Stop control system and method for internal combustion engine

A stop control system for an internal combustion engine, which is capable of accurately stopping a piston at a predetermined position during stoppage of the engine while preventing occurrence of untoward noise and vibration. After stopping the engine 3, the stop control system 1 for the engine 3 according to the present invention executes a first stage control (step 34) in which a throttle valve 13a is controlled to a first stage control target opening degree ICMDOFPRE smaller than a second predetermined opening degree ICMDOF2, in order to stop the piston at the predetermined position, before executing a second stage control (step 42) in which the throttle valve 13a is controlled to the second predetermined opening degree ICMDOF2. Further, the stop control system 1 stabilizes initial conditions at the start of the second stage control by setting a first stage control start rotational speed NEICOFPRE and a first stage control target opening degree ICMDOFPRE according to a change in a corrected target stop control start rotational speed NEICOFREFN (steps 71 and 85).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage entry of International Application No. PCT/JP2010/062901, filed Jul. 30, 2010, which claims priority to Japanese Patent Application No. 177943/2009 filed Jul. 30, 2009, the disclosure of the prior application are incorporated in their entirety by reference.

TECHNICAL FIELD

The present invention relates to a stop control system and method for an internal combustion engine, for controlling a stop position of a piston to a predetermined position by controlling an intake air amount during stoppage of the engine.

BACKGROUND ART

When stopping the engine, it is desirable that the piston is caused to stop at a predetermined position that causes no valve overlap in which an intake valve and an exhaust valve are both opened. This is because when the engine is stopped in a state where valve overlap occurs, exhaust gases in an exhaust passage flow back into an intake passage via the exhaust valve and the intake valve during stoppage of the engine, which can result in degraded engine startability at the following start of the engine and increased exhaust emissions.

On the other hand, conventionally, as a control system for controlling the opening degree of a throttle valve during stoppage of the engine, one disclosed in Patent Literature 1 is known. In this control system, during stoppage of the engine, after an ignition switch is turned off, the throttle valve is controlled to predetermined respective opening degrees of full closing, full opening, and intermediate opening, in the mentioned order, and the opening degree of the throttle valve is learned based on the opening degrees thereof detected by a throttle position sensor during the full closing and the full opening of the throttle valve. Further, after the ignition switch is turned off, prior to the above-described full closing control, the throttle valve is held at a predetermined opening degree, whereby during the full closing control, negative pressure in an intake manifold is suppressed to prevent occurrence of untoward noise during the full open control after the full closing control.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, in the control system disclosed in the Patent Literature 1, the opening degree of the throttle valve is learned to merely prevent occurrence of untoward noise, by controlling the opening degree of the throttle valve during stoppage of the engine, as described above. Therefore, it is impossible to cause the piston to stop at the predetermined position during stoppage of the engine, and hence it is inevitable that the above-described inconvenience occurs due to valve overlap.

The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a stop control system and method for an internal combustion engine, which are capable of accurately stopping a piston at a predetermined position during stoppage of the engine while preventing occurrence of untoward noise and vibration.

Solution to Problem

To attain the above object, in an aspect, the invention provides a stop control system1for an internal combustion engine3, which controls a stop position of a piston3dof the engine3to a predetermined position during stoppage of the engine3by controlling an intake air amount, comprising, an intake air amount-adjusting valve (throttle valve13ain the embodiment (the same applies hereinafter in this section)) for adjusting the intake air amount, rotational speed-detecting means (crank angle sensor24, ECU2) for detecting a rotational speed of the engine3(engine speed NE), first intake air amount control means (ECU2, step30inFIG. 5, step34inFIG. 6) for closing the intake air amount-adjusting valve when a command for stopping the engine3is issued, and thereafter executing first intake air amount control (first stage control) in which the intake air amount-adjusting valve is controlled to a first predetermined opening degree (first stage control target opening degree ICMDOFPRE) when the detected rotational speed of the engine3becomes equal to a first predetermined rotational speed (first stage control start rotational speed NEICOFPRE), and second intake air amount control means (ECU2, step33inFIG. 5, step42inFIG. 6) for executing second intake air amount control (second stage control) in which the intake air amount-adjusting valve is controlled to a second predetermined opening degree ICMDOF2larger than the first predetermined opening degree in order to stop the piston3dat the predetermined position, when the rotational speed of the engine becomes equal to a second predetermined rotational speed (corrected target stop control start rotational speed NEICOFREFN) lower than the first predetermined rotational speed after the first intake air amount control.

According to this stop control system, when the command for stopping the engine is issued, the intake air amount-adjusting valve is once closed. This reduces the amount of intake air drawn into the engine to thereby reduce the rotational speed of the engine. Then, the first intake air amount control is executed in which when the rotational speed of the engine becomes equal to the first predetermined rotational speed, the intake air amount-adjusting valve is opened to control the intake air amount-adjusting valve to the first predetermined opening degree. This introduces intake air via the intake air amount-adjusting valve, and intake pressure acts as resistance to the piston to thereby further reduce the rotational speed of the engine. Further, after that, when the rotational speed of the engine becomes equal to the second predetermined rotational speed which is smaller, the second intake air amount control is executed in which the intake air amount-adjusting valve is controlled to the second predetermined opening degree larger than the first predetermined opening degree, whereby the stop position of the piston is controlled to the predetermined position.

As described above, when opening the intake air amount-adjusting valve from a closed state so as to stop the piston at the predetermined position, the intake air amount-adjusting valve is not opened to the second predetermined opening degree which is larger, at a time, but in advance of this, it is controlled to the first predetermined opening degree which is smaller. Thus, the intake air amount-adjusting valve is stepwise opened at respective times to the first predetermined opening degree and the second predetermined opening degree by separate steps, whereby it is possible to avoid a steep rise in intake pressure during opening the intake air amount-adjusting valve, thereby making it possible to prevent occurrence of untoward noise, such as flow noise, and vibration caused by the steep rise in intake pressure. Further, in the first intake air amount control, the intake air amount-adjusting valve is not progressively opened to the first predetermined opening degree but is held at the first predetermined opening degree, so that it is possible to stabilize initial conditions at the start of the second intake air amount control, such as the intake pressure, without variation, while suppressing adverse influences of variation in the operating characteristics of the intake air amount-adjusting valve, delay, etc. This makes it possible to accurately stop the piston at the predetermined position by the second intake air amount control.

In accordance with one aspect, the present invention comprises second predetermined rotational speed-setting means (ECU2, step28inFIG. 5) for setting the second predetermined rotational speed according to a state of the engine3, and first predetermined rotational speed-setting means (ECU2, step71inFIG. 13) for setting the first predetermined rotational speed according to the set second predetermined rotational speed.

With this configuration, the second predetermined rotational speed for starting the second intake air amount control is set according to a state of the engine, and the first predetermined rotational speed for starting the first intake air amount control is set according to the set second predetermined rotational speed. Therefore, even when timing for starting the second intake air amount control is changed, the first intake air amount control is started in timing coping with the change in the start timing, whereby it is possible to stabilize the initial conditions for the second intake air amount control, thereby making it possible to ensure the accuracy of the stop control of the piston by the second intake air amount control.

In accordance with a further aspect, the present invention comprises second predetermined opening degree-setting means (ECU2, steps128,138inFIG. 24,FIG. 25) for setting the second predetermined opening degree (target second stage control opening degree ATHICOFREFX) according to a state of the engine3, and first predetermined rotational speed-setting means (ECU2, step143inFIG. 27) for setting the first predetermined rotational speed according to the set second predetermined opening degree.

With this configuration, the second predetermined opening degree of the intake air amount-adjusting valve is set according to a state of the engine, and the first predetermined rotational speed for starting the first intake air amount control is set according to the set second predetermined opening degree. Therefore, even when the second predetermined opening degree for use in the second intake air amount control is changed, the first intake air amount control is started in timing coping with the change in the second predetermined opening degree, whereby it is possible to stabilize the initial conditions for the second intake air amount control, thereby making it possible to ensure the accuracy of the stop control of the piston by the second intake air amount control.

In accordance with a further aspect, the present invention comprises first predetermined rotational speed-limiting means (ECU2, steps72,74inFIG. 13) for limiting the first predetermined rotational speed to a predetermined upper limit value NEPRELMT when the set first predetermined rotational speed is higher than the upper limit value NEPRELMT, and first predetermined opening degree-correcting means (ECU2, step75inFIG. 13) for correcting the first predetermined opening degree such that the first predetermined opening degree is increased and at the same time is corrected to a smaller value than the second predetermined opening degree ICMDOF2, when the first predetermined rotational speed is limited.

With this configuration, when the first predetermined rotational speed set according to the change in the second predetermined rotational speed is higher than the predetermined upper limit value, the first predetermined rotational speed is limited to the upper limit value. This causes the first intake air amount control to be started after waiting for the rotational speed of the engine to be reduced to the upper limit value, so that it is possible to prevent the first intake air amount control from being executed in a resonance area where the rotational speed of the engine is high, thereby making it possible to positively prevent untoward noise and vibration caused by the resonance of the engine. Further, when the first predetermined rotational speed is limited as described above, the first predetermined opening degree is corrected to a larger value, so that by compensating for the insufficient amount of the intake air amount due to delay of start of the first intake air amount control, it is possible to stabilize the initial conditions for the second intake air amount control, thereby making it possible to ensure the accuracy of the stop control of the piston.

In accordance with a further aspect, the present invention comprises second predetermined rotational speed-setting means (ECU2, step28inFIG. 5) for setting the second predetermined rotational speed according to a state of the engine3, and first predetermined opening degree-setting means (ECU2, steps81,82,85inFIG. 15) for setting the first predetermined opening degree according to the set second predetermined rotational speed.

With this configuration, the second predetermined rotational speed is set according to a state of the engine, and the first predetermined opening degree for the first intake air amount control is set according to the set second predetermined rotational speed. Therefore, even when the timing for starting the second intake air amount control is changed, the first intake air amount control is executed based on an intake air amount coping with the change in the start timing, whereby it is possible to stabilize the initial conditions for the second intake air amount control, thereby making it possible to ensure the accuracy of the stop control of the piston by the second intake air amount control.

In accordance with a further aspect, the present invention comprises second predetermined opening degree-setting means (ECU2,FIG. 24, steps128,138inFIG. 25) for setting the second predetermined opening degree (target second stage control opening degree ATHICOFREFX) according to a state of the engine3, and first predetermined opening degree-setting means (ECU2, step113inFIG. 24) for setting the first predetermined opening degree according to the set second predetermined opening degree.

With this configuration, the second predetermined opening degree is set according to a state of the engine, and the first predetermined opening degree for use in the first intake air amount control is set according to the set second predetermined opening degree. Therefore, even when the second predetermined opening degree for use in the second intake air amount control is changed, the first intake air amount control is executed based on an intake air amount coping with the change in the second predetermined opening degree, whereby it is possible to stabilize the initial conditions for the second intake air amount control, thereby making it possible to ensure the accuracy of the stop control of the piston by the second intake air amount control.

