GB2077962A

GB2077962A - Automatic control of air fuel mixture in i.c. engines

Info

Publication number: GB2077962A
Application number: GB8118295A
Authority: GB
Original assignee: Ford Motor Co Ltd; Ford Motor Co
Current assignee: Ford Motor Co Ltd; Ford Motor Co
Priority date: 1980-06-17
Filing date: 1981-06-15
Publication date: 1981-12-23
Also published as: GB2077962B; DE3123260C2; CA1174334A; JPS5735140A; DE3123260A1

Abstract

This specification discloses an apparatus and method for automatically controlling the air fuel ratio in an internal combustion engine in response to a feedback signal from an exhaust gas oxygen sensor. The sensor output signal is sampled periodically. After a given number of samples and depending on the number of rich or lean samples obtained a decision is made to change the air fuel ratio of the mixture supplied in either a rich or lean direction or to leave it unchanged. Biasing, or non-stoichiometric air fuel operation, can be achieved by changing the number of samples required to be either lean or rich before the supply mixture ratio is changed. <IMAGE>

Description

SPECIFICATION Air fuel ratio control system and method This invention relates to a system and method for controlling the airfuel ratio of an internal combustion engine.

Various fuel control systems are known in the prior art in which the quantity of fuel fed to the engine is controlled by sensors in the exhaust gas which give an indication of the air fuel ratio.

Nevertheless, it remains extremely difficult to compensate for the ever changing operating conditions of the engine, the variations among different engines and so on as to always operate the engine with a predetermined air fuel ratio. This drawback may become critical when the engine is equipped with a catalytic converter for reducing undesirable components of the exhaust gases.

A widely used technique to control the air fuel ratio in stoichiometric feedback controlled fuel metering systems is limit cycle integral control. In this technique, there is a constant movement of a fuel metering component in a direction that always tends to counter the instantaneous air fuel ratio indication given by a typical two state exhaust gas oxygen (EGO) sensor. For example, every time an EGO sensor indicates a switch from a rich to a lean air fuel ratio mode of operation, the direction of motion of a typical carburetor's metering rod reverses to create a richer air fuel ratio condition until the sensor indicates a change from a lean to rich air fuel ratio condition. Then, the direction of motion of the metering rod is reversed again this time to achieve a leaner air fuel ratio condition.

Referring to Figures la and 1 b, step like changes in the sensor output voltage initiate ramp like changes in the actuator control voltage. When using the limit cycle integral control, the desired air fuel ratio can only be attained on an average basis since the actual air fuel ratio is made to fluctuate in a controlled manner about the average value. The limit cycle integral control system can be characterized as a two state controller with the mode of operation being either rich or lean. The average deviation from the desired value is a strong function of a parameter called engine transport delay time, tau. This is defined as the time it takes for a change in air fuel ratio, implemented at the fuel metering mechanism, to be recognized at the EGO sensor, afterthe change has taken place.

The engine transport delay time is a function of the fuel metering system's design, engine speed, air flow, and EGO sensor characteristics. Because of this delay time, a control system using a limit cycle technique always varies the air fuel ratio about a mean value in a cyclical manner, a rich air fuel ratio time regime typically followed by a lean air fuel ratio time regime. The shorter the transport delay time is, the higher will be the frequency of rich to lean and lean to rich air fuel ratio fluctuation and the smaller will be the amplitudes of the airfuel ratio overshoots. It can be appreciated that a system with no engine transport delay time is the ideal. These are some of the problems this invention overcomes.

This invention recognizes that the use of a three state controller can improve the operation of a feedback controlled fuel flow system in an internal combustion engine. The control in a three state system has the ability to go rich or to go lean, and in addition, a third state of operation is possible, a do nothing state.

