Slashing fuel use by all vehicles on U.S. roads within a few decades may seem an impossible task. But a new analysis by MIT researchers shows that there are various ways to do it—all of them challenging and requiring immediate action on several fronts.
Based on vehicle fleet modeling studies, the researchers found that hybrids, plug-in hybrids, and other advanced vehicle systems could be incorporated into the U.S. fleet rapidly enough to make a significant dent in total fuel use by 2035. However, reductions in total petroleum consumption will not begin for one or two decades. Reductions will come sooner if we make two other changes: start to use technology improvements to make mainstream gasoline vehicles more fuel efficient rather than faster and bigger, and adopt measures to slow the growth in demand for vehicles and the distance they travel.
If we combine all of those approaches, total fuel use in 2035 could end up being less than half what it would be if we take no action, with greenhouse gas (GHG) emissions reduced by almost as much. The researchers emphasize the difficulty of the task. “The magnitude of the changes required to achieve these reductions is daunting, especially as current trends all run counter to those changes,” said Anup Bandivadekar, postdoctoral associate in the MIT Energy Initiative. Among the biggest challenges: changing consumer expectations. Unlike in the past, next year’s model may not accelerate faster or be bigger than last year’s model, but it will get more miles per gallon of fuel.
Current status, plausible futures
U.S. cars and light trucks consume about 44 percent of all the petroleum used nationwide and 10 percent of that used worldwide, in the process generating about 22 percent of total U.S. carbon dioxide emissions. Policy makers and others are now debating long-term goals for cutting those quantities, but knowing how realistic the goals are and designing plans to meet them are difficult. Research at MIT and elsewhere has shed light on future fuel economy and emissions improvements possible with specific technology options. But knowing the potential impact on total fuel use and emissions requires understanding how quickly those technologies are likely to get on the road, how much difference they will make, and when.
Bandivadekar, John B. Heywood, the Sun Jae Professor of Mechanical Engineering and director of MIT’s Sloan Automotive Laboratory, and others have developed a methodology that provides the needed information. “Like everyone else, we don’t have the ability to predict the future,” said Bandivadekar. “So we develop various transportation scenarios, each of which combines a number of vehicle technologies, assuming that their market shares grow at different—but plausible—rates between now and 2035. We then assess the impact of each scenario on fleet-wide fuel use and emissions.” Conversely, given a fuel use or emissions target, their methodology can determine plausible pathways for getting there.
To analyze the current situation, the researchers first had to estimate market penetration rates for each of the propulsion technologies they considered. By looking at such variables as consumer buying behavior, fueling infrastructure needs, and vehicle manufacturing requirements, they defined a range of plausible rates for each technology. They then developed three scenarios for meeting projected demand for light-duty vehicles between now and 2035.
The top figure to the right shows their “market mix” scenario. The assumption here is that there is no clear technology winner. By 2035, all the advanced technologies considered in the study—turbocharged gasoline, plug-in hybrids, gasoline hybrids, and diesels—have gained fractions of the U.S. market, but more than a third of all cars sold are still conventional gasoline internal combustion engine (ICE) vehicles. (The analysis assumes that all the technologies, including conventional ICEs, continue to improve over time.)
The bottom figure shows the impact of the market mix scenario on fleet-wide fuel use over time. The top curve (No change) shows growth in fuel use if no action is taken to curtail it. Left unchecked, fuel use will grow some 35 percent between 2005 and 2035. The next curve down (Reference) measures fuel use assuming that half of all technology improvements are used to increase fuel economy rather than to push up horsepower and acceleration—a reversal of what has happened over the past 25 years. The researchers call this variable “emphasis on reducing fuel consumption,” or ERFC, and they use the 50% ERFC case as their reference.
The third curve down shows the impact of assuming 50 percent ERFC plus adding the market mix of new technologies. By 2035, fuel use is 10.5 percent below the reference case. Interestingly, the biggest contribution to that reduction comes from gasoline hybrids, despite their relatively small market share.
To demonstrate the power of emphasizing fuel economy over performance, the researchers generated the lowest curve, which shows the impact of assuming only conventional gasoline ICE vehicles plus 100 percent ERFC. Simply continuing to improve conventional vehicles with a full focus on reducing fuel consumption would cut future fuel use by as much as 26 percent—more than shifting almost two-thirds of the total fleet to advanced-technology vehicles.
