JP2013057266A - Spark ignition type direct injection engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a fuel concentration to be uniform as much as possible in an air-fuel mixture layer when penetration of fuel spray injected into a cylinder by an outward opening valve type injector is decreased to form a gas layer in an outer peripheral portion in the cylinder as well as to form the air-fuel mixture layer in a center portion.SOLUTION: A fuel is injected from a nozzle port of an injector into the cylinder of an engine in a compression stroke so that a gas layer containing fresh air is formed in an outer peripheral portion in the cylinder and an air-fuel mixture layer is formed in a center portion. When injecting the fuel, a lift amount of an outward opening valve in initial and end stages is controlled to be larger than that between the stages.

Description

  The present invention belongs to a technical field related to a spark ignition direct injection engine provided with an outer valve-open injector.

  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 discloses a technique in which a surface that forms a combustion chamber of an engine is formed of a heat insulating material including a large number of bubbles from the viewpoint of reducing cooling loss and improving thermal efficiency. . 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, as described in Patent Document 2, if the cylinder wall surface is made of a heat insulating material to insulate the combustion chamber, the cooling loss can be reduced.

  However, as in Patent Document 2, there is a limit to the reduction of the cooling loss due to the heat insulation of the combustion chamber using the heat insulating material, and when trying to increase the geometric compression ratio to about 30 for example, the cooling loss It is difficult to drastically reduce the cost, and even if a significant reduction in cooling loss can be realized, the cost is greatly increased.

  Therefore, a gas layer containing fresh air (which may contain fresh air and burned gas) is formed at the outer peripheral portion in the cylinder of the engine, and an air-fuel mixture layer is formed at the central portion. If the gas is burned, the gas layer between the gas mixture layer and the cylinder wall surface prevents the flame of the gas mixture layer from coming into contact with the cylinder wall surface, and the gas layer serves as a heat insulating layer. The release of heat from can be suppressed. The gas layer and the gas mixture layer have such a size (length) that the fuel is injected into the cylinder by the injector during the compression stroke, and the penetration of the fuel spray does not reach the outer periphery of the cylinder. If it suppresses to, it can be formed.

  As the injector, an outer valve-opening type injector that has a basically small penetration and can freely change the penetration is preferable. In this outer valve type injector, the penetration of fuel spray injected from the nozzle port into the cylinder increases as the lift amount of the outer valve that opens and closes the nozzle port from the closed state increases. .

  However, when fuel is injected into the cylinder by the injector while the cylinder pressure is high, if the lift amount is constant at the time of the fuel injection, the fuel concentration becomes non-uniform in the air-fuel mixture layer. A trend arises. That is, the fuel spray injected at the initial stage of fuel injection has a small penetration and is resistant to the pressure in the cylinder, so it is difficult to proceed forward in the fuel injection direction, and the subsequent fuel spray is the initial fuel spray. At the same time, the initial fuel spray is pushed forward, and the tail is pulled to the rear, which continues until the final fuel spray at the time of fuel injection. As a result, a lean region is generated in the front and rear portions in the fuel injection direction, and a rich region is generated in the central portion. Thus, when the fuel concentration is uneven in the air-fuel mixture layer, the ignitability of the air-fuel mixture deteriorates.

  The present invention has been made in view of such a point, and an object of the present invention is to reduce the penetration of fuel spray injected into a cylinder by an externally opened injector, thereby reducing the outer peripheral portion in the cylinder. When the gas layer is formed at the center and the air-fuel mixture layer is formed at the center, the fuel concentration in the air-fuel layer is to be made as uniform as possible.

