The drive cycle average powertrain efficiency of current US vehicles is studied by applying a first principles model to the EPA Test Car List database. The largest group of vehicles has naturally aspirated engines and six speed planetary automatic transmissions, and defines the base technology level. For this group the best cycle average powertrain efficiency is independent of vehicle size and is achieved by the lowest power-to-weight vehicles.
For all segments of the EPA test, the fuel required per unit of vehicle work (the inverse of powertrain efficiency), is found to increase linearly with a basic powertrain matching parameter. The parameter is (D/M)(n/V), where D is engine displacement, M vehicle mass, and (n/V) the top gear engine speed over the vehicle speed. The fuel consumption penalties in the City segments due to powertrain warm-up, aftertreatment warm-up, stop-and-go operation, and power-off operation are estimated.
The advanced technologies studied are turbocharging with downsizing, cylinder deactivation, and continuously variable transmissions. Cylinder deactivation is effective for vehicles with high power-to-weight ratio, whereas continuously variable transmissions are effective for small vehicles with low power-to-weight ratio. The benefits of turbocharging with downsizing are approximately constant across the range of vehicle performance, because the higher potential benefit at high performance levels is offset by a lower observed degree of downsizing.
The model can be used to estimate vehicle fuel consumption using a small number of design and drive cycle parameters, and can predict cycle average powertrain efficiency within one percentage point for the technologies studied.
