JPH07166938A - Air-fuel ratio controller of lean burn engine - Google Patents

Abstract

PURPOSE:To improve operability and surely decrease a discharge quantity of NOx by detecting concentration of the NOx in exhaust gas, and controlling an air-fuel ratio within a range between lean limitation of burn variation and allowable limitation of the discharge quantity of the NOx. CONSTITUTION:A burn variation condition is calculated by a burn variation rate calculation means 24 from the detected result of inter-cylinder pressure in an engine main body, and a discharge quantity of NOx in an engine is calculation by a NOx discharge quantity calculation 29 from the detected result of the NOx concentration in the exhaust gas. A burn variation condition is judged (27) by a calculated burn variation rate and a standard rate of the burn variation rate subjected to map-retrieving, and the NOx discharged condition is judged (32) by the calculated NOx discharge quantity and the standard value of the NOx discharge quantity subjected to map-retrieving. An air-fuel ratio is subjected to rich correction when the burn variation rate becomes larger than the standard value of the lean limitation, and then the air-fuel ratio is subjected to lean correction when the NOx discharged quantity becomes larger than the standard value of the allowable limitation of the NOx discharge quantity under respective operation conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control system for controlling a lean air-fuel ratio of an air-fuel mixture of a lean burn engine for a vehicle.

[0002]

2. Description of the Related Art In recent years, as a fuel-efficient engine for a new generation vehicle, it has been improved to generate swirl or turbulent flow such as swirl in a combustion chamber at the time of intake, and a lean air-fuel ratio is mixed rather than a theoretical air-fuel ratio. Research and development of lean-burn engines that can be burned by air are being actively conducted. In this lean burn engine, since the air-fuel mixture is lean, the amount of HC and CO emissions is originally small. On the contrary, complete combustion progresses and N
Although the Ox emission amount increases, after a certain air-fuel ratio, the NOx emission amount also decreases with an increase in the air-fuel ratio, which is also advantageous in terms of exhaust gas. When the lean air-fuel ratio exceeds the lean limit, combustion fluctuation due to misfire increases, torque fluctuation increases, and drivability deteriorates.

Therefore, in the air-fuel ratio control of the lean burn engine, the air-fuel ratio of the air-fuel mixture is lean-controlled to effectively improve the fuel consumption. At this time, the state of combustion fluctuation is checked so that the lean limit is not exceeded. That keeps the drivability good. Further, it is desired to reliably reduce the exhaust gas by checking the actual NOx emission state and controlling the air-fuel ratio to be equal to or lower than the NOx emission allowable limit.

Conventionally, regarding the air-fuel ratio control, for example, JP-A-60-27748 and JP-A-58-3835.
There is a prior art of Japanese Patent No. 4 publication, and there is a fluctuation in engine rotation speed
The combustion fluctuation of the engine is detected by measuring the torque fluctuation by the torque sensor and the cylinder pressure fluctuation by the cylinder pressure sensor. It has been shown that lean limit control of the air-fuel ratio to the fuel efficiency best point immediately before a misfire is achieved to simultaneously achieve exhaust gas purification and fuel efficiency improvement. In addition, JP-A-58-1
In the prior art of Japanese Patent No. 3137, the NOx concentration is indirectly estimated by the in-cylinder pressure, and EGR is performed based on this NOx concentration.
Have been shown to be controlled to reduce NOx.

[0005]

By the way, in the former case of the above prior art, NOx is generated in the engine operating state.
Since it is not checked whether or not is actually reduced, the air-fuel ratio is forced to be controlled to the lean side based on the characteristics of the NOx emission amount, and the drivability is likely to deteriorate. In the latter case, the EGR and the like are controlled based on the NOx concentration, and there is a problem that it cannot be applied to the lean air-fuel ratio control of the present invention.

In view of such a point, the present invention checks the combustion fluctuation and the NOx emission state and controls the lean air-fuel ratio in an appropriate region between them to ensure fuel economy, exhaust gas and drivability. The purpose is to improve.

