On Jabiru cooling - YAFT

[RFguy]

yet another forum topic.

 

I have been looking at how UL-Power build their engines.   

https://www.kitplanes.com/the-new-old-aero-engine/

 

They say max continuous CHT 160C. ---Which is what I have been suggesting for Jabirus.  Not the 180C continuous (measured on top of the head)  insanity (in my opinion) I have good reasons that I have presented  100 times to back that up. 

However, the cooling plenum setup from the factory, IMO,  is never going to cut it .

At least for the 6 cylinder........It needs a rethink.

And crafting custom plenums and glass and metalwork is very time consuming, so few will do it. If Jabiru engines were restricted to 160C continuous, 180C 5 minutes and max 90C oil temp,  then I think they'd all be mostly fine and not throw valves, seats, pistons, rings, stems, guides . if enough were interested, we could get a custom pressed metal all surround plenum made.  but plane cowlings  and engine shapes and versions vary, so really not easy. ahh well. 

 

Edited June 22, 2021 by RFguy

[RFguy]

ULPower engines USD , approx

UL260i, 97 h.p., $20k 72kg

UL260iS, 107 h.p., $21k 72kg

UL350i, 118 h.p., $25k  78kg

UL350iS, 130 h.p., $26k 78kg

[Old Koreelah]

6 hours ago, RFguy said:

…the cooling plenum setup from the factory, IMO,  is never going to cut it .

Glen this topic could take up a whole thread. A lot of people have made their own cooling setups for Jabs.

I know of one bloke who painstakingly refined the interior of his ducts until all CHTs were even and safe.

 

The factory is reported to have tried traditional plenums and decided their fibreglass ram air ducts were better.

I like the ability of fibreglass to mound closely to engine contours to minimize leaks. The forum had a pic of a neat one which encase the whole top of a Jab engine (just wasted half an hour trying to find it).

 

Quote

At least for the 6 cylinder........It needs a rethink.

And crafting custom plenums and glass and metalwork is very time consuming, so few will do it…

 if enough were interested, we could get a custom pressed metal all surround plenum made.

My fourth version is sheet aluminium and encases the spark plugs and brings cool air to the whole length of the steel cylinders. On climb out the hottest CHT sometimes nudges my 150C alarm, then stabilizes between 130-140C on cruise. Ground testing with my multimeter probe makes me reasonably confidant of those numbers.

[RFguy]

yep and I bet that as long as the flywheel is well secured, that engine with 150C max (read on top) will go forever on mogas.... (or avgas if an anti-buildup is used ).

 

[Old Koreelah]

44 minutes ago, RFguy said:

yep and I bet that as long as the flywheel is well secured, that engine with 150C max (read on top) will go forever on mogas.... (or avgas if an anti-buildup is used ).

 

Hope you’re right. 

Over the years I’ve followed the various factory announcements about modifications, including throughbolts, deeper nuts, etc. and read all the counter views…then done nothing.

Regular oil and filter changes, managing head temperatures, avoiding shock cooling and feeding it clean fuel have served me well. The head metal inside the tapper compartments is still a healthy colour and there is no evidence of fretting or oil leaks. 

[RFguy]

I did some more fluid modelling on the factory plenums last night, following discussion in another forum.

 

The function of those baffles on the tops of the cooling plenums appears to be  to direct airflow into the middle of the fins.

They don't get any airflow otherwise. The cowl vaccum wont pull air down the sides of the fins, through the fins  because there are too many leaks elsewhere.  The air will just take the easy way in and out....  So the whole cooling scheme is handicapped. The shapes of the head finning makes it time consuming to customize a plenum.

I am going to truncate the middle (over CYL3,4) baffles that direct anyflow to those to permit more airblast to go to the rear and see how it behaves.

given that it is difficult to get a seal for everything, the only other option open to this is to shroud the top side of the head front to rear, so the air blasts through the lost...but then the rear gets all the hot air.

 

back to what we discussed a couple of months ago which was individual ducts to each head, or at least the rears.

