MEMORANDUM

FROM: Aron Butler , Assessment and Standards Division

David Good, Compliance Division Arvon Mitcham, Assessment and Standards Division Office of Transportation and Air Quality Office of Air and Radiation United States Environmental Protection Agency TO: T ier 3 Docket # EPA - HQ - OAR - 2011 - 0135 DATE: February 2 8 , 2013 SUBJECT : Analy sis of the Effects of Changing Fuel Properties on the EPA Fuel

Economy Equation and R - Factor Fuel economy

is one important measure of a vehicle’s effici ency . Under federal regulations, it is EPA’s responsibility

to set specific methods used to measure and calculate fuel economy to be used in determining automobile manufacturers ’ compliance with the Corporate Average Fuel Economy (CAFE) program, as well a s the window stickers on new vehicles that inform consumers of a variety of fuel econom y related factors . The proposed change in test fuel, if finalized, c ould have implications for the CAFE emissions compliance program and the fuel econom y labeling program. Therefore, it is important to characterize the potential change in fuel econom y as a result of the changes in certification fuels proposed in this rule. As outlined in the preamble to the 2017 and Later Model Year Gree nhouse Gas Emissions and Corporate Average Fuel Economy final rule, if the EPA test fuel used for certification and fuel economy vehicles changes to include ethanol, EPA is committed to address whether the fuel change ha s a potential to affect the stringe ncy of the standards. 1 As a result, we reviewed the parameters and inputs for the fuel economy equation to explore whether changing fuel properties have any affect the equation in its current form. Historically , there was no simple , non - invasive method to accuratel y measure the amount of liquid fuel consumed over the course of a fuel economy and emission s test . Therefore , the standard method of calculating fuel econom y has been to determine the miles per gallon based on the sum of the amount of carbon - containing compounds emitted from the tailpipe (primaril y CO 2 which can be measured with great accuracy) during a standard emission s test, and then combined with the measured properties of the test fuel (density (specific gravity) and carbon weight fraction which determine the amount of carbon - containing compounds in the fuel ) , miles per gallon metric can be determined . This calculation is known as the “volumetric fuel econom y” calculation and has also been referred to as the “carbon balance” equati on in the literature 2 as shown in Equation 1 below, where “ T.fuel ” subscript indicates properties of the test fuel and “exh” indicates properties of exhaust hydrocarbons .

1 77 FR 62777 - 62778, October 15, 2012. 2 The volumetric (carbon balance) fuel economy equation is documented in various technical sources and EPA technical reports, including 59 FR 39638, August 3, 1994.     

      

  2 4 273

0 429 0 749 0 8

3781 CO

. CO . CH * . NMHC CW F .

SG CW F FE exh fuel . T fuel . T volumetric Eq.1 T he Energy

Policy and Conservation Act (Pub. Law 94 - 163 December 22, 1975) requires EPA to :

“ ... measure fuel economy for each model and calculate average fuel economy for a manufacturer under testing and calculation procedures prescribed b y the Administrator. However , except under section 32908 [Fuel econ omy labeling (new vehicle window sticker) requirements] of this title, the Administrator shall use the same procedures for passenger automobiles the Administrator used for model year 1975 (weighted 55 percent urban cycle and 45 percent highway cycle), or p rocedures for passenger automobiles the Administrator used for model year 1975(weighted 55 percent urban cycle and 45 percent highway cycle), or procedures that give comparable results.” 3 In the 1970’ s, EPA promulgated regulations to calculate fuel econom y (mpg) based on the carbon balance (volumetric ) fuel econom y equation, combined with a specific set of fuel properties which were representative of 1975 gasoline. A carbon wieght fraction of 0.866 and specific gravity of 0.739 wer e determined to be representative of 1975 gasoline test fuel and incorporated into the carbon balance fuel economy equation, as shown in Equation 2 below (note that NMHC and CH 4 are combined into the HC term here).      



