Construction machine

A construction machine including an internal combustion engine controlled based on a torque command, an electric motor mechanically connected to the internal combustion engine, and an electric energy storage device that supplies electric power to the electric motor, the construction machine performing work by driving a hydraulic pressure generator using the internal combustion engine and the electric motor, the construction machine including: a speed control device that controls a speed of the electric motor based on a speed command; and a torque limiter that limits the torque command relative to a torque target, wherein the torque command is limited by the torque limiter in such a manner that a rate of change with time of the torque command is limited to be equal to or less than a predetermined value.

TECHNICAL FIELD

The present invention relates generally to construction machines and, more particularly, to a construction machine that performs work hydraulically by driving a hydraulic pressure generator with an internal combustion engine and an electric motor.

BACKGROUND ART

A known technique in a hybrid construction machine accurately brings an engine to a target operating state by causing a motor generator to assist the engine or to generate electricity through an as simple as possible configuration (see, for example, patent document 1). To achieve that task, the technique disclosed in patent document 1 incorporates a controller that obtains an engine speed corresponding to optimum torque of a set speed as a target speed and performs the following control so as to bring the engine close to an optimum operating state. Specifically, when the engine speed is lower than the target speed because of a large load torque on the engine, the controller causes the motor generator to operate as an electric motor according to a difference therebetween to thereby assist torque. When the engine speed is higher than the target speed because of a small load torque on the engine, the controller causes the motor generator to operate as a generator according to the difference therebetween to thereby store the generated electricity in a battery.

Another known control technique is, even with a sharp increase in a hydraulic load, to increase driving power supplied to a hydraulic pressure generator in response to the increase in the hydraulic load, while maintaining appropriate operating conditions of an internal combustion engine (see, for example, patent document 2). To achieve that task, the technique disclosed in patent document 2, while causing the internal combustion engine to drive the hydraulic pressure generator, sets a rate of increase in an output of the internal combustion engine to a predetermined value. An output upper limit value of the internal combustion engine obtained from the predetermined value of the rate of increase is then compared with a driving power requirement obtained from a hydraulic pressure output that the hydraulic pressure generator is required to produce. The output of the internal combustion engine is then controlled so as to be equal to, or smaller than, the output upper limit value when the driving power requirement exceeds the output upper limit value.

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

The technique disclosed in patent document 1 does not, however, consider a transient state when the load torque undergoes a sudden change and thus involves an unavoidable situation in which a rate of change with time in the output torque of the engine as an internal combustion engine becomes high. This requires excessive fuel injection and may produce a large amount of particulate matter (PM) or nitrogen oxide (NOx).

The technique disclosed in patent document 2 controls an electric motor based on the output requirement of the hydraulic pressure generator and thus requires the output requirement of the hydraulic pressure generator. With construction machines, however, it is difficult to identify a load on a work implement, to detect a flow rate of hydraulic fluid in detail, and thus to accurately detect or estimate the output requirement. Moreover, because the electric motor is controlled without having feedback information on states of the engine, an error involved with the output requirement hampers accurate control of the rate of change with time in the engine output torque. For these reasons, a large amount of particulate matter (PM) or nitrogen oxide (NOx) may be produced, as with patent document 1.

An object of the present invention is to provide a construction machine that can reduce particulate matter (PM) or nitrogen oxide (NOx) discharged from an internal combustion engine mounted on the construction machine.

Means for Solving the Problem

To achieve the foregoing object, the present invention provides a construction machine including an internal combustion engine controlled based on a torque command, an electric motor mechanically connected to the internal combustion engine, an electric energy storage device that supplies electric power to the electric motor and a hydraulic pressure generator. The construction machine performs work by driving the hydraulic pressure generator using the internal combustion engine and the electric motor. The construction machine includes: first control means that controls a speed of the electric motor based on a speed command; and second control means that obtains the torque command having a rate of change with time limited based on a torque target.

The present invention further provides a construction machine including an internal combustion engine, an electric motor mechanically connected to the internal combustion engine, an electric energy storage device that supplies electric power to the electric motor and a hydraulic pressure generator. The construction machine performs work by driving the hydraulic pressure generator using the internal combustion engine and the electric motor. The electric motor is speed-controlled by a speed command, and torque of the internal combustion engine is greater than torque of the electric motor when a rate of change with time in torque of the hydraulic pressure generator is low, and the torque of the electric motor is greater than the torque of the internal combustion engine when the rate of change with time in the torque of the hydraulic pressure generator is high.

