JP6350573B2 - Engine exhaust system - Google Patents

Description

  The present invention relates to an exhaust system for an engine.

  In accordance with the operating state of the engine body, a compression self-ignition combustion (hereinafter referred to as “CI combustion”) mode in which the air-fuel mixture is combusted by self-ignition, and spark ignition combustion (hereinafter referred to as “CI combustion”) in which the air-fuel mixture is ignited by an ignition plug , "SI combustion".) Engines that are switched between modes are known. It is also known to execute internal EGR in order to increase the in-cylinder temperature from the viewpoint of combustion stability in the CI combustion mode.

  By the way, in SI combustion, since thermal efficiency is low compared with CI combustion, combustion gas temperature becomes high. Therefore, immediately after switching from the SI combustion mode to the CI combustion mode, the temperature atmosphere in the cylinder is relatively high, and high-temperature exhaust gas due to SI combustion immediately before the CI combustion is introduced into the cylinder by internal EGR control. Will be. In that case, the temperature in the cylinder becomes too high, and in the CI combustion, the mixture is prematurely ignited, and the pressure increase rate (dP / dθ) in the cylinder becomes steep, which may cause combustion noise.

  On the other hand, Patent Document 1 describes that a transient mode is interposed when switching from the SI combustion mode to the CI combustion mode. In the transient mode, for example, fuel is injected into the cylinder after the middle of the compression stroke, and the temperature in the cylinder is forcibly reduced by the latent heat of vaporization of the fuel. This lowers the temperature of the exhaust gas introduced into the cylinder by the internal EGR immediately after switching to the CI combustion mode, thereby avoiding premature ignition of the air-fuel mixture and preventing the generation of combustion noise. It is.

  Patent Document 1 also describes that in CI combustion, the internal EGR is performed by opening the exhaust valve twice in each of the exhaust stroke and the intake stroke.

JP 2014-185623 A

  However, in the transient mode in which the fuel is injected into the cylinder after the middle of the compression stroke described in Patent Document 1, the fuel mixing period is shortened by the delay of the fuel injection timing, so that the combustion efficiency is lowered and the fuel consumption is reduced. There is concern that it will worsen.

  Therefore, the present invention prevents pre-ignition of the fuel and thus combustion noise during the transition period when the SI combustion mode is switched to the CI combustion mode while avoiding a decrease in combustion efficiency.

  In order to solve the above-described problems, the present invention secures the internal EGR by performing an early closing of the exhaust valve at least during the transition period when the SI combustion mode is switched to the CI combustion mode.

An engine exhaust device disclosed herein includes an engine body having a plurality of cylinders,
A plurality of independent exhaust passages whose upstream ends are connected to each of the plurality of cylinders;
An exhaust valve that opens and closes the opening on the upstream end side of each independent exhaust passage;
A collecting portion to which the downstream end side of each independent exhaust passage is connected so that the exhaust gas that has passed through each independent exhaust passage joins;
A fuel injection valve provided for each cylinder to supply fuel into each cylinder;
A spark plug provided for each cylinder to ignite the air-fuel mixture in each cylinder;
A controller for operating the engine body by controlling at least the fuel injection valve and the spark plug; and
The controller is configured to switch between a CI combustion mode in which the air-fuel mixture is combusted by self-ignition and an SI combustion mode in which the air-fuel mixture is ignited by the ignition plug and combusted according to the operating state of the engine body. It is assumed that

So the exhaust system of this engine
A part of the plurality of independent exhaust passages has a passage volume different from other independent exhaust passages,
A first mode in which the exhaust valve of each cylinder is opened and closed with a first opening / closing characteristic; and a second mode in which the closing timing in the exhaust stroke is advanced as compared with the first mode. The variable operation is switched to the third mode that opens and closes with a third opening / closing characteristic that has a valve opening period in the intake stroke in addition to the exhaust stroke, and the closing timing in the exhaust stroke is delayed compared to the second mode. With a valve mechanism,
In the spark ignition combustion mode, the controller drives the exhaust valve in the first mode, and when switching from the spark ignition combustion mode to the compressed auto ignition combustion mode, at least first in the compressed auto ignition combustion mode. the mixing said exhaust valve in the cylinder is performed the combustion of the gas is driven in the second mode, as will be driven after the driving in the second mode in the third mode, and controls the variable valve mechanism ,
Further, in the second mode, the exhaust valve is closed twice in the exhaust stroke and then opened twice in the intake stroke,
A working angle of the exhaust valve in the intake stroke in the second mode is smaller than a working angle of the exhaust valve in the intake stroke in the third mode;
In a cylinder to which an independent exhaust passage having a relatively small passage volume is connected, the closing timing in the exhaust stroke when the exhaust valve is driven in the second mode is set to an independent time when the passage volume is relatively large. It is characterized in that it is advanced with respect to the cylinder to which the exhaust passage is connected .

  Here, “switching according to the operating state of the engine body (switching between the CI combustion mode and the SI combustion mode)” means that the CI combustion mode and the SI combustion mode are changed according to the change in the load state of the engine body. Or when switching from the SI combustion mode to the CI combustion mode as the engine body changes from a cold or semi-warm state below a predetermined temperature to a warm state. When the engine body in the SI combustion mode is shifted to the CI combustion mode as the engine body moves to a region other than the idle, and when the fuel supply is started from the deceleration fuel cut, the SI combustion mode is restored. Thereafter, switching to the normal CI combustion mode may be included. Further, the CI combustion mode and the SI combustion mode may be switched without substantially changing the load state of the engine body.

  According to the engine exhaust device, in the SI combustion mode, the exhaust valve is driven in the first mode in which the first open / close characteristic is opened and closed. At the time of switching from the SI combustion mode to the CI combustion mode, the exhaust valve advances the closing timing in the exhaust stroke more than the first mode in the cylinder in which the air-fuel mixture is first combusted in the CI combustion mode. Driven in the second mode. According to this, since the combustion gas is confined in the cylinder by so-called early closing of the exhaust valve and the internal EGR is secured, the high-temperature exhaust gas in the SI combustion mode of the other cylinder before the combustion mode switching is in the cylinder. It will not be introduced.

