I am curious. Have heard that lean mixtures can burn the exhaust valves, and with nitrous, a lean mix can burn holes in pistons. For lack of fuel (combustible material) in the presence of the extra oxygen, do metal parts become the combustible material?
Why does lean mixture burn exhaust valves?
- Thread starter TallPaul
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TallPaul,
Its that lean mixtures burn at much higher tempertures than richer one. Ups the temperature in teh cylinder so much that valves and seats cant keep shape. The exhaust valve only gets to cool when on the seat with a little help from the valve guide.
That's one reason for an egr valve, it keeps the peak temperature in the cylinder down.
Hope this helps.
Dan
Its that lean mixtures burn at much higher tempertures than richer one. Ups the temperature in teh cylinder so much that valves and seats cant keep shape. The exhaust valve only gets to cool when on the seat with a little help from the valve guide.
That's one reason for an egr valve, it keeps the peak temperature in the cylinder down.
Hope this helps.
Dan
Ever seen a blacksmith foundry?? When he add more oxygen than fuel, it gets much hotter. Same goes in a engine. More oxygen to fuel give more heat.
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Everyone is correct ...but mssparks came closest to dispelling your confusion.
Don't confuse potency with combustion temperature.
now don't forget the very important point that this is only true for a gasoline powered engine. diesel fuel works in the exact opposite way, it gets hotter the more fuel you dump in, unlike a gasser which runs cooler as your mixture gets over rich.
-Bret
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Hottest combustion is at Stoichiometric, when there is exactly the right amount of oxygen to consume all of the fuel.
Anything richer (which is the side that spark Ignition engines generally run), or leaner (where diesels run) is cooler than Stoichiometric.
In both those cases, as you approach stoichiometric from either of their respective sides, you get more heat.
Anything richer (which is the side that spark Ignition engines generally run), or leaner (where diesels run) is cooler than Stoichiometric.
In both those cases, as you approach stoichiometric from either of their respective sides, you get more heat.
I guess I have a different understanding of how this works vs. some of you guys. My understanding is, lean mixtures don't burn hotter.
As you continue to lean the mixture from too rich the combustion temperature and engine power output will rise up to the point where mixture is optimum. If you continue to go futher lean, they will then fall. This point of maximum combustion temperature and power output is typically called "lean best power."
In every engine there remains a thin layer of unburned air/fuel on all the surfaces that are exposed to combustion. This layer acts as a insulator to slow heat transfer between the combustion gasses and the engines surfaces. Too lean a mixture causes this buffer layer to be burned off resulting in greater heat transfer to and overheating of these parts. Parts like exhaust valves for instance...
Anyone here ever fly in a light aircraft that has an exhaust gas temperature gauge and manual mixture control? That's how you tune them for cruise at altitude. You lean the mixture until the exhaust gas temperature falls, then richen it a slight amount to get a small decrease in egt from the highest point, but on the rich side of optimum.
I suspect jetting on a dyno for maximum power by using an egt gauge works in the same manner though I've never done it...
[ February 22, 2004, 09:55 PM: Message edited by: jsharp ]
As you continue to lean the mixture from too rich the combustion temperature and engine power output will rise up to the point where mixture is optimum. If you continue to go futher lean, they will then fall. This point of maximum combustion temperature and power output is typically called "lean best power."
In every engine there remains a thin layer of unburned air/fuel on all the surfaces that are exposed to combustion. This layer acts as a insulator to slow heat transfer between the combustion gasses and the engines surfaces. Too lean a mixture causes this buffer layer to be burned off resulting in greater heat transfer to and overheating of these parts. Parts like exhaust valves for instance...
Anyone here ever fly in a light aircraft that has an exhaust gas temperature gauge and manual mixture control? That's how you tune them for cruise at altitude. You lean the mixture until the exhaust gas temperature falls, then richen it a slight amount to get a small decrease in egt from the highest point, but on the rich side of optimum.
I suspect jetting on a dyno for maximum power by using an egt gauge works in the same manner though I've never done it...
