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Two stroke

From twowheelswiki.com

A Two Stroke engine is a type of Internal Combustion Engine. It differs from the 4 Stroke engine in that it completes the same four processes (intake, compression, combustion, 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 combustion 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 have a focussed range of revs where power is at optimum, commonly referred to as a power band and cause two stroke powered motorcycles to be a formidable competitor against much larger capacity bikes.

Contents

The two-stroke cycle

Two-stroke cycle engines operate in two strokes, instead of the four strokes of the more common Otto cycle.

  1. 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.
  2. Intake/Compression: 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. Inevitably there is some mixing, roughly speaking the degree of mixing is a function of the shape of the piston head, the configuration of the ports, and the volume of gas injected per cycle.

2 Stroke Engine vs 4 Stroke Engine

Different two-stroke design types

Although the principles remain the same, the mechanical details of various two-stroke engines may differ to a large extent and, in order to understand the operation, it is necessary to know which type of design is in question.

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. The following are some of the more common types in Two Stroke Motorcycles.

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.

Reed inlet 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 piston controlled port. 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.

Rotary inlet valve

The intake tract is opened and closed by a rotating member. In the most commonly used type, it takes the form of 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.

Another form of rotary inlet valve used on two-stroke engines employs two cylindrical members with suitable cut-outs arranged to rotate one within the other - the inlet pipe being in communication with the crankcase only when the cut-outs coincide. The crankshaft itself may form one of the members such as was done with the twin cylinder Maytag washing machine engine of the 1930s and 40s and is still used on some model aircraft engines. In yet another embodiment, the crank disc is arranged to be a very close clearance fit in the crankcase and is provided with a cut-out which lines up with an inlet passage in the crankcase wall at the appropriate time. This type was used on the Vespa motor scooter.

The advantage of a 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 rotary valve allows the opening to begin earlier and close earlier.

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.

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, Cagiva C.T.S., 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.

Direct Injection

In modern two-strokes as those used for outboard engines known as Mercury OptiMax, Evinrude E-TEC, Nissan TLDI or Yamaha HPDI, personal water craft, scooters (such as Aprilia DiTech models), snowmobiles, motorcycles, tuk-tuk and small aircraft no longer require the oil and fuel to be mixed. The oil tank is either part of the engine or for larger engines a tank on the vehicle. The oil is injected just after the reeds, lubricating the rotating assembly of the engine. The fuel is injected directly into the cylinder. In most cases the fuel is not injected until after the exhaust port has closed, eliminating short circuiting (fuel lost out the exhaust port without being combusted). Direct injection creates more power and uses less fuel than a carbureted engine would as well as having better emission ratings. In some cases the two-stroke engines have emission ratings as good or better than four-stroke engines. Evinrude was even awarded for being clean with their E-TEC DI two-stroke technology. LPG gas is possible to use, in this way, as well[1].

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.

Two Stroke Motorcycles

There have been a number of notable Two Stroke motorcycles popularised for both their on road and on track performance. Manufacturers such as Suzuki, Yamaha, Honda, Kawasaki Aprilia and Cagiva. Some notable bikes from these manufacturers include:

Acknowledgments

This article was written using Wikipedia's own entry on Two Strokes, found here[2]

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