JP2024051988A - Control method and device for internal combustion engine - Google Patents

Abstract

[Problem] A single internal combustion engine is used to burn a plurality of fuels with significantly different ignition characteristics. [Solution] A control method for an internal combustion engine that compresses a mixture of air and fuel supplied to a combustion chamber, performs premixed compression ignition combustion by self-igniting the fuel contained in the compressed mixture, and recirculates exhaust gas from the combustion chamber in the previous combustion cycle back to the combustion chamber, includes determination steps S10 and S12, each executed by a computer, for determining whether the combustion cycle is immediately after the start of the internal combustion engine, and control steps S11 and S13 for controlling the internal combustion engine to compress the mixture supplied to the combustion chamber, ignite it with a spark plug, and perform premixed flame propagation combustion by propagating the flame generated by the ignition. [Selected Figure] Fig. 18

Description

The present invention relates to a method for controlling an internal combustion engine capable of burning multiple different fuels and a control device for the internal combustion engine.

A method for controlling an internal combustion engine capable of burning a mixed fuel of gasoline and methyl alcohol has been known (see, for example, Patent Document 1). In the device described in Patent Document 1, an _O2 sensor detects the excess air ratio of exhaust gas, and based on the detection result, adjusts the ratio of gasoline and methyl alcohol supplied to the internal combustion engine so that the excess air ratio approaches 1.0.

Japanese Patent Application Laid-Open No. 58-27834

From the perspective of contributing to mitigating or reducing the impact of climate change, it is desirable to replace fossil fuels with high carbon intensity with renewable energy sources such as solar, wind, hydroelectric, geothermal, or biomass, thereby reducing carbon emissions. From this perspective, renewable fuels such as e-fuels produced using renewable energy, bioalcohol produced from biomass, and biodiesel are becoming more widespread, but the ignition characteristics of each fuel vary significantly depending on the raw materials and production method. It is difficult to burn multiple fuels with significantly different ignition characteristics in a single internal combustion engine.

One aspect of the present invention is a control method for an internal combustion engine that compresses a mixture of air and fuel supplied to a combustion chamber, performs premixed compression ignition combustion by self-igniting the fuel contained in the compressed mixture, and recirculates exhaust gas from the combustion chamber in the previous combustion cycle back to the combustion chamber, and includes a determination step, each executed by a computer, for determining whether or not the combustion cycle is immediately after the start of the internal combustion engine, and a control step for controlling the internal combustion engine to compress the mixture supplied to the combustion chamber, ignite it with an ignition plug, and perform premixed flame propagation combustion by propagating the flame generated by the ignition, if the determination step determines that the combustion cycle is immediately after the start of the internal combustion engine.

Another aspect of the present invention is a control device for an internal combustion engine that compresses a mixture of air and fuel supplied to a combustion chamber, performs premixed compression ignition combustion by self-igniting the fuel contained in the compressed mixture, and returns exhaust gas from the combustion chamber in the previous combustion cycle to the combustion chamber, and includes a controller having a calculation unit and a memory unit. The internal combustion engine has an ignition plug. The calculation unit determines whether the combustion cycle is immediately after the start of the internal combustion engine, and if it is determined that the combustion cycle is immediately after the start of the internal combustion engine, controls the internal combustion engine to compress the mixture supplied to the combustion chamber, ignite it with the ignition plug, and perform premixed flame propagation combustion by propagating the flame generated by the ignition.

The present invention makes it possible to burn multiple fuels with significantly different ignition characteristics in a single internal combustion engine.

FIG. 2 is a diagram for explaining fuel to be combusted in the internal combustion engine according to the embodiment of the present invention. 5 is a diagram for explaining a change in in-cylinder pressure before and after ignition of fuels with different octane numbers. FIG. 5 is a diagram for explaining a change in in-cylinder temperature before and after ignition of fuels with different octane numbers. FIG. FIG. 4 is a diagram for explaining the relationship between the octane number of fuel, the fuel injection amount, and the generated torque. FIG. 4 is a diagram for explaining the relationship between the octane number of a fuel and the heat generation rate of a low-temperature oxidation reaction. 1 is a block diagram showing an example of a configuration of a main part of a method for controlling an internal combustion engine according to an embodiment of the present invention; FIG. 7 is a diagram for explaining an optimal opening degree of the exhaust throttle valve in FIG. 6 . 4 is a flowchart showing an example of processing during compression ignition combustion according to a control method for an internal combustion engine according to an embodiment of the present invention. 5 is a diagram for explaining a change in in-cylinder pressure at start-up when knocking occurs. FIG. 5 is a diagram for explaining a change in in-cylinder pressure at start-up when knocking does not occur. FIG. FIG. 4 is a diagram for explaining the relationship between the octane number of a fuel and the air-fuel ratio. 6 is a diagram for explaining a change in in-cylinder pressure before and after ignition at startup when the air-fuel ratio is changed. FIG. FIG. 4 is a diagram for explaining the relationship between the air-fuel ratio and the adiabatic flame temperature. 4 is a diagram for explaining a change in the octane number of fuel supplied to an internal combustion engine when fuels having different octane numbers are refueled. FIG. FIG. 4 is a diagram for explaining the relationship between the air-fuel ratio and the compression start temperature. 5 is a diagram for explaining a change in in-cylinder pressure before and after ignition during compression ignition combustion when the air-fuel ratio is changed. FIG. FIG. 4 is a diagram for explaining the relationship between the octane number of fuel and an optimal throttle opening degree. 4 is a flowchart showing an example of processing immediately after starting according to a control method for an internal combustion engine according to an embodiment of the present invention.

Below, an embodiment of the present invention will be described with reference to Figs. 1 to 18. A control method for an internal combustion engine according to an embodiment of the present invention controls an internal combustion engine that burns multiple fuels with significantly different ignition characteristics. The average temperature of the Earth is kept warm enough for living things by greenhouse gases in the atmosphere. Specifically, greenhouse gases absorb some of the heat radiated from the Earth's surface, which is warmed by sunlight, into space and re-radiate it back to the Earth's surface, thereby keeping the atmosphere warm. When the concentration of such greenhouse gases in the atmosphere increases, the average temperature of the Earth rises (global warming).

