Simple Hybrid Electric Vehicle Operating and Fuel Consumption Model

An analytic first-order fuel consumption model is developed for FWD 2-motor HEV vehicles which on average achieve 36% EPA Combined efficiency. The premise of this paper is that this is primarily the result of new functionality specific to HEV. Detailed benchmarking data show that in such an HEV the engine not only provides traction power but simultaneously charges the battery. This combined operation of engine and electric powertrain is unique to HEV and is studied using their linear transfer functions. Charging by the engine enables extended electric driving at low traction power, which reduces engine running time and the associated overhead. The analysis predicts an engine duty cycle proportional to the traction power and inversely proportional to the engine output power: the electric driving is limited by the engine’s ability to deliver the required traction work. The model equations in terms of the major functional parameters predict engine running time fractions of 15% for the EPA urban Bag 2 test and 54% on the EPA Highway test, versus observed values of 15% and 51%. As the small HEV High Voltage battery provides little net charge over a trip, the fuel consumption is proportional to the sum of the net engine work and the reduced, proportional overhead. On low power regulatory cycles, the fuel consumption changes from ‘constant overhead’ in classical ICEV to ‘constant high efficiency’ in HEV. Cycle efficiencies of 37 and 36% are predicted for the urban Bag 2 and Highway tests for a generic vehicle.