A 2-stroke engine doesn't have valves, a 4-stroke engine does. In combustion engines the inner energy set free by combustion (e.g. of gasoline or Diesel fuel ) is changed partly into mechanical energy. There are several varieties of combustion engine. The most common is the four-stroke Otto Engine, which was invented by the German merchant and engineer Nikolaus August Otto in 1876. The working mechanism of the four-stroke Otto-Cycle: The engine's cylinder has got two valves, the intake valve and the exhaust valve, which are opened and closed by a camshaft mechanism. A crank moves the piston in the cylinder bymeans of a "connecting rod". The operation of the engine is divided into 4 parts, which are called strokes: -1st stroke (Intake): The piston sucks in the fuel-air-mixture from the carburetor into the cylinder. -2nd stroke (Compression): The piston compresses the mixture. -3rd stroke (Combustion): The spark from the spark plug inflames the mixture. The following explosion presses the piston to the bottom, the gas is operating on the piston. -4th stroke (Exhaust): The piston presses the exhaust out of the cylinder. In a two-stroke engine (e.g. in lawn-mowers or in power saws), the engine is operating every second stroke - there are no valves. It is necessary to use a mix of gasoline and oil (two-stroke oil) as fuel. This is used to lubricate the piston and the crank shaft. 1st stroke The compressed fuel-air mixture ignites and thereby the piston is pressed down. At the same time the intake port is covered by the piston. Now the new mixture in the crankcase becomes precompressed. Shortly before the piston approaches the lower dead centre, the exhaust port and the overflow conduit are uncovered. Being pressurized in the crankcase the mixture rushes into the cylinder displacing the consumed mixture (exhaust now). 2nd stroke The piston is moving up. The overflow conduit and the exhaust port are covered, the mixture in the cylinder is compressed. At the same time new fuel-air mixture is sucked into the crankcase. In both types of engine. by means of a crank shaft the up and down motion is converted into a rotational motion.