Exhaust and intake resonances

Competition engines and ancillaries - general discussion
Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Fri Mar 22, 2013 3:39 pm

'Rain on the parade' = spoil the party Rob!

I know you are exceptionally good a mathematics and have a very keen interest in this stuff - and don't get me wrong - I like what you are doing.

But I made some observations here in my own section of the site to indicate my position on this kind of prediction - before folk start quoting it to me and telling how to design my engines! Altering inlet configurations is all very well - but since I do it all commercially I have little room for manouevre on this.

You will never be able to predict 100% coincidence - a few milllimeters either way on inlet length or primary pipe length can chop off 'X' amount of BHP.


Sincerely,

G
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Fri Mar 22, 2013 4:18 pm

OK Guy. So I have to take precaution using this maths and avoid plan on the comet (direct French expression translation meaning to avoid dreaming).
I will come back to you for specific questions implying commercial matter so may be directly by e Mail.

Best regards,

Robert
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timinator
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Re: Exhaust and intake resonances

Post by timinator » Sat Mar 23, 2013 2:15 am

Hi Robert, Yes I meant harmonic. I think in terms of resonance because wave pressure hits the open valve at a specific frequency or rpm and increases the amplitude or filling of the cylinder. I won't use the term resonance in the future.
As GC pointed out empirical data is valuable in dealing with engine tuning. I have a intake runner length graph that I use to predict the best runner length for the rpm the engine will operate in. It is the product of the observations of others. I find it is often of benefit. It showed a longer runner for your engine than what you have. No math involved or insight on my part.
The exhaust system you have pictured appears to be of average design.

Robert
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Re: Exhaust and intake resonances

Post by Robert » Sat Mar 23, 2013 5:49 pm

OK Timinator. Let's speak about pressure waves instead of resonance. So the waves can be tuned according to the length of the runner and the camshaft timing in order to increase the incoming pressure (or the reverse in fact). In the equation I use there is a coefficient k giving the rank of the harmonic (k is an integer) of the positive pressure. The higher k is the lower the frequency is (the revs). In fact we could speak of "sub-harmonic" (in electronics the harmonics are multiple of the frequency).
And higher k is the lower the amplitude of the wave is. This is for the humps because of the positive pressure.
When k is k+0.5 then you get the negative pressure waves (the dips).
The figure showing the explanation for the intake side.

Bonsoir Tom,
I suppose you are interested in this thread so I "don't forget you".

Regards,

Robert
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GC_131

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Re: Exhaust and intake resonances

Post by timinator » Sun Mar 24, 2013 3:38 am

Hi Robert, Just to keep this discussion in proper proportion the volumetric efficiency (VE%) that is contributed by wave tuning is small. Usually somewhere around 1% as compared to the momentum of the moving air/fuel column (15%) or the piston moving down (80%). It is not a cause of shifts in the power curve. Total intake runner length is only a part of pressure wave tuning. Cross sectional area and the total volume of the intake runner are also important. Timing the pressure wave to arrive before the intake valve closes involves many variables.

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Re: Exhaust and intake resonances

Post by Robert » Sun Mar 24, 2013 4:10 pm

Wow ! 1 % of contribution... I did not know that it was so low... Is it true ?!
What do you mean by "momentum of the moving air/fuel column" ? Is it the gas velocity ?
Cross sectional area is crucial, since I reduced from 45 to 40 mm the intake runner (trumpet) the power increased a lot. The difficulty is to find the right flow/velocity optimum. I used a Mach index of about .6 at the valve throat. and .25 at runner.

Robert
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Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Sun Mar 24, 2013 5:34 pm

Sorry - will have to move this topic to another section.

Timinator, with respect, please cite your sources when making assertions of that kind.

No offence - just cite them.

GC
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TomLouwrier
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Re: Exhaust and intake resonances

Post by TomLouwrier » Sun Mar 24, 2013 5:52 pm

Air actually has considerable mass, so inertia. Momentum = mass * speed (en français: quantité de mouvement).

I've been thinking about that aspect for some time and my idea is that in order to have as much gain as possible we would be looking to maximize the inflowing gas' kinetic energy, so m * v(square). This tallies very well with the torque loss related to too large inlet tracts.
Mass is related to pressure and temperature, velocity is related to pressure as well and to inlet aerodynamic efficiency. All connected, but solvable.

