JP7003731B2 - Low temperature plasma generation method and compression ignition engine - Google Patents

Description

The techniques disclosed herein relate to low temperature plasma generation methods and compression ignition engines.

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.

Japanese Unexamined Patent Publication No. 2014-141919

By the way, when trying to generate low temperature plasma in a predetermined volume chamber such as a combustion chamber of an engine, an ultrashort pulse voltage and / or a high voltage must be applied between the electrodes arranged in the volume chamber. .. As the cost of the plasma generator increases, so does the power consumption.

Therefore, for example, even if an attempt is made to generate low-temperature plasma in the combustion chamber of an engine mounted on an automobile, it is difficult to mount a high-cost plasma generator on the automobile, and even if the plasma generator is mounted on the automobile, it is consumed. Since the power consumption is large, the fuel efficiency of the automobile is deteriorated.

The techniques disclosed herein facilitate the generation of cold plasmas.

The low temperature plasma can be generated by applying a voltage having a wavelength longer than that of an ultrashort pulse and / or a relatively low voltage between the electrodes, but it is difficult to generate it stably. On the other hand, high temperature plasma (or thermal plasma) can be stably generated by applying a relatively long pulse and a relatively high voltage between the electrodes. The inventors of the present application paid attention to this point and repeated diligent studies. As a result, the inventors of the present application apply a relatively high voltage between the electrodes at the time of the first discharge between the electrodes to cause insulation failure between the electrodes and generate a high temperature plasma, so that heat is generated between the electrodes. Electrons and high-temperature radicals will remain, and then a voltage with a longer wavelength than the ultra-short pulse and / or a relatively low voltage can be applied between the electrodes to stably generate low-temperature plasma. I found out what I could do.

Specifically, the technique disclosed herein relates to a method for generating low temperature plasma. The method for generating low-temperature plasma is to apply a first voltage between the electrodes arranged in the volumetric chamber, which is a voltage determined by Paschen's law and is equal to or higher than the minimum voltage at which dielectric breakdown occurs between the electrodes . It includes one step and, following the first step, a second step of applying a second voltage lower than the first voltage between the electrodes at a predetermined high voltage.

According to this configuration, in the first step, a first voltage higher than the voltage determined by Paschen's law is applied between the electrodes arranged in the volumetric chamber. Paschen's law is the experimental side of the voltage at which dielectric breakdown occurs between electrodes, and the voltage determined by Paschen's law is a function of the product of the distance between the electrodes and the pressure in the volume chamber. By applying a first voltage higher than the voltage determined by Paschen's law between the electrodes, dielectric breakdown occurs between the electrodes and high-temperature plasma is generated.

The second step is performed following the first step. In the second step, a second voltage lower than the first voltage is applied between the electrodes at a predetermined high frequency. Since dielectric breakdown occurs between the electrodes, low-temperature plasma can be stably generated by applying a relatively low second voltage between the electrodes at a predetermined high frequency. The "second voltage" can be appropriately determined as a voltage capable of generating low temperature plasma, and the "predetermined high frequency" can be appropriately determined as a frequency capable of generating low temperature plasma. Further, the second step may be performed while thermions and high-temperature radicals remain between the electrodes. That is, the second step may be started immediately after the first step, or may be started after a slight rest period after the first step.

According to this method of generating low temperature plasma, it is not necessary to apply an ultrashort pulse voltage and / or a high voltage between the electrodes in order to generate low temperature plasma. The cost of the plasma generator can be reduced, and the power consumption during low temperature plasma generation can also be reduced.

In the second step, the voltage may be applied between the electrodes by switching from the first voltage to the second voltage.

By doing so, low temperature plasma can be stably generated.

In the second step, the voltage applied between the electrodes may be gradually lowered from the first voltage to the second voltage.

That is, even if a transition period for reducing the voltage from the first voltage to the second voltage is provided, the plasma generated between the electrodes can be stably transferred from the high temperature plasma to the low temperature plasma.

