Methodology

The GREET model was used to estimate "upstream" emissions of criteria pollutants, i.e., emissions produced in the "well-to-pump" (WTP) phase of the life-cycle emissions. Since the WTP emissions are a function of fuel consumption, they were adjusted based on the relative fueleconomy.gov fuel mileage to the default fuel mileage assumed in GREET. Emissions from the vehicles themselves ("pump-to-wheels" (PTW)) were based on the California Air Resources Board (CARB) certified emissions.

Here, two otherwise identical vehicles are examined - the 2012 Volkswagen Passat TDI (diesel) certified as Tier 2 Bin 5 (T2B5) by EPA/LEV II "ultra-low emission vehicle" (ULEV II) by CARB, and the 2012 gasoline Passat (specifically, the Passat 2.5 liter certified as T2B2 by EPA and LEV II "Partial Zero-Emission-Vehicle" (PZEV II) by CARB, which means that it has been certified to California's strictest emission classification (super-ultra-low emission vehicle - SULEV), plus be certified to have evaporative emissions below a "diminimus" value considered by CARB to be zero).

The official CARB certification data sheets are available at http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2012/volkswagen_pc_a0070295r1_2d5_pz.pdf (gasoline Passat 2.5 PZEV) and http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2012/volkswagen_pc_a0070301_2d0_u2_diesel.pdf (Passat TDI (diesel))

The 3-day and SFTP 2-day diurnal evaporative emissions from the gas vehicle were averaged and were converted from grams per test to grams per mile by dividing grams/test value by 62.5 based on the following data...

Average U.S. temp = 52.85 degrees F (http://www.ncdc.noaa.gov/sotc/index.php?report=national&year=2006&month=ann)

True VP = 3.444 psi @ 50 F; 4.2188 psi @ 60 F = (4.2188 – 3.444) = 0.7748 ÷ 10 = 0.07748 X (52.85 – 50 = 2.85) = 0.22 + 3.444 = 3.66 psi @ 52.85 F (interpolated)

Average temp of SHED (2/3-day diurnal test) = 84 F (http://ecfr.gpoaccess.gov, § 86.1217-96 (c) (xi))

True VP @ 84 F = 6.1891 psi @ 80 F; 7.4184 psi @ 90 F = 6.68 psi @ 84 F

3.66 ÷ 6.68 = 0.5479 [adjustment factor]

Average US car VMT = 12,500 miles/year (http://www.epa.gov/otaq/consumer/f00013.htm)

12,500 mi/year ÷ 365 days/year = 34.25 mi/day ÷ 0.5479 = 62.5

Fuel economy values (for calculation of ORVR and VOC emissions ancillary to the operation of the vehicles, and for WTP emissions) were obtained from http://www.fueleconomy.gov. A drive cycle mix of 43% city/57% highway was used to determine fuel mileage since that's what's used in GREET and is also used by EPA in its emissions calculations.

Cold CO and HC emissions of the gasoline vehicle were not considered. Exhaust PM emissions from the gasoline Passat were assumed to be zero since no data are provided on the CARB cert sheet.

The ratio of the drive test cycles was 35%:28%:37%, FTP75:US06:SC03, respectively, as suggested in http://www.dieselnet.com/standards/us/ld_t2.php#supp for determining compliance with the full useful life SFTP.

ORVR conversion...
Gasoline - grams/gallon ÷ fuel mileage (miles/gallon).

Results

Certified exhaust emissions per CARB certs...

2012 T2B5-ULEV II Passat TDI (FTP75 @ UL)

NMHC (FTP75) = 0.020 g/mi
CO (FTP75) = 0.2 g/mi
NOx (FTP75) = 0.04 g/mi
PM = 0.002 g/mi
NOx (Highway) = 0.01 g/mi
CO (US06) = 0.01 g/mi
CO (SC03) = 0.00 g/mi
NMHC+NOx (US06) = 0.01 g/mi
NMHC+NOx (SC03 ave) = 0.01 g/mi

2012 T2B2-PZEV II Passat (FTP75 @ UL)

NMHC (FTP75) = 0.010 g/mi
CO (FTP75) = 0.4 g/mi
NOx (FTP75) = 0.002 g/mi
NOx (Highway) = 0.002 g/mi
CO (US06) = 0.4 g/mi
CO (SC03) = 0.2 g/mi
NMHC+NOx (US06) = 0.02 g/mi
NMHC+NOx (SC03 ave) = 0.004 g/mi

Incorporating WTP emissions per GREET1_2011...

