JP6056882B2 - Fuel injection control device for direct injection engine - Google Patents

Description

  The technology disclosed herein relates to a fuel injection control device for a direct injection engine.

  Patent Document 1 describes an externally-opened fuel injection valve that injects fuel in a hollow cone shape into a combustion chamber of an engine. In the externally opened fuel injection valve, the effective opening area of the nozzle port through which fuel is injected changes by changing the lift amount of the valve body. Patent Document 2 describes a VCO (Valve Covered Orifice) nozzle type fuel injection valve. The fuel injection valve of the VCO nozzle type is configured such that the needle valve is directly seated on the seat portion where the nozzle port is open and the nozzle port is closed. Depending on the lift amount of the needle valve, the size of the cavity region generated on the inner peripheral surface of the nozzle port changes. In the fuel injection valve of the VCO nozzle type, the effective opening area of the nozzle opening changes according to the lift amount of the needle valve, as in the case of the externally opened injector.

  In Patent Document 3, in a direct injection engine that is disposed on the center axis of a cylinder and has an outer valve-open type fuel injection valve that injects fuel in a hollow cone shape, fuel is injected into the cylinder at the later stage of the compression stroke. It is described that an air-fuel mixture layer and a surrounding gas layer are formed in the combustion chamber by injection. In the engine described in Patent Document 3, when the air-fuel mixture burns, the surrounding gas layer functions as a heat insulating layer, thereby reducing the cooling loss.

  Patent Document 4 describes that, in a compression self-ignition engine, a wall surface defining a combustion chamber is made of a heat insulating material to reduce cooling loss on the wall surface of the combustion chamber.

JP 2008-151043 A Japanese Patent No. 4194564 JP 2013-57266 A JP 2009-243355 A

  As described in Patent Document 3, it is easy to make the mixture layer compact when the amount of fuel injection is relatively small when an insulating gas layer is formed around the mixture layer. It is easy to form a heat insulating gas layer around. On the other hand, when the fuel injection amount increases, it becomes difficult to make the air-fuel mixture compact because the injected fuel diffuses, and the air-fuel mixture comes into contact with the wall surface of the combustion chamber, It becomes difficult to secure a heat insulating gas layer. Since the insulating gas layer is not formed, a reduction in cooling loss cannot be achieved.

  The technology disclosed herein has been made in view of such a point, and an object thereof is to reliably form an air-fuel mixture layer and an adiabatic gas layer in a combustion chamber in a direct injection engine.

  If the fuel is injected all at once when the fuel injection amount increases, as described above, the fuel diffuses and a heat insulating gas layer cannot be formed. Therefore, for example, it is conceivable to perform the fuel injection in a plurality of times, for example, after performing the first injection for injecting the fuel into the combustion chamber and then performing the second injection at a predetermined interval.

  Here, if the interval between the first injection and the second injection is narrowed, the fuel spray injected by the first injection and the fuel spray injected by the second injection tend to overlap, and a locally rich mixture is generated. By forming, smoke is generated.

  Therefore, if the interval between the first injection and the second injection is widened, the fuel spray injected by the first injection and the fuel spray injected by the second injection are suppressed from overlapping, and the locally rich mixture Can be prevented from being formed. This suppresses the occurrence of smoke. On the other hand, considering the time of fuel vaporization and mixing of fuel and air, the end time of the second injection cannot be delayed. Therefore, if the interval between the first and second injections is increased, The injection start timing for one injection must be advanced. However, if the injection start timing of the first injection is advanced, the first injection injects fuel when the pressure and temperature in the cylinder are low, so the fuel spray injected by the first injection diffuses. As a result, the air-fuel mixture easily comes into contact with the wall surface of the combustion chamber.

  The inventors of the present application pay attention to this point and assume that the fuel spray injected by the first injection and the fuel spray injected by the second injection overlap with each other on the assumption that the second injection is performed after the first injection. In order to prevent the air-fuel mixture layer from coming into contact with the wall surface of the combustion chamber, the fuel injection mode is devised.

  Specifically, in a fuel injection valve configured such that the effective opening area of the injection port for injecting fuel increases as the lift amount increases, as in, for example, an externally opened fuel injection valve, the fuel pressure is constant. Under such a condition, when the effective opening area of the nozzle hole becomes large, the fuel injection speed decreases. In addition, if the effective opening area of the nozzle hole becomes too small, the influence of friction between the wall surface of the nozzle hole and the fuel when the fuel passes through the nozzle hole becomes large, and also in this case, the fuel injection speed decreases. Accordingly, there is a lift amount at which the fuel injection speed is maximum, and the fuel injection speed is reduced regardless of whether the lift amount is larger or smaller than the maximum speed lift amount. This maximum speed lift is relatively small.

  In this way, when the effective opening area of the nozzle hole is small, the effect of friction between the wall of the nozzle hole and the fuel becomes large. Therefore, when the fuel injection is started, the lift amount of the fuel injection valve is increased to increase the effectiveness of the nozzle hole. When the opening area is increased, the resistance acting on the fuel is reduced, and the fuel injection speed is likely to increase. If the fuel injection speed is sufficiently increased and then the lift amount of the fuel injection valve is reduced to approach the maximum speed lift amount, the fuel injection amount does not decrease, but the effective opening area is reduced. The injection speed increases.

  The fuel injection valve lift amount is set to a first lift amount that is larger than the maximum speed lift amount at which the fuel injection speed is maximum, and after the fuel injection is started, the lift amount is changed while the injection is continued. The fuel is injected by switching to a second lift amount smaller than the one lift amount and equal to or greater than the maximum speed lift amount. Then, the speed of the fuel spray injected in the second half (that is, the fuel spray injected after switching to the second lift amount) is increased, so that the fuel spray injected in the second half moves away from the fuel injection valve. On the other hand, since the speed of the previously injected fuel spray is not so high, the previously injected fuel spray is also prevented from coming into contact with the wall surface of the combustion chamber.

