JP5569468B2 - Control device for spark ignition engine - Google Patents

Description

  The technology disclosed herein relates to a control device for a spark ignition engine.

  A compression ignition type gasoline engine that compresses and ignites a lean air-fuel mixture is known as a technology that achieves both improved exhaust emission and improved thermal efficiency. For example, Patent Literature 1 describes an ignition assist technique in which ignition is forcibly performed near the compression top dead center in order to stabilize compression ignition in the compression ignition type gasoline engine. Further, in Patent Document 2, the spark ignition is performed together with the first fuel injection in the period up to the first half of the compression stroke, and then the second fuel injection is performed to compress and ignite this, thereby controlling the compression ignition timing. The technology to try to do is described.

JP 2005-105974 A JP 2002-276442 A

  As described above, compression ignition combustion is advantageous for improving exhaust emission and improving thermal efficiency, and therefore there is a demand for expanding the operating range in which this compression ignition combustion is performed. However, the compression ignition combustion becomes pre-ignition combustion in which the pressure rises rapidly as the engine load increases. For this reason, an increase in combustion noise and occurrence of abnormal combustion such as knocking are caused. Therefore, the operation region in which compression ignition combustion is performed is limited to the low load side, and it is difficult to expand to the high load side.

  Also, as the engine speed increases, the reaction time of the air-fuel mixture becomes insufficient, and the combustion stability of compression ignition combustion decreases, so it is difficult to expand the operating region where compression ignition combustion is performed to the high rotation side. .

  The technology disclosed herein has been made in view of the above points, and an object of the technology is to expand an operation range in which compression ignition combustion is performed in a control device for a spark ignition engine that performs compression ignition combustion. There is.

  In this regard, in an operation region where the engine load is relatively high, for example, the fuel injection timing may be delayed to delay the compression ignition timing after the compression top dead center. As a result, the compression ignition combustion is combustion during the expansion stroke in which the cylinder pressure gradually decreases due to motoring, which is advantageous for suppressing combustion noise by suppressing the pressure increase rate (dP / dT). Therefore, retarding the injection timing is effective in expanding the operation region where compression ignition combustion is performed to the high load side. However, if the compression ignition combustion is delayed too much, the combustion stability is lowered. Therefore, it is difficult to greatly expand the operation region in which the compression ignition combustion is performed only by measures for delaying the fuel injection timing.

  In view of this, the technology disclosed herein secures the combustion stability of the compression ignition combustion by performing the ignition assist by driving the ignition plug.

Specifically, the spark ignition engine control device disclosed herein includes an engine body configured to operate in a compression ignition mode for compressing and igniting an air-fuel mixture in a cylinder when in a predetermined operation region, A spark plug arranged to face the cylinder and configured to ignite the air-fuel mixture in the cylinder; and a fuel injection valve configured to inject fuel to be supplied into the cylinder; And at least one of the exhaust gas through a passage connecting the exhaust side and the intake side of the engine body, and a controller configured to operate the engine body by controlling the fuel injection valve and the spark plug. Part of the exhaust gas remains in the cylinder by controlling the opening timing of at least one of the intake valve and the exhaust valve of the engine main body and the external EGR means for recirculating the engine part to the intake side Comprising an internal EGR means.

Then, the controller, the in the compression ignition mode, later and compression top dead when in a rotation range in the medium load operating state in which injected total fuel Ri by said fuel injection valve of the engine body Ignition assist by driving the spark plug is performed before the point, thereby promoting the blue flame reaction, after which the main combustion by compression ignition starts, and the operating state of the engine body is in a low rotation range of medium load The ignition assist is not performed, and the controller also sets the air-fuel mixture in the cylinder at the time of the ignition assist to a lean state in which flame propagation does not occur even when the ignition assist is performed, and the main combustion of such crank angle position of the combustion center is the compression top dead center or later, sets the execution timing of the injection timing and the ignition assistance of the fuel, the operating state of the engine body, low On the low load side when in the load or medium load range, the exhaust gas is introduced into the cylinder by the internal EGR means out of the external EGR means and the internal EGR means, and as the load on the engine body increases, the internal The amount of internal EGR by the EGR means is reduced .

  According to this configuration, in the compression ignition mode, ignition assist by driving the spark plug is performed after fuel injection by the fuel injection valve and before compression top dead center. Here, by setting the air-fuel mixture in the cylinder to a lean state in which flame propagation does not occur even when the ignition assist is executed, a thermal flame reaction does not immediately occur even if the ignition assist is performed. The air-fuel mixture is preferably uniform from the viewpoint of exhaust emission. As will be described later, this ignition assist improves combustion stability in a uniform lean air-fuel mixture.

  NO and O radicals are generated by the thermal plasma during ignition assist ignition, and these NO and O radicals promote the blue flame reaction after the low-temperature oxidation reaction. As a result, the heat generation rate rises to raise the temperature in the cylinder, the state in the cylinder reaches the hot flame reaction start temperature / pressure, and the hot flame reaction, that is, main combustion by compression ignition starts. Since the ignition assist promotes the blue flame reaction before the hot flame reaction, it can be rephrased as reducing the ignition condition, that is, the condition that the temperature and / or pressure state in the cylinder leads to compression self-ignition.

  Therefore, even if the crank angle position of the combustion center of gravity of the main combustion is after the compression top dead center, and the combustion stability is thereby lowered, the ignition condition is relaxed by executing the ignition assist. This makes it possible to reliably lead to compression ignition. This makes it possible to suppress the combustion noise by ensuring that the crank angle position of the combustion center of gravity of the main combustion is after the compression top dead center while ensuring the combustion stability. It is possible to expand the operating range in which combustion is performed.

