Air: Fuel, Emissions and spark timing.
11-05-2005, 11:48 AM
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Air: Fuel, Emissions and spark timing. I posted below on a truck forum when someone asked about gas milage. I was tired of hearing about three drops of acetone, K&N airfilters giving 35% gas milage increases and internal combustion engines operating on water.
I would really like to have an "advanced" discussion about it. Here is the main thing I am confused about: A guy i was talking to said he melted his pistons at 17:1 AF (sorry, i dont speak lambda well). Why? My books say he should be cold. Melting a piston is just... damn hot. especially concidering it happened at low rpm on an NA motor (long cool down period)
I would really like to have an "advanced" discussion about it. Here is the main thing I am confused about: A guy i was talking to said he melted his pistons at 17:1 AF (sorry, i dont speak lambda well). Why? My books say he should be cold. Melting a piston is just... damn hot. especially concidering it happened at low rpm on an NA motor (long cool down period)
1. It will work but only because you reduce pumping losses. at part throttle- there is almost no restriction. its bs, no matter what any hillbilly or sales rep tells you. if you get better gas mileage, it means your engine required less energy to do what it had to do or you violated the conservation of energy.
2. sure it will help, but people seem to think the mechanical fan is always 100% engaged. no, its not. its barely turning. The Efan will help, but factor in cost and you're behind.
3. If you get a wideband and tuning software, you can go as lean as 16 or 17:1, depending on how lean you engine will actually run. This will definitely help. Mess with timing and get it down right. I have no idea why people think lean past 14.7 gets hotter. Well, actually I do. Its from the carb days when they came factory rich and leaning would put you at the hotspot. People then drew a conclusion that the leaner you, the hotter you go.
Its actually getting colder. usually the 11-13 range (depending on engine) is where its the hottest.
I think max tq output comes from a few steps rich or lean of this peak heat. Its called Lean Max (best) torque or rich max torque. Also called Lean of peak (LOP) and ROP (Rich of peak). Traditionally people go lean of peak, but that’s not set in stone. That’s the only thing I’m not 100% sure of. I remember reading it, but I am not 100% sure of it. It doesn't make sense intuitively. Max heat should be max torque, but I *think* spark timing has an effect on that. Its why you can make more torque and have lower EGT. <----someone needs to confirm this paragraph.
14.7:1 is PURELY an emissions thing. No other reason for this random number. Leaner (as long as there is still combustion) will always result in less HC and more NOx. Richer always results in higher HC and lower NOx. 14.7:1 is where the curves cross.
high HC is very typical and a good indication of incompletely combustion or basically- wasted gas. If you have low HC output, its a good bet you are doing well for gas mileage, or as well as you can be doing.
I just got the new LQ9 and 4L65 in so I’m kind of broke right now. Once I have a set of headers, I fully plan on doing this.
2. sure it will help, but people seem to think the mechanical fan is always 100% engaged. no, its not. its barely turning. The Efan will help, but factor in cost and you're behind.
3. If you get a wideband and tuning software, you can go as lean as 16 or 17:1, depending on how lean you engine will actually run. This will definitely help. Mess with timing and get it down right. I have no idea why people think lean past 14.7 gets hotter. Well, actually I do. Its from the carb days when they came factory rich and leaning would put you at the hotspot. People then drew a conclusion that the leaner you, the hotter you go.
Its actually getting colder. usually the 11-13 range (depending on engine) is where its the hottest.
I think max tq output comes from a few steps rich or lean of this peak heat. Its called Lean Max (best) torque or rich max torque. Also called Lean of peak (LOP) and ROP (Rich of peak). Traditionally people go lean of peak, but that’s not set in stone. That’s the only thing I’m not 100% sure of. I remember reading it, but I am not 100% sure of it. It doesn't make sense intuitively. Max heat should be max torque, but I *think* spark timing has an effect on that. Its why you can make more torque and have lower EGT. <----someone needs to confirm this paragraph.
14.7:1 is PURELY an emissions thing. No other reason for this random number. Leaner (as long as there is still combustion) will always result in less HC and more NOx. Richer always results in higher HC and lower NOx. 14.7:1 is where the curves cross.
high HC is very typical and a good indication of incompletely combustion or basically- wasted gas. If you have low HC output, its a good bet you are doing well for gas mileage, or as well as you can be doing.
I just got the new LQ9 and 4L65 in so I’m kind of broke right now. Once I have a set of headers, I fully plan on doing this.
11-08-2005, 09:05 AM
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I am not a mechanic...but nevertheless... simply, my understanding is lean without load=lower emmisions, too lean=rough idle, lean misfires, lower HC's which could and probably kill catalytic converters. Cooler, sure less fires.
