Air: Fuel, Emissions and spark timing.
11-14-2005, 09:55 AM
#21
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Originally Posted by Texas_WS6
Let me throw something in here for you to feed on. I work in the emissions industy. 14.7:1 on gasoline is what is called stoich. Stoich is were all the fuel and air is mixed perfectly and when burned makes no emissions what so ever. Because all fuel is burnt at stoich (14.7:1) then this is were the max cylender temp is created. Lets define lean and rich now. Rich is anything with more fuel then Stoich. So 11:1 is rich of stoich. Lean is anything with less fuel then stoich say 17:1 The terms lean and rich have always been used loosly. To say you are now running 11.8:1 and you leaned it out to 13:1 is correct. But for this topic please stick with lean and rich with 14.7:1 being stoich, perfect burn, max heat.
Again, 14.7:1 is were the max heat is generated, so if you go lean or rich of stoich the cylender temps will drop. You will not cause preignition going leaner of stoich, you will simply loose cylender temp, loose power, and have missfires. Going rich of stoich will cause detination, which will either melt or crack pistons and piston rings. Once you go rich enough (around 13.1) you will not detinate anymore(stock ion). The next question is load while running on stoich. When your cruise control is on or at a constant speed there is not enough load to cause detination to happen while running at stoich (14.7:1). But when you throttle the engine and incress the load it will, so the computer goes into power enrichment, which injectes extra fuel and richens the fuel mixture up from 14.7:1 to 13:1 or so. So now you have enough fuel to keep the engine from detinating while under full load.
Lets explain detination and what causes it. First off there is no such thing as predetination. You never want detination, so when it happens in never happens before you wanted it to happen.
Detination is were the cylender pressures incress, causing the cylinder temps to incress to were all of the fuel in the cyl ignites all at once. this causes the peak cyl pressures to skyrocket past safe limits and the temps to skyrocket. This is a bad thing.
What causes this situation? Timing can cause it, ion ratio is a big factor, jacket water temp, load, octane of the fuel (octane is a way do describe a fuels flashpoint so to speek, not the amount of energy it has stored in it).
So long story short, if you set the Air Fuel Ratio of the engine to cruise at 17:1 it should not harm the engine as far as detination goes. If you set the power enrichment to be 17:1 at WOT it will probly have very little power. But when you have the cruise set for 17:1 and then go WOT, the AFR might dwell in the 15.1 to 13.1 range too long before it gets below 13:1, and could destroy your engine.
Personaly if you want better gas milage, buy a volvo and sell me your gas gusler.
Again, 14.7:1 is were the max heat is generated, so if you go lean or rich of stoich the cylender temps will drop. You will not cause preignition going leaner of stoich, you will simply loose cylender temp, loose power, and have missfires. Going rich of stoich will cause detination, which will either melt or crack pistons and piston rings. Once you go rich enough (around 13.1) you will not detinate anymore(stock ion). The next question is load while running on stoich. When your cruise control is on or at a constant speed there is not enough load to cause detination to happen while running at stoich (14.7:1). But when you throttle the engine and incress the load it will, so the computer goes into power enrichment, which injectes extra fuel and richens the fuel mixture up from 14.7:1 to 13:1 or so. So now you have enough fuel to keep the engine from detinating while under full load.
Lets explain detination and what causes it. First off there is no such thing as predetination. You never want detination, so when it happens in never happens before you wanted it to happen.
Detination is were the cylender pressures incress, causing the cylinder temps to incress to were all of the fuel in the cyl ignites all at once. this causes the peak cyl pressures to skyrocket past safe limits and the temps to skyrocket. This is a bad thing.
What causes this situation? Timing can cause it, ion ratio is a big factor, jacket water temp, load, octane of the fuel (octane is a way do describe a fuels flashpoint so to speek, not the amount of energy it has stored in it).
So long story short, if you set the Air Fuel Ratio of the engine to cruise at 17:1 it should not harm the engine as far as detination goes. If you set the power enrichment to be 17:1 at WOT it will probly have very little power. But when you have the cruise set for 17:1 and then go WOT, the AFR might dwell in the 15.1 to 13.1 range too long before it gets below 13:1, and could destroy your engine.
Personaly if you want better gas milage, buy a volvo and sell me your gas gusler.
If max heat/power we made at 14.7, you are saying that cars run richer at WOT because at 14.7 its too much heat? So richening is not for power but but rather for engine durability? this goes against everything i've read in aviation guides. Typically, their max temps come at 12:1 or so- but those are EGTs. What you are saying (combustion temps) coincides with with the chart posted above.
