JP4259546B2 - Spark ignition internal combustion engine - Google Patents

Description

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

  Equipped with a variable compression ratio mechanism that can change the mechanical compression ratio and a variable valve timing mechanism that can control the closing timing of the intake valve. A spark ignition type internal combustion engine that increases the mechanical compression ratio and delays the closing timing of the intake valve as the engine load decreases while the actual compression ratio is kept constant during the medium and high load operation. The engine is known (see, for example, Patent Document 1).

JP 2004-218522 A

  In general, in an internal combustion engine, the larger the expansion ratio, the longer the period during which the pressing force acts on the piston during the expansion stroke. Therefore, the greater the expansion ratio, the better the thermal efficiency. Therefore, in order to improve the thermal efficiency during engine operation, it is preferable to increase the mechanical compression ratio as much as possible to increase the expansion ratio.

  However, when the expansion ratio is increased in this way, most of the heat energy generated in the combustion chamber is converted into kinetic energy, so that the temperature of the exhaust gas is lowered. As a result, the pressure of the exhaust gas in the combustion chamber at the end of the expansion stroke is also lowered, so that the exhaust gas is hardly discharged from the combustion chamber. Such a tendency is particularly prominent when the expansion ratio is 20 or more.

  On the other hand, the exhaust purification catalyst provided in the engine exhaust passage generally cannot exhibit a good exhaust purification action unless the temperature is raised above a certain temperature. For this reason, in many internal combustion engines, the exhaust purification catalyst is maintained at a high temperature by the heat of the exhaust gas discharged from the engine body.

  However, as described above, when the expansion ratio is increased, the temperature of the exhaust gas is lowered, so that the temperature at which the exhaust purification catalyst is heated by the exhaust gas per unit flow rate is lowered. Further, if the expansion ratio is increased, the exhaust gas is not easily discharged from the combustion chamber, so the flow rate of the exhaust gas flowing into the exhaust purification catalyst is reduced. For this reason, when the internal combustion engine is operated with a large expansion ratio, it becomes difficult to maintain the temperature of the exhaust purification catalyst at a high temperature.

  Accordingly, an object of the present invention is to provide a spark ignition type internal combustion engine that can maintain the temperature of the exhaust purification catalyst at a relatively high temperature even when the internal combustion engine is operated in a state where the expansion ratio is large.

In order to solve the above problems, in the first invention, a variable compression ratio mechanism capable of changing a mechanical compression ratio, an actual compression action start timing changing mechanism capable of changing an actual compression action start time, an exhaust valve, comprising a, to maximize the mechanical compression ratio so that the maximum expansion ratio is obtained at the time of engine low load operation, and at the maximum expansion ratio is 20 or more, a further closing timing of the exhaust valve at the time of engine low load operation Provided is a spark ignition internal combustion engine within 10 ° before and after intake top dead center .

In order to solve the above problems, in the second invention, a variable compression ratio mechanism capable of changing a mechanical compression ratio, an actual compression action start timing changing mechanism capable of changing an actual compression action start timing, an exhaust valve ; and a changeable exhaust variable valve timing mechanism the closing timing, to maximize the mechanical compression ratio so that the maximum expansion ratio is obtained at the time of engine low load operation, and at the maximum expansion ratio is 20 or more, the engine Provided is a spark ignition type internal combustion engine in which a region in which the valve closing timing of an exhaust valve can be set during low load operation is limited to the intake top dead center side than during engine high load operation.

In the third invention, in the second invention, the closing timing of the exhaust valve is set to be within 10 ° before and after the intake top dead center at the time of engine low load operation.
According to a fourth invention, in the second invention, an intake variable valve timing mechanism capable of changing the opening timing of the intake valve is further provided, and the intake valve opening period and the exhaust valve opening period during engine low load operation. The valve closing timing of the exhaust valve and the valve opening timing of the intake valve are controlled so that the overlap period in which the valve overlaps is minimized.
According to a fifth invention, in the second invention, an intake variable valve timing mechanism capable of changing the valve opening timing of the intake valve is further provided, and the intake valve opening period and the exhaust valve opening period during engine low load operation. The valve closing timing of the exhaust valve and the valve opening timing of the intake valve are controlled so that the overlap period in which they overlap is zero.

In the sixth invention, the intake air in the first or second aspect, further capable of changing the intake valve opening timing changing mechanism the opening timing of the intake valve comprises a valve opening timing of the intake valve at the time of engine low load operation Within 10 ° before and after top dead center .
In the seventh invention, in the first or second aspect of the invention, the actual compression ratio at the time of engine low load operation is an engine medium and high load operation and the same actual compression ratio.
According to an eighth aspect, in the seventh aspect, the actual compression ratio is within a range of 9 to 11 regardless of the engine load at the time of low engine speed.
In a ninth aspect, in the eighth aspect, the actual compression ratio is increased as the engine speed increases.

In a tenth aspect of the invention, in the first or second aspect of the invention, the actual compression action start timing changing mechanism comprises an intake variable valve timing mechanism capable of changing the closing timing of the intake valve.
In an eleventh aspect, in the tenth aspect, the amount of intake air supplied into the combustion chamber is controlled by changing the closing timing of the intake valve.
In the twelfth invention, in the eleventh invention, the closing timing of the intake valve is from the intake bottom dead center until the limit closing timing at which the amount of intake air supplied to the combustion chamber can be controlled as the engine load decreases. It can be moved away.
In a thirteenth aspect according to the twelfth aspect, in the twelfth aspect, the amount of intake air supplied into the combustion chamber is the engine intake air in a region where the load is higher than the engine load when the closing timing of the intake valve reaches the limit closing timing. It is controlled by changing the valve closing timing of the intake valve without depending on the throttle valve arranged in the passage.

