JP2007239602A - Variable compression ratio engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable compression ratio engine enabling ultra lean combustion of low NOx emission not requiring a NOx conversion device such as a NOx trap catalyst in a light load range. <P>SOLUTION: This engine is provided with an operation condition detection means detecting an operation condition, an air fuel ratio control means 70 adjusting air fuel ratio of air fuel mixture, compression ratio change means 51-53 changing compression ratio of a combustion chamber 30, a fuel injection control means 70 controlling fuel injection timing at which fuel is injected to an intake port, and an operation condition control means 70 capable of changing fuel injection timing by extending lean air fuel ratio limit by increasing air fuel ratio of the combustion chamber according to load and making air fuel ratio lean close to extended lean air fuel ratio limit when the operation condition is in a light load range lighter than predetermined load. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a variable compression ratio engine.

  When the air-fuel ratio is lean, the engine consumes less fuel and improves fuel efficiency. However, when the air-fuel ratio is slightly leaner than the stoichiometric air-fuel ratio (about 15) (air-fuel ratio 16-17), the amount of nitrogen oxide (NOx) discharged becomes very large. When the air-fuel ratio is further lean, the amount of NOx emission is reduced compared to when the air-fuel ratio is about 16, but the amount of NOx emission is still large. Further, the engine cannot be operated because the in-cylinder combustion becomes unstable because the amount of fuel contained in the air-fuel mixture is too small beyond the lean air-fuel ratio limit. Therefore, in the conventional so-called lean burn engine, in-cylinder combustion does not become unstable, and in order to improve fuel efficiency, the air-fuel ratio is made lean within a range that does not exceed the lean air-fuel ratio limit. A large amount of exhausted NOx needs to be purified by a catalyst, and a NOx purification catalyst used for this purpose (for example, a NOx trap catalyst that traps NOx and then purifies NOx) is expensive. Further, since fuel or rich combustion gas is required to reduce NOx, the fuel consumption reduction effect is small.

  By the way, it is known that the compression ratio should be increased to expand the lean air-fuel ratio limit. That is, if the compression ratio is high, the combustion chamber volume at the time of ignition is small, so that it is easy to ignite even with a small amount of fuel. Therefore, in-cylinder combustion is stably performed even when the air-fuel ratio is in a leaner range.

Therefore, the engine described in Patent Document 1 makes the compression ratio variable by changing the volume of the combustion chamber with a variable volume piston provided in the cylinder head, and increases the compression ratio to increase the lean air-fuel ratio limit at low loads. This makes it possible to operate at a leaner air-fuel ratio.
Japanese Patent Laid-Open No. 60-240837

  However, the above-described conventional variable compression ratio engine can certainly increase the lean combustion stability limit by increasing the compression ratio, make the air-fuel ratio leaner, and reduce NOx. However, in such an engine, the limit is that the air-fuel ratio is at most about 24 to 25, and NOx cannot be reduced until the NOx trap catalyst or the like can be completely abolished.

  The present invention has been made paying attention to such a conventional problem, and a variable compression ratio engine that enables ultra lean combustion with a small amount of NOx emission so that a NOx purifying device such as a NOx trap catalyst is unnecessary. The purpose is to provide.

  The present invention solves the above problems by the following means. In addition, in order to make an understanding easy, although the code | symbol corresponding to embodiment of this invention is attached | subjected, it is not limited to this.

  The present invention includes an operating state detecting means (step S1) for detecting an operating state, an air / fuel ratio control means (70) for adjusting the air / fuel ratio of the air / fuel mixture, and a compression ratio change for changing the compression ratio of the combustion chamber (30). Means (51-53), fuel injection control means (70) for controlling the fuel injection timing of the fuel injected into the intake port, and when the operating state is a low load region lower than a predetermined load, depending on the load And an operating state control means (70) for increasing the compression ratio of the combustion chamber to expand the lean air-fuel ratio limit, diluting the air-fuel ratio to near the expanded lean air-fuel ratio limit, and changing the fuel injection timing. It is characterized by that.

  According to the present invention, by increasing the compression ratio in the low load region, the lean air-fuel ratio limit can be expanded and the combustion of the lean air-fuel mixture can be stabilized. At this time, by delaying the fuel injection timing so that fuel injection is performed when the intake valve is open, it is possible to prevent the injected fuel from staying in the vicinity of the intake port and being homogenized. As a result, the injected fuel is fed to the vicinity of the spark plug while maintaining the dense portion, and the combustion of the lean air-fuel mixture can be further stabilized. In this way, it is possible to realize ultra lean combustion that hardly generates NOx in the low load region.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings.
(First embodiment)
FIG. 1 is a diagram showing a first embodiment of a variable compression ratio engine according to the present invention.

