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Motorcycle engine configurations include the single cylinder, the v-twin, the 1-twin, the opposed twin, the parallel twin, the inline triple, and even the flat four and flat six, which are unique to the Honda Gold Wing. For a while there was even an inline six, but they proved to be more trouble than their bulky size was worth. But with all these engine configurations (and there are many other less-prevalent ones not listed here), every conventional motorcycle engine can be placed into one of two categories based on how it works: two-stroke engines and four-stroke engines. No matter how the pistons are arranged, they pretty much do the same thing—they move up and down. The difference is in how many times a piston moves up and down to complete one power cycle. The four-stroke engine is the easiest to explain. On the first stroke the piston is moved downward. The intake valve opens and the fuel/air mixture is drawn into the cylinder in the same way a hypodermic needle draws medicine from a bottle. On the second stroke, the piston moves up to compress the fuel/air mixture. If the timing is right, the spark plug ignites this mixture when it reaches the optimum compression ratio. The third stroke is the power stroke. After being ignited by the spark plug, the air/fuel mixture expands rapidly in what can best be described as a planned and controlled explosion. Those expanding gases drive the piston down the cylinder much the way a bullet is driven from the barrel of a gun. When it reaches the bottom, the exhaust valve opens to relieve the pressure and the crankshaft swings around for an upstroke. That upstroke drives the piston back to the top of the cylinder and helps force out any remaining fuel/air mixture through the exhaust valve. Although two-stroke engines are functionally much simpler than four-stroke engines, their design is harder to understand. Each downstroke in the two-stroke engine is both a power stroke and a drawing-in of the air/fuel mixture. Each upstroke is both an exhaust and a compression stroke. The two-stroke engine does not place the valves on top as a four-stroke engine does. Actually, the two-stroke engine does not usually have valves in the way a four-stroke engine does. Instead, it has ports in the cylinder wall for both intake and exhaust. In a typical two-stroke engine, a mixture of air, fuel, and lubricant enters the crankcase through a reed valve, which actually has nothing to do with the function of the piston moving within the cylinder except in that the reed valve responds to the vacuum created by the piston's movement. The mixture is then compressed in the crankcase by the reciprocation of a piston. When one of the pistons reaches the bottom portion of its stroke, it uncovers a port in the cylinder wall and the pressurized mixture floods the cylinder. The piston then compresses this mixture as it moves upward in preparation of ignition. Towards the top of its stroke, the mixture is ignited by a spark plug, which drives the piston down into the cylinder.
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