In a fourteenth aspect, in the thirteenth aspect, the throttle valve is kept fully open in a region where the load is higher than the engine load when the closing timing of the intake valve reaches the limit closing timing.
According to a fifteenth aspect, in the twelfth aspect, combustion is performed by a throttle valve disposed in the engine intake passage in a region where the load is lower than the engine load when the valve closing timing of the intake valve reaches the limit valve closing timing. The amount of intake air supplied into the room is controlled.
In a sixteenth aspect, in the twelfth aspect, the air-fuel ratio is increased as the load decreases in a region where the load is lower than the engine load when the closing timing of the intake valve reaches the limit closing timing.

In a seventeenth aspect according to the twelfth aspect, in the twelfth aspect, in the region where the load is lower than the engine load when the closing timing of the intake valve reaches the limit closing timing, the closing timing of the intake valve is the limit closing timing. Retained.
In an eighteenth aspect based on the first or second aspect, the mechanical compression ratio is increased to a limit mechanical compression ratio as the engine load decreases.
In a nineteenth aspect, in the eighteenth aspect, the mechanical compression ratio is maintained at the limit mechanical compression ratio in a region where the load is lower than the engine load when the mechanical compression ratio reaches the limit mechanical compression ratio.

  According to the present invention, as much exhaust gas as possible is discharged from the combustion chamber to the exhaust purification catalyst, so that the temperature of the exhaust purification catalyst is maintained at a relatively high temperature even when the internal combustion engine is operated with a large expansion ratio. can do.

FIG. 1 shows a side sectional view of a spark ignition type internal combustion engine.
Referring to FIG. 1, 1 is a crankcase, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is a spark plug disposed at the center of the top surface of the combustion chamber 5, and 7 is an intake air 8 is an intake port, 9 is an exhaust valve, and 10 is an exhaust port. The intake port 8 is connected to a surge tank 12 via an intake branch pipe 11, and a fuel injection valve 13 for injecting fuel into the corresponding intake port 8 is arranged in each intake branch pipe 11. The fuel injection valve 13 may be arranged in each combustion chamber 5 instead of being attached to each intake branch pipe 11.

  The surge tank 12 is connected to the outlet of the compressor 15a of the exhaust turbocharger 15 via an intake duct 14, and the inlet of the compressor 15a is connected to an air cleaner 17 via an intake air amount detector 16 using, for example, heat rays. A throttle valve 19 driven by an actuator 18 is disposed in the intake duct 14.

  On the other hand, the exhaust port 10 is connected to an inlet of an exhaust turbine 15b of an exhaust turbocharger 15 via an exhaust manifold 20, and an outlet of the exhaust turbine 15b is connected to a catalytic converter 22 containing an exhaust purification catalyst via an exhaust pipe 21. The An air-fuel ratio sensor 23 is disposed in the exhaust pipe 21.

  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 axis 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, and the closing timing of the intake valve 7 can be controlled in order to change the actual start timing of the compression action, and An intake variable valve timing mechanism B capable of individually controlling the opening timing of the intake valve 7 is provided, and an exhaust variable valve timing mechanism C capable of individually controlling the opening timing and closing timing of the exhaust valve 7. Is provided.

  The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36. It comprises. The output signal of the intake air amount detector 16 and the output signal of the air-fuel ratio sensor 23 are input to the input port 35 via the corresponding AD converters 37, respectively. A load sensor 41 that generates an output voltage proportional to the amount of depression 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. The Further, a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 30 ° is connected to the input port 35. On the other hand, the output port 36 is connected to the spark plug 6, the fuel injection valve 13, the throttle valve driving actuator 18, the variable compression ratio mechanism A, and the intake variable valve timing mechanism B through corresponding drive circuits 38.

  2 is an exploded perspective view of the variable compression ratio mechanism A shown in FIG. 1, and FIG. 3 is a side sectional view of the internal combustion engine schematically shown. Referring to FIG. 2, a plurality of projecting portions 50 spaced from each other are formed below both side walls of the cylinder block 2, and cam insertion holes 51 each having a circular cross section are formed in each projecting portion 50. Has been. 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 with a space between each other. Cam insertion holes 53 each having a circular cross section are formed.

  As shown in FIG. 2, a pair of camshafts 54 and 55 are provided, and on each camshaft 54 and 55, a circular cam 56 is rotatably inserted into each cam insertion hole 51. It is fixed. These circular cams 56 are coaxial with the rotational axes of the camshafts 54 and 55. On the other hand, an eccentric shaft 57 arranged eccentrically with respect to the rotation axis of each camshaft 54, 55 extends between the circular cams 56 as shown by hatching in FIG. A cam 58 is eccentrically mounted for rotation. As shown in FIG. 2, the circular cams 58 are disposed between the circular cams 56, and the circular cams 58 are rotatably inserted into the corresponding cam insertion holes 53.

  When the circular cams 56 fixed on the camshafts 54 and 55 are rotated in opposite directions as indicated by solid arrows in FIG. 3A from the state shown in FIG. 3 moves toward the lower center, so that the circular cam 58 rotates in the opposite direction to the circular cam 56 in the cam insertion hole 53 as shown by the broken arrow in FIG. As shown, when the eccentric shaft 57 moves to the lower center, the center of the circular cam 58 moves below the eccentric shaft 57.

  3A and 3B, the relative positions of the crankcase 1 and the cylinder block 2 are determined by the distance between the center of the circular cam 56 and the center of the circular cam 58. The cylinder block 2 moves away from the crankcase 1 as the distance between the center and the center of the circular cam 58 increases. 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 worm gears 61 and 62 having opposite spiral directions are attached to the rotation shaft of the drive motor 59, respectively. Gears 63 and 64 that mesh with the worm gears 61 and 62 are fixed to end portions 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 A shown in FIGS. 1 to 3 shows an example, and any type of variable compression ratio mechanism can be used.

