Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1999-08-17
2001-07-03
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S285000, C123S492000, C123S486000
Reexamination Certificate
active
06253542
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine and, more specifically, relates to an air-fuel ratio control device which controls the air-fuel ratio of the engine based on the outputs of air-fuel ratio sensors upstream and downstream of a catalytic converter.
2. Description of the Related Art
Three-way reducing and oxidizing catalytic converters are commonly used to remove pollutants such as NO
x
, HC, and CO components in the exhaust gas of an internal combustion engine. Generally, the catalyst used in such converters is able to remove the pollutants from the exhaust gas simultaneously only when the air-fuel ratio of the exhaust gas is kept in a narrow range near the stoichiometric air-fuel ratio. Therefore, in order to reduce the emission of the exhaust gas, it is important to keep the air-fuel ratio of the exhaust gas in the region near the stoichiometric air-fuel ratio.
It is known to use an electronic engine control module to control the amount of fuel being injected into an engine. In particular, it is known to use the output of an exhaust gas oxygen (EGO) sensor as part of a feedback control loop to control the air-fuel ratio. Typically, such an EGO sensor is placed upstream of the catalyst which processes the exhaust gases. In some applications, it is known to use a second EGO sensor downstream of the catalyst, partly to serve as a diagnostic measure of catalyst performance. With the presence of EGO sensors both upstream of the catalyst and downstream of the catalyst, it would be desirable to develop an improved feedback air-fuel ratio control system using signals from both of the sensors.
In the double EGO sensor system, the air-fuel ratio control is carried out based on the output of the downstream EGO sensor as well as the upstream EGO sensor. Typically, the air-fuel ratio of the engine is accurately controlled by correcting the output of the upstream EGO sensor based on the output of the downstream EGO sensor. In such a system, however, there exists a delay in the response of the downstream EGO sensor to detect a change in the exhaust gas air-fuel ratio of the engine. This delay is caused by the oxygen storage capacity of the three-way reducing and oxidizing catalyst in the catalytic converter. Thus, the response of the downstream EGO sensor to the change in the air-fuel ratio of the engine becomes slow due to the absorbing and releasing action of the oxygen by the catalyst. Because of this delay in the detection of the air-fuel ratio of the engine by the downstream EGO sensor, it is difficult to compensate the output of the upstream EGO sensor accurately based on the output of the downstream EGO sensor.
Attempts have been made to improve the air-fuel ratio correction capabilities of dual sensor control systems by substantially increasing the proportional feedback gain in the downstream EGO sensor feedback loop. Although this approach provides relatively rapid transient air-fuel ratio correction, it results in undesirable low frequency air-fuel ratio limit-cycle oscillations which reduce overall catalyst efficiency.
An example of this behavior is shown in FIG.
1
. As shown in
FIG. 1
, some time after a lean air-fuel ratio disturbance occurs (at t=10 seconds), the downstream EGO sensor output
10
switches from a rich to a lean indication. The proportional feedback term derived from this change will then command the fuel controller to increase the fuel flow rate by a fixed amount. Because of the time delay associated with the downstream feedback loop (caused primarily by the oxygen storage component in the catalyst), the effect of this command will not be detected by the downstream EGO sensor for a relatively long time. In the meantime, the integral feedback term is slowly, but continuously, increasing the fuel flow rate. After a sufficiently long time delay, the effects of the increased fuel flow will be detected by the downstream EGO sensor, and the sensor output will switch back from lean to rich. In general, however, because of the fixed fuel offset induced by the proportional term, the air-fuel ratio correction will be excessive, and the cycle repeats itself as shown by the low frequency air-fuel ratio oscillations. At the same time, the pre-catalyst or upstream air-fuel ratio
12
oscillates, although at a somewhat higher amplitude.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved air-fuel ratio feedback control to solve the aforementioned problem. In the present invention, the aforementioned problem is solved through the provision of a method of controlling the air-fuel ratio of an internal combustion engine having an exhaust passage including a catalytic converter. The method includes providing a first air-fuel ratio sensor upstream of the catalytic converter, and providing a second air-fuel ratio sensor downstream of the catalytic converter. A control module having an input connected to the first and second air-fuel ratio sensors and an output connected to actuators for controlling the engine is also provided. This establishes a first feedback loop including the first air-fuel ratio sensor and a second feedback loop including the second air-fuel ratio sensor. The method further includes detecting an output value of the second air-fuel ratio indicative of a rich or lean exhaust gas air-fuel ratio. In response to the output value, the system monitors the engine mass airflow, and controls the air-fuel ratio as a function of the engine mass airflow.
One advantage of the present invention is that it suppresses fluctuation in the air-fuel ratio. Another advantage is that it improves the efficiency of the catalytic converter.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.
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Bradley John Edward
Hamburg Douglas Ray
Omara Ahmed Abdelaziz
Denion Thomas
Ford Global Technologies Inc.
Lippa Allen J.
Trieu Thai-Ba
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