Controller and control method for internal combustion engine

A controller includes a valve control unit and a target calculation unit. The valve control unit is configured to control a fuel injection valve such that divergence decreases between an ignition delay of fuel injected into a cylinder through main injection and an ignition delay target value. The target calculation unit is configured to calculate the ignition delay target value such that the ignition delay target value decreases as estimated ignitability of the fuel in the cylinder increases during an engine operation in a region where diffusion combustion and premix combustion are both performed, the ignitability of the fuel in the cylinder being estimated based on a parameter that varies the ignitability.

RELATED APPLICATIONS

The present application claims priority of Japanese Application Number 2018-160277, filed on Aug. 29, 2018, the disclosure of which is hereby incorporated by reference herein in its entirety.

The present disclosure relates to a controller and a control method for an internal combustion engine configured to control a compression ignition internal combustion engine.

2. DESCRIPTION OF RELATED ART

International Publication No. 2013/051109 discloses an example of a combustion engine for an internal combustion engine. In the internal combustion engine, the controller causes a fuel injection valve to perform pilot injection before the piston reaches the compression top dead center and then causes the fuel injection valve to perform main injection when the piston reaches the vicinity of the compression top dead center. When fuel is injected into the cylinder through the pilot injection, premixed combustion is performed in the cylinder, thereby increasing the temperature in the cylinder. When the main injection is performed in a state in which the temperature in the cylinder is sufficiently high, diffusion combustion is performed in the cylinder.

The above-described controller estimates an ignition delay, which is the length of a period from a point in time at which the fuel injection valve starts injecting fuel to a point in time at which the combustion of the fuel is started. Further, a predetermined calculation equation including engine rotation speed and engine load ratio as variables is used to obtain an ignition delay target value, which is a target of the ignition delay. Then, the open degrees of nozzle vanes of a forced-induction device are adjusted such that the ignition delay becomes the ignition delay target value.

Increases in the open degrees of the nozzle vanes of the forced-induction device reduce the boost pressure of the forced-induction device. When the boost pressure is reduced, the ignition delay is lengthened.

Thus, the above-described controller increases the open degrees of the nozzle vanes when the ignition delay is shorter than the ignition delay target value. By contrast, the controller decreases the open degrees of the nozzle vanes when the ignition delay is longer than the ignition delay target value.

During the engine operation, combustion noise, which is the noise generated due to combustion in the cylinder, is produced. In some cases, during the engine operation in a region where the diffusion combustion and the premixed combustion are both performed, the level of combustion noise varies even if the boost pressure is adjusted to keep the ignition delay equal to the ignition delay target value.

In the region where the diffusion combustion and the premixed combustion are both performed, even if the premixed combustion is started prior to the diffusion combustion, the diffusion combustion is started while the premixed combustion is still being performed.

SUMMARY

A first aspect provides a controller for an internal combustion engine. The controller is configured to control a compression self-ignition internal combustion engine including a fuel injection valve that injects fuel into a cylinder and cause the fuel injection valve to perform main injection after causing the fuel injection valve to perform pilot injection. The controller includes a valve control unit and a target calculation unit. The valve control unit is configured to control the fuel injection valve such that divergence decreases between an ignition delay of the fuel injected into the cylinder through the main injection and an ignition delay target value, the ignition delay target value being a target of the ignition delay. The target calculation unit is configured to calculate the ignition delay target value such that the ignition delay target value decreases as ignitability of the fuel in the cylinder increases during an engine operation in a region where diffusion combustion and premix combustion are both performed, the ignitability of the fuel in the cylinder being estimated based on a parameter that varies the ignitability.

It is known that the combustion noise, which is the noise resulting from the combustion of fuel in the cylinder, increases as the premixed combustion speed increases.

By executing various experiments and simulations, the inventor has found that the premixed combustion speed decreases as the ignitability of fuel in the cylinder decreases.

According to conventional findings and the inventor's new findings, the premixed combustion speed decreases as the ignitability of fuel in the cylinder decreases, thereby increasing the combustion noise.

The inventor has also found that during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed, the proportion of the premixed combustion in the diffusion combustion and the premixed combustion increases as the ignition delay of fuel in the cylinder increases, thereby increasing the combustion noise.