  On the other hand, FIG. 4 shows an intake variable valve timing mechanism B provided for the camshaft 70 for driving the intake valve 7 in FIG. As shown in FIG. 4, the intake variable valve timing mechanism B is attached to one end of the camshaft 70 to change the cam phase of the camshaft 70, the cam phase changing portion B1, and the camshaft 70 and the intake valve 7. The cam operating angle changing portion B2 is disposed between the valve lifter 24 and changes the cam operating angle of the camshaft 70 to a different operating angle and transmits the cam operating angle to the intake valve 7. As for the cam working angle changing portion B2, a side sectional view and a plan view are shown in FIG.

  First, the cam phase changing unit B1 of the intake variable valve timing mechanism B will be described. The cam phase changing unit B1 is rotated by the crankshaft of the engine in the direction of the arrow through the timing belt, and the timing pulley 71. A cylindrical housing 72 that rotates together with the camshaft 70, a rotating shaft 73 that rotates together with the camshaft 70 and that can rotate relative to the cylindrical housing 72, and an outer periphery of the rotating shaft 73 from the inner peripheral surface of the cylindrical housing 72. A plurality of partition walls 74 extending to the surface, and vanes 75 extending from the outer peripheral surface of the rotating shaft 73 to the inner peripheral surface of the cylindrical housing 72 between the partition walls 74 are provided on both sides of each vane 75. An advance hydraulic chamber 76 and a retard hydraulic chamber 77 are formed respectively.

  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 camshaft 70 is to be advanced, the spool valve 85 is moved downward in FIG. 4, and the hydraulic oil supplied from the supply port 82 enters the advance angle hydraulic chamber 76 via the hydraulic port 79. The hydraulic oil in the retarding hydraulic chamber 77 is supplied and discharged from the drain port 84. At this time, the rotation shaft 73 is rotated relative to the cylindrical housing 72 in the arrow X direction.

  On the other hand, when the cam phase of the camshaft 70 is to be retarded, the spool valve 85 is moved upward in FIG. 4 and the hydraulic oil supplied from the supply port 82 is used for retarding via the hydraulic port 80. The hydraulic oil in the advance hydraulic chamber 76 is discharged from the drain port 83 while being supplied to the hydraulic chamber 77. At this time, the rotating shaft 73 is rotated relative to the cylindrical housing 72 in the direction opposite to the arrow X.

  If the spool valve 85 is returned to the neutral position shown in FIG. 4 while the rotation shaft 73 is rotated relative to the cylindrical housing 72, the relative rotation operation of the rotation shaft 73 is stopped, and the rotation shaft 73 The relative rotation position at that time is held. Therefore, the cam phase changing portion B1 can advance or retard the cam phase of the camshaft 70 by a desired amount. That is, the valve opening timing of the intake valve 7 can be arbitrarily advanced or retarded by the cam phase changing unit B1.

  Next, the cam working angle changing portion B2 of the intake variable valve timing mechanism B will be described. The cam working angle changing portion B2 is arranged in parallel with the camshaft 70 and is moved in the axial direction by the actuator 91. An intermediate cam 94 slidably engaged with an axially extending spline 93 formed on the control rod 90 and engaged with the cam 92 of the camshaft 70, and for driving the intake valve 7 A swing cam 96 that engages with the valve lifter 24 and slidably engages with a spiral extending spline 95 formed on the control rod 90 is provided. Is formed.

  When the camshaft 70 is rotated, the intermediate cam 94 is always swung by a certain angle by the cam 92. At this time, the rocking cam 96 is also swung by a certain angle. On the other hand, the intermediate cam 94 and the swing cam 96 are supported so as not to move in the axial direction of the control rod 90. Therefore, when the control rod 90 is moved in the axial direction by the actuator 91, the swing cam 96 is intermediate. It is rotated relative to the cam 94.

  When the cam 97 of the swing cam 96 starts to engage with the valve lifter 24 when the cam 92 of the cam shaft 70 starts to engage with the intermediate cam 94 due to the relative rotational positional relationship between the intermediate cam 94 and the swing cam 96. As shown by a in FIG. 5B, the valve opening period and lift of the intake valve 7 become the largest. On the other hand, when the swing cam 96 is rotated relative to the intermediate cam 94 in the direction of the arrow Y by the actuator 91, the cam 92 of the camshaft 70 is engaged with the intermediate cam 94 for a while. Thereafter, the cam 97 of the swing cam 96 is engaged with the valve lifter 24. In this case, as indicated by b in FIG. 5B, the valve opening period and the lift amount of the intake valve 7 are smaller than a.

  When the swing cam 96 is further rotated relative to the intermediate cam 94 in the direction of the arrow Y in FIG. 4, the valve opening period and the lift amount of the intake valve 7 are further reduced as indicated by c in FIG. . That is, the valve opening period of the intake valve 7 can be arbitrarily changed by changing the relative rotational position of the intermediate cam 94 and the swing cam 96 by the actuator 91. However, in this case, the lift amount of the intake valve 7 becomes smaller as the opening period of the intake valve 7 becomes shorter.

  Thus, the cam phase changing unit B1 can arbitrarily change the valve opening timing of the intake valve 7, and the cam operating angle changing unit B2 can arbitrarily change the valve opening period of the intake valve 7, so that the cam phase By both the change part B1 and the cam working angle change part B2, that is, by the intake variable valve timing mechanism B, the valve opening timing and the valve opening period of the intake valve 7, that is, the valve opening timing and the valve closing timing of the intake valve 7 Can be arbitrarily changed.

  The intake variable valve timing mechanism B shown in FIGS. 1 and 4 shows an example, and various types of variable valve timing mechanisms other than the examples shown in FIGS. 1 and 4 can be used.

