JP2013064388A - Carbon accumulation determination method and carbon removing method and spark-ignited direct-injection engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily determine carbon accumulation in a heat insulating layer 6 of a combustion chamber wall surface part.SOLUTION: Before the carbon is accumulated on the heat insulating layer 6, in a predetermined engine operation state, at least one of mean temperature during one burning cycle of the combustion chamber wall surface section, and, the maximum temperature during the one burning cycle of the combustion chamber wall surface section is detected as an initial value. The initial value detected is stored in a storage means. In the same engine operation state as when the initial value is detected, at least one of the mean temperature during the one burning cycle of the combustion chamber wall surface section, and, the maximum temperature during the one burning cycle of the combustion chamber wall surface section is detected. When at least one of the following conditions is satisfied, the carbon accumulation on the heat insulating layer 6 is determined: a condition in which the detected mean temperature increases to the first predetermined temperature or higher with respect to the initial value of the mean temperature; and a condition in which the maximum temperature detected in the temperature detection step decreases to the second predetermined temperature or lower with respect to the initial value of the maximum temperature.

Description

  The present invention belongs to a technical field related to a carbon deposition determination method, a carbon removal method, and 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, in Patent Document 2, from the viewpoint of reducing cooling loss and improving thermal efficiency, a surface that forms a combustion chamber of an engine has a lower thermal conductivity than the base material and is lower than the base material. Alternatively, a technique is disclosed that is constituted by a heat insulating material in which a large number of bubbles are formed inside a material having a heat capacity per unit volume substantially equal to that of the base material. The compression ratio of the engine of this Patent Document 2 is 16.

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 of the engine increases and as the specific heat ratio of the gas increases. For this reason, the combination of a high compression ratio and lean air-fuel mixture as described in Patent Document 1 is advantageous to some extent for improving thermal efficiency (illustration thermal efficiency). However, in this case, the illustrated thermal efficiency is maximized at a compression ratio of about 15, 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.

  In this regard, if the combustion chamber is insulated by providing a heat insulation layer having a low thermal conductivity and a low volume specific heat as described in Patent Document 2 on the wall surface of the combustion chamber, Loss can be reduced, and there is a high possibility that the illustrated thermal efficiency can be improved by a high compression ratio.

  However, when the total operating time of the engine becomes long, carbon accumulates in the heat insulating layer due to fuel injected into the combustion chamber, engine oil scraped up by a piston ring provided in the piston, and the like. Since the thermal conductivity and volumetric specific heat of the deposited carbon are relatively close to the base material (aluminum alloy or the like) constituting the combustion chamber wall, when carbon is deposited on the heat insulating layer, the heat insulating effect is reduced, Cooling loss will increase.

  The present invention has been made in view of such points, and the object of the present invention is to make it easy to determine that carbon has accumulated on the heat insulating layer provided on the wall surface of the combustion chamber, and When it is determined that carbon is deposited, the carbon is easily removed.

In order to achieve the above object, in the present invention, in a spark ignition direct injection engine in which a geometric compression ratio is 18 or more and a heat insulating layer is provided on the wall surface of the combustion chamber, carbon is deposited on the heat insulating layer. The heat insulation layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less. Before the carbon deposition on the heat insulation layer, in a predetermined engine operating state, at least one of the average temperature of one combustion cycle of the combustion chamber wall surface and the maximum temperature of the combustion chamber wall surface during one combustion cycle Initial value detection step for detecting the initial value as a default value, a storage step for storing the initial value detected in the initial value detection step in the storage means, and the same engine as the initial value detection after the storage step. In the operation state, the temperature detection step detects at least one of the average temperature of one combustion cycle of the wall surface of the combustion chamber and the maximum temperature during one combustion cycle of the wall surface of the combustion chamber. The condition that the average temperature rises by a first predetermined temperature or more with respect to the initial value of the average temperature, and the maximum temperature detected in the temperature detection step is a second value with respect to the initial value of the maximum temperature. A carbon deposition determination step of determining that carbon is deposited on the heat insulating layer when at least one of the conditions of lowering the temperature by a predetermined temperature or more is satisfied.

  By the above method, the thermal conductivity and volumetric specific heat of the material constituting the heat insulation layer are considerably lower than the base material (metallic material including aluminum alloy etc.) constituting the combustion chamber wall, and in particular, the volumetric specific heat is low, The temperature of the combustion chamber wall surface tends to fluctuate following the change of the gas temperature in the combustion chamber. As a result, the difference between the gas temperature and the temperature of the combustion chamber wall surface is reduced, thereby reducing the cooling loss. Is possible. As a result, the illustrated thermal efficiency can be improved with a high compression ratio of 18 or more. And, from the followability of the heat insulating layer as described above, the maximum temperature during one combustion cycle of the combustion chamber wall surface portion provided with the heat insulating layer is higher than when the heat insulating layer is not provided, The average temperature of one combustion cycle of the combustion chamber wall surface provided with is lower than that when no heat insulation layer is provided. Here, when carbon is deposited on the heat insulating layer, the thermal conductivity and volume specific heat of carbon are higher than those of the heat insulating layer and are close to the base material, so the average temperature increases and the maximum temperature decreases. To do. Therefore, the condition that the average temperature of one combustion cycle of the combustion chamber wall surface portion rises by more than the first predetermined temperature with respect to the initial value, and the maximum temperature during one combustion cycle of the combustion chamber wall surface portion is equal to or higher than the second predetermined temperature. When at least one of the conditions of decreasing is satisfied, it can be determined that carbon is deposited on the heat insulating layer. Therefore, it can be determined easily and accurately that carbon is deposited on the heat insulating layer.

