JP2005113884A - Internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve thermal efficiency by making compression ratio high at the time of stratified combustion, while suppressing knocking to be likely to appear in the vicinity of a switching point of the stratified combustion and homogeneous combustion. <P>SOLUTION: This internal combustion engine is provided with a variable compression ratio mechanism and a control device (ECU). The variable compression ratio mechanism is capable of changing the engine compression ratio. The ECU performs the stratified combustion in a low speed/low load operating range, and performs the homogeneous combustion in other operating ranges. The ECU makes the engine compression ratio in the vicinity of the switching point (torque T4) of the stratified combustion and the homogeneous combustion in the stratified combustion smaller than the engine compression ratio in the vicinity of the switching point in the homogenous combustion. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spark ignition type internal combustion engine that performs stratified combustion in a part of an operation region and performs homogeneous combustion in other regions.

  In general, the thermal efficiency of a spark ignition type internal combustion engine typified by a normal gasoline engine may be in a low-speed rotation / high-load operation region, and becomes worse as a low-load operation region or a high-speed rotation operation region. As a technique for improving the thermal efficiency of such an internal combustion engine, the throttle opening is increased to reduce pump loss (loss due to throttle valve throttling), and the air-fuel ratio of the air-fuel mixture is made lean to reduce the working gas. There is known a technique for increasing the specific heat ratio to improve the theoretical thermal efficiency.

  However, in the conventional spark ignition type internal combustion engine, when the air-fuel ratio is made lean, the combustion period becomes longer and the combustion stability is deteriorated, so that there is a limit to making the air-fuel ratio lean.

  As an internal combustion engine that makes the air-fuel ratio lean while avoiding such deterioration of combustion stability, for example, an in-cylinder direct injection internal combustion engine has been put into practical use. In this internal combustion engine, fuel is directly injected into the cylinder (combustion chamber) in the latter half of the compression stroke, thereby forming a stratified mixture and locally enriching the air-fuel ratio in the vicinity of the spark plug. As a result, stable combustion is performed, and a large leaning is realized when the cylinder interior is viewed in total. By adopting such an internal combustion engine, it is possible to improve the thermal efficiency in the low-speed rotation / low-load operation region.

  As another method for improving the theoretical thermal efficiency of an internal combustion engine, a method for increasing the engine compression ratio is known. However, in a spark ignition type internal combustion engine, knocking occurs when the compression ratio is excessively increased. Therefore, the compression ratio that is normally determined by the knocking limit in the high load operation region is set as the upper limit set compression ratio. . For this reason, in the low load operation region, the engine compression ratio cannot be increased to the optimum compression ratio.

As a technique for solving such a problem, there is a technique for making the engine compression ratio variable as disclosed in Patent Document 1. If this technology is used, the engine compression ratio can be arbitrarily changed, so that the thermal efficiency can be improved by increasing the engine compression ratio in the low-speed rotation / low-load operation region. .
JP 2001-263114 A

  In an internal combustion engine, it is effective to increase the engine compression ratio as described above in order to improve the thermal efficiency in the low speed rotation / low load operation region. In addition, if a technology that makes the engine compression ratio variable is adopted in an internal combustion engine capable of stratified combustion, such as in-cylinder direct injection, it is possible to further improve thermal efficiency and improve combustion stability by improving ignitability. Can also be planned.

  However, in an internal combustion engine capable of stratified combustion such as in-cylinder direct injection, knocking due to spontaneous ignition of a locally rich air-fuel mixture generated during stratified combustion may occur. This knocking appears remarkably in the vicinity of the load for switching between stratified combustion and homogeneous combustion (region on the stratified combustion side).

  The knocking that may occur in the vicinity of the switching point between the stratified combustion and the homogeneous combustion is not particularly considered in the technique disclosed in Patent Document 1 described above.

  In an internal combustion engine that performs stratified combustion in a low-load operation region and homogeneous combustion in other operation regions, the present invention may appear in the vicinity of a switching point (load at the time of switching) between stratified combustion and homogeneous combustion. It is to improve the thermal efficiency by increasing the compression ratio during stratified combustion while suppressing high knocking.

  The internal combustion engine according to the present invention includes a variable compression ratio mechanism and a control device. The variable compression ratio mechanism is a mechanism that can change the engine compression ratio. The control device performs stratified combustion in the low load operation region and performs homogeneous combustion in the other operation regions. In the stratified combustion, the control device makes the engine compression ratio in the vicinity of the switching point between the stratified combustion and the homogeneous combustion smaller than the engine compression ratio in the vicinity of the switching point in the homogeneous combustion.

