Why lean AFR results in extra heat?

IG.

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A well known fact. But why? I searched the web and couldn't find a clear and logical explanation.
 
Molecules are spread out and you don't get as much oxygen into the engine so many lean the fuel to compensate so that the fuel to air ratio isn't as rich.
 
That is a great question. I would imagine that it has something to do with less fuel to cool the air in the compression chamber. I know when you are running fat there is just too much fluid flying around and things do not dry off completely all the time and can foul the plug or just make it miss a lot. Lean probably ignites faster, burns hotter because there is less liguid to impede it? Just an educated guess on my part. I am interested to see what pops up here.
 
i had to read the question a few times. so you're saying on a hot day you're seeing a lean afr mixture?
 
I am assuming he means that it is a well known fact that running an engine lean is dangerous because it generates enough heat to blow a hole in a piston. He is wondering why that happens in the combustion chamber. I could be dead wrong though. :laugh:
 
I am assuming he means that it is a well known fact that running an engine lean is dangerous because it generates enough heat to blow a hole in a piston. He is wondering why that happens in the combustion chamber. I could be dead wrong though. :laugh:

You are exactly right!
 
just like starting a fire, more oxygen, hotter burn. more air than needed, too hot.

Wrong analogy - when starting a fire there is an excess of fuel and adding oxygen simply makes the ratio more appropriate.

The original question was why.
 
I'll give this a stab!

Think air cooled carberated motors..

The raw cool fuel that is being sucked into the motor is cooling the cylinder.

High compression carb'd harley's are jetted richer in the rear cylinder to compensate for less air flow

Apply that to a FI and higher tolerances than carb'd injection. You get more friction, higher heat. Hot enough to cause detanation(spell check).

If so hot, it will skip right to warping cylinders, floating valves, super heating the lubricant around bearing races in the head and tada!




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Im sure it has something to due with the fact that when air is compressed it heats up, why? I dont know, may have some thing to do with the other gases found in "air"
 
Below seems to make most sense to me (#3 and #10 above touch on it also). Copied from here: http://www.eng-tips.com/viewthread.cfm?qid=101915&page=5

When running rich, the evaporative effect of excess fuel tends to absorb combustion chamber heat, which will thus lead to lower combustion temperatures and help reduce detonation/pre-ignition.

Based on above, the opposite happens with a LEAN AFR.
 
No winners so far. I read the link, but things just make no sense there. Evaporation of the fuel may absorb a little bit of heat, but don't tell me that the normal burning generates so much heat that things would melt if not for evaporative cooling.

When the bike idles, I don't smell any gasoline coming from the exhaust - which means all of it burns out.

I will give an example which I think is relevant, but I have no idea how to explain it. If anyone seen or knows gas welding, acetylene and oxygen are mixed together and the flame is used to weld metals. What I've observed years ago that welders use the following technique to cut metal. The cutting starts with the mix of acetylene and oxygen, but once the area is red hot, the welder would shut off acetylene completely leaving only the flow of oxygen, and continue cutting. Despite the absence of the fuel, the metal gets white hot and the welder is able to produce pretty long cuts. It literally looks like bare metal is burning in oxygen. Essentially it would seem that at really high temperatures, oxygen would just eat anything away, and I strongly suspect that's what might be happening in the engine when an unburned oxygen is present.
 
I will give an example which I think is relevant, but I have no idea how to explain it. If anyone seen or knows gas welding, acetylene and oxygen are mixed together and the flame is used to weld metals. What I've observed years ago that welders use the following technique to cut metal. The cutting starts with the mix of acetylene and oxygen, but once the area is red hot, the welder would shut off acetylene completely leaving only the flow of oxygen, and continue cutting. Despite the absence of the fuel, the metal gets white hot and the welder is able to produce pretty long cuts. It literally looks like bare metal is burning in oxygen. Essentially it would seem that at really high temperatures, oxygen would just eat anything away, and I strongly suspect that's what might be happening in the engine when an unburned oxygen is present.

This is very wrong.
Actually the acetylene fuel is never turned off in oxy/act cutting. The welder would get oxygen and acetylene to the proper mix (flame) and would heat the metal. Once the metal was begining to liquify, a third oxygen lever is used to blow the liquid out of the hole to begin the cut. The Oxygen/acetylene fuel mixture is on all the time to continually heat and keep the metal molten just ahead of the cut. Once the acetylene (fuel) is off, the metal immediatly begins to cool.
 
Oxygen burns hotter than fuel, running lean burns too much oxygen. My torches at work show exactly how lean is hotter, you cut using more oxygen. Your engine runs more power lean than rich because not all the fuel will burn without enough oxygen resulting in less expansion, less expansion = less Hp/tq. If your running so fat that your cylinder is wet, get ready for hydrolock because you have a serious problem.
 
I did read the post b4 mine and by cut using more oxygen I'm referring to the heat, not using the cutting valve/lever. For those who know, a rosebud tip uses much less oxygen.
 
Sum Beach, IG is correct you can cut after the puddle of moltent metal has formed. In normal cutting operation no you don't turn of the acetylene but you can. I have done it you must have a very steady hand but it can be done. In fact if anyone remembers the old pellet torch sets that's exactly how you cut by turning off the fuel. Sum Beach you are correct about there being a second jet of air that actually does the cutting. But once you get cutting you can actaully turn the acetylene off. Try it some time it works but like I said you need a very steady hand because if you loose the puddle your done. The oxygen actually causes the metal to burn look at the slag it's burnt metal.
 
From wikipedia


In oxy-fuel cutting, a cutting torch is used to heat metal to kindling temperature. A stream of oxygen is then trained on the metal, and metal burns in that oxygen and then flows out of the cut (kerf) as an oxide slag.[2]
 
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