  The inventors of the present invention have been diligently involved in a variable compression ratio engine (hereinafter referred to as a “multi-link variable compression ratio engine”) using a multi-link mechanism that connects a piston and a crankshaft by two links as shown in FIG. Research is repeated. In this multi-link variable compression ratio engine, the piston and crankshaft are connected by a single link (connecting rod), and the piston is higher than a normal engine (hereinafter referred to as “normal engine”) with a constant compression ratio. There is a characteristic that the period of staying in the vicinity of the dead center is long (for example, Japanese Patent Laid-Open No. 2002-285857 for details).

  Further research by the inventors has revealed that if this characteristic is utilized, the lean combustion stability limit of a multi-link variable compression ratio engine can be expanded, and NOx emissions can be almost eliminated.

  The present invention has been made based on such findings of the inventors.

  First, a multi-link variable compression ratio engine will be described. FIG. 1 is a diagram showing a multi-link variable compression ratio engine employed in the present embodiment.

  As shown in FIG. 1, the engine 10 has a combustion chamber 30 inside. The engine 10 includes an intake port 56 that guides outside air to the combustion chamber 30 and an exhaust port 62 that discharges exhaust gas after combustion.

  An intake valve 55 is provided between the combustion chamber 30 and the intake port 56. The intake valve 55 adjusts the amount of intake air to the combustion chamber 30 by opening and closing between the intake port 56 and the combustion chamber 30. Further, the intake port 56 includes a fuel injection valve 41. The fuel injection valve 41 is connected to a fuel tank (not shown) and supplies fuel to the combustion chamber 30. The fuel injection valve 41 injects fuel in the form of a mist and mixes it with outside air to generate an air-fuel mixture. The air-fuel mixture supplied to the combustion chamber 30 is ignited by a spark plug 42.

  An exhaust valve 61 is provided between the combustion chamber 30 and the exhaust port 62. The exhaust valve 61 opens and closes between the exhaust port 62 and the combustion chamber 30 to discharge exhaust gas. As will be described later, the opening / closing timing of the exhaust valve 61 can be changed. In the present embodiment, the EGR amount can be adjusted by controlling the opening / closing timing of the exhaust valve 61.

  The multi-link variable compression ratio engine 10 connects the piston 32 and the crankshaft 33 with two links (an upper link (first link) 11 and a lower link (second link) 12) and a control link (third The link) 13 controls the posture of the lower link 12 to change the engine compression ratio.

  The upper link 11 has an upper end connected to the piston 32 via the piston pin 21 and a lower end connected to one end of the lower link 12 via the connection pin 22. The piston 32 receives the combustion pressure and reciprocates in the cylinder 31 a of the cylinder block 31.

  One end of the lower link 12 is connected to the upper link 11 via a connecting pin 22, and the other end is connected to the control link 13 via a connecting pin 23. Further, the lower link 12 is inserted into the substantially central connecting hole with the crankpin 33b of the crankshaft 33, and rotates around the crankpin 33b. The lower link 12 is configured to be split into two left and right members. The crankshaft 33 includes a plurality of journals 33a and a crankpin 33b. The journal 33 a is rotatably supported by the cylinder block 31 and the ladder frame 34. The crank pin 33b is eccentric by a predetermined amount from the journal 33a, and the lower link 12 is rotatably connected thereto.

  The control link 13 has a connecting pin 23 inserted at the tip thereof and is connected to the lower link 12 so as to be rotatable. The other end of the control link 13 is connected to the control shaft 25 via an eccentric connecting pin 24. The control link 13 swings about the eccentric connecting pin 24. A gear is formed on the control shaft 25, and the gear meshes with a pinion 53 provided on the rotating shaft 52 of the actuator 51. The control shaft 25 is rotated by the actuator 51, and the eccentric connecting pin 24 moves.

  Control of these mechanisms is performed by the controller 70 in accordance with the operating state. The controller 70 includes a microcomputer having a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and an input / output interface (I / O interface).

  The controller 70 controls the actuator 51 to rotate the control shaft 25 to change the compression ratio. The controller 70 controls the fuel injection amount and injection timing of the fuel injection valve 41 provided in the intake port 56. Further, the controller 70 controls the ignition timing of the ignition plug 42 provided in the cylinder head. Further, the controller 70 controls the opening / closing timing of the exhaust valve 61 to adjust the EGR amount.

  Next, fuel injection control in the present embodiment will be described. The fuel injection amount is controlled by controlling the energization time to the fuel injection valve 41. Therefore, if the energization time is lengthened, the fuel injection amount is increased. The controller 70 derives an operation state based on information acquired from various sensors, and calculates a necessary fuel injection amount. The controller 70 energizes the fuel injection valve 41 for a time necessary to inject this amount of fuel.