Of all greenhouse gases, carbon dioxide contributes greatly to global warming. Its concentration in the atmosphere is determined by the balance between carbon fixed on the ground or underground as plants and fossil fuels, and carbon present in the atmosphere as carbon dioxide. For example, when carbon dioxide in the atmosphere is absorbed by plants through photosynthesis during growth, the concentration of carbon dioxide in the atmosphere decreases, and when carbon dioxide is released into the atmosphere through the combustion of fossil fuels, the concentration of carbon dioxide in the atmosphere increases. To curb global warming, it is necessary to replace fossil fuels with renewable energy sources such as solar, wind, hydroelectric, geothermal, or biomass, and reduce carbon emissions.

From this perspective, renewable fuels such as e-fuels produced using renewable energy, bioalcohol produced from biomass, and biodiesel are becoming more widespread, but the ignition properties of each fuel vary greatly depending on the raw materials and production method. Therefore, in this embodiment, a method for controlling an internal combustion engine that burns multiple fuels with significantly different ignition properties in a single internal combustion engine is described.

Figure 1 is a diagram for explaining the fuel burned in an internal combustion engine according to an embodiment of the present invention. As shown in Figure 1, a conventional diesel engine can burn fuel (diesel) with an octane number (research octane number) of about 0 to 10, and a conventional gasoline engine can burn fuel (gasoline) with an octane number of 90 or more. The internal combustion engine according to an embodiment of the present invention is configured as a two-stroke or four-stroke engine, and burns fuel with an octane number of about 0 to 120 by homogenous charge compression ignition combustion.

In a premixed flame propagation combustion system, as in conventional gasoline engines, where the air-fuel mixture is compressed and ignited, and the flame generated by the ignition propagates, knocking (abnormal combustion) can occur when a fuel with a low octane rating and high ignition properties is supplied. In a diffusion combustion system, as in conventional diesel engines, where high-pressure fuel is injected into high-temperature compressed air and the injected fuel evaporates and diffuses and ignites as it evaporates and diffuses, stable combustion and output cannot be obtained when a fuel with a high octane rating and low ignition properties is supplied. In a premixed compression ignition combustion system, where the air-fuel mixture is compressed and the fuel in the compressed mixture ignites as it ignites, it can achieve relatively high thermal efficiency like a diesel engine and relatively clean exhaust gas like a gasoline engine.

2 and 3 are diagrams for explaining the changes in in-cylinder pressure and in-cylinder temperature before and after ignition of fuels with different octane ratings, and show the experimental results when the internal combustion engine speed is 2000 [rpm] and the IMEP (indicated mean effective pressure) is 200 [kPa]. As shown in Figs. 2 and 3, the compression ignition start timing at which the in-cylinder pressure and in-cylinder temperature rise is almost the same regardless of the octane rating of the fuel, and the in-cylinder temperature at the start of compression ignition is also almost constant regardless of the octane rating of the fuel. On the other hand, the rise in the in-cylinder pressure and in-cylinder temperature is more gradual the higher the octane rating of the fuel. This is because the higher the octane rating of the fuel and the lower the ignition quality, the slower the combustion after the start of compression ignition.

Figure 4 shows the results of an experiment to explain the relationship between the octane number of the fuel, the amount of fuel injected, and the generated torque. As shown in Figure 4, the higher the octane number of the fuel, the smaller the generated torque per amount of fuel injected. This tendency becomes more pronounced at higher loads where the amount of fuel injected is large. This is because, as shown in Figures 2 and 3, combustion becomes slower after the start of compression ignition, and the proportion of fuel that burns later than the top dead center of compression increases, decreasing the degree of constant volume and thermal efficiency.

Before the start of compression ignition, an oxidation reaction (low-temperature oxidation reaction) of a hydrocarbon (iso-octane in the example shown in the following formulae (i) and (ii)) proceeds as an initial reaction.
_iC8H18 + _OH = _iC8H17 + _{H2O ...} (i)
_iC8H17 + _O2 ( ₊ M)= _iC8H17OO (+M) _... (ii)

The timing of the start of compression ignition depends on the amount of heat generated by the low-temperature oxidation reaction as shown in the following formula (iii).
Low-temperature oxidation reaction heat generation amount [J] = fuel injection amount "g/cyc·cyl"
× Lower heating value [J/g] × Low-temperature oxidation heat generation ratio [%] ... (iii)

Figure 5 shows the results of an experiment to explain the relationship between the octane number of the fuel and the heat generation rate of the low-temperature oxidation reaction. The heat generation rate of the low-temperature oxidation reaction changes depending on the compression conditions of the internal combustion engine, such as the intake temperature. As an example, Figure 5 shows the results of an experiment in which a mixture with an intake temperature of 30 [°C] and an intake pressure of 100 kPa is compressed and ignited in an internal combustion engine with a compression ratio of 14. As shown in Figure 5, the heat generation rate [%] of the low-temperature oxidation reaction is higher as the octane number of the fuel is lower and is lower as the octane number of the fuel is higher. Therefore, the amount of heat generated by the low-temperature oxidation reaction as shown in formula (iii) is higher as the octane number of the fuel is lower and is lower as the octane number of the fuel is higher.

