JP7735876B2 - Compression ignition engine control device - Google Patents

Description

The present invention relates to a control device for a compression ignition engine in which compression ignition combustion of an air-fuel mixture is carried out in a combustion chamber.

In automotive engines and other vehicle engines, further improvements in fuel economy and exhaust performance are required. In response to this, the applicant has developed a configuration in which, as shown in Patent Document 1, the cavity formed on the piston crown has a two-tiered structure consisting of an upper cavity and a lower cavity, and fuel is distributed throughout the combustion chamber by impinging on the lip connecting these two cavities, thereby shortening combustion time, improving fuel economy, and suppressing soot generation.

Specifically, Patent Document 1 discloses an engine having a piston with the above-mentioned two-stage cavity, in which a fuel injection valve that injects fuel toward the cavity performs a main injection near top dead center of compression and a pilot injection during the compression stroke, and these main injection and pilot injection are performed at a timing when the injection axis of the fuel injection valve is pointed toward the lip. Furthermore, since this engine is prone to distributing fuel from the main injection to the lower cavity when the engine load is high, the engine is configured to distribute a greater proportion of fuel from the pilot injection to the upper cavity than when the engine load is low.

Japanese Patent Application Laid-Open No. 2020-122407

According to the configuration of Patent Document 1, as described above, when fuel from the main injection is likely to be distributed to the lower cavity, a larger proportion of fuel from the pilot injection is distributed to the upper cavity. This results in the total fuel distribution proportions from the pilot injection and main injection to each cavity being equal, and fuel distribution throughout the combustion chamber after the main injection is completed is homogenized. However, with this configuration, the distribution of fuel from the main injection is still uneven, resulting in insufficient mixing of the fuel and air, and there is room for improvement in this regard and, ultimately, in terms of improving fuel economy and exhaust performance.

The present invention was made in consideration of the above circumstances, and aims to provide a control device for a compression ignition engine that can reliably improve fuel economy and exhaust performance in an engine in which part of the combustion chamber is defined by a piston crown surface having a two-stage cavity with an upper and lower structure.

An engine control device according to one aspect of the present invention is a control device for a compression ignition engine that performs compression ignition combustion of an air-fuel mixture in the combustion chamber, the control device comprising an engine body having a cylinder, a piston that reciprocates within the cylinder, a combustion chamber formed by the cylinder and the piston's crown surface, a fuel injection valve disposed on the ceiling surface of the combustion chamber and that injects fuel along an injection axis, and a fuel injection control unit that controls the fuel injection valve. In this control device for a compression ignition engine, the piston has a lower cavity disposed in the radial center of the crown surface, an upper cavity that is disposed around the lower cavity and is shallower than the lower cavity, and a lip portion that connects the lower cavity and the upper cavity. The fuel injection control unit controls the fuel injection valve to inject fuel into the compression stroke when the engine is operating in a first operating range where the engine speed is equal to or lower than a predetermined switching speed and a second operating range where the engine speed is higher than the switching speed. The system implements distributed injection control, which causes the fuel injection valve to perform a pilot injection in which fuel is injected during the switching engine speed and at a timing when the injection shaft is pointed toward the lip portion for at least a portion of the injection period, and a main injection in which fuel is injected after the pilot injection and at a timing when the injection shaft is pointed toward the lip portion for at least a portion of the injection period. When the distributed injection control is implemented, if the engine is operating in the first operating range, first control is implemented to control the fuel injection valve so that the injection timing of the main injection is more advanced when the engine speed is high than when it is low, and if the engine is operating in the second operating range, second control is implemented to control the fuel injection valve so that the injection timing of the pilot injection and the main injection are each retarded relative to the injection timing at the switching engine speed.

With this control device, by implementing distributed injection control, both pilot injection and main injection are performed at a timing where the injection axis is directed toward the lip portion for at least part of each injection period. Therefore, the fuel for these injections can be distributed to both the upper and lower cavities.

Furthermore, when the engine is operating in the first operating range, where the engine speed is below the switching speed, the main injection timing is more advanced when the engine speed is high than when it is low. Therefore, in the above case, the fuel from the main injection can be distributed to the two upper and lower cavities at an appropriate ratio, allowing the fuel from the main injection to be more evenly dispersed throughout the combustion chamber. As a result, a homogeneous mixture that is well mixed with air can be formed throughout the combustion chamber near top dead center of compression, which reliably achieves improved fuel economy and reduced soot generation, i.e., improved exhaust performance.

Specifically, as engine speed increases, piston speed increases. Therefore, if the injection timing of the main injection were to remain constant regardless of engine speed, the piston position at the time when the main fuel from the main injection reaches the lip would be lower at high engine speeds than at low engine speeds, and the fuel from the main injection would not be biased toward the upper cavity at high engine speeds. In contrast, if the injection timing of the main injection is more advanced at high engine speeds than at low engine speeds, as described above, the piston position at this timing would be such that the fuel from the main injection that reaches the lip at each engine speed is appropriately distributed to the two upper and lower cavities, and the fuel can be distributed in appropriate proportions to the two cavities.

However, if the advancement of the main injection timing is increased, the thermal energy generated near the compression top dead center becomes excessive, increasing the amount of NOx produced. In response to this, with this control device, when the engine speed exceeds the switching speed, the injection timing of the main injection and pilot injection are retarded compared to the respective injection timings at the switching speed.

If the main injection timing is retarded, the piston position will be lower when the main fuel reaches the lip, which will tend to bias that fuel toward the upper cavity. On the other hand, if the pilot injection performed during the compression stroke is retarded, the pilot-injected fuel will tend to bias toward the lower cavity. Therefore, by retarding both the main injection and pilot injection as described above, the total fuel supplied to the combustion chamber by these injections can be more evenly distributed. Therefore, with this control device, in the second operating range higher than the switching speed, the main injection timing is retarded from the switching speed, which suppresses the generation of NOx. Furthermore, the homogenization of the total fuel by the main injection and pilot injection suppresses the generation of soot, ensuring improved exhaust performance in the second operating range.

In the above control device, it is desirable that, when the first control is performed, the fuel injection control unit controls the fuel injection valve so that the injection timing of the pilot injection is more advanced when the engine speed is high than when the engine speed is low (Claim 2).

With this control device, when the engine is operating in the first operating range, the fuel distribution ratio between the upper and lower cavities can be adjusted appropriately, in addition to the fuel from the main injection, so fuel economy and exhaust emissions can be more reliably improved.

In the above control device, it is desirable that, when the distributed injection control is performed, the fuel injection control unit causes the fuel injection valve to perform a pre-injection, which injects fuel between the pilot injection and the main injection, and controls the fuel injection valve so that the time interval between the injection timing of the pre-injection and the injection timing of the main injection is constant (Claim 3).

This control system performs a pre-injection, injecting fuel between the pilot injection and the main injection, preventing a large amount of fuel from being supplied to the combustion chamber all at once by the main injection. This prevents sudden combustion near the top dead center of compression and reduces the increase in combustion noise. Furthermore, because the time interval between the injection timing of the pre-injection and the main injection is constant, interference between the fuel spray from the pre-injection and the fuel spray from the main injection can be reduced, allowing for more appropriate distribution of the fuel from the main injection to each cavity.