Under the restriction that v should be less than about 0,4 Mach (or another value to be determined empirically) in order to keep the flow from choking, we would be able to determine the optimal inlet tract dimensions.

In theory, that is.

regards
Tom
Last edited by TomLouwrier on Sun Mar 24, 2013 7:47 pm, edited 3 times in total.
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Sun Mar 24, 2013 6:34 pm

Thank you Tom (merci beaucoup même!)

mv² ! of course. So v is more important.

Guy,

You can move the thread but say where it will be ;o)

Thank you for this forum. The learning way.

Robert
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Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Sun Mar 24, 2013 7:01 pm

Tom,

Mach number has nothing to do with turbulence, the flow is fully turbulent anyway - check out the Reynold's number.

The only relevence of the port Mach number (velocity/velocity of sound at given temperature) is that it determines the extreme limit of achievable charge velocity. Subsonic 0.45 Mach is about the best you'll get, then it chokes (won't flow any higher)..

Pressure waves have no mass and travel at Mach, (ie: Mach =1)

Taking Mach as something like 340m/s, 0.45 Mach is still orders higher than the achievable absolute (not mean) maximum piston velocity on any engine ever made. Which for one thing tells us how influential the pressure waves can be in filling the cylinder. If performance engines relied purely on the 'suction' of the piston they would never achieve high power ouputs.

(It's Mach by the way, after 19th century physicist Ernst Mach. Not "mach"!)


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TomLouwrier
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Re: Exhaust and intake resonances

Post by TomLouwrier » Sun Mar 24, 2013 7:33 pm

hi Guy,
Mach number has nothing to do with turbulence, the flow is fully turbulent anyway - check out the Reynold's number.
I should, haven't done any fluid mechanics since graduating. It never came my way. For sake of this discussion I take your word.
Using Mach here as dimension of speed only.
Subsonic 0.45 Mach is about the best you'll get, then it chokes (won't flow any higher)..
Something you mentioned in another thread, which was what I more or less referred to.
Pressure waves have no mass and travel at Mach, (ie: Mach =1)
Yes.
I'm afraid I have gone off topic again; Robert started it with resonances; I've dragged mass flow and kinetic energy into it. That may be moved to a separate thread if either of you want.
Ernst Mach
My excuses to dear Ernst. Corrected.

regards
Tom
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Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Sun Mar 24, 2013 7:35 pm

no worries dear Tom

we're all friends here.

I hope it just doesn't get too complicated here (hint - Robert!!) that I have to start looking-things up B4 I post!!

btw - Mach is a measure of velocity - not 'speed'!

Sorry TL!!


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TomLouwrier
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Re: Exhaust and intake resonances

Post by TomLouwrier » Sun Mar 24, 2013 7:38 pm

Velocity, speed??? I'll go look that up.
edit:
(Ah ok, got it now. Velocity is the vector. Spoken Dutch makes no difference between the two, you'd need the v and v(vector) notation to know.)
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Sun Mar 24, 2013 8:28 pm

Guy Croft wrote:
I hope it just doesn't get too complicated here (hint - Robert!!) that I have to start looking-things up B4 I post!!

G
More complicated is more interesting !
I would like to get a summary concerning the proportion of the effect of the waves and the other ones on the filling of the cylinder. Even a pragmatic and empirical figures. Just to know the "ball park".
OK after we can write a book... Isn't it Guy (joke).

Robert
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timinator
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Re: Exhaust and intake resonances

Post by timinator » Mon Mar 25, 2013 12:26 am

Hi GC, sorry for not citing the source.

"The Horsepower Chain" by Don Terrill on page 125. The discussion involved a Pro Stock 1317hp engine from 2001. I have no first hand knowledge of this engine or any Pro Stock engine. Mr. Terrill states the engine has a VE% of 127%. That the wave action contribution is estimated at 2% of 127% not the around 1% that I thought I had remembered. I have computers at home and work, but only one copy of the book, so in this case I was going from memory. I hope my stating this from Mr. Terrill's book does not infringe on his copyright.

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