How Does a Two-Stroke Engine Work,11 Parts, Pros & Cons [PDF]

In this article, we will cover a Two-Stroke Engine and see how a Two-Stroke Engine Works. We have provided a PDF for the same at the very end. It is one type of internal combustion engine that completes two-stroke (up-stroke and down-stroke) in one revolution of the crankshaft.

Two-stroke engine bike

There are two types of two-stroke engines are in the market. One is the 2-stroke petrol engine which runs on the Otto cycle and the other is the 2-stroke diesel engine which runs on the diesel cycle. However, there are very few practical examples of a 2-stroke diesel engine in reality because a 2-stroke diesel engine does not produce the compression ratio to burn fuel.

How many types of two stroke engine?

Generally, there are two types of two-stroke engines which are as follows

  • Two-stroke Otto cycle petrol engine
  • Two-stroke Diesel cycle diesel engine

What are the parts of two stroke engine?

A two-stroke engine consists of the following parts

  • Cylinder
  • Cylinder head
  • Piston
  • Piston rings
  • Connecting rod
  • Crank
  • Crankcase
  • Crankshaft
  • Ports
  • Spark plug (For Petrol engine)
  • Fuel Injector (For Diesel engine)
Video by saVRee


  • Cylinders are generally made of cast iron for withstanding the high temperature and pressure inside the cylinder. Here the piston reciprocates.
  • It is a type of engine that houses an exhaust and transfer port.

Cylinder head

  • The cylinder head is a topmost portion of a cylinder and houses of Spark plug for the petrol engine and the fuel injector for the diesel engine.
  • The engine head is also made of cast iron.
Two stroke engine Diagram
Photo by saVRee


  • It is a moving part inside the cylinder generally made of cast aluminum alloy for excellent thermal conductivity.
  • The top surface of the piston is called a piston crown and the bottom surface is called the piston skirt.
  • Its top surface is made deflect for two-stroke engines and made flat for four-stroke engines.

Piston rings

  • Piston rings are commonly made of cast iron.
  • There are 2 or 3 piston rings provided on the piston.
  • These piston rings help to prevent the leakage of high-pressure gas from the cylinder to the crankcase. It also helps to clean the cylinder wall.

Connecting rod

Connecting rod is mainly made of steel or aluminum alloy (T6-2024 and T651-7075) for lightness and absorb high impact. It is also made of titanium but the cost for this little high.


  • Crank is made of steel. It is revolving inside the cylinder.
  • One end of it connects with a crankshaft and another end is connected with a piston.

Crank case

  • It is the house of a crank, crankshaft, lubricating oil, and other fastened engine parts.
  • The Crankcase is made of cast iron or cast aluminum via sand casting.

Crank shaft

  • The crankshaft is made of cast iron or forged steel, steels are generally used for high-loading situations in diesel engines.
  • The crankshaft converts the up-and-down motion of the piston into horizontal rotation.


  • Ports are generally used in Two-stroke engines such as two-stroke petrol engines.
  • These are the openings for inlet charge and exhaust gases.
  • There are 3 ports are present in a two-stroke engine which are as follows
    • Inlet port
    • Transfer port
    • Exhaust port

Spark plug

  • A spark plug is used for igniting the charge inside the cylinder.
  • The spark plug is only used in Petrol engines.

Fuel injector

  • When it comes to diesel engines, the fuel is sprayed with the help of a fuel injector inside the engine cylinder.
Moped engine (50 cc)

Two-stroke engine diagram

Two stroke engine working
Photo by Alamy

two-Stroke Engine working Principle

In the two-stroke engine, there is 1 power stroke for every 1 revolution of the flywheel. There are two strokes in a two-stroke engine.

  • First stroke ( Suction and Compression stroke)
  • Second stroke (Power and Exhaust stroke)
Video By Thomas Schwenke

First stroke or Suction and Compression stroke

During the first stroke, the piston moves from the bottom dead center (BDC) to the top dead center (TDC). Meanwhile, it closes all three ports name the inlet port, transfer port, and exhaust port. Then the charge above the piston is compressed adiabatically.

And in the meantime spark plug gives the spark and the charge is ignited and produces a power stroke. The power is transferred to the crankshaft with the help of connecting rod. During this stroke when the piston moves downward, it uncovers the inlet port. Due to a partial vacuum being produced in the crankcase, the inlet port is opened and the air+fuel mixture enters the crankcase.

Two-stroke engine suction and compression stroke
Photo by GrabCAD

Second stroke or Power and Exhaust stroke

During the second stroke, the piston moves down from the top dead center (TDC) and the inlet port is closed. Due to the downward movement of the piston air+fuel mixture is pushed by the bottom of the piston so the charge from the crankcase comes out through the transfer port.

Since the exhaust port is open, the exhaust gas leaves the cylinder through it but some exhaust gas remains inside the cylinder. The special shape of the piston head deflects the fresh charge inside the cylinder. So the mixture of air fuel flows downward and pushes the exhaust gas through the exhaust port. This process is called scavenging where with the help of a fresh charge, the exhaust gas is pushed out. When the flywheel completes one revolution, then the cycle of operation is repeated.

