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 2015 model year VW Passat based on the latest EPA emission factors.


Methodology


Here, two otherwise identical vehicles are examined - the 2015 Volkswagen Passat TDI (diesel) certified as LEV III "ultra-low emission vehicle" (ULEV125) by CARB, and the 2015 gasoline Passat (specifically, the Passat 1.8 liter gasoline direct-injection (GDI) turbo certified as 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=35364&id=35302&#tab2). In the California rating system (http://www.driveclean.ca.gov/searchresults.php?year=2015&make=Volkswagen&model=Passat&x=29&y=6), the PZEV Passat also receives a "smog rating" of "9", while the ULEV Passat TDI receives a "smog rating" of "6".

The official CARB certification data sheets are available at http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2015/volkswagengroupofamerica_pc_a4130028_1d8-2d0_pz.pdf (gasoline Passat 1.8T PZEV) and http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2015/volkswagengroupofamerica_pc_a4130031r2_2d0_u3-125_diesel.pdf (Passat TDI (diesel)). Both have a "useful life" (UL) of 150,000 miles.

EPA emission factors were used to estimate "upstream" emissions of criteria pollutants, i.e., emissions produced in the "well-to-pump" (WTP) phase of the life-cycle emissions. The U.S. EPA published emissions factors for 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...





Since the WTP emissions are a function of fuel consumption, the emission factors were divided by 1,000,000 (emissions per 1,000,000 BTUs), multiplied by the energy in BTUs per gallon (129,488 BTU/gallon for ULS diesel fuel; 116,090 BTU/gallon for conventional gasoline), and divided by the fuel mileage. For example...


Conventional Gasoline upstream VOC emissions...

3.035 grams (Petroleum Extraction & Transportation) + 3.162 grams (Total Petroleum Refining) + 42.91 grams (Fuel Transport, Storage, Distribution) = 49.107 grams VOC/mmBTU

49.107 grams/mmBTU / 1,000,000 BTU = 0.000049107 grams/BTU

0.000049107 grams/BTU X 116,090 BTU/gallon = 5.7 grams VOC/gallon

5.7 grams VOC/gallon / 28.1693 miles/gallon (unrounded combined mileage of Passat 1.8T) = 0.202 grams/mile in the well-to-pump phase


Low Sulfur Diesel upstream VOC emissions...

3.035 grams (Petroleum Extraction & Transportation) + 3.063 grams (Total Petroleum Refining) + 1.261 grams (Fuel Transport, Storage, Distribution) = 7.359 grams VOC/mmBTU

7.359 grams/mmBTU / 1,000,000 BTU = 0.000007359 grams/BTU

0.000007359 grams/BTU X 129,488 BTU/gallon = 0.95 grams VOC/gallon

0.95 grams VOC/gallon / 34.7474 miles/gallon (unrounded combined mileage of Passat TDI) = 0.027 grams/mile in the well-to-pump phase...


Emissions from the vehicles themselves ("pump-to-wheels" (PTW)) were based on the California Air Resources Board (CARB) certified emissions.

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.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 the unrounded combined mileage in the 2014 EPA Fuel Economy Datafile available at http://www.fueleconomy.gov/feg/download.shtml.

Cold CO and NMOG emissions of the gasoline vehicle were not considered.

The ratio of the test duty cycles was 35%:28%:37%, FTP75:US06:SC03, respectively, as defined in the EPA document, "Draft Regulatory Impact Analysis: Tier 3 Motor Vehicle Emission and Fuel Standards." http://www.epa.gov/otaq/documents/tier3/420d13002.pdf, page 1-16, to determine the composite (combined) emissions over the respective test cycles.

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



Results


Certified exhaust emissions per CARB certification data ("Composite" = combined FTP/US06/SC03 emissions weighted at 35%/28%/37%, respectively)...


