Jab engines

[RFguy]

Given that it would appear a bunch of the structural problems are out of Gen4, wrt to gen2-3 it would appear to me, that what is left is the imbalance of the mixture and air-fuel charge.

 

This would cause uneven EGT, valve embarrassment, and imbalance wear due to varying combustion charge etc

It would seem to me that solve this , and you solve many of the engine's problem children.

 

Given that I might end up with a Jab engine, at lest in my first aircraft, I am prepared to put a fair bit of my brain on this.

 

Some of what I read above seems to indicate a new manifold and or relocated carb is required.

The carby is in a convenient spot for 'where to put it ' question but of course rather suboptimal otherwise. and even worse in the 6 cyl.

and it all changes with airflow and pressure variation.

I am envisaging some sort of online variable gas vane/airflow control per cylinder by voicecoil servo motor.

 

WHAT IS the instantaneous , peak (neg) pressure at the point about 5cm from the lead on the intake ?

 

BTW Nev we can get the torsional vibration information from the Tacho sender without any mods.

[facthunter]

Think you'd need a marked index on the rear of the prop drive flange and strobe from the other end of the crank. It would be better if the flywheel didn't exist. Nev.

[RFguy]

Hi Nev. actually the pulses from the tacho sensor are excellent for the T vibration. What you do need is a good engine position reference. Ideally another flywheel mark. as the spark on a plug might be out...

 

Nev you were well across the discussions on intake issues,have you formed any opinion the above points I raised ? IE that the mixture imbalance is an evil and responsible for much grief of this motor, or do you think those issues are secondary, or simply additive ?

[Downunder]

Hard enough to tune with one carb on a 4 cyl......and even more difficult on a 6 cyl.

Spend the rest of your life playing with runner lengths and carbs or be done with it and fit electronic fuel injection (with individual cylinder mixture adjustment).......if you want perfection.

Accept the mixture variation of a carb fed engine or put your head in the sand with egt monitoring of one cyl.

 

As far as the 912 manifold goes. The front two cylinders run rich under 5000 but all evens up from then on with even egt's.

[facthunter]

RFguy two carbs would work better with the pulses taken into account but the use of complex and lengthy manifolds makes a brief backfire a big risk. This.happens a lot with LPG engines and even splits the blocks in some Vee engines unless they inject the gas to individual cylinders. There's too much combustible fuel vapour in all that manifolding. ready to ignite and put lots of mixture in another cylinder I've always suggested a mechanical injection system to each port and not requiring any electrics. Nev

[RFguy]

Hi Nev

You mean like Bosch K jetronic ?

 

I was wondering what could be done with variable vanes and other flow insertable items.

 

But you you are going to do that, yeah just stick an injector where it needs to be for each.

 

Now, with respect to backfire concern, is this about the amount of fuel air charge (energy ) available in the whole manifold plentum being like a bomb ?

[facthunter]

Yes you have it .IT can force mixture into other cylinders and severely overpressure them .. With the injector right at the port the effect reduces to very unlikely. You don't need an airflow sensor. RPM and throttle position gets you close and manual fine tunes it. Nozzles are fine tuned for even flow (balance) at whatever is most effective. Nev

[Bruce Tuncks]

After more than 15 years I finally installed EGT and CHT on each cylinder and finally got the temps fairly equal. I tried lots of things, and the big break came when I was told on this forum that nothing much upstream of the carby would be very effective. The story and some pics are on here.

 

[Bruce Tuncks]

Injectors have been used on Jabirus. There is one of these at Tailem Bend and the owner is happy with it.  But Rod Stiff has pointed out that this makes the engine dependent on an electric system and so opens up another possibility of failure.

[RFguy]

A hybrid injector/existing carb system would seem the middle ground Bruce.  The carb gives you the throttle body.  That or some variable rotary vanes in the manifold tweaking the airflow . Not sure that my usual use of stepper motors would last long, most likely some sort of hydraulic activated vanes. Or spring loaded (in rotation) and bike gear cables pulling each vane, so isolating the stepper motors from the engine vibration. 

How bad  does the mixture go at 8500 feet with the carb ?  Can a tiny bit of choke pulled on rich it up a bit if necessary? I don't know the Bing yet. That's what I used to do in the Datsuns, but that depending on the choke jetting and control was rather coarse and clumsy.