  When the first injection and the second injection are performed, the inventors of the present application perform the fuel injection in such a manner in the first injection, and the fuel spray injected by the first injection is applied to the wall surface of the combustion chamber. Since the fuel spray is prevented from coming into contact with the fuel injection valve, the fuel spray injected by the second injection following the first injection overlaps the fuel spray injected by the first injection. As a result, the present invention has been completed.

  The technology disclosed herein relates to a fuel injection control device for a direct injection engine, which is partitioned by a ceiling portion of a cylinder head, a cylinder provided in a cylinder block, and a piston that reciprocates in the cylinder. An engine body configured to have a combustion chamber, and a fuel injection valve arranged to inject liquid fuel into the combustion chamber, and at least in the second half of a compression stroke, A fuel injection control unit configured to inject into the combustion chamber.

  The fuel injection valve is configured such that the effective opening area of the injection port for injecting the fuel increases as the lift amount increases, and the fuel injection control unit performs the first injection for a predetermined injection period. After that, the second injection is performed at a predetermined interval, and the first injection has a lift amount of the fuel injection valve as a first lift amount larger than a maximum speed lift amount at which the fuel injection speed becomes maximum. After the fuel injection is started, while the injection is continued, the lift amount is switched to a second lift amount that is smaller than the first lift amount and greater than or equal to the maximum speed lift amount, and the fuel is discharged. In the second injection, the fuel is injected with a lift amount of the fuel injection valve smaller than the first lift amount.

  According to this configuration, the fuel injection control unit performs the first injection that injects the fuel into the combustion chamber during the predetermined injection period, and the second injection after the first injection after a predetermined interval. The combustion chamber here is not limited to the space in the cylinder when the piston reaches top dead center, and is a space defined by the ceiling of the cylinder head, the cylinder, and the piston regardless of the piston position. It is a combustion chamber in a broad sense.

  The first injection starts fuel injection with the lift amount of the fuel injection valve as the first lift amount that is larger than the maximum speed lift amount at which the fuel injection speed becomes maximum. Thereby, since the effective opening area of the nozzle hole of the fuel injection valve becomes relatively large, the resistance to the fuel passing through the nozzle hole is lowered, and the fuel injection speed is quickly increased. The first injection switches the lift amount of the fuel injection valve to a second lift amount smaller than the first lift amount while continuing the injection. By reducing the effective opening area of the injection port of the fuel injection valve while the fuel injection speed is sufficiently increased, the fuel injection amount does not decrease during the injection period of the first injection, and the fuel injection speed increases. . Since the second lift amount is a lift amount that is equal to or greater than the maximum speed lift amount, the speed of the fuel does not decrease due to the resistance at the nozzle hole.

  Thus, among the fuel sprays injected into the combustion chamber by the first injection, the fuel spray injected first has a relatively low speed and receives a relatively high resistance in the combustion chamber, so that it becomes difficult to fly. The contact with the wall surface of the combustion chamber is prevented. On the other hand, since the fuel spray injected in the latter half has a relatively high speed and a low resistance, the fuel spray moves away from the fuel injection valve in a short time.

  Thus, since the fuel spray injected by the first injection moves away from the fuel injection valve, the fuel spray injected by the second injection thereafter is prevented from overlapping with the fuel spray injected by the first injection. Thereby, it is possible to prevent the formation of smoke by preventing the formation of a locally rich mixture. In this way, it is possible to suppress the generation of smoke while reliably forming the heat insulating gas layer around the air-fuel mixture layer.

  Such fuel injection is preferably performed when the fuel injection amount is relatively large. When the fuel injection amount increases as the load on the engine body increases, it is difficult to make the air-fuel mixture layer compact and to provide a heat insulating gas layer around it in the case of batch injection. By performing the injection and the second injection, it is possible to prevent the formation of smoke by preventing the formation of a locally rich mixture while forming the heat insulating gas layer. Further, by arranging the fuel spray at different positions in the combustion chamber, the air utilization rate in the combustion chamber is increased.

  When the engine speed increases, the first injection and the second injection as described above may be performed. When the number of revolutions of the engine body increases, the passage of time with respect to the change in the crank angle is shortened. Therefore, in order to ensure time for vaporization of the injected fuel and mixing of the fuel and air, the injection end timing of the second injection In other words, the pressure and temperature in the cylinder at the start of the first injection must be high in order to reliably form the adiabatic gas layer. Desirably, the injection start timing of the first injection cannot be advanced so much. The injection mode described above can prevent the fuel spray injected by the first injection and the fuel spray injected by the second injection from overlapping each other, so that the interval between the first injection and the second injection can be shortened. This makes it possible to advance the injection end timing of the second injection without advancing the injection start timing of the first injection.

  Further, as the engine speed increases, the flow in the cylinder becomes stronger and the air-fuel mixture tends to diffuse. However, as described above, in the first injection, the fuel spray injected later is the fuel spray injected earlier. The air-fuel mixture concentration increases. As a result, the air-fuel mixture is prevented from excessively diffusing in the cylinder with increased flow. As a result, when the rotational speed of the engine body is high, the combustion temperature is prevented from being reduced, which is advantageous for reducing unburned loss and cooling loss.

  The injection period of the second injection may be longer than the injection period of the first injection.

  As the fuel is injected from the fuel injection valve, the spray flow formed in the combustion chamber entrains ambient air (or gas containing air). At this time, the fuel spray is injected so as to spread from the tip of the fuel injection valve, but it is difficult for air to flow in the vicinity of the injection axis of the fuel injection valve that is inside the injected fuel spray. As the injection period becomes longer, the negative pressure becomes stronger in the vicinity of the injection axis of the fuel injection valve, and the fuel spray approaches the injection axis of the fuel injection valve due to the pressure difference between the inside and outside of the fuel spray.

