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Old 04-23-2015, 02:25 PM   #51 (permalink)
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You have to look at energy content of the fuel. Propane has only ~73% of the energy content of gasoline. That an engine still gets 5% better fuel economy on propane than gasoline should stand as testament to fuel vaporization.

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Old 04-23-2015, 03:41 PM   #52 (permalink)
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I made sure not to compare propane volume of gasoline to volume of propane. I broke it down to a BTU comparison between the 2 fuels.

The propane mod engines run higher compression, different ignition possibly different cam profile. If total vaporization was so far superior then we should see a difference much greater than a 5% improvement.

What you are trying to say is the energy content of propane is only 73% of gasoline and it still did better in some tests and that is simply not the case at all in how I made my comparison.
A BTU comparison of the 2 fuels eliminates this inherent rookie volume to energy comparison difference.
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Old 04-23-2015, 04:26 PM   #53 (permalink)
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http://www.afdc.energy.gov/fuels/fue...ison_chart.pdf
Energy content of gasoline: 112,114 - 116,090 BTU/gal
Energy content of propane: 84,250 BTU/gal

A gallon of propane has 72.5730% to 75.1467% the energy of a gallon of gasoline.

I don't know what comparison you're trying to make as regards your "BTU comparison of the 2 fuels". You'll note it's energy per volume (BTU/gallon)... removing volume gives you nothing to compare.

The propane engines run higher compression because they can... propane has a higher effective octane than gasoline.

Likewise, a relatively slower flame speed (gasoline - 16.5 m/sec, propane - 0.43 m/sec), means the spark timing in a propane engine is advanced as compared to a gasoline engine.

Propane's flammability limits are 2.5% to 9.5% in air, with 4.2% by volume for maximum heat production. Gasoline's stoichiometric ratio is 1.4285% by volume.

So despite having to use more propane per volume air (and remember, your engine is an air pump... with a pretty much fixed amount of air it'll pump for any given throttle position and corresponding engine RPM), and despite the fact that propane has lower energy content (on a volume basis, the only way to compare apples to apples), the fact that an engine running propane (notwithstanding it being reconfigured to have higher compression and more spark advance to be compatible with the fuel used) gets better fuel efficiency is a testament to the fact that gaseous fuels burn more efficiently than fuels that are in liquid form and must then be evaporated (and remember, in a gasoline engine, due to the fact that the fuel has a relatively higher condensation temperature as compared to propane, when cylinder pressure rises, some of it will condense on the relatively cooler head and cylinder wall surfaces, it's a simple matter of physics that Peter Rotgans (Peterrr) demonstrated mathematically right here on ecomodder.com).

Last edited by Cycle; 04-23-2015 at 05:30 PM..
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Old 04-23-2015, 08:16 PM   #54 (permalink)
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Quote:
Originally Posted by oil_pan_4
btu/mile
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Originally Posted by Cycle View Post
I don't know what comparison you're trying to make as regards your "BTU comparison of the 2 fuels". You'll note it's energy per volume (BTU/gallon)... removing volume gives you nothing to compare.
He was comparing the energy per mile .. BTU per mile .. fuel economy.
Not Miles per volume (gallon) fuel economy.
Not Miles per weight (Lbs) fuel economy.

1 BTU is the same amount of energy weather it comes from gasoline or propane.
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Old 04-23-2015, 08:53 PM   #55 (permalink)
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Btu

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Originally Posted by oil pan 4 View Post
Ok getting back on track.
If fuel vapor engines were so amazing why cant an engine be built that runs off say propane (which has a boiling point of around -40) a substance where you know there is no possibility that its anything other than a super heated gas at room temperature when under 1atm of pressure and get significantly better btu/mile economy than a gasoline engine?

The best propane engines I have found get maybe 5% better fuel economy than their gasoline counterpart. That difference could be due to the propane mod specific engines running higher compression and different ignition advance than their gasoline counter parts because propane burns in an engine almost like 105 octane gasoline.

I don't consider simple small engines that have been converted from gasoline to propane a fair comparison, where no other changes are made aside from fuel induction system on the converted engine. The converted engine should at the very least get an ignition timing advance too.
Gasoline = 124,000 btu per gallon
propane LPG = 91,330 btu per gallon

gasoline has about 36% more btu than propane
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Old 04-23-2015, 08:59 PM   #56 (permalink)
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Looks like everyone jumped on this at the same time
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Old 04-23-2015, 11:19 PM   #57 (permalink)
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If you people cant comprehend what BTUs per mile fuel economy is then no wonder you still believe in surface carbs and vapor this and that.

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I don't know what comparison you're trying to make as regards your "BTU comparison of the 2 fuels". You'll note it's energy per volume (BTU/gallon)... removing volume gives you nothing to compare.
I give up.

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removing volume gives you nothing to compare
Funny you say that. Engineering calculations are always done in mass or in units of energy and not volume.
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Old 04-24-2015, 12:36 AM   #58 (permalink)
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So you're just using a different means of measuring the same thing... the volume of fuel (for any given fuel) used is the same mile for mile on average.

BTU/mile still has to take into account the amount of fuel used at some point, because you're not buying your fuel in accordance with its BTU content... you're buying it by volume. You might as well claim propane has a different "cent per mile" efficiency, as well... a difference with no distinction.

And you've still not addressed my main point... that a gaseous fuel exhibits better fuel economy (on a BTU/mile or a MPG basis) than a liquid fuel, even if that gaseous fuel has a lower BTU/volume content.

Last edited by Cycle; 04-24-2015 at 12:43 AM..
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Old 04-24-2015, 09:47 PM   #59 (permalink)
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Quote:
Originally Posted by Cycle View Post
BTU/mile still has to take into account the amount of fuel used at some point
BTU/Mile already does .. it includes both the amount of fuel and the fuel's energy density ... thus making it superior to the less information from just using gallons , liters, etc.

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And you've still not addressed my main point... that a gaseous fuel exhibits better fuel economy
Nooo ... he already specifically addressed that .. his point was that the difference is too small to support conclusions like that .. the other engine modifications themselves make contributions toward that tiny gain .. how do you know what % part of the 5% gains are from the engine modifications themselves and what % are from the fuel itself ? ... What's the standard deviation of expected fluctuation from no gains at all ?

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Originally Posted by oil pan 4
The best propane engines I have found get maybe 5% better fuel economy than their gasoline counterpart. That difference could be due to the propane mod specific engines running higher compression and different ignition advance than their gasoline counter parts because propane burns in an engine almost like 105 octane gasoline.
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Old 04-24-2015, 10:58 PM   #60 (permalink)
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BTU/Mile already does .. it includes both the amount of fuel and the fuel's energy density ... thus making it superior to the less information from just using gallons , liters, etc.
How so? What volume of fuel that comprises a single BTU will drive the car for a specific distance? Especially considering that different engines will have different efficiencies, some throwing away more of those BTUs than other engines.

If we bought fuel by the BTU, his point would have more merit... but we buy our fuel by volume.

So it all comes back to volume... MPG.

As for "it's too small a difference"... what about "the amount of fuel per unit volume of air for propane is approximately twice that of gasoline just to reach that fuel's flammability range" and "propane has only ~73% of the energy content of gasoline on a per volume basis" yet "propane still exhibits a higher fuel efficiency than gasoline" don't you understand?


Last edited by Cycle; 04-24-2015 at 11:09 PM..
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