It is desirable that the above control device further includes a gear detection unit capable of detecting the gear position of a transmission provided in a vehicle equipped with the engine, and that the fuel injection control unit implements the first control and the second control when the gear position detected by the gear detection unit is equal to or greater than a predetermined number set to two or more (Claim 4).

As described above, with this invention, when distributed injection control is implemented and the engine is operating in the high engine speed range of the first operating region, there is a risk that the main injection timing will be advanced, resulting in higher combustion noise. In contrast, with this control device, distributed injection control is implemented when the gear is at a high speed greater than or equal to a predetermined number, that is, when the vehicle equipped with the engine is traveling at high speed. This prevents occupants from sensing the combustion noise and feeling uncomfortable.

The present invention provides a control device for a compression ignition engine that can reliably improve fuel economy and exhaust performance in an engine in which part of the combustion chamber is defined by a piston crown surface having a two-stage cavity.

FIG. 1 is a system diagram of a vehicle equipped with a diesel engine to which a control device for a compression ignition engine according to the present invention is applied. 2A is a perspective view of the crown surface of the piston of the diesel engine shown in FIG. 1, and FIG. 2B is a perspective view of the piston with a cross section. FIG. 3 is a block diagram showing the engine control system. FIG. 4 is a map showing the operating range of the engine. FIG. 5 is a flowchart showing the overall flow of injector control. FIG. 6 is a time chart showing an example of the fuel injection timing and the heat generation rate. FIG. 7 is a flowchart showing the flow of injector control in the medium load range. FIG. 8 is a cross-sectional view of a combustion chamber showing the flow state of fuel spray. FIG. 9 is a cross-sectional view of the combustion chamber for explaining the distribution of fuel by main injection. FIG. 10 is a cross-sectional view of the combustion chamber for explaining the distribution of fuel by pilot injection. FIG. 11 is a graph showing the relationship between the engine speed and the main injection timing correction amount. FIG. 12 is a graph showing the relationship between the engine speed and the pilot injection timing correction amount.

[Overall configuration]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a control device for a compression ignition engine according to the present invention will be described in detail below with reference to the drawings. In this embodiment, an example in which the present invention is applied to a diesel engine system will be shown.

The diesel engine system according to this embodiment is mounted on a vehicle 100 as a driving source for traveling. Figure 1 is a system diagram of the vehicle 100. Wheels 120 provided on the vehicle 100 are driven and rotated by the output of the diesel engine system. The vehicle 100 is equipped with a transmission 110 including multiple gear stages, and the output of the diesel engine system is transmitted to the wheels 120 while being shifted by the transmission 110. The transmission 110 may, for example, have eight gear stages (an eight-speed transmission).

[Overall engine configuration]
The diesel engine system includes a four-stroke engine body 1 having a plurality of cylinders 2 and powered by a supply of fuel whose main component is diesel oil, an intake passage 30 through which intake air introduced into the engine body 1 flows, an exhaust passage 40 through which exhaust gas discharged from the engine body 1 flows, an EGR device 44 that recirculates a portion of the exhaust gas flowing through the exhaust passage 40 back into the intake passage 30, and a turbocharger 46 powered by the exhaust gas passing through the exhaust passage 40.

The engine body 1 has multiple cylinders 2 (only one of which is shown in FIG. 1) lined up in a direction perpendicular to the plane of the paper on which FIG. 1 is drawn. The engine body 1 also includes a cylinder block 3 in which the cylinders 2 are formed, a cylinder head 4, and a piston 5. The cylinder head 4 is attached to the upper surface of the cylinder block 3. The piston 5 is housed in the cylinder 2 so that it can slide back and forth, and is connected to the crankshaft 7 via a connecting rod 8. As the piston 5 reciprocates, the crankshaft 7 rotates around its central axis. The structure of the piston 5 will be described in detail below.

A combustion chamber 6 is formed above the piston 5. The combustion chamber 6 is formed by the underside of the cylinder head 4 (combustion chamber ceiling surface 6U, see Figure 3), the cylinder 2, and the crown surface 50 of the piston 5. Fuel is supplied to the combustion chamber 6 by injection from an injector 15, which will be described later. The engine body 1 is a compression ignition engine in which compression ignition combustion of an air-fuel mixture takes place within the combustion chamber 6. The supplied fuel-air mixture is compressed by the piston 5 and self-ignites within the combustion chamber 6. The piston 5 is pushed down by the expansion force caused by the combustion of the air-fuel mixture, causing it to reciprocate vertically.

A crank angle sensor SN1 and a water temperature sensor SN2 are attached to the cylinder block 3. The crank angle sensor SN1 detects the rotation angle (crank angle) of the crankshaft 7 and the rotation speed (engine rotation speed) of the crankshaft 7. The water temperature sensor SN2 detects the temperature of the cooling water (engine water temperature) flowing through the cylinder block 3 and cylinder head 4.

The cylinder head 4 is formed with intake ports 9 and exhaust ports 10 that communicate with the combustion chamber 6. The underside of the cylinder head 4 is formed with an intake-side opening, which is the downstream end of the intake port 9, and an exhaust-side opening, which is the upstream end of the exhaust port 10. The cylinder head 4 is fitted with an intake valve 11 that opens and closes the intake-side opening, and an exhaust valve 12 that opens and closes the exhaust-side opening. Although not shown, the engine body 1 is a four-valve system (two intake valves and two exhaust valves), with two intake ports 9, two intake valves 11, two exhaust ports 10, and two exhaust valves 12 per cylinder 2. The intake valves 11 and the exhaust valves 12 are opened and closed by valve trains 13 and 14 provided in the cylinder head 4 in conjunction with the rotation of the crankshaft 7. Each valve train 13 and 14 incorporates an S-VT, which continuously changes the opening and closing timing of the intake valves 11 and the exhaust valves 12.

Each cylinder 2 is fitted with an injector 15 (fuel injection valve) that injects fuel from its tip into the combustion chamber 6 on the cylinder head 4. As shown in Figure 8 (described below), the injector 15 has a nozzle 151 at its tip. The injector 15 is attached to the cylinder head 4 so that the nozzle 151 protrudes downward toward the interior of the combustion chamber 6 from or near the radial center of the combustion chamber ceiling surface 6U. The nozzle 151 has an injection hole 152 that injects fuel into the combustion chamber 6. While Figure 8 shows one injection hole 152, in reality, multiple injection holes 152 are arranged at equal intervals around the circumference of the nozzle 151.

The injectors 15 are connected via fuel supply pipes to a common rail (not shown) shared by all cylinders 2. High-pressure fuel pressurized by a fuel pump (not shown) is stored in the common rail. The fuel stored in this common rail is supplied to the injectors 15 of each cylinder 2, causing each injector 15 to inject fuel at high pressure (approximately 50 MPa to 250 MPa) into the combustion chamber 6. A fuel pressure regulator 16 (not shown in Figure 1, see Figure 3) is provided between the fuel pump and the common rail to change the injection pressure, which is the pressure of the fuel injected from the injectors 15.

