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Analytic Engine and Transmission Models for Vehicle Fuel Consumption Estimation
Abstract:
A normalized analytical vehicle fuel consumption model is developed based on an input/output description of engine fuel consumption and transmission losses. Engine properties and fuel consumption are expressed in mean effective pressure (mep) units, while vehicle road load, acceleration and grade are expressed in acceleration units.
The engine model concentrates on the low rpm operation. The fuel mep is approximately independent of speed and is a linear function of load, as long as the engine is not knock limited. A linear, two-constant engine model then covers the speed/load range of interest. The model constants are a function of well-known engine properties. Examples are discussed for naturally aspirated and turbocharged SI engines and for Diesel engines.
A similar model is developed for the transmission where the offset reflects the spin and pump losses, and the slope is the gear efficiency. Examples are given for manual, planetary automatic and continuously variable transmissions.
The engine and transmission models are combined into a vehicle model where the fuel consumption is proportional to the vehicle power requirement plus the mechanical powertrain losses, with an overall factor for the marginal energy conversion efficiency. The mechanical loss term is proportional to the product of the displacement-to-mass ratio and the rpm-to-vehicle speed ratio. The model is used on vehicles in the US EPA “Test Car List” database to break down the fuel use in the Highway test into its components.