Compression ratio is defined as the ratio obtained by dividing volume at BDC by volume at TDC (clearance volume).
Compression ratio= (Swept volume + Clearance volume) ÷ Clearance volume
Significance of Compression Ratio-
Affect fuel economy:
- The higher the ratio, the more compressed the air in the cylinder is. When the air is compressed, you get a more powerful explosion from the air-fuel mixture, and more of the fuel gets used.
Affect gas emission:
- High compression ratios provide better burning of fuel and this reduce the waste gases produced by the engine , anything higher than 16:1 will be problem and create something called a knock.
Affect engine cold starting characteristics:
- Engines with high compression ratios produce excessive heat compared to lower compressed vehicles thus doing a cold start should not be a problem for these type of vehicles.
- Higher compression ratios provide much more power than lower ratios. This also allows engine to be more efficient at higher rpms. Lower compression ratios give lower output but are more easy to build, maintain and generally have better life.
Higher compression ratio advantages:
- Much better performance
- Higher relative fuel economy
- Higher usable power throughout
- Better emissions.
Lower compression ratio advantage:
- Cheaper to build, run and repair.
- Lasts much longer
- Can be air cooled.
- Less noise and vibration.
- Can be run on relatively lower quality fuel.
- Do not generate as much heat