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2stroke engines - 2stroke engineering design - 2strock-design - 2stroke cycle - 2stroke diesel engines - 2strock porting - 2stroke basics - 2stroke oil - 2stroke compression - 2stroke power valve system - 2stroke adv and disadvantages

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

1- the method of intake of fresh air/fuel mixture from the outside

Fuel intake of 2stroke engine

2- the method of scavenging the cylinder (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder.

scavenging the cylinder of 2stroke engine

These are the main variations. They can be found alone or in various combinations.

1. Piston port

See Main Article 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.

2 stroke piston port engine

2. Reed valve

See Main Article 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.

2 stroke Reed valve engine

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.

3. Disk rotary valve

See Main Article 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.

Two stroke engine with rotary-disk valve control

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.

4. Exhaust valve in head

See Main Article 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.

2 stroke exhaust valve

5. Loop-scavenged

See Main Article 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.

2 stroke exhaust valve Loop-scavenged

"Schnurle" (or "Schnürl") or Loop scavenging is by far the most used system of scavenging, named after its inventor.

6. Cross flow-scavenged

See Main Article 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.

2 stroke Cross flow-scavenged

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. With smaller size and lower piston speed the deficiencies of the cross flow design become less apparent.

7. Power valve systems

See Main Article 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

Two stroke engines use a Power valve

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.

8. Uniflow

See Main Article 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. 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.

uniflow engine

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