Jab Plenum Chambers

[Oscar]

Well you may be right, but the current info does throw some suspicion on it.That pdf alluded to earlier says they did lots of pressure readings and found that any positive pressure made a negative difference. It was in a Europa and I dont recall reading whether they had a jab (or any other) mixer box or if they did whether it had a pressure relief flap.(They did say they initially cured the problem by completely removing tubing and having a 4 sq. inch hole straight into the carby throat.

But Jabiru are now saying that using in intake attached to the NACA scoop is made better by removing it and attaching it to a small circular flush hole. No statement as to why, just that it does.

 

I guess we can speculate on what might change but the obvious is that the ram pressure will change (easily explainable) and its reasonable to suggest that if that's the cause of the problems then the pressure relief flap was not enough to drop the pressure. The presence of the flap relief might negate some of the pressure rise of the NACA system but maybe only does to certain extent and the change of the intake tubing may still be enough to drop the pressure to a more favourable level.

 

I guess all will be revealed on the weekend!!

The whole issue of ram air pressure in the airbox, MAY be very much affected by the input of the Balance Tube on Bing carbies. In the initial Gazelle test aircraft, when switching from cold to hot air intake, there was a massive effect on engine performance (for the rotax engines), they dropped power vastly when switched to hot air with no ram effect, to a dangerous level. Your chances of climbing out from a go-around with hot air 'ON' was nothing.

 

 

[seb7701]

Will be trying the taped of NACA intake at some stage for sure. I re-read the article Europa/Jabiru article from 'Custom Flight Creations' and saw that bit.

 

Am also going to have another look at things in terms of sealing gaps inside the ducts themselves. Interesting that they claimed to have re-fitted one of the ducts incorrectly and were 'off by one fin' which resulted in temps up by 30 degrees F. Anyhow, still annoyed that I have one side which struggles to get warm enough for take-off when the other may potentially have all means of crap inside the duct to try and get things manageable...

 

 

[seb7701]

Men - out of interest, what jetting is everyone using? I will be checking tomorrow (after I decide whether I have to pull the darn carb off or not??), but seems I have the odd combo of 255 main and 276 needle jet?

 

Stuck a finger or two into the exhaust pipes this arvo. While I know the pipes have a common muffler, the 'cool' side's pipe had only the slightest bit of black soot while the 'hot' side had nothing at all!! I think we need more juice...

 

 

[Jaba-who]

Well as promised I did the trials that Jabiru suggested - covered the NACA intake up with a piece of cardboard but cut a 2 1/4 inch hole in it.

 

Unfortunately the weather was not kind to me. Blowing very gusty 15 to 20 knots with lots of turbulence - and I was unable to keep it flying straight and level for more than a few seconds before I was either going up or down rapidly.

 

Took lots of shots of the Dynon so have been able to collate a lot of figures but I don't think they are very helpful because of a few points.

 

1. I see that at the same RPM but different flight profiles - Descending, straight and level or climbing there is a good 10 degrees between each with descents dropping EGT and climbing raising it.

 

2. Seems to be variable outcome. The narrow duct seems to pull the spread together some of the time.

 

3. I don't see any remarkable temperature change.

 

I'm going to spend a bit more time with the numbers - put all the Straight and levels onto a spreadsheet. probably do the same with the climbing and descending.

 

But the biggest and best change came by tilting the carby. Basically my 3 and 5 cylinders were running hot in the climb (CHTs up to about 175 -180 degress and EGTs at various times running in 700 - 740 degress C.) ) but settled down when I levelled out. Anyway - tilted carby top toward the cool side and have cured the temps. No rise in CHT to above the normal running temps ( about 150 CHT) small rise in the opposite side but they were already pretty low temps and have remained within acceptable levels.

 

So maybe I'm just going to leave it at that.

 

But I might do is wait for a smooth day and try the small hole NACA again.

 

 

[seb7701]

Well that sounds like some success! Will be interested in how the covered NACA duct goes, but the carb tilt is a curious one. I tried it to no effect, but was told that it is moreso successful with the 3300, which seems to be the case alright. In any case, that's a good drop in temp!

 

 

[seb7701]

Can anyone advise as to whether there has ever been changes to the 2200 plenum chamber (after the carb).

 

Turns out that my engine has previously done a valve on no.2 cylinder, so given that it is all new top end and still hot on no. 2, I'm wondering if there were any changes to the way that the mixture was distributed. Looks like jetting may have been a little too frugal, but about to try different jets...

 

 

[geoffreywh]

K75/100/1100 fuel pressure is 36psi

 

 

[Old Koreelah]

K75/100/1100 fuel pressure is 36psi

Sounds about right Geoff. Do more modern EFIs use much higher pressure?

 

 

[seb7701]

Just finished a flight with digital manometer fitted to each duct and a probe a couple of inches 'North' of the cowl outlet, to get differential pressure readings.

 

Well, after hours of staring at what looked like identical ducts, in turns out that they basically are!! At least I can cross diff. press. off the list...

