Jab Plenum Chambers

[Yenn]

If you suspect mixture is causing high temps, then you should consider installing EGT gauges.

 

The EGT gives a much more reliable idea of mixture problems than CHT.

 

 

[seb7701]

Spoke with Ian Bent about their exhausts, as was only seconds from buying a pair, until he very honestly stated that they are unlikely to be the cure to any problems and that they had not really done any testing on them. They were more or less produced as a result of requests to do so. They sound great thought, I'll say that!!

 

 

[horsefeathers]

H.F. - I KNEW I should have checked what plane, before making assumptions about the streak!!!!! :)Ok, well I am intrigued, as I have often wondered about some of the setups I have seen with ducted oil coolers. I am curious to know how that works, but am guessing it just send the hot air nicely 'south' towards the outlet? I too would like to see some shots if that's OK...

So the 160 will have bigger cowl inlets than me and presumable hydraulic lifter donk, so the jetting is going to be different anyway i guess, otherwise I'd be asking a needle jet size to try out. (I am having interesting results with jetting trials...) In any case, they are very enviable temps at 20 degrees OAT!

 

I have just removed both baffles from my muffler, so will be interesting to see if there is a difference.

Seb.

 

I've just found the doco re my jetting that I installed about 100 hours ago, As I said it was the current Jabiru recommendation for a 2200, 40mm Bing (they have moved away from the economy jetting apparently.)

 

Needle jet : 2.90

 

Main jet : 245

 

and the jet needle was also replaced (I only have the Jabiru item number: 4A138A0D)

 

BTW, here are my numbers this morning, with OAT of 21C

 

 

 

 

[seb7701]

Thanks mate - appreciate some jetting to compare to, as well as the snaps overall and I can see what's really going on in another similar aircraft. Unfortunately and oddly enough, I'm quite a bit hotter, would justo love to know WHY!!!!!

 

Just saw that on the last 20 degree day, whilst most of my figures were had at around 2500 AMSL, my hottest cylinder was still in the 140 range, whilst cylinder 1 & 3 were around 100 and 111deg! Go figure... Would just be nice to know where the main differences lie between the intakes etc of the J160 vs my little 'ol 470. Ah well...

 

 

[Bruce Tuncks]

There is a great article in the Sept 2013 Airsport, where a GA plenum chamber was tufted on the inside.

 

The author discovered that the flow was often in the reverse direction to what you would think. In the cruise, the flow was from the rear to the front in the inner area of the cooling fins. The front-to rear flow we imagine was confined to the very outer bit of the plenum chamber.

 

Now I reckon this makes a good bit of evidence that the Jabiru cooling ducts are actually better than the older whole top-of-engine plenum chamber, not that I have done the tuft test inside the ducts.

 

Here's what would be effective at least until take-off I think : A fan at the entrance to each duct. You can get brushless ducted fan units about the right size.

 

An axial blower is all that the old air-cooled Deutz at the farm has, and it works fine.

 

Here's a report:

 

With the warmer weather, I have noticed that the CHT readings on my 2200 engine are still ok, on cruise sitting on 120 to 130 C, and not going over 155 on take-off. But the day was a max of 30 degrees and more like 25 when I took off. So there is more to report this summer.

 

 

[Oscar]

Bruce - it just isn't that easy!.

 

The Jab. ram ducts CAN be made to work effectively; there is a guy at Camden airport who has worked for over 12 months - meticulously - on his 160 cooling - including tufting his ducts and recording that with a borescope, in flight, over a scheduled set of flight tests. He's gotten them to work - in conjunction with carefully modifying the extraction lip on the bottom of the cowl - very well indeed.

 

To give you an idea of how intricate this work is: he found that the middle-deflectors in each ram duct had to be varied between the sides by 2mm or so to balance the temps side-to-side in climb. That is because the 1-3 heads are closer to the intakes than the 2-4 heads and the 'ramp' behind the intakes is at a different position relative to the intakes, plus the differential of the prop blast from the upcoming side vs. the downgoing side. (When Alan Kerr was designing the engine installation for Jab. 2200s in Drones to meet Boeing's requirements for their proposal for a modified Israeli drone for the RAAF, he found that in ground running with a slight crosswind, the RH side of the engine was sucking the hot air from the LH side back up the rh side ram duct. You know what damage extended ground running can do - and Jabiru's POH recognised this..)

