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The 'two-stroke cycle' of an internal combustion engine differs from the more common four-stroke cycle by completing the same four processes (intake, compression, power, exhaust) 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 power stroke to perform the intake and exhaust functions. 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.
The two-stroke spark-ignition engine's invention is generally credited to Joseph Day (and Frederick Cock for the transfer-port), whereas the two-stroke ''valved'' compression-ignition engine is attributed to Dugald Clark.

Contents

Applications

The two-stroke cycle

Different two-stroke design types

Piston port

Reed valve

Disk rotary valve

Exhaust valve in head

Loop-scavenged

Cross flow-scavenged

Power valve systems

Uniflow

Two-stroke diesel engines

Lubrication

Reversibility

Nonpoint source pollution

Sources

See also

External links

Applications

The smallest gasoline engines are usually two-strokes. They are popular due to their simple design (and therefore, low cost) and very high power-to-weight ratios. However, the fuel is almost always mixed with the engine oil, which significantly increases the emission of pollutants. For this reason, two-stroke engines are being replaced with four-stroke engines in as many applications as possible.
Two-stroke engines are still commonly used in high-power, handheld applications where light weight is essential, primarily string trimmers and chainsaws. To a lesser extent, these engines may still be used for certain small, portable, or specialized machine applications. These include outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, snowmobiles, karts, model airplanes (and other model vehicles) and lawnmowers. In the past, two-stroke cycles were experimented with for use in diesel engines, most notably with opposed piston designs, low-speed units such as large marine engines, and V8 engines for trucks and heavy machinery.
Several cars used two-stroke engines in the past, including the Swedish Saab, German manufacturers DKW and Auto-Union. Production of two-stroke cars stopped in the 60s in the West, but East block countries kept producing Syrena in Poland, Trabant and Wartburg in East Germany with two stroke engines until as recently as 1988.

The two-stroke cycle

Two-stroke cycle engines operate in two strokes, instead of the four strokes of the more common Otto cycle.
# Power/exhaust: This stroke occurs immediately after the ignition of the charge. The piston is forced down. After a certain point, the top of the piston passes the exhaust port, and most of the pressurized exhaust gases escape. As the piston continues down, it compresses the air/fuel/oil mixture in the crankcase. Once the top of the piston passes the transfer port, the compressed charge enters the cylinder from the crankcase and any remaining exhaust is forced out.
# Compression/intake: The air/fuel/oil mixture has entered the cylinder, and the piston begins to move up. This compresses the charge in the cylinder and draws a vacuum in the crankcase, pulling in more air, fuel, and oil from the carburetor. The compressed charge is ignited by the spark plug, and the cycle begins again.
In engines like the one described above, where some of the exhaust and intake charge are in the cylinder simultaneously the gasses are kept separate by careful timing and aiming of the transfer ports such that the fresh gas has minimal contact with the exiting exhaust which it is pushing ahead of itself.

Different two-stroke design types

In order to understand the operation of the two-stroke engine it is necessary to know which type of design is in question because different design types operate in different ways.
The design types of the two-stroke cycle engine vary according to the method of intake of fresh air/fuel mixture from the outside, the method of scavenging the cylinder (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder.
These are the main variations. They can be found alone or in various combinations.

Piston 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.

Reed valve

The reed valve is similar to and almost as simple as the piston port but with a check valve in the intake tract. Reed valve engines deliver power over a wider RPM range than the piston port types, making them more useful in many applications, such as dirt bikes, ATVs, and marine outboard engines. Reed valved engines do not lose fresh fuel charge out of the crankcase like piston port engines do.
Reed valve engines can rotate in either direction. This has been used to back up microcars such as the Messerschmitt KR200 that lacked reverse gearing, and it allows flexibility to pull or push model airplanes with either sense pitch propellers.

Disk rotary valve

The intake tract is opened and closed by a thin disk attached to the crankshaft and spins at crankshaft speed. The fuel/air path through the intake tract is arranged so that it passes through the disk. This disk has a section cut from it and when this cut passes the intake pipe it opens, otherwise it is closed.
The advantage of a disk rotary valve is that it enables the two-stroke engine's intake timing to be asymmetrical which is not possible with two-stroke piston port type engines. The two-stroke piston port type engine's intake timing opens and closes before and after top dead center at the same crank angle making it symmetrical whereas the disk rotary valve allows the opening to begin earlier and close earlier.
Disk rotary valve engines can be tailored to deliver power over a wider RPM range or higher horse power over a narrower RPM range than either piston port or reed valve engine though they are more mechanically complicated than either one of them.

Exhaust valve in head

Instead of the exhaust exiting from a hole in the side of the cylinder, valves are provided in the cylinder head. The valves function the same way as four-stroke exhaust valves do but at twice the speed. This arrangement is common in two-stroke Diesel locomotive engines, e.g. those made by Electro-Motive Diesel.

Loop-scavenged

This method of scavenging uses an external blower to supply the charge(fresh mixture of air and fuel) , under some pressure, at the inlet manifold which pushes the burnt exhaust and gases ahead of it and out the exhaust port. Usually a piston deflector is not used as it can become heavy and tends to become overheated at high output. Here the scavenging is more efficient than in the crank scavenged engines.
"Schnurle" (or "Schnürl") or Loop scavenging is by far the most used system of scavenging, named after its inventor.

