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Good control of air-fuel ratio under all operating conditions is essential for low exhaust emissions. In an effort to achieve this goal, an engine model based observer control structure has been applied to a single-cylinder CFR engine. The model includes fuel puddle dynamics, cycle delays inherent in the four-stroke engine process, and sensor dynamics for a universal exhaust gas oxygen (UEGO) sensor. This control structure has been shown to be capable of maintaining the air-fuel ratio within 0.5% rms of the commanded stoichiometric value during throttle transients. To achieve this level of performance, accurate values of model parameters such as time constants, delay times, and fuel puddle parameters are necessary. Since these parameters tend to vary with engine speed, throttle angle, time, and temperature, a method of periodically updating these parameter values is useful.

High bandwidth control of the air-fuel ratio is necessary in order to minimize the exhaust emissions of spark-ignition engines with three-way catalytic converters. A new approach is to implement a control structure based on modern control and estimation theory. This work describes the implementation of an estimator-based controller which uses the feedback from an on-off zirconia exhaust oxygen sensor of the type currently used in production vehicles. The limit-cycle associated with the on-off oxygen sensor in conventional systems is eliminated with the estimator-based control structure. Furthermore, the in-cylinder air-fuel ratio tracks the commanded value, so that if a limit cycle is desired in some areas of the engine's operating range for better catalyst operation, its amplitude and frequency can be set arbitrarily.

Better fuel economy, reduced exhaust emissions and better drivability strongly depend on precise control of air-fuel ratio (AFR) during both steady and transient engine operations. A discrete, nonlinear fuel-injected SI engine model was developed and used for the design of AFR control algorithms. The engine model includes intake manifold air dynamics, fuel wall-wetting dynamics, and cycle delays inherent in the four-stroke engine processes. The sampling period is synchronous with crank angle (“event-based”) as opposed to the conventional time synchronous sampling scheme (“time-based”). The model was validated with test data over a wide range of engine operating conditions. The exhaust O2 sensor can only provide a delayed and lagged AFR signal to the controller. This inherent delay in the measurement will slow down the system response if conventional feedback control design is used.