In another aspect of the present invention, in a spark ignition direct injection engine in which a geometric compression ratio is 18 or more and a heat insulating layer is provided on a combustion chamber wall surface, carbon removal for removing carbon deposited on the heat insulating layer is performed. In this invention, the heat insulating layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less. Before carbon deposition on the heat insulating layer, at a predetermined engine operating state, at least one of an average temperature of one combustion cycle of the combustion chamber wall surface portion and a maximum temperature of one combustion cycle of the combustion chamber wall surface portion is an initial value. An initial value detecting step for detecting the initial value, a storing step for storing the initial value detected in the initial value detecting step in a storage means, and after the storing step, in the same engine operating state as at the time of detecting the initial value. The temperature detection step of detecting at least one of the average temperature of one combustion cycle of the combustion chamber wall surface portion and the maximum temperature during one combustion cycle of the combustion chamber wall surface portion, and the average temperature detected in the temperature detection step are The condition that the initial value of the average temperature rises by a first predetermined temperature or more, and the maximum temperature detected in the temperature detection step is a second predetermined temperature or more with respect to the initial value of the maximum temperature. A carbon deposition determining step for determining that carbon is deposited on the heat insulating layer when at least one of the conditions for lowering is satisfied, and carbon is deposited on the heat insulating layer in the carbon deposition determining step. When the determination is made that the excess air ratio λ of the entire combustion chamber is 1 or less and the combustion start of the air-fuel mixture is retarded at a predetermined timing of the expansion stroke. Te, it is intended to include a carbon sintered removed by the step of tempering removed by the carbon deposited on the heat insulating layer.

  This makes it possible to easily and accurately determine that carbon has accumulated in the heat insulation layer in the same manner as in the carbon deposition determination method described above. When this determination is made, the temperature of the combustion chamber wall surface portion can be determined. Can be heated to such a high temperature that the carbon deposited on the heat insulating layer can be burned, and the carbon can be easily burned off. By removing the carbon from the heat insulating layer in this way, the same heat insulating effect as in the initial state before the carbon is deposited can be obtained.

Yet another aspect of the present invention is an invention of a spark ignition direct injection engine in which the geometric compression ratio is 18 or more and a heat insulating layer is provided on the wall surface of the combustion chamber. The material has a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less. The temperature detection means for detecting at least one of the average temperature of the cycle and the maximum temperature in one combustion cycle of the combustion chamber wall surface, and the temperature detected by the temperature detection means before carbon deposition on the heat insulation layer is initially set Storage means for storing the value, a condition that the average temperature detected by the temperature detection means after storing the initial value is higher than a first predetermined temperature with respect to the initial value of the average temperature, and the initial value After storing the value When at least one of the conditions that the maximum temperature detected by the temperature detecting means falls below a second predetermined temperature with respect to the initial value of the maximum temperature is satisfied, carbon is deposited on the heat insulating layer. It is assumed that a carbon deposition determination unit that determines that the determination has been made is provided.

  According to this invention, it is possible to easily and accurately determine that carbon is deposited on the heat insulation layer, as in the invention of the carbon deposition determination method.

  In the spark ignition direct injection engine, when the carbon deposition determination means determines that carbon has accumulated in the heat insulation layer, the excess air ratio λ in the entire combustion chamber is set to 1 or less, and combustion of the air-fuel mixture is performed. It is preferable to further include a carbon burning means for burning the carbon deposited on the heat insulating layer by retarding the start at a predetermined time of the expansion stroke.

  By this, the carbon deposited on the heat insulation layer can be easily removed as in the invention of the carbon removal method.

The invention of another carbon removal method of the present invention is a spark ignition type direct injection engine in which a geometric compression ratio is 18 or more and a heat insulating layer is provided on the wall surface of the combustion chamber. A carbon removal method for removing carbon, wherein the heat insulating layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less. A carbon deposition determination step for determining that carbon is deposited on the heat insulation layer when a predetermined condition is satisfied, and determining that carbon is deposited on the heat insulation layer in the carbon deposition determination step. In this case, by setting the excess air ratio λ in the entire combustion chamber to 1 or less and retarding the start of combustion of the air-fuel mixture at a predetermined time in the expansion stroke, the carbon burning step for burning out the carbon deposited on the heat insulating layer is performed. It is intended to include and-flops.

  According to the present invention, for example, when a predetermined condition such as when the total operation time of the engine is equal to or longer than a predetermined time is satisfied, it is determined that carbon is deposited on the heat insulating layer. Since the carbon deposited on the layer is burned out, it can be easily determined that the carbon is deposited on the heat insulating layer, and the carbon deposited on the heat insulating layer can be easily removed.

  As described above, according to the carbon deposition determination method of the present invention, it can be easily and accurately determined that carbon is deposited on the heat insulating layer. In addition, according to the carbon removal method of the present invention, it is possible to easily and accurately determine that carbon is deposited on the heat insulating layer in the same manner as the carbon deposition determination method. The carbon deposited on the heat insulating layer can be easily removed. Furthermore, according to the spark ignition direct injection engine of the present invention, the same operational effects as those of the carbon deposition determination method can be obtained. Furthermore, according to another carbon removing method of the present invention, it can be easily determined that carbon has accumulated on the heat insulating layer, and carbon deposited on the heat insulating layer can be easily removed.

1 is a schematic view showing a spark ignition direct injection engine according to an embodiment of the present invention. It is sectional drawing which shows the structural example of a heat insulation layer. In ZrO _2, it is a graph showing the relationship between the porosity and thermal conductivity and volumetric specific heat. It is sectional drawing which expands and shows the part in which the temperature sensor of a cylinder head is provided. It is a graph which shows the temperature change and average temperature in 1 combustion cycle of a combustion chamber wall surface part. It is a flowchart which shows the control action regarding carbon deposition determination and carbon removal by an engine controller (CPU).

