Engine Control Unit

Abstract

To set an optimum injection amount for sub injection while considering the cooling loss inherent to the sub injection.SOLUTION: A control device executes processes including a step (S100) of acquiring required torque, a step (S102) of acquiring an injection pressure and a boost pressure, a step (S104) of setting a sub injection amount after main injection so that the velocity of fuel spray at its front end on the wall surface of a combustion chamber is a threshold value or lower, a step (S106) of setting a main injection amount using the sub injection amount, and a step (S108) of executing injection control.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to the control of fuel injection into cylinders of an engine. [Background technology]

[0002] 2. Description of the Related Art With respect to fuel injection into engine cylinders, a technique is known in which fuel injection is divided into a main fuel injection and a sub-injection performed after the main fuel injection, for the purpose of improving fuel economy and reducing soot.

[0003] Regarding the determination of the timing and injection amount of this secondary injection, for example, JP 2017-227169 A (Patent Document 1) discloses a technology that estimates the amount of soot contained in the exhaust gas and the fuel consumption amount relative to the secondary injection timing, and determines the timing and injection amount of the secondary injection based on the estimated amount of soot and fuel consumption amount. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-227169 A Summary of the Invention [Problem to be solved by the invention]

[0005] In order to improve the fuel economy of the engine, it is desirable to reduce the cooling loss. Therefore, for example, by increasing the injection amount of the sub-injection, the cooling loss caused by the main injection is reduced, thereby improving the fuel economy.

[0006] However, when the injection amount of the secondary injection is large, the spray penetration force of the secondary injection becomes large, and the speed at which the spray collides with the wall of the combustion chamber increases. As a result, the fuel spreads along the wall of the combustion chamber, and the cooling loss due to the secondary injection becomes large, and the cooling loss of the entire fuel injection increases, which may result in a deterioration of fuel efficiency. Therefore, it is necessary to set the optimal injection amount of the secondary injection taking into account the cooling loss due to the secondary injection.

[0007] The present invention has been made to solve the above-mentioned problems, and its object is to provide an engine control device that sets an optimal injection amount of secondary injection taking into account the cooling loss caused by secondary injection. [Means for solving the problem]

[0008] According to one aspect of the present invention, there is provided an engine control device including a cylinder and a fuel injection device that injects fuel into a combustion chamber in the cylinder. The control device includes a setting unit that sets a fuel injection amount to be injected into the cylinder in one cycle using a required torque required for the engine, and a control unit that controls the fuel injection device so that the set fuel injection amount is injected into the cylinder. The fuel injection amount includes a first injection amount corresponding to a main injection and a second injection amount corresponding to a secondary injection after the main injection. The setting unit sets the second injection amount so that the fuel injected by the secondary injection reaches the wall surface of the combustion chamber after the secondary injection is terminated, and the speed of the tip of the fuel spray when it reaches the wall surface of the combustion chamber is in a reduced state due to the termination of the secondary injection.

[0009] In this way, the speed of the tip of the fuel spray decreases due to the end of the secondary injection when it reaches the wall of the combustion chamber, preventing the fuel that collides with the wall from spreading significantly along the wall, thereby suppressing an increase in cooling loss due to the secondary injection.

[0010] In one embodiment, the setting unit sets the second injection amount so that the velocity of the tip of the fuel spray on the wall surface of the combustion chamber becomes equal to or lower than a threshold value.

[0011] In this way, the fuel that collides with the wall surface can be prevented from spreading significantly along the wall surface, and an increase in cooling loss due to the secondary injection can be suppressed.

[0012] In yet another embodiment, the setting unit sets the first injection amount using the set second injection amount.

[0013] In this way, the required torque can be satisfied while suppressing an increase in cooling loss caused by the secondary injection.

[0014] Further, in one embodiment, the control unit performs the secondary injection multiple times after the main injection.

