Control method for a vehicle powertrain with protection against low load conditions

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, a catalytic converter in an exhaust gas transfer system for the engine, and a vehicle system 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 torque command;
• calculating a minimum torque adder;
• comparing the engine torque command at a given engine speed with a minimum torque value that corresponds to the calibrated minimum engine load;
• adding the minimum torque adder to the minimum engine torque to obtain an increased engine torque command when the measured engine load is near the calibrated minimum engine load whereby engine fuel combustion stability is maintained;
• limiting the minimum torque adder to effect a calibrated maximum torque limit;
• and
• limiting the duration of operation of the powertrain when the measured engine load is near the calibrated minimum load whereby excessive thermal damage to the catalytic converter is prevented.

A method for controlling an engine in a powertrain for a vehicle, the vehicle having an internal combustion engine, gearing establishing power delivery from the engine to a driven member connected drivably to vehicle traction wheels, and a controller for coordinating power distribution to the traction wheels in accordance with operating variables including a driver demand for power, 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 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 engine load whereby engine fuel combustion stability is maintained.

A method for controlling an engine in a powertrain for a vehicle, the vehicle having an internal combustion engine, gearing establishing at least one power flow path from the engine to a driven member connected drivably to vehicle traction wheels, a catalytic converter in an exhaust gas transfer system for the engine, and a controller for controlling power distribution to the traction wheels in accordance with operating variables including a driver demand for power, 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 torque command;
• calculating a minimum torque adder;
• comparing the engine torque command with a minimum engine torque that corresponds to the calibrated minimum engine load;
• adding the minimum torque adder to the minimum engine torque to obtain an increased engine torque command when the measured engine load is near the calibrated minimum engine load whereby engine fuel combustion stability is maintained;
• limiting the torque adder to effect a maximum torque limit;
• and
• limiting the duration of operation of the powertrain when the measured engine load is near the calibrated minimum load whereby excessive thermal damage to the catalytic converter is prevented.