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 schematically shows a spark ignition direct injection engine 1 (hereinafter simply referred to as an engine 1) according to an embodiment of the present invention. In the present embodiment, the engine 1 includes various actuators attached to the engine body, various sensors, and an engine controller 100 that controls the actuators based on signals from the sensors.

  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 body of the engine 1 includes a cylinder block 12 and a cylinder head 13 mounted thereon, and a plurality of cylinders 11 (cylinders) are formed inside the cylinder block 12 (in FIG. 1). Only one is shown). Although not shown in the drawings, a water jacket through which cooling water flows is formed inside the cylinder block 12 and the cylinder head 13.

  A piston 15 is slidably inserted into each cylinder 11, and the piston 15 defines a combustion chamber 17 together with the cylinder 11 and the cylinder head 13. A concave cavity 15 a is formed at the center of 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 of the combustion chamber 17). Communicating with 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 on the lower surface of the cylinder head 13 (the ceiling surface of the combustion chamber 17). The intake port 18 is connected to an intake passage (not shown) through which fresh air introduced into the cylinder 11 flows. A throttle valve 20 for adjusting the intake flow rate is interposed in the intake passage, and the opening degree of the throttle valve 20 is adjusted in response to a control signal from the engine controller 100. On the other hand, the exhaust port 19 is connected to an exhaust passage (not shown) 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.

  The cylinder head 13 is provided with an intake valve 21 and an exhaust valve 22 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, respectively, and perform gas exchange 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. In addition, you may make it provide the variable lift mechanism (CVVL (Continuous Variable Valve Lift)) which can change a valve lift amount continuously with VVT23.

  A spark plug 31 is disposed on the cylinder head 13. The ignition plug 31 is attached and fixed to the cylinder head 13 by a known structure such as a screw. In the present embodiment, the spark plug 31 is attached and fixed in a state inclined to the exhaust side with respect to the central axis of the cylinder 11, and the tip (electrode) thereof faces the ceiling of the combustion chamber 17. The arrangement of the spark plug 31 is not limited to this. The spark plug 31 generates a spark by the ignition system 32. 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. The use of a plasma ignition type plug with high ignition energy is advantageous in improving the ignition stability.

  An injector 33 that directly injects fuel into the combustion chamber 17 is disposed on the central axis of the cylinder 11 in the cylinder head 13. The injector 33 is fixedly attached to the cylinder head 13 with a known structure such as using a bracket. The tip of the injector 33 faces the center of the ceiling of the combustion chamber 17. The arrangement of the injector 33 is not limited to this. In the present embodiment, the injector 33 is an outer valve-opening type injector having an outer valve that opens and closes a nozzle port that injects fuel into the combustion chamber. In this outer valve-opening injector, as the lift amount of the outer valve from the state in which the nozzle port is closed is larger, the penetration of fuel spray injected from the nozzle port into the combustion chamber 17 becomes larger. Adjustment of fuel spray penetration is facilitated.

  The fuel supply system 34 includes a fuel supply system that supplies fuel to the injector 33 and an electric circuit that drives the injector 33. This electric circuit receives a control signal from the engine controller 100 and operates the injector 33 to inject a desired amount of fuel into the combustion chamber 17 at a predetermined timing.

  Here, the fuel of the engine 1 is gasoline in the present embodiment, but may be gasoline containing bioethanol or the like, and any fuel as long as it is a fuel (liquid fuel) containing at least gasoline. Also good.

  The engine controller 100 is a controller based on a well-known microcomputer, and includes a central processing unit (CPU) that executes a program, a memory that is configured by, for example, RAM and ROM, and stores a program and data, And an input / output (I / O) bus for inputting and outputting 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 and a temperature signal from a temperature sensor 75 described later are received. Based on these input signals, the engine controller 100 calculates control parameters of the engine 1 such as a desired throttle opening signal, a fuel injection pulse, an ignition signal, a valve phase angle 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, and the like.

  The geometric compression ratio ε of the engine 1 is 18 or more. The upper limit value of the geometric compression ratio ε is preferably 40, and the geometric compression ratio ε is particularly preferably 25 or more and 35 or less. In the present embodiment, the engine 1 is also an engine 1 having a relatively high expansion ratio as well as a high compression ratio because of the configuration where 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 of the cylinder head 13 (a ceiling surface of the combustion chamber 17), and an intake valve 21 and an exhaust valve 22. The head is defined by a surface (lower surface) on the combustion chamber side of the head. And in order to reduce a cooling loss, the combustion chamber 17 is thermally insulated by providing the heat insulation layers 61, 62, 63, 64, and 65 on each of these surface portions (that is, the combustion chamber wall surface portion). 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 the combustion chamber wall surface portion, or may be provided on a part of the combustion chamber wall surface portion. Further, in the illustrated example, the heat insulating layer 61 on the cylinder wall surface portion is provided at a position above the piston ring 14 in a state where the piston 15 is located at the top dead center. The ring 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 of the piston 15 or a part thereof by extending the heat insulating layer 61 downward. Further, although it is not a wall surface portion that directly partitions the combustion chamber 17, a heat insulating layer may be provided on the port wall surface portion near the opening on the ceiling surface side of the combustion chamber 17 in the intake port 18 or the exhaust port 19. 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.

