Final answer:
Additives are substances added to fuel to improve its quality by enhancing properties like the octane rating, which indicates a fuel's ability to burn efficiently in an engine without causing knocking or pinging.
Step-by-step explanation:
Substances added to fuel to improve its quality are called additives. These compounds enhance certain properties of the fuel, such as increasing the octane rating. The octane rating is a measure of a fuel's ability to burn in a combustion engine without knocking or pinging, with higher ratings indicating better quality fuel. Previously used additives like tetraethyllead have been phased out due to toxicity, and alternatives like methyl t-butyl ether (MTBE) and ethanol have been developed. MTBE, while effective, has caused groundwater contamination issues leading to its restriction or ban in some areas. Ethanol is an increasingly popular octane enhancer due to its renewable sources and lower environmental impact.