Lean mixture is hotter...why?

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12,385
Location
Northern CA
quote:
Originally posted by tom slick: only gas engines run hotter, it's because there is more oxygen to burn up. diesels and turbine engines are the opposite, they run hotter when you add fuel.
Until you overfuel, then they run cooler. Same principle, use more fuel than you can burn and it cools things down. Diesels start cooling down at about 80% of stoic. Diesels alway run way on the lean side of stoic. They can't use the air as efficiently as gas engine which run great at stoic. That's why a naturally aspirated diesel is doing good to devlop 80% of the torque of a naturally aspirated gas engine of the same displacement.
 
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354
Location
Chicago
The extra unburnt fuel(when running rich) has a cooling effect by absorbing the heat of combustion. Remember at full load the combustion chambers and immediate areas are way hotter than when just cruising around To burn all the fuel in a gasoline car, all you need is the 14.7 to 1 mixture, or less, as during part throttle cruise. At full throttle stock cars have been engineered to go lower than even a 12 to 1 mixture to help 'spark' the combustion mixture easier and of course the cooling effect of evaporation.
 
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354
Location
Chicago
The extra unburnt fuel(when running rich) has a cooling effect by absorbing the heat of combustion. Remember at full load the combustion chambers and immediate areas are way hotter than when just cruising around To burn all the oxygen in a gasoline car, all you need is the 14.7 to 1 mixture, or less, as during part throttle cruise. At full throttle stock cars have been engineered to go lower than even a 12 to 1 mixture to help 'spark' the combustion mixture easier and of course the cooling effect of evaporation.
 
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1,357
Location
California, USA
Detonation can burn a hole in a piston, not running lean itself. Carbon deposits or the wrong spark plug is often the culprit. As was correctly stated by others, peak combustion temperature is at ideal mixture, which also results in highest NOx emissions. In piston aircraft engines mixture is adjustable from the cockpit and is often measured by exhaust gas temperature probes near the exaust ports, or EGT. Running "lean of peak" is acutally where you want to be at part throttle. All EFI car engines run that way for economy and emissions. You need a strong spark to avoid "lean misfire" and high HC emissions here. Rich of peak is used at full throttle for maximum power. With the poor cylinder to cylinder mixture distribution of a carburator and manifold, you cannot to this very accurately. Even mechanical port fuel injection can be way unbalanced.
 
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39,805
Location
Pottstown, PA
quote:
Originally posted by TallPaul: How is it that running lean can burn a hole in a piston (at least I have heard that can happen)?
The way it was explained to me, there's an insulating/cooling layer of unburned fuel during the combustion cycle when you're at 'stoic balance (not ust at a fuel enriched condition) If you run leaner then that ..this cooling/insulating layer is spent.
 
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1,203
Location
Oregon
quote:
Originally posted by Palut: I'm just curious as I've never been able to figure it out. Why does an engine run hotter when running lean?
Its my understanding as far as gasoline engines. Under ideal conditions (not excessively lean) a boundary layer of gasoline/air mixture just above the metal surfaces keeps the flame front from actually touching the metal surfaces. If for what ever reason such as detonation and or not enough gasoline in the mixture this boundary layer gets blown away. Metal parts would be unguarded, thus exposed to the actual flame. Hence the hotter running.
 
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Location
Austin Texas
quote:
Under ideal conditions (not excessively lean) a boundary layer of gasoline/air mixture just above the metal surfaces keeps the flame front from actually touching the metal surfaces.
Correct, this boundary layer is a dozen microns thick. This thin boundary layer keeps the 2000dF burn temperature from reaching the combustion chamber walls. When detonation occurs, the pressure rise is rapid enough to wipe this film away from the combustion chamber walls and allow the 2000dF+ gas temperatures to 'interact' with the aluminum and steel surrounding them. This leads to a lot of heat arriving in the cooling systems.
 
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1,357
Location
California, USA
A lean mixture has an excess of air to cool things down. The "boundary layer" always exists regardless of mixture, except when detonation blows it away. A large amount of excess fuel also cools, but be careful. At slightly rich, the maximum power area, produces the most heat. Cylinder head temperatures peak 25-50 degrees rich of peak exhaust gas temps.
 
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1,357
Location
California, USA
Im afraid I dont fully understand your question. Valve burning is a completely different animal than burning a hole in a piston. Exhaust valves are subject to maximum heating when open and hot gasses are flowing past, not during the power stroke, when the valves are closed and in full contact with the seat, which is what cools them. Deposits that build to the point of sticking the valve slightly open is the usual cause of burning. Severe spikes in cylinder pressure in a detonation event damages pistons. I'll try post a link to a graph of CHT and EGT vs mixture ratio if anyone is interested.
 
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39,805
Location
Pottstown, PA
Well, exchange valve for piston if you please. The focus of the question was the differentiation between a balanced (not lean) ruptured flame front and a fragmented flame front from too lean a mixture. Both result in a collision of multiple flame fronts.
 
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Location
Mizzou-land
Gary Allan, Now you have me confused. I don't understand your question. Where did the fragmented flame front enter into the discussion? With a closed, sealed valve, you can consider the valve surface (combustion side) in the same way that you consider the piston surface. Detonation, from all definitions that I know is without a flame front. During detonation, combustion is driven by the rate at which a compressed wave front moves through the medium (as opposed to the rate at which heat is transferred). My understand is that detonation occurrs as a result of combustion moving through a combustable at rate that exceeds the material's abilitily to expand. As a result, much of the material expands at near unison, post combustion - big boom...
 
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39,805
Location
Pottstown, PA
So I can run as lean as I please ..and as long as no detonation occurs ...there's absolutely no problem, correct? Nothing ..prehaps other then NOX output.
 
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1,357
Location
California, USA
That is correct. Your limit on leaning is misfire from too weak a mixture to ignite with the available spark energy. When flying simple airplanes without EGT instrumentation, we just lean until misfire, then richen only enough for smooth running. Of course that is steady state cruise. We go full rich for climb power, at lower altitudes. Carbed cars and trucks when trying to accelerate may "hesitate" when jetted lean. That is what accelerator pumps are for. Also power valves automatically richen at full or near full throttle (low vacuum). Those two may need adjustment when jetting lean.
 
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39,805
Location
Pottstown, PA
Oh ..I'm not ignorant of the power valve and the accellerator pumps of our somewhat antiquated carbs. I just wanted qualification on lean conditions having no deleterious effect on chamber internals. It was explained to me, by another aero-litterate fellow, that it was simply due to the exhaustion of the insulating layer of unburned fuel. You are the first to say that this exists in all combustion situations for the duration of the entire combustion cycle regardless of the density of the air:fuel charge. This was the first time that I've heard that detonation is the only source of temperature related damage. If you don't hear anything ..and have damage ..it is only due to inaudible detonation. [ July 07, 2006, 08:04 PM: Message edited by: Gary Allan ]
 
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