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Old 10-16-2012, 07:38 PM   #21 (permalink)
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Heats of combustion (kJ/kg) of different fuels:

Methanol (CH3OH).....20,000
Ethanol (C2H5OH).....26,800
Gasoline.............43,700
Diesel...............44,400

source: http://www.nrc.gov/reading-rm/doc-co...1805/ch3-6.pdf

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Old 10-17-2012, 05:27 PM   #22 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
Heats of combustion (kJ/kg) of different fuels:

Methanol (CH3OH).....20,000
Ethanol (C2H5OH).....26,800
Gasoline.............43,700
Diesel...............44,400

source: http://www.nrc.gov/reading-rm/doc-co...1805/ch3-6.pdf
That's not per equivalence oxygen though. Per equivalence oxygen I think the alcohol fuels have about the same heat of combustion, maybe ever slightly more, maybe slightly less I can't remember. I am pretty sure the usual quoted heats of combustion are going from liquid state fuel, so they include latent heat of vaporization. I believe gaseous ethanol combusting produces about the same amount of heat.

With insufficient timing the slower burn of ethanol could possibly give rise to higher exhaust temperatures, but I imagine this would be very hard to actually accomplish. Moreover, the most heat sensitive thing in the exhaust stream is not the valves but rather the catalyst, and manufacturers tune their engines to protect the cats.

The thing with ethanol is that because its latent heat of vaporization is significant, even compared to its heat of combustion, it's very sensitive to how the fuel is delivered and vaporized. Ideally for efficiency you'd want all the fuel to vaporize in the chamber after the intake valve seals, to reduce the work on the compression stroke. Direct injection systems inject on the intake stroke and maybe a second time on compression because they need time for the mixture to form, so no engines actually take full advantage of the cooling effect for efficiency. What you can do is cool down the surfaces in the engine a bit to quench potential hot spots that could cause knock, and cooling the intake charge a little helps too.

However some engines end up with unvaporized ethanol at the end of the compression stroke, and these will see near the full 34% loss of "fuel economy". I think this happens to some extent on a lot of current crop flex fuel engines because they're not doing so well in fuel efficiency. In this case, the exhaust temperature is definitely going to be lower.
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Old 10-17-2012, 08:30 PM   #23 (permalink)
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While I like the conversation about the Ethanol fuels, I would like if someone would focus a little more on the brand to brand comparison if possible! I would do this but I fill up once every month or two so...
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Old 10-17-2012, 09:01 PM   #24 (permalink)
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Quote:
Originally Posted by TheIVJackal View Post
While I like the conversation about the Ethanol fuels, I would like if someone would focus a little more on the brand to brand comparison if possible! I would do this but I fill up once every month or two so...
The reason you are getting talk of ethanol fuels (which almost every fuel on the market is) is because that is the main factor of variation between one gas station and another.

You will find certain stations have different additives and different ethanol blends, some include more napathylene others don't have a winter blend of gas on certain grades.

The thing is (in my experience) is that from one week to another the same gas stations can have a swing in ethanol content (spot price varies), unless they GUARANTEE ethanol free but then those that have ethanol free may add MTBE or whatever that crap is called, which on my car kills FE more than ethanol.

So what I am saying is that this is sort of a unicorn, there are ethanol test kits on the market that aren't real bad $$, there are also a few sites that exposes who uses MTBE but beyond this, empiracle don't mean squat.

To make this thread into a non-unicorn we would need some sort of chemist kit and someone willing to do analysis, trouble is in my area there is a MONOPOLY in that ALL FUEL COMES FROM ONE SUPPLIER with only 2 exceptions in the whole area where I live. Does not matter what brand they slap on, comes out of the same tank.

This means the only differences between stations are the additive package and possibly ethanol content (to a point)

AKA the same tank fills BP & SHELL, they just blend in some additives from the as-is gas from the supplier.

Cheers
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Old 05-19-2016, 01:40 PM   #25 (permalink)
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Shell vs. Unocal 76

I have a very routine, lengthy commute m-f with 60 miles of straight highway driving each way. I drive a 2014 Ford Focus SE Hatch. I've always used shell 87 in this car and was steadily getting 38 MPG per tank. I decided to try Unocal 76 87 octane since I used them years ago. I started getting 42 mpg per tank. I continued to use Unocal 76 for a long while and continuosly got over 40 mpg per tank. I recently switched back to Shell 87 Octane just out of curiosity. Sure enough mpg dropped down to 37mpg?! There is clearly a difference between Shell 87 octane and Unocal 76 87 octane. I drive the same route everyday, at the same times, with cruise control set at 70, and used each brand of gas consistently for weeks at a time for comparison. I'm confident the differences are due to the fuel.
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Old 05-19-2016, 02:09 PM   #26 (permalink)
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Time of year (summer-blend vs. winter-blend) could cause similar results.
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Old 05-19-2016, 03:42 PM   #27 (permalink)
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Good point. I did think about the summer/winter blend. I didn't consider it much since I used both during the Spring and weather here in Cali is consistently mild this time of year. I switched back to Shell in the past month (beginning of April). Since I commute a lot, I get gas rather frequently. The change in mpg was noticeable on the first tank after switching, which I thought was just my imagination, but it has been consistently 36-38 mpg since I switched back to Shell in April. Prior to April, 40+ mpg consistently using 76. Not a big deal I guess, just found it interesting enough to google the topic and find this thread lol Shell offers much better reward incentives (discounts, etc) than 76 does, which is why I gave them another shot to reduce fuel costs. But over time it probably balances out the same.
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Old 05-20-2016, 03:06 PM   #28 (permalink)
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...only your gas bills will know/prove which is right (ha,ha).
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Old 07-08-2016, 02:56 AM   #29 (permalink)
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Wish some scientist could come on here, grab fuel from a few popular stations, and test them all to discover what could be triggering the differences!
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Old 07-08-2016, 10:39 AM   #30 (permalink)
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Don't need to be a scientist.

Get an ethanol test kit.

Locally fuel marked may contain up to 10% ethanol many times doesn't.

Smaller stations sometimes have more heavy ends, a few states allow non oxygenated fuel (as I discovered out west)

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