In this article an air-fuel ratio (AFR) estimator, using torque measurements, for a spark-ignited engine is presented. Instead of estimating the in-cylinder AFR directly the estimator, which employs a stochastic linear filtering technique, generates an estimation of a predefined AFR effector (that is, it estimates a multi-valued function of the AFR). The AFR effector equation may be solved directly to give a rich and lean value for the AFR. Since only one of these values will be the required estimate, it is shown how, for a simple ratio AFR controller, that this can be found from throttle angle measurements. Simulation results on a validated engine model show that the estimator has good transient performance.
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