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Helmholtz resonators...Mitch A. you there? Fuilds guys?

Discussion in 'Technical Q&A' started by fatbillybob, Apr 9, 2004.

  1. fatbillybob

    fatbillybob F1 World Champ
    Consultant Owner

    Aug 10, 2002
    19,003
    socal
    I got to see a real race 348 with a special engine done partly by michelloto of course. One of the things I noticed was different from stock intake plenum that looked just like a oem stock unit but with longer runner and higher bulge over the runners consistent with a higher horse power modified motor. It could have been easily overlooked it looked so stock. We know what HR's do. Do you think for acedemic purposes you could find optimum volume and runner length to maximize HP/Torque at specific RPM's as in compute a curve? A clue of how to do this which is over my head is http://www.me.psu.edu/me415/SPRING02/intake/intake.html. What do you think? Also, how does such a curve get effected by variations in pressure from changes in ramair effects by different airbox configurations etc. upstream of the throttles? Mitch you did such a great job on the 355 suspension analysis how about taking an acedemic stab at this?
     
  2. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,967
    Let me start by saying that I understand headers a lot better than intakes; but since you ask:

    Generally, a Helmholtz Resonator does two things; a) to each intake port it setup a standing resonance wave such that when the intake valve opens, there is positive pressure above that port; and b) to the air feed port is combines and smooths the 4 (6) intake pulses into a single continuous demand for more air. I will get back to Helmholtz resonators in a second.

    The intake port in these systems is someting akin to a velocity stack, and in the F355 system it IS a velocity stack as the profile admits air with very little friction and resistance over a broad range of pulse frequencies. Another way to think of a VS is that it acts like a long tube at low frequencies and acts like a shorter tube at higher frequencies, in effect becoming the right length ofer a considerable frequency band.

    Lets look at the wave dynamics of the velocity stack. Near the end of the intake interval, air is flowing quite nicely into the cylinder. When the intake valve slams shut the air continues towards the now closed back side of the valve, builds up pressure and reflects as a positive pressure pulse. This positive pressure pulse travels up the velocit stack and encounters the mouth and the relatively low pressure of the HR, and reflects back towards the back side the the valve. If the subsequent valve opening occurs when the positive pressure pulse is present, the air starts to flow into the cylinder even before the pistons starts downward. Thus free TQ (volumetric efficiency) that leads to free HP.

    In order to arrive at the right time, either the VS has to be quite long, or the pulse has to travel up and down the pipe several times. In the case of the F355 The pusle traves up and down the VS 7 to 9 times between valve events at peak TQ and 4-5 times at peak HP. This wave travels at constant speed (throttles wide open) and gives the F355 that open nasal 'Haaaaa' sound we so cherish.

    OK, back at the resonator.

    With each VS 'Haaaa'ing along, the resonator needs a fairly low impedance for the VS resonance to enable one throat to resonate with another, the side to side distance (and top to bottom) of the resonator is generally used to tune this resonance. Then the resonator needs to have a relativey high impedance to the 4 basic air breathing intervals to smooth out the flow into the resonator, then end to end distance in the F355 is used to tune this anti-resonance*. Finally, the throat into the resonator and the resonator itself must have very low impedance to admision of air at all lower frequencies, this is tuned with throat size and length. The air metering device is often placed in this tube because of the smooth laminar airflow characteristics in this tube. Since the resonance of the VS is constant (WOT), and the flow through the inlte throat smooth and laminar; the HR itself can only be tuned over slightly less than 1/2 of the cars powerband.

    Whether to join the edge of the resonator to the intake port (ala M3) or to encroach into the resonator with a VS (ala F355) plays with the tuning dimensions s2s and t2b, and the width of the powerband resonance.

    By careful choice of the VS length, HR resonance, and headers; all this wave action can accentuate the powerband over a fairly wide RPM band. One would choose the HR resonance and the header resonance to occur at the same RPM, and use one of the high count VS resonances (like 9) to optimize the power below peak TQ and a lower count resonance (like 5) in the VS to enhance the power between peak TQ and peak HP.

    Some minor programming of the ECU (advance[RPM,throttle]; mixture[RPM,throttle,Oxygen]) smooths this all out into a big wide seamless powerband.

    I will end by saying that the new airboxes on the 360 CS in effect have a velocity stack added to the end of the intake throat (flush with edge of box) to broaden the laminar airflow region and allow more power up top.

    * you might ask how the side to side and top to bottom resonances build a peak, while the end to end resonance depresses a peak. Side to side, a wave simply bounces back and forth, as more intakes add sonic energy to the wave it grows in amplitude. In the long direction, one end of the resonator is open to the intake throat so there is only one wall to resonate off of, and one wall that is semi-non-reflecting. This is why the intake throat is so similar to the size of the HR, we are taking a wall and making it an open port <very carefully> sort of like the collector of headers.
     