  By making the injection period of the second injection relatively long, the fuel spray injected by the second injection is more in the direction of the spray angle than the fuel spray injected by the first injection due to the pressure difference described above. It will be located inside. In this way, the fuel spray injected by the first injection and the fuel spray injected by the second injection are more reliably prevented. Moreover, the utilization factor of the air in a combustion chamber increases.

  The fuel injection valve has a nozzle body in which the nozzle hole is formed, and an outer valve that opens and closes the nozzle hole by lifting outward, and is configured to inject the fuel from the nozzle hole in a hollow cone shape. Alternatively, the fuel injection valve may be an open valve type.

  As the lift amount increases, the outer-open fuel injection valve increases the effective opening area of the injection port for injecting fuel and injects the fuel in a hollow cone shape, so that the first injection and the second injection described above are performed. This makes it possible to prevent the formation of a locally rich mixture while forming the heat insulating gas layer.

  The outer valve-open fuel injection valve is disposed in a ceiling portion of the cylinder head such that an injection axis is along the axis of the cylinder, and the tip of the fuel injection valve is disposed on the ceiling portion. The recessed part which accommodates a part is good also as being dented from the ceiling surface.

  This makes it possible to place the fuel injection valve at a position deeper from the ceiling surface of the cylinder head, so that when the piston reaches top dead center, the top surface of the piston and the tip of the fuel injection valve Can be made as wide as possible. This is advantageous in forming an insulating gas layer around the gas mixture layer.

  In addition, as described above, the externally opened fuel injection valve injects fuel in a hollow cone shape, so that the fuel spray injected from the fuel injection valve easily adheres to the ceiling surface of the cylinder head due to the Coanda effect. In the above configuration, by providing the recess recessed from the ceiling surface of the cylinder head, the distance between the tip of the fuel injection valve and the inner peripheral surface of the recess increases, and the fuel spray injected from the fuel injection valve Adhering to the surface is suppressed.

  A cavity may be formed on the top surface of the piston, and the fuel injection valve may be configured to inject the fuel into the cavity.

  By doing so, the air-fuel mixture layer formed by the fuel injected from the fuel injection valve is formed mainly in the cavity, which is advantageous in forming the air-fuel mixture layer and the surrounding insulating gas layer.

  As described above, according to the fuel injection control device of the direct injection engine, fuel injection is started with the first lift amount larger than the maximum speed lift amount, and the first lift is in progress while continuing the injection. After performing the first injection to switch to the second lift amount smaller than the amount, the second injection is performed at a predetermined interval and with a lift amount smaller than the first lift amount, so that the mixture layer and its surroundings While forming the adiabatic gas layer, it is possible to prevent the fuel injected by the first injection and the fuel injected by the second injection from overlapping, thereby preventing the occurrence of smoke.

It is the schematic which shows the structure of a direct injection engine. It is sectional drawing which shows the structure of a combustion chamber. It is a figure which shows the relationship between the amount of lifts and the effective opening area of a nozzle hole of an outer valve-opening type fuel injection valve. It is a figure which shows the injection aspect of a fuel. (A) The figure which shows the change of the lift amount of a fuel injection valve, (b) The figure which shows the change of the injection speed of a fuel. It is a figure explaining the spread of the fuel spray in a combustion chamber. It is a figure which shows the fuel injection mode different from FIG.

  Hereinafter, embodiments will be described with reference to the drawings. The following description is exemplary.

(Entire engine configuration)
FIG. 1 shows a configuration of an engine 1 according to the embodiment. Although not shown, the crankshaft 15 of the engine 1 is connected to drive wheels via a transmission. The vehicle is propelled by the output of the engine 1 being transmitted to the drive wheels. Here, the fuel of the engine 1 is gasoline in this embodiment, but may be gasoline containing bioethanol or the like. The technology disclosed herein can be widely applied to engines using various types of liquid fuel.

  The engine 1 includes a cylinder block 12 and a cylinder head 13 placed thereon, and a plurality of cylinders 11 are formed inside the cylinder block 12 (only one is shown in FIG. 1). . The engine 1 is a multi-cylinder engine. Although not shown, a water jacket through which cooling water flows is formed inside the cylinder block 12 and the cylinder head 13. In each cylinder 11, a piston 16 connected to a crankshaft 15 via a connecting rod 14 is slidably fitted. The piston 16 divides the combustion chamber 17 together with the cylinder 11 and the cylinder head 13.

  In the present embodiment, the ceiling portion 170 (the lower surface of the cylinder head 13) of the combustion chamber 17 is provided with an intake side inclined surface 171 provided with an opening 180 of the intake port 18 and having an upward slope toward the center of the cylinder 11. The exhaust port 19 is provided with an opening 190 and an exhaust side inclined surface 172 having an upward slope toward the center of the cylinder 11. The combustion chamber 17 is a pent roof type combustion chamber. Note that the ridgeline of the pent roof may both coincide with the bore center of the cylinder 11 and may not coincide. Further, as shown in FIG. 2, the top surface 160 of the piston 16 is located at the center of the piston 16 on each of the intake side and the exhaust side so as to correspond to the intake side inclined surface 171 and the exhaust side inclined surface 172 of the ceiling portion 170. Due to the inclined surfaces 161 and 162 that have an upward slope toward the top, a triangular roof is raised. Thereby, the geometric compression ratio of the engine 1 is set to a high compression ratio of 15 or more. A concave cavity 163 is formed on the top surface 160 of the piston 16.

  Although only one is shown in FIG. 1, two intake ports 18 are formed in the cylinder head 13 for each cylinder 11. The opening 180 of the intake port 18 is provided on the intake side inclined surface 171 of the cylinder head 13 along the direction of the engine output shaft (that is, the crankshaft 15). The intake port 18 passes through the opening 180 and is in the combustion chamber 17. Communicating with Similarly, two exhaust ports 19 are formed in the cylinder head 13 for each cylinder 11. The opening 190 of the exhaust port 19 is provided on the exhaust-side inclined surface 172 of the cylinder head 13 along the direction of the engine output shaft, and the exhaust port 19 communicates with the combustion chamber 17 through the opening 190.