This makes sense. peak EGT is at 12ish:1 but peak combustion temps are at 14.7:1. The difference is because with more fuel, slower burn and some of the unburnt /later burning fuel heats up the exhuast instead of being aborbed in the walls/power.
As far as selling the truck, i'm not sure. This is the first vehicle i've modified that hasn't given me an ounce of trouble short of a single O2 sensor backing out 2000 miles later due to not being tightened enough durring the engine swap. 20 seconds with a wrench fixed that.
But hey, what isn't for sale!
11-19-2005, 10:38 PM
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TXNC5 has posted some good information based around his experiences. Good read.
There is a lot being said, some is really good and some is totally wrong. Don't want to step on any toes but considering that treyz28 started this thread I'd like to direct my attention at a few points he's made.
Since we're trying to understand "why they just won't run", why don't we elaborate on the issues a lean burn engine is faced with?
I'd categorize the three prime factors as;
1. Time losses: Inability to complete combustion before blow down
2. Lean flammability limit (LFL): Leanest a/f ratio whch will sustain flame propogation
3. Lean ignition limit (LIL): Minimal external energy that must be supplied to a critical volume to raise the a/f to minimal ignition temperatures
All of these play an important part in the fine line between normal combustion and unstable combustion/misfire. I don't want to write a book but will try and include examples in my replies.
I think the reason why so many people are confused about temperatures is in the fact that they never define WHERE/WHEN in the process they'd like to examine them.
Combustion temperatures will continue to decrease as a/f ratios lean because excess air promotes oxidation in the combustion process. If we limit the discussion to piston failure due to excess heat then we are talking about detonation and pre-ignition. Lean burn engines are very susceptible to this. Lean mixtures burn slower (LFL) and combustion temperatures are often higher later in the cycle. Metal surfaces have less cooling time before the next charge and with all these factors at work, combustion can get out of control in a hurry.
I'm sure we all know what happens when detonation occurs in regards to combustion temperatures. Comb. temps are markedly high and EGT temps are low. Where does the heat go?
Why are we surprised by thermal breakdown in this situation?
Quite the opposite.
As a/f ratios increase, flame speeds slow, time losses increase and ignition becomes more difficult. Combustion temperatures are lower and dependent on a/f ratios at various loads, the EGT's may be markedly higher. The less complete the combustion, the higher the EGTs.
"Lean" or pre-ignition/detonation? 
NOx actually reaches an equilibrium around stoich but shows a decrease at leaner a/f ratios. Hydrocarbons will descrease until unburned mixtures are exhausted. We go back to time losses, lean flammability limits and lean ignition limits.
You should be able to find a number of papers on this subject in the NACA archive.
Good luck.
There is a lot being said, some is really good and some is totally wrong. Don't want to step on any toes but considering that treyz28 started this thread I'd like to direct my attention at a few points he's made.
leaner after about 14:1 or 15:1, combustion temps go down. engines will typicall run until about 16.5:1 or 17:1. Then they just wont run.
I'd categorize the three prime factors as;
1. Time losses: Inability to complete combustion before blow down
2. Lean flammability limit (LFL): Leanest a/f ratio whch will sustain flame propogation
3. Lean ignition limit (LIL): Minimal external energy that must be supplied to a critical volume to raise the a/f to minimal ignition temperatures
All of these play an important part in the fine line between normal combustion and unstable combustion/misfire. I don't want to write a book but will try and include examples in my replies.
so perhaps someone could tell me why running 16:1 a:f with high load creates higher temps than running 14.7?
Everyone keeps telling me that if I go this lean, I will get hot and melt stuff under load (say 90mph through the mountains due to weight and aero).
I have a friend with a 2.3 turbocoupe running +16:1 who rocks 37mpg for several years. He tuned his friends s/c 5.4 mustang to 17:1. Again, no problems.
Everyone keeps telling me that if I go this lean, I will get hot and melt stuff under load (say 90mph through the mountains due to weight and aero).
I have a friend with a 2.3 turbocoupe running +16:1 who rocks 37mpg for several years. He tuned his friends s/c 5.4 mustang to 17:1. Again, no problems.