  In order to achieve the above object, according to the present invention, an open valve that opens and closes a nozzle port for injecting fuel into a cylinder, and the nozzle port is opened by lifting the open valve from a state in which the nozzle port is closed. The valve opening drive means for injecting fuel into the cylinder from the nozzle port, and the larger the lift amount of the outer valve from the closed state of the nozzle port, the larger the lift amount from the nozzle port to the cylinder. For a spark ignition type direct injection engine having an injector configured to increase the penetration of fuel spray injected into the engine, the geometric compression ratio of the engine is 18 or more and 40 or less, and the outer valve is opened. The injection control means further includes an injection control means for controlling the operation of the drive means, wherein the injection control means is such that a gas layer containing fresh air is formed at the outer peripheral portion in the cylinder of the engine and an air-fuel mixture layer is formed at the central portion. The compressed line In conjunction to inject fuel into the cylinder from the nozzle orifice, at the time of the fuel injection, the lift amount in the initial and end, has a configuration that is configured to be larger than the lift amount in between.

  With the above configuration, the gas layer located around the gas mixture layer prevents the flame of the gas mixture layer from coming into contact with the cylinder wall surface, and the gas layer serves as a heat insulating layer to release heat from the cylinder wall surface. Can be suppressed. Thereby, even if a geometric compression ratio becomes high, a cooling loss can be reduced significantly. Further, since the lift amount of the outer valve at the initial stage and the end stage is made larger than the lift quantity between them at the time of fuel injection, the penetration of the fuel spray injected at the initial stage and the end stage at the time of fuel injection becomes larger (longer). ) And the amount of fuel injected in the initial and final stages increases. As a result, the initial fuel spray tends to advance forward, and coupled with the increase in the fuel amount, when the lift amount is constant, the fuel concentration in the lean region generated in the front portion in the fuel injection direction can be increased. it can. Further, since the amount of fuel injected at the end stage increases, the fuel concentration in the lean region generated in the rear portion in the fuel injection direction when the lift amount is constant can be increased. Furthermore, the lift amount between the initial stage and the end stage can be reduced, and when the lift amount is constant, the fuel concentration in the rich region generated in the central portion in the fuel injection direction can be reduced. Therefore, the fuel concentration can be made uniform throughout the air-fuel mixture layer. Therefore, the ignitability of the air-fuel mixture can be improved.

  In the spark ignition direct injection engine, it is preferable that the injection control means is configured such that the lift amount at the end of the fuel injection is greater than or equal to the lift amount at the initial time of the fuel injection.

  That is, when the lift amount is constant, the initial fuel spray is pushed forward by the subsequent fuel spray, while the final fuel spray appears to have been crushed toward the rear, so in the fuel injection direction The fuel concentration in the rear part tends to be lower than the fuel concentration in the front part. Therefore, by making the lift amount at the end of fuel injection equal to or greater than the lift amount at the initial stage of fuel injection, the difference in concentration between the front portion and the rear portion in the fuel injection direction can be reduced. Therefore, the fuel concentration can be made more uniform throughout the air-fuel mixture layer.

  In the spark ignition direct injection engine, it is preferable that the injection control means is configured to inject fuel into the cylinder from the nozzle opening in the latter half of the compression stroke.

  That is, in the latter half of the compression stroke, the pressure in the cylinder is very high and the fuel concentration is more likely to be non-uniform.However, at the time of fuel injection, the lift amount of the open valve at the initial stage and the end stage is more than the lift amount between them. By increasing the value, it is possible to eliminate the uneven fuel concentration. Further, the fuel injection in the latter half of the compression stroke shortens the time until ignition, making it difficult for the fuel in the air-fuel mixture to mix with the gas layer in the outer peripheral portion, thereby ensuring the gas layer during combustion.

  Alternatively, when the engine load is in a high load region where the engine load is higher than a predetermined value, the injection control means is arranged so that the start of fuel combustion is performed after the compression top dead center and from the nozzle opening to the cylinder near the compression top dead center. You may be comprised so that a fuel may be injected in.

  Thus, combustion can be performed in the crank angle period in which the pressure increase rate is a negative maximum value or in the vicinity thereof, and as a result, even if the engine load is high, the vibration noise (so-called NVH) level is reduced. be able to. In addition, since the fuel concentration is uniform, the combustion period in the expansion stroke is shortened, and the illustrated thermal efficiency can be further enhanced in combination with the gas layer becoming a heat insulating layer. Furthermore, since the total amount of fuel to be injected increases at a high load, an extremely rich region is likely to occur. However, in the present invention, the fuel concentration can be made uniform, which makes the region extremely rich. The area disappears and the generation of wrinkles can be suppressed.

