Control method and control device for internal combustion engine

An internal combustion engine (1) has a variable compression ratio mechanism (2) that varies a mechanical compression ratio and a variable valve timing mechanism (7) that varies a valve timing of an intake valve (4). When there is a demand to execute reference position learning (step 21) for system calibration of the variable valve timing mechanism (7), the execution of the reference position learning is permitted on the condition that the mechanical compression ratio is higher than a threshold value VCRth (step 22). When any anomaly is present in the variable compression ratio mechanism (2), the reference position learning of the variable valve timing mechanism (7) is prohibited (steps 23 and 25).

FIELD OF THE INVENTION

The present invention relates to a control method and control device adapted, for use in an internal combustion engine equipped with a variable compression ratio mechanism capable of varying a mechanical compression ratio of the internal combustion engine and a variable valve timing mechanism capable of varying a closing timing of an intake valve, to perform reference position learning of the variable valve timing mechanism.

BACKGROUND ART

Patent Document 1 discloses a control device configured to, when a learning operation for a variable valve timing mechanism is demanded, execute the learning operation by temporarily controlling the variable valve timing mechanism to the most retarded position as a reference position and reading an output value of a cam angle sensor at that time.

Patent Document 2 discloses a variable compression ratio mechanism provided with a multi-link piston-crank unit to vary a mechanical compression ratio of an internal combustion engine by vertically moving the top dead center position of the piston. Patent Document 2 further discloses a control method in which, when any anomaly in the variable compression ratio mechanism is detected, the timing of ignition is controlled by considering the actual compression ratio to be the maximum compression ratio for prevention of knocking.

In the case of an internal combustion engine equipped with both a variable valve timing mechanism for an intake valve and a variable compression ratio mechanism, there is a possibility that a learning operation for the variable valve timing mechanism is demanded under the situation that the mechanical compression ratio is set low. In such a case, the compression end temperature in the combustion chamber of the internal combustion engine is lowered due to not only the low compression ratio, but also decrease of the effective compression ratio by retardation of the intake valve closing timing, when the variable valve timing mechanism is controlled to the most retarded position or relatively retard-side reference position for the purpose of execution of the learning operation. This results in instability of combustion.

The above problem cannot be avoided by the control method of Patent Document 2.

PRIOR ART DOCUMENTS

Patent Document

SUMMARY OF THE INVENTION

The present invention is directed to learning control in which, when there is a demand to execute a reference position learning operation of controlling a variable valve timing mechanism to a predetermined reference position and then reading a sensor value, the execution of the reference position learning operation is permitted on the condition that a mechanical compression ratio controlled by a variable compression ratio mechanism is higher than a predetermined compression ratio value.

This learning control has the effect of, even when the valve timing is temporality retarded toward the reference position for the purpose of execution of the learning operation, preventing excessive lowering of the compression end temperature by the action of the high mechanical compression ratio and thereby suppressing combustion instability during the execution of the learning operation.

DESCRIPTION OF EMBODIMENTS

Hereinafter, one embodiment of the present invention will be described below with reference to the drawings.

FIG. 1shows a system configuration of an internal combustion engine1to which the present invention is applicable. The internal combustion engine1is in the form of a four-stroke-cycle spark-ignition type internal combustion engine equipped with a variable compression ratio mechanism2using a multi-link piston-crank unit. In the internal combustion engine, a pair of intake valves4and a pair of exhaust valves5are arranged on a ceiling wall surface of each cylinder3; and a spark plug6is arranged on a center part of the ceiling wall surface surrounded by the intake valves4and the exhaust valves5. The internal combustion engine1is also equipped with a turbocharger8as illustrated in the present embodiment.

An intake-side variable valve timing mechanism7is provided on the intake valves4so as to vary and control the opening and closing timings of the intake valves4. The variable valve timing mechanism7is of any type as long as it is capable of advancing or retarding at least the intake valve closing timing. In the present embodiment, the variable valve timing mechanism7is configured to advance or retard the intake valve opening timing and the intake valve closing timing simultaneously by advancing or retarding the phase of a camshaft. As such a variable valve timing mechanism, various types are known and usable. In the present invention, the variable valve timing mechanism is not limited to the above specific configuration.

For example, the variable valve timing mechanism7includes a sprocket arranged coaxially on a front end portion of the camshaft and a hydraulic rotary actuator arranged to cause relative rotation of the sprocket and the camshaft within a predetermined angle range. The sprocket is linked to a crankshaft via a timing chain or timing belt (not shown). Accordingly, the phase of the camshaft relative to the crank angle is changed by relative rotation of the sprocket and the camshaft. The rotary actuator has an advance-side hydraulic chamber for applying a hydraulic biasing force toward the advance side and a retard-side hydraulic chamber for applying a hydraulic biasing force toward the retard side, and advances or retards the phase of the camshaft by controlling the supply of hydraulic pressure to these hydraulic chambers through a hydraulic control valve (not shown) according to a control signal from an engine controller10. By a cam angle sensor11which is responsive to a rotational position of the camshaft, the actual control position of the camshaft (corresponding to the actual valve timing) varied and controlled by the variable valve timing mechanism7is detected. The supply of hydraulic pressure through the hydraulic control valve is controlled by a closed loop control method such that the actual control position of the camshaft detected by the cam angle sensor11is adjusted to a target control position set according to operating conditions.

