Speed Density Implementation Discussion
Oct 21, 2008, 05:53 AM
#16
I would think it would be irrelevant because the baro is necessary as the air is traveling through the MAF so it can calculate the density of the air. Once it is Post MAF, it becomes "relatively speaking" irrelevant.
Basically, with a MAF system you are calculating the amount of air pre-turbo, with speed density, you are calculating it post turbo, at the intake manifold. You are essentially doing the same thing, calculating the air mass/volume, just at different locations.
For speed density to really work properly, you would need it calculated at the point just before it enters the engine, hence, the intake manifold because of the changes the turbo makes to the intake charge (Air temperature, density changes).
Basically, with a MAF system you are calculating the amount of air pre-turbo, with speed density, you are calculating it post turbo, at the intake manifold. You are essentially doing the same thing, calculating the air mass/volume, just at different locations.
For speed density to really work properly, you would need it calculated at the point just before it enters the engine, hence, the intake manifold because of the changes the turbo makes to the intake charge (Air temperature, density changes).
Oct 21, 2008, 06:03 AM
#17
Evolved Member
iTrader: (5)
Join Date: Dec 2003
Location: NW Georgia
Posts: 1,378
Likes: 0
Received 0 Likes
on
0 Posts
^ which is exactly why I am installing an IAT.
I don't know if baro is important, once the turbo has compressed the air then is it not irrelevant what the outside air pressure is?
BUT does the amount of fuel you are injecting depend on IAT as well? OR are you saying make Fuel map based on RPM/MAP and then leave the ECU to do its own IAT corrections like its doing now?
I don't know if baro is important, once the turbo has compressed the air then is it not irrelevant what the outside air pressure is?
BUT does the amount of fuel you are injecting depend on IAT as well? OR are you saying make Fuel map based on RPM/MAP and then leave the ECU to do its own IAT corrections like its doing now?
Oct 21, 2008, 06:05 AM
#18
Evolved Member
iTrader: (6)
Join Date: Jul 2005
Location: Charleston, SC
Posts: 1,228
Likes: 0
Received 0 Likes
on
0 Posts
On a side note Tephra, if you look at the backside of the manifold, right next to where the vacuum line for the brakes connects there is an unused boss there. I drilled and tapped mine there for an 1/8" NPT port, and it turned out super clean. When the GM AIT is installed, it is real close to the center of the manifold, and located back behind the manifold where you can hardly see the GM AIT chilling there. Makes for a very clean install.
Oct 21, 2008, 06:06 AM
#19
Evolved Member
iTrader: (6)
Join Date: Jul 2005
Location: Charleston, SC
Posts: 1,228
Likes: 0
Received 0 Likes
on
0 Posts
Yeah, that's what I was getting at, the best possible place would be in the intake manifold plenum, only about 8" before it goes into the actual head.
Oct 21, 2008, 06:07 AM
#20
On a side note Tephra, if you look at the backside of the manifold, right next to where the vacuum line for the brakes connects there is an unused boss there. I drilled and tapped mine there for an 1/8" NPT port, and it turned out super clean. When the GM AIT is installed, it is real close to the center of the manifold, and located back behind the manifold where you can hardly see the GM AIT chilling there. Makes for a very clean install.
My only concern is moisture settling in the sensor and potentially causing an issue if it is upside down.
Oct 21, 2008, 06:07 AM
#21
sorry you are correct... CAT not IAT
well we already have a timing correction for IAT - it wouldn't be too hard (and required since the IAT will be gone once you remove the MAF) to overwrite the IAT with CAT variable.
I will have to check for the temp correction for Fueling - no doubt there is one.
So we can easily change the LOAD axis for Fuel/Ignition to MAP, using the current map as a basis.
But where does VE or indeed SD come into play now? We havn't really done anything yet
well we already have a timing correction for IAT - it wouldn't be too hard (and required since the IAT will be gone once you remove the MAF) to overwrite the IAT with CAT variable.
I will have to check for the temp correction for Fueling - no doubt there is one.
So we can easily change the LOAD axis for Fuel/Ignition to MAP, using the current map as a basis.
