Optimum AFR under full throttle?
04 September 2005 | 02:19 PM
#6
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Many people with turbochargers believe that they need to run at very rich mixtures. The theory is that the excess fuel cools the intake charge and therefore reduces the probability of knock. It does work in reducing knock, but not because of charge cooling. The following little article shows why.
First let’s look at the science. Specific heat is the amount of energy required to raise 1 kg of material by one degree K (Kelvin, same as Celsius but with 0 point at absolute zero). Different materials have different specific heats. The energy is measured in kJ or kilojoules:
Air ~ 1 kJ/( kg * deg K)
Gasoline 2.02 kJ/( kg * deg K)
Water 4.18 kJ/( kg * deg K)
Ethanol 2.43 kJ/( kg * deg K)
Methanol 2.51 kJ/( kg * deg K)
Fuel and other liquids also have what's called latent heat. This is the heat energy required to vaporize 1 kg of the liquid. The fuel in an internal combustion engine has to be vaporized and mixed thoroughly with the incoming air to produce power. Liquid gasoline does not burn. The energy to vaporize the fuel comes partially from the incoming air, cooling it. The latent heat energy required is actually much larger than the specific heat. That the energy comes from the incoming air can be easily seen on older carbureted cars, where frost can actually form on the intake manifold from the cooling of the charge.
The latent heat values of different liquids are shown here:
Gasoline 350 kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Most engines produce maximum power (with optimized ignition timing) at an air-fuel-ratio between 12 and 13. Let's assume the optimum is in the middle at 12.5. This means that for every kg of air, 0.08 kg of fuel is mixed in and vaporized. The vaporization of the fuel extracts 28 kJ of energy from the air charge. If the mixture has an air-fuel-ratio of 11 instead, the vaporization extracts 31.8 kJ instead. A difference of 3.8 kJ. Because air has a specific heat of about 1 kJ/kg*deg K, the air charge is only 3.8 C (or K) degrees cooler for the rich mixture compared to the optimum power mixture. This small difference has very little effect on knock or power output.
If instead of the richer mixture about 10% (by mass) of water would be injected in the intake charge (0.008 kg Water/kg air), the high latent heat of the water would cool the charge by 18 degrees, about 4 times the cooling effect of the richer mixture. The added fuel for the rich mixture can't burn because there is just not enough oxygen available. So it does not matter if fuel or water is added.
So where does the knock suppression of richer mixtures come from?
If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.
So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.
Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.
Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.
This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power.
First let’s look at the science. Specific heat is the amount of energy required to raise 1 kg of material by one degree K (Kelvin, same as Celsius but with 0 point at absolute zero). Different materials have different specific heats. The energy is measured in kJ or kilojoules:
Air ~ 1 kJ/( kg * deg K)
Gasoline 2.02 kJ/( kg * deg K)
Water 4.18 kJ/( kg * deg K)
Ethanol 2.43 kJ/( kg * deg K)
Methanol 2.51 kJ/( kg * deg K)
Fuel and other liquids also have what's called latent heat. This is the heat energy required to vaporize 1 kg of the liquid. The fuel in an internal combustion engine has to be vaporized and mixed thoroughly with the incoming air to produce power. Liquid gasoline does not burn. The energy to vaporize the fuel comes partially from the incoming air, cooling it. The latent heat energy required is actually much larger than the specific heat. That the energy comes from the incoming air can be easily seen on older carbureted cars, where frost can actually form on the intake manifold from the cooling of the charge.
The latent heat values of different liquids are shown here:
Gasoline 350 kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Most engines produce maximum power (with optimized ignition timing) at an air-fuel-ratio between 12 and 13. Let's assume the optimum is in the middle at 12.5. This means that for every kg of air, 0.08 kg of fuel is mixed in and vaporized. The vaporization of the fuel extracts 28 kJ of energy from the air charge. If the mixture has an air-fuel-ratio of 11 instead, the vaporization extracts 31.8 kJ instead. A difference of 3.8 kJ. Because air has a specific heat of about 1 kJ/kg*deg K, the air charge is only 3.8 C (or K) degrees cooler for the rich mixture compared to the optimum power mixture. This small difference has very little effect on knock or power output.
If instead of the richer mixture about 10% (by mass) of water would be injected in the intake charge (0.008 kg Water/kg air), the high latent heat of the water would cool the charge by 18 degrees, about 4 times the cooling effect of the richer mixture. The added fuel for the rich mixture can't burn because there is just not enough oxygen available. So it does not matter if fuel or water is added.