An intake passage14is connected to a combustion chamber13of the internal combustion engine through the intake valves4. In the intake passage14, a fuel injection valve15for port injection is provided on each cylinder. Further, a fuel injection valve16for in-cylinder injection is provided so as to directly inject fuel into each cylinder3. In other words, the internal combustion engine is equipped with a dual-injection type fuel injection system so that the supply of fuel is controlled by properly using the fuel injection valve15for port injection and the fuel injection valve16for in-cylinder injection according to a load etc. An intake collector14is provided in the intake passage14. An electronically-controlled throttle valve19, whose opening is controlled according to a control signal from the engine controller10, is disposed in the intake passage14at a position upstream of the intake controller14. A compressor8aof the turbocharger8is disposed in the intake passage at a position upstream of the throttle valve16. An air flow meter20for detecting an intake air amount and an air cleaner21are disposed at positions upstream of the compressor8ain the intake passage14. An intercooler22is provided between the compressor8aand the throttle valve19. A recirculation valve23is provided so as to provide communication between the discharge side and intake side of the compressor8a. This recirculation valve23is opened in a deceleration state where the throttle valve19is closed.

An exhaust passage25is connected to the combustion chamber13of the internal combustion engine through the exhaust valves5. A turbine8bof the turbocharger8is disposed in the exhaust passage25. A pre-catalytic unit26and a main catalytic unit27, each of which is provided with a three-way catalyst, are disposed in the exhaust passage25at positions downstream of the turbine8b. An air-fuel ratio sensor28for detecting an air-fuel ratio is disposed at a position upstream of the turbine8bin the exhaust passage25. For control of boost pressure, a wastegate valve29is provided in the turbine8bso as to bypass a part of exhaust gas according to the boost pressure.

An exhaust gas recirculation passage30is arranged between a portion of the exhaust passage25downstream of the turbine8band a portion of the intake passage14upstream of the compressor8a, and is provided with an EGR gas cooler31and an EGR valve32.

Into the engine controller10, there are inputted detection signals of various sensors. These various sensors include not only the cam angle sensor11, the air flow mater20and the air-fuel ratio sensor28, but also a crank angle sensor34for detecting an engine rotation speed, a coolant sensor35for detecting a coolant temperature, an accelerator opening sensor36for detecting a depression amount of an accelerator pedal operated by a driver, and the like. Based on these sensor detection signals, the engine controller10optimally controls the amounts and timings of fuel injection by the fuel injection valves15and16, the timing of ignition by the spark plug6, the mechanical compression ratio of the internal combustion engine by the variable compression ratio mechanism2, the opening and closing timings of the intake valves4by the variable valve timing mechanism7, the opening of the throttle valve19, the opening of the EGR valve32and the like.

On the other hand, the variable compression ratio mechanism2is of the type using a known multi-link piston-crank unit as disclosed in Patent Document 2, Japanese Laid-Open Patent Publication No. 2004-116434 or the like. More specifically, the variable compression ratio mechanism2mainly includes: a lower link rotatably supported on a crank pin41aof the crankshaft41; an upper link45connecting an upper pin43on one end portion of the lower link42and a piston pin44aof a piston44to each other; a control link47connected at one end thereof to a control pin46on the other end portion of the lower link42; and a control shaft48pivotally supporting the other end of the control link47. The crankshaft41and the control shaft48are rotatably supported via a bearing structure (not shown) within a crankcase49aat a lower portion of a cylinder block49. The control shaft48has an eccentric shaft part whose position is changed with rotation of the control shaft48. The thus-configured variable compression ratio mechanism2vertically moves the top dead center position of the piston44with rotation of the control shaft48, thereby varying the mechanical compression ratio.

In the present embodiment, an electric actuator51is provided as a driving unit for causing the variable compression ratio mechanism2to vary and control the mechanical compression ratio. The electric actuator51is disposed on an outer wall surface of the crank case49a, and has a rotational center shaft arranged in parallel with the crankshaft41. A first arm52is fixed to an output rotation shaft of the electric actuator51, whereas a second arm53is fixed to the control shaft48. These arms are coupled by an intermediate link54. The electric actuator51and the control shaft48are thus interlocked with each other through the first arm52, the second arm53and the intermediate link54. The electric actuator51includes an electric motor and a transmission unit arranged in series along an axial direction thereof.