When the in-cylinder temperature rises due to compression and low-temperature oxidation reactions and reaches _{a predetermined activation temperature, a thermal decomposition reaction of H2O2 (} trigger reaction of compression ignition) starts as shown in the following formula (iv). The top dead center temperature _T2 after compression can be calculated based on the bottom dead center temperature _T1 before compression, the air-fuel ratio, the compression ratio ε, and the specific heat ratio κ, using the thermodynamic equation for adiabatic compression as shown in the following formula (v).
_{H2O2 (} +M)=OH+OH(+M) ...(iv)
T ₂ = T ₁ × ε ⁽ κ- ¹⁾ ... (v)

The activation temperature of the trigger reaction is generally around 1000K, regardless of the octane number of the fuel. When the trigger reaction (formula (iv)) starts, the concentration of OH radicals rises rapidly, promoting a chain reaction that generates a large amount of heat, and compression ignition begins. In other words, the time when the temperature inside the cylinder reaches the activation temperature of the trigger reaction is the start of compression ignition. The lower the octane number of the fuel is and the greater the amount of heat generated by the low-temperature oxidation reaction, the earlier the start of compression ignition will be, and the higher the octane number of the fuel is and the smaller the amount of heat generated by the low-temperature oxidation reaction, the later the start of compression ignition will be.

The earlier the compression ignition starts and the closer it is to the top dead center of compression, the greater the proportion of fuel that burns near the top dead center, the greater the amount of heat generated per crank angle, the greater the degree of constant volume and thermal efficiency, and the greater the generated torque. The later the compression ignition starts and the farther it is from the top dead center of compression, the greater the proportion of fuel that burns later than the top dead center, the smaller the amount of heat generated per crank angle, the lower the degree of constant volume and thermal efficiency, and the lower the generated torque. The generated torque is greater the lower the octane number of the fuel, the greater the amount of heat generated by the low-temperature oxidation reaction, and the earlier the compression ignition starts, and is smaller the higher the octane number of the fuel, the smaller the amount of heat generated by the low-temperature oxidation reaction, and the later the compression ignition starts.

The torque generated by each internal combustion engine is greater as the octane rating of the fuel is lower, and is smaller as the octane rating of the fuel is higher, and varies according to the fuel injection amount (total heat generation amount). The torque generated (output torque) can be measured by a rotary torque meter attached to the output shaft of the internal combustion engine. Since the torque generated corresponds to the pressure inside the cylinder, the pressure inside the combustion chamber (inside the cylinder pressure) may be measured by a pressure sensor attached to the combustion chamber of the internal combustion engine instead of the torque. Other parameters corresponding to the torque generated, such as exhaust temperature, may also be measured. By conducting tests with different fuel octane ratings and fuel injection amounts for each internal combustion engine and measuring the torque generated (inside the cylinder pressure), a characteristic map showing the relationship between the octane rating of the fuel, the fuel injection amount, and the torque generated can be created, as shown in Figure 4.

Figure 6 is a block diagram showing an example of the main configuration of a control device for an internal combustion engine according to an embodiment of the present invention. As shown in Figure 6, the control device for an internal combustion engine according to an embodiment of the present invention mainly has a controller 10. The controller 10 is configured to include a computer having a calculation unit 11 such as a CPU, a storage unit 12 such as a ROM and RAM, and peripheral circuits thereof. The storage unit 12 stores information such as programs executed by the calculation unit 11 and set values, as well as a characteristic map (Figure 4) that has been created in advance by testing and shows the relationship between the octane number of the fuel, the amount of fuel injected, and the generated torque (in-cylinder pressure).

An in-cylinder pressure sensor 2 that detects the in-cylinder pressure is connected to the controller 10. The controller 10 acquires information on the in-cylinder pressure detected by the in-cylinder pressure sensor 2 for each combustion cycle of the internal combustion engine.

The controller 10 is connected to a fuel injection control device 50 that controls fuel injection by the injector according to the torque required for the internal combustion engine. The controller 10 obtains information on the required torque and the fuel injection amount from the fuel injection control device 50 for each combustion cycle of the internal combustion engine. The controller 10 and the fuel injection control device 50 may be configured as a single device.

The calculation unit 11 of the controller 10 estimates the octane number of the fuel based on the acquired fuel injection amount information and in-cylinder pressure information, and a characteristic map (Figure 4) stored in the memory unit 12 that shows the relationship between the octane number of the fuel, the fuel injection amount, and the generated torque (in-cylinder pressure). The calculation unit 11 estimates the octane number of the fuel at any time (e.g., for each combustion cycle) during operation from when the internal combustion engine is started to when it is stopped.

The information on the octane number of the fuel estimated by the calculation unit 11 is stored in the memory unit 12. The information on the octane number of the fuel estimated by the calculation unit 11 and stored in the memory unit 12 may be accumulated as time-series information or may be updated to the latest estimated value.

The information on the octane number of the fuel estimated by the calculation unit 11 of the controller 10 is also transmitted to the fuel injection control device 50. The fuel injection control device 50 switches the characteristic map for controlling the fuel injection based on the information on the octane number of the fuel obtained from the controller 10. In other words, the fuel injection amount for the required torque is calculated based on a characteristic map of the optimal fuel injection amount for the required torque, which was created in advance by testing for each fuel with a different octane number. By switching such characteristic maps of the optimal fuel injection amount based on the information on the octane number of the fuel obtained from the controller 10, the fuel injection control device 50 can appropriately control the fuel injection.

The internal combustion engine according to the embodiment of the present invention is provided with an exhaust throttle valve 3 that limits the flow rate of exhaust from the combustion chamber according to the opening degree, and recirculates a portion of the exhaust generated in the previous combustion cycle to the combustion chamber, so that the exhaust gas remains in the current combustion cycle. The exhaust throttle valve 3 is provided, for example, in an exhaust port through which exhaust from the combustion chamber passes. An exhaust valve provided in the exhaust port for opening and closing the exhaust port may be used as the exhaust throttle valve 3. The exhaust throttle valve 3 (actuator) is connected to the controller 10 and controlled by the controller 10. The internal combustion engine according to the embodiment of the present invention may be configured as a two-stroke engine or a four-stroke engine, but it is preferable to configure it as a two-stroke engine from the viewpoint of significantly changing the exhaust gas recirculation amount in response to a wide range of octane fuels.

