It is the purpose of this paper to discuss some of the gains in knock-limited engine performance to be made through utilization of fuels of improved antiknock quality, and also to consider a method of approach to the problem of the quantitative evaluation of these gains in terms of the antiknock quality of the fuel. Thermodynamic approaches to the problem of knock-limited power and economy in a gasoline engine have often been made in terms of compression ratio or supercharge. The translation of these relationships, however, into terms of fuel antiknock quality as measured by currently used standards presents some difficulties at present. It is well-known that fuel antiknock quality is one property of a fuel that determines its potential usefulness, but the question is: how much more antiknock quality makes it how much more valuable?
By means of some semiempirical relationships supported by quite extensive engine data, it has been found possible to relate changes in fuel antiknock quality of primary reference fuels (as measured by performance numbers) and relative knock-limited power output of either throttled or supercharged engines. The performance number index of antiknock quality is also related to changes in allowable compression ratio and hence thermal efficiency.
The absolute values of power or economy to be obtained from even a nonsensitive fuel depend upon engine design and also upon the compromise desired between power and economy. Sensitive fuels are not considered quantitatively in this paper, since their relative utility depends upon engine design and operating conditions and quantitative evaluations have not been completely worked out.
The increase in performance number rating of nonsensitive fuels accompanying the addition of tetraethyllead is substantially a constant fraction of the antiknock quality of the base fuel. The utility of tetraethyllead in terms of engine performance may be evaluated on this basis.


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