The present invention still further provides a construction machine including: an internal combustion engine; an electric motor mechanically connected to the internal combustion engine; an electric energy storage device that supplies electric power to the electric motor; and a hydraulic pressure generator. The construction machine performs work by driving the hydraulic pressure generator using the internal combustion engine and the electric motor. The electric motor is speed-controlled by the speed command, and a change of a rate of change with time in torque of the internal combustion engine is higher than a change of a rate of change with time in torque of the hydraulic pressure generator when the rate of change with time in the torque of the hydraulic pressure generator is low, and the change of the rate of change with time in the torque of the internal combustion engine is lower than the change of the rate of change with time in the torque of the hydraulic pressure generator when the rate of change with time in the torque of the hydraulic pressure generator is high.

Such arrangements allow the particulate matter (PM) or the nitrogen oxide (NOx) discharged from the internal combustion engine mounted on the construction machine to be reduced.

Effect of the Invention

The present invention can reduce the particulate matter (PM) or the nitrogen oxide (NOx) discharged from the internal combustion engine mounted on the construction machine.

MODES FOR CARRYING OUT THE INVENTION

Arrangements and operations of a construction machine according to a first embodiment of the present invention will be described below with reference toFIGS. 1 to 8. The following description assumes that the construction machine is a hydraulic excavator as a representative construction machine.

A general arrangement of the construction machine according to the first embodiment of the present invention will be described with reference toFIG. 1.

FIG. 1is a side view showing the general arrangement of the construction machine according to the first embodiment of the present invention.

A hydraulic excavator200includes a track structure201and a swing structure202. The track structure201has a function of causing the construction machine to travel with a track hydraulic motor. The track structure201includes a right track structure and a left track structure, each being driven by an independent track hydraulic motor. The swing structure202is rotated relative to the track structure201by a swing mechanism113.

The swing structure202includes a boom203, an arm204, and a bucket205that perform excavating work, the boom203, the arm204, and the bucket205being disposed on the one side (e.g., on the right-hand side, looking to the front) at a front portion of the swing structure202. The boom203, the arm204, and the bucket205are driven by a hydraulic cylinder107, a hydraulic cylinder106, and a hydraulic cylinder105, respectively.

The swing structure202further includes a cab206. An operator gets on board the cab206and uses an operating lever to operate the construction machine200.

An arrangement of a drive system that drives the construction machine according to the first embodiment will be described below with reference toFIG. 2.

FIG. 2is a block diagram showing the arrangement of the drive system that drives the construction machine according to the first embodiment of the present invention.

A diesel engine101as an internal combustion engine and a first electric motor102are mechanically connected to each other to thereby drive a hydraulic pump103as a hydraulic pressure generator. It is here noted that, for example, the diesel engine101, the first electric motor102, and the hydraulic pump103are mechanically connected so as to run at an identical speed. Hydraulic fluid sent from the hydraulic pump103is distributed by a control valve104based on an operation by the operator and supplied to the hydraulic cylinders105,106and107, a left track hydraulic motor108, and a right track hydraulic motor109. The hydraulic cylinder105drives the bucket205shown inFIG. 1. The hydraulic cylinder106drives the arm204shown inFIG. 1. The hydraulic cylinder107drives the boom203shown inFIG. 1. The left track hydraulic motor108and the right track hydraulic motor109drive the left track structure and the right track structure, respectively, of the track structure201shown inFIG. 1.

The first electric motor102and a second electric motor112that drives the swing mechanism113are each a three-phase synchronous motor and a motor generator. An electric power converter110converts direct current (DC) electric power stored in an electric energy storage device111to three-phase alternating current (AC) electric power and supplies the three-phase AC electric power to, and thereby drive, the first electric motor102and the second electric motor112. The first electric motor102is also operated as a generator to charge the electric energy storage device111via the electric power converter110. The second electric motor112operates as a generator when the swing structure202rotating is to be braked, thereby charging the electric energy storage device111via the electric power converter110.

A capacitor having a relatively small capacity is used for the electric energy storage device111. In this case, a charge amount of the electric energy storage device111needs to be appropriately controlled.

