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Two Stroke Engine: Working, Types, Advantages [Petrol & Diesel]

In this article, you will learn what is Two Stroke EngineTheir Working, Types, Advantages, and Disadvantages are explained with Pictures.

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What is a Two-Stroke Engine?

two-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution.

In four-stroke engines, there is one working stroke in two revolutions of the crankshaft or a cycle of four strokes of the piston. The desire for one working stroke in every revolution of the crankshaft has led to the development of the two-stroke engine.

In these engines, there are no suction and exhaust strokes. There are only two remaining strokes the compression stroke and power stroke. These are usually called upward strokes and downward strokes. Also, instead of valves, there are inlet and exhaust ports in two-stroke engines.

A fresh charge enters the cylinder at the end of the working stroke through the inlet port. And then burnt exhaust gases are forced out through the exhaust port by a fresh charge.

Read Also: What is the function of a Cylinder Block?

Two Strokes Spark Ignition (Petrol) Engine

The principle of a two-stroke spark-ignition engine is shown in the figure. Its two strokes are as follows:

  1. Upward Stroke
  2. Downward Stroke

Upward Stroke

During the upward stroke, the piston moves from the bottom dead center to the top dead center. By compressing the charge air petrol mixture in the combustion chamber of the cylinder. Due to the upward movement of the piston, a partial vacuum is created in the crankcase.

A new charge is drawn into the crankcase through the uncovered inlet port. The exhaust port and transfer port are covered when the piston is at the top-dead-center position. The compressed charge is ignited in the combustion chamber by a spark given by the spark plug.

Downward Stroke

As soon as the charge is ignited the hot gases compress the piston which moves downward, rotating the crankshaft thus doing the useful work. During this stroke, the inlet port is covered by the piston, and the new charge is compressed in the crankcase.

Further downward movement of the piston uncovers first the exhaust port and then the transfer port. and hence the exhaust starts through the exhaust port.

As soon as the transfer port is open, the charge through it is forced into the cylinder. The charge strikes the deflector on the piston crown, rises to the top of the cylinder, and pushes out most of the exhaust gases. The piston is now at the bottom dead center position.

The cylinder is filled with a fresh charge, although it is somewhat with the exhaust gases. The cycle of events is then repeated, the piston making two strokes for each revolution of the crankshaft.

Port timing diagram for two stroke cycle engine

The figure shows a port diagram for a two-stroke petrol engine. which is self-explanatory.

P-V diagram for two stroke petrol engine

A figure shows the shape of the p-v diagram for a two-stroke petrol engine. This diagram is only for the main cylinder or top side of the piston. 

Read Also: What Are The Different Types of Engine? (PDF)

Two Stroke Compression Ignition (Diesel) Engine

two stroke cycle diesel engine

In this engine, only air is compressed inside the cylinder. and the fuel (diesel) is injected by an injector fitted in the head of the cylinder. There is no spark plug in this engine. The remaining operations of the two-stroke compression ignition engine are the same. as those of the spark-ignition engine.

The port timing diagram for a two-stroke diesel engine.

Two stroke diesel engine

The figure shows the cylinder pressures and temperatures acting on spark plugs for two-stroke and four-stroke engines.

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Advantages of Two-Stroke Engine Over Four Stroke:

  1. This engine gives one working stroke for each revolution of the crankshaft. The four-stroke gives one working stroke for every two revolutions of the crankshaft. Hence, the power developed by a two-stroke engine is twice that developed by a four-stroke engine for the same engine speed and cylinder volume.
  2. The turning moment on the crankshaft is more even in a two-stroke engine. Due to one working stroke for each revolution of the crankshaft, and so it needs a lighter flywheel in it.
  3. For the same power, this engine is more compact, and light and requires less space than a four-stroke engine. Thus it is more suited for auto-cycles, motorcycles, and scooters.
  4. A two-stroke engine is simpler in construction and mechanism. There is no valve and valve mechanism in it. The ports are easy to design and they are covered and uncovered by the movement of the piston itself.
  5. It has high mechanical efficiency due to the absence of cams, crankshaft, rockers, etc., of the valves.
  6. It gives less torsional oscillations.
  7. The two-stroke engine requires fewer spare parts due to its simple design.
  8. It can be reversed if it is of valveless type.
  9. There is a saving in work required to overcome the friction of the inlet and exhaust port.


