Ignition Timing and Knock – Effect of Advancing Spark in Petrol Engines In a spark-ignition petrol engine, what is the effect on knocking tendency if the spark timing is advanced (spark occurs earlier before top dead centre)?

Introduction / Context:
Ignition timing is a powerful lever in spark-ignition engines. Advancing or retarding the spark changes the pressure–temperature history of the unburned end-gas and therefore the likelihood of knock. Understanding this relationship is vital for engine calibration, particularly under high load where knock limits torque and compression ratio.

Given Data / Assumptions:

• Constant compression ratio and fuel octane rating.
• Load high enough that knock is a limiting factor.
• No active knock-control interventions assumed.

Concept / Approach:

Advancing the spark starts combustion earlier in the compression stroke. This increases the time the end-gas spends at elevated pressure and temperature before the flame front arrives. The longer residence at high temperature raises the chance of auto-ignition, thereby increasing knock tendency. Retarding the spark reduces the end-gas exposure and typically suppresses knock but may reduce efficiency and torque if excessive.

Step-by-Step Solution:

Verification / Alternative check:

Knock sensors in modern engines detect onset of knock and automatically retard timing to eliminate it, reflecting the strong positive relation between spark advance and knock intensity at a given fuel octane.

Why Other Options Are Wrong:

“Not affect” is incorrect because timing strongly affects knock. “Decrease” reverses the known trend. “First decrease then increase” lacks basis under fixed conditions. “Eliminate entirely” is unrealistic; spark advance cannot remove knock by itself.

Common Pitfalls:

Confusing MBT (minimum advance for best torque) timing with knock-limited timing; at high load, MBT may be beyond the knock limit, requiring retard to avoid knock even if torque drops slightly.

Final Answer:

increase