  The intake port 18 is connected to the intake passage 181. The intake passage 181 is provided with a throttle valve 55 for adjusting the intake flow rate. The exhaust port 19 is connected to the exhaust passage 191. Although not shown, an exhaust gas purification system having one or more catalytic converters is disposed in the exhaust passage 191. The catalytic converter includes a three-way catalyst.

  An intake valve 21 is disposed in the cylinder head 13 so that the intake port 18 can be shut off from the combustion chamber 17 (that is, the combustion chamber 17 is closed). The intake valve 21 is driven by an intake valve drive mechanism. The cylinder head 13 is also provided with an exhaust valve 22 so that the exhaust port 19 can be shut off from the combustion chamber 17. The exhaust valve 22 is driven by an exhaust valve drive mechanism. The intake valve 21 reciprocates at a predetermined timing to open and close the intake port 18, and the exhaust valve 22 reciprocates at a predetermined timing to open and close the exhaust port 19. The gas in the cylinder 11 is exchanged by them.

  Although not shown, the intake valve drive mechanism has an intake camshaft that is drivingly connected to the crankshaft 15, and the intake camshaft rotates in synchronization with the rotation of the crankshaft 15. Although not shown, the exhaust valve drive mechanism has an exhaust camshaft that is drivingly connected to the crankshaft 15, and the exhaust camshaft rotates in synchronization with the rotation of the crankshaft 15.

  In this example, the intake valve drive mechanism includes at least a hydraulic or electric variable valve timing (VVT) 23 capable of continuously changing the phase of the intake camshaft within a predetermined angle range. It consists of The intake valve drive mechanism may include a variable lift mechanism capable of changing the valve lift amount together with the VVT 23.

  In this example, the exhaust valve drive mechanism includes at least a hydraulic or electric VVT 24 that can continuously change the phase of the exhaust camshaft within a predetermined angular range. The exhaust valve drive mechanism may include a variable lift mechanism that can change the valve lift amount together with the VVT 24.

  The lift variable mechanism may be a CVVL (Continuous Variable Valve Lift) capable of continuously changing the lift amount. The valve mechanism for driving the intake valve 21 and the valve mechanism for driving the exhaust valve 22 may be any type, for example, a hydraulic or electromagnetic drive mechanism may be employed. .

  As shown in an enlarged view in FIG. 2, a fuel injection valve 6 that directly injects fuel into the combustion chamber 17 is attached to the cylinder head 13. The fuel injection valve 6 is disposed on the ridgeline of the pent roof where the intake side slope 171 and the exhaust side slope 172 intersect. The fuel injection valve 6 is also arranged such that its injection axis S is along the axis of the cylinder 11, and the injection tip faces the combustion chamber 17. There are both cases where the injection axis S of the fuel injection valve 6 coincides with the axis of the cylinder 11 and when it deviates from the axis of the cylinder 11.

  The cavity 163 of the piston 16 is provided so as to face the fuel injection valve 6. The fuel injection valve 6 injects fuel into the cavity 163.

  Here, the fuel injection valve 6 is an externally opened fuel injection valve. As shown in FIG. 3, the outer-open fuel injection valve 6 has a nozzle body 60 in which a nozzle 61 for injecting fuel is formed, and an outer valve 62 that opens and closes the nozzle 61. Have

  The nozzle body 60 is configured in a cylindrical shape so that fuel flows therethrough, and the nozzle 61 is provided at the tip of the nozzle body 60. The nozzle hole 61 is formed in a tapered shape whose diameter increases toward the tip side.

  The outer opening valve 62 includes a valve main body 63 exposed to the outside from the nozzle main body 60 at the tip of the nozzle main body 60, and a connection portion 64 that passes through the nozzle main body 60 from the valve main body 63 and is connected to a piezoelectric element (not shown). have. The valve main body 63 has a seating portion 65 having substantially the same shape as the tapered nozzle hole 61. A reduced diameter portion 66 is interposed between the seating portion 65 of the valve body 63 and the connection portion 64. As shown in FIG. 3, the reduced diameter portion 66 is different in inclination from the seating portion 65, and the inclination of the reduced diameter portion 66 from the proximal end toward the distal end is gentler than the inclination of the seating portion 65.

  As shown by a two-dot chain line in FIG. 3, when the seating portion 65 is in contact with the nozzle 61, the nozzle 61 is in a closed state. When the voltage is applied, the piezo element is deformed, and the outer valve 62 is lifted outward along the injection axis S. Accordingly, as shown by a solid line in FIG. When the nozzle hole 61 is in the open state, the fuel is injected in a direction inclined from the nozzle hole 61 with respect to the injection axis S and in a radial direction centering on the injection axis S. The fuel is injected in a hollow cone shape around the injection axis S. When the application of voltage to the piezo element is stopped, the piezo element returns to its original state, so that the seating portion 65 of the outer opening valve 62 comes into contact with the injection port 61, and the injection port 61 is closed again.

  As the voltage applied to the piezo element increases, the lift amount of the outer open valve 62 from the closed state of the nozzle hole 61 increases. As is apparent from FIG. 3, the larger the lift amount, the larger the opening of the nozzle 61, that is, the effective opening area. The effective opening area is defined by the distance between the nozzle hole 61 and the seating portion 65. The larger the lift amount, the larger the particle size of the fuel spray injected from the nozzle 61 into the combustion chamber 17. Conversely, the smaller the lift amount, the smaller the particle size of the fuel spray injected from the nozzle 61 into the combustion chamber 17. Assuming that the fuel pressure is the same, the injection speed decreases as the effective opening area increases. On the contrary, if the effective opening area is reduced, the injection speed is increased, but if the effective opening area is too small, the influence of the frictional resistance of the fuel received from the wall surface of the injection hole is increased, so that the injection speed is reduced. Accordingly, there is a lift amount at which the fuel injection speed is maximum, and the fuel injection speed is reduced regardless of whether the lift amount is larger or smaller than the maximum speed lift amount. This maximum speed lift is relatively small.