     2 1975 273 0 429 0 866 0 2421 CO . CO . HC . ) highway / city ( FE Eq. 2 4 As outl ined above, EPCA also mandated that the fuel econom y equation used for passenger cars should account for any future change in test fuel properties such that the test procedure would give comparable results compared to the 1975 test procedures. In January , 1982, the U.S. C ourt of Appeals for the Sixth Circuit remanded a case to EPA concerning the need for CAFE adjustments based on test procedure changes since 1975, so that EPA could initiate rulemaking proceedings concerning an y appropriate adjustme nts for the then current test procedures. F rom 1983 to 1986, EPA made various regulatory changes to account for test procedure changes since 1975 which were determined to have an effect on CAFE. 5 This included revisions to the 1975 fuel econom y equation. B eginning with model year 1988 vehicles, the fuel economy equation was revised from an equation which assumed the test fuel had average 1975 fuel properties to an equation which was based on the measured fuel properties of the test fuel which was used to perform the actual test . The fuel economy equation corrected the fuel econom y results to account for the differences in the actual fuel properties of the test fuel compared to the properties of the 1975 baseline test fuel.

3 49 USC 32904(c). 4 Federal regulations at 40 CFR 600.113 - 78(d) 5 48 FR 56526, December 21, 1983; 49 FR 48024, December 7, 1984; 50 FR 27172, July 1, 1985; 50 FR 27188, July 1, 1985; and 51 FR 37844, October 24, 1986.B eginning with model year 198 8 vehicles, EPA modified the calculation method so that the fuel

econom y calculation accounted for difference between the test fuel and the 1975 baseline fuel by account ing for differences in carbon weight fraction (CWF) and net heating value (NHV or volum etric energy

content) between the actual test fuel and the 1975 baseline fuel . 6 Accompan yi ng the NHV adjustment was a sensitivity factor, called “R”, which represent s the response of a ty

pical vehicle’s fuel econom y to small changes in the fuel’s energy content . Accordingl y, the R - factor can be determined using the general formula shown in Equation 3, where “ B.fuel ” indicates baseline fuel :

             

   1 1

fuel . B fuel . T fuel . B , volumetric fuel

. T , volumetric NHV NHV FE FE R Eq.3 Studies of fuel econom y data done in 1985 b y General Motors and others suggested an R - factor of 0.6 would be appropriate. 7 Based on this information, EPA regulations currentl y account for changes in test fuel properties compared to the 1975 baseline fuel b y calculating fuel econom y as shown in Equation 4. 8



##         

            fuel . T fuel . T exh fuel . T fuel . T EPA SG NHV . CO . CO . HC CWF SG

CWF FE 6 0 5471 273 0 429 0 10 5174 2 4 Eq.4 The constants in Equation 4 (i.e. 5,174 x 10 4 and 5,471) contain values for the baseline fuel’s NHV (Btu/gal), CWF, and specific gravity . The R - factor of 0.6 is also visible in the denominator. A theoretical anal ys es done by EPA in 1987 described how only a small fraction of the fuel’s chemical energy

is used to propel the vehicle, and that the many losses to heat and friction are nonlinear and do not depend on fuel properties. Itconcluded that while a maximum R - factor of 0.95 was possible, a value of 0.82 was more plausible. 9 Other studies examining the R - factor have been published since then. For instance, in a 1993 study conducted jointly

by members of the automobile and oil industries (known as the Auto/Oil program), the authors suggested that “[f]or the current fleet, R had a value of 0.93 ± 0.05 and for the older fleet, the value of R was 0.92 ± 0.21.” 10 6 Ibid 7 Letter to Docket #A - 83 - 44 from T.M. Fisher, Director of Automotive Emission Control at General Motors Corporation (Letter #FE 3419) to the U.S. EPA in response to the Notice of Proposed Rule Making on “Fuel Economy Test Procedures; Proposed Procedures for Adjustment to Test Results to Compensate for Changes in 1975 Test Procedure,” 48 FR 56526, December 21, 1983. 8 See the Code of Federal Regulations at 40 CFR 600.113 - 12(h) 9 ”Draft Regulatory Impact Analysis, Control of Gasoline Volatility and Evaporative Hydrocarbon Emissions from New Motor Vehicles,” Section V, July 1987. 10 Hochhauser, A., et al., "Fuel Composition Effects on Automotive Fu el Economy - Auto/Oil Air Quality Im provement Research Program," SAE Technical Paper 930138, 1993.More recentl y , EPA staff assessed the R - factor based on the EPAct/V2/E - 89 dataset. The

