Thread: lean vs. rich

Tweet

Why is a lean mixture more prone to detonation?
Why does a lean mixture burn hotter?

buy why do they burn hotter? If you have a stoichiometric mix, compared to a really lean mixture, who would the lean one burn hotter.
One molecule of fuel can still only react with 14.7 molecules of air, if there's extra air it should just pass through shouldn't it.

Originally posted by tcodi
Why is a lean mixture more prone to detonation?
Why does a lean mixture burn hotter?

Combustion needs 2 ingredients to occur. 1 Is a hydrocarbon like gas, the other is oxygen. Oxygen is found in the air around us, but only in a small percentage. (about 15-20 % oxygen). the rest of the "air" is made up of nitrogen which doesn't burn. the optimum ratio for the best combustion is about 14.5 to 1 or so. ( That's 14.5 parts air to 1 part fuel). more like 12.5 to 1 if your building a race engine. I could be off a little on those numbers but that's fairly close I believe.

So, to answer your question. Oxygen burns really hot. Maybe a chemist can stop by here and tell you why that is, cuz' I'm not 1. But, if you run your engine below the 14.7:1 meaning too much (Hot) oxygen it will run (Hot) just like an acetylene torch does when you hit the oxygen lever. More oxygen = more complete burn of the fuel = more heat. If you run a richer mixture you in turn have a lower oxygen % in the mix and more fuel so a less complete burn = lower combustion temp. Don't know if that answered your question, but it sure got me confused.

Cool (no pun intended) U got a bunch of replies whilst' i was typin'. And nope, I still don't know the scientific reason why oxygen burns hot other than it supplies more combustion to the fuel. Maybe Don can come by and give you the equation for how oxygen works on a molecular level.

Originally posted by DennyW
Oh, you wanted to know stoichiometric? This is a good artical about what you can add also,
http://www.woodward.com/engine/gaseng/gaseng2/ratio.cfm
Yes, Don, where are you??

Not to hi-jack this thread but since we're on the subject of air/fuel, I thought I would call the guy i bought my EFI injection from the other day. ............................No answer................... checked his web site ...................... gone. I'm havin' a streak of bad luck here with suppliers HUH???? The guy owes me a mapping of the injection once I get the engine specs finalized. Not a real big deal, it's a Hilborn system, so I can go somewhere else for parts. I'll probably have it mapped on the dyno on the actual engine anyway. It's just the idea it appears he pulled up stakes and left me hangin. Guess I'm gonna' hafta' do a little more thorough research on suppliers from now on.

Here's what I seem to remember. In a rich mixture, all of the fuel doesn't oxidize (burn). However, with the high heat in the combustion chamber, it can vaporize. Vaporization takes up heat, reducing temperature (latent heat of vaporization). Note that top fuel cars run so rich that nitro comes out the headers (hence the flames). They do that for the cooling effect.

One point. Oxygen doesn't burn. It promotes combustion, though.

Originally posted by Henry Rifle


One point. Oxygen doesn't burn. It promotes combustion, though.

I went through that subject on another site a few months ago. I tried to explain to the guy that oxygen doesn't burn it just allows hydrocarbons to burn.
The guy came back and said "Well..... what do you think makes an acetylene torch burn then????????HHHHMMMM I replied ....... the acetylene????....... .... Didn't get an answer. I even supplied a data sheet on oxygen stating it was a NON COMBUSTIBLE GAS. But he wasn't buyin' it.

I believe that oxygen can burn when mix with a combustible gas. I known in my field of work, I use a combustion analazier to adjust boilers fuel and air mixture, one thing we don't want to do is burn all the oxygen then we will be burning to rich which is dangerous because you will then carbon up the boiler tubes or refactory and prevent heat transfer. I don't known how this will help but the fuel and air mixture when adjusted proberly you can get it to burn efficiently and where almost all the gas is burn.
And at that point the fire or detination is at its hottest point.

Originally posted by TedIII
I believe that oxygen can burn when mix with a combustible gas.

Your right TedIII, that's the point I was tying to make to the guy. Oxygen first has to be mixed with a hydrocarbon before there can be combustion. The more oxygen the more efficient the burn of the hydrocarbon. If oxygen were a combustible fuel all on its own ........... this would be a very dangerous world. ONe spark and we would become a sun. .........HEHEHE ..Booooom.

Good point, Pro. If oxygen could burn, the earth would have lit off a long time ago.

Just for grins and chuckles, here's the data from the MSDS:

EMERGENCY OVERVIEW
Elevated oxygen levels may result in cough and other pulmonary changes. High concentrations of oxygen (greater than 75%) causes symptoms of hyperoxia which included cramps, nausea, dizziness, hypothermia, ambylopia, respiration difficulties, bradycardia, fainting spells and convulsions capable of leading to death. Nonflammable. Oxidizer, will accelerate combustion.

I believe that oxygen can burn when mix with a combustible gas.

If that were stated: "I believe that oxygen can cause a combustible gas to burn," I'll agree. Otherwise, I stick to my statement.

The latent heat of vaporization thing was a good call.
I think water can absorb ten times the energy in changing phase as it normally absorbs in a one degree temp increase.
Or maybe it's 100 times.
eh, I don't remember.

In my field of work as a chemical plant operator we feed what we call a synthensis gas generator pure O2 and methane. The generator is preheated to above 1000 deg C prior to the introduction of the O2, methane feed so that auto ignition takes place. The result of the reaction of the methane and 02 is a Carbon Monoxide/ Hydogen gas mixture. The generator runs about 1350 degrees C when it's operated properly. To control the temp of the generator we limit the amount of O2 we feed it by monitoring what we call the methane slip, which is the amount of unburned/unreacted methane left in the system after the reaction of methane and O2 takes place. If we were to feed too much O2 the reaction in the system would start making CO2, carbon dioxide instead of CO. The CO2 reaction produces a whole lot more heat than the CO reaction, if we fed too much O2 for even just a few seconds we could have a melt down on our generator.
My point here is really combustion is a reaction between oxygen and a hydrocarbon that requires an ignition source that produces pressure and heat. The more O2 the hotter the reaction will be because of the production of CO2 a much hotter reaction. If you run too rich then you won't react the Carbon to CO and then you end up with a bunch of soot or carbon build up in the system.
I also argree that in a internal combustion engine you need a certain amount of excess fuel for cooling purposes.
I am no scientist and maybe someone who can explain this better to me.

The answer, as you've acknowledged, to your original question is in Jack's first response.

Other concepts to play with in visualizing what's going on are;

-Only vapors will burn.

-When talking about atoms, e.g. oxygen, carbon, hydrogen it isn't the atom itself that burns, after combustion the compound structure that contained those atoms has changed state, but those atoms are still those atoms. They may rejoin with other atoms to form other compounds, e.g. CO2, CO, H2O, NOX, etc from combustion of gasoline (a range of various size HC molecules, which also contain other junk, combined with atomospheric air, O, N, H20, +other junk)