Methodology to Predict Fuel Economy Benefits of Secondary Axle Disconnects on Four-Wheel-Drive / All-Wheel-Drive Vehicles 2005-01-0550
Various studies have been done that equate automotive fuel economy with fixed vehicle parameters such as aerodynamic drag coefficient, tire rolling resistance, and vehicle mass. Usually, these works have presented the data in the form of tractive energy requirements for the various driving conditions. These analyses have typically used the EPA driving schedules (urban and highway) as test cases to validate the various vehicle coefficients used in the formulas.
One such document is that authored by G. Sovran and M. Bohn (SAE 810184). A limitation of this work is that the coefficients representing losses due to tire rolling resistance do not take into account losses due to other driveline rotating components, and also do not allow for any of these losses to be variable or have more than one value. This latter consideration is of particular importance in four-wheel drive (4WD) vehicles where some portion of the drivetrain is disconnected in two-wheel drive (2WD) operation, and may or may not be disconnected during any or all of the EPA driving schedule. This work expands the Sovran equations to include the effects of various forms of secondary drivetrain disconnects.