth embodiment; Fig. 43 shows an exemplified structure when the essential structure of the first through the third embodiments is applied to the vehicle with a four-wheel drive; and Fig. 44 shows an exemplified structure when the essential structure of the fifth embodiment is applied to the vehicle with a four-wheel drive. Best Mode for Carrying Out the Invention Preferable modes for embodying the present invention are described hereafter. Fig. 1 is a schematic view illustrating structure of a power output apparatus 20 as a first embodiment according to the present invention; Fig. 2 is a cross sectional view illustrating detailed structures of a clutch motor 30 and an assist motor 40 included in the power output apparatus 20 of Fig. 1; and Fig.3 is a schematic viewillustrating general structure of avehicle with the power output apparatus 20 of Fig. 1 incorporated therein. The general structure of the vehicle is described first for the convenience of description. Referring to Fig. 3, the vehicle is provided with an engine 50 driven by gasoline as a power source. The air ingested from an air supply system via a throttle valve 66 is mixed with fuel, that is, gasoline in this embodiment, injected from a fuel injection valve 51. The air/fuel mixture is supplied into a combustion chamber 52 to be explosively ignited and burned. Linear motion of a piston 54 pressed down by the explosion of the air/fuel mixture is converted to rotational motion of a crankshaft 56. The throttle valve 66 is driven to open and close by an actuator 68. An ignition plug 62 converts a high voltage applied from an igniter 58 via a distributor 60 to a spark, which explosively ignites and combusts the air/fuel mixture. Operation of theengine 50 is controlled by an electronic control unit (hereinafter referred to as EFIECU) 70. The EFIECU 70 receives information fromvarious sensors, which detect operating conditions of the engine 50. These sensors include a throttle position sensor 67 for detecting a valve travel or position BP of the throttle valve 66, a manifold vacuum sensor 72 for measuring a load applied to the engine 50, awater temperature sensor 74 formeasuringthetemperature of cooling water in the engine 50, and a speed sensor 76 and an angle sensor 78 mounted on the distributor 60 for measuring the revolving speed (the number of revolutions per a predetermined time period) and the rotational angle of the crankshaft 56. A starter switch 79 for detecting a starting condition ST of an ignition key (not shown) is also connected to the EFIECU 70. Other sensors and switches connecting with the EFIECU 70 are omitted from the illustration. The crankshaft 56 of the engine 50 is linked with a drive shaft 22 via a clutch motor 30 and an assist motor 40 (described later in detail). The drive shaft 22 further connects with a differential gear 24, which eventually transmits the torque output from the drive shaft 22 of the power output apparatus 20 to left and right driving wheels 26 and 28. The clutch motor 30 and the assist motor 40 are driven and controlledby a controller 80. The controller 80 includes an internal control CPU and receives inputs from a gearshi t position sensor 84 attached to a gearshift 82 and an accelerator position sensor 65 attached to an accelerator pedal 64, as described later in detail. The controller 80 sends and receives a variety of data and information to and from the EFIECU 70 through communication. Details of the control procedure including a communication protocol will be described later. Referring to Fig. 1, the power output apparatus 20 essentially includes the engine 50 for generating power, the clutch motor 30 with an outer rotor 32 and an inner rotor 34, the assist motor 40 with a rotor 42, and the controller 80 for driving and controlling the clutch motor 30 and the assist motor 40. The outer rotor 32 of the clutch motor 30 is mechanically connected to oneend of the crankshaft 56 of theengine 50, whereas the innerrotor 34 thereof is mechanically linked with the rotor 42 of the assist motor 40. Structures of the clutch motor 30 and the assist motor 40 are described briefly. As shown in Fig. 1, the clutch motor 30 is constructed as a synchronous motor having permanent magnets 35 attached to an inner surface of the outer rotor 32 and three-phase coils 36 wound on slots formed in the inner rotor 34. Power is supplied to the three-phase coils 36 via a rotary transformer 38. Laminated sheets of non-directional electromagnetic steel are used to form teeth and slots for the three-phase coils 36 in the inner rotor 34. A resolver 39 for measuring a rotational angle θe of the crankshaft 56 is attached to the crankshaft 56. The resolver 39 may also serve as the angle sensor 78 mounted on the distributor 60. The assist motor 40 is also constructed as a synchronous motor having three-phase coils 44, which are wound on a stator 43 fixed to a casing 45 to generate a revolving magnetic field. The stator 43 is also made of laminated sheets of non-directional electromagnetic steel. A plurality of permanent magnets 46 are attached to an outer surface of the rotor 42. In the assist motor 40, interaction between a magnetic field formed by the permanent magnets 46 and a revolving magnetic field formed by the three-phase coils 44 results in rotations of the rotor 42. The rotor 42 is mechanically linked with the drive shaft 22 working as the torque output shaft of the power output apparatus 20. A resolver 48 for measuring a rotational angle θd of the drive shaft 22 is attached to the drive shaft 22, which is further supported by a bearing 49 held in the casing 45. The inner rotor 34 of the clutch motor 30 is mechanically linked with the rotor 42 of the assist motor 40 and further with the drive shaft 22. The rotation and axial torque of the crankshaft 56 of the engine 50 are accordingly transmitted via the outer rotor 32 and the inner rotor 34 of the clutch motor 30 to the drive shaft 22 while the rotation and torque of the assist motor 40 are added to or subtracted from the transmitted rotation and torque. While the assist motor 40 is constructed as a conventional permanent magnet-type three-phase synchronous motor, the clutch motor 30 includes two rotating elements or rotors, that is, the outer rotor 32 with the permanent magnets 35 mounted thereon and the inner rotor 34 with the three-phase coils 36 attached thereto. The detailed structure of the clutch motor 30 is described according to the cross sectional view of Fig.2. The outer rotor 32 of the clutch motor 30 is attached to a circumferential end of a wheel 57 set around the crankshaft 56, by means of a pressure pin 59a and a screw 59b. A central portion of the wheel 57 is protruded to form a shaft-like element, to which the inner rotor 34 is rotatably attached by means of bearings 37A and 37B. One end of the drive shaft 22 is fixed to the inner rotor 34. A plurality of permanent magnets 35, four in this embodiment, are attached to the inner surface of the outer rotor 32 as mentioned previously. The permanentmagnets 35 are magnetizedinthedirection towards the axial center of the clutch motor 30 and have magnetic poles of alternately inverted directions. The three-phase coils 36 of the inner rotor 34 facing to the permanent magnets 35 across a little gap are wound on a total of 24 slots (not shown) formed in the inner rotor 34. Supply of electricity to the respective coils forms magnetic fluxes running through the teeth (not shown), which separate the slots from one another. Supply of a three-phase alternating current to the respective coils rotates this magnetic field. The three-phase coils 36 are connected to receive electric power supplied from the rotary transformer 38. The rotary transformer 38 includes primary windings 38A fixed to the casing 45 and secondary windings 38B attached to the drive shaft 22 coupled with the inner rotor 34. Electromagnetic induction enables electric power tobe transmitted from the primary windings 38A tothe secondary windings 38B or vice versa. The rotary transformer 38 has windings for the three phases, that is, U, V, and W phases, to allow for the transmission of three-phase electric currents. Interaction between amagnetic field formedbyone adjoiningpair of permanent magnets 35 and a revolving magnetic field formed by the three-phase coils 36 of the inner rotor 34 leads to a variety of behaviors of the outer rotor 32 and the inner rotor 34. The frequency of the three-phase alternating current supplied to the three-phase coils 36 is generally equal to a difference between the revolving speed (the number of revolutions per second) of the outer rotor 32 directly connected to the crankshaft 56 and the revolving speed of the inner rotor 34. This results in a slip between the rotations of the outer rotor 32 and the inner rotor 34. Details of the control procedures of the clutch motor 30 and the assist motor 40 will be described later, based on the flowcharts. As mentioned above, the clutch motor 30 and the assist motor 40 are driven and controlled by the controller 80. Referring back to Fig. 1, the controller 80 includes a first driving circuit 91 for driving the clutch motor 30, a second driving circuit 92 for driving the assist motor 40, a control CPU 90 for controlling both the first and the second driving circuits 91 and 92, and a battery 94 including a number of secondary cells. The control CPU 90 is a one-chip microprocessor including a RAM 90a used as a working memory, a ROM 90b in which various control programs are stored, an input/output port (not shown) , and aserial communication port (not shown) through which data are sent to and received from the EFIECU 70. The control CPU 90 receives a variety of data via the input port. The input data include a rotational angle θe of the crankshaft 56 of the engine 50 measured with the resolver 39, a rotational angle θd of the drive shaft 22 measured with the resolver 48, an accelerator pedal position AP (step-on amount of the accelerator pedal 64) output from the accelerator position sensor 65, a gearshift position SP output from the gearshift position sensor 84, clutch motor currents Iuc and Ivc from two ammeters 95 and 96 disposed in the first driving circuit 91, assist motor currents Iua and Iva from two ammeters 97 and 98 disposed in the second driving circuit 92, and a remaining charge BRM of the battery 94 measured with a remaining charge meter 99. The remaining charge meter 99 may determine the remaining charge BRM of the battery 94 by any known method; for example, by measuring the specific gravity of an electrolytic solution in the battery 94 or the whole weight of the battery 94, by computing the currents and time of charge and discharge, or by causing an instantaneous short-circuit between terminals of the battery 94 and measuring an internal resistance against the electric current. The control CPU 90 outputs a first control signal SWl fordriving six transistors Tri through Tr6 working as switching elements of the first driving circuit 91 and a second control signal SW2 for driving six transistors Trll through Tri6 working as switching elements of the second driving circuit 92. The six transistors Tri through Tr6 in the first driving circuit 91 constitute a transistor inverter and are arranged in pairs to work as a source and a drain with respect to a pair of power lines Ll and L2. The three-phase coils (U,V,W) 36 of the clutch motor 30 are connected via the rotary transformer 38 to the respective contacts of the paired transistors. The power lines Ll and L2 are respectively connected to plus andminus terminals of the battery 94. The first control signal SWl output from the control CPU 90 thus successively controls the power-on time of the paired transistors Tri through Tr6. The electric current flowing through each coil 36 undergoes PWM (pulse width modulation) to give a quasi-sine wave, which enables the three-phase coils 36 to form a revolving magnetic field. The six transistors Trll through Tri6 in the second driving circuit 92 also constitute a transistor inverter and are arranged in the same manner as the transistors Tri through Tr6 in the first driving circuit 91. The three-phase coils (U,V,W) 44 of the assist motor 40 are connected to the respective contacts of the paired transistors. The second control signal SW2 output from the control CPU 90 thus successively controls the power-on time of the paired transistors Trll through Trl6. Theelectric current flowing through each coil 44 undergoes PWM to give a quasi-sine wave, which enables the three-phase coils 44 to form a revolving magnetic field. The power output apparatus 20 thus constructed works in accordance withthe operationprinciples discussed below, especially with the principle of torque conversion. By way of example, it is assumed that the crankshaft 56 of the engine 50 driven by the EFIECU 70 rotates at a revolving speed (the number of revolutions per a predetermined timeperiod) Ne, whichis equal to a predeterminedvalue NI. In the description below, the revolving speed Ne of the crankshaft 56 is also referred to as the revolving speed Ne of the engine 50. While thetransistors Tri throughTr6 inthe firstdriving circuit 91 are in OFF position, the controller 80 does not supply any electric current to the three-phase coils 36 of the clutch motor 30 via the rotary transformer 38. No supply of electric current causes the outer rotor 32 of the clutch motor 30 to be electromagnetically disconnected from the inner rotor 34. This results in racing the crankshaft 56 of the engine 50. Under the conditionthat all the transistors Tri throughTr6 are in OFFposition, there is no regeneration of energy from the three-phase coils 36, and the engine 50 is kept at an idle. As the control CPU 90 of the controller 80 outputs the first control signal SWl to control on and off the transistors Tri through Tr6 inthe first driving circuit 91, aconstant electriccurrent flows through the three-phase coils 36 of the clutch motor 30, based on the difference between the revolving speed Ne of the engine 50 and a revolving speed Nd of the drive shaft 22 (in other words, a difference Nc (= Ne-Nd) between therevolving speed of theouter rotor 32 and that of the inner rotor 34 in the clutch motor 30) . A certain slip accordingly exists between the outer rotor 32 and the inner rotor 34 connected with each other in the clutch motor 30. At this moment, the inner rotor 34 rotates at the revolving speed Nd, which is lower than the revolving speed Ne of the engine 50. In this state, the clutch motor 30 functions as a generator and carries out the regenerative operation to regenerate an electric current via the first driving circuit 91. In order to allow the assist motor 40 to consume energy identical with the electrical energy regenerated by the clutch motor 30, the control CPU 90 controls on and off the transistors Trll through Trl6 in the second driving circuit 92. The on-off control of the transistors Trll through Trl6 enables an electric current to flow through the three-phase coils 44 of the assist motor 40, and the assist motor 40 consequently carries out the power operation to produce a torque. Referring to Fig. 4, in the power output apparatus 20, when the engine 50 is driven at a first driving point Pl, where the engine speed Ne is equal to a predetermined revolving speed Nl and an engine torque Te is equal to a predetermined value Tl, the clutch motor 30 carries out the regenerative operation to produce an energy defined by a first area Gl. The energy of the first area Gl is supplied to the assist motor 40 as an energy defined by a second area G2. The drive shaft 22 is accordingly driven at a second driving point P2, where the drive shaft speed Nd is equal to a predetermined revolving speed N2 and a drive shaft torque Td is equal to a predetermined value T2. The torque conversion is carried out in the manner discussed above, and the energy corresponding to the slip in the clutch motor 30 or the revolving speed difference Nc (= Ne-Nd) is consequently given as a torque to the drive shaft 22. In accordance with another example, it is assumed that theengine 50 is driven at the second driving point P2, where the engine speed Ne is equal to the predetermined revolving speed N2 and the engine torque Te is equal to the predetermined value T2, while the revolving speed Nd of the drive shaft 22 is equal to the predetermined revolving speedNl, which is greater thanthe revolving speedN2. Inthis state, the inner rotor 34 of the clutch motor 30 rotates relative to the outer rotor 32 in the direction of rotation of the drive shaft 22 at a revolving speed defined by the absolute value of the revolving speed difference Nc (= Ne-Nd). The clutch motor 30 accordingly functions as a normal motor and consumes electric power to supply the energy of rotational motion to the drive shaft 22. When the control CPU 90 of the controller 80 controls the second driving circuit 92 to enable the assist motor 40 to regenerate electrical energy, a slip between the rotor 42 and the stator 43 of the assist motor 40 makes the regenerative current flow through the three-phase coils 44. In order to allow the clutch motor 30 to consume theenergy regenerated by the assist motor 40, the control CPU 90 controls both the first driving circuit 91 and the second driving circuit 92. This enables the clutch motor 30 to be driven without using electric power stored in the battery 94. Referring back to Fig. 4, when the engine 50 is driven at the second driving point P2, where the revolving speed Ne=N2 and the torque Te=T2, the assist motor 40 regenerates anenergy corresponding to the sum of the second area G2 and a third area G3. The energy of the areas G2 and G3 is supplied to the clutch motor 30 as an energy defined by the sum of the first area Gl and the third area G3. The drive shaft 22 is accordingly driven at the first driving point Pl, where the revolving speed Nd=Nl and the torque Td=Tl. Other than the torque conversion discussed above, the power output apparatus 20 of the embodiment can charge the battery 94 with an excess of electrical energy or discharge the battery 94 to supplement the electrical energy. This is implemented by controlling the mechanical energy output from the engine 50 (that is, the product of the torque Te and the revolving speed Ne) , the electrical energy regenerated or consumed by the clutch motor 30, and the electrical energy consumedor regenerated by the assistmotor 40. The power (energy) output from the engine 50 can thus be transmitted to the drive shaft 22 at a higher efficiency. The concreteprocedureof torque conversion executedbythe power output apparatus 20 is described accordingtoatorquecontrol routine shown in the flowchart of Fig. 5. The torque control routine is executed repeatedly at predetermined time intervals after thedriver has started the vehicle. When the program enters the torque control routine, the control CPU 90 of the controller 80 first receives data of revolving speed Nd of the drive shaft 22 at step S100. The revolving speed Nd of the drive shaft 22 can be computed from the rotational angle θd of the drive shaft 22 read from the resolver 48. At subsequent step S101, the control CPU 90 reads the accelerator pedal position AP detected by the accelerator position sensor 65. The driver steps on the accelerator pedal 64 when feeling insufficiency of output torque. The value of the accelerator pedal position AP accordingly represents the desired output torque (that is, desired torque of the drive shaft 22) which the driver requires. The program then goes to step S102 at which the control CPU 90 computes a target output torque Td* corresponding to the input accelerator pedal positionAP. The target output torque Td* implies a target torque to be output to the drive shaft 22 and is hereinafter referred to as the Output torque command value', in this