So where does the knock suppression of richer mixtures come from?
If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.
So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.
Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.
Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.
This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power.
Trending Topics
04 September 2005 | 03:42 PM
#11
Originally Posted by DaveC1
'I have a wideband located before the turbo where the 4 primarys meet. Is this location ok'
That should be fine
That should be fine
Plus of the two AFRs mentioned i'd run 11.2:1....
Tony.
04 September 2005 | 05:10 PM
#12
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Joined: Nov 2000
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From: 32 cylinders and many cats
Some lambda sensors/kits also specifically state not to use pre-turbo because of the higher pressures giving a falsely rich reading. Quite dangerous that would be on my Techedge.
On Optimax, most Subarus seem to run well richer than 11.5:1 IMHO. Otherwise I find I can't run the boost or the ignition needs to be excessively retarded, or I can't keep the EGTs under control. I couldn't get much more power on water injection despite tuning for it, but I did gain a lot more power by pouring methanol into the fuel tank. Bear in mind with lean mixtures that there are uneven air/fuel distributions in Subaru engines and reading the composite may give a false impression.
Additionally, as well as things you can measure like EGT, give some thought to the poor piston crowns (which only on some engines are sprayed with oil from underneath), and the poor exhaust valves (which are not always sodium filled), plus the chance of warping or cracking heads. You can also seize a piston in the bore if you let forged pistons on tight clearance (eg pre 02/03 STI) get too hot.
Lots to think about considering the theoretical arguments for running in the 12s, I try to run 11.2 whilst aiming for 200 BHP/litre.
On Optimax, most Subarus seem to run well richer than 11.5:1 IMHO. Otherwise I find I can't run the boost or the ignition needs to be excessively retarded, or I can't keep the EGTs under control. I couldn't get much more power on water injection despite tuning for it, but I did gain a lot more power by pouring methanol into the fuel tank. Bear in mind with lean mixtures that there are uneven air/fuel distributions in Subaru engines and reading the composite may give a false impression.
Additionally, as well as things you can measure like EGT, give some thought to the poor piston crowns (which only on some engines are sprayed with oil from underneath), and the poor exhaust valves (which are not always sodium filled), plus the chance of warping or cracking heads. You can also seize a piston in the bore if you let forged pistons on tight clearance (eg pre 02/03 STI) get too hot.
Lots to think about considering the theoretical arguments for running in the 12s, I try to run 11.2 whilst aiming for 200 BHP/litre.
04 September 2005 | 05:31 PM
#14
Originally Posted by DaveC1
Many people with turbochargers believe that they need to run at very rich mixtures. The theory is that the excess fuel cools the intake charge and therefore reduces the probability of knock. It does work in reducing knock, but not because of charge cooling. The following little article shows why.
First let’s look at the science. Specific heat is the amount of energy required to raise 1 kg of material by one degree K (Kelvin, same as Celsius but with 0 point at absolute zero). Different materials have different specific heats. The energy is measured in kJ or kilojoules:
Air ~ 1 kJ/( kg * deg K)
Gasoline 2.02 kJ/( kg * deg K)
Water 4.18 kJ/( kg * deg K)
Ethanol 2.43 kJ/( kg * deg K)
Methanol 2.51 kJ/( kg * deg K)
Fuel and other liquids also have what's called latent heat. This is the heat energy required to vaporize 1 kg of the liquid. The fuel in an internal combustion engine has to be vaporized and mixed thoroughly with the incoming air to produce power. Liquid gasoline does not burn. The energy to vaporize the fuel comes partially from the incoming air, cooling it. The latent heat energy required is actually much larger than the specific heat. That the energy comes from the incoming air can be easily seen on older carbureted cars, where frost can actually form on the intake manifold from the cooling of the charge.
The latent heat values of different liquids are shown here:
Gasoline 350 kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Most engines produce maximum power (with optimized ignition timing) at an air-fuel-ratio between 12 and 13. Let's assume the optimum is in the middle at 12.5. This means that for every kg of air, 0.08 kg of fuel is mixed in and vaporized. The vaporization of the fuel extracts 28 kJ of energy from the air charge. If the mixture has an air-fuel-ratio of 11 instead, the vaporization extracts 31.8 kJ instead. A difference of 3.8 kJ. Because air has a specific heat of about 1 kJ/kg*deg K, the air charge is only 3.8 C (or K) degrees cooler for the rich mixture compared to the optimum power mixture. This small difference has very little effect on knock or power output.