Generally, a two-stroke petrol engine works in this principle, also for a two-stroke diesel engine the working principle is the same, but there is one little change and that is, a two-stroke diesel engine has a fuel injector instead of the spark plug. The fuel is sprayed inside the cylinder, and due to the high-temperature compressed air inside the cylinder, the fuel is ignited.

2-stroke engine animation
via Wikimedia Commons
two stroke engine working
via Wikimedia Commons

Lubrication system in Two-stroke engine

In a two-stroke petrol engine, a Petrol lubrication system/mist lubrication system is used.

In this system, a certain amount of lubrication oil is mixed with the petrol itself. The usual ratio is 2%-3% of the oil. If it is less it can damage the engine or even if it is more there will be excessive carbon deposits in the cylinder head and the engine will also give dark smoke. When the petrol mixture is entered into the cylinder due to the high temperature it evolves and leaves a thin film of lubricant over the cylinder wall, crankcase, crankshaft, and bearings.

Two-Stroke engine ports
Photo by Grab CAD

Port timing of 2-stroke engine

For 2-stroke petrol engine

  • Inlet port open- 40* before TDC
  • Inlet port close- 40* after TDC
  • Exhaust port open- 60* before BDC
  • Exhaust port close- 60* after BDC
  • Transfer port open- 50* before BDC
  • Transfer port close- 50* after BDC
  • Ignition- 15-20* before TDC

[Where ‘*’ = Degree]

Advantages of two-stroke engine

  • The power produced by the two-stroke engine is more.
  • The two-stroke engine is simple in design because no valve is present. The valves are replaced by the ports in two-stroke engines.
  • The weight of a two-stroke engine is less than a four-stroke engine because of the small flywheel size than a four-stroke engine.
  • In a two-stroke engine, there are fewer parts so there is less friction therefore high mechanical efficiency.
  • High Power to weight ratio.
  • Maintenance cost is low.

Disadvantages of two-stroke engine

  • Two-stroke engines consume more lubricating oil than four-stroke engine.
  • In a two-stroke engine power stroke is generated after every stroke, so there is lots of heat produced, therefore we have to lubricate and cool it well.
  • In the two-stroke engine, some amount of fresh charge mixes with the exhaust gas and leaves the cylinder. Due to this the engine has less output.
  • The two-stroke engine is not long-lasting due to the absence of a lubricating system as a Four-stroke engine does.

Difference between Four-stroke Engine and Two-stroke Engine

Four-stroke engineTwo-stroke engine
Power is developed every two revolutions of the crankshaft.Power is developed every one revolution of crank shaft.
Consists only of valves.Consists only of Ports.
Engine is water cooled.Engine is air cooled.
Weight of the engine is more because of heavier flywheel construction.The weight of the engine is less because of the light flywheel construction.
Maintenance cost is high.Maintenance cost is low .
The exhaust gases are fully burnt and leave as the exhaust so engine output is high.Some fresh charge mixes with the exhaust gas and leave as the exhaust so engine output is less.

Two-stroke Engine application

  • There are many motorcycles that run on 2-stroke engines which are as follows
    • Yamaha RX135/RX-Z.
    • Kinetic Honda.
    • Kawasaki-Bajaj Enduror
    • Bajaj Chetak
    • Suzuki Shogun
    • Suzuki Shaolin
    • Yezdi Roadking 250
    • Yezdi 350
Two stroek engine example
Photo by Bikiinfobd
  • Previously 2-stroke engines are used in Racing cars. But recently many countries banned 2-stroke racing cars.
  • 2-stroke engines are used in boats. Many outboard companies like Yamaha, Suzuki, and Mercury are still making 2-stroke boats.
    • The 2-stroke outboards – YAMAHA, outboard
    • Suzuki 2 Stroke Outboard
    • Mercury® Two Stroke Outboards
  • Some gardening equipment we use two-stroke engines

Is 2-stroke engines banned in worldwide

No, 2-stroke engines are not completely banned worldwide. However, there are regulations in place in various countries and regions that restrict or prohibit the use of 2-stroke engines in certain applications, particularly in motor vehicles, due to their higher emissions and lower fuel efficiency compared to 4-stroke engines. Some areas have also banned 2-stroke engines in watercraft to reduce pollution in waterways. However, 2-stroke engines are still commonly used in other applications, such as chainsaws, dirt bikes, and some small boats.


In conclusion, a two-stroke engine is a simple and efficient internal combustion engine that operates on a two-stroke cycle. It provides a high power-to-weight ratio and is commonly used in small engines such as those found in motorcycles, boats, and lawnmowers. The engine works by compressing a fuel-air mixture in the crankcase, which is then transferred to the combustion chamber where it is ignited by a spark plug.

The force of the expanding gases drives the piston downward, which in turn rotates the crankshaft and powers the engine. While two-stroke engines have advantages in terms of simplicity and power output, they are also known for producing high levels of pollution and noise. Therefore, appropriate measures must be taken to minimize their impact on the environment and human health.


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