JP3350187B2 - Air-fuel ratio control device for lean burn engine - Google Patents

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 device for controlling a lean air-fuel ratio of a mixture in a lean-burn engine for a vehicle.

[0002]

2. Description of the Related Art In recent years, as a fuel-saving engine for a new generation of vehicles, for example, a swirl or turbulent flow such as swirl has been improved in a combustion chamber upon intake, and a mixture of a leaner air-fuel ratio than a stoichiometric air-fuel ratio has been improved. Research and development of lean-burn engines capable of burning with air have been actively conducted. In this lean-burn engine, the amount of HC and CO emissions is originally reduced due to the lean mixture, and on the contrary, complete combustion proceeds and N
Although the emission amount of Ox increases, the emission amount of NOx also decreases in accordance with the increase in the air-fuel ratio after a certain air-fuel ratio, which is advantageous in terms of exhaust gas. When the lean air-fuel ratio exceeds the lean limit, combustion fluctuations due to misfire increase, torque fluctuations increase, 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 improve the fuel efficiency effectively. At this time, the state of the combustion fluctuation is checked so as not to exceed the lean limit. In this way, good operability is maintained. It is also desirable to check the actual NOx emission state and control the air-fuel ratio to be equal to or lower than the allowable NOx emission limit to reliably reduce the exhaust gas.

Conventionally, the above-mentioned air-fuel ratio control has been disclosed, for example, in Japanese Patent Application Laid-Open No. 60-27748 and Japanese Patent Application Laid-Open No. 58-3835.
There is a prior art disclosed in Japanese Patent Publication No.
By measuring the torque fluctuation by the torque sensor and the cylinder pressure fluctuation by the cylinder pressure sensor, the combustion fluctuation of the engine is detected. It is shown that the air-fuel ratio is controlled by a lean limit to the best fuel efficiency just before the misfire, thereby simultaneously purifying the exhaust gas and improving the fuel efficiency. Japanese Patent Application Laid-Open No. 58-1
In the prior art disclosed in Japanese Patent No. 3137, the NOx concentration is indirectly estimated from the in-cylinder pressure, and EGR is performed based on the NOx concentration.
And the like to reduce NOx.

[0005]

However, in the former case of the above prior art, NOx in the engine operating state is not considered.
Since it is not checked whether or not the air-fuel ratio is actually reduced, the air-fuel ratio must be controlled to the lean side based on the characteristics of the NOx emission amount, and the drivability tends to deteriorate. The latter controls EGR and the like based on the NOx concentration, and has a problem that it cannot be applied to the lean air-fuel ratio control of the present invention.

In view of the above, the present invention checks the combustion fluctuation and the NOx emission state, controls the lean air-fuel ratio to an appropriate range between the two, and ensures the fuel efficiency, exhaust gas and drivability. The purpose is to improve.

[0007]

To achieve this object, the present invention provides an air-fuel ratio control device for a lean-burn engine for controlling a lean air-fuel ratio of an air-fuel mixture, which calculates a combustion fluctuation state of the lean-burn engine. When the combustion fluctuation state becomes larger than the standard value of the combustion fluctuation state indicating the lean limit of the engine, the air-fuel ratio is made rich and the NO.
x is calculated from the detected value of NOx concentration in exhaust gas ,
A control unit is provided for leaning the air-fuel ratio when the NOx emission becomes larger than a standard value of the NOx emission indicating an allowable limit of the engine.