  In other words, when internal EGR is performed by opening the exhaust valve twice, not only exhaust gas due to CI combustion remaining in the independent exhaust passage extending from the own cylinder but also high-temperature exhaust due to SI combustion remaining from the other cylinders remaining in the collective portion. Gas is also introduced into the cylinder. For this reason, the in-cylinder temperature of the own cylinder may be higher than the assumed temperature. The influence is large in a light load region where the amount of internal EGR gas introduced is large. On the other hand, in the exhaust system of the engine, the internal EGR is performed by the gas having a relatively low temperature due to the CI combustion of the own cylinder by the early closing of the exhaust valve. Therefore, it is avoided that the in-cylinder temperature rises excessively, that is, the pre-ignition of fuel and the accompanying combustion noise are avoided.

  Further, if the volume of the independent exhaust passage is increased (for example, if the passage length is increased), it is possible to avoid that the high-temperature exhaust gas remaining in the collecting portion is introduced into the cylinder by opening the exhaust valve twice. In this case, the volume of the independent exhaust passage is increased, resulting in an increase in the size of the engine. On the other hand, the early closing of the exhaust valve described above is advantageous in making the engine compact because it is not necessary to increase the volume of the independent exhaust passage.

  Further, according to the above-described early closing of the exhaust valve, the fuel efficiency is lowered as in the case where the fuel is injected into the cylinder after the middle of the compression stroke and the in-cylinder temperature is forcibly lowered by the latent heat of vaporization of the fuel. It is also possible to avoid the deterioration of fuel consumption.

  At the time of switching from the SI combustion mode to the CI combustion mode, the exhaust valve may be driven in the second mode not only for the cylinder in which the CI combustion is performed first but also in the cylinder in which the CI combustion is subsequently performed. . Preferably, the exhaust valve of the cylinder in which the first CI combustion is performed in accordance with the other combustion mode switching is driven in the second mode except for the cylinder that is switched to CI combustion at the end of the plurality of cylinders. It is. For the cylinders finally switched to CI combustion, there is no high temperature exhaust gas due to SI combustion from other cylinders already in the collecting portion, so there is no particular need to execute internal EGR by early closing of the exhaust valve. . Of course, for the purpose of simplifying the control logic, for all cylinders, the exhaust valve is driven in the second mode when the first CI combustion is performed, and the exhaust valve is driven in the third mode from the second CI combustion. You may make it do.

  In the next CI combustion cycle after being driven in the second mode, the exhaust valve opens and closes with a third opening / closing characteristic in which the closing timing is retarded than in the second mode and the valve is opened in the intake stroke in addition to the exhaust stroke. To do. Therefore, although the pumping loss due to the second mode is small and the closing timing in the exhaust stroke is retarded, the EGR amount necessary in the CI combustion mode can be secured by opening the exhaust valve in the intake stroke. .

  Here, the “third opening / closing characteristic that opens in the intake stroke in addition to the exhaust stroke” includes the lift characteristic (that is, the valve opens again in the intake stroke after the exhaust valve is substantially closed in the exhaust stroke (that is, The exhaust valve lift curve is not limited to two lift characteristics (so-called double opening), and the exhaust valve once lifted during the exhaust stroke does not close, and the predetermined opening degree is not closed. It also includes a lift characteristic that leads to the intake stroke while being maintained (that is, a lift characteristic in which the peak of the lift curve of the exhaust valve is substantially one, but the base of the peak extends with respect to the progress of the crank angle).

In the second mode in which the exhaust valve closing timing is advanced as described above, the engine exhaust device further opens twice so that the exhaust valve is closed in the exhaust stroke and then opened again in the intake stroke. the working angle of the exhaust valve in the intake stroke in the second mode, you smaller than the working angle of the exhaust valve in the intake stroke in the third mode.

  Since it is necessary to increase the EGR rate for stable CI combustion, in order to secure the required EGR amount by the advance timing of the exhaust valve closing timing, the closing timing must be greatly advanced, Pumping loss increases. Further, if the closing timing of the exhaust valve is greatly advanced by a phase variable mechanism (hereinafter referred to as “VVT”) that changes the rotational phase of the camshaft with respect to the crankshaft, the opening timing is also greatly advanced to the expansion stroke side at the same time. To do. As a result, since the effective expansion ratio becomes small, the influence of a decrease in engine torque may be transmitted to the driver of the automobile as a feeling of strangeness.

  Therefore, the exhaust valve closing timing is not excessively advanced, and the exhaust valve is opened twice, that is, the exhaust valve is opened during the intake stroke, thereby securing an EGR amount that is insufficient only by the closing timing advance. It is. In this case, since the shortage of EGR is only compensated by opening twice, the operating angle of the exhaust valve in the intake stroke can be reduced. The purpose is to compensate for the shortage of EGR with the exhaust gas from the CI combustion remaining in the independent exhaust passage connected to the cylinder. Since the operating angle is reduced, it is possible to prevent exhaust gas generated by SI combustion from other cylinders remaining in the collective portion from being introduced into the cylinder, and thus avoiding that the temperature in the cylinder becomes higher than expected. Can do.

As described above, the exhaust device of the engine has an exhaust valve in a case where the volume of some of the independent exhaust passages connected to each of the plurality of cylinders is different from the volume of the other independent exhaust passages. the closing timing of the exhaust stroke when driven by 2 mode, Ru is more advanced the volume of independent exhaust passages become smaller cylinder.

  In a cylinder having a small independent exhaust passage, the amount of exhaust gas due to CI combustion remaining in the independent exhaust passage is small. Therefore, when the EGR amount that is insufficient only by the early closing of the exhaust valve is compensated by opening the exhaust valve twice, there is a concern that exhaust gas due to SI combustion of other cylinders remaining in the collecting portion is introduced into the cylinder. .

