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ACEEE's Green Book®
How we rate the vehiclesWe analyze automakers' test results for fuel economy and emissions as reported to the U.S. Environmental Protection Agency and the California Air Resources Board, along with other specifications reported by automakers. We estimate pollution from vehicle manufacturing, from the production and distribution of fuel and from vehicle tailpipes. We count air pollution, such as fine particles, nitrogen oxides, hydrocarbons and other pollutants according to the health problems caused by each pollutant. We then factor in greenhouse gases (such as carbon dioxide) and combine the emissions estimates into a Green Score that runs on a scale from 0 to 100. The top vehicles this year score a 57, the average is 32 and the worst gas-guzzlers score around 16. A complete discussion of the ratings is given in our technical report, Rating the Environmental Impacts of Motor Vehicles: ACEEE's Green BookŪ Methodology, available from the ACEEE publications office. The ACEEE's Green Book® MethodologyMany factors determine the environmental impact of a car or light truck. Tailpipe emissions and fuel efficiency are clearly important, but impacts also depend on the type of fuel used and the materials that go into manufacturing the vehicle. A scientific approach for estimating the environmental impacts of a product is known as lifecycle assessment, since it traces the impacts of a product from "cradle to grave": materials production and product manufacturing; emissions and other effects when the product is in use; through end-of-life effects of disposal and recycling. We developed the Green Scores and Class Rankings according to the principles of lifecycle assessment, using available data that are sufficiently standardized to be applicable to all makes and models. Three types of vehicle-specific data form the basis of the ACEEE's Green Book® ratings: tailpipe emissions, given by the emissions standard to which a vehicle is certified; fuel economy, based on EPA test cycles; and vehicle mass (curb weight). In real-world driving, tailpipe pollution (CO, HC, NOx, and PM) can be as much as 3 times higher than the nominal grams-per-mile (g/mi) emission standard to which a vehicle is certified. These excess emissions occur for a variety of reasons: inaccuracy of the tests; malfunctioning emission control systems; and deterioration of the catalytic converter and other components. Therefore, we apply adjustment factors, similar to those used in EPA's vehicle emissions calculation models, to determine the expected lifetime average emissions for vehicles meeting a given standard. The same adjustment factors are used for all makes and models certifying to a given emissions standard for a given fuel. Our adjustment factors reflect emission reductions based on EPA's recent improvements to the tests that automakers must use to certify their vehicles. Fuel economy data are used to calculate greenhouse gas emissions, fuel-cycle criteria emissions (air pollution due to producing and distributing the fuel), and those aspects of vehicle emissions that are related to fuel consumption rates (such as a portion of evaporative HC emissions). Fuel economy determines a vehicle's energy consumption rate (gallons/mile, or kWh/mile or Btu/mile for electric and alternative-fuel vehicles). This value is multiplied by national average emission factors for the various pollutants to give emission rates in grams per mile. The greenhouse gas emissions portion of these results is shown in our Green Ratings master tables as the GHG number, given in tons per year. Vehicle weight is used as the basis for estimating manufacturing impacts. Standardized, model-specific data on the environmental damage of vehicle manufacturing are not available. Therefore we use average manufacturing-sector emission factors and average breakdowns of vehicle materials by weight. These statistics determine the average emissions of each pollutant per unit of vehicle weight, which are multiplied by vehicle weight and divided by average vehicle lifetime mileage to estimate emissions related to manufacturing. We did not have sufficient data to estimate vehicle disposal and scrappage impacts, but these impacts are much smaller than manufacturing and in-use impacts, and, in general, would also be proportional to vehicle weight. For electric vehicles, we account for the weight of the replacement batteries needed over the vehicle's lifetime. Having determined the average emission rates for each major stage of the vehicle's lifecycle (including those associated with the fuel consumed), the next step is to determine the relative environmental damage done by each pollutant. An economics-based approach for assessing environmental harm involves estimates of damage costs associated with a given pollutant. Specified, for