JP7056229B2 - Premixed compression ignition engine controller - Google Patents

Description

The technique disclosed herein relates to a control device for a premixed compression ignition engine.

Patent Document 1 describes a technique for measuring a flow velocity in a cylinder by forming a low-temperature plasma state between electrodes in a 4-stroke reciprocating engine that performs lean burn operation. Specifically, in this engine, the voltage control circuit forms a low temperature plasma state between the electrodes by applying a short pulse electric field to the spark plug before the ignition timing.

Patent Document 2 describes a technique for injecting water onto the crown surface of a piston in a spark-ignition type reciprocating engine. This engine vaporizes water by the heat lost as a cooling loss during the combustion period, and improves thermal efficiency by recovering the pressure energy of the vaporized steam as work in the expansion stroke.

Japanese Unexamined Patent Publication No. 2014-141919 Japanese Unexamined Patent Publication No. 2008-175078

By the way, unlike the spark ignition type engine described in Patent Documents 1 and 2, in an engine that burns an air-fuel mixture formed in a combustion chamber by compression ignition, when low-temperature plasma is generated in the combustion chamber, the air-fuel mixture is ignited. It is possible to improve the sex. That is, the electrons generated by the low-temperature plasma discharge chemically react with oxygen, nitrogen, etc. in the combustion chamber to generate chemical species such as ozone or O radicals in the combustion chamber. Then, the ignitability of the air-fuel mixture is improved by promoting the low-temperature oxidation reaction of the air-fuel mixture with chemical species such as ozone or O radicals.

However, if the ignitability of the air-fuel mixture is improved by low-temperature plasma, combustion proceeds rapidly, and as a result, abnormal combustion such as knocking may occur. In a compression ignition type engine, it is required to improve the ignitability of the air-fuel mixture and avoid abnormal combustion at the same time.

The technique disclosed herein achieves both improvement of the ignitability of the air-fuel mixture and avoidance of abnormal combustion in the premixed compression ignition type engine.

The technique disclosed herein relates to a control device for a premixed compression ignition engine. The control device of the premixed compression ignition type engine has a combustion chamber that repeats an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, and has an engine body in which the air-fuel mixture in the combustion chamber is burned by compression ignition, and the combustion chamber. A plasma generation unit that generates plasma in the combustion chamber by applying a voltage between the pair of electrodes arranged in the combustion chamber, an injector that directly injects fuel into the combustion chamber, and water is supplied to the combustion chamber. It is equipped with a water supply unit.

Then, the plasma generation unit generates low-temperature plasma in the combustion chamber at a predetermined timing in the period from the start of the intake stroke to the end of the compression stroke, and the injector in the combustion chamber after the generation of the low-temperature plasma. The fuel is injected , and the water supply unit supplies water to the combustion chamber after the fuel is supplied.

When the plasma generator generates low-temperature plasma in the combustion chamber at a predetermined timing in the period from the start of the intake stroke to the end of the compression stroke, the electrons generated by the low-temperature plasma discharge chemically react with oxygen and nitrogen in the combustion chamber. As a result, chemical species such as ozone or O radicals are generated in the combustion chamber.

After the low temperature plasma is generated, the injector injects fuel into the combustion chamber to form an air-fuel mixture in the combustion chamber. In the air-fuel mixture, the low-temperature oxidation reaction is promoted by chemical species such as ozone or O radicals, and the ignitability of the air-fuel mixture is improved. The air-fuel mixture is stably burned by compression ignition.

After the fuel is supplied, the water supply unit supplies water to the combustion chamber. The rapid progress of combustion is suppressed, and abnormal combustion such as knocking is prevented from occurring. Therefore, the improvement of the ignitability of the air-fuel mixture and the avoidance of abnormal combustion are compatible.

In addition, since the above-mentioned configuration slows down combustion, it is advantageous in reducing combustion noise and can suppress deterioration of exhaust emission performance.

Further, by generating the low temperature plasma in the combustion chamber, it is possible to suppress the increase in the gas temperature in the combustion chamber, unlike the case where the high temperature plasma is generated. Since the combustion temperature is low, the cooling loss is reduced. Therefore, the above configuration is also advantageous for improving the fuel efficiency of the engine.

