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  • Theory on Fuel Tuning

    I'm working on mapping out my fuel for the AEM, and to do so I had to create some sort of equation to base my map off of. A lot of this is just sitting around thinking about stuff and attempting to make some use of logic and Google. Anyways, here's what I've got:

    We all know:
    Code:
    Duty Cycle = Fuel per Min / Injector Size
    Since injector size is constant on a given engine, we have to solve for fuel per minute:
    Code:
    Fuel per Min = Volume Airflow per Min * %Fuel per CC intake
    To find fuel, we need to know how much gas (air/fuel mixture) is being ingested by the engine. Engine speed is just RPM, and the cylinder will only take air in every other revolution. So, the maximum air ingested by the engine per minute is the product of the cylinder volume and half the RPM.

    This, however, neglects a bunch of other stuff. While the cylinder creates vacuum, drawing air in, not all of the cylinder fills with air (and 0 vac isn't necessarily reached within the cylinder by the time the piston bottoms out). Only a percentage of the cylinder will "fill". I compensated for this with a Volumetric Efficiency figure.
    Another factor is cam duration. The cams are only open for a percentage of the time the cylinder is drawing air in. I oversimplified by taking cam duration and dividing by 360, creating a percent modifier. Finally, things just weren't jiving, so I threw in BSFC, with admittedly no real explaination other than it generated what looked like my working fuel map. It's really a measure of fuel consumption vs power produced, however in this case it's simply fuel consumption vs max theoretical fuel intake:
    Code:
    Volume Airflow per Min = RPM/2 * Cylinder Volume * Load% * BSFC * VE * (Cam Duration / 360)
    To find the percentage of fuel per volume of gas (air/fuel mixture) intake, I simply calculated the volume percent of fuel given 1 fuel molecule and the volume of air for the number of air molecules corresponding to the air/fuel ratio:
    Code:
    %Fuel = 144cc/mol (fuel) / (144cc/mol + 24137 cc/mol * AirFuel Ratio)
    The sheet I created is here:
    http://spreadsheets.google.com/pub?k...NQzOkV_qfcesZQ

    Seems to jive with what I've got.... indicating that my top end is extremely rich (which it is). Any comments or suggestions or further calculation would be greatly appreciated. I'll probably regenerate it with a variable BSFC (to accomodate affects of throttling and such).

    Next up I'd like to put some math into a timing map.....

  • #2
    nice. take it you don't like boost comp?

    http://forum.aempower.com/forum/inde...c,11494.0.html

    Lots of tuners use it. quick and easy fuel tuning.

    tryed to change values in your sheet and it won't do anything. am i retarded i never messed with google docs. nevermind its locked. slow learner.
    Last edited by Your Mom; 04-30-2008, 11:04 AM.

    Most PM answers: F 17X9 +22, R 17X9 +35

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    • #3
      A big problem with boost comp is:
      Well, you need to get the car running and driving first before you can do this.
      I basically look at boost comp as a method of "guess and check", but it's sort of how I came up with my cell-by-cell map (tuned it at a given load level, interped down and up). However, as stated above, you'll still need some sort of method to get the car to WOT. You could potentially guess your way up to it... or you could plug in safe values into an equation and come up with numbers that will work within reason (and not dump fuel from 850cc injectors to the point where you flood the engine). Personally, I think it's a bit more comforting to have an objective reason to plug a number in, and something to correlate to in the event of problems.

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      • #4
        sure i like you logic. most of the time you just need enough to get it to a dyno. then most of the issues are worked out. if your just doing street tuning your logic makes perfect sense.

        Most PM answers: F 17X9 +22, R 17X9 +35

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        • #5
          I agree that it's better to have some theory to fall back on, but what good is it when the equations are filled with made up terms like duration/360? VE isn't a constant either.

          Your duty cycle % is basically going to outline what your HP curve will look like (assuming constant AFR target and pressure). I can tell you right now that you're not going to make peak HP at redline with 232 cams, which is what your computed map says. It'll probably level off around 5k rpms. Similarly, your injection times will go down because your torque will fall off up top.

          What I'd suggest is to find a map from someone running the same cams and similar turbo. If the injectors are different, scale the injection times based on injector size ratio. That'll be your neighborhood. This worked great for tuning my old Haltech. We started with Enthalpy's GT3040 fuel maps, did a couple low boost pulls, cleaned it up at that level and scaled the injection times upto a higher boost level. Fix that, and go back and linearly interpolate between those 2 good rows. Eventhough we the maps were based on a car with different cams and a turbo 2 sizes bigger, the fuel maps were pretty close. Using that method and because Scott knows where MBT timing should happen, it only took us 3.5 hours on the dyno to tune the car to 75%.
          She's built like a Steakhouse, but she handles like a Bistro.

          Comment


          • #6
            Originally posted by Epstein View Post
            I agree that it's better to have some theory to fall back on, but what good is it when the equations are filled with made up terms like duration/360? VE isn't a constant either.
            Duration is simply the anglular distance a cam will hold a valve open during a single rotation of the crank. Duration/ 360 should in theory be the percentage of time the cylinder is drawing air in, but as I said, it's oversimplified. Ideally it'd include lift and an equation to estimate flow percentage based on load/ rpm and the actual cam profile. Acceleration of gases and flow pulses are also not considered.