The intake passage 30 is connected to one side of the cylinder head 4 so as to communicate with the intake port 9. Air (fresh air) taken in from the upstream end of the intake passage 30 is introduced into the combustion chamber 6 through the intake passage 30 and the intake port 9. Arranged in the intake passage 30, in order from the upstream side, are an air cleaner 31, a compressor 47 of a turbocharger 46, a throttle valve 32, an intercooler 33, and a surge tank 34.

The air cleaner 31 removes foreign matter from the intake air. The throttle valve 32 opens and closes the intake passage 30 to adjust the flow rate of intake air in the intake passage 30. The compressor 47 is driven to rotate by a turbine 48 provided in the exhaust passage 40, and compresses (supercharges) the intake air flowing through the intake passage 30. The intercooler 33 cools the intake air compressed by the compressor 47. The surge tank 34 provides space for evenly distributing the intake air to multiple cylinders 2.

An air flow sensor SN3, an intake air temperature sensor SN4, an intake pressure sensor SN5, and an intake _O2 sensor SN6 are arranged in the intake passage 30. The air flow sensor SN3 is arranged downstream of the air cleaner 31 and detects the flow rate of the intake air passing through that section. The intake air temperature sensor SN4 is arranged downstream of the intercooler 33 and detects the temperature of the intake air passing through that section. The intake pressure sensor SN5 and the intake _O2 sensor SN6 are arranged near the surge tank 34 and detect the pressure and oxygen concentration of the intake air passing through that section, respectively.

The exhaust passage 40 is connected to the other side of the cylinder head 4 so as to communicate with the exhaust port 10. Burned gas (exhaust gas) generated in the combustion chamber 6 is discharged to the outside of the vehicle through the exhaust port 10 and the exhaust passage 40. The exhaust passage 40 is provided with, from upstream to downstream, a turbine 48 of a turbocharger 46 and an exhaust purification device 41. The exhaust purification device 41 incorporates an oxidation catalyst 42 and a DPF (diesel particulate filter) 43 for capturing particulate matter. A NOx catalyst that reduces and detoxifies NOx may also be located downstream of the exhaust purification device 41 in the exhaust passage 40.

An exhaust _O2 sensor SN8 and a differential pressure sensor SN9 are arranged in the exhaust passage 40. The exhaust _O2 sensor SN8 is arranged between the turbine 48 and the exhaust purification device 41 and detects the oxygen concentration of the exhaust passing through these sections. The differential pressure sensor SN9 detects the differential pressure between the upstream end and downstream end of the DPF 43.

The EGR device 44 includes an EGR passage 44A that connects the exhaust passage 40 and the intake passage 30, and an EGR valve 45 that is provided in the EGR passage 44A and adjusts the flow rate of exhaust gas (EGR gas) flowing through the EGR passage 44A. The EGR passage 44A connects the portion of the exhaust passage 40 upstream of the turbine 48 with the portion of the intake passage 30 between the intercooler 33 and the surge tank 34. An EGR cooler (not shown) is provided in the EGR passage 44A to cool the EGR gas passing through it.

[Detailed structure of the piston]
Next, the structure of the piston 5, particularly the structure of the crown surface 50, will be described in detail. Fig. 2(A) is a perspective view mainly showing the upper portion of the piston 5. Fig. 2(A) shows a piston head that forms the upper part of the piston 5 and has the crown surface 50 as its top surface. Fig. 2(B) is a perspective view with a radial cross section of the upper part (piston head) of the piston 5. Note that in Figs. 2(A) and 2(B), the cylinder axial direction A and the radial direction B of the combustion chamber are indicated by arrows. In the following description of the piston 5, the sliding direction of the piston 5 is considered to be the vertical direction, and the plane perpendicular to this is considered to be the horizontal plane.

The piston 5 includes a cavity 5C, a peripheral flat surface 55, and a side circumferential surface 56. A portion (bottom surface) of the combustion chamber wall that defines the combustion chamber 6 is formed by the piston crown 50, and the cavity 5C is provided in this crown 50. The cavity 5C is a portion of the crown 50 that is recessed downward in the cylinder axial direction A, and receives the spray of fuel injected by the injector 15. The peripheral flat surface 55 is an annular flat portion located in an area near the outer periphery of the crown 50 in the radial direction B. The cavity 5C is located in the central area of the crown 50 in the radial direction B, excluding the peripheral flat surface 55. The side circumferential surface 56 is a surface that comes into sliding contact with the inner wall surface of the cylinder 2.

Cavity 5C is a so-called egg-shaped two-tiered cavity, with upper and lower cavities each having an egg-shaped cross-section wall.

Specifically, the cavity 5C includes a lower cavity 51, an upper cavity 52, a lip portion 53, and a peak portion 54. The lower cavity 51 is a recess located in the radial center of the crown surface 50, i.e., in the central region in the radial direction B. The upper cavity 52 is an annular recess located on the outer periphery of the lower cavity 51 on the crown surface 50. The lip portion 53 connects the lower cavity 51 and the upper cavity 52 in the radial direction B. The peak portion 54 is a mountain-shaped protrusion located in the central position of the crown surface 50 (lower cavity 51) in the radial direction B. The peak portion 54 is protruding directly below the nozzle 151 of the injector 15 (Figure 8).

The lower cavity 51 includes a first upper end 511, a first bottom 512, and a first inner end 513. The first upper end 511 is located at the highest point in the lower cavity 51 and is connected to the lip portion 53. The first bottom 512 is the most recessed area in the lower cavity 51, and is annular in top view. The first bottom 512 is also the deepest part of the cavity 5C as a whole. In top view, the first bottom 512 is located close to the inner side of the lip portion 53 in the radial direction B.

The first upper end 511 and the first bottom 512 are connected by a radial recess 514 that curves outward in the radial direction B. The radial recess 514 has a portion that is recessed further outward in the radial direction B than the lip portion 53. The first inner end 513 is located at the radially innermost position in the lower cavity 51 and is connected to the lower end of the peak portion 54. The first inner end 513 and the first bottom 512 are connected by a gently curved, skirt-like surface.

The upper cavity 52 includes a second inner end portion 521, a second bottom portion 522, a second upper end portion 523, a tapered region 524, and a standing wall region 525. The second inner end portion 521 is located at the radially innermost position in the upper cavity 52 and is connected to the lip portion 53. The second bottom portion 522 is the most recessed region in the upper cavity 52. The second bottom portion 522 of the upper cavity 52 has a shallower depth in the cylinder axial direction A than the first bottom portion 512. In other words, the upper cavity 52 is a recess located above the lower cavity 51 in the cylinder axial direction A. The second upper end portion 523 is located at the highest position and radially outermost in the upper cavity 52 and is connected to the peripheral flat portion 55. The tapered region 524 extends from the second inner end portion 521 toward the second bottom portion 522 and has a surface shape that slopes downward radially outward.

The standing wall region 525 is a wall surface formed to rise relatively steeply radially outward from the second bottom portion 522. In the cross-sectional shape in the radial direction B, from the second bottom portion 522 to the second upper end portion 523, the wall surface of the upper cavity 52 is curved from the horizontal direction to the upward direction, and the standing wall region 525 is the portion of the wall surface near the second upper end portion 523 that is close to a vertical wall. The lower portion of the standing wall region 525 is located inside in the radial direction B relative to the upper end position of the standing wall region 525.