  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 wall surface of the combustion chamber. These heat insulating layers 61 to 65 are configured so that the heat of the combustion gas in the combustion chamber 17 is In order to suppress discharge through the wall surface portion of the combustion chamber, 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 portion of the cylinder 11, the cylinder block 12 is the base material, and for the heat insulating layer 62 provided on the crown surface portion of the piston 15, the piston 15 is the base material, and the cylinder head 13. For the heat insulating layer 63 provided on the lower surface portion of the cylinder, the cylinder head 13 is a base material, and for the heat insulating layers 64 and 65 provided on the surface of the intake valve 21 and the exhaust valve 22 on the combustion chamber side, Each of the valve 21 and the exhaust valve 22 is a base material. Hereinafter, these base materials are collectively referred to as a base material 7. Accordingly, the base material 7 is made of aluminum alloy or cast iron for the cylinder block 12, the cylinder head 13 and the piston 15, and is made of heat-resistant steel or cast iron for the intake valve 21 and the exhaust valve 22.

  Further, the heat insulating layer 6 is set to have a lower volume specific heat than the base material 7 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 the heat insulation structure of the combustion chamber 17, the cooling water flows in a water jacket formed in the cylinder head or cylinder block. Thus, the temperature of the wall surface of the combustion chamber is maintained substantially constant regardless of the progress of the combustion cycle.

  On the other hand, since the cooling loss is determined by cooling loss = heat transfer coefficient × heat transfer area × (gas temperature−temperature of the combustion chamber wall surface portion), the difference temperature between the gas temperature and the temperature of the combustion chamber wall surface portion is The larger the value, the larger the cooling loss. In order to suppress the cooling loss, it is desirable to reduce the difference between the gas temperature and the temperature of the combustion chamber wall surface. However, if the temperature of the combustion chamber wall surface is maintained approximately constant by cooling water, It is unavoidable that the temperature difference increases with fluctuation. Therefore, it is preferable to reduce the heat capacity of the heat insulating layer 6 so that the temperature of the combustion chamber wall surface portion changes following the fluctuation of the gas temperature in the combustion chamber 17.

In view of the above points, in the present embodiment, the heat insulating layer 6 is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less. . As a reference, the thermal conductivity of an aluminum alloy (casting alloy AC8A) is 140 W / (m · K), and its volume specific heat is 2300 kJ / (m ³ · K). The thermal conductivity of cast iron, carbon steel, stainless steel, and the like is about 10 to 40 W / (m · K), and their volume specific heat is about the same as that of the aluminum alloy. The thermal conductivity of ceramics such as ZrO ₂ and SiO _{2 is about 2 to 3} W / (m · K), and the volume specific heat is about the same as that of metal.

As a material having a low thermal conductivity and a low volume specific heat constituting the heat insulating layer 6, for example, as shown in FIG. 2, a material containing a large number of pores therein (in particular, a ceramic material such as ZrO ₂ is preferable). Can be adopted. The heat insulating layer 6 made of ceramic containing pores can be formed by coating on the base material by plasma spraying, and at that time, the porosity can be adjusted by adjusting the spraying energy. Alternatively, before the ceramic is fired, a material (for example, a polymer resin) that is burned off during ceramic firing is mixed, and the material is burned off during firing, thereby forming the heat insulating layer 6 made of ceramic containing pores therein. Is also possible.

FIG. 3 shows the relationship between porosity, thermal conductivity, and volumetric specific heat in ZrO ₂ containing pores inside (the pores are spherical and containing air). From this relationship, if the porosity of ZrO ₂ is 85% or more, the heat insulating layer 6 having the low thermal conductivity and the low volume specific heat can be obtained.

  Further, in the present embodiment, as shown in FIG. 1, a port liner 181 made of aluminum titanate having an extremely low thermal conductivity, excellent heat insulation, and excellent heat resistance is integrated with the cylinder head 13. A heat insulating layer is provided in the intake port 18 by casting. With this configuration, when fresh air passes through the intake port 18, it is possible to suppress or avoid an increase in temperature due to heat received from the cylinder head 13. As a result, the temperature of fresh air introduced into the combustion chamber 17 (initial gas temperature before compression) is lowered, so that the gas temperature at the time of combustion is lowered, and the temperature difference between the gas temperature and the temperature of the combustion chamber wall surface portion. This is advantageous in reducing the size. Lowering the gas temperature at the time of combustion can lower the heat transfer rate, which is advantageous for reducing the cooling loss. The configuration of the heat insulating layer provided in the intake port 18 is not limited to the cast-in of the port liner 181, and it is not always necessary to provide the heat insulating layer in the intake port 18.

  As described above, the cooling loss can be reduced by providing the heat insulating layer 6 having the low thermal conductivity and the low volume specific heat on the wall surface of the combustion chamber. Further, in addition to such a heat insulating structure, it is possible to further reduce the cooling loss by forming a heat insulating layer by a gas layer in the combustion chamber 17. In this case, fuel is injected from the injector 11 into the combustion chamber 17 during the compression stroke so that a gas layer containing fresh air is formed at the outer peripheral portion of the combustion chamber 17 of the engine 1 and an air-fuel mixture layer is formed at the central portion. Try to spray. That is, fuel is injected into the combustion chamber 17 by the injector 33 during the compression stroke, and the penetration of the fuel spray is suppressed to such a size (length) that the fuel spray does not reach the outer periphery of the combustion chamber 17. Thus, stratification is realized, in which an air-fuel mixture layer is formed at the center of the combustion chamber 17 and a gas layer containing fresh air is formed around it. This gas layer may be only fresh air, and may contain burned gas (EGR gas) in addition to fresh air. It should be noted that there is no problem even if a small amount of fuel is mixed in the gas layer, and the fuel layer may be leaner than the gas mixture layer so that the gas layer can serve as a heat insulating layer.