In accordance with a further aspect, the present invention comprises detection means (intake air temperature sensor22, atmospheric pressure sensor23, engine coolant temperature sensor26) for detecting at least one of a temperature of intake air drawn into the engine3(intake air temperature TA), an atmospheric pressure PA, and a temperature of the engine3(engine coolant temperature TW), and first correction means (ECU2, steps83to85inFIG. 15) for correcting at least one of the first predetermined rotational speed and the first predetermined opening degree according to at least one of the temperature of intake air, the atmospheric pressure PA, and the temperature of the engine, which are detected.

With this configuration, at least one of the temperature of intake air, the atmospheric pressure, and the temperature of the engine is detected. These three parameters all have influence on the degree of rise in the intake pressure and the rate of reduction of the rotational speed of the engine during the intake air amount control. Specifically, as the temperature of intake air and the temperature of the engine are lower, the sliding friction of the piston becomes larger, so that the rate of reduction of the rotational speed of the engine becomes larger. Further, as the atmospheric pressure is higher, or as the temperature of intake air is lower, the density of intake air becomes higher, and hence the degree of rise in intake pressure becomes higher even when the intake air amount is the same, and in accordance therewith, the rate of reduction of the rotational speed of the engine becomes larger. According to the present invention, in the first intake air amount control, at least one of the first predetermined rotational speed and the first predetermined opening degree is corrected according to at least one of these parameters which are detected. Therefore, it is possible to stabilize the initial conditions for the second intake air amount control, thereby making it possible to ensure the accuracy of the stop control of the piston while accommodating influence of differences in the degree of rise in intake pressure and the rate of reduction of the rotational speed of the engine dependent on at least one of the parameters.

In accordance with a further aspect, the present invention comprises detection means (intake air temperature sensor22, atmospheric pressure sensor23, engine coolant temperature sensor26) for detecting at least one of a temperature of intake air drawn into the engine3(intake air temperature TA), an atmospheric pressure PA, and a temperature of the engine3(engine coolant temperature TW), and second correction means (ECU2, steps26to28inFIG. 5) for correcting at least one of the second predetermined rotational speed and the second predetermined opening degree according to at least one of the temperature of intake air, the atmospheric pressure PA, and the temperature of the engine, which are detected.

With this configuration, at least one of the temperature of intake air, the atmospheric pressure, and the temperature of the engine is detected. As described above, these three parameters all have influence on the degree of rise in the intake pressure, the rate of reduction of the rotational speed of the engine, and further the stop characteristics of the piston during the intake air amount control. Therefore, at least one of the second predetermined rotational speed and the second predetermined opening degree is corrected during the second intake air amount control according to one of these parameters which are detected, whereby it is possible to accommodate influence of differences in the stop characteristics of the piston, thereby making it possible to enhance the accuracy of the stop control of the piston.

In accordance with a further aspect, the present invention comprises a stop control method for an internal combustion engine, which controls a stop position of a piston3dof the engine3to a predetermined position during stoppage of the engine3by controlling an intake air amount, comprising a step of detecting a rotational speed of the engine3(engine speed NE in the embodiment (the same applies hereinafter in this section)), a step of closing an intake air amount-adjusting valve (throttle valve13a) for adjusting the intake air amount when a command for stopping the engine3is issued, and thereafter executing first intake air amount control (first stage control) in which the intake air amount-adjusting valve is controlled to a first predetermined opening degree (first stage control target opening degree ICMDOFPRE) when the detected rotational speed of the engine3becomes equal to a first predetermined rotational speed (first stage control start rotational speed NEICOFPRE), and a step of executing second intake air amount control (second stage control) in which the intake air amount-adjusting valve is controlled to a second predetermined opening degree ICMDOF2larger than the first predetermined opening degree in order to stop the piston3dat the predetermined position, when the rotational speed of the engine becomes equal to a second predetermined rotational speed (corrected target stop control start rotational speed NEICOFREFN) lower than the first predetermined rotational speed after the first intake air amount control.

In accordance with a further aspect, the present invention comprises a step of setting the second predetermined rotational speed according to a state of the engine3, and a step of setting the first predetermined rotational speed according to the set second predetermined rotational speed.

In accordance with a further aspect, the present invention comprises a step of setting the second predetermined opening degree according to a state of the engine3, and a step of setting the first predetermined rotational speed according to the set second predetermined opening degree.

In accordance with a further aspect, the present invention comprises a step of limiting the first predetermined rotational speed to a predetermined upper limit value NEPRELMT when the set first predetermined rotational speed is higher than the upper limit value NEPRELMT, and a step of correcting the first predetermined opening degree such that the first predetermined opening degree is increased and at the same time is corrected to a smaller value than the second predetermined opening degree ICMDOF2, when the first predetermined rotational speed is limited.

In accordance with a further aspect, the present invention comprises a step of setting the second predetermined rotational speed according to a state of the engine3, and a step of setting the first predetermined opening degree according to the set second predetermined rotational speed.

In accordance with a further aspect, the present invention comprises a step of setting the second predetermined opening degree according to a state of the engine3, and a step of setting the first predetermined opening degree according to the set second predetermined opening degree.

In accordance with a further aspect, the present invention comprises a step of detecting at least one of a temperature of intake air drawn into the engine3(intake air temperature TA), an atmospheric pressure PA, and a temperature of the engine3(engine coolant temperature TW), and a step of correcting at least one of the first predetermined rotational speed and the first predetermined opening degree according to at least one of the temperature of intake air, the atmospheric pressure PA, and the temperature of the engine, which are detected.

In accordance with a further aspect, the present invention comprises a step of detecting at least one of a temperature of intake air drawn into the engine3(intake air temperature TA), an atmospheric pressure PA, and a temperature of the engine3(engine coolant temperature TW), and a step of correcting at least one of the second predetermined rotational speed and the second predetermined opening degree according to at least one of the temperature of intake air, the atmospheric pressure PA, and the temperature of the engine, which are detected.

MODE FOR CARRYING OUT INVENTION

The present invention will now be described in detail with reference to the drawings showing preferred embodiments thereof.FIG. 1schematically shows an internal combustion engine3to which is applied a stop control system1(seeFIG. 2) according to the present embodiment. This internal combustion engine (hereinafter referred to as the “engine”)3is a six-cylinder gasoline engine, for example.

Fuel injection valves6(seeFIG. 2) are mounted on respective cylinders3aof the engine3. The opening and closing of each fuel injection valve6is controlled by a control signal from an ECU2(seeFIG. 2), whereby fuel injection timing is controlled by valve-opening timing of the fuel injection valve6, and a fuel injection amount QINJ is controlled by a valve-opening time period thereof.

Cylinder heads3bof respective cylinders3aof the engine3are connected to an intake pipe4and an exhaust pipe5, cylinder by cylinder, and a pair of intake valves8and8(only one of which is shown) and a pair of exhaust valves9and9(only one of which is shown) are provide for each cylinder head3b.

As shown inFIG. 3, the cylinder head3bis provided therein with a rotatable intake cam shaft41, an intake cam42integrally formed with the intake cam shaft41, a rocker arm shaft43, and two rocker arms44and44(only one of which is shown) which are pivotally supported by the rocker arm shaft43for being brought into abutment with respective top ends of the intake valves8and8.

The intake cam shaft41is connected to a crankshaft3c(seeFIG. 1) via an intake sprocket and a timing chain (neither of which is shown), and rotates once whenever the crankshaft3crotates twice. As the intake cam shaft41is rotated, the rocker arms44and44are pressed by the intake cam42to be pivotally moved about the rocker arm shaft43, whereby the intake valves8and8are opened and closed.

Further, the cylinder head3bis provided therein with a rotatable exhaust cam shaft61, an exhaust cam62integrally formed with the exhaust cam shaft61, a rocker arm shaft63, and two rocker arms64and64(only one of which is shown) which are pivotally supported by the rocker arm shaft63for being brought into abutment with respective top ends of the exhaust valves9and9.

The exhaust cam shaft61is connected to the crankshaft3cvia an exhaust sprocket and a timing chain (neither of which is shown), and rotates once whenever the crankshaft3crotates twice. As the exhaust cam shaft61is rotated, the rocker arms64and64are pressed by the exhaust cam62to be pivotally moved about the rocker arm shaft63, whereby the exhaust valves9and9are opened and closed.

Further, the intake cam shaft41is provided with a cylinder discrimination sensor25. Along with rotation of the intake cam shaft41, the cylinder discrimination sensor25delivers a CYL signal, which is a pulse signal, to the ECU2at a predetermined crank angle position of a specific cylinder3a.

The crankshaft3cis provided with a crank angle sensor24. The crank angle sensor24delivers a TDC signal and a CRK signal, which are both pulse signals, to the ECU2along with rotation of the crankshaft3c. The TDC signal indicates that a piston3dof one of the cylinders3ais at a predetermined crank angle position in the vicinity of the top dead center (TDC) at the start of the intake stroke thereof, and in the case of the six-cylinder engine as in the present embodiment, it is delivered whenever the crankshaft3crotates through 120°. The CRK signal is delivered whenever the crankshaft3crotates through a predetermined angle (e.g. 30°. The ECU2calculates the rotational speed of the engine3(hereinafter referred to as “the engine speed”) NE based on the CRK signal. This engine speed NE represents the rotational speed of the engine3. Further, the ECU2determines which cylinders3ais in the compression stroke, based on the CYL signal and the TDC signal, and assigns cylinder numbers CUCYL1to6to the respective cylinders3a, based on results of the determination.

Furthermore, the ECU2calculates a crank angle CA based on the TDC signal and the CRK signal, and sets a stage number STG. Assuming that a reference angle position of the crank angle CA, which corresponds to a start of the intake stroke in one of the cylinders3a, is set to 0°, the stage number STG is set to 0 when the crank angle CA is within a range of 0≦CA<30, to 1 when the same is within a range of 30≦CA<60, to 2 when the same is within a range of 60≦CA<90, and to 3 when the same is within a range of 90≦CA<120. That is, the stage number STG=0 represents that one of the cylinders3ais in an initial stage of the intake stroke, and at the same time, that since the engine3has six cylinders, another of the cylinders3ais in an middle stage of the compression stroke, more specifically, is during a time period corresponding to its crank angle range of 60° to 90° after the start of the compression stroke.

The intake pipe4is provided with a throttle valve mechanism13. The throttle valve mechanism13has a throttle valve13awhich is pivotally provided in the intake pipe4and a TH actuator13bfor actuating the throttle valve13a. The TH actuator13bis a combination of a motor and a gear mechanism (neither of which is shown), and is driven by a control signal based on a target opening degree ICMDTHIGOF delivered from the ECU2. This varies the opening degree of the throttle valve13a, whereby the amount of fresh air drawn into each cylinder3a(hereinafter referred to as the “fresh air amount”) is controlled.

