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How a Four-Stroke Cycle Small Gas Engine Operates
Introduction
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Four-stroke engines are in use for an ever-increasing number of applications, from string trimmers to large trucks. Small or large, they all operate on the same principle. These engines are called four-stroke because there are four stages to the power-making process for each cylinder. Four-stroke engines are quieter, more fuel efficient and cleaner burning than the two-stroke engines they replace.
Engine Cycles
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The first stroke is called the intake stroke. During this stroke, the piston is traveling down in the cylinder and the intake valve is open. The downward motion of the piston creates a vacuum in the cylinder, which draws the fuel/air mixture through the carburetor or injection system and fills the cylinder.
Next is the compression stroke. The piston is now traveling upward in the cylinder, and both the intake and exhaust valves are closed. This forces the fuel air mixture into a very small space at the top of the cylinder when the piston achieves the top of its stroke. More power is generated when the fuel/air mix is compressed greatly, so high-performance engines have a higher compression ratio. This is an expression of the area at the top of the cylinder with the piston at the top, as compared to the area of the cylinder with the piston at the bottom of its stroke.
The third stroke is the power stroke. This is the result of the sparkplug firing and igniting the compressed fuel/air mix in the cylinder. The resulting explosion forces the piston down rapidly, causing the crankshaft to rotate. Note that four-stroke engines only produce a power stroke every other crankshaft revolution; two-stroke engines make power at every turn of the crankshaft.
The fourth and final stroke is the exhaust stroke. The exhaust valve opens, and the rising piston pushes the burned fuel/air mix out of the cylinder. As the piston nears the top of the stroke, the camshaft closes the exhaust valve and opens the intake valve to prepare the cylinder for its repeat of the intake stroke.
Valves
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The action of the intake and exhaust valves is precisely timed to the piston travel. Valve action is controlled by the camshaft. The camshaft is turned at one half of the crankshaft speed and is driven by the crankshaft, either through gears, timing chain or timing belt.
eHow Article: How a Four-Stroke Cycle Small Gas Engine Operates
