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, electrical generators, etc). The Thermodynamics cycles used in internal combustion reciprocating engines are the Otto Cycle (the ideal cycle for spark-ignition engines) and the Diesel Cycle (the ideal cycle for compression-ignition engines). The Otto Cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume. It was conceptualized by the French engineer, Alphonse Beau de Rochas in 1862 and independently, by the German engineer Nicolaus Otto in 1876

The Otto cycle

The Otto cycle is characterized by four strokes, or straight movements alternately, back and forth, of a piston inside a cylinder:

1. intake (induction) stroke

The intake stroke (A.K.A the induction stroke) in relation to an internal combustion engine is the downward stroke of the piston creating a partial vacuum that draws a fuel/air mixture into the combustion chamber.


2. compression stroke

The compression stroke is the second of four stages in an otto cycle or diesel cycle internal combustion engine.

In this stage, the air is compressed to the top of the cylinder by the piston until it is either ignited by a spark plug as in an otto engine or, as in the case of a diesel engine, reaches the point at which the fuel spontaneously combusts, forcing the piston back down.

Compression serves to increase the proportion of energy which can be extracted from the hot gas and should be as high as is practical for a given application.



3. power (combustion) stroke

A power stroke is, in general, the stroke of a cyclic motor which generates force. It is used in describing mechanical engines and molecular motors such as ATP synthase. Many types of motors can be simply described by first, intake stroke (intake of fuel, e.g. gasoline, ATP, etc.) then power stroke and last exhaust stroke (exhaustion of what's left of the fuel which is now in a low energy state), possibly with some steps in between such as the compression stroke in four-stroke cycle engines and then repeating the cycle.

In muscles, the power stroke is the stage of muscle contraction when the cross-bridge (connecting the actin in the thin filament to the myosin in the thick filament) moves towards the H-zone, thus causing the muscle fiber to contract. The energy for this process comes from ATP present in the myosin prior to contraction.

In sports, too, often a swing with a lot of force will be called a power stroke. For example, this is used in canoeing to describe a powerful motion with a paddle.



4. exhaust stroke

The exhaust stroke is the fourth of four stages in an internal combustion engine cycle. In this stage gases remaining in the cylinder from the fuel ignited during the compression step are removed from the cylinder through a valve at the top of the cylinder. The gases are forced up to the top of the cylinder as the piston rises and are pushed through the opening which then closes to allow fresh air/fuel mixture into the cylinder so the the process can repeat itself.



The cycle begins at top dead centre (TDC), when the piston is furthest away from the crankshaft. On the first stroke (intake) of the piston, a mixture of fuel and air is drawn into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s) and the following stroke (compression) compresses the fuel-air mixture.

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 or 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 stroke (exhaust) evacuates the spent exhaust gases from the cylinder past the then-open exhaust valve or valves, through the exhaust port.