Combustion temperatures will continue to decrease as a/f ratios lean because excess air promotes oxidation in the combustion process. If we limit the discussion to piston failure due to excess heat then we are talking about detonation and pre-ignition. Lean burn engines are very susceptible to this. Lean mixtures burn slower (LFL) and combustion temperatures are often higher later in the cycle. Metal surfaces have less cooling time before the next charge and with all these factors at work, combustion can get out of control in a hurry.
I'm sure we all know what happens when detonation occurs in regards to combustion temperatures. Comb. temps are markedly high and EGT temps are low. Where does the heat go?
Why are we surprised by thermal breakdown in this situation?
Originally, the aswer I got was that EGT will be reduced with lean conditions because there is a very quick burn and there its all well done burning by the time EVO (ex valve opening) occurs. However, combustion temps were higher. So basically, none of the combustion heat made it out the head, but the combustion temps were higher for lean.
As a/f ratios increase, flame speeds slow, time losses increase and ignition becomes more difficult. Combustion temperatures are lower and dependent on a/f ratios at various loads, the EGT's may be markedly higher. The less complete the combustion, the higher the EGTs.
That goes against that chart posted and a chart or two in my Internal combustion engine books.
I, still, am not convinced that lean will melt motor- but i'm still not confident.
I, still, am not convinced that lean will melt motor- but i'm still not confident.
I'm not sure I agree with some of those statements. Emissions are still produced at 14.7. Its just where HC and NOx are minimized. Lean (we'll use your def's) will reduce HC and rich with increase it. vice versa with NOx
You should be able to find a number of papers on this subject in the NACA archive.
Good luck.
11-20-2005, 01:26 AM
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All good reading, one specific part really confused me
Quite the opposite.
As a/f ratios increase, flame speeds slow, time losses increase and ignition becomes more difficult. Combustion temperatures are lower and dependent on a/f ratios at various loads, the EGT's may be markedly higher. The less complete the combustion, the higher the EGTs.
Can you please define a "time loss?"
Let me see if I have this
Peak flame speed occurs at around 14.7:1. I'm pretty sure I had that right (at some point before I started looking into this whole mess anyway!)
Deviation rich or lean slows flame speed. Lean causes more of the "heat" to go out the exhaust and show higher EGT while combustion temps actually were reduced?
Also:
I follow the whole slow flame speed so higher temps b/c piston cant cool off thing-
And this would make sense if people were racing through mountains at 15,000rpm at 17:1 af. But why would something like my silverado melt/damage a piston at 2100rpm? Wouldn't the additional load from hauling up mountains and the high drag increase flame speed helping to reduce this issue?
The lean condition causes a hot piston = pre detonation = sudden quick burn = high combustion temp = cool egt (none makes it out).
I guess the problem is that I'll never really get to even try this. thermal data logging software for pistons is pricey- more than the engine anyway.
Its really a shame we have retard of a prof we have for our intro to ICE and advanced ICE classes. I honestly cant ask him any of this stuff. Well I could, but I dont trust a man that bullshits on the basics every time a question is asked. For a university based on the automotive industry, its very sad that I had to argue- to the death- that a mechanical supercharger has a larger parasitic/pumping loss on an engine than NA. Apparently the "higher rpm needed to create equal horsepower will create large pumping losses while the SC engine simply sucks it in"
Between that, and the argument about a supercharged engine was superior to a turbocharged engine because the SC engine was always running under positive intake manifold pressure I realized it was a lost cause and switched my concentration. What a shame.
Originally Posted by Moldmaker
Quite the opposite.
As a/f ratios increase, flame speeds slow, time losses increase and ignition becomes more difficult. Combustion temperatures are lower and dependent on a/f ratios at various loads, the EGT's may be markedly higher. The less complete the combustion, the higher the EGTs.
Can you please define a "time loss?"
Let me see if I have this
Peak flame speed occurs at around 14.7:1. I'm pretty sure I had that right (at some point before I started looking into this whole mess anyway!)
Deviation rich or lean slows flame speed. Lean causes more of the "heat" to go out the exhaust and show higher EGT while combustion temps actually were reduced?
Also:
I follow the whole slow flame speed so higher temps b/c piston cant cool off thing-
And this would make sense if people were racing through mountains at 15,000rpm at 17:1 af. But why would something like my silverado melt/damage a piston at 2100rpm? Wouldn't the additional load from hauling up mountains and the high drag increase flame speed helping to reduce this issue?
The lean condition causes a hot piston = pre detonation = sudden quick burn = high combustion temp = cool egt (none makes it out).
