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Getting simplicity back into the Jabiru engine


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I can’t find it now, but years ago in discussing oil cooling on the Jab engine, someone said the original oil sump had the cooling fins on the wrong side; they should have been inside,

The idea being that would provide sufficient surface area inside the oil reservoir for heat exchange with the outside air.

 

It makes sense to me that deep fins both inside and outside the front (and perhaps also the bottom) of the sump would provide similar surface area to the external oil cooler and eliminate quite a bit of complexity. 
 

No more oil lines to check for leaks, a bit of weight saved.

The original engine was elegantly simple, so perhaps, with a redesign of the sump, we could go back there.

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The problem with that idea is no control. If it was really effective the oil would become to cold in some situations.  

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The finned sump was never enough. There was too-short an oil temp sensor which gave Stiffy the idea it was ok for awhile till he wised up.

Fins on the inside? Has this ever been tried? I can see some drawbacks. 

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Mine was the first Jabiru kit to be supplied with an oil cooler, while an earlier plane had its builder asking for one for years. This guy was a motor-bike racing mechanic and I reckon he knew his stuff.

My Jabiru ( 1998) has never had any oil-related problems, not even a leak.

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52 minutes ago, Old Koreelah said:

I can’t find it now, but years ago in discussing oil cooling on the Jab engine, someone said the original oil sump had the cooling fins on the wrong side; they should have been inside,

The idea being that would provide sufficient surface area inside the oil reservoir for heat exchange with the outside air.

 

It makes sense to me that deep fins both inside and outside the front (and perhaps also the bottom) of the sump would provide similar surface area to the external oil cooler and eliminate quite a bit of complexity. 
 

No more oil lines to check for leaks, a bit of weight saved.

The original engine was elegantly simple, so perhaps, with a redesign of the sump, we could go back there.

OK, I admire your philosophy - in the technological environment, we should always strive for simplicity (the KISS principal) . Unfortunately we must sometimes accept an increase in complexity, to achieve certain performance goals ( eg the fitting of an external oil cooler). I think we would all hope for technological improvements that  can be incorporated, that result in simpler systems.

 

Your idea external/internal sump fins would certainly present as an excellent KISS solution to a tendency to overeat the engine oil but as Thruster points out there is a significant potential down side, that might then need to be "managed" using pilot or thermostat  controlled variable air flow - complexity again.

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Finning on the inside of the sump is the dumbest idea I've heard of, and I have no idea what would possess someone to promote the idea.

The basic idea of finning is to expose as much surface area to strong airflow that is carrying the heat well away from the heat-producing area. There is no strong airflow inside the sump.

When aircooling with fins is designed to promote maximum effectiveness in cooling, the airflow is fan-forced and designed to carry serious volumes of air away. No such ability exists with a design where the fins are inside an engine sump.

 

The greatest single weak point with separate external oil coolers is the plumbing - particularly plumbing that involves rubber hoses. I have seen numerous engines destroyed when oil cooler plumbing failed. The best plumbing for separate oil coolers is plumbing that is integral with the engine and cooler and has no rubber components.

Finning is a very effective cooling measure, but it is never enough for oil cooling. Oil is the second largest source of engine cooling behind cooling air or coolant, and it needs a separate, radiator-style oil cooler to get rid of the engine heat.

Finning is just an additional method of cooling that helps inadequate oil coolers.

Edited by onetrack
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3 minutes ago, onetrack said:

Finning on the inside of the sump is the dumbest idea I've heard of, and I have no idea what would possess someone to promote the idea.

The basic idea of finning is to expose as much surface area to strong airflow that is carrying the heat well away from the heat-producing area. There is no strong airflow inside the sump.

When aircooling with fins is designed to promote maximum effectiveness in cooling, the airflow is fan-forced and designed to carry serious volumes of air away. No such ability exists with a design where the fins are inside an engine sump.

Onetrack - as I understand the concept of heat transfer it is directly related to surface area on BOTH the heat and cool side of the system.

