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How we rate the vehicles

We 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 54, the average is 35 and the worst gas-guzzlers score around 18.

We’ve made major changes to our methodology for the 2011 model year. The complete report on these changes is not yet available, but see a summary of 2011 changes here.  For those aspects of the methodology that remain unchanged, a complete discussion is given in our technical report, Rating the Environmental Impacts of Motor Vehicles: ACEEE's Green Book® Methodology.

The ACEEE's Green Book® Methodology

Many 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.

Four 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; vehicle mass (curb weight); and battery mass and composition (for hybrids and plug-in vehicles).

In real-world driving, tailpipe pollution (CO, HC, NOx, and PM) can be substantially 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. We previously applied adjustment factors, similar to those used in EPA's MOBILE model, to determine the expected lifetime average emissions for vehicles meeting a given standard. With the completion of the phase-in of the Tier 2 tailpipe pollution control program, the earlier adjustment factors are outdated, so we currently assume vehicle emissions at the level of the standards to which they are certified. We will revisit this assumption once real-world data are available for today’s cleaner 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. 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.

Vehicle and battery weights are used as the basis for estimating manufacturing and disposal impacts. Standardized, model-specific data on the environmental damage of vehicle manufacturing are not available. With this year’s methodology change, we draw from the vehicle life-cycle module of Argonne National Laboratory’s GREET model to generate weight-based estimates of impacts that vary by technology. For hybrid and electric vehicles, GREET accounts for 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, Green Book methodology initially gave 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 model year 1998 light duty vehicle, approximately half of the overall environmental harm was associated with global warming risks and the other half is associated with the health effects of conventional air pollutants. We have kept the CO2 emissions damage cost constant over time. Consequently large gains in tailpipe pollution control and relatively minor fuel economy gains yield a roughly 70-30 split of damages between greenhouse gases and conventional pollutants on average for model year 2011.

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). 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 2.42¢/mi, corresponding to a Green Score of 35.

Methodology Updates for ACEEE’s Green Book® in MY 2011

This year’s Green Scores reflect some important changes to ACEEE’s Green BookÒ rating methodology. The updates, summarized below, include the way emissions standards are applied; use of results from Argonne National Laboratory’s GREET 2.7 model in the vehicle life-cycle analysis, and an update to the treatment of plug-in vehicles.  ACEEE is preparing an updated methodology report, which will be posted on GreenerCars.org. In the meantime, those interested in further details may consult our draft methodology memo [PDF].

Emissions Factors

Green Book® emissions factors in prior years have been largely based on runs of EPA’s MOBILE model. However, EPA has recently transitioned away from the MOBILE model and the replacement model, MOVES, does not directly permit the reporting out of emissions of vehicles meeting a given certification level (Tier 2 or LEV II bin), which is required for Green Book’s vehicle ratings.  ACEEE has not identified any other source of information regarding real-world emissions performance by certification level that is sufficiently complete to serve as a source for updating those emissions factors in a satisfactory way.  Consequently, for the 2011 model year, the in-use criteria pollutant component of a vehicle’s damage cost (EDX) is based directly on the full useful life standards to which that vehicle is certified.

Vehicle-Cycle Emissions Estimates

To date, the Green Book methodology has estimated emissions associated with the materials production, manufacturing, and recycling/disposal phases of vehicle life using only vehicle weight (including battery replacements) as a proxy. This year, we incorporated results from Argonne National Laboratory’s GREET 2.7 vehicle-cycle model (available at http://greet.es.anl.gov/). GREET 2.7 permits the incorporation of greater detail on key components, such as batteries, that differentiate the vehicle cycle impacts of more advanced vehicles from those of conventional vehicles, and it explicitly includes recycling and disposal impacts. GREET 2.7 provides emissions and energy estimates for internal combustion engine (ICE), hybrid-electric, and fuel cell cars, SUVs and pickup trucks.

We derived greenhouse gas (GHG) and criteria pollutant emissions estimates for vehicles with conventional internal combustion engines (ICEs) by scaling emissions results from GREET 2.7 by vehicle weight. For hybrid and electric vehicles, battery weight (and composition) were factored into the equations as well.

Emissions Factors for PHEVs and EVs

Grams per mile emissions from a vehicle running on electricity generated off-board are calculated as the product of the vehicle’s average kilowatt hours (kWh) per mile and grams per kWh from power generation. For these vehicles, EPA listings include kWh per mile over the city and highway test cycles, i.e. the FTP and Highway Fuel Economy Test (HWFET) Cycles, respectively.

For alternative fuel vehicles, including PHEVs and EVs, manufacturers have the option of using a “Derived 5-cycle” test (which was also an option for testing any vehicle through model year 2010), in which the new label city and highway fuel economy values are calculated from the original two-cycle test values alone. For high fuel economy vehicles, the derived 5-cycle adjustments yield fairly severe corrections.

We capped the derived 5-cycle adjustments at 30 percent, as was done in the 2010 EPA/DOT proposed fuel economy labeling rule (despite the fact that this approach may be generous to plug-in vehicles and should be reconsidered in the future). As in the case of non-plug-in vehicles, we calculated the combined energy consumption for plug-ins by using a 43%/57% city/highway weighting.

Emissions from PHEV operation are the emissions associated with the operation of the ICE together with the emissions associated with the grid electricity used to power the vehicle. Thus we calculate PHEV emissions as the weighted sum of emissions associated with operation on the two power sources, where the weighting corresponds to the percentage of operation using each power source. The weighting for grid electricity is percentage of miles the vehicle is operated on electricity, or the utility factor (UF). Both emissions rate and UF may vary with drive cycle; these variations are reflected to the extent possible based on the available data.

U.S. Power Generation Characteristics

The U.S. power generation mix has changed significantly over the past decade. Coal generated power has decreased from 56 percent in 2001 to 45 percent in 2009 (EIA 2001, EIA 2009). On the other hand, power generation from natural gas has doubled from approximately 10 percent in 2001 to little more than 23 percent in 2009. Distribution efficiency increased from 2001 to 2009, while generation efficiency of coal- and oil-based power plants decreased. These effects together leave the average net efficiency of generation and distribution almost unchanged.

We updated electricity generation emissions factors for  NMOG, CH4, CO, N2O, NOx, SOx, PM10, and CO2 using the latest version of the DeLucchi Life-cycle Emissions Model (2005). Damage costs for these pollutants were left unchanged (in real dollars) for the 2011 Green Book methodology. The resultant damage cost for non-nuclear electricity in constant cents per kWh declined by 17 percent. For nuclear power, we left damage costs unchaged. The resultant overall damage cost for electricity is 0.61 cents per kilowatt-hour, a 14 percent decline from the damage cost in the 2010 methodology.

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How we rate the vehicles: methodology

Reviews of ACEEE's Green Book®