If the compression ignition start timing is too early compared to the optimal ignition timing near the top dead center, combustion noise occurs and _NOx emissions increase. Also, if it is too late compared to the optimal ignition timing, thermal efficiency decreases (fuel consumption worsens) and THC emissions increase. By recirculating high-temperature exhaust gas, the temperature inside the cylinder at the compression ignition start timing increases and the amount of heat generated by the low-temperature oxidation reaction decreases. By adjusting the exhaust throttle valve 3 to the open side and limiting the amount of exhaust gas recirculation, the temperature inside the cylinder at the compression ignition start timing can be lowered and the compression ignition start timing can be adjusted to be later, i.e., to the retard side. Also, by adjusting the exhaust throttle valve 3 to the closed side and increasing the amount of exhaust gas recirculation, the temperature inside the cylinder at the compression ignition start timing can be raised and the compression ignition start timing can be adjusted to be earlier, i.e., to the advance side.

The calculation unit 11 of the controller 10 controls the exhaust throttle valve 3 so that the opening degree of the exhaust throttle valve 3 is optimal based on the estimated octane number of the fuel. For example, if the estimated octane number of the fuel is high and the ignition quality of the fuel is low, the exhaust throttle valve 3 is adjusted to the closed side, thereby advancing the compression ignition start timing. This makes it possible to adjust the compression ignition start timing to the optimal value regardless of the octane number of the fuel, and obtain a stable output.

Figure 7 is a diagram for explaining the optimal opening of the exhaust throttle valve 3, and shows an example of characteristics that have been created in advance by testing and stored in the memory unit 12 of the controller 10. The calculation unit 11 determines the opening of the exhaust throttle valve 3 corresponding to the required load (required torque) and the estimated octane number of the fuel based on the characteristics of the optimal opening of the exhaust throttle valve 3 stored in the memory unit 12, and controls the exhaust throttle valve 3 (actuator) to achieve the determined opening. The calculation unit 11 may feedback control the exhaust throttle valve 3 (actuator) based on the required load (required torque) obtained from the fuel injection control device 50 and the in-cylinder pressure (generated torque) obtained from the in-cylinder pressure sensor 2 so that the generated torque becomes the required torque.

Figure 8 is a flowchart showing an example of processing during compression ignition combustion according to an internal combustion engine control method according to an embodiment of the present invention. The processing shown in this flowchart is executed by the calculation unit 11 of the controller 10, for example, for each combustion cycle of the internal combustion engine.

As shown in FIG. 8, first, in step S1, information on the in-cylinder pressure (generated torque) is obtained from the in-cylinder pressure sensor 2. Next, in step S2, information on the fuel injection amount is obtained from the fuel injection control device 50. Next, in step S3, the octane number of the fuel is estimated based on the information obtained in steps S1 and S2 and a characteristic map (FIG. 4) stored in the memory unit 12 that shows the relationship between the octane number of the fuel, the fuel injection amount, and the generated torque (in-cylinder pressure). The information on the octane number of the fuel estimated in step S3 is stored in the memory unit 12. The information on the octane number of the fuel estimated in step S3 is also transmitted to the fuel injection control device 50, and thus, fuel injection control in the next combustion cycle is appropriately performed based on the characteristic map for controlling fuel injection according to the octane number of the fuel.

Next, in step S4, information on the required torque is obtained from the fuel injection control device 50. Next, in step S5, the opening degree of the exhaust throttle valve 3 is determined based on the octane number of the fuel estimated in step S3, the required torque (required load) obtained in step S4, and the optimal opening degree characteristics of the exhaust throttle valve 3 stored in the memory unit 12 (Figure 7). In addition, the exhaust throttle valve 3 (actuator) is controlled to achieve the determined opening degree.

According to this embodiment, the following advantageous effects can be obtained.
(1) A method for controlling an internal combustion engine includes a method for controlling an internal combustion engine that performs homogenous charge compression ignition combustion by compressing a mixture of air and fuel supplied to a combustion chamber and self-igniting the fuel contained in the compressed mixture. The method for controlling an internal combustion engine includes information acquisition steps S1 and S2, which are each executed by a computer, for acquiring information on a fuel injection amount and information on a generated torque for each combustion cycle of the internal combustion engine, a characteristic map (FIG. 4) showing the relationship between a predetermined fuel injection amount, generated torque, and ignition property of the fuel, a fuel estimation step S3 for estimating the ignition property of the fuel based on the information on the fuel injection amount and the information on the generated torque acquired in the information acquisition steps S1 and S2, and a control step for controlling the internal combustion engine based on the ignition property of the fuel estimated in the fuel estimation step S3 (FIG. 8). In this way, by estimating the ignition property of the supplied fuel at any time and controlling the fuel injection amount, etc. according to the estimated ignition property of the fuel, a single internal combustion engine can burn a plurality of fuels with significantly different ignition properties.

(2) In the fuel estimation step S3, the octane number of the fuel is estimated as a numerical value representing the ignition quality of the fuel (Figure 4). By quantifying the ignition quality as an octane number, a general-purpose characteristic map showing the relationship between the fuel injection amount, generated torque, and the ignition quality of the fuel can be created.

(3) The internal combustion engine has an exhaust throttle valve 3 that adjusts the amount of exhaust gas recirculated from the combustion chamber to the combustion chamber depending on its opening (Figure 6). In the information acquisition step S4, information on the torque required by the internal combustion engine is acquired (Figure 8). The control method for the internal combustion engine further includes a recirculation amount determination step S5 that determines the opening amount of the exhaust throttle valve 3 based on the information on the torque required by the internal combustion engine acquired in the information acquisition step S4 and the ignition quality of the fuel estimated in the fuel estimation step S3, both of which are executed by a computer (Figure 8). In the control step S5, the internal combustion engine is further controlled based on the opening amount of the exhaust throttle valve 3 determined in the recirculation amount determination step S5.