 

Left side - 162mm H2O cruise (100kts) and 125mm climb (80kt) (2.7 x higher than outlet pressure)

 

Right side- 158mm H2O cruise (100kts) and 111mm climb (80kt) (2.0 x higher than outlet pressure)

 

Also upped the needle jet to 2.82 from 2.76. Hottest CHT (no.2) down by around 6-8 degrees with a EGT now of 670. Still looks like mixture issue! AArrgghhh!!!

 

Somebody please knock up a fuel injection setup for each cylinder!!!

 

 

[Oscar]

Seb - those are very good p-delta figures; your cooling set-up is fine. Don't obsess on the damn egt figures: the egt reporting is indicative at best, and is most useful for alerting to 'out of normal' excursions in flight for anything that doesn't have mixture control. As a general guide to mixture, the plug colour is a better guide than the egt's.

 

 

[seb7701]

Thanks Oscar - yep, assuming everything was accurate, ducts seemed nicely symmetrical! Good point re the plugs for sure.

 

Now just considering at present what to mess with in terms of intake to correct mixture issues or to put it bluntly, stuff more air in the L/H intake and do some creative ducting to get the CHT's down some more, as the initial opening of intake and deflectors etc have been the only successful measure to date. That said, the fact the no.2 is hotter than no.4 does scream out mixture issues...

 

(At 21 degrees C ambient, no.2 was cruising at 150-151 C, with no.4 at 144 C.)

 

 

[Old Koreelah]

As I mentioned before Seb, I wonder is some of that cooling air escaping before doing its job?

 

I thought I had a good seal, but my temps were still too high. After closing the doors to get darkness, I poked around with a strong torch and was surprised to find gaps in both the ram air ducts and the cowls themselves. Very carefully sealing every damned join dropped my CHTs over 20 degrees.

 

 

[Oscar]

Seb: Personally, I'd say that ALL of your cht's are quite decent: you are within about 4% spread (unless my maths is as bad as it often is..)

 

Remember: the 1-3 side cylinders are forward of the 2-4 cylinders, so the 'ramp' effect for the intakes is quite a bit different. Cooling air just does NOT behave as one wants; it's a capricious bastard. I hope that one day, CAMit will reveal what its research has shown to be the problems with Jab engine cooling.

 

To get balanced, controlled air delivery to each individual head, this is what your need ( my engine in that image):

 

 

The small unused central outlet is for the oil cooler air supply; the two blanked off outer outlets are there for testing six-cylinder engines. As you can see, each cylinder air supply has a butterfly to allow balancing of the airflow to that head. The whole assembly is fed by a 6KW or so electric motor driving a squirrel-fan assembly about 25% the size of a Volkswagen Beetle (original version)!

 

 

[seb7701]

Oscar - now that is cooling! Was it fed with the standard carby setup?

 

The ramp effect you mention is interesting - we only noticed this morning that the front of the left hand side intake is actually set back around 30mm further than the right side from the closest part of the propellor to pass by it, namely the trailing edge. Makes you wonder what air is being forced from the prop straight over the top of the cowl.

 

O.K. - Not quite even I'm afraid - what I didn't mention is that no. 1 was running at 103 degrees C and no.3 was 110C!! Imagine if I could get 2/4 doing that??

 

 

[Oscar]

seb: standard set-up AFTER the carby - no airbox. But we were testing the capability of the test rig to deliver the conditions required - not the engine performance - so we didn't bother with numbers that would be meaningless anyway if not in the actual installation.

 

 

[jetjr]

You can get them very even, just take lots of trial and error with inside top of duct deflectors

 

Jab USA have a good how to guide and with patience it does work but very small changes have big effects.

 

I used some adhesive door seals (hollow type foam setup) to seal where ducts meet top cowl and oil cooler. sure made a difference

 

I found that air loss around oil cooler was a significant problem, extended front flap and blocked off sides so air at least did something, next is to start blocking it off a little.

 

 

[seb7701]

Would be very interesting stuff Oscar - near Toowoomba isn't it? Given the issues seem to occur 'upstream' of the carby, I was curious today as to whether anyone has stuck a pod filter on the car for kicks. I am awfully curious about that - works fine for Rotax!

 

That Jet - sealing is the least of my worries unfortunately, as oil cooler is sealed to perfection and ducts no dramas. You are right with regard to the deflectors though - few little bits to be done there!!

 

 

[seb7701]

As I mentioned before Seb, I wonder is some of that cooling air escaping before doing its job?I thought I had a good seal, but my temps were still too high. After closing the doors to get darkness, I poked around with a strong torch and was surprised to find gaps in both the ram air ducts and the cowls themselves. Very carefully sealing every damned join dropped my CHTs over 20 degrees.

That's a darn good idea - the dark room/torch bit! The jetting is providing AT LEAST enough fuel at this stage, so back to the airflow and what you've said!