I guess the problem is that I'll never really get to even try this. thermal data logging software for pistons is pricey- more than the engine anyway.
Its really a shame we have retard of a prof we have for our intro to ICE and advanced ICE classes. I honestly cant ask him any of this stuff. Well I could, but I dont trust a man that bullshits on the basics every time a question is asked. For a university based on the automotive industry, its very sad that I had to argue- to the death- that a mechanical supercharger has a larger parasitic/pumping loss on an engine than NA. Apparently the "higher rpm needed to create equal horsepower will create large pumping losses while the SC engine simply sucks it in"
Between that, and the argument about a supercharged engine was superior to a turbocharged engine because the SC engine was always running under positive intake manifold pressure I realized it was a lost cause and switched my concentration. What a shame.
11-20-2005, 11:26 AM
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Originally Posted by The Pipe
Moldmaker, You said most of what I wanted to say, but you did it for me. Very good post.
I was going to make them read Charles Fayette Taylor's "Internal Combustion Engine" Volumes one and two. Then Heywood's "internal Combustion Engine Fundamentals"
I didn't want to type everything out. Most of what I understand about this topic I learned from these books.
One thing to look at is all the other great info in these books. I would say CFT's books will cover pretty much everything Heywood does.
Even if you aren't into all the formulas in these books, you can find enough very good information to make the purchase worthwhile.
I was going to make them read Charles Fayette Taylor's "Internal Combustion Engine" Volumes one and two. Then Heywood's "internal Combustion Engine Fundamentals"
I didn't want to type everything out. Most of what I understand about this topic I learned from these books.
One thing to look at is all the other great info in these books. I would say CFT's books will cover pretty much everything Heywood does.
Even if you aren't into all the formulas in these books, you can find enough very good information to make the purchase worthwhile.
Actually I own both. Havn't read them cover to cover or anything- But i've read a good amount of each.
I've been trying to link the books to what is said to happen to myth etc. I just couldn't understand for the life of me why people claim to melt pistons at lean a:f.
I've also been thinking that it makes more sense for a lower compression car to be able to get away with all this since it will be less prone to pre-igniting than a higher compression car- as long as its not under boost. (?)
11-20-2005, 09:40 PM
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Originally Posted by Moldmaker
I think the reason why so many people are confused about temperatures is in the fact that they never define WHERE/WHEN in the process they'd like to examine them.
Originally Posted by Moldmaker
I'm sure we all know what happens when detonation occurs in regards to combustion temperatures. Comb. temps are markedly high and EGT temps are low. Where does the heat go?
Originally Posted by Moldmaker
Why are we surprised by thermal breakdown in this situation?
Most people severly underestimate the role of fuel in thermal management. The only way to know for sure what 'your' engine will tolerate is to perform a test to failure analysis. That gets kind of expensive though. 
Timing is also going to play a critical role in determining 'when' and 'where' heat is going to be generated. Most people don't realize that when there are multiple flame fronts (pre-ignition or detonation) what normally takes 20* of crank rotation (with normal spark) can occur within 4*. The cylinder see lots of heat almost instantaneously, and it has nowhere to go. When these flame fronts collide, it is like thunderstorms colliding. You can see cylinder pressure jump 3 times its normal value (e.g. from 1400psi to 4000+psi).
11-21-2005, 01:02 PM
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Originally Posted by treyZ28
Can you please define a "time loss?"
To expand on that please keep in mind that the idealized otto cycle assumption of constant volume combustion becomes less valid as lambda increases.
We have covered flame speeds and it's also important to note that ignition delay -the time needed to create pre flame chemical components for rapid reaction- also slows as lambda increases. Time is working against you in a lean burn application and although ignition delay does allow one to recoup some thermal efficiency via building with higher static compression, you still want to burn the mixture before blow down.
Let me see if I have this
Peak flame speed occurs at around 14.7:1. I'm pretty sure I had that right (at some point before I started looking into this whole mess anyway!)
Deviation rich or lean slows flame speed. Lean causes more of the "heat" to go out the exhaust and show higher EGT while combustion temps actually were reduced?
Peak flame speed occurs at around 14.7:1. I'm pretty sure I had that right (at some point before I started looking into this whole mess anyway!)
Deviation rich or lean slows flame speed. Lean causes more of the "heat" to go out the exhaust and show higher EGT while combustion temps actually were reduced?
At lambda=2, how much of the fuel is burned and how much "air" is left in the combustion chamber -theoretically-?