[0007]

In order to achieve this object, the present invention is directed to a combustion fluctuation rate calculating means for detecting the cylinder internal pressure of a lean burning engine body and calculating the combustion fluctuation state of the engine, and an exhaust gas In a lean-burn engine equipped with a NOx emission amount calculation means for calculating the NOx concentration of the engine and calculating the NOx emission amount of the engine, a search is made from the combustion variation rate calculation means and a predetermined combustion variation rate standard value map. NOx from the operating state determination unit that determines the state of combustion fluctuations from the standard value search unit, the NOx emission amount calculation unit, and the standard value search unit that is searched from a predetermined NOx emission amount standard value map. When the actual combustion fluctuation rate becomes larger than the standard value of the lean limit of the combustion fluctuation rate under each operating condition, the air-fuel ratio is richly corrected by the exhaust gas judging means that judges the emission state. , The actual NOx emission amount is larger than the standard value of the allowable limits of NOx emissions in the operating condition, characterized by leaning the air-fuel ratio.

[0008]

With the above structure, in the present invention, the air-fuel ratio of the lean-burn engine is controlled to a lean air-fuel ratio with a small amount of NOx. In this case, the state of actual combustion fluctuation is judged by the cylinder pressure,
The air-fuel ratio is controlled to be richer than the lean limit, and good drivability is ensured. Also, depending on the actual NOx concentration, NOx
By determining the exhaust state, the air-fuel ratio is controlled to the lean side from the allowable limit of the NOx emission amount, so that the exhaust gas is also reliably reduced.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. The overall structure of the lean burn engine will be described with reference to FIG. Reference numeral 1 denotes a lean burn engine body, and in the intake system of the engine body 1, an air cleaner 2 has a duct 3, a throttle body 5 having a throttle valve 4,
An injector 7 that is communicated with the intake manifold 6 and injects fuel into each cylinder of the intake manifold 6 is mounted. In the intake manifold 6 of the engine body 1,
A means for generating swirls and tumbles (not shown) is provided,
A vortex or a turbulent flow is generated in the combustion chamber at the time of intake, and combustion can be performed by a mixture having a lean air-fuel ratio rather than the stoichiometric air-fuel ratio.

In a lean burn engine, the fuel is thin and the harmful components HC and CO in the exhaust gas are small, so
In particular, it is necessary to reduce NOx. Therefore, the exhaust manifold 8 of the engine body 1 has an exhaust gas purifying device,
The lean NOx catalytic converter 10 is mounted, and is configured to reduce NOx in exhaust gas mainly by a lean NOx catalyst at a high temperature for purification treatment. The lean NOx catalytic converter 10 is further communicated with the muffler 9 via the exhaust pipe 11.

Next, the control system will be described. First,
The control principle will be explained. Particularly, by detecting the in-cylinder pressure in the expansion stroke after combustion, the state of combustion fluctuation can be determined, and by detecting the NOx concentration in the exhaust system, the actual N
Ox emissions can be calculated. N for air-fuel ratio (A / F)
FIG. 2 shows the characteristics of the Ox emission amount and the combustion fluctuation rate.

That is, when the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio of 14.7 and the lean control is performed, the NOx emission amount becomes maximum near 16 and thereafter the NOx emission amount increases in accordance with the increase of the air-fuel ratio. Gradually less. So in this characteristic,
A point a where the air-fuel ratio is 19, for example, is the allowable limit of the NOx emission amount in the exhaust gas. Further, the combustion fluctuation rate continues to be small on the lean side of the air-fuel ratio, but the air-fuel ratio starts to rapidly increase from around 23, for example. Therefore, the point b where the air-fuel ratio is 24
Is the lean limit for combustion fluctuations. Therefore, it is understood that the lean air-fuel ratio should be controlled to these points a to b, that is, the region where the air-fuel ratio is 19 to 24.

Then, signals from the air flow meter 12 for detecting the intake air amount Q and the crank angle sensor 13 for detecting the engine speed N are input to the control unit 20. Further, an in-cylinder pressure sensor 14 that detects an in-cylinder pressure P is attached to each cylinder of the engine body 1, and the exhaust pipe 8 has a NOx concentration NOxc.
NOx concentration sensor 15 for detecting onc is attached,
These sensor signals are also input to the control unit 20.

