NA and Nitromethane chemistry

[fritz_269]

Old PH posts (written by fritz_269):

There's a lot of confusion here - the terms are very poorly defined in common usage - they get thown around a lot without much thought. This is just my personal take on things:

Nitrous is the really confusing bit - an argument can be made either way (calling it NA or FI). It's NA since there is no extra pressure (Force) pulling it into the cylinder - and when you're not 'on the bottle' you are definitely NA. It is FI in the sense that you manage to pull more oxygen molecules into the combustion chamber than you possibly could in a NA engine.

In reality - Nitrous is neither NA or FI. It's a different type of power adder.

In the 50s, when drag racing really got started - there was a lot of confusion about how to write the rules for the different classes. They ended up creating a new term, "power adder". Thus there are different classes where you can have zero, one, two or unlimited power adders.

For example, one power adder is one supercharger OR one nitrous plate OR one turbocharger. The nice thing about this rule is that nitrous cars are in the same class as turbo cars - both have one power adder. And the rule works well for novel or yet un-invented devices that you might stick on your car (like an O2 tank, nitromethane injection, or an electric supercharger).

I tend to think of NA as meaning a car with zero power adders. I tend to think of FI as meaning a car with one or more superchargers or turbos. NA and FI are not antonyms!

Thus, a car on the bottle is neither NA or FI.

PS> Matt: Yes, your car is NA - you have zero power adders. And 93CivicExR's has one power adder which happens to be FI.


Last edited by fritz_269 on 07-10-2001 at 12:37 PM


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As I said before - the definitions are not strict - and what texan said about FI being "any method of supplying additional air or oxygen to the motor which it would not naturally be capable of ingesting" makes a lot of sense too.

Basically, I've cheated in my definiton by stating the NA and FI are not opposites - that lets me have a big, unnamed grey area where I can stick anything that doesn't fit my nicely defined categories.

If you do want to make the two terms cover everything - then I think the way to do it would be to say that NA means no power adders and FI means one or more power adders. By that logic - your statement above becomes mostly true - particularly if you change "air or oxygen" to "oxidizer and fuel".

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About the definition of fuel:
A Fuel is something that produces energy when it is consumed. That could be wood in a fire, uranium in a bomb, hydrogen in a fuel cell, or glucose in your body.

Specifically in our application (in an internal combustion engine we use a reduction-oxidation aka "redox" reaction - sometimes called burning )
fuel + oxidizer + ignition = radiation(heat) + byproducts

Anything that can be oxidized can be a fuel, ergo any reducing agent is a fuel.

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Nitromethane is actually a reducing agent - it is a fuel and flammable in the classical sense. You can extinguish a nitromethane fire by smothering and preventing it from getting oxygen (like with a CO2 or water extinguisher). It will not burn (for long) in a sealed engine.

Despite what you may have heard (and I was under this same error until I spent some time playing with the chemistry), nitromethane does not disassociate into free oxygen at comubustion cylinder temperatures. The chemical formula is CH3NO2 - the NO2 disassociates as a whole and is just passed out the exhaust pipe as smog. It does not work like N2O (nitrous oxide).

The reason Nitromethane is such a good fuel is basically that it requires less oxygen to fully burn (oxidate) each molecule. For instance:

100 octane gasoline is mostly C8H18
2(C8H18)+25(O2) => 18(H20) + 16(CO2)
A ratio of 25/2 = 12.5 Oxygen molecules per fuel molecule

Nitromethane is CH3NO2
4(CH3NO2) + 7(O2) => 6(H2O) + 4(CO2) + 4(NO2)
A ratio of 7/4 = 1.75

So nitro requires over seven times less oxygen per molecule than does gasoline. Unfortunately, it's not quite as dense as gas, but after figuring the energy densities in, we still find that nitromethane nets you about 2.5 greater combustion power per pound of oxygen.

Methanol (CH3OH) is even better!
2(CH3OH) + 3(O2) => 4(H2O) + 2(CO2)
A ratio of 3/2 = 1.5
Figuring in the energy density, it's almost 3 times the combustion power of gasoline! But it's more dangerous to handle than nitromethane, and it's far more corrosive to the engine parts.

Whew...

******EDIT******
There are errors in the above.
Please see my next post!
******EDIT******


[quote]Originally posted by texan
Hell just by the smell of nitro, you've got to figure it's the most explosive fuel of the bunch .
[quote]


Funny thing about that is -
Volitility (flash point) of:
Methanol - 52 degrees F
Gasoline (Octane) - 55 degrees F
Nitromethane - 95 degrees F
Nitro is the safest of the bunch as far as accidental ignition. Strange, hunh? Our nose must be more sensitive to it for some reason.

In thinking about this some more I just re-did a couple of calculations and came up with a totally different answer. Now I'm a bit confused. I think I was wrong before and I've been doing exactly what I hate - spreading mis-information. I'm very sorry. I checked my work a couple of times now, and I now believe that the formula for combustion of nitromethane looks like this (where the NO2 structure does disassociate into free oxygen!):

4(CH3NO2) +3(O2) => 4(CO2) + 6(H2O) + 2(N2)
A ratio of 3/4 => 0.75!!!

I don't know what happened before - I did those calcs a some time back and convinced myself that the NO2 wouldn't disassociate. But now I'm quite sure it would at these temperatures. I'm sorry for any confusion I may have caused - particularly to myself!

