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I understand that much of the low CO2 emissions for ethanol as vehicle fuel are related to its entire lifecycle (CO2 is sequestrated when the crop is grown and released when it combusts so it is carbon neutral, except the emissions due to production and transport.).

But from looking at its combustion it releases approximately the same amount of CO2 as gasoline or petrol.

Since ethanol has about 50% less energy density than petrol (22.27 kWh/gal vs 33.41 kWh/gal), you have to use more ethanol to travel a given distance by car.

Does this mean that exhaust emissions in CO2e g per km are higher for ethanol and ethanol vehicles, compared to gasoline vehicles?

  • Ethanol has 1/3 less energy ; gasoline has 50% more energy. Environmentalists only look at the CO2 per gallon because if you considered all the energy inputs to produce ethanol it produces more CO2 per mile traveled. – blacksmith37 Sep 30 at 14:21
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There are a ton of different factors to take into consideration here. Lets make sure we keep net GHGe and tailpipe GHGe separate here.

Net GHGe

Net GHGe: Since 2015, satellite observation as well as international agriculture reporting data have started to show that the original assumptions surrounding Indirect Land Use Change (ILUC) for generation I and advanced biofuels have been grossly over calculating the effects of ILUC, as well as under and over accounting in other areas. Utilizing new data, there are several studies all pointing to a considerably lower GHGe footprint of Gen I and advanced biofuels. The most comprehensive study I've found to date has been A Life-Cycle Analysis of the Greenhouse Gas Emissions of Corn-Based Ethanol

The tl;dr here is, the current EPA model that claims ethanol lifecycle emissions are a 20% reduction from baseline gasoline still has the incorrect ILUC numbers. The real number as calculated in the above as well as several other studies [ GHGe recent evidence | ILUC data review ] is likely between a 35% and 50% reduction (with many studies finding in the 40%-45% range) from baseline gasoline. This is the net carbon accounting of EVERYTHING - Tailpipe, ILUC, vapor emissions, fuel production and transport, fertilizer - everything. It should be noted that the GHGe intensity of gen 1 and advanced biofuels are continuing to decline and will likely do so as shown below, and that none of these numbers even take into account other sources of ethanol with lower GHGe, including sugarcane and cellulosic.

figure 4-4

From left to right: 2005 gasoline baseline (self explanatory), EPARIA: 2022 (current EPA model), ICF 2014 (correction for ILUC and other accounting from EPA model), ICF: 2022 BAU (business-as-usual projection to 2022 showing addition reductions), ICF: 2022 Building Blocks (described in paper, this is a theoretical potential based on new technologies such as combined heat & power, fiber digestion, etc). Note the dramatic difference in production & ILUC numbers

Tailpipe GHGe

The first thing you'll notice in the above graph is that the tailpipe emissions of EtOH are a sliver of gasoline. This is confusing until you read the lifecycle analysis report.

The carbon dioxide emissions from corn ethanol are assumed to not increase the atmospheric CO 2 emissions as the biogenic carbon emitted is offset by the carbon uptake of new growth biomass. Life- cycle CO 2 emissions from biofuel tailpipe combustion are not included in the analysis. The biofuel tailpipe combustion CH 4 and N 2 O emissions are included however. These emission factors are based on EPA’s MOVES model results (EPA, 2015b; EPA, 2010g).

This may seem "shady" if you aren't used to the carbon accounting, but every atom of carbon (no matter the form) involved in the actual transport biofuel comes from sequestration of an identical carbon atom in the air. This is an even, 1-1 exchange. They note that other compounds that are not CO2 are included, as they may be a 1-1 carbon exchange but they may have an uneven impact (GHGe).

Now, you were asking about the actual tailpipe emissions, not the tailpipe emissions that were matched with carbon sequestration via agriculture. For this we have to do a bit of math, and use the correct numbers. Reference this same study:

About 19.64 pounds (8.91 kg) of carbon dioxide (CO 2 ) are produced from burning a gallon of gasoline that does not contain ethanol.......About 12.73 pounds (5.77 kg) of CO 2 are produced when a gallon of pure ethanol is combusted

But as always when utilizing ethanol - gasoline comparisons, the per-gallon numbers don't mean anything. Using an energy density of 121.12 & 79.86 MJ/Gal for Gasoline & EtOH respectively, we end up with 73.56 & 72.25 gCO2e/MJ for Gasoline and EtOH, respectively. that difference is effectively nothing, as if you jumped between the LHV and HHV of gasoline you can flip which fuel is technically on top. We should expect this too, because we're talking about general hydrocarbon bonds being taken from a fixed state of reduction to completely oxidized, and regardless of the species we should see similar if not the same amount of energy released from breaking those bonds in the same way.