[ February 22, 2004, 09:55 PM: Message edited by: jsharp ]
Looks like I posted before reading your post
[ February 22, 2004, 09:53 PM: Message edited by: jsharp ]
I'm afraid that is an OWT that has caused many aircraft engines to not last to TBO (time before overhaul). If you have CHT (cylinder head temperature) as well as EGT (exhaust gas temperature) you will see that just rich of peak EGT results in peak CHT. Unfortunately that is also best power. That is why you go full rich for takeoff, close to max power, but richer to stay out of the danger zone of maximum cylinder heat. Best engine life and economy is to be on the lean side of peak EGT, but before lean misfire, resulting in much lower head temperatures and best cruise economy. That is where modern automotive injection systems run for economy and lowest emissions. Burned exhaust valves are a rarity in these modern automobile engines.
JSharp,
It's called "boundary layer cooling" and most liquid rocket engines are fuel rich on the walls to keep the combustion chambers and nozzles intact ....Of course, it works a bit better when your fuel starts out at -423F
As gas or alcohol fuels evaporate, cooling is also provided from the phase change from liquid to gas ...
Why do I sometimes feel like "Dr. Science" on NPR?
It's called "boundary layer cooling" and most liquid rocket engines are fuel rich on the walls to keep the combustion chambers and nozzles intact ....Of course, it works a bit better when your fuel starts out at -423F
As gas or alcohol fuels evaporate, cooling is also provided from the phase change from liquid to gas ...
Why do I sometimes feel like "Dr. Science" on NPR?
I'll defer to you on small aircraft since my memory is vague this. I clearly remember watching the changes as a function of mixture but I don't recall also watching cht. Probably because I was the one doing it and it wasn't explained completely to me.
From this we've yet to explain the problems with exhaust valves and lean mixture though...
[ February 24, 2004, 12:43 AM: Message edited by: jsharp ]
i have always been told lean = hotter on gas engines. why do internal engine parts melt and disfigure with a lean mixture?
turbine engines run hotter the more fuel you give them. then again 80% of the air that goes in the front comes out the rear unburned.
turbine engines run hotter the more fuel you give them. then again 80% of the air that goes in the front comes out the rear unburned.
here is another piece of diesel trivia. they have no throttle. the regulate fuel to make more power and take in as much air as needed. that is why they are so repsonsive to forced induction.
Yeah, and you can keep pumping the fuel to them after the intake valve has closed, as done by Gale Banks to create a Corvette beating diesel pickup: http://www.bankspower.com/sidewinder-muscleflexing.cfm
[ February 25, 2004, 12:02 AM: Message edited by: TallPaul ]
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There is just no arguing with Jimbo and jsharp. That's too high powered rhetoric to combat.
It was somewhat in conflict with some of my training so I consulted my notes on a somewhat related topic. It was still combustion ..which is universal somewhat.
Combustion produces CO and CO2
To reduce your CO you could just lean the mixture. This works fine ..but if you're over 1500 degrees NOX forms.
Naturally we all know that we deal with NOX with EGR ...or in the case of fuel oil FGR to cool the combustion process (actually take of space with inert material to "lean" it without adding air).
My false assumption was that the high NOX production was cause by increased temperature ..and not the inherant combustion temperature of the given fuel with just the addition of air.
Interesting triangle that CO, CO2, and NOX form in combustion.
[ February 25, 2004, 11:34 PM: Message edited by: Gary Allan ]
It was somewhat in conflict with some of my training so I consulted my notes on a somewhat related topic. It was still combustion ..which is universal somewhat.
Combustion produces CO and CO2
To reduce your CO you could just lean the mixture. This works fine ..but if you're over 1500 degrees NOX forms.
Naturally we all know that we deal with NOX with EGR ...or in the case of fuel oil FGR to cool the combustion process (actually take of space with inert material to "lean" it without adding air).
My false assumption was that the high NOX production was cause by increased temperature ..and not the inherant combustion temperature of the given fuel with just the addition of air.
Interesting triangle that CO, CO2, and NOX form in combustion.
[ February 25, 2004, 11:34 PM: Message edited by: Gary Allan ]
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