When the engine body is operated at a load higher than the predetermined load, the plasma generation unit generates low-temperature plasma in the latter half of the compression stroke, and the engine body is operated at a low load equal to or lower than the predetermined load. When present, cold plasma is generated within the period from the end of the inspiratory stroke to the early stage of the compression stroke, at least within the period including the compression stroke . However, the latter half of the compression stroke is the latter half when the compression stroke is bisected into the first half and the second half, and the end of the intake stroke is when the intake stroke is bisected into the initial, middle, and final stages. The initial stage of the compression stroke is the initial stage when the compression stroke is divided into three equal parts at the initial stage, the middle stage, and the final stage.

If the generation timing of the low temperature plasma is advanced, the low temperature plasma diffuses throughout the combustion chamber. If the low temperature plasma generation timing is advanced when the engine body is operating under a high load, abnormal combustion such as premature ignition or knocking may occur. When the engine body is operating under a high load, it is possible to prevent the occurrence of abnormal combustion by delaying the generation timing of the low temperature plasma. On the other hand, when the engine body is operating at a low load, abnormal combustion such as premature ignition and knocking is unlikely to occur, while by accelerating the generation timing of low temperature plasma, the pressure in the combustion chamber is low and the distance between the electrodes is low. It is possible to generate low-temperature plasma in a state where it is easy to discharge. It suppresses the power consumption required to generate low-temperature plasma, which is advantageous for improving the fuel efficiency of the engine.

The water supply unit may supply water to the combustion chamber after the air-fuel mixture in the combustion chamber is ignited.

After the air-fuel mixture is stably compressed and ignited by generating low-temperature plasma, the rapid progress of combustion is suppressed by the water supplied after ignition. It is possible to effectively prevent the occurrence of abnormal combustion.

The water supply unit supplies water to the combustion chamber when the engine body is operating at a load higher than the predetermined load, and the engine body is operated at a low load equal to or lower than the predetermined load. It may be said that water is not supplied to the combustion chamber when it is present.

When the engine body is operating under a high load, a large amount of fuel is supplied to the combustion chamber, so that rapid combustion is likely to occur and abnormal combustion is likely to occur. When the engine body is operating with a high load, the water supply unit supplies water to the combustion chamber, so that abnormal combustion can be effectively prevented. On the other hand, when the engine body is operating at a low load, the amount of fuel supplied to the combustion chamber is small, so that abnormal combustion is unlikely to occur. When the engine body is operating at a low load, abnormal combustion can be prevented even if the water supply unit does not supply water to the combustion chamber.

According to the control device of the premixed compression ignition type engine, the improvement of the ignitability of the air-fuel mixture and the avoidance of abnormal combustion are compatible.

FIG. 1 is a diagram illustrating an engine configuration. FIG. 2 is a block diagram illustrating a configuration of an engine control device. FIG. 3 is a diagram illustrating the configuration of the plasma generator. FIG. 4 is a diagram illustrating an example of a plasma generator having a configuration different from that of FIG. FIG. 5 is a diagram showing the relationship between the time from the start of discharge and the voltage applied between the electrodes, in which low-temperature plasma and high-temperature plasma are generated in the combustion chamber. FIG. 6 is a diagram illustrating a time waveform of a voltage applied between electrodes when generating low temperature plasma. FIG. 7 is a diagram illustrating the Paschen curve. FIG. 8 is a diagram illustrating a time waveform of a voltage applied between the electrodes when generating a low temperature plasma, which is different from FIG. FIG. 9 is a diagram illustrating an operating region of the engine. The upper figure of FIG. 10 is a diagram illustrating the timing of generating plasma in the combustion chamber and the timing of injecting fuel together with the change curve of the in-cylinder pressure in the first operating state with a low load. The figure below illustrates the timing of plasma generation, fuel injection, and water injection in the combustion chamber, along with the in-cylinder pressure change curve in the second operating state with a high load. FIG. 11 is a flowchart illustrating control related to engine operation.

Hereinafter, exemplary embodiments of the control device for the premixed compression ignition engine will be described in detail with reference to the drawings. FIG. 1 is a diagram illustrating the configuration of the engine 1. FIG. 2 is a block diagram illustrating a configuration of an engine control device. In FIG. 1, the intake side is on the left side of the paper surface, and the exhaust side is on the right side of the paper surface.

The engine 1 is a 4-stroke engine in which the combustion chamber 17 is operated by repeating an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. The engine 1 is mounted on a four-wheeled vehicle. The automobile runs by driving the engine 1. The fuel of the engine 1 is gasoline in this configuration example. The fuel may be gasoline containing bioethanol or the like. The fuel of the engine 1 may be any fuel as long as it is at least a liquid fuel containing gasoline.

