One of the figures that consumers give special consideration to when they're looking for a new vehicle is the fuel efficiency. The EPA city and highway fuel economy numbers give them a pretty good idea of how vehicles compare on a per-mile basis. That's important, especially now that an ever-expanding range of fuel and propulsion systems offer consumers more options than ever.

But the energy used by the vehicle per unit of distance traveled tells only part of the story. Even consumers who wouldn't describe themselves as "green" express concern about vehicle emissions. Although an electric or fuel cell vehicle seems to provide an answer, producing the electricity or hydrogen to fuel these vehicles also has a cost in terms of energy consumption and greenhouse gas emissions. Unfortunately, the sticker on the window of a showroom car doesn't list any of these figures.

To provide information on the big picture, GM conducted a "Well-to-Wheel Study" for both North America and Europe. The studies analyzed energy use and greenhouse gas emissions for combinations of advanced fuel and propulsion systems on a total system basis: from well to tank - the production source of energy - and from tank to wheel - the vehicle itself (see Figure 1).

Figure 1. Both Well-to-Wheel studies used a systems approach to analyze energy consumption and greenhouse gas emissions for combinations of advanced fuel and propulsion systems from well to tank (the energy production source) and from tank to wheel (the vehicle itself). (Click to enlarge)

"We designed the study to help inform public and private decision makers about the impact of introducing advanced fuel and propulsion systems," says Norm Brinkman, Group Manager for Fuel Chemistry and Systems in the GM R&D Chemical and Environmental Sciences Laboratory. "These choices shouldn't be made based only on what occurs on-board the vehicle; rather, the whole energy and emissions cycle needs to be considered."

Predicting the future
To conduct the well-to-tank analysis, GM partnered with Argonne National Laboratories in North America and Ludwig Bölkow Systemtechnik in Europe, along with oil companies BP, Exxon Mobil, Shell, and Total Fina Elf. The tank-to-wheel analysis was performed entirely by GM, using a Chevrolet Silverado full-sized pickup for the North American study and an Opel Zafira minivan for the European.

"Our goal was to predict energy consumption and greenhouse gas emissions for vehicles in 2010," explains Trudy Weber, Staff Research Scientist. "We therefore needed to project how the various technologies would evolve in order to evaluate them consistently. We were looking at propulsion-fuel system combinations as diverse as conventional drives with gasoline engines, diesel hybrids, and on-board fuel-processor fuel cell vehicles."

Regardless of the technology, all vehicles were required to deliver the same performance based on parameters such as acceleration, top speed, and vehicle range (see Figure 2). "Often people relax some of these performance requirements, which makes the fuel economy gains greater than they otherwise would be," says Weber. "We maintained the vehicle performance by sizing the propulsion system appropriately and then calculating the fuel consumption and emissions output."

Figure 2. The performance targets served as the design criteria for North American vehicle concepts. Researchers optimized powertrain operation based on EPA driving cycles by using energy management and control strategies to achieve the maximum fuel economy for each vehicle concept. To provide more realistic and realizable fuel economy projections, they imposed constraints on component operation (e.g., engine, accessories, motors, batteries) to reflect vehicle driveability and comfort requirements. (Click to enlarge)

To obtain the information necessary to run the simulations and perform the analysis for the North American study, Weber worked with many different groups at GM responsible for advanced propulsion technologies (see Figure 3 for an example). She reviewed the results of the analysis and simulation work with the technology owners to ensure that it properly represented the technology's potential. Later, the entire process was repeated for Europe based on their future technology options.

Figure 3. A schematic illustrating the Fuel Processor Fuel Cell Hybrid Electric Vehicle shows the complexity of the systems that were simulated to determine energy efficiency for the tank-to-wheel portion of the study. (Click to enlarge.)

Why run the study again? The source and processing of fuels in Europe are quite different from those in North America. Europeans drive significantly fewer trucks, but the minivan enjoys a high popularity. In addition, performance requirements, which impact the size of the powertrain, vary between the two continents, as do the driving cycles mandated by regulatory agencies.

For the extensive effort that went into gathering the information required, coordinating with GM and Opel advanced technology groups, performing the simulations and analyses, and reporting the results, the Well-to-Wheel Study received GM R&D's McCuen Award in 2001.

The Bottom Line
In the end, the results reflecting the technology trends did not differ greatly between the North American and European studies - but they did make clear the larger impacts of introducing advanced propulsion and fuel systems.

From well to tank
In terms of total energy use, petroleum-based fuels and compressed natural gas produced the lowest well-to-tank energy losses. Fuels with the highest losses included liquid hydrogen, produced from central plants, hydrogen produced via electrolysis, electricity, and cellulosic ethanol.

With regard to greenhouse gases, producing petroleum-based fuels and natural-gas based methanol resulted in fewer well-to-tank emissions than producing electricity or hydrogen from fossil fuels. Producing hydrogen from natural gas results in high greenhouse gas emissions, because it requires removing all the carbon in the natural gas feedstock. In contrast, greenhouse gas emissions for ethanol pathways could be negative, because plants, trees, and grass use up different amounts of carbon as they grow.

From tank to wheel
As a baseline, the researchers used a conventional automatic transmission driven by a gasoline engine; they then compared energy consumption for the advanced powertrain concepts to determine the improvement. The simulations included diesel, ethanol, compressed natural gas, methanol, and hydrogen in corresponding internal combustion engines and fuel cell systems, plus conventional drives using state-of-the-art mechanical transmissions and hybrid electric drives. All systems had to meet the appropriate emissions standards, whether U.S. or European, that will be in place in 2010.

In terms of fuel consumption, diesel engines showed improvements over conventional technology and fuel cell systems proved even better. To provide even greater reductions, either diesel or fuel cell technology can be incorporated into a hybrid propulsion system.

Putting it all together
Overall the diesel hybrid and fuel cell vehicles used the lowest amount of total system energy. The diesel hybrid also offered a significant reduction in greenhouse gas emissions compared to the conventional gasoline engine. Ethanol-fueled vehicles yielded the lowest greenhouse gas emissions, followed by fuel cell hybrids. The fuel cell, however, did not result in as large a benefit as some expected.

"Running fuel cell vehicles doesn't produce greenhouse gases, but producing the hydrogen does, especially if it's made from natural gas," says Brinkman. "Even so, the fuel cell vehicle emits fewer greenhouse gases than the best petroleum technologies - but to really decrease these emissions, the hydrogen should be made from a renewable resource, such as wind energy. Unfortunately, cost and lack of availability makes that impractical within the next 10 years."

In the end, what became obvious to Brinkman is the potential for improvement - and the need for creative solutions. To provide further information on new propulsion and fuel technologies, a study to estimate criteria pollutants (hydrocarbons, carbon monoxide, nitrogen oxides, and particulate matter) for the United States will be released in 2003.

Links
Both Well-to-Wheel studies are available online:
North American Well-to-Wheel Study
European Well-to-Wheel Study.