  Further, when the fuel passes through the nozzle hole 61, it flows along the reduced diameter portion 66. Therefore, as the lift amount increases, the reduced diameter portion 66 moves away from the injection hole 61, so that the fuel spray angle (that is, the fuel spray angle) The smaller the lift amount, the smaller the diameter of the reduced-diameter portion 66 approaches the injection hole 61, and the greater the spray angle of the fuel (that is, the taper angle of the hollow cone).

  As shown in FIG. 2, the ceiling portion 170 of the cylinder head 13 is provided with a recess 173 that is recessed from the ceiling surface, and the tip of the fuel injection valve 6 is accommodated in the recess 173. The inner peripheral surface of the recess 173 is inclined so as to gradually increase in diameter as it goes inward of the combustion chamber 17. By disposing the tip of the fuel injection valve 6 at a position recessed from the ceiling surface of the cylinder head 13, the piston 16 reaches the top dead center while increasing the geometric compression ratio. The distance between the top surface 160 and the tip of the fuel injection valve 6 can be made as wide as possible. As will be described later, this is advantageous in forming an insulating gas layer around the air-fuel mixture layer. Moreover, since the space | interval of the front-end | tip part of the fuel injection valve 6 and the internal peripheral surface of the recessed part 173 spreads, it suppresses that the fuel spray injected from the fuel injection valve 6 adheres to the ceiling surface of the cylinder head 13 by a Coanda effect. It becomes possible.

  The fuel supply system 57 includes an electric circuit for driving the outer opening valve 62 and a fuel supply system for supplying fuel to the fuel injection valve 6. 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 outer opening valve 62 via the electric circuit to supply a desired amount of fuel. Inject into the cylinder. When the injection signal is not output (that is, when the voltage of the injection signal is 0), the nozzle 61 is closed by the outer opening valve 62. Thus, the operation of the piezo element is controlled by the injection signal from the engine controller 100. Thus, the engine controller 100 controls the operation of the piezo element to control the fuel injection from the nozzle 61 of the fuel injection valve 6 and the lift amount at the time of the fuel injection. The response of the piezo element is fast, and, for example, about 20 multistage injections are possible within 1 to 2 msec. However, the means for driving the outer opening valve 62 is not limited to the piezo element.

  The fuel supply system is provided with a high-pressure fuel pump (not shown) and a common rail. The high-pressure fuel pump pumps 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. Then, the fuel stored in the common rail is injected from the nozzle 61 by operating the fuel injection valve 6 (that is, the outer opening valve 62 is lifted). A fuel injection control unit is configured including the engine controller 100 and the fuel injection valve 6.

  As will be described later in detail, the fuel injection control unit includes a (combustible) air-fuel mixture layer, a surrounding insulating gas layer, and a combustion chamber 17 (that is, in the cavity 163), as conceptually shown in FIG. Is configured to be formed.

  The engine 1 is basically configured to burn the air-fuel mixture formed in the cylinder 11 by compression ignition (that is, controlled auto ignition (CAI)) in the entire operation region. The engine 1 includes an ignition assist system 56 for assisting the ignition of the air-fuel mixture under a predetermined environment. The ignition assist system 56 may be, for example, a discharge plug disposed facing the combustion chamber 17. That is, by applying a controlled pulsed high voltage to the electrode of the discharge plug so that an extremely short pulse discharge is generated in the combustion chamber 17, streamer discharge is generated in the combustion chamber and ozone is generated in the cylinder. To do. Ozone assists CAI. The ignition assist system is not limited to a discharge plug that generates ozone, but may be a spark plug that assists CAI by applying energy to the air-fuel mixture by performing spark discharge.

  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 51, a crank angle pulse signal from the crank angle sensor 52, an accelerator opening signal from the accelerator opening sensor 53 that detects the amount of depression of the accelerator pedal, And the vehicle speed signal from the vehicle speed sensor 54 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, fuel injection pulse, ignition assist signal, valve phase angle signal, and the like. The engine controller 100 then outputs these signals to the throttle valve 55 (more precisely, the throttle actuator that moves the throttle valve 55), the VVTs 23 and 24, the fuel supply system 57, the ignition assist system 56, and the like.

  As described above, the engine 1 has the geometric compression ratio ε set to 15 or more. The geometric compression ratio may be 40 or less, and particularly preferably 20 or more and 35 or less. Since the engine 1 is configured such that the higher the compression ratio is, the higher the expansion ratio is. Therefore, the engine 1 is also an engine 1 having a relatively high expansion ratio simultaneously with the high compression ratio. A high geometric compression ratio stabilizes CAI combustion.

  The combustion chamber 17 is defined by the inner peripheral surface of the cylinder 11, the top surface 160 of the piston 16, the lower surface (ceiling portion 170) of the cylinder head 13, and the valve head surfaces of the intake valve 21 and the exhaust valve 22. Is formed. In order to reduce the cooling loss, the combustion chamber 17 is insulated by providing a heat shielding layer on these sections. The heat shielding layer may be provided on all of these section screens, or may be provided on a part of these section screens. Further, although it is not the wall surface that directly partitions the combustion chamber 17, a heat shield layer may be provided on the port wall surface near the opening on the ceiling portion 170 side of the combustion chamber 17 in the intake port 18 or the exhaust port 19.

  These thermal barrier layers are set to have a lower thermal conductivity than the metal base material constituting the combustion chamber 17 in order to prevent the heat of the combustion gas in the combustion chamber 17 from being released through the section screen. The

  Further, the heat shielding layer preferably has a volumetric specific heat smaller than that of the base material in order to reduce cooling loss. That is, it is preferable to reduce the heat capacity of the heat shield layer 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 thermal barrier layer 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 a thermal insulation layer can be made lower.

