阅读说明：本技术 内燃机的控制方法及控制装置 (Method and device for controlling internal combustion engine ) 是由 铃木健儿 大滝绫一 山田幸誉 小野雅司 高田洋司 于 2017-06-28 设计创作，主要内容包括：内燃机(1)具有对机械压缩比进行变更的可变压缩比机构(2)和对进气阀(4)的气门正时进行变更的可变气门正时机构(7)。在加速请求时,与稳定时的目标值相比将目标压缩比设为低压缩比,将气门正时设为提前侧。可变压缩比机构(2)在规定的中间压缩比区域中容许燃烧压力变低,因此,在伴随加速的压缩比变化的过程中,在实际压缩比(rVCR)处于规定的中间压缩比区域时,通过废气旁通阀(29)的开度增大或者节气门(19)的开度减小等来限制进气压力。(An internal combustion engine (1) is provided with a variable compression ratio mechanism (2) that changes a mechanical compression ratio, and a variable valve timing mechanism (7) that changes the valve timing of an intake valve (4). At the time of the acceleration request, the target compression ratio is set to a low compression ratio and the valve timing is set to an advanced side as compared with the target value at the time of stabilization. The variable compression ratio mechanism (2) allows the combustion pressure to be made lower in a predetermined intermediate compression ratio region, and therefore, during the compression ratio change accompanying acceleration, when the actual compression ratio (rVCR) is in the predetermined intermediate compression ratio region, the intake pressure is limited by an increase in the opening degree of the wastegate valve (29), a decrease in the opening degree of the throttle valve (19), or the like.)

1. A control method of an internal combustion engine having a variable compression ratio mechanism that changes a mechanical compression ratio of the internal combustion engine,

when there is a request for acceleration, the target compression ratio of the variable compression ratio mechanism is made lower than a basic target compression ratio corresponding to the engine operating conditions at the time of stabilization,

in the process of changing the compression ratio of the variable compression ratio mechanism to the target compression ratio, the maximum combustion pressure is limited in a predetermined intermediate compression ratio region.

2. The control method of an internal combustion engine according to claim 1,

the internal combustion engine further has a variable valve timing mechanism that changes the valve timing of the intake valve,

when an acceleration request is made, the target control position of the variable valve timing mechanism is set to be more advanced than a reference target control position corresponding to a steady engine operating condition.

3. The control method of an internal combustion engine according to claim 1 or 2, wherein,

the internal combustion engine is provided with a turbocharger,

as the limit of the maximum combustion pressure, a decrease correction of the boost pressure of the turbocharger is performed.

4. The control method of an internal combustion engine according to any one of claims 1 to 3, wherein,

as the limitation of the maximum combustion pressure, a reduction correction of the throttle opening degree is performed.

5. The control method of an internal combustion engine according to any one of claims 1 to 4, wherein,

the actual compression ratio of the variable compression ratio mechanism is successively detected, and the maximum combustion pressure is limited when the actual compression ratio is in the predetermined intermediate compression ratio region.

6. The control method of an internal combustion engine according to claim 5,

an intake pressure limit value is set in advance for the value of the mechanical compression ratio in the intermediate compression ratio region,

when the actual compression ratio is in the predetermined intermediate compression ratio region, the intake pressure is limited using an intake pressure limit value corresponding to the actual compression ratio.

7. The control method of an internal combustion engine according to claim 5,

an intake pressure limit value is set in advance for a value of the mechanical compression ratio in the entire control range of the variable compression ratio mechanism, the intake pressure is limited using the intake pressure limit value corresponding to the actual compression ratio,

the value of the intake pressure limit value of the intermediate compression ratio region is set to a value smaller than the intake pressure limit values of the other regions.

8. The control method of an internal combustion engine according to any one of claims 1 to 7, wherein,

the maximum combustion pressure is limited in accordance with the characteristic of the allowable combustion pressure in the mechanism of the variable compression ratio mechanism.