  In the present embodiment, the timing of fuel injection is controlled to generate a dark portion and a thin portion in the air-fuel mixture. Then, by injecting the fuel so that the rich and thin portions of the air-fuel mixture are alternately arranged, combustion energy is generated larger than that in the case where the concentration is uniform, and the combustibility is improved. FIG. 2 is a diagram showing a mechanism for controlling the fuel injection timing to generate a rich portion and a thin portion in the air-fuel mixture. FIGS. 2 (A) and 2 (B) show normal fuel injection control, and FIG. 2 (C). (D) shows fuel injection control for generating a dark portion and a thin portion in the air-fuel mixture. 2A and 2C show the energization of the fuel injection valve 41, and FIGS. 2B and 2D show the concentration distribution of the injected fuel.

  In normal fuel injection, the fuel injection valve 41 is energized continuously for a certain period of time as shown in FIG. By doing so, the injection port of the fuel injection valve 41 is opened for a certain time, and a predetermined amount of fuel is injected. Accordingly, a relatively homogeneous air-fuel mixture is generated as shown in FIG.

  On the other hand, in this embodiment, as shown in FIG. 2C, the fuel injection valve 41 is intermittently energized. As described above, by intermittently repeating the fuel injection and stop, it is possible to generate an air-fuel mixture in which a dark portion and a thin portion as shown in FIG.

  A combustion process of an air-fuel mixture in which dark portions and thin portions are alternately arranged as shown in FIG. When ignition is performed on a portion where the air-fuel mixture is rich, relatively large combustion energy can be obtained. The combustion of the dense part can surely propagate the flame even to the air-fuel mixture in the thin part. And this combustion flame is propagated to the rich part of the air-fuel mixture, and a large combustion energy is obtained again. In the present embodiment, the combustion flame is grown by repeating such combustion. In this way, even if the amount of fuel injection is small, by producing a rich portion in the air-fuel mixture, combustion energy can be generated and combustion can be improved compared to combustion at a uniform concentration.

  The control shown in FIG. 2C is performed in an operation region where the load is relatively low, and a normal fuel as shown in FIG. 2A is used in a medium and high load region where a torque output exceeding a certain level is required. Injection control is performed.

  In the present embodiment, the fuel injection timing (end timing of fuel injection) is delayed so that fuel injection is performed while the intake valve 55 is open in the low load region. Further, in normal control, fuel injection is completed before the intake valve 55 is opened. For this reason, the generated air-fuel mixture once stays in the vicinity of the intake port 56 and then is sucked into the combustion chamber 30. Therefore, when fuel injection is performed at a normal timing, even if the air-fuel mixture is generated so that the thick portions and the thin portions are alternately arranged as described above, the fuel is stirred before being taken into the combustion chamber. Probability is high. Therefore, in the present embodiment, by injecting fuel when the intake valve 55 is open, the generated air-fuel mixture can be fed into the combustion chamber 30 without being stirred.

  3A and 3B are diagrams for explaining an opening / closing timing adjustment mechanism of the exhaust valve 61. FIG. 3A shows a state when the valve is opened, and FIG. 3B shows a state when the valve is closed. In the present embodiment, exhaust gas is left in the combustion chamber by closing the exhaust valve 61 before exhaust is completed, and the EGR effect is obtained by this exhaust gas. Therefore, in order to control the EGR amount, a mechanism for opening and closing the exhaust valve 61 independently of the rotation of the crankshaft is required.

  As a mechanism for adjusting the opening / closing timing of the exhaust valve 61, for example, a mechanism for changing the opening / closing lift amount and the opening / closing timing as disclosed in Japanese Patent Application Laid-Open No. 2004-346825, and a mechanism for adjusting the opening / closing timing of the exhaust valve 61 are disclosed. There is a drive mechanism that can change the opening and closing timing. Here, what is based on an electromagnetic drive mechanism is demonstrated easily.

  The exhaust valve 61 opens and closes the exhaust port 62 by being seated on or separated from the valve seat 63. A mover 65 is fixed to the valve stem 64 of the exhaust valve 61. The mover 65 is connected to the upper spring 66 and the lower spring 67. The upper spring 66 biases the mover 65 downward in the figure and the lower spring 67 biases the mover 65 downward in the figure to elastically support it in the axial direction. The upper spring 66 and the lower spring 67 loosely insert the valve stem 64.

  Above and below the mover 65, a valve opening electromagnetic coil 68 and a valve closing electromagnetic coil 69 are provided. The mover 65 is made of a magnetic material. When one of these electromagnetic coils is energized, the mover 65 is attracted to the electromagnetic coil by the generated magnetic force. The valve opening electromagnetic coil 68 has the lower spring 67 inserted therein, and the valve closing electromagnetic coil 69 has the upper spring 66 inserted therein.

