The compression ratio is the ratio of the maximum volume enclosed inside a cylinder and the clearance volume. The maximum value is the volume enclosed inside the cylinder when the piston is at BDC
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The compression ratio is the volume enclosed in cylinder when the piston is at BDC and the volume enclosed in cylinder when the piston is at TDC.
The compression ratio is denoted by the r.
The compression ratio can be written as,
Compression ratio = `\frac{\text{Total volume}}{\text{Clearance volume}}`
The total volume = Swept volume + Clearance volume
`\therefore ` Compression ratio = `\frac{\text{Swept volume + Clearance volume}}{\text{Clearance volume}}`
r = `\frac{V_{S}+V_{C}}{V_{C}}`
r = `\frac{(\frac{\pi}{4}D^{2}L)+V_{C}}{V_{C}}`
Where,
D = Bore diameter
L = Length of piston stroke
The engine compression ratio is directly proportional to the engine output power. Hence as the compression ratio increases, the engine power output also increases while as the compression ratio decreases the engine power output decreases.
The compression ratio of the petrol engine is around 10:1 to 14:1 while the compression ratio for the diesel engine is 18:1 to 23:1. Hence the compression ratio is varied as per the application.
In the case of SI engine, the higher compression ratio than the maximum limit causes the autoignition or Detonation problem while in the case of CI engine, a lower compression ratio than the minimum limit causes problems in fuel auto-ignition.
The compression ratio for diesel engines is higher than petrol engines since in diesel engines, the air is required to be compressed at high pressure to fuel get auto-ignites while in petrol engine spark plug is used to ignite fuel.
What is the need of compression ratio?
The compression ratio is necessary because of the following reasons:-
1] In the case of SI engine, when the air-fuel mixture enters into the engine, it properly mixes with each other and proper combustion takes place which is because of the compression ratio.
2] In the case of the CI engine, as there is no spark plug hence the auto-ignition of the fuel is starts due to the injecting fuel into compressed hot air that is achieved by the compression ratio.
3] Increasing compression ratio increases the thermal efficiency of the engine.
Hence the compression ratio is plays a significant role in the IC engine.
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