9. The control method of an internal combustion engine according to any one of claims 1 to 8, wherein,

the variable compression ratio mechanism is constituted by a multi-link piston crank mechanism having an upper link one end of which is coupled to a piston, a lower link coupled to the other end of the upper link and rotatably attached to a crank pin of the crankshaft, and a control link one end of which is coupled to the lower link and the other end of which is supported swingably with respect to the engine main body, and the variable compression ratio mechanism is configured to change a mechanical compression ratio by displacing a swing support position of the control link with respect to the engine main body.

10. A control device for an internal combustion engine, comprising:

a variable compression ratio mechanism that changes a mechanical compression ratio of the internal combustion engine in accordance with an operation of the actuator;

a sensor that detects a torque request of a driver;

a storage unit that stores a reference target compression ratio according to an engine operating condition at a steady time as a target compression ratio of the variable compression ratio mechanism;

an intake pressure changing device that changes an intake pressure of the internal combustion engine; and

and a control unit that sets the target compression ratio of the variable compression ratio mechanism to be lower than the reference target compression ratio when the acceleration request is detected by the sensor, and limits the maximum combustion pressure in a predetermined intermediate compression ratio region via the intake pressure changing device while the compression ratio of the variable compression ratio mechanism is changing to the target compression ratio.

Technical Field

The present invention relates to a control method and a control device for controlling an acceleration request in an internal combustion engine having a variable compression ratio mechanism that changes a mechanical compression ratio of the internal combustion engine.

Background

In patent document 1, in an internal combustion engine having a variable compression ratio mechanism using a multi-link piston crank mechanism, when there is a request for acceleration of the internal combustion engine, a target compression ratio of the variable compression ratio mechanism is controlled to a lower compression ratio side than a characteristic in a steady state.

By such control, the target compression ratio of the variable compression ratio mechanism is changed stepwise in accordance with the acceleration request, but the actual compression ratio of the variable compression ratio mechanism using the multi-link piston crank mechanism is changed relatively slowly. In addition, as acceleration progresses, the combustion pressure acting on the piston increases.

In some intermediate compression ratio regions, when the strength or resistance of the mechanism to the combustion pressure applied to the piston is relatively lower than in other compression ratio regions, the mechanical mechanism as the variable compression ratio mechanism may excessively exceed the allowable combustion pressure during the compression ratio change, which is not preferable.

Patent document 1: japanese patent laid-open publication No. 2005-127200

Disclosure of Invention

In the present invention, when an acceleration request is made, the target compression ratio of the variable compression ratio mechanism is made lower than a reference target compression ratio corresponding to the engine operating conditions at the time of stabilization, and the maximum combustion pressure is restricted in a predetermined intermediate compression ratio region while the compression ratio of the variable compression ratio mechanism is being changed to the target compression ratio.

That is, when the allowable combustion pressure is low in the intermediate compression ratio region, the limitation of the maximum combustion pressure in the intermediate compression ratio region can avoid adverse effects on the durability of the variable compression ratio mechanism.

Drawings

Fig. 1 is a configuration explanatory diagram showing a system configuration of an internal combustion engine according to the present invention.

Fig. 2 is a flowchart showing a control flow at the time of acceleration.

Fig. 3 is a time chart showing changes in the actual compression ratio and the intake pressure at the time of acceleration.

Fig. 4 is a flowchart showing a control flow at the time of acceleration according to embodiment 2.

Detailed Description

An embodiment of the present invention is described in detail below with reference to the drawings.

Fig. 1 shows a system configuration of an internal combustion engine 1 for an automobile to which the present invention is applied. This internal combustion engine 1 is a four-stroke cycle spark ignition type internal combustion engine having a variable compression ratio mechanism 2 using a multi-link piston crank mechanism, and a pair of intake valves 4 and a pair of exhaust valves 5 are arranged on the top wall surface of each cylinder 3, and an ignition plug 6 is arranged in the central portion surrounded by these intake valves 4 and exhaust valves 5. The internal combustion engine 1 further includes a turbocharger 8 that performs supercharging using exhaust energy.