embodiment, output torque command values Td* corresponding to the respective accelerator pedal positions AP have been set in advance and stored in the ROM 90b. In response to an input of the accelerator pedal position AP, the output torque commandvalue Td* corresponding tothe input acceleratorpedal position AP is extracted from the output torque command values Td* stored in the ROM 90b. At step S103, an amount of energy Pd (target energy) to be output to the drive shaft 22 is calculated from the extracted output torque command value Td* and the input revolving speed Nd of the drive shaft 22 accordingtotheequationof Pd= Td*xNd. The programthenproceeds to step S104 at which the control CPU 90 sets a target engine torque Te* and a target engine speed Ne* of the engine 50 based on the output energy Pd thus obtained. The energy supplied from the engine 50 is equal to the product of the torque Te and the revolving speed Ne of the engine 50, so that the relationship between the output energy Pd, the target engine torque Te*, and the target engine speed Ne* can be defined as Pd = Te*xNe*. There are, however, numerous combinations of the target engine torque Te* and the target engine speed Ne* of the engine 50 satisfying the above relationship. In this embodiment, favorable combinations of the target torque Te* and the target revolving speed Ne* of the engine 50 are experimentally or otherwise determined in advance for the respective amounts of output energy Pd. In such favorable combinations, the engine 50 is driven at highest possible efficiency and the driving state of the engine 50 is smoothly varied with a variation in output energy Pd. The predetermined favorable combinations are stored in the form of a map in the ROM 90b. In practice, the target torque Te* and the target revolving speed Ne* of the engine 50 corresponding to the output energy Pd obtained at step S103 is read from the map at step S104. The following gives a further description of the map. Fig.6 is a graph showing driving points of the engine 50 (defined by the target engine torque Te* and the target engine speed Ne*) with their efficiencies. The curve B in Fig. 6 represents a boundary of an engine-operable range, in which the engine 50 can be driven. In the engine-operable range, efficiency curves, such as curves αl through ct6, can be drawn by successively joining the driving points having the identical efficiency. In the engine-operable range, constant-output energy curves, such as curves Cl-Cl through C3-C3, can also be drawn. On each curve of constant-output energy, the energy output from the engine 50 and defined as the product of the torque Te and the revolving speed Ne is constant. The graph of Fig. 7 shows the efficiency of the respective driving points along the constant-output energy curves Cl-Cl through C3-C3 plotted against the revolving speed Ne of the engine 50. Referring to Fig. 7, the efficiency with respect to the same output energy from the engine 50 is significantly varied by the driving point of the engine 50. On the constant-output energy curve Cl-Cl, for example, the efficiency of the engine 50 reaches its maximum when the engine 50 is driven at a driving point Al (torque Tel and revolving speed Nel) . Such a driving point attaining the highest possible efficiency exists on each constant-output energy curve; a driving point A2 for the constant-output energy curve C2-C2 and a driving point A3 for the constant-output energy curve C3-C3. The curve A in Fig. 6 is obtained by joining such driving points attaining the highest possible efficiency of the engine 50 for the respective amounts of output energy Pd by a continuous curve. The map representing the relationship between each driving point (torque Te and revolving speed Ne) on the curve A and the output energy Pd is used at step S104 in the flowchart of Fig. 5 for setting the target torque Te* and the target revolving speed Ne* of the engine 50. The curve Ashould be continuous because of the followingreason. In case that discontinuous curves are used to set the driving point of the engine 50 against a variation in output energy Pd, the driving state of the engine 50 is abruptly varied with a variation in output energy Pdcrossingoverthe discontinuous driving points. Theabrupt variation may prevent the driving state from being smoothly shifted to a target level and thereby cause knocking or another undesirable condition. Each driving point on the curve A may accordingly not correspond to the driving point attaining the highest possible efficiency on the curve of output energy Pd = constant. After setting the target torque Te* and the target revolving speed Ne* of the engine 50, the program proceeds to steps S108, SHO, and SHI to control the clutch motor 30, the assist motor 40, and the engine 50 based on the target engine torque Te* and the target engine speed Ne*, respectively. As a matter of convenience of illustration, the control operations of the clutch motor 30, the assist motor 40, and the engine 50 are shown as separate steps. In the actual procedure, however, these control operations are carried out simultaneously. By way of example, the control CPU 90 simultaneously controls the clutch motor 30 and the assist motor 40 by utilizing an interrupting process, while transmitting an instruction to the EFIECU 70 through communication in order to allow the EFIECU 70 to control the engine 50 concurrently. The concrete procedures of the control are described below. Fig. 8 is a flowchart showing details of the control process of the clutch motor 30 executed at step S108 in the flowchart of Fig. 5. When the program enters the clutch motor control routine, the control CPU 90 of the controller 80 first reads the revolving speed Ne of theengine 50 atstep S112. The revolving speed Neof theengine 50 may be calculated from the rotational angle θe of the crankshaft 56 read fromthe resolver 39 or directlymeasuredwiththe speed sensor 76 mounted on the distributor 60. In case that the speed sensor 76 is used, the control CPU 90 receives data of revolving speed Ne from the EFIECU 70 connecting with the speed sensor 76 through communication. At subsequent stepS113, atarget clutch torqueor torque command value Tc* of the clutch motor 30 is then calculated according to Equation (1) given below: Tc* = kc(Ne - Ne*) + Te* (1) wherein kc represents a coefficient of proportionality. The torque command value Tc* of the clutch motor 30 is varied according to the deviation of the actual revolving speed Ne of the engine 50 from the target engine speed Ne*, in order to enable the engine 50 to be driven stably at thedriving point of the target engine torque Te* andthe target engine speed Ne*. Even when it is desirable to drivethe engine 50 atthe drivingpoint ofthe targetenginetorque Te* and the target engine speed Ne*, since the torque Te of the engine 50 corresponds to the reaction against the loading torque Tc of the clutch motor 30, the function of the engine 50 alone does not allow the engine 50 to be driven at the desired driving point. Compared with the clutch motor 30 and the assist motor 40, the driving state of the engine 50 is fluctuated more significantly. Even when the actual torque Tc of the clutch motor 30 is set equal to the target engine torque Te* and the torque Te of the engine 50 is thereby made identical with the target engine torque Te*, the revolving speed Ne of the engine 50 may not be coincident with the target engine speed Ne*. The structure of the embodiment accordingly does not set the torque command value Tc* of the clutch motor 30 equal to the target engine torque Te* of the engine 50, but introduces a correction term based on the difference between the actual revolving speed Ne of the engine 50 and the target engine speed Ne*. The graph of Fig. 9 shows the relationship between the torque command value Tc* of the clutch motor 30 and the target torque Te* and the target revolving speed Ne* of the engine 50. Referring to Fig. 9, when the revolving speed Ne of the engine 50 is greater than the target engine speedNe*, the torque commandvalue Tc* is setequal to a value greater than the target engine torque Te*, in order to allow the clutch motor 30 to reduce the revolving speed Ne of the engine 50. When the revolving speed Ne of the engine 50 is less than the target engine speed Ne*, onthe contrary, the torque commandvalue Tc* is set equal to a value smaller than the target engine torque Te*, in order to allow the clutch motor 30 to enhance the revolving speed Ne of the engine 50. Referring back to the flowchart of Fig. 8, the control CPU 90 reads the rotational angle θd of the drive shaft 22 from the resolver 48 at step S114 and the rotational angle θe of the crankshaft 56 of the engine 50 from the resolver 39 at step S115. The control CPU 90 then computes a relative angle θc of the drive shaft 22 to the crankshaft 56 by the equation of θc= θe-θd at step S116. The program proceeds to step S118, at which the control CPU 90 reads the clutch motor currents luc and Ivc, which respectively flow through the U phase and V phase of the three-phase coils 36 in the clutch motor 30 and are measured by the ammeters 95 and 96. Although the currents naturally flow through all the three phases U, V, and W, measurement is required only for the currents passing through the two phases since the sum of the currents is equal to zero. At subsequent step S120, the control CPU 90 executes transformation of coordinates (three-phase to two-phase transformation) using the values of currents flowing through the three phases obtained at step S118. The transformation of coordinates maps the values of currents flowing through the three phases to the values of currents passing through d and q axes of the permanent magnet-type synchronous motor and is executed according to Equation (2) given below: Ide f-sin(flc - 120) sinøc ' luc ' ] Iqc = 2 cos(0c - 120) cos 0c Ivc] (2) The transformation of coordinates is carried out because the currents flowing through the d and q axes are essential for the torque control in the permanent magnet-type synchronous motor. Alternatively, the torque control may be executed directly with the currents flowing through thethree phases. After the transformation to the currents of two axes, the control CPU 90 computes deviations of currents Ide and Iqc actually flowing through the d and q axes from current command values Ide* and Iqc* of the respective axes, which are calculated from the torque command value Tc* of the clutch motor 30, and subsequently determines voltage command values Vdc and Vqc with respect to the d and q axes at step S122. in accordance with a concrete procedure, the control CPU 90 executes arithmetic operations of Equations (3) and Equations (4) given below: A de = Ide * -Ide Alqc = Iqc * —Iqc (3) Vdc = Kpl ■ Aide + ∑Ki\ ■ Aide Vqc = Kp2 ■ Alqc + ∑ Ki2 ■ Alqc (4) wherein Kpl , Kp2 , Kil , and Ki2 represent coefficients , which are adjusted to be suited to the characteristics of the motor applied. Each voltage command value Vdc (Vqc) includes a part in proportion to the deviation Δl from the current command value I* (the first term in the right side of Equation (4) ) and a summation of historical data of thedeviations Δl for 'i' times (the secondtermin the right side) . The control CPU 90 then re-transforms the coordinates of the voltage command values thus obtained (two-phase to three-phase transformation) at step S124. This corresponds to an inverse of the transformation executed at step S120. The inverse transformation determines voltages Vuc, Vvc, and Vwc actually applied to the three-phase coils 36 as expressed by Equations (5) given below: Vuc cosθc - sinøc Vdc ' Vvc ■ S cos(0c - 120) sin(0c - 120) Vqc Vwc = -Vuc — Vvc (5) The actual voltage control is accomplished by on-off operation of the transistors Tri through Tr6 in the first driving circuit 91. At step S126, the on- and off-time of the transistors Tri through Tr6 in the first driving circuit 91 is PWM (pulse width modulation) controlled in order to attain the voltage command values Vuc, Vvc, and Vwc determined by Equations (5) above. The torque command value Tc* is positive when a positive torque is applied to the drive shaft 22 in the direction of rotation of the crankshaft 56. Bywayof example, it is assumed that a positivevalue is set to the torque command value Tc*. When the revolving speed Ne of the engine 50 is greater than the revolving speed Nd of the drive shaft 22 on this assumption, that is, when the revolving speed difference Nc (= Ne-Nd) is positive, the clutchmotor 30 is controlled to carry out the regenerative operation and produce a regenerative current according to the revolving speed difference Nc. when the revolving speed Ne of the engine 50 is lower than the revolving speed Nd of the drive shaft 22, that is, when the revolving speeddifference Nc (= Ne-Nd) is negative, on the contrary, the clutch motor 30 is controlled to carry out the power operation and rotate relative to the crankshaft 56 in the direction of rotation of the drive shaft 22 at a revolvingspeed defined by the absolute value of the revolving speed difference Nc. Forthe positive torque command value Tc*, both the regenerative operationandthepoweroperation of theclutchmotor 30 implement the identical switching control. In accordance with a concrete procedure, the transistors Tri through Tr6 of the first driving circuit 91 are controlled to enable a positive torque to be applied to the drive shaft 22 by the combination of the magnetic field generated by the permanent magnets 35 set on the outer rotor 32 with the revolving magnetic field generated by the currents flowing through the three-phase coils 36 mounted on the inner rotor 34 of the clutch motor 30. The identical switching control is executed for both the regenerative operation and the power operation of the clutch motor 30 as long as the sign of the torque command value Tc* is not changed. The clutch motor control routine of Fig. 8 is thus applicabletoboththeregenerative operation andthepoweroperation. Under theconditionofbrakingthedrive shaft 22 ormovingthe vehicle in reverse, the torque command value Tc* has the negative sign. The clutch motor control routine of Fig. 8 is also applicable to the control procedure under such conditions, when the relative angle θc obtained at step S116 is varied in the reverse direction. Figs. 10 and 11 are flowcharts showing details of the torque control process of the assist motor 40 executed at step SHO in the flowchart of Fig.5. When the programenters the assist motorcontrol routine, the control CPU 90 first reads the revolving speed Nd of the drive shaft 22 at step S131 and the revolving speed Ne of the engine 50 at step S132. Thecontrol CPU 90 thencalculates a revolving speed difference Nc between the input data of revolving speed Nd of the drive shaft 22 and revolving speedNe of the engine 50 (Nc =Ne-Nd) at step S133, and checks the sign of the revolving speed difference Nc thus obtained at step S134. When the revolving speed difference Nc has the positive sign, the clutch motor 30 is under regenerative control and the program proceeds to step S135, at which a power Pc regenerated by the clutch motor 30 is calculated according to Equation (6) given below. At subsequent step S136, the control CPU 90 calculates a torque command value Ta* of the assist motor 40 that consumes the regenerative power Pc, according to Equation (7) given below: Pc = Ksc x Nc x Tc* (6) Ta* = Ksa x Pc / Nd (7) wherein Ksc in Equation (6) represents the efficiency of the clutch motor 30 and Ksa in Equation (7) the efficiency of the assist motor 40. When the revolving speed difference Nc has the negative sign, on the other hand, the clutch motor 30 is under power control and the program proceeds to step S137, at which a power Pc consumed by the clutch motor 30 is calculated according to Equation (8) given below. At subsequent step S138, the control CPU 90 calculates a torque command value Ta* of the assist motor 40 that regenerates the consumed power Pc, according to Equation (9) given below. When the revolving speeddifference Nc is negative, the powerPc and thetorque command value Ta* are also negative and the assist motor 40 applies the torque in reverse of the rotation to the drive shaft 22. The assist motor 40 is accordingly under regenerativecontrol. Although the efficiency Ksc of the clutch motor 30 and the efficiency Ksa of the assist motor 40 in Equations (6) and (7) are also included in Equations (8) and (9), different efficiencies may be applied to the regenerative operation and the power operation in motors having different efficiencies of regenerative operation and power operation. Pc = (1 / Ksc) x Nc x TC* (8) Ta* = (1 / Ksa) x Pc / Nd (9) After the processing of step S136 or S138, the program proceeds to step S139, at which the torque command value Ta* thus calculated is compared with a maximum torque Tamax which the assist motor 40 can apply. When the torque command value Ta* exceeds the maximum torque Tamax, the torque command value Ta* is restricted to and set equal to the maximum torque Tamax at step S140. The control CPU 90 then reads the rotational angle θ of thedrive shaft 22 from the resolver 48 at step S141, andreceives dataof assist motor currents lua and Iva at step S142, which respectively flow through the U phase and V phase of the three-phase coils 44 in the assist motor 40 and are measured with the ammeters 97 and 98. The control CPU 90 then executes transformation of coordinates for the currents of the three phases at step S144, computes voltage command values Vda and Vqa at step S146, and executes inverse transformation of coordinates for the voltage command values at step S148. At subsequent step S150, the control CPU 90 determines the on- and off-time of the transistors Trll through Trl6 in the second driving circuit 92 for PWM (pulse width modulation) control. The processing executed at steps S144 through S150 is similar to that executed at steps S120 through S126 of the clutch motor control routine shown in the flowchart of Fig. 8. The control of the engine 50 (step SHI in the flowchart of Fig. 5) is executed in the followingmanner. In ordertoattainstationary driving at the driving point defined by the target engine torque Te* and the target engine speed Ne* set at step S104 inFig.5, the control CPU 90 regulates thetorque Te and the revolving speedNe of the engine 50. In accordance with a concrete procedure, the control CPU 90 transmits the target torque Te* and the target revolving speed Ne* of the engine 50 to the EFIECU 70 through communication, , and the EFIECU 70 controls the position of the throttle valve 66 and fuel injection from the fuel injection valve 51 based on the target engine torque Te* and the target engine speed Ne*. The position of the throttle valve 66 is controlled, for example, according to athrottle valve position control routine shown in Fig. 12, whereas the fuel injection control is carried out, for example, according to a fuel injection control routine shown in Fig. 13. These routines are repeatedly executed at predetermined time intervals. The following describes these routines in brief. When the program enters the throttle valve position control routine shown in the flowchart of Fig. 12, the EFIECU 70 first reads the position BP of the throttle valve 66 measured with the throttle valve position sensor 67 at step S152 and the revolving speed Ne of the engine 50 at step S154. The revolving speed Ne of the engine 50 supplied to the EFIECU 70 is typically measured with the speed sensor 76 mounted on the distributor 60. In case that the revolving speed Ne of the engine 50 is calculated from the rotational angle θe of the crankshaft 56 read from the resolver 39, the EFIECU 70 receives the data of rotational angle θe from the controller 80 through communication. At subsequent step S156, the EFIECU 