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Which of the following factors can control detonation in spark ignition engines? 1. Increasing engine rpm 2. Advancing spark timing 3. Making fuel-air ratio very rich

Which of the following factors can control detonation in spark ignition engines? 1. Increasing engine rpm 2. Advancing spark timing 3. Making fuel-air ratio very rich Correct Answer 1 and 3 only

Explanation:

Knocking or detonation is due to the auto-ignition of the end portion of the unburned charge in the combustion chamber.

Factors affecting knocking in SI engine:

  1. Compression ratio:- Higher compression ratio will result in the higher temperature of the charge, the tendency to knock will increase.
  2. Inlet air temperature:- The effect of inlet air temp is the same as the compression ratio. The high inlet temp of the air-fuel mixture will increase knocking.
  3. Advancing the spark:- Advancing the spark will result in more compression during sparking. The increase in temperature during the sparking will be higher. Hence tendency to knock will increase and so by retarding the spark, the tendency to knock will decrease.
  4. Coolant water temperature:- By increasing coolant water temperature, less heat will be carried from the engine which will increase the engine temperature. So the knocking will increase.
  5. Fuel-air ratio: The flame speed is affected by the fuel-air ratio. Also, the flame temperature and reaction time are different for different fuel-air ratios. If the fuel air-ratio is high it will give maximum reaction time for auto-ignition therefore the rich fuel ratio will decrease the knocking tendency in the spark-ignition engine.  
  6. Increasing the speed: Increasing the speed will cause turbulence in the cylinder, hence the flame front will reach the end charge faster in all the directions therefore there will be less knocking. 
  7. Engine Overheating: Even the excessive intake air temperature or overheating of the engines can cause pre-ignition because the mixture in the cylinder will be at a higher temperature and so closer to the ignition point.

Related Questions

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