Engine speed and knock tendency in petrol engines For a spark-ignition engine operating on a fixed-octane fuel, what is the qualitative effect of increasing engine speed on the tendency to knock?

Introduction / Context:
Knock in a petrol (spark-ignition) engine is caused by auto-ignition of the end-gas ahead of the flame front. The octane rating of the fuel indicates resistance to this auto-ignition. Engine speed influences in-cylinder time scales, turbulence, and heat transfer, thereby affecting knock tendency.

Given Data / Assumptions:

• Fuel octane number is fixed.
• Comparisons are made over a normal operating range, not at destructive overspeeds.
• Mixture strength and load are not drastically altered except due to normal volumetric efficiency effects.

Concept / Approach:
As engine speed increases, the absolute time available for end-gas to undergo pre-flame reactions before the flame front arrives becomes shorter (milliseconds decrease with higher rpm). Increased turbulence at higher speeds also accelerates flame propagation, reducing the residence time of end-gas at high temperature and pressure. Both effects reduce the probability of knock for the same octane fuel.

Step-by-Step Solution:
Higher rpm → shorter time per crank angle → less pre-flame reaction time.Higher turbulence → faster flame speed → end-gas consumed sooner.Net effect → decreased knock tendency with increasing speed for a given fuel.

Verification / Alternative check:
Ignition timing maps often advance at low–mid speeds and are retarded near knock limits at high load, but, all else equal, the shorter time scales at higher speeds give more knock margin for a fixed octane.

Why Other Options Are Wrong:

• “Increases knocking” contradicts the time-scale argument.
• “No effect” ignores well-established combustion dynamics.
• Conditional statements about redline or WOT are too restrictive and not central to the basic relationship.

Common Pitfalls:
Confusing effects of increased load (which can promote knock) with effects of increased speed at similar load.

Final Answer:
Yes, increasing speed decreases knocking tendency