Cooling Modification Link Attached

[rv969wf]

http://x-jets.com/a_cooling_efficiency.html The web link I attached is worth the time to look at for anyone looking at cooling issues. I have applied some but not all of these along with other cooling mods to reach 300F CHT's on HOT 100F++++ degree days. BTW. In the winter at say 35F degree outside temps I'm using a 3 1/2" round inlet on the right with 3" round inlet on the left cowl to maintain 350 F CHT's. AJ

[gmcjetpilot]

Interesting, I have seen most of the ref's he used but Rpt 787, "Factors affecting heat transfer in the internal-combustion engine".

Max heat transfer (to engine) is @ 7% richer than the chemically correct mixture. (Rpt 787, NACA / MIT, Dec 1940)

New engines run hotter because of the combustion coating that forms in the heat. (Rpt 787, NACA / MIT, Dec 1940)

Piston speed is a function heat transfer, so longer stroke engines (360 vs 320) will make more heat and be harder to cool. (Rpt 787, NACA / MIT, Dec 1940) Is the new ECI XIO-340 a stroked engine?

An interesting comment he made and attributed to a NASA rept, not sure which one, was about painting everything with a thin (0.002) black paint and that it's better than bare metal. 0.005 paint thickness is too much. When I use to see an engine with worn thin engine paint, I thought they did a bad job painting it. Now I know they where keeping the paint as thin as possible. So a pretty engine paint job may be bad for cooling.


The main thing that caught my attention where his comments and recommendation about the wrap around part of the (lower) baffle. He recommends a larger radius and making a duct or diffuser at both the entrance (at the top) and exit under the engine.

On a stock Van's baffle it has little or no radius leading in or out, and it has NO extend path (scoop/diffuser) to guide air. Gradual transition would improve air flow; it makes sense. He also addresses some flow guides on the bottom of the cylinder. Not sure how to support those but again it would reduce turbulence and gain efficency.

He breaks the loss down to 64% is baffle loss (50% entrance and 14% exit). 36% is lost going thru the fins. There's not much you can do with the fins but make sure there is no excessive rough and blocked fins from left over casting with some careful filing. Its like a wing, you have drag. With cooling you have drag from the job of cooling. However flying with the flaps down can be inefficent. So cooling is getting the air to flow thru with min loss while providing the volume or mass needed to cary the heat away.

He estimates a 20% improvement in cooling efficiency by making baffle changes (larger radius and gradual entrance and exit ducts). He touches on not transfering heat to the engine with the comments about combustion coatings and exterior coatings. It makes sense to use high tech coatings to keep the heat from being transferred into the engine. Piston top and other coatings as you have mentined Alan may be a brave new world, however from the reports it seems the combustion deposits do the coating after 80 hours.

With an increase in cooling efficency of the air you take in, you can reduce the cowl cooling inlets/exits further and use less air, reducing cooling drag. (NACA Annual Report 22, p. 237-249.; NACA Technical Report 555, 1937)

Interesting, I think Van's baffles (any baffle) has LOTS of room for improvement. It is getting to the point where making complicated baffle parts from composites might be attactive? Fiberglass might wear too fast. I guess you can make it thicker and add rub strips. A Carbonfiber baffle may be better but could wear or abrade the aluminum heads. Abrasion protection will be in order as well. Its kind of normal to wear into the baffles; you see that all the time with the hair-comb effect you see on aluminum baffles.

Bottom line take a hard look at your baffles, new, old or yet to be made. Good stuff Alan thanks. To get the gains of better baffles, remember you need to reduce the cowl inlets and exits.