A subtractor120calculates a difference between a charge amount command Q* and a charge amount Q of the electric energy storage device111. The charge amount command Q* is given by a host controller and is a predetermined value that corresponds to, for example, an 80% charge amount of the electric energy storage device111.

A charge amount control device114calculates and outputs a torque target so that the difference obtained by the subtractor120becomes equal to 0, specifically, the charge amount Q of the electric energy storage device111agrees with the charge amount command Q*. A torque limiter115obtains and outputs a first torque command T1* that limits a rate of change with time relative to the torque target output by the charge amount control device114. If, for example, the torque target value changes in a step fashion, the torque target value is made to change gradually, so that the rate of change with time in the torque target may be limited to a level below a predetermined value.

An engine controller116controls the diesel engine101so that output torque of the diesel engine101becomes equal to the first torque command T1*. Specifically, the engine controller116controls an amount of fuel supplied by a fuel injection valve of the diesel engine101to a combustion chamber of the diesel engine101or an EGR recirculation amount.

A subtractor117calculates a difference between a rotational speed command N* and a rotational speed N of the first electric motor. The rotational speed command N* is given by a host controller and is, for example, a predetermined value.

A speed control device118obtains a second torque command T2* based on the difference calculated by the subtractor117so that the rotational speed command N* agrees with the rotational speed N of the first electric motor and outputs the second torque command T2* to the electric power converter110. The electric power converter110controls so that torque of the first electric motor102becomes equal to the second torque command T2*.

A swing control device119obtains a third torque command T3* based on an operating amount of a swing lever operated by the operator and outputs the third torque command T3* to the electric power converter110in order to control the second electric motor112. The electric power converter110controls so that torque of the second electric motor112becomes equal to the third torque command T3*.

The electric power converter110includes first and second electric power converting portions built therein, the first electric power converting portion controlling the first electric motor102, the second electric power converting portion controlling the second electric motor112. For example, the first electric power converting portion includes a plurality of switching elements and a control part. The switching elements convert DC electric power to three-phase AC electric power. The control part performs PWM control for opening or closing the switching elements so that current flowing through the first electric motor102agrees with a current command corresponding to the abovementioned second torque command T2*. The first electric power converting portion thereby controls so that the torque of the first electric motor102becomes equal to the second torque command T2*. Additionally, when the first electric motor102operates as a generator, the control part controls the switching elements and converts an output of electric power generated by the first electric motor102to DC electric power and stores the DC electric power in the electric energy storage device111. The second electric power converting portion, having arrangements and operations identical to those of the first electric power converting portion, controls so that the torque of the second electric motor112becomes equal to the third torque command T3*. When the second electric motor112operates as a generator, the control part controls the switching elements and converts an output of electric power generated by the second electric motor112to DC electric power and stores the DC electric power in the electric energy storage device111.

Operations of the drive system incorporated in the construction machine according to the first embodiment will be described below with reference toFIGS. 3 to 8.

FIGS. 3 to 8are timing charts showing the operations of the drive system incorporated in the construction machine according to the first embodiment of the present invention.

Operations of different parts of the drive system when, for example, the arm is operated will first be described below with reference toFIG. 3.

The abscissas onFIG. 3represent elapsed time. The ordinate onFIG. 3(a)represents pump torque of the hydraulic pump103and the ordinate onFIG. 3(b)represents the rotational speed N of the first electric motor102. It is assumed that the diesel engine101, the first electric motor102, and the hydraulic pump103are mechanically connected so as to run at an identical speed. The ordinate onFIG. 3(c)represents torque of the first electric motor102and the ordinate onFIG. 3(d)represents discharge current of the electric energy storage device111. The ordinate onFIG. 3(e)represents the charge amount Q of the electric energy storage device111and the ordinate onFIG. 3(f)represents torque of the diesel engine101.FIG. 3then shows that torque of the hydraulic pump103increases as a result of an operation performed by the operator at time t1. One of the cases in which the torque of the hydraulic pump103increases as a result of an operation of the operator is when, for example, the operator operates an operating lever for the bucket205shown inFIG. 1and the torque of the hydraulic pump103is increased to drive the hydraulic cylinder105according to the operation. Other possible cases include when each one of the boom203, the arm204or the track structure201is driven.