  Next, the opening / closing operation of the exhaust valve 61 will be described. In a state where power supply to both the valve opening electromagnetic coil 68 and the valve closing electromagnetic coil 69 is interrupted, the mover 65 is placed between the electromagnetic coils 68 and 69 by the elastic force of the upper spring 66 and the lower spring 67. Located in. When the valve opening electromagnetic coil 68 is energized, as shown in FIG. 3A, the mover 65 is attracted to the valve opening electromagnetic coil 68 and moves in the direction of arrow C to open the exhaust port 62. On the other hand, when the valve closing electromagnetic coil 69 is energized, the mover 65 is attracted to the valve closing electromagnetic coil 69 and moves in the direction of arrow D as shown in FIG. 3B, thereby closing the exhaust port 62.

  By switching the energization of the electromagnetic coils 68 and 69 in this way, the exhaust valve 61 can be opened and closed independently of the rotation of the crankshaft.

  FIG. 4 is a diagram for explaining a compression ratio changing method by a multi-link variable compression ratio engine.

  By rotating the control shaft 25 and changing the position of the eccentric connecting pin 24, the engine compression ratio is changed. For example, as shown in FIGS. 4 (A) and 4 (C), when the eccentric connecting pin 24 is set to the position A, the top dead center position becomes high and the compression ratio becomes high.

  Then, as shown in FIGS. 4B and 4C, when the eccentric connecting pin 24 is set to the position B, the control link 13 is pushed upward, and the position of the connecting pin 23 is raised. As a result, the lower link 12 rotates counterclockwise about the crank pin 33b, the connecting pin 22 is lowered, and the position of the piston 32 at the piston top dead center (TDC) is lowered. Therefore, the compression ratio becomes a low compression ratio.

  FIG. 5 is a diagram showing the piston behavior, and FIG. 5 (A) is an enlarged view of a dotted line part of FIG. 5 (B).

  As described above, the multi-link variable compression ratio engine has a longer period in which the piston stays near the top dead center than the normal engine having the same compression ratio. This point will be described with reference to FIG. In FIG. 5, the piston behavior of a multi-link variable compression ratio engine having the same compression ratio as that of the normal engine is shown by a thin solid line. From this figure, it can be seen that the multi-link variable compression ratio engine has a longer period during which the piston stays near the top dead center than the normal engine having the same compression ratio.

  Furthermore, when the piston is within a predetermined distance from the top dead center and the piston is near the top dead center, the direction when the piston is near the top dead center when the compression ratio of the multi-link variable compression ratio engine is high. However, the staying period near the top dead center of the piston is longer than when it is near the top dead center when the compression ratio is low. That is, in FIG. 5B, L1> L2.

  Thus, the multi-link variable compression ratio engine has a longer period during which the piston stays near the top dead center than the normal engine. Furthermore, the higher the compression ratio, the longer the piston stays near top dead center. The fact that the piston stays in the vicinity of the top dead center means that the high compression state is maintained for a long time during combustion. If the high compression state is maintained for a long time, relatively high combustion energy can be obtained even in the case of ultra lean combustion, so that the combustibility is stabilized. Moreover, if the stroke characteristics of the piston are made similar to simple vibrations, the vibration of the entire engine can be reduced.

  Since the multi-link variable compression ratio engine has such characteristics, it has the characteristics shown in FIG. FIG. 6A is a diagram showing the relationship between the air-fuel ratio and the combustion stability. The thin line in the figure is a normal engine, and the thick line is a multi-link variable compression ratio engine.

  As can be seen from this figure, the air-fuel ratio that can ensure combustion stability in a normal engine (compression ratio of about 8 to 12) is about 22.

  On the other hand, according to the multi-link variable compression ratio engine, since the stay time near the top dead center of the piston is long, the state where the compression ratio is high becomes long, and the combustion stability limit is not easily lost. Further, by increasing the compression ratio (for example, about compression ratio 18), stable combustion can be performed up to an air-fuel ratio A / F of about 30.

  FIG. 6B is a diagram showing the relationship between the air-fuel ratio and the amount of exhausted NOx in a multi-link variable compression ratio engine. The thick line in the figure is for a high compression ratio, and the thin line is for a low compression ratio.

  From this figure, it can be seen that although the amount of NOx discharged is lower when the compression ratio is lower, if the air-fuel ratio is increased to about 30 or more, almost no NOx is discharged regardless of the compression ratio.

  From the above, using these characteristics, the present invention controls the compression ratio, air-fuel ratio, and ignition energy as follows. These controls are executed by the controller 70.