  In this internal combustion engine, the control device switches between stratified combustion and homogeneous combustion. In order to improve thermal efficiency, the engine in the low load operation region (region where stratified combustion is performed) as described above. It is necessary to increase the compression ratio compared to the middle / high load operation region (region where homogeneous combustion is performed). However, in the vicinity of the switching point between stratified combustion and homogeneous combustion, when the engine compression ratio in the stratified combustion side region is larger than the engine compression ratio in the homogeneous combustion side region, the stratification in the vicinity of the switching point The probability of knocking occurring in the combustion side region is increased.

  In contrast, in the internal combustion engine according to the present invention, the engine compression ratio on the stratified combustion side in the vicinity of the switching point between stratified combustion and homogeneous combustion is set small, and the engine compression ratio on the stratified combustion side in the vicinity of the switching point. Is smaller than the engine compression ratio on the homogeneous combustion side. As a result, the probability of knocking occurring in the stratified combustion side region in the vicinity of the switching point is reduced.

  On the other hand, even when the engine compression ratio on the stratified charge combustion side in the vicinity of the switching point is reduced to suppress knocking, the engine compression ratio is homogeneously burned in the region away from the switching point in the region where stratified combustion is performed. By making it larger than the engine compression ratio in the region, the compression ratio is increased during stratified combustion, and the thermal efficiency of the internal combustion engine is improved.

  Another internal combustion engine according to the present invention includes a variable compression ratio mechanism, an ignition device, and a control device. The variable compression ratio mechanism is a mechanism that can change the engine compression ratio. The ignition device is a device that ignites an air-fuel mixture in a combustion chamber. The control device performs stratified combustion in the low load operation region and performs homogeneous combustion in the other operation regions. In the stratified combustion, the control device retards the timing of ignition by the igniter in the vicinity of the switching point between stratified combustion and homogeneous combustion.

  Even in this internal combustion engine, switching between stratified combustion and homogeneous combustion is performed by the control device. In order to improve thermal efficiency, the engine in the low load operation region (region where stratified combustion is performed) as described above. It is necessary to increase the compression ratio compared to the middle / high load operation region (region where homogeneous combustion is performed). Therefore, if the engine compression ratio is not particularly devised, the engine compression ratio in the stratified combustion side region is higher than the engine compression ratio in the homogeneous combustion side region even near the switching point between stratified combustion and homogeneous combustion. Is bigger.

  However, in this case, the engine compression ratio becomes a relatively large value in the region on the stratified combustion side in the vicinity of the switching point, and the probability of occurrence of knocking increases.

  In contrast, in another internal combustion engine according to the present invention, in the stratified combustion, the timing of ignition by the ignition device is retarded in the vicinity of the switching point between stratified combustion and homogeneous combustion. As described above, since the ignition timing is retarded in the vicinity of the switching point in the region where stratified combustion is performed, even when the engine compression ratio is relatively large, the maximum temperature and pressure in the combustion chamber decrease, and knocking occurs. Is less likely to occur.

  On the other hand, by making the engine compression ratio in the stratified combustion region larger than the engine compression ratio in the homogeneous combustion region, a higher compression ratio is achieved during stratified combustion and the thermal efficiency of the internal combustion engine is improved.

  In the internal combustion engine according to the present invention, it is possible to improve the thermal efficiency by making the engine compression ratio in the stratified combustion region larger than the engine compression ratio in the homogeneous combustion region, and in the vicinity of the switching point between stratified combustion and homogeneous combustion. Smooth operation is ensured by suppressing knocking in the stratified combustion side region.

[First Embodiment]
<Schematic configuration of internal combustion engine>
FIG. 1 shows a four-cycle spark ignition engine that is an internal combustion engine according to a first embodiment of the present invention.

  The internal combustion engine includes a combustion chamber 21, an intake port 22, an intake valve 23, a piston 3, a fuel injection valve 24, a spark plug 25, an electronic control unit (hereinafter abbreviated as ECU) 26, an exhaust port 27, an exhaust valve 28, The throttle valve 29 and the variable compression ratio mechanism 19 shown in FIG.

  At least one intake port 22 is provided for the combustion chamber 21. The intake valve 23 is positioned downstream of the intake port 22 and at the inlet of the combustion chamber 21. The combustion injection valve 24 is a valve for directly injecting fuel into the combustion chamber 21, and at least one is provided for the combustion chamber 21. At least one exhaust valve 28 is provided for the combustion chamber 21. The exhaust valve 28 is located upstream of the exhaust port 27 and at the outlet of the combustion chamber 21. The throttle valve 29 is located upstream of the intake valve 23 in the intake port 22.

