In the context of a combustion engine, the term stroke has the following related meanings:
- A phase of the engine's cycle (eg compression stroke, exhaust stroke), during which the piston travels from top to bottom or vice-versa.
- The type of power cycle used by a piston engine (eg two-stroke engine, four-stroke engine).
- "Stroke length", the distance travelled by the piston in each cycle. The stroke length- along with bore diameter- determines the engine's displacement.
Phases in the power cycle
Commonly-used engine phases/strokes (ie those used in a four-stroke engine) are described below. Other types of engines can have very different phases.
The induction stroke is the first stroke in a four-stroke internal combustion engine cycle. It involves the downward movement of the piston, creating a partial vacuum that draws a fuel/air mixture (or air alone, in the case of a direct injection engine) into the combustion chamber.
In a reciprocating engine, it is that portion of the cycle when the pistons move from TDC (top dead center) to BDC (bottom dead center) and the fuel-air mixture is drawn into the cylinders....
This is a cylinder for a 4-stroke Petrol/Gasoline engine. The first step is to get the air-fuel mixture into the chamber. Mixture enters through an inlet port that is opened and closed by an inlet valve. This is called the Intake Manifold.
In this stage, the mixture (in the case of an Otto engine) or air (in the case of a Diesel engine) is compressed to the top of the cylinder by the piston until it is either ignited by a spark plug in an Otto engine or, in the case of a Diesel engine, by the fuel being injected into the compressed (and thereby heated) air, forcing the piston back down. In a Diesel engine, the injection of fuel usually leads top dead center by about 4 mechanical degrees, this "lead" being intended to allow complete fuel ignition to occur slightly after top dead center.
Compression serves to increase the proportion of energy which can be extracted from the hot gas and should be optimised for a given application. Too high a compression can cause detonation, which is undesirable compared with a smooth, controlled burn. Too low a compression may result in the fuel/air mixture still burning when the piston reaches the bottom of the stroke and the exhaust valve opens.
A power stroke is, in general, the stroke or movement of a cyclic motor while generating force and thus power. It is used in describing mechanical engines. This force is the result of the spark plug igniting the compressed fuel-air mixture (in Petrol Engines).
The exhaust stroke is the fourth of four stages in a four stroke 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 an exhaust 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 a fresh air/fuel mixture into the cylinder so the process can repeat itself.
Types of power cycles
The thermodynamic cycle used by a piston engine is often described by the number of strokes to complete a cycle. The most common designs of for engines are two-stroke and four-stroke. Less common designs include five-stroke engines, six-stroke engines and two-and-four stroke engines.
Two-stroke engines complete a power cycle every two strokes, which means a power cycle is completed with every crankshaft revolution. Two-stroke engines are commonly used in outdoor power tools (eg lawnmowers and chainsaws) and motorcycles.
Four-stroke engines complete a power cycle every four strokes, which means a power cycle is completed every two crankshaft revolutions. Most automotive engines are a four-stroke design.