  FIG. 7 is a main flowchart of the control logic executed by the controller 70 according to the present invention.

  In step S1, the current driving state is acquired based on information detected by various sensors. Specifically, it is determined whether the current operation load region belongs to a low load region, a medium load region, or a high load region based on an air intake amount, a fuel injection amount, or the like.

  In step S2, it is determined whether or not the driving load region acquired in step S1 belongs to a low load region. If the operating state belongs to the low load range, the process proceeds to step S3, and if not, the process proceeds to step S4.

  In step S3, low load operation control is performed. In the low load operation control, ultra lean combustion is performed so that almost no NOx is discharged. Specific processing will be described later.

  In step S4, it is determined whether or not the operation load region belongs to the medium load region. If it belongs to the middle load region, the process is shifted to step S5 to perform middle load operation control, and if it does not belong to the middle load region, the process is shifted to step S6 to perform high load operation control.

  FIG. 8 is a flowchart showing a subroutine for low load operation control. In the low load operation control, super lean combustion with an air-fuel ratio A / F of 30 or more is performed to reduce the NOx emission amount to substantially zero. In order to perform ultra-lean combustion stably, control such as a high compression ratio is performed.

  In step S31, an air-fuel ratio A / F corresponding to the load is set. At this time, the air-fuel ratio A / F is set to a value of 30 or more that hardly generates NOx.

  In step S32, a compression ratio ε corresponding to the load is set. At this time, the compression ratio ε is set to a value equal to or greater than ε1 that enables stable combustion even with ultra lean combustion.

  In step S33, the fuel injection end timing is delayed until the intake valve 55 is open. Further, the fuel injection timing is set to be delayed as the load is low. By doing so, the fuel can be fed into the combustion chamber while maintaining the portion where the concentration of the injected fuel is high.

  In step S34, the fuel injection valve 41 is energized intermittently as shown in FIG. 2, thereby injecting fuel so that a rich portion and a thin portion are alternately generated in the air-fuel mixture. Improves flammability.

  In step S35, the opening / closing timing of the exhaust valve 61 is controlled to make the EGR amount substantially zero.

  FIG. 9 is a flowchart showing a subroutine for medium load operation control. Medium load operation control is generally the same as conventional control.

  In step S51, the air-fuel ratio A / F is set to stoichiometric. This is because the ultra-lean combustion with an air-fuel ratio of 30 or more cannot achieve the required engine output.

  In step S52, a compression ratio ε corresponding to the load is set. This compression ratio ε is not a high compression ratio, but is within the range of a normal compression ratio. This is because when the combustion is performed at an air-fuel ratio of 30 or less, if the compression ratio is increased, the amount of NOx generated is increased (FIG. 6B). Therefore, the air-fuel ratio in the intermediate region from the ultra-lean region to the stoichiometric region is not operated, but when the intermediate load region is reached, the air-fuel ratio is set to stoichiometric and the operation is performed at the normal compression ratio.

  In step S53, the fuel injection timing is set to a normal timing. This is because, since the air-fuel ratio A / F is set to stoichiometric, sufficient combustion stability can be ensured even with normal fuel injection.

  In step S54, normal fuel injection in which the fuel injection valve 41 is continuously energized is set.

  In step S55, in order to reduce NOx, the opening / closing timing of the exhaust valve 61 is controlled to adjust the EGR amount according to the load.

  FIG. 10 is a flowchart showing a subroutine for high load operation control. In high-load operation control, the occurrence of knocking is suppressed.

  In step S61, the air-fuel ratio A / F is set slightly richer than stoichiometric.

  In step S62, the compression ratio ε is set. At this time, the compression ratio ε is set to be lower as the load is higher, and the compression ratio ε is set to the minimum compression ratio εmin when a certain load or more is required. This is because if the compression ratio is too high during high load operation, the temperature in the combustion chamber rises and knocking is likely to occur.

  In step S63, the fuel injection timing is set to a normal timing in the same manner as in the middle load range control.

  In step S64, normal fuel injection is performed in the same manner as in the middle load region control.

  In step S65, in order to give priority to output, the opening / closing timing of the exhaust valve 61 is controlled to make the EGR amount substantially zero.

  FIG. 11 is a diagram for explaining the operation and effect when the control according to the present invention is executed. 11A is the air-fuel ratio, FIG. 11B is the EGR, FIG. 11C is the compression ratio, FIG. 11D is the fuel injection end timing, FIG. 11E shows the amount of NOx generated.