In a cross section taken along the radial direction B, the lip portion 53 has a hump-like shape that protrudes radially inward between the lower cavity 51 located below and the upper cavity 52 located above. The lip portion 53 has a lower end portion 531, a third upper end portion 532 (the upper end portion in the axial direction of the cylinder), and a central portion 533 located in the middle between them. The lower end portion 531 is connected to the first upper end portion 511 of the lower cavity 51. The third upper end portion 532 is connected to the second inner end portion 521 of the upper cavity 52.

In the cylinder axial direction A, the lower end 531 is the lowest portion of the lip portion 53, and the third upper end 532 is the highest portion. The tapered region 524 described above also extends from the third upper end 532 toward the second bottom 522. The second bottom 522 is located below the third upper end 532. In other words, the upper cavity 52 of this embodiment does not have a bottom surface that extends horizontally outward from the third upper end 532 in the radial direction B. In other words, the third upper end 532 is not connected by a horizontal surface to the peripheral flat portion 55, but has a second bottom 522 that is recessed below the third upper end 532.

The ridge portion 54 protrudes upward, but its protruding height is the same as the height of the third upper end portion 532 of the lip portion 53, and it is recessed from the peripheral flat surface portion 55. The ridge portion 54 is located at the center of the circular lower cavity 51 when viewed from above, which gives the lower cavity 51 the form of an annular groove formed around the ridge portion 54.

[Control configuration]
Next, the control configuration of the diesel engine system will be described with reference to the block diagram of Figure 3. The diesel engine system of this embodiment is comprehensively controlled by a processor 70 (engine control device). The processor 70 is composed of a CPU, ROM, RAM, etc.

Detection signals from various sensors mounted on the vehicle are input to processor 70. In addition to sensors SN1 to SN9 described above, the vehicle is equipped with an accelerator position sensor SN10 that detects the accelerator position, an atmospheric pressure sensor SN11 that measures the atmospheric pressure in the vehicle's driving environment, an outside air temperature sensor SN12 that measures the air temperature in the vehicle's driving environment, and a gear position sensor SN13 that detects the current gear position of the transmission 110. Information detected by these sensors SN1 to SN13, such as crank angle, engine speed, engine water temperature, intake air flow rate, intake air temperature, intake air pressure, intake oxygen concentration, injection pressure of the injector 15, exhaust oxygen concentration, accelerator position, outside air temperature, air pressure, and gear position, is sequentially input to processor 70. Gear position sensor SN13 corresponds to the "gear position detection unit" in the claims.

The processor 70 controls various parts of the engine while making various judgments and calculations based on input signals from the above-mentioned sensors SN1 to SN13. That is, the processor 70 is electrically connected to the injector 15 (fuel pressure regulator 16), throttle valve 32, EGR valve 45, etc., and outputs control signals to these devices based on the results of the above calculations, etc.

By executing a predetermined program, the processor 70 operates to functionally include a determination unit 71, a fuel injection control unit 72 that controls the operation of the injector 15, and a memory unit 78.

The determination unit 71 sets the target engine torque, which is the target torque for the engine. The determination unit 71 sets the target engine torque based on the engine speed detected by the crank angle sensor SN1 and the accelerator opening detected by the accelerator opening sensor SN10.

The determination unit 71 determines the engine's operating range (determines in which operating range the engine is operating). Figure 4 is a map diagram showing operating ranges set according to engine speed and target engine torque, i.e., engine load. Engine operating ranges are broadly divided into a low-load range A1, a medium-load range A2, and a high-speed, high-load range A3 based on differences in combustion mode. The low-load range A1 is the range where the engine speed is equal to or less than a predetermined first speed N1 and the engine load is equal to or less than a predetermined first load T1. The medium-load range A2 is the range where the engine speed is equal to or less than the first speed N1 and the engine load is equal to or less than a predetermined second load T2, excluding the low-load range A1. The high-speed, high-load range A3 is the remaining range outside the low-load range A1 and the medium-load range A2. The determination unit 71 determines the engine's operating range based on the set target engine torque (engine load) and engine speed.

The determination unit 71 determines whether the current gear position of the transmission 110 is a high gear position. The determination unit 71 determines that the gear position is a high gear position when the current gear position detected by the gear position sensor SN13 is equal to or greater than a preset reference gear position (predetermined number of positions). The reference gear position is preset to a value equal to or greater than two positions (second gear) and stored in the memory unit 78. For example, if the transmission 110 has eight gear positions, the reference gear position is set to seventh position (seventh gear).

The fuel injection control unit 72 controls the fuel injection operation by the injector 15. By executing a predetermined program, the fuel injection control unit 72 operates to functionally include an injection amount setting unit 73 and an injection timing setting unit 74.

The injection quantity setting unit 73 sets the injection quantity, which is the amount of fuel injected from the injector 15. In this embodiment, in any gear position and in any of the regions A1 to A3, multi-stage injection is performed in which fuel is injected from the injector 15 in multiple injections during one combustion cycle. Thus, the injection quantity setting unit 73 sets the injection quantity for each injection performed during one combustion cycle. The injection quantity setting unit 73 sets each injection quantity according to the determination result of the determination unit 71, the engine speed, and the engine load. The injection quantity setting unit 73 sets each injection quantity so that the total of the set injection quantities, which is the sum of the injection quantities for each injection performed during one combustion cycle, matches the injection quantity required to achieve the target engine torque.

The injection timing setting unit 74 sets the injection timing for each injection (the timing at which fuel injection from the injector 15 begins). The injection timing setting unit 74 sets the injection timing for each injection based on the determination result of the determination unit 71, the engine speed, and the engine load.

The fuel injection control unit 72 controls the injector 15 so that the amount of fuel set by the injection amount setting unit 73 is injected at the injection timing set by the injection timing setting unit 74.

[Overall control flow of the injector 15]
FIG. 5 is a flowchart showing the overall flow of control of the injector 15 performed by the fuel injection control unit 72 (processor 70). The processor 70 first reads information detected by the sensors SN1 to SN13 (step S1). Next, the processor 70 (determination unit 71) determines whether the engine is operating in the medium load range A2 (step S2). If the determination is YES and the engine is operating in the medium load range A2, the processor 70 controls the injector 15 to achieve rapid multi-stage combustion (step S3). Rapid multi-stage combustion will be described later. If the determination in step S2 is NO and the engine is not operating in the medium load range A2, the processor 70 determines whether the engine is operating in the low load range A1 (step S4). If the determination is YES and the engine is operating in the low load range A1, the processor 70 controls the injector 15 to achieve premixed combustion of the air-fuel mixture (step S5). For example, the processor 70 causes the injector 15 to perform four-stage injection. On the other hand, if the determination in step S4 is NO, that is, if the engine is operating in the high-speed, high-load range A3, the processor 70 controls the injector 15 to perform diffusion combustion of the air-fuel mixture (step S6). For example, the processor 70 causes the injector 15 to perform two to six-stage injection.

[Injector control in the medium load range]
The control of the injector 15 when the engine is operating in the medium load range A2 will be described.

Figure 6 is a graph showing an example of the injection rate (amount of fuel injected from the injector 15 per unit time) and heat release rate when the engine is operating in the medium load range A2 (when the determination unit 71 determines that the engine is operating in the medium load range A2).

