JP5834646B2 - Spark ignition direct injection engine - Google Patents

Description

  The technology disclosed herein relates to a spark ignition direct injection engine.

  For example, in Patent Document 1, in order to increase the theoretical thermal efficiency of a spark-ignition gasoline engine, a cavity recessed in the lower surface of the cylinder head and a protrusion projecting from the piston crown surface divide the combustion chamber into the central combustion chamber and the main combustion chamber. The combustion chamber as a whole is set to a compression ratio as high as about 16, and the air-fuel mixture is relatively rich in the central combustion chamber, and the air-fuel mixture is relatively lean in the main combustion chamber. Thus, an engine having a lean air-fuel mixture is described for the entire combustion chamber.

  Further, for example, Patent Document 2 describes a technique in which a surface that defines a combustion chamber is formed of a heat insulating material including a large number of bubbles from the viewpoint of reducing cooling loss and improving thermal efficiency.

Japanese Patent Laid-Open No. 9-217627 JP 2009-243355 A

  By the way, in the Otto cycle, which is the theoretical cycle of a spark ignition engine, the theoretical thermal efficiency increases as the compression ratio increases and the specific heat ratio of the gas increases. For this reason, the combination of the high compression ratio and the leaning of the air-fuel mixture as described in Patent Document 1 is advantageous to some extent for improving the thermal efficiency (the illustrated thermal efficiency), but in this case, the compression ratio of 15 The illustrated thermal efficiency is maximized to some extent, and even if the compression ratio is increased further, the illustrated thermal efficiency does not increase (inversely, the higher the compression ratio is, the lower the illustrated thermal efficiency is). This is because, since the air-fuel mixture is lean, a relatively large amount of air is introduced into the cylinder. On the other hand, a large amount of air in the cylinder is greatly compressed as the compression ratio increases, and the combustion pressure and temperature are reduced. This is because it becomes significantly higher. That is, the amount of heat released through the cylinder wall and the like is increased by a high combustion pressure and combustion temperature, and the cooling loss is greatly increased. As a result, the illustrated thermal efficiency is lowered. Reduction of cooling loss is important for improving thermal efficiency in a spark ignition engine.

  The technology disclosed herein has been made in view of such a point, and an object thereof is to reduce cooling loss in a spark ignition engine.

  Since the cooling loss is proportional to the temperature difference between the combustion gas in contact with the partition wall of the combustion chamber and the partition wall, the inventors of the present application have determined that the combustion loss in the combustion chamber is suppressed. We focused on the formation of a rich air-fuel mixture layer in the central part of the gas and a lean air-fuel mixture layer on the outer periphery surrounding it. By such stratification, the outer peripheral portion does not substantially contribute to combustion, and the combustion gas is prevented from coming into contact with the partition wall. Thus, the cooling loss is reduced. Moreover, since the outer peripheral part with much air or formed with air functions as a heat insulation layer between combustion gas and a division wall, it becomes more advantageous by reduction of a cooling loss.

  Here, since the fuel injection amount is relatively small when the operating state of the engine is in the low load region, the excess air ratio λ is set to a lean value of 2 or more to improve thermal efficiency and suppress NOx generation. However, since the fuel injection amount increases when the engine operating state is in the high load region, the excess air ratio λ cannot be maintained at 2 or more. The excess air ratio λ must be set to 1 or more.

  In this case, the stratification as described above contributes to combustion by the amount of air present in the outer peripheral portion that does not contribute to combustion even if the excess air ratio λ is set to λ ≧ 1 for the entire combustion chamber. There are cases where the air-fuel ratio in the central portion is insufficient and the excess air ratio becomes λ <1. In this case, the emission performance may be reduced.

  Therefore, the technology disclosed herein has decided to increase the amount of gas introduced into the combustion chamber by performing exhaust gas recirculation (EGR) in a high load region where the excess air ratio λ ≧ 1. This reduces the amount of air at the outer peripheral portion in the combustion chamber and increases the amount of air at the central portion that contributes to combustion. As a result, the excess air ratio λ in the central portion becomes λ ≧ 1 as much as possible, and a decrease in the emission performance is avoided while reducing the cooling loss.

  Specifically, a spark ignition direct injection engine disclosed herein includes an engine body, a fuel injection valve configured to inject fuel into a combustion chamber of the engine body, and exhaust gas from the engine body to the combustion chamber. An exhaust gas recirculation means configured to recirculate to the engine, and a controller that controls fuel injection into the combustion chamber through the fuel injection valve and exhaust gas recirculation by the exhaust gas recirculation means in accordance with an operating state of the engine body; .

