JP2000297673A - Controller for diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control an excess air ratio so that it is not made smaller than a target value through calculation of the excess air ratio by variably controlling concentration and amount of oxygen of intake air, changing the amount of oxygen without varying the oxygen concentration in the intake air according to operating conditions of an engine to control the excess air ratio above the target value. SOLUTION: An excess air ratio is calculated from the amount of intake air detected by an air flow meter 12, the concentration of residual oxygen within exhaust gas detected by an oxygen concentration sensor 13, and the amount of fuel injection. The calculated excess air ratio is compared with the prescribed value. When the excess air ratio is smaller than the prescribed value, an EGR rate is calculated based on the information given from the sensors and stored. EGR rate is recalculated and compared with the stored EGR rate. If the EGR rate, which was once reduced due to a rise in boost pressure, is not increased to a level equivalent to the stored EGR rate, an EGR valve 5 is opened and an operation to increase the EGR rate is repeated. Consequently, NOx and PM are reduced simultaneously, and fuel economy is maintained satisfactorily.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control system for a diesel engine.

[0002]

2. Description of the Related Art In general, a diesel engine has better fuel efficiency than a gasoline engine, but has a drawback that it emits a large amount of nitrogen oxides (NOx) and particulate matter (PM).

[0003] As means for reducing NOx in the exhaust gas of a diesel engine, there is known a technique disclosed in No. 9503
Symposium No. As described in U.S. Pat. No. 5,933,271, there is a method of performing exhaust gas recirculation (EGR) to keep the combustion temperature of an engine low.

[0004]

However, according to this method, when the exhaust gas recirculation amount is increased to reduce the NOx emission amount, the PM emission amount is increased in a region where the fuel injection amount is large and the excess air ratio is small. Tend to increase, S
AEPaperNo. As described in 980174, when the air-fuel ratio becomes about 24: 1 or less (the excess air ratio is about 1.6) or less, the amount of PM emission sharply increases. Therefore, if an attempt is made to suppress the PM emission, the reduction of the NOx emission is restricted.

[0005] As a method of eliminating the two trade-offs and simultaneously reducing the amount of NOx and PM emissions in the exhaust of a diesel engine, there is a method of performing low-temperature premix combustion as disclosed in Japanese Patent Application Laid-Open No. 7-4287.

[0006] This is because the EGR lowers the oxygen concentration in the intake air to lower the combustion speed, delays the fuel injection timing until after compression top dead center, and injects most of the fuel during the ignition delay period. Combustion is carried out at a low temperature mainly by mixed combustion. According to this method, since the fuel is premixed combustion in which the fuel is sufficiently vaporized and then burned, the PM
NOx emission due to low emission and low temperature combustion
Emissions are reduced.

In this low-temperature premixed combustion, as the excess air ratio is reduced, both the emission amount of NOx and PM are reduced. However, when the excess air ratio becomes about 1.3 or less, the fuel efficiency tends to rapidly deteriorate. is there.

[0008] This is because when the excess air ratio is reduced, the proportion of fuel and combustion products, which are polyatomic molecules, is increased.
The specific heat ratio of the working gas in the engine decreases, and in the case of a diesel engine, ηth = <1- (1 / ε) κ-1) (σκ-1) / (κ (σ-1)) (where ε : Compression ratio, κ: specific heat ratio, σ: cut-off ratio).

The present invention has been made in view of such problems, and performs low-temperature premixed combustion in a diesel engine to change the intake oxygen amount without changing the intake oxygen concentration according to the operating conditions of the engine. It is intended to solve the above problem.

[0010]

According to a first aspect of the present invention, there is provided a diesel engine having means for lowering the combustion temperature of an engine and means for increasing the ignition delay period of fuel injected into a combustion chamber. Means for variably controlling the oxygen concentration, means for variably controlling the oxygen amount of the intake air, means for calculating the excess air ratio, and changing the oxygen amount without changing the oxygen concentration in the intake air according to the operating conditions of the engine. Combustion control means for controlling the excess air ratio to be equal to or higher than the target value.

In a second aspect based on the first aspect, the combustion control means increases the intake oxygen amount while maintaining the same intake oxygen concentration when the excess air ratio becomes equal to or less than a predetermined value. , So that the excess air ratio is equal to or higher than the target value.

