JP5899727B2 - Direct injection gasoline engine - Google Patents

Description

  The present invention belongs to a technical field related to a direct injection gasoline engine having a self-ignition combustion operation region in which fuel including at least gasoline injected into a cylinder by an injector is self-ignited and combusted.

  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.

  Therefore, as disclosed in Patent Document 2, the amount of heat released from the cylinder wall surface is reduced, that is, the cooling loss is reduced by heat insulation of the combustion chamber by configuring the cylinder wall surface with a heat insulating material. It is preferable. Thereby, the geometric compression ratio of the engine can be made higher than the geometric compression ratio of the engine described in Patent Documents 1 and 2, and high illustrated thermal efficiency can be realized.

  However, when the compression ratio of the engine is increased in this way, the maximum pressure in the cylinder increases during motoring of the engine, and the maximum value of the cylinder pressure increase rate, which is the pressure change in the cylinder with respect to the crank angle change, and The minimum value (negative maximum value) also increases.

  Here, when fuel is self-ignited and combusted with an increase in the compression ratio of the engine, if the self-igniting combustion is started before the compression top dead center, the crank angle at which the rate of increase in cylinder pressure during motoring is large Since combustion occurs at the time, the maximum value of the cylinder pressure increase rate during combustion becomes very large, and the vibration noise (so-called NVH) level increases. The vibration noise (NVH) level is more greatly influenced by the magnitude of the cylinder pressure rise rate than the magnitude of the cylinder pressure itself.

  The present invention has been made in view of such a point, and an object of the present invention is to reduce a cylinder pressure increase rate during combustion when fuel injected into a cylinder is self-ignited and combusted. The aim is to reduce the vibration noise (NVH) level as much as possible.

  In order to achieve the above object, the present invention is directed to a direct injection gasoline engine having a self-ignition combustion operating region in which fuel including at least gasoline injected into a cylinder by an injector is self-ignited and combusted. The geometric compression ratio is 18 or more and 40 or less, the injection control means for controlling the fuel injection by the injector, and the injector injected into the cylinder by the injector when the engine is in the self-ignition combustion operation region. And an ignition assist means for assisting self-ignition combustion of the fuel by applying energy to the fuel, and the injection control means compresses the fuel injection start timing when the engine is in the self-ignition combustion operation region. The ignition assist means is configured to be set within a period from the end of the stroke to the compression top dead center. The combustion period in which the combustion mass ratio of the fuel is 10% or more and 90% or less is the crank angle time point at which the cylinder pressure increase rate, which is the pressure change in the cylinder with respect to the crank angle change during motoring of the engine, becomes a negative maximum value The energy is applied to the fuel injected into the cylinder during the period from the start of the fuel injection to the beginning of the expansion stroke.

With the above configuration, the fuel injection is started within the period from the end of the compression stroke to the compression top dead center, and the period from the start of the fuel injection to the beginning of the expansion stroke (usually the period until all the fuel is completely injected) to, by the operation of the ignition assistance means, which energy is imparted to the fuel injected into the cylinder by the time the operation, its energy is fire wear fuel granted, and the ignition fuel to the base point, is injected after Fuel is ignited in a chain. By providing the ignition assist means in this manner, the self-ignition combustion of the fuel can be easily performed even after the compression top dead center. The crank angle point at which the cylinder pressure increase rate during motoring reaches a negative maximum value overlaps the combustion period (called the main combustion period) in which the combustion mass ratio is 10% or more and 90% or less. The in-cylinder pressure increase rate can be reduced, and the vibration noise (NVH) level can be reduced.

  Here, when the geometric compression ratio is 18 or more and 40 or less, the crank angle time point at which the cylinder pressure increase rate during motoring of the engine becomes a negative maximum value is after the compression top dead center. 4 ° to 15 ° CA.

  In the direct-injection gasoline engine, the ignition assist means applies the energy to the fuel injected into the cylinder so that a time when the combustion mass ratio of the fuel becomes 10% comes after compression top dead center. It is preferable that it is comprised.

  Thereby, since the main combustion period is after compression top dead center, the cooling loss can be reduced by the combustion in the expansion stroke.

  In the direct injection gasoline engine, the injection control means performs a first injection for injecting a predetermined amount of fuel when the engine is in the self-ignition combustion operation region, and after the first injection, It is configured to perform a second injection that injects the remaining fuel continuously or discontinuously with respect to one injection, and the ignition assist means is within a period from the end of the first injection to the initial stage of the second injection. Furthermore, it is preferable that the energy is imparted to the fuel by the first injection.

