Jab engines

[Mhalc]

So far the test stand testing is progressing well.  The current configuration is an old Jabiru 3300 (SN 966) and Bing 94/40 carburetor.  The supercharger is ROTREX C15-60 with a custom drive/mount assembly with a electrically controllable BOV to regulate the boost level.  

 

Current boosted output is in the 140HP range, which will be available thru ~10,000DA and 120HP thru ~16,000’DA.

 

The next series of tests will be with the Rotec 40mm TBI, followed by ground and flight testing in a Jabiru powered (Gen4 3300) Lightning. 

 

https://newtube.app/user/N530MH/tbt4GD6

 

[facthunter]

Have you got a backfire relief valve in the system? Nev.

[Mhalc]

8 hours ago, facthunter said:

Have you got a backfire relief valve in the system? Nev.

There is not a separate backfire relief valve.  

 

The design is “draw-thru” so the Blow-Off-Valve (BOV) vents the boost overboard unlike a “blow-thru” which would have a fuel/air mixture and needs to be recirculated before the blower to prevent compressor stalls during rapid deceleration.  In addition the BOV remains open until takeoff/climb/cruise power settings are selected.

Edited February 4, 2021 by Mhalc

[Yenn]

A friend of mine who is an ex LAME came to our strip today to work on an RAAus plane,so I assked him to bring his borescope and let me look at my Jab engine. It was a very basic machine with a head which only just went through the 12mm spark plug hole.

One piston looked as if it had molten aluminium on it and both of us couldn't decide if it was a detonation problem or not.

Twenty minutes later the head was off and it appeared that what we saw was just reflection from the piston crown. no sign of detonation, the bore looked perfect, valves also but I haven't had a good look at them yet. I have decided to do a top overhaul to all the cylinders, will probably be able to post some photos.

[onetrack]

I'm a little disappointed with the borescope camera (at least the one I borrowed). They're quite limited with their abilities in low-light conditions, and it's difficult to get them to reposition, unless you're not very far into the orifice you're inserting it through.

 

I guess if you went for a mega-dollar unit, you would probably have a lot better low-light ability - but the positioning and angle alteration would still appear to be a problem with all of them.

[Thruster88]

A borescope looking into a cylinder can not see a broken or stuck ring or a leaking valve unless it is just about to drop. Only the leak down or differential compression tester will highlight faults reliably and consistently.  

[facthunter]

It will show bore scratches and rust and that's about all.  Nev

[Blueadventures]

3 hours ago, facthunter said:

It will show bore scratches and rust and that's about all.  Nev

Sometimes very good definition some times bad.  A 360 camera end would be great if / when available.  The one direction view is a pain to use at times.   Look forward to using a 360 type one day. Cheers

[RFguy]

the biggest issue to providing diffused , uniform light.

 

like the difference between a pinspot light and a flouro tube. 

 

really need to illuminate in one plug hole, and camera in the other. illumination on the end of the camera stick is limiting.... Need to illluminate with a fairly omnidirectional light source (a small filament light bulb, not an LED) . dash lamp etc. solder a couple of wires to a little 12V pigtail light, some of them have leads on them. THAT will provide a completely different picture to the LED head camera. (which are sh1t IMO). (but I am a pro video guy, so I would say that) 

 

12V Mini Lamps - Small | Jaycar Electronics

 

https://www.jaycar.com.au/12v-mini-lamps-small/p/SL2685

 

 

 

[facthunter]

If rust is detected the only way to assess the engine's serviceability is to  remove the cylinder and examine the rings. Rust can completely jamb the rings in the ring lands resulting in very little sealing and then probably more damage or even failure resulting.  Nev

[Yenn]

I have seen the reslts of detonation and am sure that a good borescope would pick it up. I was looking just because a borescope was available to me and I have never used one before. My friend has a vast knowledge of aero engines and what he saw was inconclusive, it looked like a little blob on top pf the piston. It turned out to be a bit of carbon reflecting.

Just to make sure there was not a problem I removed the head and now I will remove the other three so as to be as the Irish say "To be sure to be sure" It is not a very big job with a Jab.

[RFguy]

Yenn, how many hours and what fuel ?

[Yenn]

330 hours on avgas mainly.

[RFguy]

mine was 370h / all AVGAS LL100.

Not sure which carby you have, but probable time for a bit of a top overhaul from what I read, and hear on AVGAS and a Bing.   mine could be richer than it should be...  variations in internal cowling pressure can confuse the Bing to think it is at some other altitude than it really is.....

 

[Old Koreelah]

24 minutes ago, RFguy said:

...  variations in internal cowling pressure can confuse the Bing to think it is at some other altitude than it really is.....

 

Surely you have installed the tube which connects the carb pressure sensing port to air filter box? 

[RFguy]

That's where it is, but on my list is to inspect air / carb  inlet scat hoses, pressure eq tubes etc.  Applying carb heat does nothing... and the airbox vane IS changing to heated air when pulling carb heat, so more investigation required . 

[Bruce Tuncks]

My engine is an avgas one, nearly 700 hours now. It does nothing on applying carby heat or on turning off a mag. I decided that it must be doing things which I'm not sensitive enough to notice.

[RFguy]

Pretty much like mine. no/negligible  rpm drop with 1 mag  and no change with carb heat.

I'll need to verify that the carb heat air is actually going past the heated manifold, and getting heated, and not sucking through some other gap...... or maybe the scat hose is cracked between the carb and the airfilter box and it's going around the filter !

 

 

 

[Old Koreelah]

12 hours ago, RFguy said:

Pretty much like mine. no/negligible  rpm drop with 1 mag  and no change with carb heat...

Mine too. I have a probe just upstream of the carby which shows a slow temp increase of about 25C, but have never detected any change in engine revs. It’s not easy to get enough hot air for a really effective carb heat setup; it’s supposed to raise intake temps by about 50C in a few seconds.
 

[facthunter]

With two plugs close together, the rev drop is not much. On opposite sides of the head makes a lot more difference. Continued running like this will heat the motor and have a power loss, as it's the same as retarding the timing. Nev

[RFguy]

same as retarding the timing because... I guess the complete burn will take longer and hence is equivalent to later ignition timing ? 

OldK- are you using the stock general airbox Bing  - Jab manifold heater setup ? 25C rise eh ?

 

Pulling Carb heat in our Brumby (912ULS) is quite dramatic. 200 to 300 revs at 4k rpm  at 70 kts IAS .

[pmccarthy]

Topical 😁

[Old Koreelah]

On 27/02/2021 at 12:43 PM, RFguy said:

...OldK- are you using the stock general airbox Bing  - Jab manifold heater setup ? 25C rise eh ?

Glen I have the standard under-sump intake manifold, but my own air cleaner setup: a large air box with Corolla filter diagonally across the middle, then a straight run thu firewall into carb.  Nice even EGTs.

 

Carb heat air comes from a sheet metal sleeve fitting close around the muffler. I’d get much hotter air further upstream on an exhaust pipe, but the necessary cuff would cause the pipe to run extra hot when carb heating isn’t required. Some of that extra heat would conduct back to the head...

[RFguy]

OK, understood. I'll fly up there  and say hello when I get the 230 back together.. cheers

[facthunter]

I know some jabs have effective carb heat. Why the variation?   You WILL put out less power so the head temp won't do a runner.  Cooling required is proportional to power output. That's why turboing  a motor requires much more attention to cooling. Oil especially.  On a few occasions I've flown them when it's far too hot but if the climb speed is increased you keep within the parameters if it was OK in the first place. Nev