The intake valve 4 has an intake variable valve timing mechanism 7 capable of variably controlling the opening/closing timing of the intake valve 4. In the present embodiment, the variable valve timing mechanism 7 advances or retards the opening timing and the closing timing at the same time by advancing or retarding the phase of the camshaft. Various types of variable valve timing mechanisms are known, and the present invention is not limited to a specific form of variable valve timing mechanism.

For example, the variable valve timing mechanism 7 is configured to have: a sprocket concentrically disposed at a front end portion of the camshaft; and a hydraulic rotary actuator that relatively rotates the sprocket and the camshaft within a predetermined angular range. The sprocket is linked to the crankshaft via a timing chain or a timing belt, not shown. Therefore, the phase of the camshaft relative to the crank angle is changed by the relative rotation of the sprocket and the camshaft. The rotary actuator has an advance side hydraulic chamber biased to the advance side by a hydraulic pressure and a retard side hydraulic chamber biased to the retard side by a hydraulic pressure, and is configured to advance or retard the phase of the camshaft by controlling the supply of the hydraulic pressure to these hydraulic chambers via a hydraulic control valve, not shown, by a control signal from the engine controller 10. The actual control position of the camshaft variably controlled by this variable valve timing mechanism 7 (which corresponds to the actual valve timing) is detected by a cam angle sensor 11 responsive to the rotational position of the camshaft. The supply of hydraulic pressure via the hydraulic pressure control valve is closed-loop controlled so that the actual control position detected by the cam angle sensor 11 coincides with the target control position set according to the operating conditions.

The engine controller 10 has a target control position map having the load and the rotation speed of the internal combustion engine 1 as parameters as operating conditions, and sets the target control position based on the map. The target control position is substantially the valve timing on the relatively retarded side on the low rotation speed side, and has a characteristic that the valve timing is more advanced as the rotation speed is larger.

Since the basic intake valve opening timing is set before the top dead center and the intake valve closing timing is set after the bottom dead center, if the variable valve timing mechanism 7 is advanced, the intake valve opening timing is shifted from the top dead center to the advance side, the valve overlap with the exhaust valve 5 is expanded, the intake valve closing timing approaches the bottom dead center, and the volumetric efficiency is improved. Although the valve mechanism of the exhaust valve 5 is not changed in opening/closing timing in the illustrated example, the present invention may be configured such that a variable valve timing mechanism is provided on the exhaust valve 5 side in addition to the variable valve timing mechanism 7 on the intake valve 4 side.

A port injection fuel injection valve 15 is disposed for each cylinder in an intake passage 14 connected to the combustion chamber 13 via the intake valve 4. In addition, an in-cylinder injection fuel injection valve 16 is provided for directly injecting fuel into each cylinder 3. That is, the illustrated example is a so-called dual injection type fuel injection system in which fuel is supplied by appropriately using the port injection fuel injection valve 15 and the in-cylinder injection fuel injection valve 16 in accordance with a load or the like. An electronically controlled throttle valve 19 is mounted on the intake passage 14 upstream of the intake manifold 18, the opening of the electronically controlled throttle valve 19 is controlled by a control signal from the engine controller 10, and the compressor 8a of the turbocharger 8 is located upstream of the electronically controlled throttle valve 19. An air flow meter 20 and an air cleaner 21 for detecting the intake air amount are disposed upstream of the compressor 8a in the intake passage 14. An intercooler 22 is provided between the compressor 8a and the throttle valve 19. In addition, a recirculation valve 23 is provided to communicate the discharge side and the intake side of the compressor 8 a. The recirculation valve 23 opens at deceleration when the throttle valve 19 closes.

A turbine 8b of the turbocharger 8 is mounted in an exhaust passage 25 connected to the combustion chamber 13 via the exhaust valve 5, and a pre-catalyst device 26 and a main catalyst device 27 each composed of a three-way catalyst are disposed on the downstream side thereof. An air-fuel ratio sensor 28 that detects an air-fuel ratio is disposed upstream of the turbine 8b in the exhaust passage 25. The turbine 8b has a wastegate valve 29, and the wastegate valve 29 bypasses a part of the exhaust gas in accordance with the boost pressure in order to control the boost pressure. The wastegate valve 29 is an electronically controlled wastegate valve, and the opening degree thereof is controlled by a control signal from the engine controller 10 via an actuator constituted by an electric motor.