The control unit 20 has an operating condition judging means 21 for inputting the engine speed N and the intake air amount Q, and judges the engine operating condition by both parameters. The signal of the operating condition is input to the fuel injection amount calculation means 22, and the fuel injection amount Ti is calculated according to each operating condition of the lean burn engine so that the lean air fuel ratio with less NOx is obtained, and this injection signal is given. Injector 7 at the timing
Output to.

The in-cylinder pressure P and the operating condition signal are input to the in-cylinder pressure detecting means 23 to detect the in-cylinder pressure P under each operating condition.
This in-cylinder pressure P is input to the combustion variation rate calculating means 24 to obtain the actual combustion variation rate B based on the change in the in-cylinder pressure P. The operating condition signal is input to the combustion fluctuation rate standard value search means 25, and the combustion fluctuation rate standard map 26 is referenced to search for the lean limit standard value Bmax under each operating condition. Then, the actual combustion fluctuation rate B and the lean limit standard value Bmax are input to the operating state determination means 27 to compare the two, and B> Bma.
In the case of x, the fuel injection amount calculation means 22 is instructed to enrich the air-fuel ratio.

The NOx concentration NOxconc and operating condition signals are input to the NOx concentration detecting means 28 to detect the NOx concentration NOxconc under each operating condition. This NOx
The concentration NOxconc is input to the NOx emission amount calculation means 29, and the intake air amount Q, the NOx concentration NOxconc and N are input.
The actual NOx emission amount A is calculated by multiplying the specific gravity γ of Ox. Further, the signal of the operating condition is input to the NOx emission standard value search means 30, and the standard value Amax of the allowable limit under each operating condition is searched by referring to the NOx emission standard map 31. And the actual NOx emission amount A and the standard value A of the allowable limit
max is input to the exhaust gas determination means 32 to compare the two, and when A> Amax, the fuel injection amount calculation means 22 is instructed to make the air-fuel ratio lean.

The fuel injection amount calculation means 22 calculates the fuel injection amount Ti as a lean air-fuel ratio as described above, and further increases or decreases the amount by the enrichment instruction by the combustion variation rate B or the lean instruction by the NOx emission amount A. To do. Further, the fuel amount is emphasized or the traveling property is emphasized, so that the overall amount is decreased or increased, whereby the air-fuel ratio is controlled so that the air-fuel ratio is always maintained in the region of points a to b in FIG. It should be noted that the control unit 20 is configured to determine an appropriate ignition timing according to the operating state based on various input information and output this ignition signal to the igniter.

Next, the operation of this embodiment will be described. First, during engine operation, air is drawn into the engine body 1 in accordance with the opening of the throttle valve 4, and at this time swirl flow is generated in the combustion chamber by the swirl generating means of the intake manifold 6. Also, intake air amount Q and engine speed N
The fuel injection amount Ti is calculated in accordance with each operating condition according to the above so as to have a lean air-fuel ratio with substantially less NOx, and this fuel is injected by the injector 7 at a predetermined injection timing. Therefore, the air-fuel mixture in the combustion chamber generates a vortex, and the ignition plug ignites the stratified mixture so that the vicinity of the spark plug is thicker and the periphery thereof is thinner. Good combustion, good fuel economy and drivability are obtained.

On the other hand, the exhaust gas resulting from lean combustion is discharged from the engine body 1 to the exhaust manifold 8. In this case, however, the unburned amount of HC and CO is reduced in the exhaust gas due to the lean air-fuel ratio. In particular, it is necessary to reduce NOx. The exhaust gas containing this NOx is introduced into the lean NOx catalytic converter 10, and the NOx is lean at high temperature.
The x-catalyst undergoes a reduction reaction for purification treatment. The exhaust gas purified by the lean NOx catalytic converter 10 is
Further, it passes through the muffler 9 located downstream and is discharged.

Next, the air-fuel ratio control during the operation of the lean burn engine will be described with reference to the flowchart of FIG.
First, in step S1, engine speed N and intake air amount Q
The operating condition is determined by the following, the in-cylinder pressure P is detected in step S2, the combustion variation rate B is calculated in step S3, and the lean variation standard value B of the combustion variation rate is calculated by the map in step S4.
Search for max. Then, in step S5, the actual combustion fluctuation rate B is compared with the lean limit standard value Bmax,
When the air-fuel ratio exceeds the lean limit and combustion fluctuations increase due to misfire, and B> Bmax, the process proceeds to step S6, the amount of fuel is increased, and the lean air-fuel ratio is corrected to rich. Therefore, combustion fluctuations are suppressed along with misfires, and deterioration of drivability is prevented.