  The controller is configured such that when the operating state of the engine body is in a high load region, an air-fuel mixture layer that is richer than an outer peripheral portion that surrounds the central portion is formed in the combustion chamber. The fuel injection is performed in the compression stroke, and the excess air ratio λ of the entire combustion chamber at the start of combustion is set to 1 or more, and the controller is also in a high load region where the excess air ratio λ ≧ 1. The exhaust gas is recirculated into the combustion chamber.

  Here, the “high load region” may be a high load region when the operation region of the engine body is divided into two regions of a low load region and a high load region, or may be a low load region, a medium load region, and a high load region. It is good also as a high load area | region when it divides into three area | regions of a load area | region.

  According to the above configuration, when the operating state of the engine body is in the high load region, in the compression stroke, a richer air-fuel mixture layer is formed in the center portion than in the outer peripheral portion surrounding it in the combustion chamber. Perform fuel injection. Accordingly, as described above, the outer peripheral portion that is relatively lean and does not contribute to the combustion is interposed between the combustion gas and the partition wall of the combustion chamber, so that the combustion gas is prevented from contacting the partition wall. This is advantageous for reducing the cooling loss.

  On the other hand, since the excess air ratio λ of the entire combustion chamber at the start of combustion is set to satisfy λ ≧ 1, the air in the central portion is likely to be insufficient due to the presence of air in the outer peripheral portion. In the above configuration, exhaust gas recirculation is performed in a high load region where λ ≧ 1, and exhaust gas (that is, EGR gas) is introduced into the combustion chamber. Therefore, the amount of air contained in the outer peripheral portion depends on the amount of EGR gas introduced. To reduce. This increases the amount of air in the central part in the combustion chamber, so that the air-fuel mixture in the central part that contributes to combustion approaches λ ≧ 1, and it becomes possible to avoid deterioration in emission performance. .

  The spark ignition direct injection engine further includes an intake throttle valve that is disposed on an intake passage with respect to the engine body and whose opening degree is controlled by the controller, and the controller includes the excess air ratio λ. In the high load region of ≧ 1, the opening degree of the intake throttle valve may be fully opened.

  Performing exhaust gas recirculation when the intake throttle valve is fully open and no more fresh air can be introduced, the amount of gas introduced into the combustion chamber using the higher pressure on the exhaust side than on the intake side Makes it possible to increase As a result, as described above, the excess air ratio of the air-fuel mixture in the central portion in the combustion chamber can be maintained at λ ≧ 1.

  The geometric compression ratio ε of the engine body may be set to 18 or more and 40 or less.

  While a high compression ratio engine is advantageous for improving thermal efficiency, it may cause an increase in cooling loss as described above. The engine with this configuration performs stratification to form a relatively rich air-fuel mixture layer in the center of the combustion chamber, thereby reducing the cooling loss and increasing the expansion ratio associated with the higher compression ratio. Since the energy of the combustion gas corresponding to the reduced loss is efficiently converted into mechanical work, the illustrated thermal efficiency can be increased.

  When the operating state of the engine body is in a low load region, the controller injects fuel so that the excess air ratio λ of the entire combustion chamber becomes 2 or more and 8 or less, and stops recirculation of the exhaust gas. You may do it.

  A lean burn engine with an increased excess air ratio is advantageous in improving thermal efficiency and emission performance.

  When the operating state of the engine body is in a low load region, the controller performs fuel injection so that a richer air-fuel mixture layer is formed in the center portion than in the outer peripheral portion surrounding the combustion chamber. It may be executed.

  Even when the operating state of the engine body is in the low load region, as in the high load region, by providing an outer peripheral portion that does not contribute to combustion in the combustion chamber, cooling loss is reduced, which is advantageous for improving thermal efficiency. . Also, in the low load region, it is possible to secure a sufficient amount of air for a relatively small amount of fuel, so even if the exhaust gas recirculation is stopped while providing an outer peripheral portion that does not contribute to combustion, the emission performance deteriorates. do not do.

  As described above, the spark ignition direct injection engine forms a rich air-fuel mixture layer in the central portion in the combustion chamber as compared with the outer peripheral portion surrounding it when the operating state is in the high load region. Thus, while reducing the cooling loss, the exhaust gas recirculation is performed while setting the excess air ratio λ of the entire combustion chamber at the start of combustion so that λ ≧ 1, so that the air-fuel mixture in the center portion is λ It becomes closer to ≧ 1, and it becomes possible to avoid deterioration of emission performance.