In the first step, the first voltage is applied at least once between the electrodes . That is, the first voltage may be applied between the electrodes only once and then the second voltage may be applied between the electrodes, or the first voltage may be applied between the electrodes a plurality of times and then the second voltage may be applied to the electrodes. It may be applied in between.

When the pressure in the volume chamber is high, the number of times the first voltage is applied between the electrodes is larger than when the pressure is low .

When the pressure in the volume chamber becomes high, it becomes difficult to break down the insulation between the electrodes. When the pressure in the volume chamber is high, the first voltage is applied between the electrodes more often than when the pressure is low. As a result, after the high temperature plasma is appropriately generated, the low temperature plasma can be stably generated by applying the second voltage.

The number of times the first voltage is applied is such that when the pressure in the volume chamber 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 step by step. The number of steps for changing the number of applications 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 volume chamber so that the number of times the first voltage is applied increases as the pressure in the volume chamber increases.

As the pressure in the volume chamber increases, the first voltage increases according to Paschen's law.

The volume chamber is a combustion chamber of an engine having a geometric compression ratio of 15 or more, and the engine generates chemical species such as ozone or O radicals by low-temperature plasma generated in the combustion chamber and also produces chemical species such as ozone or O radicals in the combustion chamber. The air-fuel mixture may be burned by compression ignition.

By generating chemical species such as ozone or O radicals by low temperature plasma, the low temperature oxidation reaction of the air-fuel mixture in the combustion chamber is promoted. As a result, the ignitability of the air-fuel mixture is improved, so that the air-fuel mixture in the combustion chamber can be stably burned by compression ignition. Further, by generating low-temperature plasma in the combustion chamber, it is possible to suppress the increase in the gas temperature in the combustion chamber, so that the combustion temperature can be lowered. As a result, the cooling loss is reduced, which is advantageous for improving the fuel efficiency of the engine.

The engine is a four-stroke engine in which the combustion chamber is operated by repeating an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. During the period, low temperature plasma may be generated in the combustion chamber.

By setting the generation timing of the low temperature plasma to a relatively late timing in the latter half of the compression stroke, it is possible to suppress the occurrence of premature ignition of the air-fuel mixture and abnormal combustion such as knocking. This configuration is particularly effective in suppressing the occurrence of abnormal combustion when the engine is operating at a high load and the amount of fuel supplied to the combustion chamber is large.

The first voltage may be 5 kV or more.

The second voltage may be 0.5 kV or more lower than the first voltage.

In the second step, the second voltage may be applied between the electrodes at a frequency of 10 kHz or more and 1000 kHz or less.

Further, the technique disclosed herein relates to a compression ignition type engine. The compression ignition type engine has a combustion chamber that repeats an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, a pair of electrodes arranged in the combustion chamber, and a control unit that applies a voltage between the electrodes. The control unit has a voltage determined by Paschen's law, which is lower than the first voltage after applying a first voltage between the electrodes, which is equal to or higher than the minimum voltage at which insulation breakage occurs between the electrodes. By applying a second voltage between the electrodes at a predetermined high voltage, a low temperature plasma is generated in the combustion chamber before the air-fuel mixture in the combustion chamber is burned by compression ignition.

According to this configuration, when the control unit applies the first voltage between the electrodes, dielectric breakdown occurs between the electrodes and high-temperature plasma is generated in the combustion chamber.

Further, the control unit applies the first voltage between the electrodes and then applies the second voltage between the electrodes at a predetermined high frequency, whereby low temperature plasma can be stably generated in the combustion chamber. ..

By generating low-temperature plasma in the combustion chamber before the air-fuel mixture in the combustion chamber is burned by compression ignition, chemical species such as ozone and / or O radicals are generated in the combustion chamber. Since the ignitability of the air-fuel mixture is enhanced, the air-fuel mixture can be stably burned by compression ignition. Further, by generating low temperature plasma, it is possible to suppress the increase in gas temperature in the combustion chamber. Since the combustion temperature can be lowered, the cooling loss is reduced. As a result, the compression ignition type engine having the above configuration improves the thermal efficiency of the engine.