But where does VE or indeed SD come into play now? We havn't really done anything yet
Oct 21, 2008, 06:09 AM
#23
On a side note Tephra, if you look at the backside of the manifold, right next to where the vacuum line for the brakes connects there is an unused boss there. I drilled and tapped mine there for an 1/8" NPT port, and it turned out super clean. When the GM AIT is installed, it is real close to the center of the manifold, and located back behind the manifold where you can hardly see the GM AIT chilling there. Makes for a very clean install.
I think I know what you mean - picture thou if you don't mind
I will get my 'tapper' to tap that at the same time... Just need to go to PIRTEK to get a brass screw nipple thingy
Oct 21, 2008, 06:10 AM
#24
Yeah so this is what you were saying about key-on, when the RPM is 0, you can take a reading and this is your current atmo pressure, then you can use this as the offset (adder/subtracter whatever you call it) for the boost target routines...
Oct 21, 2008, 06:13 AM
#25
The axis for the maps, ideally, should stay as what they are, which is a mass airflow/rev. That's really what our load number is...a temp and baro corrected mass airflow/rev number.
This mass airflow is currently calculated from pressure, temperature, and VE (basically our MAF scaling). Changing to a SD setup, we will need to use pressure and temp somewhere else now, like in the IM. So, a pressure sensor and IAT sensor will be needed. The VE correction will have to be from a lookup table that you can adjust, so that when new parts are added to the car, you can make appropriate changes there instead of making changes to your main fuel/ignition tables.
There are really only a few equations that we have to understand and use:
1. Volumetric airflow through an engine:
Airflow (CFM) = PR[RPM*V.E.*Cid/3456]
PR=Pressure ratio=(boost in psi+14.7)/14.7
RPM = RPM of engine
V.E. = volumetric efficiency at RPM being measured
Cid=cubic inch displacement= 122 for our 2.0L engines
2. Conversion of volumetric airflow rate to mass airflow rate, using PV=nRT:
n (lbs/min) = P (psia) x V (CFM) x 29 / (10.73[ft3·psi· °R-1·lb-mol-1] x T)
So, in #2 above, you can substitute in the whole #1 equation to get your mass airflow into your engine. As you can see, the only variables are:
Temp (can be logged/input via IAT sensor)
Pressure (can be logged/input via map sensor)
RPM (already logged/input)
VE (need a 2D/3D lookup table for this - possibly use something like MAF scaling)
I think if you setup axes like this, you don't have to worry about baro. You are using psia for your calculation, which will then calculate the amount of fuel you need. So, since your never have and boost targets, you won't need to worry about baro.
Eric
This mass airflow is currently calculated from pressure, temperature, and VE (basically our MAF scaling). Changing to a SD setup, we will need to use pressure and temp somewhere else now, like in the IM. So, a pressure sensor and IAT sensor will be needed. The VE correction will have to be from a lookup table that you can adjust, so that when new parts are added to the car, you can make appropriate changes there instead of making changes to your main fuel/ignition tables.
There are really only a few equations that we have to understand and use:
1. Volumetric airflow through an engine:
Airflow (CFM) = PR[RPM*V.E.*Cid/3456]
PR=Pressure ratio=(boost in psi+14.7)/14.7
RPM = RPM of engine
V.E. = volumetric efficiency at RPM being measured
Cid=cubic inch displacement= 122 for our 2.0L engines
2. Conversion of volumetric airflow rate to mass airflow rate, using PV=nRT:
n (lbs/min) = P (psia) x V (CFM) x 29 / (10.73[ft3·psi· °R-1·lb-mol-1] x T)
So, in #2 above, you can substitute in the whole #1 equation to get your mass airflow into your engine. As you can see, the only variables are:
Temp (can be logged/input via IAT sensor)
Pressure (can be logged/input via map sensor)
RPM (already logged/input)
VE (need a 2D/3D lookup table for this - possibly use something like MAF scaling)
I think if you setup axes like this, you don't have to worry about baro. You are using psia for your calculation, which will then calculate the amount of fuel you need. So, since your never have and boost targets, you won't need to worry about baro.
