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Question on the ram air effect

  1. Apr 23, 2015 #1
    I am now 76 years old but as a young man I served an apprenticeship with a well known motor manufacturer and we apprentices were able to carry out experiments on engines in testbeds that were connected to dynamometers. In one such experiment we attached a straight length of pipe to the air intake of an engine which caused the engine to speed up and/or produce more power. The length and diameter of the pipe was critical.

    I understand that what we observed was called the ram effect whereby the speed of the air in the pipe was forced to increase for some reason and adjustments were needed to the air fuel ratio to take advantage of this.

    Is anybody able to tell me any more about this and why the speed of the airflow in the pipe should become faster? Also how the length and diameter of the pipe would have been calculated?

    Many thanks

  2. jcsd
  3. Apr 23, 2015 #2

    Simon Bridge

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    The ram effect that I know of involves using dynamic pressure to increase airflow - as in placing an air intake in the wind.
    Was the test-bed moving or stationary?
  4. Apr 23, 2015 #3
    Thanks Simon, the test bed was stationary but I would like to add something to my previous post.

    Many years later I was working on a construction project in the Kalahari Desert when I saw something unusual. In the works yard. There was a piece of galvanised pipe about 6 ins dia and 6 to 8 feet long that was making a very unusual noise like a jet engine. I went closer and saw that the bottom of the pipe rested on the corner of a tin box and was in the shade but the rest of the pipe was in the hot sun (110f) and I was unable to touch it due to its high temperature. The noise was being generates by air that was being drawn up into the bottom of the pipe and being pushed out of the top at high speed.

    The only reason that I could see for this was that I was again observing the ram effect and the air rising up the inside of the pipe had accelerated due to this ram effect. As the sun went down and the temperature dropped the phenomenon got less and less until it stopped. The same thing happened each day when the pipe got hot.

    I am still wondering?

  5. Apr 23, 2015 #4

    Simon Bridge

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    The second example was normal air flow due to the pressure difference between the hot and cold parts of the pipe.
    You also get a speeding up of any fluid flow through a constriction - but that would have added a load to the engine.
    You can get resonant effects, since an engine does not draw air continuously but in bursts, so you can get standing waves building up in a pipe. I'm more used to this from the exhaust creating a back-pressure though.
    Not finding anything called "ram effect" that involves putting a pipe on a stationary engine.
    However - a short intake pipe can decrease the inflow turbulence, increasing inflow ...
  6. Apr 23, 2015 #5
    Thanks again Simon you are jogging my memory about my first example. I can recall (sixty years later) that it was explained to us the effect was to do with the build up of the standing waves that you mention. If the pipe had a certain length, dependent upon certain other factors, you could take advantage of this wave effect so that the waves actually pushed each other. Does this make sense?

    I am very interested in the explanation you give for my second example, is there somewhere I can learn more about this? I would like to know why the air accelerates through the pipe at high speed and if this is solely due to high temperature? Also how does the colder air at the bottom of the pipe have an effect? What do the differences in temperature need to be? Does the size of the pipe have a bearing on how high the temperature must be? Is there any way I can recreate this effect in my back garden using a heat source to create temperature difference?

    Thank you so much

  7. Apr 23, 2015 #6

    jack action

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    To learn about the subject, you should look for the keywords «pressure wave» and «gas dynamics» instead of «ram effect» (which is an older terminology that doesn't really apply anymore to explain the phenomena).

    I wrote a few posts in previous threads about the effect of pressure waves on both intake and exhaust tuning:
  8. May 10, 2015 #7


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    A pipe with a thermally induced flow might flutter at the natural frequency of the pipe. That will probably be the frequency at which the pipe length is half a wavelength. For a length of about 7 foot, with sound travelling at 1100 feet per second, I would expect a fundamental frequency of 1100 / (2 * 7) = 78 Hz. As a fundamental oscillation, that would sound like a hum or purr. If higher frequency components were present it could develop a wide spectrum of harmonics.