  When ignition is performed by the spark plug 31 in a state where the gas layer and the air-fuel mixture layer are formed as described above, the gas mixture between the air-fuel mixture layer and the wall surface of the cylinder 11 causes the flame of the air-fuel mixture layer to be in the cylinder 11. The gas layer becomes a heat insulating layer without coming into contact with the wall surface of the cylinder 11, and the release of heat from the wall surface of the cylinder 11 can be suppressed.

  It should be noted that if the cooling loss is simply reduced, the reduced cooling loss is converted into exhaust loss and does not contribute much to the improvement in the illustrated thermal efficiency. The energy of the combustion gas corresponding to the reduced cooling loss is efficiently converted into mechanical work. That is, it can be said that the illustrated thermal efficiency is greatly improved in the engine 1 by adopting a configuration that reduces both the cooling loss and the exhaust loss.

  Here, in the low load region where the engine load is equal to or less than a predetermined value, the excess air ratio λ in the entire combustion chamber 17 is set to 2 or more, or the weight ratio G / F of gas to fuel in the cylinder is set to 30 or more. The Thereby, in the low load region, RawNOx can be reduced while achieving thermal insulation by the heat insulation layer and improving the illustrated thermal efficiency. From the viewpoint of reducing RawNOx, the excess air ratio λ ≧ 2.5 is more preferable. In addition, since the illustrated thermal efficiency reaches a peak when the excess air ratio λ = 8, the range of the excess air ratio λ is preferably 2 ≦ λ ≦ 8 (more preferably 2.5 ≦ λ ≦ 8). 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).

  On the other hand, in the high load region where the engine load is higher than the predetermined value, the excess air ratio λ = 1 in the entire combustion chamber 17 is set with priority on torque (in the mixed gas layer, the excess air ratio λ <1. ). The predetermined value may increase as the engine speed increases, and may be a constant value regardless of the engine speed.

  As described above, when the total operation time of the engine 1 provided with the heat insulating layer 6 on the wall surface of the combustion chamber becomes long, the fuel that is injected into the combustion chamber 17 or the engine that is scraped up by the piston ring 14 provided in the piston 15. Carbon accumulates on the heat insulation layer 6 due to oil or the like. This carbon easily deposits when the temperature of the wall surface of the combustion chamber is within a certain temperature range (200 ° C. to 300 ° C.), but it accumulates if the time exceeding the temperature range in one combustion cycle is long to some extent. Even if it is easy to burn out. However, due to the heat insulation of the combustion chamber 17, the temperature of the wall surface of the combustion chamber becomes relatively low except during combustion (see the thick solid line graph in FIG. 5), and the deposited carbon is hardly burned off. In particular, as in this embodiment, if the intake port 18 is also provided with a heat insulating layer to lower the temperature of fresh air, the deposited carbon is more difficult to be burned off, and the carbon deposition proceeds. It becomes easy. The thermal conductivity and volumetric specific heat of the deposited carbon are higher than those of the heat insulating layer 6 and are relatively close to the base material 7. Therefore, when carbon is deposited on the heat insulating layer 6, the heat insulating effect is reduced and cooling is performed. Loss will increase.

  Therefore, in the present embodiment, when the engine controller 100 determines that carbon has accumulated on the heat insulating layer 6 as described later, and determines that carbon has accumulated, Remove (burn).

  For this purpose, a temperature sensor 75 for detecting the temperature of the combustion chamber wall surface portion is provided on the combustion chamber wall surface portion (in this embodiment, the lower surface portion of the cylinder head 13 (the ceiling portion of the combustion chamber 17)). In this embodiment, the temperature sensor 75 is a highly responsive thin film type temperature sensor, and can follow a change in gas temperature during combustion in the combustion chamber 17. That is, as shown in FIG. 4, the temperature sensor 75 is inserted into an insertion hole 13a provided in the cylinder head 13 (base material 7) so as to extend in the vertical direction. The insertion hole 13 a is covered with a heat insulating layer 63 in the same manner as the portion other than the insertion hole 13 a on the lower surface of the base material 7. Although not shown, the lower surface of the temperature sensor 75 is in contact with the heat insulating layer 63 with two electrodes in contact with the heat insulating layer 63 and a conductive wire made of Pt, Ni or the like connecting the two electrodes. It is provided to do. The resistance of the conducting wire is proportional to the temperature of the conducting wire (that is, the temperature of the combustion chamber wall surface portion). From the current value flowing between the two electrodes, the resistance value of the conducting wire, that is, the temperature of the conducting wire ( Temperature of the combustion chamber wall surface). The detection value (temperature signal) detected by the temperature sensor 75 is input to the engine controller 100, and the engine controller 100 receives this input, and the average temperature of one combustion cycle of the combustion chamber wall surface portion and the combustion chamber wall surface portion. At least one of the maximum temperatures during one combustion cycle is detected, and it is determined that carbon is deposited on the heat insulating layer 6 based on the detected temperature.

  In the present embodiment, the temperature sensor 75 is provided only on the combustion chamber wall surface of the combustion chamber 17 of any one cylinder 11 from the viewpoint of cost reduction. However, a temperature sensor 75 may be provided for each combustion chamber 17 of each cylinder 11.

  Here, how to determine that carbon is deposited on the heat insulating layer 6 will be described.

As shown in FIG. 5, before carbon deposition on the heat insulating layer 6, due to the heat insulating effect of the heat insulating layer 6, the maximum temperature in one combustion cycle of the combustion chamber wall surface portion is high, but the minimum temperature is low, and the combustion chamber The average temperature of one combustion cycle of the wall surface is low. The heat insulating layer 6 is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less, so that the gas as described above can be used. If the heat insulating layer is not formed by the layers, the temperature change of the combustion chamber wall surface before carbon deposition is substantially the same as the gas temperature change in the combustion chamber 17.