  The exhaust variable valve timing mechanism C has basically the same configuration as the intake variable valve timing mechanism B, and the opening timing and opening period of the exhaust valve 9, that is, the opening timing of the exhaust valve 9 are The valve closing timing can be arbitrarily changed.

  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 action 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 most basic features of 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 compression bottom dead center. In the example shown in FIG. 8A, the combustion chamber volume is 50 ml, and the stroke volume of the piston is 500 ml, as in the examples 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, in this situation, the present inventor has studied to increase the theoretical thermal efficiency by strictly dividing the mechanical compression ratio and the actual compression ratio, and as a result, the theoretical thermal efficiency is governed by the expansion ratio, and the theoretical thermal efficiency It was found that the actual compression ratio 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 in FIG. 7 indicates the theoretical thermal efficiency when the expansion ratio is increased with the actual compression ratio fixed at 10. Thus, the theoretical thermal efficiency when the expansion ratio is increased while maintaining the actual compression ratio at a low value, and the theoretical thermal efficiency when the actual compression ratio is increased 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 the intake 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 intake valve closing timing is delayed by the intake variable valve timing mechanism B 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 to 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.

  As described above, in general, in an internal combustion engine, the lower the engine load, the worse the thermal efficiency. Therefore, in order to improve the thermal efficiency when the vehicle is running, that is, to improve the fuel efficiency, the thermal efficiency during the engine low load operation is reduced. It is necessary to improve. 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, the super high expansion ratio cycle shown in FIG. 8 (B) is used during low engine load operation, and the normal cycle shown in FIG. 8 (A) is used during high engine load operation. This is the basic feature of the present invention.

FIG. 9 shows the overall operation control during steady operation at a low engine speed. The overall operation control will be described below with reference to FIG.
FIG. 9 shows changes in the mechanical compression ratio, the expansion ratio, the closing timing of the intake valve 7, the actual compression ratio, the intake air amount, the opening degree of the throttle valve 17 and the pumping loss according to the engine load. In the present embodiment, the average air-fuel ratio in the combustion chamber 5 is usually the air-fuel ratio sensor 23 so that unburned HC, CO and NO _x in the exhaust gas can be simultaneously reduced by the three-way catalyst in the catalytic converter 22. Feedback control to the stoichiometric air-fuel ratio based on the output signal.

  As described above, the normal cycle shown in FIG. 8A 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 or almost fully open, so that the pumping loss is zero.

  On the other hand, as shown in FIG. 9, when the engine load is lowered, the mechanical compression ratio is increased accordingly, and thus the expansion ratio is also increased. Further, at this time, the valve closing timing of the intake valve 7 is delayed as the engine load becomes lower as shown by the solid line in FIG. 9 so that the actual compression ratio is kept substantially constant. At this time as well, the throttle valve 17 is kept fully open or substantially fully open, and therefore the amount of intake air supplied into the combustion chamber 5 changes the closing timing of the intake valve 7 regardless of the throttle valve 17. Is controlled by that. Also at this time, the pumping loss becomes zero.

  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 decreased in proportion to the decrease in 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, since the air-fuel ratio in the combustion chamber 5 is the stoichiometric air-fuel ratio, the volume of the combustion chamber 5 when the piston 4 reaches the compression top dead center changes in proportion to the fuel amount. Become.

When the engine load is further reduced, the mechanical compression ratio is further increased. When the mechanical compression ratio reaches the limit mechanical compression ratio that is the structural limit of the combustion chamber 5, the engine when the mechanical compression ratio reaches the limit mechanical compression ratio is reached. In the region where the load is lower than the load L ₁ , the mechanical compression ratio is maintained at the limit mechanical compression ratio. Therefore, the mechanical compression ratio is maximized and the expansion ratio is maximized during engine low load operation. In other words, in the present invention, the mechanical compression ratio is maximized so that the maximum expansion ratio can be obtained during engine low load operation. Further, at this time, the actual compression ratio is maintained at substantially the same actual compression ratio as that during the engine medium and high load operation.

On the other hand, as shown by the solid line in FIG. 9, the closing timing of the intake valve 7 is delayed to the limit closing timing that can control the amount of intake air supplied into the combustion chamber 5 as the engine load becomes lower. closing timing of the valve 7 is the closing timing of the intake valve 7 in the region of a load lower than the engine load L ₂ when reaching the limit closing timing is held at the limit closing timing. If the closing timing of the intake valve 7 is held at the limit closing timing, the intake air amount can no longer be controlled by the change in the closing timing of the intake valve 7, so it is necessary to control the intake air amount by some other method There is.

In the embodiment shown in FIG. 9, at this time, that is, in a region where the load is lower than the engine load L ₂ when the closing timing of the intake valve 7 has reached the limit closing timing, the throttle valve 17 supplies the fuel into the combustion chamber 5. The amount of intake air to be controlled is controlled. However, if the intake air amount is controlled by the throttle valve 17, the pumping loss increases as shown in FIG.

In order to prevent such a pumping loss, the throttle valve 17 is kept fully open or almost fully open in a region where the load is lower than the engine load L ₂ when the closing timing of the intake valve 7 reaches the limit closing timing. In this state, the air-fuel ratio can be increased as the engine load decreases. At this time, it is preferable to arrange the fuel injection valve 13 in the combustion chamber 5 and perform stratified combustion.

  As shown in FIG. 9, the actual compression ratio is maintained substantially constant regardless of the engine load at the time of low engine speed. The actual compression ratio at this time is approximately within a range of ± 10%, preferably within a range of ± 5% with respect to the actual compression ratio at the time of engine medium and high load operation. In this embodiment, the actual compression ratio at the time of low engine rotation is approximately 10 ± 1, that is, between 9 and 11. However, when the engine speed increases, the air-fuel mixture in the combustion chamber 5 is disturbed, so that knocking does not easily occur. Therefore, in the embodiment according to the present invention, the actual compression ratio increases as the engine speed increases.

