JP6284141B2 - Combustion control system for homogeneous premixed compression auto-ignition engine - Google Patents

Description

  The present invention relates to a combustion control device for a homogeneous premixed compression auto-ignition engine (hereinafter referred to as “HCCI engine”), and more particularly to a combustion control device for controlling ignition timing and pressure increase rate.

  The HCCI engine presupposes that fuel is supplied as a premixed gas as in the gasoline engine, but does not use a spark plug and starts combustion by self-ignition during the compression stroke. It is expected as a future engine that can achieve both high performance and high efficiency equivalent to a diesel engine.

In Patent Document 1, an oscillation signal is sent to a plasma generation unit that generates plasma with a minimum ignition energy or less in an engine cylinder, a microwave irradiation unit that irradiates the plasma with microwaves, a microwave irradiation unit, and a plasma generation unit. The control device includes an ignition timing signal transmission device that sends an ignition timing signal to the control device, and the control device performs plasma generation and microwave irradiation at a predetermined timing before the ignition timing indicated by the ignition timing signal. Thus, a homogeneous premixed compression auto-ignition engine is described that generates plasma in a cylinder.
In Patent Document 2, an intake port, first fuel injection means for injecting a first fuel with lower ignitability into the intake port, and a second fuel with higher ignitability are directly injected into the combustion chamber. A compression ignition internal combustion engine that includes a second fuel injection unit and compresses and self-ignites a fuel mixture containing fuel supplied from both fuel injection units is described. This compression ignition internal combustion engine includes ozone supply means for supplying ozone generated by the ozone generation means to the intake port.

JP 2009-36201 A JP 2011-157940 A

In the HCCI engine, since the start of combustion depends on the progress of the chemical reaction, the ignition timing largely fluctuates with changes in the operating state represented by the rotation speed and load, and it is difficult to maintain stable operation. .
In particular, when the engine operates in a high load state, the rate of increase in pressure tends to increase rapidly, causing abnormal combustion such as knocking, and in the worst case, damage to the internal combustion engine.
For this reason, it has been studied to control the ignition timing according to the operating state by a mechanism such as increasing the intake air temperature, incorporating a mechanism that makes the valve timing variable, or adjusting the EGR amount.

  In addition, there is a method of irradiating microwaves as in Patent Document 1 or using a plurality of fuels and additives as in Patent Document 2, but the structure is complicated and causes an increase in cost. In addition, it is difficult to precisely control the ignition timing according to each operation state, and no investigation has been made on how to control the rate of pressure increase that causes abnormal combustion such as knocking.

  Accordingly, an object of the present invention is a combustion control apparatus for an HCCI engine, which is a low-cost device that can precisely control the ignition timing and effectively control the pressure increase rate in accordance with each operation state. Is to realize.

  In order to solve the above-described problems, the premixed compression auto-ignition (HCCI) engine of the present invention is provided in an air-fuel mixture supply system up to the intake port of each cylinder and discharges a pulsed discharge to the air-fuel mixture. A non-thermal equilibrium plasma generator that forms an easy ignition region in the air-fuel mixture by applying, a discharge drive circuit that applies a discharge voltage to the non-thermal equilibrium plasma generator, a crank angle sensor and a load sensor mounted on the engine And a control circuit that controls a discharge voltage, a discharge timing, and a discharge period output from the discharge drive circuit based on the detected value, and the control circuit has the easy ignition region based on the detected value of the crank angle sensor. The discharge timing is controlled so as to be sucked into the combustion chamber of each cylinder during the intake stroke of each cylinder, and the discharge voltage and the front are controlled according to the engine speed and load. By controlling at least one discharge period, so as to control the ignition timing and the pressure increase rate in each cylinder.

According to the present invention, the non-thermal equilibrium plasma generator generates an easy ignition region in synchronization with the crank angle by the discharge control device, and this is taken into the combustion chamber of each cylinder and starts combustion by self-ignition during the compression stroke. It becomes the core when doing. And by controlling the intensity and distribution of the easy ignition region in the combustion chamber according to the discharge voltage and discharge time applied to the non-thermal equilibrium plasma generator, the ignition timing and pressure rise rate in each cylinder can be reduced by adding low-cost equipment. It can be controlled effectively.
Note that the discharge control by the non-thermal equilibrium plasma generator may be performed not only on the air-fuel mixture but also on the intake air as described above.