It is a figure showing roughly composition of a spark ignition type direct injection engine. It is a block diagram which shows the structure which concerns on control of a spark ignition direct injection engine. It is a conceptual diagram which shows the structure of the gas in a combustion chamber. It is a figure which compares the composition of the gas in a combustion chamber.

  Hereinafter, an embodiment of a spark ignition direct injection engine (hereinafter also simply referred to as an engine) will be described with reference to the drawings. The following description of the preferred embodiment is merely exemplary in nature. As shown in FIGS. 1 and 2, the engine system includes an engine 1, various actuators associated with the engine 1, various sensors, and an engine controller 100 that controls the actuators based on signals from the sensors.

  The engine 1 is a spark ignition internal combustion engine, and has only a plurality of cylinders 11 although only one is shown in the figure. The engine 1 is mounted on a vehicle such as an automobile, and its output shaft is connected to drive wheels via a transmission, although not shown. The vehicle is propelled by the output of the engine 1 being transmitted to the drive wheels. The engine 1 includes a cylinder block 12 and a cylinder head 13 mounted thereon, and a cylinder 11 is formed inside the cylinder block 12. Inside the cylinder block 12 and the cylinder head 13, a water jacket 121 through which cooling water flows (however, only the water jacket in the cylinder block 12 is shown) is formed.

  The piston 15 is slidably inserted into each cylinder 11, and defines a combustion chamber 17 together with the cylinder 11 and the cylinder head 13. In this embodiment, a recess is formed in the crown surface of the piston 15.

  Although only one is shown in FIG. 1, two intake ports 18 are formed in the cylinder head 13 for each cylinder 11, and each opens to the lower surface of the cylinder head 13 (the ceiling surface that defines the upper surface of the combustion chamber 17). And communicates with the combustion chamber 17. Similarly, two exhaust ports 19 are formed in the cylinder head 13 for each cylinder 11, and each communicates with the combustion chamber 17 by opening in the ceiling surface of the cylinder head 13. The intake port 18 is connected to an intake passage 41 through which fresh air introduced into the cylinder 11 flows. A throttle valve 20 that adjusts the intake air flow rate is provided upstream of the intake passage 41 (not shown). The throttle valve 20 receives a control signal from the engine controller 100 and its opening degree is adjusted. . On the other hand, the exhaust port 19 is connected to an exhaust passage 42 through which burned gas (exhaust gas) from each cylinder 11 flows. Although not shown, an exhaust gas purification system having one or more catalytic converters is disposed in the exhaust passage 42.

  As schematically shown in FIG. 1, an exhaust gas recirculation passage (that is, an EGR passage) 43 for returning a part of the burned gas to the intake passage 41 is provided between the exhaust passage 42 and the intake passage 41. It has been. An EGR valve 44 for adjusting the recirculation amount of burned gas (in other words, EGR gas) is disposed in the middle of the EGR passage 43, and downstream of the connection position of the EGR passage 43 in the exhaust passage 42. Is provided with an exhaust throttle valve 45. While the opening degree of the exhaust throttle valve 45 is normally fully opened, as will be described in detail later, the opening degree is reduced and the back pressure is increased in a predetermined operating state. Thus, the amount of EGR gas into the combustion chamber 17 is adjusted by adjusting the opening degrees of both the EGR valve 44 and the exhaust throttle valve 45. Although illustration is omitted, on the EGR passage 43, for example, a water-cooled EGR cooler for cooling the EGR gas introduced into the combustion chamber 17 may be provided.

  The intake valve 21 and the exhaust valve 22 are arranged so that the intake port 18 and the exhaust port 19 can be shut off (closed) from the combustion chamber 17, respectively. The intake valve 21 is driven by an intake valve drive mechanism, and the exhaust valve 22 is driven by an exhaust valve drive mechanism. The intake valve 21 and the exhaust valve 22 reciprocate at a predetermined timing to open and close the intake port 18 and the exhaust port 19 to exchange gas in the cylinder 11. Although not shown, the intake valve drive mechanism and the exhaust valve drive mechanism each have an intake camshaft and an exhaust camshaft that are drivingly connected to the crankshaft. These camshafts are synchronized with the rotation of the crankshaft. Rotate. Further, at least the intake valve drive mechanism includes a hydraulic or mechanical phase variable mechanism (Variable Valve Timing: VVT) 23 that can continuously change the phase of the intake camshaft within a predetermined angle range. ing. You may make it provide the lift variable mechanism (CVVL (Continuous Variable Valve Lift)) which can change a valve lift amount continuously with VVT23.