  Therefore, in a cylinder having a small volume of the independent exhaust passage, the advance angle of the closing timing in the exhaust stroke is increased to increase the amount of internal EGR due to this early closing. Thereby, the amount of EGR caused by opening twice can be relatively reduced, which is advantageous in avoiding introduction of high-temperature exhaust gas remaining in the collecting portion into the cylinder.

  Here, in a preferred embodiment, the advancement degree of the closing timing of the exhaust valve in the second mode with respect to the first mode is changed according to the operating state of the engine, and the advancement degree as the engine load is relatively low. Is set larger.

According to the present invention, in the SI combustion mode, the exhaust valve is driven in the first mode that opens and closes with the first opening / closing characteristic, and at the time of switching from the SI combustion mode to the CI combustion mode, at least the cylinder in which the CI combustion is performed first. The exhaust valve is driven in a second mode that opens and closes with a second opening / closing characteristic in which the closing timing in the exhaust stroke is advanced than in the first mode, and after the driving in the second mode, the exhaust stroke is made more than in the second mode. Since the engine is driven in the third mode that opens and closes with the third opening / closing characteristic that opens in the intake stroke in addition to the exhaust stroke, the transition period in which the SI combustion mode is switched to the CI combustion mode is delayed. In addition, it is advantageous in preventing the pre-ignition of the fuel and thus the generation of the combustion noise while avoiding the decrease in the combustion efficiency. Further, according to the present invention, the exhaust valve is opened twice in the second mode, and the operating angle of the exhaust valve in the intake stroke in the second mode is set to the operating angle of the exhaust valve in the intake stroke in the third mode. In a cylinder to which an independent exhaust passage having a relatively small passage volume is connected, the closing time in the exhaust stroke when the exhaust valve is driven in the second mode is relatively large. Since the cylinder is advanced more than the cylinder to which the independent exhaust passage is connected, the in-cylinder temperature becomes higher than expected by preventing the exhaust gas due to SI combustion from other cylinders remaining in the collecting portion from being introduced into the cylinder. Furthermore, in a cylinder having a small volume of the independent exhaust passage, the amount of EGR caused by opening twice can be relatively reduced, and the high-temperature exhaust gas remaining in the collecting portion is introduced into the cylinder. It is advantageous to on to avoid to be.

Schematic which shows a structure of an engine. The figure which shows the structure of an exhaust manifold typically. The front view made into the partial cross section which shows a part of variable valve mechanism. The graph which shows the opening / closing characteristic of the exhaust valve which concerns on a reference form . The block diagram which concerns on control of an engine. The graph which shows the driving | operation area | region of an engine. The flowchart which shows the basic flow of exhaust valve control. The flowchart which shows the specific flow of exhaust valve control. Graph showing the opening-closing characteristic of the exhaust valve according to the implementation embodiments.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or its use.

1 and 2 show a schematic configuration of a 4-cycle direct injection multi-cylinder engine 1 according to a reference embodiment . The engine 1 is a gasoline engine that is mounted on a vehicle and supplied with a fuel containing at least gasoline. The engine body 2 includes a cylinder block 11 provided with a plurality of cylinders 18 (only one cylinder is shown in FIG. 1, but four cylinders are provided in series, for example), a cylinder head 12, and lubricating oil. And a stored oil pan 13. A piston 14 connected to the crankshaft 15 via a connecting rod 142 is fitted in each cylinder 18 so as to be able to reciprocate. A cavity 141 is formed on the top surface of the piston 14. The cylinder head 12, the cylinder 18 and the piston 14 define a combustion chamber of the engine.

  The shape of the combustion chamber is not limited to the shape illustrated. For example, the shape of the cavity 141, the top surface shape of the piston 14, the shape of the ceiling portion of the combustion chamber 19, and the like can be changed as appropriate.

  The engine 1 is set to a relatively high geometric compression ratio of 15 or more for the purpose of improving the theoretical thermal efficiency and stabilizing the compression ignition combustion described later. What is necessary is just to set geometric compression ratio suitably in the range of about 15-20.

  The cylinder head 12 is formed with an intake port 16 and an exhaust port 17 for each cylinder 18. The intake port 16 and the exhaust port 17 have an intake valve 21 and an exhaust valve that open and close the opening on the combustion chamber side. 22 are arranged respectively.

  A fuel injection valve 67 that directly injects fuel into the cylinder 18 is attached to the cylinder head 12 for each cylinder 18. The fuel injection valve 67 is disposed such that its nozzle hole faces the combustion chamber from the center portion of the ceiling surface of the combustion chamber. The fuel injection valve 67 injects fuel in an amount corresponding to the operation state of the engine body 2 at an injection timing set according to the operation state of the engine body 2.

  A fuel tank (not shown) and the fuel injection valve 67 are connected to each other by a fuel supply path. A fuel supply system 62 including a fuel pump 63 and a common rail 64 and capable of supplying fuel to the fuel injection valve 67 at a relatively high fuel pressure is interposed on the fuel supply path. The fuel pump 63 pumps fuel from the fuel tank to the common rail 64. The common rail 64 can store the pumped fuel at a relatively high fuel pressure. When the fuel injection valve 67 is opened, the fuel stored in the common rail 64 is injected from the injection port of the fuel injection valve 67. Here, although not shown, the fuel pump 63 is a plunger type pump and is driven by the engine 1. The fuel supply system 62 configured to include this engine-driven pump makes it possible to supply fuel with a high fuel pressure of 30 MPa or more to the fuel injection valve 67. The fuel pressure may be set to about 120 MPa at the maximum. The fuel supply system 62 is not limited to this configuration.

  A spark plug 25 that ignites the air-fuel mixture in the cylinder is attached to the cylinder head 12 for each cylinder 18. In this example, the spark plug 25 is disposed through the cylinder head 12 so as to extend obliquely downward from the exhaust side of the engine 1. The tip of the spark plug 25 is disposed so as to face the cavity 141 when the piston 14 is located at the compression top dead center.

  An intake passage 30 is connected to one side of the engine 1 so as to communicate with the intake port 16 of each cylinder 18. On the other hand, an exhaust passage 40 that exhausts exhaust gas (burned gas) from each cylinder 18 is connected to the other side of the engine 1.

