# Why Diesel Article



## Jim E. (Apr 23, 2002)

Interesting article.


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## wxmanCCM (Feb 17, 2010)

I am an atmospheric scientist who has worked in the air quality field for 25 years. I can make what I believe is a compelling argument that, contrary to popular opinion, there is one more reason to favor diesel, at least over gasoline-fueled vehicles/hybrids...that being lower detrimental impacts on air quality.

Most evaluate how "clean" a vehicle is by noting what "Bin" or LEV category a vehicle hits. Based on that method, a BMW 335d is not as "clean" as the "cleanest" gasoline vehicles, e.g., a Toyota Prius hybrid (the 335d is Bin 5/LEV II, the Prius is Bin 2/PZEV). However, this methodology ONLY takes into account the emissions from the exhaust (i.e., "tailpipe" ). It doesn't even take into consideration the evaporative VOC emissions from the vehicle itself, never mind the indirect evaporative VOC emissions inherently associated with the distribution, storage and "handling of gasoline, an extremely volatile substance. Diesel/biodiesel fuel is essentially nonvolatile and has virtually none of these VOC emissions. Volatile hydrocarbon (VOC) emissions are at least as problematic from an air quality perspective (e.g., "smog") as exhaust hydrocarbon emissions (and arguably more problematic than NOx, gram for gram).

Just to illustrate, the certified exhaust emissions from the 335d (certified as LEV) and Prius (certified as PZEV) are as follows according to the CARB certs...

335d (http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2010/bmw_pc_a0080268_3d0_l2_diesel.pdf)
Prius (http://www.arb.ca.gov/msprog/onroad/cert/pcldtmdv/2010/toyota_pc_a0140657_1d8_pz_hevge.pdf)

(Grams/Mile)
Emission.....................2010 335d...............2010 Prius 
NMHC/NMOG................0.023.......................0.005 
CO...............................0.2...........................0.04 
CO @ 20 F...................----...........................0.6 
NOx.............................0.04..........................0.003 
Highway NOx................0.00..........................0.002 
PM...............................0.00..........................---- 
US06 HC+NOx..............0.02..........................0.01 
US06 CO......................0.0.............................0.1 
SC03 HC+NOx.............0.01..........................0.01 
SC03 CO......................0.0.............................0.1

Since NMHC and NOx are combined in the US06 (aggressive) and SC03 (air conditioning), combining the FTP75 NMHC and NOx gives...

Emission.....................2010 335d...............2010 Prius 
NMHC+NOx.....................0.063.......................0.008 
CO...............................0.2...........................0.04 
PM...............................0.00..........................---- 
US06 HC+NOx.................0.02..........................0.01 
US06 CO......................0.0.............................0.1 
SC03 HC+NOx................0.01..........................0.01 
SC03 CO......................0.0.............................0.1

Combining the FTP75/US06/SC03 roughly equally yields

Emission.....................2010 335d...........................2010 Toyota Prius (PZEV)
HC+NOx.........................0.03135...................................0.0093
CO..................................0.07........................................0.08
PM..................................0.00.........................................0.00*

*assumed ***8211; not reported on cert

However, as mentioned, there are evaporative VOC emissions from the gasoline vehicle (Prius)...

Emission.....................2010 335d......................................2010 Prius 
3-D..................................----..........................................0.20 g/test
2-D..................................----..........................................0.28 g/test
RL...................................----...........................................0.00 g/mile
ORVR.............................----............................................0.01 g/gallon

The 2-D/3-D are evaporative emissions from gasoline vehicles while they are not running (i.e., sitting in a driveway, parking lot or garage), RL is "running loss" evaporative emissions while the vehicle is running, ORVR is evaporative emissions while refueling.

Based on conversion factors, the VOC emissions from the Prius is approximately 0.004 grams/mile.

