Science Fair Project Encyclopedia
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 four-stroke cycle is more fuel-efficient and clean burning than the two-stroke cycle, but requires considerably more moving parts and manufacturing expertise and the resulting engine is larger and heavier than a two-stroke engine of comparable power output. The later invented Wankel engine has four similar phases but does not use a stroke.
- intake (induction) stroke
- compression stroke
- power (ignition) stroke
- 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.
Starting position, intake stroke, and compression stroke. View an animation.
The air-fuel mixture is then ignited, usually by a spark plug for a gasoline or Otto cycle engine, or by the heat and pressure of compression for a Diesel cycle of compression ignition engine, 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.
Ignition of fuel, power stroke, and exhaust stroke. View an animation.
The valves are typically 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, sometimes through a pushrod, the entire chain of parts being known as the valve train ), 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.
Assuming the engine is robust enough in design not to break, the speed and therefore power output of the engine is typically limited by the ability to flow large volumes of air-fuel mixture or exhaust through the valve openings. Therefore a great deal of work goes into designing this part of an engine. Common strategies are to enlarge the valves to take up as much of the cylinder diameter as possible, to lighten the valve train by eliminating parts, to open the valves as far as possible into the cylinder, or to use multiple smaller valves with more total area. Each of these methods has its drawbacks, causing the recent development of engines with computer controlled valve operation to optimize the engine's operation at any speed and load. The illustrations show an engine with Double overhead cams, a standard strategy for many years for increasing the high-speed capability of an engine.
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. The cam follower then fails to follow the closing profile of the cam, changing the timing and therefore the engine performance detrimentally. To reduce this, lighter valves and stronger springs are used, but there is a practical limit to how low the inertial mass of the valve can be reduced, and increasing the strength of the valve return spring greatly increases the already considerable wear on the camshaft.
One solution to this problem is the desmodromic valve timing system. This eliminates the valve return spring and uses a mechanical arrangement to both directly open and directly 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), others a crank arrangement similar to the crankshaft. 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.
Recent Formula 1 engines have resorted to use of pneumatically operated valves to solve the problem of high acceleration of valve opening and closing without excessive cam wear. The cams are omitted entirely, and the opening and shutting of the valves is driven by high pressure air, controlled by computers. With this system, previously unimaginable engine speeds have become routine.
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