Gasoline is a fuel mixture. High compression internal combustion engines place gasoline under great pressure. The pressure can cause the gasoline to ignite early (pre-ignition or detonation) causing a damaging "engine knocking" (also called "pinging" or "pinking") noise. A.H. Gibson and Harry Ricardo in England and Thomas Midgley, Jr. and Thomas Boyd in the United States led early research into this effect. They discovery that lead additives ended or reduced knocking. So, most gasoline brands started adding lead in the 1920s. The lead antiknocking agents allowed the sale of even more powerful, higher compression car engines. The most popular additive was tetra-ethyl lead. However, lead causes environmental and health damage. The lead in gasoline leaves the car through the engine exhaust pipes. The wind and rain then spread the lead around the environment. Also, lead harms the catalytic converters found on virtually all US automobiles since 1975. So, the sale of gasoline with lead additives was greatly reduced in the 1980s. Most countries are stopping leaded fuel use; different additives have replaced the lead compounds. The most popular antiknock additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol). But some people believe that using aromatic hydrocarbons poses a cancer risk if they do not burn completely.
The typical antiknock agents in use are:
- Tetra-ethyl lead (phased out)
- Methylcyclopentadienyl manganese tricarbonyl (MMT)
- Iron pentacarbonyl
- Isooctane (2,2,4-Trimethylpentane)
In some parts of South America, Asia and the Middle East, leaded gasoline is still in use. Sub-Saharan Africa stopped using leaded gasoline on 1 January 2006. A growing number of countries have drawn up plans to ban leaded gasoline in the near future.
To avoid deposits of lead inside the engine, lead scavengers were added to the gasoline together with tetra-ethyl lead. The most common ones are:
- Schnaas L, Rothenberg SJ, Flores MF, et al. (July 2004). "Blood lead secular trend in a cohort of children in Mexico City (1987-2002)". Environ. Health Perspect. 112 (10): 1110–5. doi:10.1289/ehp.6636. PMC 1247386. PMID 15238286. http://ehpnet1.niehs.nih.gov/members/2004/6636/6636.html.
- "knocking - antiknock agent". Net Industries. http://science.jrank.org/pages/16348/knocking.html. Retrieved 2011-09-10.