JP2012225199A - Spark ignition internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a decrease in combustibility in a combustion chamber, concerning a spark ignition internal combustion engine including a variable compression ratio mechanism with a variable mechanical compression ratio and a variable valve mechanism with a variable valve closing timing of an intake valve.SOLUTION: The spark ignition internal combustion engine includes a variable compression ratio mechanism, a variable valve mechanism, and an ignition timing adjusting device, wherein it is formed to perform a control so that the combustibility of fuel in the combustion chamber decreases. The engine has a load threshold associated with an ignition timing to stabilize the combustibility of the fuel in the combustion chamber, and a control is performed for maintaining the ignition timing at a constant fixed ignition timing in a region of a load smaller than the load threshold. When performing the control in which the combustibility of the fuel in the combustion chamber decreases, the fixed mechanical compression ratio is decreased and also the fixed valve closing timing is advanced. The load threshold associated with the ignition timing is defined, based on a decrease amount of the fixed mechanical compression ratio and an advance angle amount of the fixed valve closing timing.

Description

  The present invention relates to a spark ignition internal combustion engine.

  In the combustion chamber of the internal combustion engine, the air-fuel mixture is ignited in a compressed state. It is known that the compression ratio when compressing the air-fuel mixture affects the output torque and the fuel consumption. By increasing the compression ratio, the output torque can be increased or the fuel consumption can be reduced. On the other hand, it is known that if the compression ratio is too high, abnormal combustion such as knocking occurs. In the prior art, an internal combustion engine is known that includes a variable compression ratio mechanism that can change the compression ratio during an operation period, and a variable valve timing mechanism that can change the opening / closing timing of the intake valve.

  For example, Japanese Unexamined Patent Application Publication No. 2007-303423 discloses an internal combustion engine that includes a variable compression ratio mechanism that can change the mechanical compression ratio and an actual compression action start timing changing mechanism that can change the actual compression action start timing. Has been. In this internal combustion engine, the mechanical compression ratio is maximized so that the expansion ratio is 20 or more during low-load operation, and the actual compression ratio during low-load operation is set to be substantially the same as that during high-load operation. It is disclosed.

  In Japanese Patent Application Laid-Open No. 07-133735, atmospheric pressure detection means for detecting atmospheric pressure, limit air-fuel ratio setting means for setting a lean-side limit air-fuel ratio according to atmospheric pressure, and target air-fuel ratio corresponding to operating conditions When the air-fuel ratio is leaner than the limit air-fuel ratio, the air-fuel ratio correction means that gives the limit air-fuel ratio as the target air-fuel ratio, and the ignition timing correction means that corrects the ignition timing to the retard side according to the correction amount of the target air-fuel ratio And a control device for an internal combustion engine including the above. This internal combustion engine suppresses the problem that the lean combustion becomes unstable at high altitudes where the atmospheric pressure is low, and the occurrence of knocking occurs by correcting the ignition timing to the retarded side according to the correction amount of the air-fuel ratio. Is disclosed.

  Japanese Patent Application Laid-Open No. 11-30134 discloses an engine having a valve timing adjustment mechanism for adjusting the opening / closing timing of an intake valve. When the engine is started, the closing timing of the intake valve is set to the most retarded state. In this state, since the compression ratio of the air in the combustion chamber becomes small, it is disclosed that the pressure in the combustion chamber during the compression stroke is kept low and the vibration of the engine is reduced. Further, during the cold period after the completion of engine start, the closing timing of the intake valve is set to the most advanced state. In this state, since the compression ratio of the air in the combustion chamber increases, it is disclosed that the pressure in the combustion chamber during the compression stroke is increased and the output of the engine is improved.

JP 2007-303423 A Japanese Patent Application Laid-Open No. 07-133735 JP-A-11-030134 JP 09-032651 A

  An internal combustion engine provided with a mechanism for changing the mechanical compression ratio disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2007-303423 and a mechanism capable of changing the actual start time of compression action, particularly in a low load region, While maintaining the mechanical compression ratio at a high value, it is possible to perform the ultra-high expansion ratio control that greatly delays the timing of closing the intake valve and suppresses the increase in the actual compression ratio while increasing the expansion ratio.

  The internal combustion engine has MBT (Minimum Advance for Best Torque) as an ignition timing at which the output torque becomes the largest. The torque output from the internal combustion engine can be increased by advancing the ignition timing and gradually approaching the MBT in the retarded angle region from the MBT. However, when the ignition timing is advanced from a predetermined ignition timing, abnormal combustion such as knocking may occur. For this reason, as the ignition timing, a timing that is advanced to such an extent that the occurrence of abnormal combustion can be suppressed is selected. In general, combustion in the combustion chamber becomes slower at a low load as compared with a high load, and the MBT moves toward the advance side (front dead center direction). That is, the MBT advances as the load decreases.

  Even in the internal combustion engine that performs the above-described ultra-high expansion ratio control, it is possible to perform control to advance the ignition timing while reducing the load. However, in a region smaller than a predetermined load in the low load region, there has been a problem that the combustibility in the combustion chamber is reduced. For example, the ignition delay varies in the combustion chamber, causing a problem that the work amount for each combustion cycle varies. That is, there has been a problem that combustion fluctuation becomes larger than a desired amount in a low load region. When the combustion fluctuation becomes large, the torque fluctuation that causes the torque output from the internal combustion engine to become unstable occurs. For example, there is a problem that the driver feels uncomfortable.

  The present invention suppresses a decrease in combustibility in a combustion chamber in a spark ignition internal combustion engine including a variable compression ratio mechanism capable of changing a mechanical compression ratio and a variable valve mechanism capable of changing a closing timing of an intake valve. The purpose is to do.

  The spark ignition internal combustion engine of the present invention includes a variable compression ratio mechanism that can change the mechanical compression ratio, a variable valve mechanism that can change the closing timing of the intake valve, and an ignition timing that can change the ignition timing in the combustion chamber. And an adjusting device. In the high load region, when the load increases, the closing timing of the intake valve is advanced to increase the amount of intake air flowing into the combustion chamber and lower the mechanical compression ratio. In the low load area, which is smaller than the high load, the mechanical compression ratio is maintained at a fixed fixed mechanical compression ratio, the intake valve closing timing is maintained at a fixed fixed valve closing timing, and the throttle is reduced when the load decreases. Control is performed to reduce the amount of intake air flowing into the combustion chamber by reducing the opening of the valve. The spark ignition type internal combustion engine is formed so as to perform control to reduce the combustibility of the fuel in the combustion chamber. It has a load threshold related to the ignition timing for stabilizing the flammability of the fuel in the combustion chamber. In the load region smaller than the load threshold, the ignition timing is maintained at a constant fixed ignition timing, and the fuel combustion in the combustion chamber When control is performed to reduce performance, the fixed mechanical compression ratio is decreased and the fixed valve closing timing is advanced, and the load related to the ignition timing is determined based on the amount of decrease in the fixed mechanical compression ratio and the advanced valve advancement amount of the fixed valve closing timing. A threshold is defined.

