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Cylinder head plenums

MarkW

Well Known Member
Friend
I have seen the typical plenums used like the James cowl and plenum but have not seen a discusion or pictures of the cylinder head only plenums like I saw on a new LSA using the new Lycoming 0-233. Very good looking setup as it doesn't cover the top of the engine, only the cylinder heads. I like the idea of covering the fuel spider and the one I saw looks like maintenance would be a lot easier along with removal.
Has anyone seen these on the I0-320 or 360.
Any ideas or comments?
I haven't even been able to Google a picture of one.
 
Here's some pics from Oshkosh 2010

I've thought about the same thing. Here are some pictures to get the creative juices flowing.

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...a discusion or pictures of the cylinder head only plenums like I saw on a new LSA using the new Lycoming 0-233......
Any ideas or comments?

Most of the examples in the photos exhibit a typical design problem...they don't seal very well at the inlet. The whole point of using a plenum is sealing, i.e. not allowing any of the cool, high pressure inlet air (roughly a range of 6 to 26 inches H2O) to reach the lower cowl volume without passing in close proximity to a hot engine surface. Every bit of air which leaks through those crappy inlet seals is pure drag. Doesn't matter very much on a 100 knot airplane, but most of us would prefer a 200 knot airplane.

Now look at the photos of the fancy carbon "plenums" on the 233. Do they seal well, for example, around the pushrod tubes? Sheesh.

Also note they are one piece; no split between cylinders. Not good.

Yes, well fitted individual plenums with sealed intakes are possible.

A full plenum (or stock baffles) circulates relatively cool air around the spider and injector lines. Fuel pressure in them is quite low, and low pressure increases the possibility of boiling.

A full plenum also increases the deltaT between much of the engine case surface and cooling mass. Even the non-finned surfaces shed heat.

Notes about Paul's Lancair; those tiny inlets were possible only because the system was heavily exhaust augmented. That aside, take note of Paul's typical focus on fundamentals...the ducting wrapped around the entire circumference of both cylinder and cylinder head. He wanted cooling air in tight contact with the maximum hot surface area. His goal (and yours) is to raise air temperature as much as possible during the pass by the engine. It means you carry away more heat with less mass...the other reason he could use the tiny inlets.
 
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I'm biased towards the full plenum as I have been running one on Grezdlitn for eight years now. I've only been compelled to remove it once, to check the plugs. The fuel injection spider and related lines don't require any maintenance
unless you are experiencing an issue.

If your going to build a plenum there are fewer steps required in building a full one as compared to two individual.

I'm with Dan on the air inlets! Strive to create a transition that utilizes 100% of the cooling air entering the inlets. I just helped with a project where we created some inlets very similar to the "Sam James" only out of fiberglass.
 
Is anyone using one on a flying RV? How well does it work?

Unfortunately there have been a lot of aviation ideas that have been good ideas or that looked good but did not work well.
 
The split cylinder plenums have been tried by many, I don't know of any that were a real success.

Paul Lipp's. The AR-6. Both of which cooled TOO much on the first attempts.

The devil is in the details. The cooling system is not just a type. Stock works for some, plenums for others, then the opposite for both by others. It should be looked at as an entire system. Answer questions like; where is the air coming from, how much, what speed, where does it need to go, how to get the most BTU's out of the cyl, how do you get it back into the free airstream?
 
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But there is no flight data listed to know how well it worked.

That is correct. I actually have the template drawings for the final design from Mark. I think there is actually "cooling drag" issues with this type of design with standard size of the inlets as you have a lot of cooling air with a limited area to flow. :)
 
I did a reasonable amount of work on cooling drag

I did a reasonable amount of work on cooling drag and published the results in this forum. After I had achieved a 4 KTAS increase in speed at 6,000 ft density altitude with a lot of lower cowl experimentation I started working with stock Van's inlet size for more speed. I reduced the size in 1/4" segments from the center outboard and the only significant change I saw was a direct increase in CHT with each inlet size reduction. There was no increase in speed. However, I did observe that the shape of the "plug" between the spinner and the inlet opening could reduce the speed.