  The spark plug 31 is attached to the cylinder head 13 by a known structure such as a screw. In this embodiment, the spark plug 31 is attached in a state inclined to the exhaust side with respect to the central axis of the cylinder 11, and the tip (electrode) faces the ceiling of the combustion chamber 17. The arrangement of the spark plug 31 is not limited to this. The ignition system 32 receives a control signal from the engine controller 100 and energizes the spark plug 31 to generate a spark at a desired ignition timing. As an example, the ignition system 32 may include a plasma generation circuit, and the ignition plug may be a plasma ignition type plug.

  In this embodiment, the fuel injection valve 33 is disposed along the central axis of the cylinder 11 and is attached to the cylinder head 13 with a known structure such as using a bracket. The tip of the fuel injection valve 33 faces the center of the ceiling of the combustion chamber 17. In this example, the fuel injection valve 33 is a piezo-type injector of an outer valve type. Although such an injector has a relatively low penetration, it is excellent in fuel atomization. As will be described in detail later, a relatively rich air-fuel mixture is compared in the central portion in the combustion chamber 17 with an outer peripheral portion surrounding it. It is advantageous to form a lean mixture (preferably only air).

  The fuel supply system 34 includes a fuel supply system that supplies fuel to the fuel injection valve 33, and an electric circuit that drives the fuel injection valve 33. The electric circuit receives a control signal from the engine controller 100 and operates the fuel injection valve 33 to inject a desired amount of fuel into the combustion chamber 17 at a predetermined timing. Here, the fuel of the lean burn engine 1 is gasoline in this embodiment, but is not limited thereto, and may be various liquefied fuels containing gasoline, for example.

  As shown in FIG. 2, the engine controller 100 is a controller based on a well-known microcomputer, and includes a central processing unit (CPU) that executes a program and, for example, a RAM and a ROM. And an input / output (I / O) bus for inputting and outputting electrical signals.

  The engine controller 100 includes at least a signal related to the intake air flow from the air flow sensor 71, a crank angle pulse signal from the crank angle sensor 72, an accelerator opening signal from the accelerator opening sensor 73 that detects the amount of depression of the accelerator pedal, A vehicle speed signal from the vehicle speed sensor 74 is received. The engine controller 100 calculates the following control parameters of the engine 1 based on these input signals. For example, a desired throttle opening signal, fuel injection pulse, ignition signal, valve phase angle signal, EGR valve opening signal, exhaust throttle valve opening signal, and the like. The engine controller 100 outputs these signals to the throttle valve 20 (throttle actuator that moves the throttle valve 20), the fuel supply system 34, the ignition system 32, the VVT 23, the EGR valve 44, the exhaust throttle valve 45, and the like.

  A characteristic feature of the lean burn engine 1 is that the geometric compression ratio ε of the engine 1 is 18 or more and 40 or less from the viewpoint of improving the indicated thermal efficiency of the engine and greatly improving the fuel consumption performance compared to the conventional one. In addition to setting the high compression ratio and setting the excess air ratio λ to 2 or more and 8 or less in at least the partial load operation region to make the air-fuel mixture lean, the heat insulating structure of the combustion chamber 17 is further combined. In the point.

  Here, the engine 1 is also an engine 1 having a relatively high expansion ratio at the same time as the high compression ratio because the compression ratio = expansion ratio. In addition, you may employ | adopt the structure (for example, Atkinson cycle and a mirror cycle) used as compression ratio <expansion ratio.

  As shown in FIG. 1, the combustion chamber 17 includes a wall surface of the cylinder 11, a crown surface of the piston 15, a lower surface (ceiling surface) of the cylinder head 13, and valve heads of the intake valve 21 and the exhaust valve 22. The combustion chamber 17 is thermally insulated by providing heat insulation layers 61, 62, 63, 64, and 65 having a configuration described later on each of these surfaces. In addition, below, when these heat insulation layers 61-65 are named generically, a code | symbol "6" may be attached | subjected to a heat insulation layer. The heat insulation layer 6 may be provided on all of these section screens, or may be provided on a part of these section screens. Further, in the illustrated example, the heat insulating layer 61 on the cylinder wall surface is provided at a position above the piston ring 14 in a state where the piston 15 is located at the top dead center. 14 is configured not to slide. However, the heat insulating layer 61 on the cylinder wall surface is not limited to this configuration, and the heat insulating layer 61 may be provided over the entire stroke or a part of the stroke of the piston 15 by extending the heat insulating layer 61 downward. Further, a heat insulating layer may be provided on the port wall surface near the opening on the ceiling surface side of the combustion chamber 17 in the intake port 18 and the exhaust port 19, although it is not the wall surface that directly partitions the combustion chamber 17. In addition, the thickness of each heat insulation layer 61-65 illustrated in FIG. 1 does not show actual thickness, but is only an illustration, and does not show the magnitude relationship of the thickness of the heat insulation layer in each surface.