Also:
I follow the whole slow flame speed so higher temps b/c piston cant cool off thing-
And this would make sense if people were racing through mountains at 15,000rpm at 17:1 af. But why would something like my silverado melt/damage a piston at 2100rpm? Wouldn't the additional load from hauling up mountains and the high drag increase flame speed helping to reduce this issue?
I follow the whole slow flame speed so higher temps b/c piston cant cool off thing-
And this would make sense if people were racing through mountains at 15,000rpm at 17:1 af. But why would something like my silverado melt/damage a piston at 2100rpm? Wouldn't the additional load from hauling up mountains and the high drag increase flame speed helping to reduce this issue?
Is it likely?
Yes it is, but once we understand the problems a lean burn engine is faced with we can counteract a potential disaster by way of tuning. Or perhaps, if serious enough about lean burn, we might design a water injection system or perhaps abandon gasoline all together.
The lean condition causes a hot piston = pre detonation = sudden quick burn = high combustion temp = cool egt (none makes it out).
I guess the problem is that I'll never really get to even try this. thermal data logging software for pistons is pricey- more than the engine anyway.
Its really a shame we have retard of a prof we have for our intro to ICE and advanced ICE classes. I honestly cant ask him any of this stuff. Well I could, but I dont trust a man that bullshits on the basics every time a question is asked. For a university based on the automotive industry, its very sad that I had to argue- to the death- that a mechanical supercharger has a larger parasitic/pumping loss on an engine than NA. Apparently the "higher rpm needed to create equal horsepower will create large pumping losses while the SC engine simply sucks it in"
Between that, and the argument about a supercharged engine was superior to a turbocharged engine because the SC engine was always running under positive intake manifold pressure I realized it was a lost cause and switched my concentration. What a shame.
I guess the problem is that I'll never really get to even try this. thermal data logging software for pistons is pricey- more than the engine anyway.
Its really a shame we have retard of a prof we have for our intro to ICE and advanced ICE classes. I honestly cant ask him any of this stuff. Well I could, but I dont trust a man that bullshits on the basics every time a question is asked. For a university based on the automotive industry, its very sad that I had to argue- to the death- that a mechanical supercharger has a larger parasitic/pumping loss on an engine than NA. Apparently the "higher rpm needed to create equal horsepower will create large pumping losses while the SC engine simply sucks it in"
Between that, and the argument about a supercharged engine was superior to a turbocharged engine because the SC engine was always running under positive intake manifold pressure I realized it was a lost cause and switched my concentration. What a shame.
"The Pipe" mentioned Taylor's books and I agree that they are essential reads for engine guys. You'll need to search out later references for this study though. I think Taylor mentions lean burn thermal management problems once or twice in his books but all of that research was built on carbureted engine technology. With carbs you have a plethora of problems, not the least of which are unequal air/fuel distribution.
TXNC5 makes some very good points too. Timing is a biggie with a lean burn engine. Remember the time losses....
Good read.
treyz28,
Got your PM & will get back to you but also wanted to include these references for you to follow.
http://www.chemeng.ucl.ac.uk/researc.../nl98_2_9.html
http://www.mech-eng.leeds.ac.uk/staf...ns/mendeb.html
http://powerlab.mech.okayama-u.ac.jp...a2001/3-04.pdf
http://www.autozine.org/technical_sc...ne/petrol1.htm
http://www.tvu.com/CycleByCycleweb.htm
Keep in mind that I'm hardly an authority on this subject. My study of lean burn spark ignition engines goes back close to ten years ago. I've never been serious about making it work, nor do I have any hands on experience worth mentioning. Please take this for what it's worth --just free insight.
Good luck.
Last edited by Moldmaker; 11-21-2005 at 02:14 PM.
11-22-2005, 09:18 AM
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Thanks for the info Moldmaker. I believe you also mentioned NACA earlier. For those that don't know, it was a predecessor to NASA. They did a fair amount of research on IC's, combustion, AFR, etc. You can search through the reports here
11-23-2005, 10:25 AM
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Hot and lean means you have excess oxygen
that's up for grabs; your "melted" piston might
in fact be aluminum burning with what's left.
That's gonna put heat right into the slug surface,
not into the gas, and accelerate itself once the
native oxide is breached. Think Thermite....
that's up for grabs; your "melted" piston might
in fact be aluminum burning with what's left.
That's gonna put heat right into the slug surface,
not into the gas, and accelerate itself once the
native oxide is breached. Think Thermite....
