Jab Plenum Chambers

[facthunter]

Jetjr, you are right about applying the P&W limits to a jabiru. I did not wish to imply that is the way to go except to indicate how far from others in the industry the situation is. Even for them. (Jabiru ) the 120 degrees figure sets a nearly unachievable aim.

 

Yes the motor is sensitive to overheating, that is clear, but the cylinder head is not the entire story. Any part of the combustion chamber surface could cause detonation . A half detached piece of carbon incandescing. The plug overheating. The piston crown being too hot. A badly seating exhaust valve glowing red hot. Deteriorated mogas fuel? Definitely a possibility Running the heads excessively cool may mask the problem. An overtemping head may be the result of detonation, not the cause of it.

 

Fuel distribution is another factor, which many have tried to address. Even an old Lyc or Conti with the mechanical injectors especially IF upgraded to GAMI (calibrated) would solve the distribution problems some Jabs have with a carburetter and manifolds. Nev

 

 

[seb7701]

I've been reading like crazy about plenums and especially cowling intakes and exhausts and boy oh boy, isn't there some material out there!!!!

 

It was coincidence that the latest Kit Planes has an article about reducing drag, including intake drag. Turns out there is a bloke out there who is often referred to by the name of David Anders, who appears to be one of the authorities on drag reduction and is the owner of a 220kt RV-4, which I would say is no mean feat.

 

Whilst I best not copyright Kitplanes, these articles include some shots of his plane (and another). Of particular interest are the curvy bits inside the lower cowl to 'arrange' the air prior to it's exit and more relevant still is the cowl exit, which he claims is much better running parallel to the airflow. The icing on the cake?? His outlet is about 75% the size of his INLETS!!

 

His intake is 34 square inches whilst outlet is 26 sq in!!!

 

Happy reading if you're keen, along with a picture of Anders' cowl outlet.

 

Twisted: Cowling design - Part 2

 

 

 

[jetjr]

Good info and those thoughts are behind the changes CAE included in their versions. Under piston cooling, thermostatic oil cooling, better materials .

 

Time will tell if Jabirus approach to fixing broken throughbolts is sound. Resonance and temps is a simple explanation for complex failures. Truely hope I am wrong

 

 

[seb7701]

Yep, there sure is some info out there and it's bit easier reading than NACA reports and WW2 cowl tests!!

 

Well, at least on the latest Jab engines. There are an interesting bunch of changes, including no head bolts and cylinders which dissipate hear three times more than the old ones but (aside from the cooling woes) I do love my solid lifter engine. From my best guess though, I would say the only problem children would be the hydraulic lifter engines WITHOUT the roller cam, which suffered from resonant vibration I think it was.

 

I may be frustrated with mine, but still think it's a simplistic work of art when I take the cowl off...especially compared to the five million hoses and wires hanging off the 912's new door...

 

 

[jetjr]

However some of the problems seen are not adressed directly at all in new models like valves, pistons and flywheel attachment. Theres some good changes for sure but still sourcing low cost parts.

 

All new manufactuers and suppliers for key parts cant assume quality of previous ones.

 

In some ways the new version could have totally new problems with areas long ago sorted out.

 

Fuel distribution also remains as it was

 

 

[seb7701]

However some of the problems seen are not adressed directly at all in new models like valves, pistons and flywheel attachment. Theres some good changes for sure but still sourcing low cost parts.All new manufactuers and suppliers for key parts cant assume quality of previous ones.

In some ways the new version could have totally new problems with areas long ago sorted out.

 

Fuel distribution also remains as it was

True, but they have run the hell out of the new donk in their test cell and I for one heard it 12 months ago copping a flogging, so we'll definitely give them a chance.

 

 

[jetjr]

Yep just like the others

 

The hydraulic lifter versions were astm certified

 

I hope it works out well too but theres a long path of promises and too many poor outcomes to ignore

 

 

[facthunter]

Putting fuel through a motor in a test rig could be more productive IF the test was a bit more orientated to more than emphasis on hours. Running ONE engine for 3,000 hours doesn't mean the next one wouldn't blow up at 10. If it had to have one head off at 800 doesn't mean it's a bad design either.