In this way, by determining the opening degree of the exhaust throttle valve 3 according to the required torque and the ignition quality of the fuel, the temperature inside the cylinder can be adjusted, the ignition timing can be optimally adjusted, and a stable output (generated torque) can be obtained regardless of the ignition quality of the fuel. The opening degree of the exhaust throttle valve 3 can be feedback controlled so that the generated torque becomes the required torque.

(4) The internal combustion engine has an in-cylinder pressure sensor 2 that detects the pressure in the combustion chamber. In the information acquisition step S1, information on the pressure in the combustion chamber detected by the in-cylinder pressure sensor 2 is acquired as information on the generated torque. In this case, the generated torque can be estimated with high accuracy based on the in-cylinder pressure.

(5) The control device of the internal combustion engine controls the internal combustion engine that compresses a mixture of air and fuel supplied to a combustion chamber and performs homogenous charge compression ignition combustion by self-igniting the fuel contained in the compressed mixture. The control device of the internal combustion engine includes a controller 10 having a calculation unit 11 and a memory unit 12 (FIG. 6). The internal combustion engine has an in-cylinder pressure sensor 2 that detects the generated torque (in-cylinder pressure) for each combustion cycle (FIG. 6). The memory unit 12 is configured to store a characteristic map (FIG. 4) that shows the relationship between a predetermined fuel injection amount, generated torque, and ignition of the fuel. The calculation unit 11 is configured to acquire information on the fuel injection amount and information on the generated torque (in-cylinder pressure) detected by the in-cylinder pressure sensor 2 for each combustion cycle of the internal combustion engine, estimate the ignition of the fuel based on the characteristic map stored in the memory unit 12 and the acquired information on the fuel injection amount and generated torque, and control the internal combustion engine based on the estimated ignition of the fuel.

The above describes a method of controlling an internal combustion engine to adjust the compression ignition start timing by exhaust gas recirculation and perform appropriate combustion, but exhaust gas recirculation using high-temperature exhaust gas from the previous combustion cycle cannot be performed immediately after starting the internal combustion engine (at the time of the first explosion). Therefore, in this embodiment, a method of controlling an internal combustion engine is described that performs flame propagation combustion to ignite the air-fuel mixture immediately after starting, thereby performing appropriate combustion at the time of the first explosion even with fuels that have low ignition properties.

As shown in FIG. 6, an internal combustion engine according to an embodiment of the present invention has an ignition plug 4 in the combustion chamber. A glow plug, which has a certain heat capacity and heats the inside of a cylinder, has low responsiveness and may raise the temperature inside the cylinder too much, which may lead to pre-ignition and knocking, especially with highly ignitable fuels. When the air-fuel mixture is ignited by a spark using the ignition plug 4, pre-ignition due to excessive heat is not achieved even when the fuel is highly ignitable. The ignition plug 4 (actuator) is connected to and controlled by the controller 10.

Figures 9 and 10 are diagrams for explaining the change in cylinder pressure during startup. Figure 9 shows the change in cylinder pressure when combustion is not performed properly and knocking occurs, and Figure 10 shows the change in cylinder pressure when combustion is performed properly and knocking does not occur. Also, Figure 11 is a diagram for explaining the relationship between the octane number of the fuel and the air-fuel ratio.

As shown in Figure 11, by setting the air-fuel ratio A/F of the mixture within the appropriate range (normal flame propagation combustion region) according to the octane number of the fuel, appropriate flame propagation combustion as shown in Figure 10 can be achieved. If the air-fuel ratio A/F of the mixture exceeds the appropriate range and the mixture becomes excessively lean, there is a high possibility that the flame generated by ignition will not propagate and will misfire (extinguish) (misfire region). If the air-fuel ratio A/F of the mixture exceeds the appropriate range and the mixture becomes excessively rich, there is a high possibility that knocking will occur as shown in Figure 9 (knocking region).

Figure 12 is a diagram for explaining the change in cylinder pressure before and after ignition (ignition) at start-up when the air-fuel ratio is changed, and shows the results of an experiment. As shown in Figure 12, when the air-fuel ratio A/F of the mixture is 15, knocking occurs and oscillations in the cylinder pressure are observed. When the air-fuel ratio A/F of the mixture is 23, misfire (flame quenching) occurs and no sudden increase in the cylinder pressure is observed. On the other hand, when the air-fuel ratio A/F of the mixture is 20, proper combustion occurs and the cylinder pressure rises rapidly without oscillation. As shown in Figure 11, the appropriate range of the air-fuel ratio A/F of the mixture changes depending on the octane number of the fuel, but the appropriate range of the air-fuel ratio A/F that can perform proper combustion over a wide range of octane numbers, as shown by the dashed line in Figure 11, is about 16 to 20.

Knocking is a sudden self-ignition combustion caused by an increase in the reaction rate of the combustion reaction, which is an exothermic oxidation reaction (equation (vi) below), resulting in excessive heat generation. In equation (vi) below, A is the pre-exponential factor, E is the activation energy, R is the gas constant, T is the in-cylinder temperature, and the units of fuel concentration and oxygen concentration are [mol/ ^m3 ].
Reaction rate = A exp(-E/RT)(fuel concentration)(oxygen concentration) ... (vi)

The reaction rate of the combustion reaction shown in formula (vi) becomes faster as the air-fuel ratio becomes larger and the mixture becomes leaner toward the stoichiometric air-fuel ratio, and the mixture becomes richer. By making the mixture leaner than the stoichiometric air-fuel ratio, the heat capacity of the excess oxygen suppresses the rise in the cylinder temperature and reduces the reaction rate, making it difficult for knocking to occur. If knocking occurs, the components of the internal combustion engine (combustion chamber) may be damaged, or the marketability of the internal combustion engine may be impaired due to combustion noise. The air-fuel ratio at the knocking limit, which is the lower limit of the appropriate range in FIG. 11, varies depending on the strength of the internal combustion engine (combustion chamber) and the allowable noise level. In addition, the air-fuel ratio at the knocking limit depends on the compression ratio, valve timing, etc., and therefore differs for each internal combustion engine. The characteristic representing the knocking limit for each internal combustion engine (the lower limit of the appropriate range in FIG. 11) is determined in advance by testing and stored in the memory unit 12 of the controller 10.