Further, since the cost of the device for generating low temperature plasma can be reduced and the power consumption at the time of generating low temperature plasma can be reduced, if the engine is mounted on the automobile, the fuel efficiency of the automobile is improved.

The control unit may switch from the first voltage to the second voltage and apply a voltage between the electrodes.

The control unit may gradually reduce the voltage applied between the electrodes from the first voltage to the second voltage.

The control unit may apply the first voltage at least once between the electrodes.

The control unit generates low-temperature plasma in the combustion chamber during the latter half of the period when the compression stroke is bisected into the first half and the second half .

When the timing of applying the first voltage is late, the control unit applies the first voltage between the electrodes more times than when the timing of applying the first voltage is early .

If the application timing of the first voltage is late in the latter half of the compression stroke, the pressure in the combustion chamber is high, so that it becomes difficult to discharge between the electrodes. When the application timing of the first voltage is late, by increasing the number of times the first voltage is applied between the electrodes, it is possible to appropriately generate the high temperature plasma in the combustion chamber and then stably generate the low temperature plasma.

The number of times the first voltage is applied is such that when the application timing is later than the predetermined timing, the number of times the first voltage is applied is larger than when the predetermined timing or earlier than the predetermined timing is applied. It may be changed step by step. The number of steps for changing the number of applications is arbitrary. Further, the number of times the first voltage is applied may be continuously changed with respect to the delay of the application timing so that the application timing increases as the application timing becomes later.

When the application timing of the first voltage is late, the control unit may raise the first voltage higher than when it is early.

Similar to the above, when the application timing of the first voltage is late and the pressure in the combustion chamber is high, the first voltage applied between the electrodes is increased to appropriately generate high-temperature plasma in the combustion chamber, and then the plasma is appropriately generated. Low temperature plasma can be generated stably.

The first voltage is changed stepwise with respect to the delay of the application timing so that the first voltage becomes higher when the application timing is later than the predetermined timing or earlier than the predetermined timing or earlier than the predetermined timing. You may. The number of steps for changing the first voltage is arbitrary. Further, the first voltage may be continuously changed with respect to the delay of the application timing so that the first voltage increases as the application timing becomes later.

The geometric compression ratio of the engine may be 15 or more.

The first voltage may be 5 kV or more.

The second voltage may be 0.5 kV or more lower than the first voltage.

The control unit may apply the second voltage between the electrodes at a frequency of 10 kHz or more and 1000 kHz or less.

According to the above-mentioned method for generating low temperature plasma and the compression ignition type engine, it is possible to facilitate the generation of low temperature plasma.

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 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. FIG. 10 is a flowchart illustrating control related to the generation of low temperature plasma of the engine.

Hereinafter, a method for generating low-temperature plasma and an exemplary embodiment of a 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. The combustion chamber 17 is an example of a volumetric chamber.

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 "the inside of 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 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 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 a program 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 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 intake valve mechanism 23, and the exhaust valve mechanism 24.

<Engine operation control>
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. When the low temperature plasma chemically reacts with oxygen in the combustion chamber 17, 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 by chemical species such as O radicals.

Here, when high-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 becomes high, and the combustion temperature becomes high. 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 the low temperature plasma in the combustion chamber 17, the thermal efficiency due to the compression ignition combustion can be improved, the cooling loss can be reduced, and the thermal efficiency of the engine 1 can be significantly 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. When trying to generate low temperature plasma in the combustion chamber 17 in order to improve the fuel efficiency of the automobile, an ultrashort pulse voltage is applied between the electrodes and / or a high voltage is applied between the electrodes. Plasma generators are disadvantageous for mounting in automobiles.