Caveats

There are two caveats here.

The first: The above Net GHGe analysis takes into account conversion of N2 into N2O, and CH4, CO etc emissions at the tailpipe. The tailpipe GHGe analysis does not. Generally speaking, Ethanol fuel results in lower or identical non-CO2 emissions than gasoline, though under certain circumstances N2O & acetaldehyde emissions can be slightly higher than gasoline, and certain toxic, organic and other ozone forming compounds can be slightly lower than gasoline.

The second caveat is that EtOH can be considerably more efficient in producing usable work on a per-MJ basis than gasoline, due to its highly desirable qualities as an internal combustion engine fuel (specifically octane rating, octane sensitivity & heat of vaporization). EERC found that in both flex fuel & non-flex fuel cars, ethanol has a dramatic impact on fuel economy when looked at on a per-BTU or MJ basis. This is effectively an increase in efficiency in internal combustion engines, especially undersized engines under heavy load. I personally tested this on my own vehicle (2014 GMC Sierra, 5.3 flex fuel) and found almost identical results to what EERC found.

EERC's results: enter image description here

My personal results: enter image description here

I had previously calculated my per-mile carbon intensity using the life cycle analysis here: enter image description here

Here I have adjusted the per-mile carbon intensity utilizing the above calculated tailpipe emissions numbers instead of the lifecycle analysis numbers:

enter image description here

I believe we're just getting the point where vehicles are capable of utilizing higher blends of ethanol combined with gasoline, and I think there is going to be a dramatic reduction of carbon footprint of the U.S. fuel supply as higher amounts of ethanol are blended in, and as auto-manufactures begin demanding higher performing liquid transport fuels. Ford is already testing out all horsepower numbers for their Ecoboost engines on 94-95 octane fuel, and many manufactures are finding the value in producing downsized-turbocharged engines.

  • Hello and welcome! There's a lot of good info here, however on longer posts sometimes it's helpful to include some headings (using '#'). It also doesn't look like you've answered the specific question -- tailpipe emissions per km for gasoline and ethanol. – LShaver Jul 26 at 18:43
  • @LShaver I agree that a short summary or headings will improve the answer, but I also feel that the percentages mentioned in the 3rd section do answer the question. The OP didn't specify that he wanted absolute numbers, he merely asked which has higher emissions. – THelper Jul 27 at 8:52
  • @THelper looks like that stat covers life cycle emissions and not tailpipe emissions, unless I've missed it. – LShaver Jul 27 at 14:05
  • @LShaver You are right. I forgot that the OP asked for tailpipe. – THelper Jul 30 at 7:02
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I don't know about per km, but this article has some info:

https://afdc.energy.gov/vehicles/flexible_fuel_emissions.html

This calculator has some info showing the different CO2 Emmissions of petrol vs ethanol: https://www.sunearthtools.com/tools/CO2-emissions-calculator.php

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Ethanol is C2H5OH. Burning it produces 2 CO2 + 3 H2O. Ratio of H2O to CO2 is 3/2 = 150%.

Octane is C8H18. Burning it produces 8 CO2 + 9 H2O. Ratio of H2O to CO2 is 9/8 = 112.5%.

In the case of ethanol, more of the gases in the exhaust are carbon and less of them are hydrogen, when compared to octane, which is fairly representative of gasoline.

So, yes, ethanol most likely produces more CO2 tailpipe emissions when compared to gasoline, for a given energy output, because more of the energy is in the form of C and less of it is in the form of H.

Also, keep in mind that while ethanol can be made from renewable sources, it would probably make more sense to bury those renewable sources underground in an anaerobic environment and continue burning oil.

No way can the entire human oil usage be substituted with renewable ethanol, because of the sheer magnitude of oil usage. Only electricity can be the solution.

  • You say "it would probably make more sense to bury those renewable sources underground in an anaerobic environment and continue burning oil." Why? Oil adds CO2 to the atmosphere, renewables do not (if you replant) – THelper Aug 2 at 6:12
  • @THelper Renewables can produce NEGATIVE indirect emissions if they are buried underground in an anaerobic environment. If renewables have more direct CO2 emissions than oil, it makes sense to bury renewables underground and burn oil, than to leave oil underground and burn renewables. – juhist Aug 2 at 14:00
  • I understand that burying wood or biochar or plastics for that matter can be a carbon sink. However if that also means using oil as fuel then it's still counter-productive. Burning oil adds 'new' CO2 to the atmosphere, wood (or ethanol from organic matter) doesn't because it took CO2 from the atmosphere first. – THelper Aug 4 at 9:45

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