 

 

[Jim McDowall]

11 hours ago, Bruce Tuncks said:

But Rod Stiff has pointed out that this makes the engine dependent on an electric system and so opens up another possibility of failure.

And millions of vehicles have used EFI for billions of kilometers travelled with a very low failure rate probably far better than that experienced with Bing carbys on Jabiru engines albeit when electronics stop they stop whereas carbys may just lower performance.

[RFguy]

and I add - "hybrid" I mean injectors at the  cylinder head end of the manifold, not one big injector. I think the most difficult aspect of DYI in this regard is managing the injector fuel pressure and plumbing. high pressure fuel leaks are hazardous.. Inline ICE has a rigid,  bolt down doubled O-ringed distribution bar. Have to look at how Subaru do their boxers. Of course there are injection kits.

Edited September 23, 2020 by RFguy

[Jim McDowall]

I wonder what the long run experience of the SDS EFI on Jabiru engines is? see http://www.sdsefi.com/jabiru.htm

[440032]

People who do not own, operate and have never built a Jabiru engine themselves are wanting to tell Jabiru how to make their product better? Now that is funny. This has been going on for over 20 years. Time to give it a rest.

[RFguy]

440032. ownership is not a pre-requisite for suggestions, objections or whatever. 

 

Organisations can get tunnel vision due to in many cases a lack of outside influence. I'm not saying this is the case with Jabiru, either, it is general. 

[facthunter]

I started flying with a love of engines but it gradually changed to  the people end of the equation, training and flight management (energy and Newtonian physics and aerodynamics.)

 Play with your engines but read the signs ."If it's shaking and popping, you'll soon be dropping". ALL engines CAN fail. A prop blade may fly off and the engine tear out of the mounts. The biggest cause of engine  failure  is NO FUEL Left.. That affects the best and the worst of engines. In all situations where the engine is critical have a pre thought plan of action. Before there I stop. After that it's different because you are airborne but you still have a PLAN.. Or you are behind it unnecessarily. At 300' I doubt I'd be on the radio.  You have higher priorities  and about 20seconds of flight time. Nev

[RFguy]

well you are right about the shaking and popping Nev. 

The Brumby Rotax  last week- the oil on the Friday I noticed had been getting black much faster than usual... and it was at 49 hours out of the 50 for the oil change ( the maintenance guys they do oil at 50 h on that aircraft) . anyway, put it to bed, and the next morning during run up tests I noticed severe vibration in the mid RPM range. One of the rotax floats in the Bing pair had sunk. sunk ALOT overnight. so taxied it back to the hanger.   And that it probably started sinking the day or two before and the oil going black was a tell tale of the mixture going rich on one side . I need to pay more attention to the tailpipe colour. and that it wouldn't make spec full static RPM.. 
-Gimme a Jab engine any day.. Or a Continental O320 etc. 

[facthunter]

When you refuel do a flight hours vs  fuel used comparo and you will pickup these changes. Black oil means RICH in a petrol motor. ACTUAL fuel rate /hr must be used for flight planning if in excess of the published figures. Best check WHY? I hate float carburetters. IF you get one flooding in flight you just might meter it on a the fuel tap/selector with a steady throttle setting if you are really switched on. Easier with a flowmeter. I'd just land ASAP when safe. Nev

[kgwilson]

The two main advantages of air cooled engines, no coolant plumbing & radiators to fail and weight.

[RFguy]

7 minutes ago, kgwilson said:

The two main advantages of air cooled engines, no coolant plumbing & radiators to fail and weight.

Well a while back , a month I used to want a Rotax. But now I've had some time with them, and after that issue, I proceeded to read about 500 pages of faults and things that happen with the 912 (you would expect me to do that, wouldnt you) ? I am no longer a big fan of it,  of all the things that can go wrong. Give me a LOW chance of a valve seat coming adrift (or a valve, or a piston LOL)  with a Jab motor   than a MEDIUM chance of some annoying maintenance  problem regularly. 

Edited September 24, 2020 by RFguy

[Old Koreelah]

6 hours ago, RFguy said:

...How bad  does the mixture go at 8500 feet with the carb ?  Can a tiny bit of choke pulled on rich it up a bit if necessary?..