As shown in FIG. 6, in the medium load range A2, the fuel injection control unit 72 performs five-stage injection. Specifically, the fuel injection control unit 72 causes the injector 15 to perform the following injections: a pilot injection P1 that injects fuel during the compression stroke; a pre-injection P2 that injects fuel during the compression stroke and later than the pilot injection P1; a main injection P3 that injects fuel near the compression top dead center (TDC) and later than the pre-injection P2; a first after-injection P4 that injects fuel during the expansion stroke and later than the main injection P3; and a second after-injection P5 that injects fuel during the expansion stroke and later than the first after-injection P4. Note that the main injection P3 is the main injection for obtaining engine torque, and its injection amount is greater than the other four injection amounts.

Figure 7 is a flowchart showing an overview of the control of the injector 15 performed by the fuel injection control unit 72 when the engine is operating in the medium load range A2. As shown in Figure 7, the fuel injection control unit 72 sets the pilot injection amount and pilot injection timing (step S12), and also sets the main injection amount and main injection timing (step S13). The fuel injection control unit 72 also sets the pre-injection amount and, as described below, sets the pre-injection timing based on the main injection timing (step S14). The fuel injection control unit 72 also sets the first after-injection amount and second after-injection amount, and, as described below, sets the first after-injection timing and second after-injection timing based on the main injection timing (steps S15 and S16).

[Control configuration when high-gear multi-stage combustion conditions are met]
Next, a description will be given of the control of the injector 15 when the conditions that the engine is operating in the medium load range A2 and the gear position is a high gear position (when the determination unit 71 determines that the gear position is a high gear position), which is a characteristic configuration of the present invention. Hereinafter, the above condition will be referred to as a high gear multi-stage combustion condition.

(Pilot injection and main injection)
The injection timings (injection start timings) of the pilot injection P1 and main injection P3 when the high gear multi-stage combustion condition is met will be described below. Hereinafter, the injection timing of the pilot injection P1 will be referred to as the pilot injection timing, and the injection timing of the main injection will be referred to as the main injection timing, as appropriate.

When high-gear multi-stage combustion conditions are met, the fuel injection control unit 72 (injection timing setting unit 74) performs distributed injection control by setting the pilot injection timing and main injection timing so that the injection axis AX of the injector 15 points toward the lip portion 53 for at least part of the injection period (the period from the injection timing to the end of injection). The timing at which the injection axis AX of the injector 15 points toward the lip portion 53 varies depending on the specifications of the injector 15, but is, for example, within a range from approximately 20° CA (CA: crank angle) before BTDC (top dead center) to approximately 20° CA after ATDC (top dead center).

This injection control allows a portion of the fuel from the pilot injection P1 and a portion of the fuel from the main injection P3 to collide with the lip portion 53 when high-gear multi-stage combustion conditions are met. As shown in Figure 6, this makes it possible to achieve rapid multi-stage combustion in which the heat generation caused by the pilot injection P1 and the heat generation caused by the main injection P3 occur in gradual succession, with the heat generation rate peaking near top dead center (TDC) of compression.

This will be explained in more detail using Figure 8. Figure 8 is a simplified partial cross-sectional view of the combustion chamber 6. Figure 8 shows the piston 5 positioned so that the injection axis AX of the injector 15 points toward the lip portion 53, and a portion of the fuel (fuel spray Df) injected from the injector 15 collides with the lip portion 53.

Fuel is injected from the injection hole 152 of the injector 15 along the injection axis AX. The injection axis AX roughly coincides with the axis of the injection hole 152. The injected fuel (fuel spray) Df diffuses at a spray angle θ. Figure 8 shows an upper diffusion axis AX1, which indicates upward diffusion relative to the injection axis AX, and a lower diffusion axis AX2, which indicates downward diffusion. The spray angle θ is the angle between the upper diffusion axis AX1 and the lower diffusion axis AX2.

When the fuel spray Df injected from the injector 15 hits the lip portion 53, the fuel spray Df separates into a portion (arrow F11) heading toward the lower cavity 51 (downward) and a portion (arrow F12) heading toward the upper cavity 52 (upward).

Fuel heading toward the lower cavity 51 flows along the wall surface of the lower cavity 51, mixing with the air in the radial center of the lower cavity 51 and the combustion chamber 6. Specifically, fuel heading in the direction of arrow F11 (downward) enters the radial recess 514 of the lower cavity 51 while mixing with the air, then changes its flow direction from downward to inward in the radial direction B, and flows along the first bottom 512 as shown by arrow F12. Furthermore, fuel flowing in the direction of arrow F12 is lifted upward and flows radially outward from the combustion chamber ceiling surface 6U as shown by arrow F13, mixing with the air in the radial center of the combustion chamber 6.

On the other hand, fuel heading toward the upper cavity 52 flows along the wall surface of the upper cavity 52, mixing with the air in the upper cavity 52. Specifically, fuel heading in the direction of arrow F21 (upward) enters the tapered region 524 of the upper cavity 52 while mixing with the air, and flows diagonally downward along the tapered region 524 as shown by arrow F22, reaching the second bottom 522. After that, it is lifted upward by the rising curved surface between the second bottom 522 and the standing wall region 525, and flows radially inward from the combustion chamber ceiling surface 6U. Furthermore, as shown by arrow F23, some of the fuel lifted upward also flows into the space radially outward of the standing wall region 252 (the squish space above the peripheral flat surface 55), where it mixes with the air in that space.

By colliding the fuel spray Df with the lip 53 and distributing it between the lower cavity 51 and the upper cavity 52 in this manner, the fuel can be dispersed throughout the combustion chamber 6. Therefore, by colliding the fuel from the pilot injection P1 (fuel injected from the injector 15 upon execution of the pilot injection P1) with the lip 53, the fuel from the pilot injection P1 is dispersed throughout the combustion chamber 6, forming a homogeneous mixture with a low fuel concentration throughout the combustion chamber 6. This allows the mixture to burn slowly, increasing the temperature throughout the combustion chamber 6. Furthermore, by colliding the fuel from the main injection P3 (fuel injected from the injector 15 upon execution of the main injection P3) with the lip 53, the fuel from the main injection P3 is dispersed throughout the combustion chamber 6, forming a homogeneous mixture with a high fuel concentration throughout the combustion chamber 6. This allows the mixture to combust rapidly following combustion associated with the pilot injection P1. Therefore, the heat release rate shown in Figure 6 is achieved. This combustion mode improves fuel economy by shortening the combustion period, and also improves exhaust performance by creating a homogeneous air-fuel mixture, which promotes mixing of air and fuel and reduces soot generation.

However, to fully achieve the above effect, the fuel must be properly distributed between the lower cavity 51 and the upper cavity 52.

Specifically, if the main injection timing is set to a predetermined timing that allows appropriate distribution of fuel between the lower cavity 51 and the upper cavity 52 at the lowest engine speed in the medium load range A2, regardless of engine speed, the higher the engine speed, the more the fuel from the main injection will be biased toward the upper cavity 52.