Add load to the equation and you add something to the above list, preignition...knock. Which wouldn't "melt" anything foresay, but surely wreck havoc in the motor, breaking ****.
Your right as far as I understand...just gotta factor load and no load very carefully.
The 14.7 came about because that's the exact amount of air to fuel that completes a complete burn....in the perfect setting of course.
Add load to the equation and you add something to the above list, preignition...knock. Which wouldn't "melt" anything foresay, but surely wreck havoc in the motor, breaking ****.
Your right as far as I understand...just gotta factor load and no load very carefully.
The 14.7 came about because that's the exact amount of air to fuel that completes a complete burn....in the perfect setting of course.
The mass of air supplied to the engine divided by the mass of fuel supplied in the same period of time. The Stoichiometric, or chemically correct, air-fuel ratio (A/F ratio) is the exact ratio necessary to burn all the carbon and hydrogen in the fuel to carbon dioxide and water with no oxygen remaining. The fuel-air ratio is the reciprocal of the air-fuel ratio.
Stable combustion conditions require the right amounts of fuel and oxygen. The combustion products are heat energy, carbon dioxide, water vapor, nitrogen, and other gases (excluding oxygen). In theory there is a specific amount of oxygen needed to completely burn a given amount of fuel. In practice, burning conditions are never ideal.
When air and gasoline are mixed together and ignited, the chemical reaction requires a certain amount of air to completely burn all of the fuel. The exact amount is 14.7 lbs of air for every pound of fuel. This is called the "Stoichiometric" Air/Fuel ratio. It's also referred to the Greek letter "lambda."
When lambda equals one, you have a 14.7:1 Stoichiometric Air/Fuel ratio and ideal combustion. When the Air/Fuel ratio is greater than 14.7:1, lambda also will be greater than one and the engine will have a lean mixture.
Lean mixtures improve fuel economy but also cause a sharp rise in oxides of nitrogen (NOX). If the mixture goes too lean, it may not ignite at all causing "lean misfire" and a huge increase in unburned hydrocarbon (HC) emissions. This can cause rough idle, hard starting and stalling, and may even damage the catalytic converter. Lean mixtures also increase the risk of spark knock (detonation) when the engine is under load.
When the Air/Fuel ratio is less than 14.7:1, lambda also is less than one and the engine has a rich fuel mixture. A rich fuel mixture is necessary when a cold engine is first started, and additional fuel is needed when the engine is under load. But rich mixtures cause a sharp increase in carbon monoxide (CO) emissions. When the relative proportions of air and fuel are "just right," the mixture burns clearly and produces the fewest emissions. The trick is balancing the mixture as driving conditions, temperatures and loads are constantly changing.
Stable combustion conditions require the right amounts of fuel and oxygen. The combustion products are heat energy, carbon dioxide, water vapor, nitrogen, and other gases (excluding oxygen). In theory there is a specific amount of oxygen needed to completely burn a given amount of fuel. In practice, burning conditions are never ideal.
When air and gasoline are mixed together and ignited, the chemical reaction requires a certain amount of air to completely burn all of the fuel. The exact amount is 14.7 lbs of air for every pound of fuel. This is called the "Stoichiometric" Air/Fuel ratio. It's also referred to the Greek letter "lambda."
When lambda equals one, you have a 14.7:1 Stoichiometric Air/Fuel ratio and ideal combustion. When the Air/Fuel ratio is greater than 14.7:1, lambda also will be greater than one and the engine will have a lean mixture.
Lean mixtures improve fuel economy but also cause a sharp rise in oxides of nitrogen (NOX). If the mixture goes too lean, it may not ignite at all causing "lean misfire" and a huge increase in unburned hydrocarbon (HC) emissions. This can cause rough idle, hard starting and stalling, and may even damage the catalytic converter. Lean mixtures also increase the risk of spark knock (detonation) when the engine is under load.
When the Air/Fuel ratio is less than 14.7:1, lambda also is less than one and the engine has a rich fuel mixture. A rich fuel mixture is necessary when a cold engine is first started, and additional fuel is needed when the engine is under load. But rich mixtures cause a sharp increase in carbon monoxide (CO) emissions. When the relative proportions of air and fuel are "just right," the mixture burns clearly and produces the fewest emissions. The trick is balancing the mixture as driving conditions, temperatures and loads are constantly changing.
11-08-2005, 09:37 AM
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Originally Posted by treyz28
I have no idea why people think lean past 14.7 gets hotter.
Originally Posted by treyz28
Leaner (as long as there is still combustion) will always result in less HC and more NOx. Richer always results in higher HC and lower NOx. 14.7:1 is where the curves cross.