70 sets a standard position BPF of the throttle valve 66 based on the output energy Pd obtained at step S103 in the flowchart of Fig. 5. In this embodiment, the positions BP of the throttle valve 66 to attain stationary driving of the engine 50 at the driving point of the target engine torque Te* and the target engine speed Ne* are experimentally or otherwise determines for the respective amounts of output energy Pd. The relationship thus obtained is previously stored as a map in a ROM (not shown) included in the EFIECU 70. The position BPcorresponding to the given output energy Pd is read from the map as the standard position BPF. At step S158, the EFIECU 70 then calculates a position command value BP* from the standard position BPF, the revolving speed Ne, and the target engine speed Ne* according to Equation (10) given below: BP* = ke(Ne* - Ne) + BPF (10) wherein ke represents a constant of proportionality. The position control value BP* is set in this manner, so that the engine 50 is stably driven at the target engine speed Ne*. After setting the position command value BP*, the EFIECU 70 subtracts theposition BP from theposition commandvalue BP* toyield a difference ΔBP at step S160. The actuator 68 then works to drive the throttle valve 66 by the difference ΔBP at step S162, and the program exits from the routine. The following describes the fuel injection control executed according tothe fuel injectioncontrol routine shown inthe flowchart of Fig.13. When the programenters the routine, the EFIECU 70 first receives data of revolving speed Ne of the engine 50 at step S164 and an amount of intake air Q at step S166. The amount of intake air Q can be calculated from the negative pressure in an intake manifold measured by the manifold vacuum sensor 72 and the revolving speed Ne of the engine 50. At step S168, the EFIECU 70 then calculates a standard amount of fuel injection TP from the resolving speedNe and the amount intake air Q input at steps S164 and S166 according to Equation (11) given below: TP = kt • Q / Ne (11) An actual amount of fuel injection TAU is then calculated at step S170 by multiplying the standard amount of fuel injection TP by required correction coefficients according to Equation (12) given below: TAU = TP • FAF • FWL CC (12) FAF, which represents an air/fuel ratio correction coefficient based on a lean-rich state of an air/fuel mixture detected by an air/fuel ratio sensor (not shown) , gradually increases by integration until the output of the air/fuel ratio sensor reaches a value corresponding to the rich state of the air/fuel mixture, and then gradually decreases by integration until the output reaches a value corresponding to the lean state. FWL represents a warm-up increase correction coefficient and takes a value equal to or greater than 1.0 when the temperature of cooling water is not higher than 60°C. α and β represent other correction coefficients relating to, for example, intake temperature correction, transient correction, and power voltage correction. After calculating the actual amount of fuel injection TAU at step S170, the program proceeds to step S172 to set a fuel injection time corresponding to the actual amount of fuel injection TAU on a counter (not shown) that determines an opening time period, for which the fuel injection valve 51 is open. The program then enters a fuel injection valve driving routine (not shown) to drive and open the fuel injection valve 51 for the opening time period preset on the counter and to enable a required amount of fuel to be injected into an intake port of the engine 50. As discussed above, the power output apparatus 20 of the embodiment selects a driving point attaining the highest possible efficiency among the respective driving points on each curve of constant energy output from the engine 50, and sets the torque Te and the revolvingspeedNe at each selecteddrivingpoint as the target engine torque Te* and the target engine speed Ne*. This enhances the efficiency of the engine 50 and thereby the efficiency of the whole power output apparatus 20. The selected driving points regarding the respective amounts of output energy can be joined with one another to form a continuous curve. The driving point of the engine 50 can thus be varied smoothly witha small amount ofvariation in output energy Pd. This structure effectively prevents the engine 50 from undesirably stalling or stopping. The power output apparatus 20 of the embodiment sets the torque command value Tc* of the clutch motor 30 to lessen the difference between the actual revolving speed Ne of the engine 50 and the target engine speed Ne*. This structure enables the engine 50 to be stably driven at the target engine speed Ne*. Adjustment of the position BP of the throttle valve 66 is also carried out to reduce the difference between the actual revolving speed Ne of the engine 50 and thetarget engine speed Ne*. Such adjustment further facilitates the stable operation of the engine 50 at the target engine speed Ne*. Although the driving points set as the target engine torque Te* and the target engine speed Ne* are continuous with respect to the amount of output energy in the power output apparatus 20 of the embodiment, they may be discontinuous as long as an abrupt change of the driving point of the engine 50 can be effectively avoided. As mentioned above, the power output apparatus 20 of the embodiment sets the torque command value Tc* of the clutch motor 30 to lessen the difference between the actual revolving speed Ne of the engine 50 and the target engine speed Ne*. The target torque Te* of the engine 50 may, however, be directly set as the torque command value Tc*. The position command value BP* of the throttle valve 66 is also set to decrease the difference between the actual revolving speed Ne of the engine 50 and the target engine speed Ne*. The standard position BPF may, however, be directly set as the position command value BP*. The power output apparatus 20 of the embodiment selects adriving point attaining the highest possible efficiency among the respective driving points on eachcurve of constant energyoutput fromthe engine 50, and sets the torque Te and the revolving speed Ne at each selected driving point as the target engine torque Te* and the target engine speed Ne*. Another possible structure sets the torque Te and the revolving speed Ne of a selected driving point, which attains the best possible emission among the respective driving points on each constant-output energy curve, as the target engine torque Te* and the target engine speed Ne*. This structure further improves the emission of the engine 50. Still another possible structure sets the torque Te and the revolving speed Ne of a selected driving point, which has the smallest possible driving noise among the respective driving points on each constant-output energy curve, as the target engine torque Te* and the target engine speed Ne*. This structure further lessens the driving noise of the engine 50. In the power output apparatus 20 of the embodiment, a map of the selecteddrivingpoint, which attains the highest possibleefficiency among the respective driving points on each constant-output energy curve, is used to set the target engine torque Te* and the target engine speed Ne*. An alternative structure provides a plurality of maps, such as a map of a driving point with the highest efficiency, that of a driving point with the best emission, and that of a driving point with the smallest driving noise, and selects an appropriate map according to the environment in which the vehicle runs. By way of example, a map of the driving point with the highest efficiency is selected while the vehicle runs in the suburbs; and a map of the driving point with the best emission is selected while the vehicle runs in the town. This structure realizes the appropriate operation of the engine 50 based on the environment in which the vehicle runs. The user may select an appropriate map, for example, with a press of a selection button. The power output apparatus 20 of the embodiment joins the selected drivingpoints, whichattain the highest possibleefficiency among the respective driving points on the curves of constant energy output from the engine 50, with one another to yield a continuous curve, so as to provide a map for setting the target torque Te* and the target revolving speed Ne* of the engine 50. In accordance with another possible structure, however, the target engine torque Te* and the target engine speed Ne* may be set out of a predetermined range of driving points of the engine 50. In case that the vehicle or another equipment mounted on the vehicle resonates in a specific range of revolving speed of the engine 50, for example, it may be desirable to avoid driving points in this specific range of revolving speed (that is, resonant revolving speed range NF) when setting the target engine torque Te* and the target engine speedNe*. The driving points of the engine 50 for setting the target torque Te* and the target revolving speed Ne* in the resonant revolving speed range NF are given as a curve D in Fig. 14. The curve D is identical with the curve A in the graph of Fig. 6, except an area in the vicinity of the resonant revolving speed range NF, that is a range in which the revolving speed Ne of the engine 50 is varied from Nefl to Nef2. Fig. 15 is an enlarged view showing the area in the vicinity of the resonant revolving speed range NF. Referring to Fig. 15, the curve D goes as points Dl, D2, D3, and D4 in the resonant revolving speed range NF and in its vicinity. A curve E-E including the points D2 and D3 is a constant-output energy curve passing through a driving point Pef, which is on the curve A in the graph of Fig. 6 at a median of the resonant revolving speed range NF. Variation in target engine torque Te* and target engine speed Ne* on the curve D shifts the driving point of the engine 50 from the point D2 to the point D3 or vice versa when the output energy Pd is varied across the curve E-E. This causes an abrupt change of the driving point of the engine 50. Unless the resonant revolving speed range NF is significantly wide, the condition of the engine 50 can smoothly shift to the new driving point without stalling or stopping the engine 50. Setting the target torque Te* and the target revolving speed Ne* of the engine 50 out of the resonant revolving speed range NF can effectively prevents the vehicle or another equipment mounted on the vehicle from resonating. In the embodiment discussed above, the target torque Te* and the target revolving speed Ne* of theengine 50 are setout of the resonant revolving speed range NF in which the vehicle or another equipment mounted on the vehicle resonates. Other possible structures may, however, set the target torque Te* and the target revolving speed Ne* of the engine 50 out of a predetermined torque range, out of a predetermined revolving speed range and predetermined torque range (that is, a predetermined range of driving points) , or out of a range of driving points in which the vehicle or another equipment mounted on the vehicle resonates with the driving noise of the engine 50. The following describes another power output apparatus 20A as a second embodiment according to the present invention. The structure of the power output apparatus 20A of the second embodiment is identical with that of the power output apparatus 20 of the first embodiment, and is thus not described specifically. The numerals and symbols used in the description of the first embodiment have the same meanings in the second embodiment, unless otherwise specified. The torque control in the poweroutput apparatus 20Aof the second embodiment is carried out byexecuting a torque control routine shown in the flowchart of Fig. 16, instead of the torque control routine in the flowchart of Fig. 5 executed by the power output apparatus 20 of the first embodiment. When the program enters the torque control routine of Fig. 16, the control CPU 90 of the controller 80 first reads the revolving speed Ne of the engine 50 at step S180 and the revolving speed Nd of the drive shaft 22 at step S182. The control CPU 90 then reads the accelerator pedal position AP measured by the accelerator position sensor 65 at step S184, and determines the output torque command value Td* based on the input accelerator pedal position AP at step S186. The control CPU 90 calculates a desired output energy Pd by multiplying theoutput torquecommandvalue Td* bythe revolving speed Nd of the drive shaft 22 at step S188, and subsequently calculates an actual output energy Pe of the engine 50 by multiplying the torque command value Tc* of the clutch motor 30 by the revolving speed Ne of the engine 50 at step S190. The program then proceeds to step S192 to calculate a difference ΔPd between the desired output energy Pd and the actual output energy Pe. The torque command value Tc* of the clutch motor 30 is used for the calculation of the energy Pe actually output romtheengine 50, because thetorqueTeof theengine 50 is not easily measured while it can be assumed that the torque Te of the engine 50 is equivalent to the torque Tc of the clutch motor 30, that is, the torque commandvalue Tc*. Thepoweroutput apparatus 20A ofthe secondembodiment alsocarries out the clutchmotorcontrol routine of the first embodiment shown in the flowchart of Fig. 8 as described later, so that the torque command value Tc* of the clutch motor 30 is set at step S113 in the flowchart of Fig. 8. The difference ΔPd is then compared with a predetermined threshold value Pref at step S194. The threshold value Pref is set as an energy difference between driving points, which can smoothly increase the energy Pe output from the engine 50 without stalling or stopping the engine 50. The threshold value Pref depends upon the characteristics of the engine and the map of the driving point. When the difference ΔPd is greater than the threshold value Pref at step S194, the program determines that the shift from the current driving point of the engine 50 to another driving point corresponding to the output energy Pd can not be implemented smoothly. The program accordingly proceeds to step S196 to add the threshold value Pref to the energy Pe actually output from the engine 50 and set a new output energy Pd, prior to the processing of step S200. When the difference ΔPd is equal to or less than the threshold value Pref, on the contrary, the program determines that the shift from the current driving point of the engine 50 to another driving point corresponding to the output energy Pd can be implemented smoothly, and directly goes to step S200. At stepS200, the targettorque Te* and thetarget revolving speed Ne* of the engine 50 are set using the preset output energy Pd and a map corresponding to that of the first embodiment shown in Fig. 6. Based on the target engine torque Te* and the target engine speed Ne* thus obtained, the clutch motor 30, the assist motor 40, and the engine 50 are controlled respectively at steps S202, S204, and S206. The concrete procedures of steps S202 through S206 are identicalwith those of steps S108 through SHI of the first embodiment shown in the flowchart of Fig. 5. Like the first embodiment, although the control operations of the clutch motor 30, the assist motor 40, and the engine 50 are shown as separate steps for thematterof convenience, these controls are carried out simultaneously in the actual procedure. This torque control enables the engine 50 to smoothly increase its output energy Pe, even when the driver steps on the accelerator pedal 64 to a relatively large depth. Fig. 17 shows the variations against time under such conditions. Referring to Fig. 17, by way of example, the driver steps on the accelerator pedal 64 to a relatively large depth at a time point tl to change the accelerator pedal position AP from a value API to another value AP2. The energy Pe actually output from the engine 50 has values Pel and Pe2 respectively corresponding to the values API and AP2 of the accelerator pedal position AP. It is assumed here that the difference between the values Pel and Pe2 is remarkably larger than the threshold value Pref. The output energy Pd is then set at step S196 in the flowchart of Fig. 16 by adding the threshold value Pref to the energy Pe actually output from the engine 50, and is used for the control procedures of the engine 50, the clutch motor 30 , and the assist motor 40. The energy Pe actually output from the engine 50 and the torque Tc of the clutch motor 30 thus increase little by little. The repetition of this processing enables the actual output energy Pe of the engine 50 to approach the value Pe2 and makes the difference betweenthe actual output energy Pe andthe value Pe2 equal to or less than the threshold value Pref. under such conditions, the output energy Pd corresponding to the step-on amount of the accelerator pedal 64 is directly used for the control operations of the engine 50, the clutch motor 30, and the assist motor 40. The actual output energy Peof theengine 50 eventuallybecomes coincident with the value Pe2 at a time point t2. When the driver steps on the accelerator pedal 64 by arelatively large amount, the power output apparatus 20A of the secondembodiment controls the engine 50, the clutch motor 30, and the assist motor 40, based on the output energy at a driving point towhich the engine 50 can smoothly shift from the current driving point, instead of the output energy Pd corresponding to the step-on amount of the accelerator pedal 64. This structure enables the driving point of the engine 50 to be smoothly shifted to the driving point giving the output energy Pd corresponding to the step-on amount of the accelerator pedal 64, thereby effectively preventing the engine 50 from stalling or stoppingdue to an abrupt change of the drivingpoint of the engine 50. Like the first embodiment, the power output apparatus 20A of the second embodiment sets the torque Te and the revolving speed Ne at a specific driving point, which attains the highest possible efficiency among the respective driving points on each constant- output energy curve of the engine 50, as the target engine torque Te* and the target engine speed Ne*. This further enhances the operation efficiency of the engine 50 and thereby improves the efficiency of the whole power output apparatus 20A. Although the power output apparatus 20A of the second embodiment uses the predetermined thresholdvalue Pref, the thresholdvaluePref may be set according to the energy currently output from the engine 50. This alternative structure enables the threshold value Pref to be set more suitably for each driving point of the engine 50. In the power output apparatus 20A of the second embodiment, when the output energy Pd is set by adding the threshold value Pref to the energy Pe actually output from the engine 50, the control procedure is carried out to enable the newly set output energy Pd to be applied to the drive shaft 22. In accordance with another possible structure, an insufficient amount of energy may be supplemented by the power stored in the battery 94. In this case, the assistmotor 40 is controlledaccording toan assist motorcontrol routine shown in the flowchart of Fig. 18, instead of the control routine of Figs. 10 and 11. In the assist motor control routine of Fig. 18, the torque command value Ta* of the assist motor 40 is set at step S210 by subtracting the torque commandvalue Tc* of theclutch motor 30 set at step S113 in the clutch motor control routine of Fig. 8 from the output torque command value Td* determined at step S186 in the torque control routine of Fig. 16. The concrete procedures of steps S211 through S220 are identical with those of steps S141 through S150 in the assist motor control routine of Figs. 10 and 11. In case that the step-on amount of the accelerator