JP3514086B2 - Combustion control device for diesel engine - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a combustion control device for a diesel engine.

[0002]

2. Description of the Related Art The oxygen concentration is reduced by EGR, the injection start timing is delayed until after compression top dead center, the gas flow is controlled in the combustion chamber, and the fuel injection is ended during the ignition delay period. Forming a mixture, so-called low
Which causes the temperature premixed combustion has been proposed (see Japanese Patent Laid-Open No. 8-86251).

[0003]

By the way, when a large amount of EGR was carried out at the time of high load as to whether the above premixed combustion could be performed even in a high load region, a large amount of high temperature EG was found.
It was found that since the intake temperature rises due to the R gas, the ignition delay period becomes shorter than the injection period, so that low temperature premix combustion does not occur, smoke increases, and fuel consumption deteriorates. In other words, in low-temperature premixed combustion, the aim is to disperse fuel as much as possible around oxygen before ignition starts, so in order to realize low-temperature premixed combustion, in addition to promoting fuel dispersion through gas flow control, The latest experiment has revealed that the most important condition is to finish the fuel injection during the ignition delay period.

Therefore, according to the present invention, the oxygen concentration is reduced to a predetermined value by performing a large amount of EGR in order to end the fuel injection during the ignition delay period even in the high load region, and the injection period is the same as the ignition delay period. The purpose is to extend the region where low temperature premixed combustion is possible to the high load region by controlling so as to be shorter than that.

[0005]

A first aspect of the invention is to reduce the oxygen concentration, delay the injection start timing until after compression top dead center, control the gas flow in the combustion chamber, and fuel during the ignition delay period. by the ending the injection to form the premixed air-fuel mixture, the combustion control apparatus for de I over diesel engine to perform a low-temperature premixed combustion, the oxygen concentration of less vital the injection start timing to less than a predetermined value in the high load region The fuel injection is terminated during the ignition delay period by controlling the injection period so that the injection period, which is a period until the end timing of the fuel injection, is the same as or shorter than the ignition delay period.

In the second invention, the injection period is shortened as the load increases in the first invention.

In a third invention, the injection period is controlled by the injection pressure in the first or second invention.

According to a fourth aspect of the present invention, the injection pressure is reduced when the oxygen concentration exceeds the predetermined value or when the injection period becomes longer than the ignition delay period in the third aspect.

According to a fifth aspect of the present invention, in the case where a nozzle capable of controlling the injection hole area or the number of injection holes is provided in the first or second invention, the injection period is determined by the injection hole diameter or the number of injection holes of the nozzle. Control.

According to a sixth aspect of the invention, in the first or second aspect of the invention, a plurality of nozzles per cylinder is provided, and when the number of nozzles in charge of injection can be controlled, the number of nozzles in charge of injection can be used to control the number of nozzles. Control the injection period.

In a seventh aspect of the invention, the oxygen concentration is reduced by increasing the EGR rate in any one of the first to sixth aspects.

[0012]

According to the first aspect of the present invention, when the low temperature premixed combustion can be performed even in the high load range, the combustion temperature can be kept low in the low temperature premixed combustion and the initial combustion speed can be suppressed. Therefore, the combustion noise becomes lower than before, and NOx is greatly reduced due to the decrease in oxygen concentration. As a result, according to the experimental results, the combustion noise and NOx could be reduced to less than half of the conventional values when compared in the entire operating range where low-temperature premixed combustion became possible.

[0013] load further increases, the have a higher injection pressure to become impossible low temperature premix combustion region, although the combustion noise increases, in the fourth invention, the low temperature <br / > Injection pressure is reduced when premixed combustion is not possible, so combustion noise can be prevented from deteriorating.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a structure for performing low temperature premixed combustion, and this structure itself is disclosed in JP-A-8-8.
It is known from, for example, Japanese Patent No. 6251.