  In the above invention, the control for reducing the combustibility of the fuel in the combustion chamber may include at least one of control for increasing the exhaust gas recirculation rate and control for increasing the air-fuel ratio during combustion in the combustion chamber. it can.

  In the above-described invention, a detection device that detects a variable corresponding to the combustibility of the fuel in the combustion chamber is provided. Control can be performed to increase the amount of decrease in the mechanical compression ratio and increase the advance amount of the fixed valve closing timing.

  In the above invention, it is preferable that the upper limit load for maintaining the intake valve closing timing at the fixed valve closing timing is set larger than the upper limit load for maintaining the mechanical compression ratio at the fixed mechanical compression ratio.

  According to the present invention, in a spark ignition internal combustion engine that includes a variable compression ratio mechanism that can change the mechanical compression ratio and a variable valve mechanism that can change the closing timing of the intake valve, the combustibility in the combustion chamber is reduced. Can be suppressed.

1 is a schematic overall view of an internal combustion engine in an embodiment. It is a general | schematic exploded perspective view of the variable compression ratio mechanism in embodiment. (A) to (C) is a schematic cross-sectional view of a variable compression ratio mechanism for explaining a change in mechanical compression ratio in the embodiment. It is the schematic explaining the variable valve timing mechanism in embodiment. It is a graph explaining opening and closing timing of the intake valve and the exhaust valve in the embodiment. (A) is explanatory drawing of a mechanical compression ratio, (B) is explanatory drawing of an actual compression ratio, (C) is explanatory drawing of an expansion ratio. It is a graph explaining the relationship between the expansion ratio of an internal combustion engine and theoretical thermal efficiency in embodiment. (A) is explanatory drawing of a normal driving cycle, (B) is explanatory drawing of a super-high expansion ratio cycle. It is a graph of the operation control of the comparative example of the internal combustion engine in embodiment. It is a graph of the 1st operation control of the internal-combustion engine in an embodiment. It is a graph of the 2nd operation control of the internal-combustion engine in an embodiment. It is a graph of the 3rd operation control of the internal-combustion engine in an embodiment. It is a graph of the 4th operation control of the internal-combustion engine in an embodiment. It is a graph of the 5th operation control of an internal-combustion engine in an embodiment. It is a graph of the 6th operation control of the internal-combustion engine in an embodiment.

  An internal combustion engine according to an embodiment will be described with reference to FIGS. The internal combustion engine in the present embodiment includes a variable compression ratio mechanism that can change the mechanical compression ratio and a variable valve mechanism that can change the closing timing of the intake valve, and increases the expansion ratio in a low load region. Super high expansion ratio control.

  FIG. 1 is a schematic overall view of an internal combustion engine in the present embodiment. The internal combustion engine in the present embodiment includes a crankcase 1, a cylinder block 2, and a cylinder head 3. A piston 4 is disposed in a hole formed in the cylinder block 2. A spark plug 6 is disposed at the center of the top surface of the combustion chamber 5 surrounded by the top surface of the piston 4 and the cylinder head 3. An intake port 8 and an exhaust port 10 are formed in the cylinder head 3. An intake valve 7 is disposed at the end of the intake port 8. An exhaust valve 9 is disposed at the end of the exhaust port 10. The intake port 8 is connected to a surge tank 12 via an intake branch pipe 11. Each intake branch pipe 11 is provided with a fuel injection valve 13 for injecting fuel into the corresponding intake port 8. Instead of being attached to each intake branch pipe 11, the fuel injection valve 13 may be arranged so as to inject fuel directly into each combustion chamber 5.

  The surge tank 12 is connected to an air cleaner 15 via an intake duct 14. A throttle valve 17 driven by an actuator 16 is disposed in the intake duct 14. Further, an intake air amount detector 18 using, for example, heat rays is disposed in the intake duct 14. On the other hand, the exhaust port 10 is connected through an exhaust manifold 19 to a catalyst device 20 containing, for example, a three-way catalyst. An air-fuel ratio sensor 21 is disposed in the exhaust manifold 19.

  The internal combustion engine in the present embodiment includes an exhaust gas recirculation device for performing exhaust gas recirculation (EGR). The exhaust gas recirculation device of the present embodiment includes an EGR gas conduit 26 as a recirculation passage. The EGR gas conduit 26 connects the exhaust manifold 19 and the surge tank 12 to each other. An EGR control valve 27 is disposed in the middle of the EGR gas conduit 26. The EGR control valve 27 is formed so that the flow rate of exhaust gas circulating from the engine exhaust passage to the engine intake passage can be adjusted. By changing the opening degree of the EGR control valve 27, the recirculation rate of the exhaust gas can be adjusted.

  On the other hand, in the embodiment shown in FIG. 1, the piston 4 is brought to the compression top dead center by changing the relative position of the crankcase 1 and the cylinder block 2 in the cylinder axial direction at the connecting portion between the crankcase 1 and the cylinder block 2. A variable compression ratio mechanism A capable of changing the volume of the combustion chamber 5 when positioned is provided. Furthermore, an actual compression action start timing changing mechanism B that can change the actual compression action start timing is provided. The actual compression operation start timing changing mechanism B in the present embodiment includes a variable valve timing mechanism as a variable valve mechanism that can control the closing timing of the intake valve 7.

  A relative position sensor 22 for detecting the relative positional relationship between the crankcase 1 and the cylinder block 2 is attached to the crankcase 1 and the cylinder block 2. The relative position sensor 22 outputs an output signal indicating a change in the distance between the crankcase 1 and the cylinder block 2. The variable valve timing mechanism B is attached with a valve timing sensor 23 that generates an output signal indicating the closing timing of the intake valve 7. A throttle opening sensor 24 for generating an output signal indicating the throttle valve opening is attached to the actuator 16 for driving the throttle valve.

  The internal combustion engine in the present embodiment includes an electronic control unit 30 as a control device. The electronic control unit 30 in the present embodiment includes a digital computer and includes a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, and an input connected to each other by a bidirectional bus 31. A port 35 and an output port 36 are provided. Output signals of the intake air amount detector 18, the air-fuel ratio sensor 21, the relative position sensor 22, the valve timing sensor 23, and the throttle opening sensor 24 are input to the input port 35 via the corresponding AD converters 37. A load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. Is done. The required load can be detected from the output of the load sensor 41. Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 30 °. The engine speed can be detected from the output of the crank angle sensor 42.

  On the other hand, the output port 36 is connected to the spark plug 6, the fuel injection valve 13, the actuator 16 for driving the throttle valve, the EGR control valve 27, the variable compression ratio mechanism A, and the variable valve timing mechanism B through a corresponding drive circuit 38. Is done. These devices are controlled by the electronic control unit 30.

