Jab Plenum Chambers

[Bruce Tuncks]

Oscar, on my lower cowl where there was a heat-browned area I fixed it by using silastic to stick an area of soft aluminium sheet, the stuff you get in baking trays. This has lasted over the years, but I have to say that your heat scorched bit is worse than mine was.

 

Your new setup looks great.

 

So does yours Seb, your top cowl looks like mine now and I reckon the work was worthwhile. But what are the Camit cooling plates? I have done the "gull-wing " plates to the underside of the cylinders, but I don't think you mean those.

 

 

[facthunter]

If you don't use the plates like what you have, you are wasting cooling airflow. Nev

 

 

[seb7701]

Bruce - the cooling kit is a nice little item from CAMIT which more or less closes in the gap between tappet covers do direct flow between the cylinders which would otherwise just be wasted.

 

Here's the link - you can't quite see them completely in my shots.

 

Cooling Kits

 

 

[Bruce Tuncks]

Thanks seb, I did close this gap years ago with some aluminium plate.

 

I sure agree that you don't want to provide a hole where cooling air is wasted.

 

 

[seb7701]

Yep, I just took the pre-fab easy way out!! Lol. Bruce - what can you tell me about the cross vanes in the cobra head? Seen a bit about this to stop the swirling mixture into the diffuser. Did it change things much? I am guessing at the moment that there may be a leaner mixture on the left side, but not quite certain.

 

 

[Bruce Tuncks]

the cross-vanes were AFTER the carby. At the entrance to the plenum chamber, I put in a cross made up of 2mm tube with tinplate vanes. The vanes were about 12mm wide by 17mm deep folded over the tube like little wings. In my case, the vanes directed the flow into no4 and away from no2.

 

There were actually only 3 vanes as one cross-arm didn't get one as I couldn't figure out which way to divert the flow there.

 

Sorry I didnt take a pic before installing it. I can draw a diagram if you want more.

 

I don't have a cobra head, mine is the old 32mm carb and they are not made for that.

 

Nothing I tried upstream of the carby helped, and even putting in diverters into the plenum chamber didnt help.

 

 

[Old Koreelah]

I'm impressed by the neat inlet ducts on your cowls, Oscar and Seb. It looks like the oil cooler inlets are smaller than the actual oil cooler face area. I presume this allows high speed air to slow down before making its way thru the fine channels of the cooler. If so, is there a formula for this to guide me?

 

 

 

 

[seb7701]

Hate to break it to you O.K., but in my case, I was given the newer style 4cyl oil cooler and a new nose bowl and simply made it work!Lol. The inlet fits the oil cooler very well as they are made for each other. I got a glass duct from Jab which slips over the opening from the inside of the cowl to bridge things a little across to the cooler and simply glassed here and there to make it the best fit for the cooler's area. There is currently only about a 10mm gap from the cooler to the inlet which I just haven't sealed around with rubber yet, but will for sure. I ran compressed air from the outside just to see and yep, it found the largest gap and leaked around the cooler wherever it could. That said, 95% would be going through the cooler and the temps at present reflect that. I am fascinated with the concept of ducting the oil cooler outlet as Oscar has - make sense....

 

 

[seb7701]

Out of all my readings, this site in the U.S. fascinated me the most - looks like cluey people. They did the ducted oil cooler bit too. (Pics included in the PDF)

 

http://www.customflightcreations.com/sitebuildercontent/sitebuilderfiles/cooling102.pdf

 

 

[Oscar]

I'm impressed by the neat inlet ducts on your cowls, Oscar and Seb. It looks like the oil cooler inlets are smaller than the actual oil cooler face area. I presume this allows high speed air to slow down before making its way thru the fine channels of the cooler. If so, is there a formula for this to guide me?

Well, the lower 'nostrils' have been reduced to two 2.25" round inlets to feed the barrel plenum - not 'howl cowl' intake style but I suppose some will think it's a bit that way!

 

Yes, the expanding and contracting shape of the oil cooler ducts are for pressure recovery; I got the dimensions from Dafydd, but basically, I THINK it's an included angle from the cooler forwards and backwards of no more than about 12.5 degrees i.e. 6.25 degrees on every face. I had two objectives for that arrangement: firstly to get the cooler exhaust out of the low-pressure area of the cowl (obviously), but secondarily to mount the cooler not OFF the engine /mounts so the oil isn't being shaken and possibly frothed. So that duct is a self-supporting structure for the cooler (there are a pair of tubes bolted across the bottom of the cooler so its weight doesn't bend the cooler flanges:

 

 

The 'nose' at the intake was moulded to fit the intake lip and seals with some bicycle inner-tube - which being already curved, fits beautifully!. The front simply plugs over the inlet. At the rear, two nut-plates allow the cooler to be detached and dropped off the bottom cowl so it can hang off its delivery tubes when the bottom cowl is removed for maintenance - a three-minute job to detach/replace.