[0015] In this way, the cooling loss due to the main injection can be further reduced while suppressing an increase in the cooling loss due to the sub-injection, thereby improving fuel efficiency. Effect of the Invention

[0016] According to the present invention, it is possible to provide an engine control device that sets an optimal injection amount of secondary injection in consideration of the cooling loss caused by the secondary injection. [Brief description of the drawings]

[0017] [Figure 1] 1 is a diagram showing an example of a configuration of an engine according to an embodiment of the present invention. [Diagram 2] FIG. 4 is a diagram for explaining main injection and sub-injection. [Diagram 3] FIG. 11 is a diagram for explaining an example of a state of injected fuel when sub-injection after main injection is not performed. [Figure 4] FIG. 4 is a diagram for explaining an example of a state of injected fuel when a sub-injection is performed after a main injection. [Diagram 5] 4 is a flowchart illustrating an example of a process executed by a control device. [Figure 6] FIG. 13 is a diagram showing an example of a change in tip velocity of a spray over time. [Figure 7] FIG. 4 is a diagram showing the relationship between cooling loss and the secondary injection amount. [Figure 8] FIG. 13 is a plot of wall tip velocity corresponding to each of a number of injection amounts. [Figure 9] FIG. 4 is a diagram showing the relationship between various cooling losses and the secondary injection amount. [Figure 10] FIG. 4 is a diagram showing the relationship between thermal efficiency and the secondary injection amount. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the present embodiment will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

[0019] 1 is a diagram showing an example of the configuration of an engine 1 according to the present embodiment. The engine 1 is, for example, a direct injection type internal combustion engine, typically a diesel engine. The engine 1 is mounted as a power source in a moving body such as a vehicle.

[0020] As shown in FIG. 1, the engine 1 includes a cylinder 2, a piston 4, a fuel injector 10, an intake valve 12, an exhaust valve 14, an intake passage 16, an exhaust passage 18, a common rail 20, a control device 100, an injection pressure sensor 102, a boost pressure sensor 104, and an accelerator position sensor 106.

[0021] The cylinder 2 is formed in a cylinder block of the engine 1. A plurality of cylinders 2 may be formed in the cylinder block of the engine 1. A piston 4 is slidably housed in the cylinder 2. One end of an intake passage 16 and one end of an exhaust passage 18 are connected to a top 3 of the cylinder 2. A combustion chamber 6 is formed by the top 3 of the cylinder 2, an upper surface of the piston 4, and an inner wall of the side of the cylinder 2. In the combustion chamber 6, a mixture of air and fuel supplied into the cylinder 2 is burned. The air flows from an intake port (not shown), which is the other end of the intake passage 16, and the fuel is directly injected into the cylinder 2 by a fuel injection device 10. A recess 5 is formed in the upper surface of the piston 4.

[0022] Fuel injection device 10 is provided at top 3 of cylinder 2 with its tip protruding into cylinder 2. Fuel injection device 10 operates to inject fuel based on a control signal from control device 100. Fuel injection device 10 injects fuel toward the wall surface of recess 5 on the top surface of piston 4.

[0023] The fuel injection device 10 is constituted by, for example, an injector having a body in which a nozzle hole is formed, and a needle provided inside the body for opening and closing the nozzle hole. The fuel injection device 10 is connected to a fuel pump and a fuel tank (neither of which are shown) via a common rail 20. The fuel pump pumps fuel in the fuel tank 12 into the common rail 20 in response to a control signal from the control device 100. The control device 100 controls the fuel pump so that the pressure of the fuel in the common rail 20 becomes a predetermined pressure. The fuel injection device 10 supplies fuel in the common rail 20 to the combustion chamber 6 in the cylinder 2 by operating the needle in response to the control signal from the control device 100 to open the nozzle hole (i.e., to communicate between the nozzle hole and the common rail 20).

[0024] The control device 100 determines the amount of fuel injected in one cycle in the fuel injection device 10 based on, for example, the operating state and required output of the engine 1. The control device 100 generates a control command value corresponding to the determined amount of fuel injection, and controls the fuel injection device 10 using the generated control command value. The control command value is, for example, a value indicating the period during which fuel is injected from the fuel injection device 10 (more specifically, the period during which the nozzle hole is in an open state).

[0025] An intake valve 12 is provided at the connection between the top 3 of the cylinder 2 and one end of the intake passage 16. The intake valve 12 operates by using an intake cam that rotates in synchronization with the rotation of a crankshaft (not shown), which is the output shaft, so as to be in an open state during a predetermined period of one cycle (e.g., the intake stroke) and in a closed state during the other periods. Air taken in through the intake port flows into the cylinder 2 when the intake valve 12 is in an open state.