Further, an intake air temperature sensor22is disposed in the intake pipe4at a location downstream of the throttle valve13a. The intake air temperature sensor22detects the temperature of intake air (hereinafter referred to as the “intake air temperature”) TA, and delivers a detection signal indicative of the detected intake air temperature TA to the ECU2.

Furthermore, delivered to the ECU2are a detection signal indicative of atmospheric pressure PA from an atmospheric pressure sensor23, and a detection signal indicative of the temperature of engine coolant of the engine3(hereinafter referred to as “the engine coolant temperature”) TW from an engine coolant temperature sensor26.

Further, a signal indicative of an on/off state of an ignition switch (SW)21(seeFIG. 2) is delivered from the ignition switch21to the ECU2. Note that during stoppage of the engine3, when the ignition switch21is turned off, supply of fuel from the fuel injection valve6to the cylinders3ais stopped.

The ECU2is implemented by a microcomputer comprising an I/O interface, a CPU, a RAM, and a ROM (none of which are specifically shown). The detection signals from the aforementioned switch and sensors21to26are input to the CPU after the I/O interface performs A/D conversion and waveform shaping thereon. Based on the detection signals from the above-mentioned switch and sensors, the ECU2determines operating conditions of the engine3in accordance with control programs stored in the ROM, and executes control of the engine3including stop control, based on the determined operating conditions.

Note that in the present embodiment, the ECU2corresponds to rotational speed-detecting means, first intake air amount control means, second intake air amount control means, second predetermined rotational speed-setting means, first predetermined rotational speed-setting means, second predetermined opening degree-setting means, first predetermined rotational speed-limiting means, first predetermined opening degree-correcting means, first predetermined opening degree-setting means, first correction means, and second correction means.

Next, stop control of the engine3according to the first embodiment, executed by the ECU2, will be described with reference toFIGS. 4 to 14. The stop control is for controlling the stop position of the piston3dto a predetermined position at which no valve overlap occurs in which the intake valve8and the exhaust valve9open at the same time, by controlling the throttle valve13atoward an open side when the engine speed NE becomes lower than a stop control start rotational speed NEIGOFTH after the ignition switch21has been turned off, to thereby control the engine speed NE in the final compression stroke immediately before stoppage of the piston3d(final compression stroke rotational speed NEPRSFTGT) to a predetermined reference value.

FIG. 4shows a process for setting a target stop control start rotational speed NEICOFREFX. The present process and processes described hereinafter are executed in synchronism with generation of the CYL signal. The present process is for setting a target value of the stop control start rotational speed for starting control of the throttle valve13atoward the open side in the stop control (second stage control, described hereinafter) as a target stop control start rotational speed NEICOFREFX, and for learning the target value. The present process is carried out once in a single stop control process.

In the present process, first, in a step1(shown as “S1” inFIG. 4; the following steps are also shown in the same way), it is determined whether or not a target stop control start rotational speed setting completion flag F_IGOFTHREFDONE is equal to 1. If the answer to this question is affirmative (YES), i.e. if the target stop control start rotational speed NEICOFREFX has already been set, the present process is immediately terminated.

On the other hand, if the answer to the question of the step1is negative (NO), i.e. if the target stop control start rotational speed NEICOFREFX has not yet been set, in a step2, it is determined whether or not the number of times of learning NENGSTP is equal to 0. If the answer to this question is affirmative (YES), i.e. if the number of times of learning NENGSTP has been reset e.g. by battery cancellation, the target stop control start rotational speed NEICOFREFX is set to a predetermined initial value NEICOFINI (step3), and then the process proceeds to a step12, referred to hereinafter.

On the other hand, if the answer to the question of the step2is negative (NO), it is determined in a step4whether or not a learning condition satisfied flag F_NEICOFRCND is equal to 1. This learning condition satisfied flag F_NEICOFRCND is set to 1 when there are satisfied predetermined learning conditions for learning the target stop control start rotational speed NEICOFREFX, including a condition that no engine stall is caused and a condition that the engine coolant temperature TW is not in a low temperature state where it is not higher than a predetermined value. If the answer to the question of the step4is negative (NO), i.e. if the learning conditions are not satisfied, the target stop control start rotational speed NEICOFREFX is not learned, but the process proceeds to a step13, referred to hereinafter.

On the other hand, if the answer to the question of the step4is affirmative (YES), i.e. if the learning conditions for learning the target stop control start rotational speed NEICOFREFX are satisfied, the process proceeds to a step5, wherein an intercept INTCPNPF is calculated using the final compression stroke rotational speed NEPRSFTGT obtained at the time of the immediately preceding stop control, the stop control start rotational speed NEIGOFTH, and a predetermined slope SLOPENPF0, by the following equation (1):
INTCPNPF=NEPRSFTGT−SLOPENPF0·NEIGOFTH  (1)

This equation (1) is based on preconditions that a correlation as shown inFIG. 9, i.e. a correlation expressed by a linear function having a slope of SLOPENPF0and an intercept of INTCPNPF holds between the stop control start rotational speed NEIGOFTH and the final compression stroke rotational speed NEPRSFTGT, and the slope SLOPENPF0is constant if the engine3is of the same type. The intercept INTCPNPF is calculated according to the above preconditions, using the stop control start rotational speed NEIGOFTH obtained during the stop control and the final compression stroke rotational speed NEPRSFTGT, by the equation (1). This determines the correlation between the stop control start rotational speed NEIGOFTH and the final compression stroke rotational speed NEPRSFTGT. Incidentally, as the friction of the piston3dis larger, the final compression stroke rotational speed NEPRSFTGT takes a smaller value with respect to the same control start rotational speed NEICOFRRT, so that the linear function is offset toward a lower side (as indicated by a two-dot chain line inFIG. 9, for example), and the intercept INTCPNPF is calculated to be a smaller value. Inversely, as the friction of the piston3dis smaller, the linear function is offset toward an upper side (as indicated by broken lines inFIG. 9, for example) for the converse reason to the above, and the intercept INTCPNPF is calculated to be a larger value.

Then, in a step6, a basic value NEICOFRRT of the target stop control start rotational speed is calculated based on the correlation determined as described above, by using the calculated intercept INTCPNPF and slope SLOPENPF0and applying a predetermined reference value NENPFLMT0of the final compression stroke rotational speed to the following equation (2) (seeFIG. 9).
NEICOFRRT=(NENPFLMT0−INTCPNPF)/SLOPENPF0  (2)

The reference value NENPFLMT0of the final compression stroke rotational speed corresponds to such a value that will cause the piston3dto stop at a predetermined position free from occurrence of valve overlap, when the final compression stroke rotational speed NEPRSF is controlled to the reference value NENPFLMT0. The reference value NENPFLMT0is determined empirically e.g. by experiment in advance, and is set to e.g. 260 rpm in the present embodiment. Therefore, by using the basic value NEICOFRRT of the target stop control start rotational speed calculated by the above-mentioned equation (2), it is possible to stop the piston3dat the predetermined position.

Next, in a step7, a map shown inFIG. 10is searched according to the atmospheric pressure PA0detected during the stop control to determine a map value DNEICOFPA, and the map value DNEICOFPA is set as a learning PA correction term dneicofrpa. In this map, the map value DNEICOFPA (=learning PA correction term dneicofrpa) is set to a larger value as the atmospheric pressure PA0is higher.

Next, in a step8, a map shown inFIG. 11is searched according to an intake air temperature TA0detected during the stop control to determine a map value DNEICOFTA, and the map value DNEICOFTA is set as a learning TA correction term dneicofrta. In this map, the map value DNEICOFTA (=learning TA correction term dneicofrta) is set to a larger value as the intake air temperature TA0is lower.

Next, a corrected basic value NEICOFREF of the target stop control start rotational speed is calculated using the basic value NEICOFRRT of the target stop control start rotational speed, the learning PA correction term dneicofrpa, and the learning TA correction term dneicofrta calculated in the steps6to8, by the following equation (3) (step9):
NEICOFREF=NEICOFRRT−dneicofrpa−dneicofrta  (3)

As described hereinabove, since the learning PA correction term dneicofrpa is set to a larger value as the atmospheric pressure PA0is higher, the corrected basic value NEICOFREF of the target stop control start rotational speed is corrected to a smaller value as the atmospheric pressure PA0is higher. Further, since the learning TA correction term dneicofrta set to a larger value as the intake air temperature TA0is lower, the corrected basic value NEICOFREF of the target stop control start rotational speed is corrected to a smaller value as the intake air temperature TA0is lower.

Next, in a step10, an averaging coefficient CICOFREFX is calculated by searching a map shown inFIG. 12according to the number of times of learning NENGSTP. In this map, the averaging coefficient CICOFREFX is set to a larger value as the number of times of learning NENGSTP is larger (0<CICOFREFX<1).

Next, in a step11, a current value NEICOFREFX of the target stop control start rotational speed is calculated using the calculated corrected basic value NEICOFREF of the target stop control start rotational speed, an immediately preceding value NEICOFREFX of the target stop control start rotational speed, and the averaging coefficient CICOFREFX, by the following equation (4):
NEICOFREFX=NEICOFREF·(1−CICOFREFX)+NEICOFREFX·CICOFREFX  (4)

As is clear from the above equation (4), the target stop control start rotational speed NEICOFREFX is calculated as a weighted average value of the corrected basic value NEICOFREF of the target stop control start rotational speed and the immediately preceding value NEICOFREFX of the target stop control start rotational speed, and the averaging coefficient CICOFREFX is used as a weight coefficient for weighted averaging. Therefore, the current value NEICOFREFX of the target stop control start rotational speed is calculated such that it becomes closer to the corrected basic value NEICOFREF of the target stop control start rotational speed as the averaging coefficient CICOFREFX is smaller, whereas it becomes closer to the immediately preceding value NEICOFREFX of the target stop control start rotational speed as the averaging coefficient CICOFREFX is larger. Further, the averaging coefficient CICOFREFX is set as described above according to the number of times of learning NENGSTP, and therefore as the number of times of learning NENGSTP is smaller, the degree of reflection of the corrected basic value NEICOFREF of the target stop control start rotational speed becomes larger, whereas as the number of times of learning NENGSTP is larger, the degree of reflection of the immediately preceding value NEICOFREFX of the target stop control start rotational speed becomes larger.

In the step12following the step3or11, the number of times of learning NENGSTP is incremented. Further, if the answer to the question of the step4is negative (NO), or after the step12, the proceeds to the step13, wherein in order to indicate that the setting of the target stop control start rotational speed NEICOFREFX has been completed, the target stop control start rotational speed setting completion flag F_IGOFTHREFDONE is set to 1, followed by terminating the present process.

