An engine is a machine designed to convert fuel into mechanical energy. The engines of all motors sound similar, but did you know there are different types of engines based on the stroke? The engines are classified into two types based on the number of strokes.
They are two-stroke engines and four-stroke engines. The engine parts such as piston, crankshaft, cylinder, and combustion chamber are the same in both engines.
Let’s delve right into the factors of difference between two stroke and four stroke engines.
Parts of an engine
The main aspects of difference lie in the classification of the parts of the engines. Here are a few parts generally found in an engine:
- Piston: Piston is a component of two-stroke and four-stroke engines. It is a piece of metal in the engine that fits tightly and moves forward and backwards, and causes the other parts of the engine to move.
- Crankshaft: A crankshaft is a shaft that converts a piston’s linear movement (up and down) into rotary motion.
- Cylinder: A cylinder is a space in the engine where fuel is burnt, and power is generated.
- Combustion chamber: This is a part of a cylinder where the fuel-air mixture is ignited.
What does ‘stroke’ refer to in an engine?
Stroke in engines is the distance covered by the piston. The combustion cycle consists of different phases such as intake, compression, expansion, and exhaust. In an internal combustion engine, stroke is how the piston moves up and down to complete these phases.
To get a clear understanding of the concept, let us go through some basics about the combustion cycle in an engine. During a combustion cycle in the cylinder, the piston moves up and down. There are two positions of the piston in the cylinder. One is the top dead centre, and the other is the bottom dead centre.
Now, let us see what a two-stroke engine and four-stroke engine mean. The top dead centre position is near the valves, and the bottom dead centre position is the farthest. One stroke is the movement of the piston from the top dead centre to the bottom dead centre.
One combustion cycle is the process of sucking fuel and air, igniting and releasing the gases.
- Two-stroke engine: In such engines, each combustion cycle is completed in two piston movements (one stroke equals two-piston movements) during one crankshaft revolution. The cycle in a two-stroke engine consists of a compression stroke where the fuel mixture is sucked into the cylinder and compressed.
The exhaust is let out during the return stroke, and a fresh mixture is absorbed into the cylinder. The spark plug is ignited once in each cycle. Power is generated once during the two strokes of the piston. The oil needs to be pre-mixed in such engines. Examples of two-stroke engines include scooters and motorcycles.
- Four-stroke engine: In a four-stroke engine, each combustion cycle requires four strokes of piston during two crankshaft revolutions. The four-stroke engine has four phases. They are intake stroke, compression stroke, power stroke or combustion stroke, and exhaust stroke. We will have a look at the four phases.
- Intake stroke: This is the first phase of the combustion cycle. During the intake stroke, the piston moves downward to draw the air-fuel mixture into the combustion chamber. The atmospheric pressure pushes the mixture through the inlet valve into the chamber.
- Compression stroke: This is the second phase in the four-stroke engine. In this phase, the air-fuel mixture is filled and compressed by this piston. This leads to the piston moving up.
- Power stroke or combustion stroke: During this third phase in the four-stroke engine, the mixture is ignited either by spark plug or self-ignition (petrol engines use spark plug and diesel engines use self-ignition method).
When ignited, the mixture expands and forces the piston downwards. The force created during the expansion is the power generated.
- Exhaust stroke: This is the last phase in the four-stroke engine. In this phase, the gases created during power stroke are released from the combustion chamber into the atmosphere through the exhaust valve.
This causes the piston to move upwards. Following this, the exhaust valve closes, and the intake valve is opened to let in the fresh air-fuel mixture, and the process keeps repeating.
The four-stroke engine is the most typically used. The vehicles that use gasoline as fuel, like trucks, cars, and a few motorcycles, use the four-stroke engine. The other examples include auto-rickshaw, motor-powered boats, water spray systems, etc.
We understood the mechanism of a two-stroke engine and a four-stroke engine. Let us now see the Difference Between Two-Stroke and Four-Stroke Engines to check out how they differ in performance and efficiency and understand the concept better.
Difference between two-stroke and four-stroke engine
| Two-stroke engine | Four-stroke engine |
| The crankshaft has one revolution during one power stroke | The crankshaft has two revolutions during two power stroke |
| Port is used for inlet and outlet of the fuel | Valve is used for the inlet and the outlet of the fuel |
| A two-stroke engine generates high torque | A four-stroke engine generates low torque |
| Two-stroke engines are light in weight | Four-stroke engines are heavy as it requires heavy fly-wheel |
| Two-stroke engines are cheap because they require less effort in manufacturing. | Four-stroke engines are expensive due to heavy fly-wheel, valve mechanisms and lubrication. |
| A two-stroke engine generates more heat, thus requiring more cooling and lubrication. | A four-stroke engine generates less heat. |
| These engines have less thermal efficiency. | These engines have higher thermal efficiency. |
| A two-stroke engine produces more smoke due to poor efficiency. | A four-stroke engine has greater efficiency, and thus the smoke produced is less. |
| Tear and wear are more due to poor lubrication. | Tear and wear are less. |
| More lubrication oil is required as some lubrication oil burns with fuel. | The function is smooth hence does not require more lubrication. |
| Two-stroke engines are noisy. | Four-stroke engines are quiet. |
| Volumetric efficiency is less. | Volumetric efficiency is high. |
| Power generation is more in two-stroke engines. | Power generation is less in four-stroke engines. |
| Two-stroke engines are utilized in scooters, motorcycles, and ship propulsion. | Four-stroke engines are utilized in cars, modern bikes, aircraft, etc. |
Conclusion
This concludes the difference between two-stroke and four-stroke engines. The main difference between two-stroke engines and four-stroke engines is in the action of the piston and crankshaft. There has been a lot of development in automotive engines over the years, but the two fuel-powered combustion engine design remains the same.
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