<Detailed Configuration of Variable Compression Ratio Mechanism of Internal Combustion Engine>
The variable compression ratio mechanism 19 of the internal combustion engine shown in FIG. 2 is a multi-link type piston-crank mechanism that can change the engine compression ratio. The variable compression ratio mechanism 19 mainly includes a lower link (second link) 2, an upper link (first link) 5, a control link (third link) 6, an eccentric cam 8, and a variable compression ratio actuator. (Swing support position changing mechanism) 16.

  The lower link 2 is externally fitted and supported on the crankpin 11 of the crankshaft 1 so as to be rotatable. The crankshaft 1 is rotatably supported by a cylinder block corresponding to the main body of the internal combustion engine via a main bearing.

  One end of the upper link 5 is rotatably connected to the piston 3 via the piston pin 4. The other end of the upper link 5 is rotatably connected to the lower link 2 via a first connecting pin 10.

  When a combustion load is applied to the piston 3, this combustion load is transmitted as rotational power from the piston 3 to the crankshaft 1 via the upper link 5 and the lower link 2.

  One end of the control link 6 is rotatably connected to the lower link 2 via the second connecting pin 9. The other end of the control link 6 is rotatably fitted and supported by the eccentric cam 8.

  The eccentric cam 8 is eccentrically fixed to the control shaft 7 (or integrally formed). The control shaft 7 is a shaft that is rotatably supported by the cylinder block. Further, at one end of the control shaft 7, a control plate 15 having a slide groove 15 a is fixed (or integrally formed).

  The compression ratio variable actuator 16 plays a role of rotating the control shaft 7 with respect to the cylinder block or holding the control shaft 7 so as not to rotate with respect to the cylinder block. The variable compression ratio actuator 16 is electrically driven, and mainly includes a substantially cylindrical thread drive 14, a rod-shaped thread driven 13, and an electric motor 14 a as a driving device (see FIG. 1; not shown in FIG. 2). It is composed of The thread drive 14 is a cylindrical member that is rotationally driven around an axis by an electric motor 14a, and a gear is formed on an inner peripheral surface thereof. The thread driven 13 meshes with the gear on the inner peripheral surface of the thread drive 14, and moves forward and backward in the axial direction (direction D1 in FIG. 2) according to the rotation of the thread drive 14. A control shaft pin 12 is provided at the tip of the thread driven 13. The control shaft pin 12 is slidably fitted in the slide groove 15 a of the control plate 15.

  By rotating the control shaft 7 according to the engine operating state by the compression ratio variable actuator 16, the piston stroke with respect to the crank angle θ changes, and the engine compression ratio changes. The variable compression ratio actuator 16 may be hydraulically driven.

  Further, even if the compression ratio variable actuator 16 is an electric type or a hydraulic drive type, there is a slight possibility that the electric motor 14a or the hydraulic drive device may break down. For this reason, the compression ratio variable actuator 16 is configured to hold the control plate 15 and the control shaft 7 so that the engine compression ratio becomes the lowest compression ratio when the electric motor 14a or the hydraulic drive device fails. That is, here, when the driving force no longer acts from the electric motor 14a that rotationally drives the thread drive 14 of the electric compression ratio variable actuator 16 shown in FIG. 2, the engine compression ratio is urged by the combustion pressure, The minimum compression ratio is set.

<Detailed configuration of ECU of internal combustion engine>
The ECU 26 receives inputs from various sensors and adjusts the fuel injection amount, ignition timing, throttle opening, etc. so that the engine torque corresponding to the accelerator opening required by the driver can be output, and the combustion state in the combustion chamber It is a control apparatus for controlling appropriately. Here, the configuration of the ECU 26 related to the present invention will be described below.

  As shown in FIG. 1, the ECU 26 includes a combustion form determination unit 31, a spark ignition control unit 32, a fuel injection control unit 33, a throttle control unit 34, and a compression ratio control unit 35. Each part 31-35 of these ECU26 is implement | achieved as a program of a microcomputer. Further, the ECU 26 receives the engine rotation signal detected by the crank angle sensor 51 and the accelerator opening signal (load) detected by the accelerator opening sensor 52. Based on the engine rotation signal and the accelerator opening signal, the combustion mode determination unit 31 determines whether the stratified combustion or the homogeneous combustion is used.