In a third aspect based on the second aspect, the combustion control means reduces the intake oxygen amount while maintaining the same intake oxygen concentration when the excess air ratio becomes equal to or higher than the target value.

In a fourth aspect based on the first to third aspects, the target value of the excess air ratio λ is set to λ = 1.3.

In a fifth aspect based on the first to fourth aspects, the means for controlling the oxygen concentration comprises an exhaust gas recirculation device for controlling an exhaust gas recirculation amount.

In a sixth aspect based on the first to fifth aspects, the means for controlling the oxygen amount comprises a turbocharger for controlling a supercharging pressure.

[0016]

According to the present invention (first to sixth aspects), the low-temperature premixed combustion is achieved by lowering the oxygen concentration of the intake air to lower the combustion temperature and increasing the ignition delay period of the fuel. At this time, the amount of intake oxygen is changed without changing the oxygen concentration of the intake air, and control is performed so that the excess air ratio does not become smaller than a target value. The emission of NOx and PM can be reduced by low-temperature premixed combustion, but the emission of NOx changes according to the oxygen concentration at that time. Therefore, by maintaining the oxygen concentration in the intake air at the target value without changing it, the NOx emission level can always be suppressed to a predetermined value or less. On the other hand, the fuel efficiency deteriorates significantly when the excess air ratio becomes smaller than a predetermined value. In response, the intake air amount is changed without changing the oxygen concentration to control the excess air ratio so as not to be smaller than the target value.

As a result, it is possible to always maintain good fuel economy while reducing NOx and PM in the exhaust gas.

In this case, in the fifth invention, the control of the oxygen concentration is performed by controlling the exhaust gas recirculation.
According to the invention, since the control of the oxygen amount is performed by controlling the supercharging pressure, the control can be easily and rationally performed by a normal exhaust gas recirculation device, a turbocharger, or the like.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, 1 is an engine body, 2 is an intake pipe, and 3 is an exhaust pipe. 4 is an EGR (exhaust gas recirculation) passage communicating the exhaust pipe 3 and the intake pipe 2, 5 is an EGR valve for adjusting a flow rate of exhaust gas (EGR gas) flowing through the EGR passage 4,
Reference numeral 6 denotes an actuator for driving the EGR valve 5.

Reference numeral 7 denotes a VN turbocharger capable of adjusting the supercharging pressure by changing the diameter of a nozzle for introducing exhaust gas into the exhaust turbine. Reference numeral 8 denotes a variable turbine nozzle thereof. Reference numeral 9 denotes an actuator for driving the turbine nozzle 8.

Reference numeral 10 denotes an engine control unit (E
CU) and 11 are fuel injection devices (fuel injection valves).
, Which is not shown in the ECU 10.
A signal representative of the operating state is input from an accelerator opening sensor, an engine speed sensor, a water temperature sensor, and the like.

In the present invention, the fuel injection timing is delayed at or after the compression top dead center, so that the ignition delay period of the fuel is lengthened, and the necessary fuel is almost injected during the ignition delay period. The fuel injection nozzle capacity is set large.

Further, ECU 10 receives detection signals from an air flow meter 12 for measuring the amount of intake air and an oxygen concentration sensor 13 for measuring the concentration of oxygen in the exhaust gas. E
The CU 10 calculates an excess air ratio and an EGR ratio using the fuel injection amount controlled according to the operation state, the intake air amount detected by the air flow meter 12, and the residual oxygen concentration in the exhaust gas detected by the oxygen concentration sensor 13. Then, in a predetermined operation region in which low-temperature premix combustion is performed based on this, while maintaining the oxygen concentration in the intake air at a predetermined value, the oxygen amount is controlled so as to reach a target excess air ratio, and NOx and PM are controlled. The aim is to improve fuel efficiency while controlling emissions.

In this embodiment, the EGR valve 5 constitutes means for controlling the oxygen concentration in the intake air, and the variable turbine nozzle 8 of the VN turbocharger 7 constitutes means for controlling the oxygen amount in the intake air.

FIG. 2 is a flowchart showing the contents of the above-described control executed by the ECU 10, and the control operation will be described in accordance with the flowchart.

