Internal combustion always requires five things: air, fuel, compression, combustion and exhaust. There are two well known means of using pistons in cylinders to obtain power from the air fuel mixture. These are the four-stroke cycle and the two-stroke cycle.

First
four-strokes, Four Cycle

In 1801, a Professor Otto, in whose time there was not yet any such thing, wrote a paper describing how he thought an internal combustion engine might work, and taught that the steps would be: intake, compression, power and exhaust. Most cars operate with four-stroke engines. As most of us learn in school, the four-strokes are: Intake, Compression, Combustion, (or power), and exhaust.

In any four-stroke engine, each cylinder has at least two valves in its head, one or more just for intake, and one or more just for exhaust. The cycle comprises four-strokes and is often called "four cycle." In order to have four movements of the piston, (up twice and down twice), the crankshaft rotates twice.

During the cycle, for every two rotations of the crankshaft, there is one rotation of the camshaft. It is the camshaft which controls the timing of the opening and closing of the valves. Let's begin with compression, and continue in order - combustion, exhaust, intake. The camshaft must be driven by the crankshaft ,by gear, pulley, or chain, so that the crankshaft makes two full rotations for every one full rotation of the camshaft.

During compression, the piston moves up, the crankshaft completes half a revolution, and all the valves are closed. At the end of compression, the combustion stroke begins, with all the valves still closed, a spark ignites the air fuel mixture, and the piston moves back down; the crankshaft turns another half revolution, the camshaft just a quarter.

As the piston approaches the bottom, the exhaust valves open. Then, the piston moves back toward the top as the cam holds open the exhaust valve, and the crankshaft turns another half revolution.

As the piston reaches the top again, the exhaust stroke finishes, and the camshaft turns to force the intake valves open, and to allow the exhaust ports to close. The piston, moving down, sucks in the air and fuel it will use on the power stroke. As the intake stroke ends with the piston at the bottom, all the valves close and compression begins again.

In Diesel type engines there is NO fuel mixed with the intake air; fuel is injected as the compression stroke is completed. Otherwise, there is no significant difference between the cycle of the gasoline or alcohol burning engine and the Diesel.

Second
two-stroke, Two Cycle

two-stroke engines are of two conventional types. First we will discuss the unvalved, which is seen on most model airplanes and lawn mowers, and many small motorcycles.

All of the essential functions of the four-stroke cycle are still present, but very much simplified. Compression takes place twice. The piston moves downward in its cylinder during the power portion of the down stroke, compressing the air fuel and oil mixture in the crank case. In multi cylinder two-stroke engines the crank case may be compartmented to segregate the cylinders.

The power portion of the down stroke ends just as the exhaust port is exposed. Soon thereafter, as the piston continues downward, an intake port is exposed, which is the upper end of a small duct, connecting the crank case to the cylinder. The compressed contents of the crank case, fuel air and oil, spray into the cylinder as the piston reaches the bottom, displacing most of the exhaust gasses, and charging the cylinder with a combustible mixture for the next cycle. Then compression begins in the cylinder, and the piston moves up, expanding simultaneously the volume of the crank case, into which air, fuel and oil are drawn into the crank case.

The piston begins to compress the contents of the cylinder while the exhaust port is still open. Unfortunately, this squeezes out some of the fuel and oil, which is why two-stroke exhaust is blue, dirty, and smelly. Finally, as the piston reaches the top, the spark ignites the remaining mixture, propelling the piston downward, the cycle completed.

Another good system
Valved two-stroke Configuration

There may be others, but the best known valved two-stroke is the Detroit Diesel, introduced in 1937, and produced into the late 1990s. It is very nearly a hybrid of four-stroke technology and two-stroke, with many of the best features of each.

The Detroit Diesel does not utilize the crank case as part of the cycle. Its crank case is isolated from the combustion process. There are ports all around the cylinder, just above the lowest point the top of the piston can descend to. They are surrounded by an air jacket, into which air is pumped by either a positive displacement blower (super charger) driven by the engine mechanically, or a centrifugal blower driven by engine exhaust, a turbo charger.

The head of the cylinder has one or more exhaust valves, but no intake valves. Fuel is supplied by Diesel fuel injectors. The valves in the head are operated by a camshaft, but open at the same point and for the same duration on every revolution.

The Cycle

With the piston at the bottom and the exhaust valve(s) open, air is forced into the cylinder by the blower, and displaces the remainder of the combustion from the previous cycle. The valves close, the piston moves upward, causing compression, and fuel is injected just as the piston almost reaches top. When the piston has been driven down to almost the height of the intake ports, the valves (at the top) open for exhaust, and the cycle begins anew.

Please click here to read more about Conventional two-stroke.