<Engine configuration>
Although the engine 1 shows only one cylinder 11 in FIG. 1, it is a multi-cylinder engine having a plurality of cylinders 11. The engine 1 includes an engine body 2 having a combustion chamber 17. The engine body 2 includes a cylinder block 12 and a cylinder head 13 mounted on the cylinder block 12.

A piston 3 is slidably inserted in each cylinder 11. The piston 3 is connected to a crankshaft (not shown) via a connecting rod 14. The piston 3 partitions the combustion chamber 17 together with the cylinder 11 and the cylinder head 13. Here, the "combustion chamber" is not limited to the meaning of the space when the piston 3 reaches the compression top dead center. The term "combustion chamber" may be used in a broad sense. That is, the "combustion chamber" may mean the space formed by the piston 3, the cylinder 11 and the cylinder head 13 regardless of the position of the piston 3. The "combustion chamber" can be paraphrased as "inside the cylinder 11".

The lower surface of the cylinder head 13, that is, the ceiling surface of the combustion chamber 17, has a so-called pent roof shape. The upper surface of the piston 3 is raised toward the ceiling surface of the combustion chamber 17. The geometric compression ratio of the engine body 2 is set to 15 or more and 30 or less. As will be described later, the engine body 2 utilizes a high geometric compression ratio to burn the air-fuel mixture in the combustion chamber 17 by compression ignition.

The cylinder head 13 is formed with an intake port 18 for each cylinder 11. The intake port 18 communicates with the combustion chamber 17. The intake port 18 is provided with an intake valve 21. The intake valve 21 opens and closes the intake port 18 between the combustion chamber 17 and the intake port 18. The engine body 2 is provided with a valve operation mechanism 23 (see FIG. 2) for the intake valve 21. The intake valve 21 is opened and closed at a predetermined timing by the valve operating mechanism 23. The valve operating mechanism 23 of the intake valve 21 may be a variable valve mechanism that makes the valve timing and / or the valve lift variable.

The cylinder head 13 is also formed with an exhaust port 19 for each cylinder 11. The exhaust port 19 communicates with the combustion chamber 17. The exhaust port 19 is provided with an exhaust valve 22. The exhaust valve 22 opens and closes the exhaust port 19 between the combustion chamber 17 and the exhaust port 19. The engine body 2 is provided with a valve operation mechanism 24 (see FIG. 2) for the exhaust valve 22. The exhaust valve 22 is opened and closed at a predetermined timing by the valve operating mechanism 24. The valve operation mechanism 24 of the exhaust valve 22 may be a variable valve mechanism that makes the valve timing and / or the valve lift variable.

An injector 6 is attached to the cylinder head 13 for each cylinder 11. The injector 6 is configured to inject fuel directly into the combustion chamber 17. The injector 6 is an example of a fuel supply unit. In the configuration example of FIG. 1, the injector 6 is arranged near the center of the cylinder 11. The location where the injector 6 is arranged is not limited to the configuration example shown in FIG. Although detailed illustration is omitted, the injector 6 is composed of, for example, a multi-injection type combustion injection valve having a plurality of injection ports. The injector 6 may be configured by an externally open valve type fuel injection valve.

A water injection valve 61 is also attached to the cylinder head 13 for each cylinder 11. The water injection valve 61 is configured to inject water into the combustion chamber 17. The water injection valve 61 is an example of a water supply unit. In the configuration example of FIG. 1, the water injection valve 61 is arranged near the center of the cylinder 11. The injector 6 and the water injection valve 61 may be displaced from each other in the direction of the cylinder row, for example. The arrangement position of the water injection valve 61 is not limited to the configuration example of FIG. Although detailed illustration is omitted, the water injection valve 61 may be configured by, for example, a multi-injection type injection valve, similarly to the injector 6. Further, the water injection valve 61 may be configured by, for example, an externally open type injection valve.

A discharge electrode 4 is attached to the cylinder head 13 for each cylinder 11. In this configuration example, the discharge electrode 4 is arranged in the combustion chamber 17 on the exhaust side of the injector 6. Although the details will be described later, the discharge electrode 4 generates plasma in the combustion chamber 17 by performing discharge. The discharge electrode 4 is an example of a pair of electrodes arranged in the combustion chamber 17.

The discharge electrode 4 has, for example, as illustrated in FIG. 3, a center electrode 41, a ground electrode 42, and an insulator 43 interposed between the center electrode 41 and the ground electrode 42. The center electrode 41 and the insulator 43 are housed in the tubular body 44. The ground electrode 42 is configured in an L shape when viewed from the side. The base of the ground electrode 42 is fixed to the lower end surface of the tubular body 44 in a conductive state. The tip of the ground electrode 42 faces the center electrode 41. The discharge electrode 4 has the same configuration as a general spark plug for spark ignition.