  3. fatbillybob

    fatbillybob F1 World Champ
    Consultant Owner

    Aug 10, 2002
    19,003
    socal
    "By careful choice of the VS length, HR resonance, and headers; all this wave action can accentuate the powerband over a fairly wide RPM band. One would choose the HR resonance and the header resonance to occur at the same RPM, and use one of the high count VS resonances (like 9) to optimize the power below peak TQ and a lower count resonance (like 5) in the VS to enhance the power between peak TQ and peak HP.

    Some minor programming of the ECU (advance[RPM,throttle]; mixture[RPM,throttle,Oxygen]) smooths this all out into a big wide seamless powerband."

    Yes this all makes perfect sence and defines HR and VS. But how can one compute decent starting values to match intake HR/VS and exhaust to work as a tuned set for racing vs street compromises?
     
  4. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    EXCELLENT!!!!!!!!!
     
  5. mk e

    mk e F1 World Champ

    Oct 31, 2003
    11,505
    The twilight zone
    Full Name:
    The Butcher
    FSAE...interesting, I build one of those. I did the design on the frame and intake and after looking at the pictures, if their intake design is on par with the basic frame design, I would do the opposite of what ever they recomend.

    Seriously though, a few years ago I read an SAE paper (that was quite old) on HR that was quite clear and to the point. I was designing my first supercharger intake, and after looking at it I came to the conclusion that ferrari had done a pretty good job and I should stay as close as possible to what they had and I moved on. If you go to the SAE web sight, you should be able to buy the paper. Also do a search for the paper/book mensioned on in the write up you found " Ohata and Ishida (Winterbourne, page 162)". Both of those souces will have the equations required to get started doing a simulation.

    On the FSAE car, we only had 4 full time team member and didn't have all the time we would have liked, so we simple pulled the analysis from a previous car which was (thie intake at least) well researched , added a few quick tweakes and moved. The intake was a header flowing backwards. The FSAE requires a 20mm flow restictor, and even with that we had a broader torque curve and were only 1 hp below the stock (FZR600) numbers which was flowing though four 32mm carbs with a HR above them. We were the highest naturally aspirated hp on the dyno that was at the competition and .4 sec faster in acceleration run than next closest.
     
  6. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,967
    Lets start like this:

    Speed of sound at sea level is 1100 ft/sec, and let's assume we want the resonator to tune at 6000 RPMs so that it 'comes in' at 4000 and 'signs off' at 8000 (which is a little broader than actuality but gives us critical working parameters.)

    The intake pulse pattern on a flat plane V8 is just like the input pulse pattern on a I4. So lets scope out a 1-3-4-2 cylinder firing order on one side and build from there.

    In the 1->3 transition, there are 2 units of distance (1-2> and 2->3) between the intake ports. In the 3->4 transition we need to jump over the 4th port and reflect off the resonator wall and then get back to the 4th port. So the wall is 1/2 a unit distance farther than 3->4 is directly. Then the length from 3-> can be 1+1/2+1/2 = 2 and its now the same as 1->3. 4_>2 is 2 units just like 1->3 is 2 units. Similarly with 3->4, 2->1 reflects off anothe wall giving 1+1/2+1/2 = 2 again. This is a standing wave that moves from end to end of the resonator. The critical thing here is to get the wave flowing over each port in the order that the intake pulses occur.

    At 6000 RPMs we have 100 rounds/sec or 10 milliseconds/round. 2 rounds complete a 4 cycle set of events and repeats the process. So the wave we are talking about has 20 milliseconds to complete a fore-to-back movement and a back to fore movement. At 6000 RPMs, the wave moves 22 feet, or 11 ft fore and 11 ft back. This is way to big for a resonator, so we use a higer frequency component and tap the wave every second, third, or higher pass in front of the port. Using 7 as our choice resonance, the end to end length of the resonator should be on-the-order of 1.57 feet or 18.57".

    The actual F355 resonator is 17" long (outside), so it looks like Ferrari chose* 7 as the number of passes over the ports between gulps of air, and either chose a slightly different resonance RPM or did some fluid mechanics to determing the actual speed of sound inside the resonator after all the velocity staks were acounted. Perhaps Ferrari choose an RPM closer to 6500 to extend the power band up towards 8500 and sacrifice the band lower than 4500.

    As far as the top to bottom and side to side distances, Measure either a) the distance from the back side of the valve to the throttle plate, or b) the distance from the throttle plate to the midpoint of the exponential flare of the Velocity stack. And then use that distance for both s2s and t2b.

    Anyway, that is how the math basically scopes out. However aerodynamics tends to be more difficult than follow the math and achieve success.

    Caio.

    * it is entirely possible that Ferrari choose 8 as the resonance number which gies 16.5" at 6000 RPMs. However, resonance phenomonom tend to come in odd multiples <and the why of that is something I have searched for for quite a while without satisfaction>. It is not beyond possiblity that Ferrari chose 9 as the resonance number and an RPM lower than 6000 RPMs.
     

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