example, in cents per gram (¢/g) of pollutant, these estimates reflect the costs to society of illnesses and premature deaths associated with pollution. Damage cost estimation involves uncertainties, of course, but it may also fail to reflect the full value we place on our health, environmental quality, and the protection of ecosystems. In spite of these limitations, damage costs provide a rational and consistent way to account for the different effects of various pollutants, and so we apply them to the emissions rates calculated from the vehicle data. It is very difficult to estimate a damage cost for CO2 and other greenhouse gases. The damage due to global warming is just beginning to occur and the worst risks are largely in the future. Therefore, we cannot look back at the harm that has already occurred—as has been done for conventionally regulated pollutants such as NOx and PM—in order to estimate damage costs. However, because of the grave risks and growing concerns about greenhouse gas emissions, we give global warming concerns equal weight to other forms of air pollution in determining our green vehicle ratings. Therefore, we assigned CO2 emissions a cost value such that, for the average 2007 light duty vehicle, approximately half of the overall environmental harm is associated with global warming risks and the other half is associated with the health effects of conventional air pollutants. Multiplying the gram-per-mile pollutant rates by their appropriate cents-per-gram damage costs (which vary by pollutant and location of emissions) yields environmental impact estimates in cents per mile (¢/mi). For conventionally regulated pollutants, adding these estimates up for a typical year of driving results in the "Health Cost" number shown in the main tables. Adding up the ¢/mi estimates for all pollutants, including greenhouse gases, gives a total impact estimate for a given vehicle, which we term its environmental damage index (EDX). The EDX is the main result of our analysis for each vehicle and it provides the common metric with which we compare different makes and models. The EDX represents environmental harm; thus, the lower the EDX, the greener the vehicle. For a green scoring system, greener vehicles should get higher scores. Therefore, we converted the EDX to a Green Score on a scale of 0-100 by grading along a curve, using a formula specified so that an EDX of zero corresponds to a Green Score of 100. Finally, to determine the class ranking symbols, we examined the range of EDX values within each vehicle class. Cutpoints were determined on the basis of the distribution unique to each class. In addition, for a model to earn a "superior" class rating, its Green Score must be better than the overall average Green Score, as well as being among the highest in its class. This year, the overall average EDX is 3.02¢/mi, corresponding to a Green Score of 30. The average car has a Green Score of 34 and the average light truck has a Green Score of 26. Diesel-powered vehicles are highly efficient. Why don't I see them in your "Greenest Vehicles" list?It is still an open question whether diesel engines can be made clean enough at a competitive price to extensively exploit their efficiency advantage in the U.S. market. Most of the diesels on the market, such as Volkswagen's Jetta TDI (turbocharged direct-injection), score "Inferior" in Green Book ratings even though they are more fuel-efficient than their gasoline counterparts. The Jetta 1.9-liter TDI diesel automatic rates 35 MPG in the city and 42 MPG on the highway, for an overall average of 38 MPG. That's about 35 percent better than the 28 MPG average for the Jetta with a 2.0-liter gasoline engine. But the diesel version is certified to a standard that allows it to emit, for every mile driven, more than eight times the amount of nitrogen oxide (NOx) emitted by the gasoline-powered Jetta, which now qualifies as a Tier 2 bin 5 vehicle in the majority of the country. Automakers are working to clean up the diesel vehicle. For example, Ford is developing a version of the Focus sedan that uses advanced control technologies targeted to meet California's ULEV II standards. It has equipped its laboratory test car with a special NOx clean-up device in which a solution of urea in water is sprayed on the catalyst to selectively reduce NOx from the exhaust stream. The vehicle also has a catalytic, soot-trapping filter to remove fine particles. Widespread use of such systems is still some years away, particularly if a new chemical such as urea needs to be widely distributed along with ultra-clean diesel fuel. Engineers at Ford and other companies trying to slash diesel emissions are making up for lost time, since today's gasoline engines benefit from over three decades of experience with ever-tighter pollution standards. |
How we rate the vehicles: methodology |