When the torque of the hydraulic pump103is increased at time t1as shown inFIG. 3(a), the rotational speed N decreases as shown inFIG. 3(b)with a resultant increase in the difference from the rotational speed command N*; this increases the second torque command T2*, which increases the torque of the first electric motor102as shown inFIG. 3(c). This causes the rotational speed N to start increasing and to recover at time t2as shown inFIG. 3(b). Specifically, even with fluctuations in pump torque, the speed control device118controls the torque of the first electric motor102and the rotational speed N is maintained at a constant level.

When the torque of the first electric motor102is increased at time t1as shown inFIG. 3(c), the discharge current of the electric energy storage device111increases at time t1to supply electric power as shown inFIG. 3(d)and the charge amount Q decreases as shown inFIG. 3(e). This increases the difference from the charge amount command Q* calculated by the subtractor120, which increases the engine torque target output by the charge amount control device114. The torque target is subject to limitation of the rate of change with time imposed by the torque limiter115and output as the first torque command T1* to the engine controller116.FIG. 3(f)shows the diesel engine torque when the rate of change with time in the first torque command T1* is limited as described above. Changes in torque after time t1follow the rate of change with time limited by the torque limiter115or lower. This eliminates the likelihood that the diesel engine101will change its torque sharply and allows the diesel engine101to avoid combustion in a condition of high equivalence ratios due to excessive fuel injection with which particulate matter tends to be produced or in a condition of excessive combustion temperatures at which nitrogen oxide tends to be produced.

When the torque of the diesel engine101increases at time t2to time t3as shown inFIG. 3(f), the torque of the first electric motor102decreases in proportion thereto as shown inFIG. 3(c). This is because of the torque of the first electric motor102being controlled so that a sum of the torque of the diesel engine101and the torque of the first electric motor102balances the pump torque to thereby keep the rotational speed N constant.

The control at time t2to time t3will be described in greater detail below. Because the charge amount Q of the electric energy storage device111decreases at time t2, the difference output by the subtractor120increases. Accordingly, the torque target value output by the charge amount control device114increases. The engine controller116controls the output torque of the diesel engine101according to the torque target value, which causes the torque of the diesel engine101to increase gradually as shown inFIG. 3(f). Meanwhile, when the output torque of the diesel engine101increases, the rotational speed of the diesel engine101increases and the rotational speed of the first electric motor102connected to the diesel engine101also increases. As a result, the speed difference output by the subtractor117increases. This causes the second torque command T2* output by the speed control device118to decrease. The torque of the first electric motor102, being controlled by the electric power converter110according to the second torque command T2*, gradually decreases as shown inFIG. 3(c).

When the torque of the diesel engine101exceeds the pump torque at time t3, the torque of the first electric motor102becomes negative, specifically, the first electric motor102performs an electric power generating operation and the diesel engine101drives the first electric motor102that performs the electric power generating operation as well as the hydraulic pump103. In addition, the electric power generated by the first electric motor102is supplied to the electric energy storage device111, which causes the charge amount Q to start increasing toward the charge amount command Q* as shown inFIG. 3(e).

At time t4, the charge amount Q shown inFIG. 3(e)substantially agrees with the charge amount command Q*. At this time, the torque of the first electric motor102is 0 as shown inFIG. 3(c)and the torque of the diesel engine101balances the pump torque with the rotational speed N controlled at the rotational speed command N*.

Operations of different parts of the drive system when the swing structure202performs a swing operation will be described below with reference toFIG. 4. The ordinates onFIGS. 4(a) to 4(f)represent the same as those onFIGS. 3(a) to 3(f).FIG. 4(g)shows the output of the second electric motor112.FIG. 4shows that the second electric motor112is started by an operation of the swing lever performed by the operator at time t1, the second electric motor112is braked by an operation of the swing lever performed by the operator at time t2, and the second electric motor112is brought to a stop at time t4.

