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Paul R

Over heating on 2200 Gen 4 Jabiru 170

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Thanks Nev, yes the prop is going up as it passes 2 and 4 which would add to the angle produced by the nose-up attitude. 

A suitably-shaped deflector attached to the cowl could be arranged to divert more upward moving more air into the ducts.  I reckon its worth a try, with a rivetted aluminium plate to start. This won't be too hard to remove if it doesn't work.

The prop doesn't have much of an airfoil shape this close in, so the main effect would be air dragged in the direction of the prop.

I have read that airflow in the cowl is not what we visualize. The experimentalists who have tufted and photographed these airflows have shown this.

There is a lancair in Australia where the rear of the spinner has been sealed with brush-fibers , I reckon to reduce the centrifugal-pump effect whereby the spinner sucks air from the crankshaft region and flings it out sideways.   

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 The centre of the prop is not providing much thrust but it must affect the airflow into the cowl even if it only spins it a bit. Nev

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On 28/07/2019 at 3:37 PM, Bruce Tuncks said:

Great figures OF. For years, I bragged about how  my max cht didn't exceed  150 but after  upgrading the instrumentation, I found the engine ran hotter than I thought and it can go to 160 on climb. At 160, I throttle back and speed up till it goes closer to 150. And my spread is worse than yours, numbers 2 and 4 are about 20 degrees hotter than 1 and 3.

Any ideas for making the port side ( 2 and 4) cooler will be most welcome.  I can't figure out why there is a difference, the ducts look the same, I have closed the gaps equally on each side, and the internal deflectors are the same on each side.

Strange that no 2 is hotter than 3.

If I saw Paul's figures going to 200 degrees, I would freak out for sure. But, as has been correctly pointed out, Lycomings allow higher figures again. I'm confused, but there is a clue in the materials. The gen 4 Jabiru and the Lycoming  use a cast alloy which appears to be similar and much harder than the old Jabiru heads. Even so, 180 would seem to be an achievable maximum.

Part of the rationale of the gen 4 engine was that the cylinders would expand on heating and reduce the tightening effect of an alloy piston in a steel cylinder, so I would have expected it to run cooler not hotter. 

The Gen 4 heads are effectively one piece thermally with the barrel and the barrel is now alloy and a far better conductor than steel, this would assist in the head at the sensor point being held cooler by the barrel than previous Generations.  A fair bit of the barrel is within the duct.

The barrel now has larger fin area. Someone may have a figure on surface areas of the gen 4 vs earlier.  There is also the relativity of a rough surface of a casting vs a shiny CNC surface of barrel and heads.

As an aside is there an advantage in black heat dispersant coating vs silver alloy finish.  Such coatings are commonly used on air cooled motorcycles.

OF

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