Optimal fuel ratio in a propane + air combustion gun

[jimmy101]

What's the optimal fuel ratio for a propane + air combustion gun? I think most spudders would say that a stoichiometric mix will give the best performance. But is that really the case? In general, maximum energy release in a combustion engine does not occur at a stoichiometric mix. In addition, the maximum burn speed rarely occurs at a stoichiometric mix.

So what's the best fuel ratio?

I've done some number crunching and it looks like, for propane + air, you probably want a rich mixture. Something in the vicinity of 4.4% to 4.8% propane. The increase in performance will probably be too small to measure but a slightly rich mixture has the advantage of being less sensitive to errors in the various volume and pressure measurements than is an exactly stoichiometric mix. It looks like the performance drops off pretty quickly with a too lean mixture, so it is better to be a bit too rich than a bit too lean.

I've got a page here with the details. Any comments, flames ... ?

[mark.f]

Nope, no flames. This is one of the reasons an automotive engine runs rough after disconnecting the power to the PCM for an extended period of time. After riding a few miles, the engine smooths out as the PCM "learns" the proper parameters again, including fuel mixture.

[DYI]

Also, propane combustion has a higher tolerance for rich mixes than for lean mixes. I've been running slightly rich mixes ever since I first fired my original hybrid about a few months ago.

[jimmy101]

Also, propane combustion has a higher tolerance for rich mixes than for lean mixes. I've been running slightly rich mixes ever since I first fired my original hybrid about a few months ago.

DYI: An idea how rich? Any estimate on the accuracy of the volumes, pressure etc.? Be interesting to see how close to ~4.5% it is.

[psycix]

Hmm and what is your theory why richer mixes tend to burn better then lean mixes?

Reaction speed with an excess of fuel is higher then the reaction speed with an excess of oxygen, increasing performance even though the resulting thermal energy is slightly lower?

[SpudFarm]

i have a related question on this.

what would happen if you got your chamber to 100%oxygen and 4.2% propane? would it be too lean?

[SpudUke5]

Yes it would be, too much oxygen or too little propane.

when you do this with stoichiometry im pretty sure that you assume that where ever the reaction is occurring has 100% oxygen.

So taken from the wiki,
C3H8 + 5O2 --> 3CO2 + 4H2O

you need 1 mole of propane to 5 moles of oxygen.

Depending on the volume of the chamber the number changes but the ratio stays the same, you use the ratio 1/5 (C3H8 to O2)


so lets use 100L chamber as an example.

for this chamber, for a any mix you would need it to contain 20% propane.

In the chamber, you would need 16.6667L of propane and 83.33333333L of pure oxygen.

[jimmy101]

Hmm and what is your theory why richer mixes tend to burn better then lean mixes?

Reaction speed with an excess of fuel is higher then the reaction speed with an excess of oxygen, increasing performance even though the resulting thermal energy is slightly lower?

No theory from me as to why it works that way. Combustion is fiendishly complex at the molecular level. GasEq uses ten product species for the combustion of propane in air (and that is the "reduced product" set). IIRC, there are something like several dozen known intermediates in the combustion of propane. Not all steps release the same amount of energy. Perhaps the slightly rich mix gives a slight preference for steps in the combustion process that require propane or one of the early combustion products instead of oxygen (or an early oxygen containing products).

Richer mixes don't tend to burn better than lean mixes. The optimal mix is a bit rich but if you make it richer than that then performance will drop off.

In the atuomotive world the disconnect between maximum efficiency and maximum power is well established. Most race cars can tweak their mixture up and down depending on what they are trying to maximize; efficency (MPG) or power.

[jimmy101]

i have a related question on this.

what would happen if you got your chamber to 100%oxygen and 4.2% propane? would it be too lean?

Yes, that is an extremely lean mixture. But would it still ignite? Off hand I would think that it would. I'm sure that the combustion limits for typical hydrocarbon fuels in pure oxygen are known.

Googling with combustion limit propane "pure oxygen" returns this as the first link;
http://repositories.cdlib.org/cgi/viewc ... ntext=lbnl
Scroll down to Table 2 and it says the flammability limits for propane in pure O2 is 2.4~61% propane.

So, it looks like the 4.2% mix will indeed ignite.

I would think that the energy production for 4% propane in air versus 4% propane in pure O2 would be pretty similar. If any one ever wanted to make a gun that had the capability of very fine adjustments in the chamber energy they could probably fuel with pure O2 and then add varying amounts of propane. Looks like you would be able to vary the energy in the chamber from about one half a normal combustion gun up to about five times more energy than a normal combustion.

[D_Hall]

No idea how it works in guns, but the "slightly rich is better" statement is true in rocket motors. That one is easy to understand, however.... Slightly rich mixtures result in unburned hydrogen, hydrogen monoxide and carbon monoxide. All of those gases have lower molecular weights than their fully oxidized, completed combustion counterparts (water and CO2). Result? Faster gas flows through choked nozzles.

No idea how this applies to spud guns, however.

[psycix]

No idea how it works in guns, but the "slightly rich is better" statement is true in rocket motors. That one is easy to understand, however.... Slightly rich mixtures result in unburned hydrogen, hydrogen monoxide and carbon monoxide. All of those gases have lower molecular weights than their fully oxidized, completed combustion counterparts (water and CO2). Result? Faster gas flows through choked nozzles.

No idea how this applies to spud guns, however.

That is a good theory.
Since gas flow is one of the most important things in spudguns, I think this indeed applies to spudguns.

[jimmy101]

No idea how it works in guns, but the "slightly rich is better" statement is true in rocket motors. That one is easy to understand, however.... Slightly rich mixtures result in unburned hydrogen, hydrogen monoxide and carbon monoxide. All of those gases have lower molecular weights than their fully oxidized, completed combustion counterparts (water and CO2). Result? Faster gas flows through choked nozzles.

No idea how this applies to spud guns, however.

That is a good theory.
Since gas flow is one of the most important things in spudguns, I think this indeed applies to spudguns.

Perhaps, but the gas flow rate in a spudgun is a heck of a lot slower than in a rocket motor. So, I'm not sure if the molecular weight affect would be noticable in a spudgun.

It looks like there isn't any hard data on the affect of mixture on spudgun performance. My analysis would not have found an affect caused by changes in the distribution of molecular weights since it doesn't really enter into the model. Using GasEQ, the total mass in the system is constant and it doesn't move.

I wonder though, if your fuel is propane might the affect be reversed from what D_Hall is talking about? Incomplete combustion may give molecules with MW significantly greater than CO2 and H2O, perhaps even greater than propane itself. Though I suppose you would need really incomplete combustion to have molecules that big left.

I would guess that (if the affect is true in an actual spudgun) that it comes down to two things; (1) the difference between thermodynamics and kinetics (you can't use one to predict the other and both have affects on the operation of the gun) and (2) swapping efficiency for performance. Thermodynamically, the less efficient use of a greater amount of fuel can create more energy. Kinetically, an excess of one reagent in a reaction makes the reaction proceed faster than it does with a stoichiometric mix.

Anybody know if modern gun powders are stoichiometric mixes? Or are they rich, lean, or variable depending on the desired burn characteristics?