backstreet: Two-stroke engine

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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.

A two-stroke engine, in this case with an expansion pipe illustrating the effect of a reflected pressure wave on the fuel charge. This feature is essential for maximum charge pressure (volumetric efficiency) and fuel efficiency. It is used on most high-performance engine designs.
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).
Contents
1 Applications
2 Different two-stroke design types
2.1 Piston controlled inlet port
2.2 Reed inlet valve
2.3 Rotary inlet valve
2.4 Crossflow-scavenged
2.5 Loop-scavenged
2.6 Uniflow-scavenged
2.7 Stepped Piston Engine
3 Power valve systems
4 Direct Injection
5 Two-stroke Diesel engines
6 Lubrication
7 Reversibility
8 Sources
9 See also
10 External links
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Applications
Throughout the 20th century, many small motorized devices such as chainsaws and outboard motors were powered by two-stroke designs. They are popular due to their simple design (and resulting low cost) and higher power-to-weight ratios. However, in most designs to date the lubricating oil is mixed with the fuel, which significantly increases the emission of pollutants (due to the oil's incomplete combustion). For this reason, two-stroke engines have been replaced with four-stroke engines in many applications.

A two-stroke minibike.
Two-stroke engines are commonly used in high-power, handheld applications such as string trimmers and chainsaws. The light overall weight, and light-weight spinning parts give important operational and even safety advantages. Only a two-stroke (with a specialized fuel-system) can run a chainsaw and be used in any position.
To a lesser extent, these engines may still be used for small, portable, or specialized machine applications such as outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, tuk-tuks, snowmobiles, karts, ultralights, model airplanes (and other model vehicles) and lawnmowers. The two-stroke cycle is used in many diesel engines, most notably large industrial and marine engines, as well as some trucks and heavy machinery.
A number of main-stream automobile manufacturers have used two-stroke engines in the past, including the Swedish Saab and German manufacturers DKW and Auto-Union. The Japanese manufacturer Suzuki did the same in the 1970s.[1] Production of two-stroke cars ended in the 1960s in the West, but Eastern Bloc countries continued until around 1991, with the Trabant and Wartburg in East Germany and Syrena in Poland. Lotus of Norfolk, UK, has a prototype direct-injection two-stroke engine intended for alcohol fuels called the Omnivore which it is demonstrating in a version of the Exige.[2]
Different two-stroke design types

A Cox Baby Bee 0.049 cubic inch (0.8 cubic cm.) reed valve engine disassembled. It uses glow plug ignition. The weight is 64 grams.
Although the principles remain the same, the mechanical details of various two-stroke engines differ depending on the type. The design types of the two-stroke engine vary according to the method of introducing the charge to the cylinder, the method of scavenging the cylinder (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder.
Piston controlled inlet port
Piston port is the simplest of the designs. All functions are controlled solely by the piston covering and uncovering the ports as it moves up and down in the cylinder. A fundamental difference from typical four-stroke engines is that the crankcase is sealed and forms part of the induction process in gasoline and hot bulb engines. Diesel engines have mostly a roots blower or piston pump for scavinging.
Reed inlet valve
Main article: Reed valve
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...(and so on)