In the lean burn engine 1, the geometric compression ratio ε is set to 18 ≦ ε ≦ 40 as described above. The theoretical thermal efficiency η _th in the Otto cycle, which is the theoretical cycle, is η _th = 1−1 / (ε ^κ−1 ), and the theoretical thermal efficiency η _th increases as the compression ratio ε increases. Further, the higher the specific heat ratio κ of gas, in other words, the higher the excess air ratio λ, the higher the theoretical thermal efficiency η _th .

  However, the illustrated thermal efficiency of the engine (more precisely, the engine having no combustion chamber insulation structure) peaks at a predetermined geometric compression ratio ε (for example, about 15), and the geometric compression ratio ε is more than that. However, the illustrated thermal efficiency does not increase, and conversely, the illustrated thermal efficiency decreases. This is because, when the geometric compression ratio is increased while the fuel amount and the intake air amount are kept constant, the higher the compression ratio, the higher the combustion pressure and the combustion temperature. That is, the cooling loss due to heat released through the surface defining the combustion chamber 17 is determined by cooling loss = heat transfer coefficient × heat transfer area × (gas temperature−zone screen temperature), and the combustion gas pressure and The higher the temperature, the higher the heat transfer rate. Therefore, the higher the combustion pressure and the combustion temperature will increase the cooling loss accordingly. As a result, in the lean burn engine, the higher the geometric compression ratio, the lower the illustrated thermal efficiency. In this way, even if it is attempted to increase the indicated thermal efficiency of the engine by increasing the geometric compression ratio while making the air-fuel mixture lean, the increase in cooling loss results in a peak in the indicated thermal efficiency that is significantly lower than the theoretical thermal efficiency. End up.

  On the other hand, in the lean burn engine 1, the heat insulating structure of the combustion chamber 17 is combined so that the illustrated thermal efficiency is increased at a high geometric compression ratio ε. That is, the heat loss of the combustion chamber 17 is reduced to reduce the cooling loss, thereby increasing the indicated thermal efficiency.

  On the other hand, merely reducing the cooling loss by insulating the combustion chamber 17 converts the reduced cooling loss into the exhaust loss and does not contribute much to the improvement in the illustrated thermal efficiency. As described above, the high expansion ratio accompanying the high compression ratio efficiently converts the combustion gas energy corresponding to the reduced cooling loss into mechanical work. That is, it can be said that the lean burn engine 1 greatly improves the illustrated thermal efficiency by adopting a configuration that reduces both the cooling loss and the exhaust loss.

  Here, the excess air ratio λ will be examined. When the excess air ratio λ is lower than 2, the maximum combustion temperature in the combustion chamber 17 becomes high, and RawNOx can be discharged from the combustion chamber 17. As described above, since the lean burn engine 1 aims to reduce exhaust loss as well as cooling loss, the exhaust temperature is relatively low, which is disadvantageous for catalyst activation. Therefore, it is desirable to avoid or suppress the discharge of RawNOx from the combustion chamber 17, and for that purpose, the excess air ratio λ is preferably set to 2 or more. In other words, it is desirable to set the excess air ratio λ in a range where the maximum combustion temperature in the combustion chamber 17 is a predetermined temperature (for example, 1800 K (Kelvin) as a temperature at which RawNOx can be generated) or less. The engine controller 100, for example, within the operation region of the partial load of the engine 1 is accompanied by an increase in the load (in other words, as the excess air ratio λ decreases due to an increase in the fuel injection amount), and the maximum combustion temperature becomes a predetermined temperature. When exceeding the above, it is desirable to operate the engine 1 by increasing the excess air ratio λ.

  On the other hand, according to the study by the inventors of the present application, the illustrated thermal efficiency peaks when the excess air ratio λ = 8. Therefore, the range of the excess air ratio λ is preferably 2 ≦ λ ≦ 8. In the high load operation region including the full load of the engine 1, the excess air ratio λ may be further reduced so that, for example, λ = 1 or λ ≧ 1 by torque priority. The numerical range of the excess air ratio λ is a preferable range of the engine 1 in a medium load and low load operation region.