[0026] The piston 4 is connected to a crankshaft, which is an output shaft of the engine 1, via a connecting rod (not shown). When an injection operation is performed using the fuel injection device 10 in a predetermined crank angle section including the rotation angle of the crankshaft (hereinafter also referred to as the crank angle) at which the piston 4 reaches the top dead center (TDC), the mixture of air and fuel formed in the combustion chamber 6 is compressed in the compression stroke and burns by self-ignition. Then, in the expansion stroke, the piston 4 moves so that the piston 4 is pushed down by the combustion pressure (pressure wave) generated by the combustion of the mixture. The up and down movement of the piston 4 is converted into a rotational movement by the crankshaft, and the combustion pressure acting on the piston 4 acts on the crankshaft as an output torque, causing the crankshaft to rotate.

[0027] An exhaust valve 14 is connected to the connection between the cylinder 2 and one end of an exhaust passage 18. The exhaust valve 14 operates by using an exhaust cam that rotates in synchronization with the rotation of the crankshaft so as to be in an open state during a predetermined period of one cycle (for example, during the exhaust stroke) and in a closed state during the other periods. The gas combusted in the combustion chamber 6 of the cylinder 2 is discharged as exhaust gas into the exhaust passage 18 when the exhaust valve 14 is in an open state.

[0028] An exhaust treatment device (not shown) is provided in the exhaust passage 18. Predetermined components (such as nitrogen oxides) contained in the exhaust are purified by the exhaust treatment device. The exhaust gas flowing through the exhaust passage 18 is discharged to the outside from the other end of the exhaust passage 18 via the exhaust treatment device.

[0029] A turbocharger (not shown) is provided in the intake passage 16 and the exhaust passage 18. The turbocharger includes a compressor provided in the intake passage 16 and a turbine provided in the exhaust passage 18 (neither of which are shown). The compressor is provided with compressor blades that are rotatably supported. The turbine is provided with turbine blades that are rotatably supported and connected to the compressor blades via a shaft. Therefore, when the turbine blades are rotated by exhaust energy supplied from the cylinder 2 to the turbine in the exhaust passage 18, the compressor blades rotate via the shaft, and the intake air is compressed in the compressor. The intake air compressed (supercharged) in this manner is cooled in an intercooler (not shown) and supplied from the intake passage 16 to the cylinder 2.

[0030] The control device 100 includes a CPU (Central Processing Unit), a memory, and input and output ports (none of which are shown). The CPU executes various processes. The memory includes a ROM (Read Only Memory) that stores programs indicating the various processes executed by the CPU and data used when executing the various processes, and a RAM (Random Access Memory) that stores the processing results of the CPU, etc. Through the input port and the output port, information is exchanged between the control device 100 and devices external to the control device 100. An injection pressure sensor 102, a supercharging pressure sensor 104, and an accelerator opening sensor 106 are connected to the input port.

[0031] The injection pressure sensor 102 detects the pressure (hereinafter, referred to as the injection pressure) Pf of the fuel injected by the fuel injection device 10. The injection pressure sensor 102 transmits to the control device 100 a signal indicating the detected injection pressure Pf.

[0032] The supercharging pressure sensor 104 detects the pressure (hereinafter referred to as supercharging pressure) Pt of the intake air supplied to the cylinder 2. The supercharging pressure sensor 104 transmits to the control device 100 a signal indicating the detected supercharging pressure Pt.

[0033] The accelerator opening sensor 106 detects the depression amount (hereinafter also referred to as accelerator opening) Aac of an accelerator pedal (not shown) and transmits a signal indicating the detected accelerator opening Aac to the control device 100.

[0034] The output ports are connected to various electric devices to be controlled (for example, the fuel injection device 10). That is, the control device 100 receives signals from various sensors connected to the input ports, and transmits control signals to the various electric devices to be controlled in accordance with various processing results based on the received signals, thereby controlling the various electric devices.

[0035] In the present embodiment, the control device 100 includes, as functional blocks, a setting unit 122 and an injection control unit 124. The setting unit 122 and the injection control unit 124 may be configured as software or hardware.

[0036] The setting unit 122 obtains a required torque required for the engine 1 based on various signals from the input ports, and sets the amount of fuel injected into the cylinder 2 in one cycle using the obtained required torque.

[0037] The injection control unit 124 controls the fuel injection device 10 so that the set fuel injection amount is injected into the cylinder 2 .

[0038] In the fuel injection device 10 of the engine 1 having the above configuration, fuel injection is divided into a main injection of fuel and sub-injections performed before and after the main injection, for the purpose of improving fuel efficiency and reducing soot.