The operation region in which the ignition assist is performed is a middle rotation region of a medium load, and the ignition assist is not performed when it is in a low rotation region of a medium load. In other words, the crank angle position of the combustion center of gravity of the main combustion is the compression top dead center in the operating region in the compression ignition mode where the load on the engine body increases and the compression ignition combustion becomes the combustion of premature ignition with a strong pressure rise. The ignition assist is extremely effective for ensuring combustion stability while suppressing combustion noise in the following manner. Here, the “medium load” region may be a medium load region when the operation region of the engine body is divided into low, medium, and high in the load direction.

  In addition, it is possible to compensate for the fact that the engine speed increases and the reaction time of the air-fuel mixture becomes insufficient due to the relaxation of the ignition condition by the ignition assist. For this reason, the ignition assist is also effective in ensuring combustion stability in the middle speed region of the engine and in a higher rotational speed region.

  The controller may set the air-fuel mixture in the cylinder at the ignition assist time so that the excess air ratio λ is 1.7 or more, or G / F is 25 or more. This air-fuel mixture is in a lean state in which flame propagation does not occur even when ignition assist is executed, and is effective in improving the combustion stability of main combustion.

  The controller may perform the ignition assist in a load region lower than the medium load region when the temperature of the engine body is equal to or lower than a predetermined temperature. That is, when the temperature of the engine body is not warmed up to a predetermined temperature or less, the temperature in the cylinder is low, which is disadvantageous for compression ignition. As described above, the ignition assist is advantageous for improving the combustion stability by relaxing the ignition condition. Therefore, when the engine body is not warmed up, the ignition assist is executed even in the load region lower than the medium load region. It becomes possible to ensure combustion stability. That is, the use of the ignition assist makes it possible to expand the operating conditions for performing the compression ignition combustion with respect to the temperature.

The controller sets the air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio when the operating state of the engine main body is in a high rotation range of a medium load, and drives the spark plug to cause the engine main body by spark ignition combustion. You may drive.

Further, the spark plug includes a plurality of ignition coils, and when the operating state of the engine body is in a middle rotation range of a medium load, the controller is configured to operate in a low rotation side operation range in the middle rotation range. The ignition assist is performed by normal ignition that drives any one of the plurality of ignition coils, and the plurality of ignition coils are sequentially driven in the high rotation side operation region in the middle rotation region. The ignition assist may be performed by continuous ignition.

  As described above, in the compression ignition mode, the control device for the spark ignition engine performs an ignition assist after the fuel injection by the fuel injection valve and before the compression top dead center to promote the blue flame reaction. Since the ignition condition is relaxed, it is possible to suppress the combustion noise by ensuring that the crank angle position of the combustion center of gravity of the main combustion is after the compression top dead center while ensuring the combustion stability. As a result, the operating range in which compression ignition combustion is performed is expanded.

It is the schematic which shows the structure of a spark ignition type gasoline engine. It is a block diagram concerning control of a spark ignition type gasoline engine. It is a figure which illustrates the operating area of an engine. It is a figure which compares the state of SI combustion by a high pressure retarded injection, and the state of conventional SI combustion. It is a timing chart which shows the difference in operation | movement of an intake valve and an exhaust valve with respect to the difference in an engine load, and the difference in an ignition timing and an injection timing. It is a figure which shows the relationship of the pressure increase rate (dP / dT) with respect to engine load, and an excess air ratio ((lambda)). It is a figure which shows the difference of the pressure increase rate (dP / d (theta)) with respect to the fuel injection timing being different. It is a figure which illustrates the reaction process of compression ignition combustion when performing ignition assistance. (A) The figure explaining the thermal effect by ignition assistance, (b) The figure explaining the chemical effect by ignition assistance. It is a figure which shows the relationship between the difference in an ignition method, and combustion stability.

  Hereinafter, an embodiment of a control device for a spark ignition gasoline engine will be described with reference to the drawings. The following description of preferred embodiments is exemplary. 1 and 2 show a schematic configuration of an engine (engine body) 1. The engine 1 is a spark ignition gasoline engine that is mounted on a vehicle and supplied with fuel containing at least gasoline. The engine 1 is provided with a cylinder block 11 provided with a plurality of cylinders 18 (only one shown), a cylinder head 12 provided on the cylinder block 11, and a cylinder block 11 below the cylinder block 11. And an oil pan 13 in which oil is stored. 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 having a reentrant shape is formed on the top surface of the piston 14. The cavity 141 faces a direct injection injector 67 described later when the piston 14 is positioned near the compression top dead center. The cylinder head 12, the cylinder 18 and the piston 14 having the cavity 141 define a combustion chamber. 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, and the like can be changed as appropriate.

  The engine 1 is set to a relatively high geometric compression ratio of 14 or more for the purpose of improving the theoretical thermal efficiency, stabilizing the compression ignition combustion described later, and the like. In addition, what is necessary is just to set a geometric compression ratio suitably in the range of about 14-20.

  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 include an intake valve 21 and an exhaust valve that open and close the opening on the combustion chamber side. 22 are arranged respectively.

  Among the valve systems that drive the intake valve 21 and the exhaust valve 22, respectively, on the exhaust side, the operation mode of the exhaust valve 22 is switched between a normal mode and a special mode, for example, a hydraulically operated variable mechanism (see FIG. 2). Hereinafter, a VVL (Variable Valve Lift) 71 is provided. Although the VVL 71 is not shown in detail in its configuration, two types of cams having different cam profiles, a first cam having one cam crest and a second cam having two cam crests, and its first And a lost motion mechanism that selectively transmits an operating state of one of the second cams to the exhaust valve. When the operating state of the first cam is transmitted to the exhaust valve 22, the exhaust valve 22 operates in a normal mode in which the valve is opened only once during the exhaust stroke (see FIGS. 5B, 5C, and 5D). On the other hand, when the operating state of the second cam is transmitted to the exhaust valve 22, the exhaust valve 22 opens during the exhaust stroke and also opens during the intake stroke. It operates in a special mode that opens twice (see FIG. 5A). The normal mode and the special mode of the VVL 71 are switched according to the operating state of the engine. Specifically, the special mode is used in the control related to the internal EGR. In order to enable switching between the normal mode and the special mode, an electromagnetically driven valve system that drives the exhaust valve 22 by an electromagnetic actuator may be employed. Also, the execution of internal EGR is not realized only by opening the exhaust twice. For example, the internal EGR control may be performed by opening the intake valve 21 twice or by opening the intake valve twice, or by providing a negative overlap period in which both the intake valve 21 and the exhaust valve 22 are closed in the exhaust stroke or the intake stroke. Internal EGR control that causes the fuel gas to remain in the cylinder 18 may be performed.

