How a two-stroke engine works (2024)

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

A two-stroke engine

Some smaller engines, however, notably those fitted to some mopeds ormotorcycles, operate on a two-stroke cycle - the piston is on a power strokeevery time it moves down the cylinder so the crankshaft turns only once duringeach cycle. A few cars have used this engine too, such as the Wartburg Knightand some early Saabs.


The earliest two-strokes were of the uniflow type. With this design, thefuel/air mixture is forced into the cylinder by a rotary blower (supercharger)driven by the engine. There is no inlet valve: instead, there is an elongatedhole, called a port, in the side of the cylinder near the bottom of thepiston's stroke. The port is opened or closed as the piston passes up and downthe cylinder. The exhaust gases usually pass out through a conventionalcam-operated poppet valve.

The cycle starts with a down-stroke in which burning fuel pushes the pistondown. When the piston uncovers the inlet port at the bottom of its stroke, fueland air is pushed in above it. On the upstroke the exhaust gas is forced outand fuel is compressed, ready to be fired. To allow this to happen, the exhaustvalve opens just before the descending piston uncovers the inlet port, so thereis no resistance to the incoming charge.

The two stroke cycle

Modern version

Most modern two-stroke engines work slightly differently. Instead of havinga blower to force the fuel/ air mixture into the cylinders, they use what isknown as crankcase compression.

This type of engine needs no conventional valves. The inlet ports lead intothe bottom of the cylinder which is open to the crankcase: higher up thecylinder on the opposite side are another set of ports leading to the exhaustpipe. A transfer port leads back up to the cylinder from the crankcase,entering at a slightly higher level than the inlet port, but a little lowerthan the exhaust port.

During the upstroke the piston uncovers the inlet port and allows thefuel/air mixture to rush into the crankcase, underneath the piston. Sometimesthere is a cut-out in the side of the piston through which the mixture can passto reach the crankcase.

When the piston reaches the top of the cylinder, the compressed fuel/airmixture is fired by a spark plug, forcing the piston down on the powerstroke.

As the piston descends, it compresses the fuel/air mixture in the crankcase,and it also uncovers the exhaust poit closely followed by the transfer port.The exhaust gases start to escape as the exhaust port is uncovered, and arefurther scavenged (forced out) by the fuel/air mixture coming in from thetransfer port under slight pressure from the crankcase.

To help scavenge the exhaust gases out of the cylinder, the top of thepiston is often shaped to deflect the incoming mixture upwards. The mixturethen doubles back when it strikes the cylinder head, flows down the exhaustport side and pushes the exhaust gases out.

The momentum of the gases from the transfer ports, which will have been opensince near the bottom of the downstroke, continues to expel the exhaustproducts until the exhaust ports are closed. This system of expelling exhaustgases is known as loop scavenging.

Exhaust design

The design of the exhaust is more critical in a two-stroke engine than it is in a four-stroke engine. The burnt exhaust gases are not positively forced out by the upward-travelling piston, so it is essential that the exhaust system offers the minimum amount of resistance to the gases' path.

With most two-strokes, the inward rushing inlet charge helps to sweep the residual exhaust gases out of the cylinder. The problem is that some of the inlet charge — unburnt fuel — can be lost to the atmosphere because both the inlet and exhaust ports are open together for some time. However, the design of the exhaust pipe and silencer can be exploited to minimize this effect.

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


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

How a two-stroke engine works (5)

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How a two-stroke engine works (2024)


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