IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change Well-to-wheels analysis of technical mitigation options

Life cycle analysis (LCA) is the most systematic and comprehensive method for the assessment of the environmental impacts of transport technologies. However, non-availability, uncertainty or variability of data limit its application. One key difficulty is deciding where to draw the boundary for the analysis; another is treating the byproducts of fuel production systems and their GHG emission credits. Also in some cases, LCA data varies strongly across regions

For automobiles, the life-cycle chain can be divided into the fuel cycle (extraction of crude oil, fuel processing, fuel transport and fuel use during operation of vehicle) and vehicle cycle (material production, vehicle manufacturing and disposal at the end-of-life). For a typical internal combustion engine (ICE) vehicle, 70–90% of energy consumption and GHG emissions take place during the fuel cycle, depending on vehicle efficiency, driving mode and lifetime driving distance (Toyota, 2004).

Recent studies of the Well-to-wheels CO2 emissions of conventional and alternative fuels and vehicle propulsion concepts include a GM/ANL (2005) analysis for North America, EU-CAR/CONCAWE/JRC (2006) for Europe and Toyota/Mizuho (2004) for Japan. Some results are shown in Fig. 5.12. Some of the differences, as apparent from Figure 5.12 for ICE-gasoline and ICE-D (diesel) reflect difference in the oil producing regions and regional differences in gasoline and diesel fuel requirements and processing equipment in refineries.

Figure 5.12

Figure 5.12: Comparison of three studies on Well-to-wheels CO2 emission analyses

Note: See text for an explanation of the legend. All the results are normalized by the value of ICE-G (gasoline).

Source: EUCAR/CONCAWE/JRC, 2006; GM/ANL, 2005; Toyota/Mizuho, 2004.

The Well-to-wheel CO2 emissions shown in Fig. 5.12 are for three groups of vehicle/fuel combinations – ICE/fossil fuel, ICE/biofuel and FCV. The full well-to-wheels CO2 emissions depend on not only the drive train efficiency (TTW: tank-to-wheel) but also the emissions during the fuel processing (WTT: well-to-tank). ICE-CNG (compressed natural gas) has 15–25% lower emissions than ICE-G (Gasoline) because natural gas is a lower-carbon fuel and ICE-D (Diesel) has 16–24% lower emissions due to the high efficiency of the diesel engine. The results for hybrids vary among the analyses due to different assumptions of vehicle efficiency and different driving cycles. Although Toyota’s analysis is based on Prius, and using Japanese 10–15 driving cycle, the potential for CO2 reduction is 20–30% in general.

Table 5.3 summarizes the results and provides an overview of implementation barriers. The lifecycle emissions of ICE vehicles using biofuels and fuel cell vehicles are extremely dependent on the fuel pathways. For ICE-Biofuel, the CO2 reduction potential is very large (30–90%), though world potential is limited by high production costs for several biomass pathways and land availability. The GHG reduction potential for the natural gas-sourced hydrogen FCV is moderate, but lifecycle emissions can be dramatically reduced by using CCS (carbon capture and storage) technology during H2 production (FCEV-H2ccs in Table 5.3). Using renewable energy such as C-neutral biomass as a feedstock or clean electricity as an energy source (FCEV-RE-H2) also will yield very low emissions.

Table 5.3: Reduction of Well-to-wheels GHG emissions for various drive train/fuel combinations

 Drive train/Fuel GHG reduction (%) Barriers 
ICE Fossil fuel Gasoline (2010) 12-16   
Diesel 16-24 Emissions (NOx, PM) 
CNG 15-25 Infrastructure, storage 
G-HEV 20-52 Cost, battery 
D-HEV 29-57 Cost 
Biofuel Ethanol-Cereal 30-65 Cost, availability (biomass, land), competition with food 
Ethanol-Sugar 79-87 
Biodiesel 47-78 
Advanced biofuel (cellulosic ethanol) 70-95 Technology, cost, environmental impact, competition with usage of other sectors 
H2 H2-ICE 6-16 H2 storage, cost 
Cost, infrastructure, deregulation 
FCV FCEV 43-59 
FCEV+H2ccs 78-86 Technology (stack, storage), cost, durability 
FCEV+RE-H2 89-99 

Source: EUCAR/CONCAWE/JRC, 2006, GM/ANL, 2005 and Toyota/Mizuho (2004).