  In the present embodiment, in addition to the heat shield structure of the combustion chamber, a heat insulating layer is formed in the combustion chamber 17 by a gas layer so that the cooling loss is greatly reduced.

  Specifically, the cavity 163 is formed from the injection tip of the fuel injection valve 6 after the compression stroke so that a gas layer containing fresh air is formed at the outer peripheral portion in the combustion chamber 17 and an air-fuel mixture layer is formed at the center. By injecting the fuel inward, as shown in FIG. 2, a gas mixture layer is formed in the central portion of the cavity 163 in the vicinity of the fuel injection valve 6, and the heat insulating gas containing fresh air around it. Stratification is realized in which layers are formed. The air-fuel mixture layer referred to here may be defined as a layer composed of a combustible air-fuel mixture, and the combustible air-fuel mixture may be, for example, an air-fuel mixture having an equivalent ratio φ = 0.1 or more. As the time elapses from the start of fuel injection, the fuel spray diffuses. Therefore, the size of the mixture layer is the size at the time of ignition. Ignition can be determined, for example, when the combustion mass ratio of the fuel becomes 1% or more. The air-fuel mixture ignites near the compression top dead center.

  The adiabatic 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 heat insulation gas layer, and it is sufficient that the fuel insulation layer is leaner than the gas mixture layer so that the heat insulation gas layer can serve as a heat insulation layer.

  If the air-fuel mixture is subjected to CAI combustion in the state where the heat insulating gas layer and the air-fuel mixture layer are formed as described above, the heat of the air-fuel mixture layer is caused by the heat insulating gas layer between the air-fuel mixture layer and the wall surface of the cylinder 11. Without coming into contact with the wall surface of the cylinder 11, the heat insulating gas layer becomes a heat insulating layer, and 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.

  In order to form such an air-fuel mixture layer and a heat insulating gas layer in the combustion chamber 17, it is desirable that the gas flow in the combustion chamber 17 is weak at the timing of fuel injection. For this reason, the intake port is configured to have a straight shape in which swirl does not occur or hardly occurs in the combustion chamber 17, and the tumble flow is also weakened as much as possible.

  As described above, when the air-fuel mixture layer and the heat insulating gas layer are formed in the combustion chamber 17, when the fuel injection amount is relatively small, for example, when the operating state of the engine 1 is in the light load region, the air-fuel mixture is Since it is easy to make the layer compact, it is relatively easy to form the gas mixture layer and the heat insulating gas layer. On the other hand, when the fuel injection amount increases, it is difficult to make the air-fuel mixture layer compact by injecting the fuel all at once, and it becomes difficult to form a heat insulating gas layer around the air-fuel mixture layer.

  In this regard, the fuel injection into the combustion chamber 17 is not performed all at once, but is performed by dividing into multiple injections, while increasing the air utilization rate in the combustion chamber 17 and It is advantageous in forming an insulating gas layer, but by dividing the fuel injection into a plurality of injections, fuel sprays overlap each other to form a locally rich mixture, and smoke It may cause occurrence.

  The engine 1 has a devised fuel injection mode so that a locally rich mixture is not formed locally. FIG. 4 illustrates the fuel injection mode. The horizontal axis in FIG. 4 indicates the crank angle, and the vertical axis indicates the lift amount of the fuel injection valve 6. The engine controller 100 performs the first injection through the fuel injection valve 6. The first injection is a fuel injection within the compression stroke period. The first injection is fuel injection with a relatively large first lift amount, and the injection period is relatively short. The first injection also switches the lift amount of the fuel injection valve 6 from the first lift amount to a smaller second lift amount while continuing the injection. Thus, the injection speed is increased without reducing the fuel injection flow rate.

  FIG. 5 shows a change in the lift amount of the fuel injection valve 6 (FIG. 5A) and a change in the fuel injection speed (FIG. 5B). As described above, in the outer-open valve type fuel injection valve 6, when the lift amount is increased, the effective opening area of the injection hole 61 is increased. As a result, the influence of the frictional resistance that the fuel receives from the wall surface of the nozzle 61 when passing through the nozzle is reduced. On the other hand, assuming that the fuel pressure is the same, the larger the effective opening area, the lower the fuel injection speed. Conversely, if the effective opening area decreases, the fuel injection speed increases. However, if the effective opening area decreases too much, the influence of the frictional resistance that the fuel receives from the wall surface of the injection hole 61 increases, and the injection speed decreases. . Accordingly, the fuel injection speed becomes maximum at a predetermined lift amount, and the fuel injection speed is reduced regardless of whether the fuel injection speed is larger or smaller than the predetermined lift amount. In the example shown in FIGS. 4 and 5, the second lift amount is a lift amount that maximizes the fuel injection speed (that is, the maximum speed lift amount). Note that the second lift amount can be set as appropriate above the maximum speed lift amount.

When fuel injection is started, the fuel injection speed can be quickly increased by increasing the effective opening area and reducing the influence of resistance. As shown by the solid line in FIG. 5 (a), the lift amount starts injection of fuel at a relatively large first lift amount (see time T ₀ through T _1), as indicated by the solid line in FIG. 5 (b) In addition, the fuel injection speed increases rapidly. On the other hand, as shown by the broken line in FIG. 5A, if the lift amount is reduced at the start of fuel injection, the increase in the fuel injection speed is delayed as shown by the broken line in FIG. 5B.