<Operation of Internal Combustion Engine Controlled by ECU>
FIG. 3 shows an operation region of a four-cycle spark ignition engine that is an internal combustion engine according to the present embodiment. As shown in FIG. 3, the ECU 26 stratifies in a low-speed rotation / low-load operation region, that is, in an operation region where the load is lower than a predetermined torque corresponding to each engine speed (shown by a one-dot chain line in FIG. 3). The intake air flow rate is controlled by adjusting the throttle opening so that the combustion is performed and the homogeneous combustion is performed in the other operation region, that is, the operation region where the load is higher than the predetermined torque (see FIG. 4). . Further, in the region where stratified combustion is performed, the ECU 26 performs fuel injection toward the combustion chamber 21 during the compression stroke by the fuel injection valve 24, thereby forming a stratified mixture in the combustion chamber 21. As a result, the combustion chamber 21 as a whole achieves a significant lean air-fuel ratio, while the air-fuel ratio in the vicinity of the spark plug 25 is locally enriched and stable combustion is performed.

  FIG. 4 shows an example of the engine compression ratio and the intake air flow rate (throttle opening) corresponding to each load (torque). FIG. 4 shows the relationship between the load at a certain engine speed, the engine compression ratio, and the intake air flow rate.

  The ECU 26 controls the engine compression ratio and the intake air flow rate as shown in FIG. First, the combustion mode determination unit 31 of the ECU 26 determines whether to operate in any one of stratified combustion and homogeneous combustion from the accelerator opening signal and the engine rotation signal (engine speed signal). The throttle control unit 34 adjusts the throttle valve 29 from the accelerator opening signal and the engine rotation signal so that the intake air flow rate corresponding to the load (torque) is obtained in each combustion method region. The compression ratio control unit 35 also controls the electric motor 14a of the variable compression ratio mechanism 19 so as to obtain an engine compression ratio corresponding to a load (torque) in each combustion method region from the accelerator opening signal and the engine rotation signal. To do. Further, the ignition timing and the fuel injection amount are also appropriately controlled by the spark ignition control unit 32 and the combustion injection control unit 33 according to respective instruction values stored in advance.

  Next, control of the ECU 26 at each load will be described.

  Under the full load condition (when the torque T7 is full load in FIG. 4), the compression ratio control unit 35 controls the variable compression ratio mechanism 19 so that the engine compression ratio becomes the set minimum compression ratio. Further, the throttle opening degree under the full load condition is controlled by the throttle control unit 34 so as to be fully opened.

  The region where the load is smaller than the full load torque T7, that is, the region of torque T5 to torque T6 in FIG. In this region, the engine compression ratio is increased as the load decreases. The engine compression ratio at this time is set to an optimum engine compression ratio defined from the viewpoint of preventing knocking, based on an instruction value stored in advance in the compression ratio control unit 35 in the ECU 26. Also in the homogeneous combustion region of torque T5 to torque T6, the ignition timing, the fuel injection amount, and the throttle opening are controlled by prestored instruction values.

  When the load is smaller than the region of torque T5 to torque T6 in FIG. 4, the combustion form determination unit 31 issues an instruction to switch the combustion form from homogeneous combustion to stratified combustion, and the combustion form changes to stratified combustion. . Specifically, the combustion mode is switched from homogeneous combustion to stratified combustion when the torque T4 is applied. Before and after switching of the combustion mode, the engine compression ratio changes greatly. As shown in FIG. 4, the engine compression ratio in the stratified combustion side region near the torque T4 is smaller than the engine compression ratio in the homogeneous combustion side region near the torque T4. In other words, when switching to stratified combustion, the engine compression ratio is set to a value smaller than the highest set compression ratio at the time of homogeneous combustion.

  As described above, the reason why the engine compression ratio is changed to a small value when switching to stratified combustion is to avoid knocking in the high load region in the region where stratified combustion is performed, that is, the region of torque T3 to torque T4 in FIG. It is in. In the high load region in the region where stratified combustion is performed, since the ratio of the local rich mixture increases, if the engine compression ratio is too high, knocking due to the spontaneous combustion of the end gas in the flame propagation of the rich mixture will occur. It tends to occur. As a method for avoiding this knocking, in the present embodiment, the engine compression ratio is reduced when switching to stratified combustion as described above.

  Further, when the combustion mode is switched from homogeneous combustion to stratified combustion with the load of torque T4, the throttle control unit 34 controls the throttle valve 29 so that the throttle opening is fully opened (see FIG. 4). Thereby, the operation by stratified combustion is performed in a state where the pump loss due to the throttle valve 29 is reduced.

  In the region where the load is further smaller than the region near the torque T4 in FIG. 4, that is, in the stratified combustion region of the torques T2 to T3 in FIG. 4, the compression ratio increasing process for increasing the engine compression ratio again as the load decreases is compressed. This is done by the ratio control unit 35. As the engine compression ratio increases, the ignitability improves, so the lean limit during stratified combustion is expanded. By expanding the lean limit, the region where the intake air flow rate must be reduced (the region where the throttle opening must be reduced) is expanded to a lower load side than the conventional internal combustion engine that performs stratified combustion. Is done. For this reason, a highly efficient operation region in which the throttle opening is fully opened and the pump loss is small is expanded.