FIG. 1 is a diagram illustrating a schematic diagram of an HCCI engine according to an embodiment. FIG. 2 is a diagram showing the relationship between the discharge timing by the non-thermal equilibrium plasma generator and the intake stroke of the engine. FIG. 3 is a diagram showing the relationship between the engine load and the engine speed and the control target values of the discharge voltage and discharge time by the non-thermal equilibrium plasma generator. FIG. 4 is a process flow diagram when feedback control is performed at the optimal combustion timing. FIG. 5 is a schematic diagram of the experimental apparatus. FIG. 6 is a diagram showing the structure of the dielectric barrier discharge used in the experiment. FIG. 7 is a diagram showing the difference in ignition characteristics depending on the presence or absence of discharge by the non-thermal equilibrium plasma generator. FIG. 8 is a diagram showing a difference in ignition characteristics due to a difference in discharge voltage between non-thermal equilibrium plasma generators. FIG. 9 is a diagram showing a difference in pressure rise rate characteristics due to a difference in application timing of the pulse discharge voltage by the non-thermal equilibrium plasma generator.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First, the basic principle of the present invention will be described.
FIG. 1 is a schematic diagram of an HCCI engine according to the present embodiment. A non-thermal equilibrium plasma generator 2 such as dielectric barrier discharge (DBD), streamer discharge, glow discharge is installed in an intake system upstream of an intake port 1. Then, non-thermal equilibrium plasma is generated with respect to the air-fuel mixture flowing inside or the intake air.
Non-thermal equilibrium plasma has a high electron temperature of several tens of thousands of degrees, but the generated ions, neutral molecules, and atoms are in a non-thermal equilibrium state where the temperature is almost room temperature. The temperature of the plasma generator 2 does not increase extremely. For this reason, normally, there is very little risk that the air-fuel mixture will ignite inside the intake system. However, in preparation for long-term operation, a cooling device may be incorporated in the non-thermal equilibrium plasma generator 2 to maintain it below a certain temperature. preferable.

A discharge drive circuit 3 for applying a discharge voltage to the non-thermal equilibrium plasma generator 2 is controlled by a control circuit 4 to which detection signals such as an engine load (accelerator opening degree, etc.), a crank angle, etc. are input, and the discharge voltage timing, discharge voltage, The discharge time is controlled.
Here, when a DBD or the like is used as the non-thermal equilibrium plasma generator 2, for example, a high-voltage sine wave of 60 kHz is discharged from the non-thermal equilibrium plasma generator at a predetermined timing with a predetermined time width by a control circuit. .

Next, a chemical change that the non-thermal equilibrium plasma gives to the air-fuel mixture and the intake air will be described.
(1) In the case of an air-fuel mixture When the air-fuel mixture flowing in the intake system is irradiated with non-thermal equilibrium plasma, a low-temperature oxidation reaction of some fuel molecules is induced or hydrogen is dissociated due to electron collision by the non-thermal equilibrium plasma. During the compression stroke, the low-temperature oxidation reaction is started early, and an easy ignition region serving as a nucleus of self-ignition is formed in the combustion chamber.
(2) Intake Air In the case of a direct injection engine or the like, non-thermal equilibrium plasma is irradiated to intake air that does not contain fuel, oxygen is dissociated from the plasma, and an ozone rich region is formed. This ozone region dissociates into oxygen radicals by heating accompanying the compression stroke and acts on the fuel molecules directly injected into the combustion chamber, so that an easy ignition region serving as a nucleus of self-ignition is formed in the combustion chamber.

  In order to cause the easy ignition region or ozone region formed by the intake system to be sucked into the combustion chamber during the intake stroke of each cylinder, as shown in FIG. 2, the timing for applying the discharge voltage to the non-thermal equilibrium plasma generator, Synchronize with the intake stroke of the cylinder.