FIG. 13 is a diagram for explaining the relationship between the air-fuel ratio and the adiabatic flame temperature. As shown in FIG. 13, the adiabatic flame temperature of hydrocarbons changes depending on the air-fuel ratio A/F. When the mixture becomes excessively lean, the adiabatic flame temperature of hydrocarbons (FIG. 13) and the laminar burning velocity (formula (vii) below) decrease, making it difficult to maintain and propagate the flame, leading to misfire (extinction). This is because the balance between heat radiation and heat generation from the flame is lost and heat cannot be maintained. In formula (vii) below, α is the thermal diffusivity, P is the cylinder pressure, E is the activation energy, R is the gas constant, and T _ad is the adiabatic flame temperature.
Laminar burning velocity ∝α×P ^n-2 (−E/RT _ad ) ...(vii)

The air-fuel ratio at the flame propagation limit varies from one internal combustion engine to another because it depends on the compression ratio, valve timing, etc., but the air-fuel ratio at the flame propagation limit for a typical gasoline engine is about 20. The characteristics representing the flame propagation limit for each internal combustion engine (the upper limit of the appropriate range in Figure 11) are determined in advance by testing and stored in the memory unit 12 of the controller 10.

The calculation unit 11 of the controller 10 determines whether the current combustion cycle is the first combustion cycle (initial explosion) immediately after starting the internal combustion engine, and if it is determined that it is the initial explosion, it determines the target air-fuel ratio and controls the internal combustion engine including the spark plug 4 to perform flame propagation combustion. The target air-fuel ratio at the initial explosion is determined to a value near the limit at which knocking does not occur (approximately 16 to 20) based on the characteristics representing the knocking limit stored in the memory unit 12 and the octane number of the fuel. Information on the target air-fuel ratio determined by the calculation unit 11 of the controller 10 is sent to the fuel injection control device 50, which controls the fuel injection according to the target air-fuel ratio.

Figure 14 is a diagram for explaining the change in the octane number of fuel supplied to an internal combustion engine when fuels with different octane numbers are refueled. The horizontal axis of Figure 14 indicates the operating time of the internal combustion engine. At time t1, the internal combustion engine is stopped and started, and fuel with a lower octane number than the fuel stored in the fuel tank is refueled between the time of stopping and starting. At time t3, the internal combustion engine is stopped and started, and fuel with a higher octane number than the fuel stored in the fuel tank is refueled between the time of stopping and starting.

The fuel for several cycles immediately after the internal combustion engine is started (times t1 to t2 and t3 to t4 in Figure 14) is supplied by the fuel that has already been pumped from the fuel tank into the fuel pipe before the internal combustion engine is stopped. Therefore, even if refueling occurs between the time the internal combustion engine is stopped and the time it is started, and the octane rating of the fuel in the fuel tank changes, the octane rating of the fuel supplied to the combustion chamber in the several cycles immediately after starting will match the octane rating estimated before the internal combustion engine was stopped. Therefore, the target air-fuel ratio at the time of first combustion can be appropriately determined based on the octane rating of the fuel estimated before the internal combustion engine was stopped.

Figure 15 is a diagram for explaining the relationship between the air-fuel ratio and the compression start temperature, and shows an example of the optimal air-fuel ratio in several combustion cycles (from the first explosion (first explosion) to the fifth explosion in the figure) after the first explosion. As shown in Figure 15, in the second and subsequent combustion cycles (hereinafter sometimes referred to as the "transition cycle"), the exhaust gas from the previous combustion cycle is recirculated, gradually increasing the in-cylinder temperature (compression start temperature and temperature near top dead center). For this reason, if the air-fuel ratio A/F of the mixture determined at the time of the first explosion is maintained as it is, knocking may occur. In the second and subsequent combustion cycles, the air-fuel ratio A/F determined at the time of the first explosion is gradually increased and the mixture is gradually leaned, allowing stable flame propagation combustion to continue without knocking.

After the initial explosion, the compression start temperature reaches a temperature range where stable compression ignition combustion is possible in several combustion cycles (the fifth in the figure) by adjusting the compression ignition start timing through exhaust gas recirculation, and then it is possible to transition to compression ignition combustion that does not require ignition by the spark plug 4.

Figure 16 is a diagram illustrating the change in cylinder pressure before and after ignition during compression ignition combustion when the air-fuel ratio is changed, and shows the results of an experiment. As shown in Figure 16, when the air-fuel ratio A/F of the mixture is 19, the peak of the cylinder pressure almost coincides with the compression top dead center, and the ignition timing is too early, resulting in an excessive pressure rise rate. In this case, there is an increased possibility that components of the internal combustion engine (combustion chamber) will be damaged, and the marketability of the internal combustion engine will be impaired due to combustion noise. When the air-fuel ratio A/F of the mixture is 24, the ignition timing is too late, causing misfires due to the temperature drop during the expansion stroke, and no sudden rise in cylinder pressure is observed.

On the other hand, when the air-fuel ratio A/F of the mixture is 21 or 22, proper combustion occurs, and the peak of the cylinder pressure occurs at an appropriate time slightly later than the top dead center of compression. Therefore, the appropriate range of the air-fuel ratio A/F for proper combustion after transition to compression ignition combustion is approximately 20 to 22. The appropriate air-fuel ratio during compression ignition combustion depends on the compression ratio and intake temperature, etc., and therefore differs for each internal combustion engine. The appropriate air-fuel ratio during compression ignition combustion for each internal combustion engine is determined in advance by testing and stored in the memory unit 12 of the controller 10.

