Torque management of engines with variable cam timing

This paper describe the variable cam timing (VCT) system which addresses both the drivability and emission performance by utilising an electric hydraulic mechanism to rotate the camshaft relative to the crankshaft in order to retard the cam timing with respect to the intake and exhaust strokes of the engine. By retarding the exhaust valve closing further into the intake stroke, more exhaust gas is drawn into the cylinder providing internal exhaust gas recirculation. In this manner, the amount of residual gas trapped in the cylinder at the end of the exhaust stroke is controlled by cam timing, suppressing NOx formation and reducing the pumping losses. Furthermore, this residual contains some unburned hydrocarbons; consequently, retaining it in the cylinder through two combustion cycles also reduces hydrocarbon emissions. In addition to the reduction of NOx and HC emissions, variable cam timing permits the engine designer to optimize cam timing over a wide range of engine operating conditions.