[0039] FIG. 2 is a diagram for explaining main injection and auxiliary injection. The horizontal axis of FIG. 2 indicates the crank angle CA. The vertical axis of FIG. 2 indicates whether the fuel injection device 10 is in an operating state or in an inoperating state (i.e., whether the injection hole is in an open state or not). The control device 100 sets the injection periods of, for example, an auxiliary injection (hereinafter referred to as a pilot injection) performed before the main injection performed before TDC, the main injection, and the auxiliary injection performed after the main injection. The control device 100 controls the fuel injection device 10 so that the injection operation is performed during the set injection period according to the set injection period.

[0040] FIG. 2 shows an example in which pilot injection is performed in a first section from crank angles CA(0) to CA(1) before TDC. FIG. 2 also shows an example in which main injection is performed in a second section from crank angles CA(2) to CA(3) after TDC. FIG. 2 also shows an example in which auxiliary injection after main injection is performed in a third section from crank angles CA(5) to CA(6). FIG. 2 also shows an example in which main injection is performed in a fourth section from crank angles CA(2) to CA(4) when auxiliary injection after main injection is not performed. Note that a section of the fourth section that is longer than the third section (i.e., the section from crank angles CA(3) to CA(4)) is shown by a dashed line.

[0041] As shown in FIG. 2, by splitting the fuel injection into a main injection and a secondary injection after the main injection, the amount of fuel injected in the main injection is reduced compared to the case where secondary injection after the main injection is not performed. This reduces the cooling loss caused by the main injection, which occurs when the fuel injected in the main injection comes into contact with the wall surface of the combustion chamber 6, thereby improving fuel efficiency.

[0042] Fig. 3 is a diagram for explaining an example of the state of injected fuel when sub-injection after main injection is not performed, and Fig. 4 is a diagram for explaining an example of the state of injected fuel when sub-injection after main injection is performed.

[0043] R1 in Fig. 3 shows an example of a diffusion region of fuel injected by main injection when no secondary injection is performed after the main injection. As shown in Fig. 3, the fuel injected by main injection collides with the wall surface of the recess 5 and spreads along the wall surface, forming a diffusion region shown by R1 in Fig. 3.

[0044] In contrast to this, R2 in Fig. 4 shows an example of the diffusion region of fuel injected by main injection when a secondary injection is performed after a main injection. Furthermore, R3 in Fig. 4 shows an example of the diffusion region of fuel injected by secondary injection when a secondary injection is performed after a main injection.

[0045] As shown in Fig. 4, the amount of fuel injected by the main injection is smaller than the diffusion region R1 (dashed line) of the fuel by the main injection when the secondary injection is not performed, and therefore the contact area with the wall surface of the recess 5 is smaller, as shown by R2 in Fig. 4. As a result, the cooling loss caused by the main injection is reduced, and fuel efficiency is improved.

[0046] Therefore, in order to improve fuel economy in the engine 1, the injection amount of this secondary injection is increased, which further reduces the cooling loss caused by the main injection, thereby improving fuel economy.

[0047] However, when the injection amount of the secondary injection is large, the spray penetration force of the secondary injection increases, and the speed at which the spray collides with the wall of the combustion chamber increases. As a result, the fuel from the secondary injection spreads along the wall of the combustion chamber, which increases the cooling loss caused by the secondary injection, and the cooling loss in the entire fuel injection increases, which may result in a deterioration of fuel efficiency. Therefore, it is necessary to set the optimal injection amount of the secondary injection taking into account the cooling loss caused by the secondary injection.

[0048] Therefore, in this embodiment, the control device 100 (more specifically, the setting unit 122) sets the injection amount of the secondary injection so that the fuel injected by the secondary injection reaches the wall surface of the combustion chamber 6 after the secondary injection is terminated, and so that the speed of the tip of the fuel spray when it reaches the wall surface of the combustion chamber 6 is in a reduced state due to the termination of the secondary injection.

[0049] In this way, the speed of the tip of the fuel spray decreases due to the end of the secondary injection when it reaches the wall of the combustion chamber 6, so that the fuel that collides with the wall is prevented from spreading significantly along the wall, thereby suppressing an increase in cooling loss due to the secondary injection.

[0050] Hereinafter, an example of a control process executed by the control device 100 will be described with reference to Fig. 5. Fig. 5 is a flowchart showing an example of a process executed by the control device 100. The control device 100 executes the process shown in the flowchart in Fig. 5 every time a predetermined time has elapsed.

