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Four Types of Hybrids
Written by PFM Staff
All hybrids use a gasoline engine and an electric motor with its own high-voltage battery. However, at least four variations of a hybrid exist. These differ dramatically on initial cost, complexity, modes of powering the vehicle and the resulting fuel economy.
The “full” hybrid uses a gas engine and a battery-powered motor in parallel. The gas engine shuts off while coasting at low speeds to a stop and at idle. Under slow acceleration from a stop, the vehicle is powered by only the electric motor up to speeds around 25 mph. Under heavy acceleration, the gas engine starts up and both the gas engine and electric motor power the vehicle.
GM calls the “full” hybrid a “parallel” hybrid. The fuel savings come during engine shut off during coasting at low speeds and at a stop and during mild low speeds acceleration when the vehicle is powered only by the battery.
A “mild” hybrid uses a gas engine for propulsion and a big electric motor to start the engine but not to power the vehicle. The gas engine shuts off while coasting at low speeds to a stop and at idle. Once shut off, the gas engine starts as soon as the brake pedal is released.
GM calls this stop-start hybrid its Flywheel Alternator Starter, FAS. The gas engine starts up to power the vehicle from a stop. The fuel savings come only during the engine shut off during coasting and at a stop.
The two newer systems, improvements on each of the first two systems, are called the “two-mode” hybrid and the Belt Alternator Starter, BAS.
The two-mode system is an upgrade to the full hybrid. It uses a gas engine and two, smaller electric motors. These three power sources (gas engine, two electric motors) work solo and in combination with one another, depending on vehicle speed, throttle position and the gear the trans is in, among many other factors. Significantly more complex than the earlier full hybrid, the two-mode hybrid delivers better fuel economy.
Like the full hybrid, the gas engine on the two-mode hybrid shuts off during coasting and at a stop. During moderate acceleration, the two-mode hybrid uses one or both of the electric motors. Under heavy acceleration, the gas engine starts.
The Belt Alternator Starter (BAS) is an upgrade to the Flywheel Alternator Starter (FAS), the original stop-start hybrid. The FAS gas engine shuts off during coasting and at a stop but does not help to power the vehicle. The BAS engine only shuts off after a full stop but slightly assists the vehicle during acceleration. The engine restarts as soon as the brake is released. The gas engine is used to propel the vehicle under most situations.
Under heavy acceleration, the same battery and electric motor that starts the engine is also designed to kick in, ever so slightly assisting acceleration. The electric motor spins the engine to start, and on heavy demand spins the already running engine to help the acceleration. Both the original FAS system and the upgraded BAS system rely on the gas engine for all (FAS) or most (BAS) of the propulsion.
The full (parallel) and two-mode hybrids are expensive options but yield the most fuel savings. The mild (FAS) and BAS hybrids are less expensive but yield less fuel savings.
In all these hybrids, a separate 36-volt electrical system is used to operate all of the gas engine accessories. With the gas engine on or off, the 36-volt system powers the steering, and assists the brakes. The air conditioning changes to recirculate mode during the time the gas engine is off.
All these hybrids use regenerative braking. The regenerative braking is just that, braking. You can feel the car being slowed down by an action other than yours. This is noticeable but not distracting and does not affect drivability.
The same system that powers the gas engine to start or under acceleration acts as a generator to change the main battery during deceleration. The Malibu Hybrid can regenerative brake (charge) and belt assist at all speeds and in all gears. In the Malibu Hybrid, almost any full throttle-off results in regenerative braking. Of course, all foot-pedal braking results in regenerative braking.
Published in Police Fleet Manager, Jan/Feb 2009
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