When irradiating the mixture or intake air with non-thermal equilibrium plasma, for example, in synchronization with the intake stroke of cylinder # 1, the non-thermal equilibrium plasma generator is discharged to form an easy ignition region or ozone region in the mixture. This needs to be taken into the combustion chamber through the intake port and intake valve of cylinder # 1.
As shown in FIG. 2, the discharge timing is preferably matched with the timing so that the “easy ignition region” or “ozone region” is introduced into the combustion chamber immediately before the intake valve is closed.
This is because if the discharge timing is set in accordance with the opening of the intake valve, the easy ignition region or the ozone region may be dispersed due to the flow inside the cylinder, which may reduce the action as a core of self-ignition. It is.
In addition, since the easy ignition region is formed in a limited space region of each cylinder, it is necessary to devise not only the valve opening / closing timing but also the shape of the valve portion to prevent the easy ignition region from spreading fluidly.

Here, when the distance from the installation position of the non-thermal equilibrium plasma generator to the intake port of cylinder # 1 is L, and the average flow velocity of the air-fuel mixture during this time is V, the time until the easy ignition region reaches the intake port is , L / V. Therefore, it is necessary to start discharge by the non-thermal equilibrium plasma generator ahead of L / V from the timing when the intake valve is opened (crank angle sensor). For example, in order to inject the easy ignition region into the combustion chamber just before the end of the intake stroke (just before the intake valve closes), the discharge timing is calculated backward from the timing just before the end of the intake stroke.
However, since V is particularly related to the rotational speed and load, it is necessary to call V from a database determined in advance by experiments or the like to determine the discharge voltage application timing of the non-thermal equilibrium plasma generator in accordance with these parameters. .
The same applies to the case where the ozone region is formed by irradiating the intake air with non-thermal equilibrium plasma.

Next, how the ignition timing and the pressure increase rate are controlled by the discharge voltage and discharge time to the non-thermal equilibrium plasma generator will be described.
When the mixture is irradiated with non-thermal equilibrium plasma, when the discharge voltage by the non-thermal equilibrium plasma generator is high, the easy ignition region is activated and the ignition timing can be advanced.
When the discharge time by the non-thermal equilibrium plasma generator is long, the “easy ignition region” is expanded, so that the ignition timing can be advanced.
As described above, the easy ignition region generated by applying the discharge to the air-fuel mixture and irradiating the plasma is introduced into the cylinder, and during the compression stroke, the easy ignition region becomes a nucleus and self-ignition occurs.
As the rotational speed increases, the flow rate increases and the intake valve opening time decreases, so the energy density on the easy ignition region decreases, so to compensate for this and promote ignition, the discharge voltage is increased. There is a need.

On the other hand, in a high-load operation state, the rate of increase in pressure increases rapidly, and there is a high possibility that abnormal combustion such as knocking will occur, so it is necessary to shorten the application time of the discharge voltage.
This makes it possible to control the ignition timing to a desired value without localizing the easy ignition region and causing abnormal combustion.
Conversely, at low loads where self-ignition is unlikely to occur, or at start-up when the engine is at a low temperature, warm-up, etc., spread the easy ignition area to the entire combustion chamber to achieve the desired ignition. In order to obtain timing and stable combustion, it is necessary to lengthen the application time of the discharge voltage.
From the above, the discharge control parameters in the operation map with the horizontal axis representing the engine load and the vertical axis representing the rotational speed are set as shown in FIG.

Depending on the load and the number of rotations, the operating state can be broadly divided into four areas: low load low rotation area, low load high rotation area, high load low rotation area, and high load high rotation area. At the load, the premixed gas becomes lean, so that the ignition tends to be unstable and retarded. Therefore, by increasing the pulse width of the applied discharge voltage or applying a pulse in the first half of the intake stroke so that the entire chemical reaction proceeds, the easy ignition region is uniformly distributed throughout the cylinder. .
On the other hand, in the high rotation region, since the time from the intake valve closing to the compression and top dead center is shorter than in the low rotation region, the applied voltage is increased so that the chemical reaction proceeds faster.

In addition, in the high load region, the premixed gas becomes relatively dense, and thus there is a tendency to cause rapid combustion after compression self-ignition. Therefore, the pulse width of the discharge voltage is shortened so that the chemical reaction progresses from a spatially limited region, or the application timing of the discharge voltage is set at the end of the intake stroke in each cylinder (intake valve). The timing is retarded so that it is introduced into the combustion chamber at the closing timing).
In other words, it is basically necessary to increase the voltage for high revolutions and shorten the applied pulse width to increase the engine load. In actual operation, the intake air temperature may increase further. Basically, it leads to early ignition, so it can be dealt with by lowering the voltage.

