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JabSP6

How to improve the reliability of the 6 Cylinder Jab

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things run more even with heated air on

 

Havent really tried but just using air from inside cowl might work

 

Im convinced its ram and swirl which changes as velocity changes through intake system - in which case every build will be quite possibly different

 

 

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How do you know it did,not make any difference what where you trying to achieve did you say you have not flown 

 

it yet. How do you know if it is running lean or rich.

 

 

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Even fuel distribution

 

EGT are how you measure it - one per cylinder

 

I have flown  - some 100hrs and no difference

 

Been fighting this uneven EGT, as have others, through three different engines and 500+ hrs

 

 

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How do you know it did,not make any difference what where you trying to achieve did you say you have not flown 

 

it yet. How do you know if it is running lean or rich.

 

Sorry I didn’t make it clear. 

 

I did the initial blockage with card and cut a hole the same circular size as the duct. But it was in same site as the naca duct. 

 

Then flew it for about ten hours with all data collated ( 6 x egt, chts) charting temps against  altitudes and rpms and OATs. All compared to similar data done for previous 10 hours. 

 

Talked to Jamie at Jabiru about it. 

 

He told me they did pressure transducer tests all along the cowel and the reason they moved it so far back was there was still some Higher pressure forward due to ram air on the cowel surface. They moved it back till they found the spot where the pressure drops off. Perhaps mine would get better if it was moved. 

 

So since I’ve been doing other stuff that meant glass work and painting, I’d give it another go. Since it’s not a huge amount of extra work I’ve done the move. 

 

But in the new position I have yet to fly it it. So as yet can’t say if there’ll be any benefit. 

 

something that I’ve found though is that so many times one person will swear that some mod made a difference but others ( including myself) will do it and it makes no difference.  Equally I have done mods that made a difference and I have done significant testing to prove it but when we did the same in another jab it made no difference. 

 

There are just so many differences in people’s set ups. 

 

 

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I don’t know the advantages of the intake Change good I would think. I had no experience with the 

 

3300 only 2200. I changed the air cleaner used a auto filter I had the early corse fin heads at times I thought 

 

the heads where to cool. I have since sold the plane I believe it’s still trouble free flying.

 

 

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I don’t know the advantages of the intake Change good I would think. I had no experience with the 

 

3300 only 2200. I changed the air cleaner used a auto filter I had the early corse fin heads at times I thought 

 

the heads where to cool. I have since sold the plane I believe it’s still trouble free flying.

 

Jabiru maintain that changing the intake structure and location makes a positive difference in most set ups. ( both 3300 as well as 2200. )

 

The standard air filter is just an auto one. At one time I had over rich in some cylinders ( can’t remember whether it was all or some or which) and had black soot exhaust. In discussions with stiffy we tried changing to effectively no filter. Essentially to lower the fuel:air ratio. What we used Was actually a very open large hole foam (filter foam made to withstand oil and petrol) coated in filter oil.  It made a difference but didn’t cure it completely.

 

Repitched the prop to a finer prop and that made the most difference. 

 

 

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Modifications to the air intake on the side of the cowl (& its position) made a big difference to EGT spread on my Bing carbed 2200. So I concur with Hyundai.

 

I ended up settling on a port mid height (ie. widest part of cowl just under the join) about 75mm forward of the firewall.  I also have a small deflector on the forward side of the hole that extends outward about 1 cm.  After lots of experimentation this seemed to give the best results at cruise  & WOT throttle settings. Although I have to throttle back at WOT (rpm ~ 3100 FF 23lph) to avoid exceeding 700C egt on my rear cylinders (particularly #4) at the cost of about 50rpm,  & at cruise (15lph) my #4 runs at around 640 to 650C when the others are around 710-720C. No data on manifold pressure unfortunately.

 

Why the inlet matters I don't know as I would have thought the air filter and the little rubber flap on the air filter box would remove any significant pressure effect on the carb. Carb inlet vanes made bugger all difference, except when a 'vane' obstructed one of the pressure sensing ports of the Bing.

 

My limited experience on a Rotec carbed 2200 with a J&H filter attached directly to the inlet drawing warm cowl air with no carb heat 'a la' many Rotax 912s is totally different.  EGTs are evenly spread irrespective of carb rotation. Carb temp has never been less than 30C even on a 5C OAT day, and has seen 50C+. I do worry about vapour problems as there is no float bowl in this setup. 

 

 

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MARKDUN

 

thanks for your comments I had a different air cleaner on mine with that pressure flap and it made little 

 

diferance. I am trying to convince everyone to install a air fuel ratio gauge.

