One variation is to have the wheels driven only by the electric motor or motors, current coming from batteries. Batteries can be smaller than in an all-electric car because they only have to supply current for short periods. The hybrid car still has the range of a conventional petrol or diesel car. A second variation is to have a relatively small petrol engine drive the wheels through a mechanical transmission. An electric motor provides assistance when high power is needed - overtaking and climbing hills.
Some engine power is diverted to charging the batteries at times of low power demand.
It is said to achieve a low consumption figure of 3.85 litres per 100km. A hybrid car can use a smaller internal combustion engine which spends most of its time operating in the more efficient part of its range. Also see the Toyota Prius . This could be achieved by having an exhaust stroke that is longer than the compression stroke. At first sight this seems to be a geometric impossibility, but it can be managed by sacrificing some of the upward movement of the piston - leaving the inlet valves open for a while.