Effects of alkyllead antiknock agents and scavengers on carbon deposit formation in engines

The most effective and economically favorable means for increasing the knock resistance of automotive gasolines is the addition of antiknock agents, specifically, tetraethyllead (TEL) and tetramethyllead (TML). In gasolines containing large amounts of aromatic hydrocarbons, TML is more effective than TEL. Moreover, TML has a lower boiling point than TEL (110 degree and 200 degree C, respectively), and hence is distributed among the cylinders more uniformly, thus providing better utilization of the antiknock properties of the gasoline. In order to ensure that the combustion products are discharged from the chamber, special substances known as scavengers are added to the antiknock agents. When the scavenger is burned, certain compounds are formed with lead or its oxides, these compounds having high vapor pressures and low melting points so that they are readily discharged from the combustion chamber with the exhaust gas. Organic halogen compounds are used as scavengers. Bromine-containing compounds are the most effective. However, because of the shortage of bromine, mixtures of bromine and chlorine compounds have also been used in practice.