  An air cleaner 31 that filters intake air is disposed at the upstream end of the intake passage 30. A surge tank 33 is disposed near the downstream end of the intake passage 30. The intake passage 30 on the downstream side of the surge tank 33 is an independent intake passage branched for each cylinder 18, and the downstream end of each of the independent intake passages is connected to the intake port 16 of each cylinder 18.

  Between the air cleaner 31 and the surge tank 33 in the intake passage 30, there are an inter warmer 34 that heats the air using the heat of the cooling water, and a throttle valve 36 that adjusts the amount of intake air to each cylinder 18. It is arranged. An interwarmer bypass passage 35 that bypasses the interwarmer 34 is also connected to the intake passage 30, and an interwarmer bypass valve for adjusting the flow rate of air passing through the passage 35 is connected to the interwarmer bypass passage 35. 351 is disposed. It is possible to adjust the temperature of fresh air introduced into the cylinder 18 by adjusting the ratio of the passage flow rate of the inter-warmer bypass passage 35 and the passage flow rate of the inter-warmer 34 through the adjustment of the opening degree of the inter-warmer bypass valve 351. It is.

  The upstream portion of the exhaust passage 40 is constituted by an exhaust manifold. As shown in an example of a four-cylinder engine in FIG. 2, the exhaust manifold branches for each of cylinders # 1 to # 4 and is connected to the outer end of the exhaust port 17 shown in FIG. 43 and a collecting portion 44 to which the downstream end side of each independent exhaust passage 43 is connected so that the exhaust gas that has passed through each independent exhaust passage 43 merges.

  As shown in FIG. 1, a direct catalyst 41 and an underfoot catalyst 42 are connected as exhaust purification devices for purifying harmful components in the exhaust gas downstream of the exhaust manifold in the exhaust passage 40. Yes. Each of the direct catalyst 41 and the underfoot catalyst 42 includes a cylindrical case and, for example, a three-way catalyst disposed in a flow path in the case.

  A portion between the surge tank 33 and the throttle valve 36 in the intake passage 30 and a portion upstream of the direct catalyst 41 in the exhaust passage 40 are used for returning a part of the exhaust gas to the intake passage 30. They are connected via a passage 50. The EGR passage 50 includes a main passage 51 in which an EGR cooler 52 for cooling the exhaust gas with engine coolant is disposed, and an EGR cooler bypass passage 53 for bypassing the EGR cooler 52. ing. The main passage 51 is provided with an EGR valve 511 for adjusting the amount of exhaust gas recirculated to the intake passage 30, and the EGR cooler bypass passage 53 has a flow rate of exhaust gas flowing through the EGR cooler bypass passage 53. An EGR cooler bypass valve 531 for adjustment is provided.

As shown in FIG. 3, the exhaust valve 22 is provided with a variable valve mechanism that switches its operation mode. In this reference embodiment , two exhaust valves 22 are provided for each cylinder 18. The variable valve mechanism has, for example, a hydraulically operated VVL. Although the detailed illustration of the configuration of the VVL is omitted, the cam element 26 having the first cam 23 and the second cam 24 having different cam profiles, and one of the cams 23 and 24 of the cam element 26. The electromagnetic operation devices 27 and 28 for selectively transmitting the operation of the cam to the exhaust valve 22 are included.

  The cam element 26 includes a pair of cam portions provided corresponding to each of the two exhaust valves 22, and first and second cams 23 and 24 are provided in each cam portion. At both ends of the cam element 26, movement cams 29 are provided for moving the cam element 26 in the axial direction in cooperation with the operating devices 27 and 28.

  The cam element 26 is spline-fitted to the shaft portion 38 a of the cam shaft 38 supported by the bearing 37. A detent mechanism for selectively positioning the cams 23 and 24 with respect to the rocker arm 39 of the exhaust valve 22 is provided at a fitting portion between the cam element 26 and the shaft portion 38 a of the cam shaft 38.

  The operating devices 27 and 28 include main bodies 27a and 28a having electromagnetic actuators therein, pins 27b and 28b protruding toward the moving cam 29 by energization of the actuators, and pins 27b and 28b. And a return spring (not shown) for returning to the side.

  When the actuator of one operating device 27 is energized, the pin 27 b protrudes and engages with one moving cam 29 of the cam element 26. As the cam shaft 38 rotates, the pin 27 b slides on the cam surface of the moving cam 29, so that the cam element 26 moves in the axial direction and the exhaust valve 22 is opened and closed by the first cam 23. Positioned by detent mechanism. After the positioning, the energization is cut off, and the pin 27b returns to the original position by the bias of the return spring. When the actuator of the other operating device 28 is energized, the pin 28 b protrudes and engages with the other moving cam 29 of the cam element 26. As the cam shaft 38 rotates, the pin 28b slides on the cam surface of the moving cam 29 so that the cam element 26 moves in the axial direction and the exhaust valve 22 is opened and closed by the second cam 24. Positioned by detent mechanism. After this positioning, the energization is cut off, and the pin 28b returns to the original position by the bias of the return spring.

  The variable valve mechanism includes a VVT capable of changing the rotational phase of the camshaft 38 relative to the crankshaft 15. The VVT may adopt a known structure of a hydraulic type, an electromagnetic type, or a mechanical type as appropriate, and illustration of the detailed structure is omitted.

  The first cam 23 is a cam having one cam crest. When the operation of the first cam 23 is transmitted, the exhaust valve 22 is driven in a first mode that opens and closes with a first opening / closing characteristic. That is, the exhaust valve 22 opens near the bottom dead center of the expansion stroke and closes near the top dead center of the exhaust stroke, as shown by the solid line in FIG. The second cam 24 is a cam having two large and small cam peaks. When the operation of the second cam 24 is transmitted, the exhaust valve 22 is driven in a third mode that opens and closes with a third opening / closing characteristic (opened twice). That is, as shown by a solid line in FIG. 4C, the exhaust valve 22 opens near the bottom dead center of the expansion stroke, closes near the top dead center of the exhaust stroke, and then opens again during the intake stroke.

