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Author: Subject: Cooling an "air" cooled engine - How smart are you?
russbost

posted on 19/7/18 at 09:53 AM Reply With Quote
Cooling an "air" cooled engine - How smart are you?

Sounds simple - Yeah, trust me it's not - see the pic below

[img] Jabiru engine
Jabiru engine
[/img]

This is our 6 cyl 3.3l aero engine in the plane I own a share of, it's a Jabiru, if that means anything to any of you.

You are looking at the engine from underneath, cyl's 1,3,5 are on the RHS as pictured, 2,4 & 6 on the left; fuel is supplied by a single Bing carburettor fitted where the towel is currently blocking the entry - that plenum has a vertical "strut" on the entry designed to split airflow equally left & right & has various vanes/fins within designed to get airflow to each cyl (yes, I know it''s a crap design!)

We, along with other Jabiru owners have always had difficulty in getting consistent cooling equally to each side, the LHS (2,4,6) always tends to run warmer than the RHS (by as much as 40 degrees). The engine does have an oil cooler, but that only really controls oil temp, has very little to do with cyl head temps which are what we have to monitor The engine has air ducts/cowls which fit around the top of the heads & direct air around the cooling fins - these have to be a little different side to side, because as you can see the cyl's are staggered

Reading the Jabiru forums everyone agrees that the engine is not just air cooled it is air/fuel cooled & having experimented with mixture, we would certainly agree that running richer lowers both EGT's & CHT's (Exhaust gas temps & Cylinder head temps)

Now; we've done manometer checks within the cooling ducts at entry & exit & pressures are as near the same as we can tell with the limited resources available, so there would not appear to be any significant difference in the airflow

Even with all the ducts & cowls removed if the engine is run up on the ground the LHS always warms up significantly faster than the RHS - EGT's are very close to equal, cyl head temps miles different & airflow around all cyl's with the ducts is fairly random, just from the propwash

There are suggestions on the Jabiru forums that one side runs richer than the other, our plug colours would deny this as would our EGT's so why should one side run hotter than the other, I also fail to see how with only one atomiser putting vapourised fuel into ALL air drawn thro' the carb how more fuel could end up consistently one side or t'other???

Is it feasible that as cyl's 2,4 6 are "fed" first from the single carb that they are actually getting MORE of the same mixture, a bit like having twin carbs, but with one carb bigger than the other, but delivering equal air/fuel mix. This would probably keep EGT's very similar, but would presumably give those 3 cyl's more power & more power = more heat, or is that idea just plain daft

We can overcome the basic high cyl head temps issue by running a richer mixture at WOT when the demands on engine & cooling are greatest, but that still leaves us with a significant imbalance between the 2 sides, which means it's impossible to run both sides at the ideal running temps

Any ideas or input welcome it's got me tearing my (already grey!) hair out





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Schrodinger

posted on 19/7/18 at 10:29 AM Reply With Quote
I'm not really into air cooled engines but IIRC they do need good oil flow as part of the cooling so could it be that the oil is not getting to/from the hot bank as fast as it should?





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Dick

posted on 19/7/18 at 11:03 AM Reply With Quote
Is it not possible to fit afr/ lambar sensors to the left and right side exhaust so you could check fuelling on the 2 banks. You would then have a better idea if its cooling or management of fuel
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theconrodkid

posted on 19/7/18 at 11:20 AM Reply With Quote
assuming all the parts are the same and just the location changes, how about the exhaust, a restriction there will cause it to run hot, have you been onto the engine manafacturers ?





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Mr Whippy

posted on 19/7/18 at 11:56 AM Reply With Quote
Remembering that the prop wash is in a spiral one intake will have the cooling air entering the intake from the top and the other from the bottom, although the flow may seem to be equal in pressure the flow direction sure isn't so poor ducting could be causing issues there or the assumption that both sides should have identical ducting. Tufts of wool attached to the visible internal cowling should help identify turbulent air flow or stagnant pockets. Even a spot infrared temperature gun should help identify overheating sections.

Also having an over-size intake can actually cause overheating if the exit is undersized. I was always told the exit should be about 3 times larger than the intake to accommodate the expansion of the hot air. You don't show the engine baffle arraignment, I'm assuming its leak tight and close fitting passing from the top to the bottom. The rear cylinders are always going to have a hard time unless fed by their own dedicated baffle. I can't see in between the barrels but VW / Porsche fitted plates between the barrels to force air sideways rather than directly through the fins to cool the underside. Not that that always worked as there was one cylinder that cooked and use to drop valves.

