EBay Temps

[skippydiesel]

A brain teaser for the Forum Brain Trust.

 

After a lot of work, my engine oil/coolant temperature are all within Rotax (912ULS) recommended specifications ✔️

 

Unfortunatly my engine bay (EBay)/under cowl temperatures are high. Were about 25 C above ambient, have got it down to about 15 C above. Can't figure out how to get more cooling air into the EBay or hot air out. I understand Rotax recomend no more than 7 C above ambient(?)

 

Why does this matter? 

High air/carb inlet temps reduce engine efficiency. May cause fuel vaporisation (vapour lock) and damage electrical systems. I have seen 65C on engine shutdown.

 

For best engine efficiency, I was using a home built airbox, to deliver nice cool air to my carburettors. Unfortunately the airbox, hanging of the carburettor ,was negating the vibration damping of the rubber manifolds and causing fuel to be ejected from the carb float chambers. I have replaced the air box with Roatx standard conical filters (sucking hot air).

 

Fixes (?) I have tried;

• Flown with cowl top inspection hatch removed - nill effect in flight, may have improved ventilation on the ground.
• Increased air outlet significantly - some benefit to temp reduction but likly have increased drag (a little).
• Ducted air from front of cowl to vicinity of carbs - some benefit.
• Ducted air from vicinity of carbs to exit air - some benefit.

 

Refitting air box  - very likely, will have to find some way of supporting it, independent of carbs,  so as to prevent previous problem.

Refitting the air box will improve engine performance but will do nothing for the Ebay temp and its potentially negative effects.

 

I have toyed with the idea of additional exit air holes (proven a top vent will have little/no effect) wondering if side (lower cowl) vents might work.

 

Your constructive thoughts will be most appreciated.😈

 

 

[onetrack]

Fit an electric fan to boost airflow. All long-life, reliable air-cooled engines have cooling fans of some sort. Even Briggs and Stratton and Honda garden equipment engines utilise a flywheel-driven fan. It always amazes me that both conventional aircraft engines and air-cooled motorcycle engines have no forced air cooling. Radials are the only exception, they have basic forced air cooling system.

[IBob]

Skippy, the Savannah has a pronounced lip at the front of air outlet, to provide negative pressure there.

It also has an airbox, but mounted to the rear engine mount, so not hanging off the carbs.
For combustion air intake it has a NACA scoop......but I discovered that doesn't work so well at high angles of attack, was also pulling air from under the cowl. Was able to fix that by extending the airbox inlet to almost reach the scoop....while still leaving a small gap for drainage on the ground.
 

Also: where specifically are you measuring the temperatures? Air temperatures tend to be very location specific, by which I mean there will be quite different temperatures in various places under the cowl.

Edited Thursday at 07:15 AM by IBob

[skippydiesel]

Hmmm!

 

In reverse order;

 

"Also: where specifically are you measuring the temperatures? Air temperatures tend to be very location specific, by which I mean there will be quite different temperatures in various places under the cowl."

 

EBay temp is measured top left of firewall, not far from the left carb conical filete. I did place additional  temporary temp sensors on cone filters - gave temps within 2C of fixed unit.

 

In recognition of the potential for some sort of hot air pooling:

• I install, one each side, 50mm ID ducts, from front of cowl (cool ram air) too close to cone filters - small reduction in air temp.
• Then one each side 70 mm ID ducts running from close to cone filters, to exit air just below firewall, hoping for air to be drawn away from top rear of EBay - small reduction in air temp

Fan? - Did try a fan back when I had oil/coolant cooling issues, now solved by relocation of radiator & oil cooler (fan on shelf). I guess I could try putting it over the carbs to pull more (cooling)  air over them - will check into the feasibility. 

 

My preference is always for a KISS solution where possible - not discounting it but to me a fan = complexity/weight😈

[spacesailor]

I never understood, 

Why

Very few ( if any ) air-outlet's  are at the top of the ebay , were the cooling air would be forced up through

The cooling fins .

When the fan stops the hot air naturally rises.  As it does every-were .

spacesailor

[Red]

Spacesailor, there was bloke designed such a system for light aircraft engines, but I cant remember his name ill post if I find it.

 

the idea had more to do with cooling the undersides of the cylinders more as that was the hot spot nearest the exhaustson his chosen engine, the only downside i remember was the potential for a screen covered in oil if you had a leak.

 

Skippy I've never measure the temp in my engine bay, are you measuring this in the cruise?

 

p.s. 25c above ambient doesnt seem at all worrying to me, but like I' said earlier, not something I've investigated in the Rotax installs I've had.

Edited Thursday at 08:41 AM by Red

[onetrack]

Here's a webpage with plenty of useful aircraft engine cooling tips. It pays to cover all the areas mentioned, and look for potential problems. Checking the accuracy of your temperature-measuring instruments is important.

 

Engine Cooling Tips

WWW.EAA.ORG

---

Experimental Aircraft Association

 

[Thruster88]

The tecnam p92 has a nice very light fibre glass airbox that encapsulates the two air filters and has inlets for cold and carb heat. I can’t remember if it is mounted or just hanging.

 

In your quest for performance and efficiency a cowl flap may be required. Climb wide open into the cool air, close and go fast.

[skippydiesel]

Red - EBay temps quoted, are in Cruise eg OAT 20C/ EBay 35C. After flight, taxi, shutdown, can get to 65C

 

Onetrack -Temperatures verified by using three additional,  temporary, IN/OUT digital thermometers. Thanks for the "Cooling Tips" unfortunately is mostly about engine oil/coolant/CHT cooling. All of mine are well within Rotax specifications however EBay temps are, in my view, far too high. Read my opening statement.

 

Thruster - Cowl flap, inoperative at the moment - waiting for replacement linear actuator. Not just my "...quest for performance and efficiency..." also trying to minimise potential heat damage to components (mainly electrical) and fuel vaporisation (vapour lock)

 

Spacesailor - "Very few ( if any ) air-outlet's  are at the top of the ebay ," Tried this (read opening statement). Could be, like my experince, ineffective.  Which could be something to do with external air pressure/flow, internal airflow/direction/pressure or combination thereof????  Will certainly help with, after flight, ground temps.

 

Thanks Gents. Keep the suggestions 😈