  As described above, according to the spark ignition direct injection engine of the present invention, compression is performed so that a gas layer containing fresh air is formed at the outer peripheral portion in the cylinder of the engine and an air-fuel mixture layer is formed at the center. In the stroke, fuel is injected into the cylinder from the nozzle port of the injector, and at the time of fuel injection, the lift amount of the outer open valve at the initial stage and the end stage is made larger than the lift amount in the meantime. The fuel concentration can be made uniform throughout the layer, and as a result, the ignitability of the air-fuel mixture can be improved.

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 internal structure of an injector. It is a graph which shows the change of the voltage of the injection signal with respect to a crank angle (or time), and the change of the lift amount of an outer valve opening. It is a graph which shows the change of the fuel concentration in a fuel injection direction.

  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, 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. In this embodiment, the combustion chamber 17 is a so-called pent roof type, and the ceiling surface (the lower surface of the cylinder head 13) has a triangular roof shape composed of two inclined surfaces on the intake side and the exhaust side. The crown surface of the piston 15 has a convex shape corresponding to the ceiling surface, and a concave cavity 15a is formed at the center of the crown surface. The shape of the ceiling surface and the crown surface of the piston 1 may be any shape as long as a high geometric compression ratio described later is possible. For example, the ceiling surface and the crown surface of the piston 1 ( Both of the portions excluding the cavity 15a may be configured by a surface perpendicular to the central axis of the cylinder 11, and the ceiling surface forms a triangular roof as described above, while the crown surface (cavity) of the piston 1 The portion excluding 15a) may be constituted by a plane perpendicular to the central axis of the cylinder 11.

  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 on the lower surface of the cylinder head 13 (the inclined surface on the intake side on the ceiling surface of the combustion chamber 17). By doing so, it communicates with the combustion chamber 17. Similarly, two exhaust ports 19 are formed in the cylinder head 13 for each cylinder 11, and each opens to the lower surface of the cylinder head 13 (the inclined surface on the exhaust side of the ceiling surface of the combustion chamber 17). Communicate. 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 tip of the spark plug 31 is located in the vicinity of a nozzle port 41 of an injector 33 described later. 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 cylinder (inside 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.

  As shown in FIG. 2, the injector 33 is an outer valve-opening type injector having an outer valve 42 that opens and closes a nozzle port 41 that injects fuel into the cylinder. The nozzle port 41 is formed in a tapered shape whose diameter increases toward the distal end side at the distal end portion of the fuel pipe 43 extending along the central axis of the cylinder 11. The end of the fuel pipe 43 on the base end side is connected to a case 45 in which a piezo element 44 serving as an external valve opening drive means is disposed. The outer opening valve 42 includes a valve main body 42 a and a connecting portion 42 b that is connected from the valve main body 42 a through the fuel pipe 43 to the piezo element 44. A portion of the valve body 42a on the side of the connecting portion 42b has substantially the same shape as the nozzle port 41, and when the portion is in contact (sitting) with the nozzle port 41, the nozzle port 41 is closed. . At this time, the tip side portion of the valve main body 42 a is in a state of protruding to the outside of the fuel pipe 43.

  The piezo element 44 lifts the outer open valve 42 from the state in which the nozzle opening 41 is closed by pressing the outer open valve 42 toward the combustion chamber 17 in the central axis direction of the cylinder 11 by deformation due to application of voltage. Then, the nozzle port 41 is opened. At this time, fuel is injected from the nozzle port 41 into the cylinder in a cone shape (specifically, a hollow cone shape) centered on the central axis of the cylinder 11. The taper angle of the cone is 90 ° to 100 ° in this embodiment (the taper angle of the inner hollow portion is about 70 °). When the application of voltage to the piezo element 44 is stopped, the piezo element 44 returns to the original state, and the outer opening valve 42 closes the nozzle port 41 again. At this time, the compression coil spring 46 disposed around the connecting portion 42 b in the case 45 facilitates the return of the piezo element 44.

