APPENDIX A:
Method for Evaluating Vehicle PerformanceThe Office of Technology Assessment’s (OTA’s) analysis of vehicular performance and fuel
economy hinges on examining the vehicle on the Environmental Protection Agency (EPA) driving
cycle, using average (“lumped parameter”) estimates of key variables such as motor efficiency and
battery efficiency over the urban or highway portions of the cycle. Ideally, a performance analysis
of complex vehicles such as hybrids should be based on detailed engine and motor maps coupled
with models that are capable of capturing the second-by-second interactions of all of the
components. Such models have been developed by the auto manufacturers and others.
Nevertheless, OTA believes that the approximate performance calculations described here give
results that are adequate for our purposes. Also, the detailed models require a level of data on
technology performance that is unavailable for all but the very near-term technologies.
ENERGY CONSUMPTION IN CONVENTIONAL AUTOMOBILESIt is relatively easy to derive a simple model of energy consumption in conventional
automobiles that provides insight into the sources and nature of energy losses. In brief, the engine
converts fuel energy to shaft work. This shaft work is used to overcome the tractive energy
required by the vehicle to move forward, as well as to overcome driveline losses and supply
accessory drive energy requirements. The tractive energy can be separated into the energy
required to overcome aerodynamic drag force, rolling resistance, and inertia force. It is useful to
consider energy consumption on the EPA urban and highway test cycles, which provide a
reference for comparing fuel economy.The engineering model used in this study follows the work by GM
scientists Sovran and Bohn.^1 Defining the average engine brake specific fuel
test cycle as bsfc, fuel consumption FC
2
is given by:
Research Laboratory
consumption over the