In a piston engine, the cylinder does four separate jobs in the same space; in a rotary engine, the intake, compression, combustion and exhaust all happen in different places. Another big difference between piston and rotary engines is that rotary engines have fewer moving parts: three in a two-rotor engine. Meanwhile, a typical four-cylinder piston engine has a minimum of 40 parts that move, including pistons, camshafts, valves and timing belts.
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