When the combustion is controlled to the rich side where the combustion fluctuation is small by B ≦ Bmax, the routine proceeds from step S5 to step S7, the actual NOx concentration NOxconc is detected, and the NOx emission amount A is calculated in step S8. Further, in step S9, the standard value Amax of the allowable limit of the NOx emission amount according to the operating condition is searched, and in step S10, both are compared. Therefore, when the NOx emission amount A exceeds the allowable limit and the exhaust gas deteriorates and A> Amax, step S1
Continuing to step 1, the amount of fuel is reduced and the air-fuel ratio is corrected to lean. Therefore, the lean air-fuel ratio reduces the amount of NOx emissions and improves the exhaust gas.

When A≤Amax, it is determined that the exhaust gas is in a good state and the air-fuel ratio at points a and b in FIG. 2 is appropriate. In this case, the process proceeds from step S10 to step S12 to check the mode of emphasizing the fuel efficiency or the traveling property. If the fuel economy is important, the process proceeds to step S11, the air-fuel ratio is controlled to the lean side, and the maximum fuel economy is improved. Further, when the traveling property is emphasized, the routine proceeds to step S6, where the air-fuel ratio is controlled to the rich side, so that the air-fuel ratio becomes dark and the vibration and the like are improved.

In this way, the lean air-fuel ratio of the lean burn engine is always controlled in the region between the lean limit b of the combustion fluctuation and the allowable limit a of the NOx emission amount, thereby ensuring good drivability and exhaust gas at the same time. To be done. The NOx in the exhaust gas is controlled to be equal to or lower than the allowable limit, so that the lean NOx catalytic converter 10 in the exhaust system always purifies NOx stably.

Although the embodiment of the present invention has been described above, the present invention is not limited to this.

[0025]

As described above, according to the present invention,
In a lean burn engine, the actual NOx concentration in the exhaust gas is detected, the NOx emission state is determined, and the air-fuel ratio is controlled in the region between the lean limit of combustion fluctuation and the allowable limit of NOx emission. Operability and NO in exhaust gas
x can also be reliably reduced. Further, the air-fuel ratio control region is expanded to the rich side, and vibration can be reduced when the traveling performance is emphasized. For each operating condition, the combustion fluctuation is compared with its standard value to determine the operating state, and the NOx emission amount is compared with its standard value to determine the exhaust gas state, and the air-fuel ratio is controlled to rich or lean. Therefore, the control accuracy is high.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment suitable for an air-fuel ratio control device for a lean burn engine according to the present invention.

FIG. 2 is a diagram showing a relationship between a NOx emission amount and a combustion fluctuation rate with respect to an air-fuel ratio.

FIG. 3 is a flowchart of air-fuel ratio control.

[Explanation of symbols]

 1 Engine Body 14 Cylinder Pressure Sensor 15 NOx Concentration Sensor 20 Control Unit

Claims (1)

[Claims] 1. A NOx emission amount of an engine is detected by detecting a combustion variation rate calculating means for calculating a combustion variation state of the engine by detecting an in-cylinder pressure of a lean burning engine body and a NOx concentration in exhaust gas. In a lean-burn engine provided with a NOx emission amount calculating means for calculating, a state of combustion fluctuation is judged from the combustion fluctuation rate calculating means and a standard value searching means searched from a predetermined combustion fluctuation rate standard value map. The operating state determination unit, the NOx emission amount calculation unit, and the exhaust gas determination unit that determines the NOx emission state from the standard value search unit searched from the predetermined NOx emission amount standard value map. When the combustion fluctuation rate becomes larger than the standard value of the lean limit of the combustion fluctuation rate under each operating condition, the air-fuel ratio is rich-corrected and the actual NO
An air-fuel ratio control device for a lean-burn engine, wherein the air-fuel ratio is lean-corrected when the x emission amount exceeds a standard value of an allowable limit of the NOx emission amount under each operating condition.