Eric
Last edited by l2r99gst; Oct 21, 2008 at 06:18 AM.
Oct 21, 2008, 06:21 AM
#28
good info. i'm just gonna sit back and listen. i'm still trying to understand bez's patch, SD theory, and the ecu itself.
on a side note, i'm happy that the MAF code is pretty consistant through all of the ecu renditions. should make porting easy when the time has come to convert.
about the IAT, location is key as bez stated that we need IAT temps and not manifold temps, but thats for the future when the early odds and ends are worked out.
on a side note, i'm happy that the MAF code is pretty consistant through all of the ecu renditions. should make porting easy when the time has come to convert.
about the IAT, location is key as bez stated that we need IAT temps and not manifold temps, but thats for the future when the early odds and ends are worked out.
Oct 21, 2008, 06:26 AM
#29
presumable the best location for IAT (or CAT) is where mitsu designed it to go? ie next to the map directly in the intake manifold.
Mr Turco - Both options are (at this stage) as easy/hard as each other. I would prefer to get a solution that's the best rather than easiest. At the moment I am leaning towards MAP based axis using the stock (albeit modified) IAT (CAT) corrections for both Fuel and Timing. (Let me find the fuel correction first thou)
Mr Turco - Both options are (at this stage) as easy/hard as each other. I would prefer to get a solution that's the best rather than easiest. At the moment I am leaning towards MAP based axis using the stock (albeit modified) IAT (CAT) corrections for both Fuel and Timing. (Let me find the fuel correction first thou)
Oct 21, 2008, 08:14 AM
#30
Sorry for the length, but I’m hoping to add a little insight.
There are two main types of MAP based ECUs that I have come across.
Table look-up systems
Calculated airflow system
In table look-up system, the ECU takes the MAP reading and the Engine RPM reading and looks up the injector pulsewidth from a 3-D injector pulse width table. From here, you have correction tables to deal with intake air temperature compensation, coolant temp comp, accel comp, etc. The intake air temp comp will be near a line that slopes down and matches closely with the ideal gas law. As temp increases with respect to absolute temperature, your density drops. The pulsewidth table is where the vast majority of the tuning is done. The ignition advance is handled in the same look-up table manner and has similar corrections applied based on various parameters.
In a calculated ariflow system, you have a background process that calculates the airflow into the engine by looking at engine speed, air temperature, and MAP readings. The math is based simply on the ideal gas law and you have user-definable constants used in the equation that are engine specific. These constants account primarly for the volume of the motor, injector size and the type of fuel used (stochiometric A/F ratio value typically used). Along with this system, you also have a target A/F ratio table and a VE table. The target A/F ratio table is done in either lambda values or in actual desired A/F ratios (this will affect the constants used above) and is a 3-D table with RPM and MAP as the x and y axis, respectively. These values are used to calculate the IPW that is needed based on the desired A/F ratio and the amount of air entering the engine calculated in the main equation. The VE table is a scaling system to account for the engine volumetric efficiency. Simply, if your table value is 0.85 at 6000 RPM and 300kPa, it takes the calculate airflow from the ideal gas equation from the main process, and multiplies it by 0.85 under those given engine conditions. There are also IAT and coolant temp corrections, however, they are generally pretty flat since you have already compensated for the density change due to air temperature in your main process equation.
From a tuning perspective, the VE based system is much better, IMO. When you change mods, but want to keep the A/F ratio relatively constant, you adjust the VE table to get back to your target A/F ratio. If you want to change A/F ratio and have the VE table already setup, you just change the A/F table. The ignition advance is handled similarly to the first system where it is more of a look-up type system since it is less dependent on temperature. Systems that uses this method include AEM (“boost comp”), MOTEC, Autronic, Bosch and many OEM MAP based systems.
From the setup perspective, I see the first method being the easiest to implement. The only systems that I can think of that use this method are the AEM (non-boost comp), Hydra Nemisis, and possibly the older Haltech systems (although I don’t have much Haltech experience and it’s been a while so I can’t remember 100%).
