The Effect of Exhaust Variable Cam Phaser Transients on Equivalence Ratio Control in an SI 4 Stroke Engine

It is a well established fact that variable camshaft phasing can be used to reduce engine out emissions during steady state operation. However, a transition in camshaft timing which often accompanies engine speed / load changes, can have a significant dynamic impact on volumetric efficiency. The change in volumetric efficiency can be highly unpredictable, and can last for several engine cycles following the endpoint of cam movement. Since precise control of catalyst feedgas equivalence ratio is crucial to meeting increasingly stringent automotive emission requirements, the steady state benefits of cam phasing can be overshadowed by the loss of A/F control during transient operation if the engine control algorithms are not properly designed and calibrated. This paper investigates the impact of exhaust variable cam phaser transients on equivalence ratio control, and the mechanism by which volumetric efficiency is affected.