Spark-ignition (SI) engines — effect of advancing ignition timing on knock In a petrol/gasoline spark-ignition engine, if the spark timing is advanced (spark occurs earlier before top dead centre, BTDC), how does this change influence the detonation/knocking tendency of the end-gas under otherwise similar operating conditions?

Introduction / Context:
Knock (detonation) in spark-ignition engines is the spontaneous auto-ignition of the end-gas after normal flame initiation. Ignition timing is a first-order control variable that strongly affects peak pressure, end-gas temperature, and the time available for auto-ignition reactions. Understanding how advancing the spark changes knock tendency is core to calibration and engine diagnostics.

Given Data / Assumptions:

• Conventional spark-ignition engine operating on gasoline-like fuel.
• Operating point (speed, load, coolant temp) otherwise unchanged.
• Advancing timing means firing the spark earlier BTDC.

Concept / Approach:
Advancing the spark increases the time for the flame front to propagate before TDC. This raises the cylinder pressure and temperature earlier in the cycle. The unburned end-gas is thus exposed to higher pressure/temperature for longer, which accelerates pre-flame reactions and reduces auto-ignition delay. Therefore, earlier timing generally increases knock tendency, while retarding timing tends to reduce it (at the expense of efficiency and exhaust temperature).

Step-by-Step Solution:
Advance timing → earlier heat release near TDC.Earlier heat release → higher peak pressure and temperature in the end-gas region.Higher T and p → shorter chemical ignition delay of end-gas.Shorter delay + more residence time → higher probability of knock.

Verification / Alternative check:
Engine maps show that the knock-limited spark advance (KLSA) is retarded under high load/higher intake temperatures to avoid detonation, confirming that advancing timing increases knock susceptibility.

Why Other Options Are Wrong:
reduce: opposite of observed behavior. not affect: ignores thermochemistry and pressure-temperature history. lead to complete elimination of knock at high load: advancing timing usually worsens knock at high load.

Common Pitfalls:
Confusing knock with pre-ignition; pre-ignition is an abnormal early ignition source, not end-gas auto-ignition. Also, small timing changes at light load may not trigger audible knock, but the tendency still moves in the same direction.

Final Answer:
increase