 

 

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An AF gauge means little if the fuel distribution isnt even

 

your only looking at the average and could easily have one of more cylinders running very lean

 

 

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MARKDUN

 

thanks for your comments I had a different air cleaner on mine with that pressure flap and it made little 

 

diferance. I am trying to convince everyone to install a air fuel ratio gauge.

 

 

 

An AF gauge means little if the fuel distribution isnt even

 

your only looking at the average and could easily have one of more cylinders running very lean

 

I have to concur with Jetjr here.

 

(Of course I am assuming you are referring to the typical AF ratio sensor system where it selectively samples gas from one site only.

 

As  has been stated here and on many sites and threads on this forum - the major problem with Jabiru engines is that there is not uniform mixing in the after carby plenum chamber.

 

This then means that depending on the amount of mixing that has occurred gas that is aspirated by the individual cylinders can vary dramatically.

 

This is actually similar on many engines, its just that the massive heatsinks of Lycs and continentals can cope happily with some cylinders running lean. 

 

Jabs are a fine light engine and the cause of many problems is they can't tolerate the same as the heavy old ones.

 

There are a bunch of reasons why they run uneven:

 

Typically the richest (laden with the heaviest droplets of fuel) air has the highest inertia and is carried forward to the front cylinders.

 

The air on the sides of the column of flow through the throat of the carby tends to be the leanest and this air is the least dense and has least inertia and is preferentially directed toward the rear cylinder intakes.

 

There's a further complicating factor - The direction of air rotation in the carby intake SCAT hose. Typically it rotates in a clockwise direction and as it passes through the carby picks up the fuel (in an uneven concentration) as as it passes out the carby tends to throw the densest fuel laden air to the right.

 

Then there's a further complication:

 

In the intake plenum chamber there's an airfoil shaped post in front of the carby outlet. The role of that is to try to stop the swirl of air - but it also can produce complex coanda effects (gas flow sticking to the surface and then leaving at a more pronounced angle in the original direction) which may over-enhance the negative effect in some situations.

 

Without getting into what can be done to ameliorate these -  The point is that all these things serve to either improve or mess up the mixing depending on lots of factors.

 

Often they impede mixing.

 

The lack of mixing is what finally ends up with significant variation of ratios at each cylinder.

 

So a Fuel Air ratio gauge is not going to tell you which cylinders are rich and which are lean unless you have one at each and every inlet because as JetJR said an average tells you nothing about the individuals cylinders.

 

 

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Short of some major changes nothing much will change that. Stiffy was always reluctant to change to a more complex system for a lot of reasons. 

 

The only real way to change things is to separate the carby from the plenum. This will allow the air+fuel droplets to change into a real mixture. Right now the fuel droplets simply don't have time to atomize and mix before getting to the plenum. This means puddles of fuel in unpredictable places and uneven mixture that changes seemingly random.

 

Limbach used to have a setup where the carby was on top of the engine, and the fuel/air mixture had to travel half a meter through an aluminum tube to get to the plenum below the engine. This worked a lot better.

 

 

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...Carb temp has never been less than 30C even on a 5C OAT day, and has seen 50C+. I do worry about vapour problems as there is no float bowl in this setup. 

 

Interesting. My current setup has carb intake temps that high, which worried me enough to do lots of work to track down the cause- without any great success. I must admit EGT spreads are not too bad and it runs very well, but uses a bit more fuel than previously. Can anyone comment on the downside of 49C air going into the carb?

 

 

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Jaba

 

There are no droplets of fuel if that was the case the carby is not doing its job. Also it’s not possible one side of 

 

the engine to run lean and other run rich. With all your knowledge you should be able to build the perfect jab engine.

 

i modified my engine to run optimal but you keep saying this probably won’t work or maybe that won’t work.

 

 

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 A hotter intake temp means less power developed. That's a commonly agreed fact. It applies to all carnot cycle engines including jets..  Nev

 

 

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Jaba

 

There are no droplets of fuel if that was the case the carby is not doing its job. Also it’s not possible one side of 

 

the engine to run lean and other run rich.

 

With all your knowledge you should be able to build the perfect jab engine.

 

i modified my engine to run optimal but you keep saying this probably won’t work or maybe that won’t work.

 

 ooooh  kaaay!  ?

 

 

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Jaba is right about the droplets of fuel in my experience. Of course the job of the carby is to convert the fuel to vapour in the intake plenum, but nothing is perfect.

