The Knock Rating of Motor Fuels.

Abstract

The knock rating of a fuel is essentially a direct comparison of the intensity of the knock produced by i t with that of a standard fuel, these intensities being measured according to a specified procedure. Other methods, such as chemical analysis or bomb explosions, have been found to be inadequate for predicting the knock characteristics of a fuel when in actual service. The fuels used as a standard of comparison are normal heptane and isooctane (2,2,4-tr imethylpentane) , first proposed by Edgar? These fuels are pure hydrocarbons and thus can always be duplicated; they are similar to each other and to gasolines in their physical and chemical properties, and when blended can duplicate the range of antiknock qualities of gasolines likely to be used for motor cars, since isooctane is considerably better and normal heptane poorer in this respect than ordinary motor fuels. Thus blends of these substances establish a standard scale for comparison of any motor gasoline. In order that the results of different laboratories may correlate, not only is a standard reference fuel necessary, but the engine and the operating procedure must be standardized, because operating conditions affect the relative knock intensities of fuels. Therefore, the Cooperative Fuel Research, single-cylinder, variable compression, vapor-cooled engine was designed specifically for this purpose. In order to obtain the knock rating of a fuel this engine is operated at 900 rpm with a mixture temperature of 300~ and the knock intensity of the fuel measured? The blend of normal heptane and isooctane which matches the fuel in knock intensity under the same operating conditions is then determined, and the fuel is said to have an octane number which is the percentage of isooctane in the isooctane-heptane mixture which it matched. A specified knock intensity is used which is obtained by varying the compression ratio and is measured by means of a bouncing pin. Secondary reference fuels which have been suitably calibrated against normal heptane and isooctane blends are generally used instead of thc expensive primary reference fuels. This method was developed by the Cooperative Fuel Research Committee, on which are represented the Automotive Industry, the Petroleum Industry, and the National Bureau of Standards, and is the result of a great deal of co6perative work done by individual laboratories throughout the world. In 1932, the Committee had developed the octane number scale, the knock-testing engine, and a procedure. The procedure at that time differed from the present procedure in that the engine was operated at 600 rpm instead of 900 rpm and the mixture temperature was approximately room temperature instead of 300~ Also the spark timing was retarded 3.5 degrees with respect to that now in use. This method, now called the C.F.R. research method, was modified because results obtained with it did not correlate with results obtained in service.