<Day 3 at the ASPO-USA meeting The Oil Drum DrumBeat: September 27, 2008>

35 comments on The Transition to Winter Gasoline--Revisited

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dr_dr on September 26, 2008 - 12:23pm Permalink | Subthread | Parent | Comments top

This is great topic! (note:...I just crashed, so if this shows up twice, forgive me)

I have a couple of questions...
1) Why is the RVP of ethanol 18? When I look up the vapor pressure ( wiki_ethanol ) it looks like at 100 F the vapor pressure is just over 100 mmHg, or 2-2.5 psi. Am I missing something, or is this a result of the significant deviation that leads to ethanol/benzene azeotropes? Everything I've looked up supports ethanol raising the RVP...except the numbers.

2) All this vapor pressure stuff is dependant on mole fractions, so a very small amount of butane by mass or volume can have a huge effect. Thus a 10% butane mix (mass) will be much greater than 10% by mole fraction, thus much more than 5.2 psi...is there really much butane in any but the coldest gasolines?

3) More of a thermo question. How does decreasing energy content relate to mileage? For example, let's say a hydrocarbon fuel has 100 j/mL, and ethanol has 60 j/mL (made up numbers for easy math). If the unadulterated fuel gives you 20 mpg, what would a 50/50 fuel give you? The energy content would be about 80 J/mL, and a simple calculation would suggest that that translates to 16 mpg (20% reduction).
-my question is whether it is this simple. I suggest that it should be worse, as work that goes into friction and heat will largely stay constant, meaning that the fraction of energy that goes into moving the car takes most of the loss. If half the fuel's energy moves the car then the original fuel used 50 j/mL for friction and heat. If we switched to the mix, 50 J taken from the 80 leaves only 30 for proplusion, for a 40% loss (50 to 30), or a drop in mileage to 12 mpg. Maybe there are advantages (less wasted heat?) that mitigate this?

thanks
-dr

btw...great to hear you live at ASPO, Mr. Rapier

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dr_dr on September 26, 2008 - 12:47pm Permalink | Subthread | Parent | Comments top

Thanks for that first link....If I had waited a few minutes before my previous posting I mighta saved a little time!

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Robert Rapier on September 26, 2008 - 1:53pm Permalink | Subthread | Parent | Comments top

Why is the RVP of ethanol 18?

It isn't, but my guess is that what he was reporting above was something like an 'effective RVP' when blending. Based on RVP only, ethanol should lower the RVP when added to gasoline. However it raises RVP as noted.

is there really much butane in any but the coldest gasolines?

It is at about 2% in the winter, and can get up to 12% or so in the winter depending on what else is in the blend.

Maybe there are advantages (less wasted heat?) that mitigate this?

Compression ratio can be made higher if octane is higher. Ethanol increases octane. In theory, you could build a car that gets much better gas mileage than you would expect from a straight BTU calculation by raising compression ratio. Think about diesel. It has only about 10-15% more BTUs than gasoline, but diesel engines get 30% or so better fuel efficiency. Why? Higher compression ratios.

btw...great to hear you live at ASPO, Mr. Rapier

After hearing me, Debbie Cook said that she would always read my essays now envisioning a southern accent. But what I didn't tell her is that I actually write with a British accent. :-)

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majorian on September 26, 2008 - 6:55pm Permalink | Subthread | Parent | Comments top

The RVP of denatured ethanol is 3psi , but the RVP of ethanol above a gasohol mix is 18( vapor pressure varies per Henry's Law). The reference below says 17 but I've seen 18 psi in other places. So for purposes of what I am talking about the RVP of ethanol to be added to gasoline mix is 18psi(17psi) and I standby it.

The RVP of gasoline product prior to blending can very greatly depending on where it comes from in the refinery--allkylate or reformate gasoline has a different RVP than FCC cracked gasoline and the amount of these 'gasolines' depends on the kind of oil supplied to the refinery. Reformate or FCC gasoline typically has RVPs of less than 5, so blending in butane, ether or ethanol is necessary.
The reason you can't reduce the RVP below 5 psi is that you'd have starting problems from low rates of fuel evaporation.

http://www.ethanolrfa.org/objects/pdf/newRFA%20Fuel%20Ethanol%20960501.pdf

R^2,
I don't understand your comment about polarity. MTBE ether is also polar. Do you recommend putting lead into gasoline?
At any rate Feinstein wanted to waive the whole oxygenate requirement. Oxygenates improve performance and reduce the production of carbon monoxide, isn't this important?

As far as the 10% allowance for ethanol goes, the EIA example assumes 9psi RVP gasoline prior to blending. I suppose a refinery could make one pre-blend for ethanol and a different RVP for straight gas to be used if the EPA requirements were too tight.

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Robert Rapier on September 26, 2008 - 9:01pm Permalink | Subthread | Parent | Comments top

I don't understand your comment about polarity. MTBE ether is also polar.

Minor compared to ethanol. Ethanol is much less like a hydrocarbon, thus it doesn't mix as well.

Do you recommend putting lead into gasoline?

You throw out a lot of straw men? I have blended millions of barrels of gasoline. I have blended very high octane gasoline. I never used ethanol, MTBE, or lead.

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dr_dr on September 27, 2008 - 12:23am Permalink | Subthread | Parent | Comments top

Good stuff, thanks.

BAck to my mpg question. Assuming my '99 Tacoma is not re-tuned when I put gasoline with ethanol in it, what happens to my mileage? I assume the car is smart enough to adjust the air mix and the timing, but then what? 10% ethanol drops the energy content by 3% relative to the straight hydrocarbon fuel. In practice does anyone know (via careful tests) what that does to mileage? This seems an important part of the arguments for or against using ethanol as motor fuel. I recognize that adding ehtanol likely means removing other high octane components from the original fuel so exact comparisons are of limited value, but...?

Is there some rough equation that relates energy content to mileage?

(mileage) = EC * a where EC is energy content and "a" is a proportionality constant
or
(mileage) = (EC - b)*a where b is energy lost to friction/heat

-d

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Robert Rapier on September 27, 2008 - 9:11am Permalink | Subthread | Parent | Comments top

In practice does anyone know (via careful tests) what that does to mileage?

There have been a number of independent tests done, and they show the expected decline in mpg. Most of these are on E85 though:

http://www.consumerreports.org/cro/cars/new-cars/news/2006/ethanol-10-06...

I have around here somewhere a University of Wisconsin-Milwaukee study from the 90's that looked at 5% ethanol blends, and concluded that they dropped fuel efficiency by around 2%. The American Coalition of Ethanol commissioned a study recently and came out with the conclusion that fuel efficiency generally drops, but by less than the BTUs would warrant. The ethanol guys like that study, but I don't suppose they would like a study funded by the American Petroleum institute that concluded just the opposite.

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35 comments on The Transition to Winter Gasoline--Revisited

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