[Bruce Tuncks]

I like the idea of 150 to 160 being the max temps for a Jabiru engine. This is what I achieve in practice as on a hot day the temps can exceed the 150 "max" on climbout. But anything over 160 and it is time to panic.

The first Jab engines were redlined at 200 C and this was way too hot, as evidenced by the distortions in the cylinder heads. Then they lowered the max down to 175 degrees which is still too hot in my opinion.

BUT cooler running on avgas turns up another problem.

Edited June 22, 2021 by Bruce Tuncks

[Old Koreelah]

I’m just now at airport and took a pic of my hottest head:

 

 

[planesmaker]

I maintain a Jab engine, when originally asked I stipulated it must have CHT on every head. I set the alarm at 155. I check the max temps every 25hrs and reset. Engine is now coming up to 1000 hrs. Yes I’ve had to remove heads etc for various reasons, stuck rings(avgas), failed lifters etc. but the Jab engine must be kept cool. 180+ that Jabiru specify as max is way too hot. 

[RFguy]

Tom, what sort of lifter fault did you see ?

[Bruce Tuncks]

I have wondered about "gull-wing " baffles on the top of the cylinder walls and heads. Following the Limbach example, I put these on the underside of the cylinders, with the idea of keeping air in contact with the fins for longer. Now I am thinking of swapping them to the top or adding them to the top as well and having both top and lower baffles.

Any comments?

[Old Koreelah]

Bruce think about it from the air’s point of view; it’s rushing in at high speed and expected to suddenly change direction and slip down thru narrow orifices to cool all the hot bits. Gull wings below the barrels seem to be sensible, but probably best to make them enormous, to hold cooling airflow close to the hot metal for as long as possible, but always allowing that rocket-nossle-shaped outlet underneath.

 

 

[Kenlsa]

Bruce, I got a set from Ian at Camit and they were to go under not over. Worked a treat helping to average out the cht’s 

Ken

[Bruce Tuncks]

Yesterday, I bought a J230D with a 3300 Camit engine. It has a dynon CHT/EGT setup with a rotary switch to select the cylinder. In flight, the indication did not go much beyond 100 C on any cylinder. I find this hard to believe, and completely different to what RF guy saw.

Yes, it was a cold day. My plan is to take out a sender and put it into boiling water.

Any comments?

[RFguy]

What was the OAT Bruce? I could look it up if you are not sure, what altitude what location ?

I see 110-125C CHT at 50% fuel flow cruise and OAT=5.

[APenNameAndThatA]

Isn’t 90C max for too cool for oil because the water does not get boiled out? 

[RFguy]

we're talking CHT, not oil

[Old Koreelah]

3 hours ago, Bruce Tuncks said:

…My plan is to take out a sender and put it into boiling water.

Any comments?

Congratulations on the plane, Bruce!

I can’t believe an air-cooled head runs that cool.

Hot water won’t test the actual operating temps (well above 100C).

I removed all 4 of my sensors and used a tiny bolt to clamp them tightly together. Then I zapped them with a hot air gun whiie watching the MGL gauge. They went up and down the temp scale in unison, proving they all read the same.

I also used two different probes to calibrate their readings, both while removed and installed. 
 

Ever since I insulated the first 150mm of the CHT cables with shrinkwrap they’ve read quite a bit higher. 

Lots of testing with independent probes tells me they’re pretty close to the actual hottest parts of the head, near the exhaust port. I’m happy to err in the side of caution, keeping climbout temps under 150C and cruising about 140C.

 

I’m also trying to get my simple water injector ready for summer flying.

[Bruce Tuncks]

Thanks guys.  The OAT would have been about 5 degrees, it was 16/9/2021 and between Kadina and Edenhope say 4pm.

I'll look some more and do what old K suggests.

Thats also a good point about the oil temp. It was in the green, but not very hot if I am remembering correctly.

The CHT's were in the green too, near the bottom of the zone and just too much cooler than my old Jabiru SK to be believable.

[APenNameAndThatA]

11 hours ago, RFguy said:

we're talking CHT, not oil

Do kindly refer back to your original post.