The production of NOx largely depends on the combustion temperature,
Lowering the combustion temperature is effective in reducing this. In the low-temperature premixed combustion, the EGR connecting the exhaust passage 2 and the intake passage 3 is performed in order to realize the low-temperature combustion by reducing the oxygen concentration by EGR.
The R passage 4 is provided with a diaphragm type EGR valve 6 that responds to the control negative pressure from the negative pressure control valve 5.

The negative pressure control valve 5 is a control unit 4
It is driven by the duty control signal from 1.
Thereby, a predetermined EGR rate according to the operating condition is obtained. For example, as shown in FIG. 3, the EGR rate is set to a maximum of 100% in the low rotation and low load range, and the EGR rate is decreased as the rotation speed and the load increase.
Since the exhaust temperature rises on the high load side, if a large amount of EGR gas is recirculated, the effect of reducing NOx is reduced due to the rise in intake air temperature, or the ignition delay period of the injected fuel is shortened and premixed combustion cannot be realized. Therefore, the EGR rate is gradually reduced.

A cooling device 7 for EGR gas is provided in the EGR passage 4. This is composed of a water jacket 8 formed around the EGR passage 4 in which a part of the engine cooling water is circulated, and a flow rate control valve 9 provided in the cooling water inlet 7a and capable of adjusting the circulation amount of the cooling water. Therefore, according to the command from the control unit 41, the cooling degree of the EGR gas increases as the circulation amount increases via the control valve 9.

A swirl valve (not shown) having a predetermined notch is provided in the intake passage near the intake port for promoting combustion. When the swirl valve is closed by the control unit 41 in the low rotation and low load region, the flow velocity of the intake air sucked into the combustion chamber increases and swirl is generated in the combustion chamber.

The combustion chamber is a large diameter toroidal combustion chamber (not shown). This is a cylindrical shape from the crown to the bottom of the piston without restricting the inlet of the piston cavity, and at the center of the bottom, resistance is given to the swirl that flows from the outside of the piston cavity while swirling from the outside of the compression stroke. A cone is formed so as to further improve the mixing of air and fuel. With this cylindrical piston cavity that does not throttle the inlet, the swirl generated by the swirl valve described above diffuses from the inside of the piston cavity to the outside of the cavity as the piston descends during the combustion process. The swirl is sustained.

The engine is equipped with a common rail type fuel injection device 10. The configuration of the common rail type fuel injection device 10 is also publicly known (see Proceedings of the 13th Internal Combustion Engine Symposium, pp. 73 to 77), and will be outlined with reference to FIG.

This fuel injection device 10 mainly comprises a fuel tank 11, a fuel supply passage 12, a supply pump 14, a common rail (accumulation chamber) 16, and a nozzle 17 provided for each cylinder.
After the fuel pressurized by the supply pump 14 is temporarily stored in the pressure accumulating chamber 16 through the fuel supply passage 15, the high pressure fuel in the pressure accumulating chamber 16 has the same number of nozzles 1 as the number of cylinders.
It is divided into seven.

The nozzle 17 includes a needle valve 18, a nozzle chamber 19,
Fuel supply passage 20 to nozzle chamber 19, retainer 21, hydraulic piston 22, return spring 23 for urging needle valve 18 in the valve closing direction (downward in the figure), fuel supply passage 24 to hydraulic piston 22, this passage 24 A three-way valve (solenoid valve) 25, etc. installed in the nozzle
When the three-way valve 25 in which the high pressure fuel is guided to both the upper portion of the hydraulic piston 22 and the nozzle chamber 19 is OFF (ports A and B communicate, ports B and C are cut off), the pressure receiving area of the hydraulic piston 22 is Is larger than the pressure receiving area of the needle valve 18, the needle valve 18 is in the seated state, but the three-way valve 25 is ON.
When the state (ports A and B are cut off, ports B and C are in communication), the fuel above the hydraulic piston 22 is returned to the fuel tank 11 via the return passage 28, and the fuel pressure acting on the hydraulic piston 22 is reduced. . As a result, the needle valve 18 rises and fuel is injected from the injection hole at the tip of the nozzle. Three-way valve 2
When 5 is returned to the OFF state again, the high pressure fuel in the pressure accumulating chamber 16 is guided to the hydraulic piston 22 and the fuel injection ends. That is, if the fuel injection start timing is adjusted by the switching timing of the three-way valve 25 from ON to OFF and the fuel injection amount is adjusted by the ON time, and the pressure in the pressure accumulating chamber 16 is the same,
The fuel injection amount increases as the ON time increases. Reference numeral 26 is a check valve, and 27 is an orifice.