  Thus, by setting the predetermined amount appropriately (for example, by setting the mass percentage to 1% or less with respect to the total injected fuel), it is possible to form a minute air-fuel mixture, and this minute air-fuel mixture By concentrating energy on (fuel by the first injection), the fuel by the first injection can be surely ignited. The fuel from the first injection serves as a trigger for ignition, and the fuel from the subsequent second injection ignites in a chained manner. Therefore, the ignitability of the fuel can be improved.

  In the direct injection gasoline engine, it is preferable that the ignition assisting unit is configured to give the energy to the fuel injected into the cylinder by plasma ignition.

  That is, plasma ignition can give more energy to the fuel than spark ignition, which is advantageous in improving the ignition stability of the fuel.

  As described above, according to the direct injection gasoline engine of the present invention, when the engine is in the self-ignition combustion operation region, energy is imparted to the fuel injected into the cylinder by the injector, and the self-ignition combustion of the fuel is performed. An ignition assist means for assisting is provided, and when the engine is in the self-ignition combustion operation region, the fuel injection start timing is set within a period from the end of the compression stroke to the compression top dead center, and the ignition assist means is The crank angle point at which the rate of increase in the cylinder pressure, which is the pressure change in the cylinder relative to the change in the crank angle during motoring, becomes a negative maximum value is the combustion period in which the fuel combustion mass ratio is 10% or more and 90% or less In order to overlap, the energy is given to the fuel injected into the cylinder during the period from the start of the fuel injection to the beginning of the expansion stroke. With the arrangements to so that, to be able to reduce the cylinder pressure rise rate during combustion, it is possible to reduce the vibration noise (NVH) levels.

1 is a schematic view showing a direct injection gasoline 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 lift amount of the valve opening with respect to a crank angle, and the change of the cylinder internal pressure rise rate. It is a graph which shows the change of the cylinder internal pressure rise rate at the time of motoring of an engine in case the geometric compression ratios of an engine are 20, 30, and 40. It is a graph which shows the result of having investigated how the rate of pressure increase in a cylinder at the time of combustion changes by changing the combustion start time when the geometric compression ratio of an engine is 40.

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

  FIG. 1 schematically shows a direct injection gasoline 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 of being inclined toward the exhaust side with respect to the central axis of the cylinder 11, and the tip 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. In the present embodiment, the ignition plug 31 is a plasma ignition type plug, and the ignition system 32 includes a plasma generation circuit. The spark plug 31 generates plasma by discharge by the ignition system 32 and injects the plasma into the cylinder from the tip of the spark plug 31 to ignite the fuel. Further, as will be described later, the spark plug 31 plays a role of assisting the self-ignition combustion of the fuel in the self-ignition combustion operation region in which the fuel self-ignites and burns. The ignition system 32 receives a control signal from the engine controller 100 and energizes the ignition plug 31 to generate plasma at a desired ignition timing. The spark plug 31 is not limited to a plasma ignition type plug, but may be a spark ignition type plug that is commonly used.

  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 proximal end of the fuel pipe 43 is connected to a case 45 in which a piezo element 44 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 first injection and the second injection described later can be easily realized. However, the means for driving the outer valve 42 is not limited to the piezo element 44.

  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. 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.

  If ignition (or assist of self-ignition combustion) is performed with the spark plug 31 in a state where the gas layer and the mixture layer are formed as described above, the gas layer between the mixture layer and the wall surface of the cylinder 11 The flame of the air-fuel mixture layer does not come into contact with the wall surface of the cylinder 11, and the gas layer becomes a heat insulating layer, so that 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 a predetermined value, the excess air ratio λ in the entire cylinder (in the combustion chamber 17) is 2 or more, or the weight ratio G / F of gas to fuel in the cylinder is 30. Set as above. 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.

  The engine 1 has a self-ignition combustion operation region in which the fuel injected into the cylinder by the injector 33 is self-ignited and combusted. In the present embodiment, the self-ignition combustion operation region includes the entire low load region and a region excluding the full load or the vicinity thereof in the high load region, but is not limited thereto.

  When the engine 1 is in the self-ignition combustion operation region, the engine controller 100 sets the fuel injection start timing by the injector 33 within a period from the end of the compression stroke to the compression top dead center. In the present embodiment, as shown in the lower part of FIG. 3, fuel injection is started at the end of the compression stroke. Therefore, the engine controller 100 constitutes the injection control means of the present invention.

In addition, when the engine 1 is in the self-ignition combustion operation region, the engine controller 100 imparts energy to the fuel that has been injected into the cylinders by the plasma ignition by the spark plug 31 so that the fuel is injected. Assists self-ignition combustion. Thus, the energy is fire wear fuel granted, and the ignition fuel to the base point, the injected fuel is gradually ignited like a chain later. The above-mentioned energy is applied when the crank angle point at which the cylinder pressure increase rate (ΔP / Δθ), which is the pressure change (ΔP) in the cylinder with respect to the crank angle change (Δθ) during engine motoring, becomes a negative maximum value. In the period from the start of the fuel injection to the initial stage of the expansion stroke (in this embodiment, compression is increased so that the combustion mass ratio of the fuel becomes 10% or more and 90% or less. Near the dead center). Therefore, the engine controller 100 and the spark plug 31 constitute the ignition assist means of the present invention.