 

The guy at Camden also found that his extraction lip on the bottom of the cowl was perhaps 25mm longer than ideal - because it was stalling at high climb angles and choking the extraction airflow..

 

The flow reversal inside the traditional plenum, is quite well-known. In fact, the usual gap of the cowl around the prop. shaft (mandated because the clearance between the cowl and the engine MUST be at least 1/2" to allow for engine movement) actually creates a 'pumping' area of low pressure.

 

The answer to cooling is absolutely NOT just stuffing air in - it has to be able to move past the hot areas with sufficient energy to extract heat.. That means getting the low-pressure area working well - the p-delta. As a crude analogy: you as a farmer would know what happens when you get a serious downpour. You dig ditches to direct that away from your sheds etc. BUT - if the ditches block up, or aren't sufficient, the bloody water floods out everywhere.. Now, assume that you WANTED that water to flow around specific areas... Huge amounts of water flow are coming in, but it's not resulting in huge amounts of water flow in the areas you'd like it to be - its just going everywhere. (and usually, flooding the bloody hayshed and creating a self-combusting mass of your stored hay.. been there, done that).

 

You'd remember, I am sure, that in the 'good old days' of us trying to improve the performance of our cars: extractors were the FIRST thing we'd add. We wouldn't even think about porting and polishing, extended intakes, larger carbies, even hot cams ahead of extractors and a larger and less back-pressure exhaust. Well, basically with cooling of an air-cooled engine, it's the same!. Air In and air OUT is the prime requisite.

 

And the air OUT needs to flow around the heat-intensive areas.

 

 

[Bruce Tuncks]

Thanks Oscar, yes I do have a difference between 1-3 and 2-4. Now I'll have another look at that.

 

I wasn't all that serious about the blowers idea, but they would be a lot easier and lighter than going to water-cooled. I reckon the Deutz tractor proves that air-cooled can be superior. At the age of this tractor, there would have been corrosion problems before now with a water ( I know I should say liquid) cooled engine.

 

I well remember the maintenance guy of the day at Tocumwal dreaming about how good life would be without the problems of water-cooled Rotaxes. He had to remove an engine every time a water-pump leaked.

 

 

[bexrbetter]

I reckon the Deutz tractor proves that air-cooled can be superior. .

Moot point, diesel does not require the same (higher) level of cooling.

 

Even Porsche, the longest holdout, had to eventually go to water cooling, heads first, all engine later.

 

 

[Paul davenport]

Just a thought has any one experimented with the inlet ramps on the cooling system, on looking at Lo Presti and the like they all seem to have smoothed out the flow in . Jabs blunts and crude looking head and cylinder covers seem to be counterintuitive to smooth airflow in ?

 

 

[Bruce Tuncks]

That's right Paul. Every discontinuity gives a pressure loss. There is, or was, a good paper on this on the Limbach website.

 

In my case, I increased the size of the cowl inlets to match the ducts and also did some other smoothing. I reckon it worked, although I have never measured the delta-p directly.

 

 

[Old Koreelah]

Good point, Paul. Smoothing the inflow should help, but there's precious little room to do this. It might be easier on engines with a prop extension. As Oscar points out, the large clearance gaps required for engine movement complicates airflow control.

 

Measuring air pressure differential can be instructive, if fiddly. I'm about to remove my simple gauge because all the coloured liquid keeps getting sucked out and has stained the exit vent. It's done its job and told me there's plenty of suction downstream.

 

 

[jetjr]

I added ramps to inlets on old engine.....did close to nothing to help cooling.

 

Others say different.

 

 

[Paul davenport]

From my investigation to date (and I cannot remember where I found it ) the top and bottom of the Cowl inlet are of equal importance ,something to do with a pressure wave and pressure recovery ,where the cowl inlets accept far more air than the engine and low pressure area can get rid of causing drag and a spilling out of air from the cowl (correct me if I am wrong but that's the best way I can describe it) . I have some expanding foam and at the earliest chance I intend to try it out. Getting it to stay put with out moving and blocking off the cooling system is a concern

 

 

[Paul davenport]

Just saw your post JJ seems to me that there doesn't seem to be two jabs the same,what works for you may not work for me or others cooling science seems to be a black art indeed

 

 

[jetjr]

Absolutely and the POH says this on the first page.