Nice engine but the fins look a bit chunky on the heads, would have liked them to be tapered thinner.

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907

posted on 19/7/18 at 12:11 PM Reply With Quote
I did an exhaust for an H/O 8 cylinder version than was installed in a Spitfire lookalike, hence the 6 outlets per side at the top.

This engine is now running water cooled cylinder heads.

Paul G



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russbost

posted on 19/7/18 at 12:20 PM Reply With Quote
in answer to the first 3 Q's, there is nothing in the top end of the engine to allow any significant oil cooling, it's a pushrod engine & only fairly limited flow to the rocker covers, no one has ever mentioned anything connected with oil cooling the engine other than airflow thro' the oil cooler affecting the overall flow over the engine as a whole.

Fitting lambda sensors or anything to properly monitor air/fuel on each bank would be ideal, but unfortunately would require a "Mod" with LAA engineering, we can only do stuff which would be regarded as temporary & test on the ground, anything fitted more permanently or to be used in actual flight needs a Mod applied for & is a proper PITA!

We've had the exhaust to bits in the past, no restrictions other than the crap (typical aeronautical - think pre '50's - design!) - the manufacturer are no help, they take the attitude that every plane is different & will be assembled differently & it's an Australian manufacturer - which makes it all the more strange that they didn't design it with decent cooling in the first place - these are the 2nd generation barrels & heads, the early ones had b*gger all fins on worth talking about - strangely they suffered with overheating problems ...............





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russbost

posted on 19/7/18 at 12:35 PM Reply With Quote
quote:
Originally posted by Mr Whippy
Remembering that the prop wash is in a spiral one intake will have the cooling air entering the intake from the top and the other from the bottom, although the flow may seem to be equal in pressure the flow direction sure isn't so poor ducting could be causing issues there or the assumption that both sides should have identical ducting. Tufts of wool attached to the visible internal cowling should help identify turbulent air flow or stagnant pockets. Even a spot infrared temperature gun should help identify overheating sections.

Also having an over-size intake can actually cause overheating if the exit is undersized. I was always told the exit should be about 3 times larger than the intake to accommodate the expansion of the hot air. You don't show the engine baffle arraignment, I'm assuming its leak tight and close fitting passing from the top to the bottom. The rear cylinders are always going to have a hard time unless fed by their own dedicated baffle. I can't see in between the barrels but VW / Porsche fitted plates between the barrels to force air sideways rather than directly through the fins to cool the underside. Not that that always worked as there was one cylinder that cooked and use to drop valves.

Nice engine but the fins look a bit chunky on the heads, would have liked them to be tapered thinner.


Yup, aware of all the oddities of spiral airflow & you may well be right that that is where the problem lies, but, if that were the case I would be surprised that it was still similarly unbalanced in cooling when run with all the ducts & cowls off.

It's not much fun working around the engine when running due to that big whizzy thing on the front being a constant threat to the unwary - you tend to make mistakes only once (so I'm told!) ..........., but the tufts of wool idea might work, we could then video from the front of the ducts to avoid getting anywhere near. I do also have an infra red "gun", but TBH it doesn't appear to be a localised problem, other than the fact it's local to the one bank!

The individual cylinders do have baffles in the ducting, which we have played around with in the past & got a much more even balance between cylinders, but it doesn't help with the overall balance between the sides & they also have plates between the barrels such as you describe.

It feels as though we need to take cooling from one side & transfer it to the other, there just isn't any easy way of doing that without getting into the whole "Mod" scenario as described above. I feel sure if we can get it balanced on the ground it will be fine in the air & if we could achieve that, however it was done, then would actually be worth the aggro of applying for the necessary mod to make a permanent update

Your comment about the chunky fins is rather prophetic - guess what the phase 3 heads looked like ! Annoyingly, we can "upgrade" to the later heads, but at the same time they changed the exhaust, upper & lower cowls & the ducts - I think we'd be looking at around £4k & a load of work, which seems somewhat excessive for what appears on the surface such a simple issue ..................





