This is kind of a back door way of figuring out your engines air flow for calculating VE and so on. The math seems correct and makes sense to me.
I know it doesnt take into account boost pressure and so forth but all boost really does is increase your engine VE anyways.
One would think all you would need to do is log:
AFR Through the curve.
Injector Duty cycle/Pulsewidth
RPM
You wont need Baro readings, ambient temp readings or intake temp readings or engine temp readings because that is already calculated in the pcm to adjust VE based on those readings
This should work in both closed loop and open loop, because the programmed VE of the engine isnt changed but the physical VE is changed based on mods. Which is where your LTFT take place. They are striving to achieve 14.7 to one so therefore they adjust pulsewidth to make that target AFR. All you would have to do is update your VE tables with the new calculated ones from this set of equations. In theory your PCM adaptive should then get closer to 0 for better drivability.
While this is interesting, this way of estimating VE is only usefull if NO VE existed and you had to build one from scratch. Megasquirt guys have to do this often, which I suspect is where this originated. IMO, the VE map below 200KPa is very, very accurate in the stock ECU.
Problems with this is that its only a gross estimate of mass air flow below or at Barometric pressure. Why?:
1) Injector mass fuel rate will be changing(dropping) as manifold pressure increases.
2) Actual fuel mass flow rate can differ up to 5% with manufacturing tollerances
3) VE defined by these Eq is relative to BP,,as you stated. BUT,, VE listed in the ECU is relative to the Manifold pressure,,,BIG BIG difference. The problem appling this VE estimate to a turbocharged engine is this equation:
AirFlow in3/ min = AirFlowlbs/hr x 386.58
The conversion coefficient 386.58, used to convert Mass flow into Volumeteric flow, has a unit of pressure built into it. This pressure is static and most likely 101Kpa. If you can't vary the pressure the VE value is bogus above this pressure.
Also, the ECU is model based. So instead of using simple look-up tables, a' la AEM, it uses an equation 800% more complex than the one you linked to, to calculate mass air flow into and our of the engine. Imo SCT PRP guys should limit their screwing around with the VE map.
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Last edited by duster360 : 10-17-2008 at 09:01 AM.
The problem appling this VE estimate to a turbocharged engine is this equation:
AirFlow in3/ min = AirFlowlbs/hr x 386.58
It is not that hard to convert the constant into an equation taking into account the pressure and temperature.
Under low pressure, if I remember right, air is behaving like a perfect gaz. Which means you can know pretty much everything you want from the equation:
P.V = n.R.T
P: Pressure, V: Volume, n: atomic mass, R: perfect gaz constant, T: temperature
It would be easy to built a quick excel table using this formula.
It is not that hard to convert the constant into an equation taking into account the pressure and temperature.
Under low pressure, if I remember right, air is behaving like a perfect gaz. Which means you can know pretty much everything you want from the equation:
P.V = n.R.T
P: Pressure, V: Volume, n: atomic mass, R: perfect gaz constant, T: temperature
It would be easy to built a quick excel table using this formula.
I like his idea
Its not that easy,,The Ideal Gas Law is only for static gases in a finite volume. A moving gas loses pressure and density as its velocity increases, independant of its temperature.
Regardless of being inaccurate, using this as a VE estimate in also unneccesary. The VE table in the stock ECU is very accurate as it is.
Yes the stock VE is very accurate on a stock motor. Once you change out a ported intake/exhaust, different turbo, a ported head with bigger cams, different design/compression pistons the VE changes in the motor.
I'm basically trying to come up with a way/find a way to make VE adjustments now before I put my built motor in the car. So that its not all trial and error. (which I'm not a real big fan of doing on a project such as this)
I have the speed density calculation from Chrysler(when i went through class) but I don't think that will have much merit in this case since Speed Density is for calculating injector pulsewidth verses VE.
Even if you had an equation model that applies the proper "adjustments" for all the variables you listed, you still would need actual data from those modified parts.
Since the ecu is model based, it can be used on many different engines. This is great but the calibration engineers still need the actual "flow data" to plug into the equation. The stock model has adjustments(called coeficients) for:
Intercooler pressure drop
Air cleaner pressure drop
PCV valve flow
IAC valve air flow
EGR valve flow(not apply to turbo but its in there)
These are all mapped out vs RPM FIRST in the lab, so the VE calculations can be properly adjusted. If you have no such data then, imo, you are stuck with trial and error.
Be aware, what you are really trying to accomplish is terribly complex. This is usually left to more expensive engine simulation software($500-$1200) Read this paper on engine modeling and see if it makes sense. MODELING OF A TURBOCHARGED SI ENGINE
All the "Flow Coefficients" they are talking about are specific to your parts and are what you need to make a decent VE estimate.
Lotus and Performance Trends both offer trial versions of their simulations I suggest you give them a try.
This is one reason I loved the "autotuner" feature within Megasquirt when I had it on my car. Basically you would tune it to the best you could then set autotune...it would auto change your VE table for you. You would still have to fine tweak it, but at least it did most of the work for you
This is one reason I loved the "autotuner" feature within Megasquirt when I had it on my car. Basically you would tune it to the best you could then set autotune...it would auto change your VE table for you. You would still have to fine tweak it, but at least it did most of the work for you
The stock ECU has a built in "AutoTuner" too,,,Its called Long Term Fuel Trim.
Things you need to log or calculate; RPM, pressure ratio, A/F ratio, total weight of fuel being injected at each cylinder. this last one is the pain. the enhanced obd2 on the ngc will output this information though. So lets say I log a run and it tells me that at 3000RPM, I have a pressure ratio of 1.5, A/f of 12.5, and the pcm injected a total of 68765 micrograms of fuel for cylinder one.
1. Calculate your engines volume in cubic inches and divide by number of cylinders. This will give you the total cubic inches of air each cylinder will injest at 100% vE. Then multiply this number by the pressure ratio you are working at.
this tells us that at a pressure ratio of 1.5, a 2.4 liter engine should injest 54.921375 cubic inches of air if it would have a 100% VE at that pressure.
2. Now calculate the weight of that air. using the ideal gas law, we know that the weight of air(standard conditions) is .07674 pounds per square foot. So, lets convert our cubic inches into cubic feet, and then find the weight of the air in it.
This tells us that at a pressure ratio of 1.5 and 100% ve , each cylinder should injest .0024390430078125 pounds of air. Now lets calculate the fuel for this.
3. take the ridiculously long number we just calculated and divide it by your a/f ratio you are looking for. this gives us the weight of fuel in pounds. We then need to convert this to milligrams.
This is the weight of the fuel in milligrams needed for 100% ve at a pressure ratio of 1.5.
4. Now, we divide the amount our data logger says is being injected, which is usually in micrograms by the amount we just calculated. 1 milligram equals 1000 micrograms.
Example:
(68765 / 72828.2 = .94420842) OR 94.420842% VE at 3000rpm with a pressure ratio of 1.5.
Now, you can take this a step further and use the multipliers for temp and pressure to be more exact, but I'm not going to go into that.
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Last edited by sour kraut : 10-22-2008 at 10:08 AM.
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Useful Air Flow calculation.
Linear Mode