“There’s all this fascination with vehicle technology—more hybrids, more diesels, and so on. But this result shows that you can achieve a greater reduction in fuel use at a potentially lower cost just by focusing on reducing fuel consumption rather than increasing performance and size,” said Bandivadekar. The challenge, of course, is to design policies or incentives that will help consumers understand and accept this change in what new models have to offer.
The researchers’ other two scenarios assume a different future. In the second one, battery development stalls, hybrids remain expensive, and turbocharged gasoline and diesel vehicles do well, taking over 75 percent of the market by 2035. The outcome: fuel use in 2035 drops by 12 percent from the reference case—not so different from the market mix scenario, despite the differing composition of the fleet.
The third scenario assumes that hybrids and plug-hybrids succeed, and by 2035 they make up fully 55 percent of the total market. The resulting fuel use is lower than the reference case by 18 percent—the best result of the three. That outcome is consistent with the MIT team’s assessment of hybrids, which are likely to improve more rapidly than more established technologies and will benefit from any improvements in conventional technologies.
If the hybrid-strong scenario is combined with 100 percent ERFC, fuel use in 2035 is almost 40 percent lower than it would be if no action is taken. “Now you’re talking really big reductions,” said Bandivadekar. “Despite enormous growth in demand, fuel use in 2035 would be lower than it was in 2000.”
In all three scenarios, fleet fuel use peaks around 2020. Thus, even with 50 percent ERFC and a substantial penetration of advanced vehicles, growth in fleet-wide fuel use over the next decade has already been committed.
Environmental concerns
What are the implications of the researchers’ “plausible” scenarios for the future growth of GHG emissions? As an example, they looked at the market mix scenario. Under that scenario, GHG emissions peak in 2024 and are 9.5 percent below the reference case by 2035. In contrast, fuel use peaks in 2020 and is 10.5 percent below the reference case in 2035. Those results emphasize that cuts in fuel use do not translate directly into cuts in emissions. Plug-in hybrids, for example, use electricity in place of petroleum, but generating that electricity gives off emissions. Considering the overall energy picture is critical to assessing changes in environmental impacts.
Looking at the exact type of fuel used is also important. As the U.S. tries to both reduce emissions and increase the security of its energy supplies, it is looking to two petroleum replacements: biofuels (corn ethanol and cellulosic ethanol) and oil recovered from Canadian tar sands. Switching to the former decreases GHG emissions; switching to the latter increases them. By examining future supplies and demands for those fuels, Bandivadekar estimated plausible ranges for their use in vehicles between now and 2035. Depending on how they are combined, their use could (on net) decrease GHG emissions in the market mix scenario by 2 to 6 percent by 2035. The researchers note that the increase in emissions due to using more tar sands can easily offset the decrease due to using more biofuels. They therefore recommend that policy makers be careful to focus on measures that will improve both energy security and carbon emissions at the same time.
The power of cutting demand
In general, the analyses show that significant reductions in fuel use and emissions come only with major efforts to change fleet-wide vehicles and fuels. A big part of the challenge is the incessant growth in demand. Moderating that growth would alter the picture dramatically. To demonstrate, Bandivadekar prepared a scenario assuming that the growth rate of vehicle sales is halved and that drivers travel no farther in their vehicles than they do today. With just those two assumptions—no change in ERFC or vehicle technology—fuel use in 2035 drops by more than 18 percent from the No change scenario. Add 100 percent ERFC and that reduction is fully 39 percent, still without the addition of advanced vehicles.
The overall message? “If our goal is to achieve deep, long-term reductions in fuel use and emissions we should do all three things—increase the ERFC, increase the market penetration rate of advanced propulsion technologies, and find ways to reduce the rate of growth in demand. With that combination we can get very deep cuts by 2035,” said Bandivadekar. “To make those things happen, we need strong, long-term policies, and we need to adopt them now because the longer we wait the higher the starting point is and the more difficult the task.”
This research was supported by the Martin Family Society Fellowship for Sustainability, the Ford-MIT Alliance, Concawe, Eni S.p.A., Shell Hydrogen, and Environmental Defense. For more information see: Bandivadekar, Anup P. Evaluating the Impact of Advanced Vehicle and Fuel Technologies in U.S. Light-Duty Vehicle Fleet (PDF). PhD thesis, MIT Engineering Systems Division. February 2008.