An exhaust gas recirculation passage 30 for recirculating a part of the exhaust gas to the intake system is provided between a position on the downstream side of the turbine 8b of the exhaust passage 25 and a position on the upstream side of the compressor 8a of the intake passage 14. The exhaust gas recirculation passage 30 includes an EGR gas cooler 31 and an EGR valve 32.

In addition to the cam angle sensor 11, the air flow meter 20, and the air-fuel ratio sensor 28, detection signals of a crank angle sensor 34 for detecting the engine speed, a water temperature sensor 35 for detecting the cooling water temperature, an accelerator opening sensor 36 for detecting the depression amount of an accelerator pedal operated by a driver as a sensor for detecting a torque request of the driver, a boost pressure sensor 37 for detecting the boost pressure (intake air pressure) in the intake manifold 18, and the like are input to the engine controller 10. The engine controller 10 optimally controls the fuel injection amount and injection timing of the fuel injection valves 15 and 16, the ignition timing of the ignition plug 6, the mechanical compression ratio of the variable compression ratio mechanism 2, the opening/closing timing of the intake valve 4 of the variable valve timing mechanism 7, the opening degree of the throttle valve 19, the opening degree of the waste gate valve 29, the opening degree of the EGR valve 32, and the like based on these detection signals.

On the other hand, the variable compression ratio mechanism 2 is configured mainly of a lower link 42, an upper link 45, a control link 47, and a control shaft 48, in which the lower link 42 is rotatably supported by a crankpin 41a of a crankshaft 41, the upper link 45 connects an upper pin 43 at one end of the lower link 42 to a piston pin 44a of a piston 44, one end of the control link 47 is connected to a control pin 46 at the other end of the lower link 42, and the control shaft 48 swingably supports the other end of the control link 47, by using a known multi-link piston crank mechanism described in patent document 1, japanese patent application laid-open No. 2004-116434, and the like. The crankshaft 41 and the control shaft 48 are rotatably supported in a crankcase 49a at a lower portion of the cylinder block 49 via a bearing structure, not shown. The control shaft 48 has an eccentric shaft portion whose position changes with the rotation of the control shaft 48, and more specifically, an end portion of the control link 47 is rotatably fitted to the eccentric shaft portion. That is, the swing support position of the control link 47 is displaced as the control shaft 48 rotates. In the variable compression ratio mechanism 2 having such a structure, the top dead center position of the piston 44 is displaced up and down with the rotation of the control shaft 48, and therefore, the mechanical compression ratio is changed.

In the present embodiment, as a drive mechanism for variably controlling the compression ratio of the variable compression ratio mechanism 2, an electric actuator 51 having a rotation center axis parallel to the crankshaft 41 is disposed on the outer wall surface of the crankcase 49a, and the electric actuator 51 and the control shaft 48 are interlocked via a 1 st arm 52 fixed to the output rotation axis of the electric actuator 51, a 2 nd arm 53 fixed to the control shaft 48, and an intermediate link 54 connecting the 1 st arm 52 and the 2 nd arm 53. The electric actuator 51 includes an electric motor and a transmission mechanism arranged in series in the axial direction.

The actual value of the mechanical compression ratio variably controlled by the variable compression ratio mechanism 2 as described above, that is, the actual compression ratio is detected by the actual compression ratio detection sensor 56. The actual compression ratio detection sensor 56 is configured by, for example, a rotary potentiometer, a rotary encoder, or the like that detects the rotation angle of the control shaft 48 or the rotation angle of the output rotary shaft of the electric actuator 51. Alternatively, the actual compression ratio may be detected without using a separate sensor by obtaining the amount of rotation of the electric motor from a command signal transmitted to the electric motor constituting the electric actuator 51 and determining the angle of rotation of the control shaft 48 from the amount of rotation.