Bob Axsom
 
plenums

Seems like the cyl. plenums would be less than optimal. The set I saw on a 0-233 LSA were very sweet looking. Per Dans statement they may have only been the tops. Just tops would be easier to build but much less effective. The smaller engine and less horsepower would require less mass. This would mean a larger engine like the IO-320 would be more difficult to do. If it could be worked out it looks like a market for someone to start selling prefab.
 
I think there is actually "cooling drag" issues with this type of design with standard size of the inlets as you have a lot of cooling air with a limited area to flow.

In our conventional cooling scheme, a plenum is a sealing device, nothing more. In the context of cooling drag (not necessarily cooling capacity), a full plenum or two partial plenums would be no different if they seal equally well.

A "limited area to flow" decreases cooling mass flow. If the system throttle is at the inlet (per Bob's experiment above) the decrease in drag due to reduced mass flow is offset by an increase due to momentum loss, i.e. low exit velocity. If the throttle is at the exit, the decrease in mass flow is accompanied by higher exit velocity, the result being reduced cooling drag.

http://www.vansairforce.com/community/showpost.php?p=556582&postcount=117
 
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I believe

what Dan and Bob are saying (and both please correct me if I am mistaken, I'm continually trying to learn here) is that poorly managed airflow through the inlet/plenum/exit system is more detrimental than mis matched inlet size.

"Poorly managed" would include leaky seals between inlet and plenum, leaky plenum, leaky cylinder baffles, slow/turbulent exit flow, etc etc.

These will all have a greater detrimental effect on cooling efficiency and cooling drag than mismatched inlets because the drag created by inlet spillage is relatively small.

This of course assumes that you have a spill tolerant inlet shape, such as Dan's or Van's stock inlet.

The best gains are from getting the air that comes in to do the most amount of work (heat transfer) and get it out efficiently. If it is too much air (more than is necessary) you'll have a bit more drag than you need and lower temps than you need. By restricting that air mass flow (by restricting exit size), you will reduce the mass air flow to what is necessary, and have a net reduction in drag. That is because the air that is forced to spill at the inlets causes very little drag. Less than the gain you will see from having lesser, well managed, air flow through the system.

That's my theory, and after reading a lot of threads and references, talking with some fast and cool guys, and building a couple inlet plenums and exit fairings, I'm stickin to it.
 
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Experimental=Education=enjoyment

OK so -
I need to change some names around to fit my brain. I hope they are the same.
Mass = volume
Momentum = velocity
Keep the volume of cooling air to a minimum by increasing efficiency of cooling air by having a well sealed, smooth path. If you are able to reduce volume, do it by reducing exit at the bottom of the cowl. This increases the velocity of the exit cooling air.
One of my main questions is, If you design and tweek a cooling system to work properly at top end, is that worse case? Or would there be concerns at idle on the ramp? I do understand heat=HP so I tend to believe that top end would be worse case but I also worry about inlet air volume being very low at idle.
 
Cylinder Head Plenums...

...and to follow up with Dan, also with temperature. If it's doing its job properly, the air will be hotter (and take up more volume) on its way out than it did on its way in. So, technically, to prevent an adverse pressure gradient the hot side of the plenum should be larger than the cold side.

This is probably the neatest area with which to experiment. But since I did CFD for food years ago, I'm going to experiment with something else.
 
Mark

mass is essentially equivalent to the total flow rate of the air - pounds per hour - how much air is going through the system.

Volume of that air will vary some with temperature. Don't confuse it with volume of the plenum.

I can live with velocity=momentum. They are not technically the same but using that equivalency won't get you into trouble.

You are right, with a non-variable system (no adjustable cowl flap) you are going to have to accept some compromises. In my experience, taxi is not a problem, it does not last long enough to be a concern and heat generated (HP) is small.

The biggest problem (worst case) is climbs for CHT and oil temp - high horspower with low mass flow. This can be a problem but your climb times are generally short in these airplanes. I try to optimze for fast crusie. That is where I spend the most time. I have lower cruise temps than I need but never had a problem with high temps in the climbs. That might be different if I squeak down the flow rate (exit throttling) to increase cruise temps and further reduce mass flow (and drag). It's all a compromise.

Build on now and deal with the tweaks latter.
 
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Sorry to drag up a very old thread here, but my Dad built an aluminium plenum over the io-320 in his Mustang II, which i have recently purchased and note that we are experiencing a situation where we struggle to get the engine warm, even on a hot day. In winter we have to restrict the airflow through the oil cooler just to get it to normal operating temperature!
 