  When carbon is deposited on the heat insulating layer 6, the maximum temperature decreases while the average temperature increases according to the amount of deposition. The temperature change of the combustion chamber wall surface when the carbon deposition amount is 100 μm is relatively close to the temperature change of the combustion chamber wall surface when the heat insulating layer is not provided. Therefore, it can be seen that carbon is deposited on the heat insulating layer 6 by at least one of the amount of decrease in the maximum temperature from before carbon deposition and the amount of increase in the average temperature from before carbon deposition.

  Therefore, in the present embodiment, the average temperature of one combustion cycle of the combustion chamber wall surface portion and the combustion in a predetermined engine operating state before carbon deposition on the heat insulating layer 6 (for example, immediately after the engine is first operated) At least one of the maximum temperatures in one combustion cycle of the chamber wall surface portion is detected as an initial value, and the initial value is stored in the memory of the engine controller 100. The predetermined engine operating state is preferably an operating state that is used with high probability during operation of the engine 1.

  After storing the initial value, at least one of the average temperature of one combustion cycle of the combustion chamber wall surface portion and the maximum temperature of the combustion chamber wall surface portion in one combustion cycle is detected in the same engine operating state as when the initial value was detected. To do. The engine operating state that is the same as that at the time of detecting the initial value is an operating state in which the engine speed, the engine load, and the cooling water temperature (detected by a water temperature sensor not shown) are the same as those at the time of detecting the initial value. If the predetermined engine operating state is the operating state when the engine is warmed up, the same engine operating state as when the initial value is detected is the same operating state as when the engine speed and load are detected. Also good.

  Then, the condition that the detected average temperature rises by more than a first predetermined temperature with respect to the initial value of the average temperature, and the detected maximum temperature is second with respect to the initial value of the maximum temperature. When at least one of the conditions of lowering by a predetermined temperature or more is established, it is determined that carbon is deposited on the heat insulating layer 6. The first predetermined temperature may be, for example, a temperature corresponding to an increase amount of the average temperature from before carbon deposition when the carbon deposition amount is 10 μm. Similarly, the second predetermined temperature is What is necessary is just to set it as the temperature corresponding to the amount of decrease of the maximum temperature from before carbon deposition when the carbon deposition amount becomes 10 μm.

  In the present embodiment, as described above, the temperature sensor 75 is provided only on the combustion chamber wall surface of the combustion chamber 17 of any one of the cylinders 11, but it is determined that carbon has accumulated on the heat insulating layer 6. In this case, it is assumed that carbon is deposited on the heat insulating layer 6 provided on the combustion chamber wall surfaces of the combustion chambers 17 of all the cylinders 11. In addition, when the temperature sensor 75 is provided for each combustion chamber 17 of each cylinder 11, the carbon deposition determination may be performed for each combustion chamber 17 of each cylinder 11.

  In the present embodiment, since the temperature sensor 75 is a highly responsive thin film type temperature sensor, the engine controller 100 can accurately detect the maximum temperature during one combustion cycle of the combustion chamber wall surface. The average temperature of one combustion cycle on the wall surface of the combustion chamber can also be accurately detected (strictly, the average temperature is detected by calculating the average of one combustion cycle at a temperature that changes every moment). Thus, it can be determined that carbon is deposited on the heat insulating layer 6 based on one or both of the average temperature and the maximum temperature. However, the temperature sensor 75 is not limited to one having high response, and may be one that cannot accurately detect the maximum temperature. When such a temperature sensor is used, it may be determined that carbon is deposited on the heat insulating layer 6 based on the average temperature. Even if the maximum temperature cannot be accurately detected, the average temperature can be accurately detected because the maximum temperature is instantaneous.

  When it is determined that carbon is deposited on the heat insulating layer 6 as described above, the carbon deposited on the heat insulating layer 6 is burned out by performing a carbon burning operation. In this carbon burning operation, all the cylinders 11 (only the cylinders 11 in which it is determined that carbon has accumulated on the heat insulating layer 6 when the temperature sensor 75 is provided for each combustion chamber 17 of each cylinder 11). In this embodiment, the excess air ratio λ in the entire combustion chamber 17 is set to 1 or less (in this embodiment, λ = 1 according to the excess air ratio λ in the high-load operation region), and the start of combustion of the air-fuel mixture is determined in a predetermined expansion stroke. Retard on time. Accordingly, the temperature of the wall surface of the combustion chamber is maintained at a temperature at which carbon burns (temperature exceeding 300 ° C.), and the carbon deposited on the heat insulating layer 6 is burned off. In this embodiment, when the operation region when it is determined that carbon is deposited on the heat insulating layer 6 is the high load region, the carbon burning operation is performed immediately, but when the above determination is performed. When the operation region is the low load region, the carbon burning operation is not performed until the operation region is shifted to the high load region, and after the transition to the high load region where the excess air ratio λ in the entire combustion chamber 17 is 1, Perform carbon burning operation. Thus, the carbon deposited on the heat insulating layer 6 can be removed, and the same heat insulating effect as in the initial state before the carbon is deposited can be obtained. In the carbon burning operation, the heat insulating layer is not formed by the gas layer as described above.

  The control operation (the operation after storing the initial value) regarding the carbon deposition determination and the carbon removal by the engine controller 100 (CPU) will be described with reference to the flowchart of FIG. In this flowchart, it is determined that carbon is deposited on the heat insulating layer 6 based on the average temperature of one combustion cycle in the wall surface of the combustion chamber.