  Thus, when the VVT is operated while the operation of the first cam 23 is transmitted to the exhaust valve 22, the exhaust valve 22 is driven in the second mode that opens and closes with the second opening / closing characteristic. become. That is, as shown by the solid line in FIG. 4 (B), the exhaust valve 22 is closed earlier than the first mode (FIG. 4 (A)) when the closing timing in the exhaust stroke is advanced in the cam rotation direction. In the exhaust stroke, a negative overlap period in which both the intake valve 21 and the exhaust valve 22 are closed is provided. The length of the negative overlap period is changed according to the internal EGR rate, and becomes longer as the internal EGR rate is higher. That is, the advancement degree of the closing timing in the exhaust stroke of the exhaust valve 22 in the second mode is changed according to the operating state of the engine, and the advancement degree becomes larger as the engine load becomes relatively lower. In the second mode, the opening timing of the exhaust valve 22 is also advanced to the expansion stroke side.

  In FIG. 4, the lift characteristic of the intake valve 21 is indicated by a broken line. In the second mode, a negative overlap period similar to the negative overlap period provided in the exhaust stroke is also provided in the intake stroke. . The reason for this is that a negative overlap period is provided in the latter half of the exhaust stroke, and the internal EGR gas is compressed when the piston reaches the top dead center of the exhaust stroke. In this state, when the intake valve is opened from the top dead center of the intake stroke, the internal EGR gas flows out from the cylinder into the intake passage. Therefore, by providing a negative overlap period also in the first half of the intake stroke, the internal EGR gas existing in the cylinder during the intake stroke is prevented from exiting in the intake passage direction.

In the present embodiment , the intake valve 21 is also provided with two intake valves 21 for each cylinder 18, and similarly to the exhaust valve 22 side, a variable valve mechanism having VVL and VVT for switching its operation mode is provided. Yes.

  The engine 1 configured as described above is controlled by a powertrain control module (hereinafter referred to as PCM) 10. The PCM 10 includes a microprocessor having a CPU, a memory, a counter timer group, an interface, and a path connecting these units. This PCM 10 constitutes a controller.

  As shown in FIGS. 1 and 5, the PCM 10 receives detection signals from various sensors SW1 to SW17 and a gear stage detection unit 43 that detects the currently selected gear stage of the automatic transmission.

  The various sensors include the following sensors. That is, the air flow sensor SW1 that detects the flow rate of fresh air, the intake air temperature sensor SW2 that detects the temperature of fresh air, and the interwarmer 34 disposed downstream of the interwarmer 34 on the downstream side of the air cleaner 31 A second intake temperature sensor SW3 for detecting the temperature of fresh air, an EGR gas temperature sensor SW4 for detecting the temperature of the external EGR gas disposed in the vicinity of the connection portion of the EGR passage 50 with the intake passage 30, and an intake port 16 And an intake port temperature sensor SW5 for detecting the temperature of the intake air just before flowing into the cylinder 18, an in-cylinder pressure sensor SW6 for detecting the pressure in the cylinder 18 attached to the cylinder head 12, and an exhaust passage 40 Exhaust temperature sensors for detecting the exhaust temperature and the exhaust pressure, respectively, disposed near the connection portion of the EGR passage 50 SW7, exhaust pressure sensor SW8, a linear O2 sensor SW9, which is disposed upstream of the direct catalyst 41 and detects the oxygen concentration in the exhaust, is disposed between the direct catalyst 41 and the underfoot catalyst 42, A lambda O2 sensor SW10 for detecting the oxygen concentration in the exhaust gas, a water temperature sensor SW11 for detecting the temperature of engine cooling water, a crank angle sensor SW12 for detecting the rotation angle of the crankshaft 15, and an operation amount of an accelerator pedal (not shown) of the vehicle. The fuel pressure supplied to the fuel injection valve 67 attached to the common rail 64 of the fuel supply system 62, the accelerator opening sensor SW13 for detecting the accelerator opening corresponding to the above, the intake and exhaust cam angle sensors SW14 and SW15, and the fuel supply system 62. A fuel pressure sensor SW16 to be detected and a vehicle speed sensor SW17.

  The PCM 10 performs various calculations based on these detection signals to determine the state of the engine 1 and the vehicle, and accordingly, the fuel injection valve 67, the spark plug 25, the exhaust-side VVL 71 and VVT 72, and the intake-side Control signals are output to the actuators of the VVL 73 and VVT 74, the fuel supply system 62, and various valves (throttle valve 36, intercooler bypass valve 351, EGR valve 511, and EGR cooler bypass valve 531). In the present embodiment, the target acceleration of the vehicle is determined based on the accelerator opening detected by the accelerator opening sensor SW13, the vehicle speed detected by the vehicle speed sensor SW17, and the gear stage detected by the gear stage detection means. The target load of the engine is determined accordingly.

  FIG. 6 shows an example of the operation region when the engine body 2 is warm. In the engine body 2, for the purpose of improving fuel efficiency and exhaust emission performance, in the low load range where the engine load is relatively low, the ignition plug 25 is not ignited, and the combustion of the air-fuel mixture is performed by compression self-ignition. Perform CI combustion. However, as the load on the engine body 2 increases, the combustion becomes too steep in the CI combustion, causing problems such as combustion noise, for example. Therefore, in the high load region where the engine load is relatively high, the CI combustion is stopped and switched to SI combustion using the spark plug 25. As described above, the engine 1 is configured to switch between the CI combustion mode and the SI combustion mode in accordance with the operation state of the engine main body 2, particularly the engine load. In addition, the boundary line of combustion mode switching is not limited to the example of a figure.

  In addition, since a high temperature and high pressure environment is necessary for CI combustion of the air-fuel mixture, CI combustion is not suitable, for example, during idling immediately after a cold start before the engine warms up. In such a case, the idle operation is performed by SI combustion until the engine is warmed up, and the CI combustion mode is switched after the warm-up is completed.

