Question on tuning AFR's for boost.
09-18-09, 10:10 PM
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Question on tuning AFR's for boost.
I started toying with tuning because I thought it would be a good skill to have, and while it was challenging to take things slow at first, I'm glad I have learned how to. I stabbed the tuning at vacuum but am waiting to do so for boost. My questions are, WHY are AFR's richer under boost? Is it simply because there is more pressure there? Are there any good formula's for a target AFR? Would setting the boost in 5 lb increments per tune be a good strategy? Will the POWER FC be atleast close to accurate at 5 lbs?
Thanks, everyone.
Thanks, everyone.
09-19-09, 11:52 AM
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Well of course LOL. I was just wondering why they need to be richer under boost... 14:1 for 14.7psi of air... then add 1 bar so it's 28:2 which is still 14:1... And yes I for sure need a book. think I'll check out chuck's notes.
09-19-09, 12:56 PM
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Richer mixture is just band-aid in turbocharged engines, be it rotary or piston engine... With all that excess fuel, you are just trying cool intake air and mainly combustion chamber, prevent preignition, detonation... but gasoline isn´t exactly good in it... For example, with water injection, you could tune air-fuel to figures used for N/A engines like 13,5:1, whereas on pump gas for given power level rather 10.x figures...
09-19-09, 02:25 PM
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[QUOTE=SirCygnus;9506146]more air, more fuel.
QUOTE]
not really what he was asking.. if the ratio is the same then you automatically get more fuel for your more air. like the other guy said more fuel is used to absorb extra heat which would otherwise cause detonation. this extra heat is cuased from the extra pressures from the turbo and more fuel being burnt. in essence the unburned fuel is doing the same as would water/meth injection. just not as effeciently or environmentaly friendly
QUOTE]
not really what he was asking.. if the ratio is the same then you automatically get more fuel for your more air. like the other guy said more fuel is used to absorb extra heat which would otherwise cause detonation. this extra heat is cuased from the extra pressures from the turbo and more fuel being burnt. in essence the unburned fuel is doing the same as would water/meth injection. just not as effeciently or environmentaly friendly
09-20-09, 09:41 AM
#6
The richer mixture does cool combustion temps to a point, which is important for cat life especially. But it also slows the speed at which the flame propagates throughout the combustion chamber. This reduces the chance of hotspots forming and other types of uncontrolled burns which can damage the engine or at least heat up the chamber. The type of damage in question is called pre ignition (uncontrolled burn before the plug fires) and detonation (uncontrolled burn after the spark plug has fired).
You CAN be too rich, an article from innovate Motorsports:
full article: http://www.innovatemotorsports.com/resources/rich.php
now that's for piston engines but the basic principles apply, although the rotary combustion chamber has an unusual shape that requires more fuel.
You CAN be too rich, an article from innovate Motorsports:
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... 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...
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...
now that's for piston engines but the basic principles apply, although the rotary combustion chamber has an unusual shape that requires more fuel.
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09-20-09, 11:45 PM
#9
As a general rule, ignition advance increases with rpm but decreases with absolute load (often confused with calculated load, even though they are different). Here is how the SAE defines absolute load:
and here is a factory timing map on a turbocharged Subaru engine
the main measurement of engine load is done with a MAF sensor on this vehicle. Look at each row of rpm. At a given rpm, when more load is applied (more air in the combustion chamber), ignition advance decreases. Now look at each load column. At a given level of load (anything over 1.00 is boost), ignition timing increases with rpm. It gets a little confusing though on speed-density based systems, because airflow is not directly measured. Pressure is measured and airflow/absolute load is assumed. The changing loads at a given rpm/pressure combination (volumetric efficiency changes) are reflected in the shape of the timing advance curves:
That's the leading timing map I'm using right now on straight 93 octane fuel with my PFC. Y axis is in kg/cm^2. 10000 = 0psi, 20000 = 14.22 , 30000 = 28.44 . I run 16psi with a large street port and T04R, so I'm between the rows labeled 20000 and 22000.