[0008]

According to 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 the actual combustion fluctuation is determined by the in-cylinder pressure.
The air-fuel ratio is controlled to be richer than the lean limit, and good operability is ensured. Also, depending on the actual NOx concentration, NOx
By judging the emission state, the air-fuel ratio is controlled to be leaner than the permissible limit of the NOx emission amount, so that the exhaust gas is surely reduced.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 illustrates the overall configuration of the lean burn engine. Reference numeral 1 denotes a lean-burn engine body. In an 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 which is communicated through 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 swirl and tumble generating means (not shown) is provided,
A vortex or a turbulent flow is generated in the combustion chamber at the time of intake, and it is configured to be able to burn with a mixture having a leaner air-fuel ratio 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 reduced.
In particular, it is necessary to reduce NOx. For this reason, the exhaust manifold 8 of the engine body 1 is provided with an exhaust gas purifying device.
The lean NOx catalytic converter 10 is mounted, and is configured to purify mainly the NOx of exhaust gas by causing a reduction reaction at a high temperature by the lean NOx catalyst. Then, the lean NOx catalytic converter 10 is further connected to the muffler 9 via the exhaust pipe 11.

Next, the control system will be described. First,
The control principle will be described. In particular, by detecting the in-cylinder pressure during 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 emission can be calculated. N to 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 larger than the stoichiometric air-fuel ratio of 14.7 and the lean control is performed, the NOx emission amount becomes maximum near the air-fuel ratio of 16 and thereafter increases according to the increase of the air-fuel ratio. It gradually decreases. So in this characteristic,
The point a where the air-fuel ratio is 19 is, for example, the allowable limit of the amount of NOx emitted from 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 increase rapidly from, for example, around 23. 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 in these points a and b, that is, the air-fuel ratio is in the range of 19 to 24.

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

The control unit 20 has an operating condition determining means 21 to which the engine speed N and the intake air amount Q are inputted, and determines the engine operating condition based on both parameters. The signal of the operating condition is input to the fuel injection amount calculating 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 a small amount of NOx is obtained. Injector 7 at timing
Output to

The signals of the in-cylinder pressure P and the operating conditions are input to the in-cylinder pressure detecting means 23 to detect the in-cylinder pressure P under each operating condition.
The in-cylinder pressure P is input to the combustion fluctuation rate calculation means 24, and the actual combustion fluctuation rate B is obtained based on the change in the in-cylinder pressure P. The operating condition signal is input to the combustion variation standard value searching means 25, and the standard value Bmax of the lean limit under each operating condition is searched with reference to the combustion variation standard map 26. Then, the actual combustion fluctuation rate B and the standard value Bmax of the lean limit are input to the operating state determination means 27 and compared with each other, and B> Bma
In the case of x, the fuel injection amount calculation means 22 is instructed to enrich the air-fuel ratio.

The signals of the NOx concentration NOxconc and the operating conditions are input to 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 calculating means 29, and the intake air amount Q, the NOx concentration NOxconc, and the N
The actual NOx emission amount A is calculated by multiplying the specific gravity γ of Ox. The operating condition signal is input to the NOx emission standard value searching means 30, and the standard value Amax of the allowable limit under each operating condition is searched with reference 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 and compared between them. If A> Amax, the fuel injection amount calculation means 22 is instructed to make the air-fuel ratio lean.

The fuel injection amount calculating means 22 calculates the fuel injection amount Ti into the lean air-fuel ratio as described above, and further increases or decreases the fuel injection amount Ti by the combustion variation rate B or the leaning instruction by the NOx emission amount A. I do. In addition, the amount of fuel is reduced or increased as a whole in a mode in which importance is placed on fuel efficiency or driving performance, whereby the air-fuel ratio is controlled so that the air-fuel ratio is always maintained in the region between points a and b in FIG. 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 operation of the engine, air is drawn into the engine body 1 in accordance with the opening of the throttle valve 4, and at this time, swirl or the like is generated in the combustion chamber by the swirl generating means of the intake manifold 6. The intake air amount Q and the engine speed N
The fuel injection amount Ti is calculated so as to be substantially a lean air-fuel ratio with little NOx, and this fuel is injected by the injector 7 at a predetermined injection timing. Therefore, the mixture of air and fuel in the combustion chamber generates a vortex, and the mixture is ignited by the ignition plug in a stratified mixture so that the area around the spark plug is dense and the surrounding area is thin, so that the mixture with a lean air-fuel ratio is formed. Good combustion results in good fuel economy and driving performance.