  FIG. 2 is an exploded perspective view of the variable compression ratio mechanism shown in FIG. FIG. 3 shows a schematic sectional view of the internal combustion engine for explaining the operation of the variable compression ratio mechanism. Referring to FIG. 2, a plurality of protrusions 50 are formed below the both side walls of the cylinder block 2 so as to be spaced apart from each other. A cam insertion hole 51 having a circular cross section is formed in each protrusion 50. On the other hand, a plurality of protrusions 52 are formed on the upper wall surface of the crankcase 1 so as to be fitted between the corresponding protrusions 50 at intervals. Each of these protrusions 52 is also formed with a cam insertion hole 53 having a circular cross section.

  The variable compression ratio mechanism in the present embodiment includes a pair of camshafts 54 and 55. On each of the camshafts 54 and 55, a circular cam 58 that is rotatably inserted into each of the cam insertion holes 53 is fixed. These circular cams 58 are coaxial with the rotational axes of the camshafts 54 and 55. On the other hand, on both sides of each circular cam 58, as shown in FIG. 3, eccentric shafts 57 that are eccentrically arranged with respect to the rotation axes of the cam shafts 54 and 55 extend. Another circular cam 56 is eccentrically mounted on the eccentric shaft 57 so as to be rotatable. As shown in FIG. 2, these circular cams 56 are arranged on both sides of each circular cam 58, and these circular cams 56 are rotatably inserted into the corresponding cam insertion holes 51. A cam rotation angle sensor 25 that generates an output signal representing the rotation angle of the cam shaft 55 is attached to the cam shaft 55. The output of the cam rotation angle sensor 25 is input to the electronic control unit 30.

  When the circular cams 58 fixed on the camshafts 54 and 55 are rotated in opposite directions as shown by arrows in FIG. 3A from the state shown in FIG. The circular cam 56 rotates in a direction opposite to the circular cam 58 in the cam insertion hole 51 so as to move away from each other, and as shown in FIG. Position. Next, when the circular cam 58 is further rotated in the direction indicated by the arrow, the eccentric shaft 57 is at the lowest position as shown in FIG.

  3A, 3B, and 3C show the positional relationship among the center a of the circular cam 58, the center b of the eccentric shaft 57, and the center c of the circular cam 56 in each state. It is shown.

  3A to 3C, the relative positions of the crankcase 1 and the cylinder block 2 are determined by the distance between the center a of the circular cam 58 and the center c of the circular cam 56. The cylinder block 2 moves away from the crankcase 1 as the distance between the center a of the cylinder and the center c of the circular cam 56 increases. That is, the variable compression ratio mechanism A changes the relative position between the crankcase 1 and the cylinder block 2 by a crank mechanism using a rotating cam. When the cylinder block 2 moves away from the crankcase 1, the volume of the combustion chamber 5 increases when the piston 4 is positioned at the compression top dead center. Therefore, by rotating the camshafts 54 and 55, the piston 4 is compressed at the top dead center. The volume of the combustion chamber 5 when it is located at can be changed.

  As shown in FIG. 2, in order to rotate the camshafts 54 and 55 in opposite directions, a pair of worms 61 and 62 having opposite spiral directions are attached to the rotation shaft of the drive motor 59, respectively. Worm wheels 63 and 64 that mesh with the worms 61 and 62 are fixed to the ends of the camshafts 54 and 55, respectively. In this embodiment, by driving the drive motor 59, the volume of the combustion chamber 5 when the piston 4 is located at the compression top dead center can be changed over a wide range.

  The variable compression ratio mechanism in the present embodiment moves the cylinder block relative to the lower structure including the crankcase. However, the variable compression ratio mechanism is not limited to this form. Any mechanism that can change the ratio can be employed.

  FIG. 4 shows the variable valve timing mechanism B attached to the end of the camshaft 70 for driving the intake valve 7 in FIG. The variable valve timing mechanism B in the present embodiment includes a timing pulley 71 that is rotated in the direction of an arrow by a crankshaft of an engine via a timing belt, a cylindrical housing 72 that rotates together with the timing pulley 71, and an intake valve drive A rotating shaft 73 that rotates together with the camshaft 70 and is rotatable relative to the cylindrical housing 72; a plurality of partition walls 74 that extend from the inner peripheral surface of the cylindrical housing 72 to the outer peripheral surface of the rotating shaft 73; A vane 75 extending from the outer peripheral surface of the rotary shaft 73 to the inner peripheral surface of the cylindrical housing 72 is provided between the partition walls 74, and an advance hydraulic chamber 76 and a retard angle are provided on both sides of each vane 75, respectively. A hydraulic chamber 77 is formed.

  The hydraulic oil supply control to the hydraulic chambers 76 and 77 is performed by a hydraulic oil supply control valve 78. The hydraulic oil supply control valve 78 includes hydraulic ports 79 and 80 connected to the hydraulic chambers 76 and 77, a hydraulic oil supply port 82 discharged from the hydraulic pump 81, a pair of drain ports 83 and 84, And a spool valve 85 for controlling communication between the ports 79, 80, 82, 83, and 84.

  When the cam phase of the intake valve driving camshaft 70 should be advanced, the spool valve 85 is moved to the right in FIG. 4 and the hydraulic oil supplied from the supply port 82 is advanced via the hydraulic port 79. The hydraulic oil in the retard hydraulic chamber 77 is discharged from the drain port 84 while being supplied to the angular hydraulic chamber 76. At this time, the rotary shaft 73 is rotated relative to the cylindrical housing 72 in the direction of the arrow.

  On the other hand, when the cam phase of the intake valve driving camshaft 70 should be retarded, the spool valve 85 is moved to the left in FIG. And the hydraulic oil in the advance hydraulic chamber 76 is discharged from the drain port 83. At this time, the rotating shaft 73 is rotated relative to the cylindrical housing 72 in the direction opposite to the arrow.

  If the spool valve 85 is returned to the neutral position shown in FIG. 4 while the rotating shaft 73 is rotating relative to the cylindrical housing 72, the relative rotating operation of the rotating shaft 73 is stopped. Is held at the relative rotational position. Therefore, the variable valve timing mechanism B can advance or retard the cam phase of the intake valve driving camshaft 70 by a desired amount.

  In FIG. 5, the solid line shows the time when the cam phase of the intake valve driving camshaft 70 is advanced the most by the variable valve timing mechanism B, and the broken line shows the cam phase of the intake valve driving camshaft 70 being the most advanced. It shows when it is retarded. Therefore, the valve opening period of the intake valve 7 can be arbitrarily set between the range indicated by the solid line and the range indicated by the broken line in FIG. Therefore, the closing timing of the intake valve 7 can also be set to an arbitrary crank angle within the range indicated by the arrow C in FIG.