  Further, it is preferable to apply the energy so that the time when the combustion mass ratio of the fuel becomes 10% comes after the compression top dead center (that is, the main combustion period starts after the compression top dead center).

  As shown in FIG. 4, the cylinder pressure increase rate during motoring reaches its maximum value before compression top dead center, becomes zero at compression top dead center, becomes negative after compression top dead center, and eventually becomes negative. The maximum value (minimum value). The crank angle point at which the cylinder pressure increase rate during motoring reaches a negative maximum value varies depending on the geometric compression ratio ε. It becomes 4 ° to 15 ° CA after hatching (the hatched range in FIG. 4).

  FIG. 5 shows the results of examining how the rate of increase in the cylinder pressure during combustion changes at different combustion start times (geometric compression ratio ε is 40). The combustion start time is 20 ° CA before compression top dead center (−20 ° CA), 10 ° CA before compression top dead center (−10 ° CA), compression top dead center (0 ° CA), and after compression top dead center. There are five types: 10 ° CA (+ 10 ° CA) and 20 ° CA (+ 20 ° CA) after compression top dead center. The cylinder pressure increase rate during motoring is indicated by a two-dot chain line. If the start of combustion is before the compression top dead center, combustion occurs at a crank angle at which the cylinder pressure increase rate during motoring is large, so the maximum value of the cylinder pressure increase rate during combustion increases. On the other hand, when combustion starts at the compression top dead center, combustion occurs at the crank angle when the cylinder pressure increase rate during motoring is small. The maximum value of the cylinder pressure rise rate becomes considerably small. Even when the start of combustion is considerably after the compression top dead center, the maximum value of the cylinder pressure increase during combustion is reduced, but this is because the combustion efficiency deteriorates. Therefore, as described above, the crank angle point at which the cylinder pressure increase rate during motoring becomes a negative maximum value overlaps with the main combustion period within the period from the start of fuel injection to the beginning of the expansion stroke. Then, energy is applied to the fuel injected into the cylinder.

  The fuel injection when the engine 1 is in the self-ignition combustion operation region is particularly preferably performed as follows. That is, the first injection for injecting a predetermined amount of fuel from the start of fuel injection to the vicinity of compression top dead center (TDC) is performed. The predetermined amount, which is the amount of fuel injected by the first injection, is preferably set to a relatively small amount of 1% or less by mass with respect to all the injected fuel, as will be described later. Therefore, the lift amount of the outer open valve 42 at the time of the first injection is considerably smaller than the lift amount of the outer open valve 42 at the time of the second injection described later (however, excluding the initial stage of the second injection). Therefore, the penetration of the fuel spray is small, and the fuel by the first injection is located in the vicinity of the tip of the spark plug 31 as a fine air-fuel mixture.

  Subsequently, after the first injection, a second injection for continuously injecting the remaining fuel is performed on the first injection. The initial stage of the second injection is in the vicinity of the compression top dead center, and is the time when the self-ignition combustion assist is performed as will be described later. The lift amount of the outer open valve 42 at this time is higher than that at the time of the first injection. Although it is small, after that, the lift amount of the outer opening valve 42 suddenly increases and all the remaining fuel is injected.

The engine controller 100 performs plasma ignition by the spark plug 31 within a period from the end of the first injection to the initial stage of the second injection (in the present embodiment, near the compression top dead center (the initial stage of the second injection)). Energy is applied to the fuel by the first injection (the small air-fuel mixture) to assist the self-ignition combustion of the fuel by the first injection and the fuel by the second injection. As described above, by performing plasma ignition at the initial stage of the second injection (before a large amount of injection), energy can be concentrated on the minute air-fuel mixture. Thus, the fine mixture mass is ignited immediately after the compression dead center, the fine mixture mass becomes a trigger for ignition, fuel by a subsequent second injection is gradually ignited a chain reaction. As a result, the cylinder pressure increase rate during combustion changes as indicated by the solid line in the upper graph of FIG. 3 (the two-dot chain line is the cylinder pressure increase rate during motoring).

  In the present embodiment, in the upper graph of FIG. 3, it is possible to prevent the change curve of the cylinder pressure increase rate during combustion from entering the hatched range. That is, when the cylinder pressure increase rate during combustion exceeds P1, the vibration noise (NVH) level deteriorates. Further, if the combustion is too slow, the combustion efficiency is deteriorated. If the combustion is too early, the in-cylinder pressure and the temperature are increased, and the emission is deteriorated and the combustion efficiency is also deteriorated. However, in the present embodiment, such a problem does not occur, the vibration noise (NVH) level can be reduced, and the deterioration of emission and combustion efficiency can be suppressed.

