A two-stroke, or two-cycle, engine is a type of internal combustion engine which completes a power cycle in only one crankshaft revolution and with two strokes, or up and down movements, of the piston in comparison to a “four-stroke engine“, which uses four strokes. This is accomplished by the end of the combustion stroke and the beginning of the compression stroke happening simultaneously and performing the intake and exhaust (or scavenging) functions at the same time.
Two-stroke engines often provide high power-to-weight ratio, usually in a narrow range of rotational speeds called the “power band”. Compared to 4-stroke engines, they have a greatly reduced number of moving parts, are more compact and significantly lighter.
The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk, who in 1881 patented his design, his engine having a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day.
Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications such as chainsaws and motorcycles. They are used in diesel compression ignitionengines in large and weight insensitive applications, such as ships, locomotives and electricity generation. The heat transfer from the engine to the cooling system is less in a two-stroke engine than in a four-stroke. This adds to the overall engine efficiency. Two-stroke engines have higher exhaust emissions than four-stroke engines
–> Two piston movements to complete its engine cycle.
–> ports are used
–> fuel consumption is more
–> oil is added to fuel
–> elevate emission of hydrocarbon
–> less weight
–> easy to clear water out of engine.
A four-stroke engine (also known as four-cycle) is an internal combustion engine in which the piston completes four separate strokes which comprise a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction. While risqué slang among some automotive enthusiasts names these respectively the “suck,” “squeeze,” “bang” and “blow” strokes. they are more commonly termed
- INTAKE: this stroke of the piston begins at top dead center. The piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port.
- COMPRESSION: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the cylinder head.
- POWER: this is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed air–fuel mixture in a gasoline engine is ignited, by a spark plug in gasoline engines, or which ignites due to the heat generated by compression in a diesel engine. The resulting pressure from the combustionof the compressed fuel-air mixture forces the piston back down toward bottom dead centre.
- EXHAUST: during the exhaust stroke, the piston once again returns to top dead centre while the exhaust valve is open. This action expels the spent fuel-air mixture through the exhaust valve(s).
–> Four piston movements to complete its engine cycle.
–> valves are used
–> Fuel consumption is less when compares to two stroke.
–> no oil is added to fuel
–> clean emissions.
–> good fuel efficiency
–> Quiet when compared to rwo stroke.
–> Increased weight
–> more involved to clear out of engine.