  On the other hand, as described above, the expansion ratio is 26 in the ultra-high expansion ratio cycle shown in FIG. This expansion ratio is preferably as high as possible, but if it is 20 or more, a considerably high theoretical thermal efficiency can be obtained. Therefore, in the present invention, the variable compression ratio mechanism A is formed so that the expansion ratio is 20 or more.

  In the example shown in FIG. 9, the mechanical compression ratio is continuously changed according to the engine load. However, the mechanical compression ratio can be changed stepwise according to the engine load.

On the other hand, as shown by the broken line in FIG. 9, the intake air amount can be controlled without depending on the throttle valve 17 by advancing the closing timing of the intake valve 7 as the engine load becomes lower. Therefore, if expressed in FIG. 9 so as to include both the case indicated by the solid line and the case indicated by the broken line, in this embodiment, the valve closing timing of the intake valve 7 is set in the combustion chamber as the engine load decreases. limit closing timing capable of controlling the amount of intake air fed to L ₂ will be moved in the direction away from intake bottom dead center BDC to.

Next, the closing timing of the exhaust valve 9 will be described focusing on the low load operation in which the super high expansion ratio cycle shown in FIG. 8B is executed.
In general, during a low load operation in which an ultra-high expansion ratio cycle is performed, the amount of heat generated by combustion of the air-fuel mixture in the combustion chamber 5 is small, so the temperature of the exhaust gas discharged from the combustion chamber 5 is low. Easy to be. In addition, in the internal combustion engine, the larger the expansion ratio, the longer the period during which the pressing force acts on the piston during the expansion stroke, so that much of the heat energy generated by the combustion of the air-fuel mixture in the combustion chamber is reduced. This is converted into kinetic energy of the piston, and accordingly, the temperature of the combustion gas in the combustion chamber 5 at the end of the expansion stroke is lowered. For this reason, when the ultra-high expansion ratio cycle shown in FIG. 8B is executed, the temperature of the exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20 during the exhaust stroke is extremely low. Such a tendency is particularly noticeable when the expansion ratio is set to 20 or more. The combustion chamber is used when an ultra-high expansion ratio cycle with an expansion ratio of 20 or more is executed and when a normal cycle with an expansion ratio of about 12 is used. The temperature of the exhaust gas discharged from 5 differs by about 100 ° C.

On the other hand, in many internal combustion engines, in order to purify harmful components (for example, HC, CO, NO _x, etc.) contained in the exhaust gas, an oxidation catalyst, a three-way catalyst, a NO _x storage reduction catalyst, etc. are disposed in the engine exhaust passage. An exhaust purification catalyst is provided. Such an exhaust purification catalyst cannot effectively purify harmful components in the exhaust gas unless the temperature becomes higher than the activation temperature. Here, in many internal combustion engines, since the temperature of the exhaust gas is considerably higher than the activation temperature, the temperature of the exhaust purification catalyst is maintained at or above the activation temperature by flowing the exhaust gas into the exhaust purification catalyst.

  However, when the ultra-high expansion ratio cycle shown in FIG. 8B is executed, the temperature of the exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20 is only slightly higher than the activation temperature. Even if the exhaust gas flows into the exhaust purification catalyst, it becomes difficult to maintain the temperature of the exhaust purification catalyst at or above its activation temperature. Therefore, in order to maintain the temperature of the exhaust purification catalyst at the activation temperature or higher when the ultra-high expansion ratio cycle is being executed, it is necessary to flow as much exhaust gas as possible into the exhaust purification catalyst.

  Now, with reference to FIG. 10, consider the relationship between the closing timing of the exhaust valve 9 and the flow rate of the exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20. FIG. 10A shows a case where the exhaust valve 9 is closed substantially at the intake top dead center, FIG. 10B shows a case where the exhaust valve 9 is closed earlier than the intake top dead center, and FIG. The lift changes of the exhaust valve 9 and the intake valve 7 when the valve 9 is closed later than the intake top dead center are shown.

  As shown in FIG. 10B, when the exhaust valve 9 is closed earlier than the intake top dead center, the volume of the combustion chamber 5 when the exhaust valve 9 is closed is that the piston is positioned at the intake top dead center. When the exhaust valve 9 is closed, the exhaust gas corresponding to the volume of the combustion chamber 5 when the valve is closed remains in the combustion chamber 5 after the exhaust valve 9 is closed. For this reason, a relatively large amount of exhaust gas remains in the combustion chamber 5 even after the exhaust valve 9 is closed, so that the exhaust gas in the combustion chamber 5 cannot be sufficiently discharged to the exhaust manifold 20. Therefore, the flow rate of the exhaust gas flowing into the exhaust purification catalyst is small.

  On the other hand, as shown in FIG. 10C, when the exhaust valve 9 is closed later than the intake top dead center, the exhaust valve 9 is opened even at the intake top dead center. When the top dead center is reached, most of the exhaust gas in the combustion chamber 5 flows out into the exhaust port 10. However, if the exhaust valve 9 is opened even after the intake top dead center, a part of the exhaust gas once flowing into the exhaust port 10 flows into the combustion chamber 5 again as the piston 4 descends. End up.

  In particular, when the ultra-high expansion ratio cycle is being executed, the combustion gas in the combustion chamber 5 expands considerably during the expansion stroke, so the pressure of the combustion gas at the end of the expansion stroke is relatively low. For this reason, the momentum of the exhaust gas flowing out from the combustion chamber 5 to the exhaust port 10 in the exhaust stroke is weak. Therefore, when the piston 4 descends after reaching the intake top dead center, a part of the exhaust gas that has flowed out to the exhaust port 10 once again burns It becomes easy to flow into the chamber 5.

