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Thread: Another Helmholtz Resonance Post

  1. #1

    Another Helmholtz Resonance Post

    I'm fairly certain I understand the theory behind Helmholtz resonance but haven't figured out which distance I should be working with. As I understand it, Helmholtz resonance allows the pressure wave that is formed by the closing of the intake valves in one runner to bounce back up the runner, with some amount of that pressure traveling to the runner where intake stroke is happening. The ideal case is for the wave to reach the valves of the piston in the intake stroke right before the valves close for increased VE, and this distance can be set by selecting an RPM to optimize to and calculating the length of the runners to allow the pressure wave rebound to occur at the right time.

    Now correct me if I'm wrong, but the pressure wave must travel from the closed valves, up the runner, across the bellmouths, down the intake-stroke runner, past the open valves, and into the combustion chamber. Would it make sense to be working with the shortest distance from the valves of the closed valves to the open valves because the pressure wave only provides a quick burst of pressure right as it arrives at the open valves? Or the centerline of the runners with the same valves to "average out" the entire pressue wave? Or should I be using the distance from the closed valves to 1/4 (or 1/3, or 1/4...) of the stroke length because the air must actually enter the combustion chamber? I'm sure Ricardo could plug and chug some numbers but I think understanding why the 1-D case works prefers certain assumptions is more helpful.
    Columbia University
    '13-'14: Intake & Exhaust

  2. #2
    Senior Member
    Join Date
    Oct 2009
    San Antonio, TX

    Helmholtz theory can be applicable to silencer design (intake and exhaust), but it is not the right intake duct performance design criteria for a high-speed 4-stroke engine. Helmholtz has been used in intake tuning for low and medium-speed engines (below approximately 2000 rpm), but open-closed end organ pipe theory provides the predominant tuning effect at higher speeds. Your post does not seem to be concerned with cylinder or plenum volume, so you might already be there and just using the wrong nomenclature. http://www.fsae.com/forums/showthrea...8-Intake-Info&

    The only application of what you're describing that I'm aware of, with one cylinder charging another with a positive reflected wave at port closing when the other is opening, are naturally aspirated Mazda 2-rotor Wankels. Look up the RX-8 intake design and you'll see that it's a complex network of several Y-branches with valving to enable different Y-junctions with speed and variable port area/timing changes. This works because it operates with the firing frequency of a 2-cylinder 2-stroke and the second rotor needs the first rotor's ramming wave less than 180 degrees later. On a 4-cylinder with 1-2-4 manifolding to accomplish what you're describing, the tuned lengths would be much longer. I'm curious how it would work though and it wouldn't hurt to parameterize a few things in WAVE and give it a try.

    More common is to treat the plenum as an atmosphere from which the cylinders draw and assume that there is no acoustic cross-talk from cylinder to cylinder. Using organ pipe theory or someone's constants based on testing and organ pipe theory (my testing and simulations always matched Blair's), the ramming depends on the speed of sound and the length of the duct from the bellmouth to the valves, following the centerline of any bends. This is the length over which the wave will travel and reflect when the intake valves are closed. You will have your choice of several lengths that create several ramming peaks at different speeds that you can use to shape the torque curve as you desire.
    Matt Birt
    Engine Calibration and Performance Engineer, Enovation Controls
    Former Powertrain Lead, Kettering University CSC/FSAE team
    1st place Fuel Efficiency 2013 FSAE, FSAE West, Formula North
    1st place overall 2014 Clean Snowmobile Challenge

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