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skippydiesel

Cowl Air Temperatures

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Obvious to all regular readers on this Forum - I am an addicted fiddler. Often my ideas work/have a benefit. Sometimes I screw up badly. Sooo I seek your council on the following:

 

Background -

 

A few months back I noticed a fellow aviators aircraft (very similar to mine) is fitted with a carburettor air inlet temperature read out.

 

Intrigued, I purchased one of those stand alone digital fridge/room temperate gauges (same thing can be had as an indoor/ outdoor thermometer). Did a temporary fit (velcro, electricians tape & zip tied) the gauge to my Zephyr. Made sure the little remote thermocouple/sensor was placed close to the air filter of left carbi.

 

Went flying (several times) and have noted in flight air temps at carbi to be  plus 10 to 20 degrees C above ambient (OAT). Now that is why I have never experienced carbi ice. Also after shut down temps can rise alarmingly (this a knew from having fuel vaporisation/lock issues) as high as 60 + C  -something I have managed by opening my oil/coolant inspection hatch to encourage exhausting of hot air.

 

My Zephyr has a front mounted radiator coolant vent (works very well) and two round general air inlets. Air exhaust is via a large  fixed "belly/lower cowl" vent. Oil, head & coolant temps are all well managed.

 

The "Itch" -

 

As happens, this information has started to niggle at me  - Can I "improve" the situation by bring down the carbi air inlet/cowling  temp?

 

What if any benefits will result  - better combustion, longer engine component life??????

 

Down side - if successful more likely to get carbi icing (my aircraft is not fitted with carbi heat) - will have to expend  $$ (minimal), may have negative impact on existing air flow (direction/rate), create more drag, change/alter/spoil the look of my aircraft by fitting additional air vents.

 

This is what I have come up with (to date):

 

Bunnings Aerospace do a line of neat little pressed aluminium vents (pics not to scale left one 100 x 75, right 100 x 150) Each louvre has an approximate 48 mm x  3 mm opening

 

a79fb6e7-2bd2-4314-89cf-3d81d98e2cb0.jpg80baf781-27f8-4119-b87a-f36f7ba8e2f5.jpg

 

Have been contemplating fitting the larger of the two at centre rear of top cowling, in line with carburettor air filters

 

OR

 

The smaller of the two x 2 one above each carburettor.

 

I think I would want to try and preserve much of the current engine compartment downward air flow direction as possible, so I would face the vent (s) forward in anticipation/hope of a forced down flow of cool air over the carbi inlets. After engine shut down the air flow would reverse with hot air exiting through the top went (s).

 

What think you all ??????

 

 

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You may gain a bit but a proper sealed airbox is really the only effective solution...

 

If you look at "marine vents", you will have alot more choice...

 

https://www.ebay.com.au/sch/i.html?_from=R40&_trksid=p2323012.m570.l1313.TR2.TRC0.A0.H0.Xmarine+vent.TRS0&_nkw=marine+vent&_sacat=0

 

 

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 You lose power with hotter intake air but it vapourises fuel better. Lots of engines release a lot of heat after shutdown because the hott(er) parts have a conductive  path to the colder ones. and you usually have a stinking hot muffler beneath the engine as well.. Most aircraft have the hot air coming out of the bottom of the cowl. This doesn't help at all as hot air rises. Park  and run up into wind always if possible.  Nev

 

 

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You may gain a bit but a proper sealed airbox is really the only effective solution...

 

If you look at "marine vents", you will have alot more choice...

 

https://www.ebay.com.au/sch/i.html?_from=R40&_trksid=p2323012.m570.l1313.TR2.TRC0.A0.H0.Xmarine+vent.TRS0&_nkw=marine+vent&_sacat=0

 

In the immortal words of one of our top leaders "please explain" "proper sealed airbox"

 

Had a look at your kindly supplied web page - thanks for that - more ideas.