  According to the present embodiment, in the low load range (Yes in S2), the air-fuel ratio A / F is set to 30 or more, and ultra lean combustion is performed (S31; FIG. 11 (A)). Even in such ultra lean combustion, the combustibility can be stabilized by increasing the compression ratio ε (S32; FIG. 11C). In the present embodiment, fuel mixture is intermittently generated to generate an air-fuel mixture in which dark portions and thin portions are alternately arranged. By burning such an air-fuel mixture, larger combustion energy can be generated than when an air-fuel mixture having a uniform concentration is combusted. The vertical axis in FIG. 11D indicates the end timing of fuel injection as described above, and the timing becomes slower as it goes upward in the figure. Therefore, the period during which the intake valve 55 is open is between “intake valve open” and “intake valve closed” on the vertical axis. In this embodiment, fuel injection is performed during the period when the intake valve 55 is open in the low load range (S33), and the end timing of fuel injection is delayed as the load decreases (FIG. 11D). By doing so, it can be sent to the combustion chamber while maintaining a rich portion of the air-fuel mixture, and the combustion stability can be further improved. In this way, ultra lean combustion can be realized, and the NOx emission amount can be made substantially zero (FIG. 11E).

  In addition, by reducing the air-fuel ratio as the load decreases in this way and increasing the compression ratio to ensure combustion stability, throttling for load adjustment is not required, non-throttle operation can be realized, and pump loss is reduced. Can be reduced.

  On the other hand, in this embodiment, the EGR amount is set to substantially zero, but an amount of EGR gas corresponding to the load may be introduced. For example, the temperature in the combustion chamber can be increased by increasing the amount of EGR as the load is lower, so that the combustion stability can be improved without significantly increasing the compression ratio, and the pump loss can be reduced.

  According to the present embodiment, when the engine is in the middle load range (Yes in S4), the engine output is insufficient, so that the ultra lean combustion cannot be performed. Therefore, the necessary engine output is ensured by setting the air-fuel ratio ε to stoichiometric (S51; FIG. 11A). Moreover, the combustibility can be further stabilized by increasing the compression ratio as the load is lower (S52; FIG. 11C). Furthermore, by introducing EGR gas according to the load (S54; FIG. 11B), NOx can be purified only by the three-way catalyst (FIG. 11E).

  Furthermore, by introducing EGR gas (S55; FIG. 11 (B)), the engine throttle opening is opened and the pump loss can be reduced. Further, knocking can be prevented by setting the compression ratio in the medium load region to be lower than the compression ratio in the low load region.

  According to this embodiment, when the load is high (No in S4), the compression ratio can be set lower than the compression ratio of the medium load to prevent knocking (S62; FIG. 11 (C)). .

(Second Embodiment)
FIG. 12 is a diagram showing the control of the second embodiment of the variable compression ratio engine according to the present invention.

  In the following embodiments, the same reference numerals are given to the portions that perform the same functions as those of the above-described embodiments, and overlapping descriptions are omitted as appropriate.

  In the present embodiment, as in the first embodiment, in the low load region, the air-fuel ratio A / F is set to 30 or more to perform ultra lean combustion (FIG. 12A), and the compression ratio ε is increased. At the same time (FIG. 12 (C)), stable fuel combustibility is ensured by performing fuel injection that generates light and shade in the air-fuel mixture. Even in the low load region, in the region where the load is relatively low, the fuel injection is terminated while the intake valve 55 is open, as in the first embodiment (FIG. 12D). Further, in a region where the load is relatively high even in the low load region, control according to the load is performed while setting the end timing of fuel injection later than that in the middle and high load regions (FIG. 12D).

  In the middle and high load range, the same control as in the first embodiment is performed.

  According to the present embodiment, as in the first embodiment, it is possible to improve the combustion stability by feeding into the combustion chamber while maintaining the rich portion of the air-fuel mixture in the low load region, and to substantially reduce the NOx emission amount to zero. (FIG. 12E). Further, in a relatively high load region even in a low load region, the time during which the air-fuel mixture stays can be shortened by delaying the end timing of fuel injection, and sufficient combustion stability can be ensured. By doing so, it is possible to suppress a rapid increase in the NOx emission amount when the operation state transitions from the low load region to the medium load region.

(Third embodiment)
FIG. 13 is a view showing a third embodiment of the variable compression ratio engine according to the present invention.

  In the present embodiment, the control direction and size of the control shaft 25 are changed as compared with the first embodiment. That is, in the first embodiment, the compression ratio is increased as the control shaft 25 is rotated to the right. In the present embodiment, the compression ratio is increased as the control shaft 25 is rotated counterclockwise (FIG. 13A). Further, in the present embodiment, the control shaft 25 is larger, and the distance from the center of the control shaft 25 to the eccentric connecting pin 24 is longer (FIG. 13A). By setting such dimensions, the piston behavior can be set more characteristically, that is, the behavior near the top dead center can be set extremely, and the piston ascending speed can be made faster than the descending speed. That is, the degree of L31 <L32 increases.

