A machine with moving parts that converts power into motion is called as an Engine, and the best-suited example is automobiles. (cars, truck, bus,2-wheeler etc.) Since a single type of engine cannot be used in all automobiles, there are various types in which engines are designed according to the requirements. The basic components of engine such as piston, cylinders, crankshaft, camshaft, connecting rod, valves, spark plugs remain same for most of the engines however Engines are classified into different categories depending upon the type of fuel used, displacement, ignition methods, firing order, no. of cylinders and their arrangement, number of strokes and so on. Today let us see the difference between a 2-stroke engine and a 4-stroke engine.
During an engine’s combustion cycle the piston moves up and down inside the cylinder. Burning of the air-fuel mixture takes place inside the cylinder. The position of the piston in the cylinder is denoted by “Top dead centre” (TDC) and “Bottom dead centre” (BDC). The top dead centre position is the position in which the piston is closest to the valves and at the bottom dead centre position the piston is farthest from the valves. A stroke is said to be completed when the piston moves from TDC to BDC and vice versa. A combustion cycle consists of 4 events namely- Suction, Compression, Power and Exhaust.
4-stroke engine-
In a 4-stroke internal combustion engine, the four events are completed in 4 strokes of the piston or say in 2 revolutions of the crankshaft. The first process that takes place is the suction stroke in which the inlet valve is opened and the air-fuel mixture is allowed into the cylinder. In this stroke, the piston is at the bottom dead centre position. As the cylinder is filled with the air-fuel mixture the inlet valve closes and now the piston moves upwards compressing the mixture. This is called as the compression stroke. When the piston is at top dead centre position a spark is ignited by the spark plug to burn the mixture. After ignition of the mixture tremendous amount of energy is generated causing very high pressure inside the cylinder which pushes the piston in the downward direction. This is called the power stroke. The connecting rod transmits the power from the piston to the crankshaft and the crankshaft rotates. Now during the exhaust stroke, the piston moves upward again and the exhaust valve is opened for the gases to escape making the cylinder vacant for the fresh mixture to enter.
2-stroke engine-
In a 4-stroke internal combustion engine, the four events are completed in 2 strokes of the piston or say in 1 revolution of the crankshaft. There are no valves in this case. Movement of gas takes place through holes called as ports in the cylinder. When the piston moves up it covers two of its ports, the exhaust port and the transfer port which mostly placed opposite to each other. As a result, the air-fuel mixture is gets trapped in the cylinder. Further upward movement of the piston compresses the mixture and unblocks the suction port due to which fresh mixture gets entered in the cylinders. Just when the stroke is about to end the mixture is ignited by the spark plug making causing the suction and the compression stroke together. Burning of mixture produces a tremendous amount of energy causing the piston to move in a downward direction. The downward direction of the piston causes the suction port to close and opens up the exhaust port along with the transfer port. Now fresh charge in the crankcase moves into the cylinder through transfer port driving out the exhaust gases. This combine stroke is the power-exhaust stroke.
Let us discuss some pros and cons of both 2-stroke as well as 4-stroke engines-
Efficiency being a major factor, 4-stroke engine is more efficient as fuel is consumed once every 4-strokes.
On the contrary, 4-strokes are much heavier than the 2-stroke ones (almost 50% heavier).
The 4-stroke engine is quieter (more refined) than the other.
A 2-stroke engine produces more torque at higher RPM, whereas the 4-stroke produces more torque at lower RPM.
2-stroke engines tend to wear out faster.
In 2-stroke engines sometimes incomplete burnt oil is released into the environment so 4-stroke engines are more eco-friendly as the fuel is burnt completely.
2-stroke engines are much simpler in design and easier to fix, unlike the 4-stroke engine which has many parts and complex design.
2-stroke engines require pre-mixing of oil and fuel which is not in the case of the 4-stroke engine.
2-stroke engines are easier to start in cold conditions.
The power output of 2-stroke engine is less in comparison to the 4-stroke engine.
Applications-
The 2-stroke engines were mostly used in 2-wheeler, chainsaws, weed trimmers, outboard motors, off-road motor cycles and racing motorcycles, scooters and mopeds.
The 4-stroke engines are mostly used in cars, trucks, tractors, buses etc.
Disclaimer- Photos are taken from Google.
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