Well-to-Wheels Emissions Analysis of Volkwagen Passat - Gas vs. Diesel





Introduction



This analysis quantifies the well-to-wheels (WTW) emissions of the gasoline and the diesel (TDI) versions of the 2012 VW Passat based on the latest version of the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET1_2012) Model (http://greet.es.anl.gov/).



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 PZEV version of the gasoline Passat receives a "smog rating" of "9" by the EPA (on a scale of 1-10), while the ULEV version of the Passat TDI receives a "smog rating" of "6" (http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=31585&id=31768&#tab2). In the California rating system (http://www.driveclean.ca.gov/index.php), the PZEV Passat receives a "smog score" of "9", while the Passat TDI receives a "smog score" of "5".


The official CARB certification data sheets are available at http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2012/volkswagen_pc_a0070295r2_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 57.5 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, section 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 = 11,493 miles/year (http://www.epa.gov/cleanenergy/energy-resources/refs.html)

11,493 mi/year / 365 days/year = 31.5 mi/day / 0.5479 = 57.5


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 duty test cycles was 35%:28%:37%, FTP75:US06:SC03, respectively, as suggested in http://www.dieselnet.com/standards/us/ld_t2.php#supp to determine the "combined" emissions over the respective test cycles.

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



Results


Certified exhaust emissions per CARB certs...


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

NMOG (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
NMOG+NOx (US06) = 0.01 g/mi
NMOG+NOx (SC03) = 0.01 g/mi



2012 T2B2/PZEV II Passat (FTP75 @ UL)

NMOG (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
NMOG+NOx (US06) = 0.02 g/mi
NMOG+NOx (SC03) = 0.004 g/mi


Incorporating WTP emissions per GREET1_2012...

2012 Passat TDI (32.80 miles per gallon gasoline equivalent (mpgge) @ 43%/57%)

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.0280.0200.048
CO0.0410.20.241
NOx0.1470.040.187
PM0.0230.023**0.046
NMOG+NOx (US06)0.1750.010.185
NMOG+NOx (SC03)0.1750.010.185
CO (US06)0.0410.010.051
CO (SC03)0.0410.000.041
SOx0.0800.0020.082


2012 Passat 2.5 PZEV (26.76 mpgge @ 43%/57%)

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.1170.018*0.135
CO0.0510.40.451
NOx0.1870.0020.189
PM0.0310.021**0.052
NMOG+NOx (US06)0.3040.028*0.332
NMOG+NOx (SC03)0.3040.012*0.316
CO (US06)0.0510.40.451
CO (SC03)0.0510.20.251
SOx0.1040.0060.110



(@ WTW emissions per GREET1_2012 (combined FTP/US06/SC03))

(Grams/Mile)


Emission2012 Passat TDI2012 Passat 2.5 PZEV 
 
NMOG+NOx0.2030.323
CO0.110.38
PM0.0460.052
SOx0.0820.110


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_2012





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)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.0280.0500.078
CO0.0410.3840.425
NOx0.1470.1520.299
PM0.0230.0210.044
SOx0.0800.0020.082



(@ WTW emissions per GREET1_2012 (combined HC and NOx in the FTP for the PZEV Passat 2.5 (SFTP certified emissions not included), combined HC and NOx in the NEDC for the Euro 5 Passat TDI))

(Grams/Mile)


EmissionEuropean Passat TDI2012 Passat 2.5 PZEV 
 
NMOG+NOx0.3770.320
CO0.4250.451
PM0.0440.052
SOx0.0820.110



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

(@ WTW emissions per GREET1_2012)

(BTU/mile or Grams/Mile)


Attribute2012 Passat TDI2012 Passat 2.5 PZEV 
 
Total Energy42235311
Petroleum Used37384469
GHG Emissions351416


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


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 (36.585 mpg @ 43%/57%)

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.0260.0200.046
CO0.0410.20.241
NOx0.1220.040.162
PM0.0130.009**0.022
NMOG+NOx (US06)0.1480.010.158
NMOG+NOx (SC03)0.1480.010.158
CO (US06)0.0410.010.051
CO (SC03)0.0410.000.041
SOx0.0630.0020.065


2012 Passat 2.5 PZEV (26.76 mpg @ 43%/57%)

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.2130.018*0.231
CO0.0530.40.453
NOx0.1580.0020.160
PM0.0180.007**0.025
NMOG+NOx (US06)0.3710.028*0.399
NMOG+NOx (SC03)0.3710.012*0.383
CO (US06)0.0530.40.456
CO (SC03)0.0530.20.256
SOx0.0820.0060.088



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

(Grams/Mile)


Emission2012 Passat TDI2012 Passat 2.5 PZEV 
 
NMOG+NOx0.1750.390
CO0.110.38
PM0.0220.025
SOx0.0650.088


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

**Includes PM emissions generated from brake and tire wear per GREET1_2012

All PM assumed to be PM2.5




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)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.0260.0500.076
CO0.0410.3840.425
NOx0.1220.1520.274
PM0.0130.0070.020
SOx0.0630.0020.065



(@ WTW emissions per EPA Emission Factors (combined HC and NOx in the FTP for the PZEV Passat 2.5 (SFTP certified emissions not included), combined HC and NOx in the NEDC for the Euro 5 Passat TDI))

(Grams/Mile)


EmissionEuropean Passat TDI2012 Passat 2.5 PZEV 
 
NMOG+NOx0.3500.390
CO0.4250.453
PM0.0200.025
SOx0.0650.088



* - 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.