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Thread: Restrictor Choke RPM

  1. #21
    The amount of fuel injected is pretty arbitrary and depends mostly on how much you tell your Engine-ECU to inject under the given circumstances. So Wesley is right that it's too complicated.

    Your calculation is off again, I think. Shouldn't it be something like <max possible rpm> = <max restrictor throughput> / 225[cc/rpm].

    Since that value assumes a continuous flow, it is a very rough estimate. Taking valve timing into the equation the, result will be different.
    Cheers,

    Josef Duschl
    www.munichmotorsport.de Alumni
    2007&2008 DAQ & Steering Wheel
    Solving the problems, you wouldn't have without computers.

  2. #22
    So, using the oft-quoted 72.6g/s flow rate and assuming air density to be at atmospheric (1.2kg/m^3) within the engine at 100%VE,

    1.2 kg/m^2 = .0012 g/cm^3

    [total mass flow] = [cylinder fills per second][displacement per rev][density of air]

    72.6 g/s = (RPM/60 s*m^-1) * (225cm^3 * .0012 g/cm^3)

    RPM = 16,133 @ theoretical choked flow.

    Now, clearly that is a very high estimate. But there are a number of factors that were mentioned previously that make it impossible for the engine to create that kind of flow through the restrictor.

    It is on this fact that I disagree with almost every design judge, in that flowbench data is just about useless when it comes to restrictors. An engine is a transient environment. While you can do studies with flowbenches to minimize flow loss through the restrictor, you are never going to see 72 g/s unless you have an infinitely large plenum to create a constant pressure differential. The calculations tell us our F4i should choke at about 14.5k, but in reality it's at about 12k. I'd imagine, similarly, that the Aprilia chokes at somewhere around 13, which, while it is outside the operating range, as you approach mach, your flow losses skyrocket. So you lose power not just at choked flow, but up to it.
    Wesley
    OU Sooner Racing Team Alum '09

    connecting-rods.blogspot.com

  3. #23
    If you have access to an accurate, calibrated AFR meter, then one way of measuring air mass flow directly during steady state dyno running is to measure fuel mass flow accurately and multiply by AFR.

    I've seen this done by OEMs to measure VE.

    Regards, Ian

  4. #24
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    Wesley,

    Using the equation you have there an F4i would choke at 12,100 RPM which seems to agree with your observation.
    I agree that you can't actually reach fully choked flow over a complete engine cylcle. But you will choke your restrictor and see 72g/s over portions of the cycle and as a result VE will drop because MAP will have to decrease. This will occur before you reach the theoretical choke RPM but is a function of many variables and can be mittigated somewhat. I would have to look up the data but I seem to recall reaching 68~70g/s.
    Josh Gillett
    Oregon State FSAE '04-'06

  5. #25
    Right, as the pressure waves peak, you'll probably see choked flow, but it will be oscillating madly, and you'll average less than that.

    And I guess my math is broken... I blame MS Calc.
    Wesley
    OU Sooner Racing Team Alum '09

    connecting-rods.blogspot.com

  6. #26
    Right, as the pressure waves peak, you'll probably see choked flow, but it will be oscillating madly, and you'll average less than that.
    The higher the rpm the more even the massflow through the restrictor gets, but you are right it will never reach choked conditions for a complete cycle.



    Krautsalat

  7. #27
    Assuming 100% VE is a rather broken assumption, even if you have a 4 cyl. And the less cylinders you have, the worse an assumption it'll be. The VE you end up with will depend heavily on your intake volume, which will tend to smooth out the pulses in engine air demand.

    Still it's not a half bad thing to calculate. You can mostly neglect fuel. Your AFR is massive, 13:1 or so and fuel is more dense so it takes up even less as a volume ratio. Your other inaccuracies (pulses, etc) will swamp any effect fuel will have.

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