In a parallel hybrid, the vehicle has an internal combustion engine and an electric motor. The two systems are connected to the drivetrain by a differential gear. When one source is powering the vehicle, the other source idles and rotates freely by the use of a one-way clutch. Each power source is capable of independently propelling the vehicle. This is the most common design used to create hybrid vehicles. The system automatically determines which source it will use based upon the power demands as you drive. These vehicles have smaller battery packs due to the fact that the engine can recharge the batteries. They deliver their best fuel economy during highway driving.
Mild hybrids are vehicles with electric motors that are not powerful enough to propel the vehicle. The small onboard electric motor is used to power auxiliary devices when the vehicle is idling, braking or coasting. The fossil fuel engine quickly restarts when the demand for power resumes. Since these vehicles do not have electrically powered drivetrains, they do not provide the same level of fuel economy as a full hybrid model. Mild hybrids are beneficial for urban drivers because they do provide some additional fuel savings during city driving. The vehicle's engine does not operate during various portions of the stop-and go trek.
Series hybrid vehicles feature two power sources arrayed in a straight line. As a result, only the electric motor propels the vehicle in these model hybrids. The motor receives electricity from a battery pack or a generator powered by a small internal combustion engine. Unlike a parallel hybrid, the engine in a series hybrid is too small to power the vehicle. The engine recharges the battery pack or supplies additional power to the drive motor and satisfies peak load demand. This is the same design concept utilized in diesel locomotives. Series hybrids are also called Range-Extended Electric Vehicles.