  As described above, when the exhaust valve 9 is closed later than the intake top dead center, the exhaust gas that has once flowed out to the exhaust port 10 returns to the combustion chamber 5 again. The exhaust gas cannot be sufficiently discharged to the exhaust manifold 20, and the flow rate of the exhaust gas flowing into the exhaust purification catalyst becomes small.

  Therefore, in this embodiment, when the ultra-high expansion ratio cycle shown in FIG. 8B is executed, that is, when the mechanical compression ratio is high, the closing timing of the exhaust valve 9 is too early than the intake top dead center. In order to prevent the exhaust valve 9 from being too late, the region in which the valve closing timing of the exhaust valve 9 can be set is limited to the intake top dead center side.

  FIG. 11 is a diagram showing a region in which the valve closing timing of the exhaust valve 9 can be set according to the mechanical compression ratio. As shown in FIG. 11, the settable region of the exhaust valve 9 is a region between the settable maximum advance amount and maximum retard amount. As can be seen from the figure, the maximum advance angle that can be set for the closing timing of the exhaust valve 9 is smaller (slower) as the mechanical compression ratio becomes higher, and conversely, the maximum delay that can be set for the closing timing of the exhaust valve 9 The amount is reduced (faster) as the mechanical compression ratio increases. For this reason, the region in which the valve closing timing of the exhaust valve 9 can be set is smaller as the mechanical compression ratio is higher, that is, the region is limited as the mechanical compression ratio is higher. For example, as shown in FIG. 11, when the mechanical compression ratio is low, the region where the closing timing of the exhaust valve 9 can be set is ΔTOC1, whereas when the mechanical compression ratio is high, the closing timing of the exhaust valve 9 is The settable area is ΔTOC2 (ΔTOC2 <ΔTOC1).

  Alternatively, when the ultra-high expansion ratio cycle shown in FIG. 8B is executed, that is, when the mechanical compression ratio is high, the closing timing of the exhaust valve 9 may be earlier or later than the intake top dead center. In order to surely prevent this, as shown in FIG. 10A, the valve closing timing of the exhaust valve 9 may be set substantially at the intake top dead center.

  As described above, when the mechanical compression ratio is high, the region where the closing timing of the exhaust valve 9 can be set is limited to the intake top dead center side, or the closing timing of the exhaust valve 9 is almost set to the intake top dead center. As a result, the exhaust gas in the combustion chamber 5 can be sufficiently discharged to the exhaust manifold 20, and the flow rate of the exhaust gas flowing into the exhaust purification catalyst can be increased.

  That is, since the exhaust valve 9 is closed near the intake top dead center, as shown in FIG. 10B, the exhaust valve 9 is closed compared to the case where the exhaust valve 9 is closed earlier than the intake top dead center. The volume of the combustion chamber 5 when the valve is closed is small, so that the amount of exhaust gas remaining in the combustion chamber 5 after the exhaust valve 9 is closed can be reduced. Further, since the exhaust valve 9 is closed in the vicinity of the intake top dead center, the exhaust port 10 is compared with the case where the exhaust valve 9 is closed later than the intake top dead center as shown in FIG. The amount of exhaust gas that flows back into the combustion chamber 5 out of the exhaust gas that flows into the interior can be reduced. Therefore, when the exhaust valve 9 is closed in the vicinity of the intake top dead center as shown in FIG. 10A, the exhaust valve 9 is inhaled as shown in FIGS. 10B and 10C. Compared to the case where the valve is closed away from the top dead center, the exhaust gas in the combustion chamber 5 is sufficiently discharged to the exhaust manifold 20 and the flow rate of the exhaust gas flowing into the exhaust purification catalyst is increased. Can do. As a result, the exhaust purification catalyst can be maintained at the activation temperature or higher even during low load operation in which the ultra-high expansion ratio cycle is executed.

  Note that the description of “substantially intake top dead center” indicates within about 10 ° before and after the intake top dead center, preferably within about 5 ° before and after the intake top dead center.

  Further, when the mechanical compression ratio is increased, the combustion chamber volume at the intake top dead center is reduced, and thus the exhaust valve 9 interferes with the piston 4 depending on the closing timing of the exhaust valve 9.

  FIG. 10 shows a piston interference line indicating the limit at which the exhaust valve 9 or the intake valve 7 interferes with the piston 4. When the lift curve of the exhaust valve 9 intersects with the piston interference line, the exhaust valve 9 interferes with the piston 4. Will do. Here, in FIG. 10C, the lift curve of the exhaust valve 9 intersects with the piston interference line. This means that when the exhaust valve 9 is closed later than the intake top dead center, the exhaust valve 9 and the piston 4 interfere with each other, depending on the degree of delay.

  On the other hand, according to this embodiment, when the mechanical compression ratio is high, the region where the closing timing of the exhaust valve 9 can be set is limited to the intake top dead center side, and in particular, the closing timing of the exhaust valve 9 is set. The maximum possible retard amount is reduced. For this reason, as shown in FIG. 10A, the exhaust valve 9 is prevented from interfering with the piston 4 even when the mechanical compression ratio is increased.

  By the way, when there is a valve overlap in which the valve opening period of the intake valve 7 and the valve opening period of the exhaust valve 9 overlap, the amount of exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20 in accordance with the valve overlap period. Changes. Hereinafter, with reference to FIG. 12, the relationship between the overlap period in which the valve opening period of the intake valve 7 and the valve opening period of the exhaust valve 9 overlap and the amount of exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20 will be described. Think. FIG. 12A shows the lift changes of the exhaust valve 9 and the intake valve 7 when the overlap period is zero, and FIG. 12B shows the lift changes of the exhaust valve 9 and the intake valve 7 when the overlap period is long.