The electric actuator 51 is drive-controlled by the engine controller 10 so that the actual compression ratio determined as described above becomes the target compression ratio according to the operating conditions. For example, the engine controller 10 has a target compression ratio map having the load and the rotation speed of the internal combustion engine 1 as parameters as operating conditions, and sets the target compression ratio based on the map. The target compression ratio is basically a high compression ratio on the low load side, and becomes a low compression ratio to suppress knocking or the like as the load becomes higher.

The variable compression ratio mechanism 2 using the above-described multi-link piston crank mechanism has a characteristic that the strength or resistance of the mechanism with respect to the combustion pressure applied to the piston 44 is relatively lowered in an intermediate compression ratio region between the highest compression ratio and the lowest compression ratio due to the relationship of the link geometry. That is, if the combustion pressure acting on the piston 44 is defined as "allowable combustion pressure" including an appropriate safety factor so that the mechanical mechanism of the variable compression ratio mechanism 2 is not damaged, the allowable combustion pressure has a characteristic of being lower in the intermediate compression ratio region than the allowable combustion pressures in the vicinity of the highest compression ratio and the lowest compression ratio.

Therefore, if the maximum combustion pressure exceeds the allowable combustion pressure transitionally even during operation, it is not preferable in terms of durability of the variable compression ratio mechanism 2 and the like. In the present invention, as described above, the maximum combustion pressure does not exceed the allowable combustion pressure in the intermediate compression ratio region where the allowable combustion pressure is lowered during acceleration.

The control executed by the engine controller 10 during acceleration will be specifically described below based on the flowchart of fig. 2. The routine shown in the flowchart is repeatedly executed at appropriate intervals (for example, at minute time intervals).

In step 1 (noted as S1, etc. in fig. 1), the actual compression ratio rVCR detected by the actual compression ratio detection sensor 56 is read. In step 2, it is determined whether or not there is a request for acceleration. The acceleration request is determined based on, for example, the amount of change or the change speed of the accelerator pedal opening detected by the accelerator opening sensor 36. Specifically, the presence or absence of a sudden acceleration request of a predetermined level or more is sequentially determined by another routine not shown, and the diagnostic result is referred to in step 2. Further, the magnitude of the acceleration request (magnitude of the amount of change in the accelerator pedal opening degree, magnitude of the change speed) may be determined together.

If it is determined in step 2 that there is no acceleration request, the routine proceeds to step 3, and normal control is performed during steady state. That is, the target control position ttvc of the variable valve timing mechanism 7 is controlled to the reference target control position at the time of stabilization according to the current operating condition (load and rotation speed), and the target compression ratio ttcr of the variable compression ratio mechanism 2 is similarly controlled to the reference target compression ratio at the time of stabilization according to the current operating condition (load and rotation speed). Then, the process proceeds from step 3 to step 4, where the intake pressure limiting control described later is not performed.

When there is an acceleration request, the process proceeds from step 2 to step 5, where the target control position ttvc of the variable valve timing mechanism 7 is corrected to the advance side of the reference target control position at the time of stabilization according to the operating conditions (load and rotation speed) at that time. The target compression ratio ttcr of the variable compression ratio mechanism 2 is corrected to a lower compression ratio side than the reference target compression ratio at the time of stabilization according to the operating conditions (load and rotation speed) at that time. With such setting, occurrence of knocking is suppressed and output torque is improved. In particular, in the embodiment including the turbocharger 8, although the response delay of the turbocharger 8 is present at the initial stage of acceleration, the output torque during the response delay period is improved by the above setting, and good acceleration performance can be obtained. The advance correction amount of the target control position ttvc of the variable valve timing mechanism 7 and the correction amount of the target compression ratio ttcr of the variable compression ratio mechanism 2 to the low compression ratio side at the time of such an acceleration request may be constant amounts, or may be set according to the magnitude of the acceleration request.

