Fuel Economy Improvement via Hybridization vs. Vehicle Performance Level

Although many of the studies that use vehicle simulation models to estimate fuel economy gains for a range of hybrid vehicles have attempted to control for the comparability of performance between conventional and hybrid vehicles, different rules and simulation models have been used. This paper reviews the estimates of city, highway, and corporate average fuel economy gain vs. varying measures of performance change for a set of those studies. We examine the causes for the wide range in estimates when hybridizing a vehicle, establish a database, and provide detailed discussions of relationships using several of the studies. Statistical models developed on the basis of the data reveal the causes of variation in mpg gain among conventional/hybrid pairs that have the same 0-60 mph acceleration times. Our study reveals that potential mpg gain via hybridization is greater as the 0-60 mph acceleration time of the pair of compared vehicles drops (and power-to-weight ratios increase). We demonstrate that engine downsizing is necessary to obtain large benefits, and that an increase in electric motor power relative to engine power - up to a point - improves the fuel economy of hybrids.