  As shown in FIG. 2, the exhaust side valve system having the VVL 71 is arranged on the intake side as shown in FIG. 2. 72) and a lift variable mechanism (hereinafter referred to as CVVL (Continuously Variable Valve Lift)) 73 capable of continuously changing the lift amount of the intake valve 21. . The VVT 72 may employ a hydraulic, electromagnetic, or mechanical structure as appropriate, and illustration of the detailed structure is omitted. The CVVL 73 can also adopt various known structures as appropriate, and the detailed structure is not shown. With the VVT 72 and the CVVL 73, the intake valve 21 can change its valve opening timing, valve closing timing, and lift amount, as shown in FIGS.

  Further, for each cylinder 18, a direct injection injector 67 that directly injects fuel into the cylinder 18 and a port injector 68 that injects fuel into the intake port 16 are attached to the cylinder head 12.

  The direct injection injector 67 is disposed so that its nozzle hole faces the combustion chamber from the central portion of the ceiling surface of the combustion chamber. The direct injection injector 67 directly injects an amount of fuel corresponding to the operation state of the engine 1 at an injection timing according to the operation state of the engine 1 into the combustion chamber. In this example, the direct injection injector 67 is a multi-injector type injector having a plurality of injection holes, although detailed illustration is omitted. Thereby, the direct injection injector 67 injects the fuel so that the fuel spray spreads radially. When the piston 14 is positioned near the compression top dead center, the fuel spray injected radially from the central portion of the combustion chamber flows along the wall surface of the cavity 141 formed on the top surface of the piston. Then, it reaches around the spark plug 25 described later. It can be paraphrased that the cavity 141 is formed so that the fuel spray injected at the timing when the piston 14 is located near the compression top dead center is contained therein. The combination of the multi-hole injector 67 and the cavity 141 is advantageous in that it shortens the time until the fuel spray reaches around the spark plug 25 after fuel injection and shortens the combustion period. is there. In addition, the direct injection injector 67 is not limited to a multi-injection type injector, and an external valve-opening type injector may be adopted as the direct injection injector.

  As shown in FIG. 1, the port injector 68 is arranged facing an independent passage communicating with the intake port 16 to the intake port 16 and injects fuel into the intake port 16. One port injector 68 may be provided for each cylinder 18, and if two intake ports 16 are provided for one cylinder 18, the port injector 68 is provided for each of the two intake ports 16. Also good. The format of the port injector 68 is not limited to a specific format, and various types of injectors can be appropriately employed.

  The fuel tank (not shown) and the direct injection injector 67 are connected to each other by a high-pressure fuel supply path. A high-pressure fuel supply system 62 that includes a high-pressure fuel pump 63 and a common rail 64 and supplies fuel to the direct injection injector 67 at a relatively high fuel pressure is interposed on the high-pressure fuel supply path. The high-pressure fuel pump 63 pumps fuel from the fuel tank to the common rail 64, and the common rail 64 stores the pumped fuel at a high fuel pressure. When the direct injection injector 67 is opened, the fuel stored in the common rail 64 is injected from the injection port of the direct injection injector 67. Here, although not shown, the high-pressure fuel pump 63 is a plunger type pump, and is driven by the engine 1 by being connected to a timing belt between a crankshaft and a camshaft, for example. The high-pressure fuel supply system 62 configured to include this engine-driven pump makes it possible to supply fuel with a high fuel pressure of 40 MPa or more to the direct injection injector 67. The pressure of the fuel supplied to the direct injection injector 67 is changed according to the operating state of the engine 1 as will be described later. The high-pressure fuel supply system 62 is not limited to this.

  Similarly, the fuel tank (not shown) and the port injector 68 are connected to each other by a low-pressure fuel supply path. On this low pressure fuel supply path, a low pressure fuel supply system 66 for supplying fuel with a relatively low fuel pressure to the port injector 68 is interposed. Although not shown in detail, the low-pressure fuel supply system 66 includes an electric or engine-driven low-pressure fuel pump and a regulator, and is configured to supply a predetermined pressure of fuel to each port injector 68. . Since the port injector 68 injects fuel into the intake port, the pressure of the fuel supplied by the low pressure fuel supply system 66 is set lower than the pressure of the fuel supplied by the high pressure fuel supply system 62.

  A spark plug 25 that ignites the air-fuel mixture in the combustion chamber is also attached to the cylinder head 12. 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 facing the combustion chamber in the vicinity of the tip of the direct injection injector 67 disposed in the central portion of the combustion chamber. Although illustration is omitted, the ignition plug 25 is a multi-ignition coil system including a plurality (for example, four) of ignition coils in this example, and drives any one of the plurality of ignition coils. It is possible to switch between normal ignition, continuous ignition that sequentially drives a plurality of ignition coils, and multiple ignition that simultaneously drives a plurality of ignition coils.

  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 for discharging burned gas (exhaust gas) from the combustion chamber of 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 downstream of the surge tank 33 is an independent passage branched for each cylinder 18, and the downstream end of each independent passage 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, a water-cooled intercooler / warmer 34 that cools or heats the air and a throttle valve 36 that adjusts the amount of intake air to each cylinder 18 are arranged. It is installed. An intercooler bypass passage 35 that bypasses the intercooler / warmer 34 is also connected to the intake passage 30. The intercooler bypass passage 35 is connected to an intercooler for adjusting the flow rate of air passing through the passage 35. A bypass valve 351 is provided. The temperature of fresh air introduced into the cylinder 18 is adjusted by adjusting the ratio between the flow rate of the intercooler bypass passage 35 and the flow rate of the intercooler / warmer 34 through the adjustment of the opening degree of the intercooler bypass valve 351.