  Note that the lean air-fuel mixture sets the throttle valve 20 on the open side, which can contribute to the improvement of the indicated thermal efficiency by reducing the gas exchange loss (pumping loss).

  Next, the heat insulation structure of the combustion chamber 17 will be described in more detail. As described above, the heat insulating structure of the combustion chamber 17 is configured by the heat insulating layers 61 to 65 provided on the respective screens that define the combustion chamber 17, and these heat insulating layers 61 to 65 are the combustion in the combustion chamber 17. In order to suppress the release of the heat of the gas through the section screen, the thermal conductivity is set lower than that of the metal base material constituting the combustion chamber 17. Here, for the heat insulating layer 61 provided on the wall surface of the cylinder 11, the cylinder block 12 is the base material, and for the heat insulating layer 62 provided on the crown surface of the piston 15, the piston 15 is the base material. For the heat insulating layer 63 provided on the ceiling surface, the cylinder head 13 is a base material, and for the heat insulating layers 64 and 65 provided on the valve head surfaces of the intake valve 21 and the exhaust valve 22, respectively, the intake valve 21 and the exhaust valve 22 are provided. Are the base materials. Therefore, the material of the base material is aluminum alloy or cast iron for the cylinder block 12, the cylinder head 13 and the piston 15, and the heat-resisting steel or cast iron for the intake valve 21 and the exhaust valve 22. However, as described above, since the lean burn engine 1 has reduced exhaust loss, the exhaust gas temperature has been greatly reduced. It is also possible to use a material that could not be used or was difficult to use (for example, an aluminum alloy).

  In addition, the heat insulating layer 6 preferably has a volumetric specific heat smaller than that of the base material in order to reduce cooling loss. That is, the gas temperature in the combustion chamber 17 varies with the progress of the combustion cycle, but in a conventional engine that does not have a heat insulation structure of the combustion chamber, cooling water flows in a water jacket formed in the cylinder head or cylinder block. Thus, the temperature of the surface defining the combustion chamber 17 is maintained substantially constant regardless of the progress of the combustion cycle.

  On the other hand, as described above, the cooling loss is determined by cooling loss = heat transfer coefficient × heat transfer area × (gas temperature−temperature of the section screen), so that the difference between the gas temperature and the wall surface temperature is large. The higher the loss, the greater the cooling loss. In order to suppress the cooling loss, it is desirable to reduce the difference temperature between the gas temperature and the temperature of the section screen. However, as described above, when the temperature of the section screen of the combustion chamber 17 is maintained substantially constant, It is inevitable that the temperature difference will increase as the temperature changes.

  Therefore, it is preferable that the heat insulating layer 6 has a small heat capacity, and the temperature of the section screen of the combustion chamber 17 changes following the fluctuation of the gas temperature in the combustion chamber 17.

  Further, reducing the heat capacity of the heat insulating layer 6 is advantageous for reducing exhaust loss. That is, if the heat capacity of the heat insulating layer is large, the temperature of the section screen does not decrease even when the temperature in the combustion chamber 17 decreases, but the combustion chamber 17 has a heat insulating structure. The temperature inside is kept high. This results in an increase in exhaust loss and hinders improvement in the thermal efficiency of the engine 1.

  On the other hand, reducing the heat capacity of the heat insulating layer 6 reduces the temperature of the section screen following that when the temperature in the combustion chamber 17 decreases. Therefore, it is possible to avoid maintaining the temperature in the combustion chamber 17 at a high temperature, which can be advantageous in suppressing exhaust loss in addition to the above-described suppression of cooling loss due to temperature followability.

As an example of the heat insulating layer 6, the heat insulating layer 6 includes the wall surface of the cylinder 11, the crown surface of the piston 15, the ceiling surface of the cylinder head 13, and the valve head surfaces of the intake valve 21 and the exhaust valve 22, that is, the combustion chamber. 17 to the partition face defining the, for example, formed by plasma spraying, zirconia (ZrO _2), or may be formed by coating of partially stabilized zirconia (PSZ). Zirconia or partially stabilized zirconia has a relatively low thermal conductivity and a relatively low volumetric specific heat, so that the thermal conductivity is lower than that of the base material and the heat capacity is the same as or lower than that of the base material. Layer 6 is constructed.