Thus, in the above-described configuration, the ignition delay target value of fuel injected into the cylinder through the main injection is calculated based on the ignitability of fuel in the cylinder, which is estimated based on the parameters that vary the ignitability of fuel in the cylinder. That is, the ignition delay target value is calculated such that the ignition delay target value decreases as the estimated ignitability of fuel in the cylinder increases. The fuel injection valve is controlled such that divergence decreases between the ignition delay of the fuel injected into the cylinder through the main injection and the ignition delay target value.

As described above, as the ignitability of fuel increases, the premixed combustion speed increases and the combustion noise tends to increase. In the above-described configuration, the ignition delay target value decreases as the ignitability of fuel increases. Thus, even if the ignitability of fuel increases, an increase in the proportion of the premixed combustion in the diffusion combustion and the premixed combustion can be limited by reducing the ignition delay target value. That is, even if the ignitability of fuel increases, an increase in combustion noise can be limited. This limits a change in the level of combustion noise resulting from a change in the premixed combustion speed, that is, a change in the ignitability. Thus, when the engine operation state is kept at a certain state, a change in the level of combustion noise can be limited even if a change occurs in the parameters that vary the ignitability of fuel in the cylinder.

Thus, the above-described configuration limits the variation in the level of combustion noise during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed.

As the fuel partial pressure in the cylinder increases, the ignitability of fuel in the cylinder tends to increase. That is, the fuel partial pressure in the cylinder is one example of the above-described parameters. Thus, the target calculation unit may be configured to estimate the ignitability of the fuel in the cylinder such that the ignitability increases as the fuel partial pressure in the cylinder increases.

Also, as the oxygen partial pressure in the cylinder increases, the ignitability of fuel in the cylinder tends to increase. That is, the oxygen partial pressure in the cylinder is one example of the above-described parameters. Thus, the target calculation unit may be configured to estimate the ignitability of the fuel in the cylinder such that the ignitability increases as the oxygen partial pressure in the cylinder increases.

Also, as the temperature in the cylinder increases, the ignitability of fuel in the cylinder tends to increase. That is, the temperature in the cylinder is one example of the above-described parameters. Thus, the target calculation unit may be configured to estimate the ignitability of the fuel in the cylinder such that the ignitability increases as the temperature in the cylinder increases.

The controller for the internal combustion engine may include an index calculation unit that calculates an index of the ignitability of the fuel based on the fuel partial pressure in the cylinder, the oxygen partial pressure in the cylinder, and the temperature in the cylinder. In this case, it is preferred that the target calculation unit be configured to calculate the ignition delay target value based on the index calculated by the index calculation unit.

τ0is the index, Pfuel is the fuel partial pressure in the cylinder, O2 is the oxygen partial pressure in the cylinder, T is the temperature in the cylinder, M(T) is a function that includes the temperature in the cylinder as a variable, and A, B, and C are model constants. Using, for example, the following equation, the index calculation unit can calculate the above-described index on which the parameters are reflected. The index calculated in this manner is the length of the ignition delay of fuel when a single injection is performed. The index decreases as the ignitability of fuel increases. Calculating the ignition delay target value based on the index allows the ignition delay target value to decrease as the ignitability of fuel increases.

A decrease in the fuel injection amount of the pilot injection lengthens the ignition delay of fuel injected into the cylinder through the main injection. Thus, the valve control unit may be configured to approximate the ignition delay of fuel injected into the cylinder through the main injection to the ignition delay target value by adjusting the fuel injection amount of the pilot injection.

The ignition delay of fuel injected into the cylinder through the main injection can be lengthened by retarding the start point in time of the pilot injection, that is, by adjusting the start point in time of the pilot injection. Thus, the valve control unit may be configured to approximate the ignition delay of fuel injected into the cylinder through the main injection to the ignition delay target value by adjusting the start point in time of the pilot injection.

A second aspect provides a method for controlling an internal combustion engine. The controller controls a compression ignition internal combustion engine including a fuel injection valve that injects fuel into a cylinder. The method includes performing pilot injection with the fuel injection valve, performing main injection after the pilot injection, controlling the fuel injection valve such that divergence decreases between an ignition delay of the fuel injected into the cylinder through the main injection and an ignition delay target value, the ignition delay target value being a target of the ignition delay, and calculating the ignition delay target value such that the ignition delay target value decreases as ignitability of the fuel in the cylinder increases during an engine operation in a region where diffusion combustion and premix combustion are both performed, the ignitability of the fuel in the cylinder being estimated based on a parameter that varies the ignitability.

