Hot Air

[Skippydiesel]

Hi AlexZ -

My temperature issue is in flight - I suspect that the efficiency of the combustion processes being slightly compromised by the carburettors taking in relativly hot air (20C above ambient).

Openings in the top cowling will certainly improve on ground/engine stopped cooling but it they don't work in the air then my particular concern is not being addressed.

[Kai]

Well,

If you are concerned about the engine being fed too hot combustion air: what about your fresh air intake? According to the Rotax installation manual for the 912 ULS the engine is supposed to be provided with both a hot air intake to combat eventual icing conditions, as well as an ambient cold air intake.

By the way, the SAE norm for testing engine power output, is specified as 27C. Anything above that, and you should expect a power drop.

[Skippydiesel]

Hi Kai,

My engine came with coolant heated rings (anti carb ice) on the engine side of the carburettor /induction system - supposed to be very good.

My last 912 ULS had no provision for carburettor icing - in 12 years and 900 plus hours, never had an icing problem - it had a consistent 10C above OAT.

Back to my perceived problem (OAT +20C hot air in the vicinity of the carburettor air intakes). I have noticed a marked improvement in my engines performance, when OAT is near/below 0C.
I don't expect to have this sort of carb inlet temperature all the time but giving the engine its best operating environment, can only result in it giving me its best performance.

[Kai]

Hi Kai,

My engine came with coolant heated rings (anti carb ice) on the engine side of the carburettor /induction system - supposed to be very good.

My last 912 ULS had no provision for carburettor icing - in 12 years and 900 plus hours, never had an icing problem - it had a consistent 10C above OAT.

Back to my perceived problem (OAT +20C hot air in the vicinity of the carburettor air intakes). I have noticed a marked improvement in my engines performance, when OAT is near/below 0C.
I don't expect to have this sort of carb inlet temperature all the time but giving the engine its best operating environment, can only result in it giving me its best performance.

I can offer no argument!

However, as I read your posting- am I to understand you have not provided any means of ambient cold combustion air supply to the engine?

If this is the case it is my observation that summer will be nocking on the door Down Under within a few months- summer is vastly different there from what we have in the northern hemisphere. I was down in Wagga Wagga in january around year 2000: the themometer registeret 49C! Add to that your 20C under the bonnet: you are truly experimental!

[Bryan Cotton]

Back to my perceived problem (OAT +20C hot air in the vicinity of the carburettor air intakes). I have noticed a marked improvement in my engines performance, when OAT is near/below 0C.
I don't expect to have this sort of carb inlet temperature all the time but giving the engine its best operating environment, can only result in it giving me its best performance.

Which brings us back to the solution some of us AeroVee guys have done. It really works. I've seen a performance increase and others have reported the same.

If possible I would like to avoid significant modification to the cowling (not fond of composite work).

Define significant - you need to do something, and I'd say the mods that I've done and similar mods our peers have done are not that significant to implement.

[Skippydiesel]

"Define significant - you need to do something, and I'd say the mods that I've done and similar mods our peers have done are not that significant to implement."

Significant - In this instance anything more than the minimum necessary to achieve the same/similar goal - cooler air in the upper firewall region/carburettor intakes.

If strategically placed & sized exit air holes will achieve the goal, I would rather do this simple fix, than fabricate an inlet system of doubtful efficacy (in my situaton).

I already have significant forward facing air inlets - I speculate that the problem is an area of slow moving (stail) air, likly relativly high pressure in the rear/upper cowling area. IF cutting the aforementioned holes, in the right location, will facilitate air movement then it is likly this will result in cooler air.

The question are:

Is the idea of exit air holes in the upper rear/upper sides of cowling reasonable?
Where best to cut the holes?
What initial size should the holes be?

[Skippydiesel]

Builderflyer,

Thanks for the comments and the excellent photo.

"I recently added two 2" (50mm) diameter vents above the coils"
Did you start with a smaller diameter hole and move up to 50 mm?
If straight to 50mm - is this a guess or do you have some reason for this size?

"During flight there is no noticeable difference in temps at the back of the engine"
Why do you think this is?

"My disappointment so far is the small amount of heat I feel exiting the vent holes after shutdown..........expected more."

Is your Sonex a tail wheel model? if so could be that the rear vents are lower than the front - air will favour the higher/larger vents.
If a nose wheel - once again, air flow will favour the line of least resistance (front inlets now outlets).

I have an inspection door, in my upper cowling - often open it after landing - usually get a good hot air exit air flow.

[bipbuilder]

To help the engine cool after flight I put a fan under the cowling blowing up through the exit. This reverses the air flow and gets a lot of heat to exit through the cooling inlets. I put the fan on a timer for 30 minutes. And no I'm not worried about shock cooling as it's the same temperature air as when flying and going slower.

[Bryan Cotton]

To help the engine cool after flight I put a fan under the cowling blowing up through the exit. This reverses the air flow and gets a lot of heat to exit through the cooling inlets.

I do the same! I also put the prop vertical when I do this so it doesn't get heated by the air coming out backwards. I put the fan on top of a bucket pointing up.

Out in the wild I point into the wind and put the prop vertical.

[Skippydiesel]

Pointing into the wind, is considered good engine cooling management at any time.

Using a fan sounds like overkill and as you point out not available when away from home - opening a top cowling inspection door, will usually result in excellent convection flow.