Then, after the fuel injection speed is sufficiently increased, the first injection has a lift amount smaller than the first lift amount, that is, the maximum lift amount, as shown by a solid line in FIG. Set the speed lift amount. By doing so, the effective opening area of the nozzle hole 61 is reduced, so that the injection speed of the fuel passing through the nozzle hole 61 is increased without reducing the injection flow rate (see times T _{2 to} T ₃ ). As shown by the solid line in FIG. 5B, the fuel injection speed reaches the maximum speed, and the fuel injection speed is maintained at the second lift amount by maintaining the lift amount of the fuel injection valve 6 as it is. (Refer to times T _{3 to} T ₅ ). Then, leading to the closing (see the time _T 5 ~T _6). The timing for switching the lift amount may be set to an appropriate timing so that a sufficient amount of fuel is injected after the fuel injection speed is sufficiently increased as described above and after the lift amount is switched.

The broken lines in FIGS. 5A and 5B are examples in which the lift amount of the fuel injection valve 6 is set to the maximum speed lift amount, but as described above, the lift amount at the start of fuel injection is small. As a result of the increase in the fuel injection speed being delayed, it takes a long time to reach the maximum speed (see times T _{0 to} T ₅ ).

When the lift amount of the fuel injection valve 6 is continued with the first lift amount as indicated by the one-dot chain line in FIG. 5A, the fuel injection speed is as shown by the one-dot chain line in FIG. 5B. It becomes constant at a predetermined speed without increasing the speed (see times T _{2 to} T ₄ ). Even in this case, the injection speed increases as the lift amount of the fuel injection valve 6 is reduced in order to close the fuel injection valve 6.

  FIG. 6 conceptually shows the spread of the fuel spray injected into the combustion chamber 17 in accordance with the fuel injection mode shown in FIG. When the lift amount of the fuel injection valve 6 is continued with the first lift amount, the fuel injection speed does not increase. Therefore, as shown by a two-dot chain line in FIG. The rear end in the direction is close to the tip of the fuel injection valve 6. On the other hand, since the fuel injection speed is increased as described above by switching the lift amount to a small value while continuing the fuel injection, as shown by the white arrow in FIG. The fuel spray injected in the second half of the first injection moves away from the fuel injection valve 6.

  By setting the fuel injection mode of the first injection as the first lift amount at the start of injection and then the second lift amount, the fuel spray injected earlier among the fuel sprays injected by the first injection is Since the speed is relatively low and the resistance is relatively high in the combustion chamber 17, it becomes difficult to fly and contact with the wall surface of the combustion chamber 17 is prevented. On the other hand, the fuel spray injected in the latter half of the first injection has a relatively high speed and a low resistance, so that the fuel spray moves away from the fuel injection valve 6 in a short time.

  After the end of the first injection, the second injection is performed at a predetermined interval. Part or all of the second injection is fuel injection in the latter half of the compression stroke. The latter half here is the latter half when the compression stroke period is equally divided into two periods, the first half and the second half. As described above, since the fuel spray injected by the first injection is away from the fuel injection valve 6, as shown in FIG. 6, the fuel spray injected by the second injection is different from the fuel spray injected by the first injection. Overlap is avoided. This prevents the air-fuel mixture from becoming excessively concentrated locally.

  The second injection is a fuel injection that is a third lift amount that is smaller than the first lift amount and that does not switch the lift amount halfway. The third lift amount is also preferably larger than the maximum speed lift amount (see FIG. 4). The third lift amount may be larger than the second lift amount. In the second injection, since the lift amount of the fuel injection valve 6 is relatively large, an increase in the fuel injection speed is suppressed. The second injection is performed at a timing delayed from the first injection. As a result, the fuel spray injected by the second injection is disposed closer to the fuel injection valve 6 than the fuel spray injected by the first injection, as shown in FIG. The positions of the fuel spray and the fuel spray injected by the second injection are shifted in the injection direction.

  In the second injection, the injection period is set longer than the injection period of the first injection. As a result, the fuel spray injected by the second injection comes closer to the injection axis S of the fuel injection valve 6. That is, the spray flow formed in the combustion chamber 17 as the fuel is injected entrains the surrounding air. Air hardly flows into the inside of the fuel spray that is injected in a hollow cone shape from the tip of the fuel injection valve 6. Therefore, when the injection period becomes longer, the negative pressure increases in the vicinity of the injection axis S of the fuel injection valve 6, and the fuel difference between the inside and outside of the fuel spray causes the fuel as shown by the solid line arrow in FIG. 6. The spray comes closer to the injection axis S of the fuel injection valve 6. As a result, the position of the fuel spray injected by the first injection and the position of the fuel spray injected by the second injection are also shifted in the angle direction of the spray angle. More specifically, the fuel spray injected by the second injection in the radial direction orthogonal to the axial direction of the fuel spray injected by the first injection is the fuel spray injected by the first injection. It comes to be located on the inner side in the radial direction. Thus, since the fuel sprays are prevented from overlapping each other, it is possible to surely prevent the air-fuel mixture layer from becoming excessively concentrated. As a result, it is possible to suppress the occurrence of smoke.

  Further, in the first injection, since the injection speed of the previously injected fuel is suppressed from the maximum speed, the fuel spray is prevented from coming into contact with the wall surface of the combustion chamber 17. As a result, it is possible to reliably provide the air-fuel mixture layer and the surrounding insulating gas layer. That is, it becomes possible to reduce cooling loss.

  Such an injection mode may be performed when the fuel injection amount increases as described above. For example, when the operating state of the engine 1 is in a low load or medium load region exceeding the light load region. , You may go. That is, in the light load region where the fuel injection amount is small, the fuel injection is performed collectively at a predetermined injection timing, and in the low load or medium load region where the load is higher than the light load region, as shown in FIG. Split injection including the first injection and the second injection may be performed.

