How Does a Two-Stroke Engine Work? [With Animation, Notes, and PDF]

Hey, Readers today we will discuss a Two-Stroke Engine, let’s see How Does a Two-Stroke Engine Work? Also at the end of the article, I will give you PDF downloadable link.

It is one type of internal combustion engine that completes two-stroke (up-stroke and down-stroke) in one revolution of the crankshaft.

I will describe up-stroke and down-stroke later on in the following article. 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 another one 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 get the compression ratio to burn the fuel.

Don’t worry I will discuss all those new terms like compression ratio etc.

How many types of two stroke engine?

In general, there are two types of two-stroke engines.

And those are:

  1. Two-stroke Otto cycle petrol engine
  2. Two-stroke Diesel cycle diesel engine

I have already discussed that two-stroke diesel engines are not available in the market because the needed compression ratio is not achieved by the two-stroke diesel engine. Also, there is a couple of more reason that I will discuss later on.

What are the parts of two stroke engine?

A two-stroke engine consists of the following parts:

  1. Cylinder
  2. Cylinder head
  3. Piston
  4. Piston rings
  5. Connecting rod
  6. Crank
  7. Crankcase
  8. Crankshaft
  9. Ports
  10. Spark plug (For Petrol engine)
  11. Fuel Injector (For Diesel engine)

Let me discuss it one by one.


Cylinders are generally made of cast iron for withstanding the high temperature and pressure inside the cylinder. Here the piston reciprocates. For a two-stroke engine, it 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 fuel injector for the diesel engine. The engine head is also made of cast iron.


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 are 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:

  1. Inlet port
  2. Transfer port
  3. 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 pump inside the engine cylinder.


Two-stroke engine diagram:

2-stroke engine diagram

2-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.

  1. First stroke ( Suction and Compression stroke)
  2. Second stroke (Power and Exhaust stroke)

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 the three ports name 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 are 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.

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 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, due to the high temperature compressed air inside the cylinder, the fuel is ignited.

2-stroke engine animation
derivative work: Turbojet (talk)Arbeitsweise_Zweitakt.gif: Topory, A7N8X [CC BY-SA 3.0], via Wikimedia Commons
two stroke engine working
By Agster –, CC BY-SA 3.0,

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, bearings.

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]

Firstly discuss the advantages of 2-stroke engines then go to the disadvantages.

Advantages of two-stroke engine:

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

Let’s talk about some disadvantages.

Disadvantages of two-stroke engine

  1. Two stroke engine consume more lubricating oil than a four-stroke engine.
  2. In a two-stroke engine power stroke is generated after every stroke, so there is lots of heat produce, therefore we have to lubricate and cool it well.
  3. In the two-stroke engine, some amount of fresh charge mixes with the exhaust gas and leaves the cylinder. So that the engine has less output.
  4. Two stroke engine is not last long due to there is no lubricating system as Four-stroke engine.

Difference between Four-stroke Engine and Two-stroke Engine:

Four-stroke engine

  1. Power is developed every two revolutions of the crankshaft.
  2. Consists only of valves.
  3. Engine is water cooled.
  4. Weight of the engine is more because of heavier flywheel construction.
  5. Maintenance cost is high.
  6. The exhaust gases are fully burnt and leave as the exhaust so engine output is high.

Two-stroke engine

  1. Power is developed every one revolution of crank shaft.
  2. Consists only of Ports.
  3. Engine is air cooled.
  4. Weight of the engine is less because of light flywheel construction.
  5. Maintenance cost is low .
  6. Some fresh charge mixes with the exhaust gas and leave as the exhaust so engine output is less.

Two-stroke Engine application:

  • There many motorcycles which run on 2-stroke engines. These are the models:
  1. Yamaha RX135/RX-Z.
  2. Kinetic Honda.
  3. Kawasaki-Bajaj Enduror
  4. Bajaj Chetak
  5. Suzuki Shogun
  6. Suzuki Shaolin
  7. Yezdi Roadking 250
  8. Yezdi 350
  • Previously 2-stroke engines are used in Racing cars. But recently many countries banned 2-stroke racing cars.
  • 2-stroke engines are use in boats. There are many outboards companies like yammaha , Suzuki, Mercury is still making 2-stroke boats.
  1. The 2-stroke outboards – YAMAHA,outboard
  2. Suzuki 2 Stroke Outboard
  3. Mercury® Two Stroke Outboards
  • Some gardening equipment we use two-stroke engines

Is 2-stroke engines banned in worldwide?

NO! Actually recently we are not using these types of engines in Bikes and Cars for the above-mentioned disadvantages.

Also, Two-stroke engines polluted air much more than Four-stroke engines. That’s why many states or countries banned two-stroke engines.

SO this is all about 2-Stroke Engine, I hope you liked it, I also have an article on Four-Stroke Engine, you may have interested to read that too.

And don’t forget to share this article on your favorite social media platforms, and spread the knowledge.


  1. Scavenging the two-stroke engine
  2. AVL two-stroke diesel engine
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Anup Kumar Dey

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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