 

I have installed a CAMIT TOCA to keep the oil temp where it should be without excessive warm-up time.

 

The whole thing is a tight fit under the engine but with enough room ( I HOPE!) to allow engine rocking without touching:

 

 

 

[Oscar]

Out of all my readings, this site in the U.S. fascinated me the most - looks like cluey people. They did the ducted oil cooler bit too. (Pics included in the PDF)http://www.customflightcreations.com/sitebuildercontent/sitebuilderfiles/cooling102.pdf

Yep, that's a very good source of useful information. I made up a trial set of under-cylinder barrel baffles as per the Diagrams #3 and #4 in 'glass, to try on the engine test stand and see how the airflow was, but we didn't have a chance to run those tests. The closeness of the Jab pushrods makes getting the 'ideal' exit volume across the barrel length ( which is a function derived from the fin diameters, so I had to best guess the exit volumes around the pushrods) to maintain fairly even airflow right down the length of the barrel, to avoid barrel warping through differential heat, but here's what I came up with (on my dummy engine again).:

 

 

I'm not much of a metal-basher, and to do those in alloy would require a set of solid moulds and more skills than I know I have, but maybe one day I'll find someone with the skills ( might take them across to Barry Manktelow and watch him break into knee-slapping laughter one day!) bit it'd be easier with CAMit barrels which don't have those annoying cut-out towards the base of the barrel. I know that Ian Bent was finding the temps at the top (head-end) fins was way more than he liked, and I think he was researching better barrel-cooling, but I haven't followed up that with him.

 

 

[Old Koreelah]

Stop stressing about your workmanship, Oscar. It's miles neater than mine. Getting something that works is more important than good looks. Your cylinder wraps look OK to me and should be an improvement on the standard ram air ducts, which only cover the top half of the barrels. I'm amazed how so many Jabs seem to get by with them.

 

I plan to make my updraught cooling ducts out of sheet alloy. No pesky pushrod tubes to get in the way of nicely-sculpted exits. The major problem is sealing around all that plumbing under the heads, but I think I've worked out how to do that.

 

 

[Oscar]

Stop stressing about your workmanship, Oscar. It's miles neater than mine. Getting something that works is more important than good looks. Your cylinder wraps look OK to me and should be an improvement on the standard ram air ducts, which only cover the top half of the barrels. I'm amazed how so many Jabs seem to get by with them.I plan to make my updraught cooling ducts out of sheet alloy. No pesky pushrod tubes to get in the way of nicely-sculpted exits. The major problem is sealing around all that plumbing under the heads, but I think I've worked out how to do that.

Updraught is damn attractive. My only worry about it for a Jab engine, is the potential heat to the ancillaries - in my case, the starter and the CAMit alternator, and hot-air exit onto the battery and the only possible location for the ngl EMIS module. I had not considered updarught as an option when I built the new cowlings, but when you think about it, the lower 'nostrils' are very suitable intakes for updraught cooling are in the ideal position in the high-climb attitude. For that reason, I am not modifying my moulds with the large 'lower' nostrils, I've made inserts into the lower cowl to fit the separated barrel plenum feed(s).

 

Everything I have developed has to be considered a 'work in progress'; when we can do flight testing, I hope to be able to report the results.

 

 

[Old Koreelah]

Updraught is damn attractive. My only worry about it for a Jab engine, is the potential heat to the ancillaries - in my case, the starter and the CAMit alternator, and hot-air exit onto the battery and the only possible location for the ngl EMIS module...

All those ancillary items are currently cooled by air that's been down thru the hot bits, so what would be different feeding them air that's been up thru the hot bits?

What's ngl EMIS?

 

 

[Oscar]

Fat fingers: MGL EMIS...

 

 

[Old Koreelah]

Except for the probes plumbed into the engine, my MGL is all contained in the 80mm Guage on the panel. Yours must be different. My battery is also located behind the firewall away from the heat and fuel.

 

 

[Oscar]

The Extreme series has an RDAC which needs to be mounted inside the cowl so it measures ambient and auto-corrects the cht's.

 

 

[seb7701]

All I can say is 'Wow' Oscar - unbelievable attention to detail there! I would love to know exactly how the oil cooler exhaust affects the low pressure area. I have just picked up a digital manometer, so will be interesting collecting some data from the 'downstairs' section of my cowls.

 

My biggest mystery is still why I have 2 ice cold cylinders on one side and two hot ones on the other. Given what has been achieved with cyl 1&3, I can only think the lower cowl/ low pressure area in my setup is just fine at removing hot air, (unless there is something strange going on directly under one bank of cylinders, but I doubt it...) When both ducts are near identical, it's got me buggered.

 

I have EGT's all fairly similar, with maximums of 650-690ish seen on the last flight, while the max on my problem child, no.2, was 713 degrees on the last flight. I am going to try dropping the clip on the needle down one notch to richen things up a little, as I am currently looking at around only 13LPH, but it may prove to stuff the other cylinders up. We'll see....