The gap between the center electrode 41 of the discharge electrode 4 and the ground electrode 42 faces the inside of the combustion chamber 17 and is located near the ceiling surface of the combustion chamber 17.

The discharge electrode 4 constitutes a plasma generator 49 that generates low-temperature plasma in the combustion chamber 17 in the engine 1. As shown in FIG. 3, for example, the plasma generation device 49 includes a discharge electrode 4 and an application unit 45 for applying a voltage to the discharge electrode 4. The application unit 45 illustrated in FIG. 3 is configured in an alternating current manner. The application unit 45 includes a frequency generation circuit 451 that generates an alternating current of a desired frequency, and a booster circuit 452 that boosts the alternating current of a desired frequency generated by the frequency generation circuit 451 to a desired voltage. ing. The application unit 45 applies a predetermined voltage at a predetermined frequency between the electrodes of the discharge electrode 4 according to a control signal from the ECU 10 described later. The application unit 45, together with the ECU 10, constitutes a control unit that applies a voltage between the electrodes. The plasma generator 49 is an example of a plasma generator.

The discharge electrode is not limited to the configuration shown in FIG. For example, FIG. 4 illustrates a modified example of the discharge electrode. The discharge electrode 40 omits the L-shaped ground electrode, and the tubular body 44 surrounding the insulator 43 constitutes the ground electrode. The discharge electrode 40 may be configured by a so-called creeping spark plug.

Further, the application unit is not limited to the AC type shown in FIG. 3, and may be configured as a DC type application unit 46 as shown in FIG. 4, for example. The direct current type application unit 46 includes a high voltage generation circuit 461 that generates a desired high voltage direct current, and a switching circuit 462 that converts the high voltage direct current into a pulse waveform of a desired frequency. There is. The application unit 46 also applies a predetermined voltage at a predetermined frequency between the electrodes of the discharge electrode 40 according to a control signal from the ECU 10 described later. The application unit 46, together with the ECU 10, constitutes a control unit that applies a voltage between the electrodes.

It is also possible to combine the discharge electrode 4 shown in FIG. 3 and the application unit 46 shown in FIG. 4, and to combine the application unit 45 shown in FIG. 3 and the discharge electrode 40 shown in FIG. 4, respectively. ..

An intake passage (not shown) is connected to the intake port 18 of the engine body 2. A throttle valve 51 is interposed in the intake passage (see FIG. 2). Further, an exhaust passage (not shown) is connected to the exhaust port 19 of the engine body 2. A catalytic converter that purifies the exhaust gas discharged from the combustion chamber 17 is arranged in the exhaust passage. Further, an EGR passage for returning a part of the exhaust gas to the intake passage is connected to the exhaust passage and the intake passage, respectively. An EGR valve 52 for adjusting the amount of exhaust gas recirculation is interposed in the EGR passage (see FIG. 2).

The engine 1 includes an ECU (Engine Control Unit) 10 for operating the engine main body 2. The ECU 10 is a controller based on a well-known microcomputer. As shown in FIG. 4, the ECU 10 is composed of a central processing unit (CPU) 101 for executing a program, and for example, a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores programs and data. It includes a memory 102 for storing and an input / output bus 103 for inputting / outputting electric signals.

The throttle valve 51, the EGR valve 52, the injector 6, the plasma generator 49, the water injection valve 61, the intake valve mechanism 23, and the exhaust valve mechanism 24 are each connected to the ECU 10. At least sensors 71 to 74 are also connected to the ECU 10. Each of the sensors 71 to 74 outputs a detection signal to the ECU 10.

The sensor includes an airflow sensor 71 arranged in the intake passage. The air flow sensor 71 detects the flow rate of fresh air flowing through the intake passage.

The sensor includes a water temperature sensor 72 attached to the engine body 2 and a crank angle sensor 73. The water temperature sensor 72 detects the temperature of the cooling water. The crank angle sensor 73 detects the rotation angle of the crankshaft.

The sensor includes an accelerator opening sensor 74 attached to the accelerator pedal mechanism. The accelerator opening sensor 74 detects the accelerator opening.

