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 (NOx) emitted to the exhaust system. Apart from this positive effect to fe edgas emissions, camshaft timing can cause large air-to-fuel ratio excursio ns if not coordinated with the fuel command. Large air-to-fuel ratio excurs ions can reduce the catalytic converter efficiency and effectively cancel t he 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 c ontroller alleviates the limitation.