Definition: – Anti Knocking agents are the additives added in small quantities in petrol engine to reduce the knocking tendency.
Introduction: – Initially A.H. Gibson and harry Ricardo do their research on anti knock agents. Very much known anti knock agent was tetraethyl lead (TEL). Later on Derek Bryce-Smith and Clair Cameron Patterson discover the environmental and Health issues with TEL because of lead. Now a day’s most of countries are phasing out leaded fuel and focusing on various anti knock additives. There are three major group of anti knock agents.
3 organo-metallic compounds.
1 Aromatics: – Benzene is one of the known aromatic compounds. U.S. EPA limits the overall benzene concentration in refined gasoline. Because benzene have been identified as carcinogen. Toluene and xylene are the main aromatic organic solvents usually found in anti knock additives. Toluene is a clear, water insoluble liquid. The properties of toluene and xylene are nearly identical. They are both octane boosters. They are also not bad for human health because it does not show carcinogenic.
2 Oxygenates: – Ethanol is most widely blended with gasoline. Similar type of octane enhancement can be obtained by methanol, isopropyl alcohol and tertiary butyl alcohol. The either is most widely used as a gasoline additive by refiners is MTBE (methyl tertiary butyl ether).
MTBE is soluble with water therefore its use as an anti knock agent is controversial. ETBE (ethyl tertiary butyl ether) are good octane enhancing ether additives.
Oxygenates additives reduce the knocking but there are other draw with this. Like they are having low calorific value and reduce the mileage also.
3 organometallic compounds: – MMT (methyl cyclopentadieny manganese tri-carbonyl), dicyclopentadienyl iron, nickel carbonyl are metallic anti knock agent. They are toxic in nature. These
additives having the problems with catalytic exhaust system, premature spark plug failure and engine wear. MMT is banned by U.S.A. in 1978 because of its toxic nature. Ferrocene (Fe (C5H5)2) having the unusual chemical structure. But it is safer then TEL. In Europe, iron pent carbonyl was once used as an anti-knock agent in petrol in place of tetraethyl lead. Iron pent carbonyl has been found to be a strong flame speed inhibitor in oxygen based flames.
Conclusion: – oxygenates additives are the best anti knock agents. It can increase octane rating and induce the complete fuel combustion. Aromatic compounds are also good anti knocking agents compare to the paraffin, olefins etc. Whereas organometallic compounds have environmental and health issues of human.