Two-stroke
From Quad Hub Wiki
The two-stroke internal combustion engine differs from the more common four-stroke engine by completing the same (thermodynamic) cycle in only two strokes of the piston, rather than four. This is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to simultaneously perform the intake and exhaust functions, which is called scavenging. This allows a power stroke for every revolution of the crank, instead of every second revolution as in a four-stroke engine. For this reason, two-stroke engines provide high specific power, so they are valued for use in portable, lightweight applications such as chainsaws as well as large-scale industrial applications like locomotives.
Invention of the two-stroke cycle is attributed to Dugald Clerk around 1880 whose engines had a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Joseph Day (and Frederick Cock for the piston-controlled inlet port).
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Diagram
Reed Inlet Valve Design
This is similar to and almost as simple as the piston port but substitutes a reed type check valve in the intake tract for the piston-controlled port. Reed valve engines deliver power over a wider speed range than the piston port types, making them more useful in applications such as dirt bikes, ATVs, and marine outboard engines. Reed-valved engines do not lose fresh fuel charge out of the crankcase as do piston-port engines.
Power valve systems
Many modern two-stroke engines employ a power valve system. The valves are normally in or around the exhaust ports. They work in one of two ways, either they alter the exhaust port by closing off the top part of the port which alters port timing or by altering the volume of the exhaust which changes the resonant frequency of the expansion chamber. The result is an engine with better low-speed power without sacrificing high-speed power.
Lubrication
Two-stroke engines usually do not have an internal lubrication system; the moving parts are lubricated by a small flow of lubricating oil (usually a special two-stroke oil) which enters the combustion chamber pre-mixed with the fuel. This oil is usually not completely burned during the power stroke, and therefore a portion of it clings to the cylinder walls, passes into the crankcase, and eventually reaches all moving parts of the engine. Handheld devices using this method of lubrication have the advantage of being able to operate in any orientation since there is no oil reservoir (which requires gravity for proper function).
Depending on the engine's application, the oil can be mixed with the fuel manually each time fuel is added, or an oil pump can automatically mix fuel and oil from separate tanks as those liquids flow toward the engine.