I have no idea how the coding works but if it's possible, the second (VE based system) will be the better product in the end.
Barometric pressure is useful still in turbo motors. When properly implemented, it can help accommodate the VE lose associated with the higher pressure ratios that the turbo must run at altitude. However, I would think of this as more a secondary goal, since most people don't change barometric pressure on a commute much more than a few hundred feet. I will say this though, I believe it was the RX-7 that used the MAP sensor on start up to find barometric pressure. It is pretty common to hear of RX-7s that pop while going up a mountain because there is nothing extra to see the baro is dropping and the car would run into detonation because of the higher exhaust back pressures. Best bet is to make it a goal to incorporate a separate baro sensor in the long run. If you look in the other thread though, I posted the values from the FSM and it looks to be a pretty standard 1 bar MAP sensor. It should be pretty easy to implement a replacement sensor.
There are two main types of MAP based ECUs that I have come across.
Table look-up systems
Calculated airflow system
In table look-up system, the ECU takes the MAP reading and the Engine RPM reading and looks up the injector pulsewidth from a 3-D injector pulse width table. From here, you have correction tables to deal with intake air temperature compensation, coolant temp comp, accel comp, etc. The intake air temp comp will be near a line that slopes down and matches closely with the ideal gas law. As temp increases with respect to absolute temperature, your density drops. The pulsewidth table is where the vast majority of the tuning is done. The ignition advance is handled in the same look-up table manner and has similar corrections applied based on various parameters.
In a calculated ariflow system, you have a background process that calculates the airflow into the engine by looking at engine speed, air temperature, and MAP readings. The math is based simply on the ideal gas law and you have user-definable constants used in the equation that are engine specific. These constants account primarly for the volume of the motor, injector size and the type of fuel used (stochiometric A/F ratio value typically used). Along with this system, you also have a target A/F ratio table and a VE table. The target A/F ratio table is done in either lambda values or in actual desired A/F ratios (this will affect the constants used above) and is a 3-D table with RPM and MAP as the x and y axis, respectively. These values are used to calculate the IPW that is needed based on the desired A/F ratio and the amount of air entering the engine calculated in the main equation. The VE table is a scaling system to account for the engine volumetric efficiency. Simply, if your table value is 0.85 at 6000 RPM and 300kPa, it takes the calculate airflow from the ideal gas equation from the main process, and multiplies it by 0.85 under those given engine conditions. There are also IAT and coolant temp corrections, however, they are generally pretty flat since you have already compensated for the density change due to air temperature in your main process equation.
From a tuning perspective, the VE based system is much better, IMO. When you change mods, but want to keep the A/F ratio relatively constant, you adjust the VE table to get back to your target A/F ratio. If you want to change A/F ratio and have the VE table already setup, you just change the A/F table. The ignition advance is handled similarly to the first system where it is more of a look-up type system since it is less dependent on temperature. Systems that uses this method include AEM (“boost comp”), MOTEC, Autronic, Bosch and many OEM MAP based systems.
From the setup perspective, I see the first method being the easiest to implement. The only systems that I can think of that use this method are the AEM (non-boost comp), Hydra Nemisis, and possibly the older Haltech systems (although I don’t have much Haltech experience and it’s been a while so I can’t remember 100%).
I have no idea how the coding works but if it's possible, the second (VE based system) will be the better product in the end.
Barometric pressure is useful still in turbo motors. When properly implemented, it can help accommodate the VE lose associated with the higher pressure ratios that the turbo must run at altitude. However, I would think of this as more a secondary goal, since most people don't change barometric pressure on a commute much more than a few hundred feet. I will say this though, I believe it was the RX-7 that used the MAP sensor on start up to find barometric pressure. It is pretty common to hear of RX-7s that pop while going up a mountain because there is nothing extra to see the baro is dropping and the car would run into detonation because of the higher exhaust back pressures. Best bet is to make it a goal to incorporate a separate baro sensor in the long run. If you look in the other thread though, I posted the values from the FSM and it looks to be a pretty standard 1 bar MAP sensor. It should be pretty easy to implement a replacement sensor.