First, in step a1, the air flow meter 1
The excess air ratio λ is calculated from the amount of intake air detected by the control unit 2, the residual oxygen concentration in the exhaust gas detected by the oxygen concentration sensor 13, and the fuel injection amount.

In step a2, the obtained excess air ratio λ is compared with a predetermined value (= 1.3). If the excess air ratio λ is smaller than the predetermined value, the process proceeds to step a3. In steps a3 and a4, an EGR rate α (EGR gas amount / new air amount) is calculated based on information from each sensor and the like as in step a1, and this EGR rate α is stored.

The EGR rate is determined according to the NOx reduction target determined by the operating conditions at that time.

At step a5, the VN turbocharger 7
Of the variable turbine nozzle 8 is slightly increased to increase the supercharging pressure. As a result, the fresh air amount increases, and as a result, the intake oxygen amount increases.

However, if the EGR amount remains unchanged, the EGR rate will decrease. Since the intake oxygen concentration is inversely proportional to the EGR rate, if the EGR rate decreases, the intake oxygen concentration will increase. Therefore, in step a6, the EGR valve 5
Is slightly opened to increase the EGR gas amount, thereby reducing the intake oxygen concentration.

In step a7, the EGR rate is calculated again, and in step a8, the EGR rate is compared with the EGR rate α stored in step a4. EGR once decreased due to increase in supercharging pressure
If the rate has not increased until it becomes equal to the stored EGR rate, the process returns to step a6 and the operation of opening the EGR valve 5 and increasing the EGR rate is repeated.

Conversely, if the EGR rate is equal to the stored EGR rate, the excess air rate is equal to that before the operation of the control flow, and the process returns to step a1. Returning to step a1, if the excess air ratio λ has reached a predetermined value, this control flow ends. If the excess air ratio has not reached the predetermined value, the control flow is repeated.

In the present invention, the injection timing of the fuel injected from the fuel injection device 11 is delayed, and a part of the exhaust gas is recirculated into the intake air to lower the oxygen concentration in the intake air.
Low-temperature premixed combustion with a long ignition delay period is realized, so that both NOx and PM can be reduced.

On the other hand, the excess air ratio λ changes in the direction indicated by arrow A in FIG. 3 by adjusting the supercharging pressure while keeping the oxygen concentration in the intake air, in other words, the EGR ratio constant. Control is performed so that λ = 1.3.

The fuel efficiency remains almost the same and good in the region where the excess air ratio λ is 1.3 or more. However, when λ becomes 1.3 or less, the fuel efficiency rapidly deteriorates as the value becomes smaller. However, as described above, since λ is controlled to 1.3, the fuel economy can be maintained in a good state.

The NOx concentration in the exhaust gas is almost proportional to the oxygen concentration because oxygen is involved in the generation.
Therefore, it is not appropriate to discuss the magnitude of the NOx emission amount from the engine based on the excess air ratio corresponding to the intake oxygen amount if the fuel injection amount is constant.

Actually, the NOx emission amount is almost proportional to both the intake oxygen concentration and the intake oxygen amount. In the present invention, since the intake oxygen concentration does not change, the NOx concentration does not increase, and the NOx emission slightly increases due to the increase in the intake air amount. It is kept at a low level.

In the present embodiment, although the variable turbine nozzle 8 of the VN evening charger 7 is used as a means for controlling the intake oxygen amount, the present invention is not limited to this. The control may be performed by increasing / decreasing the supercharging pressure by a wastegate using a turbocharger having no.

As for the intake oxygen concentration, the control means is EG
The control is not limited to the increase and decrease of the EGR gas amount by the R valve 5, but may be controlled by manipulating the oxygen concentration in the fresh air with an oxygen permeable membrane. Further, although the EGR valve 5 and the variable turbine nozzle 8 are controlled independently, both may be controlled integrally.

By the way, when the excess air ratio becomes 1.3 or more during low-temperature premixed combustion, deterioration of fuel efficiency is not a problem. Control may be performed so as to reduce the intake oxygen amount while keeping the intake oxygen concentration constant.

This control is added to the second embodiment, and the control flow will be described with reference to FIG. However, the description will focus on the differences from the control flow of FIG.