2015 LEV III ULEV125 Passat TDI

NMOG+NOx (Composite @ UL) = 0.034 g/mi
CO (Composite @ UL) = 0.1 g/mi
PM (FTP75 @ UL) = 0.004 g/mi
NMOG+NOx (Highway @ UL) = 0.017 g/mi



2015 LEV II PZEV Passat 1.8T

NMOG (FTP75 @ UL) = 0.004 g/mi
CO (FTP75 @ UL) = 0.2 g/mi
NOx (FTP75 @ UL) = 0.01 g/mi
PM (FTP75 @ UL) = 0.004 g/mi
NOx (Highway @ UL) = 0.003 g/mi
CO (US06 @ 4,000 miles) = 1.5 g/mi
CO (SC03 @ 4,000 miles) = 0.2 g/mi
NMOG+NOx (US06 @ 4,000 miles) = 0.03 g/mi
NMOG+NOx (SC03 @ 4,000 miles) = 0.03 g/mi


Incorporating WTP emissions per EPA emission factors...


2015 Passat TDI (34.7474 mpg (from unrounded EPA value))

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC+NOx0.1550.0340.189
CO0.0430.10.143
PM0.0140.009**0.023
SOx0.0660.002***0.068
 


2015 Passat 1.8T PZEV (28.1693 mpg (from unrounded EPA value))

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.2020.012*0.214
CO0.0500.20.250
NOx0.1500.010.160
PM0.0170.009**0.026
NMOG+NOx (US06)0.3520.033*0.390
NMOG+NOx (SC03)0.3520.038*0.390
CO (US06)0.0501.51.550
CO (SC03)0.0500.20.250
SOx0.0780.005***0.083
 

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

(Grams/Mile)


Emission2015 Passat TDI2015 Passat 1.8T 
 
NMOG+NOx0.1890.383
CO0.1430.61
PM0.0220.026
SOx0.0680.083
 


*Includes evap emissions from vehicle - ~=0.0079 g/mi @ FTP and SC03; 0.0034 g/mi @ US06

**Includes PM emissions generated from brake and tire wear per GREET1_2014

***PTW SOx emissions per GREET1_2014

All PM assumed to be PM2.5


[HC = NMOG/VOC for purposes of the chart]



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 1.8T gasoline version is certified PZEV. In fact, based on these EPA emission factors, the Passat TDI has lower WTW criteria pollutant emissions across-the-board than the PZEV version of the gasoline Passat.




Argonne National Laboratory released the latest version of the GREET model (GREET1_2014) in October 2014. GREET1_2014 is available on the ANL GREET web page (http://greet.es.anl.gov/).

Using GREET1_2014 to estimate upstream criteria pollutants, and using the same methodology as described above, with the exception that a 43% city/57% highway drive cycle mix is used (consistent with what EPA uses in its MOVES modeling) and including emissions from well drilling and well infrastructure, the following are the WTW emission results of the Passats. [Note - upstream (WTP) emissions include well drilling and well infrastructure]



Results


Certified exhaust emissions per CARB certs ("Composite" = combined FTP/US06/SC03 emissions weighted at 35%/28%/37%, respectively)...


2015 LEV III ULEV125 Passat TDI

NMOG+NOx (Composite @ UL) = 0.034 g/mi
CO (Composite @ UL) = 0.1 g/mi
PM (FTP75 @ UL) = 0.004 g/mi
NMOG+NOx (Highway @ UL) = 0.017 g/mi



2015 LEV II PZEV Passat 1.8T

NMOG (FTP75 @ UL) = 0.004 g/mi
CO (FTP75 @ UL) = 0.2 g/mi
NOx (FTP75 @ UL) = 0.01 g/mi
PM (FTP75 @ UL) = 0.004 g/mi
NOx (Highway @ UL) = 0.003 g/mi
CO (US06 @ 4,000 miles) = 1.5 g/mi
CO (SC03 @ 4,000 miles) = 0.2 g/mi
NMOG+NOx (US06 @ 4,000 miles) = 0.03 g/mi
NMOG+NOx (SC03 @ 4,000 miles) = 0.03 g/mi


Incorporating WTP emissions per GREET1_2014...