 

Also in the black art file is a guy who had long term starting trouble, all gone when "vacuum bleed valve" (Hacman), fitted and open?? - Yet I have never had an issue.

 

Hacman basically equalises downstream and carb bowl air

 

As per pictures here with round hole for air inlet instead of NACA........mine has NACA duct fitted in different location to many others Ive seen, further back and higher.

 

Bing is super sensitive to incoming air pressure and even propwash could be enough to lean it out a little.

 

Informed people tell me the flap on top of air cleaner housing is supposed to prevent this pressure but may not work too well.

 

This train of thought/development might be on track to cure a few bugs.

 

 

[horsefeathers]

There is a great article in the Sept 2013 Airsport, where a GA plenum chamber was tufted on the inside.The author discovered that the flow was often in the reverse direction to what you would think. In the cruise, the flow was from the rear to the front in the inner area of the cooling fins. The front-to rear flow we imagine was confined to the very outer bit of the plenum ....

Which got me to wondering,if there is some reverse.airflow into the back of one of the ram air ducts,would extending the rear of the ducts down by a few inches towards the low pressure area help reduce/eliminate the reverse airflow?

 

 

[Oscar]

Absolutely and the POH says this on the first page.Also in the black art file is a guy who had long term starting trouble, all gone when "vacuum bleed valve" (Hacman), fitted and open?? - Yet I have never had an issue.

 

Hacman basically equalises downstream and carb bowl air

 

As per pictures here with round hole for air inlet instead of NACA........mine has NACA duct fitted in different location to many others Ive seen, further back and higher.

 

Bing is super sensitive to incoming air pressure and even propwash could be enough to lean it out a little.

 

Informed people tell me the flap on top of air cleaner housing is supposed to prevent this pressure but may not work too well.

 

This train of thought/development might be on track to cure a few bugs.

jj: the flap on the top of the airbox is there to prevent backfires from crushing the air filter and thus stuffing up further intake air to the engine- not to regulate intake air pressure. The whole hot-air-intake design that bypasses the air fillter, is done that way so that there is a back-up air delivery system in case the air filter ( or the SCAT intake tube) collapses.

 

 

[jetjr]

Yes that's what I thought but Rod says different

 

Standard Jab hot air setup doesn't bypass air filter or the main scat hose

 

 

[Oscar]

Hmm - then it's been changed, it did on my (certificated) ST1... via a bloody awfully dodgy airbox, maybe that's why it is so awful!.

 

 

[Bruce Tuncks]

I don't think extending the duct at the rear end would help, horsefeathers. But this is just my intuition at work, and we are learning that intuition is no substitute for actual measurements. A simple "U" tube gauge like Old Koreelah used would show if the idea was right.

 

 

[Old Koreelah]

I don't think extending the duct at the rear end would help, horsefeathers. But this is just my intuition at work, and we are learning that intuition is no substitute for actual measurements. A simple "U" tube gauge like Old Koreelah used would show if the idea was right.

If you're interested, this is my pressure gauge. It's about 5mm food-grade vinyl tube from any hardware shop, with about 20ml of food-dyed metho in it. I learned the hard way to mount it where it can be easily seen in flight.

 

One end taps into the intake end of the cowl, the other near the outlet. To avoid the "pitot effect", I drilled some 1.5mm holes near each end of the tube. It worked well, showing about 60mm pressure variation at cruise, but during one climb I open the owl flaps wide and all the metho got sucked out.

 

 

 

[scsirob]

jj: the flap on the top of the airbox is there to prevent backfires from crushing the air filter and thus stuffing up further intake air to the engine- not to regulate intake air pressure. The whole hot-air-intake design that bypasses the air fillter, is done that way so that there is a back-up air delivery system in case the air filter ( or the SCAT intake tube) collapses.

I'm confused.. How would those flaps prevent the air filter from being crushed when the flaps are on the intake side of the filter? Any backfire pressure will have to travel back through the filter to get there.

