Air Fuel Ratio Control of a Spark Ignition Engine



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Lean-burn operation in spark-ignition engines has shown promise in improving fuel economy and reducing harmful emissions in comparison with traditional stoichiometric operation. Close reference-tracking of the set air-fuel ratio profile is very crucial to healthy engine operation. What makes air-fuel ratio control challenging is the presence of a large variable time delay in the system's closed-loop, resulting mainly from the large distance traveled by the air-fuel mixture between the injection point and the exhaust. This thesis proposes modifications to an IMC-Smith predictor design employed to control the air-fuel ratio in a lean-burn engine. Matlab's Simulink provides a convenient platform to build dynamic models and simulate controllers, and for that reason, it is chosen to validate the proposed controller design and compare its performance to that of a PI controller and that of an IMC-Smith controller. Simulation results reveal the inadequacy of a basic PI controller in providing good reference tracking to a lean-burn profile. The proposed design shows very similar performance to a basic IMC-Smith controller in terms of overshoot and disturbance. However, its reduced settling time in comparison with the IMC-Smith controller (difference of up to 1.5s) renders it a more effective design at providing the desired level of reference-tracking.