FIGS. 5 and 6show a process for setting a target opening degree ICMDTHIGOF that serves as a target of the opening degree of the throttle valve13a. In this process, after turning off the ignition switch21, fully-closing control for controlling the target opening degree ICMDTHIGOF of the throttle valve13ato 0, first stage control for setting the target opening degree ICMDTHIGOF to a first predetermined opening degree, and second stage control for setting the target opening degree ICMDTHIGOF to a second predetermined opening degree larger than the first predetermined opening degree are performed in the mentioned order according to the engine speed NE.

In the present process, first, in a step21, it is determined whether or not a second stage control execution flag F_IGOFFTH2is equal to 1. This second stage control execution flag F_IGOFFTH2is set to 1 during execution of the above-described second stage control, and otherwise set to 0. If the answer to the question of the step21is affirmative (YES), the present process is immediately terminated.

On the other hand, if the answer to the question of the step21is negative (NO), it is determined in a step22whether or not a fuel cut flag FIGOFFFC is equal to 1. If the answer to this question is negative (NO), i.e. if interruption of fuel supply to the engine3has not been completed yet after turning off the ignition switch21, a first stage control execution flag F_IGOFFTH1and the second stage control execution flag F_IGOFFTH2are set to 0 (steps23and24), respectively, and the target opening degree ICMDTHIGOF is set to 0 (step25), followed by terminating the present process.

On the other hand, if the answer to the question of the step22is affirmative (YES), i.e. if the interruption of fuel supply to the engine3has been completed, the above-mentioned map shown inFIG. 10is searched according to the atmospheric pressure PA currently detected to thereby determine the map value DNEICOFPA, and the map value DNEICOFPA is set as a setting PA correction term dneicofpax (step26).

Next, in a step27, the above-mentioned map shown inFIG. 11is searched according to the intake air temperature TA currently detected to thereby determine the map value DNEICOFTA, and the map value DNEICOFTA is set as a setting TA correction term dneicoftax.

Next, in a step28, a corrected target stop control start rotational speed NEICOFREFN is calculated using the target stop control start rotational speed NEICOFREFX set in the step11inFIG. 4, the setting PA correction term dneicofpax, and the setting TA correction term dneicoftax calculated as described above, by the following equation (5):
NEICOFREFN=NEICOFREFX+dneicofpax+dneicoftax  (5)

As described hereinabove, since the setting PA correction term dneicofpax is set to a larger value as the atmospheric pressure PA is higher, the corrected target stop control start rotational speed NEICOFREFN is corrected to a larger value as the atmospheric pressure PA is higher. This is for the following reason:

As the atmospheric pressure PA is higher, the density of intake air is higher and the resistance of intake air to the piston3dis larger, so that the rate of reduction of the engine speed NE becomes larger. Further, after a control signal based on the target opening degree ICMDTHIGOF is delivered, there occurs a delay before the opening degree of the throttle valve13abecomes commensurate with the control signal, and a further delay occurs before an intake air amount becomes large enough to be commensurate with the opening degree of the throttle valve13a. Therefore, by correcting the corrected target stop control start rotational speed NEICOFREFN to a larger value as the atmospheric pressure PA is higher, and starting the second stage control in earlier timing, it is possible to properly avoid the adverse influence of the operation of the throttle valve13aand the delay of intake air, described above.

On the other hand, since the setting TA correction term dneicoftax is set to a larger value as the intake air temperature TA is lower, the corrected target stop control start rotational speed NEICOFREFN is corrected to a larger value as the intake air temperature TA is lower. As the intake air temperature TA is lower, the sliding friction of the piston3dis larger and the density of intake air is higher, which increases the rate of reduction of the engine speed NE. Therefore, by correcting the corrected target stop control start rotational speed NEICOFREFN to a larger value as the intake air temperature TA is lower and starting the second stage control in earlier timing, it is possible to properly avoid the adverse influence of the operation of the throttle valve13aand the delay of intake air.

Next, in a step29, a first stage control target opening degree ICMDOFPRE is calculated.FIG. 13shows a subroutine of a process for calculating the first stage control target opening degree ICMDOFPRE. In the present process, first, in a step71, a value obtained by adding a predetermined value DNEICOFPRE to the corrected target stop control start rotational speed NEICOFREFN (=NEICOFREFN+DNEICOFPRE) is calculated as a first stage control start rotational speed NEICOFPRE.

Next, it is determined whether or not the calculated first stage control start rotational speed NEICOFPRE is larger than a predetermined upper limit value NEPRELMT (step72). This upper limit value NEPRELMT corresponds to a value at which the engine3might resonate if the first stage control is started in a state where the engine speed NE is higher than the upper limit value NEPRELMT, and is set to 600 rpm, for example.

If the answer to the question of the step72is negative (NO), i.e. if NEICOFPRE≦NEPRELMT holds, the first stage control target opening degree ICMDOFPRE is set to a predetermined basic value ICMDPREB (step73), followed by terminating the present process.

On the other hand, if the answer to the question of the step72is affirmative (YES), i.e. if the first stage control start rotational speed NEICOFPRE calculated in the step71is higher than the upper limit value NEPRELMT, it is determined that the engine3might resonate, and to avoid the resonance, the first stage control start rotational speed NEICOFPRE is set to the upper limit value NEPRELMT, for limitation (step74). Further, the first stage control target opening degree ICMDOFPRE is set to a value obtained by adding a predetermined correction term DICMD to the basic value ICMDPREB (step75), followed by terminating the present process. Note that the corrected first stage control target opening degree ICMDOFPRE (=ICMDPREB+DICMD) is smaller than both a second predetermined opening degree ICMDOF2and a third predetermined opening degree ICMDOF3, which are set as a target opening degree for use in the second stage control, described hereinafter.

Referring again toFIG. 5, in a step30following the step29, it is determined whether or not the engine speed NE is smaller than the calculated first stage control start rotational speed NEICOFPRE. If the answer to this question is negative (NO), i.e. if NE≧NEICOFPRE holds, the above-described steps23to25are executed to thereby continue the full closing control of the throttle valve13a, followed by terminating the present process.

On the other hand, if the answer to the question of the step30is affirmative (YES), i.e. if the engine speed NE is smaller than the first stage control start rotational speed NEICOFPRE, it is determined whether or not the first stage control execution flag F_IGOFFTH1is equal to 1 (step31). If the answer to this question is negative (NO), i.e. if the first stage control has not been executed yet, the target opening degree ICMDTHIGOF is set to the first stage control target opening degree ICMDOFPRE calculated in the step29(step34), and the first stage control of the throttle valve13ais started. Further, to indicate that the first stage control is being executed, the first stage control execution flag F_IGOFFTH1is set to 1 (step35), followed by terminating the present process.

On the other hand, if the answer to the question of the step31is affirmative (YES), i.e. if the first stage control is being executed, it is determined whether or not the stage number STG is 0 (step32). If the answer to this question is negative (NO), i.e. if none of the cylinders3aare in the middle stage of the compression stroke, the above-described steps34and35are executed, followed by terminating the present process.

On the other hand, if the answer to the question of the step32is affirmative (YES), i.e. if the stage number STG is 0, more specifically, if any of the cylinders3ais in the middle stage of the compression stroke, it is determined whether or not the engine speed NE is smaller than the corrected target stop control start rotational speed NEICOFREFN calculated in the step28(step33). If the answer to this question is negative (NO), i.e. if NEICOFREFN≦NE<NEICOFPRE holds, the above-described steps34and35are executed to thereby continue the first stage control, followed by terminating the present process.

On the other hand, if the answer to the question of the step33is affirmative (YES), i.e. if the stage number STG is 0, and at the same time if the engine speed NE is lower than the corrected target stop control start rotational speed NEICOFREFN, the process proceeds to a step36, wherein the engine speed NE obtained at the time is stored as an actual stop control start rotational speed NEIGOFTH, and the atmospheric pressure PA and intake air temperature TA currently detected are stored as the atmospheric pressure PA0and intake air temperature TA0detected during the stop control, respectively, (steps37and38). The stored stop control start rotational speed NEIGOFTH is used in the aforementioned equation (1), and the atmospheric pressure PA0and the intake air temperature TA0are used in the steps7and8inFIG. 4for calculating the learning PA correction term dneicofrpa and the learning TA correction term dneicofrta, respectively.

In a step39following the step38, the difference between the corrected target stop control start rotational speed NEICOFREFN and the actual stop control start rotational speed NEIGOFTH (=NEICOFREFN−NEIGOFTH) is calculated as a difference DNEIGOFTH.

Next, in a step40, it is determined whether or not the above difference DNEIGOFTH is smaller than a predetermined first reference value DNEIGOFTHL. If the answer to this question is affirmative (YES), it is judged that the difference DNEIGOFTH is small, and hence to indicate the fact, a rotational speed difference flag F_DNEIGOFTH is set to 0 (step41), and the target opening degree ICMDTHIGOF is set to the second predetermined opening degree ICMDOF2for use in the second stage control (step42). This second predetermined opening degree ICMDOF2is larger than the first stage control target opening degree ICMDOFPRE for use in the first stage control. Then, to indicate that the second stage control is being executed, the second stage control execution flag F_IGOFFTH2is set to 1 (step43), followed by terminating the present process.

On the other hand, if the answer to the question of the step40is negative (NO), i.e. if DNEIGOFTH≧DNEIGOFTHL holds, it is judged that the difference between the corrected target stop control start rotational speed NEICOFREFN and the actual stop control start rotational speed NEIGOFTH is large, and hence to indicate the fact, the rotational speed difference flag F_DNEIGOFTH is set to 1 (step44). Then, it is determined whether or not the difference DNEIGOFTH is not smaller than a predetermined second reference value DNEIGOFTHH which is larger than the first reference value DNEIGOFTHL (step45). If the answer to this question is affirmative (YES), i.e. if DNEIGOFTH≧DNEIGOFTHH holds, the process proceeds to the step42, wherein the target opening degree ICMDTHIGOF is set to the second predetermined opening degree ICMDOF2, and the above-mentioned step43is executed, followed by terminating the present process.

On the other hand, if the answer to the question of the step45is negative (NO), i.e. if DNEIGOFTHL≦DNEIGOFTH<DNEIGOFTHH holds, the target opening degree ICMDTHIGOF is set to a third predetermined opening degree ICMDOF3(step46), and the step43is executed, followed by terminating the present process. This third predetermined opening degree ICMDOF3is larger than the first stage control target opening degree ICMDOFPRE, and is smaller than the second predetermined opening degree ICMDOF2.

FIGS. 7 and 8show a process for calculating the final compression stroke rotational speed NEPRSFTGT. In the present process, first, in a step51, it is determined whether or not the second stage control execution flag F_IGOFFTH2is equal to 1. If the answer to this question is negative (NO), i.e. if the second stage control is not being executed, the final compression stroke rotational speed NEPRSFTGT is set to 0 (step52), followed by terminating the present process.