 

 

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 You lose power with hotter intake air but it vapourises fuel better. Lots of engines release a lot of heat after shutdown because the hott(er) parts have a conductive  path to the colder ones. and you usually have a stinking hot muffler beneath the engine as well.. Most aircraft have the hot air coming out of the bottom of the cowl. This doesn't help at all as hot air rises. Park  and run up into wind always if possible.  Nev

 

 

 

Thanks Nev -

 

When I first acquired my Zepher I quickly adopted a number of heat reduction/mitigating strategies however there is no getting away from that build up  of hot air once the engine is shut down. I taxi slowly, turn into wind on stopping and slowly reduce power and finally  (after shut down) open my small inspection hatch - this  all helps to keep that hot air build up to a minimum. 

 

My main interest however is any real world/significant benefits that I may expect from reducing the engine compartment in flight temperatures or at least delivering cool air to the carbi's  - what think you ??

 

 

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My main interest however is any real world/significant benefits that I may expect from reducing the engine compartment in flight temperatures or at least delivering cool air to the carbi's  - what think you ??

 

Engines lose about 3% power for each 1000' alt increase. From our BAK  I we know that  1 degrees C is 120' of density altitude, so your +10-20 C inlet temp would result in a 3-7 HP  loss.   

 

 

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Engines lose about 3% power for each 1000' alt increase. From our BAK  I we know that  1 degrees C is 120' of density altitude, so your +10-20 C inlet temp would result in a 3-7 HP  loss.   

 

Yeah ! that's what I thought - very similar to putting on carbi heat (from my GA days). 

 

 

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It appears you get about the same result using the "perfect gas law"

 

Air Density = P x Ma / (Ru x T),

 

where density is in pounds/cu ft, P = pressure in psf, Ma= Molecular weight of air (28.9),  Ru is universal gas constant, (1545) and T is absolute temperature (this is ambient temperature Fahrenheit plus 460).   (You have to forgive me, I work in old British units since I'm in the US.  Can convert if anybody cares, but then I'd have to look stuff up)  

 

Let's assume your OAT is 70F ( 21C), and your Carbi temperature is 106F (41C), which is 20C higher.   Since you only are interested in the air density change due to temperature change, you can forget most of the constants, and just make a ratio of the absolute temperature change.   So, 

 

Density change = density1/density2 =  (T1+460)/(T2+460)  = (70+460) / (106+460) = .936  (or a 6.4% reduction)

 

So theoretically, the 20C (36F) rise cuts air density (and engine power) by about 6%.   I don't know your engine hp, but if it's around 100 hp, that's about the same as by the rule of thumb.

 

 

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Hi Langred - several ways to "skin a cat".

 

Its pretty obvious that there is some loss of combustion efficiency/hp, however what I am interested in is weighing up the "pros & cons" of attempting to change this. 

 

The initial question is about the merit(s) of retro fitting some form of air vent with the intention of supplying cooler air to the carburettors.  So have I (you) considered all the potential impacts -

 

Pros::

 

Improved combustion efficiency - leads to better performance and/or reduced fuel consummation.

 

Cons:

 

As I dont have a carbi heat  system fitted - may increase chance of carbi ice - leads to heart failure and possible damage to aircraft.

 

May change internal engine compartment air flow for the worse - overheating and engine damage.

 

May increase drag - lower performance/increased fuel consumption

 

Downgrade the appearance of my Zephyr

 

Small expenditure 

 

Related:

 

Vent orientation

 

Face forward -

 

If located in the right location (how to determine this is a topic in itself) high pressure cool/ambient air will be directed onto the carbi air inlet AND existing downward direction of internal air flow will be preserved.

 

Flight through rain (not a good thing in this class of aircraft) may result in significant water entry to engine compartment

 

Face back - 

 

May result in worm air being exhausted drawing in cooling air from existing forward cowling inlets leading to general cooling of engine compartment. May also disrupt existing air flow over oil cooler etc leading to overheating.

 

In the event of a fluid leak  fluid more likely to exit top of cowl & spread over canopy obscuring vision, although this might be the least of my worries.