  In the first step S1, it is determined whether or not the engine 1 is in a predetermined engine operating state (the same engine operating state as when the initial value is detected). When the determination in step S1 is NO, the operation of step S1 On the other hand, if the determination in step S1 is yes, the process proceeds to step S2.

  In step S2, the average temperature of one combustion cycle of the combustion chamber wall surface portion is detected. In the next step S3, it is determined whether or not the detected average temperature has risen above a first predetermined temperature with respect to the initial value stored in the memory.

  When the determination in step S3 is NO, it is determined that no carbon has accumulated on the heat insulating layer 6 or carbon deposited on the heat insulating layer 6 has been removed, the process proceeds to step S4, and the counter value n (at the time of engine start) Is reset to 0, and then the process returns to step S1. On the other hand, when the determination in step S3 is YES, the process proceeds to step S5.

  In step S5, it is determined that carbon has accumulated on the heat insulating layer 6, and in the next step S6, it is determined whether or not the counter value n has reached a predetermined value (10 in the present embodiment). When the determination in step S6 is NO, the process proceeds to step S7, and it is determined whether or not the operating state of the engine 1 is in the high load operation region. When the determination of step S7 is NO, the operation of step S7 is repeated, while when the determination of step S7 is YES, the process proceeds to step S8 to perform the carbon burning operation. The carbon burning operation is performed, for example, by a predetermined number of cycles in the combustion cycle (the number of cycles in which most of the carbon deposited on the heat insulating layer 6 is burned off), and during that time, the temperature of the combustion chamber wall surface is the temperature at which carbon burns (Temperature exceeding 300 ° C.) In the next step S9, 1 is added to the counter value n, and then the process returns to step S1.

  When the determination in step S6 is YES, the process proceeds to step S10, a warning lamp provided on the display unit of the instrument panel of the vehicle is turned on, and then the control operation is terminated.

  When carbon is deposited on the heat insulating layer 6 by the control operation by the engine controller 100 and the average temperature of one combustion cycle of the combustion chamber wall surface portion rises above the first predetermined temperature with respect to the initial value, In step S8, a carbon burning operation is performed. The carbon is usually removed from the heat insulating layer 6 by this carbon burning operation. Therefore, the determination in step S3 after returning to step S1 via step S9 is NO. On the other hand, if the determination is YES, this means that the carbon was not burned off in the carbon burning operation. In this case, the carbon burning operation is performed again, and this is repeated. However, if the carbon is not burned out after repeating a predetermined number of times (in this embodiment, 10 times), a warning lamp is turned on because an abnormality such as a failure of the temperature sensor 75 has occurred, and the vehicle driver Encourage inspection at the service factory. If the determination in step S3 is NO until the predetermined number of times is repeated, this means that the carbon has been burned out, and the counter value n is reset to zero.

  If the average temperature is detected immediately after the end of the carbon burning operation, there is a possibility of erroneous detection due to the influence of the high temperature due to the carbon burning operation. In the meantime, it is preferable to open a predetermined number of cycles or more (the number of cycles that eliminates the influence of temperature during the carbon burning operation). However, when detecting the maximum temperature in one combustion cycle of the wall surface of the combustion chamber, it is not necessary to do so because the maximum temperature itself is not affected by the temperature during the carbon burning operation.

  In the present embodiment, the temperature sensor 75 and the engine controller 100 (CPU) detect at least one of the average temperature of one combustion cycle of the combustion chamber wall surface portion and the maximum temperature during one combustion cycle of the combustion chamber wall surface portion. The memory of the engine controller 100 constitutes a storage means, and the CPU of the engine controller 100 constitutes a carbon deposition determining means and a carbon burning means.

  Therefore, in the present embodiment, the condition that the average temperature of one combustion cycle of the combustion chamber wall surface portion detected after storing the initial value rises by more than the first predetermined temperature with respect to the initial value, and storing the initial value. When at least one of the following conditions is established, the maximum temperature during one combustion cycle of the wall surface of the combustion chamber decreases by a second predetermined temperature or more with respect to the initial value, the heat insulating layer 6 Since it is determined that carbon has been deposited, it can be easily and accurately determined that carbon has been deposited on the heat insulating layer 6.

  In addition, when it is determined that carbon is deposited on the heat insulating layer 6, the carbon deposited on the heat insulating layer 6 is burned out by performing a carbon burning operation. Can be easily removed, and the same heat insulating effect as in the initial state before the carbon is deposited can be obtained.

  The present invention is not limited to the embodiment described above, and can be substituted without departing from the spirit of the claims.

  For example, in the above embodiment, the condition that the average temperature of one combustion cycle of the combustion chamber wall surface portion detected after storing the initial value rises by more than the first predetermined temperature with respect to the initial value, and storing the initial value When at least one of the following conditions is established, the maximum temperature during one combustion cycle of the wall surface of the combustion chamber decreases by a second predetermined temperature or more with respect to the initial value, the heat insulating layer 6 Although it is determined that carbon has accumulated, for example, when the total operating time of the engine exceeds a predetermined time, or when there is a high possibility that carbon will accumulate (at an engine speed greater than a predetermined speed) It may be determined that carbon is deposited on the heat insulating layer 6 when a predetermined condition such as when the cumulative operation time in the (operation) state exceeds a predetermined time is satisfied. And when this determination is performed, the carbon deposited on the heat insulation layer 6 may be burned out by the carbon burning operation.

The material constituting the heat insulating layer 6 is any material as long as the thermal conductivity is 1.0 W / (m · K) or less and the volume specific heat is 500 kJ / (m ³ · K) or less. There may be. However, basic functions as the engine 1 such as heat resistance and pressure resistance need to be provided.

