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TDI (Diesel) Emissions This is a discussion about emissions from TDI's. Pro's cons of Diesels (including biodiesel) effects on the environment and how they compare to Gasoline and other fuel sources for Internal combustion engines.

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Old April 14th, 2007, 20:30   #1
Ernie Rogers
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Default Comparing CO2 emissions

Global warming is the big deal these days, and people are casting about, asking, what kind of car has the lowest greenhouse gas emissions? I have the answer.

Here is a table, derived from data obtained from Wang and others at Argonne National Laboratory. These emissions are totals of all greenhouse gases, but expressed in terms of grams of CO2 per mile traveled. The numbers are for a car that gets 50 miles per gallon, normalized to "gasoline equivalent." For example, the CNG car burns the same BTUs of CNG per mile as the gasoline car does.

Gasoline or hybrid....... 236 grams CO2 per mile
Diesel....................... 228
Compressed NG.......... 261
Ethanol (E85).............186
Biodiesel................... 116
Electric (100 mpg)...... 264

Remember, these cars all use about the same amount of energy per mile. The electric car is the very worst of the bunch. (Which means that plug-in hybrids, PHEVs, which use electricity are also worse than all the others.)

You should notice that the electric car is rated at "100 mpg" instead of 50. This is necessary to even out the energy use. You see, electric cars are always rated at the point of charging. So about half of their energy use, from the power plant to the battery charger, is neglected. Thus the "100 mpg" electric car is really a 50 mpg car. If I don't make this adjustment, then the EV's CO2 is TWICE as high as the number above.

Remember, the numbers are for cars of equivalent mpgs (or energy use). Diesels will do even better than above because they usually get much better mileage than the others. In fact, this applies similarly to electric cars-- If a gas or diesel car is converted to an EV, assuming state-of-the art parts, the EV will actually use less energy than the others, and then it does emit slightly less CO2. No such EVs exist today, the really good ones were all crushed by GM. (The EV1.) The best available EV today is probably the RAV4 EV which is rated at 114 miles per gallon equivalent, and puts out 232 grams of CO2 per mile. In order for an EV to score better than a TDI getting 50 mpg on biodiesel, it would have to have a mileage rating greater than 228 miles per gallon.

I mentioned that the data comes from Wang at ANL. The data are available on-line, it's called the GREET model. I took the information from the www.auto.xprize.org web site. You might want to look into this. They are intending to award over $10 million to the best 100 mpg cars, based on a race in 2009.

Ernie Rogers

Last edited by Ernie Rogers; April 14th, 2007 at 20:48.
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Old April 15th, 2007, 13:15   #2
Ernie Rogers
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Default The CO2 numbers are Well-to-Wheels

Hi, Mopar,

I know you mean well, but we taxpayers spent millions to have Argonne National Lab carefully tally every bit of energy that went into making that ethanol. The data are for "Well-to-Wheels," which in this case means they added up even the energy to drive to town to get fuel for the tractor. (I don't think they got as extreme about it as Pimentel and Patzek-- they included the sandwich ate by the farmer during his lunch break.)

So, yes, the CO2 numbers include everything, "WTW." If you want to delete your message, then I won't be able to prove that you asked.
(I loved your disclaimer.)

Ernie Rogers

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Originally Posted by MrMopar
Your numbers for E85 are (possibly) pure BS. They leave out all of the CO2 expended from energy that is used to produce and distill the ethanol. With natural gas used to heat the distillation process, that's more CO2. If electric, that's CO2 emitted from coal-fired power plants to produce the electricity. Etc, etc, etc.

My only point is that everything is at least a little bit intangible, and doesn't have accurate final numbers when complete energy usage is taken into account.
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Old April 15th, 2007, 13:39   #3
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I cannot accept the figure for CNG to be higher than gasoline than Diesel, even on a energy equivalent, well-to-wheel basis.

I can do a "tank-to-wheel" CO2 calculation rather easily, and I have actually done so for some fuels and can do so for the others, but well-to-wheel is so vaguely defined that the factors and assumptions can almost be pulled out of the air to make whatever result desired. I mean, what significant "well-to-tank" energy input is required in the CNG infrastructure other than compressing the gas at various stages of delivery from the well to the consumer?

I know of a reputable organization that has designed and built a prototype CNG-fuelled Audi A6 and claims a net 50% reduction in CO2 emissions on the NEDC test cycle with comparable performance to a gasoline engine of the same rated power. Who's to be believed, then?

Don't assume that just because an organization like ARL says something, that it means it's impartial, objective and infallible. Most of the time there are alterior motives and one is always advised to follow the money.
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Old April 15th, 2007, 23:05   #4
Ernie Rogers
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Default Care to run for president?

Hey, Meister,

You could have my vote if you want to run for president-- you can do math, and probably know how to read the words in books too.

Yes, the CO2 number for CNG looks very suspicious. You and I are too busy really to go around correcting everybody's mistakes, so let's go away with our suspicions intact. Before I go, I will offer a possible explanation for the high number. This is a well-to-wheels number. I didn't read the calculations behind it. I am imagining that along the path from the gas well to the pipeline, and finally to the fuel tank, that there might be a lot of spillage of NG. I think they count NG as maybe 50 to 100 times the weight as CO2 because of its higher infrared cross-section. That's the only possible explanation I can see.