When the second electric motor112starts rotating at time t1, the rotational speed starts increasing, which increases the output of the second electric motor112as shown inFIG. 4(g). Accordingly, to prevent the charge amount Q of the electric energy storage device111from being decreased due to an increase in the discharge current of the electric energy storage device111, the torque target output by the charge amount control device114increases. This increases the torque of the diesel engine101as shown inFIG. 4(f). Meanwhile, to prevent the rotational speed N from increasing due to the increase in the torque of the diesel engine101, the speed control device118decreases the second torque command T2*. This makes the torque of the first electric motor102negative as shown inFIG. 4(c). The first electric motor102then performs the electric power generating operation and the discharge current of the electric energy storage device111is prevented from increasing, so that the charge amount Q can be prevented from decreasing. Specifically, the torque of the diesel engine101increases in proportion to the output of the second electric motor112. The first electric motor102then generates electric power with the increased torque, so that the rotational speed N and the charge amount Q are controlled so as to agree with the rotational speed command N* and the charge amount command Q*, respectively. It is noted that the rate of change with time in the torque target associated with the increase in the output of the second electric motor112at this time is equal to, or lower than, the limited value and the first torque command T1* agrees with the torque target.

Through the foregoing control, at time t1to time t2, the torque of the diesel engine101increases as shown inFIG. 4(f)and the torque of the first electric motor102decreases (the amount of electric power generated increases) as shown inFIG. 4(c), in proportion to the increase in the output of the second electric motor112as shown inFIG. 4(g).

When deceleration of the second electric motor112is started at time t2, the output suddenly changes from powering to regeneration. The output of the second electric motor112undergoes a sudden change from positive to negative as shown inFIG. 4(g). Accordingly, in order to absorb electric power regenerated by the second electric motor112and electric power generated by the first electric motor102, the discharge current of the electric energy storage device111is decreased as shown inFIG. 4(d), specifically, charging of the electric energy storage device111is started as shown inFIG. 4(e)to increase the charge amount Q. As the charge amount Q increases, the torque target is decreased by the charge amount control device114. At this time, the torque target tends to change sharply because of the precipitous change in the output of the second electric motor112; however, because of the rate of change in the first torque command T1* being limited by the torque limiter115, the torque of the diesel engine101does not change precipitously, as shown inFIG. 4(f).

This prevents the diesel engine101from changing its torque precipitously and allows the diesel engine101to avoid combustion in a condition of high equivalence ratios due to excessive fuel injection with which particulate matter tends to be produced or in a condition of excessive combustion temperatures at which nitrogen oxide tends to be produced.

The torque of the first electric motor102with its rotational speed N controlled at a constant level increases with the decreasing torque of the diesel engine101as shown inFIG. 4(c)and the amount of electric power generated by the first electric motor102decreases slowly. Thus, the charge amount Q continues to increase for some while.

The torque of the first electric motor102continues to increase and the first electric motor102shifts from an electric power generating state to a powering state. Then, when power consumption exceeds the electric power regenerated by the second electric motor112at time t3, the charge amount Q starts decreasing as shown inFIG. 4(e). When the charge amount Q decreases, the torque target is increased by the charge amount control device114and, as shown inFIG. 4(f), the torque of the diesel engine101increases. To prevent the rotational speed N from increasing due to the increase in the torque of the diesel engine101, the speed control device118decreases the torque of the first electric motor102and the discharge current of the electric energy storage device111decreases as shown inFIG. 4(d).

As a result, the following conditions develop at time t5: specifically, the torque of the first electric motor102is 0 as shown inFIG. 4(c), the discharge current of the electric energy storage device111is 0 as shown inFIG. 4(d), the charge amount Q agrees with the charge amount command Q* as shown inFIG. 4(e), and the torque of the diesel engine101agrees with the pump torque.

As described above, even when the second electric motor112performs powering and regenerative operations as a result of a swing operation, the rotational speed N is controlled so as to agree with the rotational speed command N*, the rate of change with time in the torque of the diesel engine101can be limited, and the charge amount Q of the electric energy storage device111is controlled so as to agree with the charge amount command Q*.

Changes with time of the pump torque, the rotational speed N, the torque of the first electric motor102, and the torque of the diesel engine101when the rate of change with time in the pump torque is changed will be described below with reference toFIGS. 5 to 8.

FIGS. 5 to 8are each concerned with a specific condition of the rate of change with time in the pump torque, the conditions being labeled as condition1to condition4. Conditions1and2show cases with low rates of change with time in the pump torque and conditions3and4show cases with high rates of change with time in the pump torque. Conditions2and4are concerned with rates of change with time in the pump torque twice as high as those of conditions1and3, respectively.