  The above-described embodiments are merely examples, and the scope of the present invention should not be interpreted in a limited manner. The scope of the present invention is defined by the scope of the claims, and all modifications and changes belonging to the equivalent scope of the claims are within the scope of the present invention.

  INDUSTRIAL APPLICABILITY The present invention is useful for a spark ignition direct injection engine having a geometric compression ratio of 18 or more and a heat insulating layer provided on the combustion chamber wall surface.

DESCRIPTION OF SYMBOLS 1 Spark ignition direct injection engine 6 Heat insulation layer 17 Combustion chamber 75 Temperature sensor (temperature detection means)
100 Engine controller (temperature detection means) (storage means)
(Carbon deposition judgment means) (Carbon burning means)

Claims (5)

In a spark ignition direct injection engine in which a geometric compression ratio is 18 or more and a heat insulating layer is provided on a wall surface of a combustion chamber, a carbon deposition determining method for determining that carbon is deposited on the heat insulating layer,
The heat insulation layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less.
Prior to carbon deposition on the heat insulation layer, in a predetermined engine operating state, at least one of an average temperature of one combustion cycle of the combustion chamber wall surface portion and a maximum temperature during one combustion cycle of the combustion chamber wall surface portion is set as an initial value. An initial value detection step to detect;
A storage step of storing in the storage means the initial value detected in the initial value detection step;
After the storing step, at least one of an average temperature of one combustion cycle of the combustion chamber wall surface portion and a maximum temperature of the combustion chamber wall surface portion during one combustion cycle is detected in the same engine operating state as when the initial value was detected. A temperature detection step;
The condition that the average temperature detected in the temperature detection step rises by more than a first predetermined temperature with respect to the initial value of the average temperature, and the maximum temperature detected in the temperature detection step is the maximum temperature And a carbon deposition determination step for determining that carbon is deposited on the heat insulating layer when at least one of the conditions of lowering the second predetermined temperature or more with respect to the initial value is satisfied. Characteristic carbon deposition judgment method. In a spark ignition direct injection engine having a geometric compression ratio of 18 or more and a heat insulation layer provided on a combustion chamber wall surface, a carbon removal method for removing carbon deposited on the heat insulation layer,
The heat insulation layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less.
Prior to carbon deposition on the heat insulation layer, in a predetermined engine operating state, at least one of an average temperature of one combustion cycle of the combustion chamber wall surface portion and a maximum temperature during one combustion cycle of the combustion chamber wall surface portion is set as an initial value. An initial value detection step to detect;
A storage step of storing in the storage means the initial value detected in the initial value detection step;
After the storing step, at least one of an average temperature of one combustion cycle of the combustion chamber wall surface portion and a maximum temperature of the combustion chamber wall surface portion during one combustion cycle is detected in the same engine operating state as when the initial value was detected. A temperature detection step;
The condition that the average temperature detected in the temperature detection step rises by more than a first predetermined temperature with respect to the initial value of the average temperature, and the maximum temperature detected in the temperature detection step is the maximum temperature A carbon deposition determination step for determining that carbon is deposited on the heat insulating layer when at least one of the conditions of lowering the second predetermined temperature or more with respect to the initial value is established;
When it is determined in the carbon deposition determination step that carbon is deposited on the heat insulating layer, the excess air ratio λ in the entire combustion chamber is set to 1 or less, and the start of combustion of the air-fuel mixture is retarded at a predetermined time in the expansion stroke. And a carbon burning step for burning the carbon deposited on the heat insulating layer. A spark ignition type direct injection engine having a geometric compression ratio of 18 or more and having a heat insulating layer provided on a wall surface of the combustion chamber,
The heat insulation layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less.
Temperature detecting means for detecting at least one of an average temperature of one combustion cycle of the combustion chamber wall surface portion and a maximum temperature in one combustion cycle of the combustion chamber wall surface portion in the same engine operating state;
Storage means for storing, as an initial value, the temperature detected by the temperature detection means before carbon deposition on the heat insulating layer;
The condition that the average temperature detected by the temperature detecting means after storing the initial value rises by more than a first predetermined temperature with respect to the initial value of the average temperature, and the temperature detecting means after storing the initial value When at least one of the conditions that the maximum temperature detected by the above is lower than the second predetermined temperature with respect to the initial value of the maximum temperature is satisfied, it is determined that carbon is deposited on the heat insulating layer. A spark ignition direct injection engine characterized by comprising a carbon deposition determination means. The spark ignition direct injection engine according to claim 3,
When it is determined by the carbon deposition determination means that carbon has accumulated in the heat insulating layer, the excess air ratio λ in the entire combustion chamber is set to 1 or less, and the start of combustion of the air-fuel mixture is retarded at a predetermined time in the expansion stroke. A spark ignition direct injection engine, further comprising carbon burning means for burning carbon deposited on the heat insulation layer. In a spark ignition direct injection engine having a geometric compression ratio of 18 or more and a heat insulation layer provided on a combustion chamber wall surface, a carbon removal method for removing carbon deposited on the heat insulation layer,
The heat insulation layer is made of a material having a thermal conductivity of 1.0 W / (m · K) or less and a volume specific heat of 500 kJ / (m ³ · K) or less.
A carbon deposition determination step for determining that carbon is deposited on the heat insulating layer when a predetermined condition is satisfied;
When it is determined in the carbon deposition determination step that carbon is deposited on the heat insulating layer, the excess air ratio λ in the entire combustion chamber is set to 1 or less, and the start of combustion of the air-fuel mixture is retarded at a predetermined time in the expansion stroke. And a carbon burning step for burning the carbon deposited on the heat insulating layer.

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