  Therefore, the control circuit is first connected to a memory that records a map of the discharge timing (crank angle), discharge voltage, and discharge time in a database according to the number of revolutions and load, and the accelerator opening that is input , Throttle opening, intake air amount (Q) divided by rotation speed (N) (Q / N), information from load sensor represented by intake pipe negative pressure, detection signal of crank angle, etc. From these maps, the discharge timing, discharge voltage, and discharge time are called and the basic control amount is set.

In addition to this, during actual operation, in-cylinder pressure, misfire and knocking are detected, and in the case of misfire, the discharge time is increased, and when the ignition timing is late, the discharge voltage is increased. In addition, when knocking is detected, it is preferable to perform learning control such as correcting the control amount of the discharge voltage and discharge time or correcting the data of each map in a direction in which the applied pulse width is shortened. . For such learning correction, the oxygen concentration of the exhaust gas or a detection value of a knocking sensor or the like can be used.
Further, since the ignition timing changes depending on the temperature of the intake air, the pressure, etc. in addition to the cooling water temperature and the combustion chamber temperature, in addition to the basic control described above, as shown in the flowchart of FIG. Detected by the combustion chamber pressure sensor, crank angular velocity fluctuation rate, vibration sensor mounted on the engine, etc. and fed back to the optimal combustion timing, and learning control of the basic control amount using the obtained feedback control amount is adopted It is preferable to do.

Here, an experimental result using a rapid compression / expansion apparatus (RCEM) capable of reproducing the compression / expansion stroke of the engine will be described.
In this experimental device, as shown in FIG. 5, DBD-1 is upstream (intake air side) and downstream (air mixture side) of a vaporizer (a device that forms a mixture from measured intake air and fuel). DBD-2 was attached, and it was observed how the combustion changed by changing the presence / absence of discharge by each DBD, the discharge time, and the discharge timing.

  In this experiment, dielectric barrier discharge (DBD) is used as a non-thermal equilibrium plasma generator, and holes are formed in the copper mixture introducing pipe 5 connected to the upstream side and the downstream side of the vaporizer as shown in FIG. The simple thing which inserted silicon resin covering wiring 6 in about 5 cm length and sealed with the adhesive was used as DBD-1 and DBD-2 in FIG.

When a high voltage sine wave of about ± 3 kV and 60 kHz is applied to the silicon resin-coated wiring, the silicon resin becomes a dielectric barrier, and discharge is generated between the metal pipe and the silicon resin wiring.
In the DBD drive circuit, the transmitter operates by the input of the TTL signal and boosts it with a transformer to output a high voltage. The high voltage sine wave is continuously output in the time zone when the TTL signal is ON. Has been.

FIG. 7 shows the state of ignition depending on whether DBD-1 and DBD-2 are applied. In this experiment, in a rapid compression / expansion device, a cylinder having a capacity of 1 liter was filled for 1 minute in a state where the piston was at the bottom dead center, and then a compression stroke and an explosion stroke were performed.
The experimental conditions are as follows.
Fuel: normal heptane, compression ratio: 5.8, λ (air-fuel ratio): 2.6, initial temperature: 72 ° C
Under these experimental conditions, as a result of repeating the experiment of applying a discharge voltage of ± 2.5 kV to DBD1 (upstream of the vaporizer) over the entire filling time over the entire filling time by the rapid compression / expansion device, the pressure rise due to the start of combustion Is the range of the rising curve indicated by the two dotted lines on the left.
That is, in any case, a pressure increase accompanying ignition is started around 0.03 seconds, the pressure gradually decreases until it exceeds 0.05 (s), and then rapidly increases until it reaches 0.07 (s). It can be confirmed that the pressure decreased, the fluctuation between experiments was small, and a stable ignition timing and pressure increase rate were obtained.

  On the other hand, when no discharge voltage is applied to DBD 1, as shown by the solid line on the right side, the pressure increase at the start of combustion varies from 0.04 to 0.05 (s), and fluctuations are large between experiments, resulting in unstable ignition. It is the time and the rate of pressure increase.