  1. In a two-stroke cycle Otto engine, the fuel consumption is high. Because the fresh charge is likely to be wasted by escaping through the exhaust port.
  2. The actual compression starts when the ports are completely closed by the upward movement of the piston, after a few degree revolutions of the crankshafts. Thus, the actual compression ratio and hence the thermal efficiency of a two-stroke engine is less than that of the four-stroke for the same dimensions.
  3. The charge is diluted by the burnt gases due to incomplete scavenging.
  4. It makes greater noise.
  5. It consumes more lubricating oil.
  6. There is greater wear and tear of moving parts.

Read Also: Connecting Rod: Parts, Types, Functions, and More

Applications of Two-Stroke Engine

Two-stroke engines are used in a variety of applications, including:

  1. Motorcycles, mopeds, scooters, and snowmobiles all use two-stroke engines, which have a smaller engine capacity and cost less to operate.
  2. These engines are employed in handheld mechanical appliances such as chainsaws because of their ability to function without the requirement for a reservoir.

Comparison Between The Four-Stroke Engine and The Two-Stroke Engine.

Four-Stroke EngineTwo-Stroke Engine
In a four-stroke engine, one working stroke for every two revolutions of the crankshaft. In this one working stroke for each revolution of the crankshaft.
The turning moment on the crankshaft is not even due to one working stroke for every two revolutions of the crankshaft. hence it requires a heavy flywheel and the engine runs unbalanced. The turning moment on the crankshaft is more even due to one working stroke for each revolution of the crankshaft. Hence it requires a lighter flywheel and the engine runs balanced.
The engine is heavy The engine is light.
The cost of the engine is high The cost of the engine is low.
Less mechanical efficiency due to more friction in many parts of the four-stroke engine. Mechanical efficiency is more due to less friction in a few parts.
The output is more due to full fresh charge intake and full burnt gases exhaust. The output is low due to the mixing of fresh charges with burnt gases.
The engine runs cold.The engine runs hot.
It is water-cooled.It is air-cooled.
Less fuel consumption in the four-stroke engine Fuel consumption is more.
The engine requires more space.The engine requires less space.
A lubricating system is complicated The lubricating system is simple.
The engine produces less noise.Noise is more.
The engine is used in cars, buses, and trucks.The engine is used for mopeds, scooters, and motorcycles.
The engine consists of inlet and exhaust valves. The engine consists of inlet and exhaust ports.
More thermal efficiency.Low thermal efficiency.
The engine consumes less lubricating oil. These engines consume more lubricating oil.
Less wear and tear of moving parts in the engine. Greater wear and tear of moving parts.


So now, I hope I’ve covered everything about this topic. If you still have any doubts or questions on this topic, you can contact us or ask in the comments. If you like this article then please share it with your friends.

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Images in this blog are from britannica.com, cycleworld.com


What is a two-stroke engine?

An internal combustion engine that requires two piston strokes to accomplish one crankshaft rotation is known as a two-stroke engine. In a single piston stroke, the full rotational cycle is completed.

How does a two-stroke engine work?

In a two-stroke engine, the piston moves up and down, or through two strokes, in one crankshaft rotation. This is how fuel is converted into motion. During the compression stroke, the piston rises and draws air, gasoline, and oil into the crankcase via an intake valve.

What is an example of a 2-stroke engine?

These engines are usually found in smaller, lower-power engines such as scooters, dirt bikes, jet skis, smaller outboard motors, and lawn equipment such as lawnmowers and chainsaws.

How do two-stroke engines get identified?

A two-stroke engine can be distinguished from a four-stroke engine from the outside by lacking its unique inlet and exhaust valves. A two-stroke engine’s chamber houses its inlet and exhaust ports.

About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

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