This is because, while the time it takes for the fuel spray to travel from the nozzle 151 to the lip 53 is approximately constant regardless of engine speed, the higher the engine speed, the faster the piston 5 moves; and, with main injection P3, which occurs near top dead center of compression, the piston 5 is descending at the time the main fuel spray reaches the lip 53. In other words, if the main injection timing is set to the above-mentioned predetermined time, as shown in Figure 9, the position of the piston 5 when the fuel spray Df from main injection P3 reaches the lip 53 will be the dashed line position when the engine speed is low, but will be the solid line position, below the dashed line position, when the engine speed is high. Therefore, when the engine speed is high, the fuel spray Df collides with a higher part of the lip 53, and more fuel is distributed to the upper cavity 52.

In contrast, if the main injection timing is set to be more advanced (earlier in crank angle) when the engine speed is high than when it is low, the position of the piston 5 can be raised higher when the fuel from the main injection P3 reaches the lip portion 53, thereby preventing the fuel from being concentrated in the upper cavity 52.

Furthermore, pilot injection P1 is performed during the compression stroke, before main injection P3, when the piston 5 is rising. As a result, if the pilot injection timing is maintained at a constant time regardless of engine speed, the position of the piston 5 when the fuel spray Df from pilot injection P1 reaches the lip portion 53 will be the position shown by the dashed line in Figure 10 when the engine speed is low, but will be the position shown by the solid line in Figure 10, which is above the dashed line, when the engine speed is high. Therefore, if the pilot injection timing is set to a constant time regardless of engine speed, when the engine speed is high, the fuel spray Df will impinge on a lower part of the lip portion 53, and more fuel will be distributed to the lower cavity 51.

In contrast, if the pilot injection timing is set to be more advanced when the engine speed is high than when it is low, the piston 5 can be positioned lower when the fuel from the pilot injection P1 reaches the lip portion 53, thereby preventing the fuel from being concentrated in the lower cavity 51.

From the above, it is considered preferable to set the main injection timing and pilot injection timing so that the injection axis AX of the injector 15 points toward the lip portion 53 for at least part of each injection period, and so that the timing becomes more advanced as the engine speed increases.

However, the inventors of the present application have discovered that advancing the main injection timing as engine speed increases results in an increase in the amount of NOx produced. Specifically, as engine speed increases, the wall temperature of the combustion chamber 6 rises, resulting in an increase in the amount of NOx produced. However, if the main injection timing is advanced as engine speed increases, the amount of NOx produced increases even more than the above increase. This is thought to be because advancing the main injection timing increases the amount of heat generated near the compression top dead center, causing the temperature inside the combustion chamber 6 to rise.

After extensive research into the above problem, the inventors discovered that when engine speeds are high, retarding the pilot injection timing rather than advancing the main injection timing can improve exhaust performance, although fuel economy performance may be slightly reduced.

Specifically, if the pilot injection timing is retarded, the fuel from pilot injection P1 collides with the lip 53 when the piston 5 is in a higher position. As a result, the fuel from pilot injection P1 is biased toward the lower cavity 51. When the fuel from pilot injection P1 is biased toward the lower cavity 51, the degree of homogeneity of that fuel throughout the combustion chamber 6 decreases. Therefore, the temperature rise throughout the combustion chamber 6 before the start of main injection P3 is suppressed, lengthening the combustion period of the mixture after the start of main injection P3 and slightly reducing fuel economy. However, if the main injection timing is retarded in this case and the fuel from main injection P3 is biased toward the upper cavity 52, the total fuel from pilot injection P1 and main injection P3 is distributed throughout the combustion chamber 6, suppressing soot generation, and retarding the main injection timing suppresses NOx generation.

Based on the above findings, in this embodiment, when the high gear multi-stage combustion condition is met and the engine is operating in a range where the engine speed is equal to or lower than the predetermined switching speed N10, the fuel injection control unit 72 (injection timing setting unit 74) sets the main injection timing to a time when the injection axis AX of the injector 15 points toward the lip portion 53 for at least a portion of the injection period of the main injection P3, and sets the main injection timing to be more advanced as the engine speed increases. Furthermore, when the high gear multi-stage combustion condition is met and the engine is operating in a range where the engine speed is higher than the switching speed N10, the main injection timing is set to a time when the injection axis AX of the injector 15 points toward the lip portion 53 for at least a portion of the injection period of the main injection P3, and sets the main injection timing to be more retarded than the injection timing at the switching speed N10.

Furthermore, when the high gear multi-stage combustion condition is met and the engine is operating in a range where the engine speed is below the predetermined switching speed N10, the fuel injection control unit 72 (injection timing setting unit 74) sets the pilot injection timing to a time when the injection axis AX of the injector 15 is directed toward the lip portion 53 for at least a portion of the injection period of the pilot injection P1, and sets the timing to be more advanced as the engine speed increases. Furthermore, when the high gear multi-stage combustion condition is met and the engine is operating in a range where the engine speed is higher than the switching speed N10, the fuel injection control unit 72 (injection timing setting unit 74) sets the pilot injection timing to a time that is more retarded than when the engine speed is at the switching speed N10.

Specifically, when the high-gear multi-stage combustion condition is met, the fuel injection control unit 72 (injection timing setting unit 74) first sets the main injection timing (hereinafter referred to as the reference main injection timing) at the lowest engine speed N0 (hereinafter referred to as the reference speed) in the medium load range A2 based on the engine load, and the pilot injection timing (hereinafter referred to as the reference pilot injection timing) at the reference speed. The reference main injection timing and the reference pilot injection timing are each set in advance for each engine load and stored in the memory unit 78. The reference main injection timing and the reference pilot injection timing are each set to a timing at which the injection axis AX of the injector 15 points toward the lip portion 53 for at least a portion of the injection period.

Next, the fuel injection control unit 72 (injection timing setting unit 74) sets a main injection timing correction amount, which is the amount of correction for the main injection timing, and a pilot injection timing correction amount, which is the amount of correction for the pilot injection timing, based on the engine load and engine speed. The fuel injection control unit 72 (injection timing setting unit 74) then corrects the reference main injection timing with the main injection timing correction amount and sets the corrected value as the main injection timing, and corrects the reference pilot injection timing with the pilot injection timing correction amount and sets the corrected value as the pilot injection timing. The main injection timing correction amount and pilot injection timing correction amount are each set in advance for the engine load and engine speed and stored in the memory unit 78.

Figure 11 is a graph showing the relationship between the main injection timing correction amount and engine speed. Figure 12 is a graph showing the relationship between the pilot injection timing correction amount and engine speed. In Figures 11 and 12, the vertical axis represents the injection timing advance amount (advance side correction amount). Note that Figures 11 and 12 show the above relationship at a specified engine load, but the relationship between each correction amount and engine speed when high gear multi-stage combustion conditions are met has a similar tendency at other engine loads.

As shown in Figure 11, in the region where the engine speed is below the switching speed N10, the main injection timing correction amount (advance amount) increases as the engine speed increases, and is set to the maximum amount Tm_10 (hereinafter referred to as the maximum advance correction amount Tm_10) when the engine speed is at the switching speed N10. On the other hand, in the region where the engine speed is higher than the switching speed N10, the main injection timing correction amount (advance amount) is gradually reduced from the maximum advance correction amount Tm_10 as the engine speed increases.