Based on these detection signals, the ECU 10 determines the operating state of the engine body 2 and calculates the control amount of each device. The ECU 10 outputs a control signal related to the calculated control amount to the throttle valve 51, the EGR valve 52, the injector 6, the plasma generator 49, the water injection valve 61, the intake valve mechanism 23, and the exhaust valve mechanism 24.

<Engine operation control>
FIG. 9 illustrates an operating region of the engine 1 determined by the engine speed (horizontal axis) and the engine load (vertical axis). The engine 1 is configured to burn the air-fuel mixture in the combustion chamber 17 by compression ignition in the entire operating region. The engine 1 generates low-temperature plasma in the combustion chamber 17 so that the air-fuel mixture is stably compressed and ignited. The electrons generated by the low temperature plasma discharge chemically react with oxygen, nitrogen, etc. in the combustion chamber 17, and chemical species such as ozone or O radicals are generated in the combustion chamber 17. Then, the ignitability of the air-fuel mixture is improved by promoting the low-temperature oxidation reaction of the air-fuel mixture with chemical species such as ozone or O radicals.

Here, when high temperature plasma is generated in the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17, the gas temperature in the vicinity of the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17 rises. It gets higher and the combustion temperature gets higher. When low-temperature plasma is generated in the combustion chamber 17, the gas temperature near the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17 is suppressed from rising, and the combustion temperature can be lowered. This is advantageous in reducing the cooling loss.

Therefore, by generating low temperature plasma in the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17, the thermal efficiency due to the compression ignition combustion is improved, the cooling loss is reduced, and the thermal efficiency of the engine 1 is greatly improved. Can be improved.

<Generation of low temperature plasma>
FIG. 5 shows the time from the start of discharge (horizontal axis) at which low-temperature plasma and high-temperature plasma are generated when the inside of the combustion chamber 17 is at atmospheric pressure or above atmospheric pressure, and the voltage applied between the electrodes (vertical axis). ) Is shown. The low temperature plasma shortens the time from the start of discharge (that is, applies an ultrashort pulse voltage between the electrodes) and / or applies a high voltage between the electrodes, as shown in the region 5A surrounded by a broken line in FIG. Although it can be generated by applying the voltage to the plasma generator, the cost of the plasma generator for realizing the voltage increases and the power consumption also increases. An ultrashort pulse voltage is applied between the electrodes when trying to generate low temperature plasma in the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17 in order to improve the fuel efficiency of the automobile. And / or a plasma generator that applies a high voltage between the electrodes is disadvantageous for mounting in an automobile.

As shown in the region 5B surrounded by the broken line in FIG. 5, the low temperature plasma can be generated even when the time from the start of discharge is relatively long and / or the voltage is relatively low. Then, it is difficult to stably generate low temperature plasma.

Therefore, the plasma generation method disclosed here first generates high-temperature plasma in the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17 (see the region of reference numeral 5C in FIG. 5). As a result, thermions and high-temperature radicals remain between the electrodes, and in that state, as shown by reference numeral 5B, a predetermined high-frequency, relatively low voltage is applied between the electrodes to generate low-temperature plasma. Generate (see the white arrow in FIG. 5).

By doing so, low-temperature plasma can be stably generated without applying an ultrashort pulse voltage between the electrodes or applying a high voltage between the electrodes. Further, since it is not necessary to apply an ultrashort pulse voltage between the electrodes or a high voltage between the electrodes, the cost of the plasma generator is reduced. Further, since the power consumption at the time of generating the low temperature plasma is low, it is possible to prevent the fuel consumption performance from being deteriorated when the low temperature plasma is mounted on the automobile.

Specifically, FIG. 6 illustrates a time waveform of a voltage applied between the electrodes of the discharge electrode 4 by the application unit 45. As illustrated in FIG. 6, the application unit 45 first applies a first voltage higher than a predetermined voltage between the electrodes of the discharge electrode 4 (that is, the first step). Here, the predetermined voltage is a voltage determined by Paschen's law. Paschen's law is the experimental side regarding the voltage at which dielectric breakdown occurs between electrodes, and as shown in FIG. 7, the predetermined voltage determined by Paschen's law is the distance between the electrodes (that is, the gap length) and the combustion chamber 17 It is a function of the product of the pressure inside. According to the Paschen curve, the longer the distance between the electrodes and / or the higher the pressure, the higher the predetermined voltage. By applying a first voltage between the electrodes, which is higher than the voltage determined by Paschen's law (see 5C in FIG. 5), dielectric breakdown occurs between the electrodes, and high-temperature plasma can be generated in the combustion chamber 17. The first voltage may be, for example, 5 kV or more when the geometric compression is set to 15 or more and the first voltage is applied between the electrodes during the latter half of the compression stroke, as will be described later.