  The variable valve timing mechanism B shown in FIG. 1 and FIG. 4 shows an example. For example, the variable valve timing that can change only the closing timing of the intake valve while keeping the opening timing of the intake valve constant. Various types of variable valve timing mechanisms, such as mechanisms, can be used.

  Next, the meanings of terms used in the present application will be described with reference to FIG. 6 (A), (B), and (C) show an engine having a combustion chamber volume of 50 ml and a piston stroke volume of 500 ml for the sake of explanation. , (B), (C), the combustion chamber volume represents the volume of the combustion chamber when the piston is located at the compression top dead center.

  FIG. 6A explains the mechanical compression ratio. The mechanical compression ratio is a value mechanically determined only from the stroke volume of the piston and the combustion chamber volume during the compression stroke, and this mechanical compression ratio is expressed by (combustion chamber volume + stroke volume) / combustion chamber volume. In the example shown in FIG. 6A, this mechanical compression ratio is (50 ml + 500 ml) / 50 ml = 11.

  FIG. 6B describes the actual compression ratio. This actual compression ratio is a value determined from the actual piston stroke volume and the combustion chamber volume from when the compression action is actually started until the piston reaches top dead center, and this actual compression ratio is (combustion chamber volume + actual (Stroke volume) / combustion chamber volume. That is, as shown in FIG. 6B, even if the piston starts to rise in the compression stroke, the compression operation is not performed while the intake valve is open, and the actual compression is performed from the time when the intake valve is closed. The action begins. Therefore, the actual compression ratio is expressed as described above using the actual stroke volume. In the example shown in FIG. 6B, the actual compression ratio is (50 ml + 450 ml) / 50 ml = 10.

  FIG. 6C illustrates the expansion ratio. The expansion ratio is a value determined from the stroke volume of the piston and the combustion chamber volume during the expansion stroke, and this expansion ratio is expressed by (combustion chamber volume + stroke volume) / combustion chamber volume. In the example shown in FIG. 6C, this expansion ratio is (50 ml + 500 ml) / 50 ml = 11.

  Next, the super expansion ratio cycle used in the present invention will be described with reference to FIGS. FIG. 7 shows the relationship between the theoretical thermal efficiency and the expansion ratio, and FIG. 8 shows a comparison between a normal cycle and an ultrahigh expansion ratio cycle that are selectively used according to the load in the present invention.

  FIG. 8A shows a normal cycle when the intake valve closes near the bottom dead center and the compression action by the piston is started from the vicinity of the intake bottom dead center. In the example shown in FIG. 8A as well, the combustion chamber volume is set to 50 ml, and the stroke volume of the piston is set to 500 ml, similarly to the example shown in FIGS. 6A, 6B, and 6C. As can be seen from FIG. 8A, in a normal cycle, the mechanical compression ratio is (50 ml + 500 ml) / 50 ml = 11, the actual compression ratio is almost 11, and the expansion ratio is also (50 ml + 500 ml) / 50 ml = 11. That is, in a normal internal combustion engine, the mechanical compression ratio, the actual compression ratio, and the expansion ratio are substantially equal.

  The solid line in FIG. 7 shows the change in the theoretical thermal efficiency when the actual compression ratio and the expansion ratio are substantially equal, that is, in a normal cycle. In this case, it can be seen that the theoretical thermal efficiency increases as the expansion ratio increases, that is, as the actual compression ratio increases. Therefore, in order to increase the theoretical thermal efficiency in a normal cycle, it is only necessary to increase the actual compression ratio. However, the actual compression ratio can only be increased to a maximum of about 12 due to the restriction of the occurrence of knocking at the time of engine high load operation, and thus the theoretical thermal efficiency cannot be sufficiently increased in a normal cycle.

  On the other hand, under such circumstances, it is considered to increase the theoretical thermal efficiency while strictly dividing the mechanical compression ratio and the actual compression ratio. As a result, the theoretical thermal efficiency is governed by the expansion ratio, and the actual compression ratio is compared to the theoretical thermal efficiency. Has little effect. That is, if the actual compression ratio is increased, the explosive force is increased, but a large amount of energy is required for compression. Thus, even if the actual compression ratio is increased, the theoretical thermal efficiency is hardly increased.

  On the other hand, when the expansion ratio is increased, the period during which the pressing force acts on the piston during the expansion stroke becomes longer, and thus the period during which the piston applies the rotational force to the crankshaft becomes longer. Therefore, the larger the expansion ratio, the higher the theoretical thermal efficiency. The broken line ε = 10 in FIG. 7 indicates the theoretical thermal efficiency when the expansion ratio is increased with the actual compression ratio fixed at 10. The increase in the theoretical thermal efficiency when the expansion ratio is increased while maintaining the actual compression ratio ε at a low value, and the theoretical thermal efficiency when the actual compression ratio increases with the expansion ratio as shown by the solid line in FIG. It can be seen that there is no significant difference from the amount of increase.

  Thus, if the actual compression ratio is maintained at a low value, knocking does not occur. Therefore, if the expansion ratio is increased while the actual compression ratio is maintained at a low value, the theoretical thermal efficiency is reduced while preventing the occurrence of knocking. Can be greatly increased. FIG. 8B shows an example where the variable compression ratio mechanism A and variable valve timing mechanism B are used to increase the expansion ratio while maintaining the actual compression ratio at a low value.

  Referring to FIG. 8B, in this example, the variable compression ratio mechanism A reduces the combustion chamber volume from 50 ml to 20 ml. On the other hand, the variable valve timing mechanism B delays the closing timing of the intake valve until the actual piston stroke volume is reduced from 500 ml to 200 ml. As a result, in this example, the actual compression ratio is (20 ml + 200 ml) / 20 ml = 11, and the expansion ratio is (20 ml + 500 ml) / 20 ml = 26. In the normal cycle shown in FIG. 8A, the actual compression ratio is almost 11 and the expansion ratio is 11, as described above. Compared with this case, only the expansion ratio is shown in FIG. 8B. It can be seen that it has been increased to 26. This is why it is called an ultra-high expansion ratio cycle.

  Generally speaking, in an internal combustion engine, the lower the engine load, the worse the thermal efficiency. Therefore, in order to improve the thermal efficiency during engine operation, that is, to improve fuel efficiency, it is necessary to improve the thermal efficiency when the engine load is low. Necessary. On the other hand, in the ultra-high expansion ratio cycle shown in FIG. 8B, since the actual piston stroke volume during the compression stroke is reduced, the amount of intake air that can be sucked into the combustion chamber 5 is reduced. The expansion ratio cycle can only be adopted when the engine load is relatively low. Therefore, in the present invention, when the engine load is relatively low, the super high expansion ratio cycle shown in FIG. 8B is used, and during the high engine load operation, the normal cycle shown in FIG. 8A is used.