11-08-2005, 10:14 AM
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Well, 14.7:1 is definitely emissions related. If we combine 14.7 parts air to every one part of fuel, we will have the minimal amount of ‘harmful’ emissions; thus, the EPA is happy. You can most definitely run mixtures that are leaner that 14.7:1. That in itself should indicate ‘better’ gas mileage because we are using less fuel per unit of air. The whole issue with running lean is a matter of engine load and thermal management. It does not take significant power to keep a vehicle cruising along a highway (relatively flat and no strong headwind). However, keep in mind that a Silverado with a 5.3L would definitely have to work ‘harder’ (in percentage terms) than say a 6.0L in a new Corvette. This is simply a matter of aerodynamics. Therefore, just because a Vette may be able to cruise with a 16:1 AFR, doesn’t mean that a Silverado can do so as well.
It is indeed very difficult (haven’t seen/heard about anybody doing it yet) to melt a piston at low load while running lean. As the mixture is continually leaned out, the engine will begin to stumble as combustion becomes ‘less’ complete and eventually there is no combustion at all. I have leaned cars out tremendously on the dyno (with no/low load) and at idle with no issue.
I will give you an even better example. I do some teaching for EFI University. They did some testing on a Porsche motor running 20lbs of boost. For endurance racing, they found that the motor could live as long as they wanted/needed it running 11.7:1. For sprint races (20-30 minutes), they found that they could lean it out to approximately 12.2:1 with no problems. For more of a drag racing application (15-20 seconds max), they found that the motor could survive with 12.5:1. However, at 12.7:1 the motor was toast after just one pull on the dyno.
When referencing heat, I do agree there is some folklore regarding what exhaust temperatures can tell us. An exhaust temperature is not going to always tell you the ‘truth’ about what is going on the combustion chamber. A really rich mixture can create extremely high exhaust temps by the fact that combustion can still occur in the exhaust itself. Also, rich mixtures are used to cool or manage heat particularly in boosted applications. The excess fuel absorbs heat, and much of it transferred and dissipated through the exhaust.
I would agree with the concept that the engine will generate the most heat when it producing the most torque. This is when the engine is most efficient and working the hardest. The question how long can the engine survive (not the exhaust) at that temperature.
So, what was the guy doing when he melted his piston? Was pulling a trailer up a hill and trying to run 17:1? Is it a heavy duty truck that he was driving into a 20 mph headwind? As an FYI, 17:1 is a little to lean in my experiences even with the awesome Gen III.
BTW, here is good link to showing emissions related to AFR.
It is indeed very difficult (haven’t seen/heard about anybody doing it yet) to melt a piston at low load while running lean. As the mixture is continually leaned out, the engine will begin to stumble as combustion becomes ‘less’ complete and eventually there is no combustion at all. I have leaned cars out tremendously on the dyno (with no/low load) and at idle with no issue.
I will give you an even better example. I do some teaching for EFI University. They did some testing on a Porsche motor running 20lbs of boost. For endurance racing, they found that the motor could live as long as they wanted/needed it running 11.7:1. For sprint races (20-30 minutes), they found that they could lean it out to approximately 12.2:1 with no problems. For more of a drag racing application (15-20 seconds max), they found that the motor could survive with 12.5:1. However, at 12.7:1 the motor was toast after just one pull on the dyno.
When referencing heat, I do agree there is some folklore regarding what exhaust temperatures can tell us. An exhaust temperature is not going to always tell you the ‘truth’ about what is going on the combustion chamber. A really rich mixture can create extremely high exhaust temps by the fact that combustion can still occur in the exhaust itself. Also, rich mixtures are used to cool or manage heat particularly in boosted applications. The excess fuel absorbs heat, and much of it transferred and dissipated through the exhaust.
I would agree with the concept that the engine will generate the most heat when it producing the most torque. This is when the engine is most efficient and working the hardest. The question how long can the engine survive (not the exhaust) at that temperature.
So, what was the guy doing when he melted his piston? Was pulling a trailer up a hill and trying to run 17:1? Is it a heavy duty truck that he was driving into a 20 mph headwind? As an FYI, 17:1 is a little to lean in my experiences even with the awesome Gen III.
BTW, here is good link to showing emissions related to AFR.
11-08-2005, 10:55 AM
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At idle there is less work being performed. Less air, less fuel, less friction, etc. Under a load heat increases regardless of mixture due to increasing frequency of combustion cylces, but the wrong mixture compounds that heat buildup. A lean mixture under load will create excess HC's in the exhaust combined with higher combustion pressures and cause the heat to stack up in the pipes. That extra heat can also cause pre-ignition and detonation. Since the process is cyclical, the lean condition will continue to aggravate the situation and temperatures will rise.