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Skippy - But cooling fins inside an engine are only releasing heat into the engine - which is where it's coming from in the first place. The radiated heat is not getting carried clear of the engine. You're only recirculating the heat.

 

It's the same with liquid cooling systems, where you have poor fan shrouding. The heat being radiated from the radiator core is recirculated into the radiator with inadequate fan shrouding, and you end up with cooling problems.

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As I understood OK he is advocating fins  (increased surface area ) on both the oil and air side - thus maximising heat transfer from the hot to the cool side. I would guess that there may be problems actually fabricating such a system.

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Hi OldK

yeah I generally agree, although one'd need to do the numbers to see  if you can get the surface area that an oil cooler does.

Oil has a low thermal conductivity, you need alot of oil in contact with the heat exchanger.

And long thin fins dont work as well as you might think,  because the fins have thermal resistance. 

 

As for hoses- I like the clamps used for turbo charger hoses, T bolt constant clamp force type, rather than the common worm drive clamps.

 

I think there is more to be gained by working a bit harder getting air up on the underside the the steel cylinder bores. mine are clean- not much airflow there.  cooler bores permits more heat to get out of the piston. -glen

 

 

 

 

Edited by RFguy
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The oil cooler increases both air area and and oil area. Fins inside would help it sludge up and it's likely the local flow near it would be slowed. Fins reduce flow velocity near their base. Not ALL engines require an oil cooler. IF you don't need one you are better off. Warming up can be slowed also.  They also sludge up and can be virtually impossible to clean out. Cowl gills make it possible to cope with widely varying temps and engine outputs  You will reduce drag as well. Nev

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The fins were removed from the 2200 engine but have been retained on the 3300. Overheating was an issue with early Jabirus mainly due to the airflow having too small an exhaust area and a fibreglass lip kit was produced to assist with sucking exhaust air out. The accepted minimum is 3 times exhaust area to intake. The original Jab oil coolers were thin and small and attached to the sump so not as efficient as they could have been. When i installed my 3300A engine I spent quite some time working on  cooling and there is an excellent, if very technical paper created by NASA in the 80s on cooling or air cooled aero engines.

 

I installed a fairly large 7 row Positech oil cooler at the bottom of the firewall with a separate and sealed from the rest of the engine NACA intake and exhaust. The exhaust for the engine air is over a metre wide & 50mm high with a suction lip for 70% of the width. The result is that the engine runs cool even on 35 -40 degree days but in very cold weather I need to cover part of the oil cooler with some gaffer tape. The possible problem side is that the oil lines from the filter to the cooler are a metre long so need a lot of firesleeve.

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And just to be difficult ... the standard J2200 installed in Thruster T600s in the UK have to have a perspex wind deflecting cover in front of the sump ... to stop the oil OVERcooling in the midl UK conditions.

 

A seperate oil cooler and the ability to manage airflow through that is the ideal.

 

In trikes in the UK it is usual to have a very simple summer/winter oil cooler control - a neoprene velcro sleeve to go over the oil cooler and you cover/uncover as much of the finning as you need based on flying conditions.  Simple and light.  We tried a 'simple' reduced oil cooler for the R912 of a plain section of metal tube with welded on penny washer 'fins' and it worked well and took minimal space but was too fiddly to manufacture.

Edited by kasper
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When I was working, we had very expensive software that could model heat flow in and around various substances. It was really impressive to watch and when we validated the model with real world stuff, I was impressed how close the two were.

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A mate spoke to Jabiru at length when they didn’t have a cooler as ours was overheating  (he was a person who did this for a living, think rocket propulsion and turbines) and tried to work out the size of internal finning required.

Long story short, fins inside absorb heat from the oil and this heat then travels to the coolest part of the structure (sump) which on the outside has a rough surface so any high temp travels to the tips of the casting and tries to equalise with the cooler atmosphere.

 

Same deal when making a heat sink from ally sheet; when you cut it with shears, don’t file or polish it as the very small rough edges bleed off the heat much quicker than a polished surface.