 

How else would you explain how cylinders 1 and 2 originally had lower EHT's than 3 and 4? I imagine those droplets overshooting the branches going off to 3 and 4, and hitting the end of the plenum to be probably now vapour which is then sucked into 1 and 2. (sorry but the engine is a 4) 

 

In fact I made some  little vanes just downstream of the carby to deflect these droplets into 3 and 4 and there was a big change for the better. (thanks to  jetjr for the comment about how nothing much upstream of the carby would work )

 

But my vanes are not perfect either and while the EGT's are quite even now, the CHT's for 1 and 3 ( stbd side) run about 20 degrees cooler than 2 and 4.  Yep, 3 now is cooler on CHT than 2, even though  2 is at the front. But the whole engine is much more even now than before the exercise so it was all worthwhile. I would still like to reduce the difference to 10 degrees though.

 

One thing I considered was putting more distance between the carby and the intake plenum, it is interesting to know that Rotax does just this. I have also wondered about putting an electrical agitator ( tiny fan ) into the plenum but both these things would be very hard to do.

 

 

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If you can measure the distance between the piston face (measured with the plenum off) to the cylinder flange face plus the mean distance from that face to the carby flange face of the manifold for each cylinder plus the distance, if any from the manifold flange face to tha carby flange face, and what rpm you want your power delivered at, plus perhaps a carby internal measurement I’ll let you know about, I should be able to tell you if the flow is being impeded or enhanced at that rpm and what length to add to optimise combustion. 

 

I realise measuring inside the manifold is a piece of string, so just for this exercise you could use a length of string to give us a manifold length for each cyl.  That would tell us if the engine is sucking or blowing, and if the temp figures correspond, then some serious improvement is possible.

 

 

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Fuel-air mixing is reliant on intake swirl. Intake swirl in the combustion chamber is well known and well-studied, but intake swirl in the intake manifold is less-studied.

 

Intake manifold swirl can be improved by the addition of an Al-Rousan intake manifold loop. The dimensions of the loop are critical as regards improving fuel-air mixing.

 

Improvement of Gasoline Engine Performance by Introducing Swirl Mixing In the Intake Manifold

 

 

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That is interesting onetrack. It confirms the droplet theory.

 

Unfortunately the Jabiru intake plenum is too close to the carby for such a thing to work.  

 

Turbs, are you thinking about tuned inlet pipe lengths? I haven't got the figures anymore, but I did work out that the inlet pipes on my engine were much less than the displacement of each piston. That means the flow is through the carby through the plenum and the pipe and into the sucking cylinder for the time the inlet valve is open.

 

 

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I'd like to look at the sonic waves to optimise the bounce. Overpacking the cylinder with gas can increase power, but filling the cylinder with mixture and then sucking it back out again before the valve is fully closed costs power.

 

 

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Jaba

 

There are no droplets of fuel if that was the case the carby is not doing its job. Also it’s not possible one side of 

 

the engine to run lean and other run rich.

 

You are kidding, right?? *THE* documented way to even out left-to-right EGT's in Jabs is to tilt the carby slightly to the left or right. The only reason this has any effect is because the poorly mixed stream hits the manifold divider, guiding more or less fuel to the left or right bank depending on offset of the carb atomizer and air stream. If the fuel was mixed properly, any tilt would have zero effect.

 

I wish I could find the video again, but Pete Krotje, long-time USA dealer for Jabiru, had a transparant manifold made, to study what goes on. It shows solid fuel droplets coming from the carby. There are several documented mods for the carby to improve atomization, but distance from carb to manifold simply is too short for the fuel to atomize properly.

 

 

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  Yes ,  there are even drops in it when it goes into the cylinder. They affect the flame travel rate.. Full atomisation is unlikely. You could experiment with a test using LPG where you aren't dealing with  a liquid  at all. Nev

 

 

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...

 

Repitching the prop to a finer prop and that made the most difference. 

 

JW a few years ago a well-respected LAME told me that he'd had good results in the battle to protect head bolts from detonation by  "unloading the prop", which I presume is a finer pitched prop.

 

 

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Ok I give up I will let you guys figure it out with all the scientific words and explanation. 

 

 

 

 

 

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After finally installing the new position for the air intake and changing the NACA scoop to a perpendicular circular inlet - and more importantly FINALLY getting a day without rain - I managed to do some flying today for first time in months - (sorry to all you guys down south in drought but I think we have had about 25 days without rain this year and I'm thoroughly over it)

Back to story.

 

My range of temps which was from 110 ( CHT. 6) to 165 (CHT 3) with CHTS 3 & 5 climbing to 175-ish on first climb out.

Has now changed to 125-ish to 140-ish across the lot. With none going over 160 in climb out.

 

This is the best I think I've ever had them so finally I'm happy.

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