The actual value of the mechanical compression ratio varied and controlled as mentioned above by the variable compression ratio mechanism2, that is, the actual compression ratio is detected by an actual compression ratio sensor56. The actual compression ratio sensor56has, for example, a rotary potentiometer or rotary encoder that detects a rotational angle of the control shaft48or a rotational angle of the output rotation shaft of the electric actuator51. The actual compression ratio can alternatively be detected, without using a separate sensor, by calculating the rotation amount of the electric motor from a control signal outputted to the electric motor of the electric actuator and then determining the rotational angle of the control shaft48based on the calculated rotation amount.

The electric actuator51is driven and controlled by the engine controller11such that the actual compression ratio as determined above is adjusted to a target compression ratio set according to operating conditions. For example, the engine controller10has a target compression ratio map using a load and rotation speed of the internal combustion engine1as parameters and sets the target compression ratio based on this map. Basically, the target compression ratio is set high in a low load region. As the load becomes higher, the target compression ratio is set lower for prevention of knocking etc.

Next, the control of the variable valve timing mechanism7executed by the engine controller10will be explained below by referring to flowcharts ofFIGS. 2 to 4. Herein, the routines of these flowcharts are repeatedly executed at appropriate intervals (e.g. very small time intervals).

FIG. 2shows a flowchart for setting a VTC control compression ratio aVCR as one of control parameters for the variable valve timing mechanism7(VTC). In step1(referred to as S1in the figure; the same applies to the other steps), it is judged whether or not any anomaly is present in the variable compression ratio mechanism2(VCR). The target of judgment as to the presence or absence of any anomaly includes not only the mechanical configuration of the variable compression ratio mechanism2but also the related hardware such as sensor and actuator and the control system software. In other words, anomaly diagnosis is performed on the entire variable compression ratio system including the variable compression ratio mechanism2in this step. Typical examples of the anomaly are a defect in the electric actuator51of the variable compression ratio mechanism2, a defect in the actual compression ratio sensor56and the like. The presence or absence of these anomalies is diagnosed successively or at proper timings under self-diagnosis function achieved by another routine process (not shown). In step1, the judgment is made by referring to the results of the diagnosis.

When the variable compression ratio system is in normal operation, the routine proceeds to step2. In step2, the actual compression ratio rVCR detected by the actual compression ratio sensor56at the current time is set as it is as the VTC control compression ratio aVCR. On the other hand, the routine proceeds to step3when any anomaly is present in the variable compression ratio system. In step3, a maximum compression ratio VCRmax achievable by the variable compression ratio mechanism2is set as the VTC control compression ratio aVCR. The reliability of the detected actual compression ratio value rVCR is low when any anomaly is present in the variable compression ratio system. In such an abnormal state, the mechanical compression ratio is considered to be the maximum compression ratio vCRmax in the control of the variable valve timing mechanism7, in order to reliably avoid interference of the intake valves4and the piston44in the vicinity of the top dead center.

FIG. 3shows a flowchart for the main routine of the control of the variable valve timing mechanism7. In step11, the target engine torque tTq and the engine rotation speed Ne are read as engine operating conditions. The above-mentioned VTC control compression ratio aVCR is also read as an additional parameter to avoid interference of the intake valves4and the piston44. The target engine torque tTq corresponds to the load of the internal combustion engine1, and can be determined from the accelerator opening (accelerator pedal depression amount) detected by the accelerator opening sensor36, the intake air amount detected by the air flow meter20and the like. In step12, the target control position tVTC of the variable valve timing mechanism7is set based on the target engine torque tTq, the engine rotation speed Ne and the VTC control compression ratio aVCR. The target control position tVTC is set to an optimum value determined from the target engine torque tTq and the engine rotation speed Ne, within the range determined from the VTC control compression ratio aVCR to avoid interference of the intake valves4and the piston44. In step13, the variable valve timing mechanism7is controlled according to the target control position tVTC.

As mentioned above, the VTC control compression ratio aVCR is set to the actual compression ratio rVCR when the variable compression ratio system is in normal operation. By this setting, interference of the intake valves4and the piston4is reliably avoided even in the case where there is a delay in response to changes of the actual compression ratio during a transient state such as acceleration of the internal combustion engine1. When any anomaly is present in the variable compression ratio system, on the other hand, the VTC control compression ratio aVCR is set to the maximum compression ratio VCRmax. By this setting, interference of the intake valves4and the piston4is reliably avoided even in the state where the actual compression ratio is indefinite.