 

the maths didn’t work out and the factory had already worked thru that internal fin question and found a cooler was a better option.

 

All been done before!

Ken

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Also heat transfers more readily from a liquid to a solid because more molecules are in contact.

Solid to air needs more area.Even with a radiator or an oil cooler there is a smooth fluid channel or tube and a finned 

outer surface in contact with the air. 

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Interetsed to hear thoughts on this point: Increasing the volume of oil carried by the engine.
I think I have mentioned before how my old 900cc 4 cyl Yamaha engine had 2.7l oil, 2.9l if changing the filter as well.

My Griso 1100cc and V-Strom 1000 cc, (although the V-Strom is also water cooled) both carry nearly 3 litres of oil, for an engine half the size of the Jab.

 

Surely more circulating oil would assist in cooling the hot bits???



 

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Extra oil mainly makes warm up take longer It's best the engine warm up relatively quickly.. Greater quantity alone does not guarantee a cooler running engine. Oil itself is a poor conductor of heat . So is air. They best dissipate heat by physically carrying it away by flow (circulation) . A larger quantity  of oil does work as a heat sink but not a very effective one. Older engines with babbit bearings and low oil flow rates don't get much help from the oil's cooling effect.  Some bearing metals add a lot to the engines heat because of poor frictional properties.. Nev

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According to my notes on engines, the purpose of oil is to lubricate and carry heat away from the hot bits.

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Jabiru7252, my point exactly, the more oil, the greater the opportunity/time for it to lose the heat it has carried away.

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Instead of internal fins there could be convoluted shapes in the casting to (say) double the internal surface area of the sump. Just a ripple would do. But more weight. Another trade off.

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Goodaye all

 

Internally finned sump needs to be taller as the crank and rods need to clear the fins.

A windage tray may be need to scrape the flung oil and keep the oil at the bottom.

 

Heat does run from hot to cold here, so it could work but the extra stuff required and size may be a problem.

Couple of years ago l did see a finned cooler that clamped to the spin on oil filter, not sure about its effectivness.

 

It is easier to control a external oil cooler.

 

regards Bruce

 

regards Bruce

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Oil does not just lubricate and relocate heat, it also absorbs, holds  & neutralises  contaminants. The build up of the latter is the main reason for oil replacement. Many cars have had their service intervals extended by having a larger sump capacity - more oil more ability to absorb & neutralise contaminants, the longer the time between oil changes.

 

I see examples of how little oil an engine actually needs quite regularly - I  "play" around with a range of small 4/ engines , lawn mowers, pumps, generators, the occasional tractor or skid steer - some that are brought to me for repair/service, have virtually no oil showing on the dip stick and the little that is drained out is very black & foul smelling. After a good service most, not all, will carry on regardless, just as if they had been treated with TLC all their working lives - the others , clouds of blue smoke  from fresh oil entering the combustion chamber, tells the story of  damaged bores, piston rings, valve stems/seals. 

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I can recall a certain model of Komatsu front end loader that was introduced in the late 1980's with a smaller capacity engine than the previous model, with the new engine turbocharged, and a lot more highly stressed than the previous model.

The oil capacity of the new engine was also considerably less than the previous engine, and the HP/Kw output was substantially more.

Within a relatively short space of time, this new engine was blowing up all too often, at relatively low hours.

Komatsu engineers were called in, and the result was the fitting of a much larger sump, with substantially increased oil capacity.

The premature engine failures stopped at that point, and the engine regained a reputation for reliability.

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1 hour ago, pmccarthy said:

Instead of internal fins there could be convoluted shapes in the casting to (say) double the internal surface area of the sump. Just a ripple would do. But more weight. Another trade off.

As my engine approaches half it’s TBO, it might be prudent to plan for the big dismantle job.

If the sump can be removed, why not install/weld in a set of fine, deep fins both inside and out, to increase heat transfer? Very little extra weight.
If we do our sums right, we might be able to remove all that external plumbing,.

 

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