That would make it worse, RF. The Bing runs richer above its optimum altitude; I’ve seen some frightening fuel flow numbers up very high.

[RFguy]

Oh it runs richer? OK.  mmmmmm.  even at  wide open settings ? yeah I guess if the error is under metering the airflow, rich is what you will get. 

Edited September 24, 2020 by RFguy

[Old Koreelah]

3 minutes ago, RFguy said:

Oh it runs richer? OK.  mmmmmm.  even at  wide open settings ?

Too plurry right!

[Markdun]

38 minutes ago, facthunter said:

I started flying with a love of engines but it gradually changed to  the people end of the equation, training and flight management (energy and Newtonian physics and aerodynamics.)

 Play with your engines but read the signs ."If it's shaking and popping, you'll soon be dropping". ALL engines CAN fail. A prop blade may fly off and the engine tear out of the mounts. The biggest cause of engine  failure  is NO FUEL Left.. That affects the best and the worst of engines. In all situations where the engine is critical have a pre thought plan of action. Before there I stop. After that it's different because you are airborne but you still have a PLAN.. Or you are behind it unnecessarily. At 300' I doubt I'd be on the radio.  You have higher priorities  and about 20seconds of flight time. Nev

I’ve had few silent moments, all with VW derivatives and nearly always due to the idiot mechanic, me.  First one was an EFATO from a broken head bolt at 100’, giving one good cylinder in a half vw motor.  Zigged and zagged between the eucs and herefords.  Cause was secondhand head bolts with a nick in them from the original vw cooling shrouds.  Second was a broken rocker bolt in the half vw at 500’ over a dry lake.  Third was a broken crankshaft at 4000’. I had previously noticed oil leaking from the prop hub seal ...but did I do anything? Fourth was several broken prop bolts causing a bit of a shaking in an 1835cc vw from wrongly using ht bolts.  Landed in a rapeseed field after shutting engine down before it ripped itself off the front completely. Learnt to use malleable bolts and Bellville washers on the prop, but obviously not enough as my fifth engine out was, ‘is that a bit of an increase in vibration?, yes, better go land. Oh, the vibration has gone, and what was that shiny spinning thing that went whizzing off...must be the prop.  Gee, the engine is running real smooth now.  Now where is that airstrip?’   I’ve never had the ‘too much air in fuel tank’ problem, and I’ve generally considered an engine out as a ‘where have all the thermals gone’ issue that requires an out landing and fly accordingly....ie. only over terrain I feel comfortable landing on.  Nev is correct....have a plan for when the engine fails.  I would add....practice, practice, practice dead stick landings and speed/energy/glide path control. I do miss the spoiler/air brakes of gliders in my Jab powered Corby!
I’ve been much more frightened on a commercial charter in a twin engine Cessna flying to Yuendumu with 8 govt VIPs on board.  The plane was critically short of fuel because the pilot the previous day flew several hundred miles off course to the wrong airstrip and he thought that aggressive leaning would get us there.  The last 100nm with lots of tapping of fuel gauges and then flashing red lights on the panel the pilot was flying above 4wd tracks between the sand dunes.  We definitely took to the whisky at chairman’s lounge at the Alice, and the pilot of the red rat jet we then took to Brisbane kindly agreed to my suggestion he inform the passengers over the speakers that the plane would fly to the correct airstrip, Brisbane, and had more than enough fuel for the trip.  My VIPs in first class cheered.

Mark

[turboplanner]

1 hour ago, RFguy said:

well you are right about the shaking and popping Nev. 

The Brumby Rotax  last week- the oil on the Friday I noticed had been getting black much faster than usual... and it was at 49 hours out of the 50 for the oil change ( the maintenance guys they do oil at 50 h on that aircraft) . anyway, put it to bed, and the next morning during run up tests I noticed severe vibration in the mid RPM range. One of the rotax floats in the Bing pair had sunk. sunk ALOT overnight. so taxied it back to the hanger.   And that it probably started sinking the day or two before and the oil going black was a tell tale of the mixture going rich on one side . I need to pay more attention to the tailpipe colour. and that it wouldn't make spec full static RPM.. 
-Gimme a Jab engine any day.. Or a Continental O320 etc. 

And so a leaking float decides which engine you fly behind? What about the pilot that flew after an RPM check failure?