A third aspect provides a controller for an internal combustion engine. The controller is configured to control a compression ignition internal combustion engine including a fuel injection valve that injects fuel into a cylinder and configured to cause the fuel injection valve to perform pilot injection and then perform main injection. The controller includes processing circuitry configured to perform a process for controlling the fuel injection valve such that divergence decreases between an ignition delay of the fuel injected into the cylinder through the main injection and an ignition delay target value, the ignition delay target value being a target of the ignition delay and a process for calculating the ignition delay target value such that the ignition delay target value decreases as ignitability of the fuel in the cylinder increases during an engine operation in a region where diffusion combustion and premix combustion are both performed, the ignitability of the fuel in the cylinder being estimated based on a parameter that varies the ignitability.

DETAILED DESCRIPTION

A controller60for an internal combustion engine10according to an embodiment will now be described with reference toFIGS. 1 to 7.

FIG. 1shows the controller60of the present embodiment and the internal combustion engine10, which is controlled by the controller60. The controller60includes processing circuitry. The internal combustion engine10is a compression ignition internal combustion engine. The internal combustion engine10includes cylinders11and an exhaust-driven forced-induction device12. The internal combustion engine10includes an intake passage21. The intake passage21includes, in order from the upstream end in the flow direction of air, an air cleaner22, a compressor13for the forced-induction device12, an intercooler23, and a throttle valve24. In the intake passage21, air filtered by the air cleaner22is delivered in a state of being compressed by a compressor wheel13a, which is incorporated in the compressor13. The air compressed in this manner is cooled by the intercooler23. An intake air amount, which is the amount of air introduced into the cylinders11through the intake passage21, is adjusted by controlling the open degree of the throttle valve24.

The internal combustion engine10includes fuel injection valves26. The number of the fuel injection valves26is the same as that of the cylinders11. Each fuel injection valve26directly injects fuel into the corresponding cylinder11. Each fuel injection valve26is supplied with fuel from a fuel supply device27. The fuel supply device27includes a supply pump29and a common rail30. The supply pump29is used to pump fuel stored in the fuel tank through the supply passage28. The fuel pressurized by the supply pump29is temporarily stored in the common rail30. The fuel in the common rail30is supplied to each fuel injection valve26. When the fuel is injected from the fuel injection valves26into the cylinders11, the fuel is exposed to the compressed air and burns.

The exhaust gas generated through the combustion of the fuel in the cylinders11are discharged to the exhaust passage36. The exhaust passage36includes, in order from the upstream end in the flow direction of exhaust gas, a turbine14for the forced-induction device12and an exhaust removal device37. The exhaust removal device37captures particulate matter in exhaust gas to remove the exhaust gas.

The turbine14is equipped with a turbine wheel14a. The turbine wheel14ais coupled to the compressor wheel13aby a coupling shaft15. Thus, when the turbine wheel14ais rotated by the momentum of flow of exhaust gas, the compressor wheel13ais rotated in synchronization with the rotation of the turbine wheel14a. As a result, the compressor13pressurizes air. The turbine14includes an exhaust gas blowing port for the turbine wheel14a. This port is provided with a variable nozzle16, which varies the opening area of the exhaust gas blowing port depending on a change in the nozzle open degree. Adjusting the nozzle open degree of the variable nozzle16adjusts the flow rate of exhaust gas blown against the turbine wheel14a.

The internal combustion engine10includes an EGR device40, which causes some of exhaust gas flowing through the exhaust passage36to recirculate through the intake passage21as EGR gas. The EGR device40includes an EGR passage41, which causes exhaust gas to be taken out of a portion of the exhaust passage36located upstream of the turbine14, and an EGR flow rate adjustment device42, which adjusts the flow rate of EGR gas through the EGR passage41toward the intake passage21. The EGR passage41connects a portion of the intake passage21located downstream of the throttle valve24to a portion of the exhaust passage36located upstream of the turbine14. The EGR passage41is provided with an EGR cooler43, which cools EGR gas flowing through the EGR passage41. When the valve of the EGR flow rate adjustment device42is open, the EGR gas that has flowed from the exhaust passage36into the EGR passage41is cooled by the EGR cooler43and then introduced into the intake passage21through the EGR flow rate adjustment device42.