On the other hand, the exhaust gas from the lean combustion is discharged from the engine body 1 to the exhaust manifold 8. In this case, the unburned portion 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 when the NOx becomes
Purification treatment is performed by a reduction reaction with the x catalyst. The exhaust gas purified by the lean NOx catalytic converter 10 is
The exhaust gas passes through the muffler 9 further 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, the engine speed N and the intake air amount Q
The in-cylinder pressure P is detected in step S2, the combustion fluctuation rate B is calculated in step S3, and the standard value B of the lean limit of the combustion fluctuation rate is calculated by a map in step S4.
Search for max. Then, in step S5, the actual combustion fluctuation rate B is compared with the standard value Bmax of the lean limit,
When the air-fuel ratio exceeds the lean limit and the combustion fluctuation increases due to misfire and B> Bmax, the routine proceeds to step S6, in which the amount of fuel is increased and the lean air-fuel ratio is corrected to be rich. Therefore, the combustion fluctuation is suppressed together with the misfire, and the deterioration of drivability is prevented.

If the control is performed on the rich side where the combustion fluctuation is small due to B ≦ Bmax, the process proceeds from step S5 to step S7, where the actual NOx concentration NOxconc is detected, and the NOx emission amount A is calculated in step S8. In step S9, the standard value Amax of the allowable limit of the NOx emission amount according to the operating condition is searched, and the two values are compared in step S10. Then, when the NOx emission amount A exceeds the allowable limit and the exhaust gas deteriorates and A> Amax, step S1
Proceeding to 1, the air-fuel ratio is corrected lean by reducing the amount of fuel in the opposite manner as described above. Therefore, the leaner air-fuel ratio reduces the emission amount of NOx and improves the exhaust gas.

When A ≦ Amax, the exhaust gas is also in a good state, and it is determined that the air-fuel ratio is appropriate at points a and b in FIG. In this case, the process proceeds from step S10 to step S12 to check the mode for emphasizing fuel efficiency and driving performance. If emphasis is placed on fuel efficiency, the process proceeds to step S11, where the air-fuel ratio is controlled to the lean side to maximize the fuel efficiency. If the driving performance is emphasized, the process proceeds to step S6, where the air-fuel ratio is controlled to the rich side, whereby the air-fuel ratio becomes rich 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 a range between the lean limit b of the combustion fluctuation and the allowable limit a of the NOx emission, thereby ensuring good operability and exhaust gas simultaneously. Is done. The NOx in the exhaust gas is controlled to be equal to or less 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,
Since the actual NOx concentration in the exhaust gas is detected in the lean burn engine, the NOx emission state is determined, and the air-fuel ratio is controlled to a range between the lean limit of the combustion fluctuation and the allowable limit of the NOx emission amount. NO in exhaust gas with drivability
x can be surely reduced. In addition, the air-fuel ratio control region is expanded to the rich side, and vibration can be reduced in a case where traveling performance is emphasized. The air-fuel ratio is controlled to be rich or lean by comparing the combustion fluctuation and its standard value for each operating condition to determine the operating state, and comparing the NOx emission with the standard value to determine the exhaust gas state. Therefore, the control accuracy is high.

[Brief description of the 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 an NOx emission amount and a combustion variation rate with respect to an air-fuel ratio.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 14 In-cylinder pressure sensor 15 NOx concentration sensor 20 Control unit

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Claims (1)

(57) [Claims] An air-fuel ratio control device for a lean-burn engine for controlling a lean air-fuel ratio of an air-fuel mixture, wherein a combustion fluctuation state of a lean-burn engine is calculated, and the combustion fluctuation state indicates a lean limit of the engine. with enriching the air-fuel ratio when it becomes larger than the standard value, the NOx emissions of the engine concentrated NOx in the exhaust gas
Calculated from degrees detected values, the NOx emissions, the lean burn engine, characterized by comprising a control unit for lean air-fuel ratio when it becomes greater than the NOx emission standard value indicating the tolerance limits of the engine Air-fuel ratio control device.