 

Once a motor has reached equilibrium temps and run 2 hours or so to do it, another 10 doesn't mean much unless you introduce other factors( measurements of strain, overspeed test. etc If there's much variation in the engines being produced it's not of much value. When lots of a particular motor do good life THAT has meaning but that's a time and numbers thing. It establishes its reputation in the field. where it can also lose it. IF they make changes to just one part it's not valid either unless.... If the part was so superior to the original it could be claimed that its failure is completely unlikely the logic of and validity of the test could then still be argued to be valid. These tests are costly and time consuming and perhaps don't achieve a lot. Perhaps a better more effective test regime could be devised. I would suggest that is very likely. Will energy be applied in that direction? I think NOT. Bureaucracy doesn't stick it's neck out. More of what we had in the past is good enough, and is safer for them. Nev

 

 

[derekliston]

My engine temps are all in the black. I only have a problem with oil temp. Jabiru say 100C max continuous and 118C max but no info about how long that can be maintained. I don’t worry about CHT or EGT spreads as long as they are within limits.

[Bruce Tuncks]

I don't like how no2 is at 107 and no4 is 129 cht. How about diverting some air from no2 to no4...

 

 

[seb7701]

Terrific temps Derek - I am jealous!!! The oil probably is a frag high for 22 deg OAT, but still not scary until ambient gets higher. Which oil cooler are you running - the 4 cylinder (black auto type)?

 

Feel free to throw up some cowl / intake photos if you feel so inclined - I've seen some really great work written up about Jab installs on Zenith forums.

 

 

[derekliston]

I don't like how no2 is at 107 and no4 is 129 cht. How about diverting some air from no2 to no4...

Yes I know, but since 4 is directly behind 2 that is difficult to achieve. As long as they are all within limits I am happy. Up until I started flying this aeroplane, none of what I flew had all four cylinders, if any, monitored. Would have been interesting sometimes to see temps on C152, C172 or Cherokee 140 (maybe better that we couldn’t!)

 

 

[derekliston]

Terrific temps Derek - I am jealous!!! The oil probably is a frag high for 22 deg OAT, but still not scary until ambient gets higher. Which oil cooler are you running - the 4 cylinder (black auto type)?Feel free to throw up some cowl / intake photos if you feel so inclined - I've seen some really great work written up about Jab installs on Zenith forums.

Yes, the four cylinder black one as supplied by Jab. I have considered installing a bigger one but just want to fly, not work on the plane. Also I can’t track down a suitable one (not that I have tried very hard)

 

 

[facthunter]

Most if not ALL conventional flat four/ sixes have baffles between the cylinders and a full seal around the top and rear of the engine with the cowl. The standard installation would have been well researched and fully tested before going into production and therefore not many problems evidence themselves generally in use. Nev

 

 

[Old Koreelah]

Yes, the four cylinder black one as supplied by Jab. I have considered installing a bigger one but just want to fly, not work on the plane. Also I can’t track down a suitable one (not that I have tried very hard)

Derek I've got a slightly larger oil cooler spare. I tried it last year, but the NACA inlet I installed it behind was a dismal failure, so I went back to the original factory-supplied one (which I believe is from a VW Golf).

If you're interested send me a PM.

 

 

[seb7701]

Whilst the black 4cyl cooler should do the trick if properly set up, I would say the 6cyl oil cooler would be the quick fix if it didn't require too much mucking around for your particular install. The other temps (CHT's) are not worth thinking about with those numbers - stick to flying it and enjoying!!!

 

 

[derekliston]

Most if not ALL conventional flat four/ sixes have baffles between the cylinders and a full seal around the top and rear of the engine with the cowl. The standard installation would have been well researched and fully tested before going into production and therefore not many problems evidence themselves generally in use. Nev

Whilst I don’t disagree with that, it is only true when the aeroplane is new and or perfectly maintained. I have seen plenty of fairly iffy baffling over the years.