The controller60receives signals from various types of sensors such as an intake pressure sensor101, an intake temperature sensor102, an air flow meter103, a water temperature sensor104, a boost pressure sensor105, a crank angle sensor106, and a fuel pressure sensor107.

The intake pressure sensor101detects an intake pressure Pim, which is the pressure of air flowing at a section of the intake passage21located downstream of the throttle valve24, and outputs a signal corresponding to the detected intake pressure Pim. The intake temperature sensor102detects an intake temperature Thim, which is the temperature of air flowing at a section of the intake passage21located downstream of the intercooler23, and outputs a signal corresponding to the detected intake temperature Thim. The air flow meter103detects an intake air amount GA, which is the flow rate of air flowing at a section of the intake passage21located upstream of the compressor13, and outputs a signal corresponding to the detected intake air amount GA. The water temperature sensor104detects a water temperature Thw, which is the temperature of engine coolant flowing through the cylinder block of the internal combustion engine10, and outputs a signal corresponding to the detected water temperature Thw. The boost pressure sensor105detects a boost pressure BP of the forced-induction device12and outputs a signal corresponding to the detected boost pressure BP. The boost pressure sensor105detects a gauge pressure as the boost pressure BP. The reference of the gauge pressure is the atmospheric pressure. The crank angle sensor106detects an engine rotation speed NE, which is the rotation speed of an output shaft of the internal combustion engine10, and outputs a signal corresponding to the detected engine rotation speed NE. The fuel pressure sensor107detects a common rail pressure Pcr, which is the pressure of fuel in the common rail30, and outputs a signal corresponding to the detected common rail pressure Pcr.

The controller60controls the engine operation based on the output signals of the sensors101to107.

The controller60includes a valve control unit61, an index calculation unit62, and a target calculation unit63as functional units.

The valve control unit61controls the driving of the fuel injection valve26. More specifically, when causing fuel to burn in the cylinder11, the valve control unit61causes the fuel injection valve26to perform pilot injection and main injection. The pilot injection is fuel injection performed before the pistons moving back and forth in respective cylinders11reach the compression top dead center. The main injection is fuel injection performed after the pilot injection and performed before the pistons reach the vicinity of the compression top dead center. When fuel is injected into the cylinder11through the pilot injection, premixed combustion is performed in the cylinder11, thereby increasing the temperatures in the cylinder11. With the temperatures in the cylinders11increased in such a manner, the main injection is performed. As a result, diffusion combustion is performed in the cylinder11. In some cases, the diffusion combustion is started in a state in which the premixed combustion, which has been started earlier, is still being performed. The region where the diffusion combustion is started in a state in which the premixed combustion is still being performed is referred to as a region where the premixed combustion and the diffusion combustion are both performed.

When the engine operation is performed in the region where the premixed combustion and the diffusion combustion are both performed, the valve control unit61controls the fuel injection valve26to approximate an ignition delay τ of fuel injected into the cylinder11through the main injection to an ignition delay target value τtrg. The ignition delay τ is the length of a period from a point in time at which the fuel injection valve26starts injecting fuel to a point in time at which the combustion of the fuel is started. The ignition delay target value τtrg is a target of the ignition delay.

The index calculation unit62is configured to calculate an index τ0of the ignitability of fuel in the cylinder11based on parameters that vary the ignitability of the fuel in the cylinder11. The ignitability of fuel refers to how easily fuel is ignited. The index τ0calculated by the index calculation unit62is the length of time of the ignition delay of fuel when the index calculation unit62causes the fuel injection valve26to perform single injection. The index τ0decreases as the ignitability of fuel in the cylinder11increases.

The parameters that vary the ignitability of fuel in the cylinder11include, for example, the intake temperature Thim, the intake pressure Pim, the recirculation amount of EGR gas, the boost pressure BP, the water temperature Thw, which is the temperature of engine coolant, an outside air temperature, and an outside atmospheric pressure.

