At this point, the engine is turned on and the vehicle operates in CS mode, similar to a gasoline hybrid. Other PHEVs (the PHEV-10) use a “blended” strategy, where the engine is engaged when additional power is needed for acceleration or hill climbing as well as when the battery is discharged.
Vehicle simulation models were not used in this study. However, several recent studies have simulated a range of vehicles on a self-consistent basis, including gasoline ICEVs, HEVs, PHEVs, EVS, and HFCVs (Kromer and Heywood, 2007; Elgowainy et al., 2009; Simpson, 2006; Plotkin and Singh, 2009). These studies employ varying assumptions about PHEV design and control strategies.
To span the range of control strategies, the committee modeled a PHEV-40 with an all-electric drive strategy and a PHEV-10 with a blended strategy. Both PHEVs are midsize sedans with 100 kW power output. The committee drew on the results of the referenced studies to approximate the performance of the PHEVs modeled. This was accomplished in four steps.
The committee used a chart similar to Figure C.8 which estimates the utility factor—the fraction of miles that could be traveled on electricity in the United States—as a function of the PHEV’s all-electric range, or battery size. For a PHEV-10, 23 percent of the nation’s miles traveled could be on electricity. For a PHEV-40, the utility factor is 63 percent.
The committee took the energy-use values for PHEVs in CD and CS modes from the referenced reports. The energy-use values were then combined with the estimated fraction of miles spent in CD and CS modes from Step 1 to estimate electricity and fuel use over the whole drive cycle.
Figure C.9 illustrates the energy consumption of gasoline and electricity over the combined FTP/HWFET drive cycle for various types of advanced hybrid and plug-in hybrid vehicles. As battery size increases, gasoline consumption falls and electricity increases. The overall energy efficiency of the vehicle is higher with larger batteries.
PHEV energy use over a drive cycle depends on the degree of blending assumed during CD mode. For an all-electric strategy, petroleum consumption over a drive cycle is lower than for a blended strategy. This is illustrated in Figure C.10.
Electricity use is about the same for various degrees of blending, but gasoline use increases at higher blending ratios. Blended-26 percent represents the maximum possible blending. Blended-55 percent represents all-electric operation.
Only one study (Kromer and Heywood, 2007) evaluated both blended and all-electric-range operation, and the committee used that study for estimating PHEV energy use. Although a PHEV-40 was not specifically evaluated in the study, linear interpolation between PHEV-30 and PHEV-60 results provided estimated energy use for PHEV-40s.
PHEV gasoline and electricity energy use are expressed as fractions of the gasoline energy used in an HEV, as shown in Table C.1. These ratios put PHEV energy use on the same basis as the 2008 Hydrogen Report.