Figure 17 is a diagram for explaining the relationship between the octane number of fuel and the optimal throttle opening. The heat generation amount of the low-temperature oxidation reaction increases as the fuel injection amount increases (equation (iii)). Therefore, in order to transition from flame propagation combustion to compression ignition combustion earlier, i.e., after the initial explosion and with fewer combustion cycles, the throttle opening may be adjusted to the open side so that more air-fuel mixture is supplied to the combustion chamber. However, the lower the octane number of the fuel and the higher the possibility of knocking, the more the throttle opening is restricted.

The calculation unit 11 of the controller 10 judges whether the current combustion cycle is the second or subsequent combustion cycle (transition cycle) after the start of the internal combustion engine. If it is judged to be a transition cycle, the target air-fuel ratio is determined so that the mixture gradually becomes leaner, and the internal combustion engine including the ignition plug 4 is controlled to perform flame propagation combustion. Information on the target air-fuel ratio determined by the calculation unit 11 of the controller 10 is transmitted to the fuel injection control device 50, which performs fuel injection control according to the target air-fuel ratio. The target air-fuel ratio of the transition cycle is determined so as to gradually become leaner based on the target air-fuel ratio of the previous combustion cycle, the octane number of the fuel stored in the memory unit 12, the characteristics representing the knocking limit and the flame propagation limit, and the appropriate air-fuel ratio during compression ignition combustion for each internal combustion engine.

Figure 18 is a flowchart showing an example of processing immediately after starting according to a control method for an internal combustion engine according to an embodiment of the present invention. The processing shown in this flowchart is executed by the calculation unit 11 of the controller 10, for example, for each combustion cycle of the internal combustion engine.

As shown in FIG. 18, first, in step S10, it is determined whether the current combustion cycle is the first combustion cycle (initial explosion) immediately after the start of the internal combustion engine. If the result in step S10 is positive, the process proceeds to step S11, where a target air-fuel ratio is determined based on the characteristics representing the knocking limit stored in the memory unit 12 and the octane number of the fuel, and the internal combustion engine including the spark plug 4 is controlled to perform flame propagation combustion. On the other hand, if the result in step S10 is negative, the process proceeds to step S12, where it is determined whether the current combustion cycle is the second or subsequent combustion cycle (transition cycle) after the start of the internal combustion engine. If the result in step S12 is positive, the process proceeds to step S13, where a target air-fuel ratio is determined so that the mixture becomes gradually leaner, and the internal combustion engine including the spark plug 4 is controlled to perform flame propagation combustion. On the other hand, if the result in step S12 is negative, the process proceeds to step S14, where the internal combustion engine is controlled to perform compression ignition combustion.

This embodiment also provides the following advantages:

(6) A method for controlling an internal combustion engine includes controlling an internal combustion engine that compresses a mixture of air and fuel supplied to a combustion chamber, performs premixed compression ignition combustion by self-igniting the fuel contained in the compressed mixture, and returns exhaust gas from the combustion chamber in the previous combustion cycle to the combustion chamber. The method for controlling an internal combustion engine includes judgment steps S10 and S12, each executed by a computer, for judging whether the combustion cycle is immediately after the start of the internal combustion engine, and control steps S11 and S13, each executed by a computer, for controlling the internal combustion engine to compress the mixture supplied to the combustion chamber, ignite it with an ignition plug, and perform premixed flame propagation combustion by propagating the flame generated by the ignition.

In an internal combustion engine that performs homogeneous charge compression ignition combustion during normal operation and adjusts the in-cylinder temperature and ignition timing using exhaust gas recirculation, the air-fuel mixture is ignited immediately after starting and flame propagation combustion is performed, allowing proper combustion at the time of initial explosion even with fuels with low ignition characteristics. In this case, the air-fuel mixture is ignited by a spark using an ignition plug rather than a glow plug with a large heat capacity, so pre-ignition due to excessive heat is not achieved. This makes it possible to burn multiple fuels with significantly different ignition characteristics in a single internal combustion engine.

(7) The method for controlling an internal combustion engine further includes information acquisition steps S1 and S2, which are each executed by a computer, for acquiring information on the fuel injection amount and information on the generated torque for each combustion cycle of the internal combustion engine, a fuel estimation step S3 for estimating the octane number of the fuel based on a characteristic map (FIG. 4) showing the relationship between a predetermined fuel injection amount, generated torque, and the octane number of the fuel, and the information on the fuel injection amount and the generated torque acquired in the information acquisition steps S1 and S2, and an air-fuel ratio determination step S11 and S13 for determining a target air-fuel ratio of the mixture (FIG. 11) based on the octane number of the fuel estimated in the fuel estimation step S3 before the internal combustion engine is stopped (FIG. 18) when it is determined in the determination steps S10 and S12 that the combustion cycle is immediately after the start of the internal combustion engine. In the control steps S11 and S13, the internal combustion engine is controlled based on the target air-fuel ratio determined in the air-fuel ratio determination steps S11 and S13.

The fuel for the few cycles immediately after starting the internal combustion engine is supplied by the fuel that has already been pumped from the fuel tank into the fuel pipe before the internal combustion engine is stopped. Therefore, even if refueling occurs between the time the internal combustion engine is stopped and the time it is started, and the octane rating of the fuel in the fuel tank changes, the octane rating of the fuel supplied to the combustion chamber in the few cycles immediately after starting will match the octane rating estimated before the internal combustion engine was stopped (Figure 16). Therefore, the target air-fuel ratio of the mixture immediately after starting can be appropriately determined based on the octane rating of the fuel estimated before the internal combustion engine was stopped.