pedal 64 is remarkably varied and the engine 50 can not output the power corresponding to the step-on amountof the accelerator pedal 64, that is, even in the transient period while the driving point of the engine 50 is being shifted to a new target driving point for outputting the power corresponding to the step-on amount of the accelerator pedal 64, this control procedure of the assist motor 40 enables the torque set as the output torque command value Td* corresponding to the step-on amount of the accelerator pedal 64 to be output to the drive shaft 22. The following describes still another poweroutput apparatus 20B as a third embodiment according to the present invention. The structure of the power output apparatus 2OB of the third embodiment is identical with that of the power output apparatus 20 of the first embodiment, and is thus not described specifically. The numerals and symbols used in the description of the first embodiment have the same meanings in the third embodiment, unless otherwise specified. The power output apparatus 20B of the third embodiment executes the torque control routine of Fig. 5 carried out by the power output apparatus 20 of the first embodiment. The control procedure of the clutch motor 30 executed at step S108, however, follows aclutchmotor control routine shown in the flowcharts of Figs. 19 and 20, in place of the clutch motor control routine of Fig. 8. Like the power output apparatus 20 of the first embodiment, the control procedures of the assist motor 40 at step SHO and of the engine 50 at step SHI in the torque control routine of Fig. 5 respectively follow the assist motor control routine of Figs. 10 and 11 and the throttle valve position control routine of Fig. 12 and the fuel injection control routine of Fig. 13. The following mainly describes the difference between the clutch motor control routine of Fig. 8 executed by the power output apparatus 20 of the first embodiment and that of Figs. 19 and 20 executed by the power output apparatus 2OB of the third embodiment. When the program enters the routine, the control CPU 90 of the controller 80 reads the torque Tc which the clutch motor 30 applies to the drive shaft 22, that is, thetorque command value Tc* currently set in the clutch motor 30, from theRAM 90a at step S230. The control CPU 90 then receives data of rotational speed ω of the crankshaft 56 of the engine 50 (hereinafter referred to as the rotational speed of the engine 50) at step S232. The rotational speed ω of the engine 50 may be calculated from the rotational angle θe of the crankshaft 56 which has been read from the resolver 39, or alternatively calculated from the revolving speed Ne of the engine 50 (ω = 2π x Ne) . The program then proceeds to step S234, at which the control CPU 90 subtracts previous data of rotational speed ω (previous ω) input in a previous cycle of this routine from the current data of rotational speed ω of the engine 50, and divides the difference by an interval Δt of activating this routine, so as to determine a change rate ω' of rotational speed of the engine 50. This routine can be normally executed even immediately after a start of the vehicle, since the previous ω is initialized to zero in an initialization routine (not shown) executed prior to this routine. After calculating the change rate ω' of rotational speed of the engine 50, the programproceeds tostep S236 to calculateanestimated torque Tef, which the engine 50 is assumed to currently output, according to Equation (13) given below: Tef = Tc + 1 x ω' (13) The value '1' inthe right side ofEquation (13) represents themoment of inertia around the crankshaft 56, the outer rotor 32 linked with the crankshaft 56, or the like. Equation (13) is led as an equation of motion, based onthe equilibriumof forces acting onthecrankshaft 56. Namely the torque Te of the engine 50 acting on the crankshaft 56 is equal to the sum of the torque Tc of the clutch motor 30 and the force (lx ω') expressed as the motion of acceleration in the system. At subsequent step S238, the control CPU 90 reads a revolving speed (estimated target revolving speed) Nef* corresponding to the estimated torque Tef of the engine 50 from the map of Fig. 6 for determining thedrivingpoint oftheengine 50. Forexample, as shown in Fig. 21, the estimated target revolving speed Nef* is determined as a value corresponding to the estimated torque Tef on a curve A of driving points attaining the highest possible efficiency of the engine 50. The torque command value Tc* of the clutch motor 30 is then calculated at step S240 from the estimated torque Tef and the estimated target revolving speed Nef* according to Equation (14) given below: Tc* = Tef + kc(Ne - Nef*) + jki(Ne- Nef*)dt (14) The second term in the right side of Equation (14) represents a correction termbasedonthedifferencebetweenthe actual revolving speed Ne of the engine 50 and the estimated target revolving speed Nef*, wherein kc denotes a constant. The third term in the right side of Equation (14) represents an integral term to cancel the stationary deviation of the revolving speed Ne of the engine 50 from the estimated target revolving speed Nef*, wherein ki denotes a constant. The clutch motor 30 is controlled with the torque command value Tc* of the clutch motor 30 thus obtained, so that the engine 50 is controlled to be driven at a specific driving point where the torque Te is equal to the estimated torque Tef and the revolving speed Ne is equal to the estimated target revolving speed Nef*. After setting the torque command value Tc* of the clutch motor 30, the control CPU 90 of the controller 80 executes the processing of steps S244 through S256 in the flowchart of Fig.20. The concrete procedures are identical with those of steps S114 through S126 in the clutch motor control routine of Fig. 8 carried out by the power output apparatus 20 of the first embodiment, and are thus not specifically described here. Fig. 22 shows the operations in the power output apparatus 20B of thethird embodimentwhen thedriver steps on the acceleratorpedal 64 to a relatively large depth. By way of example, it is assumed that the driver steps on the accelerator pedal 64 by a relatively large amount at a time point tl to change the accelerator pedal position AP from a value API to another value AP2. The change of the accelerator pedal position AP causes the target torque Te* and the target revolving speed Ne* of the engine 50 to be newly set based on the new accelerator pedal position AP by the processing of steps S100 through S104 in the torque control routine ofFig.5. Theengine 50 is subsequently controlled at step SHI in the same routine, based on the target engine torque Te* and the target engine speed Ne*. In order todrive theengine 50 at the driving point defined bythe target engine torque Te* and the target engine speed Ne*, the throttle valve position control routine of Fig. 12 is carried out to change the position BP of the throttle valve 66, whereas the fuel injection control routine of Fig. 13 is carried out to vary the amount of fuel injection from the fuel injection valve 51. The standard position BPF of the throttle valve 66 based on the accelerator pedal position AP is set equal to a value BP2. Immediately after achange of the acceleratorpedal positionAP, there is a difference between the actual revolving speed Ne of the engine 50 and the target engine speed Ne*. The position BP of the throttle valve 66 is corrected according to the difference and temporarily made greater than the value BP2. The position BP of the throttle valve 66 then gradually decreases to the value BP2 as the revolving speed Ne of the engine 50 approaches the target engine speed Ne*. As discussed previously, the engine 50 has apoorer responsethan motors andcanthus notbedriven at atargetdriving point immediately after a change of the position BP of the throttle valve 66. When the torque Tc of the clutch motor 30 acting as a loading torque of the engine 50 is set equal to thetarget engine torque Te* immediately after a change in accelerator pedal position AP, the engine 50 may stall or even stop in some cases. The power output apparatus 20B of the third embodiment carries out the clutch motor control routine of Figs. 19 and 20 to calculate the estimated torque Tef, which the engine 50 is assumed to currently output, from the revolving speed Ne of the engine 50 and set the torque command value Tc* of the clutch motor 30 based on the estimated torque Tef and the estimated target revolving speed Nef* led from the estimated torque Tef. The engine 50 accordingly shifts its driving point to a target driving point (point P* ) along the curve A shown in Fig. 21. Referring to Fig. 22, for example, at an arbitrary time point t2 between the time point tl when the accelerator pedal position AP is varied and a time point t3 when the engine 50 is driven at the driving point of the target engine torque Te* and the target engine speed Ne*, the engine 50 is driven at a driving point defined by the estimated torque Tef, which the engine 50 is assumed to currently output, and by the estimated target revolving speed Nef* corresponding to the estimated torque Tef and read from the map of Fig. 6. As discussed above, the power output apparatus 20B of the third embodiment estimates the torque currently output from the engine 50, and controls the torque Tc of the clutch motor 30 in order to enable the engine 50 to be driven at the driving point attaining the highest possible efficiencywith respect to theestimatedtorque. Theengine 50 can thus be driven at driving points of the highest possible efficiency. In case that the target driving point of the engine 50 is a fixed stationary state, the engine 50 is stably driven at the target driving point. When the target driving point is changed, the engine 50 is smoothly shifted to the new target driving point along the path of high-efficiency driving points. With a change of the target driving point, the position BP of the throttle valve 66 is immediately varied to a valuecorresponding to the new target driving point, so that the engine 50 can be quickly shifted to the newtarget driving point. Like the first embodiment, the power output apparatus 2OB of the third embodiment sets the torque Te and the revolving speed Ne at a specific driving point, which attains the highest possible efficiency among the respective driving points on each constant- output energy curve of the engine 50, as the target engine torque Te* and the target engine speed Ne*. This further enhances the operation efficiency of the engine 50 and thereby improves the efficiency of the whole power output apparatus 20B. In the power output apparatus 20B of the third embodiment, the control procedure of the assist motor 40 executed at step S110 in the torque control routine of Fig. 5 follows the assist motor control routine shown in Figs. 10 and 11. it may, however, alternatively follow the assist motor control routine shown in the flowchart of Fig. 18. In the latter case, when there is a difference between the energy output from the engine 50 and the energy to be output to the drive shaft 22, for example, while the engine 50 is being shifted to a new target driving pointwith a significant change in accelerator pedal position AP, the battery 94 is charged with the excess energy or the insufficient energy is discharged from the battery 94. Even in the transient period when the engine 50 has not yet reached the new target driving point with a significant change in accelerator pedal positionAP, this structure enables the torque set as theoutput torque command value Td* corresponding to the step-on amount of the accelerator pedal 64 to be output to the drive shaft 22. In the structure of the power output apparatuses 20, 20A, and 20B of the first through the third embodiments discussed above, the clutch motor 30 and the assist motor 40 are separately attached to the different positions of the drive shaft 22. Like a power output apparatus 20C illustrated in Fig. 23 as a modification of the power output apparatus 20, however, the clutch motor and the assist motor may be joined integrally with each other. A clutch motor 30C of the power output apparatus 20C includes an inner rotor 34C connecting with the crankshaft 56 and an outer rotor 32C linked with the drive shaft 22. Three-phase coils 36C are attached to the inner rotor 34C, and permanent magnets 35C are set on the outer rotor 32C in such a manner that the outer surface and the inner surface thereof have different magnetic poles. An assist motor 40C includes the outer rotor 32C of the clutch motor 30C and a stator 43 with three-phase coils 44 mounted thereon. In this structure, the outer rotor 32C of the clutch motor 30C also works as the rotor of the assist motor 40C. Since the three-phase coils 36C are mounted on the inner rotor 34C linked with the crankshaft 56, the rotary transformer 38 for supplying electric power to the three-phase coils 36C of the clutch motor 30C is attached to the crankshaft 56. In the power output apparatus 20C, the voltage applied to the three-phase coils 36C on the inner rotor 34C is controlled against the inner-surface magnetic pole of the permanent magnets 35C set on the outer rotor 32C. This enables the clutch motor 30C to work in the same manner as the clutch motor 30 of the power output apparatuses 20, 20A, and 20B of the first through the third embodiments having the clutch motor 30 and the assist motor 40 separately attached to the drive shaft 22. The voltage applied to the three-phase coils 44 on the stator 43 is controlled against the outer-surface magnetic pole of the permanent magnets 35C set on the outer rotor 32C. This enables the assist motor 40C to work in the same manner as the assist motor 40 of the power output apparatus 20. All the operations of the power output apparatuses 20, 20A, and 20B of the first through the third embodiments discussed above are accordingly applicable to the poweroutput apparatus 20C of modifiedstructure. Theoperations include the torque control process based on the routine of Fig. 5 or Fig. 16, the clutch motor control process based on the routine of Fig. 8 or Figs. 19 and 20, the assist motor control process based on the routine of Figs. 10 and 11 or Fig. 18, and modifications thereof. The outer rotor 32C functions concurrently as one of the rotors in the clutch motor 30C and as the rotor of the assist motor 40C, thereby effectively reducing the size and weight of the power output apparatus 20C. The following describes another power output apparatus 20D as a fourth embodiment according to the present invention. Fig. 24 schematically illustrates structure of the power output apparatus 20D of the fourth embodiment. Referring to Fig.24, the power output apparatus 20D of the fourth embodiment has a similar structure to that of the power output apparatus 20 of the first embodiment, except that the assist motor 40 is attached to the crankshaft 56 placed between the engine 50 and the clutch motor 30. The same part as that of the power output apparatus 20 of the first embodiment shown in Fig. 1, such as the controller 80, is thus omitted from the drawing of Fig. 24. The power output apparatus 20D of the fourth embodiment can be mounted on the vehicle in the same manner as shown in Fig. 3. The constituents of the power output apparatus 20D of the fourth embodiment that are identicalwiththose of the poweroutput apparatus 20 of the first embodiment are expressed by like numerals and not specifically described here. The numerals and symbols used in the description of the first embodiment have the same meanings in the fourth embodiment, unless otherwise specified. The power output apparatus 20D of the fourth embodiment works in the manner discussed below. By way of example, it is assumed that the engine 50 is driven at a driving point Pl (torque Te = Tl and revolving speed Ne = Nl) on the constant-output energy curve of Fig. 4 definedbythe torque andthe revolving speed, andthat the revolving speed Nd of the drive shaft is equal to a value N2. When the assist motor 40 attached tothe crankshaft 56 applies a torqueTa (Ta=T2-T1) to the crankshaft 56, energy expressed as the sum of areas G2 and G3 in Fig. 4 is given to the crankshaft 56, so that the torque on the crankshaft 56 becomes equal to a value T2 (= Tl+Ta) . When the torque Tc of the clutch motor 30B is controlled to have the value T2, the torque Tc (= Tl+Ta) is transmitted to the drive shaft 22, while electric power based on the revolving speed difference Nc between the revolving speed Ne of the engine 50 and the revolving speed Nd of the drive shaft Nd (that is, energy expressed by the sum of areas Gl and G3) is regenerated by the clutch motor 30. Thetorque Ta of the assist motor 40 is then set to be just compensated by the electric power regenerated by the clutch motor 30, and the regenerative power is supplied to the second driving circuit 92 via the power lines Ll and L2, so that the assist motor 40 is driven with the regenerative power. In accordance with another example, it is assumed that the engine 50 is driven at a driving point P2 of Fig. 4 (torque Te = T2 and revolving speed Ne = N2) and that the revolving speed Nd of the drive shaft 22 is equal to the value Nl. When the torque Ta of the assist motor 40 is controlled to have the value (T2-T1), the assist motor 40 carries out the regenerative operation and regenerates energy (electric power) expressedbythe areaG2 in Fig.4 fromthe crankshaft 56. In the clutch motor 30, on the other hand, the inner rotor 34 rotates relative to the outer rotor 32 in the direction of rotation of the drive shaft 22 at the revolving speed difference Nc (= N1-N2) . The clutch motor 30 accordingly functions as a normal motor and gives energy expressed by the area Gl corresponding to the revolving speed difference Nc to thedrive shaft 22 as theenergy of rotational motion. The torque Ta of the assist motor 40 is then set to enable theelectric power consumed by the clutch motor 30 to be just compensated by the electric power regenerated by the assist motor 40, so that the clutch motor 30 is driven with the electric power regenerated by the assist motor 40. Like the power output apparatus 20 of the first embodiment, in the power output apparatus 20D of the fourth embodiment, the torque Ta of the assist motor 40 and the torque Tc of the clutch motor 30 are controlled to hold Equations (15) and (16) given below. This enables the energy output from the engine 50 to be freely subjected to torque conversion and output to the drive shaft 22. The relations of Equations (15) and (16) represent an ideal state having the efficiency of 100%. Inthe actual state, TcxNd and Tabecome a little smaller. Te x Ne = Tc x Nd (15) Te + Ta = Tc (16) The power output apparatus 20D of the fourth embodiment can execute thetorque control routine of Fig.5, the clutchmotor control routine of Fig. 8, the assist motor control routine of Figs. 10 and 11, the throttle valve position control routine of Fig. 12, and the fuel injection control routine of Fig. 13, which are carried out by the power output apparatus 20 of the first embodiment. The power output apparatus 20D can also execute the torque control routine of Fig. 16 carried out by the power output apparatus 20A of the second embodiment and the clutch motor control routine of Figs. 19 and 20 carried outby thepower output apparatus 20Bof thethirdembodiment. The power output apparatus 20D can further execute the assist motor control routineof Fig.18 whichmay becarried out by the poweroutput apparatus 20A of the second embodiment or the power output apparatus 20Bof the thirdembodiment. Whenthetarget torqueTe* andthetarget revolving speed Ne* of the engine 50 are set at stepS104 in thetorque control routine of Fig.5 or at step S200 inthe torque control routine of Fig. 16, the driving point which attains