  In addition, since the higher the compression ratio is, the higher the compression ratio, the higher the compression ratio, the faster the piston rises. That is, L31 <L41. In addition, the piston lowers more slowly when the compression ratio is higher. That is, L32> L42.

  According to this embodiment, at a high compression ratio, the period of time until the piston rises quickly and reaches top dead center is short. Therefore, pre-ignition (self-ignition) can be prevented. Further, since the piston descends slowly, the stay time near the top dead center of the piston is long, and combustion stability can be ensured. On the other hand, since the lowering speed of the piston is high at a low compression ratio, knocking at the late stage of combustion can be prevented.

(Fourth embodiment)
14 is a view showing a piston structure of a fourth embodiment of the variable compression ratio engine according to the present invention, and FIG. 14 (A) is a perspective view (representing a state in which a part of the piston is cut). 14B is a cross-sectional view taken along line BB in FIG. 14A, and FIG. 14C is a cross-sectional view taken along line CC in FIG. FIG. 15 is a view showing the piston behavior.

  The piston 32 shown in FIG. 14 may be used for the variable compression ratio engine. That is, the piston skirt of the piston 32 is greatly shortened as shown in FIG.

  If such a piston 32 is used, the counterweight can pass through the side of the piston pin as shown in FIG. Therefore, by setting the upper link 11 to the minimum length and bringing the bottom dead center position of the piston 32 closest to the crankshaft 33, the piston stroke can be expanded accordingly. In order to achieve such a configuration, the strength of the piston skirt is a problem, but as shown in FIG. 15B, the upper link is formed at the top dead center position of the piston 32 using the characteristics of the multi-link mechanism. By making the link 11 substantially upright, the lateral load (thrust load) applied to the piston 32 can be reduced. Thereby, the strength of the piston skirt is ensured.

  The present invention is not limited to the embodiment described above, and various modifications and changes can be made within the scope of the technical idea, and it is obvious that these are equivalent to the present invention.

  For example, in the above embodiment, the EGR amount is adjusted by the so-called internal EGR method that is adjusted according to the opening / closing timing of the exhaust valve. However, the EGR passage that connects the exhaust passage and the intake passage is opened / closed by the EGR valve. The EGR amount may be adjusted by a so-called external EGR method.

1 is a diagram showing a first embodiment of a variable compression ratio engine according to the present invention. FIG. It is a figure explaining fuel injection control by the present invention. It is a figure explaining the opening-and-closing timing adjustment mechanism of an exhaust valve. It is a figure explaining the compression ratio change method by a multiple link type variable compression ratio engine. It is a figure which shows piston behavior. It is a figure which shows the characteristic of a multiple link type variable compression ratio engine. 4 is a main flowchart of specific control logic according to the present invention. It is a flowchart which shows the subroutine of low load operation control. It is a flowchart which shows the subroutine of medium load operation control. It is a flowchart which shows the subroutine of high load operation control. It is a figure explaining the effect | action and effect when performing control of 1st Embodiment of the variable compression ratio engine by this invention. It is a figure explaining the effect | action and effect when performing control of 2nd Embodiment of the variable compression ratio engine by this invention. It is a figure which shows the piston behavior of 3rd Embodiment of the variable compression ratio engine by this invention. It is a figure which shows the piston structure of 4th Embodiment of the variable compression ratio engine by this invention. It is a figure which shows piston behavior.

Explanation of symbols

10 Multi-link variable compression ratio engine 11 Upper link (first link)
12 Lower link (second link)
13 Control link (3rd link)
30 Combustion chamber 32 Piston 33 Crankshaft 41 Fuel injection valve 42 Spark plug 51 Actuator (compression ratio changing means)
52 Rotating shaft (Compression ratio changing means)
53 Pinion (compression ratio changing means)
55 Intake valve 56 Intake port 61 Exhaust valve 62 Exhaust port 70 Controller (operating state control means)
Step S1 Operating state detection means Steps S31, S51, S61 Air-fuel ratio control means Step S34 Fuel injection control means

Claims (21)

