Organosilicon compounds as antiknock additives

Studies have been made of the effects of over 60 organosilicon compounds and, for comparison, of some purely organic compounds, on the lower two-stage ignition limit of 2-methylpentane. Experiments with pure hydrocarbons and binary hydrocarbon mixtures have shown that good correlations exist between this two-stage ignition limit and the research octane number obtained in engines, so that determinations of this limit provide a convenient and inexpensive method of assessing knock ratings of fuels. Laboratory measurements of ignition limits can thus also be used to determine the likely antiknock activity of additives, such as organosilicon compounds which during combustion react to give significant amounts of solid siliceous deposits in an engine and thus tend to give spurious values of research octane number. Silanes, alkoxysilanes, and siloxanes have, in general, only small effects on the combustion parameters of 2-methylpentane, as do silylated aliphatic and heterocyclic nitrogen compounds. However, silylated phenols and anilines, and in particular aminophenols, markedly raise the ignition limit of the hydrocarbon; they also decrease the intensity of the cool flame and lengthen the preceding induction period. Although none of the organosilicon compounds studied are likely to be viable antiknock additives, the work has nevertheless thrown light on the ways in which these compounds interfere with combustion processes.