Ok guys I have a question that I found in Thermo II. In this class we were studying Adiabatic Flame Temperature "Yeah I know I can't spell lol, sorry" and we talked about Stoichmetric A/F situation. The teacher said that the AFT is the highest available temperature and occurs at Stoichmetric. Then he said that if you go any richer or leaner than stoichmetric then the temperature would fall. I was ok with it falling due to being richer but my question lied with the temp falling with respect to the leaning of the condition. So I asked him why this would occur. He said that because air is not pure oxygen things like nitrogen would act as a heat sink and would capture some of the heat. Other words once all the fuel available is oxidized
"stoichmetric situation" then adding oxygen would not help raise the temp, but the extra nitrogen and etc would capture some of the heat. So this made since and caused another question to arise. I have always been told that if you run an engine to lean then you will burn up rings. Why is this because according to thermo shouln't the temperature be less in a lean condition? I am sorry for all the miss spelled words. Thanks for any help.
,Chase

 

i have a turbo car that makes more power when i get it close to stoich. Alot of ppl here run a way too rich of a mixture to be on the safe side from detonation. Imo when u go lean there is less fuel in the chamber or tiny droplets of liquid (vapor) to absorb the heat in the chamber to combat heat. As u know it is harder to heat or cool a liquid than a gas.

 

That is the only thing my teacher could come up with also. He told me he was not sure though but that was an educated guess. Any one else?
Thanks

 

Extra fuel takes away heat in the combustion chamber and moves it into the exhaust and then atmosphere. Without that extra fuel, the chamber will get really hot. This is only at high engine load though.

That's what I learned.

Maybe what your instructor is saying is that being at EXACT stoic ratio, the maximum amount of fuel and air is burned. Any extra air molecule could be replaced with a gas, and vice versa. The more air fuel mixture you burn in a closed chamber, the higher the temps will climb.

 

Not just rings, but pistons and cylinder head, too.

There is an insulating boundary(very thin) of air/fuel that is protecting the parts from high temperatures. Introduce detonation(much less than audible knock) from lean cylinder conditions and it disturbs/disrupts/penetrates the protective boundary layer, resulting in the high temperatures reaching the parts. It'll burn stuff left and right.

 

Keep in mind that the maximum flame temperature is reached at stoic. conditions. This is still true for the combustion of gasoline.

You are then attempting to apply this fact to “tribal knowledge” of internal combustion engines. Tribal knowledge states that “if you go leaner, the engine will overheat and you will break stuff”.


Stoic A/F ratio for gas is something like ~14.6
Most engines will run with an A/F ratio around 12, which is on the fuel rich side of stoic. When you "lean" out an engine, you are getting it closer to the magical stoic A/F ratio, and thus higher flame temperatures.

An additional factor is the cooling effect of the excess fuel as we burn it. Take away fuel from the mix (lean), and the cooling effect of the fuel is reduced – causing temperatures of combustion chamber components to go up even more.

Basically, leaning out the mix gets you closer to the stoic burn conditions which has a higher flame temperature. It also reduces the cooling effect of the fuel itself on combustion components.

Gearheds usually don’t talk in terms of “adiabatic flame temperature”, so they stick with what they know – cause and effect.

Lean out the mix and the pistons melt…

 

Thanks guys these are the answers I am looking for.

 

When I proposed this question to my Internal Combustions teacher, he stated that the reason the temperature increases during a lean condition is due to Maximum Break Torque (MBT) timing. If the mixture is lean, the timing needs to be retarded in order to combat a pressure rise before TDC. This pressure rise will decrease the heat transfer rate and increase cylinder temperatures. Modern day cars do NOT run at ~12:1 A/F (unless you were talking about WOT or maximum load). At idle and part throttle applications, they are running at stoichiometric (14.7:1) or leaner for best fuel economy (lean cruising conditions can be 15-16:1 A/F, which is a good indication of what can be allowed with timing adjustments). Under WOT situations is when you will see a richer A/F to introduce evaporative cooling (cooling the cylinder off and increasing power) due to the excess fuel. Running stoichiometric or leaner at WOT would cause a power loss and will increase temperatures, but it isn't because of the theory that is being taught to you in class. Theory is much different than real world applications, and I found it hard understanding the stuff that went against common knowledge (like what you've brought up in this thread). Hope this helps.

 

Ok – fair enough. But nobody is talking about an engine at idle (no load) conditions. An engine at low or no load conditions can easily run at stoic (14.7) or even leaner than stoic to save fuel. You can generally go extreme rich or extreme lean on an engine at idle as far as you want and not melt pistons. The engine combustion process begins to break down and the engine just begins to stumble and fall on its face or bog.

It’s the conditions under load that are critical for the A/F ratio. Richer mixtures are required under load to make optimum power and limit combustion chamber temperatures. Try taking an engines A/F ratio up near 14 on a dyno under WOT load and see what happens.

