Full hybrid is often used when the vehicle can launch forward at low speeds without consuming any gasoline. The Toyota, Lexus and Ford hybrids can do this. The Honda hybrids can’t. Some General Motors hybrids are full hybrids and others are not. (Did we lose you yet?)
Mild hybrid cars move from a standstill only if the internal combustion engine is engaged, and use the electric motor primarily to assist the gas engine when extra power is needed. Both full and mild hybrids require use of the gas engine when reaching higher speeds (of about 20 – 25 mph or more, depending on how the car is driven.)
Mild hybrid systems can broken down into subcategories:
- The Stop/Start hybrid system, used on GM trucks for example, shuts the engine off when it would otherwise idle and restarts it instantly on demand.
- The Integrated Starter Alternator with Damping (ISAD) hybrid system allows the electric motors to help move the vehicle in addition to providing stop/start capability.
- The Integrated Motor Assist (IMA) hybrid system is similar to the ISAD but has a larger electric motor and more electricity to help move the vehicle.
Open to Interpretation
Dave Reuter, a technical consultant for HybridCars.com, includes the Honda system in the list of full hybrids, based on its voltage level, electrical energy storage on board, and regen braking capacity. Dave feels that that the Chevy truck is the only real mild hybrid system presently on the market. Dave says, “Most mild hybrids are start/stop units,” referring to capability for hybrids to slip into electric mode when coming to a stop, rather than idling. The Union of Concerned Scientists uses the term "hallow hybrid," to refer to GM’s stop/start hybrids, including the Saturn Vue and Aura Green Line.
In other words, the fullness or mildness of a hybrid is a contiuum, not an absolute. The goal is to use every means possible to increase the efficiency and performance of the car, while relying on the gasoline internal combustion engine as little as possible. The degree to which the vehicle uses its electric power sources, or reduces resistance or weight for that matter, is the degree of its “fullness” as a hybrid.
Parallel versus Series Hybrid
If the fullness and mildness dichotomy is confusing to you, then the parallel versus series definitions will be impossible and/or contentious. Let’s keep it simple, and allow the debate over terms to take place in our discussion forum.
In a parallel hybrid, the fuel tank supplies gasoline to the engine, while at the same time, a set of batteries supplies power to an electric motor. Both the electric motor and the gas engine can provide propulsion power. By contrast, in a series hybrid, the gasoline engine turns a generator, and the generator can either charge the batteries or power an electric motor that drives the transmission. Thus, the gasoline engine never directly powers the vehicle. Today’s hybrids are all parallel hybrids, although some would argue that the Prius has characteristics of a parallel and a series hybrid. The folks from GM don’t want to use the term "series hybrid" to refer to their Chevy Volt concept vehicle (for marketing purposes), but that’s what it is. Or to be more precise, it’s a "plug-in series hybrid." What’s that?
Just when the American public is finally starting to understand that you don’t have to plug hybrid cars in, here comes the plug-in hybrid. With the plug-in hybrid, you still will not be required to plug the car in, but you’ll have the option. As a result, drivers will get all the benefits of an electric car, without the biggest drawback: limited range. You’ll be able to go all-electric for the ninety percent of your driving which takes place close to home. When the electric charge runs out, a downsized gas engine kicks in and your car drives like a regular hybrid.