To directly answer this question: you can't fire the plug too early because 1) the explosion will actually work against the engine, or 2) when combustion pressure peaks, the piston/rotor will not be in a position to have maximum leverage on the crank/eshaft in order to take full advantage of the explosion. The optimum time to fire the plug depends on flame front speed/propagation. That in turn is a function of fuel quality, physical characteristics of the engine, and running conditions. See this article, "Spark Timing Myths Debunked" : http://www.innovatemotorsports.com/resources/myths.php
and here is a factory timing map on a turbocharged Subaru engine
the main measurement of engine load is done with a MAF sensor on this vehicle. Look at each row of rpm. At a given rpm, when more load is applied (more air in the combustion chamber), ignition advance decreases. Now look at each load column. At a given level of load (anything over 1.00 is boost), ignition timing increases with rpm. It gets a little confusing though on speed-density based systems, because airflow is not directly measured. Pressure is measured and airflow/absolute load is assumed. The changing loads at a given rpm/pressure combination (volumetric efficiency changes) are reflected in the shape of the timing advance curves:
That's the leading timing map I'm using right now on straight 93 octane fuel with my PFC. Y axis is in kg/cm^2. 10000 = 0psi, 20000 = 14.22 , 30000 = 28.44 . I run 16psi with a large street port and T04R, so I'm between the rows labeled 20000 and 22000.
If a later crank angle produces more power, why does ignition tuning not follow this? (more advance, more power)
09-21-09, 01:42 AM
#10
Now take a look at these fuel maps, paying attention mostly to the progression of fuel delivered to the engine rather than the numbers themselves. I have the base map tab selected with "Base x INJ" mode. It shows the total pulsewidth at given rpm vs boost (the basemap values with the INJ map figured in):
default PFC map:
notice how fuel delivery drops quickly after 6000 rpm? That's because of stock turbos and stock ports. Now here's my map in the equivalent area:
First, note that I haven't precisely tuned every single cell, especially the high boost ones under 4000 rpm (because I can't hit 15+psi at that rpm on this turbo and porting combo). Now see how fuel delivery continues to 6800 and then barely tapers off by 8000? that's due to a much higher flowing turbo and the larger ports. Remember though that the PFC interpolates between rows of pressure units. This map gives me an AFR in the mid 11's, getting slightly leaner up top where torque is start to fall off.
One method for figuring out when to taper off fuel is to set all your pulsewidth values in the high rpm cells of a given row to the same value. So set the 6000, 6400, all the way up to 8000 to a value like 22.0 (don't use that number just because I threw it out there). Now watch the AFR curve you are logging with your datalogit. The rpm where it gets richer is where your volumetric efficiency is dropping off. That's where you taper the fuel because the engine is past peak efficiency. So say it's at 6400 rpm. You keep the 6000 and 6400 rpm cells near 22.0 or whatever you started with. Then from 6800 rpm on you use the INJ map to drop fuel delivery by say 1% for each cell. The AFR curve should flatten out; if it were at 11.0:1 and then steadily richened up, dialing back injector pulsewidth should bring your AFR back near 11 again. Then you can do more pulls after that and adjust individual cells to smooth things out further. A similar type of method is often employed on the AEM EMS; they have what's called a boost compensation table.
There are also Excel spreadsheets floating around that can do all this for you to an extent. But that is just one tool you may have available to use and can't always guarantee optimum results. Sometimes it takes a level of trial and error to see what the motor likes.
default PFC map:
notice how fuel delivery drops quickly after 6000 rpm? That's because of stock turbos and stock ports. Now here's my map in the equivalent area:
First, note that I haven't precisely tuned every single cell, especially the high boost ones under 4000 rpm (because I can't hit 15+psi at that rpm on this turbo and porting combo). Now see how fuel delivery continues to 6800 and then barely tapers off by 8000? that's due to a much higher flowing turbo and the larger ports. Remember though that the PFC interpolates between rows of pressure units. This map gives me an AFR in the mid 11's, getting slightly leaner up top where torque is start to fall off.
