The 4 stroke engines, called also Otto Engines, has 4 phases to make a complete cycle. 1 intake- intake valve is open to let fuel and air mixture into combustion chamber while the piston comes down from TDC. Here an stoichiometrical mixture is formed by 14.1 air and 1 fuel. 2 compression- the intake valve closes, the piston comes back to TDC and here the mixture is compressed to a certain ratio. 3. Power- fuel and air mixture is ignited by the spark plug increasing the pressure in combustion chamber forcing the piston to come down . 4. Exhaust - the combustion gas is formed, the piston comes back from the inertia of previous stroke, the exhaust opens to facilitate exhaust gases to leave the engine. Thus, the crankshaft makes two complete rotations in order to complete 1 cycle.