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11-08-2005, 12:27 PM
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At idle, a leaner mixture (17:1) will not generate as much heat as a richer mixture (14.7:1) because there is less combustion. Idle quality deteriorates but since rpms are low and compression pressures are low heat doesn't "stack up."
11-08-2005, 01:24 PM
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Originally Posted by treyZ28
i understand why its cooler at idle.
but why will it be cooler at idle with 17:1 vs 14.7 and then hotter under load at 17:1 vs 14.7
but why will it be cooler at idle with 17:1 vs 14.7 and then hotter under load at 17:1 vs 14.7
When cylinder temps go way up and EGT goes down, it is a sure sign of detonation. At 17:1 with load, it won't take long for the cylinder temp to rise, create hot spots, and start detonating. Once that cycle begins, it can be very hard to stop. You can verify this with any of the stock PCM's. Find a long, steep hill and try to induce some knock. Observe how long it takes the computer to get it under control and more importantly how much timing it has to pull to do so. Along with load, timing and the amount of time for the events to occur (RPM) both affect cylinder head and EGT. There are significant differences between idle and 3,200 RPM.
11-08-2005, 01:29 PM
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lets not get lost in details of 17:1 or 16.5 or 16. lets just say "much leaner than 14.7." (a friend is running 16.7 or something like that)
problem is, according to "the book" leaner keeps getting colder after 14.7 until misfire. so why does leaner get hotter, much hotter, under load?
problem is, according to "the book" leaner keeps getting colder after 14.7 until misfire. so why does leaner get hotter, much hotter, under load?
11-08-2005, 02:20 PM
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Originally Posted by treyZ28
problem is, according to "the book" leaner keeps getting colder after 14.7 until misfire. so why does leaner get hotter, much hotter, under load?
What is the affect of having less fuel in the combustion chamber? Cylinder temperatures can actually rise due to the lack of thermal management provided by excess fuel. Why do boosted applications use rich AFR's, alcohol, and/or water injection? It is not for efficiency but rather thermal management.
11-08-2005, 02:58 PM
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Originally Posted by TXNC5
Cylinder temperatures can actually rise due to the lack of thermal management provided by excess fuel. Why do boosted applications use rich AFR's, alcohol, and/or water injection? It is not for efficiency but rather thermal management.
I'm still wondering how a piston melted at 17:1 while unloaded per the first post. Localized hot spot and ignition maybe?
11-08-2005, 03:10 PM
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I always assumed the heat came from power production.
The intercooling effect of fuel is really that huge?
and correct, its lower egt- not combustion temp.
The intercooling effect of fuel is really that huge?
and correct, its lower egt- not combustion temp.
11-08-2005, 03:24 PM
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Originally Posted by treyZ28
I always assumed the heat came from power production.
The intercooling effect of fuel is really that huge?
The intercooling effect of fuel is really that huge?
11-08-2005, 03:37 PM
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Originally Posted by SSpeedracer
I'm still wondering how a piston melted at 17:1 while unloaded per the first post. Localized hot spot and ignition maybe?
11-08-2005, 04:04 PM
#17
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 compression). 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, compression 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 compression). 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, compression 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.
11-08-2005, 04:05 PM
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There is a huge difference between 14.7:1 and 15:1 when you take into account how little air and fuel (volume)
cube motor. Also think about the amount of fuel needed vs. air flow between
idle and full throttle @ peak RPM.
The differencecs in combustion efficiency at idle and peak torque is going to
be huge. A miscalculation in AFR can be very destructive.
Many years ago, I was walking through the pit of a Pro-Mod team. I was checking
out the intake manifold and noticed there were two injectors on the #2 runner
when all others had one injector.
Anyone care to guess why?
11-08-2005, 04:11 PM
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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 compression). 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, compression 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 compression). 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, compression 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.
Octane not only describes the fuels flashpoint, but burn rate also.
Pre-ignition is the result of high CC temperatures, not the converse.
11-14-2005, 09:47 AM
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Loaded post copied, but here goes
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.
Check slide 34
http://me.queensu.ca/courses/MECH435...n%20Theory.ppt
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.
Bryan at PCM4Less warned me against this stating his attempts at doing this resulted in the loss of a piston due to excessive heat.
Problem is both my friend and bryan are extremely knowlegable. Bryan needs no introduction, but I assure you John has a VERY firm grasp on the dynamics of the internal combustion engine.
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.
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.
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.
Check slide 34
http://me.queensu.ca/courses/MECH435...n%20Theory.ppt
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.
Bryan at PCM4Less warned me against this stating his attempts at doing this resulted in the loss of a piston due to excessive heat.
Problem is both my friend and bryan are extremely knowlegable. Bryan needs no introduction, but I assure you John has a VERY firm grasp on the dynamics of the internal combustion engine.
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.
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.