Driving state detecting means for detecting the driving state;
Air-fuel ratio control means for adjusting the air-fuel ratio of the air-fuel mixture;
Compression ratio changing means for changing the compression ratio of the combustion chamber;
Fuel injection control means for controlling the fuel injection timing of the fuel injected into the intake port;
When the operating state is a low load region lower than a predetermined load, the compression ratio of the combustion chamber is increased according to the load to expand the lean air-fuel ratio limit, and the air-fuel ratio is increased to near the expanded lean air-fuel ratio limit. Operating condition control means that can be diluted and change the fuel injection timing;
A variable compression ratio engine comprising: A piston connected to the crankshaft via a plurality of links;
The compression ratio changing means regulates the operation of at least one of the plurality of links and adjusts the top dead center position of the piston to change the compression ratio.
The variable compression ratio engine according to claim 1. The plurality of links are a first link coupled via a piston pin, and a second link pivotably coupled to the first link and rotatably mounted on the crankshaft,
The compression ratio changing means is connected to the second link and regulates the operation of the second link to change the compression ratio.
The variable compression ratio engine according to claim 2. Compared to a normal engine having a piston connected to the crankshaft through a single connecting rod, the staying period near the top dead center of the piston is long.
The variable compression ratio engine according to claim 3. The higher the compression ratio, the longer the stay period near top dead center.
The variable compression ratio engine according to claim 4. The time to reach a top dead center after moving a predetermined distance is shorter than the time to move a predetermined distance from the top dead center,
The variable compression ratio engine according to any one of claims 3 to 5, wherein the variable compression ratio engine is provided. The lower the compression ratio, the shorter the time to move the predetermined distance and reach the top dead center is shorter than the time to move the predetermined distance from the top dead center.
The variable compression ratio engine according to claim 6. Compared to a normal engine having a piston connected to a crankshaft via a single connecting rod, the stroke characteristics of the piston are characteristics close to simple vibrations.
The variable compression ratio engine according to any one of claims 3 to 7, wherein the variable compression ratio engine is provided. The operation state control means alternately generates a dark portion and a thin portion in the air-fuel mixture by intermittently injecting fuel in the low load region.
The variable compression ratio engine according to any one of claims 1 to 8, wherein the engine is a variable compression ratio engine. The operating state control means delays the fuel injection timing as the load is lower in the low load region.
The variable compression ratio engine according to any one of claims 1 to 9, wherein the engine is a variable compression ratio engine. The operating state control means delays the fuel injection timing so that fuel injection is performed while the intake valve is open when the operating state is in a load range lower than the predetermined load.
The variable compression ratio engine according to any one of claims 1 to 10, wherein the engine has a variable compression ratio. The operating state control means expands the lean air-fuel ratio limit to a compression ratio higher than a compression ratio in a load range higher than the predetermined load in the low load range.
The variable compression ratio engine according to any one of claims 1 to 11, wherein the engine is a variable compression ratio engine. The operating state control means dilutes the air-fuel ratio of the air-fuel mixture to near the lean air-fuel ratio limit in the low load region, thereby reducing the nitrogen oxides exhausted from the engine to the extent that the NOx purification catalyst is unnecessary.
The variable compression ratio engine according to any one of claims 1 to 12, wherein the variable compression ratio engine is provided. The operating state control means makes the nitrogen oxide discharged from the engine substantially zero by diluting the air-fuel ratio of the air-fuel mixture to near the lean air-fuel ratio limit in the low load range.
The variable compression ratio engine according to any one of claims 1 to 13, wherein the engine is a variable compression ratio engine. The operating state control means sets the air-fuel ratio of the air-fuel mixture to 30 or more in the low load range.
The variable compression ratio engine according to any one of claims 1 to 14, wherein the engine is a variable compression ratio engine. The operating state control means expands the lean air-fuel ratio limit by increasing the compression ratio as the load is lower in the low load region, and dilutes the air-fuel ratio to near the expanded lean air-fuel ratio limit.
The variable compression ratio engine according to any one of claims 1 to 15, wherein the engine is a variable compression ratio engine. It further comprises exhaust gas introduction means for introducing exhaust gas into the mixture,
The operating state control means sets the air-fuel ratio to a substantially stoichiometric air-fuel ratio in a load range higher than the predetermined load, and introduces exhaust gas into the mixture.
The variable compression ratio engine according to any one of claims 1 to 16, wherein the variable compression ratio engine is provided. The operating state control means, the lower the load in the load region higher than the predetermined load, the higher the compression ratio,
The variable compression ratio engine according to claim 17. The exhaust gas introducing means includes exhaust valve phase changing means for changing the opening / closing timing of the exhaust valve,
The exhaust gas is exhaust gas remaining in the cylinder by the early closing operation of the exhaust valve.
The variable compression ratio engine according to any one of claims 16 to 18, wherein the engine is a variable compression ratio engine. The exhaust gas introduction means includes an exhaust gas recirculation passage for recirculating a part of the exhaust gas flowing through the exhaust passage to the intake passage,
The exhaust gas is exhaust gas recirculated through the exhaust gas recirculation passage.
The variable compression ratio engine according to any one of claims 16 to 18, wherein the engine is a variable compression ratio engine. The amount of exhaust gas introduced is larger as the load is lower.
The variable compression ratio engine according to any one of claims 16 to 20, wherein the variable compression ratio engine is characterized by the above.