the highest possible efficiency among the respective driving points on each constant- output energy curve of the engine 50 can be set as the target driving point of the engine 50 as discussed above with the drawings of Figs. 6 and 7. The primary difference of the power output apparatus 20D of the fourth embodiment from the power output apparatuses 20 and 20A of the first and the second embodiments is the arrangement of the clutch motor 30 and the assist motor 40. The difference in arrangement inverts the powercontrol and regenerative control of theclutchmotor 30 and the assist motor 40 that are determined by the relationship between the revolving speed Ne of the engine 50 and the revolving speed Nd of the drive shaft 22. This also varies the setting of the torque command value Tc* of the clutch motor 30 at step Sll3 in the clutch motor control routine of Fig. 8 and the setting of the torque command value Ta* of the assist motor 40 at step S136 or S138 in the assist motor control routine of Fig. 10. When neglecting the efficiency of the clutch motor 30 and the assist motor 40, the power output apparatus 20Dofthe fourth embodiment candetermine thetorque command value Tc* of the clutch motor 30 at step S113 in the flowchart of Fig.8 accordingto Equation (17) givenbelow, instead of theabove Equation (1) , and determinethe torque command value Ta* of the assist motor 40 at steps S131 through S140 in the flowchart of Fig. 10 according to Equation (18) given below: Tc* = kc(Ne-Ne*) + Te* + Ta* (17) Ta* = Td* -Tc* (18) The primary difference of the power output apparatus 20D of the fourth embodiment from the power output apparatus 2OB of the third embodiment is also the arrangement of the clutch motor 30 and the assist motor 40. The difference in arrangement varies the calculation of the estimated torque Tef of the engine 50 at step S236 in the clutch motor control routine of Fig. 19 and the setting of the torque command value Tc* of the clutch motor 30 at step S240. The calculation of the estimated torque Tef of the engine 50 at step S236 follows Equation (19) given below instead of the above Equation (13), whereas Equation (20) given below is used instead of the above Equation (14) for the calculation of the torque command value Tc* of the clutch motor 30 at step S240. Tef = Tc+ I xω' -Ta (19) Tc* = Tef + Ta + kc{Ne - Nef* ) + j ki( Ne - Nef* )dt (20) As discussed above, the power output apparatus 20D of the fourth embodiment uses Equations (17) and (18) to implement the torque control routine of Fig. 5, the clutch motor control routine of Fig. 8, the assist motor control routine of Figs. 10 and 11, and other related routines carried out by the power output apparatus 20 of the first embodiment. The torque Te and the revolving speed Ne at a specific driving point, which attain the highest possibleefficiency among the respective driving points on each constant-output energy curve of the engine 50, are set as the target engine torque Te* and the target engine speed Ne*. This enhances the efficiency of the engine 50 and thereby improves the efficiency of the whole power output apparatus 20D. The driving points set as the target engine torque Te* and the target engine speed Ne* are continuouswith respect to the amount of output energy. The driving point of the engine 50 can thus be shifted smoothly with a little change in output energy Pd. The power output apparatus 20D of the fourth embodiment can also execute the torque control routine of Fig. 16, which is carried out by the power output apparatus 20A of the second embodiment. When the driver steps on the accelerator pedal 64 by a relatively large amount, the power output apparatus 20D controls the engine 50, the clutch motor 30, and the assist motor 40, based on the output energy at a driving point to which the engine 50 can smoothly shift from the current driving point, instead of the output energy Pd corresponding to the step-on amount of the accelerator pedal 64. This structure enables the driving point of the engine 50 to be smoothly shifted to the driving point giving the output energy Pd corresponding to the step-on amount of the accelerator pedal 64, thereby effectively preventing the engine 50 from stalling or stopping due to an abrupt change of the driving point of the engine 50. The power output apparatus 20D of the fourth embodiment uses Equations (19) and (20) to implement the torque control routine of Fig.5 and the clutch motor control routine of Figs. 19 and 20 carried out by the power output apparatus 20B of the third embodiment. The power output apparatus 20Destimates thetorque currentlyoutput from the engine 50, and controls the torque Tc of the clutch motor 30 in order to enable the engine 50 to be driven at the driving point attaining the highest possible efficiency with respect to the estimated torque. The engine 50 can thus be driven at driving points of the highest possible efficiency. In case that the target driving point of the engine 50 is a fixed stationary state, the engine 50 is stablydriven at the targetdriving point. Whenthetarget driving point is changed, the engine 50 is smoothly shifted to the newtarget driving point alongthepath of high-efficiency drivingpoints. With a change of the target driving point, the position BP of the throttle valve 66 is immediately varied to a value corresponding to the new target driving point, so that the engine 50 can be quickly shifted to the new target driving point. The torque command value Tc* of the clutch motor 30 is set to lessen the difference between the actual revolving speed Ne of the engine 50 and the target engine speed Ne*. The engine 50 can thus be driven stably at the target engine speed Ne*. The position BP of the throttle valve 66 is also adjusted to decrease the difference between the revolving speed Ne of the engine 50 and the target engine speed Ne*. This further ensures stable operation of the engine 50 at the target engine speed Ne*. In the power output apparatus 20D of the fourth embodiment shown in Fig.24, the assist motor 40 is attached tothe crankshaft 56 placed between the engine 50 and the clutch motor 30. Like another power output apparatus 20E illustrated in Fig. 25, however, the engine 50 may be interposed between the clutch motor 30 and the assist motor 40, both of which are linked with the crankshaft 56. One modification of the power output apparatus 20D of the fourth embodiment is given in Fig. 26 as a power output apparatus 2OF, in which a clutch motor 30F and an assistmotor 40F are integrally joined with each other. Referring to Fig.26, in the power output apparatus 20F, an outer rotor 32F of the clutch motor 30F also works as a rotor of the assistmotor 4OF. Thevoltage applied tothethree-phasecoils 36 on the inner rotor 34 is controlled against the inner-surface magnetic pole of permanent magnets 35F set on the outer rotor 32F. This allows the clutch motor 3OF to work in the same manner as the clutch motor 30 of the power output apparatus 20D shown in Fig. 24. The voltage applied to the three-phase coils 44 on the stator 43 is controlled against the outer-surface magnetic pole of the permanent magnets 35F set on the outer rotor 32F. This allows the assist motor 40F to work in the same manner as the assist motor 40 of the power output apparatus 20D. The power output apparatus 2OF accordingly carries out the same operations and exerts the same effects as those in the power output apparatus 20D of the fourth embodiment discussed above. In addition to the effects of the power output apparatus 20D of the fourth embodiment, the power output apparatus 20F of the modified structure has further effects of reducingthe size andweight of the whole power output apparatus 20F. In the power output apparatuses 20 and 20A through 2OF of the first through the fourth embodiments and their modifications, the torque Te and the revolving speed Ne at a specific driving point, which attains the highest possible efficiency among the respective driving points on each constant-output energy curve of the engine 50, are set as the target engine torque Te* and the target engine speed Ne*. The target torque Te* and the target revolving speed Ne* of the engine 50 may alternatively be set to allow the engine 50 as well as the clutch motor 30 and the assist motor 40 to be driven at adrivingpoint havingthe highest possiblecomprehensiveefficiency, which takes into account the efficiency of the clutch motor 30 and the assist motor 40 in addition to the efficiency of the engine 50. By way of example, the efficiency of the engine 50 is given as ηe, the efficiency of the clutch motor 30 and the first driving circuit 91 as ηc, and the efficiency of the assist motor 40 and the second driving circuit 92 as ηa. A comprehensive efficiency η is then expressed by multiplying the efficiency ηe of the engine 50 by an efficiency (transmission efficiency) of the power output apparatus 20 for transmitting the power from the engine 50 to the drive shaft 22 as given by Equation (21) below. The driving points of the engine 50, the clutch motor 30, and the assistmotor 40 attaining the highest possible comprehensive efficiency η with respect to each amount of output energy Pd are determined experimentally or otherwise, and stored in advance as a map in the ROM 90b. The driving points corresponding to the output energy Pd, which depends upon the operation of the accelerator pedal 64, are read from the map. This structure further enhances the efficiency of the whole power output apparatus. Tex Nd+ Te(Ne - Nd)ηc x ηa TJ = ηe x (21) Te x Ne The following describes still another poweroutput apparatus 110 as a fifth embodiment according to the present invention. Fig. 27 schematically illustrates structure of the power output apparatus 110 of the fifth embodiment; Fig.28 is an enlarged view illustrating an essential part of the power output apparatus 110 of Fig. 27; and Fig. 29 shows a vehicle with the power output apparatus 110 of Fig. 27 incorporated therein. The vehicle of Fig. 29 with the power output apparatus 110 of the fifth embodiment incorporated therein has a similar structure to that of the vehicle of Fig. 3 with the power output apparatus 20 of the first embodiment incorporated therein, except that aplanetary gear 120 and motors MG1 and MG2 are attached to a crankshaft 156, instead of the clutch motor 30 and the assist motor 40. The same constituents are expressed bylike numerals +100 and not specifically described here. The numerals and symbols used in the description of the power output apparatus 20 the first embodiment have the same meanings in the description of the power output apparatus 110 of the fifth embodiment, unless otherwise specified. Referring to Figs. 27 and 28, the power output apparatus 110 primarily includes an engine 150, a planetary gear 120 having a planetary carrier 124 mechanically linked with a crankshaft 156 of the engine 150, a first motor MG1 linked with a sun gear 121 of the planetary gear 120, a second motor MG2 linked with a ring gear 122 of the planetary gear 120, and a controller 180 for driving and controlling the first and the second motors MGl and MG2. The planetary gear 120 includes the sun gear 121 linked with a hollow sun gear shaft 125 which the crankshaft 156 passes through, the ring gear 122 linked with a ring gear shaft 126 coaxial with the crankshaft 156, a plurality of planetary pinion gears 123 arranged between the sun gear 121 and the ring gear 122 to revolve around the sun gear 121 while rotating on its axis, and the planetary carrier 124 connecting with one end of the crankshaft 156 to support the rotating shafts of the planetary pinion gears 123. In the planetary gear 120, three shafts, that is, the sun gear shaft 125, the ring gear shaft 126, and the crankshaft 156 respectively connecting with the sun gear 121, the ring gear 122, and the planetary carrier 124, work as input and output shafts of the power. Determination of the power input to or output from any two shafts among the three shafts automatically determines the power input to or output from the residual one shaft. The details of the input and output operations of the power into or from the three shafts of the planetary gear 120 will be discussed later. A power feed gear 128 for taking out the power is linked with the ring gear 122 and arranged on the side of the first motor MGl. The power feed gear 128 is further connected to a power transmission gear 111 via achain belt 129, sothat the power is transmitted between the power feed gear 128 andthe power transmission gear 111. As shown in Fig. 29, the power transmission gear 111 is further linked with a differential gear 114. The power output from the power output apparatus 110 is thus eventuallytransmittedtoleft andright driving wheels 116 and 118. The first motor MGl is constructed as a synchronous motor- generator and includes a rotor 132 having a plurality of permanent magnets 135 on its outer surface and a stator 133 having three-phase coils 134 wound thereon to forma revolvingmagnetic field. The rotor 132 is linked with the sun gear shaft 125 connecting with the sun gear 121 of the planetary gear 120. The stator 133 is prepared by laying thin plates of non-directional electromagnetic steel oneupon another and is fixed to a casing 119. The first motor MGl works as a motor for rotating the rotor 132 through the interaction between a magnetic field produced by the permanent magnets 135 and a magnetic field produced by the three-phase coils 134, or as a generator for generating an electromotive force on either ends of the three-phase coils 134 through the interaction betweenthe magnetic field produced by the permanent magnets 135 and the rotation of the rotor 132. The sun gear shaft 125 is further provided with a resolver 139 for measuring its rotational angle θs. Like the firstmotor MGl, the secondmotor MG2 is alsoconstructed as a synchronous motor-generator and includes a rotor 142 having a plurality of permanent magnets 145 on its outer surface and a stator 143 having three-phase coils 144 wound thereon to form a revolving magnetic field. The rotor 142 is linked with the ring gear shaft 126 connecting with the ring gear 122 of the planetary gear 120, whereas the stator 14 is fixed to the casing 119. The stator 143 of the motor MG2 is also produced by laying thin plates of non- directional electromagnetic steel one upon another. Like the first motor MGl, the second motor MG2 also works as a motor or a generator. The ring gear shaft 126 is further provided with a resolver 149 for measuring its rotational angle θr. Referring to Fig. 27, the controller 180 incorporated in the power output apparatus 110 of the fifth embodiment is constructed in the same manner as the controller 80 of the power output apparatus 20 of the first embodiment. The controller 180 includes a first driving circuit 191 for driving the first motor MGl, a second driving circuit 192 for driving the second motor MG2, a control CPU 190 for controlling both the first and the second driving circuits 191 and 192, and a battery 194 including a number of secondary cells. The control CPU 190 further includes a RAM 190a used as a working memory, a ROM 190b in which various control programs are stored, an input/output port (not shown), and a serial communication port (not shown) through which data are sent to and received from an EFIECU 170. The control CPU 190 receives a variety of data via the input port. The input data include the rotational angle θs of the sun gear shaft 125 measured with the resolver 139, the rotational angle θr of the ring gear shaft 126 measured with the resolver 149, an accelerator pedal position AP output from an accelerator position sensor 165, a gearshift position SP output from a gearshift position sensor 184, currents lul and Ivl fromtwoammeters 195 and 196 disposed in the first driving circuit 191, currents Iu2 and Iv2 from two ammeters 197 and 198 disposed in the second driving circuit 192, and a remaining charge BRM of the battery 194 measured with a remaining charge meter 199. The control CPU 190 outputs a first control signal SWl fordriving six transistors Tri through Tr6 working as switching elements of the first driving circuit 191 and a second control signal SW2 for driving six transistors Trll through Trl6 working as switching elements of the second driving circuit 192. The six transistors Tri through Tr6 in the first driving circuit 191 constitute a transistor inverter and are arranged in pairs to work as a source and a drain with respect to a pair of power lines Ll and L2. The six transistors Trll through Trl6 in the second driving circuit 192 also constitute a transistor inverter and are arranged in the same manner. The three-phase coils 134 of the first motor MGl are connected to the respective contacts of the paired transistors in the first driving circuit 191, whereas the three-phase coils 144 of the second motor MG2 are connected to those in the second driving circuit 192. The power lines Ll and L2 are respectively connected to plus and minus terminals of thebattery 194. The control signals SWl and SW2 output from the control CPU 190 thus successively control the power-on time of the paired transistors Tri through Tr6 and the paired transistors Trll through Trl6. The electric currents flowing through the three-phase coils 134 and 144 undergo PWM (pulse width modulation) to give quasi-sine waves, which enable the three-phase coils 134 and 144 to form revolving magnetic fields. The power output apparatus 110 of the fifth embodiment thus constructed works in accordance with the operation principles discussed below, especially with the principle of torqueconversion. By way of example, it is assumed that the engine 150 is driven at a driving point Pl having the revolving speed Ne and the torque Te and that the ring gear shaft 126 is driven at another driving point P2 having different revolving speed Nr and torque Tr but the same energy as an energy Pe output from the engine 150. This means that the poweroutput fromthe engine 150 is subjected totorque conversion and applied to the ring gear shaft 126. The relationship between the torque and the revolving speed of the engine 150 and the ring gear shaft 126 under such conditions is shown in the graph of Fig. 4. According to the mechanics, the relationship between the revolving speed and the torque of the three shafts in the planetary gear 120 (that is, the sun gear shaft 125, the ring gear shaft 126, and the planetary carrier 124) can be expressed as nomograms illustrated in Figs. 30 and 31 and solved geometrically. The relationship between the revolving speed and the torque of the three shafts in the planetary gear 120 may be analyzed numerically through calculation of energies of the respective shafts, without using the nomograms. For the clarity of explanation, the nomograms are used in the fifth embodiment. In the graph of Fig. 30, the revolving speed of the three shafts is plotted as ordinate and the positional ratio of the coordinate axes of the three shafts as abscissa. When a coordinate axis S of the sun gear shaft 125 and a coordinate axis R of the ring gear shaft 126 are positioned on either ends of a line segment, a coordinate axis C of the planetary carrier 124 is given as an interior division of the axes S and R at the ratio of 1 to p, where p represents a ratio of the number of teeth of the ring gear 122 to that of the sun gear 121 and expressed as Equation (22) given below: the number of teeth of the sun gear p = - — (22) the number of teeth of the ring gear As mentioned above, the engine 150 is driven at the revolving speed Ne, while the ring gear shaft 126 is driven at the revolving speed Nr. The revolving speed Ne of the engine 150 can thus be plotted on the coordinate axis C of the planetary carrier 124 linked with the crankshaft 156 of the engine 150, and the revolving speed Nr of the ring gear shaft 126 on the coordinate axis R of the ring gear shaft 126. A straight line passing through both the points is drawn, and a revolving speed Ns of the sun gear shaft 125 is then given as the intersection of this straight line and the coordinate axis S. This straight line is hereinafter referred to as dynamic collinear line. The revolving speed Ns of the sun gear shaft 125 can be calculated from the revolving speed Ne of the engine 150 and the revolving speed Nr of the ring gear shaft 126 according to a proportional expression given as Equation (23) below. In the planetary gear 120, the determination of the rotations of any two shafts among the sun gear 121, the ring gear 122, and the planetary carrier 124 results in automatically setting the rotation of the residual one shaft. 