Variable camshaft timing engine control

Retarding camshaft timing in an engine equipped with a dual equal camshaft timing phaser reduces the unburned hydrocarbons (HC) and oxides of nitrogen (NO/sub x/) emitted to the exhaust system. Apart from this positive effect to feedgas emissions, camshaft timing can cause large air-to-fuel ratio excursions if not coordinated with the fuel command. Large air-to-fuel ratio excursions can reduce the catalytic converter efficiency and effectively cancel the benefits of camshaft timing. The interaction between the camshaft timing and the air-to-fuel ratio results in an inherent tradeoff between reducing feedgas emissions and maintaining high catalytic converter efficiency. By designing and analyzing a decentralized and a multivariable controller, we describe the design limitation associated with the decentralized controller architecture and we demonstrate the mechanism by which the multivariable controller alleviates the limitation.