Energy density:
1) diesel (56,324 kJ/kg )
2) gasoline (47,895 kJ / kg )
3) methanol (22,725 kJ / kg)
4) nitromethane (12,002 kJ / kg)

A better "stoichiometric factor" would be the the molecular weight divided by the ratio I came up with before. This gives you the stoichiometric ratio by weight:
Stoichiometric factor:
1) nitromethane ( 61 / 0.75 = 81)
2) methanol ( 32 / 1.5 = 21 )
3) gasoline (114 / 12.5 = 9 )
4) diesel (?)

If we multiply the energy density times the stoichiometric factor, we get a number porportional to the amount of energy we can produce with one kilogram of oxygen.
Energy by oxygen weight
1) nitromethane (12002 *81 = 972,162 )
2) methanol ( 22725 * 21 = 477,225 )
3) gasoline ( 47,895 * 9 = 431,055)
4) diesel (?)

SO Nitromethane can produce 130% more energy than gasoline per kilogram of air. And contrary to my earlier post - methanol is a measly 11% better than 100 octane gasoline!

Methanol does have extra advantage though - it has a high heat of vaporization (396 BTU/lb) compared to gasoline (117 BTU/lb) - which means that it carries three times more heat out of the combustion chamber with each exhaust stroke. And when the engine runs that much cooler - you can increase compression without detonation and thus volumetric efficiency goes up. Nitromethane isn't so bad with 268 BTU/lb.

Side note:
Diesel and gasoline are witches brew - I couldn't even pretend to guess what's really in them anymore. Standard gasoline is generally a mix of heptane, octane, and nonane. Standard diesel fuel is classically tetradecane (C14H30) - but diesel is very unrefined. In general, for aliphatic carbon chains, as the chain gets longer, RON increases, the energy density increases, the "stoichiometric factor" decreases, and the heat of vaporization increases.

I'm pretty sure I've got it right now. Thanks for your patience.

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OK - here's the actual numbers (with units! ):

Oxygen (O2) is about 23% of the atmosphere by mass (21% by volume)
So, one kg of air contains about 0.23 kg of O2,
and 0.23kg / (32 g/mole) = 7.2 moles of O2 per kg of air

Fitting our balanced equation,
2(CH3OH) + 3(O2) => 4(H2O) + 2(CO2)
we can combust 2 moles of CH3OH for every 3 moles of O2. So to fully use up one kg of air, we need:

7.2 moles O2 * 2 moles CH3OH / 3 moles O2 = 4.8 moles CH3OH
and thus
4.8 moles CH3OH * 32 g/mole = 153 g of CH3OH

Therefore our mass stoichiometric ratio is:
1,000 g air / 153 g CH3OH = 6.51 mass air / mass CH3OH

Conversely:
153g CH3OH / 1,000 g air = 0.153 mass CH3OH / mass air

Ta da!

If you want to use my earlier "stoichiometric factor" to get the mass ratio (with atmospheric free Oxygen) just multiply it by the factor by (23% / 32 ) = 0.00719 to get mass CH3OH / mass air

So, here are the new numbers, in their correct, happy form:

Stoichiometric mass ratio (mass air / mass fuel):
1) nitromethane ( 1.72 )
2) methanol ( 6.51 )
3) gasoline (iso-octane) ( 15.4 )
4) diesel** (iso-tetradecane) ( 20.3 )

Fuel energy per mass air:
1) nitromethane ( (12,000 kJ/kg fuel) / (1.72 air/fuel) = 6978 kJ/kg air )
2) methanol ( (22,725 kJ/kg fuel) / (6.51 air/fuel) = 3490 kJ/kg air )
3) gasoline ( (47,895 kJ/kg fuel) / (15.4 air/fuel) = 3110 kJ/kg air )
4) diesel** ( (56,324 kJ/kg fuel / (20.3 air/fuel) = 2775 kJ/kg air )

For those who don't know, a kJ is a kilo-joule, a measure of energy equal to 2398 calories or 0.278 Watt-hour.

Fortunately for my ego, the proportionalities are close to what I reported before. (I attribute any differences to round-off error - I haven't carried many decimal places.) Nitromethane can produce 125% more energy per kg of air than 100 octane gasoline can. And methanol is only 11% better than the gas.

Make sense to everyone now?

**Note: I calculate diesel using only tetradecane (molecular weight of 198) as follows:
(C14H30) + 29(O2) => 30(H2O) + 14(CO2)
So the molecular stoich ratio is 29/1.
The mass stoich is (deep breath!):
(32 g/mol O2 / 23% (g O2 / g air)) * (29 mol O2 / 1 mol C14H30) / (198 g/mol C14H30) = 20.3 mass air / mass C14H30
**Note part II: Actual diesel fuel is a huge mix of chemicals, from C3H8 to C25H52 and beyond - it's exact stoich ratio will depend heavily on the mixture - I would assume that to get to the 13.9 mass stoich ratio that texan reports means that diesel fuel has a fair component of smaller than C14H30 molecules as well as aromatic compounds that do not break during combustion.

SilverY2KCivic - aren't you glad you asked this question!?!

[texan]

Hey, I remember this post! Good compilations of the old posts here, it's nice to see you're still around and doing your thing fritz.

[fritz_269]

Originally posted by texan
Hey, I remember this post! Good compilations of the old posts here, it's nice to see you're still around and doing your thing fritz.

Happy to see you too texan!