On the other hand, if the answer to the question of the step51is affirmative (YES), i.e. if the second stage control is being executed, it is determined in a step53whether or not an initialization completion flag F_TDCTHIGOFINI is equal to 1. If the answer to this question is negative (NO), the cylinder number CUCYL assigned at the time is shifted to an immediately preceding value CUCYLIGOFTHZ thereof (step54). Further, a TDC counter value CTDCTHIGOF for measuring the number of times of occurrence of TDC after the start of the second stage control is reset to 0 (step55), and to indicate that the above-mentioned initialization has been completed, the initialization completion flag F_TDCTHIGOFINI is set to 1 (step56). Then, the process proceeds to a step60, described hereinafter.

On the other hand, if the answer to the question of the step53is affirmative (YES), i.e. if the above-mentioned initialization has already been performed, it is determined whether or not the immediately preceding value CUCYLIGOFTHZ of the cylinder number and the cylinder number CUCYL assigned at the time are equal to each other (step57). If the answer to this question is affirmative (YES), the process proceeds to the step60, described hereinafter.

On the other hand, if the answer to the question of the step57is negative (NO), i.e. if CUCYLIGOFTHZ≠CUCYL holds, it is determined that TDC has occurred, and the TDC counter value CTDCTHIGOF is incremented (step58). Then, the cylinder number CUCYL assigned at the time is shifted to the immediately preceding value CUCYLIGOFTHZ thereof (step59), and then the process proceeds to the step60.

In the step60, it is determined whether or not the stage number STG is 0, and in a step61, it is determined whether or not the engine speed NE is equal to 0. If the answer to the question of the step60is negative (NO), i.e. if none of the cylinders3aare in the middle stage of the compression stroke, or if the answer to the question of the step61is affirmative (YES), i.e. if the engine3has been completely stopped, the present process is terminated.

On the other hand, if the answer to the question of the step60is affirmative (YES), i.e. if one of the cylinders3ais in the middle stage of the compression stroke, and at the same time if the answer to the question of the step61is negative (NO), i.e. if the engine3has not been completely stopped, it is determined in a step62whether or not a provisional value NEPRSF of the final compression stroke rotational speed is larger than the engine speed NE obtained at the time. If the answer to this question is negative (NO), i.e. if NEPRSF≦NE holds, the present process is terminated.

On the other hand, if the answer to the question of the step62is affirmative (YES), i.e. if NEPRSF>NE holds, the engine speed NE is stored as the provisional value NEPRSF of the final compression stroke rotational speed (step63), and then it is determined in a step64whether or not a final compression stroke rotational speed calculation completion flag F_SETPRSFTGT is equal to 1. If the answer to this question is affirmative (YES), i.e. if calculation of the final compression stroke rotational speed NEPRSFTGT has already been completed, the present process is terminated.

On the other hand, if the answer to the question of the step64is negative (NO), i.e. if the calculation of the final compression stroke rotational speed NEPRSFTGT has not been completed yet, it is determined whether or not the TDC counter value CTDCTHIGOF is equal to a predetermined value NTDCIGOFTH (STEP65). This predetermined value NTDCIGOFTH is determined in advance by determining empirically e.g. by experiment how many times of occurrence of TDC after the start of the second stage control will bring about the final compression stroke, and is set to e.g. 3 in the present embodiment.

If the answer to the question of the step65is negative (NO), it is judged that the final compression stroke has not been reached, and hence the process proceeds to the step52, wherein the final compression stroke rotational speed NEPRSFTGT is set to 0, followed by terminating the present process.

On the other hand, if the answer to the question of the step65is affirmative (YES), it is determined that the final compression stroke has been reached, and the provisional value NEPRSF stored in the step63is calculated as the final compression stroke rotational speed NEPRSFTGT (step66). Further, the final compression stroke rotational speed calculation completion flag F_SETPRSFTGT is set to 1 (step67), followed by terminating the present process. In the following stop control, the final compression stroke rotational speed NEPRSFTGT thus calculated is applied to the aforementioned equation (1), and is used for setting the target stop control start rotational speed NEICOFREFX.

FIG. 14shows an example of an operation obtained by a stop control process of the engine3according to the above-described first embodiment. In a case indicated by solid lines in the figure, when the ignition switch (SW)21is turned off, the supply of fuel from the fuel injection valve6is stopped, whereby the engine speed NE is lowered. Further, at this time, the target opening degree ICMDTHIGOF is set to 0, whereby the opening degree of the throttle valve13a(throttle valve opening ATH) is controlled such that the throttle valve13ais fully closed, and in accordance therewith, the intake pressure PBA is reduced. After that, when the engine speed NE becomes lower than the first stage control start rotational speed NEICOFPRE, the first stage control is started, and the target opening degree ICMDTHIGOF is set to the first stage control target opening degree ICMDOFPRE, whereby the throttle valve opening ATH is controlled toward the open side, and in accordance therewith, the intake pressure PBA increases.

Then, when the engine speed NE becomes lower than the corrected target stop control start rotational speed NEICOFREFN, the first stage control is terminated, and the second stage control is started. At this time point, the intake pressure PBA has increased up to a desired initial value PBAREF. Along with the second stage control, the target opening degree ICMDTHIGOF is set to the second predetermined opening degree ICMDOF2, whereby the throttle valve opening ATH becomes larger. In accordance therewith, the intake pressure PBA increases from the initial value PBAREF to the atmospheric pressure PA. As a consequence, the final compression stroke rotational speed NEPRSFTGT becomes approximately equal to the reference value NENPFLMT0, whereby it is possible to accurately stop the piston3dat the predetermined position to prevent valve overlap.

On the other hand, in a case indicated by broken lines in the figure, the corrected target stop control start rotational speed NEICOFREFN is set to a smaller value than in the above-described case indicated by solid lines, and accordingly the first stage control start rotational speed NEICOFPRE is set to a smaller value (step71inFIG. 13). This causes the second stage control to be started in later timing than in the above-described case indicated by solid lines, and in accordance with, the first stage control is also started in later timing. As a consequence, the intake pressure PBA at the start of the second stage control is approximately equal to the desired initial value PBAREF. Therefore, similarly to the case indicated by solid lines, it is possible to accurately stop the piston3dat the predetermined position.

Further, in a case indicated by one-dot chain lines in the figure, the corrected target stop control start rotational speed NEICOFREFN is set to a larger value than in the above-described case indicated by solid lines, and accordingly, inversely to the case indicated by broken lines, the first stage control start rotational speed NEICOFPRE is set to a larger value (step71inFIG. 13). This causes the second stage control to be started in earlier timing than in the case indicated by the solid lines, and in accordance therewith, the first stage control is also started in earlier timing. As a consequence, the intake pressure PBA at the start of the second stage control is approximately equal to the desired initial value PBAREF. Therefore, similarly to the case indicated by solid lines, it is possible to accurately stop the piston3dat the predetermined position.

As described above, according to the present embodiment, during stoppage of the engine3, when opening the throttle valve13afrom the fully-closed state (step25inFIG. 6) in order to control the stop position of the piston3d, first, the target opening degree ICMDTHIGOF of the throttle valve13ais set to the first stage control target opening degree ICMDOFPRE by the first stage control (step34inFIG. 6), and then is set to the second predetermined opening degree ICMDOF2or the third predetermined opening degree ICMDOF3, larger than the first stage control target opening degree ICMDOFPRE, by the second stage control (steps42and46inFIG. 6).

As described above, by opening the throttle valve13ain two stages, it is possible to avoid a steep rise in the intake pressure PBA during opening the throttle valve13a, thereby making it possible to prevent occurrence of untoward noise, such as flow noise, and vibration caused by the steep increase in the intake pressure PBA. Further, in the first stage control, the target opening degree ICMDTHIGOF of the throttle valve13ais not progressively increased but is held at the first stage control target opening degree ICMDOFPRE, and hence it is possible to stabilize initial conditions, such as the intake pressure PBA, at the start of the second stage control, while suppressing adverse influences of variation in the operating characteristics of the throttle valve13aand delay in operation. This makes it possible to accurately stop the piston3dat the predetermined position by the second stage control.

Further, when the corrected target stop control start rotational speed NEICOFREFN is changed according to the correlation between the stop control start rotational speed NEIGOFTH and the final compression stroke rotational speed NEPRSFTGT, the first stage control start rotational speed NEICOFPRE is set to a value obtained by adding the predetermined value DNEICOFPRE to the changed corrected target stop control start rotational speed NEICOFREFN (step71inFIG. 13). Therefore, even when timing for starting the second stage control is changed, the first stage control is started in timing coping with the change in the start timing, whereby it is possible to stabilize the initial conditions for the second stage control, thereby making it possible to ensure the accuracy of the stop control of the piston3dby the second stage control.

Furthermore, if the first stage control start rotational speed NEICOFPRE set according to the corrected target stop control start rotational speed NEICOFREFN is larger than the upper limit value NEPRELMT, the first stage control start rotational speed NEICOFPRE is limited to the upper limit value NEPRELMT (steps72and74inFIG. 13). This causes the first stage control to be started after waiting for the engine speed NE to be lowered to the upper limit value NEPRELMT, so that it is possible to avoid execution of the first stage control in a resonance area where the engine speed NE is high, thereby making it possible to positively prevent untoward noise and vibration caused by the resonance of the engine3.

Further, when the first stage control start rotational speed NEICOFPRE is limited as described above, the first stage control target opening degree ICMDOFPRE is corrected to a larger value (step75inFIG. 13), so that by compensating for the insufficient amount of the intake air amount due to delay of start of the first stage control, it is possible to stabilize the initial conditions for the second stage control, thereby making it possible to ensure the accuracy of the stop control of the piston3d.

Further, since the target stop control start rotational speed NEICOFREFX is corrected according to the actual atmospheric pressure PA and intake air temperature TA to calculate the corrected target stop control start rotational speed NEICOFREFN (steps26to28inFIG. 5), it is possible to more properly set the corrected target stop control start rotational speed NEICOFREFN, thereby making it possible to further enhance the accuracy of the stop control of the piston3d.

Note that although in the above-described first embodiment, the first stage control start rotational speed NEICOFPRE is calculated by adding the predetermined value DNEICOFPRE to the corrected target stop control start rotational speed NEICOFREFN, this value may be further corrected by the atmospheric pressure PA and the intake air temperature TA. Specifically, first, the aforementioned map shown inFIG. 10is searched according to the atmospheric pressure PA to determine the map value DNEICOFPA, and the map value DNEICOFPA is set as a setting PA correction term dneicofpax1. Further, the aforementioned map shown inFIG. 11is searched according to the intake air temperature TA to determine the map value DNEICOFTA, and the map value DNEICOFTA is set as a setting TA correction term dneicoftax1. Then, the first stage control start rotational speed NEICOFPRE is calculated using the determined map values by the following equation (6):
NEICOFPRE=NEICOFREFN+DNEICOFPRE+dneicofpax1+dneicoftax1  (6)

By the setting the maps inFIGS. 10 and 11, the above-mentioned setting PA correction term dneicofpax1is set to a larger value as the atmospheric pressure PA is higher, and the setting TA correction term dneicoftax1is set to a larger value as the intake air temperature TA is lower.