 

 

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 Your MASS airflow will give the % of power loss. . Universal gas laws cover or density altitude.  In any internal combustion engine the temp change during the cycle is related to efficiency. You motor will definitely go better with colder air. Other considerations  come into it too Dust, water, impact ice.. Most radials have a big intake scoop at the top of the engine facing forward. Most engines can handle a fair bit of water at high power and some can have water/methanol as a boost for take off. but YOU shouldn't be THERE in your trusty U/L.

 

   Carb heat often enrichens the engine and also restricts the airflow so may seriously reduce the power available. To be effective it needs to be on when the power is on  say cruise and used as an ALL or "not at all" thing.  If you need it you use all of it or you may make the situation worse. At full power it may cause detonation in a susceptible engine  Nev

 

 

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All good stuff Nev but what is you opinion regarding my latest fantasy ??? You know Go or No Go. Likely/unlike to make a discernible change/improvement. Leave well alone.  Why fix a functioning, already efficient aircraft. Etc etc. 

 

 

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I put an air scoop on the Hummel to cool the magnito, forward facing , and when the prop wash stops  it can vent upwards.

 

Little bit of tin work to put upturned vent with baffle riveted under to direct the air were needed, only small.

 

Made BIG difference to temp's before & after, as no airflow near Mag, before. (mag on top, rear of motor).

 

spacesailor 

 

 

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You need to make the distinction between maximum power and maximum efficiency. Maximum power is what we need to takeoff to get off the ground quickly and away from the tress at the end of the runway. In that situation cold air to the engine increases the power output.

 

Once we are in cruise we no longer need maximum power and we throttle back. Now we are concerned with maximum efficiency. Warmer intake air increases the engines efficiency, lowering the the required fuel burn for a given amount of power.  Some of the very long distance flight done in the long Eze's were done with a little bit of carb heat applied in the cruise, not to ward of icing but to improve efficiency.

 

 

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You need to make the distinction between maximum power and maximum efficiency. Maximum power is what we need to takeoff to get off the ground quickly and away from the tress at the end of the runway. In that situation cold air to the engine increases the power output.

 

Once we are in cruise we no longer need maximum power and we throttle back. Now we are concerned with maximum efficiency. Warmer intake air increases the engines efficiency, lowering the the required fuel burn for a given amount of power.  Some of the very long distance flight done in the long Eze's were done with a little bit of carb heat applied in the cruise, not to ward of icing but to improve efficiency.

 

An interesting observation. I have always understood cold air is denser/carries more Oxygen for a given volume entering the combustion chamber. More  O2 allows for a more effective/complete fuel burn, more energy released to turn over the Rotax. Now you tell me that is not the whole story and I might actually be benefiting from the warm air under the cowling.

 

 

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I had never heard this. I know a motorbike runs best with cold air, thought any engine would be the same.

 

 

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Must test this.

 

Run motor on it's two litre's of fuel, on Hot day 40c +.

 

Run motor on same fuel (98 mogas) on the cold day, to see if revs. on tacho change & duration of timed endurance changes.

 

Rain will postpone this test. (raining now )

 

spacesailor

 

 

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  Some of the very long distance flight done in the long Eze's were done with a little bit of carb heat applied in the cruise, not to ward of icing but to improve efficiency.

 

I wonder why none of the car manufacturers are adopting this, they all seem to have cold air intakes on EFI engines. Maybe their engineers are not very smart.

 

 

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I had never heard this. I know a motorbike runs best with cold air, thought any engine would be the same.

 

It runs best from a maximum power out point of view on a cold day, but the ammount of fuel required to generate that power is high. It is more power that makes motorbike or car feel better. To know that the fuel consumption is higher you have to do detailed measurements and so it is harder to determine the influence of higher intake temperature on fuel consumption.

 

Search for brake specific fuel consumption(BSFC).

