A Hybrid-electric vehicle (HEV) has both a petrol engine and an electric mortor. The petrol engine is the main power source. It is smaller and lighter than the engine of conventional cars. the electric mortor provides extra power when needed. In some HEVs, it is connected to the wheels by the same transmission. In addition to a fuel tank, the HEV carries a pack of advanced batteries. There is also a processor which decides when to use the motor and engine.