  The upstream portion of the exhaust passage 40 is constituted by an exhaust manifold having an independent passage branched for each cylinder 18 and connected to the outer end of the exhaust port 17 and a collecting portion where the independent passages gather. . A direct catalyst 41 and an underfoot catalyst 42 are connected downstream of the exhaust manifold in the exhaust passage 40 as exhaust purification devices for purifying harmful components in the exhaust gas. 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.

  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 2, detection signals from various sensors SW <b> 1 to SW <b> 16 are input to the PCM 10. 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, the downstream side of the intercooler / warmer 34, and the intercooler / warmer 34 downstream of the air cleaner 31. A second intake air temperature sensor SW3 for detecting the temperature of fresh air after passing through the EGR gas temperature sensor SW4, which is disposed in the vicinity of the connection portion of the EGR passage 50 with the intake passage 30 and detects the temperature of the external EGR gas. An intake port temperature sensor SW5 that is attached to the intake port 16 and detects the temperature of the intake air just before flowing into the cylinder 18, and an in-cylinder pressure sensor SW6 that is attached to the cylinder head 12 and detects the pressure in the cylinder 18. The exhaust passage 40 is disposed in the vicinity of the connection portion of the EGR passage 50, and the exhaust temperature and the exhaust respectively. Exhaust temperature sensor SW7 and exhaust pressure sensor SW8 for detecting a force, and is disposed on the upstream side of the direct catalyst 41, the linear O ₂ sensor SW9, direct catalyst 41 and underfoot catalyst 42 for detecting the oxygen concentration in the exhaust gas The lambda O ₂ sensor SW10 that detects the oxygen concentration in the exhaust gas, the water temperature sensor SW11 that detects the temperature of the engine coolant, the crank angle sensor SW12 that detects the rotation angle of the crankshaft 15, and the vehicle An accelerator opening sensor SW13 for detecting an accelerator opening corresponding to an operation amount of an accelerator pedal (not shown), intake side and exhaust side cam angle sensors SW14 and SW15, and a common rail 64 of the high pressure fuel supply system 62 are attached. Further, a fuel pressure sensor SW for detecting the fuel pressure supplied to the direct injection injector 67 16.

  The PCM 10 performs various calculations based on these detection signals to determine the state of the engine 1 and the vehicle, and in response to this, the direct injection injector 67, the port injector 68, the spark plug 25, the VVT 72 on the intake valve side, Control signals are output to actuators of the CVVL 73, the VVL 71 on the exhaust valve side, the high-pressure fuel supply system 62, and various valves (throttle valve 36, intercooler bypass valve 351, EGR valve 511, and EGR cooler bypass valve 531). Thus, the PCM 10 operates the engine 1.

  FIG. 3 shows an example of the operation region of the engine 1. For the purpose of improving fuel efficiency and exhaust emission, the engine 1 is compressed and self-ignited without ignition by the spark plug 25 in a low load region where the engine load is relatively low after the engine 1 is warmed up. Compressed ignition combustion is performed by performing combustion. However, as the load on the engine 1 increases, in the compression ignition combustion, the combustion becomes too steep and causes problems such as combustion noise. Therefore, in the engine 1, in a high load region where the engine load is relatively high, the compression ignition combustion is stopped, and the engine 1 is switched to the spark ignition combustion using the spark plug 25. Therefore, the engine 1 is configured to switch between a CI (Compression Ignition) mode in which compression ignition combustion is performed and an SI (Spark Ignition) mode in which spark ignition combustion is performed according to the operating state of the engine 1.

  As will be described in detail later, in the CI mode, for example, a relatively homogeneous lean mixing is achieved by injecting fuel into the cylinder 18 by the direct injection injector 67 at a relatively early timing, for example, during an intake stroke or a compression stroke. The air-fuel mixture is compressed and self-ignited in the vicinity of the compression top dead center. In the relatively low load region in the CI mode, the internal EGR control is used together from the viewpoint of stabilizing the compression ignition combustion by increasing the in-cylinder temperature. On the other hand, in the relatively high load region (corresponding to the medium load region) in the CI mode, the internal EGR control is stopped because the cylinder internal temperature increases and the compression ignition combustion becomes steep as the engine load increases. Switch to external EGR control. Alternatively, in the low load region in the medium load region, the internal EGR control and the external EGR control are used in combination, and the internal EGR amount is reduced as the load increases. In addition, as will be described in detail later, in the CI mode in the medium load region where external EGR control is performed, the stability of the compression ignition combustion decreases due to the lack of the reaction time of the air-fuel mixture in the medium rotation region where the engine speed increases. Therefore, for the purpose of improving combustion stability, ignition assist by driving the spark plug 25 is used in combination. Then, in the high engine speed region where the engine speed further increases, the compression ignition combustion becomes difficult from the viewpoint of combustion stability, so the SI mode is set (region below the one-dot chain line in FIG. 3).

  In contrast, in the SI mode, basically, for example, during the intake stroke or the compression stroke, the direct injection injector 67 injects fuel into the cylinder 18 to form a homogeneous or stratified air-fuel mixture and improve the compression. The mixture is ignited by performing ignition in the vicinity of the dead center. In the SI mode, the engine 1 is also operated at the theoretical air fuel ratio (λ = 1). This makes it possible to use a three-way catalyst, which is advantageous for improving the emission performance.