The abscissas onFIGS. 5 to 8represent time. The ordinate onFIG. 5(a)represents the pump torque of the hydraulic pump103and the ordinate onFIG. 5(b)represents the rotational speed N of the first electric motor102. It is assumed that the diesel engine101, the first electric motor102, and the hydraulic pump103are mechanically connected so as to run at an identical speed. The ordinate onFIG. 5(c)represents the torque of the first electric motor102and the ordinate onFIG. 5(f)represents the torque of the diesel engine101. The broad dotted line onFIG. 5(f)is a reference line that serves as a guide easily determining an inclination of a torque line. The ordinates onFIGS. 6(a) to 6(c) and 6(f)toFIGS. 8(a) to 8(c) and 8(f)represent the same as those represented by the ordinates onFIGS. 5(a) to 5(c) and 5(f).

In condition1(FIG. 5) and condition2(FIG. 6), because of the low rates of change with time in the pump torque as shown inFIGS. 5(a) and 6(a), the torque of the diesel engine101can follow the increase in the pump torque as shown inFIGS. 5(f) and 6(f). This eliminates the need for making the torque of the first electric motor102large and, as shown inFIGS. 5(c) and 6(c)and the torque of the first electric motor102is smaller than the torque of the diesel engine101at the peak of the torque of the first electric motor102.

In contrast, in condition3(FIG. 7) and condition4(FIG. 8), because of the high rates of change with time in the pump torque as shown inFIGS. 7(a) and 8(a), the torque of the diesel engine101cannot follow the increase in the pump torque. To maintain the rotational speed N, the torque of the first electric motor102needs to be made large as shown inFIGS. 7(c) and 8(c)and becomes larger than the torque of the diesel engine101at the peak of the torque of the first electric motor102.

Attention is now focused on the rate of change with time in the pump torque of the diesel engine101when the rate of change with time in the pump torque changes from condition1to condition2in conditions1and2having the low rates of change with time in the pump torque. The rate of change with time in the torque of the diesel engine101in condition2(FIG. 6) changes greatly relative to that in condition1(FIG. 5). In contrast, in conditions3and4having the high rates of change with time in the pump torque, because of the limitation imposed by the torque limiter115, the rate of change with time in the torque of the diesel engine101in condition4(FIG. 8) changes a little relative to that in condition3(FIG. 7) when the rate of change with time in the pump torque changes from condition3to condition4. Specifically, when the rates of change with time in the pump torque are low (conditions1and2), the increase in the rate of change with time in the torque of the diesel engine101relative to the increase in the rate of change with time in the pump torque is high; and when the rates of change with time in the pump torque are high (conditions3and4), the increase in the rate of change with time in the torque of the diesel engine101relative to the increase in the rate of change with time in the pump torque is low.

In either case, because of the functioning of the speed control device118, the rotational speed N is controlled so as to agree with the rotational speed command N*. Specifically, in the construction machine according to the first embodiment, the torque of the diesel engine101is controlled according to the pump torque when the rate of change with time in the pump torque is low; because the rate of change with time in the torque of the diesel engine101is low at this time, the diesel engine101does not develop a condition in which particulate matter and nitrogen oxide tend to be produced.

In contrast, when the rate of change with time in the pump torque is high, the rate of change with time in the torque of the diesel engine101is limited and is not controlled according to the pump torque. Thus, in this case, too, the rate of change with time in the torque of the diesel engine101is limited, so that the diesel engine101does not develop a condition in which particulate matter and nitrogen oxide tend to be produced.

As described heretofore, in the first embodiment, the particulate matter (PM) or the nitrogen oxide (NOx) discharged from the internal combustion engine mounted on the construction machine can be reduced and the charge amount of the electric energy storage device that supplies electric power to the electric motor can be appropriately controlled.

Additionally, the charge amount of a capacitor having a small capacity, if used for the electric energy storage device, can also be appropriately controlled.

Arrangements and operations of a construction machine according to a second embodiment of the present invention will be described below with reference toFIGS. 9 to 11. A hydraulic excavator as the construction machine according to the second embodiment has a general arrangement identical to that shown inFIG. 1.

An arrangement of a drive system that drives the construction machine according to the second embodiment will be described below with reference toFIG. 9.