  In the region where the load is smaller than the region of torques T2 to T3 in FIG. 4, that is, in the extremely low load region in the vicinity of torque T1 in FIG. 4, if the throttle opening is fully opened, the intake air flow rate with respect to the input fuel amount is excessive. Accordingly, the lean limit is reached, and therefore, for the purpose of preventing misfire and performing stable combustion, the operation is performed at a throttle opening that is set in advance so as to obtain an air flow rate corresponding to the lean limit. That is, the throttle control unit 34 shifts the throttle valve 29 to a slightly closed state rather than a fully opened state in an extremely low load region near the torque T1.

<Characteristics of the internal combustion engine according to the first embodiment>
(1)
In this internal combustion engine, an ECU 26 controls a variable compression ratio mechanism 19 that can change the engine compression ratio. Further, the ECU 26 causes stratified combustion in the low-speed rotation / low-load operation region and homogeneous combustion in the other operation regions.

  In such a configuration, since the ratio of the local rich mixture increases in the high load region in the region where stratified combustion is performed, if the engine compression ratio is too high, the end gas of the flame propagation of the rich mixture will increase. Knocking due to self-ignition tends to occur.

  On the other hand, the ECU 26 of this internal combustion engine uses an engine compression ratio in the vicinity of the load of the torque T4, which is a switching point between the stratified combustion and the homogeneous combustion, in the stratified combustion. It is smaller than the ratio. That is, the engine compression ratio in the region on the torque T3 side of the torque T4 that is the high load region in the stratified combustion region is greater than the engine compression ratio in the region on the torque T5 side of the torque T4 that is the low load region in the homogeneous combustion. (See FIG. 4).

  Thus, since the engine compression ratio is changed to a small value when the load is reduced and the combustion mode is switched from homogeneous combustion to stratified combustion, the high load region in the region where stratified combustion is performed, i.e. Knocking in the region of torque T3 to torque T4 of 4 is avoided. In the load region where stratified combustion and homogeneous combustion are switched, that is, in the region near the torque T4, smooth operation without knocking can be performed.

  In addition, this internal combustion engine is an internal combustion engine that can be operated smoothly without knocking, and the engine compression ratio in the low load region excluding the region near the torque T4 in the region of stratified combustion is set large. It has become a highly efficient internal combustion engine with an expanded lean limit and reduced pump loss.

(2)
In this internal combustion engine, the throttle valve 29 for adjusting the intake air flow rate into the combustion chamber 21 is also controlled by the throttle control unit 34 of the ECU 26. The throttle control unit 34 has an intake air flow rate in an extremely low load operation region in the vicinity of the torque T1 in stratified combustion, as compared with an operation region (torque T2 to torque T4 region) other than the extremely low load operation region in stratified combustion. I try to make it smaller. Specifically, while the throttle valve 29 is fully opened in the region of torque T2 to torque T4, the throttle valve 29 is not fully opened but slightly closed in the extremely low load operation region near the torque T1. Has been moved to.

  As a result, in this internal combustion engine, the intake air flow rate corresponding to the lean limit air-fuel ratio is supplied to the combustion chamber 21 even in the extremely low load region in the region where stratified combustion is performed, and the highest engine performance is achieved without misfire. Operation is possible at the efficiency point.

(3)
In this internal combustion engine, the compression ratio variable actuator 16 of the variable compression ratio mechanism 19 is electric and includes an electric motor 14a. In the variable compression ratio mechanism 19, assuming that the motor 14a has failed, the variable compression ratio actuator 16 controls the control plate 15 so that the engine compression ratio becomes the lowest compression ratio when the motor 14a fails. And the structure which hold | maintains the control shaft 7 is taken. For this reason, when the driving force is no longer applied from the electric motor 14a that rotationally drives the thread drive 14 of the electric compression ratio variable actuator 16 shown in FIG. 2, the engine compression ratio is urged by the combustion pressure to be set to the lowest setting. Compression ratio.

  Therefore, when the motor 14a, which is the drive unit of the variable compression ratio mechanism 19, fails, the operation is performed at the minimum set compression ratio in the entire operation region, and engine damage due to knocking or overheating is prevented. . Thus, this internal combustion engine is an internal combustion engine that can continue to operate safely even when the motor 14a of the variable compression ratio mechanism 19 fails.