  The geometric compression ratio of the engine 1 is set to 14 or more (for example, 18) as described above. Since the high compression ratio increases the compression end temperature and the compression end pressure, the CI mode is advantageous in stabilizing the compression ignition combustion. On the other hand, since the high compression ratio engine 1 is switched to the SI mode in the high load region, the higher the engine load, the faster the ignition or the knocking (hereinafter, these may be collectively referred to as knocking). There is an inconvenience that abnormal combustion tends to occur.

  Therefore, in this engine 1, when the operating state of the engine is in a high load range, abnormal combustion is avoided by executing SI combustion in which the fuel injection form is greatly different from the conventional one. Specifically, this fuel injection mode is a period that is significantly retarded from the latter half of the compression stroke to the early stage of the expansion stroke with a fuel pressure that is significantly higher than in the past (hereinafter, this period is referred to as the retard period). The fuel is injected into the cylinder 18 by the direct injection injector 67. This characteristic fuel injection mode is hereinafter referred to as “high pressure retarded injection”.

  FIG. 4 shows the heat generation rate (upper figure) and the progress of the unburned mixture reaction in the SI combustion by the high pressure retarded injection (solid line) and the conventional SI combustion (broken line) in which fuel injection is performed during the intake stroke. It is a figure which compares the difference in a degree (lower figure). The horizontal axis in FIG. 4 is the crank angle. As a premise for this comparison, the operating state of the engine 1 is a high load region in the low speed region, and the amount of fuel to be injected is the same in the case of SI combustion by high pressure retarded injection and conventional SI combustion.

  First, in the conventional SI combustion, a predetermined amount of fuel is injected into the cylinder 18 during the intake stroke (broken line in the upper diagram). In the cylinder 18, a relatively homogeneous air-fuel mixture is formed after the fuel injection until the piston 14 reaches the compression top dead center. In this example, ignition is executed at a predetermined timing indicated by a white circle after the compression top dead center, thereby starting combustion. After the start of combustion, as shown by the broken line in the upper diagram of FIG. 4, the combustion ends through the peak of the heat generation rate. Here, the period from the start of fuel injection to the end of combustion corresponds to the reaction possible time of the unburned mixture (hereinafter sometimes simply referred to as the reaction possible time). During this time, the reaction of the unburned mixture gradually proceeds. The dotted line in the figure shows the ignition threshold, which is the degree of reactivity with which the unburned mixture reaches ignition. Conventional SI combustion has a very long reaction time, during which the unburned mixture reacts. Since it continues to advance, the reactivity of the unburned mixture before and after ignition exceeds the ignition threshold, causing abnormal combustion such as premature ignition or knocking.

  On the other hand, the high pressure retarded injection aims to shorten the reaction possible time, thereby avoiding abnormal combustion. That is, as shown in FIG. 4, the reaction possible time is the period during which the direct injection injector 67 injects the fuel ((1) injection period) and the combustible air-fuel mixture around the spark plug 25 after the completion of the injection. The sum of the period until formation ((2) mixture formation period) and the period until combustion ended by ignition ((3) combustion period), that is, (1) + (2) + (3). The high-pressure retarded injection shortens the injection period, the mixture formation period, and the combustion period, thereby shortening the reaction time.

  That is, a higher turbulence energy in the cylinder 18 is advantageous for shortening the combustion period. As described above, fuel injection at a high fuel pressure increases the turbulent energy in the cylinder 18, but if the injection timing is during the intake stroke, the time until the ignition timing is long, and the compression after the intake stroke The disturbance is attenuated due to the compression of the cylinder 18 in the stroke, and the disturbance energy in the cylinder during the combustion period becomes relatively low. That is, even if fuel is injected into the cylinder 18 at a high fuel pressure, the combustion period is hardly shortened as long as the injection timing is during the intake stroke.

  On the other hand, injecting fuel into the cylinder at a timing within the retard period, that is, at a relatively late timing and at high fuel pressure, starts combustion while suppressing attenuation of turbulence in the cylinder. to enable. For this reason, the turbulent energy in the cylinder during the combustion period increases. This shortens the combustion period and stabilizes combustion.

  Moreover, a high fuel pressure relatively increases the amount of fuel injected per unit time. For this reason, when compared with the same fuel injection amount, a high fuel pressure shortens the injection period compared with a low fuel pressure.

  Furthermore, the high fuel pressure is advantageous for atomizing the fuel spray injected into the cylinder, and makes the flight distance of the fuel spray longer. Thus, high fuel pressure shortens the mixture formation period.

  Therefore, the shortening of the injection period and the mixture formation period described above make it possible to set the fuel injection timing (more precisely, the injection start timing) to a relatively late timing. That is, the high fuel pressure enables fuel injection within the retard period.

  Thus, in the spark ignition mode, by performing fuel injection into the cylinder at a high fuel pressure and within the retard period, rapid combustion with a short combustion period is realized, thereby stabilizing the combustion.

  As described above, the high pressure retarded injection shortens the injection period, the mixture formation period, and the combustion period, and as a result, as shown in FIG. 4, the fuel injection start timing SOI to the combustion end timing θend are not changed. It is possible to significantly shorten the reaction time of the fuel mixture as compared with the case of fuel injection during the conventional intake stroke. As a result of shortening the reaction possible time, in the conventional intake stroke injection at a low fuel pressure, the reaction progress of the unburned mixture at the end of combustion exceeds the ignition threshold, and abnormal combustion occurs. As a result, the high-pressure retarded injection suppresses the progress of the reaction of the unburned air-fuel mixture at the end of combustion, and can avoid abnormal combustion.

  Next, with reference to FIG. 5, a description will be given of an operation state of the intake valve 21 and the exhaust valve 22 and a control example of the fuel injection timing and the ignition timing corresponding to the operating state of the engine 1. Here, in FIGS. 5A, 5B, 5C, and 5D, the engine load increases in the order of (a) <(b) <(c) <(d). (A) and (b) are low and medium load regions corresponding to the CI mode, and (c) is a high load region corresponding to the SI mode. (D) is a fully open load region corresponding to the SI mode.