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russbost

posted on 19/7/18 at 12:38 PM Reply With Quote
quote:
Originally posted by 907
I did an exhaust for an H/O 8 cylinder version than was installed in a Spitfire lookalike, hence the 6 outlets per side at the top.

This engine is now running water cooled cylinder heads.

Paul G



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That looks lovely!

I believe there's an aftermarket water cooling option, but, aside from the cost & time fitting & needing to apply for a Mod to do so, it adds weight & would move our CoG forward, which would not be a good thing & I don't particularly like adding water cooling to what should be an air cooled engine!





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Simon

posted on 19/7/18 at 08:00 PM Reply With Quote
Iirc (and this has nowt to do with aircooled but may help in a roundabout way), Suzuki found that the inner cylinders on the Hayabusa ran hotter than the outer so run the inner cylinders slightly richer. No idea how true but may be a way to control temps (if you can do that sort of thing with your engine)
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Angel Acevedo

posted on 20/7/18 at 01:45 AM Reply With Quote
Hi Russbost,
Some questions...
Are the differences in temperature the same (or similar) at all power settings or airspeeds?
Have you changed CHT probe cables from side to side? To see if temperatures switch sides?
Is baffling tight to the cowling on both sides?

From some pics on the internet I see that the oil filter is on the left side of the engine, but I don´t think that will produce 40 deg difference.
Do you have pics of your engine bay?





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Pigsy

posted on 20/7/18 at 06:35 AM Reply With Quote
Re: Suzuki found that the inner cylinders on the Hayabusa ran hotter than the outer so run the inner cylinders slightly richer.

There are plenty of 3 cylinders Triumph motorcycle, 3 row injection throttle bodies available on ebay, or the like of ebay. If you placed one per bank you would get individual cylinder mixture control. You can fit different sized injectors. Alternatively, fit an oil cooler per bank of cylinders, or one that is much larger. Can you fit them to be sited remotely? VW air cooled flat 4, or Porsche flat 6 engine parts may be adaptable.
How about swapping it with a Porsche flat 6 engine? Get the simpler normally aspirated engine with the distributor etc.

I hope that this is of help.

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Angel Acevedo

posted on 20/7/18 at 06:37 AM Reply With Quote
quote:
Originally posted by Pigsy
Re: Suzuki found that the inner cylinders on the Hayabusa ran hotter than the outer so run the inner cylinders slightly richer.

There are plenty of 3 cylinders Triumph motorcycle, 3 row injection throttle bodies available on ebay, or the like of ebay. If you placed one per bank you would get individual cylinder mixture control. You can fit different sized injectors. Alternatively, fit an oil cooler per bank of cylinders, or one that is much larger. Can you fit them to be sited remotely? VW air cooled flat 4, or Porsche flat 6 engine parts may be adaptable.
How about swapping it with a Porsche flat 6 engine? Get the simpler normally aspirated engine with the distributor etc.

I hope that this is of help.


Don´t think he can do that, it´s a plane, not a car...





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jollygreengiant

posted on 20/7/18 at 06:52 AM Reply With Quote
IF the engine rotation is clockwise from above, could it be poor splash lubrication from the crankshaft, or poor drainage down the RHS banks from the push rod tubes.??? JMTP'sW.





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Mr Whippy

posted on 20/7/18 at 11:12 AM Reply With Quote
Mind too that the oil does have a large part to play in the cooling of the engine, though it doesn't explain the imbalance... I'm assuming you have tried swapping round the temp sensors and wiring to check it's a reliable reading? that they are mounted to identical area's on the engine? If you have access to the heads when running I would have taken average readings again with a infrared thermometer. Maybe borrowing a IR camera could highlight problem areas on the engine??
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02GF74

posted on 20/7/18 at 04:25 PM Reply With Quote
My guess is that it is the air flow from the prop. I assume you run the engine with prop fitted, can you ru. Without it?

If not, make a cylindrical cowl around the engine to control air flow. Will this be symmetrical around d the prop, then where do the cylinder heads lie in the space, is one side going ebe better cooled.

Depending on which direction the prooeis spinning, can hot air from one bank be directed to the other so it has hotter air flow g onto it?





