A diesel locomotive is powered by a diesel engine. The diesel engine may drive the locomotive's wheels directly via a gearbox, shaft or chain (called mechanical transmission) or by using a hydraulic transmission system (diesel-hydraulic). Most of the locomotives in the world are diesel-electric, due to their ease of use and reliability. They generate electricity and use it to power the wheels. They can be more powerful than steam locomotives and do not need an expensive power grid like electric locomotives.