If the excess air ratio λ is equal to or greater than the predetermined value (= 1.3) in step a2, the process proceeds to step b1. At steps b1 and b2, the EGR rate is calculated in the same manner as described above, and the EGR rate α is stored. Next, step b3
Then, the variable turbine nozzle 8 of the VN turbocharger 7 is slightly opened to reduce the supercharging pressure.

As a result, the intake oxygen amount decreases as a result of the decrease in the fresh air amount. On the other hand, if the opening degree of the EGR valve 5 is not changed, the EGR gas amount increases, the EGR rate increases,
That is, a decrease in the intake oxygen concentration occurs.

In step b4, the EGR valve is slightly closed to reduce the EGR gas amount and increase the intake oxygen concentration. In step b5, the EGR rate is calculated again, and in step b6, the EGR rate is compared with the EGR rate α stored in step b2.

If the EGR rate is not equal to the stored EGR rate α, the process returns to step b4 and the EGR valve 5
And the operation of decreasing the EGR rate is repeated. E
If the GR rate is equal to the stored EGR rate, the process returns to step a1 and the control flow is repeated.

When the added control is performed,
As shown by an arrow B in FIG. 3, the excess air ratio λ changes from a state where λ is large toward λ = 1.3. At this time, the intake oxygen amount (fresh air amount) is reduced without changing the intake oxygen concentration, so that the NOx emission amount is reduced. Regarding the fuel efficiency, the excess air ratio λ = 1.3 at which the rapid deterioration starts
, It is kept good.

As described above, according to the present invention, low-temperature premixed combustion is performed in a diesel engine, and control is performed to change the intake oxygen amount without changing the intake oxygen concentration according to the operating conditions of the engine. The effect of maintaining good fuel economy while simultaneously reducing PM is obtained.

The present invention is not limited to the embodiments described above, and it will be easily understood that various changes can be made within the scope of the technical idea of the present invention.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a flowchart showing the control contents.

FIG. 3 is an explanatory diagram showing a relationship between an excess air ratio and exhaust characteristics.

FIG. 4 is a flowchart showing control contents according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 2 Intake pipe 3 Exhaust pipe 4 EGR passage 5 EGR valve 6 EGR valve drive actuator 7 VN turbocharger 8 Variable turbine nozzle 10 Engine control unit (ECU) 11 Fuel injection device 12 Air flow meter 13 Oxygen concentration sensor

Continued on front page F-term (reference) DB03 DC09 EA01 EA04 EA05 EA08 FA17 FA18 FA24 GA02 HA01Z HA16X HD05X HD07X HD07Z HE01Z HE08Z HF08Z 3G301 HA02 HA11 HA13 JA02 JA25 KA05 LA00 MA01 MA11 MA18 NE01 NE12 NE13 PA01Z PD03Z PD15Z PE01Z PE08Z PF03

Claims (6)

[Claims] 1. A diesel engine comprising: means for lowering the combustion temperature of an engine; and means for increasing the ignition delay period of fuel injected into a combustion chamber; means for variably controlling the oxygen concentration of intake air; Means for variably controlling the amount of oxygen in the air, means for calculating the excess air ratio, and changing the amount of oxygen without changing the oxygen concentration in the intake air according to the operating conditions of the engine so that the excess air ratio exceeds the target value. A control device for a diesel engine, comprising a combustion control means for performing control as described above. 2. The combustion control means increases the intake oxygen amount while maintaining the same intake oxygen concentration when the excess air ratio falls below a predetermined value, so that the excess air ratio becomes greater than a target value. The control apparatus for a diesel engine according to claim 1, wherein the control is performed in such a manner. 3. The control device for a diesel engine according to claim 2, wherein said combustion control means reduces the intake oxygen amount while maintaining the same intake oxygen concentration when the excess air ratio becomes equal to or higher than the target value. . 4. The diesel engine control device according to claim 1, wherein the target value of the excess air ratio λ is set to λ = 1.3. 5. The control device for a diesel engine according to claim 1, wherein said means for controlling the oxygen concentration comprises an exhaust gas recirculation device for controlling an exhaust gas recirculation amount. 6. The control device for a diesel engine according to claim 1, wherein said means for controlling the amount of oxygen comprises a turbocharger for controlling a supercharging pressure.