Next, with reference to FIG. 9, the operation control of the comparative example in this Embodiment is demonstrated. FIG. 9 is a graph schematically illustrating general operation control of the super-high expansion ratio control. FIG. 9 shows changes in the ignition timing, the intake valve closing timing, the expansion ratio (mechanical compression ratio), the actual compression ratio, and the throttle valve opening according to the engine load at a certain engine speed. Incidentally, FIG. 9, unburned HC, the average air-fuel ratio output signal of the air-fuel ratio sensor 21 in the combustion chamber 5 so as to be able to reduce at the same time CO and NO _X in the exhaust gas by the three-way catalyst in the catalytic device 20 This shows a case where feedback control is performed to the theoretical air-fuel ratio based on the above.

  As described above, the normal cycle shown in FIG. 8 (A) is executed during engine high load operation. Therefore, as shown in FIG. 9, at this time, the mechanical compression ratio is lowered, so the expansion ratio is low, and the closing timing of the intake valve 7 is advanced as shown by the solid line in FIG. At this time, the amount of intake air is large, and at this time, the opening degree of the throttle valve 17 is kept fully open, so that the pumping loss is zero.

  On the other hand, as shown in FIG. 9, when the engine load becomes low, the closing timing of the intake valve 7 is delayed in order to reduce the intake air amount. Further, at this time, the mechanical compression ratio is increased as the engine load is lowered so that the actual compression ratio is kept substantially constant. Therefore, the expansion ratio is also increased as the engine load is lowered. At this time, the throttle valve 17 is kept fully open, and therefore the amount of intake air supplied into the combustion chamber 5 is controlled by changing the closing timing of the intake valve 7 regardless of the throttle valve 17. Has been.

  As described above, when the engine load is reduced from the engine high load operation state, the mechanical compression ratio is increased as the intake air amount is decreased while the actual compression ratio is substantially constant. That is, the volume of the combustion chamber 5 when the piston 4 reaches the compression top dead center is reduced in proportion to the reduction of the intake air amount. Therefore, the volume of the combustion chamber 5 when the piston 4 reaches the compression top dead center changes in proportion to the intake air amount. At this time, in the example shown in FIG. 9, the air-fuel ratio in the combustion chamber 5 is the stoichiometric air-fuel ratio, so the volume of the combustion chamber 5 when the piston 4 reaches the compression top dead center is proportional to the fuel amount. Will change.

  As the engine load is further reduced, the mechanical compression ratio further increases. In the internal combustion engine shown in FIG. 9, the mechanical compression ratio reaches a limit mechanical compression ratio (upper limit mechanical compression ratio) that becomes the structural limit of the combustion chamber 5 when the engine load is reduced to the medium load L1 that is slightly close to the low load. When the mechanical compression ratio reaches the limit mechanical compression ratio, the mechanical compression ratio is held at the limit mechanical compression ratio in a region where the load is lower than the engine load L1 when the mechanical compression ratio reaches the limit mechanical compression ratio. Accordingly, the mechanical compression ratio is maximized and the expansion ratio is maximized at the time of low engine load operation and low engine load operation, that is, at the engine low load operation side. In other words, the mechanical compression ratio is maximized so that the maximum expansion ratio is obtained on the engine low load operation side.

  On the other hand, in the embodiment shown in FIG. 9, when the engine load decreases to L1, the closing timing of the intake valve 7 becomes the limit closing timing that can control the amount of intake air supplied into the combustion chamber 5. When the closing timing of the intake valve 7 reaches the limit closing timing, the closing timing of the intake valve 7 is reduced in a region where the load is lower than the engine load L1 when the closing timing of the intake valve 7 reaches the closing timing. It is held at the limit closing timing.

  When the closing timing of the intake valve 7 is held at the limit closing timing, the amount of intake air can no longer be controlled by changing the closing timing of the intake valve 7. In the embodiment shown in FIG. 9, at this time, that is, in a region where the load is lower than the engine load L1 when the closing timing of the intake valve 7 reaches the limit closing timing, the intake valve 7 is supplied into the combustion chamber 5 by the throttle valve 17. The amount of intake air to be controlled is controlled, and the opening degree of the throttle valve 17 is made smaller as the engine load becomes lower.

  In the embodiment according to the present invention, the closing timing of the intake valve 7 moves away from the intake bottom dead center BDC until the limit closing timing at which the amount of intake air supplied into the combustion chamber can be controlled as the engine load decreases. Will be.

  As described above, the expansion ratio is 26 in the ultra-high expansion ratio cycle shown in FIG. The higher the expansion ratio, the better. However, as can be seen from FIG. 7, a considerably high theoretical thermal efficiency can be obtained if it is 20 or more with respect to the practically usable lower limit actual compression ratio ε = 5. Therefore, in this embodiment, the variable compression ratio mechanism A is formed so that the expansion ratio is 20 or more.

  In the present embodiment, the mechanical compression ratio that is maintained constant in the low load side region as described above is referred to as a fixed mechanical compression ratio, and the intake valve that is maintained constant in the low load side region is closed. The valve timing is referred to as fixed valve closing timing. In the operation example of FIG. 9, the mechanical compression ratio is maintained at the fixed mechanical compression ratio in the region below the engine load L1, and the closing timing of the intake valve is maintained at the fixed closing timing.

  The internal combustion engine in the present embodiment includes an ignition timing adjusting device that can change the ignition timing in the combustion chamber. With respect to the ignition timing in the present embodiment, the MBT advances as the engine load decreases. Therefore, the ignition timing can be advanced as the engine load decreases.

  However, in an internal combustion engine that performs ultra-high expansion ratio control, the timing for closing the intake valve is delayed in the low load region as described above. That is, the intake valve is closed at a time close to the compression top dead center. On the other hand, the ignition timing is advanced as the engine load decreases. For this reason, in the low load region, the air-fuel mixture may be ignited without being sufficiently compressed after the intake valve is closed. In addition, the temperature of the air-fuel mixture may be ignited without sufficiently increasing. Such a situation is particularly disadvantageous with respect to the growth of the initial flame. For example, in a combustion chamber, the combustibility of fuel decreases, and the ignition delay may vary. As a result, in the comparative example shown in FIG. 9, a region where the combustion fluctuation is larger than the target value appears in the low load region.

  FIG. 10 shows a graph of the first operation control in the internal combustion engine of the present embodiment. In the graph of FIG. 10, a region of a load greater than the minimum load is shown. The horizontal axis represents the engine load, which corresponds to the torque output from the internal combustion engine or a variable corresponding to the average effective pressure. In FIG. 10, the operation control of the comparative example is described by a broken line, and the first operation control is described by a solid line.

  The internal combustion engine of the present embodiment has a load threshold related to the ignition timing, and performs control to maintain the ignition timing at a fixed ignition timing in a load region smaller than the load threshold. In the operation control of the comparative example, the ignition timing is advanced as the engine load becomes smaller. In the first operation control, in a load region smaller than a predetermined load threshold Lta, Control to stop the advance of the ignition timing.

