Economies throughout the developing world have been undergoing rapid growth over most of the last 20 years, notably China and India. Increasing economic growth is leading to increased global transportation demand. Since most means of transportation currently use petroleum-based fuels, there is an increasing demand for petroleum resources. At the same time, security issues and environmental concerns are calling for lessening dependence on petroleum. With these growing demands on a global basis, it is not surprising that alternative fuel technologies in the automotive sector are rising in importance.
This paper reviews major light vehicle (car and light truck) fuel and fuel saving technologies either currently available or on the horizon that can be employed to reduce global petroleum demand including: diesel, alcohols (butanol and ethanol), biodiesel, straight vegetable oil, biomass-based diesel and gasoline, coal- or natural gas-based diesel and gasoline, natural gas, hybrids, plug-in hybrids, electric vehicles, hydrogen fuel cell vehicles, electrification of accessories, vehicle light weighting, advanced drive train technologies, as well as vehicle use technologies.
Today, diesel is the main alternative fuel to gasoline worldwide. While it is still petroleumbased, the technology for its use is widely available and increasing its use can reduce oil consumption. Diesel engines can provide 25 percent more fuel efficiency and more torque at lower rpm than gasoline engines. Due to thicker castings and higher quality components needed to withstand the higher pressures and torque of diesel combustion, comparative diesel engines tend to be more expensive to produce and have higher upfront costs for consumers though the lower fuel costs from increased efficiency can usually payback those increased upfront costs in a few years.
While diesel engines account for approximately half of the European market, they have not been popular in the United States, accounting for less than one percent of light vehicle sales. Some of the reasons for this discrepancy have been the U.S. diesel fuel’s historically higher sulfur content (compared to Europe), strict U.S. air pollution regulations for nitrous oxides and particulates, and European tax advantages for diesel-fuelled vehicles (making diesel fuel cheaper versus gasoline in Europe). In addition, U.S. consumers have not been interested because they remember the noisy, smoky, unreliable diesel engines offered in the 1980’s; therefore bad associations remain in consumer perceptions regarding diesels, despite significant improvements made in diesel technologies over the past 25 years.
However, federal rules will require refiners to sell only ultra-low-sulfur diesel fuel in the United States by December 1, 2014. This change was already substantially met for on-road diesel by late 2006. Combined with improvements in diesel engine technology, it enables diesel engines to meet tough new emission standards that began for on-road diesels at the beginning of 2007.
The Environmental Protection Agency (EPA) estimates that if one-third of U.S. light vehicles had diesel engines, the United States would save 1.4 million barrels of oil per day, roughly the amount of oil the United States currently imports per day from Saudi Arabia. Unfortunately there are several challenges to future diesel sales. These challenges include: the additional costs associated with developing the technology to comply with strict state and federal emission standards; high relative market prices for diesel fuel; overcoming diesels’ poor reputation with consumers; and, marginally by the still developing nationwide availability of ultra-low-sulfur fuel.
Alcohols – Ethanol, Butanol
Ethanol is usually produced by fermenting plant sugars but can also be created by thermalchemical reaction, or perhaps more directly through biological means. Most ethanol in the United States is made from corn. It can also be made from cellulosic materials such as trees, grasses, and forestry residues. For many years corn ethanol has been used as an additive to gasoline but it is also available as a primary fuel, most commonly as a blended mix of roughly 85 percent ethanol and 15 percent gasoline (E85). There is limited availability of E85 on a national scale. At the beginning of 2007, there were only about 1,100 E85 fuelling stations nationwide. There are currently 2,200 stations, which is still quite low. By comparison, there are roughly 170,000 gasoline stations in the United States today.
Ethanol is currently not cost-competitive with gasoline or diesel as a primary fuel. The cost of corn feedstock accounts for approximately 75 percent of corn ethanol’s production costs. Competing demand for agricultural land uses and the corn itself, primarily as livestock feed, hampers major feedstock cost reductions. Government subsidies play a huge role in generating the necessary crops. For example, the federal government provides a 45-cent per gallon subsidy to help make ethanol cost competitive along with a 2.5 percent ad valorem tariff and a 9-cent per gallon effective duty on imports.
Ethanol has a lower energy content than regular gasoline returning fewer miles to a gallon in a similarly sized gasoline engine. Due to its higher octane, smaller, higher compression engines optimized for ethanol could maintain similar power and mileage results versus larger gasolinebased engines. (Engines optimized for ethanol can achieve over 40 percent thermal efficiency. Gasoline engines currently reach about 25 percent thermal efficiency. 2) Turbochargers or superchargers fitted in slightly smaller engines could allow manufacturers to attain some of that optimization in a flexible format. They could do that by allowing engineers to change cylinder pressures depending upon the fuel. Variable compression engines could also provide some of that optimization while maintaining flexible fuelling capability but the technology is not currently commercialized in production automobiles.
Due to the much more abundant feedstocks (because any type of plant matter is potentially a feedstock), cellulosic ethanol could eventually be produced in much higher quantities than corn ethanol. However, large reductions in the production costs of cellulosic ethanol are needed to make it commercially viable. Cellulosic ethanol production is currently more costly than corn ethanol production because the cellulose must first be broken down into fermentable sugars that can be converted into ethanol, an extra processing step or it has to be treated via currently more expensive thermal-chemical reactions. In addition, cellulosic materials have less energy content per pound than corn, thus requiring larger quantities of feedstock to produce equivalent output yields. The increase in quantity for equivalent yields results in significantly higher transport and handling costs.
Moreover, because ethanol retains water, it is more corrosive than petroleum-based fuels. The widespread commercialization of ethanol will require substantial retrofitting of the refueling infrastructure. Investments will be needed to upgrade pipelines, storage tanks, and filling stations. For instance, gasoline stations may have to replace their storage tanks, at an estimated cost of $100,000 per tank. In addition, most current vehicle fuel systems and engines cannot effectively handle high concentrations of ethanol. Generally U.S. automotive fuel systems are engineered for less than 10 percent by volume (E10) fuel. Raising the level in the general gasoline pool may present problems for standard gasoline engine fuel systems.4 Therefore widespread use of ethanol as a primary fuel would require a turnover in the vehicle fleet or substantial retrofitting.
Fleet turnover would not be an issue for other alcohols such as butanol. Butanol can be made from the same feedstocks as other alcohols such as ethanol. Butanol does not retain water and can be readily used in our current fuelling infrastructure. Butanol is also much more similar to gasoline in energy content per gallon requiring little additional engine optimization, but like cellulosic ethanol it is not currently cost effective to produce. Several companies are working to make commercial butanol production cost competitive.
Biodiesel has similar properties to petroleum diesel but can be produced from vegetable oils or animal fats. While similar to petroleum-based diesel fuel, biodiesel is chemically distinct. Because it is chemically different, biodiesel presents problems to the complex emissions reductions equipment in modern diesel powered vehicles. In addition, vehicle manufacturers are not as familiar with the long-term effects of biodiesel on the reliability of their engines. Due to warranty concerns, many vehicle manufacturers only certify their products for use with five percent biodiesel blends (B5). Biodiesel manufacturers have developed industry standards to help maintain product quality and they are working with vehicle manufacturers to help certify engines for higher level blends.
Like corn-based ethanol, the cost of biodiesel feedstocks (largely soybean oil in the U.S) is the largest component of production costs. The cost of soybean oil is not expected to decrease significantly in the near-term owing to competing demands for animal feedstocks and land for feedstock production.
Several firms are exploring algae production for fuels including biodiesel and ethanol. Although production has not been economical to date, algae production could make sufficient vegetable oil available for significant bio-diesel or alcohol production in the future.