JOINT EFFORT YIELDS NEW WELL-TO-WHEELS EFFICIENCY ANALYSIS

A joint effort by General Motors Corporation, BP, ExxonMobil, Shell Oil and Argonne National Laboratory has resulted in a comprehensive report Well-To-Wheel Energy Use and Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems, published in April.

The study was undertaken because there are differing, yet strongly held views among the various "stakeholders" in the advanced fuel/propulsion system debate. In order for the introduction of advanced technology vehicles and their associated fuels to be successful, four important stakeholders must view their introduction as a "win":

• Society
• Automobile manufacturers and their key suppliers
• Fuel providers and their key suppliers
• Auto and energy company customers

If all four of these stakeholders, from their own perspectives, are not positive regarding the need for and value of advanced fuels/vehicles, the vehicle introductions will fail. The study is intended to help inform public and private decision makers regarding the impact of the introduction of such advanced fuel/propulsion system pathways from a societal point of view.

The study focuses on the United States light-duty vehicle market in 2005 and beyond, when it is expected that advanced fuels and propulsion systems could begin to be incorporated in a significant percentage of new vehicles. Given the current consumer demand for light trucks, the benchmark vehicle considered in this study is the Chevrolet Silverado full-size pickup.

According to the sponsors, this study differs from prior well-to-wheel analyses in a number of important ways including:

• The study considers fuels and vehicles that might, albeit with technology breakthroughs, be commercialized in large volumes and at reasonable prices.
• The well-to-wheel analysis involved participation by the three largest privately owned fuel providers: BP, ExxonMobil and Shell.
• The 15 vehicles considered in the study include conventional and hybrid-electric vehicles with both spark-ignition and compression-ignition engines, as well as hybridized and non-hybridized fuel-cell vehicles with and without onboard fuel processors. All 15 vehicles were configured to meet the same performance requirements.
• The 13 fuels considered in detail (selected from 75 different fuel pathways) include low-sulfur gasoline, low-sulfur diesel, crude oil-based naphtha, Fischer-Tropsch (FT) naphtha, liquid/compressed gaseous hydrogen based on five different pathways, compressed natural gas, methanol, and neat and blended (E85) ethanol.

The following vehicle architectures and fuels were included in the study:

• Conventional (CONV) vehicle with Spark-Ignition (SI) gasoline engine (baseline)
• CONV vehicle with Compression-Ignition Direct Injection (CIDI) diesel engine
• CONV vehicle with SI E85 (85 percent ethanol and 15 percent gasoline) engine
• CONV vehicle with SI Compressed Natural Gas (CNG) engine
• Charge-Sustaining (CS) parallel Hybrid Electric Vehicle (HEV) with gasoline engine
• CS parallel HEV with CIDI diesel engine
• CS parallel HEV with SI E85 engine
• Gasoline Fuel Processor (FP) Fuel Cell Vehicle (FCV)
• Gasoline FP Fuel Cell (FC) HEV
• Methanol FP FCV
• Methanol FP FC HEV
• Ethanol FP FCV
• Ethanol FP FC HEV
• Gaseous Hydrogen (GH2)/Liquid Hydrogen (LH2) FCV
• GH2/LH2 FC HEV

The vehicle fuel economy results (on a gasoline-equivalent basis) are summarized in Table 1.

Table 1

General observations based on Table 1 include:

• FC systems use less energy than conventional powertrains because of the intrinsically higher efficiency of the FC stack.
• Hybrid systems show consistently higher fuel economy than conventional vehicles because of regenerative braking and engine-off during idle and coast periods.
• In the case of the FC and FP systems, the gains resulting from hybridization are lower because the "engine-off" mode is present in both systems.
• Hydrogen-based FC vehicles exhibit significantly higher fuel economy than those that employ an FP.

Key GHG findings are summarized in Figure 1 and include the following:

• The ethanol-fueled vehicles, as expected, yield the lowest GHG emissions per mile.
• The next lowest are the two H2 HC HEVs (GH2 refueling station).
• The H2 FC HEVs are followed by the methanol, naphtha, and gasoline FP HEVs and the diesel CIDI HEV, in that order.
• The diesel CIDI HEV offers a significant reduction in GHG emissions (27 percent) relative to the gasoline conventional SI vehicle.

Figure 1

Considering both total energy use and GHG emissions, the key findings of the study are as follows:

• Among all of the crude oil- and Natural Gas (NG)-based pathways studied, the diesel CIDI HEV, gasoline and naphtha FP FC HEVs, and GH2 FC HEVs, were nearly identical and best in terms of total system energy use (BTU/mile). Among these pathways, however, expected GHG emissions were lowest for the H2 FC HEV and highest for the diesel CIDI HEV.
• Compared to the gasoline SI (conventional), the gasoline SI and diesel CIDI HEVs, as well as the diesel CIDI (conventional) yield significant total system energy use and GHG emission benefits.
• The methanol FP FC HEV offers no significant energy use or emissions reduction advantages over the crude oil-based or other NG-based FC HEV pathways.
• Ethanol-based fuel/vehicle pathways have by far the lowest GHG emissions of the pathways studied.
• It must be noted that for the HE100 FP FC HEV pathway to reach commercialization, major technology breakthroughs are required for both the fuel and the vehicle.
• On a total system basis, the energy use and GHG emissions of CNG conventional and gasoline SI conventional pathways are nearly identical.
• The crude oil-based diesel vehicle pathways offer slightly lower total system GHG emissions and considerably better total system energy use than the NG-based FTD CIDI vehicle pathways. (Note that criteria pollutants are not considered here.)
• LH2, FT naphtha, and electrolysis-based H2 FC HEVs have significantly higher total system energy use and the same or higher levels of GHG emissions than the gasoline and crude naphtha FP FC HEVs and the GH2 FC HEVs.