In the example of Figure 11, when the engine speed is below the switching speed N10, the main injection timing correction amount (advance amount) increases approximately proportionally with increasing engine speed. When the engine speed is higher than the switching speed N10 and below a predetermined speed N12, the main injection timing correction amount (advance amount) decreases approximately proportionally from the maximum advance correction amount Tm_10 as the engine speed increases. Furthermore, when the engine speed is higher than the predetermined speed N12, the main injection timing correction amount (advance amount) decreases approximately proportionally with increasing engine speed (although at a slower rate than in the range from the switching speed N10 to the predetermined speed N12), and the main injection timing correction amount (advance amount) becomes 0 at the first speed N1 (the maximum speed in the medium load range A2). The maximum advance correction amount Tm_10 is, for example, a value of 5° CA or less.

By setting the main injection timing correction amount (advance amount) as described above, in the range where the engine speed is equal to or less than the switching speed N10, the main injection timing is set to a more advanced timing as the engine speed increases, and in the range where the engine speed is higher than the switching speed N10, the main injection timing is set to a more retarded timing than when the engine speed is at the switching speed N10. Also, in the range where the engine speed is higher than the switching speed N10, the main injection timing is set to a more advanced timing when the engine speed is high than when it is low.

As shown in Figure 12, in the region where the engine speed is below the switching speed N10, the pilot injection timing correction amount (advance amount) increases as the engine speed increases, and is set to the maximum amount Tpl_10 (hereinafter referred to as the maximum advance correction amount Tpl_10) when the engine speed is at the switching speed N10. On the other hand, in the region where the engine speed is higher than the switching speed N10, the pilot injection timing correction amount (advance amount) is gradually reduced from the maximum advance correction amount Tpl_10 to a smaller advance amount Tpl_12 as the engine speed increases, and when the engine speed exceeds a predetermined speed N12, it is maintained at the timing Tpl_12 for that speed N12.

In the example of Figure 12, when the engine speed is below the switching speed N10, the pilot injection timing correction amount (advance amount) increases approximately in proportion to the increase in engine speed. Furthermore, when the engine speed is higher than the switching speed N10 and below a predetermined speed N12, the pilot injection timing correction amount (advance amount) decreases approximately in proportion to the increase in engine speed. Above the speed N12, the pilot injection timing correction amount is maintained at the predetermined speed N12 regardless of the engine speed. For example, the maximum advance correction amount tpl_10 for the pilot injection P1 is set to a value similar to the maximum advance correction amount Tm_10 for the main injection P1 (a value of 5° CA or less).

By setting the pilot injection timing correction amount (advance amount) as described above, in the range where the engine speed is below the switching speed N10, the pilot injection timing is set to a more advanced timing as the engine speed increases, and in the range where the engine speed is higher than the switching speed N10, the pilot injection timing is set to a more retarded timing than when the engine speed is at the switching speed N10. Furthermore, in the range where the engine speed is higher than the switching speed N10, the pilot injection timing is set to a more advanced timing when the engine speed is high than when it is low.

Here, as mentioned above, implementation of the above-mentioned injection control results in slightly lower fuel economy in regions higher than the switching speed N10. For this reason, in this embodiment, the switching speed N10 is set to the maximum speed in the normal rotation range so that high fuel economy can be achieved during normal operation (frequent operation). For example, the switching speed N10 is set to 1800 rpm when the first speed N1, which is the maximum speed in the medium load range A2, is 2200 rpm. The region of the medium load range A2 where the engine speed is equal to or lower than the switching speed N10 corresponds to the "first operating region" in the claims, and the region of the medium load range A2 where the engine speed is higher than the switching speed N10 corresponds to the "second operating region" in the claims.

Furthermore, among the above injection controls, the control in which the main injection timing is set to a more advanced timing when the engine speed is high than when it is low in the range where the engine speed is below the switching speed N10 corresponds to the "first control" in the claims. Furthermore, the control in which the pilot injection timing and main injection timing are set to a more retarded timing than the respective injection timings at the switching speed N10 in the range where the engine speed is higher than the switching speed N10 corresponds to the "second control" in the claims.

(Pre-injection, first after-injection, and second after-injection)
The pre-injection P2, the first after-injection P4, and the second after-injection P5 when the high gear multi-stage combustion condition is met will be described.

Pre-injection P2 is an injection designed to prevent excessive combustion noise. In other words, if a large amount of fuel is supplied to the combustion chamber 6 at once by the main injection P3, the fuel-air mixture will combust too quickly, which can easily result in excessive combustion noise. Therefore, a portion of the fuel to be burned near the top dead center of the compression stroke is supplied to the combustion chamber 6 by pre-injection P2 prior to the main injection P3.

When the high-gear multi-stage combustion condition is met, the fuel injection control unit 72 (injection timing setting unit 74) sets the injection timing of the pre-injection so that the time interval between the injection timing of the pre-injection and the injection timing of the main injection is constant regardless of the engine speed.

The first after-injection P4 and the second after-injection P5 are both injections for burning soot. That is, after the fuel from the main injection P3 is burned, fuel is supplied to the combustion chamber 6 and burned, thereby burning the soot generated by the combustion of the fuel from the main injection P3. When the high gear multi-stage combustion condition is met, the fuel injection control unit 72 (injection timing setting unit 74) sets the interval between the main injection timing and the injection timing of the first after-injection and the time between the main injection timing and the injection timing of the second after-injection based on the engine speed and engine load, and sets the injection timing of the first after-injection and the injection timing of the second after-injection to be respectively delayed by the respective intervals from the previously set main injection timing. For example, when the high gear multi-stage combustion condition is met, the fuel injection control unit 72 (injection timing setting unit 74) sets the injection timing of the first after-injection P4 so that the time interval between the main injection timing and the injection timing of the first after-injection P4 is constant regardless of the engine speed. On the other hand, the injection timing of the second after-injection P5 is set so that the time interval between the injection timing of the first after-injection P4 and the injection timing of the second after-injection P5 is shorter when the engine speed is high than when it is low.

[Control configuration under low gear multi-stage combustion conditions]
A brief description will be given of the control of the injector 15 when the condition that the engine is operating in the medium load range A2 and the gear position is a low gear position is satisfied (when the determination unit 71 determines that the gear position is a low gear position). Hereinafter, the above condition will be referred to as a low gear multi-stage combustion condition.

When the low-gear multi-stage combustion condition is met, as when the high-gear multi-stage combustion condition is met, the fuel injection control unit 72 performs the above-described distributed injection control, setting the pilot injection timing and main injection timing so that the injection axis AX of the injector 15 points toward the lip portion 53 for at least a portion of the injection period. This allows rapid multi-stage combustion to be achieved even when the low-gear multi-stage combustion condition is met. However, when the low-gear multi-stage combustion condition is met, the fuel injection control unit 72 does not perform the following control: when the engine speed is equal to or less than the switching speed N10, the fuel injection control unit 72 sets the pilot injection timing and main injection timing to more advanced timing when the engine speed is high than when it is low; and when the engine speed is higher than the switching speed N10, the fuel injection control unit 72 does not set the pilot injection timing and main injection timing to more retarded timing than the corresponding injection timing at the switching speed. For example, when the low-gear multi-stage combustion condition is met, the fuel injection control unit 72 sets the pilot injection timing and main injection timing to approximately constant timing regardless of the engine speed.

