Internal-combustion engines – Charge forming device – Fuel injection system
Patent
1995-10-12
1996-12-31
Argenbright, Tony M.
Internal-combustion engines
Charge forming device
Fuel injection system
123673, F02D 4114
Patent
active
055884105
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
1. Background Art
The present invention relates to an air-fuel ratio control method for engines which are driven by changing the air-fuel ratio between a stoichiometric air-fuel ratio and a lean air-fuel ratio.
2. Description of the Prior Art
Engines which are driven mainly under an air-fuel ratio greater than a stoichiometric air-fuel ratio, namely, a lean air-fuel ratio, have been provided because of such advantages as reduction of exhaust gas and improvement of fuel consumption.
However, such engines are also run at the stoichiometric air-fuel ratio when they are slowly accelerated at the request of exhaust gas and drivability.
A significant torque gap occurs between such an operation under the stoichiometric air-fuel ratio and an operation under the lean air-fuel ratio, as shown in FIG. 4 which illustrates a relationship between the air-fuel ratio and the torque. Thus, the engine stalls due to the torque gap when operation under the stoichiometric air-fuel ratio proceeds to operation under the lean air-fuel ratio.
For this reason, a conventional suction system comprises air bypass valves. These air bypass valves are opened to increase the intake air flow when the operation under the stoichiometric air-fuel ratio is changed to the operation under the lean air-fuel ratio. At the same time, the intake air flow is finely adjusted by means of an idle speed control valve (hereinafter referred to as ISC valve) in order to prevent the torque from dropping.
However, in a high load area where the pressure in the suction pipe (negative pressure) is low, the air does not increase to a satisfactory level. Accordingly, it is impossible to correct the torque gap completely, which sometimes hesitates upon changing of the air-fuel ratio.
The above mentioned problem may be solved by, for example, changing the air-fuel ratio slowly. This method is, however, not practical because of the resultant increase in Nox as shown in FIG. 5 caused during the changing from the stoichiometric air-fuel ratio to the lean air-fuel ratio.
Japanese Patent Laid-open (Tokkaisho) No. 62-17341 discloses a technique in which the lean air-fuel ratio is changed to the stoichiometric air-fuel ratio in engine for each respective cylinder. However, this technique may be subject to a drop of torque, causing the engine to stall upon changing of the air-fuel ratio.
Accordingly, it is an object of the present invention to reduce torque fluctuation caused by changing the air-fuel ratio in an engine driven by changing the air-fuel ratio between a stoichiometric air-fuel ratio and a lean air-fuel ratio, and further to reduce the output of NOx.
DISCLOSURE OF INVENTION
According to the first aspect of the present invention, there is provided an air-fuel ratio control method for selecting a stoichiometric air-fuel ratio and a lean air-fuel ratio as a target air-fuel ratio for an engine to control the air-fuel ratio of the engine depending on the selected target air-fuel ratio, wherein the air-fuel ratios of respective cylinders of the engine are changed individually for the respective cylinders with the change timing deviated from each other when the target air-fuel ratio is changed from one to the other between the stoichiometric air-fuel ratio and the lean air-fuel ratio, and the air flow to the engine is increased during the changing of the air-fuel ratio.
In the air-fuel ratio control method according to the first aspect of the present invention, changing of the air-fuel ratio between the stoichiometric air-fuel ratio and the lean air-fuel ratio is achieved gradually for respective cylinders with the changing of the timing deviated from each other, so that it is possible to prevent generation of NOx. Further, the air flow into the engine is increased along with the changing of the air-fuel ratio, so that it is possible to prevent reduction of engine torque during the changing of the air-fuel ratio. This prevents the sudden engine hesitation.
According to the second aspect of the present invention, in the structure of the above mention
REFERENCES:
patent: 4027637 (1977-06-01), Aono
patent: 5429104 (1995-07-01), Kondou
Kamura Hitoshi
Maeda Tetsuo
Takeyama Akihiro
Tamura Hiroki
Yamazaki Naoki
Argenbright Tony M.
Mitsubishi Denki & Kabushiki Kaisha
Mitsubishi Jidosha Kogyo Kabushiki Kaisha
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