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russbost

posted on 20/7/18 at 04:55 PM Reply With Quote
quote:
Originally posted by Simon
Iirc (and this has nowt to do with aircooled but may help in a roundabout way), Suzuki found that the inner cylinders on the Hayabusa ran hotter than the outer so run the inner cylinders slightly richer. No idea how true but may be a way to control temps (if you can do that sort of thing with your engine)


Unfortunately not, it has just the one carb delivering fuel to all 6 pots





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russbost

posted on 20/7/18 at 05:38 PM Reply With Quote
quote:
Originally posted by Angel Acevedo
Hi Russbost,
Some questions...
Are the differences in temperature the same (or similar) at all power settings or airspeeds?
Have you changed CHT probe cables from side to side? To see if temperatures switch sides?
Is baffling tight to the cowling on both sides?

From some pics on the internet I see that the oil filter is on the left side of the engine, but I don´t think that will produce 40 deg difference.
Do you have pics of your engine bay?


The difference in temps ranges from around 20 deg C across opposing cyls. ie, comparing 1 with 2, 3 with 4 etc, to around 40 deg at worst, they are probably closest at climbout when we typically see around 200 on the LH bank & around 180 on the RH, they differ most when in straight, level cruise when you may have 160 compared to 120 (the low side, ironically, is actually too cool!)

We did individual checks with both infra red & a single under plug additional lead as a check on existing, we're happy readings are accurate to certainly within 5 degrees

Baffling/cowls are equally tight both sides, over the years various small gaps have been filled, doesn't seem to make a scrap of difference. I've attached some pics below - you'll see the oil filter is completely outside all the ducting. We were doing a service on the engine today &, as an experiment, completely taped over the inlet entry ports so the heads & barrels had only still air, we only ran briefly for obvious reasons & stationary at relatively low RPM, but there was still a significant difference between the 2 sides with the LHS (as viewed from cockpit) warming up substantially quicker by around 15 - 25 deg across comparative cyls

1st, the engine "naked"
[img] Jabiru engine 1
Jabiru engine 1
[/img]

Here you can see the plenums/ducting
[img] Jabiru engine 3
Jabiru engine 3
[/img]

Looking down inside the LHS plenum, this is the side that gets hotter
[img] Jabiru engine 4
Jabiru engine 4
[/img]

Looking down the RHS plenum - due to no.1 cyl being further forward than no. 2 the air entry is very different
[img] Jabiru engine 5
Jabiru engine 5
[/img]

Finally with upper & lower cowls fitted, & the front ducts completely taped over - I fail to see why one side should warm up b4 the other when there's no air movement!

[img] Dead Ducts!
Dead Ducts!
[/img]





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russbost

posted on 20/7/18 at 05:41 PM Reply With Quote
quote:
Originally posted by jollygreengiant
IF the engine rotation is clockwise from above, could it be poor splash lubrication from the crankshaft, or poor drainage down the RHS banks from the push rod tubes.??? JMTP'sW.


Looking at the engine from the front rotation is anti - clock, which I would have thought would create MORE splash to lower bores/pistons on the side that is the hotter, which again makes no sense at all ........... As said b4 from all the info we've gleaned oil cooling does not seem to feature





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russbost

posted on 20/7/18 at 05:55 PM Reply With Quote
I think I've answered Mr Whippy's Q's with the info above, & one bit at the end of this one

02GF74 unfortunately can't run without the prop, but I think what we did by taping over the ducts is similar as it removes prop air almost entirely from the equation

Re can hot air be transferred from one side to the other, not to any significant degree due to the plenums/ducting, & given the prop rotation, you are getting air travelling in an upward spiral on the LH bank (the hot side) & a downward spiral on the cooler bank, it would seem logical that having the air forced directly down onto the heads rather than up into the cowl & along (that's what the temporary baffle at the front of the LH bank is for, with no baffle there no. 2 runs much hotter & no. 6 much cooler)

You're coming up with some interesting & sensible stuff guys, but, unfortunately we've already tried most of it, I was just hoping someone might spot something obvious we'd missed - air cooled engines are not a strongpoint for me

Possibly Mr Whippy's idea with bits of wool to detect laminar flow & an infra red camera (does anyone have one they' like to lend me??? ) might just throw up something useful, but, problem is, if we have good laminar flow on the cool side & disruptive flow on the hot side, what the hell do we do to straighten out that disrupted flow?