Combustion chamber temperatures are peak under max loading conditions due to a number of factors, including increased friction, combustion cycle frequency, cylinder pressure, ect. It is necessary to take advantage of the evaporative cooling effects of a rich mixture to help limit combustion temperatures under load.

The O2 and other sensors typically will keep the A/F mix around stoic (14.7) under low and no-load conditions to maximize fuel economy and reduce emissions. The computer will then begin to richen the mix as load is applied. At WOT the O2 sensors will be ignored and the system goes into an “open loop” condition to provide the optimum fuel rich condition.

The key concept here is that adiabatic flame temperatures are indeed the highest under stoic. conditions. However, combustion chamber peak temperatures occur under peak load and require a rich mixture to maximize power and minimize combustion chamber temps.

Do not confuse max adiabatic flame temps with max combustion chamber temps.

 

Thats just what I was going to say that the temperature in the combustion chamber is a function of the temperature of the combustion reaction, but they are not one in the same. It would be true that the as the conditions approach stoich the reaction would generate the most heat (as would any exothermic reaction) because it is a balanced reaction, if anymore reactants were introduced they would not contribute to the reaction and therefore not create more heat.

 

The biggest part of the game is flame travel speed. It is exactly why mixtures leaner than 14.7 can be "hotter" to internal parts. Expansion ratio is greatest within the first 48 degrees or so of crank rotation ATDC. When most of the combustion event hasn't occured in this time of quickly expanding volume the reaction has no option but to release energy in the form of heat (instead of expansion). Same reason why late ignition timing can also burn exhaust valves, etc.

 

Some great posts on here, but apparently we've lost track of what he is asking.

His teacher was discussing adiabatic flame temperature.

He was thinking this applied to internal combustion.

Unfortuntely, the air/fuel mixture is swirling and moving quite rapidly in a combustion chamber and therefore completely changes what air/fuel ratio the highest flame temperature is achieved at.

Remember, adiabatic is measured (calculated actually, it is impossible to accurately measure adiabatic temperature with current technology) in a zero air/zero flame movement enviroment. This is not the case in an internal combustion engine.

But, yes. In the perfect enviroment, the highest flame temp is reached at stoich. Just not in an internal combustion engine or any combustion device in which the air, fuel, or both are in motion.

This is something you have to deal with when designing boilers and furnaces. To increase your rate of temperature rise, you lean the mixture out to increase the flame temp. We then run a little extra air once the temps are reached for "overair" (emissions control). But, like I said, this is only possible due to the fact that the combustion mixture is in high motion.

 

thermo and textbooks aside, my understanding is the reason going lean burns up rings and pistons is because too much power is extracted for a given displacement. Take a 350 c.i.d chevy v-8 outputting 350 hp, that would be 1 hp per cubic inch of displacement. Then look at a briggs & stratton lawn mower engine, around 10 cubic inches and let's say it outputs 5 hp on a good day, that's 2 cubic inches per hp, 2x greater. And also think about a turbo 4 cylinder getting more an 1 hp per cubic inch, or any forced induction or nitrous motor where broken/burned rings pistons and valves are common.
For greater amounts of power per cubic inch displacement, the greater the byproduct of heat from combustion and when under load there's a much narrower operating window regarding air/fuel ratio and timing because those two are used to manage heat.

regarding air fuel mixture and timing, lean mixtures burn slower than rich mixtures. Lean mixtures need more timing advance for complete combustion and maximum cylinder pressure for that given combustion cycle to happen near piston top dead center and not long after top dead center.
If you did not advance timing for a lean (greater than stoich) mixture, then there would be less cylinder pressure anytime before and at top dead center versus aricher mixture (but with less timing advance). It is incorrect to say "If the mixture is lean, the timing needs to be retarded in order to combat a pressure rise before TDC."

Whether or not the lean mixture regardless of timing, or a rich air/fuel mixture with too much advance and cylinder pressure burns rings and pistons would mostly depend on the amount of power extracted per displacement and the heat from combustion. Going back to the briggs and stratton 5hp from 10 cubes, you could lean that out to the point where you're just getting less power or use a richer air/fuel ratio and max timing advance for greatest cylinder pressure at top dead center. It wouldn't matter and you would never hurt it because the amount of power for that given displacement and heat generated is within limits of what the rings, piston, and valves can handle, and you don't need to varying air/fuel ratio (going richer) and retard timing (less cylinder pressure) to manage heat.

worth reading:
http://www.corvette-restoration.com/.../Timing101.pdf

 

I agree with you that the mixture needs more timing advance for complete combustion and maximum cylinder pressure. What I was stating, OTOH, was as cylinder pressure goes up (from the increase in timing advance), you lose some of your heat transfer and the cylinder temperatures will go up. The complete combustion will cause a higher temperature, as well (full release of the mixture's energy). I only stated that less timing advance would help with a pressure rise in the cylinder (directly correlated with in-cylinder combustion temperatures). Too much timing (or too lean of a mixture) causes detonation. Detonation is caused by high cylinder pressures and temperatures causing a premature mixture ignition and flame propagation. I may have been unclear in my previous post, but it made sense to me.