LABORATORY QUALITIES AS PREDICTORS OF ROAD OCTANE NUMBER 570101
The rapid trend in recent years toward higher octane fuels and high-performance engines has suggested a reappraisal of the ability of current laboratory knockrating methods to define the antiknock behavior of commercial gasolines in modern multicylinder passenger car engines. Road antiknock performance data have been obtained on 91 premium gasolines in five 1957 test cars and three cars equipped with experimental 11-to-1 compression ratio engines. In addition, 16 prototype gasolines were studied in the three high compression ratio cars.
The data were analyzed by establishing a linear relationship between the road ratings and the laboratory data to determine the relative importance of each variable in enhancing the accuracy of the prediction equation. Of the many fuel laboratory variables, combinations of Research and Motor octane number, fuel composition and volatility were investigated as predictors of road performance.
For any given combination the results were rather consistent from car to car, but varied greatly with car speed. Research octane number was found to be the best single predictor of low-speed road performance (2000 rpm), and Motor octane number the best single predictor of high-speed performance (3000 rpm and higher).
If two laboratory variables are used as predictors, a combination of Research and Motor octane numbers serves best for predicting low-speed road performance. At high speed, the combination of Motor octane number and percent aromatics was found to be the best two-variable predictor.
Low-speed correlations based on Research and Motor octane number were not improved when olefin content was included as a third predicting variable. At high speed, however, the use of a hydrocarbon-type variable in addition to Research and Motor octane numbers resulted in a significant reduction of the standard error of prediction.
The use of more than three variables as predictors did not improve the accuracy of prediction at low speed and improved the accuracy at high speeds to a degree which is not considered to be of practical value.
An increase in Research octane number was found to be of more importance than an increase in Motor octane number in improving low-speed road ratings. At high speeds, however, the contribution of Motor octane number was decidedly greater than that of Research octane number.
When Research and Motor octane numbers were held constant, an increase in aromatic content or a decrease in olefin content improved the high-speed road rating of a fuel