Next, after applying the first voltage between the electrodes, the application unit 45 applies a second voltage lower than the first voltage between the electrodes of the discharge electrode 4 at a predetermined high frequency (that is, the first voltage). 2 steps). The second voltage is a voltage at which low-temperature plasma is generated, and a predetermined high frequency is also a frequency at which low-temperature plasma is generated. The second voltage may be a voltage 0.5 kV or more lower than the first voltage. Further, the predetermined high frequency may be a frequency of 10 kHz or more and 1000 kHz or less. By applying a second voltage between the electrodes at a predetermined high frequency, low temperature plasma can be generated in the combustion chamber 17 (see 5B in FIG. 5). The application of the second voltage may be continued for a predetermined period.

The first voltage may be applied between the electrodes at least once. Here, if the pressure in the combustion chamber 17 is high, it becomes difficult to discharge between the electrodes. Therefore, depending on the pressure in the combustion chamber 17, when the pressure is high, the number of times the first voltage is applied may be two or more as illustrated by the two-dot chain line in FIG. By doing so, high-temperature plasma can be stably generated in the gap between the center electrode 41 and the ground electrode 42 in the combustion chamber 17, and then, by applying a second voltage between the electrodes, a low temperature can be generated. It becomes possible to stably generate plasma. When the number of times the first voltage is applied is two or more, the application frequency of the first voltage may be the same as or substantially the same as the application frequency of the second voltage, or the application frequency of the second voltage. The frequency may be lower than.

The number of times the first voltage is applied is the pressure in the combustion chamber 17 so that when the pressure in the combustion chamber 17 is higher than the predetermined pressure, the number of times the first voltage is applied is larger than when the pressure is lower than the predetermined pressure. It may be changed stepwise with respect to high and low. The number of steps for changing the number of times the first voltage is applied is arbitrary. Further, the number of times the first voltage is applied may be continuously changed with respect to the height of the pressure in the combustion chamber 17 so as to increase as the pressure in the combustion chamber 17 increases.

Further, when the pressure in the combustion chamber 17 is high, instead of or increasing the number of times the first voltage is applied, the first voltage may be further increased. By doing so, high-temperature plasma can be stably generated in the combustion chamber 17, and then, by applying a second voltage between the electrodes, low-temperature plasma can be stably generated.

The first voltage is changed stepwise with respect to the pressure in the combustion chamber 17 so that when the pressure in the combustion chamber 17 is higher than the predetermined pressure, it becomes higher than when the pressure is lower than the predetermined pressure. May be good. The number of steps for changing the first voltage is arbitrary. Further, the first voltage may be continuously changed with respect to the height of the pressure in the combustion chamber 17 so as to increase as the pressure in the combustion chamber 17 increases.

Further, when generating the low temperature plasma, as shown in FIG. 6, the plasma generator 49 may switch the voltage applied between the electrodes from the first voltage to the second voltage and apply the voltage, for example. As illustrated in FIG. 8, the voltage applied between the electrodes may be gradually lowered from the first voltage to the second voltage. By providing a transition period for lowering the voltage, the plasma generated in the combustion chamber 17 can be transferred from the high temperature plasma to the low temperature plasma.

<Plasma generation and fuel and water supply during engine operation>
Next, the plasma generation timing, the fuel injection timing, and the water injection timing during engine operation will be described with reference to FIGS. 9 and 10. As described above, the engine 1 is configured to burn the air-fuel mixture in the combustion chamber 17 by compression ignition in the entire operating region.

The horizontal axis of FIG. 10 indicates the crank angle. The upper and lower views of FIG. 10 exemplify the change in pressure in the combustion chamber 17 with the progress of the crank angle, respectively. In the upper figure of FIG. 10, the engine 1 is operated in the first operation state of relatively low load and medium rotation (see FIG. 9, corresponding to low load operation when operating at a low load of a predetermined load or less). The lower figure of FIG. 10, which exemplifies the plasma generation timing and the fuel injection timing at the time of the operation, shows a relatively high load and a second operating state of medium rotation (higher than a predetermined load). Illustrates the plasma generation timing, the fuel injection timing, and the water injection timing when the engine 1 is operating under high load operation (corresponding to high load operation).

