HEV Fuel Consumption Analysis, including Transmission Losses

A first-order HEV fuel consumption model is developed by solving for the transition between electric drive at low and negative traction power and engine drive and charging at high traction power. Turning the engine on above the 'breakeven power' minimizes fuel consumption: indirect electric driving from engine charging is more efficient below it, and direct engine operation above it. This is derived analytically and observed in benchmarking data on different drive cycles. The engine breakeven bmep is a function of engine loss and electric round trip efficiency. The location of the breakeven power on the cumulative traction work vs. time distribution enables the estimation of the engine running time at high traction power levels and of the engine work needed for extended electric driving. The approach is generalized to HEVs with substantial transmission and driveline (T&D) losses, such as the 'P2' Rear Wheel Drive (RWD) hybrid vehicles, with a motor sandwiched between the engine and the automatic transmission. The T&D losses increase the total work required from the engine and cause longer engine running times. The analytic modeling is enabled by the major sub-systems - engine, motor, transmission, and axle - having linear transfer functions. The HEV model extends that for ICEV and BEV, adding all the indicated loads on the power source - internal, traction, accessories - over a cycle and dividing by the combined system marginal efficiency. Examples show that it describes the energy consumption of a range of powertrain systems in different vehicles and operating cycles.