For the same engine volume, the 2 stroke engines are more powerful than 4 stroke engines because the 2 stroke engine gives power stroke for each revolution of the crankshaft while the 4 stroke engine provides the power stroke for every two revolutions of the crankshaft.
To understand it more clearly it is necessary that we should know the working of 2 stroke engine and 4 stroke engine.
Contents:
Working of 2 stroke engine:
A single 2 stroke cycle completes with 2 strokes of the piston which are shown below.
- Upward stroke (Suction & Compression)
- Downward stroke (Power stroke, exhaust, fresh charge transfer)
Let’s discuss each stroke in detail.
1] Upward stroke (Compression & Suction):
During the upward stroke of the piston, the compression and suction processes are carried out simultaneously.
At the time of upward stroke, the piston travels from BDC to TDC and during this stroke, the exhaust port and transfer port are covered by the piston.
As the piston moves upward, it creates a vacuum inside the crankcase. Thus the fresh charge from the inlet port enters into the crankcase.
At the same time, as the piston moves upward, it compresses the charge trapped inside the cavity made by the cylinder and piston. And just before the piston reaches the TDC, the spark plug ignites the air-fuel mixture.
2] Downward stroke (Power stroke, exhaust, fresh charge transfer):
During downward stroke, the piston moves from TDC to BDC and it consists of the following two processes:- Expansion of combustion products, Replacing the exhaust gases by fresh charge.
After the combustion of charge, the hot gases push the piston in a downward direction to get expand. At that time, the exhaust and transfer port are covered by the piston surface.
But after moving the piston in a downward direction, at a certain distance from TDC, the transfer port and exhaust port become open to the cylinder.
And due to the further downward movement, the piston pushes the fresh charge present in the crankcase into the cylinder cavity through the transfer port. As the fresh charge enters the cylinder, the exhaust gases moved outside of the cylinder through an exhaust port.
Working of 4 stroke engine:
One complete cycle of a 4 stroke engine completes within 4 strokes of the piston or two revolutions of the crankshaft. The complete cycle of 4 stroke engine consists of the following strokes:-
1] Suction stroke:- During the suction stroke, the piston travels from TDC to BDC to create a partial vacuum inside the cylinder.
During this stroke, the suction valve remains open and because of the vacuum inside of a cylinder, the fresh charge enters into the cylinder.
2] Compression stroke:- At the time of compression stroke, both valves remain in closed condition. During this stroke, the piston moves from BDC to TDC to compress the charge and just before the piston reaches TDC, the spark plug ignites the charge.
3] Expansion/power stroke: The hot gases inside the cylinder push the piston from TDC to BDC.
4] Exhaust stroke:- At the start of the exhaust stroke, the exhaust valve becomes open and then the piston starts to move from BDC to TDC to expel exhaust gases outside of the cylinder.
Which engine is more powerful 2 stroke or 4 stroke?
The one complete cycle of a 4 stroke engine consists of one power stroke and 3 idle strokes. In other words, for the 2 revolutions of the crankshaft, the 4 stroke engine generates one power stroke.
While the one complete cycle of 2 stroke engine consists of one power stroke and one idle stroke. or for each revolution of the crankshaft, the 2 stroke engine generates one power stroke.
Thus for the same engine speed, the two stroke engine generates almost double the number of power strokes than the 4 stroke engine.
Therefore the 2 stroke engine burns fuel almost two times faster than the 4 stroke engine and hence the two stroke generates more power than that of a four stroke engine.
Example:- Consider engine A as a 2 stroke engine and engine B as a 4 stroke engine and assume that both engines have the same size and run at the same speed (n = 1500 rpm).
As 2 stroke engine makes a single stroke during each revolution of the crankshaft then the total power strokes made by 2 stroke engine is given by,
Number of power stroke (Engine A) = Engine speed = n = 1500 Power strokes/minute
And as 4 stroke engine makes a single power stroke for every two revolutions of the crankshaft then the total power stroke made by the 4 stroke engine is given by,
Number of power strokes (Engines B) = `\mathbf{\frac{\text{Engine speed (n)}}{2}}` = `\mathbf{\frac{1500}{2}}` = 750 Power strokes/minute
Thus within the same amount of time, engine A produces almost double the number of power strokes than engine B, therefore the power developed by engine A is also greater than engine B.
Therefore a 2 stroke engine produces more power than a 4 stroke engine.
Final words:
The 2 stroke engine makes the power strokes for each revolution of the crankshaft and the 4 stroke engine makes the power stroke for every two revolutions of the crankshaft. Thus for the same crankshaft speed, the 2 stroke engine burns the charge almost double that of a 4 stroke engine.
Thus the 2 stroke engine generally produces more power than the 4 stroke engine. Even though two-stroke engines create more power, they have fewer uses since they consume more fuel than four-stroke engines.
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