For example, the index calculation unit62calculates the index τ0using the following Arrhenius equation (1). In equation (1), Pfuel is a fuel partial pressure in the cylinder11at the point in time the main injection ends, O2 is an oxygen partial pressure in the cylinder11at the point in time the main injection ends, and T is the temperature in the cylinder11at the point in time the main injection starts. M(T) is a function that includes the temperature T in the cylinder11as a variable. That is, the function M(T) allows a larger value to be obtained as the temperature T in the cylinder11becomes higher. For example, the exponential function of the following equation (2) can be used for the function M(T). In this case, a model constant D is set such that the calculation result of equation (2) becomes larger as the temperature T in the cylinder11becomes higher. For example, the model constant D is set to a negative value. Further, A, B, and C in equation (1) are model constants that have been set in advance through experiments and simulations. More specifically, the model constant B is set such that the index τ0decreases as the fuel partial pressure Pfuel increases. The model constant C is set such that the index τ0decreases as the oxygen partial pressure O2 increases. For example, the model constants B and C are set to positive values. The model constant A is set such that the index τ0decreases as the product of the B-th power of the fuel partial pressure Pfuel, the C-th power of the oxygen partial pressure O2, and the M(T). For example, the model constant A is set to a positive value.

The fuel partial pressure Pfuel is calculated as the product of a fuel concentration Cfuel in the cylinder11and an in-cylinder pressure Pcy, which is the pressure in the cylinder11. The fuel concentration Cfuel is a value corresponding to a spray equivalence ratio Φ at a point in time at which the main injection ends. The spray equivalence ratio Φ at the point in time at which the main injection ends is calculated based on a command value of the injection amount when the fuel injection valve26is caused to perform the main injection.

The spray equivalence ratio Φ is the equivalence ratio in the spray of fuel injected from the fuel injection valve26into the cylinder11. For example, the spray equivalence ratio Φ can be obtained by dividing the stoichiometric air-fuel ratio by a spray air-fuel ratio. The spray air-fuel ratio is the air-fuel ratio in the spray of fuel injected from the fuel injection valve26into the cylinder11. The spray air-fuel ratio can be obtained by dividing the amount of air in the spray by the amount of fuel in the spray. The amount of air in the spray is calculated based on a volume V of spray at the point in time at which the main injection ends and an oxygen concentration Cox in the cylinder11.

The method for calculating the volume V of spray will now be described with reference toFIG. 2. As shown inFIG. 2, the spray of fuel injected from the fuel injection valve26into the cylinder11is hypothetically conical. In this case, the volume V of spray can be calculated using a known Hiroyasu's equation. The following relational equations (3) and (4) are used to calculate a spray penetration S. The relational equation (3) is used when a fuel injection time t is less than a split time tc. The relational equation (4) is used when the fuel injection time t is greater than or equal to the split time tc. The split time tc is the time for the fuel injected from the fuel injection valve26to transition from liquid to gas.

In the relational equations (3) and (4), ΔP is the difference between the common rail pressure Per and the in-cylinder pressure Pcy. The in-cylinder pressure Pcy can be estimated based on the amount of air filling the cylinder11and the position of the piston in the cylinder11. When the cylinder11is provided with a sensor that detects the pressure in the cylinder11, the detection value of the sensor may be employed as the in-cylinder pressure Pcy. Further, in the relational equations (3) and (4), ρf is a fuel density, ρa is an air density, and d0is the diameter of an injection hole of the fuel injection valve26.

The following relational equation (5) is used to calculate a spray angle θ. In the relational equation (5), μa is the viscosity coefficient of air that has been set in advance.

The following relational equation (6) is used to calculate the volume V of spray.
V=−⅓·π·tan θ2·s3(6)

The oxygen concentration Cox is calculated based on the amount of air introduced into the cylinder11and the amount of EGR gas introduced into the cylinder11. For example, the intake air amount GA, which is detected by the air flow meter103, can be used as the amount of air introduced into the cylinder11. The proportion of oxygen in the air is larger than the proportion of oxygen in EGR gas. Thus, the oxygen concentration Cox is calculated such that the oxygen concentration Cox decreases as the amount of EGR gas recirculating in the intake passage21through the EGR device40increases.

When the valve open degree of the EGR flow rate adjustment device42and the flow rate of exhaust gas in the exhaust passage36are kept to be fixed, the recirculation amount of EGR gas recirculating in the intake passage21through the EGR device40can be calculated based on the flow rate of exhaust gas in the exhaust passage36and the valve open degree of the EGR flow rate adjustment device42. The flow rate of exhaust gas is a value corresponding to the intake air amount GA and the engine rotation speed NE.