  Here, in the lean burn engine 1, when the excess air ratio is set to λ = 1 in the high load operation region including the full load during the warm period, the air-fuel mixture in the combustion chamber 17 is driven by the ignition plug 31. In a spark ignition mode in which the excess air ratio λ is set to 2 to 8 (or G / F is set to 30 to 120), in other operation regions (in other words, medium load to low load operation regions) The compression ignition mode for compressing and igniting the air-fuel mixture in the combustion chamber 17 may be set. Note that the compression ignition mode may be set over the entire operation region of the engine 1.

  Thus, in the lean burn engine 1, as conceptually shown in FIG. 3, a relatively rich air-fuel mixture is formed in the central portion 171 in the combustion chamber 17, while the outer periphery surrounding the central portion 171 is formed. A relatively lean mixture, preferably a fuel-free air layer, is formed in the portion 172. By doing so, the air-fuel mixture in the central portion 171 contributes to combustion, and the air-fuel mixture (or air) in the outer peripheral portion 172 does not contribute to combustion. This suppresses the combustion gas from coming into contact with the partition wall of the combustion chamber 17, and the air layer of the outer peripheral portion 172 functions as a part of the heat insulating layer described above, thereby further reducing the cooling loss. For this purpose, the fuel injection valve 33 is an outer-opening type piezo-type injector with low penetration and excellent fuel atomization, and the fuel injection timing by the fuel injection valve 33 is set in the latter half of the compression stroke. Set. The latter term of the compression stroke referred to here is the latter term when the compression stroke is divided into the first, middle and latter periods. By doing so, it is possible to atomize the fuel while preventing the injected fuel from reaching the wall surface of the combustion chamber 17 and before the injected fuel diffuses in the combustion chamber 17. It becomes possible to start combustion. Thus, a relatively rich air-fuel mixture is formed in the central portion 171 in the combustion chamber 17, while a relatively lean air-fuel mixture (or air layer) is formed in the outer peripheral portion 172 surrounding the central portion 171. Such stratification is realized.

  Here, as described above, the excess air ratio is set to, for example, λ = 1 in the high load operation region, while the excess air ratio λ is set to 2 to 8 in the medium load and low load operation regions. In the high load region where λ = 1, the excess air ratio of the central portion 171 in the combustion chamber 17 may be in a rich state where λ <1. That is, as shown on the left side in FIG. 4, in the load region where λ = 1, for example, the throttle valve 20 is set to fully open and the maximum amount of fresh air is introduced into the combustion chamber 17, while the combustion chamber The fuel injection amount is set so that λ = 1 as a whole. Here, as described above, when a relatively lean air-fuel mixture (that is, a heat insulating layer) is formed in the outer peripheral portion 172 in the combustion chamber 17, only the air exists in the outer peripheral portion 172 that does not contribute to combustion (FIG. 4). In the left figure, the hatched portion), air is insufficient in the central portion 171, and the excess air ratio of the central portion 171 contributing to combustion becomes excessively rich with λ <1. This state may cause a deterioration in emission properties.

  Therefore, in the lean burn engine 1, as shown on the right side in FIG. 4, the exhaust gas recirculation is performed through the EGR passage 43 by opening the EGR valve 44 to a predetermined opening degree in the high load region of λ = 1. Then, EGR gas is introduced into the combustion chamber 17. As a result, as shown in the right side of FIG. 4 with hatching, the amount of air in the central portion 171 increases by the amount of EGR gas contained in the outer peripheral portion 172, and the excess air ratio in the central portion 171 is increased. Becomes λ = 1 or close thereto. As a result, deterioration of emission performance can be avoided in advance while cooling loss is reduced by stratification.

  Here, in the high load region where λ = 1, the pressure on the exhaust side becomes higher than the pressure on the intake side, so that the exhaust gas is recirculated into the combustion chamber 17 even when the throttle valve 20 is fully opened. EGR gas can be introduced, but if necessary, the exhaust throttle valve 45 is closed to a predetermined opening to increase the back pressure and introduce a desired amount of EGR gas into the combustion chamber 17. Also good. The exhaust throttle valve 45 can be omitted. Further, in the high load region where λ = 1, from the viewpoint of reducing the cooling loss by lowering the temperature in the combustion chamber 17, as described above, an EGR cooler is arranged in the EGR passage 43, so that the EGR gas is as low in temperature as possible. Is preferably introduced into the combustion chamber 17.