  The load threshold Lta can be set, for example, in a region where the expansion ratio is constant, that is, a region where the mechanical compression ratio is maintained at a fixed mechanical compression ratio. The load threshold value Lta can be set in a region where the closing timing of the intake valve is constant, that is, a region where the valve closing timing is maintained at the fixed valve closing timing. In the present embodiment, the load threshold value Lta can be set in a region where the mechanical compression ratio is equal to or less than the engine load L1 at which the limit mechanical compression ratio is obtained.

  By performing the first operation control, in order to stop the advance of the ignition timing in a low load region where the combustibility in the combustion chamber becomes unstable, the compression and temperature rise of the air-fuel mixture after the intake valve is closed Can be promoted. For this reason, the fall of combustibility can be suppressed. The load threshold value Lta in the present embodiment is set to an engine load at which the combustion fluctuation almost reaches the target value. For this reason, it can suppress that a combustion fluctuation | variation exceeds target value.

  It is preferable to adopt a small value for the load threshold value related to the ignition timing in a region where the target value of the combustibility stability can be achieved. With this control, it is possible to increase the region where the ignition timing is advanced as the load decreases, and to improve the thermal efficiency.

  In the case of performing the first operation control, for example, a map of a mechanical compression ratio, an intake valve closing timing and an ignition timing using the engine speed and required load of the internal combustion engine as functions is prepared in advance, and the electronic control unit 30 Can be remembered. In this map, the ignition timing can be set constant in the load region below the load threshold Lta. For example, the internal combustion engine can detect the engine speed and the required load, and perform operation control by reading the ignition timing in addition to the mechanical compression ratio and the closing timing of the intake valve.

  FIG. 11 shows a graph of the second operational control in the present embodiment. In the second operation control, in addition to the first operation control that maintains the ignition timing constant in a predetermined region of low load, control is performed to advance the fixed valve closing timing of the intake valve. In FIG. 11, the first operation control is described with a broken line, and the second operation control is described with a solid line. In the low load region, abnormal combustion such as knocking is less likely to occur than in the high load region. For this reason, the actual compression ratio in a combustion chamber can be raised rather than a high load.

  In the second operation control, the control for maintaining the closing timing of the intake valve constant in a region equal to or less than the engine load Lx larger than the engine load L1 in which the closing timing of the intake valve is made constant in the first operation control. It is carried out. That is, control is performed to maintain the closing timing of the intake valve at the fixed closing timing at an engine load Lx larger than the engine load L1 at which the mechanical compression ratio reaches the fixed mechanical compression ratio when the load is reduced. Yes. The fixed valve closing timing in the second operation control is advanced from the fixed valve closing timing in the first operation control. For this reason, the actual compression ratio is higher than that in the first operation control in the region below the engine load Lx. In the region below the engine load Lx, the closing timing of the intake valve becomes earlier than the first operation control, so that the compression of the air-fuel mixture and the temperature rise in the combustion chamber can be promoted, and the combustibility is reduced. Can be suppressed.

  In addition, since the reduction in combustibility can be suppressed, the fixed ignition timing can be advanced from the first operation control. In the first operation control, the ignition timing is kept constant in the region below the load threshold Lta. On the other hand, the load threshold related to the ignition timing of the second operation control can be moved to a load threshold Ltb that is smaller than the load threshold Lta, as indicated by an arrow 110. As the engine load decreases, the region in which the ignition timing is advanced is wider than in the first operation control, so that the thermal efficiency of the internal combustion engine is improved.

  The load threshold Ltb for the second operation control can be determined according to the advance amount of the fixed valve closing timing of the intake valve. For example, as the fixed valve closing timing of the intake valve is advanced, the load threshold Ltb can be reduced, and the region in which the ignition timing is advanced as the engine load decreases can be increased.

  FIG. 12 shows a graph of the third operational control in the present embodiment. In the third operation control, the operation control is performed when the control for reducing the combustibility in the combustion chamber is performed. An example of control that lowers the combustibility of fuel in the combustion chamber is control that increases the exhaust gas recirculation rate. In the third operation control, in addition to the first operation control, control is performed to retard the fixed ignition timing in accordance with a decrease in fuel combustibility.

  The internal combustion engine of the present embodiment has an exhaust gas recirculation device. The recirculation rate (EGR rate) of the internal combustion engine in the present embodiment can be determined as the ratio of the flow rate of the recirculated exhaust gas to the flow rate of all the gases flowing into the combustion chamber. Referring to FIG. 1, when the recirculation rate is changed, the flow rate of the recirculated exhaust gas is changed by driving the EGR control valve 27 arranged in the EGR gas conduit 26. For example, by increasing the opening degree of the EGR control valve 27, the recirculation flow rate increases and the recirculation rate increases.

  In FIG. 12, the first operation control in the present embodiment is described by a broken line, and the third operation control is described by a solid line. In the third operation control, the state when the exhaust gas recirculation rate is made larger than that in the first operation control is shown. For example, the higher the exhaust gas recirculation rate, the slower the combustion in the combustion chamber and the lower the combustibility. In order to perform stable combustion in the combustion chamber, it is preferable that the pressure when igniting in the combustion chamber is higher and the temperature is higher.

  In the third operational control of the present embodiment, control is performed to increase the load threshold related to the ignition timing for stabilizing combustion in the combustion chamber. In the operation control example of FIG. 12, as indicated by an arrow 111, the load threshold is moved to a load threshold Ltc that is larger than the load threshold Lta of the first operation control. The ignition timing is kept constant in the region below the load threshold Ltc. The fixed ignition timing is delayed from the first operation control.

  In the third operation control, when the control for reducing the combustibility in the combustion chamber is performed, the load threshold is set large to retard the fixed ignition timing. With this control, it is possible to increase the load region in which the advance of the ignition timing is stopped as the engine load decreases. Since the fixed ignition timing is delayed, the compression of the air-fuel mixture and the temperature increase in the combustion chamber can be promoted in a wider load region. When exhaust gas recirculation control is introduced, a decrease in combustibility in the combustion chamber can be suppressed.

  In the third operation control, the exhaust gas recirculation rate functions as a variable corresponding to the combustibility of the fuel in the combustion chamber. During the operation period of the internal combustion engine, it is possible to detect the variable corresponding to the combustibility and perform control to increase the load threshold as it is determined that the decrease in combustibility in the combustion chamber is large. For example, the exhaust gas recirculation rate is detected, and the load threshold Ltc can be controlled to increase as the exhaust gas recirculation rate increases. As the exhaust gas recirculation rate increases, the engine load region for maintaining the ignition timing constant can be increased, and the retard amount of the fixed ignition timing can be increased.