Therefore, the first stage control start rotational speed NEICOFPRE is corrected such that it becomes larger as the atmospheric pressure PA is higher and as the intake air temperature TA is lower. This makes it possible to set the first stage control start rotational speed NEICOFPRE in a more fine-grained manner according to the actual atmospheric pressure PA and intake air temperature TA, to more properly control an intake pressure PBA at the start of the second stage control, and therefore it is possible to further enhance the accuracy of the stop control of the piston3d.

Further, although in the first embodiment, the second predetermined opening degree ICMDOF2is a fixed value, the second predetermined opening degree ICMDOF2may be corrected and set using the atmospheric pressure PA and the intake air temperature TA. Specifically, first, a map shown inFIG. 22is searched according to the atmospheric pressure PA to determine a map value DATHICOFPA, whereby the map value DATHICOFPA is set as a setting PA correction term dathicofpax, and a map shown inFIG. 23is searched according to the intake air temperature TA to determine a map value DATHICOFTA, whereby the map value DATHICOFTA is set as a setting TA correction term dathicoftax. Then, the second predetermined opening degree ICMDOF2is calculated using a basic value ICMDOF2B of the second predetermined opening degree and the setting PA correction term dathicofpax and the setting TA correction term dathicoftax, by the following equation (7):
ICMDOF2=ICMDOF2B+dathicofpax+dathicoftax  (7)

In the map shown inFIG. 22, the map value DATHICOFPA is set to a larger value as the atmospheric pressure PA is lower, and in the map shown inFIG. 23, the map value DATHICOFTA is set to a larger value as the intake air temperature TA is higher.

Therefore, the second predetermined opening degree ICMDOF2is corrected such that it becomes larger as the atmospheric pressure PA is lower and as the intake air temperature TA is higher. This makes it possible to set the second predetermined opening degree ICMDOF2in a more fine-grained manner according to the actual atmospheric pressure PA and intake air temperature TA, and therefore it is possible to further enhance the accuracy of the stop control of the piston3d.

Next, a process for calculating the first stage control target opening degree ICMDOFPRE according to a second embodiment of the present invention will be described with reference toFIG. 15. This calculation process is executed in place of theFIG. 13calculation process according to the first embodiment. In the first embodiment, the first stage control start rotational speed NEICOFPRE is changed according to a change in the corrected target stop control start rotational speed NEICOFREFN. As distinct therefrom, in the present embodiment, the first stage control target opening degree ICMDOFPRE is changed without changing the first stage control start rotational speed NEICOFPRE.

In the present process, first, in a step81, the difference between the predetermined first stage control start rotational speed NEICOFPRE and the corrected target stop control start rotational speed NEICOFREFN calculated in the step28inFIG. 5is calculated as a rotational speed difference DNE12.

Next, an NE correction term DICMDPRENE is calculated by searching a map shown inFIG. 16according to the calculated rotational speed difference DNE12(step82). In this map, the NE correction term DICMDPRENE is set to a larger value as the rotational speed difference DNE12is smaller.

Next, a PA correction term DICMDPREPA is calculated by searching a map shown inFIG. 17according to the atmospheric pressure PA (step83). In this map, the PA correction term DICMDPREPA is set to a larger value as the atmospheric pressure PA is lower.

Next, a TA correction term DICMDPRETA is calculated by searching a map shown inFIG. 18according to the intake air temperature TA (step84). In this map, the TA correction term DICMDPRETA is set to a larger value as the intake air temperature TA is higher.

Finally, the first stage control target opening degree ICMDOFPRE is calculated by adding the NE correction term DICMDPRENE, the PA correction term DICMDPREPA, and the TA correction term DICMDPRETA, which are calculated in the steps82to84, to a predetermined basic value ICMDPREB (step85), by the following equation (8), followed by terminating the present process.
ICMDOFPRE=ICMDPREB+DICMDPRENE+DICMDPREPA+DICMDPRETA  (8)

Such correction is carried out for the following reasons: As the difference between the first stage control start rotational speed NEICOFPRE and the corrected target stop control start rotational speed NEICOFREFN (=rotational speed difference DNE12) is smaller, a time period taken for the first stage control becomes shorter, and hence the intake pressure PBA at the start of the second stage control becomes liable to be short. Therefore, as described above, by setting the NE correction term DICMDPRENE to a larger value and correcting the first stage control target opening degree ICMDOFPRE to a larger value, as the rotational speed difference DNE12is smaller, the intake air amount and the intake pressure PBA are increased, whereby it is possible to hold the intake pressure PBA at the start of the second stage control substantially constant.

Further, as the atmospheric pressure PA is higher, the density of intake air is higher, so that in the case of the intake air amount being the same, the intake pressure PBA becomes more difficult to increase. Therefore, as described above, as the atmospheric pressure PA is higher, the PA correction term DICMDPREPA is set to a larger value to increase the intake air amount and the intake pressure PBA, whereby it is possible to hold the intake pressure PBA at the start of the second stage control substantially constant.

Further, as the intake air temperature TA is lower, the sliding friction of the piston3dis larger and the density of intake air is higher, so that the rate of reduction of the engine speed NE becomes larger, and timing for starting the second stage control becomes earlier. This makes the time period for the first stage control shorter to make the intake pressure PBA at the start of the second stage control liable to be short. Therefore, as the intake air temperature TA is lower, the TA correction term DICMDPREPA is set to a larger value to increase the intake air amount and the intake pressure PBA, whereby it is possible to hold the intake pressure PBA at the start of the second stage control substantially constant.

FIG. 19shows an example of an operation obtained by a stop control process of the engine3according to the above-described second embodiment. In a case indicated by solid lines in the figure, when the ignition switch21is turned off, the target opening degree ICMDTHIGOF is set to 0, whereby the throttle valve opening ATH is controlled such that the throttle valve13ais fully closed, and the intake pressure PBA is reduced. After that, when the engine speed NE becomes lower than the first stage control start rotational speed NEICOFPRE, the first stage control is started, and further when the engine speed NE becomes lower than the corrected target stop control start rotational speed NEICOFREFN, the second stage control is started. At this time, the intake pressure PBA has increased up to the desired initial value PBAREF.

In contrast, in a case indicated by broken lines in the figure, the corrected target stop control start rotational speed NEICOFREFN is set to a smaller value than in the above-described case indicated by solid lines, and in accordance therewith, the first stage control target opening degree ICMDOFPRE is set to a smaller value (step82inFIG. 15). This causes the second control to be started in later timing than in the case indicated by solid lines, and accordingly makes the time period for the first stage control longer while reducing the intake air amount. As a consequence, the intake pressure PBA at the start of the second stage control is approximately equal to the initial value PBAREF.

Further, in a case indicated by one-dot chain lines in the figure, the corrected target stop control start rotational speed NEICOFREFN is set to a larger value than in the above-described case indicated by solid lines, and accordingly, the first stage control target opening degree ICMDOFPRE is set to a larger value (step82inFIG. 15). This causes the second control to be started in earlier timing than in the case indicated by solid lines, and accordingly, makes the time period for the first stage control shorter while reducing the intake air amount. As a consequence, the intake pressure PBA at the start of the second stage control is approximately equal to the initial value PBAREF.

As described hereinabove, according to the present embodiment, when the corrected target stop control start rotational speed NEICOFREFN is changed, the first stage control target opening degree ICMDOFPRE is set according to the rotational speed difference DNE12between the predetermined first stage control start rotational speed NEICOFPRE and the changed corrected target stop control start rotational speed NEICOFREFN such that it is set to a larger value as the rotational speed difference DNE12is smaller (FIG. 15steps81and82,FIG. 16). Therefore, even when the timing for starting the second stage control is changed, the first stage control is executed by the intake air amount coping with the change in the timing, whereby it is possible to stabilize the initial conditions for the second stage control, thereby making it possible to ensure the accuracy of the stop control of the piston3dby the second stage control.

Further, since the first stage control target opening degree ICMDOFPRE is corrected according to the actual atmospheric pressure PA and intake air temperature TA (steps83to85inFIG. 15), it is possible to more properly set the first stage control target opening degree ICMDOFPRE, and therefore it is possible to further stabilize the initial conditions for the second stage control, thereby making it possible to further enhance the accuracy of the stop control of the piston3d.

Next, a third embodiment of the present invention will be described with reference toFIGS. 20 to 26. In the first and second embodiments, the target stop control start rotational speed NEICOFREFX, which is a target value of the stop control start rotational speed for starting the second stage control, is set and learned. As distinct therefrom, in the present embodiment, a target second stage control opening degree ATHICOFREFX in the second stage control is set and learned.

FIG. 20shows a process for setting this target second stage control opening degree ATHICOFREFX. In the present process, first, in a step91, it is determined whether or not a target second stage control opening degree-setting completion flag F_IGOFATHREFDONE is equal to 1. If the answer to this question is affirmative (YES), i.e. if the target second stage control opening degree ATHICOFREFX has already been set, the present process is immediately terminated.

On the other hand, if the answer to the question of the step91is negative (NO), i.e. if the target second stage control opening degree ATHICOFREFX has not been set yet, it is determined in a step92whether or not the number of times of learning NENGSTP is equal to 0. If the answer to this question is affirmative (YES), the target second stage control opening degree ATHICOFREFX is set to a predetermined initial value ATHICOFINI (step93), and then the process proceeds to a step102, described hereinafter.

On the other hand, if the answer to the question of the step92is negative (NO), it is determined in a step94whether or not the aforementioned learning condition satisfied flag F_NEICOFRCND is equal to 1. If the answer to this question is negative (NO), i.e. if the learning conditions are not satisfied, the target second stage control opening degree NEICOFREFX is not learned, and then the process proceeds to a step103, described hereinafter.

On the other hand, if the answer to the question of the step94is affirmative (YES), i.e. if the conditions for learning the target second stage control opening degree ATHICOFREFX are satisfied, the process proceeds to a step95, wherein the intercept INTCPNPF is calculated using the final compression stroke rotational speed NEPRSFTGT obtained during the immediately preceding stop control, the second stage control opening degree ATHIGOFTH, and the predetermined slope SLOPENTF0, by the following equation (9):
INTCPNTF=NEPRSFTGT−SLOPENTF0·ATHIGOFTH  (9)

This equation (9) is based on preconditions that a correlation as shown inFIG. 21, i.e. a correlation expressed by a linear function having a slope of SLOPENTF0and an intercept of INTCPNTF holds between the second stage control opening degree ATHIGOFTH and the final compression stroke rotational speed NEPRSFTGT, and the slope SLOPENTF0is constant if the engine3is of the same type. The intercept INTCPNTF is calculated according to the above preconditions, using the second stage control opening degree ATHIGOFTH and the final compression stroke rotational speed NEPRSFTGT obtained during the stop control, by the equation (9), whereby the correlation between the second stage control opening degree ATHIGOFTH and the final compression stroke rotational speed NEPRSFTGT is determined. Incidentally, as the friction of the piston3dis larger, the final compression stroke rotational NEPRSFTGT takes a larger value with respect to a basic value ATHICOFRRT of the same target second stage control opening degree, so that the linear function is offset toward an upper side (as indicated by broken lines inFIG. 21, for example), and the intercept INTCPNTF is calculated to be a larger value. Inversely, as the friction of the piston3dis smaller, the linear function is offset toward a lower side (as indicated by one-dot chain lines inFIG. 21, for example) for the converse reason to the above, and the intercept INTCPNTF is calculated to be a smaller value.

