prevalent. Reformulated gasolines are designed to lower the emissions of pollutants that contribute to near-ground ozone formation. Overall emissions of ozone precursors from gasoline-fueled motor vehicles have substantially decreased in recent decades, largely as a result of government mandates and industry's development and use of new emission controls on motor vehicles. The contribution of on-road vehicles to the total inventory of ozone precursor emissions is expected to continue to decline in the future (NRC, 1999b). If it does, the impact of using oxygenates in reformulated gasoline to mitigate near-ground ozone concentrations would also decline. Thus, the magnitude of the effect of reformulated gasoline on the downward trend is uncertain. A recent NRC report addresses this subject in detail (NRC, 1999b).
Vehicle emissions of carbon monoxide are major contributors to air pollution. A key factor influencing vehicle emissions is the air-to-fuel ratio.2 Additional oxygen in the combustion mixture of fuel and air in the engine decreases the amount of carbon monoxide emitted from the tailpipe. In older vehicles with open loop controls, the addition of ethanol to the fuel is necessary to increase the oxygen level in the combustion chamber and lower carbon monoxide emissions. In newer vehicles, regardless of whether there is oxygen in the fuel, new technologies have contributed to decreases in tailpipe emissions. Onboard diagnostic systems are now in place that can detect malfunctioning emission control systems. In addition, older high-emitting vehicles are disappearing with fleet turnover. Hence, as new vehicles with onboard diagnostic systems become dominant in the vehicle fleet, the benefit of oxygenates is expected to decline.
Although ethanol can lower exhaust emissions somewhat, problems can occur with high evaporative emissions. The Reid vapor pressure of the mixture increases with ethanolgasoline blends, making evaporative emissions more difficult to control. This may be partially offset by the lower reactivity of the alcohol after release into the atmosphere, which creates less ozone. Nevertheless, high-level blends of ethanol (e.g., E85) have lower evaporative emissions than vehicles fueled with low-level ethanol blends (e.g., E10), making this less of an issue for dedicated ethanol vehicles. Because of the low vapor pressure, however, dedicated ethanol vehicles have a difficult cold start-up, which can result in an increase of emissions of hydrocarbon and aldehyde at start-up.
In summary, the benefit in terms of air quality from reduced vehicle emissions from ethanol-gasoline blends relative to petroleum-based fuels may not be substantial enough to be a significant market driver. The use of ethanol-gasoline blends as a transportation fuel is more likely to be influenced by economic, regulatory, and political factors.
Many scientists believe that the full fuel-cycle impacts of growing, harvesting, processing, and consuming biofuels could add very little carbon dioxide (a greenhouse gas) to the atmosphere. The carbon dioxide released by the consumption of biofuels in vehicles would be offset by the uptake of carbon dioxide by the plants (e.g., grasses or trees) used as feedstock to manufacture the fuel. Because some of the plant biomass would be used for running the biofuel processing plant, some would be left over and could be converted to electricity (thus reducing carbon dioxide from other generators of electricity). The latest and most detailed estimates indicate that the net reduction in greenhouse gases, relative to a gasoline-consuming vehicle, could range from 60 to 90 percent (Brown et al., 1998; Delucchi, 1991; Wang et al., 1998). Only solar hydrogen has shown as much potential for reducing net additions of carbon dioxide to the atmosphere. There is considerable debate, however, on the magnitude of the net carbon dioxide reductions of biofuels. The entire life cycle of the fuel, including feedstock production, combustion, and transportation stages, has been considered in analyses of greenhouse gas emissions for bioethanol manufactured from corn starch, woody crops, and herbaceous crops (Wang et al., 1998). More studies are needed, however, to estimate potential greenhouse benefits, if any, from the production of bioethanol from corn residues.
The systemic effects on the ecosystem of a cellulosic biomass industry might be beneficial to the environment, depending on the ecological factors and the intensity and mode of biomass removal (see Tolbert and Wright, 1998). The collection of forest residues, for example, could reduce the accumulation of kindling that feeds forest fires. Crown fires remove large amounts of carbon from forest ecosystems and make them susceptible to extensive nutrient loss through soil erosion. Therefore, the removal of forest residues, in conjunction with stand-thinning, could substantially improve the health of trees by reducing competition for resources, especially in arid areas, and usually increases the pest resistance and growth of remaining trees. Thinning also reduces habitat for insect and disease populations, such as bark beetles, major forest pests that often develop epidemic populations in dense, stressed stands of trees. Harvesting agricultural crop residues could also potentially reduce the breeding habitat and create less amenable conditions for the reproduction for
Controls of air-to-fuel ratio can be divided into two classifications: open-loop and closed-loop controls. Generally, with open-loop control, air-to-fuel ratios are predetermined (typically stoichiometric or richer) but changed by ambient and operating conditions. With closed-loop control, the air-to-fuel ratio is automatically adjusted to achieve a given goal, in this case maintaining the stoichiometric mixture necessary to destroy carbon monoxide, oxides of nitrogen, and volatile organic compounds (NRC, 1996).