Spark-ignition (petrol) engines – anti-knock additives The knocking tendency in spark-ignition engines can be reduced by adding anti-knock dopes such as tetra-ethyl lead (TEL) together with ethylene dibromide (as a scavenger).

Introduction / Context:
Knock in spark-ignition (SI) engines is the spontaneous auto-ignition of unburned end-gas ahead of the flame front, producing pressure oscillations and potentially damaging the engine. Fuel chemistry strongly influences knock resistance, typically measured by octane rating.

Given Data / Assumptions:

• Conventional SI engine with homogeneous charge.
• Use of historical anti-knock additives (TEL with ethylene dibromide scavenger).
• Focus on principle rather than modern emissions regulations.

Concept / Approach:
Anti-knock additives increase a fuel’s auto-ignition resistance by slowing pre-flame radical chain reactions in the end-gas. Tetra-ethyl lead was historically effective at raising octane number. Ethylene dibromide aided in scavenging lead deposits by forming volatile lead halides. Although modern fuels avoid lead for environmental reasons, the statement about their knock-reducing effect is correct.

Step-by-Step Solution:
Define knock: auto-ignition of end-gas leads to sharp pressure waves.Relate additive action: TEL interrupts radical pathways, delaying auto-ignition.Scavenger role: ethylene dibromide helps remove lead compounds to limit fouling.Outcome: higher octane rating → reduced knock tendency.

Verification / Alternative check:
Octane number tests (e.g., CFR engine) historically showed substantial octane increase with TEL addition, confirming reduced knock at given compression ratios and spark timing.

Why Other Options Are Wrong:

• Applicability is specific to SI knock; diesel knock is a different phenomenon tied to ignition delay.
• Effect is not limited to very rich mixtures or a narrow rpm band.

Common Pitfalls:
Conflating modern unleaded gasoline practice with historical statements; environmental bans do not negate TEL’s anti-knock chemistry in principle.

Final Answer:
Agree