(Control when switching from SI mode to CI mode)
Since SI combustion has lower thermal efficiency than CI combustion, the temperature of the combustion gas is relatively high. On the other hand, in the CI combustion mode, the internal EGR gas is introduced into the cylinder 18 by the exhaust valve control in the third mode (double opening) shown in FIG. Thus, the temperature state in the cylinder 18 is increased.

  In SI combustion, exhaust gas that has passed through an EGR cooler provided outside the engine is introduced into the cylinder, and its EGR rate is less than about 30%. In CI combustion, thermal efficiency is higher than SI combustion. Since the exhaust gas temperature is also relatively low, it is necessary to perform internal EGR that introduces exhaust gas into the cylinder without using an EGR cooler. In addition, a large amount of internal EGR gas is required for stable CI combustion. Since the temperature of the air-fuel mixture tends to be lower as the engine load is lower, in the CI combustion region, for example, the target EGR rate is set higher as the target load of the engine is lower in the range of about 30 to 50% in EGR rate. . In addition, when operation in a relatively low load region is continued in the CI combustion operation region, the target EGR rate may be set to a high value of, for example, 50 to 80%.

  Here, immediately after switching from the SI combustion mode to the CI combustion mode where the combustion gas temperature is relatively high, the temperature of the exhaust gas is higher than when the CI combustion is continuously performed. In this case, when the internal EGR is executed by opening the exhaust valve 22 twice, the high-temperature exhaust gas generated by the SI combustion in the other cylinder 18 is introduced into the own cylinder 18, and the desired CI combustion state May not be obtained, and combustion noise may occur.

  In FIG. 2, consider a case where the # 1 cylinder 18 is first switched from the SI combustion mode to the CI combustion mode. As schematically shown in the figure, the independent exhaust passage 43 of the # 1 cylinder 18 has exhaust gas due to CI combustion (lower right oblique line), and the independent exhaust passage 43 of the other cylinder 18 has exhaust gas due to SI combustion ( There is a slanting line on the left. Further, in the collecting portion 44, the exhaust gas due to the CI combustion from the # 1 cylinder 18 and the exhaust gas having a relatively high temperature due to the SI combustion from the other cylinders 18 are mixed.

  In this case, when the exhaust valve 22 is opened in the intake stroke by opening the exhaust valve 22 of the # 1 cylinder 18 twice, not only the CI combustion exhaust gas existing in the independent exhaust passage 43 of the # 1 cylinder 18 but also the collecting portion 44. SI combustion exhaust gas from the other cylinder 18 is introduced into the # 1 cylinder 18. For example, if the cylinder volume is 500 cc and the EGR rate is 30%, 150 cc of EGR gas is required per cylinder. Therefore, if the volume of the independent exhaust passage 43 of the # 1 cylinder 18 is, for example, 120 cc, the high-temperature SI combustion exhaust gas from the other cylinders 18 is introduced to the # 1 cylinder 18 slightly less than 30 cc. Therefore, the in-cylinder temperature of the # 1 cylinder 18 may be higher than expected.

Therefore, in the engine 1 according to the reference embodiment , the exhaust valve control in the second mode is executed in the transition period of switching from the SI combustion mode to the CI combustion mode.

  FIG. 7 shows a basic flow of exhaust valve control executed by the PCM 10. In step S1 after the start, the operating state of the engine body 2 is read. In the subsequent step S2, the control target of the exhaust valve 22 by the variable valve mechanism is determined based on the operating state, and in the subsequent step S3, the exhaust valve 22 is controlled by the variable valve mechanism based on the control target.

  A specific flow of the exhaust valve control is shown in FIG. In step A1 after the start, the operation state parameters of the engine body 2 (signals from the airflow sensor SW1, the crank angle sensor SW12, the accelerator opening sensor SW13, the vehicle speed sensor SW17, the gear position detection means 43, etc.) are read. In subsequent step A2, it is determined whether or not the engine is operating in the SI combustion mode. That is, the engine speed is calculated based on the signal from the crank angle sensor SW12, the target acceleration is obtained based on the vehicle speed, the gear position, and the accelerator opening, and the required torque (engine load) to the engine main body 2 according to the target acceleration. Is calculated. Then, based on the engine speed and the engine load, it is determined whether the engine operating state is in the CI region or the SI region with reference to the control map of FIG. However, even in the CI region, when the engine coolant temperature is low, SI combustion is executed. Therefore, it is determined whether or not the engine is operating in the SI combustion mode by taking into account the engine coolant temperature. .

  If the engine is operating in the SI combustion mode, the routine proceeds to step A3 where the first exhaust valve control is executed. When the engine is not operating in the SI combustion mode, that is, when the engine is operating in the CI combustion mode, the routine proceeds to step A8 described later, and third exhaust valve control is executed.

  The first exhaust valve control in step A3 is valve control in the first mode shown in FIG. Based on the data such as the engine load, a target value (target EGR amount) of the internal EGR gas amount is read from a preset target EGR map related to SI combustion. This target EGR map is electronically stored in the memory (for example, ROM, RAM, etc.) of the PCM 10. In this target EGR map, an optimal target EGR amount corresponding to the engine operating state in SI combustion is experimentally obtained in advance, and this value is set as a map in association with the engine load and the engine speed. . The target EGR rate in SI combustion is about 0 to 20%.

  Then, the control amounts of the intake and exhaust VVTs 72 and 74 corresponding to the target EGR amount are determined. The overlap amounts of the intake and exhaust valves 21 and 22 are controlled so as to be the target EGR amount by operating the VVTs 72 and 74 to advance or retard according to the determined control amount. In this case, in the exhaust VVL 71, the positioning of the cam element 26 is controlled so that the exhaust valve 22 is opened and closed by the first cam 23 having one cam crest. For the control amounts of the VVTs 72 and 74, optimum values corresponding to the target EGR amount, the engine load, and the engine rotation speed are experimentally obtained in advance, set as a VVT map, and read from the VVT map. .

  In subsequent step A4, it is determined whether or not the operating state of the engine body is in an operating region in which CI combustion is executed. This determination is performed with reference to the control map of FIG. 6 based on the engine speed and the engine load, as in step A2.