  As the voltage applied to the piezo element 44 increases, the lift amount (hereinafter simply referred to as lift amount) of the outer open valve 42 from the state in which the nozzle port 41 is closed increases. The larger the lift amount, the larger the opening of the nozzle port 41, the greater the penetration of fuel spray injected from the nozzle port 41 into the cylinder, and the longer the amount of fuel injected per unit time. The particle size of the fuel spray increases and increases. The response of the piezo element 44 is fast, and the lift amount can be changed during fuel injection. The fuel injection time is, for example, about 2 ms at a light load, but the lift amount can be changed even during the injection time. Is possible.

  The fuel supply system 34 includes an electric circuit for driving the outer opening valve 42 (piezo element 44) and a fuel supply system for supplying fuel to the injector 33. The engine controller 100 outputs an injection signal having a voltage corresponding to the lift amount to the electric circuit at a predetermined timing, thereby operating the piezo element 44 and the outer valve 42 via the electric circuit, A desired amount of fuel is injected into the cylinder. When the injection signal is not output (when the voltage of the injection signal is 0), the nozzle port 41 is closed by the outer opening valve 42. Thus, the operation of the piezo element 44 is controlled by the injection signal from the engine controller 100. Therefore, the engine controller 100 constitutes an injection control means for controlling the operation of the piezo element 44. Thus, the engine controller 100 controls the operation of the piezo element 44 to control the fuel injection from the nozzle port 41 of the injector and the lift amount during the fuel injection.

  The fuel supply system is provided with a high-pressure fuel pump (not shown) and a common rail, and the high-pressure fuel pump pumps the fuel supplied from the fuel tank via the low-pressure fuel pump to the common rail. The pumped fuel is stored at a predetermined fuel pressure. The fuel stored in the common rail is injected from the nozzle port 41 by the operation of the injector 33 (the open valve 42 is lifted).

  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, And the vehicle speed signal from the vehicle speed sensor 74 is received, respectively. 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 then outputs these signals to the throttle valve 20 (throttle actuator that moves the throttle valve 20), the fuel supply system 34 (the above electric circuit), the ignition system 32, the VVT 23, and the like.

  The geometric compression ratio ε of the engine 1 is 18 or more and 40 or less. 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 (ceiling surface) of the cylinder head 13, and a valve head surface of each of the intake valve 21 and the exhaust valve 22. , Are partitioned. 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 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.

  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 constituted 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 gas in the combustion chamber 17. Therefore, the heat conductivity is set to be 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. Accordingly, the base material is aluminum alloy or cast iron for the cylinder block 12, cylinder head 13 and piston 15, and heat-resistant steel or cast iron for the intake valve 21 and exhaust valve 22.

  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 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 surface defining the combustion chamber 17 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 section screen), the cooling temperature increases as the temperature difference between the gas temperature and the wall surface temperature increases. The loss will increase. In order to suppress the cooling loss, it is desirable to reduce the difference between the gas temperature and the temperature of the section screen. However, when the temperature of the section screen of the combustion chamber 17 is maintained substantially 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 section screen of the combustion chamber 17 changes following the fluctuation of the gas temperature in the combustion chamber 17.

The heat insulating layer 6 may be formed, for example, by coating a ceramic material such as ZrO ₂ on the base material by plasma spraying. The ceramic material may contain a number of pores. If it does in this way, the thermal conductivity and volume specific heat of the heat insulation layer 6 can be made lower.

  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 the fresh air introduced into the cylinder 11 (initial gas temperature) is lowered, so that the gas temperature at the time of combustion is lowered and the temperature difference between the gas temperature and the section screen of the combustion chamber 17 is reduced. Become advantageous. Lowering the gas temperature at the time of combustion can lower the heat transfer rate, which is advantageous for reducing the cooling loss. In addition, the structure of the heat insulation layer provided in the intake port 18 is not limited to the casting of the port liner 181.

  In the present embodiment, in addition to the heat insulation structure of the combustion chamber 17 and the intake port 18 described above, a heat insulation layer is formed by a gas layer in the cylinder (inside the combustion chamber 17), thereby greatly reducing the cooling loss. ing.

