Patent ID: 7467033

Claim:
A method for controlling an engine in a powertrain for a hybrid electric vehicle, the vehicle having an internal combustion engine, at least one electric motor-generator, a battery, the battery and the motor-generator being electrically coupled, gearing establishing power flow paths from the engine and the motor-generator to a driven member connected drivably to vehicle traction wheels, and a controller for coordinating power distribution from the engine and the motor-generator in accordance with operating variables including a driver demand for power and battery state of charge, the method comprising the steps of: calibrating a minimum load for the engine below which the engine fuel combustion tends to be unstable at a given engine speed; determining an engine torque command by the vehicle system controller; measuring engine load corresponding to the engine torque command; calculating a minimum torque adder; comparing the engine torque command at a given engine speed with a minimum engine torque that corresponds to the calibrated minimum engine load; and adding the minimum torque adder to the minimum engine torque when the measured engine load is near the calibrated minimum load whereby engine fuel combustion stability is maintained.