[Second Embodiment]
Subsequently, a second embodiment of the present invention will be described. The internal combustion engine according to the second embodiment has the same structure as the internal combustion engine according to the first embodiment, and the control of the ECU 26, specifically, the control of the engine compression ratio in each of stratified combustion and homogeneous combustion is different. ing.

  FIG. 5 shows an example of the engine compression ratio and the intake air flow rate corresponding to each load (torque) in the second embodiment. Here, the increase rate of the engine compression ratio accompanying the decrease in load is set to be larger during stratified combustion than during homogeneous combustion.

  The knocking that usually appears prominently in the vicinity of the torque T4, which is a load for switching between stratified combustion and homogeneous combustion, is caused by a locally rich mixture existing in the stratified mixture. On the other hand, as the load decreases, the ratio of the rich mixture to the total gas amount in the cylinder (inside the combustion chamber 21) decreases. For this reason, it can be said that knocking hardly occurs naturally in the low load side region (region of lower load than the region near the torque T4) in the region where stratified combustion is performed. In view of this, here, the engine compression ratio during stratified combustion is larger than the engine compression ratio during homogeneous combustion as a whole, and the rate of increase in engine compression ratio with respect to load reduction during homogeneous combustion is larger than that during stratified combustion. The rate of increase of the engine compression ratio with respect to load reduction is increased. Thereby, in the internal combustion engine according to the second embodiment, the operating range in which the theoretical thermal efficiency is improved is expanded as compared with the internal combustion engine according to the first embodiment.

  Thus, the internal combustion engine according to the second embodiment is also an internal combustion engine that can be smoothly operated without knocking, and the lean limit is expanded and the pump loss is reduced as compared with the internal combustion engine according to the first embodiment. It has become a highly efficient internal combustion engine. In the internal combustion engine according to the second embodiment, the combustion stability is also improved due to the improved ignitability during stratified combustion.

  Note that, in the internal combustion engine according to the second embodiment, as shown in FIG. 5, the rate of increase in the engine compression ratio with respect to load reduction during stratified combustion is increased as compared with the internal combustion engine according to the first embodiment. Therefore, the maximum combustion gas temperature tends to increase, and the amount of NOx produced tends to increase slightly. However, the generation amount of NOx can be suppressed within an allowable range by EGR (Exhaust Gas Recirculation) or the like. That is, in the internal combustion engine according to the second embodiment, the thermal efficiency can be further increased as compared with the internal combustion engine according to the first embodiment, but it may be slightly inferior from the viewpoint of the amount of NOx generated. is there. The superiority or inferiority of the internal combustion engine according to the first embodiment and the internal combustion engine according to the second embodiment is determined according to the type and level of required performance, and cannot be generally determined.

[Third Embodiment]
Subsequently, a third embodiment of the present invention will be described. The internal combustion engine according to the third embodiment has the same structure as the internal combustion engine according to the first embodiment, and the control of the ECU 26 is similar to the internal combustion engine according to the second embodiment. The control of the ECU 26 of the internal combustion engine according to the third embodiment is different from the internal combustion engine according to the second embodiment in terms of knocking suppression in the vicinity of the switching load between stratified combustion and homogeneous combustion.

  In the third embodiment, the internal combustion engine according to the first embodiment or the second embodiment is used as a method of suppressing knocking that is likely to occur in the vicinity of the switching load (torque T4) between stratified combustion and homogeneous combustion. The method of retarding the ignition timing is used instead of the method of reducing the engine compression ratio.

  FIG. 7 shows the difference between the ignition timing in the region near the switching load (torque T4) for each load (torque) and the normal ignition timing in the other regions. Here, the spark ignition control unit 32 of the ECU 26 retards the ignition timing in the region near the torque T4 that is the switching load in the region where stratified combustion is performed as compared with the normal ignition timing in the region where homogeneous combustion is performed. Yes. Thus, by retarding from the normal ignition timing, the combustion start timing and the combustion period are retarded in the region on the stratified combustion side in the region near the torque T4, and in the cylinder (inside the combustion chamber 21). Maximum temperature / pressure drops. Thereby, knocking can be avoided in the vicinity region of the torque T4 on the stratified combustion side.

  Here, since the knocking in the vicinity of the stratified combustion side of the torque T4 is suppressed by a method of retarding the ignition timing, the switching load between the stratified combustion and the homogeneous combustion (torque T4) as shown in FIG. No control is performed to reduce the engine compression ratio in the stratified combustion side region in the vicinity of. That is, the engine compression ratio at the point of maximum load in the region where stratified combustion is performed is larger than the engine compression ratio at the point of minimum load in the region where homogeneous combustion is performed. For this reason, a high compression ratio is achieved in the entire region where stratified combustion is performed, and the operating region where the theoretical thermal efficiency is high can be expanded.

