The Four-stroke cycle reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Four-stroke cycle

The four-stroke cycle of an internal combustion engine is the cycle most commonly used for automotive and industrial purposes today (cars and trucks, generators, etc). It was invented by Nikolaus Otto in 1876, and is also called the Otto cycle. The cycle is characterized by four strokes, or straight movements in a single direction, of the piston:

1. intake (induction) stroke
2. compression stroke
3. power (ignition) stroke
4. exhaust stroke

One way to remember the four strokes and their functions is the series "suck, squeeze, pop, phooey", or alternatively "suck, squeeze, bang, blow". The four "strokes" are also present at each stage of a jet engine, where they are performed simultaneously rather than as a sequence.

On the first downward stroke of the piston, a mixture of fuel and air is drawn into the cylinder through the intake valve or valves. The intake valve then closes, and the following upward stroke compresses the fuel-air mixture, which is then ignited, usually by a spark plug, at approximately the top of the compression stroke. The resulting expansion of burning gases then forces the piston downward for the third stroke, and the fourth and final upward stroke evacuates the spent exhaust gases from the cylinder through the then-open exhaust valve or valves.

The valves are operated by a camshaft, which is a rod with a series of oblong protrusions called lobes. As the camshaft rotates, the lobes push against the valves (usually via an intermediate component known as a tappet or lifter), causing them to open at the appropriate time. The valves are spring-loaded, closing after the protruding camshaft lobe releases the valve. Each valve opens only once during the four-stroke cycle; that is, the camshaft makes one rotation for every two rotations of the crankshaft.

The four-stroke cycle is more efficient than the two-stroke cycle, but requires considerably more moving parts and manufacturing expertise.

External link

• Animated Engines: Four Stroke Engine

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