Unable to find answer to plenum question despite search...

[CosRush]

Before I post this question, i'd like to say that I have serached through the forums using many different keywords to try and get an answer to my question.

I apologise in advance if the answer is somewhere on here and that I have just missed it.

Information preceeding my question:-

My stock Ford Cosworth Turbo inlet plenum has an incredibly restictive elbow.

images:

Whilst this restriction still does not limit the power able to be produced (upto 500Bhp), and improvements to the restriction in its design can only possibly yield better flow i.e. less friction on the air thereby requiring less Turbo boost pressure for the same volume of air.
I hope that makes sense what im trying tosay.
The incoming air does enter the plenum from the underside just off central so does give a relatively equal flow to all inlet runners.

So.....have things moved on since this original design?

Yes they have. In Sweden where the engines have been extensively used for racing a new design plenum was divised (termed a 'swedish plenum').

images:

The inlet was at one end of the chamber at right angles to the inlets.
The volume was tapered to the rear (i believe in order to maintain the velocity of the air to the furthest inlet ports away from the Throttle, by virtue of the taper increasing the airflow velocity)

The plenum volume itself is also increased. Testing has shown that these flow very well on 500Bhp+ engine but show a slight throttle response drop on lesser engines. I put this down to the increase in volume over the standard.

Now my question........

I have here one of the Swedish Plenums, so if any more information is required in the form of measuremments thats not a problem.

However, this plenum is too big for my particular application. I am looking at having a new one made and would like ot know what factors determine the Taper rate from front to rear???
Taking this top view how would I work the taper required???

I realise that perhaps Maths aside, the best solution would be to make a prototype and test it, make adjustments and retest until a satisfactory result was achieved, but this would be very time consuming and also costly (hence why major manufactures spend a vast amount of money on the designs). Ive seen many 'Homemade' ones and some have shown really very good results far better than the stock item, but I will be straight and say that unless the maths are very simple to calulate the taper, no thought was given to any airflow differences between the inlet runners.
I base this on the very uncomplicated design of those ive seen.

Thanks in advance to anybody who can help

 

[thundair]

I have done a little flow dynamics in the past but cannot recommend any formula to help, other than Bernoulli's work.
Some empirical data is no radii less than 10mm. Air chambers I have built as of late is to allow the velocity to slow down by opening the walls (bigger chamber) then based on desired reversion wave create a velocity stack with 1* convergence. If it is not possible to assign individual stacks then just converge into the throttle body.

Go here

http://speedtalk.com/forum/viewforum.php?f=1&sid=4c2fac2254d5fcd12c512a2d872903e0

http://en.wikipedia.org/wiki/Bernoulli's_equation

I don't know anything but the people that do.

 

[Fabrico]

Something that should be considered is plenum volume vs engine displacement/RPM. Many plenums, from stock to top level racing, do not have a taper. The smaller the plenum the higher the velocity and the more it must be involved in directing air. A larger plenum will let the air slow for a brief moment before being grabbed by each cylinder.

 

[CosRush]

Thanks to both of you for the replies.

I'll investigate it further.

 

[patprimmer]

Quite the contrary for a turbo engine, a large plenum makes more steady state state power, but takes longer to respond to throttle changes as the plenum must be filled to full boost to get maximum power.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[womble]

"I am looking at having a new one made and would like ot know what factors determine the Taper rate from front to rear???"

Only fabrication issues. There is nothing really complicated about it. It's just a form of a slot distribution pipe as used in some process industries. It just allows an even distribution of airflow across the length of the plenum which ensures each runner gets an equal amount of airflow and helps reduce charge robbing.
The taper should reduce from the throttle body/inlet diameter to zero ideally, though that can only be approximated by a casting. If your fabricating it, just get it tapered down as much as you can. For the slot:

If inlet is 60mm dia, inlet area is ~ 2827mm^2. If length of slot is to be say, 300mm, then slot width is 2827/300 = 9.5mm approx.

With an inlet area to slot area ratio of 1:1, as in the example just mentioned, the airflow will exit the slot at an angle. This angle will depend on the discharge coefficient of the slot and the ratio of areas. The angle is found using the following formula,

COT theta = (Cd*As)/Ad

Where,

Cd - Discharge coefficient of the slot
As - Slot area
Ad - Inlet area

If a Cd of 0.5 is assumed for the example above the air will be comming out of the slot at ~ 60 degrees. The Cd will depend on slot entrance conditions, depth of slot, if the slot walls are parallel, etc. Not something that can be calculated out without driving you nuts, but easier to determine by testing.

