[SaabJohan]

A rotary engine does three powerstrokes per 3 crank revs, the pistons are running with a third the rev of the crank. This gives them a power to volume ratio similar to the twostroke engine which also have one combustion for each rev. But the rotary does also have a lower thermal efficiency (higher cooling area) and higher exhaust emissions.

There is also an engine type which looks something like this -<:>-, basicly two V blocks which has their own crankshafts (the : are supposed to be the two cranks), but the engine has two cylinders for each combustion chamber (the - are the combustion chambers).