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 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 higher than the voltage determined by Paschen's law between the electrodes (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. When 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 combustion chamber 17, and then, by applying a second voltage between the electrodes, low-temperature plasma can be stably generated. 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.

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.

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 during engine operation>
Next, the plasma generation timing and the fuel injection timing during engine operation will be described with reference to FIG. 9. The horizontal axis of FIG. 9 indicates the crank angle. FIG. 9 illustrates a change in pressure in the combustion chamber 17 as the crank angle progresses. It is assumed that the engine 1 is operated with a relatively high load higher than a predetermined load.

The plasma generator 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). Since the load of the engine 1 is high, the amount of fuel supplied into the combustion chamber 17 is large. If the timing of generating low-temperature plasma in the combustion chamber 17 is too early, abnormal combustion such as premature ignition and knocking may occur. Therefore, the low temperature plasma may be generated in the combustion chamber 17 in the latter half of the compression stroke. Further, in order to avoid abnormal combustion, the plasma generator 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).

However, in the compression stroke, the pressure in the combustion chamber 17 gradually increases as the piston 3 rises toward the top dead center, but if the plasma generation start timing is too late, the pressure in the combustion chamber 17 increases. Therefore, it is disadvantageous for discharging. 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 a chemical species such as ozone or O radical by generating a low temperature plasma 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 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 stably compresses and ignites near the compression top dead center and burns.

FIG. 10 shows a control flow related to the generation of low temperature plasma during the 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, the ECU 10 sets the generation time of the low temperature plasma according to the operating state of the engine 1. For example, when the load of the engine 1 is high, the generation time of the low temperature plasma may be delayed in order to avoid abnormal combustion. On the other hand, when the load of the engine 1 is low, abnormal combustion is unlikely to occur, so that the low temperature plasma may be generated earlier.

In step S3, the ECU 10 determines whether or not the generation start time of the low temperature plasma is late. When the determination is YES, the control process proceeds to step S4, and when the determination is NO, the control process proceeds to step S5.

In step S4, since the low temperature plasma generation start time is late, the pressure in the combustion chamber 17 is relatively high, so that the ECU 10 raises the first voltage and / or increases the number of times the first voltage is applied. .. As a result, high-temperature plasma is stably generated, and then low-temperature plasma is stably generated. On the other hand, in step S5, since the low temperature plasma generation start time is early, the pressure in the combustion chamber 17 is relatively low, so that the ECU 10 lowers the first voltage and / or the number of times the first voltage is applied. To reduce. As a result, low-temperature plasma can be stably generated and power consumption can be reduced.

The first voltage and / or the number of times the first voltage is applied is earlier than the predetermined time or the predetermined time when the plasma generation start time (that is, the application timing of the first voltage) is later than the predetermined time. The first voltage and / or the number of times the first voltage is applied may be changed stepwise with respect to the delay of the start time of plasma generation so as to be higher / higher than the time. The number of steps for changing the first voltage and / or the number of times the first voltage is applied is arbitrary. Further, the first voltage and / or the number of times the first voltage is applied is continuously changed with respect to the delay of the start time of plasma generation so as to increase / increase as the start time of plasma generation becomes late. May be good.

The ECU 10 and the application unit 45 apply the first voltage between the electrodes of the discharge electrode 40 in step S5, and apply the second voltage between the electrodes of the discharge electrode 40 in step S6.

