In this paper it has been demonstrated as the closed loop air fuel ratio (A/F) control system, equipped with a linear oxygen sensor (UEGO), can be improved with an innovative reformulation of the fuel injection control by means of a model based approach. The A/F control, within a rigorous speed-throttle logic (which avoids the use of the air flow-meters and of the pressure sensors inside the intake manifolds), can be designed using a Luenberger-type observer which includes the dominant dynamics of air, fuel and oxygen lambda sensor. The control has been developed and experimented on a research one-cylinder spark ignited engine (AVL 5401). The air mass reference can be supplied either by a stationary map, or, in substitution, by a physical gas dynamic code (Method of Transfer Function, MTF) which is able to compute cycle by cycle the pressure pumping fluctuation and the cylinder filling (volumetric efficiency) giving rise to an effective estimate of the volumetric efficiency during transients, avoiding in this way the use of experimental stored data. The experimental campaign on AVL 5401 has tested the control performances in transient conditions determined by moderate and severe unsteady maneuvers concerning both the load and the engine speeds, demonstrating the robustness of the proposed observer-based control strategy and the possibility to replace the map with the MTF code. The control is able to keep the A/F in the most part of transients around the stoichiometric, with transient control errors bounded within 3–4% in presence of critical throttle maneuvers which are more severe than in the usual operative conditions.

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