Next, an experimental example using DBD2 is shown. The experimental condition is λ: 2.5 using the same fuel and compression ratio as before. FIG. 8 shows the relationship between the applied discharge voltage and the ignition timing / pressure waveform.
When the discharge voltage is 6.2 kVpp, the pressure rise accompanying the ignition occurs the fastest, and the ignition timing is around 0.042 (S), and when it is 5.9 kVpp, the ignition timing is slightly delayed.
When the discharge voltage is lowered to 5.4 kVpp, the ignition timing is around 0.05 (s), and when it is lowered to 5.0 kVpp, it is around 0.055 (s), and is thin when no discharge voltage is applied. As indicated by the dotted line, no ignition occurred.

  Next, FIG. 9 shows the relationship between the pulse discharge voltage application time and the rate of increase in the pressure waveform. The pressure waveform when a ± 3.2 kV discharge voltage is applied for 1 second 15 seconds before the completion of filling is indicated by a solid line, and the pressure waveform when irradiated for 1 second immediately before the completion of filling is indicated by a dotted line.

  When a discharge voltage is applied 15 seconds before the completion of filling, the pressure rises rapidly, and a vibration waveform peculiar to knocking appears clearly in the peak portion. This is because the easy ignition region was filled at an early stage, so that it was dispersed in the combustion chamber and combustion occurred simultaneously in various places.

  On the other hand, when the discharge voltage is applied immediately before the completion of filling, the pressure rises more slowly than when the discharge voltage is applied 15 seconds before the completion of filling, and the vibration waveform at the peak portion also has an amplitude. Is very small. This is because, since the easy ignition region is filled at the end of the intake stroke, stable combustion that spreads around the easy ignition region as a nucleus occurs without being dispersed in the combustion chamber.

  As described above, according to the present invention, the low-cost equipment addition that controls the discharge voltage and the discharge period to be applied to the non-thermal equilibrium plasma generator according to the engine speed and load can be achieved. By controlling the strength and distribution of the easy ignition region, the ignition timing and pressure rise rate in each cylinder can be controlled effectively, so it is expected to be widely adopted for the practical application of premixed compression ignition engines. it can.

DESCRIPTION OF SYMBOLS 1 Intake port 2 Non-thermal equilibrium plasma generator 3 Discharge drive circuit 4 Control circuit 5 Copper mixture introduction pipe 6 Silicon resin coating wiring

Claims (4)

In premixed compression ignition engines,
A non-thermal equilibrium plasma generator that is provided in an air-fuel mixture supply system up to the intake port of each cylinder and forms an easy ignition region in the air-fuel mixture by applying a pulsed discharge to the air-fuel mixture;
A discharge drive circuit for applying a discharge voltage to the non-thermal equilibrium plasma generator;
A control circuit that controls a discharge voltage, a discharge timing, and a discharge period output from the discharge drive circuit based on a detected value of a crank angle sensor and a load sensor mounted on the engine,
The control circuit controls the application timing of the discharge voltage with respect to the closing timing of the intake valve in accordance with the engine load. The higher the engine load, the more the application timing of the discharge voltage becomes higher in the easy ignition region. A premixed compression auto-ignition engine that is retarded so that it is introduced into the combustion chamber at a time close to the closing timing of the intake valve . In premixed compression ignition engines,
A non-thermal equilibrium plasma generator that is provided in an intake air supply system up to the intake port of each cylinder and forms an ozone region in the intake air by applying a pulsed discharge to the intake air;
A discharge drive circuit for applying a discharge voltage to the non-thermal equilibrium plasma generator;
A control circuit that controls a discharge voltage, a discharge timing, and a discharge period output from the discharge drive circuit based on a detected value of a crank angle sensor and a load sensor mounted on the engine,
The control circuit controls the application timing of the discharge voltage with respect to the closing timing of the intake valve in accordance with the engine load. The higher the engine load, the more the application timing of the discharge voltage becomes, A premixed compression auto-ignition engine that is retarded to be introduced into the combustion chamber at a time close to the valve closing timing .   3. The premixed compression auto-ignition engine according to claim 1, wherein the control circuit increases the discharge voltage applied to the non-thermal equilibrium plasma generator as the engine load is lower and the rotational speed is higher. The control circuit corrects the control amount of the discharge voltage and the discharge period based on at least one detection value of an in-cylinder pressure sensor, a knocking sensor, and an O ₂ sensor that detects an oxygen concentration in exhaust gas. The premixed compression auto-ignition engine according to any one of claims 1 to 3, further comprising means.

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