[Effect, etc.]
As described above, according to the control device for a compression ignition engine according to the above embodiment, when the high gear multi-stage combustion condition is established, both the pilot injection timing and the main injection timing are set to timings in which the injection axis AX is directed toward the lip portion 53 during at least a part of each injection period. Therefore, the fuel related to these injections can be made to collide with the lip portion 53 and distributed to both the upper cavity 52 and the lower cavity 51.

Furthermore, when the high-gear multi-stage combustion conditions are met and the engine is operating at an engine speed below switching speed N10, the pilot injection timing and main injection timing are set to be more advanced when the engine speed is high than when it is low. This allows the fuel from both pilot injection P1 and main injection P3 to be distributed to the upper and lower cavities 51, 52 at appropriate ratios. This therefore improves both fuel economy and exhaust performance.

Furthermore, when the high-gear multi-stage combustion condition is met and the engine is operating in a range where the engine speed is higher than the switching speed, the pilot injection timing is set to a time that is more retarded than the time at the switching speed, and the main injection timing is set to a time that is more retarded than the time at the switching speed. Therefore, when the engine speed is higher than the switching speed, the generation of soot and NOx can be reliably suppressed, and exhaust performance can be reliably improved.

In addition, in the above embodiment, pre-injection P2 is performed between pilot injection P1 and main injection P3. This makes it possible to achieve the above-mentioned effects while suppressing an increase in combustion noise. Furthermore, when high-gear multi-stage combustion conditions are met, the time interval between the injection timing of pre-injection P2 and the injection timing of main injection is constant regardless of engine speed. This prevents interference between the fuel spray from pre-injection P2 and the fuel spray from main injection P3, ensuring that the fuel from main injection P3 is properly distributed and separated into each cavity 51, 52.

Here, when the high gear multi-stage combustion condition is met and the engine speed is equal to or less than or close to the switching speed N10, the main injection timing is set to the advanced side, which tends to increase the combustion noise. In contrast, in the above embodiment, the above injection control is performed when the gear is in the high gear, that is, when the vehicle 100 is traveling at high speed. This allows the combustion noise to blend in with the driving noise, preventing occupants from detecting the combustion noise and feeling uncomfortable.

[Modification]
In the above embodiment, when the high gear multi-stage combustion condition is met, the pilot injection timing as well as the main injection timing are controlled so that, when the engine speed is equal to or lower than the switching speed N10, the timing is advanced as the engine speed increases, and when the engine speed is higher than the switching speed N10, the timing is retarded relative to the timing at which the engine speed reaches the switching speed N10. Here, since the injection amount of the main injection P3 is greater than the injection amount of the pilot injection P1, an inappropriate distribution of fuel from the main injection P3 to the upper and lower cavities 51, 52 has a greater impact on fuel economy and exhaust performance. For this reason, the above control may be limited to the main injection P3, and the pilot injection timing may be maintained at a constant timing regardless of the engine speed.

In the above embodiment, when the transmission is in a high-speed gear, control is performed to switch the pilot injection timing and main injection timing between when the engine speed is below the switching speed N10 and when it is higher than N10 (control in which, when the engine speed is below N10, the pilot injection timing and main injection timing are set to be more advanced when the engine speed is high than when it is low, and when the engine speed is higher than N10, the pilot injection timing and main injection timing are set to be more retarded than the injection timing at the switching speed). However, regardless of the transmission gear, the above control may also be performed when the engine is operating in the medium load range A2. However, as mentioned above, if control is performed to advance the main injection timing as the engine speed increases, combustion noise tends to increase when the engine speed is close to or below N10. Therefore, the above control, which includes advancing the main injection timing as the engine speed increases, is preferably performed while the engine is operating in a high-speed gear.

Furthermore, the control when the engine is operating in the medium load range A2 and the transmission is in a low gear, the control when the engine is operating in the low load range A1, and the control when the engine is operating in the high-speed, high-load range A3 are not limited to the above.

In addition, in the above embodiment, the current stage number (current gear stage) of the transmission 110 is detected using the gear stage sensor SN13, but the stage number (gear stage) may also be calculated from the vehicle speed, engine speed, etc.

Furthermore, the above-mentioned pre-injection P2, first after-injection P4, and second after-injection P5 may be omitted.

1 Engine body 2 Cylinder 5 Piston 6 Combustion chamber 6U Combustion chamber ceiling surface (ceiling surface)
15 Injector (fuel injection valve)
5C Cavity 50 Crown surface 51 Lower cavity 52 Upper cavity 53 Lip portion 70 Processor (control device)
72 Fuel injection control unit P1 Pilot injection P2 Pre-injection P3 Main injection

Claims (4)

A control device for a compression ignition engine, which includes an engine body having a cylinder, a piston that reciprocates within the cylinder, a combustion chamber formed by the cylinder and a crown surface of the piston, a fuel injection valve disposed on a ceiling surface of the combustion chamber and that injects fuel along an injection axis, and a fuel injection control unit that controls the fuel injection valve, and in which compression ignition combustion of an air-fuel mixture is performed within the combustion chamber,
The piston has a lower cavity provided at the radial center of a crown surface of the piston, an upper cavity provided around the lower cavity and shallower than the lower cavity, and a lip portion connecting the lower cavity and the upper cavity,
The fuel injection control unit
When the engine is operated in a first operating range where the engine speed is equal to or lower than a predetermined switching speed and in a second operating range where the engine speed is higher than the switching speed, a distributed injection control is performed in which the fuel injection valve performs a pilot injection in which fuel is injected during a compression stroke and at a timing where the injection shaft is directed toward the lip portion for at least a part of an injection period, and a main injection in which fuel is injected after the pilot injection and at a timing where the injection shaft is directed toward the lip portion for at least a part of an injection period,
When the distribution injection control is performed,
When the engine is operated in the first operating range, a first control is performed to control the fuel injection valve so that the injection timing of the main injection is more advanced when the engine speed is high than when the engine speed is low,
a control device for a compression ignition engine, wherein, when the engine is operated in the second operating range, a second control is performed to control the fuel injection valve so that the injection timings of the pilot injection and the main injection are each retarded relative to the injection timings at the switchover rotation speed. 2. The control device for a compression ignition engine according to claim 1,
a fuel injection control unit that controls the fuel injection valve so that the injection timing of the pilot injection is more advanced when the engine speed is high than when the engine speed is low, when the first control is performed. 3. The control device for a compression ignition engine according to claim 1,
the fuel injection control unit, when performing the distributed injection control, causes the fuel injection valve to perform a pre-injection in which fuel is injected between the pilot injection and the main injection, and controls the fuel injection valve so that the time interval between the injection timing of the pre-injection and the injection timing of the main injection is constant. The control device for a compression ignition engine according to any one of claims 1 to 3,
a gear position detection unit capable of detecting a gear position of a transmission provided in a vehicle equipped with the engine,
a fuel injection control unit that performs the first control and the second control when the gear position detected by the gear position detection unit is equal to or greater than a predetermined number of gear positions set to two or more.