Control measures for SI Engine knocking

Control measures for SI Engine knocking

If somehow, we can control the formation of secondary flame front than there is very less tendency of SI engine knocking.

In particular there are eight ways to control it.

1. Compression ratio/cylinder dimension: 

Increase in compression ratio causes increase in cylinder temperature after compression stroke from BDC to TDC. Resulting in increase in end charge temperature and hence there is a tendency of engine knocking.

Conclusion: In SI engine compression ratio varies from 6 to 12 preferably on lower side.

2. Inlet air Tempearture: 

Increase in inlet air temperature causes increase in end charge temperature and hence increase in SI engine knocking tendency.

Conclusion: In SI engines use of supercharger is not preferred because it will increase 

the inlet air temperature.

3. Speed: 

As the speed of SI engine increases the convective heat transfer coefficient increase, results in decrease end charge temperature. Hence SI engine knocking tendency decreases.

Conclusion: SI engines are meant for high speed.

4. Load/Torque: 

As the load on SI engine increases, for constant power the speed will decrease. Therefore, to maintain the mean speed the fuel supply must have to increase hence increase in end charge temperature. It implies that SI knocking tendency increases.

Conclusion: SI engines are meant for low torque.

5. Turbulence/Swirl: 

High turbulence implies decrease in end charge temperature the reason is same as point 3. Also the high turbulence causes the flame front travel at higher speed so that it will reach to the end charge as fast 

as possible.Means there is no time left for the secondary flame front. It implies that the SI knocking tendency decrease.

6. Advance of spark timing: 

More the Advancement in spark timing more the heat is entering into the cylinder which results in increase in end charge temperature. Hence the SI engine knocking tendency increase.

7. Chemical property: 

Complex/Aromatic structure must be used so that preignition can be prevented.

Conclusion: Good SI engine fuel is bad CI engine fuel and vice-versa.

8. Octane number: 

Octane rating is a measure of fuel’s ability to resist knocking during combustion. In India fuel with 87 octane rating is used.