  Here, the energy required for igniting the fuel (the minute air-fuel mixture) by the first injection with a delay after a predetermined crank angle from the start of energy application increases as the predetermined crank angle decreases, If the predetermined crank angle is constant, the mass ratio with respect to the predetermined amount of all injected fuel and the required energy are in a proportional relationship. That is, if the fuel injection amount (the predetermined amount) by the first injection is increased, a larger amount of energy is required to ignite the minute air-fuel mixture. On the other hand, the energy that can be applied by plasma ignition is 100 mJ to 200 mJ. Considering this, the predetermined crank angle is, for example, about 5 ° CA, and the predetermined amount is 1% or less in mass percentage with respect to the total injected fuel. Is preferred. In addition, when energy is applied by spark ignition, the energy that can be applied is about 35 mJ, and therefore the predetermined amount is preferably 0.3% or less in terms of mass percentage with respect to all the injected fuel.

  Therefore, in the present embodiment, when the engine 1 is in the self-ignition combustion operation region, the fuel injection start timing is set within the period from the end of the compression stroke to the compression top dead center, and the inside of the cylinder during engine motoring is set. Within the period from the start of the fuel injection to the beginning of the expansion stroke so that the crank angle point at which the pressure increase rate becomes the negative maximum value overlaps with the combustion period in which the fuel mass ratio is 10% or more and 90% or less. By applying energy to the fuel injected into the cylinder to assist the self-ignition combustion of the fuel, it is possible to reduce the rate of increase in the cylinder pressure during combustion, and vibration noise (NVH) The level can be reduced.

  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 said embodiment, although 2nd injection was made into continuous injection with respect to 1st injection, it is good also as non-continuous injection with respect to 1st injection. That is, the divided injection by the first injection and the second injection is performed. Also in this case, during the period from the end of the first injection to the initial stage of the second injection (the initial stage of the second injection needs to be the initial stage of the expansion stroke), the first ignition is performed by plasma ignition (or spark ignition) by the spark plug 31. Although energy may be applied to the fuel by one injection, it is more preferable to apply energy between the first injection and the second injection.

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

  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.

  The present invention is useful for a direct-injection gasoline engine having a self-ignition combustion operation region in which fuel including at least gasoline injected into a cylinder by an injector is self-ignited and combusted.

1 Direct-injection gasoline engine 11 Cylinder
31 Spark plug (ignition assist means)
33 Injector 100 Engine controller (injection control means) (ignition assist means)

Claims (4)

A direct-injection gasoline engine having a self-ignition combustion operation region in which fuel including at least gasoline injected into a cylinder by an injector is self-ignited and burned,
The geometric compression ratio of the engine is 18 or more and 40 or less,
Injection control means for controlling fuel injection by the injector;
When the engine is in the self-ignition combustion operation region, it includes an ignition assist means that gives energy to the fuel injected into the cylinder by the injector and assists the self-ignition combustion of the fuel,
The injection control means is configured to set a fuel injection start timing within a period from the end of the compression stroke to the compression top dead center when the engine is in the self-ignition combustion operation region,
In the ignition assisting means, the main combustion period in which the fuel combustion mass ratio is 10% or more and 90% or less has a negative cylinder pressure increase rate, which is a pressure change in the cylinder with respect to a crank angle change during motoring of the engine. In the period from the start of the fuel injection to the beginning of the expansion stroke, the fuel injected into the cylinder is overlapped so as to overlap with the crank angle of 4 ° to 15 ° CA after the compression top dead center. A direct-injection gasoline engine configured to apply the energy. In direct injection gasoline engine according to claim 1 Symbol placement,
The ignition assist means is configured to apply the energy to the fuel injected into the cylinder so that the time when the combustion mass ratio of the fuel becomes 10% is after compression top dead center. Characteristic direct-injection gasoline engine. The direct injection gasoline engine according to claim 1 or 2 ,
The injection control means performs a first injection for injecting a predetermined amount of fuel when the engine is in the self-ignition combustion operation region, and continuously with respect to the first injection after the first injection. Or configured to cause the second injection to inject the remaining fuel discontinuously,
The direct-injection gasoline engine, wherein the ignition assist means is configured to give the energy to the fuel by the first injection within a period from the end of the first injection to the initial of the second injection. . In the direct injection gasoline engine according to any one of claims 1 to 3 ,
The direct injection gasoline engine, wherein the ignition assist means is configured to give the energy to the fuel injected into the cylinder by plasma ignition.

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