If instead of the richer mixture about 10% (by mass) of water would be injected in the intake charge (0.008 kg Water/kg air), the high latent heat of the water would cool the charge by 18 degrees, about 4 times the cooling effect of the richer mixture. The added fuel for the rich mixture can't burn because there is just not enough oxygen available. So it does not matter if fuel or water is added.
So where does the knock suppression of richer mixtures come from?
If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.
So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.
Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.
Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.
This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power.
First let’s look at the science. Specific heat is the amount of energy required to raise 1 kg of material by one degree K (Kelvin, same as Celsius but with 0 point at absolute zero). Different materials have different specific heats. The energy is measured in kJ or kilojoules:
Air ~ 1 kJ/( kg * deg K)
Gasoline 2.02 kJ/( kg * deg K)
Water 4.18 kJ/( kg * deg K)
Ethanol 2.43 kJ/( kg * deg K)
Methanol 2.51 kJ/( kg * deg K)
Fuel and other liquids also have what's called latent heat. This is the heat energy required to vaporize 1 kg of the liquid. The fuel in an internal combustion engine has to be vaporized and mixed thoroughly with the incoming air to produce power. Liquid gasoline does not burn. The energy to vaporize the fuel comes partially from the incoming air, cooling it. The latent heat energy required is actually much larger than the specific heat. That the energy comes from the incoming air can be easily seen on older carbureted cars, where frost can actually form on the intake manifold from the cooling of the charge.
The latent heat values of different liquids are shown here:
Gasoline 350 kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Most engines produce maximum power (with optimized ignition timing) at an air-fuel-ratio between 12 and 13. Let's assume the optimum is in the middle at 12.5. This means that for every kg of air, 0.08 kg of fuel is mixed in and vaporized. The vaporization of the fuel extracts 28 kJ of energy from the air charge. If the mixture has an air-fuel-ratio of 11 instead, the vaporization extracts 31.8 kJ instead. A difference of 3.8 kJ. Because air has a specific heat of about 1 kJ/kg*deg K, the air charge is only 3.8 C (or K) degrees cooler for the rich mixture compared to the optimum power mixture. This small difference has very little effect on knock or power output.
If instead of the richer mixture about 10% (by mass) of water would be injected in the intake charge (0.008 kg Water/kg air), the high latent heat of the water would cool the charge by 18 degrees, about 4 times the cooling effect of the richer mixture. The added fuel for the rich mixture can't burn because there is just not enough oxygen available. So it does not matter if fuel or water is added.
So where does the knock suppression of richer mixtures come from?
If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.
So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.
Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.
Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.
This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power.
04 September 2005 | 06:47 PM
#15
Dave's answer isnt wrong - its just that it is a textbook answer to a very non-specific question at the start.
If the question is about Subaru turbo engine tuning then the actual AFRs found by the likes of Bob, Joh etc should be considered instead.
Nick
If the question is about Subaru turbo engine tuning then the actual AFRs found by the likes of Bob, Joh etc should be considered instead.
Nick
04 September 2005 | 10:38 PM
#16
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From: Wellingborough, Northamptonshire
Originally Posted by banny sti
A very concise and comprehensive account of what goes on in an internal combustion engine. Posts like this are informative and arm us enthusiasts with knowledge so tuners beware. Incidentally Bob instead posting a snide remark correct Dave and let us all know where he is wrong.....
11.2 seems to be a good compromise with cast pistons, I would be prepared to go leaner on a car with forged pistons, but only as lean as 11.5 with 1.5 bar of boost. If you have an EGT gauge as John says, you can learn a lot. The ideal will depend on application too, long runs flat out on an autobahn will require richer AFRs if the engine is to survive.
Paul
05 September 2005 | 03:04 AM
#17
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From: Turkiye, Florida
So I think I'm in the dangerous side because I'm running 11.4 with cast pistons but EGT readings seems fine. I'm just trying to learn and experience and I'd like to ask about the limits. Yesterday I made hundereds of 4th gear launchs for to optimize the fueling map but I still dont belive that I'm on the safe side of it. No high EGTs and no detonation. Is that all we should consider about while mapping?
