Most aircraft piston engines are the reciprocating type (i. e., positive displacement, intermittent combustion): The smaller ones have an air-cooled, two-stroke cycle; the
Figure 10.8. Aircraft piston engine with installation components
larger ones (typically, more than 200 HP) have a liquid-cooled, four-stroke cycle. There are a few rotary-type positive-displacement engines (e. g., Wankel) – attractive in principle but they have sealing problems. Cost-wise, rotary-type positive- displacement engines are not yet popular; cost will decrease with increased production. Figure 10.8 shows an aircraft piston engine with installation components.
To improve high-altitude performance (with low air density), supercharging is used. Figure 10.8 shows a vane-supercharging type for precompression. Also, AVGAS differs slightly from MOGAS. Recently, some engines for the homebuilt category have been allowed to use MOGAS. Recently for small aircraft application, diesel fuel-powered piston engines have appeared in the market.
Piston engines are the oldest type used for powering aircraft. Over the life cycle of an aircraft, gas turbines are more cost effective for engine sizes of more than 500 HP. Currently, general-aviation aircraft are the main users of piston engines. Small recreational aircraft invariably are powered by piston engines.