An air fuel ratio feedback fuel control system for an internal combustion engine includes a sensing means for sensing the quantity of oxygen in the exhaust emissions of the engine at a given repetition rate. The control means is coupled to the sensing means for controlling the rate of supply of fuel to the engine in accordance with at least one of the parameters sensed by the sensing means. The control means also responds to the sampled output so that there are three decision states which act to make the air fuel ratio richer, make the air fuel ratio leaner, and not alter the air fuel ratio thereby reducing the oscillations and magnitude of variations of the desired air fuel ratio.

The invention will now be described with reference to the accompanying drawings, in which: Figure 7a is a graphical representation of the EGO sensor output voltage with respect to time in accordance with a prior art limit cycle controlled technique; Figure it is a graphical representation of the actuator control voltage with respect to time corresponding to the prior art sensor output voltage of Figure la; Figure 2a is a sensor output voltage with respect to time in accordance with an embodiment of this invention; Figure 2b is a graph representation of an actuator control voltage with respect to time related to Figure 2a and including an indication of the sampling times; Figure 3 is a block diagram of logic flow in accordance with an embodiment of this invention; and Figure 4 is a partly schematic and partly block diagram of the connection of an engine fuel control system which incorporates an air fuel ratio feedback control in accordance with an embodiment of this invention.

In accordance with an embodiment of this invention, the EGO sensor signal is statistically sampled and the fuel metering system is adjusted to produce the open loop-like stoichiometric air fuel ratio calibration. Thus, this mode of operation is essentially totally independent of engine transport delay time.

Specifically, as shown in Figures 2a and 2b, the EGO sensor signal could be sampled for rich and lean air fuel ratio indication every 30 milliseconds. Then after ten samples, taking 0.3 second to accumulate, a decision is made using the following criteria. If two or less out of 10 indications were rich, the controller changes the airfuel ratio in the rich direction one step. If two or less out of ten indications were lean, the controller changes the air fuel ratio in the lean direction one step. If the lean or rich air fuel indications are between three and seven out of ten indications then nothing is done. This latter state would approximate the open loop calibration condi tion. Such a decision pattern for three state control operation represents a stoichiometric air fuel ratio calibration.

If a nonstoichiometric air fuel ratio calibration is desired, the condition for changing in the rich or lean air fuel ratio direction can be made asymmetrical.

That is, if two or less out of ten indications are rich, make the controller change the air fuel ratio in the rich direction as before. In contrast, however, we can require thai for a change in the lean direction four out of ten indications be required to be lean. Note that the frequency of EGO sensor fluctuations in Figure 2a is much higher than the case in Figure la due to the fact that the cylinder to cylinderfluctuations control the switching rather than the limit cycle mode of operation. In addition, a technique in accordance with an embodiment of this invention could show added benefit of operating engines at higher catalyst efficiencies by approximating a perfect open loop calibration more closely.

Referring to Figure 3, a logic control flow chart in accordance with an embodiment of this invention begins with a block 31 at which a sample of an output of an exhaust gas oxygen sensor is taken. A typical rate at which these samples are taken is, for example, every 30 milliseconds. Of course, this rate may be varied depending upon the particular engine control strategy selected.

At block 32, the sample taken at block 31 is added to previous samplings thereby forming a chain or train of samples. At block 33, a decision is made as to whether or not there are enough samples in the train of samples to make a decision. For example, the decision level may be ten samples and, if ten samples have not been accumulated, the logic goes to block 34 to wait for the next exhaust gas oxygen sensor sample. On the other hand, if 10 samples have been accumulated, a calculation is made at block 35 to determine the number of rich samples versus lean samplestAt block 36 a decision is made whether the fraction of rich to lean indications is in the range requiring action. If no action is required, the logic flows to block 37 and the procedure is to wait for the next set of exhaust gas oxygen sensor indications.

On the other hand;if action is required, a decision is made at block 38. If the indication is too rich, such as, for example, there are more than 7 out of 10 samples indicating rich air fuel ratio condition, the process goes to block 39. At block 39 there is a calculation of the air fuel ratio step size. That the size of the change in the air fuel ratio if a change is indicated. The step size can be based on an engine parameter. For example, the engine parameter may be the rate of change of manifold absolute pressure.

