How Two-Stroke Engines Work

This essay discusses the history of two strokes, talks about its differences from four-stroke power plants, and discusses the advantages and disadvantages of this system.

There is no more distinctive sound in the world of internal combustible engines than the two-stroke. There is no mistaking the sound of a high-revving stroker, whether in a lawn mower, leaf blower, boat or motorcycle. For some the sound evokes memories of greasy café racers pulling ahead of mighty inline four motorcycles, but others associate the "ringa-ding-ding" with the not unpleasant odor of autumn, when leaf blowers and chainsaws work overtime to keep up with fall.

What is a two-stroke engine and why does it sound (and smell) so distinctive? Why are so many outdoor motors, sump pumps, chainsaws, lawnmowers, leaf blowers and motorcycles powered by this engine design, and what's the fuss about pollution? This essay will discuss the history of the two-stroke engine, talk about how it differs from more traditional four-stroke power plants, and discuss some of the advantages and disadvantages of this system.

In 1878 Sir Dugald Clark developed the first two-stroke engine. Clark was an engineering genius and later contributed designs for England during World War II, but he did not patent the engine design. In 1904 Alfred Scoff patented the first two-stroke, a v-twin, and later developed a rotary valve engine in 1912.

Refinements followed, although the basic design remained. Some engines used two exhaust ports, while others made use of three or even a rotary configuration, but all relied on the same economy of parts, and the 1960's Suzuki and Yamaha both won World Championship using two-stroke engines.

In the 1960s and early 1970s motorcycle companies poured their resources into the two-stroke, realizing how efficient and innovative the design was. Yamaha developed an "Autolube" system to make the system more user friendly - thereby reducing the need for pre-mixes gasoline / oil mixtures - while Suzuki produced classics such as the T500 and Kawasaki released its fearsome H1 and H2 super bikes.

In the 1970s interest in two-stroke motorcycles began to wan under political pressures. Yamaha's 1979 RD400 Daytona looked as though it would be the last of the breed, but the company released a clean-air version of its two-stroke in the RZ350 (and later a very-limited edition RD500) and Suzuki countered by producing the RG250, arguably the fastest, most refined two-stroke motorcycle engine ever produced. The Italian motorcycle company Aprilia used the RG's engine in its RS250 until fairly recently, when the bike was discontinued.



Although no longer used in street motorcycles, the two-stroke engine is a favorite on off-road motorcycles, four-wheelers, skimobiles, jet skis, and other high-performance machines and is used in lawnmowers, chainsaws, leaf blowers and other lawn and garden tools.

The lubrication system on a two-stroke is different than a four-stroke. In a two-stroke engine oil is usually injected into the fuel / air mix or added to the fuel itself, and is burnt with the fuel. This spent oil accounts for the familiar smell associated with two-strokes as well as the cloud of blue-ish smoke that exists their exhausts.

Automobiles traditionally use four-cycle engines and make use of a camshaft to regulate when vales open and close. In a traditional four-cycle engine the intake phase is followed by the compression phase, explosion phase and exhaust phase. The movement of the piston in the cylinder is coordinated so that each phase, or "stroke," is separate from the one that follows, and valves are opened or shut according to the phase the engine is in. For example, during the intake phase, the intake valve opens to allow the gas / air mixture to enter the cylinder. During the compression phase all valves are shut, then the spark plug ignites the mixture (in the explosion cycle) and the cylinder shoots upward. The exhaust valve opens allowing the spent gasses to escape the cylinder.

There are no traditional valves in a two-stroke engine. In a two-stroke the engines fires once every revolution. This makes the engine highly efficient and lightweight compared to four-stroke systems. Rather than entering through valves, the fuel/air mixture enters through an intake port and exhaust exits out of an exhaust port. In place of traditional valves the two-stroke engine uses the piston's position to force out exhaust or suck in fuel mixture.

Reeds are vital to a two-stroke system. The reeds are placed between the intake manifold and the carburetor, open and close to allow the fuel / air mixture to enter the case of the engine and trap it, and ensure the proper exchange of gasses in the engine. This procedure might sound complex, but it is, in fact, extremely effective and easy to understand.

The whole cycle can be explained as follows:

1) As the piston moves from bottom dead center to top dead center it creates a

vacuum to draw the fuel / air mixture through the carburetor and past the reed valve assembly.

2) The piston moves down from top dead center to bottom dead center. The reed closes, causing the pressure to build in the cylinder. The movement of the piston uncovers the intake port and pressurized the fuel / air mixture.

3) The piston now moves up from bottom dead center to top dead center, effectively ending a cycle and starting another. The spark plug ignites the compressed mixture, sending piston back down.

4) At this point the piston uncovers the exhaust port, allowing the spent gasses to escape. As it continues to bottom dead center, it uncovers the intake port and allows the fuel / air mixture through the carburetor and past the reed valve assembly.

A highly tuned two-stroke engine produces a tremendous amount of power per cubic centimeter, and is still effectively raced against larger capacity machines in dirt racing, where lightweight is valued more than raw horsepower. Unfortunately, two-strokes do emit more pollutants than four-strokes, are louder, and tend to use more gasoline. It is unlikely the design can be sufficiently altered to meet current emission control standards, so fewer and fewer two-strokes will be made in the future.

© High Speed Ventures 2011