A stroke is a process in which the piston covers the distance when it moves from the top dead centre (TDC) to the bottom dead centre (BDC), or vice versa, to produce power.
A stroke is the engine’s primary function, leading to the combustion process within an engine.
Key Takeaways
- 2-stroke and 4-stroke are types of internal combustion engines.
- 2-stroke engines have a more straightforward design and produce more power per cycle than 4-stroke engines but are less fuel-efficient and produce more pollution.
- 4-stroke engines are more fuel-efficient and produce less pollution but are more complex and produce less power per cycle than 2-stroke engines.
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2 Stroke vs 4 Stroke
2-stroke engines have a lighter flywheel and a high power-to-weight ratio. Still, they are less efficient and produce more pollution than 4-stroke engines. 2-stroke engines are mainly used in scooters and mopeds, 4 stroke engines are used in heavy or off-road vehicles.
Comparison Table
| Parameter of comparison | 2 Stroke | 4 Stroke |
|---|---|---|
| No. of crankshaft revolution | One | Two |
| Torque Generation | High torque | Low torque |
| Fuel’s Outlet and Inlet | Uses ports | Uses valves |
| Thermal Efficiency | Less thermal efficient | Higher thermal efficient |
| Cost | Cheaper | Expensive |
| Scavenging Process | No | Yes |
| Noise Production | Less | High |
| Space | More space required | Less space required |
What is 2 Stroke?
Two-stroke engines are mainly used in scooters and mopeds, but nowadays, they are also used in small engine tools and some trimmers.
Its name indicates its functioning. To power the engine, there are only two strokes that the engine performs, i.e., the compression stroke and the combustion stroke. Therefore, these engines are called 2-stroke engines.
In every piston revolution, the spark plugs fire, and power is produced once every 2- strokes of the piston.
To complete the entire combustion cycle, 2 stroke engines perform five functions: intake, compression, ignition, combustion, and exhaust. This entire cycle is completed after two piston strokes.
The carburettor creates pressure when the fuel is pulled from it. A vacuum is created as the piston travels downwards to the cylinder. This opens the exhaust ports, and the pressurized fuel mixture goes into the cylinder.
The piston then moves upwards to the cylinder to reach the spark plug. The spark plug generates the spark, which will heat the fuel later and then send the piston back toward the cylinder and open up the exhaust port.
The extra wear out goes out of the exhaust port on the engine as soon as the piston moves back upwards to the cylinder, and then new fuel is introduced into the cylinder.
In every single revolution of the crankshaft, the crankshaft rotates 360 degrees. In 2-stroke engines, there is an exhaust port in the combustion chamber.
Hence, it does not need valves, as the intake and exhaust are part of the compression and combustion of the piston.
Also, these engines do not have a separate chamber for the oil. Therefore, the oil must be mixed with fuel properly.
2-stroke engines combust the fuel mixture in every rotation, unlike other 4-stroke engines, which are faster than 4-stroke engines because they do not have valves and fire every piston revolution.
2 stroke engines have fewer parts, making them inexpensive and easy to manage. Thus, it also occupies less space. Their design is based on a high power-to-weight ratio.
Two-stroke engines can be positioned in any direction, as you don’t need to worry about the oil flow from any of the valves.
A few applications of 2-stroke engines are:
- Equipment used in lawn and garden, bikes, jet skis
- Small outboard motors
- Radio-controlled model plans
What is 4 Stroke?
4 stroke engine is also a type of internal combustion engine. 4-stroke engines have intake and exhaust valves, unlike 2-stroke engines with ports for pulling in fuel and air.
The successive strokes in 4-stroke engines are suction, compression, power, and exhaust.
Four strokes are needed to complete the cycle in 4-stroke engines. The crankcase is an essential part of the engine, in which the engine is kept separated from the chamber so that combustion can occur.
And therefore, you only need to lubricate the gears and other engine bearings by filling the oil into the oil reservoir.
When the first stroke occurs, it opens the intake valve, the piston travels down towards the cylinder and pulls the fuel and air mixture from the carburettor. This stroke which is performed here is known as the intake stroke.
After the intake stroke, the piston moves upward to reach the cylinder and closes the intake valve. The spark plug gives off a spark to ignite the fuel mixture just before the piston reaches the top dead centre.
The compression stroke takes place when the piston goes to the top of the piston. As soon as the fuel ignites, it forces the piston back towards the cylinder. Power stroke gives the power to the engine its power.
The exhaust valve gets open when the piston is forced down. As soon as the valves open, the piston travels upwards to reach the cylinder. Once it reaches the cylinder, the exhaust gases are emitted through the exhaust port available in the engine.
The piston will again move up towards the cylinder and begin the process. Here you can see both compression and combustion strokes.
All the successive strokes are completed in the 2 revolutions of the crankshaft. And in this, the crankshaft rotates at 720 degrees.
4-stroke engines have well-reinforced parts. The performance of 4-stroke engines is good. 4-stroke engines are used in heavy-duty appliances and vehicles like buses, cars, and trucks.
4-stroke engines are less noisy and provide high power output. Its maintenance cost is also low compared to other engines.
Main Differences Between 2 Stroke and 4 Stroke
- The main difference between 2-stroke and 4-stroke engines is how they operate, i.e., the movement of the crankshaft and piston is the significant difference between these two engines.
- Their performance, maintenance cost, and efficiency are the significant aspects of their difference and uses.
- 4-stroke engines are less noisy and provide high power output. Their maintenance cost is also low as compared to 2-stroke engines.
- The applications of both engines are also different. 2-stroke engines are used in small and light equipment and engines, whereas 4-stroke engines are used in heavy and offload equipment and engines.
- The space occupied by 4-stroke engines is relatively high and not easy to manage, whereas 2-stroke engines are small and occupy less space.
- https://bura.brunel.ac.uk/bitstream/2438/9742/1/Fulltext.pdf
- https://www.archives-pmr.org/article/S0003-9993(99)90098-9/pdf
- https://www.sae.org/gsdownload/?prodCd=981038
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Piyush Yadav has spent the past 25 years working as a physicist in the local community. He is a physicist passionate about making science more accessible to our readers. He holds a BSc in Natural Sciences and Post Graduate Diploma in Environmental Science. You can read more about him on his bio page.
