A cam engine is a reciprocating engine where instead of the conventional crankshaft, the pistons deliver their force to a cam that is then caused to rotate. The output work of the engine is driven by this cam. A variation of the cam engine, the swashplate engine (also the closely related wobble-plate engine), was briefly popular. Cam engines are generally thought of as internal combustion engines, although they have also been used as hydraulic and pneumatic motors. Hydraulic motors, particularly the swashplate form, are widely and successfully used. Internal combustion engines, though, remain almost unknown.