 

 

[Yenn]

If you are considering updraught for cooling an aero engine, also be aware that an oil or fuel leak anywhere on the engine could well result in oil or fuel all over the screen. That is probably the best reason for downdraught cooling.

 

 

[Old Koreelah]

Depends on where the cowl exits are. Mine are on the sides above the wing LE, but I think a carefully designed updraught setup could exhaust underneath just like everyone else.

 

 

[Oscar]

All I can say is 'Wow' Oscar - unbelievable attention to detail there! I would love to know exactly how the oil cooler exhaust affects the low pressure area. I have just picked up a digital manometer, so will be interesting collecting some data from the 'downstairs' section of my cowls.My biggest mystery is still why I have 2 ice cold cylinders on one side and two hot ones on the other. Given what has been achieved with cyl 1&3, I can only think the lower cowl/ low pressure area in my setup is just fine at removing hot air, (unless there is something strange going on directly under one bank of cylinders, but I doubt it...) When both ducts are near identical, it's got me buggered.

 

I have EGT's all fairly similar, with maximums of 650-690ish seen on the last flight, while the max on my problem child, no.2, was 713 degrees on the last flight. I am going to try dropping the clip on the needle down one notch to richen things up a little, as I am currently looking at around only 13LPH, but it may prove to stuff the other cylinders up. We'll see....

seb: I've spent too many years working on: a) British or French cars; and b: yachts, to be anything but anal-retentive about easy maintenance. I KNOW that Murphy is waiting for when I have to replace the widget-gasket that involves undoing a bespoke-thread widget-retainer, it will fall down into: a: the drainage hole designed to allow water ingress into the chassis rail, or b: the bottom of the bilge which is two metres below the floor at that point and 50mm wide, to be complacent. It's either avoid that, or be forced to kill somebody. It's been emotional at times..

 

Your seriously different chts side-to-side has me stuffed, also. The chances of two cht probes being out of whack is not believable. I'd be fascinated to see what you find with the digital manometer: is it something like this one, that automatically registers p-delta if you have a pair of inlets at the high and low pressure sides?: AU Air Pressure Meter ±13.79kPa Digital Manometer Gauge & Differential Pressure

 

 

[facthunter]

The upwards flow is great in theory. The actual pressure change due to warm air rising effect would be immeasurably small, but would work better after shutdown. Most installations do it the other way because the exhaust system is below the engine and the engine moves in the mounts with the exhaust and the cowl doesn't (unless it's a radial) and you need to seal the place the air flows FROM, or it doesn't work. You have a problem with oil and flames at the side or top with flow from below if something goes wrong. Of course Jabiru cylinders run hot near the head. (Particularly near the exhaust valve, where the cylinder wall is often BLUE.). To me that indicates they need baffles. Nev

 

 

[seb7701]

Oscar, yep, that's the one. Just have to find a longer length of the correct hose diameter and will find out how she goes. I did see a youtube video of someone verifying the calibration of this model, so I thought I would give it a run. Will be interesting alright. I would love the CHT probes to be reading out of whack but did verify them with an infrared thermometer a while back and was within 1 degree. Worse still, the readings all seem really quite viable. Noted today that EGT's on initial climb out are all neck and neck, but CHT's are as described. From then on, the EGT's on 2&4 are correspondingly high.

 

Had a peep in the plug holes today and seems that rings on 1,2 & 3 are not bedded in just yet, as the pistons looked a little oily looking as opposed to no. 4 which looks just right. Despite having followed all instructions to a tee, I am guessing I might need to put some more load on it in climb to try and get thing sorted. Climbs have been 80kt in general with plenty of climbing/level/throttle variations, yet here we are, but anyhow. 8hrs on the clock so far, so not sure where ring seal etc should be at this stage, but hopefully not too late?

 

 

[Oscar]

Seb - remember, this is fairly old technology in the metals etc. in use!. I grew up when running in a car ( or bike) was at least 1000 MILES of light - medium - medium-heavy - never 'flogging it' duty, varying revs, an oil change etc. ... She should be settling down, I think, by about 25 hours very nicely.

 

 

[Old Koreelah]

...Had a peep in the plug holes today and seems that rings on 1,2 & 3 are not bedded in just yet, as the pistons looked a little oily looking as opposed to no. 4 which looks just right. Despite having followed all instructions to a tee, I am guessing I might need to put some more load on it in climb to try and get thing sorted. Climbs have been 80kt in general with plenty of climbing/level/throttle variations, yet here we are, but anyhow. 8hrs on the clock so far, so not sure where ring seal etc should be at this stage, but hopefully not too late?

Seb I note in Post 68 that you're using 13 litres per hour. That's probably okey, but at the lower end of tolerance for running in. (I managed to glaze my bores, presumably by never burning more than 10-13 litres per hour. Like the factory says, don't baby it.)