benefits of leaded gasoline

Adding tetraethyl lead to gasoline increases its octane rating. Octane rating is essentially a measure of how difficult it is to ignite the gas.

The reason it for wanting for gas to be harder to ignite is to eliminate a phenomenon known as knocking. When you compress the volatile gas/air mixture in an engine cylinder, especially when it's hot, it can ignite spontaneously. This is, in fact how diesel engines work -- the fuel is ignited by compression, not by a spark plug. Knocking occurs when the fuel in a gasoline engine is ignited by compression before the spark plug fires. (The knocking sound is actually the sound of the fuel igniting at the wrong time.) Higher octane gas, being harder to ignite, is less likely to cause knocking.

Why is knocking bad? Well, besides the annoying sound, it causes a loss of power because the energy generated by that bit of gas is largely wasted if the piston isn't at the top of its stroke when it ignites.

All that being said, there are better (i.e. not deadly poisonous) ways to increase octane ratings. One that was in relatively widespread use before GM and DuPont introduced leaded gasoline, is ethanol.

Higher octane can help to prevent detonation ( 'knocking' or 'pinging' ), as well as preignition. These two problems are not merely noises heard from the engine, but are symptoms of something going very wrong, and these problems can literally destroy an engine, breaking piston rings (or apex seals in the case of a rotary engine), holing pistons, breaking connecting rods, and otherwise causing major damage.

Tetraethyl lead, in addition to its octane-increasing benefits, also lubricates valve seats and other portions of the engine that are not exposed to crankcase oil. Newer engines have special hardened valve seats that do not require this lubrication. However, running an older engine on unleaded gas can cause damage to the valve seats.

Modern high-compression and highly-tuned engines are more susceptible to detonation, especially on modern lower-octane gasoline. As an engine is leaned out to maximise power output and fuel economy, detonation also becomes more likely.

Tetraethyl lead, though beneficial in several ways to the operation of internal combustion engines, has several drawbacks, notably its toxicity. There are products that can duplicate both the octane-increasing and the lubrication function of tetraethyl lead, and these are being further researched.