  Specifically, the engine controller 100 performs compression so that a gas layer containing fresh air is formed at the outer peripheral portion of the cylinder (in the combustion chamber 17) of the engine 1 and an air-fuel mixture layer is formed at the center. In the stroke, an injection signal is output to the electric circuit of the fuel supply system 34 in order to inject fuel into the cylinder from the nozzle port 41 of the injector 11. That is, in the compression stroke, the fuel is injected into the cylinder by the injector 33 and the penetration of the fuel spray is suppressed to a size (length) so that the fuel spray does not reach the outer periphery of the cylinder. Stratification is realized in which an air-fuel mixture layer is formed at the center and a gas layer containing fresh air is formed around the air-fuel mixture layer. 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.

  The fuel injection timing by the injector 33 is preferably in the latter part of the compression stroke (particularly, the latter half when the compression stroke is divided into about three equal parts from the previous period, the middle period, and the latter period). This shortens the time until ignition, makes it difficult for the fuel in the air-fuel mixture to mix with the gas layer on the outer periphery thereof, and ensures the gas layer during combustion.

  In particular, when the engine load is in a high load region higher than a predetermined value, fuel is injected into the cylinder from the nozzle port 41 near the compression top dead center so that the start of fuel combustion is after the compression top dead center. It is preferable to do so. As a result, combustion can be performed in the crank angle period in which the pressure increase rate is a negative maximum value or in the vicinity thereof. As a result, even if the engine load is high, the vibration noise (NVH) level can be reduced. it can.

  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. As a result, the cooling loss can be greatly reduced.

  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 the predetermined value, the excess air ratio λ in the entire cylinder (inside the combustion chamber 17) is 2 or more, or the weight ratio G / F of gas to fuel in the cylinder is It is set to 30 or more. 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, the excess air ratio λ = 1 in the entire cylinder is set to give priority to torque (in the air-fuel mixture 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.

  If the lift amount of the outer opening valve 42 is constant (the voltage of the injection signal is constant) from the start to the end of injection during the fuel injection by the injector 33 (see the two-dot chain line in FIG. 3), There is a tendency for the fuel concentration to become non-uniform. That is, the fuel spray injected at the initial stage of fuel injection has a small penetration and is resistant to the pressure in the cylinder, so it is difficult to proceed forward in the fuel injection direction, and the subsequent fuel spray is the initial fuel spray. At the same time, the initial fuel spray is pushed forward, and the tail is pulled to the rear, which continues until the final fuel spray at the time of fuel injection. As a result, a lean region is generated in the front and rear portions in the fuel injection direction, and a rich region is generated in the central portion (see the two-dot chain line in FIG. 4). In addition, the initial fuel spray is pushed forward by the subsequent fuel spray, while the final fuel spray appears to be blurred by the rear side, so that the fuel concentration in the rear part in the fuel injection direction is The fuel concentration tends to be lower than the partial fuel concentration (see the two-dot chain line in FIG. 4). Thus, when the fuel concentration is uneven in the air-fuel mixture layer, the ignitability of the air-fuel mixture deteriorates.

  Therefore, in the present embodiment, the engine controller 100 changes the voltage of the injection signal during fuel injection (during fuel injection) in both the low load region and the high load region, and lifts the open valve 42. Change the amount.

  Specifically, as shown by a solid line in FIG. 3, during fuel injection, the voltage (lift amount) at the initial stage and the end stage is made larger than the voltage (lift amount) between them. Further, the voltage (lift amount) at the end of fuel injection is set to be equal to or higher than the voltage (lift amount) at the initial stage of fuel injection.

  Thereby, the penetration of the fuel spray injected at the initial stage and the final stage at the time of fuel injection increases (becomes longer), and the amount of fuel injected at the initial stage and the final stage increases. As a result, the initial fuel spray tends to advance forward, and coupled with the increase in the fuel amount, when the lift amount is constant, the fuel concentration in the lean region generated in the front portion in the fuel injection direction can be increased. it can. Further, since the amount of fuel injected at the end stage increases, the fuel concentration in the lean region generated in the rear portion in the fuel injection direction when the lift amount is constant can be increased.