After calculation of the air fuel ratio step size at block 39, an implementation at block 41 changes the air fuel ratio in the lean direction. Returning to block 38, if the indication is more than 7 out of 10 samples indicates lean air fuel ratio condition, a calculation is done at block 40 to determine the air fuel ratio step size. As in the calculation at block 39, this calculation is based upon an engine parameter such as the rate of change of manifold absolute pressure. Afterthe calculation of the step size at block 40, there is an implementation of the step in the air fuel ratio so that the air fuel ratio is changed in the rich direction at block 42.

Referring to Figure 4, in accordance with an embodiment of this invention, an engine 50 has fuel flow control sub-system 51 for physically controlling the amount of fuel flowing. An exhaust gas oxygen sensor 52, a manifold absolute pressure sensor 53, and a throttle position sensor 54 provide inputs to a microprocessor based controller 55 for determining the desired air fuel ratio for engine 50. The desired airfuel ratio is implemented by fuel flow control sub-system 51 in response to an output provided by controller 55.

Claims

1. An air fuel ratio control system for an internal combustion engine comprising: sensing means for sensing the quantity of oxygen within the exhaust emission of the engine at a given repetition rate; control means coupled to receive the output of said sensing means for controlling the rate of supply of fuel to the engine as a function of the quantity of oxygen sensed by said sensing means, any change in the rate of supply of fuel to the engine being accomplished by a step change, the size of the step being a function of a parameter describing engine operation; and control means responding to said sampled output so that there are three decision states which act to make the airfuel ratio richer, make the air fuel ratio leaner and not alter the air fuel ratio, thereby reducing oscillation and magnitude of variations about the desired air fuel ratio.

2. An air fuel ratio control system as recited in claim 1 wherein said control means includes: a biasing means for counting a predetermined number of inputs to said control means describing a first type of air fuel ratio with respect to stoichiometry in comparison to the total number of inputs and generating an output so that the air fuel ratio established by the fuel supply system is of said first type and offset from stoichiometry.

3. An air fuel ratio control system as recited in claim 2further comprising: a throttle position sensor and a manifold absolute, pressure sensor, both coupled to said control means, for providing information characterizing th8 parameter describing engine operation used in determining the step size of a change in the rate of supply of fuel.

4. An air fuel ratio control system as recited in claim 3 wherein: said sensing means is adapted to sense every 30 milliseconds the quantity of oxygen within the exhaust emission of the engine; and said control means is adapted to change the air fuel ratio in the rich direction if two or less out of ten samples are rich, change the airfuel ratio in the lean direction if two or less out of ten samples are lean, and not change the air fuel ratio if the lean or rich air fuel indications are between three and seven out of ten indications.

5. A method of controlling the air fuel ratio of an internal combustion engine including the step of: sampling the quantity of oxygen within the exhaust emission of the engine; sensing parameters characterizing engine operation; and controlling the rate of supply of fuel to the engine by steps in accordance with at least one of the parameters sensed the direction of the steps being such that a first given percentage or less of rich samples causes the air fuel ratio to be driven rich; a second given percentage or less of lean samples causes the air fuel ratio to be driven lean and an intermediate percentage causes no change in the air fuel ratio.

6. A method of controlling the air fuel ratio as recited in claim 5 further comprising the step of biasing the operating point of the internal combustion engine by: increasing the number of samples of the type either rich or lean, in which direction the engine operation is being biased with respect to samples of the other type; and changing the size of the air fuel ratio step by amounts related to other engine parameters such as, for example, the rate of change of manifold absolute pressure and engine coolant temperature to provide air fuel ratio control during transient engine operating conditions.

7. A method of controlling the air fuel ratio as recited in claim 6 further comprising the step: varying the rate of sampling the quantity of oxygen within the exhaust emission of the engine thereby varying the frequency with which corrections can be made to the air fuel ratio.

8. An air fuel ratio control system for an internal combustion engine substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

9. A method of controlling the air fuel ratio of an internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.