  First, Fig.5 (a) shows the time of the driving | running state of the engine 1 in a low load region. Since this operation region is in the CI mode, the exhaust valve 22 is opened twice during the intake stroke under the control of the VVL 71 (see the solid line Ex2 in the figure. The solid line is the exhaust valve 22). ), And the broken line indicates the lift curve of the intake valve 21), whereby the internal EGR gas is introduced into the cylinder 18. The introduction of internal EGR gas increases the compression end temperature and stabilizes compression ignition combustion. However, since the temperature in the cylinder 18 naturally increases as the engine load increases, the internal EGR amount is reduced from the viewpoint of avoiding premature ignition (that is, knocking). As illustrated in FIG. 5, the internal EGR amount may be adjusted by adjusting the lift amount of the intake valve 21 under the control of the CVVL 73. Although not shown in FIG. 5, the internal EGR amount may be adjusted by adjusting the opening degree of the throttle valve 36. The fuel injection timing is set during the intake stroke, and the direct injection injector 67 injects fuel into the cylinder 18 to form a homogeneous lean air-fuel mixture in the cylinder 18. The fuel injection amount is set according to the load of the engine 1.

  FIG. 5A shows an example in which the valve opening period of the exhaust valve 22 during the intake stroke is set in the first half of the intake stroke. The valve opening period of the exhaust valve 22 may be set in the latter half of the intake stroke. When the valve opening period is set to the first half of the intake stroke, the exhaust valve 22 may be left open from the exhaust stroke sandwiching the exhaust top dead center to the first half of the intake stroke.

  FIG. 5B shows a case where the operating state of the engine 1 is in an intermediate load region where the engine load is higher than that in FIG. Although this operation region is also in the CI mode, the engine load is high and the temperature in the cylinder is relatively high. Therefore, if the internal EGR gas is introduced into the cylinder 18, the temperature in the cylinder becomes too high. Therefore, when the engine load is in the middle load range, the exhaust valve 22 is stopped twice to stop the introduction of the internal EGR gas, and the external EGR gas is introduced into the cylinder 18 instead. The fuel injection timing is set to an appropriate timing during the intake stroke or the compression stroke. At this timing, the direct injection injector 67 injects fuel into the cylinder 18 to form a homogeneous or stratified lean mixture in the cylinder 18. The point that the fuel injection amount is set according to the load of the engine 1 is the same as in FIG. As will be described in detail later, by delaying the fuel injection timing, the start timing of compression ignition is delayed, and the combustion center of gravity of compression ignition combustion can be set after the compression top dead center. This is advantageous in avoiding an increase in combustion noise by suppressing the pressure increase where the combustion is caused by premature ignition with a strong pressure increase as the engine load increases.

  FIG. 5C shows a case where the operating state of the engine 1 is in a high load range. This operation region is the SI mode, and in this operation region, the opening of the exhaust valve 22 is stopped twice. In the SI mode, the filling amount is adjusted so that the air-fuel ratio λ = 1. The adjustment of the filling amount may be performed by slow closing of the intake valve 21 in which the throttle valve 36 is fully opened while the closing timing of the intake valve 21 is set after the intake bottom dead center by the control of the VVT 72 and the CVVL 73. Good. This is advantageous for reducing pump loss. The adjustment of the filling amount may also be performed by adjusting the opening amount of the EGR valve 511 and adjusting the amount of fresh air introduced into the cylinder 18 and the amount of external EGR gas while the throttle valve 36 is fully opened. . This is effective for reducing pumping loss and cooling loss. In addition, introduction of external EGR gas contributes to avoiding abnormal combustion and also has an advantage of suppressing generation of Raw NOx. Further, as the adjustment of the filling amount, the slow closing control of the intake valve 21 and the control of the external EGR may be combined. In particular, on the low load side in the high load region, in order to prevent the EGR rate from being too high, the charging amount is adjusted by the slow closing control of the intake valve 21 while introducing the external EGR into the cylinder 18. Also good.

  The form of fuel injection is the high-pressure retarded injection described above. Therefore, the direct injection injector 67 directly injects the fuel into the cylinder 18 with a high fuel pressure during the retard period from the latter half of the compression stroke to the early stage of the expansion stroke. The high-pressure retarded injection may be configured by one injection (that is, batch injection), but as shown in FIG. 5C, two injections of the first injection and the subsequent second injection are performed. It may be configured to perform within the retard period (that is, divided injection). Since the first injection can ensure a relatively long air-fuel mixture formation period, it is advantageous for fuel vaporization atomization. Since a sufficient mixture formation period is secured by the first injection, the injection timing of the second injection can be set to a more retarded timing. This is advantageous for improving the turbulent energy in the cylinder and for shortening the combustion period. When performing split injection, it is preferable to set the fuel injection amount of the second injection to be larger than the fuel injection amount of the first injection. By doing so, the turbulent energy in the cylinder 18 is sufficiently increased, which is advantageous for shortening the combustion period and for avoiding abnormal combustion. Such split injection is performed only on the relatively high load side where the fuel injection amount increases in the high load region, and batch injection is performed on the low load side in the high load region where the fuel injection amount is relatively small. It may be. Further, the number of divisions is not limited to two, and may be set to three or more.

  Thus, in the SI mode, ignition by the spark plug 25 is executed in the vicinity of the compression top dead center after the end of fuel injection.

  FIG. 5D shows a case where the operating state of the engine 1 is in the fully open load region. This operating region is the SI mode, as in FIG. 5C, and stops the exhaust valve 22 from being opened twice. Further, since it is in the fully open load region, the external EGR is also stopped by closing the EGR valve 511.

