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Plenum Chamber Design

  1. Feb 7, 2010 #1
    Gday all
    I am currently supercharging a 1000cc 4 cylinder motorcycle of mine, i am up to the stage of designing the plenum chamber, as usual there are heaps of designs yet not much in the way of actuall theory and mathematics on how these people came up with them.
    Who knows if all they did was wack a box together of aluminium sheeting and bolt it onto an engine.
    So if anyone has an idea please let me know
    for instance
    1) is there a ratio of plenum volume to engine volume
    2) what is the best sized inlet diameter
    3) should the inlet be on the side? (in which case i am in trouble as i have an engine frame in the way) or is underneath the centre 2 outlets fine?
    4) should the box be tapered or does a rectangular box do the job just as well

    any advice or opinions would be much appreciated
  2. jcsd
  3. Mar 7, 2010 #2
    To do it right requires computational fluid dynamics.
    Most probably take a hack at it and add boost to fix poor design.

    But there are some sites that have runner length and diameter as a function of RPM.
    1) The plenum needs to be some ratio of the runner volume - but I do not know what it is - but I seen it somewhere.

    2) Inlet diameter should be at least as big as the individual runners, since you have 4 cylinders going in 720 and the cam is ~270 degrees you probably want to have twice the area of the runner or ~1.5x bigger in diameter - but bigger does not hurt.

    3) doubt it matters - but take 4x EGT and/or 4x Lambda to ensure that the air flow is consistent across the engine. Or you could put in some diverters.

    4) If you have a throttle body before the plenum, then you may want the plenum small for throttle response.
    The trumpets should extend into the plenum, to keep the individual runners from interacting, as there is some effect of flow following along the walls.
    I doubt the overall shape is too critical other than having some room to the sides to allow for air to get sucked into each runner. data per #3 would help.
  4. Mar 18, 2010 #3
    it seems to me more info is really needed.
    is the system blow thru carb style, or carbs are on the intake of the blower?
    is the plenum you are building between the carbs/blower, blower/intake, or blower/carb/intake (blow thru style)
    intercooler?fuel injection?boost pressure?
    to a large extent, as long as the blower is large enough, runner flow won't be as critical as a natural asp. engine

  5. Mar 29, 2010 #4

    jack action

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    Not really. Here is the concept behind tuned intake. At some point during the cycle, there is a pressure wave coming from the cylinder going toward the inlet of the runner. As soon as the pressure wave encounters an area change, it will return another pressure wave back to the engine. If you make the runner the appropriate length, it will return that pressure wave to the appropriate moment to help filling the cylinder. The smaller the runner's area the stronger the pressure wave. The larger the area change at runner's inlet, the larger the pressure wave sent back. The biggest area change possible is having the runner's inlet directly to the atmosphere.

    So your plenum is some sort of "sub-atmosphere". But what count the most is the area change. an area ratio of 6:1 or larger should be good enough to simulate the atmosphere. As for depth, use something like, at least, twice the runner's diameter, enough to let the airflow disperse itself.

    I will agree with Holmz point # 2). Again, think that the closer you will simulate the real atmosphere, the stronger will be your pressure wave.

    If your plenum is large enough it shouldn't matter where is the inlet. But if what is coming in the plenum is an air/fuel mixture (as oppose to only air), there could be an unbalance air-fuel ratio from cylinder to cylinder if everything is not symmetric, as fuel droplets can separate from the airflow along the way.

    All you need is to have a nice radius entrance for your runners and inlet (a diffuser). I know there is an optimal radius to tube diameter and if memory don't fail me it's around 0.15-0.20.
  6. Apr 1, 2010 #5
    Chrysler corp used a lot of "ram tuning design" in their engines. The slant six is probably one of the best breathing NA engines built. but nothing shows the concept as well as the first 300's with the factory duel carb'd cross ram from the 60"s

    heres a pic 1/2 way down the page


    mopar did some amazing stuff in the early days

  7. Apr 1, 2010 #6
    They must have done that work empirically?

    Back to Jack Action's post...
    The pressure wave is different from a flow resonance.
    The pressure wave is analogous to hearing a fart in a room at the speed of sound, but the flow is analogous to the fact that the smell/aroma does not travel as fast as the sound.

    The end of the trumpet that is at/near atmosphere has less to do with the effect.
    It is the diameter and length of the tube that determine the resonance, and the diameter probably has more of an effect on the Q.
    The air has mass (inertia), and as piston draws a vacuum on the valve end of the plenum runner the air starts to accelerate towards and through the valve, somewhere steady state is reached. The as the piston starts to come up the cylinder gets pressured higher than the air in the runner, and the flow slows, and eventually reverses. Ideally the timing and the runner length is tuned such that when it stop (just before it reverses) the valve closes.

    The question that Phillip1978 was asking was regarding the plenum.
    I would think that the runner would behave differently at differing air pressures and maybe reynolds number come into play - which I do not understand. For sure most forced induction engine has runners that seem short compared to NA engines.