  Further, when the lift amount is constant, the fuel concentration in the rear portion in the fuel injection direction is equal to or lower than the fuel concentration in the front portion, but the voltage at the end of fuel injection (lift amount) is set to the initial value at the time of fuel injection. By making the voltage (lift amount) or more at, the difference in concentration between the front portion and the rear portion in the fuel injection direction can be reduced. Normally, the length in the fuel injection direction of the low concentration region in the rear portion is longer than the length in the fuel injection direction in the low concentration region of the front portion, so the voltage (lift amount) at the end of fuel injection May be made larger than the initial voltage (lift amount) at the time of fuel injection.

  Further, the lift amount between the initial stage and the end stage can be reduced, and when the lift amount is constant, the fuel concentration in the rich region generated in the central portion in the fuel injection direction can be reduced.

  It should be noted that the initial and final stages of fuel injection and each lift amount during that time change according to the engine operating condition (especially the cylinder pressure during fuel injection), and the fuel spray penetration varies depending on the engine operating condition. The fuel spray is adjusted to a predetermined size (length) so as not to reach the outer periphery of the cylinder.

  Therefore, by changing the lift amount of the outer valve 42 as described above, the fuel concentration becomes uniform in the fuel injection direction as shown by the solid line in FIG. be able to. Therefore, the ignitability of the air-fuel mixture can be improved.

  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, in addition to the heat insulating structure of the combustion chamber 17 and the intake port 18, a heat insulating layer is formed by a gas layer in the cylinder (inside the combustion chamber 17). The present invention can also be applied to an engine that does not employ this heat insulating structure.

  In the above-described embodiment, the injector 33 is a piezo-type outer valve-opening injector that drives the outer valve 42 by the piezo element 44. However, the outer valve that injects fuel from the nozzle port 41 into the cylinder. The driving means is not limited to the piezo element 44. However, the piezo element 44 is preferable in terms of good response.

  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 an outer valve-opening injector, and particularly useful for reducing a cooling loss while increasing a geometric compression ratio.

1 Spark ignition direct injection engine 11 Cylinder
33 Injector 41 Nozzle port 42 Open valve 44 Piezo element (External valve drive means)
100 Engine controller (injection control means)

Claims (4)

An external opening valve that opens and closes a nozzle port that injects fuel into the cylinder, and the external opening valve is lifted from a state in which the nozzle port is closed to open the nozzle port, thereby injecting fuel from the nozzle port into the cylinder. The outer valve opening drive means, and the greater the lift amount of the outer valve from the state in which the nozzle port is closed, the greater the penetration of fuel spray injected from the nozzle port into the cylinder. A spark ignition direct injection engine having a configured injector,
The geometric compression ratio of the engine is 18 or more and 40 or less,
Further comprising injection control means for controlling the operation of the outer valve drive means,
The injection control means supplies fuel from the nozzle port into the cylinder during the compression stroke so that a gas layer containing fresh air is formed at the outer peripheral portion of the engine in the cylinder and an air-fuel mixture layer is formed at the center. A spark ignition direct injection engine characterized by being configured to inject, and at the time of the fuel injection, the lift amount in the initial stage and the end stage is made larger than the lift amount in the meantime. The spark ignition direct injection engine according to claim 1,
The spark ignition direct injection engine, wherein the injection control means is configured such that the lift amount at the end of the fuel injection is greater than or equal to the lift amount at the initial stage of the fuel injection. The spark ignition direct injection engine according to claim 1 or 2,
The spark ignition type direct injection engine, wherein the injection control means is configured to inject fuel into the cylinder from the nozzle opening in the latter half of the compression stroke. The spark ignition direct injection engine according to claim 1 or 2,
When the engine load is in a high load region where the engine load is higher than a predetermined value, the injection control means is arranged so that the start of fuel combustion is at or after the compression top dead center from the nozzle opening into the cylinder. A spark ignition direct injection engine configured to inject fuel.

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