1 + p Ns= Nr -(Nr - Ne) (23) The torque Te of the engine 150 is then applied (upward in the drawing) to the dynamic collinear line at the coordinate axis C of the planetary carrier 124 as a line of action. The dynamic collinear line against the torque can be handled as a rigid body to which a force is applied as a vector. Based on the technique of dividing the force into different lines of action having the same direction, the torque Te acting on the coordinate axis C is divided into a torque Tes on the coordinate axis S and a torque Ter on the coordinate axis R. The magnitudes of the torques Tes and Ter are given by Equations (24) and (25) below: Tes = 7Vx ■ (24) 1+p 1 Ter = Te x (25) 1+p The equilibrium of forces on the dynamic collinear line is essential for the stable state of the dynamic collinear line. In accordance with a concrete procedure, a torque Tml having the same magnitude as but the opposite direction to the torque Tes is applied to the coordinate axis S, whereas a torque Tm2 having the same magnitude as but the opposite direction to a resultant force of the torque Ter and the torque that has the same magnitude as but the opposite direction to the torque Tr output to the ring gear shaft 126 is applied to the coordinate axis R. The torque Tml is given by the irst motor MGl, and the torque Tm2 by the second motor MG2. The first motor MGl applies the torque Tml in reverse of its rotation and thereby works as a generator to regenerate an electrical energy Pml, which is given as the product of the torque Tml and the revolving speed Ns, from the sun gear shaft 125. The second motor MG2 applies the torque Tm2 in the direction of its rotation and thereby works as a motor to output an electrical energy or power Pm2, which is given as the product of the torque Tm2 and the revolving speed Nr, to the ring gear shaft 126. In case that the electrical energy Pml is identical with the electrical energy Pm2, all the electric power consumed by the second motor MG2 can be supplied by the electric power regenerated by the first motorMGl. Inorder toattain such a state, all the inputenergy should be output; that is, the energy Pe output from the engine 150 should be equal to an energy Pr output to the ring gear shaft 126. Namely the energy Pe expressed as the product of the torque Te and the revolving speed Ne is made equal to the energy Pr expressed as the product of the torque Tr and the revolving speed Nr. Referring to Fig. 4, the power that is expressed as the product of the torque Te and the revolving speed Ne and output from the engine 150 driven at the driving point Pl is subjected to torque conversion and output to the ringgear shaft 126 as the powerof the same energybutexpressed as the product of the torque Tr and the revolving speed Nr. As discussed previously, the power output to the ring gear shaft 126 is transmitted to a drive shaft 112 via the power feed gear 128 and the power transmission gear 111, and further transmitted to the driving wheels 116 and 118 via the differential gear 114. A linear relationship is accordingly held between the power output to the ring gear shaft 126 and the power transmitted to the driving wheels 116 and 118. The power transmitted to the driving wheels 116 and 118 can thus be controlled by adjusting the power output to the ring gear shaft 126. Although the revolving speed Ns of the sun gear shaft 125 is positive in the nomogram of Fig. 30, it may be negative according to the revolving speed Ne of the engine 150 and the revolving speed Nr of the ring gear shaft 126 as shown in the nomogram of Fig. 31. In the latter case, the first motor MGl applies the torque in the direction of its rotation and thereby works as a motor to consume the electrical energy Pml given as the product of the torque Tml and the revolving speed Ns. The second motor MG2, on the other hand, applies the torque in reverse of its rotation and thereby works as a generator to regenerate the electrical energy Pm2, which is given as the product of the torque Tm2 and the revolving speed Nr, from the ring gear shaft 126. In case that the electrical energy Pml consumed by the firstmotor MGl is madeequal to the electrical energy Pm2 regenerated by the second motor MG2 under such conditions, all the electric power consumed by the first motor MGl can be supplied by the electric power regenerated by the second motor MG2. The operation principlediscussedabove is on the assumptionthat the efficiency of power conversion by the planetary gear 120, the motors MGl and MG2, and the transistors Tri through Tri6 is equal tothevalue '1 ' , which represents 100%. inthe actual state, however, the conversion efficiency is less than the value ' 1 ' , so that the energy Pe output from the engine 150 should be a little greater than the energy Pr output to the ring gear shaft 126 or alternatively the energy Pr output to the ring gear shaft 126 should be a little smaller than the energy Pe output from the engine 150. By way of example, the energy Pe output from the engine 150 may be calculated by multiplying the energy Pr output to the ring gear shaft 126 by the reciprocal of the conversion efficiency. In the state of the nomogram of Fig. 30, the torque Tm2 of the second motor MG2 is calculated bymultiplying theelectric power regenerated bythe first motor MGl by the efficiencies of both the motors MGl and MG2. In the state of the nomogram of Fig. 31, on the other hand, the torque Tm2 of the second motor MG2 is calculated by dividing the electric power consumed by the first motor MGl by the efficiencies of both the motors MGl and MG2. In the planetary gear 120, there is anenergy loss or heat loss due to a mechanical friction or the like, though the amount of energy loss is significantly smaller than the whole amount ofenergyconcerned. Theefficiency of the synchronousmotors used for the first and the second motors MGl and MG2 is substantially equal to the value ' 1 ' . Known devices such as GTOs applicable to the transistors Tri through Tri6 have extremely small ON-resistance. The efficiency of power conversion thus becomes practically equal to the value '1'. For the matter of convenience, in the following discussion of the fifth embodiment, the efficiency is assumed to be equal to the value '1' (=100%) unless otherwise specified. As clearly understood from the operation principle of the power output apparatus 110 of the fifth embodiment discussed above, controlling the motors MGl andMG2 enables theengine 150 to bedriven at any driving point that can output the energy identical with the energy to be output to the ring gear shaft 126. The planetary gear 120 (having the appropriate gear ratio) and the first motor MGl in the power output apparatus 110 of the fifth embodiment have similar functions tothoseof the clutchmotor 30 in the poweroutput apparatus 20 of the first embodiment, while the second motor MG2 works in the similar manner to the assist motor 40. The map of Fig. 6 , which is used to set the target torque Te* and the target revolving speed Ne* of the engine 50 in the torque control of the power output apparatuses 20 and 20A through 20C of the first through the third embodiments, can thus be applicable to set the target torque Te* and the target revolving speedNe* of the engine 150 in the torque control of the power output apparatus HO of the fifth embodiment. By taking into account the gear ratio of the planetary gear 120, the power output apparatus 110 of the fifth embodiment can execute the torque control routine of Fig.5, the clutch motorcontrol routine of Fig. 8, the assist motor control routine of Figs. 10 and 11, the throttle valve position control routine of Fig. 12, and the fuel injection control routine of Fig. 13, which are carried out by the power output apparatus 20 of the first embodiment. The power output apparatus 110 can also execute the torque control routine of Fig. 16 carried out by the power output apparatus 20A of the second embodiment and the clutch motor control routine of Figs. 19 and 20 carried out by thepower output apparatus 20Bof the thirdembodiment. The power output apparatus 110 can further execute the assist motor control routine of Fig.18 whichmay becarried out by thepower output apparatus 20A of the second embodiment or the power output apparatus 20B of the third embodiment. The following describes a typical operation of the power output apparatus 110 of the fifth embodiment, which is similar to that of the power output apparatus 20B of the third embodiment. The torque control in the power output apparatus 110 of the fifth embodiment is carried out accordingto a torque control routine shown in the flowchart of Fig. 32. When the program enters the torque control routine, the control CPU 190 of thecontroller 180 first reads the revolving speed Nr of the ring gear shaft 126 at step S300. The revolving speed Nr of the ring gear shaft 126 may be calculated from the rotational angle θr of the ring gear shaft 126 read from the resolver 149. The control CPU 190 then reads the accelerator pedal position AP detected by the accelerator position sensor 165 at step S302, and determines a torque command value Tr* or a target torque to be output tothe ring gear shaft 126, based on the input accelerator pedal position AP at step S304. Not the torque to be output to the driving wheels 116 and 118 but the torque to be output to the ring gear shaft 126 is calculated here from the accelerator pedal position AP. This is because the ring gear shaft 126 is mechanically linked with the driving wheels 116 and 118 via the power feed gear 128, the power transmission gear 111, and the differential gear 114 and the determination of the torque to be output to the ring gear shaft 126 thus results in determining the torque to be output to the driving wheels 116 and 118. In the fifth embodiment, a map representing the relationship between the torque command value Tr*, the revolving speed Nrof the ringgear shaft 126, andthe acceleratorpedal position AP is prepared in advance and stored in the ROM 190b. The torque command value Tr* corresponding to the input accelerator pedal position AP and the input revolving speed Nr of the ring gear shaft 126 is read from the map. The control CPU 190 subsequently calculates an energy Pr to be output to the ring gear shaft 126 from the torque command value Tr* thus obtained and the input revolving speed Nr of the ring gear shaft 126 (Pr = Tr*x Nr) at step S306. The program then proceeds to step S308 to set a target torque Te* and a target revolving speed Ne* of the engine 150 based on the energy Pr to be output to the ring gear shaft 126. As mentioned above, the map of Fig. 6 used in the power output apparatus 20 of the first embodiment is applicable to set the target torque Te* and the target revolving speed Ne* of the engine 150. The map of Fig.6 enables a specific driving point of theengine 150 that attains the highest possible efficiency with respect toeach amount of energy Pr and allows a smooth variation in driving state of the engine 150 with a variation in energy Pr to be set as the target engine torque Te* and the target engine speed Ne*. After setting the target torque Te* and the target revolving speed Ne* of the engine 150, the programproceeds to steps S310, S312, and S314 to respectivelycontrol the first motorMGl, the secondmotor MG2, and the engine 150 based on the target engine torque Te* and the target engine speed Ne* thus obtained. In the fifth embodiment, although the control operations of the first motor MGl, the second motor MG2, and the engine 150 are shown as separate steps for the matter of convenience, these controls are carried out simultaneously in the actual procedure. Figs. 33 and 34 are flowcharts showing details of the control process of the first motor MGl executed at step S310 in the flowchart of Fig.32. When the programenters the control routine, the control CPU 190 of the controller 180 first reads the torque Tml which the first motor MGl currently applies to the sun gear shaft 125 (that is, a torque command value Tml* currently set in the first motorMGl) at step S320, and receives data of revolving speed Ns of the sun gear shaft 125 at step S322. The revolving speed Ns of the sun gear shaft 125 can be calculated from the rotational angle θs of the sun gear shaft 125 read from the resolver 139. At subsequent step S324, a change rate ωs' of rotational speed of the sun gear shaft 125 is calculated from the input revolving speed Ns of the sun gear shaft 125 according to Equation (26) given below. The calculation subtracts previous data of revolving speed NS of the sun gear shaft 125 (previous NS) input at step S322 in a previous cycle of this routine from the current data of revolving speed NS, multiplies the difference by 2π , and divides the product by an interval Δt of activating this routine, so as to determine the change rate ωs' of rotational speed of the sun gear shaft 125. The numerator in the right side of Equation (26) includes '2π' since the relationship between the rotational speed ωs and the revolving speed Ns of the sun gear shaft 125 is defined as ωs = 2π x Ns [rad/sec]. Like the third embodiment, this routineofthe fifthembodimentcanbenormally executed even immediately after a start of the vehicle, since the previous Ns is initializedto zero in an initialization routine (not shown) executed prior to this routine. , 2π( Ns - previous Ns) ω$' = — i ^ (26) Δt The control CPU 190 then reads the revolving speed Nr of the ring gear shaft 126 at step S326, and calculates a change rate cor' of rotational speed of the ring gear shaft 126 at step S328 in a similar manner to the processing of step S324. After calculating the change rate ωs' of rotational speed of the sun gear shaft 125 and the change rate ωr' of rotational speed of the ring gear shaft 126, the program proceeds to step S330 to calculate an estimated torque Tef, which the engine 150 is assumed tocurrently output, according to Equation (27) given below: wherein 'Ie' in the right side of Equation (27) represents the moment of inertia around the engine 150 and the crankshaft 156, and 'Ig' represents the moment of inertia around the rotor 132 of the first motor MGl and the sun gear shaft 125. Equation (27) is led from the equation of motion based on the equilibrium of forces on the dynamic collinear line in the nomograms of Figs. 30 and 31. At subsequent step S332, the control CPU 190 reads a revolving speed (estimated target revolving speed) Nef* corresponding to the estimated torque Tef of the engine 150 from the map of Fig. 6 for determining the driving point of the engine 150. Like the third embodiment, for example, as shown in Fig. 21, the estimated target revolving speed Nef* is determined as a value corresponding to the estimated torque Tef on the curve A of driving points attaining the highest possible efficiency of the engine 150. A target revolving speed Ns* of the sun gear shaft 125 is then calculated at step S334 from the estimated target revolving speed Nef* thus obtained and the input revolving speed Nr of the ring gear shaft 126 according to Equation (28) given below: Ns* = ^- Nef* -- Nr (28) P P Equation (28) is readily obtained by calculating the ratio of revolving speeds of the respective coordinate axes S, C, and R in the nomograms of Figs. 30 and 31. At subsequent step S336, the control CPU 190 calculates a torque command value Tml* of the first motor MGl from the estimated torque Tef, the estimated target revolving speed Nef*, and the target revolving speed Ns* of the sun gear shaft 125 according to Equation (29) given below: Tml* =—— x Tef+kc(Ns-Ns*)+f ki(Ns-Ns*) dt (29) 1+p J The second term in the right side of Equation (29) represents a correction term based on the difference between the actual revolving speed Ns of the sun gear shaft 125 and the target revolving speed Ns*, wherein ke denotes a constant. The third term in the right side of Equation (29) represents an integral term to cancel the stationary deviation of the revolving speed Ns of the sun gear shaft 125 from the target revolving speed Ns*, wherein ki denotes a constant. The first motor MGl is controlled with the torque command value Tml* of the first motorMGl thus obtained, so that the engine 150 is controlled to be driven at a specific driving point where the torque Te is equal to the estimated torque Tef and the revolving speed Ne is equal to the estimated target revolving speed Nef*. The engine 150 can be driven at the specific driving point of the estimated toque Tef and the estimated target revolving speedNef* by controlling the revolving speed Ns of the sun gear shaft 125 to the target revolving speed Ns*. This is ascribed to the following reasons. As discussed above with the nomograms of Figs. 30 and 31, in the planetary gear 120, the determination of the revolving speeds of any two shafts among the sun gear shaft 125, the ring gear shaft 126, and the planetary carrier 124 results in automatically setting the revolving speed of the residual one shaft. The revolving speed Nr of the ring gear shaft 126 mechanically linked with the driving wheels 116 and 118 is given as input data. Controlling either the revolving speed Ns of the sun gear shaft 125 or the revolving speed Ne of the engine 150 thus determines the rotational conditions of the three shafts in the planetary gear 120. The revolving speed Ne of the engine 150 should be controlled, in order to allow the engine 150 to be driven at the high-efficient driving point of the estimated torque Tef andthe estimated target revolving speed Nef*. Thetorque Te and the revolving speed Ne of the engine 150 are, however, varied by the loading torque of the engine 150, even when the position BP of thethrottlevalve 166 andthe amount of fuel injection are adjusted finely. It is thereby rather difficult to control the driving point of the engine 150 in an independent manner. The revolving speed Ns of the sun gear shaft 125 can, onthe otherhand, becontrolled readily and precisely by controlling the revolving speed of the first motor MGl. In the fifth embodiment, the control of the revolving speed Ne of the engine 150 is thus implemented by controlling the revolving speed Ns of the sun gear shaft 125 with the first motor MGl. Referring to the flowchart of Fig. 34, the control CPU 190 subsequently receives the rotational angle θs of the sun gear shaft 125 from the revolver 139 at step S338, and detects phase currents lul and Ivl of the first motor MGl with the ammeters 195 and 196 at step S340. The control CPU 190 then executes transformation of coordinates for the phase currents at step S342, computes voltage command values Vdl and Vql at step S344, and executes inverse transformation of coordinates for the voltage command values at step S346. At subsequent step S348, the control CPU 190 determines the on- and off-time of the transistors Tri through Tr6 in the first driving circuit 191 of the controller 180 for driving andcontrolling the first motor MGl and carries out the PWM (pulse width modulation) control. The processing executed at steps S342 through S348 is similar to that