The series-parallel hybrid system uses an electric motor to drive the vehicle at low loads and low speeds and the gasoline engine when loads and speeds increase. The electric motor and the gasoline engine can work individually, or together, depending on the power required to drive the vehicle. In addition, as the system drives the wheels, the combustion engine drives a generator to simultaneously generate electricity to recharge the battery when necessary.
A control unit determines the best balance of engine and electric power to achieve the most efficient vehicle operation. The combustion engine operates within its most efficient range resulting in a vehicle that reduces exhaust emissions by 80% to 90% compared to conventional vehicles. It also produces about half the amount of carbon dioxide.
The electric motor uses power from a high voltage battery, which is charged by the internal combustion engine, and by reclaiming the waste energy of decelerating or braking.
The internal combustion engine in a hybrid vehicle is designed to operate only within its most efficient operating range, typically between 2000 and 4500 rpm. Its construction has lighter internal components than would otherwise be required, resulting in an efficient engine with low friction qualities. It may also incorporate other fuel efficient technologies such as shutting down selected engine cylinders, multi runner intake manifolds, and throttleless intake manifolds.
A powerful permanent magnet brushless motor/generator is mounted adjacent to the engine. It draws power from the battery to drive the vehicle at low load and speed situations, as well as assisting the engine to provide more power during acceleration. During deceleration and braking it functions as a generator, reclaiming kinetic energy by converting it to electrical energy and storing it in the battery.
Mounted between the engine and motor is an epicyclic power divider that also functions as a constantly variable transmission. It allocates varying amounts of drive from the engine and the motor to the wheels, selecting the best balance for the present driving condition.
A sealed nickel metal hydride 200-300 volt battery provides higher power output and is lighter than other battery types. Its output may be raised to 500 volts by the control unit to power the electric motors.
A small permanent magnet brushless motor/generator replaces the engine flywheel. It functions as the engine’s starter motor and battery charger. It also provides additional power for the larger main motor/generator.
The motor control unit controls the delivery of electric current to and from the battery and the electric motors. It has its own cooling system to control its operating temperature. It also contains an inverter that is used to change the alternating current produced by both motors into direct current to charge the battery and to convert the direct current from the battery into alternating current to power the electric motors.
Hybrid vehicle driving
When the vehicle is moving off from a stationary position, and when travelling at low to moderate speeds, the main electric motor/generator drives the vehicle. At these speeds the internal combustion engine operation is less efficient and is normally used only to charge the battery.
During normal driving, the combustion engine starts and drives the generator & the power divider. Power from the generator is used to drive the electric motor. The motor control unit controls the power divider so the drive remains at its most efficient.
When accelerating, as well as using power from the combustion engine, the control unit draws power from the battery and directs it to the electric motor/generator, providing more power to the wheels than the combustion engine could supply on its own.
During deceleration and braking the combustion engine is turned off. Inertia from the wheels drives the electric motor/generator, using the current produced to charge the battery. Normally this energy would be wasted as heat, however in the hybrid it is recovered and reused. The retarding effect caused by this provides the same deceleration as normal engine braking would.
Gentle application of the brake pedal does not apply the brakes. The braking effect is achieved by using the electric motor/generator as a retarder, which converts the vehicles momentum into electrical energy to charge the battery.
When the vehicle is stationary and the brakes applied, the combustion engine is stopped and power is not applied to the electric motor/generator. No fuel is burnt and no emissions are produced. Releasing the brakes applies power to the electric motor. Moving the accelerator pedal activates the electric motor and, if required, the combustion engine to drive the vehicle.