[0051] In step (hereinafter, step will be referred to as S) 100, the control device 100 (i.e., setting unit 122) acquires a torque (hereinafter, referred to as a required torque) required in the engine 1. For example, the control device 100 may set the required torque using an accelerator opening Aac detected by the accelerator opening sensor 106, or may set a power required in the engine 1 using the accelerator opening Aac, and set a value calculated by dividing the power by a current travel speed as the required torque.

[0052] In S102, the control device 100 acquires the injection pressure Pf and the boost pressure Pt. For example, the control device 100 acquires the injection pressure Pf using the detection result of the injection pressure sensor 102. Furthermore, the control device 100 acquires the boost pressure Pt using the detection result of the boost pressure sensor 104.

[0053] In S104, the control device 100 sets the injection amount of the secondary injection after the main injection (hereinafter also referred to as the secondary injection amount). For example, the control device 100 calculates the time until the tip of the spray of the injected fuel reaches the wall surface of the combustion chamber 6 (hereinafter referred to as the arrival time) using the injection pressure Pf and the boost pressure Pt.

[0054] For example, assuming that injection continues until the tip of the injected fuel spray reaches the wall surface of the combustion chamber 6, from the start of injection to the breakup time tb at which the fuel breaks up into droplets and becomes atomized, for example, the following relational expression (1) (for example, Arai, Proc. ICLASS 2015) holds.

[0055] S = Kv × (2 × ΔP / ρl) 0.5 ×t···(1) Here, S indicates the travel distance of the tip of the fuel, Kv indicates a speed estimation coefficient, ΔP indicates the difference (Pf-Pt) between the injection pressure Pf and the boost pressure Pt, ρl indicates the fuel density, and t indicates time.

[0056] Furthermore, after the division time tb, for example, the following relational expressions (2) and (3) hold.

[0057] S = Kp × (Dn) 0.5 ×(ΔP / ρa) 0.25 ×t 0.5 (2) tb=Kbt×ρl×Dn / (ΔP×ρa) 0.5 (3) Here, S represents the reach of the tip of the atomized fuel, Kp represents the evaluation coefficient of the penetration force of the spray, Dn represents the nozzle hole diameter, ρa represents the gas density inside cylinder 2, and Kbt represents the evaluation coefficient of the breakup time.

[0058] Therefore, the arrival time tsp from the breakup time tb to the wall surface of the combustion chamber 6 can be calculated by the following relational expression (4) (for example, Inagaki, Transactions of the Society of Automotive Engineers of Japan, Vol. 38, No. 5, 2017).

[0059] tsp=(R / Kp) 2 ×Dn×(ρa / ΔP) 0.5 (4) Here, R indicates the length from the fuel injection device 10 to the wall surface of the combustion chamber 6. The wall surface of the combustion chamber 6 indicates the wall surface of the combustion chamber 6 in the direction of fuel injection from the fuel injection device 10 (specifically, the wall surface of the recess 5 on the upper surface of the piston 4). The various coefficients Kv, Kp, and Kbt are predetermined values ​​that are set by being adapted experimentally or by design.

[0060] The control device 100 sets a plurality of injection amounts that can be injected within the arrival time tsp. For example, the control device 100 calculates a velocity V of the tip of the fuel spray when the fuel reaches the wall surface (hereinafter referred to as tip velocity) that corresponds to each of the plurality of injection amounts that can be injected within the arrival time tsp.

[0061] Until the end of injection, the control device 100 calculates the tip speed V by, for example, time-differentiating S in the above-mentioned formulas (1) and (2). On the other hand, after the end of injection, the control device 100 calculates the tip speed V by using the following formula (5).

[0062] V=V inj / ((ψ×(tt inj )×V inj +1) · · · (5) V inj is the tip speed at the end of injection, and t inj indicates the injection period, and φ is calculated using the following equation (6).

[0063] φ=ρa×As×Cd / (2×M)···(6) As indicates the spray projected area, Cd indicates the resistance coefficient, and M indicates the spray mass. The spray projected area As and the resistance coefficient Cd are predetermined values ​​that are set by experiment or design. Furthermore, the control device 100 calculates the spray mass M from, for example, the volume of the injected fuel and the fuel density ρl. The control device 100 calculates the change in tip speed V of each of the multiple set injection amounts over time.

[0064] The control device 100 identifies an injection amount corresponding to a tip speed that is equal to or lower than a threshold value V(0) among a plurality of tip velocities at the wall surface corresponding to each of the plurality of injection amounts. The threshold value V(0) is a predetermined value that is set experimentally or by design. When there are a plurality of identified injection amounts, the control device 100 sets the injection amount that has the greatest effect of improving fuel efficiency among the injection amounts as the sub-injection amount after the main injection.