2012 Passat TDI (32.37 miles per gallon gasoline equivalent (mpgge) @ 43%/57%)

(Grams/Mile)

Emission.....................WTP.............PTW................WTW Total

HC/VOC....................0.028..............0.020.................0.048
CO..............................0.041..............0.2.....................0.241
NOx............................0.148..............0.04...................0.188
PM..............................0.022..............0.023**..............0.045
HC+NOx (US06).......0.176..............0.01...................0.186
HC+NOx (SC03).......0.176..............0.01...................0.186
CO (US06).................0.041..............0.01...................0.051
CO (SC03)................0.041..............0.00...................0.041
SOx............................0.076..............0.002.................0.078

2012 VW Passat 2.5 PZEV (25.32 mpgge @ 43%/57%)

(Grams/Mile)

Emission....................WTP...............PTW................WTW Total

HC/VOC...................0.125...............0.018*..................0.143
CO.............................0.056...............0.4........................0.456
NOx...........................0.200...............0.002....................0.202
PM............................0.032...............0.021**..................0.053
HC+NOx (US06).....0.325...............0.028*...................0.353
HC+NOx (SC03).....0.325...............0.012*...................0.337
CO (US06)...............0.056..............0.4..........................0.456
CO (SC03)...............0.056..............0.2..........................0.256
SOx...........................0.110..............0.006.....................0.116

(@ WTW emissions per GREET1_2011 (comb. FTP/US06/SC03))

(Grams/Mile)

Emission.....................2012 Passat TDI.......................2012 Passat 2.5 (PZEV)

HC+NOx.........................0.204.................................................0.344
CO..................................0.11....................................................0.38
PM..................................0.045..................................................0.053
SOx.................................0.078.................................................0.116

All PM assumed to be PM₁₀

*Includes evap emissions from vehicle - ~=0.008 g/mi

**Includes PM emissions generated from brake and tire wear per GREET1_2011

Even using the less emissions-restrictive European version of the Passat 2.0 CR TDI (http://carfueldata.direct.gov.uk/search-new-or-used-cars.aspx?vid=27128), the PZEV gasoline version is still not remarkably lower in overall WTW emissions...

@ Euro 5 Cert – Passat 2.0 CR (as certified in the New European Driving Cycle (NEDC))

(Converted to Grams/Mile)

Emission.....................WTP.............PTW................WTW Total

HC/VOC....................0.028..............0.050.................0.078
CO..............................0.041..............0.384................0.425
NOx............................0.148..............0.152.................0.300
PM..............................0.022..............0.021.................0.043
SOx............................0.076..............0.002.................0.078

(@ WTW emissions per GREET1_2011 (combined HC and NOx in the FTP for the PZEV Passat 2.5, combined HC and NOx in the NEDC for the Euro 5 Passat TDI))

(Grams/Mile)

Emission.....................European Passat TDI.......................2012 Passat 2.5 (PZEV)

HC+NOx...........................0.378.................................................0.345
CO....................................0.425..................................................0.456
PM....................................0.043..................................................0.053
SOx...................................0.078.................................................0.116

Of course, the TDI has the traditional benefits of a diesel...

(@ WTW emissions per GREET1_2011)

(BTU/mile or Grams/Mile)

Attribute.......................2012 Passat TDI.........................2012 Passat 2.5 (PZEV)

Total Energy.........................4285................................................5633
Petroleum Used..................3789................................................4725
GHG Emissions...................356...................................................442

Conclusion

As can be seen, "clean diesel" vehicles are generally no "worse" with respect to total well-to-wheels emissions than the cleanest gasoline vehicles due to the nature of the very low volatility of diesel fuel and overall lower emissions in the WTP phase, even though the Passat TDI is "only" certified T2B5/ULEV II, while the 2.5 gasoline version is certified T2B2/PZEV. In fact, total well-to-wheels emissions are lower across-the-board in the case of the VW Passat TDI.

Analysis of WTW Emissions Based on EPA Emission Factors (added 02/17/2012)

The U.S. EPA published emissions factors for well-to-pump (WTP) emissions for gasoline and diesel fuel in a support document to the recently-issued "Proposed Rulemaking for 2017-2025 Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards" (EPA, “Draft Joint Technical Support Document: Proposed Rulemaking for 2017-2025 Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards.” http://www.epa.gov/otaq/climate/documents/420d11901.pdf (Table 4-12, page 4-42 [366 of 479])). The following is a reproduction of Table 4-12...