executed at steps S120 through S126 in the clutch motor control routine of the first embodiment shown in the flowchart of Fig. 8. Fig. 35 is a flowchart showing details of the control process of the second motor MG2 executed at step S312 in the flowchart of Fig. 32. When the program enters the control routine, the control CPU 190 of the controller 180 first calculates a torque command value Tm2* of the second motor MG2 according to Equation (30) given below at step S350. Equation (30) gives the torque command value Tm2* of the second motor MG2, in order to enable all the power output from the engine 150 to be subjected to torque conversion by the planetary gear 120 and the motors MGl and MG2 and to be output to the ring gear shaft 126. This is on the assumption that, when the engine 150 is driven at a specific driving point defined by the estimated torque Tef and the estimated target revolving speed Nef*, the dynamic collinear line in the nomogram is well-balanced. r r . „ * Tef x Nef * „, , 1 Tm2* = — Tef x (30) Nr 1+ P The control CPU 190 subsequently receives the rotational angle θr of the ring gear shaft 126 from the revolver 149 at step S352, and detects phase currents Iu2 and Iv2 of the second motor MG2 with the ammeters 197 and 198 at step S354. The control CPU 190 then executes transformation of coordinates for the phase currents at step S356, computes voltage command values Vd2 and Vq2 at step S358, and executes inverse transformation of coordinates for the voltage command values at step S360. At subsequent step S362, the control CPU 190 determines the on- and off-time of the transistors Trll through Tri6 in the second driving circuit 192 of the controller 180 for driving and controlling the second motor MG2 and carries out the PWM control. The processing executed at steps S356 through S362 is similar to that executed at steps S342 through S348 in the control procedure of the first motor MGl illustrated in the flowcharts of Figs. 33 and 34. As discussed above, the power output apparatus 110 of the fifth embodiment estimates the torque currently output from theengine 150, and controls the torque Tml of the first motor MGl in order to enable the engine 150 tobe driven at thedriving point attaining the highest possible efficiencywithrespectto theestimatedtorque. Theengine 150 can thus be driven at driving points of the highest possible efficiency. In case that the target driving point of the engine 150 is a fixed stationary state, the engine 150 is stably driven at the target driving point. When the target driving point is changed, the engine 150 is smoothly shifted to the newtarget driving point along the path of high-efficiency driving points. With a change of the target driving point, the position BP of the throttle valve 166 is immediately varied to a valuecorresponding to the newtarget driving point, so that the engine 150 can be quickly shifted to the new target driving point. The power output apparatus 110 of the fifth embodiment sets the torque Te and the revolving speed Ne at a specific driving point, which attains the highest possible efficiency among the respective driving points on each constant-output energy curve of the engine 150, as the target engine torque Te* and the target engine speed Ne*. This further enhances the operation efficiency of the engine 150 and thereby improves the efficiency of the whole power output apparatus 110. A variety of processes applied to the hardware structures of the power output apparatuses 20, 20A, and 20B of the first through the third embodiments including the clutch motor 30 and the assist motor 40 are also applicable to the hardware structure of the power output apparatus 110 of the fifth embodiment including the planetary gear 120 and the two motors MGl and MG2 instead of the clutch motor 30 and the assist motor 40, by taking into account the balance on the dynamic collinear line in the nomograms of Figs. 30 and 31. The processes carried out by the power output apparatus 20B of the third embodiment are discussed above as an example of the applicable processes. By taking into account the balance on the dynamic collinear line in the nomograms of Figs. 30 and 31, any processes carried out by the power output apparatus 20 of the first embodiment are also applicable to the power output apparatus 110 of the fifth embodiment, which thereby implements the same functions and exerts the same effects as those of the first embodiment. Inthe same manner, by taking into account the balance on the dynamic collinear line in the nomograms of Figs. 30 and 31, any processes carried out by the power output apparatus 20A of the second embodiment are also applicable to the power output apparatus 110 of the fifthembodiment, which thereby implements the same functions and exerts the same effects as those of the second embodiment. in the power output apparatus 110 of the fifth embodiment, the power output to the ring gear shaft 126 is taken out of the place between the first motor MGl and the second motor MG2 via the power feed gear 128 connecting with the ring gear 122. As shown by another power output apparatus 11OA of Fig. 36 given as a possible modification, however, the power may be taken out of the casing 119, from which the ring gear shaft 126 is extended. Fig. 37 shows still another poweroutput apparatus HOB as another possiblemodification, wherein the engine 150, the planetary gear 120, the second motor MG2, and the first motor MGl are arranged in this order. In this case, a sun gear shaft 125B may not have a hollowstructure, whereas a hollow ring gear shaft 126B is required. This modified structure enables the power output to the ring gear shaft 126B to be taken out of the place between the engine 150 and the second motor MG2. The following describes still another power output apparatus HOC as a sixth embodiment according to the present invention. Fig. 38 shows an essential part of the power output apparatus HOC of the sixth embodiment. Referring to Fig. 38, the power output apparatus HOC of the sixth embodiment has a similar structure to that of the power output apparatus 110 of the fifth embodiment, except that the rotor 142 of the second motor MG2 is attached to the crankshaft 156 and that the two motors MGl and MG2 have a different arrangement. The same part as that of the power output apparatus 110 of the fifth embodiment shown in Fig. 27, such as the controller 180, is thus omitted from the drawing of Fig.38. The power output apparatus HOC of the sixth embodiment can be mounted on the vehicle in the same manner as shown in Fig. 29. The constituents of the power output apparatus HOC of the sixth embodiment that are identical with those of the power output apparatus 110 of the fifth embodiment are expressed by like numerals and not specifically described here. The numerals and symbols used in the description of the fifth embodiment have the same meanings in the sixth embodiment, unless otherwise specified. Referring to Fig. 38, in the power output apparatus HOC of the sixth embodiment, the engine 150, the second motor MG2, theplanetary gear 120, and the first motor MGl are arranged in this order. The rotor 132 of the first motor MGl is attached to a sun gear shaft 125C connecting with the sun gear 121 of the planetary gear 120. Like the power output apparatus 110 of the fifth embodiment, the planetary carrier 124 is attached to the crankshaft 156 of the engine 150. The rotor 142 of the second motor MG2 and a resolver 157 for detecting a rotational angle θe of the crankshaft 156 are further attached to the crankshaft 156. A ring gear shaft 126C linked with the ring gear 122 of the planetary gear 120 has anotherresolver 149 mounted thereon for detecting a rotational angle θr of the ring gear shaft 126C and is connected to the power feed gear 128. The arrangement of the power output apparatus HOC of the sixth embodiment is different from that of the power output apparatus 110 of the fifth embodiment. In both the arrangements, however, the three-phase coils 134 of the first motor MGl are connected with the first driving circuit 191 of the controller 180, and the three-phase coils 144 of the second motor MG2 with the second driving circuit 191. Although not being illustrated, the resolver 157 is connected to the input port of the control CPU 190 of the controller 180 via a signal line. The power output apparatus HOC of the sixth embodiment works in the manner discussed below. By way of example, it is assumed that the engine 150 is driven at the driving point Pl of the revolving speed Ne and the torque Te and that the ring gear shaft 126C is driven at the driving point P2 that is defined by the revolving speed Nr and the torque Tr and gives energy Pr (Pr = Nr x Tr) identical with energy Pe (Pe = Ne x Te) output from the engine 150. In this example, the power output from the engine 150 is thereby subjected to torque conversion and applied to the ring gear shaft 126C. Figs. 39 and 40 are nomograms under such conditions. Equations (31) through (34) given below are led by taking into account the balance on a dynamic collinear line in the nomogram of Fig. 39. Equation (31 ) is derived fromthe balance between the energy Pe input from the engine 150 and the energy Pr output to the ring gear shaft 126C. Equation (32) is given as a sum of energy input into the planetary carrier 124 via the crankshaft 156. Equations (33) and (34) are obtained by dividing the torque acting on the planetary carrier 124 into torques on the coordinate axes S and R working as lines of action. Te xNe = Tr x Nr (31) Tc = Te + Tm2 (32) Tes = Tc x- P (33) 1+p 1 Ter = Tcx (34) 1+p The equilibrium of forces on the dynamic collinear line is essential for the stable state of the dynamic collinear line. For that purpose, the torque Tml should be made equal to a torque Tes, and the torque Tr equal to a torque Ter. Based on such relations, the torques Tml andTm2 are expressed as Equations (35) and ( 36 ) given below: Tml = Tr x p (35) Tm2 = Tr x(l+ p) - Te (36) The first motor MGl applies the torque Tml determined by Equation (35) to the sun gear shaft 125C, while the second motor MG2 applies the torque Tm2 determined by Equation (36) to the crankshaft 156. This enables the power output from the engine 150 and defined by the torque Te and the revolving speed Ne to be converted to the power defined by the torque Tr and revolving speed Nr and output to the ring gear shaft 126. Under the condition of the nomogram of Fig. 39, the first motor MGl applies the torque in reverse of the rotation of the rotor 132 and thereby functions as a generator to regenerate electrical energy Pml expressed as the product of the torque Tml and the revolving speed Ns. The second motor MG2, on the other hand, applies the torque in the direction of rotation of the rotor 142 and thereby functions as a motor to consume electrical energy Pm2 expressed as the product of the torque Tm2 and the revolving speed Nr. Although the revolving speed Ns of the sun gear shaft 125C is positive in the nomogram of Fig. 39, it may be negative according to the revolving speed Ne of the engine 150 and the revolving speed Nr of the ring gear shaft 126C as shown in the nomogram of Fig. 40. In the latter case, the first motor MGl applies the torque in the direction of rotation of the rotor 132 and thereby functions as a motor to consume the electrical energy Pml expressed as the product of the torque Tml and the revolving speed Ns. The second motor MG2, on the other hand, applies the torque in reverse of the rotation of the rotor 142 and thereby functions as a generator to regenerate the electrical energyPm2, which is expressed as the product of thetorque Tm2 and the revolving speed Nr, from the ring gear shaft 126C. Like the power output apparatus 110 of the fifth embodiment, the operation principle of the power output apparatus HOC of the six embodiment discussed above is on the assumption that the efficiency of power conversion by the planetary gear 120, the motors MGl and MG2, and the transistors Tri through Trl6 is equal to the value '1', which represents 100%. In the actual state, however, the conversion efficiency is less than the value *1 ' , so that the energy Pe output from the engine 150 should be a little greater than the energy Pr output to the ring gear shaft 126C or alternatively the energy Pr output to the ring gear shaft 126C should be a little smaller than the energy Pe output from the engine 150. As discussed previously, an energy loss in the planetary gear 120 due to a mechanical friction or the like is significantly small and the efficiency of the synchronous motors used for the first and the second motors MGl and MG2 is substantially equal to the value ' 1 ' . The efficiency of power conversion thus becomes practically equal to the value ' 1 ' . In the following discussion of the sixth embodiment, the efficiency is assumed to be equal to the value '1' (=100%) unless otherwise specified. As clearly understood from the operation principle of the power output apparatus HOC of the sixth embodiment discussed above, controlling the motors MGl and MG2 enables the engine 150 to bedriven at any driving point that can output the energy identical with the energy to be output to the ring gear shaft 126C. The planetary gear 120 and the first motor MGl in the power output apparatus HOC of the sixth embodiment have similar functions to those of the clutch motor 30 in the power output apparatus 20D of the fourth embodiment, while the second motor MG2 works in the similar manner to the assist motor 40. The map of Fig. 6 , which is used to set the target torque Te* and the target revolving speed Ne* of the engine 50 in the torque control of the power output apparatus 20D of the fourth embodiment, can thus be applicable to set the target torque Te* and the target revolving speed Ne* of the engine 150 in the torque control of the power output apparatus HOC of the sixth embodiment. As discussed previously, all the processes carried out by the power output apparatuses 20 and 20A through 20C of the first through the third embodiments and their modification are applicable to the power output apparatus 110 of the fifth embodiment, by taking into account the balance on the dynamic collinear line in the nomograms of Figs. 30 and 31. In the same manner, all the processes carried out by the power output apparatus 20D of the fourth embodiment, that is, all the processes in the power output apparatuses 20 and 20A through 20C of the first through the third embodiments and their modification applied to the structure where the assist motor 40 is attached to the crankshaft 56 of the engine 50, are also applicable to the power output apparatus HOC of the sixth embodiment, by taking into account the balance on the dynamic collinear line in the nomograms of Figs. 39 and 40. Namely the power output apparatus HOC of the sixth embodiment implements the same functions and exerts the same effects as those of the power output apparatuses 20 and 20A through 20C of the first through the third embodiments and their modification. Although the second motor MG2 is interposed between the engine 150 and the first motor MGl in the power output apparatus HOC of the sixth embodiment, the engine 150 may be interposed between the first motor MGl and the second motor MG2 as shown by another power output apparatus HOD of Fig. 41 having a modified structure. In the power output apparatus HOC of the sixth embodiment, the power output to the ring gear shaft 126C is taken out of the place between the first motor MGl and the second motor MG2 via the power feed gear 128 linked with the ring gear 122. As shown by still another power output apparatus HOE of Fig. 42 given as another possible modification, however, the power may be taken out of the casing 119, from which a ring gear shaft 126E is extended. The present invention is not restricted to the above embodiments or applications, but there may be many modifications, changes, and alterations without departing from the scope or spirit of the main characteristics of the present invention. Some examples of possible modification are given below. For example, any one of the power output apparatuses 20 and 20A through 20C of the first through the third embodiments may be applied to the vehicle with a four-wheel drive (4WD) as shown in Fig. 43. In the structureof Fig.43, the assistmotor 40, which is mechanically linked withthe drive shaft 22 in the structure of Fig. 1, is separated from the drive shaft 22 and independently disposed in a rear-wheel portion of the vehicle in order to drive rear driving wheels 27 and 29. One end of the drive shaft 22 is linked with a differential gear 24 via a gear 23, so as to drive front driving wheels 26 and 28. The control procedures of the first through the third embodiments are also applicable to the structure of Fig. 43. Fig. 44 shows another example, in which the power output apparatus 110 of the fifth embodiment is applied to the vehicle with a four-wheel drive (4WD). In the structure of Fig. 44, the second motor MG2, which is attached to the ring gear shaft 126 in the structure of Fig. 27, is separated from the ring gear shaft 126 and independently disposed in a rear-wheel portion of thevehicle in order to drive rear driving wheels 117 and 119. The power feed gear 128 linked with the ring gear shaft 126 is connected to a differential gear 114 in a front-wheel portion of the vehicle via the chain belt 129 and the power transmission gear 111, so as to drive front driving wheels 116 and 118. The control procedures of the fifth embodiment are also applicable to the structure of Fig. 44. The gasoline engine driven by means of gasoline is used as the engine 50 or the engine 150 in the above embodiments. The principle of the invention is, however, applicableto other internalcombustion engines and external combustion engines, such as Diesel engines, turbine engines, and jet engines. Permanent magnet (PM)-type synchronous motors are used for the clutch motor 30 andthe assist motor 40 in the first throughthe fourth embodiments and for the first motor MGl and the second motor MG2 in the fifth and the sixth embodiments. Any other motors which can implement both the regenerative operation and the power operation, such as variable reluctance (VR)-type synchronous motors, vernier motors, d.c. motors, induction motors, superconducting motors, and stepping motors, may, however, be usedaccording tothe requirements. The rotary transformer 38 used in the first through the fourth embodiments as means for transmittingthe electric powertotheclutch motor 30 may be replaced by a slip ring-brush contact, a slip ring-mercury contact, a semiconductor coupling of magnetic energy, or the like. Transistor inverters are used for the first and the second driving circuits 91 and 92 of the first through the fourthembodiments and for the fist and the second driving circuits 191 and 192 of the fifth and the sixth embodiments. Other available examples include IGBT (insulated gate bipolar mode transistor) inverters, thyristor inverters, voltage PWM (pulse width modulation) inverters, square-wave inverters (voltage inverters and current inverters) , and resonance inverters. The battery 94 in the first through the fourth embodiments or the battery 194 in the fifth and the sixth embodiments may include Pb cells, NiMH cells, Li cells, or the like cells. A capacitor may be used in place of the battery 94 or the battery 194. Although the power output apparatus is mounted on the vehicle in all the above embodiments, it may be mounted on other transportation means like ships and airplanes as well as a variety of industrial machines. It should be clearly understood that the above embodiments discussed above are only illustrative and not restrictive inany sense. The scope and spirit of the present invention are limited only by the terms of the appended claims.