  If the operating state of the engine body is not in the operating region where CI combustion is performed, the first exhaust valve control (step A3) related to SI combustion is continued. If the operating state of the engine body is in the operating region where the CI combustion is performed, the routine proceeds to step A5, where it is determined whether or not the CI combustion can be performed. That is, when the engine coolant temperature is low, CI combustion is not suitable from the viewpoint of combustion stability. Therefore, the PCM 10 determines whether the engine coolant temperature is equal to or higher than a predetermined value based on the detection result from the water temperature sensor SW11. Is determined. That is, the temperature in the cylinder 18 is indirectly determined from the temperature of the engine coolant.

  If the engine coolant temperature is lower than the predetermined value, the first exhaust valve control (step A3) related to SI combustion is continued. When the engine coolant temperature is equal to or higher than the predetermined value (when CI combustion can be performed), the routine proceeds to step A6, where the second exhaust valve control is performed. This second exhaust valve control is a valve control in the second mode shown in FIG.

In the case of the present embodiment , the second exhaust valve control is performed not only on the cylinder 18 in which the CI combustion is performed first. Since the engine 1 of the present embodiment is a four-cylinder engine, the second exhaust valve control is executed up to the third cylinder 18 in which switching from the SI combustion mode to the CI combustion mode is performed in order. Then, the third exhaust valve control is executed for the fourth cylinder 18 and subsequent cylinders that have been switched to the CI combustion mode. That is, in the transition period from the SI combustion mode to the CI combustion mode, the second exhaust valve control is executed when the first CI combustion is performed for the three cylinders 18.

  In the second exhaust valve control, as in the case of the first exhaust valve control, the target value (target EGR amount) of the internal EGR gas amount from the preset target EGR map related to the CI combustion based on the data such as the engine load. ) Is read out.

  Then, the control amounts of the intake and exhaust VVTs 72 and 74 corresponding to the target EGR amount are determined. The open / close timings of the intake / exhaust valves 21 and 22 are controlled so as to reach the target EGR amount by operating the VVTs 72 and 74 to advance or retard according to the determined control amount. In this case, the exhaust valve 22 is controlled to advance more greatly as the target EGR amount increases in order to ensure the internal EGR by early closing. As the control amounts of the VVTs 72 and 74, optimum values corresponding to the target EGR amount, the engine load, and the engine rotation speed are experimentally obtained in advance, set as a VVT map, and read from the VVT map.

  In the subsequent step A7, it is determined whether or not the switching from the SI combustion mode to the CI combustion mode has been completed for all the cylinders 18. Then, when the switching has not been completed for all the cylinders 18, the second exhaust valve control (step A6) is continued. When the switching is completed for all the cylinders 18, the process proceeds to step A8, and the third exhaust valve control is executed.

  The third exhaust valve control is valve control in the third mode shown in FIG. Accordingly, in order to open the exhaust valve 22 twice, in the exhaust VVL 71, the positioning of the cam element 26 is controlled so that the exhaust valve 22 is opened and closed by the second cam 24 having two cam peaks.

  In the third exhaust valve control, as in the second exhaust valve control, the target value (target EGR amount) of the internal EGR gas amount is determined from the preset target EGR map related to the CI combustion based on the data such as the engine load. Read out. From the target EGR amount, the control amounts of the intake and exhaust VVTs 72 and 74 corresponding to the target EGR amount are determined, and the respective VVTs 72 and 74 are respectively advanced or retarded so that the target EGR amount is obtained. The opening / closing timing of the exhaust valves 21 and 22 is controlled. As the control amounts of the VVTs 72 and 74, optimum values corresponding to the target EGR amount, the engine load, and the engine rotation speed are experimentally obtained in advance, set as a VVT map, and read from the VVT map.

  As described above, in the transition period from the SI combustion mode to the CI combustion mode, the necessary internal EGR amount is secured by the early closing of the exhaust valve 22 in the cylinder 18 in which the CI combustion is performed. Accordingly, it is possible to avoid introduction of high-temperature exhaust gas from the other cylinder 18 before that into the cylinder 18 due to SI combustion. Therefore, the in-cylinder temperature of the cylinder 18 does not rise excessively, that is, it becomes easy to set the in-cylinder temperature to a temperature appropriate for CI combustion. Therefore, in the CI combustion in the next combustion cycle, it is possible to avoid premature fuel ignition and the accompanying combustion noise.

  Thus, according to the early closing of the exhaust valve, the fuel efficiency is lowered as in the case where the fuel is injected into the cylinder after the middle of the compression stroke and the in-cylinder temperature is forcibly lowered by the latent heat of vaporization of the fuel. It is also possible to avoid the deterioration of fuel consumption. Further, it is not necessary to increase the volume of the independent exhaust passage 43 so that the high-temperature exhaust gas remaining in the collecting portion is not introduced into the cylinder, which is advantageous for making the engine compact.

Further, in the above-described reference embodiment , the internal EGR is performed not only by the cylinder first switched to the CI combustion mode but also by the early closing of the exhaust valve 22 to the third cylinder 18, so that the pre-ignition of the fuel is performed in the CI combustion. It can certainly be avoided. However, as the switching to the CI combustion mode proceeds, the ratio of the high-temperature exhaust gas due to SI combustion existing in the gathering portion 44 decreases. Therefore, only the cylinder 18 where the CI combustion is performed first is performed first, or the CI combustion is performed first. The exhaust valve 22 may be quickly closed only in the cylinder 18 in which the combustion is performed and the cylinder 18 in which the CI combustion is performed next. Thereby, the pumping loss due to early closing can be reduced.

( Embodiment )
In the reference embodiment , a cam having one cam peak is used as the first cam 23 for driving the exhaust valve 22. However, in the embodiment, a cam having two large and small cam peaks is used as the first cam 23. is there. The open / close characteristics of the exhaust valve 22 in this case are shown in FIG.