When at least one of the valve open degree of the EGR flow rate adjustment device42and the flow rate of exhaust gas changes, a response delay from the change occurs in a change in the recirculation amount of EGR gas. In the present embodiment, when at least one of the valve open degree and the flow rate of exhaust gas changes, a map is used to estimate to what degree the change in the recirculation amount is delayed. Thus, when at least one of the valve open degree and the flow rate of exhaust gas changes, the map is used to estimate the recirculation amount.

The oxygen partial pressure O2 in the cylinder11in the relational equation (1) is calculated as the product of the oxygen concentration Cox and the in-cylinder pressure Pcy in the cylinder11.

Further, the temperature T in the cylinder11when the main injection starts can be estimated based on the intake temperature Thim and the water temperature Thw. When the cylinder11is provided with a sensor that detects the temperature in the cylinder11, the detection value of the sensor may be employed as the temperature T in the cylinder11.

As described above, Pfuel, O2, and T in the relational equation (1) vary depending on the temperature of air flowing through the intake passage21, the pressure of air flowing through the intake passage21, the recirculation amount of EGR gas, and the water temperature Thw. Thus, the fuel partial pressure Pfuel, the oxygen partial pressure O2, and the temperature T in the cylinder11are also examples of parameters that vary the ignitability of fuel in the cylinder11. Additionally, the index τ0, which is calculated using the above-described equation (1), is a value based on the parameters that vary the ignitability of fuel in the cylinder11.

The target calculation unit63is configured to estimate the ignitability of fuel in the cylinder11based on the index τ0, which is calculated by the index calculation unit62. The target calculation unit63calculates the ignition delay target value τtrg such that the ignition delay target value τtrg decreases as the ignitability estimated based on the index τ0increases. In the present embodiment, the following relational equation (7) is used to calculate the ignition delay target value τtrg. Thus, the ignition delay target value τtrg can be increased monotonically relative to an increase in the index τ0. That is, the ignition delay target value τtrg can be decreased monotonically relative to an increase in the ignitability estimated based on the index τ0. F11 and F12 in the relational equation (7) are constants that have been set based on experiments and simulations. For example, the constant F11 is a positive value.
τtrg=F11·τ0+F12  (7)

The flow of processes of fuel injection during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed will now be described with reference toFIG. 3.

Prior to the flow of processes shown inFIG. 3, description will be made on the method for estimating whether the engine operation is performed in the region where the diffusion combustion and the premixed combustion are both performed. In the present embodiment, the estimation is performed based on the ignition delay of fuel injected into the cylinder11through the main injection. For example, as shown inFIG. 6, when the ignition delay τ, which is an estimated value of the ignition delay of fuel injected into the cylinder11through the main injection, is less than a given time τTh, it can be estimated that the engine operation is performed in the region where the diffusion combustion and the premixed combustion are both performed. By contrast, when the ignition delay τ is greater than or equal to the given time τTh, it can be estimated that the engine operation is performed in a region where only the premixed combustion is performed, not in the region where the diffusion combustion and the premixed combustion are both performed. Thus, since it can be estimated that the engine operation is performed in the region where the diffusion combustion and the premixed combustion are both performed when the ignition delay τ is less than the given time τTh, a series of processes shown inFIG. 3is executed.

The ignition delay τ can be calculated based on, for example, the boost pressure BP, the intake air amount GA, the water temperature Thw, the intake temperature Thim, the start point in time of the main injection, and the fuel injection amount of the main injection.

As shown inFIG. 3, various types of parameters for calculating the index τ0are obtained in step S11. Next, in step S12, the index calculation unit62calculates the index τ0using the above-described equation (1). Subsequently, in step S13, the target calculation unit63calculates the ignition delay target value τtrg using the above-described relational equation (7).

Then, in step S14, the valve control unit61controls the driving of the fuel injection valve26such that the ignition delay τ becomes the ignition delay target value τtrg. In the present embodiment, in step S14, the valve control unit61adjusts the fuel injection amount of the pilot injection executed prior to the main injection, that is, the time of energizing the fuel injection valve26during the pilot injection. For example, when the ignition delay τ is shorter than the ignition delay target value τtrg, the valve control unit61decreases the fuel injection amount of the pilot injection. By contrast, when the ignition delay τ is longer than the ignition delay target value τtrg, the valve control unit61increases the fuel injection amount of the pilot injection. The series of processes is then ended.

The operation and advantages of the present embodiment will now be described with reference toFIGS. 4 to 7.

