Transient Mixture Strength Excursions - An Investigation of Their Causes and the Development of a Constant Mixture Strength Fueling Strategy

This study investigates the causes for tailpipe mixture strength excursions during transient operation of an electronically controlled, central fuel injected engine. The investigation was made using a Ford 5.0L V8 engine instrumented to enable continuous monitoring of the tailpipe air-fuel ratio. Transient excursions of up to 20% are observed under warm engine conditions. Two causes of the excursions were identified: system time delays and fuel droplet deposition on induction system surfaces.

A programmable control strategy was developed to compensate for observed lean excursions during accelerations. This was done by modifying the injector signal to provide the necessary fuel enrichment. Transient requirements were optimized across a range of engine operations. The results have been summarized in two sets of curves-one defining the enrichment fuel volume and the other specifying the form of enrichment fuel supply. Lower induction system thermal conditions associated with cold start necessitated modification of these curves. The results of these studies can be identically applied to decelerations. The strategy developed here is suitable for any “wet manifold” engine.