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 Wankel engine has four similar phases but does not use a stroke. The cycle is characterized by four strokes, or straight movements in a single direction, of a piston inside a cylinder:

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

The cycle begins at top dead center, when the piston is at its topmost point. On the first downward stroke (intake) 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 (compression) compresses the fuel-air mixture.

The air-fuel mixture 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 (power), and the fourth and final upward stroke (exhaust) 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 or cams . 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 illustrations show an engine with Double overhead cams.

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

Desmodromic valve timing

In the vast majority of four-stroke engines, the valves are closed simply by return springs. As the rotational speed of the engine increases, the time taken for the spring to pull the valve shut can become significant, changing the timing and therefore the engine performance detrimentally.

One solution to this problem is the desmodromic valve timing system. This uses a mechanical arrangement to both open and close the valve positively. Much higher engine speeds can then be obtained.

Some designs use an additional cam and rocker, others a cam which has a channel milled into its vertical face which the follower runs in (as opposed to following the outside profile only, which can allow the follower to deviate from the profile under extreme conditions).

The drawback of the system is its increased complexity and therefore cost. One manufacturer using this system is Ducati, for some of its motorcycle engines.

External link

• Animated Engines: Four Stroke Engine (http://www.keveney.com/otto.html)