  In the case of performing the third operation control, for example, a map of a mechanical compression ratio, an intake valve closing timing and an ignition timing, which is a function of the opening of the EGR control valve in addition to the required load and the engine speed, is electronically controlled in advance. It can be stored in the unit. In this map, in the region below the load threshold Ltc, it can be determined that the ignition timing is maintained at the fixed ignition timing. In the operation control, the required load, the engine speed and the opening degree of the EGR control valve can be detected, and the ignition timing and the like can be determined based on the detected required load and the like.

  The control for reducing the combustibility in the combustion chamber can be exemplified by control for increasing the target air-fuel ratio in the combustion chamber in addition to control for increasing the recirculation rate of the exhaust gas. For example, in an internal combustion engine that performs lean burn operation, the air-fuel ratio during combustion (combustion air-fuel ratio) is higher than the stoichiometric air-fuel ratio. When the fuel is diluted, the combustibility in the combustion chamber is reduced. As the air-fuel ratio at the time of combustion in the combustion chamber is larger than the stoichiometric air-fuel ratio, the combustibility in the combustion chamber decreases. For this reason, in order to perform stable combustion, it is preferable to ignite at a higher pressure and a higher temperature.

  As with the control for increasing the exhaust gas recirculation rate, for example, the control for increasing the air-fuel ratio during combustion detects the air-fuel ratio during combustion in the combustion chamber, and the higher the air-fuel ratio during combustion (lean) Thus, it is possible to perform control to increase the load threshold Ltc and increase the retard amount of the fixed ignition timing. As the air-fuel ratio at the time of combustion, the air-fuel ratio of the exhaust gas can be detected, or the target air-fuel ratio can be detected.

  As described above, as the combustibility in the combustion chamber decreases, it is possible to increase the load threshold related to the ignition timing and perform the control to retard the fixed ignition timing. With this control, even if control for reducing combustibility is introduced, it is possible to suppress deterioration in combustibility in the combustion chamber.

  The third operation control is not limited to the control that gradually changes the load threshold based on the variable corresponding to the combustibility in the combustion chamber, and has a determination value related to the combustibility in the combustion chamber and corresponds to the combustibility. When it is determined that the variable exceeds the determination value and the flammability has decreased, the load threshold regarding the ignition timing may be increased.

  FIG. 13 shows a graph of the fourth operation control of the internal combustion engine in the present embodiment. In the fourth operation control, when control for reducing the combustibility in the combustion chamber such as exhaust gas recirculation control is introduced, in addition to the third operation control, the fixed mechanical compression ratio is reduced, Control to advance the fixed valve closing timing. In FIG. 13, the third operation control is described by a broken line, and the fourth operation control is described by a solid line.

  In the fourth operation control, the fixed mechanical compression ratio is reduced as compared with the third operation control when the control for reducing the combustibility in the combustion chamber is introduced. When the engine load is decreased from a high load, the increase in the mechanical compression ratio, that is, the increase in the expansion ratio is stopped at the engine load Ly. In the region below the engine load Ly, the mechanical compression ratio is maintained at a fixed mechanical compression ratio. The engine load Ly is larger than the engine load L1, and the load region maintained at the fixed mechanical compression ratio is larger than that in the third operation control. Even at the closing timing of the intake valve, the closing timing of the intake valve is maintained at a fixed closing timing in a region below the engine load Ly. The fixed valve closing timing is advanced from the third operation control.

  In the above-described third operation control, control for retarding the fixed ignition timing is performed to stabilize combustion in the low load region, and thermal efficiency is reduced. In the fourth operation control of the present embodiment, when the exhaust gas recirculation rate is high, the fixed mechanical compression ratio is lowered and the fixed valve closing timing is advanced more than when the recirculation rate is low. .

  By performing this control, the fixed valve closing timing of the intake valve can be advanced while the actual compression ratio in the combustion chamber is maintained at a substantially constant compression ratio. The fixed valve closing timing can be moved to the bottom dead center side, and the advance margin of the ignition timing can be increased. As a result, the load threshold value Ltd of the fourth operation control can be made smaller than the load threshold value Ltc of the third operation control, as indicated by an arrow 112. The fixed ignition timing can be advanced from the third operation control. As the engine load decreases, the region where the ignition timing is advanced can be increased, and the thermal efficiency can be improved as compared with the third operation control.

  By the way, generally, when the expansion ratio is lowered, the thermal efficiency is lowered. In the fourth operation control, since the fixed mechanical compression ratio is lowered, the expansion ratio is also lowered. However, by advancing the fixed ignition timing, the overall thermal efficiency can be improved as compared with the third operation control even in consideration of the decrease in thermal efficiency due to the decrease in the expansion ratio.

  As described above, when the control for reducing the combustibility of the fuel in the combustion chamber is performed, the fixed mechanical compression ratio is lowered and the fixed valve closing timing is advanced. A load threshold for the ignition timing is determined based on the amount of decrease in the fixed mechanical compression ratio and the amount of advancement of the fixed valve closing timing. Is going. By performing this control, it is possible to suppress a decrease in the thermal efficiency of the internal combustion engine even if a control for suppressing a decrease in combustibility in the combustion chamber is performed.

  In the fourth operation control, the variable corresponding to the combustibility in the combustion chamber can be the exhaust gas recirculation rate. The internal combustion engine in the present embodiment includes a detection device that detects the exhaust gas recirculation rate, and can determine the degree of decrease in combustibility based on the detected recirculation rate. It can be determined that the greater the detected recirculation rate, the greater the decrease in combustibility. The larger the detected recirculation rate, the larger the amount of decrease in the fixed mechanical compression ratio and the larger the advance amount of the fixed valve closing timing. Further, based on the fixed mechanical compression ratio and the fixed valve closing timing, the ignition timing can be determined so that the ignition timing is constant in the region below the load threshold Ltd.

  Similarly, when the control for reducing the combustibility in the combustion chamber includes the control for increasing the air-fuel ratio at the time of combustion, the air-fuel ratio at the time of combustion in the combustion chamber is detected, and based on the air-fuel ratio at the time of combustion. The combustibility in the combustion chamber can be determined. It can be determined that the greater the detected air-fuel ratio during combustion, the greater the decrease in combustibility. As the detected air-fuel ratio at the time of combustion increases, it is possible to increase the amount of decrease in the fixed mechanical compression ratio and to increase the advance amount of the fixed valve closing timing. Furthermore, the fixed ignition timing can be determined based on the fixed mechanical compression ratio and the fixed valve closing timing.

  In the fourth operational control of the present embodiment, when the control for reducing the combustibility is performed, the amount of decrease in the fixed mechanical compression ratio and the fixed closing of the intake valve are controlled according to the magnitude of the decrease in combustibility. Although the advance amount of the valve timing is determined, the present invention is not limited to this mode, and a switching determination value for determining the combustibility in the combustion chamber is provided, and it is determined that the combustibility in the combustion chamber is inferior to the switching determination value The fixed mechanical compression ratio can be decreased by a predetermined amount, and the fixed valve closing timing can be advanced by a predetermined amount. On the other hand, when it is determined that the combustibility of the fuel in the combustion chamber is superior to the switching determination value, it is possible to inhibit the control to lower the fixed mechanical compression ratio and advance the fixed valve closing timing.

