An engine is the means by which a moped is powered.

Common engine parts include:

The two-stroke cycle of an internal combustion engine differs from the more common four-stroke engine 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 space below the piston for air intake and compression, thus allowing the chamber above the piston to be used for just the power and exhaust strokes. This causes there to be 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. On the other hand, large two stroke diesels have been in use in industry (i.e. locomotive engines) since the early twentieth century.


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. The biggest disadvantage is that the engine lubricant is almost always mixed in with the fuel, thus significantly increasing 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.

The two-stroke cycle

Two-stroke cycle engines operate in two strokes of the piston 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. 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 aiming of the transfer ports such that the fresh gas has minimal contact with the exiting exhaust which it is pushing ahead of itself.

Two-stroke Designs

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 (which are holes in the side of the cylinder) 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.

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.


This method of scavenging uses carefully aimed transfer ports to loop fresh mixture up one side of the cylinder and down the other pushing the burnt exhaust ahead of it and out the exhaust port. It features a flat or slightly domed piston crown for efficient combustion. "Schnurle" 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. 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

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.


Two-stroke engines often have a simple lubrication system in which oil is mixed with the fuel, (then known as 'petroil' from "petrol" + "oil") and therefore reaches all moving parts of the engine. For this reason, for handheld devices, they have the advantage of working in any orientation, as there is no oil reservoir dependent upon gravity. See 'Design Issues' below. Mopeds without oil injectors require that the fuel and oil be mixed before leaving the gas tank. This mixture of fuel and two stroke oil is known as premix.

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. There is also oil injection which is common in most of today's two cycle engines.


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 this. Engines not designed to run in reverse are still capable of doing it; however running one in reverse for long periods might cause internal damage. This is due to piston throw and piston pin offset, a design feature of all modern piston engines that reduces piston slap. Ignition timing will also be severely retarded in reverse and oil pumps will not function backwards.

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