17 Electric and Hybrid Vehicles TABLE 1.6 Energy Consumption, Efficiency and Emissions for Passenger Cars Using GREET SI ICEV (Baseline CG and RFG) Plug-in SI HEV (Gasoline and Electricity) BEV (Electricity) Total energy (Wh) 257,551 526,261 1,632,131 WTT efficiency 79.5% 66.5% 38.0% TTW efficiency 21.9% 23% 48.51% WTW efficiency 17.41% 15.29% 18.43% CO 2 (g/million BTU) 17,495 57,024 219,704 CH 4 (g/million BTU) 109.120 145.658 296.031 N 2 O (g/million BTU) 1.152 1.535 3.111 VOC: total (g/million BTU) 27.077 25.630 19.679 CO: total (g/million BTU) 15.074 23.553 58.448 NO x :
total (g/million BTU) 50.052 87.100 239.571 The TTW efficiency for conventional ICEVs is inherently low (around 25%), because of the low efficiency of the gasoline or diesel engine and the losses in the powertrain components. In the case of HEVs, the engine is downsized and controlled to operate at higher-efficiency regions most of the time, which results in lower fuel consumption. The TTW efficiency of HEVs is higher than that of an ICEV and is estimated to be around 50% [9]. The efficiency of an electric powertrain is around 80%–90%. The TTW efficiency in BEVs is high due to their large-capacity batteries and electric-only mode of operation.
However, when WTT efficiencies are included in the compari- sons, the overall WTW efficiencies are not significantly different for any type of the vehicles. The WTW efficiencies of HEVS and EVs are similar to ICEVs because of the lower WTT efficiencies. The WTT efficiency for the PHEV depends on the proportions of the grid energy and the gasoline energy used during a drive cycle. The WTW efficiencies and emission impacts can be evaluated for a comparison using the GREET model developed at the Argonne National Laboratory [10]. GREET model is essentially a multidimensional spreadsheet model available for public use for analyzing fuel cycles from the source to wheel for a vehicle.
GREET is the acronym for Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation . The model has more than 100 fuel production pathways and more than 70 vehicle systems. Table 1.6 shows the WTW efficiencies and emissions for conventional ICEV, BEV and PHEV analyzed for a typical mid-size sedan using the GREET model [9,11]. The CG and RFG represents conventional gasoline and reformulated gasoline vehicles. The WTT efficiency was found to be 66.5% for PHEVs (grid-connected hybrids) using 33% grid energy and 67% gasoline for operation. On the other hand, the WTT efficiency for conventional and regular hybrid vehicles is 79.5%.
The reason for low efficiency of PHEV is due to the fact that electricity is mostly generated from con- ventional energy sources that have very low efficiency. The greenhouse gas (CO 2 , CH 4 , N 2 O, CO, VOC and NO x ) emissions are also found to be higher in PHEVs than in conventional gasoline vehicles according to the results in Table 1.6. The TTW efficiency for the plug-in hybrid is not significantly different from the baseline vehicle since the IC engine usage is still quite high. Poor efficiencies in both WTW and TTW result in an overall low WTW efficiency for plug-in hybrids. More on efficiency calculation is presented in the next section.
1.8 EV/ICEV COMPARISON The relative advantages and disadvantages of alternative vehicles over conventional ICEVs can be better appreciated from a comparison of the two on the basis of efficiency, pollution, cost and dependence on oil.