  For example, when a switching determination value for the exhaust gas recirculation rate is provided and the third operation control is performed when the recirculation rate is equal to or lower than the switching determination value, the recirculation rate becomes larger than the switching determination value. May perform the fourth operation control. Similarly, the fourth operation control may be performed when an air-fuel ratio switching determination value at the time of combustion is provided and the air-fuel ratio at the time of combustion becomes larger than the switching determination value.

  FIG. 14 shows a graph of the fifth operational control in the present embodiment. In the fifth operation control, when control for reducing combustibility in the combustion chamber is introduced, in addition to the fourth operation control in the present embodiment, the fixed valve closing timing of the intake valve is further advanced. Control. In FIG. 14, the fourth operation control is indicated by a broken line. The fifth operation control is indicated by a solid line.

  In the fifth operation control, the engine load Lz is an upper limit load for maintaining the closing timing of the intake valve at the fixed closing timing. In the fifth operation control, the engine load Lz is set larger than the engine load Ly at which the mechanical compression ratio reaches the fixed mechanical compression ratio. At the engine load Lz higher than the engine load Ly, the closing timing of the intake valve is kept constant.

  The fixed valve closing timing of the fifth operation control is advanced from the fixed valve closing timing of the fourth operation control. Since the abnormal combustion is less likely to occur at a low load than at a high load, the actual compression ratio can be increased as in the case of the second operation control. In the fourth operation control, the fixed valve closing timing can be advanced to increase the actual compression ratio in the region below the engine load Lz. Since the closing timing of the intake valve is advanced, compression of the air-fuel mixture and temperature increase can be promoted in the combustion chamber. For this reason, as indicated by an arrow 113, the load threshold value Lte related to the fixed ignition timing can be made smaller than the load threshold value Ltd in the fourth operation control. As the engine load decreases, the region where the ignition timing is advanced can be increased. The thermal efficiency can be improved as compared with the fourth operation control while suppressing a decrease in combustibility.

  FIG. 15 shows a graph of the sixth operational control in the present embodiment. In the sixth operation control, in addition to the third operation control that changes the load threshold depending on the magnitude of the exhaust gas recirculation rate, etc., a fixed valve that maintains the intake valve closing timing constant. Control to advance the timing. The fixed mechanical compression ratio is controlled so as not to decrease as in the third operation control. In the sixth operation control, as indicated by an arrow 114, the load threshold value Ltf relating to the closing timing of the intake valve can be made smaller than the load threshold value Ltc of the third operation control. Since the region where the ignition timing is advanced can be increased as the engine load decreases, the thermal efficiency can be improved as compared with the third operation control.

  In the present embodiment, an explanation has been given of an external EGR device including a recirculation passage and a recirculation valve as an exhaust gas recirculation device that recirculates exhaust gas. The device may include an internal EGR device. The internal EGR device can be configured to change the amount of exhaust gas generated in the previous combustion cycle supplied to the combustion chamber in the current combustion cycle. For example, the internal EGR device may include a device that changes the overlap period during which the intake valve and the exhaust valve are simultaneously open.

  In the present embodiment, the control for reducing the combustibility has been described by taking the control for recirculating exhaust gas and the control for increasing the air-fuel ratio at the time of combustion as examples. The present invention can be applied when performing arbitrary control for reducing the combustibility in the combustion chamber. For example, in an internal combustion engine in which a swirl control valve for improving combustibility in a combustion chamber is arranged, when some swirl control valves fail and control is performed to stop the generation of swirl flow in the combustion chamber, The present invention can be applied.

  Each of the above embodiments can be combined as appropriate. In the respective drawings described above, the same or corresponding parts are denoted by the same reference numerals. In addition, said embodiment is an illustration and does not limit invention. In the embodiment, the change shown in a claim is included.

DESCRIPTION OF SYMBOLS 1 Crankcase 2 Cylinder block 4 Piston 5 Combustion chamber 6 Spark plug 9 Exhaust valve 17 Exhaust valve 17 Throttle valve 21 Air-fuel ratio sensor 22 Relative position sensor 23 Valve timing sensor 25 Cam rotation angle sensor 26 EGR gas conduit 27 EGR control valve 30 Electronic control unit 56 Circular cam 57 Eccentric shaft 58 Circular cam 59 Drive motor 70 Cam shaft 72 Housing 73 Rotating shaft 85 Spool valve A Variable compression ratio mechanism B Variable valve timing mechanism

Claims (4)

A variable compression ratio mechanism that can change the mechanical compression ratio, a variable valve mechanism that can change the closing timing of the intake valve, and an ignition timing adjustment device that can change the ignition timing in the combustion chamber,
In the high load region, when the load increases, the intake valve closing timing is advanced to increase the amount of intake air flowing into the combustion chamber and reduce the mechanical compression ratio.
In the low load area, which is smaller than the high load, the mechanical compression ratio is maintained at a fixed fixed mechanical compression ratio, the intake valve closing timing is maintained at a fixed fixed valve closing timing, and the throttle is reduced when the load decreases. It is configured to control to reduce the amount of intake air flowing into the combustion chamber by reducing the opening of the valve,
It is formed to perform control to reduce the flammability of fuel in the combustion chamber,
It has a load threshold related to the ignition timing for stabilizing the combustibility of the fuel in the combustion chamber, and in the region of a load smaller than the load threshold, the ignition timing is maintained at a fixed ignition timing,
When control is performed to reduce the flammability of fuel in the combustion chamber, the fixed mechanical compression ratio is lowered and the fixed valve closing timing is advanced, and the fixed mechanical compression ratio reduction amount and the fixed valve closing timing advance amount are set. A spark ignition type internal combustion engine characterized in that a load threshold relating to ignition timing is determined based on the ignition timing.   The control for reducing the combustibility of the fuel in the combustion chamber includes at least one of control for increasing the exhaust gas recirculation rate and control for increasing the air-fuel ratio during combustion in the combustion chamber. Item 2. The spark ignition internal combustion engine according to Item 1. A detection device for detecting a variable corresponding to the combustibility of the fuel in the combustion chamber;
A variable corresponding to the combustibility is detected, and the amount of decrease in the fixed mechanical compression ratio is increased and the advance amount of the fixed valve closing timing is increased as it is determined that the decrease in combustibility in the combustion chamber is large. The spark ignition type internal combustion engine according to claim 1, wherein the spark ignition type internal combustion engine is provided.   The upper limit load for maintaining the closing timing of the intake valve at the fixed closing timing is set to be larger than the upper limit load for maintaining the mechanical compression ratio at the fixed mechanical compression ratio. 4. The spark ignition internal combustion engine according to claim 3.

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