Final answer:
Ethyl dibromide was added to petrol engines to replace tetraethyl lead. MTBE was another alternative, but its use has been limited due to environmental concerns, leading to increased use of ethanol.
Step-by-step explanation:
The compound added to the petrol engine to overcome the problems caused by tetraethyl lead (TEL) is ethyl dibromide. Tetraethyllead, with the formula [(C2H5)4Pb], was previously used as an antiknock additive in gasoline but was completely phased out in 1986 due to its toxicity.
Ethyl dibromide has been used to address engine knock after the phasing out of leaded gasoline, but it should be noted that MTBE (methyl tertiary-butyl ether), another alternative, was also introduced as an octane enhancer.
MTBE has a high octane rating and minimal corrosion to engine and fuel system parts, making it an effective substitute. Unfortunately, MTBE has been implicated in groundwater contamination resulting from leaks, leading to its limited use in certain regions. This has paved the way for further alternative enhancers such as ethanol, which is renewable and less hazardous.