Hybrid Vehicle Drivetrain - Types By Drive Train Structure - Parallel Hybrid

Parallel hybrid systems, which are most commonly produced at present, have both an internal combustion engine (ICE) and an electric motor coupled. If they are joined at an axis in parallel, the speeds at this axis must be identical and the supplied torques add together. Most electric bicycles are of this type. When only one of the two sources is being used, the other must either also rotate in an idling manner, be connected by a one-way clutch, or freewheel. With cars, the two sources may be applied to the same shaft- for example with the electric motor lying between the engine and transmission. The speeds are thus equal and the torques add up, with the electric motor adding or subtracting torque to the system as necessary. The Honda Insight uses this system.

An alternative parallel hybrid layout is the 'through the road' type. Here a conventional drivetrain powers one axle, with an electric motor or motors driving the other. The batteries can be recharged through regenerative braking, or by loading the electrically driven wheels during cruise. Power is thus transferred from the engine to the batteries through the road surface. This layout also has the advantage of providing four-wheel-drive in some conditions. An example of this principle is a bicycle fitted with a front hub motor, which assists the cyclist's pedal power at the rear wheel.

Parallel hybrids can be further categorized depending upon how balanced the different portions are at providing motive power. In some cases, the combustion engine is dominant (the electric motor turns on only when a boost is needed) and vice versa. Others can run with just the electric system operating. But because current parallel hybrids are unable to provide all-electric (ICE=OFF) propulsion, they are often categorized as mild hybrids (see below).

Because parallel hybrids can use a smaller battery pack as they rely more on regenerative braking and the internal combustion engine can also act as a generator for supplemental recharging, they are more efficient on highway driving compared to urban stop-and-go conditions or city driving. Honda's Insight, Civic, and Accord hybrids are examples of production parallel hybrids. General Motors Parallel Hybrid Truck (PHT) and BAS Hybrids such as the Saturn VUE and Aura Greenline and Chevrolet Malibu hybrids are also considered as utilizing a parallel architecture.