By setting the target compression ratio tVCR as described above, the variable compression ratio mechanism 2 is gradually changed from the mechanical compression ratio at that time to the target compression ratio tVCR via the electric actuator 51, and in the next step 6, it is judged whether or not the actual compression ratio rVCR at that time is within the prescribed intermediate compression ratio region (between the 1 st intermediate compression ratio VCR1 and the 2 nd intermediate compression ratio VCR2) where the combustion pressure is allowed to become low. If the actual compression ratio rVCR at this time is outside the range of the prescribed intermediate compression ratio region (i.e., rVCR ≧ VCR1 or rVCR ≦ VCR2), the routine proceeds from step 6 to step 4 without performing the intake pressure limiting control described later.

In the case where VCR1 > rVCR > VCR2 in step 6, the routine proceeds from step 6 to step 7, where the intake pressure limit value Plim is set based on the value of the actual compression ratio rVCR at that time. The intake pressure limit value Plim corresponds to an upper limit value of the intake pressure that is set so that the maximum combustion pressure in the combustion cycle of the internal combustion engine 1 does not exceed the allowable combustion pressure at the current actual compression ratio rVCR, the engine controller 10 has a table that assigns the intake pressure limit value Plim in advance using the mechanical compression ratio as a parameter, and in step 7, the intake pressure limit value Plim for the current actual compression ratio rVCR is found with reference to the table. The intake pressure limit value Plim may be the value of the intake pressure (boost pressure) itself downstream of the throttle valve 19, or may be an alternative parameter comparable to the intake pressure.

Next, proceeding from step 7 to step 8, the intake air pressure limiting control is executed based on the intake air pressure limiting value Plim set in step 7. That is, at the time of the acceleration request, the intake pressure sharply rises to satisfy the torque request, but in the case where the intake pressure exceeds the intake pressure limit value Plim associated with the actual compression ratio rVCR, the intake pressure is limited to the intake pressure limit value Plim. For example, as the intake pressure changing means, the intake pressure downstream of the throttle valve 19 is limited to the intake pressure limit value Plim by increasing and correcting the opening degree of the wastegate valve 29 of the turbocharger 8 to decrease the boost pressure, or by decreasing and correcting the opening degree of the throttle valve 19. Therefore, the maximum combustion pressure in the combustion cycle does not exceed the allowable combustion pressure at the actual compression ratio rVCR at that time.

That is, in the compression ratio change process in which the actual compression ratio rVCR is gradually decreased from the mechanical compression ratio before acceleration to the target compression ratio tVCR at the time of the acceleration request, the intake pressure is limited by the increase of the bypass amount of the turbocharger 8 and the decrease of the opening degree of the throttle valve 19 during the passage of the actual compression ratio rVCR through the intermediate compression ratio region between the 1 st intermediate compression ratio VCR1 and the 2 nd intermediate compression ratio VCR2 so that the maximum combustion pressure does not exceed the allowable combustion pressure of the variable compression ratio mechanism 2 at the actual compression ratio rVCR at that time.

Fig. 3 is a timing chart showing a comparison between a change in the compression ratio (actual compression ratio rVCR) and a change in the intake pressure at the time of the above-described acceleration request. In this example, in a steady state where the internal combustion engine 1 is operating at a lower load and the target compression ratio tVCR is high, an acceleration request is generated at time t 1. Since the target compression ratio tccr is stepped down with this acceleration request, the variable compression ratio mechanism 2 is driven toward the target compression ratio tccr via the electric actuator 51. The change in the actual compression ratio rVCR of the variable compression ratio mechanism 2 using the multi-link piston crank mechanism is relatively slow, and even if the target compression ratio tVCR is lowered stepwise, the actual compression ratio rVCR is gradually lowered as shown in the drawing. Further, in one embodiment, it takes about 1 to 2 seconds from the time point (t1) at which the acceleration request is detected until the actual compression ratio rVCR finally converges to the target compression ratio tVCR. Thus, in the process of changing the actual compression ratio rVCR to the final target compression ratio tVCR, the actual compression ratio rVCR passes through a prescribed intermediate compression ratio region (VCR1-VCR2) that allows the combustion pressure to become low.

