Inlet air temperature

[skippydiesel]

We all know that cool air at the carburettor intake delivers more oxygen to the combustion chamber, than warm/hot air,  and greater power output.

 

Many Rotax 9 motivated aircraft deliver warmed, by passing through the radiator(s) and or over the top of the engine, air to the carburettors.

 

Most, if not all, of the above systems are not fitted with carbie heat - I know on my Zephyr, in cruise, the air entering the filters is approximately 10 degrees above ambient,  the benefit a reduced risk of carbie ice but at what engine performance cost?

 

Some aircraft have dedicated (cool) engine air delivery systems, so as to maximise engine performance  - many of these are fitted with  a carbie heat facility.

 

My questions are:

 

• What reduction in engine performance might be expected from, say a 10 degree, above ambient air temperature ??
•  
• Is the added complexity and some weight,  of a dedicated cool air delivery system (with carbie heat) worth the effort/cost ??

[Thruster88]

Your engine will lose 3% of its rated power with a 10°C increase in temperature. From our BAK we know that each 1°C increase raises density altitude 120 feet, at 8000 feet a naturally aspirated piston engine produces about 75% of its sea level rating.

[skippydiesel]

3 hours ago, Thruster88 said:

Your engine will lose 3% of its rated power with a 10°C increase in temperature. From our BAK we know that each 1°C increase raises density altitude 120 feet, at 8000 feet a naturally aspirated piston engine produces about 75% of its sea level rating.

Sooo what of my second question ?

[pluessy]

It depends how you value the 3% of lost power. If it is non-measurable in flight performance: no. If it is making a difference, maybe yes.

[facthunter]

IF you are going to use temp  you can't use %.. Temp starts at minus 273C. The principle is valid. '

    If say you lose 3% of your power due to extra heat. It may have a substantial effect as you may have only had 10% above drag to start with.. In my experience, carb heat applied will substantially affect power so much that a Take off is unlikely to be safely executed  where the carb heat meets the design requirements laid down. Hot ambient and high altitude fields give high density altitudes for the plane and engine and refer to P charts for type. Nev

[Thruster88]

12 hours ago, skippydiesel said:

Sooo what of my second question ?

Only you can answer that one. My opinion is it would be a waste of time and  financial resources whilst increasing weight and complexity.  

[Thruster88]

Just now, facthunter said:

IF you are going to use temp  you can't use %.. Temp starts at minus 273C. The principle is valid. '

    If say you lose 3% of your power due to extra heat. It may have a substantial effect as you may have only had 10% above drag to start with.. In my experience, carb heat applied will substantially affect power so much that a Take off is unlikely to be safely executed  where the carb heat meets the design requirements laid down. Hot ambient and high altitude fields give high density altitudes for the plane and engine and refer to P charts for type. Nev

I only used temperature to determine the approximate loss of inlet air density. 

[facthunter]

ALL Large engines have totally unheated air in normal operations because it would be uneconomical to have it any other way.. Engines LOVE cold air coming in. Nev

Edited March 10, 2021 by facthunter
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[facthunter]

T88. YES I see that NOW.  I must have read it too fast.   Nev

[IBob]

As a side comment here, the Savannah, which has an air box with temperature probe, has a NACA scoop in the cowl top for cold air.

This works fine in normal flight, but not so much in a hard climb, where I was seeing rising airbox temperatures indicating that air was being drawn from under the cowl via the 30mm gap between cowl scoop and airbox inlet.
I was able to correct this with a simple aluminium extension clamped to the airbox inlet (while still leaving a small gap to drain rainwater while parked).

 

[Bruce Tuncks]

Skippy, you got some good replies there. Can you do an experiment? On a nice dry day, arrange a temporary cool air supply to the engine and see if there is a difference in straight and level speed?

On my Jabiru, I can't tell the difference with carb heat on or off, but then I'm not very good at picking stuff like that.

[skippydiesel]

2 hours ago, IBob said:

As a side comment here, the Savannah, which has an air box with temperature probe, has a NACA scoop in the cowl top for cold air.

This works fine in normal flight, but not so much in a hard climb, where I was seeing rising airbox temperatures indicating that air was being drawn from under the cowl via the 30mm gap between cowl scoop and airbox inlet.
I was able to correct this with a simple aluminium extension clamped to the airbox inlet (while still leaving a small gap to drain rainwater while parked).

 

Nice set up ! - 

Do you have carbie heat ?

Can we see some more photos - from each side, cowl on showing air scoop/inlet - inside of cowl to see air delivery set up

[IBob]

30 minutes ago, skippydiesel said:

Nice set up ! - 

Do you have carbie heat ?

Can we see some more photos - from each side, cowl on showing air scoop/inlet - inside of cowl to see air delivery set up

Hi Skippy, yes the Savannah has carbie heat (or more accurately, warm air to the carbs).

 

The rectangular aluminium box you see there is the air box, with air filter.

The inlet tube to the right aligns with the NACA scoop in the cowl, and as you can see in the second pic, I have extended it by attaching a sheet aluminium tube with a hose clip.

The inlet tube to the left, with the scat tube attached in the second pic, is the warm air, collected from behind the engine muffler.

These two inlet tubes have butterfly valves in them, 90deg out on the same pivot, so when one is open the other is closed. You can see them in the first pic.

 

It works fine, with a little attention paid initially to the moving parts to get positive valve action.
It doesn't deliver super hot air and the collection point below does need positioning to maximise that.

Also the temp sensor in the airbox has some thermal mass and is quite slow to respond, which is fine once you get used to it: you get warm air straight away (provided the muffler is hot) but it doesn't show up immediately on the temp gauge.

I don't notice a big difference in engine power with and without.

 

I don't have any pics looking down on the cowl. You can just make out the NACA scoop on the top of the unpainted cowl in the first pic here.
The second pic shows a side view of the insides.

 

[skippydiesel]

Very neat installation IBob.

 

Airbox -Did you make/design/have made up/supplied with kit ?

 

[IBob]

Supplied with kit.

[skippydiesel]

On 11/03/2021 at 10:00 AM, Thruster88 said:

Only you can answer that one. My opinion is it would be a waste of time and  financial resources whilst increasing weight and complexity.  

Fair comment - I guess I have always been interested in having my engines (whatever they power) operating to their full potential. I like to go that little bit extra to achieve this. This is not to be confused with performance enhancing, which almost always reduces service life and reliability. In my youth this sort of objective was known as "Blueprinting" .

 

True there would be a small weight penalty, in the fitting of an air box & ducting.

As for complexity, this would only be in the dual action "butterfly" valve (cold /hot air) and a cable/ button to the instrument panel.

Financial cost - yep! pretty much everything to do with aviation seems to have a disproportionate cost.