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