[0065] FIG. 6 is a diagram showing an example of the change in tip speed of the spray over time. The vertical axis of FIG. 6 indicates the tip speed. The horizontal axis of FIG. 6 indicates time. LN1 (thin line) of FIG. 6 shows an example of the change in tip speed when the amount of auxiliary injection is A(n). LN2 (dashed line) of FIG. 6 shows an example of the change in tip speed when the amount of auxiliary injection is A(n+1) (>A(n)). LN3 (thick line) of FIG. 6 shows an example of the change in tip speed when the amount of auxiliary injection is A(n+2) (>A(n+1)). The difference between A(n+1) and A(n) and the difference between A(n+2) and A(n+1) are assumed to be the same. "X" in FIG. 6 indicates the time when the tip of the fuel spray contacts the wall surface of the combustion chamber 6 when fuel is injected in the combustion chamber 6, and the change after that time is shown as the change in tip speed calculated according to the above-mentioned formula (5) assuming that there is no wall surface.

[0066] As shown in LN1 in Fig. 6, when the secondary injection amount is A(n), the tip speed decreases over time, and when the secondary injection ends, the tip speed at time t(0) becomes an inflection point, and then drops sharply after time t(0). This is because the force that expands the spray from the source of the spray to the tip of the spray weakens when the injection operation is stopped. In other words, the tip speed enters a decreasing state due to the end of the secondary injection after the inflection point. Then, at time t(4), the tip of the fuel spray comes into contact with the wall of the combustion chamber 6.

[0067] As shown in LN2 of FIG. 6, when the secondary injection amount is A(n+1), the tip speed decreases with time, similarly to LN1 of FIG. 6, and when the secondary injection is terminated, the tip speed at time t(1) becomes an inflection point, and then drops rapidly after time t(1), resulting in a decrease caused by the termination of the secondary injection. At this time, when the secondary injection amount is A(n+1), the injection period is longer than when the injection amount is A(n), so time t(1) is later than time t(0). Then, at time t(3), the tip of the fuel spray contacts the wall surface of the combustion chamber 6. When the secondary injection amount is A(n+1), the tip speed remains higher for a longer period than when the injection amount is A(n), so time t(3) is earlier than time t(4).

[0068] As shown by LN3 in Figure 6, when the injection amount of the secondary injection is A(n+2), the tip speed decreases over time, similar to LN1 and LN2 in Figure 6. And when the injection amount of the secondary injection is A(n+2), the injection period is longer than when the injection amount is A(n+1), so the tip of the fuel spray contacts the wall surface of the combustion chamber 6 at time t(2) before the secondary injection ends.

[0069] The control device 100 sets, for example, the injection amount A(n) corresponding to the tip speed that is equal to or less than the threshold value V(0) among the tip speeds at the wall surface corresponding to each of the multiple injection amounts A(n), A(n+1), and A(n+2) as the injection amount of the sub-injection. Note that, in FIG. 6, for convenience of explanation, a case where the injection amount that is equal to or less than the threshold value V(0) is specified from the tip speeds at the three wall surfaces of the injection amounts A(n), A(n+1), and A(n+2) is described as an example, but the number of injection amounts to be used for calculating the tip speed at the wall surface is not limited to three, and tip speeds at the wall surface corresponding to four or more injection amounts may be calculated. The multiple injection amounts may be set, for example, by the formula A(n)=A0+ΔA×n. A0 and ΔA are predetermined values.

[0070] For example, when there are a plurality of injection amounts corresponding to a tip speed equal to or lower than the threshold value V(0), the control device 100 sets the largest injection amount among them as the sub-injection amount.

[0071] Fig. 7 is a diagram showing the relationship between the cooling loss and the secondary injection amount. LN4 in Fig. 7 shows the change in the total cooling loss due to one fuel injection into cylinder 2 with respect to the change in the secondary injection amount.

[0072] As shown in Fig. 7, the overall cooling loss decreases as the sub-injection amount increases. Therefore, when there are multiple injection amounts corresponding to a tip speed that is equal to or lower than the threshold value V(0), the largest injection amount among them is set as the sub-injection amount, thereby making it possible to reduce the cooling loss.