    In a plenum for a single cylinder engine there is much greater pulsating of air flow and pressure in the plenum than on multi cylinder engines. But there is a ratio of plenum size that aids in flow into the plenum in the same way that the runner increases volumetric efficiency into the engine.

    Phillip1978 - short of modeling it with some sort of finite element, or finite difference technique, the best you can hope to do is to try a couple of boxes, and measure A:F (AFR) and maybe EGT to aid in empirically working it out. You can pull the EGT thermistors out and bung the holes when you are happy. Airplanes run these to monitor it all in-situ.
  8. Apr 1, 2010 #7

    jack action

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    This is a very poor simplification of ram-tuning.

    You can read http://www.howstuffworks.com/question517.htm" to just scratch the surface of pressure wave dynamics.

    From HowStuffWorks:
    From Wikipedia:
    The strength of the pressure wave that bounces back is primarily determined by the area change at the runner's entrance, hence my answer to question #1.

    That is what I answered. I gave the area needed at runners' entrance and the depth perpendicular to that area. Area X Depth = Volume of Plenum. Anymore precision in my answer would demand a complete computerized mathematical analysis based on actual data.
    Last edited by a moderator: Apr 25, 2017
  9. Apr 1, 2010 #8

    jack action

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    After checking, I confirm that r/D > 0.15-0.20.
  10. Apr 2, 2010 #9
    I am thinking that maybe there is a confusion on my part with the term "pressure wave".
    Are you referring to a "wave of pressure" or "wave of flow which has pressure" and which is moving sub-sonic, or a pressure wave that is at the speed of sound?

    In the http://en.wikipedia.org/wiki/Cylinder_head_porting#Wave_dynamics" - It shows a graph, but looks like a modeled pressure, otherwise shouldn't there be higher frequency ripples from the vale opening and closing events?
    Last edited by a moderator: Apr 25, 2017
  11. Apr 2, 2010 #10

    jack action

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    Pressure wave at the speed of sound.

    On the wikipedia link, you can see the blue line that is the pressure present at the back of the valve in the runner. The red line is the pressure measured at the runner's entrance.

    Atmospheric pressure is 1 bar.

    You can see that both measurements are going over and under atm pressure when the intake valve is closed (left side of the graph) and that they're out of phase.

    The low pressure created by the downward motion of the piston sends a low pressure wave near the valve (blue line at around 435°), that pressure wave travels down the runner to arrive at around 500° at the runner's entrance (red line).

    When that LOW pressure wave senses the large area change (runner's entrance-to-atmosphere, area change = [tex]\infty[/tex]), it sends back a HIGH pressure wave that will traveled down the runner and that arrives at the valve at around 600° on the blue line.

    Since at that point pressure in the cylinder (not shown on graph) is high and the valve is almost closed, it acts like a wall (area change = 0) and the pressure is "bouncing back", going again to the runner's entrance. In fact, the piston's upward motion contributes to the higher pressure wave sent back.

    Once at the entrance, this high pressure wave is reflected as a low pressure wave. The whole process repeats itself until a high pressure wave arrives at the next intake valve opening, during valve overlap (the green area), which also helps filling the cylinder (at that point, the piston is going up, pushing the exhaust through the exhaust valve)

    Once the piston is going down again, that destroys any residual pressure wave by creating a new strong one (back at the next 435°).

    The phasing is determined by runner's length and RPM; Amplitude is determined by the runner's cross sectional area and the atmosphere's "cross sectional area" (which in this case is [tex]\infty[/tex]). And for the case discussed in this thread, the atmosphere's "cross sectional area" would be replaced by the plenum's cross sectional area. If its size is too close to the runner's cross sectional area, the pressure wave will "see" that as a longer runner (runner's length + plenum's "length") which will screw up the phasing of the pressure wave pattern (The intake system will be tuned for a lower RPM).

    Other interesting fact, high pressure wave travels faster than low pressure wave. You can see that with the orange lines and arrows on the graph. That is because the high pressure wave temperature is higher than the one for the low pressure wave, hence higher speed of sound. Furthermore, the speed of the pressure wave adds up with the speed of the air flow. So when going against the air flow, the speed of the pressure wave is reduced.

    Although it would be weaker as the plenum gets bigger, but we did not cover the fact that the pressure wave that is reflected in the plenum will also come back to the runner ... and inside the other cylinder's runner too!

    That is all based on 2-D calculations, throw in the third dimension and the fun begins.

    This is why it is almost impossible to predict what is going to happen in an intake or exhaust system without accurate modeling and calculations. But understanding the principles will help us choose initial designs that may work.
    Last edited by a moderator: Apr 25, 2017
  12. Apr 3, 2010 #11
    I appreciate your being patient with me.
    But I am still at a loss to understand this, and I generally do not put 100% faith in wikis.

    In a pipe organ the pressure waves are going at speed of sound but there is little flow of air in the pipe.

    Lets say the engine is running at 6000 rpm, or 100RPS, and the speed of sound is 1000 fps.
    The engine events are once pre 2 revs, so 50Hz instead of 100Hz.
    The valve event of 270 degrees would be about 0.0075s, or 133 Hz. At the speed of sound this is 7.5 feet.

