1. Field of the Invention
This invention relates to emission control of internal combustion engines. In particular, the invention relates to an air/fuel ratio closed loop fuel control of an internal combustion engine equipped with two exhaust gas oxygen (EGO) sensors and a three way catalytic converter. The EGO sensors are located upstream and downstream of the catalyst.
2. Prior Art
It is known that catalyst efficiency is greatly affected by the ratio of air to fuel in the mixture supplied to an engine. If the air/fuel ratio is kept in a narrow range at stoichiometric ratio, catalyst conversion efficiency is high for both oxidation and reduction conversions. Air/fuel stoichiometric ratio is defined as the ratio containing air and fuel in such proportions that in perfect combustion both would be completely consumed, and air/fuel ratio LAMBDA is defined as the amount by weight of air divided by the amount by weight of fuel over air/fuel stoichiometric ratio. The purpose of any closed loop fuel control system is to keep the air/fuel ratio in this narrow range known as a conversion window.
It is also known that a control system utilizing one EGO sensor located either before or after a catalyst does not maintain the air/fuel ratio consistently inside the conversion window. Control systems with one EGO sensor located before a catalyst have acceptable time response characteristics but exhibit long term drift due to EGO sensor contamination and aging. On the other hand, control systems with one EGO sensor located after a catalyst have unacceptable time response characteristics but exhibit good long term stability and can indicate a narrow conversion window. Therefore, a control system utilizing advantages of both EGO sensors, i.e., good time response of an upstream EGO sensor and high accuracy of a downstream EGO sensor, is advantageous.
A number of closed loop fuel control systems utilizing both EGO sensors have been proposed but none are completely satisfactory. U.S. Pat. Nos. 3,939,654 issued to Creps and 4,027,477 issued to Storey describe a dual EGO sensor closed loop fuel control system having two control loops. The first control loop includes an upstream EGO sensor and a proportional or proportional with phase lead controller. The second control loop includes a downstream EGO sensor and a dual integrator controller. This arrangement of the control system precludes the usage of integral or proportional and integral controllers in both control loops simultaneously because such a control system is inherently unstable and can not be made stable by calibration. The drawback of these systems is a low accuracy of the first control loop with a proportional controller. The control accuracy may even become unacceptable when the second control loop is not operational as is the case during initial operation before the downstream EGO sensor reaches its operational temperature.
Other teachings, represented by U.S. Pat. Nos. 4,831,838 issued to Nagai et al and 4,840,027 issued Okumura et al, utilize a proportional and integral (PI) controller in the first control loop with an upstream EGO sensor. In one embodiment of these patents, calibratable parameters of the PI controller may be modified based on the output of a downstream EGO sensor, the modified parameters being a skip amount, or jumpback, and an integration amount, or ramp. Some other control system parameters such as time delay and reference voltage may also be modified based on the output of a downstream EGO sensor.
In another embodiment of the same patents, the output of a downstream EGO sensor is used to generate a second air/fuel ratio correction amount which is used as a multiplier in the main fuel equation (the main fuel equation will be introduced later). In both embodiments, a correction introduced by a downstream EGO sensor has a very low frequency limit cycle superimposed on a relatively high frequency limit cycle produced by an upstream EGO sensor control loop. It results in an undesirable effect known in the control field as beat frequency. Moreover, the initial response of a downstream EGO sensor is so slow that elaborate procedures are incorporated to mitigate this disadvantage. Accordingly, both approaches to a dual EGO fuel control system have been found to be unsatisfactory.
Another known control technique using dual EGO sensors, one upstream and one downstream of the catalyst, is a cascade control wherein a signal from the downstream EGO sensor is applied to a summer with a reference signal. The output of the summer is applied to a first proportional and integral controller. A signal from the upstream EGO sensor is applied to a summer and a reference, the reference being the output of the first proportional and integral controller. The output of the second summer is applied to a second proportional and integral controller which then generates the feedback signal to control engine air/fuel ratio.
In another scheme similar to the one just mentioned, both of the summers have an applied reference signal. The output of the first proportional and integral controller is not applied to the second summer but instead controls the parameters of the second proportional and integral controller. This is known as parametric control because the parameters of the second controller are controlled by the output of the first controller. Both this system and the previous system are relatively slow in operation. With respect to the later, parametric control, when a parameter is changed, such as the jumpback, ramp, or delay of a control function, it may well take minutes for the effect to be felt. Further, the system is slow to start. It would be desirable to overcome these problems.
Applicant's invention has a much faster response and uses a single proportional and integral controller having inputs from both the upstream and the downstream EGO sensor.