  In general, when the intake valve 7 and the exhaust valve 9 are open at the same time, a part of the exhaust gas in the combustion chamber 5 and a part of the exhaust gas once flowing out from the combustion chamber 5 to the exhaust port 10 flow into the intake port 8. There are things to do. Thus, when a part of the exhaust gas flows into the intake port 8, the amount of exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20 is reduced accordingly.

  Accordingly, when the overlap period is long as shown in FIG. 12B, a large amount of exhaust gas often flows into the intake port 8 and is thus discharged from the combustion chamber 5 to the exhaust manifold 20. Exhaust gas often decreases. For this reason, in such a case, the flow rate of the exhaust gas flowing into the exhaust purification catalyst is reduced.

  Therefore, in this embodiment, when the ultra-high expansion ratio cycle shown in FIG. 8B is executed, that is, when the mechanical compression ratio is high, the overlap period can be set as shown in FIG. The valve closing timing of the exhaust valve 7 and the valve opening timing of the intake valve 9 are controlled so as to be the smallest in this range. Therefore, for example, in an internal combustion engine having a settable overlap period of 10 ° to 60 °, the overlap period is set to 10 ° when the mechanical compression ratio is high, and the settable overlap period is set to 0 ° to 50 °. In the internal combustion engine, the overlap period is set to 0 ° when the mechanical compression ratio is high.

  Thus, by minimizing the overlap period when the mechanical compression ratio is high, the exhaust gas flowing into the intake port 8 is reduced, so that the exhaust gas discharged from the combustion chamber 5 to the exhaust manifold 20 is Therefore, the flow rate of exhaust gas flowing into the exhaust purification catalyst increases.

  The overlap period when the mechanical compression ratio is high may not necessarily be the minimum as long as it is shorter than the overlap period when the mechanical compression ratio is low. Therefore, for example, the overlap period when the mechanical compression ratio is high may be 10 ° or less even if it is not the minimum in the settable range.

  Further, as described above, when the mechanical compression ratio is increased, the combustion chamber volume at the intake top dead center is reduced. Therefore, the intake valve 7 interferes with the piston 4 depending on the opening timing of the intake valve 7.

  FIG. 12 shows a piston interference line indicating the limit at which the exhaust valve 9 or the intake valve 7 interferes with the piston 4. When the lift curve of the intake valve 7 intersects with the piston interference line, the intake valve 7 interferes with the piston 4. Will do. Here, in FIG. 12B, the lift curve of the intake valve 7 intersects with the piston interference line. This means that if the overlap period is increased, the intake valve 7 and the piston 4 interfere with each other. That is, in this embodiment, as described above, the closing timing of the exhaust valve 9 is almost the intake top dead center, so that the large overlap period means that the opening timing of the intake valve 7 is greatly advanced. Means. Thus, if the valve opening timing of the intake valve 7 is greatly advanced, the intake valve 7 and the piston 4 interfere with each other.

  On the other hand, according to the present embodiment, when the mechanical compression ratio is high, the overlap period is minimized, so that the valve opening timing of the intake valve 7 is substantially at or near the intake top dead center. For this reason, as shown in FIG. 12A, the intake valve 7 is prevented from interfering with the piston even when the mechanical compression ratio becomes high.

  FIG. 13 shows a control routine for operation control of the spark ignition type internal combustion engine of the present embodiment. Referring to FIG. 13, first, at step 101, the engine load L and the engine speed Ne are acquired. Next, in step 102, the target actual compression ratio is calculated from the map shown in FIG. As shown in FIG. 14A, the target actual compression ratio increases as the engine speed Ne increases. Next, at step 103, the mechanical compression ratio CR is calculated from the map shown in FIG. That is, the mechanical compression ratio CR required to make the actual compression ratio the target actual compression ratio is stored in advance in the ROM 32 as a function of the engine load L and the engine speed Ne in the form of a map as shown in FIG. The mechanical compression ratio CR is calculated from this map.

  Further, the valve closing timing IC of the intake valve 7 necessary for supplying the required intake air amount into the combustion chamber 5 is a map as shown in FIG. 14C as a function of the engine load L and the engine speed Ne. Is stored in the ROM 32 in advance. In step 104, the valve closing timing IC of the intake valve 7 is calculated from this map.

Then, in step 105, the engine load L is whether less than a predetermined value L ₃ is determined. Here, for example, the predetermined value L ₃ is equal to an engine load in which if the engine load is further reduced, the temperature of the exhaust purification catalyst may be lowered below its activation temperature as the temperature of the exhaust gas decreases. Value. If it is determined in step 105 that the engine load L is smaller than the predetermined value L ₃ , the process proceeds to step 106. In step 106, the valve closing timing EC of the exhaust valve 9 is set to the intake top dead center, and then in step 107, the overlap period ΔOL is minimized and the routine proceeds to step 110.

On the other hand, if it is determined in step 105 that the engine load is equal to or greater than the predetermined value L ₃ , the process proceeds to step 108. In step 108, the valve closing timing EC of the exhaust valve 9 is calculated from the map shown in FIG. 15A. Next, in step 109, the overlap period ΔOL is calculated from the map shown in FIG. That is, the valve closing timing EC and the overlap period ΔOL of the exhaust valve 9 are stored in advance in the ROM 32 as a function of the engine load L and the engine speed Ne in the form of a map as shown in FIGS. From this map, the valve closing timing EC and the overlap period ΔOL of the exhaust valve 9 are calculated. Thereafter, the process proceeds to step 110.

  In step 110, the variable compression ratio mechanism A is controlled so that the mechanical compression ratio becomes the mechanical compression ratio CR, the closing timing of the intake valve 7 becomes the closing timing IC, and the overlapping period is the overlapping period ΔOL. The intake variable valve timing mechanism B is controlled so that Further, the exhaust variable valve timing mechanism C is controlled so that the closing timing of the exhaust valve 9 becomes the closing timing EC.

