Four-stroke cycle

The four-stroke (4 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.

The Otto cycle is characterized by four strokes, or straight movements in a single direction, of a piston inside a cylinder:

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.

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Fuel ignites

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3 - Power stroke

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4 - Exhaust stroke

Ignition of fuel, power stroke, and exhaust stroke. View an animation.

Contents

1 Valve Timing

1 Valve train

1.1 Desmodromic valve timing

1.2 Pneumatic valves

2 External link

Valve Timing

In its original configuration, the four-stroke engine relies entirely on the piston's motion to intake fuel and air, and to force out exhaust. As the piston descends on the intake stroke, a partial vacuum is created within the cylinder which draws in the fuel/air mixture. The intake valve then closes, the piston ascends, and the mixture is compressed and ingited, causing the piston to descend again. As the exhaust valve opens, the piston forces the exhaust out. This was the technique used in early four-stroke engines. It was soon discovered, however, that at RPMs approaching 100 or greater, the exhaust gasses could no longer change direction quickly enough to exit through the exhaust valves by the piston's motion alone.

At high RPMs, consistent flow through the intake manifold and exhaust is maintained by allowing the intake and exhaust valves to be open simultaneously at top dead center. Soon after ignition of the fuel/air charge, as the piston descends, it becomes less useful to retain the hot, high-pressure gasses within the cylinder. To this end, the exhaust valve is typically opened at about twenty degrees of crankshaft rotation before bottom dead center. This allows the exhaust more time to escape without significant power loss.

As the piston ascends through the exhaust stroke, the intake valve will be opened, also approximately twenty degrees before top dead center. Ideally, the exhaust flow will cause a lower pressure within the cylinder, pulling the fuel/air mixture in more easily. Resultingly, both valves may be open simultaneously for more than forty-five degrees of rotation, a technique called valve overlap. Under ideal conditions, the fresh fuel/air charge will pull remaining exhaust gasses out the cylinder before the exhaust valve closes, leaving only a clean fuel/air mixture. Aiding the exhaust flow in this way is called scavenging. The disadvantage is lower fuel efficiency due to waste fuel/air going through the exhaust. It can, however, cool the exhaust valve, which is important at high speeds, such as races.

Exhaust noise and emissions equipment may impede smooth exhaust flow out of the cylinder; When exhaust ports are close together in a common manifold, the pressure wave of another exhausting cylinder may interfere with the first, trapping exhaust gasses. The same effect occurs in the intake manifold, generally being too restrictive for optimum power production. Also, excessive pressure in the cylinder may cause an exhaust back-flow into the intake manifold when the intake valve opens.

Accomplishing maximum volumetric efficiency for a given engine is not a formulaic process. Different intake and exhaust equipment is tested at different speeds and loads. The end result is always a compromise, and automobile manufacturers will usually choose the most cost effective solution.

Valve train

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.

Pneumatic valves

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 nitrogen, controlled by computers. With this system, previously unimaginable engine speeds have become routine.