  With the above configuration, smooth operation without knocking is possible even at a load (torque T4) for switching between stratified combustion and homogeneous combustion, and the engine compression ratio in the entire region where stratified combustion is performed is set large. Therefore, the lean limit is expanded and a highly efficient internal combustion engine with a small pump loss is realized.

  At present, it is often easier to secure the response speed for changing the ignition timing than to secure the response speed for changing the engine compression ratio by the variable compression ratio mechanism 19.

[Fourth Embodiment]
Subsequently, a fourth embodiment of the present invention will be described. The internal combustion engine according to the fourth embodiment has the same structure as the internal combustion engine according to the first embodiment, and the control of the ECU 26 is similar to the internal combustion engine according to the third embodiment. In the control of the ECU 26 of the internal combustion engine according to the third embodiment, as shown in FIG. 6, the increase rate of the engine compression ratio with respect to the load decrease during stratified combustion is higher than the increase rate of the engine compression ratio with respect to the load decrease during homogeneous combustion. However, in the control of the ECU 26 of the internal combustion engine according to the fourth embodiment, as shown in FIG. 8, on the contrary, during the stratified combustion than the increase rate of the engine compression ratio with respect to the load decrease during the homogeneous combustion. The increase rate of the engine compression ratio with respect to the load drop is reduced.

  The control of the engine compression ratio by the ECU 26 of the internal combustion engine according to the fourth embodiment is control that places importance on reducing the amount of NOx generated. At the time of stratified combustion, there is a tendency that NOx generated from a high temperature portion in the combustion chamber 21 due to combustion of a rich air-fuel mixture locally increases. However, here, the rate of increase in the engine compression ratio with respect to the load drop during stratified combustion is set to be smaller than the rate of increase in the engine compression ratio with respect to the load drop during homogeneous combustion. Compared to the case where the value is not changed according to the combustion mode, the average temperature in the combustion chamber 21 is lowered, and the NOx generation amount is also reduced.

  As described above, the internal combustion engine according to the fourth embodiment is a clean internal combustion engine with low emission. However, as much as it is clean, the internal combustion engine is slightly inefficient compared to the internal combustion engine according to the third embodiment.

[Other Embodiments]
(A)
In each of the above embodiments, in the description of the engine compression ratio setting with respect to the load, the description has been made by replacing the load with torque. Instead of this torque, as a parameter for controlling the load, an accelerator opening that is a fuel injection amount or a driver's pedal depression amount may be used as an index.

(B)
In each of the above embodiments, the rate of increase of the engine compression ratio accompanying the decrease in load, that is, the slopes in FIGS. 4 to 6 and FIG. 8 is linearly expressed for each of the regions where stratified combustion and homogeneous combustion are performed. doing. Instead of such a linear inclination, an inclination having a curved change may be used. In that case, the slope in the tangential direction of the curve is defined as the increase in the engine compression ratio.

(C)
In each of the above embodiments, an internal combustion engine that forms a stratified mixture by directly injecting fuel into the combustion chamber 21 is premised. However, a so-called port injection system in which a fuel injection valve is installed in the intake port 22 is adopted. The application of the present invention is also effective for an internal combustion engine that realizes stratification by performing fuel injection when the intake valve during the intake stroke is open.

(D)
Knocking at the time of stratified combustion in the region near the switching load (torque T4) between stratified combustion and homogeneous combustion is suppressed by reducing the engine compression ratio in the first and second embodiments, In the third and fourth embodiments described above, the ignition timing is retarded, but control that combines the two methods of lowering the engine compression ratio and retarding the ignition timing is performed. It is also possible to cause the ECU 26 to perform the operation.

[Summary of Internal Combustion Engine According to Each Embodiment]
In the internal combustion engine that performs the stratified combustion in the low speed rotation / partial load operation region and the homogeneous combustion in the other operation region while changing the engine compression ratio by the variable compression ratio mechanism 19 with the configuration as described above, Knocking that appears in the vicinity of a load that switches between stratified combustion and homogeneous combustion can be suppressed to ensure smooth drivability.

  Also, while taking control to increase the engine compression ratio as the load decreases, the rate of increase in the engine compression ratio due to load reduction during stratified combustion differs from the rate of increase in engine compression ratio due to load decrease during homogeneous combustion. By doing so, it is possible to individually set optimum engine compression ratios that differ depending on the combustion mode. As a result, the theoretical thermal efficiency can be improved by making the most of lean combustion, and a highly efficient internal combustion engine that makes use of the pump loss reduction effect of stratified combustion can be realized.

