Conversion of heat into mechanical work
All motorized vehicles other than purely electric vehicles use heat engines for propulsion.
A heat engine is a device that changes heat energy into mechanical work. It works on the first law of thermodynamics. The basic idea behind a heat engine, whether a steam engine, jet engine or internal combustion engine, is that mechanical work can be obtained only when heat flows from a high temperature to a low temperature. In every heat engine only some of the heat can be transformed into work.
At the heart of every engine is a working substance. In a steam engine the working substance is water, in both its vapor and liquid forms. In an automobile engine the working substance is gasoline–air mixture. If an engine is to do work on a sustained basis, the working substance must operate in a cycle, that is the working substance must pass through a closed series of thermodynamic processes, called strokes, returning again and again to each state in its cycle.
In considering heat engines, we talk about reservoirs. Heat flows out of a high-temperature reservoir and into a low–temperature one.