According to the text referring to this graphic, "...EPA modified the GREET model to change certain assumptions about emissions during crude petroleum extraction and transportation, as well as to update its emission rates to reflect adopted and pending EPA emission standards. EPA also incorporated emission factors for the air toxics estimated in this analysis: benzene, 1,3-butadiene, acetaldehyde, acrolein, and formaldehyde. The resulting emission factors are shown in Table 4-12...." [page 4-41]

Based on these emission factors and using the GREET values for gasoline and diesel fuel BTUs per gallon (129,488 BTU/gallon for ULS diesel fuel; 116,090 BTU/gallon for conventional gasoline), the following are the calculated WTW emissions of the 2012 Passat PZEV and the 2012 Passat TDI (using the same methodology as outlined previously)...

Incorporating WTP emissions per EPA Emission Factors...

2012 Passat TDI (32.37 mpgge @ 43%/57%)

(Grams/Mile)

Emission.....................WTP.............PTW................WTW Total

HC/VOC....................0.026..............0.020.................0.046
CO..............................0.042..............0.2.....................0.242
NOx............................0.123..............0.04...................0.163
PM..............................0.013..............0.009**..............0.022
HC+NOx (US06).......0.149..............0.01...................0.159
HC+NOx (SC03).......0.149..............0.01...................0.159
CO (US06).................0.042..............0.01...................0.051
CO (SC03)................0.042..............0.00...................0.041
SOx............................0.064..............0.002.................0.066

2012 VW Passat 2.5 PZEV (25.32 mpgge @ 43%/57%)

(Grams/Mile)

Emission....................WTP...............PTW................WTW Total

HC/VOC...................0.225...............0.018*..................0.243
CO.............................0.056...............0.4........................0.456
NOx...........................0.167...............0.002....................0.169
PM............................0.018...............0.007**..................0.025
HC+NOx (US06).....0.392...............0.028*...................0.420
HC+NOx (SC03).....0.392...............0.012*...................0.404
CO (US06)...............0.056..............0.4..........................0.456
CO (SC03)...............0.056..............0.2..........................0.256
SOx...........................0.087..............0.006.....................0.093

(@ WTW emissions per EPA Emission Factors (comb. FTP/US06/SC03))

(Grams/Mile)

Emission.....................2012 Passat TDI.......................2012 Passat 2.5 (PZEV)

HC+NOx.........................0.177.................................................0.411
CO..................................0.11....................................................0.38
PM..................................0.022..................................................0.025
SOx.................................0.066.................................................0.093

*Includes evap emissions from vehicle - ~=0.008 g/mi

**Includes PM emissions generated from brake and tire wear per GREET1_2011

All PM assumed to be PM_2.5

Using the less emissions-restrictive European version of the Passat 2.0 CR TDI (http://carfueldata.direct.gov.uk/search-new-or-used-cars.aspx?vid=27128)...

(Converted to Grams/Mile)

Emission.....................WTP.............PTW................WTW Total

HC/VOC....................0.026..............0.050.................0.076
CO..............................0.042..............0.384................0.426
NOx............................0.123..............0.152.................0.275
PM..............................0.013..............0.007.................0.020
SOx............................0.064..............0.002.................0.066

(@ WTW emissions per EPA emission factors (combined HC and NOx in the FTP for the PZEV Passat 2.5, combined HC and NOx in the NEDC for the Euro 5 Passat TDI*))

(Grams/Mile)

Emission.....................European Passat TDI.......................2012 Passat 2.5 (PZEV)

HC+NOx...........................0.351.................................................0.412
CO....................................0.426..................................................0.456
PM....................................0.020..................................................0.025
SOx...................................0.066.................................................0.093

* - the NEDC and FTP are different test cycles, but a comparison by the California Air Resources Board (CARB) did not find any major differences in the emission results (CARB, "California's Informal Participation in the Particle Measurement Programme (PMP) - Light Duty Inter-Laboratory Correlation Exercise (ILCE_LD)." Final Research Report, October 2008, pages 23-24)

Based on these EPA emission factors, even the Euro-spec Passat TDI has lower WTW criteria pollutant emissions across-the-board than the PZEV version of the gasoline Passat.