Why is tetra ethyl lead added to some aviation fuel?

Tetraethyl lead is added to some aviation fuel primarily to decrease the risk of detonation or knocking during combustion. In high-performance engines, especially those operating at high compression ratios, the fuel-air mixture can ignite prematurely due to high pressure and temperature before the spark plug fires. This premature ignition, known as detonation, can lead to engine knocking, which can cause significant damage over time.

By introducing tetraethyl lead, the fuel's anti-knock properties are enhanced, allowing the engine to run at higher compression without experiencing harmful detonation. This enhancement results in more effective and efficient combustion, leading to improved engine performance and longevity. Other options, such as decreasing octane rating, increasing calorific value, or specific gravity, do not align with the primary purpose of adding tetraethyl lead to aviation fuels. The primary goal is to ensure safe and efficient engine operation under demanding conditions.