Whether it's the one in your lawn mower or car, all piston engines operate on the same basic principle. They use a four-step process of intake, compression, combustion, and exhaust to convert fuel to energy. Air enters the engine through the intake manifold, where it's mixed with fuel, then compressed and ignited in the cylinder. Finally the resulting gas is released as exhaust. The action of combustion causes the pistons to move and turn the crankshaft, which eventually results in movement of the wheels. A piston engine is composed of several parts, and all should be in good working order for efficient operation of the engine. Problems with interior parts may often be difficult to diagnose, but some external symptoms can indicate engine damage. An obvious sign of possible engine problems could be excessive smoke in your exhaust or a loud metallic scraping or grinding noise coming from the engine. Contact a qualified local mechanic for more information on piston engines.