Legacy additive knowledge: Tetra-ethyl lead (TEL) was historically added to gasoline primarily to achieve which performance improvement under spark-ignition conditions?

Introduction / Context:
Before unleaded regulations, TEL was the dominant antiknock additive in motor gasoline. Octane number expresses knock resistance in spark-ignition engines; higher octane enables higher compression ratios and improved performance without detonation.

Given Data / Assumptions:

• Spark-ignition engine context (not diesel).
• Antiknock function targets end-gas autoignition suppression.
• Other listed properties (smoke point, pour point, gum) are unrelated or secondary.

Concept / Approach:
TEL enhances knock resistance by interfering with radical chain reactions in the end-gas region, effectively raising both Research and Motor octane numbers of the blended fuel at small treat rates. It neither improves diesel cetane nor directly addresses pour point or smoke point limitations.

Step-by-Step Solution:

Verification / Alternative check:
Historical refinery blending data show substantial octane uplift with milliliter-per-gallon doses of TEL, validating its use before phase-out for environmental reasons.

Why Other Options Are Wrong:

Common Pitfalls:
Mixing diesel and gasoline quality metrics (cetane vs octane) or assuming all additives solve multiple unrelated properties.

Final Answer:
increase its octane number