(8) In the air-fuel ratio determination step S13, if it is determined in the determination steps S10 and S12 that the combustion cycle is immediately after the start of the internal combustion engine, the target air-fuel ratio of the mixture is determined so that it gradually becomes leaner with each combustion cycle of the internal combustion engine (FIG. 12). In the second and subsequent combustion cycles after the first explosion, the high-temperature exhaust gas generated in the previous combustion cycle is recirculated, so that the compression start temperature rises with each combustion cycle, increasing the possibility of knocking. In the second and subsequent combustion cycles after the first explosion, transient control is performed to gradually lean the air-fuel ratio until the compression start temperature stabilizes and reaches a temperature range where compression ignition is possible, allowing an appropriate transition from flame propagation combustion to compression ignition combustion without knocking.

(9) The control method for an internal combustion engine further includes an intake amount determination step S11, which is executed by a computer, and when it is determined in the determination step S10 that the combustion cycle is immediately after the start of the internal combustion engine, the intake amount determination step S11 determines the throttle opening for adjusting the intake amount to the combustion chamber to be larger as the octane number of the fuel estimated in the fuel estimation step S3 before the internal combustion engine is stopped is higher, based on the octane number of the fuel estimated in the fuel estimation step S3 before the internal combustion engine is stopped and the target air-fuel ratio determined in the air-fuel ratio determination step S11 (FIG. 18). In the control step S11, the internal combustion engine is further controlled based on the throttle opening determined in the intake amount determination step S11. In other words, in the case of a fuel with a high octane number and low possibility of knocking, the throttle is opened to increase the intake amount, thereby increasing the exhaust temperature and the compression ignition start temperature, and the transition to compression ignition combustion can be made earlier.

(10) The control device for an internal combustion engine compresses a mixture of air and fuel supplied to a combustion chamber, performs premixed compression ignition combustion by self-igniting the fuel contained in the compressed mixture, and controls the internal combustion engine to return exhaust gas from the combustion chamber in the previous combustion cycle to the combustion chamber. The control device for an internal combustion engine includes a controller 10 having a calculation unit 11 and a memory unit 12 (FIG. 6). The internal combustion engine has an ignition plug 4 (FIG. 6). The calculation unit 11 is configured to determine whether the combustion cycle is immediately after the start of the internal combustion engine, and when it is determined that the combustion cycle is immediately after the start of the internal combustion engine, to control the internal combustion engine to compress the mixture supplied to the combustion chamber, ignite it with the ignition plug 4, and perform premixed flame propagation combustion by propagating the flame generated by the ignition.

The above description is merely an example, and the present invention is not limited to the above-mentioned embodiment and modifications, as long as the characteristics of the present invention are not impaired. It is also possible to arbitrarily combine one or more of the above-mentioned embodiment and modifications, and it is also possible to combine modifications together.

2 Cylinder pressure sensor, 3 Exhaust throttle valve, 4 Spark plug, 10 Controller, 11 Calculation unit, 12 Memory unit, 50 Fuel injection control device

Claims (5)

A control method for an internal combustion engine, which performs homogeneous charge compression ignition combustion by compressing a mixture of air and fuel supplied to a combustion chamber and self-igniting the fuel contained in the compressed mixture, and returns exhaust gas from the combustion chamber in a previous combustion cycle to the combustion chamber, comprising:
Each of the methods is executed by a computer.
a determination step of determining whether or not the combustion cycle is immediately after starting the internal combustion engine;
and a control step of controlling the internal combustion engine so that, when it is determined in the determination step that the combustion cycle is immediately after the start of the internal combustion engine, the mixture supplied to the combustion chamber is compressed, ignited by an ignition plug, and a premixed flame propagation combustion is performed in which the flame generated by the ignition propagates. 2. The method for controlling an internal combustion engine according to claim 1,
Each of the methods is executed by a computer.
an information acquisition step of acquiring information on a fuel injection amount and information on a generated torque for each combustion cycle of the internal combustion engine;
a fuel estimation step of estimating an octane number of the fuel based on a characteristic map indicating a relationship between a predetermined fuel injection amount, the generated torque, and an octane number of the fuel, and on information on the fuel injection amount and information on the generated torque acquired in the information acquisition step;
and an air-fuel ratio determination step of determining a target air-fuel ratio of the mixture based on the octane number of the fuel estimated in the fuel estimation step before the internal combustion engine is stopped, when the determination step determines that the combustion cycle is immediately after the start of the internal combustion engine,
In the control step, the internal combustion engine is controlled based on the target air-fuel ratio determined in the air-fuel ratio determination step. 2. The method for controlling an internal combustion engine according to claim 1,
a control method for an internal combustion engine, characterized in that, in the air-fuel ratio determination step, when it is determined in the determination step that the combustion cycle is immediately after start-up of the internal combustion engine, a target air-fuel ratio of the mixture is determined so as to gradually become leaner for each combustion cycle of the internal combustion engine. 4. The method for controlling an internal combustion engine according to claim 3,
Each of the methods is executed by a computer.
and an intake amount determination step of determining, when it is determined in the determination step that the combustion cycle is immediately after the start of the internal combustion engine, an opening degree of an intake control valve for adjusting the amount of intake air into the combustion chamber to be larger as the octane number of the fuel estimated in the fuel estimation step before the internal combustion engine is stopped and a target air-fuel ratio determined in the air-fuel ratio determination step are higher,
The control step further comprises controlling the internal combustion engine based on an opening degree of the intake regulating valve determined in the intake amount determining step. A control device for an internal combustion engine, which performs homogeneous charge compression ignition combustion by compressing a mixture of air and fuel supplied to a combustion chamber and self-igniting the fuel contained in the compressed mixture, and returns exhaust gas from the combustion chamber in a previous combustion cycle to the combustion chamber,
A controller having a calculation unit and a storage unit,
The internal combustion engine has a spark plug.
The calculation unit is
determining whether or not the combustion cycle is immediately after starting the internal combustion engine;
A control device for an internal combustion engine, characterized in that, when it is determined that the internal combustion engine is in a combustion cycle immediately after startup, the control device controls the internal combustion engine so as to perform premixed flame propagation combustion, compressing the mixture supplied to the combustion chamber, igniting it by the spark plug, and propagating the flame generated by the ignition.

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