Hybrid vehicle drivetrains transmit power to the driving wheels for hybrid vehicles. A hybrid vehicle has multiple forms of motive power.
Hybrids come in many configurations. For example, a hybrid may receive its energy by burning gasoline, but switch between an electric motor and a combustion engine.
Electrical vehicles have a long history combining internal combustion and electrical transmission as in a diesel–electric power-train although they have mostly been used for rail locomotives. A diesel–electric powertrain fails the definition of hybrid because the electric drive transmission directly replaces the mechanical transmission rather than being a supplementary source of motive power. One of the earliest forms of hybrid land vehicle was the 'trackless' trolleybus experiment in The United States (New Jersey) that ran from 1935 to 1948, which normally used traction current delivered by wire. The trolleybus was fitted with an internal combustion engine (ICE) to power the mechanical drivetrain directly, not to generate electricity for the traction motor. This enabled the vehicle to be used for revenue service where there was no contact wire. Since the 1990s trolleybus hybrids have been introduced with small power plants to provide a low speed capability for emergency and maintenance but not to support general revenue service.
The powertrain includes all of the components used to transform stored potential energy. Powertrains may either use chemical, solar, nuclear or kinetic and make them useful for propulsion. The oldest example is the steam locomotive. A common modern example is the electric bicycle. Hybrid electric vehicles combine a battery or supercapacitor supplemented by an ICE that can recharge the batteries or power the vehicle. Other hybrid powertrains use flywheels to store energy.
Among the different types of hybrid vehicles, only the electric/ICE type was commercially available as of 2017. One variety operated in parallel to provide power from both motors simultaneously. Another operated in series with one source exclusively providing the power and the second providing electricity. Either source may provide the primary motive force, with the other augmenting the primary.
Other combinations offer efficiency gains from superior energy management and regeneration that are offset by expense, complexity and the battery limitations. Combustion-electric (CE) hybrids have battery packs with far larger capacity than a combustion-only vehicle. A combustion-electric hybrid has batteries that are light that offer higher energy density that are far more costly. ICEs require only a battery large enough to operate the electrical system and ignite the engine.
Types by design
Parallel hybrid
Parallel hybrid systems have both an internal combustion engine and an electric motor that can both individually drive the car or both coupled up jointly giving drive. This is the most common hybrid system as of 2016.
If they are joined at an axis (in parallel), the speeds at this axis must be identical and the supplied torques will add together (most electric bicycles are of this type). When only one of the two sources is in use, the other must be connected via a one-way clutch or freewheel so it can rotate freely.
With cars the two sources may be applied to the same shaft (for example with the electric motor connected between the engine and transmission), turning at equal speeds and the torques adding up with the electric motor adding or subtracting torque to the system as necessary. (The first two generations of Honda Insight use this system.)
Parallel hybrids can be further categorized by the balance between the different motors are at providing motive power: the ICE may be dominant (engaging the electric motor only in specific circumstances) or vice versa; while in others can run on the electric system alone but because current parallel hybrids are unable to provide electric-only or internal combustion-only modes they are often categorized as mild hybrids (see below).
Parallel hybrids rely more on regenerative braking and the ICE can also act as a generator for supplemental recharging. This makes them more efficient in urban 'stop-and-go' conditions. They use a smaller battery pack than other hybrids. Honda's early Insight, Civic, and Accord hybrids using IMA 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 also employ a parallel hybrid architecture.