Hybridizing with Engine Power Reduction

Although many 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. The fuel economy gain estimates versus performance change for a set of those studies are reviewed, and causes for the wide range in estimates of fuel economy gain when hybridizing a gasoline-fueled vehicle are examined. Data are compiled, and relationships are discussed in detail for 18 cases: three studies of four or more cases each. The data are used to develop statistical models to evaluate the causes of variation in fuel economy gain within and across the studies. Three studies (each evaluating one case only) that predict lower fuel economy gains than the first three are also examined. Explanations and adjustments are provided to illustrate that the differences across the studies are less than a cursory reading would imply. Most notably, potential fuel economy gain via hybridization increases as the 0-to-60-mph times of the compared vehicle pairs drop (and power-toweight ratios increase). Also a reduction in engine power is necessary to obtain large benefits. The most complex statistical model developed indicates that an increase in electric motor power relative to engine power improves the fuel economy of hybrids, up to a point. Finally, results indicate that continuous top-speed and grade-climbing ability will drop if a less powerful engine is used.