Almost all aircraft piston engines are four-stroke engines, with one power stroke per piston every other crankshaft revolution and a great deal of intricate moving parts which are only too happy to break. Two-stroke engines, in contrast, have a number of features that would seem to make them excellent choices for aviation use:
- They have one power stroke per piston per crankshaft revolution, twice as many as a four-stroke engine, and also have far fewer moving parts, making them smaller and lighter than four-stroke engines of the same piston size and count; these two factors add up to give them a phenomenally high power-to-weight ratio, much better than that of a comparable four-stroke engine.
- The same mechanical simplicity that contributes to their high power-to-weight ratio also makes them cheaper to build and sell, and (all else being equal) more reliable than four-strokes.
- Most two-stroke engines are lubricated by mixing lubricant into the fuel rather than using a traditional oil sump; this allows them to operate happily in any orientation, including upside down.
The sole disadvantage of two-strokes is their greater fuel consumption and emissions, but:
- Good design can reduce this problem to insignificant proportions;
- Higher fuel costs could easily be offset by the reduced maintenance costs of a more reliable engine1 and the ability (due to the engine's greater power) to use fewer engines per aircraft, and;
- When it comes to piston-engined-aircraft emissions, there are far worse concerns.
Yet, despite these advantages, to the best of my knowledge, only one two-stroke aircraft engine has ever entered mass-production (the highly-unusual Junkers Jumo 204/205/207 series), with a second (the Rolls-Royce Crécy) being cancelled during static-fire testing.
Why are two-strokes essentially nonexistent as aircraft engines?
1: Especially given that maintenance costs are often (though not always) a greater expense than fuel costs to begin with (scroll to page 20); the linked document is discussing jetliners, but the preponderance of maintenance costs would be much more pronounced for piston-engined aircraft, as piston engines are considerably more efficient and considerably less reliable than turbine engines (the latter due mainly to their massive mechanical complexity).