EPAct/V2/E - 89 test program was sponso red by EPA, DOE/NREL , and CRC to assess the fuel effects of various fuel parameters on exhaust emissions of Tier - 2 - compliant passenger cars and light trucks . This resulting dataset contains 956 emission tests conducted on 15 high - sales vehicles of 2008 model year conducted b y Southwest Research Institute between March 2009 and June 2010. More details on the vehicles, fuels, and test procedures can be found in the program report. 11 While the focus of the EPAct/V2/E - 89 test program was not to evaluate R - factor, the resulting dataset included volumetric fuel economy results for all emission tests (using E quation 1 ) as well as NHV measurements for all test fuels . Since R - factor is defined as sensitivity of fuel economy to NHV, it cou ld be empirically determined from the test program results . Ethanol content has a significant effect on NHV, an issue of special relevance given the proposed increase in ethanol content in certification fuel used during emissions and fuel economy testing. Figure 1 shows NHV for all test fuels used in the EPAct/V2/E - 89 test program . **Figure 1. Net heating value versus oxygen content for EPAct/V2/E -**  **89 fuels** .

For the remainder of this analys is we divide the dataset based on whether the test fuel was E0 , E10 , or E20 . R - factor was calculated using Equation 5 , where the averages were taken for all tests on that fuel ty

pe . 12

11 See technical report “EPAct/V2/E - 89: Assessing the Effect of Five Gasoline Properties on Exhaust Emissions from Light - Duty Vehicles Certified to Tier 2 Standards, Final Report on Program Design and Data Collection”, Report Number EPA - 420 - R - 13 - 004, March 20 13; also available in this docket (Tier 3 Docket #EPA - HQ - OAR2011 - 0135). 12 This equation follows the form of Eq uation (2) in footnote reference 10 . A s imilar equation was used when comparing the E20 tot eh E0 test results for the vehicles. The BTU/gal for the E0 fuels in this test program is similar to the BTU/gal used for the 1975 baseline fuel.             

   1

1

0 10 0 10

E E E E NHV NHV FE FE R Eq. 5 Since the fuel economy value s vary significantly

by test vehicle, it was useful to calculate an R valu e for each test vehicle individually . Figure 2 shows the results for composite - cycle weighted values (combined result s of cold and hot portions of the emission test) by test vehicle for both E10 and E20 groups compared to E0. Also shown on Figure 2 are s imple averages computed across the test fleet, with both values falling between 0.8 and 0.9. T hough not shown, the average R - factor s determined from the cold start portion s of the test were very similar to the hot running results. **Figure 2. Empirical R** **- factor results for EPAct/V2/E**  **- 89 dataset by vehicle.** 

An important consideration in doing empirical analy ses for R - factor is the sensitivity of the result to measurement variability. Since the differences in fuel economy and fuel NHV being examined are ty

picall y small relative to their values, Equation 5 can produce widel y varyi ng quotients from test to test as evidenced b y the variability in results between vehicles for the same fuel comparison and fuel comparisons for the same vehicle . This makes it difficult to reply on data of limited sample size or studies where other confounding changes may be occurring. The EPAct/V2/E - 89 dataset is large, with at least 16 tests behind the averages for each ethanol level in Equation 5 when producing each vehicle result on Figure 2 . These results pointing to an R - fact or between 0.8 and 0.9 are generall y consistent with findings of the Auto/Oil program and EPA’s 1987 theoretical anal ys is. 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1 1. 1

CCO B CI MP CSIL DCA L F1 50 FEXP FFO C HCIV HO DY JL IB NA LT SOU T TC AM TC OR TS IE 

## EP **Ac t**  **Phase 3 F**  **uel Ec**  **onom y**  **R -** **values b**  **y ve**  **hicle**

wt d E1 0 vs E0 wt d E2 0 vs E0 E10 av g = 0. 86 E20 av g = 0. 82