A line PLIM in fig. 3 indicates the characteristic of the allowable combustion pressure of the variable compression ratio mechanism 2, more specifically, the characteristic of the allowable intake pressure indicated by converting the allowable combustion pressure into the intake pressure. As described above, in the variable compression ratio mechanism 2 using the multi-link piston crank mechanism, due to the relationship of the link geometry, the combustion pressure is allowed to decrease in the prescribed intermediate compression ratio region (the upper limit thereof is the 1 st intermediate compression ratio VCR1, and the lower limit thereof is the 2 nd intermediate compression ratio VCR 2). Therefore, from the viewpoint of the allowable combustion pressure, the allowable intake pressure is also lowered. That is, the region shown by hatching in fig. 3 indicates the allowable decrease in the combustion pressure. As described above, the intake pressure limit value Plim is set based on the characteristic of the line Plim that decreases in the predetermined intermediate compression ratio region. In other words, the allowable intake pressure PLIM and the intake pressure limit value PLIM in the intermediate compression ratio regions of the VCRs 1 to 2 are substantially equal.

As indicated by line Pin, the intake pressure rises after time t1 in response to the acceleration request. That is, the intake air pressure ratio is relatively rapidly increased by an increase in the opening degree of the throttle valve 19 in response to the acceleration request, an operation (increase in the rotational speed) of the turbocharger 8, an advance in the opening/closing timing of the intake valve 4, and the like. Therefore, depending on various conditions, the intake pressure may exceed the allowable intake pressure PLIM corresponding to the allowable combustion pressure in the intermediate compression ratio region as indicated by the broken line Pin 2. In the above-described embodiment, in the case where the intake pressure thus exceeds the allowable intake pressure PLIM, the intake pressure limiting control is executed by increasing the opening degree of the wastegate valve 29 or decreasing the opening degree of the throttle valve 19, and the intake pressure is limited based on the allowable intake pressure PLIM (i.e., the intake pressure limiting value PLIM) as indicated by the solid line Pin. This prevents the maximum combustion pressure actually generated from exceeding the allowable combustion pressure of the variable compression ratio mechanism 2, and can suppress adverse effects on the durability and the like of the variable compression ratio mechanism 2.

In this way, in the above embodiment, since it is possible to avoid the phenomenon in which the maximum combustion pressure transiently exceeds the allowable combustion pressure at the time of the acceleration request, it is not necessary to excessively increase the strength and rigidity of each part constituting the variable compression ratio mechanism 2, and it is possible to have a structure having the minimum necessary strength and rigidity. For example, in order to increase the strength and rigidity of each part, the size and weight of the components are increased, and this is accompanied by disadvantages such as a decrease in responsiveness at the time of changing the compression ratio and an increase in power consumption of the electric actuator 51. In the above-described embodiment, improvement in durability of the variable compression ratio mechanism 2 can be achieved without such disadvantage.

Further, in both the region where the actual compression ratio rVCR is greater than or equal to the 1 st intermediate compression ratio VCR1 and the region where the actual compression ratio rVCR is less than or equal to the 2 nd intermediate compression ratio VCR2, the combustion pressure is allowed to be high. For example, even if the intake pressure is raised to the maximum with acceleration, if the actual compression ratio rVCR is less than or equal to the 2 nd intermediate compression ratio VCR2, the intake pressure limiting control is not required because the allowable combustion pressure is high.

In addition, the target compression ratio ttcr can be set to a value within the range of the predetermined intermediate compression ratio region (VCR1 to VCR2) in the steady operation without depending on the acceleration request, but if the acceleration is not requested, the intake pressure limit control is not necessary because the maximum combustion pressure does not become a high intake pressure exceeding the allowable combustion pressure.

Fig. 4 shows a flowchart of embodiment 2. Steps 1 to 5 of the present example are not particularly changed from the steps of the flowchart of FIG. 2.