 

 

[jetjr]

There are a few early versions of the air box, Seems Oscar had a different one to current offering

 

This line of thought means that internal cowl pressures can effect mixtures........ and might explain why playing with oil cooler inlet hole or cowl flaps can alter EGT.

 

 

[Old Koreelah]

There are a few early versions of the air box, Seems Oscar had a different one to current offeringThis line of thought means that internal cowl pressures can effect mixtures........ and might explain why playing with oil cooler inlet hole or cowl flaps can alter EGT.

Years ago when installing my Jab engine it wouldn't rev out. My recollection of conversations with Don Richter at the factory, was that the engine doesn't like air being rammed into it. The carb's little pressure tube has to sense outside air pressure. After plugging it into the air cleaner box and installing a flap valve to release any ram air pressure (both downstream of the filter) the engine ran well.

 

 

[Oscar]

O K, things get complicated.

 

My aircraft was the first to be certificated (and get VH-reg) that Jabiru produced. That meant that it had to meet all the BCAR S standard for the airframe and engine installation - remember, installation is NOT an engine certification issue: the engine has to meet the requisite standard ( for the Jab. engines, and the Rotax 912A model, JAR 22H.) That is ONE set of performance standards, from which the TBO (amongst other things) gets set - but it also produces the 'limits' for operation: max revs, max temp,max running time at various power setting etc..

 

Then, the installation has to produce, for the POH, a set of performance parameters that determine the actual aircraft performance limits. Mostly, those relate to keeping the engine within its certificated (or certified) limits for that installation. (However, by way of further information, the allowable climb profile for the Seabird Seeker is actually limited by the NOISE limits: pusher aircraft tend to produce more noise as a result of the prop. passing the wings and producing a noise pulse, than from the actual engine exhaust).

 

What this means, in practical terms, is that the POH climb speeds and revs limits for, say, a J160 with a 2200A engine, is NOT automatically transferable to another aircraft with a different installation. Each installation has to be tested and the aircraft performance profile adjusted as a result of testing.

 

Now, WRT to the airbox conundrum that has been exposed here: as a certificated aircraft, my Jab. had to meet BCAR S standards. In respect to the engine air delivery, that required that the hot air setting bypassed the filter and airbox intake SCAT tube, so that if either caused a blockage, by pulling the hot air on there is a direct input of air to the carburettor. I assume that the rationale for this is that, hot (and therefore less dense, resulting in lower combustible fuel/air mixture) is delivered to the carby: some air is better than none..)

 

The airbox on my aircraft has two 'rubber' flaps.

 

One, is attached to the hot-air ./cold air switch-over mechanism. It appears to be a bit of around 6mm thick conveyor belt-type material. In the 'normal' (or 'cold air) position, it delivers air through the filter that exits into the airbox outer housing, from where the delivery SCAT tube to the carby is taken off. When 'hot air' is pulled, the air coming from around the muffler is delivered directly into the outer area of the airbox, entirely bypassing both the intake SCAT tube and the filter and thence to the carby. In simplistic terms: if your intake scoop ( or NACA duct, depending on the model) gets completely blocked by e.g swallowing a sparrow, by pulling the 'hot air' control, you will still get air coming into the airbox and therefore to the carby.

 

The second flap is a small - about 75mm x 25mm piece- of what looks suspiciously like a bit of car inner-tube on my airbox- that sits on the top of the airbox outside the filter: so a backfire pressure passes around the filter and is relieved by that top flap. Now, I have no idea as to whether that was tested to ensure that the air filter was not severely damaged by a backfire - but it does ( I assume) meet the BCAR S requirements for an alternate air delivery in the case the the filter is damaged and blocked.

 

So: in summary: the airbox (which appears to have been thrown together by a blind idiot working under the lash, it is a truly horrible bit of work) on my aircraft., evidently met the standard required for 'certificated' acceptance. If later Jabiru airboxes operate differently, I can only assume that the ASTM standard does not require the 'redundancy' of the FAR / BCAR S standard. I will not be putting that airbox back onto my aircraft; I will build something more closely resembling what has been used on Lycomings etc. for years, very successfully. That will incorporate backfire relief 'downstream' of the filter.