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Angel Acevedo

posted on 20/7/18 at 05:56 PM Reply With Quote
I think I may have the answer...
The soft baffling looks a tad too soft...
Inflight it may be collapsing back, therefore not enough cooling air being directed over the cylinders.
HTH and Solves your problem.
Angel Acevedo





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Pigsy

posted on 20/7/18 at 06:07 PM Reply With Quote
Re: Don´t think he can do that, it´s a plane, not a car...

I appreciate fully that this is an aeroplane and not a car. Are you not aware of how many light aircraft use VW beetle flat four, air cooled engines (from their car range) where you can fit extra large oil coolers. By enrichening the fuel passing into the bores, if you keep the same engine, you will reduce the running temperatures. It is perhaps too simple for some. Using two sets of Triumph fuel injection throttle bodies from their Motorcycles, you will find that they will fit straight on to the induction pipes of your flat six air cooled engine. You will not need the Bing carburettor at all. A small aftermarket fuel injection with its own ecu is available. You need to look for it. With this you will have a far greater control over your fuel mixture.

The Porsche engine has been used many times, but then, hey, what do I know.

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Angel Acevedo

posted on 20/7/18 at 06:22 PM Reply With Quote
quote:
Originally posted by Pigsy
Re: Don´t think he can do that, it´s a plane, not a car...

I appreciate fully that this is an aeroplane and not a car. Are you not aware of how many light aircraft use VW beetle flat four, air cooled engines (from their car range) where you can fit extra large oil coolers. By enrichening the fuel passing into the bores, if you keep the same engine, you will reduce the running temperatures. It is perhaps too simple for some. Using two sets of Triumph fuel injection throttle bodies from their Motorcycles, you will find that they will fit straight on to the induction pipes of your flat six air cooled engine. You will not need the Bing carburettor at all. A small aftermarket fuel injection with its own ecu is available. You need to look for it. With this you will have a far greater control over your fuel mixture.

The Porsche engine has been used many times, but then, hey, what do I know.


Didn´t mean to be rude,
But for sure my reply was not clear:
Even though what you propose is feasible, it will require approval fron aviation authorities as already stated by Russbost.
Best regards
Angel Acevedo





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russbost

posted on 20/7/18 at 06:48 PM Reply With Quote
Pigsy, I'd not replied as felt Angel's answer covered it. But, yes, a different engine could be fitted, but with aero stuff this just opens up a whole new bunch of nightmares. Firstly there is the weight & balance & CoG which it will certainly screw up - these things rarely seem to accidentally work out better. Then there's the matter of mounting it, suitable exhaust etc & then you have a fuel injection system designed to work typically between 0 & 3 or 4,000 feet on Mogas rather than at up to 10,000ft on avgas. If I was going for an engine swap then I think a ZZR1400 would be choice, I think it might need a reduction box tho'!!!

As said we could chuck £4 or £5k at it with the later heads, plenums, cowls & exhaust & given that there's 5 of us sharing that's not even that much money, certainly not in aviation terms, it just seems like using a sledge hammer to crack a relatively small nut ..........

Angel, a nice thought re the soft baffling, which allows the plenums to mate with the top cowl ducts, but we've had several different versions in the past, none have made any difference, also, that wouldn't explain the issues on the ground with little or no airflow & still a huge difference side to side, added to which when the cooling is at it's best, ie straight level cruise, is surely when the soft rubber would be most likely to be disturbed, at around 125mph rather than ground speeds

I'll keep looking/trying stuff .............





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View User's Profile E-Mail User Visit User's Homepage View All Posts By User U2U Member   russbost 's Aim
steve m

posted on 20/7/18 at 06:57 PM Reply With Quote
Russ,

What is baffling me, is which bank is at fault, is it the hot one is too hot, and shouldn't be, or the cool one is to cold, and should be hotter?

Does the engine have separate EGT gauges, as in one for each bank, and if it does, are you able to verify that both are reading the same temps ?can they be swapped with wiring to the opposite sides, to allow a comparison ?

Is carb heat fitted? and if so is it working off one bank or both,

Are the cold air feed ducts the same size, and placed equally behind the propeller,

Hot air needs to exit the cowling equally from both sides, does the hot side have any obstructions in its air exit, like startermotor
battery, or any large obstruction, that would impede its exit

steve





Thats was probably spelt wrong, or had some grammer, that the "grammer police have to have a moan at




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