That is, the actual compression ratio rVCR is read (step 1), whether or not there is an acceleration request is determined (step 2), and if there is no acceleration request, normal control is performed in steps 3 and 4.

If there is a request for acceleration, the process proceeds from step 2 to step 5, where the target control position ttvc of the variable valve timing mechanism 7 is corrected to be earlier than the reference target control position at the time of stabilization corresponding to the operating condition (load and rotation speed) at that time, and the target compression ratio ttcr of the variable compression ratio mechanism 2 is corrected to be closer to the low compression ratio side than the reference target compression ratio at the time of stabilization corresponding to the operating condition (load and rotation speed) at that time.

Next, the routine proceeds to step 7A, where the intake pressure limit value Plim is set based on the value of the actual compression ratio rVCR. The intake pressure limit value Plim corresponds to an upper limit value of the intake pressure that is set so that the maximum combustion pressure in the combustion cycle of the internal combustion engine 1 does not exceed the allowable combustion pressure at the actual compression ratio rVCR at that time, the engine controller 10 has a table in which the intake pressure limit value Plim is assigned in advance with the mechanical compression ratio as a parameter, and in step 7, the intake pressure limit value Plim for the actual compression ratio rVCR at that time is found with reference to the table.

Here, in the present embodiment, not only the predetermined intermediate compression ratio regions (VCR1 to VCR2), but also the intake pressure limit value Plim corresponding to the value of the mechanical compression ratio in the entire control range of the variable compression ratio mechanism 2 is set in advance as a table. In the table, the intake pressure limit value Plim is set in accordance with a characteristic of the allowable intake pressure that is obtained by converting the allowable combustion pressure of the variable compression ratio mechanism 2, which is indicated by, for example, a line Plim in fig. 3, into the intake pressure. That is, in a high compression ratio region larger than or equal to the 1 st intermediate compression ratio VCR1 and a low compression ratio region smaller than or equal to the 2 nd intermediate compression ratio VCR2, the intake pressure limit value Plim is set to a value based on the highest intake pressure determined by the turbocharger 8 or the like, and in an intermediate compression ratio region between the 1 st intermediate compression ratio VCR1 and the 2 nd intermediate compression ratio VCR2, the intake pressure limit value Plim is set to a relatively low value based on the characteristics of the allowable combustion pressure or the allowable intake pressure Plim.

In step 8A following step 7A, the intake air pressure limiting control is executed based on the intake air pressure limiting value Plim set in step 7A. That is, at the time of the acceleration request, the intake pressure sharply rises to satisfy the torque request, but in the case where the intake pressure exceeds the intake pressure limit value Plim associated with the actual compression ratio rVCR, the intake pressure is limited to the intake pressure limit value Plim. For example, the boost pressure is reduced by increasing the opening degree of the wastegate valve 29 of the turbocharger 8, or the intake pressure downstream of the throttle valve 19 is limited to the intake pressure limit value Plim by decreasing the opening degree of the throttle valve 19. Thus, the maximum combustion pressure in the combustion cycle does not exceed the allowable combustion pressure at the actual compression ratio rVCR at that time.

Here, in the flowchart of embodiment 2, the process of step 8A is executed without depending on the value of the actual compression ratio rVCR, but as described above, in the high compression ratio region greater than or equal to the 1 st intermediate compression ratio VCR1 and the low compression ratio region less than or equal to the 2 nd intermediate compression ratio VCR2, the intake pressure limit value Plim is a higher value, and therefore, the limitation of the intake pressure is not substantially performed. In the intermediate compression ratio region where the actual compression ratio rVCR is between the 1 st intermediate compression ratio VCR1 and the 2 nd intermediate compression ratio VCR2, intake pressure limit control substantially the same as that of the foregoing embodiment is performed in correspondence with the intake pressure limit value Plim. Therefore, in the present embodiment, the region determination of the actual compression ratio rVCR in step 6 in the flowchart of the foregoing embodiment 2 is not required.