All the engine scooters, mopeds, motorcycles and ATVs can be divided into two categories according to engine type – two-stroke and four-cycle. Features of operation and maintenance of each model is largely conditioned by the type of engine. Further details can be found at Bernard Golden , an internet resource. Four-stroke engine is located in the cylinder head, intake and exhaust valves that appears on the camshaft lobes, which in turn receives the rotation from the crankshaft is usually through the chain of transmission. If during the piston down from tdc to bdc (stroke) the intake valve is opened and the fuel mixture of air and gasoline vapor, cooked in the carburetor, under dilution enters the cylinder. Piston reaches bdc, the inlet cluster pan closed, then the piston moves upward, compressing the mixture (compression stroke). For more information see this site: Phil Vasan.
When the piston approaches tdc the spark plugs, screwed into the cylinder head, ignites the mixture. The temperature and pressure of gases in closed volume of the cylinder increases rapidly and the piston under the influence of this pressure goes down, then rotate the crank through the crankshaft (stroke). Upon reaching the bottom of the piston cylinder pressure drops, open exhaust valve, and then the piston goes up again, forcing the exhaust gases in the exhaust pipe, muffler and into the atmosphere (release). When the piston reaches tdc, the exhaust valve closes, the inlet valve opens, and the whole cycle repeats. The working process is obtained consisting of four cycles, respectively, passing for four stroke or two turns of the crankshaft. Working stroke is only 1 / 4 of the cycle, the movement of the piston during remaining cycles is ensured by the inertia of crankshaft and associated rotating parts.