Then, in a step96, the basic value ATHICOFRRT of the target second stage control opening degree is calculated based on the correlation determined as described above, by using the calculated intercept INTCPNTF and slope SLOPENTF0and applying the predetermined reference value NENPFLMT0of the final compression stroke rotational speed to the following equation (10) (seeFIG. 21).
ATHICOFRRT=(NENPFLMT0−INTCPNTF)/SLOPENTF0  (10)

By using the basic value ATHICOFRRT of the target second stage control opening degree calculated by the above-mentioned equation (10), it is possible to stop the piston3dat the predetermined position.

Next, in a step97, a map shown inFIG. 22is searched according to the atmospheric pressure PA0detected during the stop control to determine the map value DATHICOFPA, and the map value DATHICOFPA is set as the learning PA correction term dathicofrpa.

Then, in a step98, a map shown inFIG. 23is searched according to the intake air temperature TA0detected during the stop control to determine a map value DATHICOFTA, and the map value DATHICOFTA is set as a learning TA correction term dathicofrta.

By the setting the maps inFIGS. 22 and 23, the above-described learning PA correction term dathicofrpa is set to a smaller value as the atmospheric pressure PA0is higher, and the learning TA correction term dathicofrta is set to a smaller value as the intake air temperature TA0is lower.

Next, a corrected basic value ATHICOFREF of the target second stage control opening degree is calculated using the basic value ATHICOFRRT of the target second stage control opening degree, the learning PA correction term dathicofrpa, and the learning TA correction term dathicofrta, which are calculated in the steps96to98, by the following equation (11) (step99):
ATHICOFREF=ATHICOFRRT−dathicofrpa−dathicofrta  (11)

As described hereinabove, since the learning PA correction term dathicofrpa is set to a smaller value as the atmospheric pressure PA0is higher, the corrected basic value ATHICOFREF of the target second stage control opening degree is corrected to a larger value as the atmospheric pressure PA0is higher. Further, since the learning TA correction term dathicofrta is set to a smaller value as the intake air temperature TA0is lower, the corrected basic value ATHICOFREF of the target stop control start rotational speed is corrected to a larger value as the intake air temperature TA0is lower.

Next, in a step100, the averaging coefficient CICOFREFX is calculated by searching the map shown inFIG. 12according to the number of times of learning NENGSTP.

Next, in a step101, a current value ATHICOFREFX of the target second stage control opening degree is calculated using the calculated corrected basic value ATHICOFREF of the target stop control start rotational speed, an immediately preceding value ATHICOFREFX of the target second stage control opening degree, and the averaging coefficient CICOFREFX, by the following equation (12):
ATHICOFREFX=ATHICOFREF·(1−CICOFREFX)+ATHICOFREFX·CICOFREFX  (12)

As is clear from the above equation (12), the target second stage control opening degree ATHICOFREFX is calculated as a weighted average value of the corrected basic value ATHICOFRRT of the target second stage control opening degree and the immediately preceding value ATHICOFREFX of the target second stage control opening degree, and the averaging coefficient CICOFREFX is used as a weight coefficient for weighted averaging. Further, the averaging coefficient CICOFREFX is set as described above according to the number of times of learning NENGSTP, and therefore as the number of times of learning NENGSTP is smaller, the degree of reflection of the corrected basic value ATHICOFREF of the target second stage control opening degree becomes larger, whereas as the number of times of learning NENGSTP is larger, the degree of reflection of the immediately preceding value ATHICOFREFX of the target second stage control opening degree becomes larger.

In the step102following the step93or101, the number of times of learning NENGSTP is incremented. Further, if the answer to the question of the step94is negative (NO), or after the step102, the process proceeds to the step103, wherein the target second stage control opening degree-setting completion flag F_IGOFATHREFDONE is set to 1, followed by terminating the present process.

FIG. 24shows a process for calculating the first stage control target opening degree ICMDOFPRE. In the present process, first, in a step111, the above-mentioned map shown inFIG. 22is searched according to the atmospheric pressure PA currently detected to thereby determine the map value DATHICOFPA, and the map value DATHICOFPA is set as a setting PA correction term dathicofpax1.

Next, in a step112, the above-mentioned map shown inFIG. 23is searched according to the intake air temperature TA currently detected to thereby determine the map value DATHICOFTA, and the map value DATHICOFTA is set as a setting TA correction term dathicoftax1.

Then, in a step113, the first stage control target opening degree ICMDOFPRE is calculated using a basic value ICMDPREA, the target second stage control opening degree ATHICOFREFX, the initial value ATHICOFINI, a predetermined value KATH, and the setting PA correction term dathicofpax1and setting TA correction term dathicoftax1calculated as described above, by the following equation (13), followed by terminating the present process.
ICMDOFPRE=ICMDPREA−(ATHICOFREFX−ATHICOFINI)·KATH-−dathicofpax1−dathicoftax1(13)

As is clear from the above equation (13), the first stage control target opening degree ICMDOFPRE is set to a smaller value as the target second stage control opening degree ATHICOFREFX is larger. The fact that the target second stage control opening degree ATHICOFREFX is set to a large value by the learning of the target second stage control opening degree ATHICOFREFX described above represents a state where a time period required for the first stage control is liable to be long since the friction of the piston3dis small to make the piston3ddifficult to be stopped. Therefore, the first stage control target opening degree ICMDOFPRE is set to a smaller value as the target second stage control opening degree ATHICOFREFX is larger (seeFIG. 28), whereby the intake air amount is reduced to suppress the rate of rise of the intake pressure PBA during the first stage control. This makes it possible to properly control the intake pressure PBA at the start of the second stage control, irrespective of the target second stage control opening degree ATHICOFREFX.

Further, as the atmospheric pressure PA is lower and as the intake air temperature TA is higher, the piston3dbecomes more difficult to be stopped. On the other hand, by setting the maps inFIGS. 22 and 23, in the equation (13), the setting PA correction term dathicofpax1is set to a larger value as the atmospheric pressure PA is lower, and the setting TA correction term dathicoftax1is set to a larger value as the intake air temperature TA is higher.

Therefore, the first stage control target opening degree ICMDOFPRE is corrected such that it becomes smaller as the atmospheric pressure PA is lower and as the intake air temperature TA is higher. This makes it possible to set the first stage control target opening degree ICMDOFPRE in a more fine-grained manner according to the actual atmospheric pressure PA and intake air temperature TA, to more properly control the intake pressure PBA at the start of the second stage control, and therefore it is possible to further enhance the accuracy of the stop control of the piston3d.

FIGS. 25 and 26show a process for setting the target opening degree ICMDTHIGOF of the throttle valve13a. In the present process, first, in a step121, it is determined whether or not the second stage control execution flag F_IGOFFTH2is equal to 1. If the answer to this question is affirmative (YES), i.e. if the second stage control is being executed, the present process is immediately terminated.

On the other hand, if the answer to the question of the step121is negative (NO), in a step122, it is determined whether or not the fuel cut flag F_IGOFFFC is equal to 1. If the answer to this question is negative (NO), the first stage control execution flag F_IGOFFTH1and the second stage control execution flag F_IGOFFTH2are set to 0, respectively (steps123and124), and the target opening degree ICMDTHIGOF is set to 0 (step125), followed by terminating the present process.

On the other hand, if the answer to the question of the step122is affirmative (YES), the above-mentioned map shown inFIG. 22is searched according to the atmospheric pressure PA currently detected to determine the map value DATHICOFPA, whereby the map value DATHICOFPA is set as a setting PA correction term dathicofpax (step126).

Next, in a step127, the above-mentioned map shown inFIG. 23is searched according to the intake air temperature TA currently detected to thereby determine the map value DATHICOFTA, and the map value DATHICOFTA is set as a setting TA correction term dathicoftax.

Next, in a step128, a corrected target second stage control opening degree ATHICOFREFN is calculated using the target second stage control opening degree ATHICOFREFX calculated in the step101inFIG. 20, the calculated setting PA correction term dathicofpax and setting TA correction term dathicoftax, by the following equation (14):
ATHICOFREFN−ATHICOFREFX+dathicofpax+dathicoftax  (14)

As the atmospheric pressure PA is lower, the density of intake air is lower and the resistance of intake air to the piston3dis smaller, so that the rate of reduction of the engine speed NE becomes smaller. Further, after the control signal based on the target opening degree ICMDTHIGOF is delivered, there occurs a delay before the opening degree of the throttle valve13abecomes commensurate with the control signal, and a further delay occurs before the intake air amount becomes large enough to be commensurate with the opening degree of the throttle valve13a. Therefore, by correcting the corrected target second stage control opening degree ATHICOFREFN to a larger value as the atmospheric pressure PA is lower, to thereby increase the intake air amount, it is possible to properly avoid the adverse influence of the operation of the throttle valve13aand the delay of intake air, described above.

On the other hand, since the setting TA correction term dathicoftax is set to a larger value as the intake air temperature TA is higher, the corrected target second stage control opening degree ATHICOFREFN is corrected to a larger value as the intake air temperature TA is higher. As the intake air temperature TA is higher, the sliding friction of the piston3dis smaller, and the density of intake air is lower, which reduces the rate of reduction of the engine speed NE. Therefore, by correcting the corrected target second stage control opening degree ATHICOFREFN to a smaller value as the intake air temperature TA is lower to thereby reduce the intake air amount, it is possible to properly avoid the adverse influence of the operation of the throttle valve13aand the delay of intake air.

Then, in a step129, it is determined whether or not the engine speed NE is smaller than a predetermined first stage control start rotational speed NEICOFPRE (e.g. 550 rpm). If the answer to this question is negative (NO), i.e. if NE≧NEICOFPRE holds, the above-described steps123to125are executed, followed by terminating the present process.

On the other hand, if the answer to the question of the step129is affirmative (YES), i.e. if the engine speed NE is smaller than the first stage control start rotational speed NEICOFPRE, it is determined whether or not the first stage control execution flag F_IGOFFTH1is equal to 1 (step130). If the answer to this question is negative (NO), i.e. if the first stage control has not been executed yet, the target opening degree ICMDTHIGOF is set to the first stage control target opening degree ICMDOFPRE calculated in the step113inFIG. 24(step133), and the first stage control execution flag F_IGOFFTH1is set to 1 (step134), followed by terminating the present process.

