The Importance of Fuel Properties in Correlating Carbon Balance and Volumetric Fuel Consumption

The Australian Standard - Methods of test for fuel consumption of passenger cars, their derivatives and multipurpose passenger cars, AS2077-1982 - is based on the method of determining petrol consumption accepted by the United States Environmental Protection Agency. In addition to volumetric and gravimetric fuel measurement techniques, the Standard also allows for the carbon balance method to be used to calculate fuel consumption from exhaust gas analysis data for a vehicle simulating city and highway driving conditions on a chassis dynamometer. Comparison of carbon balance fuel consumption results with data from a fuel flow meter fitted to the vehicle during tests, has shown consistent differences. Fuel consumption from the carbon balance method for hot start city cycle tests averaged 7% lower than volumetric results and 4% lower for the highway cycle.

This work finds that chemical properties of the test fuel, the fuel density and the distance travelled by the vehicle during the tests, as used in the AS2077 procedure, can account for a significant fraction of these differences. The effects of constant volume sampling techniques on city and highway cycle results are also briefly discussed.