In a series design, the internal combustion engine is not directly connected to the drivetrain at all, but powers an electrical generator instead. This is similar to the operation of diesel-electric train locomotives, except that as of 2006, the overwhelming majority of diesel-electric locomotives do not store auxiliary power in batteries for use in propulsion. A series hybrid is similar to an electric car which is recharged by electricity from a stationary fossil fuel power plant, except that the power plant is carried on board.
Electricity from the generator is fed to the motor or motors that actually move the car, and excess energy can be used to charge batteries. When large amounts of power are required, electricity comes from both the battery pack and the engine-generator section. Because electrical motors can operate quite efficiently over a wide range of speeds, this design removes or reduces the need for a complex transmission. The internal combustion engine can also be finely tuned to operate at its most efficient speed whenever it is running, for a great gain in efficiency. Separate small electric motors installed at each wheel are featured in some prototypes and concept cars; this allows the possibility of easily controlling the power delivered to each wheel, and therefore simplifies traction control, all wheel drive, and similar features.
The advantage of this type of hybrid is the flexibility afforded by the lack of a mechanical link between the internal combustion engine and the wheels. A weakness of a series hybrid system, however, is that series hybrids require separate motor and generator portions, which can be combined in some parallel hybrid designs; the combined efficiency of the motor and generator will be lower than that of a conventional transmission, offsetting the efficiency gains that might otherwise be realized. Still, series hybrids are useful in driving cycles that incorporate many stops and starts, such as for delivery vehicles, or stop and go city driving. It is likely that some fuel cell cars will use a series-style setup, with the fuel cells replacing the engine-generator section; this would eliminate the loss of efficiency inherent in converting the mechanical output of an internal combustion engine to electrical power.