To be as fair as possible, I've tried to calculate evaporative emissions from the 335d based on evap emissions from the 335i are applying a correction factor (diesel fuel is about 1/383 as volatile as gasoline at ambient conditions based on EPA's AP-42)...

0.32 (average of 3-day/2-day diurnal) ÷ 60 = 0.006 g/mi ÷ 383 (relative volatility of diesel fuel) = 0.000014 g/mi 
0.01 (running loss) ÷ 383 = 0.000026 g/mi 
0.000014 + 0.000026 = 0.00004 g/mi

Incorporating the evaporative VOC emissions from the vehicles yields...

Emission.....................2010 335d...........................2010 Toyota Prius (PZEV)
HC+NOx.........................0.03139....................................0.0135
CO..................................0.07........................................0.08
PM..................................0.00.......................................0.00

However, there are also "upstream" evaporative VOC emissions associated with "handling" of gasoline. According to EPA there are currently about 475,000 tons/year of VOC emissions from this process. Assuming an annual consumption of 140 billion gallons of gasoline (per EIA) yields about 3 grams VOC per gallon of gasoline. Based on AP-42, the VOC emissions from diesel fuel is about 0.039 grams/gallon.

Incorporating these "fugitive" VOC emissions into the vehicle emissions yield...

Emission.....................2010 335d...........................2010 Toyota Prius (PZEV)
HC+NOx.........................0.0328........................................0.0675
CO..................................0.07............................................0.08
PM..................................0.00............................................0.00

So, if my math is correct, the Prius, usually considered the "greenest" car on the road, actually has higher emissions than the 335d if the evaporative VOC emissions are included.


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## d geek (Nov 26, 2008)

nice analysis! :thumbup: thanks


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## Marine5302 (Sep 13, 2009)

+1


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## santo (Dec 23, 2009)

You had me at " there is one more reason to favor diesel"


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## Neutrinolad (Jun 23, 2009)

You had me at "atmospheric scientist." But the data help.


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## lalitkanteti (Nov 15, 2009)

Long time ago I did read somewhere (ofcourse without substantial data backing it) that 335d is greener than Prius. Now I have proof as well. I already took a print out to show people who always raise their eyebrows when I tell them I have a diesel which is cleaner than many hybrids around.


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## wxmanCCM (Feb 17, 2010)

A more complete explanation of my methodology and documentation is available at http://webpages.charter.net/lmarz/emissions1.html . I also have compiled results of various studies of diesel/gas emissions at http://webpages.charter.net/lmarz/emissions.html if anyone is interested.


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## wxmanCCM (Feb 17, 2010)

I am also not convinced that we will see any significant advantage for EVs over diesel vehicles either, in spite of general consensus to the contrary.

Based on emission calculations using 2008 annual electric power generation per EIA (4,119,000,000,000 kWh (http://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.html)) and 2008 annual emissions from electric utilities per EPA (http://www.epa.gov/ttnchie1/trends/), I calculate relative emissions to be...

Emission.........................2010 335d..............3 Series EV*
HC+NOx............................0.033.....................0.044
CO...................................0.07.......................0.01
PM...................................0.0004**................0.004 (FTP75)

*Adjusted by a factor of 2.7 to account for power train efficiency improvements over diesel engines per CARB "Low Carbon Fuel Standard"

**per EPA measured PM emissions in FTP75 test cycle of 335d

Unless/until electricity is generated by a substantially higher percentage of renewable/nuclear, you're just displacing the emission source from line/area sources to large point sources (power plants).


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## Marine5302 (Sep 13, 2009)

wxmanCCM said:


> I am also not convinced that we will see any significant advantage for EVs over diesel vehicles either, in spite of general consensus to the contrary.
> 
> Based on emission calculations using 2008 annual electric power generation per EIA (4,119,000,000,000 kWh (http://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.html)) and 2008 annual emissions from electric utilities per EPA (http://www.epa.gov/ttnchie1/trends/), I calculate relative emissions to be...
> 
> ...


Excellent point.


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