The efficiency and drag parameters in the instantaneous fuel consumption model are explained by comparing the model with the original power-based model developed at Sydney University, and relating the two models to a conceptual model. Various efficiency factors internal to the vehicle system can be modelled as contributing to the efficiency parameter, or explicitly as power components. The single efficiency parameter in the original power model includes engine drag and all other internal inefficiency components of the vehicle system. On the other hand, the two efficiency parameters in the Australian Road Research Board (ARRB) model have been derived in such a way that they do not include the engine/internal drag in the steady-state driving mode. A fuel consumption model that uses a drag force component measured by coast-down (in neutral) should employ a nonconstant efficiency factor (i.e. a factor dependent on speed and acceleration rates). Otherwise, a satisfactory level of accuracy cannot be achieved, particularly if the prediction of fuel consumption during different modes of driving is required. If all power terms are modelled explicitly, then a basic (constant) engine efficiency parameter can be employed. The basic efficiency factors found from engine maps are of the order of 0.06 to 0.08, which are very close to the values obtained for the ARRB model. This confirms the accuracy of the calibration method used for the ARRB model.