  The internal combustion engine according to the present invention has an effect of suppressing knocking in a region on the stratified combustion side in the vicinity of the switching point between stratified combustion and homogeneous combustion and ensuring smooth operability. It is useful as an internal combustion engine that performs stratified combustion in a load operation region and performs homogeneous combustion in other operation regions.

It is a system configuration figure of an embodiment in the present invention. It is a block diagram which shows the variable compression ratio mechanism in this invention. It is a figure which shows the combustion form with respect to the operation area | region of embodiment in this invention. It is a figure explaining the engine compression ratio setting etc. of 1st Embodiment in this invention. It is a figure explaining the engine compression ratio setting etc. of 2nd Embodiment in this invention. It is a figure explaining the engine compression ratio setting etc. of 3rd Embodiment in this invention. It is a figure explaining the ignition timing setting of 3rd Embodiment in this invention. It is a figure explaining the engine compression ratio setting etc. of 4th Embodiment in this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Crankshaft 2 Lower link 3 Piston 4 Piston pin 5 Upper link 6 Control link 7 Control shaft 8 Eccentric cam 9 2nd connection pin 10 1st connection pin 11 Crank pin 13 Thread dripping 14 Thread drive 16 Variable compression ratio actuator 19 Variable compression ratio mechanism 21 Combustion chamber 22 Intake port 23 Intake valve 24 Fuel injection valve 25 Spark plug 26 Electronic control unit (ECU)
27 Exhaust port 28 Exhaust valve 29 Throttle valve 31 Combustion mode determination unit 32 Spark ignition control unit 33 Fuel injection control unit 34 Throttle control unit 35 Compression ratio control unit

Claims (10)

A variable compression ratio mechanism capable of changing the engine compression ratio;
A control device for causing stratified combustion in a low load operation region and performing homogeneous combustion in other operation regions;
An internal combustion engine comprising:
In the stratified combustion, the control device makes the engine compression ratio in the vicinity of the switching point between stratified combustion and homogeneous combustion smaller than the engine compression ratio in the vicinity of the switching point in homogeneous combustion.
Internal combustion engine. A variable compression ratio mechanism capable of changing the engine compression ratio;
An ignition device for igniting an air-fuel mixture in the combustion chamber;
A control device for causing stratified combustion in a low load operation region and performing homogeneous combustion in other operation regions;
An internal combustion engine comprising:
In the stratified combustion, the control device retards the timing of ignition by the igniter in the vicinity of a switching point between stratified combustion and homogeneous combustion.
Internal combustion engine. The control device temporarily retards the timing of ignition by the ignition device when switching from homogeneous combustion to stratified combustion,
The internal combustion engine according to claim 2. The control device retards the timing of ignition by the ignition device at a stroke when switching from homogeneous combustion to stratified combustion, and then gradually advances the timing of ignition by the ignition device as the load decreases.
The internal combustion engine according to claim 2 or 3. The control device performs compression ratio increase processing for increasing the engine compression ratio as the load decreases in each of stratified combustion and homogeneous combustion,
The degree of increase in the compression ratio of the compression ratio increasing process in homogeneous combustion is different from the degree of increase in the compression ratio of the compression ratio increasing process in homogeneous combustion.
The internal combustion engine according to any one of claims 1 to 4. The degree of increase in the compression ratio in the compression ratio increase process in stratified combustion is greater than the degree of increase in the compression ratio in the compression ratio increase process in homogeneous combustion.
The internal combustion engine according to claim 5. The degree of increase in the compression ratio of the compression ratio increase process in stratified combustion is smaller than the degree of increase in the compression ratio of the compression ratio increase process in homogeneous combustion.
The internal combustion engine according to claim 5. A suction mechanism for sucking air into the combustion chamber;
In the extremely low load operation region in stratified combustion, the control device reduces the intake air amount to the combustion chamber by the intake mechanism as compared to an operation region other than the extremely low load operation region in stratified combustion.
The internal combustion engine according to any one of claims 1 to 7. The variable compression ratio mechanism has a drive device, and is configured so that the engine compression ratio becomes the lowest compression ratio when the drive device fails.
The internal combustion engine according to any one of claims 1 to 8. The variable compression ratio mechanism is
A first link connected to the piston via a piston pin;
A second link coupled to the first link via a first coupling pin and rotatably coupled to a crankpin portion of the crankshaft;
A third link coupled to the first link or the second link via a second coupling pin and supported swingably on the body of the internal combustion engine;
A swing support position changing mechanism for changing the swing support position of the third link;
Including a multi-link type piston-crank mechanism, which corrects the position of the piston so that the engine compression ratio becomes the target compression ratio,
The internal combustion engine according to claim 9.

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