  The form of fuel injection is basically high-pressure retarded injection, and is constituted by two injections into the cylinder 18 during the retard period of the first injection and the second injection, as shown in the figure. . The high pressure retarded injection may be batch injection. In this fully open load region, injection during the intake stroke may be added for the purpose of improving the intake charge efficiency. In this intake stroke injection, the intake air charging efficiency is improved by the intake air cooling effect accompanying the fuel injection, which is advantageous in improving the torque. Therefore, when the operating state of the engine 1 is in the fully open load region, three fuel injections of the intake stroke injection and the first and second injections are executed, or the intake stroke injection and the batch injection are performed. Two fuel injections are performed.

  Here, as described above, the high pressure retarded injection in which the fuel is directly injected into the cylinder 18 by the direct injection injector 67 has a very high fuel pressure. Therefore, if fuel is directly injected into the cylinder 18 during the intake stroke with such a high fuel pressure, a large amount of fuel may adhere to the wall surface in the cylinder 18 and cause problems such as oil dilution. There is. Therefore, in this intake stroke injection, fuel is injected into the intake port 16 not through the direct injection injector 67 but through the port injector 68 that injects fuel with a relatively low fuel pressure. By doing so, the above-mentioned problems such as oil dilution can be avoided.

  Then, as shown in FIG. 3, in the medium load region of the engine 1, the CI mode in which external EGR control is performed and the combustion injection timing is delayed according to the load of the engine 1, thereby increasing the pressure in the cylinder. Avoid the rate of increase becoming too high. This will be described with reference to FIGS. First, FIG. 6 shows the relationship between the pressure increase rate (dP / dT) in the cylinder with respect to the engine load and the excess air rate (λ) on the premise of compression ignition combustion. According to this, since the fuel injection amount increases as the engine load increases, the excess air ratio λ gradually decreases (see the black square in FIG. 6). On the other hand, the pressure increase rate dP / dT in the cylinder gradually increases as the engine load increases, that is, as the air-fuel mixture becomes richer (see white circles in FIG. 6). Thus, at a predetermined engine load or higher, the pressure increase rate dP / dT exceeds the allowable value, that is, the combustion noise exceeds the allowable value. Therefore, in order to expand the CI mode to an engine load exceeding the allowable value of the pressure increase rate dP / dT, a measure for reducing the pressure increase rate dP / dT is required.

  FIG. 7 is a diagram illustrating a difference in the pressure increase rate (dP / dθ) in the cylinder when the fuel injection timing is varied. According to this, when the fuel injection timing is set to a relatively early timing, the compression ignition start timing is advanced so as to approach the compression top dead center, and the combustion speed is also increased accordingly. That is, the pressure increase rate tends to exceed the allowable value. On the other hand, when the fuel injection timing is set to a relatively late timing, the start timing of the compression ignition is delayed and the compression top dead center is separated. For this reason, compression ignition combustion is performed during the expansion stroke in which the pressure in the cylinder gradually decreases as the piston descends, and the combustion speed decreases, which is advantageous for suppressing the peak of the pressure increase rate. Furthermore, when the peak position of the pressure increase rate in the compression ignition combustion corresponds to the crank angle position of the valley bottom in the motoring waveform, the maximum value of the pressure increase rate also decreases accordingly. The motoring waveform becomes 0 at the compression top dead center, and then becomes the valley bottom. Thus, retarding the fuel injection timing makes it possible to suppress the pressure increase rate so as not to exceed the allowable value, and can be used for expanding the CI mode.

  Although illustration is omitted, the pressure increase rate dP / dT in the cylinder increases according to the engine speed, which is disadvantageous in terms of combustion noise. It is effective to set the crank angle position of the combustion center of gravity of (main combustion) so as to be after compression top dead center. On the other hand, an increase in the engine speed is disadvantageous in terms of combustion stability because the reaction time of the air-fuel mixture becomes insufficient. Therefore, in the engine 1, as shown in FIG. 3, ignition assistance by driving the spark plug 25 is used in the middle rotation region in the middle load region of the engine 1.

  FIG. 8 illustrates a reaction process of compression ignition combustion when ignition assist is performed. The ignition assist by driving the spark plug 25 is executed after the fuel injection by the direct injection injector 67 or the port injector 68 and before the compression top dead center. The fuel injection timing is set to an appropriate timing during the intake stroke or the compression stroke.

  Here, the ignition assist is normal ignition that drives only one of the plurality of ignition coils having the ignition plug 25. When the ignition assist is executed, the air-fuel mixture in the cylinder is preferably in a lean state in which no flame propagates even when ignited by the spark plug 25. Specifically, the excess air ratio λ is 1. It is preferable that the lean state is 7 or more or G / F is 25 or more. In addition, it is desirable that the air-fuel mixture be a homogeneous mixture from the viewpoint of exhaust emission. The ignition assist is executed because the plasma generated at the time of ignition is extremely high, so that the temperature of the gas around the spark plug 25 is raised, and as a result, as shown in FIG. This state has the effect of changing from the broken line to the solid line in FIG.

  Further, in the ignition assist, as shown in FIG. 8, NO and O radicals generated by the thermal plasma at the time of ignition promote the blue flame reaction after the low temperature oxidation reaction. As a result, the heat generation rate rises to increase the temperature in the cylinder, reach the hot flame reaction start temperature / pressure, and start the hot flame reaction, that is, main combustion by compression ignition. Since the ignition assist promotes the blue flame reaction before the hot flame reaction as described above, as shown in FIG. 9B, the ignition condition (ignition) in which the temperature and / or pressure state in the cylinder reaches the compression self-ignition. It can be paraphrased that the line) is relaxed from the broken line to the solid line in FIG.

  Since the ignition assist lowers the ignition line, compression ignition combustion (main combustion) is performed in the middle rotation region of the engine 1 by introducing external EGR and adjusting the fuel injection timing as shown in FIG. Even if the crank angle position of the combustion center of gravity is after the compression top dead center, it is possible to ensure the combustion stability of the compression ignition combustion. In other words, by executing the ignition assist, it becomes possible to avoid an increase in combustion noise while ensuring combustion stability, and the restrictions on combustion stability and combustion noise can be relaxed and the CI mode can be expanded. It becomes possible.