[0073] Returning to FIG. 5, in S106, the control device 100 sets the main injection amount. The control device 100 sets the main injection amount so as to satisfy the required torque, for example. The control device 100 may set the fuel injection amount for the cylinder 2 using a map showing the relationship between the required torque and the fuel injection amount and the required torque, and may set the main injection amount by subtracting the injection amount corresponding to the pilot injection and the auxiliary injection amount set in S104 from the set fuel injection amount. The map showing the relationship between the required torque and the fuel injection amount is adapted by experiments or the like so as to include a fuel injection amount that does not contribute to the generation of torque such as pilot injection while satisfying the required torque, and is set in advance and stored in the memory of the control device 100.

[0074] At S108, the control device 100 (i.e., the injection control unit 124) executes injection control. Specifically, the control device 100 detects the crank angle using a crank angle sensor (not shown), and injects fuel using the fuel injection device 10 according to the detected crank angle. The control device 100 executes pilot injection at a first timing (crank angle), executes main injection at a subsequent second timing, and executes auxiliary injection at a subsequent third timing. The control device 100 sets the first timing, second timing, and third timing according to the operating state of the engine 1, for example.

[0075] The operation of the control device 100 based on the above-described structure and flowchart will now be described.

[0076] During operation of the engine 1, a required torque is obtained based on the accelerator opening Aac and the moving speed (S100). When the required torque is obtained, the injection pressure Pf and the boost pressure Pt are obtained using the injection pressure sensor 102 and the boost pressure sensor 104 (S102).

[0077] Using the acquired injection pressure Pf and boost pressure Pt, the tip speed at which the fuel reaches the wall surface is calculated when each of a plurality of injection amounts of fuel that can be injected as the secondary injection is injected.

[0078] FIG. 8 is a plot of tip speeds at the wall corresponding to multiple injection amounts. The vertical axis of FIG. 8 represents tip speed V. The horizontal axis of FIG. 8 represents injection amount. Calculation results of tip speeds when fuel corresponding to each injection amount reaches the wall are shown as plots in FIG. 8.

[0079] Among the calculated tip speeds, the injection amounts at which the tip speed is equal to or less than the threshold value V(0) are identified, and the largest of the identified injection amounts (injection amount A(n) in FIG. 8) is set as the sub-injection amount (S104). Then, the main injection amount is set using the set sub-injection amount (S106), and injection control is performed (S108).

[0080] As described above, with the engine control device of this embodiment, the speed of the tip of the fuel spray decreases due to the end of secondary injection when it reaches the wall of the combustion chamber 6 (the wall of the recess on the upper surface of the piston 4), so that the fuel that collides with the wall is prevented from spreading significantly along the wall, and an increase in cooling loss due to secondary injection can be suppressed.

[0081] FIG. 9 is a diagram showing the relationship between various cooling losses and the sub-injection amount. The vertical axis in FIG. 9 shows cooling losses. LN6 in FIG. 9 shows the change in the total cooling loss due to one fuel injection into cylinder 2 versus the change in the sub-injection amount. LN7 in FIG. 9 shows the change in cooling loss due to main injection in one fuel injection versus the change in the sub-injection amount. LN8 in FIG. 9 shows the change in cooling loss due to sub-injection in one fuel injection versus the change in the sub-injection amount.

[0082] As the secondary injection amount increases, the cooling loss due to the secondary injection increases as shown in LN8 in Fig. 9, but the cooling loss due to the main injection decreases by more than the increase in the cooling loss due to the secondary injection as shown in LN7 in Fig. 9. As a result, the total cooling loss decreases as shown in LN6 in Fig. 9.

[0083] On the other hand, if the amount of secondary injection becomes large and the injection amount is such that the fuel reaches the tip of the fuel spray before the injected fuel is finished, the increase in cooling loss due to the secondary injection exceeds the decrease in cooling loss due to the main injection, and the overall cooling loss increases.

[0084] Therefore, as described above, if the tip velocity of the fuel spray when the fuel reaches the wall surface of the combustion chamber 6 is lower than the threshold value V(0) and the largest injection amount is set as the secondary injection amount, it is possible to set the injection amount close to the value at which the overall cooling loss is minimized as the secondary injection amount. Therefore, it is possible to reduce the overall cooling loss while suppressing an increase in the cooling loss due to the secondary injection, thereby improving fuel efficiency.

[0085] Therefore, it is possible to provide an engine control device that sets an optimal injection amount of the secondary injection in consideration of the cooling loss caused by the secondary injection.

[0086] Furthermore, by setting the injection amount of the main injection using the set injection amount of the secondary injection, it is possible to set the injection amount of the main injection that satisfies the required torque while suppressing the increase in cooling loss caused by the secondary injection.

