What is two-stroke engine and works

Two-stroke engine

Here is the complete guide about two-stroke engine. Here you can get two-stroke engine working, Advantages and Disadvantages ect.

Almost all car engines work on the four-stroke cycle, so called because it takes four strokes of the piston induction, compression, ignition and exhaust – to produce one firing of the fuel /air mixture. This means that the crankshaft rotates twice to complete each cycle.

Some smaller engines, however, notably those fitted to some mopeds or motorcycles, operate on a two-stroke cycle – the piston is on a power stroke every time it moves down the cylinder so the crankshaft turns only once during each cycle. A few cars have used this engine too, such as the Wartburg Knight and some early Saabs.

Two-stroke engine

A two-stroke (or two-cycle) engine may be a sort of internal combustion engine that completes an influence cycle with two strokes (up and down movements) of the piston during just one crankshaft revolution. this is often in contrast to a four-stroke engine, which requires four strokes of the piston to finish a power cycle during two crankshaft revolutions. during a two-stroke engine, the top of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust (or scavenging) functions occurring at an equivalent time.

Two-stroke engines often have a high power-to-weight ratio, power being available during a narrow range of rotational speeds called the facility band. Compared to four-stroke engines, two-stroke engines have fewer moving parts.

Two-stroke engine working

Just in case some of you aren’t sure how two-stroke engines work, here is some review. In a four-stroke engine, each of the four essential steps of the power-producing cycle is given its own piston stroke:

  • Compression
  • Power
  • Exhaust
  • Intake

A two-stroke engine performs all the same steps, but in only two piston strokes. the only two-stroke engines do that by using the crankcase and therefore the underside of the moving piston as a fresh charge pump. Such engines carry the official name “crankcase-scavenged two-strokes.” As the two-stroke’s piston rises on compression, its underside pulls a partial vacuum within the crankcase. An intake port of some kind (cylinder wall port, reed valve or rotary disc valve) opens, allowing air to rush into the crankcase through a carburetor.

As the piston nears Top dead center, a spark fires the compressed mixture. As during a four-stroke, the mixture burns and its energy becomes heat , raising the pressure of the burned mixture to many psi. This pressure drives the piston down the bore, rotating the crankshaft. As the piston continues down the bore, it begins to show an exhaust port within the cylinder wall. As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it within the crankcase.

As the piston descends more, it begins to show two or more fresh-charge ports, which are connected to the crankcase by short ducts. As pressure within the cylinder is now low and pressure within the crankcase higher, fresh charge from the crankcase rushes into the cylinder through the fresh-charge (or “transfer”) ports. These ports are shaped and aimed to minimize direct loss of fresh charge to the exhaust port. Even within the best designs, there’s some loss, but simplicity has its price! This process of filling the cylinder while also pushing leftover exhaust gas out the exhaust port is named “scavenging.”

While the piston is near Bottom dead center, mixture continues to move from the crankcase, up through the transfer ports, and into the cylinder. As the piston rises, it first covers the transfer ports, leaving only the exhaust port still open. If there have been no thanks to stop it, much of the fresh charge would now be pumped out the exhaust. But there’s an easy way to stop it—using exhaust pressure waves within the exhaust. If we shape and dimension the pipe right, a reflection of the first pressure pulse, generated because the exhaust port opened, will recover to the port even as fresh charge is being pumped out of it. This pressure wave stuffs the fresh charge back to the cylinder even as the rising piston covers the exhaust port.

Because fuel-air mixture is constantly being pumped by the crankcase, it’s not practical to lubricate piston and crank by pumped circulating oil—it would be swept away by the mixture rushing in and out. Therefore, we must either mix a touch oil with the fuel (2 to 4 percent) or inject it very sparingly into the bearings with a tiny metering pump. the very fact that there’s so little oil dictates that such simple two-stroke engines must employ rolling bearings, whose need for oil is extremely small.

More complicated two-stroke engines exist. rather than using the crankcase and underside of the piston as a fresh-charge pump, we will use a separate rotary blower, directly connected to the transfer ports within the cylinders. we do not need to place the exhaust port within the cylinder wall—it can take the form of four overhead poppet exhaust valves, because it does in two-stroke marine, rail, and truck diesels. Because such engines don’t use their crankcases as fresh charge pumps, they will employ long-lasting plain bearings, lubricated conventionally by pumped recirculating oil. Two-stroke diesels are scavenged with pure air, not a fuel-air mixture. Their fuel is injected only in any case ports have closed, preventing any loss. Certain crankcase scavenged two-strokes do the same, and are called “DI,” or Direct Injection two-strokes. they will be made as fuel-efficient and low in exhaust emissions as four-strokes. The world’s most effective piston engines are actually the giant, slow-turning marine diesels that carry the world’s international shipping trade—they are twice as efficient because the usual four-stroke spark-ignition engines found in cars and motorcycles.