FIG. 4shows a flowchart for reference position learning of the variable valve timing mechanism7. The reference position learning refers to a processing operation of, for control system calibration of the variable valve timing mechanism7, e.g. temporarily moving the hydraulic rotary actuator to a mechanically-limited reference position and reading a detection value of the cam angle sensor11in a state where the rotary actuator has been controlled to the reference position. In the present embodiment, the learning operation is executed by retarding the rotary actuator most to its physical limit position and learning this most retarded position as the reference position. It is alternatively feasible to provide a lock mechanism immediately before the mechanically-limited most retarded position and learn a most retarded position of the rotary actuator limited by the lock mechanism as the reference position.

In step21, it is judged whether or not there is a demand for a learning operation for the variable valve timing mechanism7. The demand for the learning operation is outputted by another routine process when a predetermined condition is satisfied after a start of driving operation of the internal combustion engine. The judgment is made according to the presence or absence of the output in step21. It is preferable to execute at least one leaning learning operation during one trip. The demand for the learning operation may be outputted immediately after a start of operation (e.g. immediately after a start of automatic operation) of the internal combustion engine1.

When there is no demand for the learning operation, the routine proceeds from step21to step25. In step25, the driving operation of the internal combustion engine1, that is, the normal control of the variable valve timing mechanism7is continued without the execution of the reference position learning operation.

When it is judged in step21that the learning operation is demanded, the routine proceeds to step22. In step22, the VTC control compression ratio aVCR is compared with a predetermined compression ratio threshold value VCRth. When the VTC control compression ratio aVCR is lower than or equal to the compression ratio threshold value VCRth, the routine proceeds from step22to step25so that the learning operation is not executed. In other words, the execution of the reference position learning operation is prohibited when the VTC control compression ratio aVCR is lower than or equal to the compression ratio threshold value VCRth. If the variable valve timing mechanism7is moved to the most retarded position for the purpose of execution of the learning operation under the situation that the mechanical compression ratio is controlled to a low value, there is a possibility of combustion instability with lowering of the compression end temperature in the combustion chamber due to not only the low mechanical compression ratio but also decrease of the effective compression ratio by retardation of the intake valve closing timing. For this reason, the learning operation is not executed when the VTC control compression ratio aVCR corresponding to the actual compression ratio rVCR is lower than or equal to the compression ratio threshold value VCRth. The compression ratio threshold value VCRth is set to a mechanical compression ratio level at which combustion instability does not occur even when the variable valve timing mechanism7is controlled to the reference position i.e. the most retarded position for the purpose of execution of the learning operation.

When the VTC control compression ratio aVCR is higher than the compression ratio threshold value VCRth, the routine proceeds to step23. In step23, it is judged whether or not the variable compression ratio mechanism2(VCR) is in normal operation. This step is carried out by performing self-diagnosis as to the presence or absence of any anomaly in the mechanical configuration of the variable compression ratio mechanism2, the related hardware such as sensor and actuator and the control system software, and then, referring to the results of the self-diagnosis as in the case of the above-mentioned step1. When any anomaly is present in the variable compression ratio system, the routine proceeds to step25so that the learning operation is not executed. In other words, the learning operation for the variable valve timing mechanism7is prohibited when any anomaly is present in the variable compression ratio system. This is because, when any anomaly is present in the variable compression ratio system, there is a possibility that the actual mechanical compression ratio of the internal combustion engine1is lower than or equal to the compression ratio threshold value VCRmax although the VTC control compression ratio aVCR has been set to the maximum compression ratio VCRmax in step3.

When it is judged in step23that the variable compression ratio mechanism2is in normal operation, the routine proceeds to step24. In step24, the learning operation for the variable valve timing mechanism7is executed. More specifically, the detection value of the cam angle sensor11is read in a state where the variable valve timing mechanism7is temporarily moved to the most retarded position as the reference position as mentioned above. After the completion of the learning operation, the routine returns to the normal control.

As mentioned above, the learning operation for the variable valve timing control mechanism7is permitted on the conditions that: the variable compression ratio mechanism2is in normal operation; and the actual mechanical compression ratio is higher than the compression ratio threshold value VCRth in the present embodiment. In the presence of an anomaly in the variable compression ratio mechanism2, interference of the intake valves4and the piston4is reliably avoided by considering the mechanical compression ratio (VTC control compression ratio aVCR) to be the maximum compression ratio VCRmax in the control of the variable valve timing mechanism7; and the learning operation for the variable valve timing mechanism7is prohibited irrespective of the value of the VTC control compression ratio aVCR. This makes it possible to, in the presence of any anomaly in the variable compression ratio mechanism2, prevent instability of combustion caused by retardation of the valve timing for execution of the learning operation.

The reference position for the learning operation is not limited to the most retarded position as in the above embodiment. For example, in the case where a lock mechanism is provided to the rotary actuator of the variable valve timing mechanism7, an arbitrary control position of the rotary actuator limited by the lock mechanism can be utilized as the reference position.