First, when the engine 1 is operating in the first operating state, the load is relatively low, so that the amount of fuel supplied to the combustion chamber is small. Therefore, abnormal combustion such as premature ignition and knocking is unlikely to occur. Therefore, when the engine 1 is operating in the first operating state, the plasma generation timing is set earlier than when the load is high, which will be described later. Specifically, the plasma generation device 49 generates low-temperature plasma in the combustion chamber 17 at an arbitrary timing within the period from the end of the intake stroke to the initial period of the compression stroke. Here, the end of the inspiratory stroke may be the end of the inspiratory stroke when it is divided into three equal parts (that is, −240 ° to −180 ° ATDC). Further, the initial stage of the compression stroke may be the initial stage when the compression stroke is divided into three equal parts at the initial stage, the middle stage, and the final stage (that is, −180 ° to −120 ° ATDC). By generating the low temperature plasma in the combustion chamber 17 at a relatively early timing, the low temperature plasma diffuses throughout the combustion chamber 17.

After the low temperature plasma is generated, the injector 6 injects fuel into the combustion chamber 17 near the compression top dead center. By delaying the fuel injection timing to near the compression top dead center, it is possible to avoid the occurrence of abnormal combustion. As described above, by generating the low temperature plasma in the combustion chamber 17, the low temperature oxidation reaction of the air-fuel mixture is promoted, so that the ignitability of the air-fuel mixture is enhanced. As a result, the air-fuel mixture is stably burned by compression ignition near the compression top dead center.

When the engine 1 is operating in the first operating state, water is not injected by the water injection valve 61. This is because abnormal combustion is unlikely to occur when the load of the engine 1 is low, and there is little need to prevent the occurrence of abnormal combustion by injecting water.

On the other hand, when the engine 1 is operating in the second operating state, the load is relatively high, so that abnormal combustion such as premature ignition and knocking is likely to occur. Therefore, when the engine 1 is operating in the second operating state, the plasma generation timing is delayed as compared with the case of low load. Specifically, the plasma generation device 49 generates plasma in the combustion chamber 17 in the latter half of the compression stroke. Here, the latter half of the compression stroke means the period of the latter half when the compression stroke is bisected into the first half and the second half (that is, −90 ° to 0 ° ATDC). In order to avoid abnormal combustion, the plasma generator 49 may generate low temperature plasma at the end of the compression stroke, as shown in FIG. Here, the end of the compression stroke means the end period when the compression stroke is divided into three equal parts: the initial stage, the middle stage, and the final stage (that is, −60 ° to 0 ° ATDC). As the piston 3 rises toward the top dead center in the compression stroke, the pressure in the combustion chamber 17 gradually increases. Therefore, when the plasma generation timing is delayed, the low temperature plasma generated in the combustion chamber 17 is generated in the discharge electrode. Close, in other words, gather in the central part of the combustion chamber 17. It is advantageous in avoiding abnormal combustion.

However, if the plasma generation start timing is too late in the compression stroke, the pressure in the combustion chamber 17 is high, which is disadvantageous for discharge. In particular, since the engine 1 has a high geometric compression ratio, the pressure in the combustion chamber 17 becomes considerably high as the compression top dead center is approached.

Therefore, the start of plasma generation may not be slower than −30 ° ATDC. When the plasma generation is started before −30 ° ATDC, high temperature plasma can be generated when the pressure in the combustion chamber 17 is relatively low, and then low temperature plasma can be generated. If the application of the second voltage between the electrodes is continued as it is, even if the pressure in the combustion chamber 17 increases as the crank angle progresses, the generation of low temperature plasma can be continued by applying a relatively low voltage. Can be done.

The plasma generation device 49 may also finish the generation of low temperature plasma before a predetermined period before the compression top dead center (TDC). As described above, the engine 1 generates chemical species such as ozone or O radicals by generating electrons by low temperature plasma discharge in the combustion chamber 17, and promotes the low temperature oxidation reaction of the air-fuel mixture. Therefore, the low temperature plasma is generated before the fuel is supplied into the combustion chamber 17. The plasma generator 49 may complete the generation of cold plasma, for example, by −10 ° ATDC.

The injector 6 injects fuel into the combustion chamber 17 after the low temperature plasma is generated. By generating low-temperature plasma in the combustion chamber 17, the low-temperature oxidation reaction of the air-fuel mixture is promoted, so that the ignitability of the air-fuel mixture is enhanced. As a result, the air-fuel mixture stably compresses and ignites near the compression top dead center and burns.