 

I wonder why none of the car manufacturers are adopting this, they all seem to have cold air intakes on EFI engines. Maybe their engineers are not very smart.

 

Or perhaps the EFI means that the mixing of the fuel and air is better reducing the benefits of having a system of heating the intake air.

 

 

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Must test this.

 

Run motor on it's two litre's of fuel, on Hot day 40c +.

 

Run motor on same fuel (98 mogas) on the cold day, to see if revs. on tacho change & duration of timed endurance changes.

 

Rain will postpone this test. (raining now )

 

spacesailor

 

To do the test well you need to somehow have the same output power from the engine under the two conditions. The hotter test will need the throttle to be slightly more open to produce the same power.

 

 

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I wonder if the fuel distribution between cylinders improves with temperature.

 

My best guess at the uneven EGT's on my Jab 2200 was that droplets of liquid fuel were overshooting the take-offs to cylinders 3 and 4, hitting the end of the short fuel plenum, and  helping to make cylinders 1 and 2 richer.

 

If this is true, the EGT variation will be worse on colder days, because the fuel vaporization will be less and the droplets will be more. This is something I will now check out, bugger I should have thought of it before.  

 

 

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To do the test well you need to somehow have the same output power from the engine under the two conditions. The hotter test will need the throttle to be slightly more open to produce the same power.

 

The prop will require less power on a hot day for a fixed rpm. This why there is only one static rpm in the POH and no allowance for altitude or density altitude. Orange NSW on a 35 C day, DA 6000' would still give me 2450rpm but the power is way down compared to sea level.

 

Any efficiency tests would have to be done on a good dyno to measure the power produced.

 

 

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Skippy, those vents in the pics look like they have shallow projections into the airflow. If the entire thing sits within the boundary layer, then not much ram air will come in.

 

A guy at Gawler once tried a forward-facing pitot tube sited 6mm away from the wing surface of his glider, on the upper surface not far forward of the trailing edge.

 

He had this connected to the static, through an electronic low-speed ASI. The idea was that the higher the ASI reading, the less the drag .

 

Well he got no ASI reading at all. The air where he put the probe was well within the boundary layer. 

 

 

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Some of this makes sense to me - Cold air, more O2, more fuel burnt, more power generated. Not so sure the converse is entirely true. If you are after max power I can see that the engines ability to burn fuel when breathing cold air is going to be greater than hot air - so yes, at max  throttle opening more fuel will be consumed per hour (higher fuel consumption). However at a power setting less than max, it seems to me that the cold air should deliver less /better fuel consumption for a given power setting as the  fuel burn should be more complete/effective.

 

 

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Skippy, those vents in the pics look like they have shallow projections into the airflow. If the entire thing sits within the boundary layer, then not much ram air will come in.

 

A guy at Gawler once tried a forward-facing pitot tube sited 6mm away from the wing surface of his glider, on the upper surface not far forward of the trailing edge.

 

He had this connected to the static, through an electronic low-speed ASI. The idea was that the higher the ASI reading, the less the drag .

 

Well he got no ASI reading at all. The air where he put the probe was well within the boundary layer. 

 

Yes Bruce - this is one of my concerns/considerations - will the work and disfigurement of my cowling result in any significant benefit. True I could fit a taller/larger scoop (to be sure to catch high speed air) but then there will be issues of drag and even higher impact on my little Z's good looks.

 

I must confess - after the initial rush of blood to to the vacant space between my ears, my passion for this is cooling. It may be that there would be little or no discernible benefit.

 

 

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  The engines  efficiency is greater when the temp change is maximum. That's provable mathematically and applies to all Carnot cycle engines. The hotter intake air may well improve your vapourisation so you could perhaps lean it out a bit if you could (but usually can't.) Applying carb heat often enrichens the fuel air ratio as it is an extra restriction up stream of the carburetter. It also reduces your volumetric efficiency so for a given HP output you would need more revs and have more pumping  and frictional losses. which all obey the squared law. (Double revs  4 times the friction).  Nev

 

 

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