At present, consumers use 80 million barrels a day (MBD) of petroleum (a barrel contains 42 U.S. gallons). Two thirds of this goes to transportation.
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MBD for transportation overall
MBD for land transport for people
MBD for air transport for people and freight en JOHN B. HEYWOOD is Sun Jae Professor of Mechanical Engineer-,- ing and director of the Sloan Automotive Lab at the Massachu-l— setts Institute of Technology. He was educated at the University <C ofCambridgeandatM.I.T.,wherehejoinedthefacultyin 1968. 1X1 He is author of the widely used textbook Internal Combustion i— I Engine Fundamentals (McGraw-Hill, 1988) and is a member of the National Academy of Engineering and the American Academy of Arts and Sciences.
We need to find ways to motivate buyers to use the potential for reducing fuel consumption and greenhouse gas emissions to actually save fuel and contain emissions.
In the near term, if vehicle weight and size can be reduced and if both buyers and manufacturers can step off the ever increasing horsepower/performance path, then in the developed world we may be able to slow the rate of petroleum demand, level it off in 15 to 20 years at about 20 percent above current demand, and start on a slow downward path. This projection may not seem nearly aggressive enough. It is, however, both challenging to achieve and very different from our current trajectory of steady growth in petroleum consumption at about 2 percent a year.
In the longer term, we have additional options. We could develop alternative fuels that would displace at least some petroleum. We could turn to new propulsion systems that use hydrogen or electricity. And we could go much further in designing and encouraging acceptance of smaller, lighter vehicles.
The alternative fuels option may be difficult to implement unless the alternatives are compatible with the existing distribution system. Also, our current fuels are liquids with a high-energy density: lower-density fuels will require larger fuel tanks or provide less range than today's roughly 400 miles. From this perspective, one alternative that stands out is non-conventional petroleum (oil or tar sands, heavy oil, oil shale, coal). Processing these sources to yield "oil," however, requires large amounts of other forms of energy, such as natural gas and electricity. Thus, the processes used emit substantial amounts of greenhouse gases and have other environmental impacts. Further, such processing calls for big capital invest ments. Nevertheless, despite the broader environmental consequences, nonconventional petroleum sources are already starting to be exploited; they are expected to provide some 10 percent of transportation fuels within the next 20 years.
Biomass-based fuels such as ethanol and biodiesel, which are often considered to emit less carbon dioxide per unit of energy, are also already being produced. In Brazil ethanol made from sugarcane constitutes some 40 percent of transport fuel. In the U.S. roughly 20 percent of the corn crop is being converted to ethanol. Much of this is blended with gasoline at the 10 percent level in so-called reformulated (cleaner-burning) gasolines. The recent U.S. national energy policy act plans to double ethanol production from the current 2 percent of transportation fuel by 2012. But the fertilizer, water, and natural gas and electricity currently expended in eth-anol production from corn will need to be substantially decreased. Production of ethanol from cellulosic biomass (residues and wastes from plants not generally used as a food source) promises to be more efficient and to lower greenhouse gas emissions. It is not yet a commercially viable process, although it may well become so. Biodiesel can be made from various crops (rapeseed, sunflower, soybean oils) and waste animal fats. The small amounts now being made are blended with standard diesel fuel.
It is likely that the use of biomass-based fuels will steadily grow. But given the uncertainty about the environmental impacts of large-scale conversion of biomass crops to fuel (on soil quality, water resources and overall greenhouse gas emissions), this source will contribute but is unlikely to dominate the future fuel supply anytime soon.
Use of natural gas in transportation varies around the world from less than 1 percent to 10 to 15 percent in a few countries where tax policies make it economical. In the 1990s natural gas made inroads into U.S. municipal bus fleets to achieve lower emissions; diesels with effective exhaust cleanup are now proving a cheaper option.
What about new propulsion system technology? Likely innovations would include significantly improved gasoline engines (using a turbocharger with direct fuel injection, for ex-
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