2015 Passat TDI (32.60 miles per gallon gasoline equivalent (mpgge) @ 43%/57%)

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC+NOx0.1740.0340.208
CO0.0910.10.191
PM100.0130.023**0.036
PM2.50.0100.010**0.020
SOx0.1110.0020.113
 


2015 Passat 1.8T PZEV (29.46 mpgge @ 43%/57%)

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.1210.012*0.133
CO0.1300.20.330
NOx0.1990.010.209
PM100.0210.026**0.047
PM2.50.0140.012**0.026
NMOG+NOx (US06)0.3200.033*0.353
NMOG+NOx (SC03)0.3200.038*0.358
CO (US06)0.1301.51.63
CO (SC03)0.1300.20.33
SOx0.1780.0050.183
 



(@ WTW emissions per GREET1_2014 (combined FTP/US06/SC03))

(Grams/Mile)


Emission2015 Passat TDI2015 Passat 1.8T 
 
NMOG+NOx0.2080.351
CO0.1610.694
PM100.0360.047
PM2.50.0200.026
SOx0.1120.183
 


*Includes evap emissions from vehicle - ~=0.0079 g/mi @ FTP and SC03; 0.0034 g/mi @ US06

**Includes PM emissions generated from brake and tire wear per GREET1_2014; GREET1_2014 default exhaust PM emissions used



[HC = NMOG/VOC for purposes of the chart]



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

(@ WTW emissions per GREET1_2014)

(BTU/mile or Grams/Mile)


Attribute2014 Passat TDI2014 Passat 1.8T 
 
Total Energy41984998
Petroleum Used36223891
GHG Emissions339377
 


Conclusion


Regardless of the source of the emission factors (EPA or GREET1_2014), the Passat TDI has lower WTW criteria pollutant emissions across-the-board than the most efficient version of the PZEV-certified gasoline Passat (1.8T).




One argument made is that "upstream" (i.e., well-to-pump) emissions occur primarily in rural areas and thus do not expose a large portion of the population to those emissions. The GREET model estimates upstream (WTP) urban emissions along with total emissions. The following are the urban emissions calulated by GREET1_2014. Vehicle (pump-to-wheels) emissions are assumed to be the same. WTP urban emissions in GREET are calulated based on the EPA unrounded "city" values.


2015 Passat TDI (26.74 miles per gallon "city" gasoline equivalent (mpgge))

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC+NOx0.0370.0340.071
CO0.0130.10.113
PM100.0030.023**0.026
PM2.50.0020.010**0.012
SOx0.0320.0020.034
 


2015 Passat 1.8T PZEV (24.19 mpgge "city")

(Grams/Mile)


EmissionWTPPTWWTW Total 
 
NMOG/VOC0.0670.012*0.079
CO0.0240.20.224
NOx0.0400.010.050
PM100.0050.026**0.031
PM2.50.0030.012**0.015
NMOG+NOx (US06)0.1070.033*0.140
NMOG+NOx (SC03)0.1070.038*0.145
CO (US06)0.0241.51.524
CO (SC03)0.0240.20.224
SOx0.0620.0050.067
 



(@ WTW emissions per GREET1_2014 (combined FTP/US06/SC03))

(Grams/Mile)


Emission2015 Passat TDI2015 Passat 1.8T 
 
NMOG+NOx0.0710.138
CO0.1130.588
PM100.0260.031
PM2.50.0120.015
SOx0.0340.067
 


*Includes evap emissions from vehicle - ~=0.0079 g/mi @ FTP and SC03; 0.0034 g/mi @ US06

**Includes PM emissions generated from brake and tire wear per GREET1_2014; GREET1_2014 default exhaust PM emissions used



[HC = NMOG/VOC for purposes of the chart]



Thus, the Passat TDI still has lower WTW emissions across-the-board in urban areas than the cleanest gasoline Passat available (Passat 1.8T), even though the PZEV version of the Passat 1.8T gets a partial zero emissions credit while the Passat TDI doesn't.