This paper discusses recent data which give a comparison between the low speed road antiknock performance of fuels containing tetramethyl lead and that of fuels with tetraethyl lead. It is shown that the substitution of tetramethyl lead for tetraethyl lead in fuels having overhead (60-90% distilled) fractions of low octane quality (relative to the octane quality of the total fuel) results in a marked improvement in low speed road antiknock behavior.
The difference in volatility between the two antiknocks is shown to influence the distribution of octane quality throughout the boiling range of the gasoline. The octane distribution, in turn, is characterized by a variable of the fuel called “Delta R” which has been found to predict the observed low speed advantage for tetramethyl lead. “Delta R” is defined as the Research O.N. of the fuel minus that of an appropriate overhead fraction. This advantage for tetramethyl lead is also predicted qualitatively based on the mechanism of “fuel segregation” which is described briefly.
Road antiknock data for both European and American cars are presented which show that the advantage for TML is more pronounced in the European models. This is probably a reflection of the increased tendency toward fuel segregation by those cars equipped with manual transmissions and having a somewhat different induction system design.