The fuel injection device 10 is further provided with a pressure adjusting valve 31 in the passage 13 for returning the fuel discharged from the supply pump 14 in order to adjust the pressure of the pressure accumulating chamber. The adjusting valve 31 opens and closes the flow path of the passage 13, and adjusts the pressure of the pressure accumulating chamber by adjusting the amount of fuel discharged to the pressure accumulating chamber 16. The fuel injection rate changes depending on the fuel pressure (injection pressure) in the pressure accumulating chamber 16, and the higher the fuel pressure in the pressure accumulating chamber 16, the higher the fuel injection rate.

In the control unit 41 to which signals from the accelerator opening sensor 33, the sensor 34 for detecting the engine speed and the crank angle, the sensor 35 for cylinder discrimination, and the water temperature sensor 36 are input, the engine speed and the accelerator opening are set. The target fuel injection amount and the target pressure of the pressure accumulating chamber 16 are calculated according to the degree, and the fuel of the pressure accumulating chamber 16 is adjusted via the pressure adjusting valve 31 so that the pressure accumulating chamber pressure detected by the pressure sensor 32 matches the target pressure. Feedback control the pressure.

Further, the ON time of the three-way valve 25 is controlled in accordance with the calculated target fuel injection amount, and the O of the three-way valve 25 is controlled.
By controlling the switching timing to N, the predetermined injection start timing according to the operating conditions is obtained. For example, as shown in FIG. 4, the fuel injection timing (injection start timing) is extended to the piston top dead center (TDC) so that the ignition delay period of the injected fuel becomes longer on the low rotation and low load side of the high EGR rate. It's delayed. Due to this delay, the temperature in the combustion chamber at the ignition timing is set to a low temperature state and the premixed combustion ratio is increased, thereby suppressing the occurrence of smoke in the high EGR rate region. On the other hand, the injection timing is advanced as the rotation speed and the load increase. This is because even if the ignition delay time is constant, the ignition delay crank angle (a value obtained by converting the ignition delay time into a crank angle) increases in proportion to the increase in the engine speed, and at a low EGR rate, The injection timing is advanced in order to obtain the ignition timing.

Returning to FIG. 1, a turbocharger is provided in the exhaust passage 2 downstream of the opening of the EGR passage 4. This is provided with a variable vane 53 driven by a step motor 54 at the scroll inlet of the exhaust turbine 52, so that the control unit 41 allows the variable vane 53 to obtain a predetermined boost pressure from a low rotation range. The low rotation side controls the vane angle (tilt state) that increases the flow velocity of the exhaust gas introduced into the exhaust turbine 52, and the high rotation side controls the vane angle (fully open state) that allows the exhaust gas to be introduced into the exhaust turbine 52 without resistance. Further, when the predetermined condition is met, the variable vane 53
Are controlled at vane angles that reduce the boost pressure.

Now, whether premixed combustion can be carried out even in a high load region, and when a large amount of EGR is carried out at a high load, the intake temperature rises due to a large amount of high temperature EGR gas, so the ignition delay period is the injection period. It was found that the temperature was shorter than that of the above, and therefore, low temperature premix combustion was not performed, smoke was increased, and fuel consumption was deteriorated. In other words, in low-temperature premixed combustion, the aim is to disperse fuel as much as possible around oxygen before ignition starts, so in order to realize low-temperature premixed combustion, in addition to promoting fuel dispersion through gas flow control, The latest experiment has revealed that the most important condition is to finish the fuel injection during the ignition delay period.