  As shown in FIG. 9A, in the first exhaust valve control (first mode) executed in the SI combustion mode, the exhaust valve 22 operates in accordance with the operation of the first cam 23 having two cam peaks. After closing near the top dead center, it opens twice in the intake stroke. The point that the exhaust valve 22 opens twice is the same as that of the second cam 24 (third mode in FIG. 9C), but the operating angle of the exhaust valve in the intake stroke is the same as that of the first cam 23. Is smaller than the second cam 24.

  As shown in FIG. 9B, in the second exhaust valve control (second mode) executed in the transition period from the SI combustion mode to the CI combustion mode, the operation of the exhaust valve 22 is performed by the operation of the exhaust VVT 72. The opening and closing time is advanced. Therefore, the exhaust valve 22 is closed early in the exhaust stroke, and a negative overlap period is provided.

In the embodiment shown in FIG. 9, as in the reference embodiment shown in FIG. 4, in the second mode, in order to suppress the internal EGR gas existing in the cylinder during the intake stroke from exiting in the intake passage direction. As shown by the broken line in the lift characteristic of the intake valve 21, a negative overlap period similar to the negative overlap period provided in the exhaust stroke is also provided in the intake stroke.

  According to the embodiment shown in FIG. 9, in the transition period of the combustion mode switching, the required internal EGR amount can be ensured by the early closing and twice opening of the exhaust valve 22. Here, the advance timing (early closing) of the closing timing of the exhaust valve 22 by the VVT 72 is effective for securing the necessary EGR amount, but the opening timing of the exhaust valve 22 is also advanced to the expansion stroke side at the same time. The effective expansion ratio becomes smaller. Therefore, the closing timing of the exhaust valve 22 is not excessively advanced, and an EGR amount that is insufficient only by the advance timing of the closing timing is secured by opening the exhaust valve 22 twice.

  In the second mode, the operating angle of the exhaust valve 22 in the intake stroke is reduced in order to prevent exhaust gas due to SI combustion from other cylinders remaining in the collecting portion from being introduced into the cylinder. Since the shortage of EGR is only compensated by opening twice, the working angle can be reduced.

  As shown in FIG. 2, the case where the volumes of the plurality of independent exhaust passages 43 are not the same (the independent exhaust passages 43 of the cylinders 18 of # 2 and # 3 are larger than the independent exhaust passages 43 of the cylinders 18 of # 1 and # 4). Will be described. In this case, the # 2 and # 3 cylinders 18 having a small passage volume are closed in the exhaust stroke when the exhaust valve 22 is driven in the second mode than the cylinders # 1 and # 4 having a large passage volume. The profile of each first cam 23 is determined so that the timing is advanced.

  In the cylinders # 2 and # 3, since the volume of the independent exhaust passage 43 is small, the amount of exhaust gas due to CI combustion remaining in the independent exhaust passage 43 is small. Therefore, when the exhaust valve 22 is opened twice, there is a concern that exhaust gas due to SI combustion of the other cylinder 18 remaining in the collecting portion 44 is introduced into the cylinder.

Therefore, in the cylinders 18 of # 2 and # 3 where the volume of the independent exhaust passage 43 is small, the advance angle of the closing timing in the exhaust stroke of the exhaust valve 22 is increased to increase the amount of internal EGR due to this early closing. reduce relatively EGR amount by degrees opening, Ru der which high-temperature exhaust gas remaining in the collection portion is prevented from being introduced into the cylinder 18.

1 Engine 2 Engine body 10 PCM (controller)
18 cylinder 22 exhaust valve 25 spark plug 43 independent exhaust passage 44 collecting portion 67 fuel injection valve

Claims (2)

An engine body having a plurality of cylinders;
A plurality of independent exhaust passages whose upstream ends are connected to each of the plurality of cylinders;
An exhaust valve that opens and closes the opening on the upstream end side of each independent exhaust passage;
A collecting portion to which the downstream end side of each independent exhaust passage is connected so that the exhaust gas that has passed through each independent exhaust passage joins;
A fuel injection valve provided for each cylinder to supply fuel into each cylinder;
A spark plug provided for each cylinder to ignite the air-fuel mixture in each cylinder;
By controlling at least the fuel injection valve and the spark plug, and a controller for operating the engine body,
The controller includes a compression auto-ignition combustion mode in which the air-fuel mixture is combusted by self-ignition, and a spark ignition combustion mode in which the air-fuel mixture is ignited and burned by the ignition plug in accordance with an operating state of the engine body. An engine exhaust system configured to switch between,
A part of the plurality of independent exhaust passages has a passage volume different from other independent exhaust passages,
A first mode in which the exhaust valve of each cylinder is opened and closed with a first opening / closing characteristic; and a second mode in which the closing timing in the exhaust stroke is advanced as compared with the first mode. The variable operation is switched to the third mode that opens and closes with a third opening / closing characteristic that has a valve opening period in the intake stroke in addition to the exhaust stroke, and the closing timing in the exhaust stroke is delayed compared to the second mode. With a valve mechanism,
In the spark ignition combustion mode, the controller drives the exhaust valve in the first mode, and when switching from the spark ignition combustion mode to the compressed auto ignition combustion mode, at least first in the compressed auto ignition combustion mode. the mixing said exhaust valve in the cylinder is performed the combustion of the gas is driven in the second mode, as will be driven after the driving in the second mode in the third mode, and controls the variable valve mechanism ,
Further, in the second mode, the exhaust valve is closed twice in the exhaust stroke and then opened twice in the intake stroke,
A working angle of the exhaust valve in the intake stroke in the second mode is smaller than a working angle of the exhaust valve in the intake stroke in the third mode;
In a cylinder to which an independent exhaust passage having a relatively small passage volume is connected, the closing timing in the exhaust stroke when the exhaust valve is driven in the second mode is set to an independent time when the passage volume is relatively large. An exhaust system for an engine, which is advanced with respect to a cylinder to which an exhaust passage is connected . In claim 1,
The advance angle of the closing timing of the exhaust valve in the second mode with respect to the first mode is changed according to the operating state of the engine,
An exhaust system for an engine, wherein the advancement degree is set to be larger as the engine load is relatively lower.

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