[0087] Modifications will be described below. In the above embodiment, the control device 100 has been described as calculating the tip speed at each injection amount from the injection pressure Pf and the boost pressure Pt using the above formula, but for example, a map showing the relationship between the injection pressure Pf, the boost pressure Pt, and the auxiliary injection amount may be set in advance, and the auxiliary injection amount may be set from the injection pressure Pf detected by the injection pressure sensor 102, the boost pressure Pt detected by the boost pressure sensor 104, and the map. The map is a map showing the relationship between the auxiliary injection amount at which the tip speed becomes equal to or less than the threshold value V(0) and which is the largest injection amount among the selectable injection amounts, the injection pressure Pf, and the boost pressure Pt, and may be set by experiment or the like and stored in advance in a memory.

[0088] Furthermore, in the above embodiment, the control device 100 has been described as setting the largest injection amount among the selectable injection amounts when the tip speed becomes equal to or less than the threshold value V(0) as the secondary injection amount, but the injection amount with the highest thermal efficiency among the selectable injection amounts may also be set as the secondary injection amount.

[0089] Fig. 10 is a diagram showing the relationship between thermal efficiency and the secondary injection amount, where the vertical axis represents thermal efficiency and the horizontal axis represents the secondary injection amount.

[0090] As shown in Fig. 10, the thermal efficiency increases as the sub-injection amount increases from zero. When the sub-injection amount is A1, the thermal efficiency reaches a maximum value E(0), and as the sub-injection amount increases beyond A1, the thermal efficiency decreases.

[0091] Therefore, the control device 100 may set the injection amount close to A1 among the selectable injection amounts when the tip speed is equal to or lower than the threshold value V(0) as the sub-injection amount. Note that A1 may be a predetermined value, or may be a value set according to the operating state of the engine 1.

[0092] Furthermore, in the above embodiment, the control device 100 has been described as performing a single sub-injection after the main injection to improve fuel efficiency, but such sub-injection may be performed two or more times, which can further improve fuel efficiency.

[0093] The above-described modified examples may be implemented in whole or in part in combination. The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0094] 1 engine, 2 cylinder, 3 top, 4 piston, 5 recess, 6 combustion chamber, 10 fuel injector, 12 intake valve, 14 exhaust valve, 16 intake passage, 18 exhaust passage, 20 common rail, 100 control device, 102 injection pressure sensor, 104 boost pressure sensor, 106 accelerator opening sensor, 122 setting unit, 124 injection control unit.

Claims

1. A control device for an engine including a cylinder and a fuel injection device that injects fuel into a combustion chamber in the cylinder, a setting unit that sets a fuel injection amount to be injected into the cylinder in one cycle using a required torque required for the engine; a control unit that controls the fuel injection device so that the set fuel injection amount is injected into the cylinder, the fuel injection amount includes a first injection amount corresponding to a main injection and a second injection amount corresponding to a sub-injection after the main injection, the setting unit sets the second injection amount so that the fuel injected by the secondary injection reaches a wall surface of the combustion chamber after the secondary injection is terminated, and the velocity of a tip of the fuel spray when it reaches the wall surface of the combustion chamber is in a reduced state due to the termination of the secondary injection, the setting unit calculates a time required for the tip of the spray of the injected fuel to reach the wall surface of the combustion chamber, assuming that injection is continued, sets a plurality of injection amounts that can be injected within the time, calculates a speed of the tip of the fuel spray when the fuel reaches the wall surface of the combustion chamber for each of the plurality of injection amounts as a tip velocity, and sets an injection amount among the plurality of injection amounts whose tip velocity is equal to or less than a threshold value and which is the largest as the second injection amount; the fuel injection amount further includes a pilot injection amount corresponding to the secondary injection prior to the main injection, a control device for an engine, wherein the first injection amount is set to an amount obtained by subtracting the pilot injection amount and the second injection amount from the fuel injection amount after the second injection amount is set.

2. 2. The engine control device according to claim 1, wherein the setting unit sets the second injection amount so that a velocity of a tip of the spray of the fuel injected by the secondary injection enters the decreasing state having an inflection point after the secondary injection is ended.

3. The engine control device according to claim 1 , wherein the setting unit sets the first injection amount using the second injection amount that has been set.

4. 4. The engine control device according to claim 1, wherein the control unit performs the sub-injection a plurality of times after the main injection.

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