Cross flow-scavenged

In a cross flow engine the transfer ports and exhaust ports are on opposite sides of the cylinder and a baffle shaped piston dome directs the fresh mixture up and over the dome pushing the exhaust down the other side of the baffle and out the exhaust port. Before loop scavenging was invented almost all two-strokes were made this way. The heavy piston with its very high heat absorption along with its poor scavenging and combustion characteristics make it an unsuitable design for most applications. Cross flow scavenging is still often used in small engines because it is less expensive to manufacture and allows a more compact design for multiple cylinder configurations.^[1] With smaller size and lower piston speed the deficiencies of the cross flow design become less apparent.

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 such as Ski-doo R.A.V.E Yamaha YPVS, Suzuki AETC system or by altering the volume of the exhaust which changes the resonant frequency of the expansion chamber, such as Honda V-TACS system. The result is an engine with better low end power without losing high rpm power.

Uniflow

In a uniflow engine, the air enters through a port at one end of the cylinder and the exhaust exits through a valve, or port, at the other end. The gas-flow is therefore in one direction only, hence the name uniflow.
The first type is represented by the "exhaust valve in head" design, see above. The second type is represented by the "opposed piston" design in which there are two pistons in each cylinder, working in opposite directions. An example of an opposed piston engine is the Napier Deltic.

Two-stroke diesel engines

Unlike a gasoline engine, which requires a spark plug to ignite the fuel/air charge in the cylinder, a diesel engine relies solely on the heat of compression for ignition. Fuel is injected at high pressure into the superheated compressed air and instantly ignites. Therefore, scavenging is performed with air alone.
In order to allow the usage of a conventional oil-filled crankcase and pressure lubricated main and connecting rod bearings, a two-stroke diesel is scavenged by a mechanically driven blower (often a Roots positive displacement blower) or a hybrid turbo-supercharger, rather than by crankcase pumping. Generally speaking, the blower capacity is carefully matched to the engine displacement so that a slight positive pressure is present in each cylinder during the scavenging phase (that is, before the exhaust valves are closed). This feature assures full expulsion of exhaust gases from the previous power stroke, and also prevents exhaust gases from backfeeding into the blower and possibly causing damage due to contamination by particulates.
It should be noted that the scavenging blower is not a supercharger, as its purpose is to supply airflow to the cylinders in proportion to their displacement and engine speed. A two-stroke diesel supplied with air from a blower alone is considered to be naturally aspirated. In some cases, turbocharging may be added to increase mass air flow at full throttle—with a corresponding increase in power output—by directing the output of the turbocharger into the intake of the scavenging blower, an arrangement that was found on some Detroit Diesel two-stroke engines.
A conventional, exhaust-driven turbocharger cannot be used by itself to produce scavenging airflow, as it is incapable of operating unless the engine is already running. Hence it would be impossible to start the engine. The common solution to this problem is to drive the turbocharger's impeller through a gear train and overrunning clutch. In this arrangement, the impeller turns at sufficient speed during engine cranking to produce the required airflow, thus acting as a mechanical blower. At lower engine speeds, the turbocharger will continue to act as a mechanical blower. However, at higher power settings the exhaust gas pressure and volume will increase to a point where the turbine side of the turbocharger will drive the impeller and the overrunning clutch will disengage, resulting in true turbocharging.

Lubrication

Two-stroke engines often have a simple lubrication system in which a special two-stroke oil is mixed with the fuel, (then known as 'petroil' from "petrol" + "oil") and therefore reaches all moving parts of the engine. Handheld devices using this method of lubrication have the advantage of operating in any orientation since there is no oil reservoir which would be dependent upon gravity for proper function. Depending on the design of the engine system, 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.
The engine uses cylinder port valves which are incompatible with piston ring seals. This causes lubricant from the crank to work its way into the combustion chamber where it burns. Research has been conducted on designs that attempt to reduce the combustion of lubricant. This research could potentially produce an engine having very valuable properties of both high specific-power and low pollution.

Reversibility

With proper design, a two-stroke engine can be arranged to start and run in either direction, and many engines have been built to do so.

Nonpoint source pollution

According to the United States Environmental Protection Agency, some forms of water recreational activities contribute to nonpoint source pollution or "pollution runoff," and "the old 2 cycle motors have been said to cause more pollution in two hours than a car running for an entire year."[1].
Because fuel leaks through the exhaust port each time a new charge of air/fuel is loaded into the combustion chamber, oil pollution is a problem at many National Parks and outdoor recreation areas that allow four-wheelers, snowmobiles, dirt bikes, and small watercraft.[2]
To address these problems, some organizations have begun to offer biodegradable two-stroke engine oil, and newer models are said to be more efficient.

Sources

★ How Stuff Works: Two-Stroke Engine
1.

See also

★ Napier Deltic

★ Junkers Jumo 205

★ Twingle engine

★ Wärtsilä-Sulzer RTA96-C

External links

★ Animated Engines: Two Stroke

★ How Stuff Works: Two-Stroke Engine

★ The Fuel and Engine Bible - A good resource for different engine types and fuels

★ 2-stroke engines CDX ''e''Textbook

★ Racejuice.org (Two stroke pocketbike technical resource)

★ Envirofit International