Significance of Daily VMT Variation over Time and among Drivers on Assessment of PHEV Energy Impact

摘要

Accurate assessment of the impact of plug-in hybrid electric vehicles (PHEVs) on petroleumand electricity consumption is a necessary step toward effective policies. Although it is widelyacknowledged that variations in daily vehicle-miles-travelled (VMT) over time and amongdrivers affect the PHEV energy impact, their significance has not been well understood. Thispaper examines the cause and significance of such an effect through three methods: a graphicalillustration, a mathematical derivation and an empirical study. The first two methods both revealthat ignoring daily VMT variation always causes underestimation of petroleum consumption andoverestimation of electricity consumption by PHEVs and that both biases increase as theassumed PHEV charge-depleting (CD) range moves closer to the average daily VMT. Theempirical analysis based on national travel survey data shows that the assumption of uniformdaily VMT over time and among drivers can lead to an underestimation of the expected PHEV40(PHEV with a 40-mile CD range) fuel use by nearly 68% and an overestimation of the expectedPHEV40 electricity consumption by nearly 48%. Also for PHEV40s, considering daily VMTvariation over time but not among drivers, similar to how the “utility factor” curve is derived inSAE J2841, can lead to an underestimation of the expected fuel use by more than 24% and anoverestimation of the expected electricity consumption by about 17%. Both petroleum useunderestimation and electricity use overestimation increase with larger-battery PHEVs.