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 lifecycle 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 2011 Volkswagen Jetta TDI (diesel) certified as Tier 2 Bin 5 (T2B5) by EPA/LEV II "ultra-low emission vehicle" (ULEV II) by CARB, and the 2011 gasoline Jetta (specifically, the Jetta 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/2011/volkswagen_pc_a0070287_2d5_pz.pdf (gasoline Jetta 2.5 PZEV) and http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2011/volkswagen_pc_a0070292_2d0_u2_diesel.pdf (Jetta 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 60 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 [adj factor]

Average US VMT = 12,000 miles/year (http://www.epa.gov/OMS/climate/420f05004.htm)

12,000 mi/year ÷ 365 days/year = 32.88 mi/day ÷ 0.5479 = 60

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.

Cold CO and HC emissions of the gasoline vehicle were not considered. Tailpipe PM emissions from the gasoline Jetta 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).

A drive cycle mix of 43% city/57% highway was used since that's what's used in GREET and is also used by EPA in its emissions calculations.

Results

Certified exhaust emssions per CARB certs...

2011 T2B5-ULEV II Jetta TDI (FTP75 @ UL)

NMHC (FTP75) = 0.015 g/mi
CO (FTP75) = 0.5 g/mi
NOx (FTP75) = 0.05 g/mi
PM = 0.002 g/mi
NOx (Highway) = 0.04 g/mi
CO (US06) = 0.03 g/mi
CO (SC03) = 0.03 g/mi
NMHC+NOx (US06) = 0.13 g/mi
NMHC+NOx (SC03 ave) = 0.12 g/mi

2011 T2B2-PZEV II Jetta (FTP75 @ UL)

NMHC (FTP75) = 0.006 g/mi
CO (FTP75) = 0.3 g/mi
NOx (FTP75) = 0.003 g/mi
NOx (Highway) = 0.01 g/mi
CO (US06) = 0.6 g/mi
CO (SC03) = 0.2 g/mi
NMHC+NOx (US06) = 0.04 g/mi
NMHC+NOx (SC03 ave) = 0.004 g/mi

Incorporating WTP emissions per GREET v1.8d...

2011 Jetta TDI (32.13 miles per gallon gasoline equivalent (mpgge) @ 43%/57%)

(Grams/Mile)

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

HC/VOC....................0.029..............0.015.................0.044
CO..............................0.042..............0.5.....................0.542
NOx............................0.149..............0.05...................0.199
PM..............................0.024..............0.023**..............0.047
HC+NOx (US06).......0.178..............0.13.................. 0.308
HC+NOx (SC03).......0.178..............0.12.................. 0.298
CO (US06).................0.042..............0.03...................0.072
CO (SC03)................0.042..............0.03...................0.072
SOx............................0.077..............0.002.................0.079

2011 VW Jetta 2.5 (26.09 mpgge @ 43%/57%)

(Grams/Mile)

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

HC/VOC...................0.121...............0.015*..................0.136
CO.............................0.054...............0.3........................0.354
NOx...........................0.192...............0.003....................0.195
PM............................0.033...............0.021**..................0.054
HC+NOx (US06).....0.313...............0.049*...................0.362
HC+NOx (SC03).....0.313...............0.013*...................0.326
CO (US06)...............0.054..............0.6..........................0.654
CO (SC03)...............0.054..............0.2..........................0.254
SOx...........................0.101..............0.004.....................0.105

(@ WTW emissions per GREET 1.8d.1 (comb. FTP/US06/SC03))

(Grams/Mile)

Emission.....................2011 Jetta TDI.......................2011 Jetta 2.5 (PZEV)

HC+NOx.........................0.282.................................................0.338
CO..................................0.24....................................................0.40
PM..................................0.047..................................................0.054
SOx.................................0.079.................................................0.105

All PM assumed to be PM10

*Includes evap emissions from vehicle - ~=0.009 g/mi

**Includes PM emissions generated from brake and tire wear per GREET version 1.8d.1

Even using the less emissions-restrictive European version of the Jetta 2.0 CR TDI (http://carfueldata.direct.gov.uk/search-new-or-used-cars.aspx?vid=27672), the PZEV gasoline version is still not remarkably lower in overall WTW emissions...

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

(Grams/Mile)

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

HC/VOC....................0.029..............0.031.................0.060
CO..............................0.042..............0.216................0.258
NOx............................0.149..............0.24...................0.389
PM..............................0.024..............0.021.................0.045
SOx............................0.077..............0.002.................0.079

(@ WTW emissions per GREET 1.8d.1 (combined HC and NOx in the FTP for the PZEV Jetta 2.5, combined HC and NOx in the NEDC for the Euro 5 Jetta TDI))

(Grams/Mile)

Emission.....................European Jetta TDI.......................2011 Jetta 2.5 (PZEV)

HC+NOx...........................0.449.................................................0.338
CO....................................0.258..................................................0.35
PM....................................0.045..................................................0.054
SOx...................................0.079.................................................0.105

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

(@ WTW emissions per GREET 1.8d.1)

(BTU/mile or Grams/Mile)

Attribute.......................2011 Jetta TDI.........................2011 Jetta 2.5 (PZEV)

Total Energy.........................4235.................................................5091
Petroleum Used...................3855.................................................4626
GHG Emissions....................355....................................................425

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 Jetta 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 Jetta.