In the parallel hybrid, the engine and motor (run by battery) are mechanically connected to wheels and traction can be provided simultaneously by both the power units. When the engine is unable to meet the power requirement of the vehicle (such as under acceleration), energy from the battery supplements the vehicle demand. Engine is thus, subjected to transient demands and consequently fuel efficiency is poorer and emission penalties occur.

Mixed hybrid

In the mixed hybrid, an alternator run by the engine continues to charge batteries. The power to the wheels flow directly from the engine as well as from the batteries charged by the alternator simultaneously as in the parallel hybrids. Toyota Prius car, one of the most successful HEVs is a mixed hybrid vehicle.

Plug-in hybrid

Most cars run less than 50 to 60 km/day in cities. Thus,  a near zero emission vehicle can be designed if a  small  size battery pack provides this range during city driving and when the vehicle needs to run more distance  the IC engine drives the vehicle. The batteries are charged every day by the mains supply. Such hybrids are called as Plug-in- Hybrid Electric Vehicle (PHEV).

Already a few million HEVs are in use. The gasoline engines used on HEV run on Atkinson cycle to improve thermal efficiency with large reduction in pumping losses. The Atkinson cycle is implemented by late closing of intake valve (72 º to 105º after bdc) while keeping the expansion ratio close to 13:1.  The power output of the engine is increased by supercharging.

• The fuel efficiency improvements of nearly 50% in city driving and 30% on combined city and highway driving have been obtained.
•  HEVs have met the SULEV emission standards (NMHC = 0.01, CO = 1.0, NO_x = 0.02 g/mile).
• HEV powered by diesel engine have obtained 25 % better fuel economy than the comparable diesel vehicle.  The NO_x and PM emissions are lower by nearly   45 and 65 %, respectively.
• The diesel hybrids produce up to 50% less CO₂ than the gasoline engines and 30 to 35% less than the diesel engines making the diesel-hybrid more fuel efficient and environment friendly than the gasoline engine hybrid.