Or you could just use CFD...

Personally I like a 1:1 Ad/As ratio for least restriction and just start the slot before the first runner entrance, say 1-2 runner diameters.

 

[womble]

I re-read this thread when I got home last night and I see you ment a different type of manifold than the "swedish" style I thought you ment i.e. WRC/Rallycross style.
When I wrote the first post I was unable to see your pictures due to our internet restrictions.

The manifold without the taper will tend to favour no.4 cylinder (at the rear of the engine). This is a manifold with a good taper.

 

[CosRush]

Thanks to all for the replies.

Womble,

The Standard plenum actually flows slightly more air to No.3 than all the rest.
The inlet section to the plenum is not actually central but slightly offset towards No.3, and this being the inlet means the shortet path toNo.3 allows for a path of least resistance over the other 3.

This has been extensively tested over many years.

I fully understand that with the throttle inlet at right angles and on the extreme end of the row of inlets as in the Sedish style plenum, on induction you want all the airflow into each cylinder to arrive at the same time since the distance between the throttle and the valve is effectively different (if the plenum was parallel in this instance) therefore leaving No.4 cylinder (furthest away) a greater disntance to travel than that of cylinder No.1 (closest to throttle).
Therefore i can only conclude that the taper serves to increase the velocity of the air further (convergent duct).
Thus hopefully (if designed correctly) to allow all gas flow to arrive at the valves at the same time on the stroke irrespective of the different lengths they have to travel.

Anybody wish to comment on this???

I have tried a search on some free CFD programs but as yet have not found any. If anybody knows where these can be found please let me know.

 

[GregLocock]

OpenFOAM is free. You'll need a Linux installation, it works very well with Ubuntu

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Deltona]

I've also been looking into plenum design and wondering about the behaviour of pressurised air and if it is different to that of non pressurised air or what happens on initial boost, Is it different when the whole system is pressurised? This leads onto flowbench testing, are the results relevant because the system is not under high pressure?
I also pondered over buying a 'Swedish Style' plenum, but then thought that there may be better designs, one of the guys earlier mentioned a WRC type of plenum, this is one on a Focus WRC car:

http://i103.photobucket.com/albums/m127/Deltona_II/ec127d1e.jpg

I would really love to know what the slot passage like in between the inlet pipe and plenum, does it look like this perhaps?

http://i103.photobucket.com/albums/m127/Deltona_II/add599d2.jpg

http://i103.photobucket.com/albums/m127/Deltona_II/442372fd.jpg

http://i103.photobucket.com/albums/m127/Deltona_II/723e51c4.jpg

What about this one for equalizing the flow?

http://i103.photobucket.com/albums/m127/Deltona_II/DeltaThrottlebodies3100.jpg

Looks ideal, unless the pipes come into the plenum from a bend, that may mess things up a bit.....

Just some food for thought.........

 

[CosRush]

Thanks for the reply and the pictures......

The 1st and 2nd pictures you have linked are exactly what i'd been considering making.

Here is a picture of one a friend of mine made.........

It worked out very well and looks awesome considering he made it by hand.

http://img.photobucket.com/albums/v385/CosRush/pic1.jpg

And here it is welded up and polished.

http://img.photobucket.com/albums/v385/CosRush/pic2.jpg

And quite simple to make too.

He told me he had done some comprehensive maths to work out what was needed but in all honesty I'd suspect a simple flow bench would yield results easier.
You only need equal flow/pressure to each runner.......

 

[jbthiel]

What is the purpose of the slot in Deltona's second manifold?
(add599d2.jpg, 442372fd.jpg, 723e51c4.jpg) I can see trying to equalize the flow, but wouldn't this slot be restrictive? If they were trying to minimize the effective plenum volume that could have been done other ways.

 

[CosRush]

I would suggest that instead of a slot, the whole interface of the plenum body and the inlet be opened up to increase maximum possibly flow.

 

[CosRush]

Deltona,

Where did you obtain your plenum body pieces from??? were they custom made???

I see you have been looking at the Ford Sierra Cosworth inlet plenum.
Is that because you were intending using it or modifying it in some way.

It is indeed the original 2wd inlet plenum you have pictures of that I have, but am trying to replace with a better flowing one (the restriction in the elbow section).