FIG. 4shows the relationship between a premixed combustion speed and the level of combustion noise, which is the noise resulting from the combustion of fuel in the cylinder11. As shown inFIG. 4, the combustion noise increases as the premixed combustion speed increases. This is because flame spreads at once in the cylinder11as the premixed combustion speed increases. As the speed of flame spreading in the cylinder11increases, the combustion noise tends to increase.

FIG. 5shows the relationship between the ignitability of fuel injected into the cylinder11and the premixed combustion speed. The graph shown inFIG. 5is the result obtained through experiments and simulations. The graph ofFIG. 5reveals that the premixed combustion speed decreases as the ignitability of fuel in the cylinder11decreases. That is, as the index τ0increases, the premixed combustion speed decreases.

FIG. 6is a graph showing the relationship between the ignition delay τ and the level of combustion noise. Referring toFIG. 6, when the ignition delay τ is less than the given time τTh in the cylinder11, the diffusion combustion and the premixed combustion are both performed. By contrast, when the ignition delay τ is greater than or equal to the given time τTh, only the premixed combustion is performed in the cylinder11. The graph ofFIG. 6reveals that the combustion noise increases as the ignition delay τ increases during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed. This is because during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed, the proportion of the premixed combustion in the diffusion combustion and the premixed combustion increases as the ignition delay of fuel in the cylinder increases, thereby increasing the combustion noise. More specifically, as the ignitability of fuel in the cylinder11decreases, the ignition delay of fuel in the cylinder tends to increase. Further, as the ignitability of fuel in the cylinder11decreases, the premixed combustion speed tends to decrease. In addition, as the premixed combustion speed decreases, the proportion of the premixed combustion in the diffusion combustion and the premixed combustion tends to increase. Thus, since the premixed combustion speed decreases as the ignition delay increases, the proportion of the premixed combustion in the diffusion combustion and the premixed combustion increases. This increases the combustion noise.

The relationship between the combustion noise and the ignition delay τ during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed can be represented by the following approximate equation (8). In equation (8), P1, P2, and P3 are constants.
Combustion Noise∝P1·τP2+P3  (8)

As described above, the index τ0is inversely proportional to the premixed combustion speed. Thus, the relationship between the combustion noise and the ignition delay τ can be represented as shown by the above-described equation (8). Accordingly, the relationship between the combustion noise, the ignition delay τ, and the index τ0can be represented as shown by the following equation (9).
Combustion Noise∝(P1·τP2+P3)/τ0  (9)

When the combustion noise is a fixed value Const, equation (9) can be represented as equation (10). In a case in which the ignition delay τ when the combustion noise is the fixed value Const is the ignition delay target value τtrg, the ignition delay target value τtrg can be represented by the following equation (11).

As obvious from equation (11), the level of combustion noise can be kept at a fixed value by increasing the ignition delay target value τtrg as the index τ0increases. Further, the above-described relational equation (7) can be obtained by setting the constant P2 in equation (11) to one.

In the present embodiment, the relational equation (7) obtained in this manner is used to calculate the ignition delay target value τtrg. The solid line shown inFIG. 7represents the relationship between the index τ0and the ignition delay target value τtrg calculated using the relational equation (7). The broken line shown inFIG. 7represents the ignitability of fuel, that is, comparative example 1, in which the ignition delay target value τtrg is set regardless of the index τ0.

In comparative example 1, when the operating state of the internal combustion engine10is fixed, that is, when the engine rotation speed NE and an engine load ratio KL are fixed, the ignition delay target value τtrg remains unchanged even if the parameters varying the ignitability of fuel (i.e., index τ0of ignitability) change. As a result, when the parameters change, the level of combustion noise varies.

In the present embodiment, the ignitability of fuel is estimated based on the index τ0. Further, the ignition delay target value τtrg is calculated such that the ignition delay target value τtrg decreases as the ignitability increases. Then, the fuel injection valve26is controlled such that the ignition delay τ of fuel injected into the cylinder11through the main injection is approximated to the ignition delay target value τtrg. That is, the fuel injection valve26is controlled such that the deviation decreases between the ignition delay τ of fuel injected into the cylinder11through the main injection and the ignition delay target value τtrg. This reduces variation in the level of combustion noise that is caused when the above-described parameters vary.

