Knocking tendency in SI engine reduces with an increase in engine speed. This can be explained by the following points:

Explanation:

- Knocking tendency in SI engine refers to the occurrence of spontaneous combustion of the air-fuel mixture before the spark plug fires, which results in a sudden increase in pressure and temperature inside the engine cylinder. This can cause damage to the engine components and reduce its efficiency.
- The knocking tendency is influenced by several factors such as the composition of the air-fuel mixture, compression ratio, engine speed, etc. Among these factors, engine speed has a significant impact on knocking tendency.
- At low engine speeds, the air-fuel mixture has more time to react and undergo pre-ignition due to the high temperature and pressure inside the engine cylinder. This results in a higher knocking tendency.
- On the other hand, at high engine speeds, the air-fuel mixture has less time to react and undergo pre-ignition, as the piston moves up and down faster. This reduces the knocking tendency and improves engine performance.
- Moreover, at high engine speeds, the pressure and temperature inside the engine cylinder are more uniform, which reduces the likelihood of spontaneous combustion of the air-fuel mixture. This also contributes to a lower knocking tendency.

Conclusion:

- In summary, the knocking tendency in SI engine reduces with an increase in engine speed due to the reduced time available for pre-ignition of the air-fuel mixture and the more uniform pressure and temperature inside the engine cylinder.