If that is the cause, clearly it shouldn't be held against the car-- the oil and gas companies should clean up their act.

Ernie Rogers

Quote:
Originally Posted by TDIMeister
I cannot accept the figure for CNG to be higher than gasoline than Diesel, even on a energy equivalent, well-to-wheel basis.

I can do a "tank-to-wheel" CO2 calculation rather easily, and I have actually done so for some fuels and can do so for the others, but well-to-wheel is so vaguely defined that the factors and assumptions can almost be pulled out of the air to make whatever result desired. I mean, what significant "well-to-tank" energy input is required in the CNG infrastructure other than compressing the gas at various stages of delivery from the well to the consumer?

I know of a reputable organization that has designed and built a prototype CNG-fuelled Audi A6 and claims a net 50% reduction in CO2 emissions on the NEDC test cycle with comparable performance to a gasoline engine of the same rated power. Who's to be believed, then?

Don't assume that just because an organization like ARL says something, that it means it's impartial, objective and infallible. Most of the time there are alterior motives and one is always advised to follow the money.
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Old April 15th, 2007, 23:19   #5
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Pneumatic operation unit steps are notoriously inefficient. Compressing a gas to liquid takes a LOT of wasted energy. And yes, there will be diffusive losses too. I don't know that these could account for the apparent discrepancies; I just thought I'd bring it up as food for thought.
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Old April 16th, 2007, 03:32   #6
wxman
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Quote:
Originally Posted by Ernie Rogers
...Before I go, I will offer a possible explanation for the high number. This is a well-to-wheels number. I didn't read the calculations behind it. I am imagining that along the path from the gas well to the pipeline, and finally to the fuel tank, that there might be a lot of spillage of NG. I think they count NG as maybe 50 to 100 times the weight as CO2 because of its higher infrared cross-section. That's the only possible explanation I can see....
Ernie,

Appears your explanation may be correct. According to the GREET model (accesable from a link at http://www.auto.xprize.org/downloads...s_20070402.pdf ("spreadsheet" link on page 13 of 37)), the actual global warming potential of CH4 is 23 times higher than CO2 (GREET1.7 Fuel_Specs). Also the emission factor of CH4 is 10 times higher (1000%) for CNG than gasoline (Inputs & Calculations). Looks like this may be why CNG is relatively higher in CO2 equivalents.
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Old April 16th, 2007, 11:55   #7
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Compressed natural gas remains in gaseous form throughout its transit from wells through pipelines to the final consumer. There is no liquefaction involved unless the specific desired product is LNG.

The overall energy input and losses associated with natural gas to get it from well to consumer is appreciably less than the combined processes that yield highly refined products like gasoline and Diesel fuel from crude oil.
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Old April 16th, 2007, 12:05   #8
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Ernie, the only way I can see the numbers making any sense is, as you alluded, the greenhouse potential of CH4 over and above CO2 is taken account. I did not read the article in detail, but someone who has can chime in if that was indeed how the numbers were fudged... errr... arrived upon.

It wasn't so long ago that volatile organic gases of which methane and other light hydrocarbons are a major component, were regarded as petroleum byproducts and simply released to the atmosphere or flash-burned in impressive flames shooting out of chimneys and seen at almost every oil refinery. I wonder if the those raw hydrocarbons were ever taken stock for their greenhouse gas emissions.
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Old April 16th, 2007, 16:29   #9
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Quote:
Originally Posted by TDIMeister
Ernie, the only way I can see the numbers making any sense is, as you alluded, the greenhouse potential of CH4 over and above CO2 is taken account. I did not read the article in detail, but someone who has can chime in if that was indeed how the numbers were fudged... errr... arrived upon.

It wasn't so long ago that volatile organic gases of which methane and other light hydrocarbons are a major component, were regarded as petroleum byproducts and simply released to the atmosphere or flash-burned in impressive flames shooting out of chimneys and seen at almost every oil refinery. I wonder if the those raw hydrocarbons were ever taken stock for their greenhouse gas emissions.
CH4 has a larger IR vibrational cross section as it has both stretch (asymetric and symetric) as well as bending vibrations...it is much more efficient than CO2 at storing IR energy. It also absorbs at a wavelength that is more intense from sunlight, IIRC. (don't have time to consult CRC now.) So burning those gasses to produce CO2 is a lot better than just releasing them raw, though still far from ideal.

But in the process of compressing it, you're going to have some diffusive losses from seals and o-rings and such as no process is perfect.
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Old April 16th, 2007, 21:56   #10
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Did they take into account the fixed CO2 that would occur with an established renewable fuel program (i.e., storage in the biomass)?

As for the CNG / spillage question, I have seen some (ridiculous) numbers for the greenhouse potential of evaporated products of gasoline lost at point of filling...suggesting that just this component contributes more to global warming than all of diesel combustion! Seems a bit much, and certainly doesn't account for the numbers seen here...
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Old April 17th, 2007, 13:26   #11
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These numbers appear to indicate that gasoline production/transportation/other (well to the car) adds about 58 g of CO2 per mile, while diesel production/transportation/other adds about 25 g of CO2 based on the stated assumptions. Any idea where that difference comes from? Or is my math wrong?
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