  Moreover, since the overlap of the fuel spray injected by the first injection and the fuel spray injected by the second injection is prevented, the time interval between the first injection and the second injection can be narrowed. . Therefore, the fuel injection mode shown in FIG. 4 may be performed when the operating state of the engine 1 is in a region where the rotational speed is relatively high. In other words, as the rotational speed of the engine 1 increases, there is a demand to advance the injection end timing of the second injection in order to ensure the time for vaporization of the injected fuel and mixing of the fuel with air. In order to reliably form the gas layer, it is desirable that the pressure and temperature in the cylinder 11 when starting the first injection be high, and the injection start timing of the first injection is advanced so much. I can't. However, since the injection mode shown in FIG. 4 can shorten the time interval between the first injection and the second injection, the injection of the second injection can be performed without advancing the injection start timing of the first injection. It is possible to advance the end time. That is, when the rotational speed of the engine 1 increases, the mixture layer and the heat insulating gas layer are formed while preventing the formation of a locally rich mixture, and the vaporization of the injected fuel is further performed. In addition, it is possible to ensure time for mixing the fuel with air.

  Further, particularly in the operation region on the high rotation side, since the fuel injection amount is relatively small in the low load region, the flow in the cylinder 11 is increased, so that the air-fuel mixture is excessively diffused and the combustion temperature is increased. There is a risk of lowering and increasing unburned loss and increasing unburned components. With respect to this point, in the fuel injection mode shown in FIG. 4, the fuel spray injected by the first injection is a mixture having a relatively high concentration because the fuel spray injected in the latter half catches up with the previously injected fuel spray. Form. Therefore, the combustion temperature is prevented from greatly decreasing, which is advantageous for reducing unburned loss and cooling loss.

  FIG. 7 shows a fuel injection mode different from FIG. In the fuel injection mode shown in FIG. 7, the first injection is the same as the fuel injection mode shown in FIG. On the other hand, the second injection is different from the fuel injection mode shown in FIG. Specifically, the second injection in the injection mode shown in FIG. 7 is configured by multistage injection including a plurality of fuel injections. As described above, the fuel injection valve 6 having a piezo element has a high response and can perform about 20 multistage injections within 1 to 2 msec. The lift amount of the second injection configured by multi-stage injection is smaller than the first lift amount of the first injection and the maximum speed lift amount (or the second lift amount), as in the injection mode shown in FIG. Is a large third lift amount. Further, the injection period of the second injection is longer than the injection period of the first injection, similarly to the injection mode shown in FIG. Even when the second injection is a multistage injection, the injected fuel spray comes close to the fuel injection valve 6 and the injection axis S of the fuel injection valve 6 as shown in FIG. In the example of FIG. 7, the interval between the fuel injections constituting the second injection is substantially zero, but a predetermined interval may be provided between the fuel injection and the fuel injection.

  Further, when the required injection amount further increases, the third injection may be further performed after the first injection and the second injection shown in FIG. 4 or FIG.

  Further, in the above example, an externally opened fuel injector is employed as the fuel injector 6, but the fuel injector 6 applicable to the technology disclosed herein is an externally opened fuel injector. Not limited to valves. For example, a VCO (Valve Covered Orifice) nozzle type injector can also change the effective opening area of the nozzle by adjusting the degree of cavitation generated at the nozzle. Accordingly, as with the fuel injection valve of the open valve type, the fuel injection mode shown in FIG. 4 or 7 forms an air-fuel mixture layer at the center of the cavity 163 and an insulating gas layer around the outer periphery thereof, It is possible to suppress the formation of a locally rich mixture.

  Further, in the above example, the heat shielding structure of the combustion chamber and the intake port is adopted and the heat insulating gas layer is formed in the combustion chamber. However, the technology disclosed herein is applied to an engine that does not adopt the heat shielding structure. It can also be applied to.

1 Engine (Engine body)
100 Engine control unit (fuel injection control unit)
11 Cylinder 12 Cylinder Block 13 Cylinder Head 16 Piston 163 Cavity 17 Combustion Chamber 173 Recess 6 Fuel Injection Valve 60 Nozzle Body 61 Injection Port 62 Outer Valve

Claims (5)

An engine body having a combustion chamber defined by a ceiling portion of a cylinder head, a cylinder provided in a cylinder block, and a piston that reciprocates in the cylinder;
A fuel injection control unit having a fuel injection valve arranged to inject liquid fuel into the combustion chamber and configured to inject the fuel into the combustion chamber at least in the second half of a compression stroke; With
The fuel injection valve is configured such that the larger the lift amount, the larger the effective opening area of the nozzle that injects the fuel,
The fuel injection control unit performs the first injection for a predetermined injection period, then performs the second injection at a predetermined interval,
In the first injection, the fuel injection valve lift amount is set to a first lift amount larger than the maximum speed lift amount at which the fuel injection speed is maximum, and then the fuel injection is started, and then the injection is continued. The fuel is injected while switching the lift amount to a second lift amount that is smaller than the first lift amount and greater than or equal to the maximum speed lift amount.
The second injection is a fuel injection control device for a direct injection engine, in which the fuel is injected with a lift amount of the fuel injection valve smaller than the first lift amount. The fuel injection control device for a direct injection engine according to claim 1,
The fuel injection control device for a direct injection engine, wherein an injection period of the second injection is longer than an injection period of the first injection. The fuel injection control device for a direct injection engine according to claim 1 or 2,
The fuel injection valve has a nozzle body in which the nozzle hole is formed, and an outer valve that opens and closes the nozzle hole by lifting outward, and is configured to inject the fuel from the nozzle hole in a hollow cone shape. A fuel injection control device for a direct injection engine, which is a fuel injection valve of an open valve type. The fuel injection control device for a direct injection engine according to claim 3,
The outer-opening fuel injection valve is disposed in the ceiling portion of the cylinder head such that the injection axis is along the axis of the cylinder,
A fuel injection control device for a direct injection engine, wherein a concave portion that accommodates a tip portion of the fuel injection valve is formed in the ceiling portion so as to be recessed from the ceiling surface. The fuel injection control device for a direct injection engine according to claim 3 or 4,
A cavity is formed on the top surface of the piston,
The fuel injection control device of a direct injection engine, wherein the fuel injection valve is configured to inject the fuel into the cavity.

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