Any information you have regarding the designs, the source of any parts for custom fabrication greatfully received

Find me on

http://www.Passionford.com

 

[Deltona]

"jbthiel (Mechanical) Wrote:
What is the purpose of the slot in Deltona's second manifold?
(add599d2.jpg, 442372fd.jpg, 723e51c4.jpg) I can see trying to equalize the flow, but wouldn't this slot be restrictive? If they were trying to minimize the effective plenum volume that could have been done other ways. "

"CosRush (Aerospace) Wrote:
I would suggest that instead of a slot, the whole interface of the plenum body and the inlet be opened up to increase maximum possibly flow. "

The way I see it (theory:unproven) is this:

To get equal flow to each runner from an end feed plenum is very difficult, to address this the manufacturers of my plenum have cut a slot, this equalizes the flow, BUT, as you point out, this upsets the flow and causes a restriction, loose this restriction and loose the equal flow to each runner. It's a case of not being able to have your cake and eating it.....
This is why we need some pics of the INSIDE of your friends home made plenum (which looks very good BTW)

I just wish I was where I am now (design wise) when I saw the Focus WRC plenum, photos weren't allowed as it was in the MSport facility, but I could have blagged a look i'm sure If I had known, It would have given us some ideas...

 

[jbthiel]

On the home made manifold (add599d2.jpg, 442372fd.jpg, 723e51c4.jpg) I can see your point about equalizing flow. The inlet pipe does not taper until past #3, so flow will be biased. BUT I think it could have been done more efficiently than having a sharp edged slot.

As for the Cosworth manifold my hunch is that it is open. The momentum of the air is deflected by the taper in the inlet section. Plus the air has to make two 90 deg turns to get into the intake runners. I find it interesting that the inlet tube opens up to the plenum right at the #1 runner - I wonder if this was done on purpose to balance the flow OR were they space limited. My first inclination would be to start the opening about 2" farther forward so the air for #1 doesn't have to go back towards the front.

As for balancing flow I would first consider an air splitter or deflectors inside of the plenum. This type of thing is done on the 70's Trans Am style of cross-ram manifolds with the large flat plenums. They had lots of problems with air and fuel distribution.

 

[Deltona]

Air splitters/deflector plates? I like that idea, I've just designed an intercooler with some of those in, but never thought of putting some in the inlet plenum.....

One thing I really need to know: Does constantly highly pressurised air in a boosted engines intake pipes/plenum behave differently to that in a normally atmospherically pressurised engine? In my mind they behave differently, am I wrong?

 

[jbthiel]

I try to think of it as just being more dense. Both a NA and turbo motor have a pressure drop between the throttle and intake valve, the turbo just has less. The biggest thing to remember is that an engine may be a nominal 2.0L - but with the turbo it is actually flowing much more (3+ liters?) while the NA is flowing around 1.8L max.

Not talking about anyone here - but one of my pet peeves is people who think that turbo's trump all evils. That there is no need to consider intake restrictions, port the heads, etc. They think that it doesn't matter with boost. I see this especially with diesel people who are used to being air rich anyway. But if you increase the intake efficiency you just increased the engines efficiency. BTW - Gale Banks has been preaching this message for years.

 

[CosRush]

Jbthiel,

My sentiments exactly.............

It matters not whether the engine is N/A or boosted, at the end of the day the N/A engine still gets 1 bar of atmospheric pressure where as a boosted engine might get 1.5bar.

Any restriction to flow in design i.e. unsmooth casting lines in manifolds/plenums, tight radius' all effect the gas flow by the same amount.
It matters not the whether thats at 1,2 or 3 bar.

What people in the Turbo world often seem to believe is that because the intake is pressurised above atmospheric pressure this overcomes any small losses, which whilst it is true also means that both the turbo and intake system are working harder than they need to.

Gas flowing to any of the intake system on both N/A and Turbo engine will yield improvements in gas flow and in a turbo engine this has shown itself as producing the same power for less pressure.
or producing more power with the same boost.

Effecient flow is the key to reducing friction to the gas flow and this ultimately affects the velocity,pressure and temperature of the gas.
Its here that the velocity is reduced and the temperature increased as friction is caused by the gas flow through a lesser efficient intake, both of which you want to avoid as much as possible.

 

[CosRush]

Jbthiel said: "As for balancing flow I would first consider an air splitter or deflectors inside of the plenum. This type of thing is done on the 70's Trans Am style of cross-ram manifolds with the large flat plenums. They had lots of problems with air and fuel distribution."

Ive also seen custom plenums with the addition of balance tubes between the inlet and the runner area, such that the pressure in the inlet as it enters the plenum is split so as to provide additional air directly to the runners in a similar way (but in reverse obviously) to an 4-1 exhuast manifold and help equalise the pressure difference between them. Worked very well too.