FIG. 9is a block diagram showing the arrangement of the drive system that drives the construction machine according to the second embodiment of the present invention. Like or equal parts are identified by the same reference numerals as those used inFIG. 2.

Based on a difference between a rotational speed command N* and a rotational speed N′ of a diesel engine101obtained by a subtractor130, a second speed control device131calculates a torque target such that the rotational speed N′ of the diesel engine101agrees with the rotational speed command N*. The second speed control device131then outputs the torque target to a torque limiter115.

A high-pass filter132produces an output of a speed control device118from which a low-frequency component including a DC component is removed. A subtractor133subtracts an output of a charge amount control device114from the output of the high-pass filter132representing the output of the speed control device118from which the low-frequency component including the DC component is removed. The subtractor133then outputs the result as a second torque command T2*.

It is noted that the diesel engine101and a first electric motor102are mechanically connected to each other and thus run at an identical speed that will hereinafter be represented by a rotational speed N.

Operations of the drive system according to the second embodiment will be described below.

When torque of a hydraulic pump103changes, the second speed control device131limits fluctuations in the rotational speed N; still, the torque limiter115limits the rate of change in torque of the diesel engine101. This prevents the diesel engine101from developing a condition in which particulate matter or nitrogen oxide tends to be produced. Meanwhile, because of the rate of change in the torque of the diesel engine101being limited, it is difficult to sufficiently limit the fluctuations in the rotational speed N only with the second speed control device131. Thus, the fluctuations in the rotational speed N is limited transiently by the speed control device118. In addition, because the low-frequency component is removed by the high-pass filter132in a steady state, control of the charge amount Q by the charge amount control device114is performed.

Operations of the drive system incorporated in the construction machine according to the second embodiment will be described below with reference toFIGS. 10 to 11.

FIGS. 10 and 11are timing charts showing operations of the drive system incorporated in the construction machine according to the second embodiment of the present invention.

FIG. 10shows operations of different parts of the drive system when, for example, the arm is operated and the pump torque is changed. The operations are the same as those described with reference toFIG. 3.

In this case, the rotational speed N is controlled so as to agree with the rotational speed command N*, the rate of change with time in the torque of the diesel engine101is limited, and the charge amount Q of an electric energy storage device111is controlled so as to agree with the charge amount command Q*.

FIG. 11shows operations of different parts of the drive system when a second electric motor112is operated. The operations are the same as those described with reference toFIG. 4.

In this case, the rotational speed N is controlled so as to agree with the rotational speed command N*, the rate of change with time in the torque of the diesel engine101is limited, and the charge amount Q of the electric energy storage device111is controlled so as to agree with the charge amount command Q*.

As described heretofore, in the second embodiment, too, the particulate matter (PM) or the nitrogen oxide (NOx) discharged from the internal combustion engine mounted on the construction machine can be reduced and the charge amount of the electric energy storage device that supplies electric power to the electric motor can be appropriately controlled.

Additionally, the charge amount of a capacitor having a small capacity, if used for the electric energy storage device, can also be appropriately controlled.

In the above-described embodiments, a predetermined constant value is given as the rotational speed command N*. The rotational speed command N* may, however, be decreased for a light hydraulic pump load or increased for a heavy hydraulic pump load. Varying the rotational speed command N* in this manner still allows the rotational speed N to follow the rotational speed command N* because of the control performed based on the difference therebetween.

The rate of change with time of the torque limiter115, while it has been described to be constant, may still be varied depending on the operating condition of the diesel engine101within a range in which the particulate matter or the nitrogen oxide does not increase to a level more than a predetermined amount. Additionally, an input to, and an output from, the torque limiter115are made to agree with each other such that the particulate matter or the nitrogen oxide does not increase to a level more than a predetermined amount and the torque limiter115may be configured so as to limit the rate of change with time in the output if the particulate matter or the nitrogen oxide increases to a level more than the predetermined amount with the input made to agree with the output.

Additionally, the diesel engine101, the first electric motor102, and the hydraulic pump103are mechanically connected so as to run at an identical speed. The connection may nonetheless be achieved via a transmission, in which case, the rotational speed command N*, the rotational speed N′, and the rotational speed N need to be converted in consideration of a gear ratio.

DESCRIPTION OF REFERENCE NUMERALS