A Fundamental Study of the Relationship Between Altitude and Research Octane Number

The influence of altitude (ambient pressure) on the auto-ignition phenomenon is a fuel property that has been studied indirectly in terms of octane-response studies involving large fleets of test vehicles. The magnitude of the measured altitude-octane response has varied from study to study and the phenomenon lacks a definitive answer. The present paper presents a simple theoretical analysis based on the response of the end-gas auto-ignition to pressure which is used to predict the altitude-octane effect. A series of tests were conducted using a standard CFR engine operating at an altitude of 1485 meters above sea-level to evaluate and confirm the theoretical findings which indicated clearly that the altitude-octane effect is an intrinsic fuel property. The magnitude of the effect was shown to vary depended on the fuel itself, but was typically in the range -1.0 to -1.5 ON/300 m for modern commercial fuels. Against this background, the altitude response of the combined automotive engine and fuel was investigated via the results of a recent 54-vehicle fleet study that was conducted in South Africa. A novel statistical analysis technique was used to determine the magnitude of the underlying altitude-octane effect, which was determined as -0.88 ON/300 m.