Question

My understanding of knocking is that in a spark ignition engine as the fuel is being compressed it heats up. The heat causes some of the oxygen to break up into oxygen radicals the O∙O∙ radicals react with the fuel grabbing parts of the fuel and creating radicals. These radicals react with molecular Oxygen thus releasing oxygen radicals to go on and continue the chain reaction.

I can understand how adding ethanol to fuel would fix this as O∙+H₃CCH₂OH⟶H₂O+H₃CCHOO would mop up the oxygen radicals. But how does tetraethyl lead fix this? Or is my understanding of knocking and cold flame just wrong?

Answer 1

Engine knocking is caused by premature combustion and can be reduced by antiknock agents such as ethanol and tetraethyl lead. Although tetraethyl lead effectively increases the octane rating and prevents knocking, it has been phased out due to toxicity, making way for other alternatives like ethanol.

Step-by-step explanation:

Engine knocking, also known as pinging, occurs when the fuel-air mixture in a spark ignition engine combusts unevenly, leading to premature combustion. This can be due to a low octane rating of the fuel or engine malfunctions, resulting in reduced power and potential engine damage.

Ethanol is known to help reduce knocking by reacting with oxygen radicals, which are a part of the chain reaction leading to premature combustion. On the other hand, tetraethyl lead was used as an antiknock agent by forming stable lead compounds during combustion that would not initiate or propagate this chain reaction. Because of its toxicity, the use of tetraethyl lead has been phased out, in favor of other antiknock agents like MTBE and ethanol.