Most of the examples in the photos exhibit a typical design problem...they don't seal very well at the inlet. The whole point of using a plenum is sealing, i.e. not allowing any of the cool, high pressure inlet air (roughly a range of 6 to 26 inches H2O) to reach the lower cowl volume without passing in close proximity to a hot engine surface. Every bit of air which leaks through those crappy inlet seals is pure drag. Doesn't matter very much on a 100 knot airplane, but most of us would prefer a 200 knot airplane...

Concerning the challenge of sealing the intakes, it might be worth looking at the approach taken by Glasair, Mooney, and others which employ a fwd dam behind the ring carier. Not only does this solve problem of sealing individual inlets, it also takes care of the "backwash" path from the lower cowl, past the ring carier and out the spinner area. Yes, this can be somewhat solved with a rub seal as you have done, but the "great wall" baffle solves both in one fell swoop.


55-baffles.jpg


Im looking very hard at this right now for my Rocket.
 
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Concerning the challenge of sealing the intakes, it might be worth looking at the approach taken by Glasair, Mooney, and others which employ a fwd dam behind the ring carier.

Mooney Acclaim below. Keep in mind that this only works with a low Vi/Vo inlet.



 
it also takes care of the "backwash" path from the lower cowl, past the ring carier and out the spinner area.

Will some one please explain this concept to me so I can understand it a bit better as how it negates cooling? If I understand it correctly, some of the [hot] air from the lower cowl will be forced out of the front and the area around the spinner. What I don't understand is that how does that translate in less cooling of the engine. Also any pix of how this seal has been accomplished would be great.

As always thanks in advance.
 
I have a home made aluminum plenum (with very careful duct shapes) with no rubber intake seals and have no cooling trouble. Actually the cooling is better than expected. All cylinders typically stay within 20 degrees of each other ( low 300s) with the coolest being the back two.

So far I don't see the need for the seal. The air coming through the inlet is at the highest speed (lowest pressure) it will be at any point within the cooling system. This would mean if there's any leackage, it would be FROM the lower cowl INTO the inlet area.

I like having no seal to maintain or get in the way when taking the cowl on/off.

YMMV

Bevan
 
Will some one please explain this concept to me so I can understand it a bit better as how it negates cooling?

First, at this time, it's been proposed that air flow around the propshaft and spinner may not be a simple flow out of the lower cowl volume. It will require measurement to be sure. More later.

Another theory says there is a flow, and some of it gets into the cooling inlets. Ditto.

For the moment, let's assume there is a flow from the lower cowl volume to the freestream, via the spinner gap. A propshaft seal would increase pressure within the lower cowl volume slightly, increasing exit velocity, and thus reducing cooling drag. Note that such a pressure increase would push CHT upward.
 
For the moment, let's assume there is a flow from the lower cowl volume to the freestream, via the spinner gap. A propshaft seal would increase pressure within the lower cowl volume slightly, increasing exit velocity, and thus reducing cooling drag. Note that such a pressure increase would push CHT upward.

In my simplistic way of thinking, higher pressure within the lower cowl ought to result in higher CHT so why sealing the gap unless the theory as you states is that the warmer air, instead of cooler air gets injected into the inlet area. And realistically, how much of that warm air will get circulated back into the engine area?
 
In my simplistic way of thinking, higher pressure within the lower cowl ought to result in higher CHT ....

Pretty sure I said that. Given that mass flow through the cylinder fins is entirely driven by deltaP, you would be right,
 
years ago van said that there was a large boundary layer at the back of the spinner because the shank of the prop is not doing good things to the air at that point. the air exiting the gap between the spinner and cowl causes needless drag. stan mileys' work actually showed reverse flow at the inboard edges the inlets (he tufted the entire inlet) due to the differential prop thrust from its' hub to the tip of the blade. so it didn't appear that heat could be reentering. that's why i decided to move the cowl inlets outboard, use axio symmetric inlet to have the least pressure differential across them, and seal the space between the spinner and hub to prevent air flow to decrease drag due to what van said. an efficient cooling system doesn't have to worry about air leaking out there in order to decrease the lower cowl pressure. dan is right it's all about delta P. so seal EVERY leaking spot in the upper plenum.
 
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