Two-stroke engines: parts, cycles, working principles and applications

A two-stroke (or two-cycle) engine is a type of internal combustion engine which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaft revolution. This is in contrast to a "four-stroke engine", which requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust (or scavenging) functions occurring at the same time.

Two-stroke engines often have a high power-to-weight ratio, power being available in a narrow range of rotational speeds called the "power band". Compared to four-stroke engines, two-stroke engines have a greatly reduced number of moving parts, and so can be more compact and significantly lighter.


1. Piston

In an engine, a piston is used to transfer the expanding force of gases to mechanical rotation of crankshaft via a connecting rod. The piston is able to do this because it is secured tightly within the cylinder using piston rings to minimize the clearance between cylinder and piston.

2. Crankshaft

A crankshaft is a part which is able to convert the reciprocating motion to rotational motion.

3. Connecting rod

A connecting rod transfers motion from a piston to crankshaft which acts as a lever arm.

4. Counterweight

The counterweight on the crankshaft is used to reduce the vibrations due to imbalances in the rotating assembly.

5. Flywheel

The flywheel is a rotating mechanical device which is used to store energy.

6. Inlet & Outlet ports

It allows to enter fresh air with fuel & to exit the spent air-fuel mixture from the cylinder.

7. Spark Plug

A spark plug delivers electric current to the combustion chamber which ignites the air-fuel mixture leading to an abrupt expansion of gas.


1. Intake

The fuel/air mixture is first drawn into the crankcase by the vacuum that is created during the upward stroke of the piston. The illustrated engine features a poppet intake valve; however, many engines use a rotary value incorporated into the crankshaft.

2. Crankcase compression

During the downward stroke, the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

3. Transfer/Exhaust

Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixtures is usually expelled as well.

4. Compression

The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).

5. Power

At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. (At the same time, another crankcase compression stroke is happening beneath the piston.)


Down Stroke

First, the piston is moved downside from TDC to BDC to let the fresh air enter into the combustion chamber. The fresh air-fuel mixture gets into the combustion chamber through the crankcase. Crankshaft rotation – 180°.

Up Stroke

Here happens all the magic. The piston is pushed up from BDC to TDC. The fuel-air mixture gets compressed & spark plug ignites the mixture. As the mixture gets expanded, the piston moves down. During the upstroke, the inlet port is opened. While this inlet port is opened, the mixture gets sucked inside the crankcase. When the mixture is pushed up into the combustion chamber during the previous upstroke, a partial vacuum is created as no mixture is left behind in the crankcase. This mixture is ready to go into the combustion chamber during downstroke but remains in the crankcase until the piston goes up till TDC. Crankshaft rotation – 360°

Two strokes get completed along with one power cycle.

From the 2nd downstroke onwards the exhaust gases get expelled out from one side while a fresh mixture enters into the combustion chamber simultaneously due to partial vacuum created in the combustion chamber after removal of exhaust gases. This is the beauty of the engine. Both things happen at the same time which makes it a 2 stroke engine.

Two-stroke diesel cycle goes like this:

1. When the piston is at the top of its travel, the cylinder contains a charge of highly compressed air. Diesel fuel is sprayed into the cylinder by the injector and immediately ignites because of the heat and pressure inside the cylinder. This is the same process described in How Diesel Engines Work.

2. The pressure created by the combustion of the fuel drives the piston downward. This is the power stroke.

3. As the piston nears the bottom of its stroke, all of the exhaust valves open. Exhaust gases rush out of the cylinder, relieving the pressure.

4. As the piston bottoms out, it uncovers the air intake ports. Pressurized air fills the cylinder, forcing out the remainder of the exhaust gases.

5. The exhaust valves close and the piston starts traveling back upward, re-covering the intake ports and compressing the fresh charge of air. This is the compression stroke.

6. As the piston nears the top of the cylinder, the cycle repeats with step 1.


1. For the same dimensions, the power developed is twice to that of four-stroke engines
2. Power required for exhaust and suction stroke is saved
3. More uniform turning moment compared to four-stroke engines hence a lighter flywheel is required
4. For the same power output, the two-stroke engine occupies less space
5. Two-stroke engines are lighter than the same power four-stroke engine
6. Construction and working of a two-stroke is simple
7. Less maintenance is required
8. High power to weight ratio

Disadvantages of two-stroke engines

1. High-speed two-stroke engines are less efficient due to reduced volumetric efficiency
2. In the case of the two-stroke engine running on Otto cycle, a considerable amount of fresh charge is lost due to poor volumetric efficiency
3. Effective compression is less die to the provision of ports for intake and exhaust
4. More consumption of lubricating oil
5. At high loads, the two-stroke engine does not run smoothly due to dilution of charge


Due to increased regulation of air pollution, the use of two-stroke engines is decreased but they are still used in many low power applications. Such as

1. Outboard motors
2. High-performance, small-capacity motorcycles, mopeds, and dirt bikes
3. Under-bones, scooters, tuk-tuks, snowmobiles, karts
4. Ultralight airplanes, and model airplanes and other model vehicles
5. They are also common in power tools used outdoors, such as lawnmowers, chainsaws, and weed-whackers