On the other hand, if the answer to the question of the step130is affirmative (YES), i.e. if the first stage control is being executed, it is determined whether or not the stage number STG is 0 (step131). If the answer to this question is negative (NO), the above-described steps133and134are executed, followed by terminating the present process.

On the other hand, if the answer to the question of the step131is affirmative (YES), i.e. if the stage number STG is 0, it is determined whether or not the engine speed NE is smaller than a predetermined stop control start rotational speed NEICOFREFN (e.g. 500 rpm) (step132). If the answer to this question is negative (NO), i.e. if NEICOFREFN≦NE<NEICOFPRE holds, the above-described steps133and134are executed to thereby continue the first stage control, followed by terminating the present process.

On the other hand, if the answer to the question of the step132is affirmative (YES), i.e. if the stage number STG is 0, and at the same time if the engine speed NE is lower than the stop control start rotational speed NEICOFREFN, the process proceeds to a step135, wherein the corrected target second stage control opening degree ATHICOFREFN calculated in the step128is stored as a second stage control opening degree ATHIGOFTH for the stop control, and the atmospheric pressure PA and the intake air temperature TA, which are currently detected, are stored as an atmospheric pressure PA0and an intake air temperature TA0detected for the stop control (steps136and137), respectively. The stored second stage control opening degree ATHIGOFTH is applied to the aforementioned equation (9), and the atmospheric pressure PA0and the intake air temperature TA0are used in theFIG. 20steps97and98, for calculating the learning PA correction term dathicofrpa and the learning TA correction term dathicofrta, respectively.

Next, in a step138, the target opening degree ICMDTHIGOF is set to the corrected target second stage control opening degree ATHICOFREFN set in the step128. Further, the second stage control execution flag F_IGOFFTH2is set to 1 (step139), followed by terminating the present process.

After that, the final compression stroke rotational speed NEPRSFTGT is calculated in the process shown inFIGS. 7 and 8. In the following stop control, the calculated final compression stroke rotational speed NEPRSFTGT is applied to the aforementioned equation (9), and is used for setting the target second stage control opening degree ATHICOFREFX.

As described hereinabove, according to the present embodiment, when the target second stage control opening degree ATHICOFREFX is changed, the first stage control target opening degree ICMDOFPRE is set to a smaller value as the target second stage control opening degree ATHICOFREFX is larger (seeFIG. 28). Therefore, even when the target second stage control opening degree ATHICOFREFX is changed, the first stage control is executed by the intake air amount dependent on the change in the target second stage control opening degree ATHICOFREFX, whereby it is possible to stabilize the intake pressure PBA at the start of the second stage control, thereby making it possible to ensure the accuracy of the stop control of the piston3dby the second stage control.

Further, since the first stage control target opening degree ICMDOFPRE is corrected according to the actual atmospheric pressure PA and intake air temperature TA, it is possible to more properly set the first stage control target opening degree ICMDOFPRE, and therefore it is possible to further stabilize the intake pressure PBA at the start of the second stage control, thereby making it possible to further enhance the accuracy of the stop control of the piston3d.

Note that in the above-described third embodiment, the first stage control start rotational speed NEICOFPRE is a fixed value, the first stage control start rotational speed NEICOFPRE may be corrected and set using the atmospheric pressure PA and the intake air temperature TA. Specifically, first, a map shown inFIG. 10is searched according to the atmospheric pressure PA to determine a map value DNEICOFPA, whereby the map value DNEICOFPA is set as the setting PA correction term dneicofpax, and a map shown inFIG. 11is searched according to the intake air temperature TA to determine a map value DNEICOFTA, whereby the map value DNEICOFTA is set as the setting TA correction term dneicoftax. Then, the second predetermined opening degree ICMDOF2is calculated using a basic value NEICOFPREB of the first stage control start rotational speed and the setting PA correction term dneicofpax and the setting TA correction term dneicoftax, by the following equation (15):
NEICOFPRE=NEICOFPREB+dneicofpax+dneicoftax  (15)

In the map shown inFIG. 10, the map value DNEICOFPA is set to a larger value as the atmospheric pressure PA is higher, and in the map shown inFIG. 11, the map value DNEICOFTA is set to a larger value as the intake air temperature TA is lower.

Therefore, the first stage control start rotational speed NEICOFPRE is corrected such that it becomes larger as the atmospheric pressure PA is higher and as the intake air temperature TA is lower. This makes it possible to set the first stage control start rotational speed NEICOFPRE in a more fine-grained manner according to the actual atmospheric pressure PA and intake air temperature TA, and therefore it is possible to further enhance the accuracy of the stop control of the piston3d.

Next, a variation of the third embodiment will be described with reference toFIG. 27. In the third embodiment, the first stage control start rotational speed NEICOFPRE used in the step129inFIG. 25is a fixed value. As distinct therefrom, in this variation, the first stage control start rotational speed NEICOFPRE is calculated according to the target second stage control opening degree ATHICOFREFX.

In the present embodiment, first, in a step141, the above-mentioned map shown inFIG. 10is searched according to the atmospheric pressure PA to thereby determine the map value DNEICOFPA, and the map value DNEICOFPA is set as a setting PA correction term dneicofpax1for the first stage control start rotational speed.

Next, in a step142, the above-mentioned map shown inFIG. 11is searched according to the intake air temperature TA to determine the map value DNEICOFTA, whereby the map value DNEICOFTA is set as a setting TA correction term dneicoftax1for the first stage control start rotational speed.

Next, in a step143, the first stage control start rotational speed NEICOFPRE is calculated using a predetermined basic value NEICPREB, the target second stage control opening degree ATHICOFREFX, the initial value ATHICOFINI, and a predetermined coefficient KATHNE, as well as the setting PA correction term dneicofpax1and setting TA correction term dneicoftax1calculated as described above, by the following equation (16):
NEICOFPRE=NEICPREB−(ATHICOFREFX−ATHICOFINI)·KATHNE+dneicofpax1+dneicoftax1  (16)

followed by terminating the present process.

As is clear from the above equation (16), the first stage control start rotational speed NEICOFPRE is set to a smaller value as the target second stage control opening degree ATHICOFREFX is larger. The fact that the target second stage control opening degree ATHICOFREFX is set to a large value by the learning of the target second stage control opening degree ATHICOFREFX described above represents a state where the time period required for the first stage control is liable to be long since the friction of the piston3dis small to make the piston3ddifficult to be stopped. Therefore, the first stage control start rotational speed NEICOFPRE is set to a smaller value as the target second stage control opening degree ATHICOFREFX is larger (seeFIG. 29), whereby the first stage control is started in later timing. As a consequence, it is possible to properly control the intake pressure PBA at the start of the second stage control irrespective of the target second stage control opening degree ATHICOFREFX.

Further, as the atmospheric pressure PA is lower and as the intake air temperature TA is higher, the piston3dbecomes more difficult to be stopped. On the other hand, by setting the maps inFIGS. 10 and 11, in the equation (16), the setting PA correction term dneicofpax1is set to a smaller value as the atmospheric pressure PA is lower and the setting TA correction term dneicoftax1is set to a smaller value as the intake air temperature TA is higher.

Therefore, the first stage control start rotational speed NEICOFPRE is corrected such that it becomes smaller as the atmospheric pressure PA is lower and as the intake air temperature TA is higher. This makes it possible to set the first stage control start rotational speed NEICOFPRE in a more fine-grained manner according to the actual atmospheric pressure PA and intake air temperature TA, to thereby more properly control the intake pressure PBA at the start of the second stage control. Therefore, it is possible to further enhance the accuracy of the stop control of the piston3d.

Note that the present invention is by no means limited to the embodiments described above, but can be practiced in various forms. For example, although in the above-described embodiments, the throttle valve13ais used as the intake air amount-adjusting valve for adjusting the intake air amount during stoppage of the engine3, in place of the throttle valve13a, there may be used intake valves the lift of which can be changed by a variable intake lift mechanism.

Further, although in the above-described embodiments, the correction of the target stop control start rotational speed NEICOFREFX or the first stage control target opening degree ICMDOFPRE is performed according to the atmospheric pressure PA and the intake air temperature TA, the correction may be performed according to a parameter indicative of the temperature of the engine3, such as the engine coolant temperature TW, in addition to or in place of the atmospheric pressure PA and the intake air temperature TA. In this case, as the engine coolant temperature TW is lower, the sliding friction of the piston3dis larger, and hence the target stop control start rotational speed NEICOFREFX or the first stage control target opening degree ICMDOFPRE is corrected to a larger value. Further, such correction may be carried out on the first stage control start rotational speed NEICOFPRE and/or the second predetermined opening degree ICMDOF2for use in the second stage control.

Further, in the above-described embodiments, when the ignition switch21is turned off, judging that a command for stopping the engine3is issued, the stop control is executed, but in a case where an idle stop is executed in which the engine3is automatically stopped when predetermined stop conditions are satisfied, the stop control may be executed after satisfaction of the stop conditions.

Furthermore, although in the above-described embodiment, the present invention is applied to the gasoline engine installed on a vehicle, this is not limitative, but it can be applied to various engines other than the gasoline engine, e.g. a diesel engine, and further, it can be applied to engines other than the engines for a vehicle, e.g. engines for ship propulsion machines, such as an outboard motor having a vertically-disposed crankshaft. Further, it is possible to change details of the construction of the embodiment within the spirit and scope of the present invention.

INDUSTRIAL APPLICABILITY

As described heretofore, the stop control system according to the present invention is useful in accurately stopping the piston at a predetermined position while preventing occurrence of untoward noise and vibration during stoppage of the engine.

REFERENCE SIGNS LIST

1stop control system for internal combustion engine2ECU (rotational speed-detecting means, first intake air amount control means, second intake air amount control means, second predetermined rotational speed-setting means, first predetermined rotational speed-setting means, second predetermined opening degree-setting means, first predetermined rotational speed-limiting means, first predetermined opening degree-correcting means, first predetermined opening degree-setting means, first correction means, second correction means)3engine (internal combustion engine)3dpiston13athrottle valve (intake air amount-adjusting valve)22intake air temperature sensor (detection means)23atmospheric pressure sensor (detection means)24crank angle sensor (rotational speed-detecting means)26engine coolant temperature sensor (detection means)NE engine speed (rotational speed of internal combustion engine)PA atmospheric pressureTA intake air temperature (temperature of intake air)TW engine coolant temperature (temperature of internal combustion engine)NEICOFPRE first stage control start rotational speed (first predetermined rotational speed)NEICOFREFN corrected target stop control start rotational speed (second predetermined rotational speed)ICMDOFPRE first stage control target opening degree (first predetermined opening degree)ICMDOF2second predetermined opening degreeNEPRELMT upper limit value