Fuel consumption on different drive cycles:
A unified approach based on average power/weight 2020-01-1278
In previous work we have shown that fuel consumption on a particular drive cycle is proportional to traction work, with an offset for powertrain losses. The finding applies to different drive cycles, but with different offsets. Following Soltic (2011), it is shown that if fuel usage and traction work are both expressed in terms of cycle average power, a wide range of drive cycles collapse to a single transfer function. Data for vehicles of different weights further collapses when normalized for weight, i.e. by working in power/weight (P/W). The fuel P/W is primarily a function of traction P/W, and secondarily of displacement/weight.
The useful work or power definition is then expanded beyond the traction power to include electrical power for customer functions, and power to drive the air conditioning. With this expanded definition the linear powertrain transfer function can be applied not only to strictly defined regulatory drive cycles and procedures, but also to ‘real driving’ conditions that cover a much broader range of situations. When applied to hybrid electric vehicles, the method clearly shows how the power split system largely eliminates the fuel consumption overhead on the low power/low speed cycles.
The approach works because the input/output transfer functions of the main powertrain sub-systems are approximately linear: the engine has substantial offset losses and about 40% fuel-power conversion efficiency, the transmission and axle have mainly offset losses, and the electric motors have high conversion efficiencies and low offset losses. As it is grounded in fundamentals and it unifies fuel consumption analysis across a variety of technologies, vehicle applications, and driving situations, the cycle average power approach is a substantial extension of our ability to understand, analyze, and predict fuel consumption.
Patrick Phlips, William Ruona, Thomas Megli, Mrudula Orpe