 

 

[jetjr]

Or you can run a little rich and use leaning valve, heard some good sucess stories inc easier starting for some reason

 

 

[seb7701]

Well, today's discovery was nothing short of a revelation!! Following the discussions about blocking off NACA ducts etc, I did just that, but instead then just pulled the hose off the air box which fed from the NACA duct. The air into the air box was then just drawn from inside the cowl but was still filtered. Well, all I can say is holy crap!

 

My little 4 bar graph of EGT's went from having 2&4 clearly elevated to having 1,2 & 3 level pegged with no.4 only being different because it was 25 degrees lower than normal!!!!

 

So, even out the EGT's? Yes, it does. Dropped about 7-9 degrees off no. 2 and a clear 25 degrees off number 4, which was totally unexpected.

 

Forgive the fuzzy photo, but it was from the other day, while the second clear shot is from this morning. Kinda illustrates the point...

 

Next step, gonna try silicone hose instead of the cobra head....just in case!

 

Incidentally, had a little look at the NACA duct. The funny thing is that looking along the side of the fuse (in the vicinity of the firewall) the fuselage 'swells' outwards, right where the NACA duct is. That means that the air runs along the side of the fuse and actually gets forced straight into the NACA duct as it is positioned right on the outward curve, which I think defeats the whole purpose of a NACA duct!!

 

We're making headway...

 

 

 

 

[Jaba-who]

That's good news.

 

I had a fairly lack lustre experience with my attempt.

 

However, I did as Jabiru suggested and covered the NACA inlet with a big piece of cardboard and cut a 2 1/4 inch hole in it.

 

I then ran it at varied RPMs and plotted it out in a graph.

 

I also did the same with the normal NACA duct.

 

Essentially it was fairly unchanged (with my 3300 engine).

 

One cylinder (forget which at present). One cylinder EGTs was about 5 or so degrees hotter at lower RPMs (2700 - 2800) and the overall EGT spread was about (5 degrees C) closer at the normal operating end (2800 - 2900). But the temps are all about reasonable anyway. The CHTs were essentially unchanged.

 

All in all, a minimal better change but not enough to justify the considerable workload in cutting off the old NACA inlet, turning it into a flush circular inlet. I guess I may try the trick of leaving the loose end in the engine bay. But that sounds basically just like leaving the carb heat on ( since you have to be sucking in warmed air from around the air that passes the engine. ) . Which I already know drops the EGTs ...But that drops the power as well.

 

I also made some side plates and blocked off the spaces between the rocker covers.

 

Again. As good as no change in the CHTs when I did.

 

 

[seb7701]

Consider just leaving the hose off - I have a temp probe in the engine bay as we had a spare left over. Originally had it in top of the airbox, but was floating around the engine bay lately adjacent to the firewall - only read 17 degrees, so I think you'll find that there is plenty of airflow to supply cool-ish air and nothing as warm as carb heat. Would love to hear what happens if you tried the same thing!

 

 

[Old Koreelah]

Consider just leaving the hose off - I have a temp probe in the engine bay as we had a spare left over. Originally had it in top of the airbox, but was floating around the engine bay lately adjacent to the firewall - only read 17 degrees, so I think you'll find that there is plenty of airflow to supply cool-ish air and nothing as warm as carb heat. Would love to hear what happens if you tried the same thing!

Very interesting results, fellas. I wonder, Seb, if your more even EGTs are a result of the engine drawing air from a less turbulent source than directly from the prop wash?

 

 

[planesmaker]

Or is the NACA duct actually creating a vacuum? And engine is starving for air? I don't know just a thought.

 

 

[seb7701]

O.K - turbulence yes, but from prop wash, well, not really convinced. Well, I'll get a photo for you, but as I say, that duct is essentially right on a curve in the fuse, meaning the NACA intention of being a non-force-fed intake is somewhat lost. I had a very interesting conversation with a very trusted source the other day who stressed that all issues come from 'upstream' of the carb, starting from the fact that air is shoved into the airbag from outside and would start by being thrown against the airbox's far wall and the turbulence and pressure starting from there and only getting worse as it trips over the less that preferable surface of the scat hose and into the carb. Planesmaker - the force fed air (I think) increases pressure, which in turn slows the velocity, which can't be good for feeding a carb, so you are kinda right.

 

I do have one underlying query of my own and that is why this engine is so sensitive to an issue which doesn't present itself elsewhere. That being said, most people probably don't have individual monitors on their 912, but I hear that they are actually quite uneven. In any case, their issues are solved by a carb for each pair of cylinders.

 

We have a Tecnam here which is simply fed by pod filtered carbs inside the cowling, so there really isn't anything diabolical about what I have tried.

 

I am interested though, in the difference between current test and removing internal flat filter and simply putting a pod on the old NACA intake. Just comes down to which filter offers the most air, which will then require some re-jetting. I did chew 19LPH on my test, but I am still climbing the bejeezus out of it at regular intervals (temp permitting) given the fact that I still only have 13 hours on the clock. That could easily add 3 litres to the predicted hourly rate I would think!

 

During my discussions the other day, this example came up - the intake for a Robbo helicopter.

 

Simple and works!