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Twin or Single Throttle Body Blade Comparison

  1. Nov 2, 2007 #1
    Hi all,

    I have been looking for an answer to this and I cannot find one. So here is my question...

    The surface area of 57mm twin round throttle body set-up was 5100 mm^2 and the single 75mm throttle body set-up was 4415 mm^2.

    Now to the actual question...

    Does smaller (twin) throttle bodies allow the gas pedal to be less "touchy" under part throttle conditions.

    In other words, assuming both throttle bodys (twin and single) have the same surface area on the blade, would the twin be more manageable under part throttle conditions.

    This seems automotive, but I figured you professionals would better be able to help me.

    I hope this made sense. Thank you guys and gals.
     
    Last edited: Nov 2, 2007
  2. jcsd
  3. Nov 5, 2007 #2

    brewnog

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    I suppose it all boils down to the pressure loss across the throttles. I don't feel that either would have an inherent advantage for part-load drivability.
     
  4. Feb 7, 2009 #3
    Assuming the surface area of the twin blades is exactly equal to the surface area of the single blade, it all boils down to the actual flow that each produces. As the air horn ahead of the throttles affects flow, and the diameter of the throttle shafts also affects flow, you have 2 variables that will affect how sensitive each is. As the smaller pair of throttle blades should use a smaller diameter shaft, I would say there would be a tiny air flow advantage using the twin blade setup. So my guess would be the twin throttles would be slightly more sensitive. Such a small difference between the two however, you would never feel the difference.
     
    Last edited: Feb 7, 2009
  5. Feb 7, 2009 #4
    Hello 5.0stang, this should resolve your question.

    The main purpose of the twin throttle blades is to provide greater fuel atomization efficiency at lower engine RPM. This becomes evident when examining the piston down-stroke times during intake at low verses high engine RPM operation.

    For instance:

    650 RPM = 650 RPM / 60 seconds = 10.8333 RPS (crankshaft Revolutions Per Second)

    1 second / 10.8333 RPS = .0923 seconds (the time required to complete a single crankshaft revolution at 650 RPM), half of which is the down-stroke time at 650 RPM therefore,

    .0923 seconds / 2 = .04615 seconds (actual piston down-stroke time at 650 RPM)

    Since 6,500 RPM is 10 times faster than 650 RPM,

    6,500 RPM = .004615 seconds (actual piston down-stroke time at 6,500 RPM)

    At lower RPM, the intake air and fuel enters the cylinder at just 1/10 the velocity that it would at 6,500 RPM. This lower intake velocity isn’t as efficient in atomizing the fuel compared to the 10 times greater piston down-stroke velocity experienced at 6,500 RPM.

    So, to help atomize the fuel more efficiently at lower engine RPM to produce greater power and produce a more complete combustion of the fuel, the secondary blade remains partially closed. This partial air restriction causes the intake air drawn by the cylinder during the piston’s intake down-stroke to increase in velocity thereby better atomizing the fuel to produce a more efficient A/F mixture.

    When full engine power is required, both throttle blades are opened to allow the greatest volume of unrestricted intake air to be drawn into the cylinders. As the engine’s RPM increases, the fuel is naturally being drawn in at greater velocity (as well as injected in most cases these days), so the need to increase intake air velocity isn’t required at higher engine RPM. The only thing required for max power at high engine RPM is to derestrict air intake as much as possible, so max air volume intake is possible to mix with a max volume of fuel.
     
  6. Feb 24, 2009 #5

    S_Happens

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    Your progressive linkage carb answer does not apply to a multi-port fuel injection setup, which is probably the case that the OP is asking about.

    The case which I am assuming the OP is talking about has both blades fixed on a single shaft so that they operate identically.

    To the OP- The answer is not as straight forward as you think. The throttle opening on current fuel injected vehicles is not linear (it is progressive as well, but both blades operate in sync). This means that you could have different throttle cam profiles for single or dual blades that would match up actual flow very closely, if there exists a discrepancy between the two setups.

    Your question assumes a linear or at least identical opening profile for a single or dual blade setup. If you want to answer that question then it shouldn't be too hard to get into the ballpark (plus if you just ballpark you can do things like ignore shaft dimensions and use nonprogressive opening).

    First find dimensions for twin and single so that the surface areas are equal. Next you can calculate opening area for each type at a fixed throttle position. A very small throttle percentage would make the actual opening very close to the "swept circumference" (I doubt this term is correct, but I'm using it to describe the travel of the edge of the blades through opening) for easy calculation.

    I'm not very concerned with the actual answer at 1:30 AM, so I figured I'd add the situation that I thought applied to the original question and give a little nudge in the right direction. It's been a long time, but maybe the OP will return.
     
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