The engine – the driving motor – of diesel locomotives is an internal combustion heat engine.Usually it is a four-stroke engine, but in order to achieve higher performance, two-stroke ones were also in use earlier. A peculiar feature of the diesel engine is that its supporting strength and exerted performance is dependent on its revolutions. At a low revolution per minute it cannot be burdened or used directly for driving vehicles. To eliminate this problem, a separate machinery unit has to be built in between the diesel engine and the axles to be driven. This unit is the so-called gear box, in other words a transmission gear. The system of transmission gear to be used is usually selected in accordance with the task of the vehicle. For diesel engines, usually three types of transmission gear are used. In the case of diesel-mechanical transmission, between the diesel engine and the axles to be driven, a cog wheel gear drive is built in, the transmission of which is adjustable gradually. This solution is typical for lower performance locomotives and multiple units. In the case of diesel-hydrokinetic transmission, energy transmission is performed by way of a flowing liquid. In the case of diesel-electric transmission, the driving engine rotates an electricity generator. The energy produced by the generator is used to drive the electric engine coupled to the axle of the locomotive.