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 10 ECU (control unit)
17 Combustion chamber 2 Engine body 4 Discharge electrode (electrode)
40 Discharge electrode (electrode)
45 Applying unit (control unit)
46 Applying unit (control unit)
49 Plasma generator 6 Injector (fuel supply unit)

Claims (17)

The first step of applying a first voltage between the electrodes arranged in the volume chamber, which is a voltage determined by Paschen's law and is equal to or higher than the minimum voltage at which dielectric breakdown occurs between the electrodes .
Following the first step, a second step of applying a second voltage lower than the first voltage between the electrodes at a predetermined high frequency is provided .
In the first step, the first voltage is applied at least once between the electrodes.
A method for generating low-temperature plasma in which the number of times the first voltage is applied between the electrodes is larger when the pressure in the volume chamber is high than when the pressure is low . In the method for generating low temperature plasma according to claim 1,
The second step is a method of generating low-temperature plasma by switching from the first voltage to the second voltage and applying a voltage between the electrodes. In the method for generating low temperature plasma according to claim 1,
The second step is a method for generating low-temperature plasma in which the voltage applied between the electrodes is gradually lowered from the first voltage to the second voltage. The method for generating low-temperature plasma according to any one of claims 1 to 3 .
The volume chamber is a combustion chamber of an engine having a geometric compression ratio of 15 or more.
The engine is a method of generating low-temperature plasma in which ozone and / or O radicals are generated by the low-temperature plasma generated in the combustion chamber and the air-fuel mixture in the combustion chamber is burned by compression ignition. In the method for generating low temperature plasma according to claim 4 ,
The engine is a 4-stroke engine in which the combustion chamber is operated by repeating an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke.
The engine is a method for generating low-temperature plasma that generates low-temperature plasma in the combustion chamber during the latter half of the period when the compression stroke is bisected into the first half and the second half. The method for generating low-temperature plasma according to any one of claims 1 to 5 .
A method for generating low-temperature plasma in which the first voltage is 5 kV or more. In the method for generating low temperature plasma according to claim 6 ,
The method for generating low-temperature plasma in which the second voltage is 0.5 kV or more lower than the first voltage. The method for generating low-temperature plasma according to any one of claims 1 to 7 .
The second step is a method for generating low-temperature plasma in which the second voltage is applied between the electrodes at a frequency of 10 kHz or more and 1000 kHz or less. A combustion chamber that repeats the intake stroke, compression stroke, expansion stroke, and exhaust stroke,
A pair of electrodes arranged in the combustion chamber and
A control unit for applying a voltage between the electrodes is provided.
The control unit is a voltage determined by Paschen's law, and is lower than the first voltage after applying a first voltage between the electrodes, which is equal to or higher than the minimum voltage at which dielectric breakdown occurs between the electrodes. By applying a voltage between the electrodes at a predetermined high frequency, a low temperature plasma is generated in the combustion chamber before the air-fuel mixture in the combustion chamber is burned by compression ignition.
The control unit generates low-temperature plasma in the combustion chamber during the latter half of the period when the compression stroke is bisected into the first half and the second half.
The control unit is a compression ignition type engine in which when the application timing of the first voltage is late, the number of times the first voltage is applied between the electrodes is larger than when the application timing is early . In the compression ignition type engine according to claim 9 .
The control unit is a compression ignition engine that switches from the first voltage to the second voltage and applies a voltage between the electrodes. In the compression ignition type engine according to claim 9 .
The control unit is a compression ignition engine that gradually reduces the voltage applied between the electrodes from the first voltage to the second voltage. In the compression ignition type engine according to any one of claims 9 to 11 .
The control unit is a compression ignition engine that applies the first voltage at least once between the electrodes. In the compression ignition type engine according to claim 9 .
The control unit is a compression ignition type engine in which when the application timing of the first voltage is late, the first voltage is higher than when it is early. In the compression ignition type engine according to any one of claims 9 to 13 .
A compression ignition engine with a geometric compression ratio of 15 or more. In the compression ignition type engine according to any one of claims 9 to 14 ,
The first voltage is a compression ignition engine having a voltage of 5 kV or more. In the compression ignition type engine according to claim 15 ,
The second voltage is a compression ignition engine that is 0.5 kV or more lower than the first voltage. In the compression ignition type engine according to any one of claims 9 to 16 .
The control unit is a compression ignition engine in which the second voltage is applied between the electrodes at a frequency of 10 kHz or more and 1000 kHz or less.

Images