1 is an overall view of a spark ignition internal combustion engine. It is a disassembled perspective view of a variable compression ratio mechanism. 1 is a schematic side sectional view of an internal combustion engine. It is a figure which shows a variable valve timing mechanism. It is a figure which shows the lift amount of an intake valve and an exhaust valve. It is a figure for demonstrating a mechanical compression ratio, an actual compression ratio, and an expansion ratio. It is a figure which shows the relationship between theoretical thermal efficiency and an expansion ratio. It is a figure for demonstrating a normal cycle and a super-high expansion ratio cycle. It is a figure which shows changes, such as a mechanical compression ratio according to an engine load. It is a figure which shows the lift change of an intake valve and an exhaust valve. It is a figure which shows the area | region which can set the valve closing timing of an exhaust valve according to mechanical compression ratio. It is a figure which shows the lift change of an intake valve and an exhaust valve. It is a flowchart for performing operation control. It is a figure which shows a target actual compression ratio etc. It is a figure which shows the map etc. of the valve closing timing of an exhaust valve.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Crankcase 2 Cylinder block 3 Cylinder head 4 Piston 5 Combustion chamber 7 Intake valve 9 Exhaust valve A Variable compression ratio mechanism B Intake variable valve timing mechanism C Exhaust variable valve timing mechanism

Claims (19)

It has a variable compression ratio mechanism that can change the mechanical compression ratio, an actual compression action start timing changing mechanism that can change the actual compression action start time, and an exhaust valve. Spark ignition is performed so that the mechanical compression ratio is maximized so that the maximum expansion ratio is 20 or more and the closing timing of the exhaust valve is within 10 ° before and after the intake top dead center during low engine load operation. Internal combustion engine. A variable compression ratio mechanism capable of changing the mechanical compression ratio, an actual compression action start timing changing mechanism capable of changing the actual compression action start timing, and an exhaust variable valve timing mechanism capable of changing the exhaust valve closing timing. provided, it maximizes the mechanical compression ratio so that the maximum expansion ratio is obtained at the time of engine low load operation, and at the maximum expansion ratio is 20 or more, the engine low load during operation of the closing timing of the exhaust valve can be set A spark ignition type internal combustion engine in which the critical region is restricted to the intake top dead center side than during engine high load operation. The spark ignition type internal combustion engine according to claim 2, wherein the closing timing of the exhaust valve is within 10 ° before and after the intake top dead center during engine low load operation.   An intake variable valve timing mechanism that can change the opening timing of the intake valve is further provided so that the overlap period in which the intake valve open period and the exhaust valve open period overlap during engine low load operation is minimized. The spark ignition type internal combustion engine according to claim 2, wherein a valve closing timing of the exhaust valve and a valve opening timing of the intake valve are controlled.   An intake variable valve timing mechanism that can change the valve opening timing of the intake valve is further provided so that the overlap period in which the valve opening period of the intake valve overlaps the valve opening period of the exhaust valve becomes zero during low engine load operation. The spark ignition internal combustion engine according to claim 2, wherein the closing timing of the exhaust valve and the opening timing of the intake valve are controlled. The intake valve opening timing changing mechanism capable of changing the opening timing of the intake valve is further provided, and the opening timing of the intake valve is set within 10 ° before and after the intake top dead center at the time of engine low load operation. 2. The spark ignition internal combustion engine according to 2. Engine the actual compression ratio at the time of low load operation is an engine medium and high load operation and the same actual compression ratio, spark-ignition internal combustion engine according to claim 1 or 2.   The spark ignition internal combustion engine according to claim 7, wherein the actual compression ratio is set in a range of 9 to 11 regardless of the engine load at a low engine speed.   The spark ignition internal combustion engine according to claim 8, wherein the actual compression ratio is increased as the engine speed increases.   The spark ignition type internal combustion engine according to claim 1 or 2, wherein the actual compression action start timing changing mechanism comprises an intake variable valve timing mechanism capable of changing a closing timing of the intake valve.   The spark ignition internal combustion engine according to claim 10, wherein the intake air amount supplied into the combustion chamber is controlled by changing a closing timing of the intake valve.   The valve closing timing of the intake valve is moved in a direction away from the intake bottom dead center until the limit valve closing timing at which the amount of intake air supplied to the combustion chamber can be controlled as the engine load decreases. Spark ignition internal combustion engine.   In the region where the load of the intake valve is higher than the engine load when the closing timing of the intake valve reaches the above limit closing timing, the amount of intake air supplied into the combustion chamber does not depend on the throttle valve disposed in the engine intake passage. The spark ignition internal combustion engine according to claim 12, which is controlled by changing a closing timing of the intake valve.   The spark ignition internal combustion engine according to claim 13, wherein the throttle valve is held in a fully open state in a region where the load is higher than the engine load when the closing timing of the intake valve reaches the limit closing timing.   In the region where the load is lower than the engine load when the intake valve closing timing reaches the above limit closing timing, the amount of intake air supplied into the combustion chamber is controlled by the throttle valve disposed in the engine intake passage. The spark ignition internal combustion engine according to claim 12.   The spark ignition internal combustion engine according to claim 12, wherein the air-fuel ratio is increased as the load decreases in a region where the load is lower than the engine load when the closing timing of the intake valve reaches the limit closing timing.   The spark according to claim 12, wherein the closing timing of the intake valve is held at the limit closing timing in a region where the load is lower than the engine load when the closing timing of the intake valve reaches the limit closing timing. Ignition internal combustion engine.   The spark ignition internal combustion engine according to claim 1 or 2, wherein the mechanical compression ratio is increased to a limit mechanical compression ratio as the engine load decreases.   19. The spark ignition internal combustion engine according to claim 18, wherein the mechanical compression ratio is maintained at the limit mechanical compression ratio in a region where the load is lower than the engine load when the mechanical compression ratio reaches the limit mechanical compression ratio.

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