|Trends in average hybrid performance: curb weight, net hybrid system peak power, and fuel consumption. Variability across models is indicated by the error bars. Click to enlarge.
The market trend to heavier, more powerful hybrids is eroding the fuel consumption advantage of hybrid technology, according to a study done by researchers at the University of British Columbia.
The study, which compares hybrids for sale in the United States in 2007 to equivalent conventional vehicles, appears online in Environmental Research Letters.
Researchers Milind Kandlikar and Conor Reynolds used linear regression models to analyze how vehicle weight and power affect the fuel consumption of hybrid electric vehicles (HEVs) and internal combustion engine vehicles (ICEVs) independently, and then compared the set of currently available HEVs (2007 model year) against a functionally equivalent set of conventional ICEVs. The equivalent set are ICEVs of the same make and model, and with similar power.
From 2000 to 2006, the sales-weighted average hybrid-electric vehicle in the US fleet has changed significantly, driven largely by the introduction of new sports-utility and high-performance HEV models. The average curb weight has increased by 30%. Propelling this larger weight is a hybrid-electric system that delivers 60% more power. The gasoline engine component of this system is 43% larger in terms of engine displacement. Some of the observed net power increase is explained by the need to provide a larger vehicle with acceptable performance.
Over the same period, however, the manufacturer-reported acceleration times also increased: the average HEV in 2004 reaches 96.6 km h–1 (60 miles per hour) from a standing start in 20% less time than the average in 2003. Because vehicle weight and power both strongly influence fuel consumption, it is not surprising that average fuel consumption has gone up by 15% with the shift towards higher-performance HEVs.
Kandlikar and Reynolds chose two parameters to characterize the changing nature of HEV technology: the ratio of net system power to vehicle curb weight and the ratio of power to engine displacement.
Until 2003, with only the Prius and the Insight on the market, the parameters stayed approximately constant. Since then, the increased power per unit vehicle weight indicates a trend towards new vehicles having higher performance—similar to what has occurred across the entire light-duty fleet, conventional and hybrid. An increase in the power to engine displacement ratio indicates that the newer HEV technology has significantly improved.
Even while bulking up, HEVs still offer a benefit compared to their conventional counterparts. The fuel consumption penalty imposed by increased vehicle weight is significantly lower in HEVs than in equivalent ICEVs, according to the study, with a 100 kg change in vehicle weight increasing fuel consumption by only 0.4 l/100 km in HEVs, compared with 0.7 l/100 km in ICEVs.
Three different equivalence models in the study (based on different comparisons of HEVs against ICEVs) yielded average fuel consumption benefits ranging from 2.7 to 3.25 l/100 km for the hybrids, with varying effects of changes in weight and power.
When the hybrids are compared with their ICEV equivalents (grouped into cars and SUVs), the average fuel consumption benefit of an HEV was 2.65 l/100 km. However, the analysis found that an HEV that is 100 kg heavier than an identical ICEV, holding everything else constant, has a fuel consumption penalty of 0.15 l/100 km. Likewise, an HEV that is 10 kW more powerful than its ICEV counterpart results in a fuel consumption penalty of 0.27 l/100 km.
“How hybrid-electric vehicles are different from conventional vehicles: the effect of weight and power on fuel consumption”; C Reynolds and M Kandlikar; Environ. Res. Lett. 2 (January–March 2007) 014003 doi:10.1088/1748-9326/2/1/014003