    In a 1 foot trumpet the wave would go back and forth 500 times/second (or 0.002s), and a one way time of 0.001s.

    I am still stuck on the inertia of the air, rather than the pressure - can you explain this to me in way other than the wiki as the pressure wave frequencies and the length of the trumpets seem to be at odds.

    I recall when I did a test blowing across a coke bottle with my daughter, that it was the volume of the bottle that was the driver not the length of the bottle.
    <looks for link>
    I found a link: http://stason.org/TULARC/physics/acoustics-faq/6-5-Why-does-blowing-over-a-bottle-make-a-note.html"
    Is this "springiness and mass of the air" not the physics of what is happening in the runners from the plenum?
    Last edited by a moderator: Apr 25, 2017
  13. Apr 3, 2010 #12

    jack action

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    Calculations for the pressure wave moving in the runner is similar to the one you describe. But it is not the valve duration (270°) that counts. The "strong" signal is when the piston is at around mid-course (@ 435° on the wiki graph). From there, the high pressure wave must return when the valve closes (where the pressure rises to 1.6 bar on the wiki graph). Then, this new high pressure at the back of the inlet valve will travel down the pipe, back and forth (just like in the organ pipe you've mentioned), until the next valve opening, where - if tuned properly - a high pressure wave will arrive at that time (the green area on the wiki graph).

    I've seen the Helmholtz resonator theory (the one related to your coke bottle experiment) applied to engine tuning (although without math, just as a concept) and the volume was not the plenum, it was the cylinder. And the problem is that the cylinder volume is constantly changing as the piston goes down.

    Furthermore, I have not study sound waves much but I know this when compared to pressure waves:

    Yes, they both travel at the speed of sound, but the pressure wave has much, way much, energy than a sound wave. As you can see on the wiki graph, pressure waves can go as high as 1.6 times the atm pressure. For sound waves (like someone talking), it is something like 1.0001 times the atm pressure (I don't remember the actual number of zeros).

    So, like I said in a previous post, higher pressure means higher temperature, hence higher local speed of sound. This is what complicates a lot the precision of the calculations and why this simple theory cannot be applied directly to engine tuning.
    Last edited by a moderator: Apr 25, 2017
  14. Apr 5, 2010 #13
    does ram tuning actually matter on a blown engine?
    positive pressure from the turbo/blower already makes the cylinder start to fill as soon as the valve opens
    ram tuned runners are usually long, but the runners on a blower engine are much shorter

  15. Apr 5, 2010 #14
    But why?
    What is the physics behind the shorter runners?

    The only reason I can think of is that the filling at low RPM is adequate, but at high RPM it is still lacking so short runners to optimize for high RPM, and low RPM sorts itself out...
  16. Apr 5, 2010 #15
    my suspicion is that anytime the pressure is positive in the intake, the effects of ram tuning decrease.

  17. Apr 5, 2010 #16

    jack action

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    Pressure waves will occur no matter what is the pressure in the plenum (It's a pressure ratio thing, so they're just as effective, no matter the initial pressure). So, yes, you can still tune the length of the runners to further increase the volumetric efficiency of the engine.

    On a blown engine, it might not be too important as the pressure in the intake system is greater than the one in the exhaust system. But on an engine with a turbo, the pressure will rise in both systems, which presents similar proportion of the pressure when compare to NA engine (where both systems are at atm pressure; pressure ratio = 1), so tuning might be helpful.

    If the runners are short on a blown engine, it is usually a question of packaging. In that case the runners might not be tuned. In that case you don't care about plenum volume or runner's length and diameter, as long as nothing is too restrictive.
  18. Apr 6, 2010 #17
    I knew the waves would exist in all applications with "poppet valves" but I have serious doubts that on an engine with a turbo or blower that it really makes a difference.

  19. Jan 25, 2011 #18
    Hi there guys. i hope you dont mind me hijacking the thread but i am studying motorcycle engineering at kingston university, and am lookin at plenum chamber design for my dissertation. very little solid information seems to be availble, and i was just wondering if anyione can point me int he right direction with regards to books or similar that i can read on the subject?

    apart from the equations and numerical side of things, i was thinking of starting with box section to make a plenum, and using CFD to fine things right down to the best design possible.
    Hopefully this will provide greater insight into this subject which seems to almost be a black art.

    many thanx.

  20. Jan 25, 2011 #19
    In the 70's there was a book called "the two stroke tuners manual", which covered the theory very well. I just ordered a book last weekend, but it is not here and I doubt it is what you want.

    It only seems like a black art because people do not do CFD at home for their engines, nor is there a solid understanding of what the physics is by many people that dabble in the field.

    What sort of CFD code will you be using?
  21. Jan 25, 2011 #20
    to be honest mate I have no clue about the CFD yet. I have arranged a meeting with a lecturer about getting started so hopefully I will be able to provide more info in a few weeks. in the mean time I will look for the two stroke tuners manual. thanks for the info.
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