I have nothing to compare. I'm just trying to experiment/learn with my poor English. For example I'm running 16.5 AFR while spooling up the turbo but I cant be sure. I just read on internet that leaning afr will help the turbo to spool up quickly and i tried it. But I still can't be sure if i'm doing wrong or not because I have no experience what I do.
so please share your knowledge =)
cheers
by the way here is a good thread about afrs
http://forums.nasioc.com/forums/showthread.php?t=673369
I have nothing to compare. I'm just trying to experiment/learn with my poor English. For example I'm running 16.5 AFR while spooling up the turbo but I cant be sure. I just read on internet that leaning afr will help the turbo to spool up quickly and i tried it. But I still can't be sure if i'm doing wrong or not because I have no experience what I do.
so please share your knowledge =)
cheers
by the way here is a good thread about afrs
http://forums.nasioc.com/forums/showthread.php?t=673369
05 September 2005 | 10:30 AM
#18
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From: 32 cylinders and many cats
I think 11.4 will be OK if the boost levels aren't crazy, intercooler is good, turbo isn't overstretched, don't do top end etc.
I prefer it to come rich of stoich during spool up on a smooth ramp to your final full throttle mixture. I don't deliberately retard the timing either because I don't like how it feels.
I prefer it to come rich of stoich during spool up on a smooth ramp to your final full throttle mixture. I don't deliberately retard the timing either because I don't like how it feels.
05 September 2005 | 05:46 PM
#20
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I wasn't trying to look clever. It was an article I found that I thought might be helpfull. Anyway, glad I brought it up because i've just recieved my wideband air flow meter and i will be starting to set my AFR soon. I'll be going quite a bit richer than I would have. I read one article that said not to make any correction at 20% throttle position or below because your in closed loop mode. What are anyones opinion on that?
05 September 2005 | 06:26 PM
#21
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From: 32 cylinders and many cats
If you are using an S-AFC or piggyback then you will be fighting the ECU's closed loop below the throttle transition point. The ECU may then depending on design apply corrections to other parts of the map or you'll get step changes or adaptation faults.
Note that if you change the MAF voltage then you will also influence the ignition timing as well. Unfortunately the usual requirements are incompatible - beefier fuel system requires lower MAF voltage to get the fuelling right, bigger turbo running big boost often requires higher MAF voltage to get the ignition right. MAF tube diameters complicate it further, best to reflash the OEM ECU or use a standalone.
Note that if you change the MAF voltage then you will also influence the ignition timing as well. Unfortunately the usual requirements are incompatible - beefier fuel system requires lower MAF voltage to get the fuelling right, bigger turbo running big boost often requires higher MAF voltage to get the ignition right. MAF tube diameters complicate it further, best to reflash the OEM ECU or use a standalone.
05 September 2005 | 07:27 PM
#23
Originally Posted by Zen Performance
Perhaps bob can be forgiven, seeing as he actually maps cars for a living, rather than reading theoretical info from a text book.
11.2 seems to be a good compromise with cast pistons, I would be prepared to go leaner on a car with forged pistons, but only as lean as 11.5 with 1.5 bar of boost. If you have an EGT gauge as John says, you can learn a lot. The ideal will depend on application too, long runs flat out on an autobahn will require richer AFRs if the engine is to survive.
Paul
11.2 seems to be a good compromise with cast pistons, I would be prepared to go leaner on a car with forged pistons, but only as lean as 11.5 with 1.5 bar of boost. If you have an EGT gauge as John says, you can learn a lot. The ideal will depend on application too, long runs flat out on an autobahn will require richer AFRs if the engine is to survive.
Paul
06 September 2005 | 06:53 PM
#25
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From: 32 cylinders and many cats
I was going to use one to trim the AFR on a project car with the OEM ECU, fuel cut defender, Dawes, 20G, 2.5, but I was increasing the injector size, engine size and MAF cross sectional area all in proportion so that it would have run the correct timing.
06 September 2005 | 06:55 PM
#26
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From: Under your bonnet
if I tried mapping a flat 4 turbo to .89 it wouldn't have made it off the rollers.Atmo engine yes, but
turbo..84 is begining to push it from what I've seen
Andy
Last edited by Fuzz; 06 September 2005 at 07:01 PM.
06 September 2005 | 07:06 PM
#28
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From: Under your bonnet
Originally Posted by garethbrunt
So when mapped on a power Fc, this is the figure that I should be around after mapping?
I have a wideband located before the turbo where the 4 primarys meet. Is this location ok?!
I have a wideband located before the turbo where the 4 primarys meet. Is this location ok?!
Five minute job to drill a hole and weld a boss in.
Andy
Last edited by Fuzz; 06 September 2005 at 07:09 PM.