    Was there a light weight flap valve at the bottom of the tube? in the tin box? That could make a sound like a pulse jet. http://en.wikipedia.org/wiki/Pulsejet

    Was there a slot at the bottom of the tube, maybe where it rested on the tin box? That would make a continuous sound in the same way as a flute, tin whistle or organ pipe.
  9. May 10, 2015 #8
    Thank you for your response Baluncore. As far as I remember the pipe was about 8 feet ,long and 6 ins dia and was completely open at both ends. I tried to put a cap on top of the pipe but it was almost impossible due to the force of the air leaving the pipe. I have checked the Pulsejet information but it is my theory that the temperature of the inner wall of the pipe fluctuated a minute amount due to changes to the temperature of the incoming air.This was sufficient to cause a slight fluctuation in the air speed in the pipe which in turn caused a standing wave effect. The only way to check this is to create the same conditions again. I will try this on a scale model.
  10. May 10, 2015 #9


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    If it was a simple chimney, with airflow generated by the heating of air in the hot pipe, the energy available could only be that of the sunlight on the pipe. 8' x 6” = 4 sq ft maximum. How many watts of insolation could 4 sq ft gather? What colour was the pipe, black or silver ?

    I do not believe that variation in pipe wall temperature is important. There would only be slow heat transfer between the tube wall and the air. Heat transfer would take longer than one cycle of the jet engine. For that reason I think the sound must be generated by turbulence at the air entry to the pipe, which might then be selectively reinforced by acoustic reflection from the open end of the pipe.

    I find it hard to believe that it was difficult to cap. The pressure generated by the temperature difference would be quite low. Energy could be stored in the momentum of the rising air but that would not amount to much since little mass of air was flowing. Also the flow path would be inefficient, most of the energy would go into getting air to flow in at the bottom. It would be a little different if there was a horn at the bottom to eliminate flow contraction, but that would kill the fluctuating turbulence at the pipe entry which is I believe, where the sound is generated.
  11. May 10, 2015 #10
    Thanks Baluncore. I also found it hard to believe that it was so difficult to cap - hence my interest. As I say, the only way to find out what went on is to do the experiment using the various different possibilities that may have existed in the original set up.
  12. May 11, 2015 #11


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    There seem to be contradictions in some definitions. This is as far as I have been able to go so far.
    A flame driven pipe organ is a Pyrophone.
    A steam driven pipe organ is a Calliope.
    An air driven Calliope is a Calliaphone.

    "The fire organ uses the laws of thermoacoustics to produce sound. The pyrophone, like traditional pipe organs, has one pipe for each note and is activated by a piano keyboard. The sound that originates from the fire organ is created by the temperature difference across a set of channels in extremely close proximity. Propane flames on one end and liquid nitrogen on the other maintain the temperature of the fire organ. "

    "The calliope is similar to the pyrophone. The difference between the two is that the calliope is an external combustion instrument and the pyrophone is an internal combustion instrument."


    I have not searched for the sources of noise generated in the ducts of heating, ventilation and air conditioning.
  13. May 25, 2015 #12

    jim hardy

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    Might bob have noticed the effect of standing waves, as somebody earlier remarked?
    Tuned exhaust is well known to racers, and intakes can be similarly "tuned" to frequency of intake valve opening.

    to wit the Chrysler "Ram Induction" intake manifold of around 1960..

    inertia of air in those long tubes just might provide a little boost at organ-pipe resonant frequency... ..........
  14. May 25, 2015 #13
    Thank you for that comment Jim. As young apprentices in the 50's we were able to get a ram effect on an engine in a test bed. However it only worked if the pipe was straight and the length of the pipe had an exact ratio to its diameter. What you have described is more like the "gefferator" that was fitted to the early Austin Seven. This was a valve fitted into the exhaust manifold that allowed air back through the exhaust valve during the valve overlap period when there was negative pressure in the manifold. But I doubt this would work on modern oversquare engines.
  15. Jul 21, 2015 #14


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    I think the ram effect happens when a mass of air in the inlet pipe is accelerated by the inlet vacuum so it has a velocity. When the inlet valve starts to close, due to the momentum of the air mass, it keeps travelling towards the engine, and forces its way in via the part closed valve. In this way, it is acting to prolong the induction cycle and will therefore increase the charge in the combustion chamber. It is quite similar to supercharging. The distinction from a tuned inlet is quite indistinct as far as I can see.
  16. Jul 21, 2015 #15


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    Last edited: Jul 21, 2015
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