Accordingly, in the present embodiment, the variation in the level of combustion noise during the engine operation in the region where the diffusion combustion and the premixed combustion are both performed is limited.

The ignition delay τ of fuel injected into the cylinder11through the main injection changes depending on the start point in time of the pilot injection. More specifically, the ignition delay of fuel injected into the cylinder11through the main injection can be lengthened by retarding the start point in time of the pilot injection and narrowing the interval between the point in time of the pilot injection and the point in time of the main injection. Thus, the point in time of the pilot injection may be retarded when the ignition delay τ is shorter than the ignition delay target value τtrg, and the point in time of the pilot injection may be advanced when the ignition delay τ is longer than the ignition delay target value τtrg.

When the ignition delay τ of fuel injected into the cylinder11through the main injection is different from the ignition delay target value τtrg, the fuel injection amount of the pilot injection and the start point in time of the pilot injection may be both adjusted.

The divergence between the ignition delay τ of fuel injected into the cylinder11through the main injection and the ignition delay target value τtrg may be decreased by changing the start point in time of the main injection. In this case, adjustment of the fuel injection amount of the pilot injection and adjustment of the start point in time of the pilot injection to decrease the divergence between the ignition delay τ and the ignition delay target value τtrg may be omitted.

In the above-described embodiment, the ignition delay target value τtrg is calculated using the above-described relational equation (7), which is a linear function. However, as long as the ignition delay target value τtrg can be decreased monotonically relative to a decrease in the index τ0, an equation that differs from the above-described relational equation (7) may be used to calculate the ignition delay target value τtrg. For example, a quadratic function such as the following relational equation (12) may be used to calculate the ignition delay target value τtrg. In the relational equation (12), F21, F22, and F23 are constants that have been set based on experiments and simulations. The relational equation (12) can be obtained by setting the constant P2 in the above-described equation (11) to 0.5.
τtrg=F21·τ02+F22·τ0+F23  (12)

When the ignition delay target value τtrg is calculated using the relational equation (9), the ignition delay target value τtrg changes relative to changes in the index τ0as shown inFIG. 8.

The function M(T) in the Arrhenius equation (1) may be different from the above-described equation (2) as long as the value of the calculation result can be increased as the temperature T in the cylinder11increases.

In the above-described embodiment, the Arrhenius equation (1) is used to calculate the index τ0. However, as long as the index τ0can be set to a value corresponding to the ignitability of fuel in the cylinder11, equation (1) does not have to be used to calculate the index τ0.

For example, as long as the index τ0can be decreased as the fuel partial pressure Pfuel in the cylinder11increases at the point in time at which the main injection ends, the index τ0may be calculated without using equation (1).

Further, as long as the index τ0can be decreased as the oxygen partial pressure O2 in the cylinder11increases at the point in time at which the main injection ends, the index τ0may be calculated without using equation (1).

In addition, as long as the index τ0can be decreased as the temperature T in the cylinder11increases at the point in time at which the main injection starts, the index τ0may be calculated without using equation (1).

Instead of estimating the ignitability of fuel based on the index τ0, the ignitability may be directly estimated from the parameters that vary the ignitability of fuel in the cylinder11. For example, the ignitability may be estimated based on the fuel partial pressure Pfuel in the cylinder11at the point in time at which the main injection ends. For example, the ignitability may be estimated based on the oxygen partial pressure O2 in the cylinder11at the point in time at which the main injection ends. Further, the ignitability may be estimated based on the temperature T in the cylinder11at the point in time at which the main injection starts.

The controller60is not limited to a device that includes a CPU and a memory and executes software processing. For example, at least part of the processes executed by the software in the above-described embodiment may be executed by hardware circuits dedicated to execution of these processes (such as ASIC). That is, the controller may be modified as long as it has any one of the following configurations (a) to (c). (a) A configuration including a processor that executes all of the above-described processes according to programs and a program storage device such as a ROM that stores the programs. (b) A configuration including a processor and a program storage device that execute part of the above-described processes according to the programs and a dedicated hardware circuit that executes the remaining processes. (c) A configuration including a dedicated hardware circuit that executes all of the above-described processes. A plurality of software processing circuits each including a processor and a program storage device and a plurality of dedicated hardware circuits may be provided. That is, the above processes may be executed in any manner as long as the processes are executed by processing circuitry that includes at least one of a set of one or more software processing circuits and a set of one or more dedicated hardware circuits.