This category is for gas turbine engines, not mixed phase (steam) or liquid (hydroelectric) turbines. All turbines are pure rotary systems. In gas turbines, the four stages of operation and combustion (compression, ignition, expansion, exhaust) occur concurrently, in a spatial sequence of different, specialized areas and parts of the engine. This is unlike Otto and Diesel cycle engines, where the four stages of operation occur in a time sequence (strokes).