In other words, when the oxygen concentration is reduced to less than 18% in the high load region and the injection period is controlled so that the injection period is the same as or shorter than the ignition delay period, the low temperature premixed combustion reaches the high load region. It was found that it would be possible to do.

This will be explained with reference to FIG. 5. In FIG. 5, A, B, and C are in the EGR region, and the load of B is higher than that of A and C is higher than that of B. That is, as the load increases, E
Since the GR gas temperature rises and the ignition delay period becomes shorter, the injection pressure is increased in accordance with the shortening of the ignition delay period (at the same time, the injection end timing by the three-way valve 25 is advanced).
As a result, the injection period is controlled to be the same as or shorter than the ignition delay period (see the bottom of FIG. 6). In addition,
A, B, C, D, and E on the horizontal axis in FIG. 6 correspond to A, B, C, D, and E in FIG.

As a result, it is possible to reduce both the combustion noise and NOx compared to the conventional case until the load C (see the uppermost stage and the second stage in FIG. 6). This means that, as shown in FIG. 5, the low temperature premixed combustion region is expanded to the high load side as compared with the conventional case.

As described above, according to the present invention, by enabling the low temperature premixed combustion even in the high load region, the combustion temperature can be suppressed low in the low temperature premixed combustion, and the initial combustion speed can be suppressed. Combustion noise is lower than before,
Due to the large amount of EGR, NOx was also greatly reduced. Comparing the entire low-temperature premixed combustion region according to the present invention, as shown in FIG. 7, both combustion noise and NOx were reduced to less than half of the conventional values (experimental results). Note that P
M (smoke) does not change so much (the decrease in PM in FIG. 7 corresponds to the difference in EGR).

On the other hand, in FIG. 6, D is close to the full load and E
Although it is a region where GR is cut, even in this D, if the injection pressure is as high as C as in a, combustion noise and NO
Since x also becomes large, the injection pressure is once lowered to a value larger than that at C at the load of D as indicated by a black circle, thereby preventing deterioration of combustion noise and NOx.

Even in the conventional example in which the combustion noise does not decrease as much as in the present invention, the combustion noise becomes smaller as the loads C, D, and E increase (see the uppermost stage in FIG. 6). This is the effect of pressure supply.

The flow chart of FIG. 8 embodies the control contents of the oxygen concentration and the injection pressure performed by the control unit, and is executed at regular intervals.

In step 1, the oxygen concentration is set to 1 in the EGR region.
The EGR rate Qegr to be 8% or less is calculated from the rotation speed and the load. The EGR region where the oxygen concentration is 18% or less in FIG. 3 is a region where the EGR rate is 40% or more.

Here, the EGR region includes a region where low temperature premixed combustion is possible as in the conventional case and a region which is on a higher load side than this region and where low temperature premixed combustion is possible according to the present invention (Fig. 5). However, for simplicity,
The description will be made assuming that the EGR region and the premixed combustion region (both regions thereof) match.

The upper limit value of 18% of the oxygen concentration is the optimum value for the engine used in the experiment, and it goes without saying that the value will be different if the engine is different.

In step 2, the injection pressure P is calculated from the rotational speed and the load, for example, by searching the map shown in FIG.

Here, the value of the injection pressure P with respect to the load is shown in FIG.
As seen in the bottom row, the higher the load in the EGR region, the higher the load, and the higher the load becomes.
In the cut region, the value decreases once and then increases again at full load.

The injection pressure P thus obtained and the above E
The GR rate Qegr is stored in a predetermined address in step 3, and this processing is ended.