  Further, in the medium load and low load regions where the excess air ratio λ is set to 2 to 8, particularly in the low load region, stratification is performed in the combustion chamber 17, while exhaust gas recirculation through the EGR passage 43 is stopped. . In the operation region where the excess air ratio is set to lean as a whole in the combustion chamber 17, a sufficient amount of air can be secured for a relatively small amount of fuel supplied into the combustion chamber 17. Even if the exhaust gas recirculation is stopped while forming, the emission property does not deteriorate. In the middle load region, exhaust gas recirculation may be performed as appropriate.

  The technique disclosed here is not limited to the application to the high compression ratio lean burn engine 1 having the heat insulation structure of the combustion chamber 17 as described above. For example, the heat insulation structure of the combustion chamber 17 is omitted. May be. Even in this case, since the heat insulation function of the combustion chamber 17 is obtained by stratifying the inside of the combustion chamber 17 at the center portion and the outer peripheral portion as described above, the cooling loss is reduced, and the indicated thermal efficiency of the engine 1 is improves.

  Moreover, since the technique disclosed here is effective in reducing the cooling loss, it is not limited to application to the lean burn engine 1 having a high compression ratio, and can be widely applied to a spark ignition direct injection engine.

1 Engine (Engine body)
17 Combustion chamber 100 Engine controller 20 Throttle valve (intake throttle valve)
33 Fuel injection valve 43 EGR passage (exhaust gas recirculation means)
44 EGR valve (exhaust gas recirculation means)
45 Exhaust throttle valve (exhaust recirculation means)

Claims (4)

The engine body,
A fuel injection valve configured to inject fuel into a combustion chamber of the engine body;
Exhaust gas recirculation means configured to recirculate exhaust gas of the engine body into the combustion chamber;
A controller for controlling fuel injection through the fuel injection valve into the combustion chamber and exhaust gas recirculation by the exhaust gas recirculation means according to the operating state of the engine body,
When the operating state of the engine body is in a high load region, the controller performs a compression stroke so that a richer air-fuel mixture layer is formed in the center portion than in the outer peripheral portion surrounding the combustion chamber. In the fuel injection, the excess air ratio λ of the entire combustion chamber at the start of combustion is set to 1 or more,
The controller also recirculates the exhaust gas into the combustion chamber in a high load region where the excess air ratio λ ≧ 1.
An intake throttle valve that is disposed on the intake passage with respect to the engine body and whose opening degree is controlled by the controller;
The controller is a spark ignition type direct injection engine that fully opens an opening of the intake throttle valve in a high load region where the excess air ratio λ ≧ 1. The engine body,
A fuel injection valve configured to inject fuel into a combustion chamber of the engine body;
Exhaust gas recirculation means configured to recirculate exhaust gas of the engine body into the combustion chamber;
A controller for controlling fuel injection through the fuel injection valve into the combustion chamber and exhaust gas recirculation by the exhaust gas recirculation means according to the operating state of the engine body,
When the operating state of the engine body is in a high load region, the controller performs a compression stroke so that a richer air-fuel mixture layer is formed in the center portion than in the outer peripheral portion surrounding the combustion chamber. In the fuel injection, the excess air ratio λ of the entire combustion chamber at the start of combustion is set to 1 or more,
The controller also recirculates the exhaust gas into the combustion chamber in a high load region where the excess air ratio λ ≧ 1.
A spark ignition direct injection engine in which the geometric compression ratio ε of the engine body is set to 18 or more and 40 or less. The engine body,
A fuel injection valve configured to inject fuel into a combustion chamber of the engine body;
Exhaust gas recirculation means configured to recirculate exhaust gas of the engine body into the combustion chamber;
A controller for controlling fuel injection through the fuel injection valve into the combustion chamber and exhaust gas recirculation by the exhaust gas recirculation means according to the operating state of the engine body,
When the operating state of the engine body is in a high load region, the controller performs a compression stroke so that a richer air-fuel mixture layer is formed in the center portion than in the outer peripheral portion surrounding the combustion chamber. In the fuel injection, the excess air ratio λ of the entire combustion chamber at the start of combustion is set to 1 or more,
The controller also recirculates the exhaust gas into the combustion chamber in a high load region where the excess air ratio λ ≧ 1.
When the operating state of the engine body is in a low load region, the controller injects fuel so that the excess air ratio λ of the entire combustion chamber becomes 2 or more and 8 or less, and stops recirculation of the exhaust gas. A spark ignition direct injection engine. In the spark ignition direct injection engine according to claim 3 ,
When the operating state of the engine body is in a low load region, the controller performs fuel injection so that a richer air-fuel mixture layer is formed in the center portion than in the outer peripheral portion surrounding the combustion chamber. A spark ignition direct injection engine to run.

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