The earliest two-strokes were of the uni flow type. With this design, the fuel/air mixture is forced into the cylinder by a rotary blower ( supercharger ) driven by the engine. there’s no inlet valve : instead, there’s an elongated hole, called a port, within the side of the cylinder near the bottom of the piston’s stroke. The port is opened or closed because the piston passes up and down the cylinder. The exhaust gases usually pass out through a standard cam-operated poppet valve.

The cycle starts with a down-stroke during which burning fuel pushes the piston down. When the piston uncovers the inlet port at the bottom of its stroke, fuel and air is pushed in above it. On the upstroke the exhaust gas is forced out and fuel is compressed, able to be fired. to permit this to happen, the valve opens just before the descending piston uncovers the inlet port, so there’s no resistance to the incoming charge.

The two stroke cycle

  • As the piston is compressing the fuel/air mixture on its upward stroke, a fresh intake charge is being sucked into the crankcase.
  • The compressed mixture, fired by a correctly timed electric spark, burns and expands, driving the piston down.
  • The burned gases leave the cylinder via the now uncovered exhaust port, and the fresh intake charge rushes into the cylinder (via the transfer port), helping to push out the exhaust gases. As the piston starts its upward travel again, it begins to suck a fresh fuel/air charge into the crankcase.

Modern version

Most modern two-stroke engines work slightly differently. rather than having a blower to force the fuel/ air mixture into the cylinders, they use what’s referred to as crankcase compression. This type of engine needs no conventional valves. The inlet ports lead into rock bottom of the cylinder which is open to the crankcase: above the cylinder on the other side are another set of ports resulting in the exhaust pipe. A transfer port leads back up to the cylinder from the crankcase, entering at a rather higher level than the inlet port, but a touch less than the exhaust port. During the upstroke the piston uncovers the inlet port and allows the fuel/air mixture to rush into the crankcase, underneath the piston. Sometimes there’s a cut-out within the side of the piston through which the mixture can pass to succeed in the crankcase. When the piston reaches the top of the cylinder, the compressed fuel/air mixture is fired by a spark plug , forcing the piston down on the facility stroke.

As the piston descends, it compresses the fuel/air mixture within the crankcase, and it also uncovers the exhaust poit closely followed by the transfer port. The exhaust gases start to flee because the exhaust port is uncovered, and are further scavenged (forced out) by the fuel/air mixture coming in from the transfer port under slight pressure from the crankcase. To help scavenge the exhaust gases out of the cylinder, the top of the piston is often shaped to deflect the incoming mixture upwards. The mixture then doubles back when it strikes the plate , flows down the exhaust port side and pushes the exhaust gases out.

The momentum of the gases from the transfer ports, which can are open since near rock bottom of the down stroke, continues to expel the exhaust products until the exhaust ports are closed. this technique of expelling exhaust gases is known as loop scavenging.

Exhaust design

The design of the exhaust is more critical during a two-stroke engine than it’s during a four-stroke engine. The burnt exhaust gases aren’t positively forced out by the upward-traveling piston, so it’s essential that the exhaust offers the minimum amount of resistance to the gases’ path.

With most two-strokes, the inward rushing inlet charge helps to comb the residual exhaust gases out of the cylinder. the matter is that a number of the inlet charge unburnt fuel are often lost to the atmosphere because both the inlet and exhaust ports are open together for a few time. However, the planning of the exhaust pipe and silencer are often exploited to attenuate this effect.

When an exhaust charge leaves the cylinder, it sends a pulse —a blast wave — down the exhaust pipe, which is reflected back from the top of the pipe. By paying careful attention to the design of the exhaust, engineers can arrange a system which will use the returning exhaust pulse to push the inlet charge, which is trying to follow the exhaust gases down the exhaust pipe, back to the cylinder.


In most engines the crankcase and sump contain the oil to lubricate the engine’s moving parts. But with a crankcase compression two-stroke, the crankcase cannot do this because it is needed for initial compression of the fuel and air.

Advantages of two stroke engines

  • Two stroke engines don’t have valves which are easy to construct and lowers their weight
  • Two stroke engines fire once every revolution while four stroke every other revolution,
  • This two stroke engines lower output in horse power
  • Two stroke engines can add any position, since oil flow isn’t a priority with any valves to stress about.

Disadvantages of two stroke engines

  • Two stroke engines don’t last as long as four stroke engines; there’s no lubrication system during a two stroke engine so parts wear out tons faster.
  • Two stroke oil is expensive; you would burn a gallon every 1000 miles if it were in a car
  • Two stroke engines use more fuel
  • Two stroke engines produce a lot of pollution, and therefore the way the engine is meant that a part of the air/fuel leaks out of the chamber through the exhaust port, which is why you see a small thin film, or sheen, of oil around any two stroke outboard motor, and this leaking oil may be a real mess for the environment. this is often the reason why two stroke engines are used only in application where the motor isn’t used very often and a fantastic power-to-weigh ratio is important.


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