In the embodiment, the common rail type fuel injection device whose injection pressure is easily controlled is used and the injection period is controlled by the injection pressure. However, the present invention is not limited to this. For example, in the case where a nozzle capable of controlling the nozzle area (nozzle diameter) or the number of nozzle holes is provided (preprint 964, Automotive Engineering Society Academic Lecture, published in October 1996, “Direct nozzle area variable mechanism”). Characteristics of injection diesel fuel injection nozzle "), the injection period can be controlled by the injection hole area or the number of injection holes of that nozzle, and the number of nozzles in charge of injection can be controlled by providing multiple nozzles per cylinder. In some cases, the injection period may be controlled by the number of nozzles in charge of injection.

[Brief description of drawings]

FIG. 1 is a control system diagram of a first embodiment.

FIG. 2 is a schematic configuration diagram of a common rail fuel injection device.

FIG. 3 is a characteristic diagram of a basic EGR rate.

FIG. 4 is a characteristic diagram of basic injection start timing.

FIG. 5 is a region diagram showing a low temperature premixed combustion region of the first embodiment.

FIG. 6 is a characteristic diagram of combustion noise, NOx, smoke, and injection pressure with respect to load.

FIG. 7 is a characteristic diagram of NOx, PM, and combustion noise when compared with a conventional example in the entire low temperature premixed combustion region of the first embodiment.

FIG. 8 is a flowchart for explaining setting of an EGR rate and an injection pressure according to the first embodiment.

FIG. 9 is a characteristic diagram of injection pressure.

[Explanation of symbols]

4 EGR passage 6 EGR valve 8 EGR gas cooling device 10 Common rail fuel injection system 33 Accelerator position sensor 34 Crank angle sensor 41 Control unit

Continuation of front page (51) Int.Cl. ⁷ Identification code FI F02M 25/07 F02M 25/07 A 550 550R 61/14 310 61/14 310U 61/18 330 61/18 330C (56) References 8-218920 (JP, A) JP-A-8-296469 (JP, A) JP-A-8-246935 (JP, A) JP-A-6-272636 (JP, A) JP-A-7-4287 (JP, A) A) Japanese Unexamined Patent Publication No. 8-86251 (JP, A) Actual Development No. 5-36069 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) F02D 41/04 385 F02D 41/38 F02D 43/00 301 F02D 45/00 310 F02M 25/07 F02M 25/07 550 F02M 61/14 310 F02M 61/18 330

Claims (7)

(57) [Claims] 1. A premixed mixture by reducing the oxygen concentration, delaying the injection start timing until after compression top dead center, controlling the gas flow in the combustion chamber, and ending the fuel injection during the ignition delay period. To form low temperature premixed combustion.
The combustion control apparatus of I over diesel engine, or the same as the oxygen concentration period in which the injection period is the ignition delay period until end timing of the fuel injected from the small vital the injection start timing to less than a predetermined value in the high load region or combustion control apparatus <br/> de I over diesel engine, characterized in that to terminate the fuel injection during the ignition delay time by controlling the injection period to be shorter. 2. The combustion control device for a diesel engine according to claim 1, wherein the injection period is shortened as the load increases. 3. The combustion control device for a diesel engine according to claim 1, wherein the injection period is controlled by the injection pressure. 4. The combustion of a diesel engine according to claim 3, wherein the injection pressure is reduced when the oxygen concentration exceeds the predetermined value or when the injection period is longer than the ignition delay period. Control device. 5. The injection period is controlled by the nozzle diameter or the number of nozzles of the nozzle when the nozzle is capable of controlling the nozzle area or the number of nozzles. A combustion control device for a diesel engine according to 1. 6. A plurality of nozzles per cylinder are provided, and when the number of nozzles in charge of injection can be controlled, the injection period is controlled by the number of nozzles in charge of injection. 1. The combustion control device for a diesel engine according to 1 or 2. 7. The combustion control device for a diesel engine according to any one of claims 1 to 6, wherein the oxygen concentration is reduced by increasing the EGR rate.