  Here, FIG. 10 shows the relationship of the combustion stability to the difference in the ignition method. In FIG. 10, as an ignition method, normal ignition for driving only one of a plurality of ignition coils having an ignition plug 25, double ignition for sequentially driving two ignition coils, and three ignition coils. Comparison is made between triple ignition for sequentially driving and four ignition for sequentially driving four ignition coils. Therefore, the discharge period becomes longer in the order of normal ignition, 2 ignitions, 3 ignitions, 4 ignitions, and according to the figure, the longer the discharge period, the better the combustion stability. Therefore, as shown in FIG. 3, in the operation region on the higher rotation side than the operation region using the ignition assist based on the normal ignition described above, the ignition assist based on the quadruple ignition that provides improved combustion stability is performed. Use. As a result, since it is on the high rotation side, it is possible to ensure the combustion stability by the ignition assist by the quadruple ignition even in the operation region that is disadvantageous in terms of combustion stability. As a result, the operating range of the CI mode is further expanded.

  The CI mode (including normal ignition and quadruple ignition) using such ignition assist has an advantage of being advantageous in reducing NOx because the air-fuel mixture in the cylinder is brought into a homogeneous lean state.

  Here, it is preferable to set the ignition assist timing so that the end timing of the discharge period substantially coincides with the start timing of the low-temperature oxidation reaction in terms of promoting the blue flame reaction. For this reason, the start timing of the ignition assist is changed according to the ignition methods such as the normal ignition and the four-way ignition described above.

  In addition, since the compression ignition timing is changed by promoting the blue flame reaction by the ignition assist, the combustion center of gravity of the main combustion (thermal flame reaction) is compressed by adjusting the fuel injection timing and / or the ignition assist timing. What is necessary is just to make it become the predetermined crank angle position after a dead center.

  In the medium load region of the engine 1, the SI mode is set in the high engine speed region where the engine speed is higher than that in the medium engine speed region in which the ignition assist by the four-way ignition is performed. Alternatively, a CI mixture mode may be employed in which a relatively rich air-fuel mixture is formed and the ignition plug 25 is driven to increase the temperature and pressure in the cylinder, and then the air-fuel mixture in the cylinder is subjected to compression ignition combustion. This CI mode is different from the CI mode using the ignition assist described above because the blue flame reaction is not promoted by the ignition assist.

  The map shown in FIG. 3 corresponds to a map when the engine 1 is warmed up. When the engine 1 is warmed up, ignition assist is used in the medium load region of the engine 1. Here, since the ignition assist has an effect of lowering the ignition line and improving the combustion stability of the compression ignition combustion, for example, when the engine 1 is not warmed up to a predetermined temperature, usually the compression ignition combustion However, the compression ignition combustion may be performed by using the ignition assist. When the engine 1 is not warmed up, compression ignition combustion using ignition assist may be performed particularly in the low load region of the engine 1.

1 Engine (Engine body)
10 PCM (controller)
18 cylinder 25 spark plug 67 direct injection injector (fuel injection valve)
68 Port injector (fuel injection valve)

Claims (5)

An engine body configured to operate in a compression ignition mode for compressing and igniting an air-fuel mixture in a cylinder when in a predetermined operation region;
An ignition plug disposed facing the cylinder and configured to ignite an air-fuel mixture in the cylinder;
A fuel injection valve configured to inject fuel to be supplied into the cylinder;
A controller configured to operate the engine body by controlling at least the fuel injection valve and the spark plug;
An external EGR means for recirculating a part of the exhaust gas to the intake side through a passage connecting the exhaust side and the intake side of the engine body;
Internal EGR means for causing a part of the exhaust gas to remain in the cylinder by controlling the opening timing of at least one of the intake valve and the exhaust valve of the engine body ,
In the compression ignition mode, the controller
Wherein when the operating state of the engine body is in rotation range in the medium load performs ignition assistance by the driving of the spark plug in a later and before the compression top dead center and injecting the entire fuel Ri by said fuel injection valve, whereby Promote the blue flame reaction, after which the main combustion by compression ignition starts,
When the operating state of the engine body is in a low rotation range of medium load, the ignition assist is not performed,
The controller also sets the air-fuel mixture in the cylinder at the time of the ignition assist to a lean state in which flame propagation does not occur even when the ignition assist is executed, and the crank angle position of the combustion center of gravity of the main combustion is the compression Set the fuel injection timing and ignition assist execution timing to be after the top dead center ,
On the low load side when the operating state of the engine body is in a low load to medium load range, exhaust gas is introduced into the cylinder by the internal EGR means out of the external EGR means and the internal EGR means, and the engine A control device for a spark ignition engine that reduces the amount of internal EGR by the internal EGR means as the load on the main body increases . The control device for a spark ignition engine according to claim 1,
The controller is a control device for a spark ignition engine that makes the air-fuel mixture in the cylinder at the ignition assist point an excess air ratio λ of 1.7 or more or a G / F of 25 or more. The control device for a spark ignition engine according to claim 1 ,
The controller is a control device for a spark ignition engine that performs the ignition assist in a load region lower than the medium load region when the temperature of the engine body is equal to or lower than a predetermined temperature. The control device for a spark ignition engine according to claim 1,
The controller sets the air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio when the operating state of the engine main body is in a high rotation range of a medium load, and drives the spark plug to cause the engine main body by spark ignition combustion. A spark ignition engine control device that drives the engine. The control device for a spark ignition engine according to claim 1,
The spark plug includes a plurality of ignition coils,
When the operating state of the engine main body is in the middle rotation range of the medium load, the controller controls any one of the plurality of ignition coils in the low rotation side operation range in the middle rotation range. A spark ignition engine control apparatus that performs ignition assist by normal ignition that drives the engine, and performs ignition assist by continuous ignition that sequentially drives the plurality of ignition coils in the high rotation side operation region in the middle rotation region.

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