Claims: CLAIMS 1. A power output apparatus for outputting power to a drive shaft, said power output apparatus comprising: an engine having an output shaft; energy adjustment means having a first shaft connected with said output shaft of said engine and a second shaft connected with said drive shaft, said energy adjustment means adjusting a difference in energy between power input into or output from said first shaft and power input into or output from said second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between said drive motor and said drive shaft; target power setting means for setting a target power output to said drive shaft; driving state setting means for setting a target driving state of said engine that outputs energy corresponding to the target power set by said target power setting means, based on a predetermined condition; and control means for controlling said engine, so as to enable said engine to be driven in the target driving state set by said driving state settingmeans, and for controlling said energy adjustmentmeans and said drive motor, so as to enable power output from said engine to be subjected to torque conversion and output as the target power to said drive shaft. 2. A power output apparatus in accordance with claim 1, wherein the predetermined condition in said driving state setting means comprises a condition for enhancing an energy efficiency of said engine that outputs energy corresponding to the target power. 3. A power output apparatus in accordance with claim 1, wherein the predetermined condition in said driving state setting means comprises acondition forenhancingacomprehensive efficiency, which is calculated by multiplying anenergy efficiency of said enginethat outputs energy corresponding to the target power by a transmission efficiency of said energy adjustment means and said drive motor when the power output from said engine is subjected to torque conversion and output to said drive shaft. 4. A power output apparatus in accordance with claim 1, wherein the predetermined condition in said driving state setting means comprises a condition for continuously varying a driving state of said engine with a variation in target power. 5. A power output apparatus in accordance with claims 1, 2, 3 or 4, wherein said energy adjustment means comprises a twin-rotor motor comprising a first rotor connected with said first shaft and a second rotor connected with said second shaft, said second rotor being rotatable relative to said first rotor, whereby power is transmitted between said first shaft and said second shaft via an electromagnetic coupling of said first rotor with said second rotor, said twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of said first rotor with said second rotor and a difference in revolving speed between said first rotor and said second rotor. 6. A power output apparatus in accordance with claim 5, wherein said drive motor comprises said second rotor included in said twin-rotor motor and a stator for rotating said second rotor. 7. A power output apparatus in accordance with claims 1, 2, 3 or 4, wherein said energy adjustment means further comprises: three-shaft-type power input and output means connected with said first shaft, said second shaft, and a third shaft, said three-shaft-type power input and output means for, when powers input into or output from any two shafts among said three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with said third shaft, wherein power is transmitted between said third shaft and said shaft motor. 8. Apoweroutput apparatus inaccordancewith claim1, saidpower output apparatus further comprising driving state detecting means for detecting a driving state of said engine, wherein said control means further comprises means for controlling said energy adjustment means, so as to enable saidengine to be driven in the target driving state, based on the driving state of said engine detected by said driving state detecting means. 9. Apower output apparatus inaccordancewith claim 1, saidpower output apparatus further comprising driving state detecting means for detecting a driving state of said engine, wherein said control means further comprises tentative target driving state setting means for, whena statedeviation of the driving state detected by said driving state detecting means from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the predetermined condition and setting the selected driving state as a tentative target driving state, the tentative target driving state set by said tentative target driving state setting means being used in place of the target driving state for operation control of said engine and control of said energy adjustment means and said drive motor, until the state deviation enters the predetermined range. 10. A power output apparatus in accordance with claim 9, wherein said tentative target driving state setting means further comprises predetermined range settingmeans for setting thepredeterminedrange based on the driving state detected by said driving state detecting means. 11. A power output apparatus in accordance with claim 9, wherein said control means further comprises means for controlling said energy adjustment means, so as to enable said engine to be driven in the target driving state, based on the driving state detected by said driving statedetectingmeans, whenthe statedeviation is within the predetermined range. 12. A power output apparatus in accordance with claim 9, said power output apparatus furthercomprising storage batterymeans being charged with electrical energy taken out of said energy adjustment means, being charged with electrical energy taken out of said drive motor, being discharged to release electrical energy used in said energy adjustment means, and being discharged to release electrical energy used in said drive motor, wherein said control means further comprises means for, when the tentative target driving state is used in place of the target driving state for the operation control of said engine and the control of said energy adjustment means and said drive motor, utilizing the electrical energy stored into or released from said storage battery means and controlling said drive motor, so as to enable said drive motor to input or output a specific power into or from said drive shaft, said specific power corresponding to an energy difference between the target power and the power output rom said engine that is driven in the tentative target driving state. 13. A power output apparatus in accordance with claim 1, wherein said control means further comprises: driving state estimating means for estimating a driving state of said engine when said target power setting means sets a different target power; and estimated-condition control means for controlling said energy adjustment means and said drive motor based on the estimated driving state of said engine. 14. A poweroutput apparatus in accordance withclaim 13, wherein said driving state estimating means further comprises means for estimating thedriving state of saidengine basedon arevolvingspeed of said output shaft of said engine and a state of said energy adjustment means. 15. A poweroutput apparatus in accordance withclaim 13, wherein said estimated-condition control means further comprises means for controlling said energy adjustment means and said drive motor, so as to enable an estimatedpower output from said engine corresponding to the driving state of said engine estimated by said driving state estimating means to be subjected to torque conversion and output as the target power to said drive shaft. 16. A power output apparatus in accordance with claim 13, said poweroutput apparatus furthercomprising storage batterymeans being charged with electrical energy taken out of said energy adjustment means, being charged with electrical energy taken out of said drive motor, being discharged to release electrical energy used in said energy adjustment means, and being discharged to release electrical energy used in said drive motor, wherein said estimated-condition control means further comprises means for utilizing the electrical energy stored into or released from said storage battery means and controlling said drive motor, so as to enable said drive motor to input or output a specific power into or fromsaiddrive shaft, said specific powercorresponding to an energy difference between the target power and the estimated power output from said engine corresponding to the driving state of said engine estimated by said driving state estimating means. 17. Apoweroutputapparatus foroutputtingpowertoadrive shaft, said power output apparatus comprising: an engine having an output shaft; energy adjustment means having a first shaft connected with said output shaft of said engine and a second shaft connected with said drive shaft, said energy adjustment means adjusting a difference in energy between power input into or output from said first shaft and power input into or output from said second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between said drive motor and said output shaft of said engine; target power setting means for setting a target power output to said drive shaft; driving state setting means for setting a target driving state of said engine that outputs energy corresponding to the target power set by said target power setting means, based on a predetermined condition; and control means for controlling said engine, so as to enable said engine to be driven in the target driving state set by said driving state settingmeans, and for controlling said energy adjustmentmeans and said drive motor, so as to enable power output from said engine to be subjected to torque conversion and output as the target power to said drive shaft. 18. A power output apparatus in accordance withclaim 17, wherein the predetermined condition in said driving state setting means comprises a condition for enhancing an energy efficiency of said engine that outputs energy corresponding to the target power. 19. A power output apparatus in accordance withclaim 17, wherein the predetermined condition in said driving state setting means comprises a condition for enhancing acomprehensive efficiency, which is calculated by multiplying anenergy efficiency of said enginethat outputs energy corresponding to the target power by a transmission efficiency of said energy adjustment means and said drive motorwhen the power output from said engine is subjected to torque conversion and output to said drive shaft. 20. A power output apparatus in accordance withclaim 17, wherein the predetermined condition in said driving state setting means comprises a condition for continuously varying a driving state of said engine with a variation in target power. 21. A power output apparatus in accordance with claims 17, 18, 19, or 20, wherein saidenergyadjustmentmeans comprises atwin-rotor motor comprising a first rotor connected with said first shaft and a second rotor connected with said second shaft, said second rotor being rotatable relative to said first rotor, whereby power is transmitted between said first shaft and said second shaft via an electromagnetic coupling of said first rotor with said second rotor, said twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of said first rotor with said second rotor and a difference in revolving speed between said first rotor and said second rotor. 22. A poweroutput apparatus in accordance withclaim 21, wherein said drive motor comprises said first rotor included in said twin-rotor motor and a stator for rotating said first rotor. 23. A power output apparatus in accordance with claims 17, 18, 19, or 20, wherein said energy adjustment means further comprises: three-shaft-type power input and output means connected with said first shaft, said second shaft, and a third shaft, said three-shaft-type power input and output means for, when powers input into or output from any two shafts among said three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with said third shaft, wherein power is transmitted between said third shaft and said shaft motor. 24. A power output apparatus in accordance with claim 17, said power output apparatus further comprising driving state detecting means for detecting a driving state of said engine. wherein said control means further comprises tentative target driving state setting means for, whena statedeviation of thedriving state detected by said driving state detecting means from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the predetermined condition and setting the selected driving state as a tentative target driving state, the tentative target driving state set by said tentative target driving state setting means being used in place of the target driving state for operation control of said engine and control of said energy adjustment means and said drive motor, until the state deviation enters the predetermined range. 25. A poweroutput apparatus in accordance withclaim 17, wherein said control means further comprises: driving state estimating means for estimating a driving state of said engine when said target power setting means sets a different target power; and estimated-condition control means for controlling said energy adjustment means and said drive motor based on the estimated driving state of said engine. 26. A power output apparatus in accordance with claim 25, wherein said estimated-condition control means further comprises means for controlling said energy adjustment means and said drive motor, so as to enable an estimatedpower output from said engine corresponding to the driving state of said engine estimated by said driving state estimating means to be subjected to torque conversion and output as the target power to said drive shaft. 27. A power output apparatus in accordance with claim 25, said poweroutput apparatus furthercomprising storagebatterymeans being charged with electrical energy taken out of said energy adjustment means, being charged with electrical energy taken out of said drive motor, being discharged to release electrical energy used in said energy adjustment means, and being discharged to release electrical energy used in said drive motor, wherein said estimated-condition control means further comprises means for utilizing the electrical energy stored into or released from said storage battery means and controlling said drive motor, so as to enable said drive motor to input or output a specific power into or from said output shaft of said engine, said specific power corresponding to an energy difference between the target power and the estimated power output from said engine corresponding to the driving state of said engine estimated by said driving state estimating means. 28. A driving system comprising: an engine having an output shaft; energy a justment means having a first shaft connected with said output shaft of said engine and a second shaft connected with a drive shaft of said driving system, said energy adjustment means adjusting a difference in energy between power input into or output from said first shaft and power input into or output from said second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between said drive motor and said drive shaft; target power setting means for setting a target power output to said drive shaft; driving state setting means for setting a target driving state of said engine that outputs energy corresponding to the target power set by said target power setting means, based on a first condition for enhancing an energy efficiencyof said engine that outputs energy corresponding to the target power and a second condition for making a vibration due to an operation of said engine out of a range of resonance frequency of said driving system; and control means for controlling said engine, so as to enable said engine to be driven in the target driving state set by said driving state settingmeans, and for controlling said energy adjustmentmeans and said drive motor, so as to enable power output from said engine to be subjected to torque conversion and output as the target power to said drive shaft. 29. A driving system in accordance with claim 28, wherein said energy adjustment means comprises a twin-rotor motor comprising a first rotor connected with said first shaft and a second rotor connected with said second shaft, said second rotor being rotatable relative to said first rotor, whereby power is transmitted between said first shaft and said second shaftviaan electromagnetic coupling of said first rotor with said second rotor, said twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of said first rotor with said second rotor and a difference in revolving speed between said first rotor and said second rotor. 30. A driving system in accordance with claim 29, wherein said drive motor comprises said second rotor included in said twin-rotor motor and a stator for rotating said second rotor. 31. A driving system in accordance with claim 28, wherein said energy adjustment means further comprises: three-shaft-type power input and output means connected with said first shaft, said second shaft, and a third shaft, said three-shaft-type power input and output means for, when powers input into or output from any two shafts among said three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with said third shaft, wherein power is transmitted between said third shaft and said shaft motor. 32. A driving system comprising: an engine having an output shaft; energy adjustment means having a first shaft connected with said output shaft of said engine and a second shaft connected with a drive shaft of said driving system, said energy adjustment means adjusting a difference in energy between power input into or output from said first shaft and power input into or output from said second shaft by regulating input and output of corresponding electrical energy; a drive motor, wherein power is transmitted between said drive motor and said output shaft of said engine; target power setting means for setting a target power output to said drive shaft; driving state setting means for setting a target driving state of said engine that outputs energy corresponding to the target power set by said target power setting means, based on a first condition for enhancing an energy efficiencyof said engine that outputs energy corresponding to the target power and a second condition for making a vibration due to an operation of said engine out of a range of resonance frequency of said driving system; and control means for controlling said engine, so as to enable said engine to be driven in the target driving state set by said driving state settingmeans, and for controlling said energy adjustmentmeans and said drive motor, so as to enable power output from said engine to be subjected to torque conversion and output as the target power to said drive shaft. 33. A driving system in accordance with claim 32, wherein said energy adjustment means comprises a twin-rotor motor comprising a first rotor connected with said first shaft and a second rotor connected with said second shaft, said second rotor being rotatable relative to said first rotor, whereby power is transmitted between said first shaft and said second shaftvia anelectromagnetic coupling of said first rotor with said second rotor, said twin-rotor motor inputting and outputting electrical energy based on the electromagnetic coupling of said first rotor with said second rotor and a difference in revolving speed between said first rotor and said second rotor. 34. A driving system in accordance with claim 33, wherein said drive motor comprises said first rotor included in said twin-rotor motor and a stator for rotating said first rotor. 35. A driving system in accordance with claim 32, wherein said energy adjustment means further comprises: three-shaft-type power input and output means connected with said first shaft, said second shaft, and a third shaft, said three-shaft-type power input and output means for, when powers input into or output from any two shafts among said three different shafts are determined, automatically setting a power input into or output from a residual shaft based on the powers thus determined; and a shaft motor connected with said third shaft, wherein power is transmitted between said third shaft and said shaft motor. 36. A method of controlling a power output apparatus for outputting power to a drive shaft, said method comprising the steps of: (a) providinganengine having anoutput shaft; energyadjustment means having a first shaft connected with said output shaft of said engine and a second shaft connectedwith said drive shaft, saidenergy adjustment means adjusting adifference inenergy between power input into or output from said first shaft and power input into or output from said second shaftbyregulating input andoutputof corresponding electrical energy; and a drive motor, wherein power is transmitted between said drive motor and said drive shaft; (b) setting a target power output to said drive shaft; (c) setting a target driving state of said engine that outputs energy corresponding to the target power set in said step (b) , based on a specific condition of selecting a specific driving point that attains a highest possible efficiency among a plurality of available driving points of said engine that outputs energy corresponding to the target power; and (d) controlling said engine, so as to enable said engine to be driven in the target driving state set in said step (c), and for controlling said energy adjustment means and said drive motor, so as to enable power output from said engine to be subjected to torque conversion and output as the target power to said drive shaft. 37. A method in accordance with claim 36, wherein said step (d) further comprises the steps of: (e) detecting a driving state of said engine; (f) when a state deviation of the driving state of said engine detected in said step (e) from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the specific condition andsettingthe selecteddriving state as atentativetarget driving state; and (g) using the tentative target driving state set in said step (f) in place of the target driving state, in order to control said engine, said energy adjustment means, and said drive motor, until the state deviation enters the predetermined range. 38. A method in accordance with claim 36, wherein said step (d) further comprises the steps of: (h) when a different target power is set, estimating a driving state of said engine based on a revolving speed of said output shaft of said engine and a state of said energy adjustment means; and (i) controlling saidenergy adjustmentmeans and saiddrivemotor, so as to enable power output from saidengine tobe subjectedtotorque conversion and output to said drive shaft, based on the estimated driving state of said engine. 39. A method of controlling a power output apparatus for outputting power to a drive shaft, said method comprising the steps of: (a) providinganengine having an output shaft; energy adjustment means having a first shaft connected with said output shaft of said engine anda second shaft connectedwith saiddrive shaft, saidenergy a justment means adjusting adifference inenergy between power input into or output from said first shaft and power input into or output from saidsecond shaft by regulating input andoutput of corresponding electrical energy; and a drive motor, wherein power is transmitted between said drive motor and said drive shaft; (b) setting a target power output to said drive shaft; (c) setting a target driving state of said engine that outputs energy corresponding to the target power set in said step (b) , based on a specific condition of selecting a specific driving point that attains ahighest possible comprehensive efficiency among aplurality of available driving points of said engine that outputs energy corresponding to the target power, said comprehensive efficiency being calculated by multiplying an energy efficiency of said engine by a transmission efficiency of said energy adjustment means andsaid drive motor when the power output from said engine is subjected to torque conversion and output to said drive shaft; and (d) controlling said engine, so as to enable said engine to be driven in the target driving state set in said step (c), and for controlling said energy adjustment means and said drive motor, so as to enable power output from said engine to be subjected to torque conversion and output as the target power to said drive shaft. 40. A method in accordance with claim 39, wherein said step (d) further comprises the steps of: (e) detecting a driving state of said engine; (f) when a state deviation of the driving state of said engine detected in said step (e) from the target driving state is out of a predetermined range, selecting a driving state within the predetermined range based on the state deviation and the specific condition and settingthe selecteddriving state as atentativetarget driving state; and (g) using the tentative target driving state set in said step (f) in place of the target driving state, in order to control said engine, said energy adjustment means, and said drive motor, until the state deviation enters the predetermined range. 41. A method in accordance with claim 39, wherein said step (d) further comprises the steps of: (h) when a different target power is set, estimating a driving state of said engine based on a revolving speed of said output shaft of said engine and a state of said energy adjustment means; and (i) controllingsaidenergyadjustmentmeansandsaiddrivemotor, so as toenablepower output from saidengine tobe subjectedtotorque conversion and output to said drive shaft, based on the estimated driving state of said engine.

Inventors: KAWABATA YASUTOMO, KOTANI TAKESHI, MAEDA TOMOHARU, NII YOSHIHIDE, SASAKI SHOICHI, YAMADA EIJI, KAWABATA, YASUTOMO, KOTANI, TAKESHI, MAEDA, TOMOHARU, NII, YOSHIHIDE, SASAKI, SHOICHI, YAMADA, EIJI
Applicants: KAWABATA YASUTOMO, KOTANI TAKESHI, MAEDA TOMOHARU, NII YOSHIHIDE, SASAKI SHOICHI, TOYOTA MOTOR CO LTD, YAMADA EIJI, KAWABATA, YASUTOMO, KOTANI, TAKESHI, MAEDA, TOMOHARU, NII, YOSHIHIDE, SASAKI, SHOICHI, TOYOTA JIDOSHA KABUSHIKI KAISHA, YAMADA, EIJI
Application year: 1996
Main classification: B60L11
IPC codes: B60K1, B60K6, B60K17, B60L11, B60W10, B60W20, H02K7, H02K51, H02P9, H02P15