A diesel-electric transmission system includes a diesel engine connected to an electrical generator, creating electricity that powers electric traction motors. No clutch is required. Before diesel engines came into widespread use, a similar system, using a petrol (gasoline) engine and called petrol-electric or gas-electric, was sometimes used. Early examples were buses by Tilling Stevens built in the UK in 1906. In some high-efficiency applications, electrical energy may be stored in rechargeable batteries, in which case these vehicles can be considered as a class of hybrid electric vehicle.