One method for figuring out when to taper off fuel is to set all your pulsewidth values in the high rpm cells of a given row to the same value. So set the 6000, 6400, all the way up to 8000 to a value like 22.0 (don't use that number just because I threw it out there). Now watch the AFR curve you are logging with your datalogit. The rpm where it gets richer is where your volumetric efficiency is dropping off. That's where you taper the fuel because the engine is past peak efficiency. So say it's at 6400 rpm. You keep the 6000 and 6400 rpm cells near 22.0 or whatever you started with. Then from 6800 rpm on you use the INJ map to drop fuel delivery by say 1% for each cell. The AFR curve should flatten out; if it were at 11.0:1 and then steadily richened up, dialing back injector pulsewidth should bring your AFR back near 11 again. Then you can do more pulls after that and adjust individual cells to smooth things out further. A similar type of method is often employed on the AEM EMS; they have what's called a boost compensation table.
There are also Excel spreadsheets floating around that can do all this for you to an extent. But that is just one tool you may have available to use and can't always guarantee optimum results. Sometimes it takes a level of trial and error to see what the motor likes.
09-21-09, 02:45 AM
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You should read both posted articles. Maximum leverage occurs around 45° ATDC, you want pressure peak to occur a bit earlier around 20° ATDC. We usually see more power with more advance, because we are using stupidly rich mixtures, with their slower burn you are just adjusting peak pressure to occur at right point, but this approach is inefficient and totally silly. You will be advancing timing to the point where combustion will fight against movement of rotor/piston, excessive heating of combustion chamber-preignition, detonation-you will end up with what you tried avoid... On the other hand, when you use proper air-fuel ratio and retarded timing, you will have more power, torque, less unburned HC, lower BSFC, win win situation... of course normal timing principles apply here, more RPMs/more advance, Higher load/less advance, increased VE, boost pressure, compression ratio=higher charge density/less advance
09-21-09, 02:37 PM
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I started toying with tuning because I thought it would be a good skill to have, and while it was challenging to take things slow at first, I'm glad I have learned how to. I stabbed the tuning at vacuum but am waiting to do so for boost. My questions are, WHY are AFR's richer under boost? Is it simply because there is more pressure there? Are there any good formula's for a target AFR? Would setting the boost in 5 lb increments per tune be a good strategy? Will the POWER FC be atleast close to accurate at 5 lbs?
Thanks, everyone.
Thanks, everyone.
afrs are richer under boost, for a few reasons. one is that to burn EVERYTHING it helps to have a little extra fuel, the combustion chamber shape on a rotary isn't ideal.
secondly extra fuel will resist preignition, and lower EGT's.
the AMOUNT of extra fuel you need depends on a lot of little factors, like intake temps, boost pressure, VE of the engine, spark plug heat range, oil and water temps, etc etc.
09-23-09, 11:49 AM
#14
Very interesting thread.
Why the innovate article says that the mixture burns faster at 12.5-13 afr and the book "How to tune and modify engine management system" says that it burns faster at 11.1 afr? I'm a bit confused. Someone can explain?
http://www.innovatemotorsports.com/resources/myths.php
http://books.google.ca/books?id=RYsg...0ratio&f=false
Why the innovate article says that the mixture burns faster at 12.5-13 afr and the book "How to tune and modify engine management system" says that it burns faster at 11.1 afr? I'm a bit confused. Someone can explain?
http://www.innovatemotorsports.com/resources/myths.php
http://books.google.ca/books?id=RYsg...0ratio&f=false
09-23-09, 12:27 PM
#15
I read through the passage in question. I don't know where that number 11:1 comes from. There is no footnote or endnote cited and very little details were given. It contradicts conventional wisdom and so it's pretty hard to take the claim seriously with so little information about it. Was this a lab test in some kind of sealed container? Or was this test done with an actual engine, and if so what kind of engine?
look through the parts on valve timing in the spark advance section of the Hartman book. It explains driveability tuning on a ported rotary engine to an extent. You need a lot more fuel and timing at low loads to get the car to run smoothly, especially if you want to idle at a lower speed. With the large streetport I have, under 900 rpm it gets a lot rougher due to the lack of vacuum.
look through the parts on valve timing in the spark advance section of the Hartman book. It explains driveability tuning on a ported rotary engine to an extent. You need a lot more fuel and timing at low loads to get the car to run smoothly, especially if you want to idle at a lower speed. With the large streetport I have, under 900 rpm it gets a lot rougher due to the lack of vacuum.
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