The water injection valve 61 injects water into the combustion chamber 17 after fuel injection by the injector 6. The water injection valve 61 may inject water into the combustion chamber 16 after the air-fuel mixture is ignited. As illustrated in the lower figure of FIG. 10, the water injection valve 61 may inject water into the combustion chamber 17 after the compression top dead center. The rapid progress of combustion due to the injection of water is suppressed, and abnormal combustion such as knocking is prevented from occurring. Therefore, when the engine 1 is operated with a high load, the improvement of the ignitability of the air-fuel mixture and the avoidance of abnormal combustion are compatible.

FIG. 11 shows a control flow related to plasma generation, fuel injection, and water injection during operation of the engine 1. The control flow is executed by the ECU 10. In step S1 after the start, the ECU 10 reads the detection signals of each sensor and the like to determine the operating state of the engine 1. In the following step S2, if the operating state of the engine 1 is the first operating state, the control process proceeds to step S3, and if the operating state of the engine 1 is the second operating state, the control process proceeds to step S5.

In step S3, the ECU 10 generates low-temperature plasma in the combustion chamber 17 by the plasma generator 49 at an arbitrary timing within the period from the end of the intake stroke to the beginning of the compression stroke. In a subsequent step S4, the ECU 10 injects fuel into the combustion chamber 17 by the injector 6 before the compression top dead center. The air-fuel mixture formed in the combustion chamber 17 is stably ignited and burned at a predetermined timing as a result of the low-temperature oxidation reaction being promoted by chemical species such as ozone and O radicals.

On the other hand, in step S5, the ECU generates low-temperature plasma in the combustion chamber 17 by the plasma generation device 49 at an arbitrary timing at the end of the compression stroke. In a subsequent step S6, the ECU 10 injects fuel into the combustion chamber 17 by the injector 6 before the compression top dead center. Also at this time, the air-fuel mixture formed in the combustion chamber 17 is stably ignited and burned at a predetermined timing as a result of the promotion of the low-temperature oxidation reaction by chemical species such as ozone and O radicals.

Then, in step S7, after the start of combustion, the ECU 10 injects water into the combustion chamber 17 by the water injection valve 61. Rapid combustion is prevented, and abnormal combustion such as knocking is prevented.

The technique disclosed herein is not limited to the above configuration. The above-mentioned plasma generator is not limited to the application of generating low-temperature plasma in the combustion chamber 17 of the engine 1 mounted on an automobile, but can be widely applied to other applications.

1 Engine 17 Combustion chamber 2 Engine body 4 Discharge electrode (electrode)
40 Discharge electrode (electrode)
49 Plasma generator (plasma generator)
6 Injector (fuel supply section)
61 Water injection valve (water supply unit)

Claims (3)

An engine body having a combustion chamber that repeats an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, and the air-fuel mixture in the combustion chamber burns by compression ignition.
A plasma generating unit that generates plasma in the combustion chamber by applying a voltage between the pair of electrodes arranged in the combustion chamber.
An injector that directly injects fuel into the combustion chamber,
A water supply unit that supplies water to the combustion chamber is provided.
The plasma generation unit generates low-temperature plasma in the combustion chamber at a predetermined timing in the period from the start of the intake stroke to the end of the compression stroke.
The injector injects fuel into the combustion chamber after the generation of the low temperature plasma.
After supplying the fuel, the water supply unit supplies water to the combustion chamber.
When the engine body is operated at a load higher than the predetermined load, the plasma generation unit generates low-temperature plasma in the latter half of the compression stroke, and the engine body is operated at a low load equal to or lower than the predetermined load. When present, cold plasma is generated during the period from the end of the intake stroke to the beginning of the compression stroke, at least within the period including the compression stroke .
However, the latter half of the compression stroke is the latter half when the compression stroke is bisected into the first half and the second half, and the end of the intake stroke is when the intake stroke is bisected into the initial, middle, and final stages. The control device for the premixed compression ignition engine, which is the final stage of the compression stroke, which is the initial stage when the compression stroke is divided into three equal parts at the initial stage, the middle stage, and the final stage . In the control device for the premixed compression ignition engine according to claim 1.
The water supply unit is a control device for a premixed compression ignition engine that supplies water to the combustion chamber after the air-fuel mixture in the combustion chamber is ignited. In the control device for the premixed compression ignition engine according to claim 1 or 2.
The water supply unit supplies water to the combustion chamber when the engine body is operating at a load higher than the predetermined load, and the engine body is operated at a low load equal to or lower than the predetermined load. A control device for a premixed compression ignition engine that does not supply water to the combustion chamber when the engine is in operation.

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