            I know VE and BSFC are both variable (and honestly, I don't think BSFC belongs in the equation). I'd like to come up with a decent equation for each to create a more accurate model. Really, there are a lot more variables that should be included, but overcomplicating it from the start makes it more difficult to verify current equations through actual testing. As it stands right now, there are way too many assumptions to really be accurate. Yet, the current estimates provide a fairly accurate representation of my off-boost map.

            Your duty cycle % is basically going to outline what your HP curve will look like (assuming constant AFR target and pressure). I can tell you right now that you're not going to make peak HP at redline with 232 cams, which is what your computed map says. It'll probably level off around 5k rpms. Similarly, your injection times will go down because your torque will fall off up top.
            It actually levels off earlier. This is an older dyno (back when I had an EManage Ultimate):


            There are a bunch of factors that contribute to that: small ass turbo (GT2871r), small cams, long intake runners, etc.


            What I'd suggest is to find a map from someone running the same cams and similar turbo. If the injectors are different, scale the injection times based on injector size ratio. That'll be your neighborhood. This worked great for tuning my old Haltech. We started with Enthalpy's GT3040 fuel maps, did a couple low boost pulls, cleaned it up at that level and scaled the injection times upto a higher boost level. Fix that, and go back and linearly interpolate between those 2 good rows. Eventhough we the maps were based on a car with different cams and a turbo 2 sizes bigger, the fuel maps were pretty close. Using that method and because Scott knows where MBT timing should happen, it only took us 3.5 hours on the dyno to tune the car to 75%.
            I could probably pull off a decent map just street driving and fine tuning off of datalogs, then interpolating for the rest of the map (which is how I've pre-dyno tuned in the past). The theory and calculation are more of an after-the-fact attempt at being scientific about tuning fuel. Once that's modeled, I think timing would be a bit easier to play with as well.

            Truthfully, the model I currently have sucks. The only way it'll get any better is through replacing loose assumptions with better equations that represent each portion of the engine, and testing them out against what actually works. While I know that it currently isn't the best way to tune an engine, there's nothing wrong with trying to make it easier in the future.

            Things I'd like to implement are:
            - Turbocharger efficiency table
            - VE table
            - Intake model (runner length, diameter, plenum size, etc)

            Any guesses or theories are more than welcome! I'll try to get an editable version up if anyone wants to play with it.

            Comment


            • #7
              Well the way you have duration figured into the equation all wrong. Multiplied directly by the cylinder volume? The cam duration doesn't limit/scale the cylinder air volume. Not at low RPMS.... I'm not saying that it can't be done, but good luck modeling engine dynamics and fluids in Excel.

              For the love of God, put some S13 240/248 cams in there!
              She's built like a Steakhouse, but she handles like a Bistro.

              Comment


              • #8
                Yea, I've got software and ability to do some CFD on it... just trying to do it through equations for now :P

                I know cam duration isn't the limiting factor at low RPMs, as they should be factored solely into the VE equation (which should then be factored into the actual air flow rate equation). Still working it out while avoiding actual work :P I have a senior flow engineer for a co-driver, so I'll see if he can help with doing an actual model for flow efficiency.

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                • #9
                  Good luck! You know there are programs that exist for that sort of thing. Finding the airflow through the motor is the hard part. It's relatively trival to calculate fueling once you have the flow numbers.
                  She's built like a Steakhouse, but she handles like a Bistro.

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                  • #10
                    I don't think I'd trust CFD for even a ballpark figure unless you had a VERY accurate model from the TB to the turbine inlet... and some good data about exhaust conditions.
                    '18 Chevrolet Volt - Electric fun hatch for DD duty!


                    DefSport Koni Sleeve and Spring Perch Buy!!!
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                    • #11
                      I don't know too much about the AEM EMS in particular. I know it has all the bells and whistles, but I think it should have your standard speed density algorithms using:

                      RPM, displacement
                      VE
                      manifold air pressure
                      air temperature

                      ...to determine air mass flow. If you use this method, the only thing you really have to mess around with is the VE table, and typically VE will have values between 60 and 90 percent anywhere on the table. When you try starting up for the first time, if you put 75 VE all across the map, you should only have up to 25% error from your target AFR(Instead of 14 AFR, you'd have 11-18 - still enough to keep her running)

                      Just use the ideal gas law for air temp compensation curve.

                      P.S: I know this thread is old, and you've probably got everything up and running fine, but this is still useful information. I will try to make an excel spreadsheet tomorrow. I will have the time, but I might forget to check this thread to remind myself.

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                      • #12
                        i made an excel file. i think it might be useful for a lot of people.

                        what you do is fill out your engine and fuel parameters (leave bsfc and fuel density alone, unless you want to use something other than gasoline)

                        then you fill out the ve table(difficult, try finding a map already filled out with a similar turbo) and the desired afr map(easy). this will give you the pulsewidth and duty cycle maps on the second page, in case your engine management requires those parameters instead.

                        i also threw in a temperature compensation curve, which is extremely important for the ecu's airflow model. don't mess with that unless you want extra enrichment at higher temperature. it doesn't affect the tp or duty cycle tables on pg 2, though.

                        finally there's a "real time" simulation where you enter boost, ve, temp at a particular point in time. this will give you adjusted pulsewidth(taking air temperature into account), as well as tq/hp based on bsfc.

                        don't mess with anything in red!
                        Attached Files

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