 

 

[Bruce Tuncks]

yes derek has a good point. A lycoming I know has too much difference in CHT with one rear cylinder going 190 C and we are trying to even things out.

 

On my Jabiru, I put in a deflector in front of the no2 to divert some air up past no 2 to eventually get to no 4 and this worked, with no2 going hotter and no4 a bit cooler. .It is easy to get a cylinder to go hotter and hard to get it cooler.

 

I agree a flat 4 has a problem with the rear heads being in the lee of the front ones. Radial engine anyone?

 

 

[facthunter]

Radial engines have a problem too with low speed climbing and high AoA. The Cowl is set for an average or cruise optimum design. My point is that at the design development stage a certified plane has the engine/ plane/propeller combo sorted out. The in between cylinder baffles don't give a lot of trouble. The cowl seals should be inspected frequently. I'm a believer in cowl flaps/gills, then the operator can adjust for the ambient and load altitude climb descent situations and manage the engine for a multitude of conditions, save fuel and the engine as well. A Jabiru is an engine where temps are a bit more critical than most Lyc-cont type engines. Trying to keep an aircooled engine head temps around say 120 C and even is a bit of an ask. The max permitted temp on the P&W R-2000 was 235 C. That is not where you would choose to have it of course but probably about 200 would be fine. Nev

 

 

[seb7701]

Yes, me again...

 

Among my consideration of enlarging intakes and outlets, I have been considering cheating a little with oil temps and fitting a 6cyl oil cooler in lieu of the black current style 4cyl cooler (or maybe fitting my old skinny style one in addition?). On the days approaching the 30deg mark, I see mid 90’s and maybe the 100 from time to time.

 

Would I be right to guess that lowering the oil temp could make it easier to cool the CHT’s or am I way off?

 

 

[Old Koreelah]

Yes, me again...Among my consideration of enlarging intakes and outlets, I have been considering cheating a little with oil temps and fitting a 6cyl oil cooler in lieu of the black current style 4cyl cooler...

Seb I may have what you need. See post 315 above.

 

 

[Bruce Tuncks]

Seb, cooling the oil more will have to reduce the cht's. But it can't be by much, the oil going to the head is not that big a volume.

 

 

[seb7701]

Thanks Bruce. I figured it would have to have some impact, but the volume aspect makes sense....it won't be a magic fix....dammit!!

 

 

[facthunter]

The effect would vary. You can't run the oil temp too low or it will retain water. (Become cloudy) and does damage by corrosion. On an aircooled motor, cool the heads by cooling the heads. Nev

 

 

[Oscar]

The effect would vary. You can't run the oil temp too low or it will retain water. (Become cloudy) and does damage by corrosion. On an aircooled motor, cool the heads by cooling the heads. Nev

Retained water in the oil can boil (turn to steam) at high-temp bearing points, expressing the oil film and causing scouring damage. In the CAMit development research, several instances of this were found in Main bearings of Jab engines sent to CAMit for a 'CAMit rebuild" , (identified by one of the experts in bearing shells in Australia) which was previously thought to be over-compression in the crank-case halves.

 

The role of oil-cooling as a medium of extracting heat from engines has been used very effectively in motorcycle engines ( the GSX-R Suzuki inline 4 and the Honda VFR v4 are examples) - but the engine has to be designed for this. The Jab. engine is not.

 

To get the best result from increased oil-cooling on a Jab. engine, one needs to have the CAMit TOCA ( thermostatically-controlled Oil Cooler Adaptor) installed. It maintains flow to the oil-cooler ABOVE the minimum desirable temperature ( set for the CAMit TOCA at about 96C). Then, with a decent oil-cooler set-up, you will operate within a band of 96C and tops 100 or so C in extreme conditions.

 

Sadly, the CAMit TOCA is no longer available, but there is a British version which I believe is available to order.

 

I have a CAMit TOCA and an Aero-classics 7-row cooler on my engine. I don't have flight testing on the efficacy of that yet - but I won't be prised loose of the CAMit TOCA by a large hammer.