Cowl Air Temperatures

[spacesailor]

WELL,

 

"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."

 

Far to complimented for my test.:  NO Dyno, !.

 

Was going to see if the measured amount of fuel (98 ron mogas), lasted longer on a hot or cold day, and if any tacho increase.

 

Nice & cool at 0800 hrs, 25c, but soon up to 35 c by 1130 hrs.

 

Also had granddaughter with Great-grand-son this morning then another daughter coming this arvo.

 

 

[skippydiesel]

  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

So you disagree with Nobody (nom de plume above)

 

 

[Old Koreelah]

... 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.

You have them too, Skippy?

 

I've had a few too many brainwaves, particularly regarding my carby intake system. This morning I finally got to test the latest version, which sucks air thru a 60mm hole pretty much back where I had it originally.

 

In the cool, smooth 25C air after last night's storm, the engine purred away beautifully, but the air temp upstream of the carby still crept up to 41C. That's mobs hotter than my original system. CHTs and EFT spreads are acceptable, but I guess It's running richer than need be.

 

Why so hot, after I relocated the intake duct from the corner of the engine bay? Perhaps the air is getting warmed as it passes the exhaust pipe and hot cowling. 

 

 

[Yenn]

A sealed air box is just a flow path from outside air to the carbie that keeps the air separate from other air under the cowl, basicly a tube with inbuilt filter if required from outside to the carbie throat, of injector body if that is what you use.

 

The power of an internal combustion engine is related to the increase in temperature of the gases passing through. If the air is hot to start with it will be equally hotter afterwards to get the same power.

 

 

[onetrack]

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.

Thruster - The cold air intake on cars today is more usually an aftermarket addition, as most car manufacturers today have no controls on air intake temperatures.

 

 

[Thruster88]

Thruster - The cold air intake on cars today is more usually an aftermarket addition, as most car manufacturers today have no controls on air intake temperatures.

I mean the factory setups drawing from the inner guard  area compared to the HQ Holden etc sucking out of the engine bay.

 

 

[spacesailor]

Or the after-market "snorkel" to get that air-intake out of the dust off the dirt roads,

 

especially when in a convoy and said dust is as high as the windows.

 

spacesailor

 

 

[skippydiesel]

Only city people drive on dirt roads in close "convoy" - much cheaper to let leading vehicles have sufficient distance to allow dust plume to move off road.

 

Another tip for the city folk - when on dusty, road drive with windows up, full fan  with outside air selected (air con optional). Vehicle approaching temporarily select recycle air until  past dust.

 

 

[facthunter]

 Yes, get a dose of dust in your motor and it won't go for long, or be any use after it stops.  Nev

 

 

[Nobody]

  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

Yes the engines theoretical maximum efficiency is the diminished by a hotter intake but that is only past of the story. The theoretical efficiency of a heat engine with a hot temperature of 1000k and a cold temp of 300k is 70%, Our aircraft engines are only about 30% efficient showing that things like friction, heal loss to the cylinder wall and pumping losses are very significant. Increasing the hot temperature by 30 degrees celcius to 330k would drop the theoretical maximum efficiency to 67% but the in many instances the real achieved efficiency is higher.

 

The attached table is from the Cessna 182S POH, the yellow highlight is mine. This is for 2000 feet pressure altitude. From the table at 2400 RPM and 21 inches of MP the power output is 67% and the fuel flow is 11.8 GPH when the temperature is 20 degrees below standard. If the temperature is 20 degrees above standard and the engine setting remain the same then the power drops to 63% and the fuel flow to 11.1 GPH. If we increase the MP to 22 inches to produce the same power as when the temperature was cooler the fuel flow has dropped to 11.7 GPH.  This table suffers a little from rounding, it is only for a whole inch of MP and so sometimes there is no change (and for one or two instances the pattern isn't born out) but generally this shows that higher temperatures result in less power but more efficiency.

 

Now 0.1 GPH isn't a lot but for the long distance record breakers that can mean the difference between claiming the record or having to divert and make another attempt.

 

 

 

[Old Koreelah]

If anyone is interested, in the last couple of days I made a few test flights with different carb air intake configurations. 

 

None made much difference to the high intake temperature readings I'd been concerned about.

 

As someone suggested a few weeks back, turns out the probe was giving false readings. Being mounted right next to the exit tube, it was getting conducted heat as the plywood air filter box gradually heated up.

 

 The lesson here is how much heat is conducted from the engine bay thru the firewall, and how hot even insulators like wood can get.

 

Now that the sensor is hanging in free air inside the air filter box, the temp readings are barely above ambient.

 

 

[Kiwi]

Also regarding the original post.....

 

My Ran S6S has a top mounted radiator with louvers in the top cowl,  below is the carbs then the exhaust. 

 

 

The radiator is twice the size of the standard Rotax one but my CHT's are higher than a Savannah (as a example) so I don't believe that the location of the louvers on the top cowl work the best.

 

I also have had issues with the carb vent location due to airflow hitting them from different locations (disturbed /turbulent airflow)  which was one of the reasons I fitted a airbox.

 

Kiwi

 

 

[Bruce Tuncks]

Don't stop trying things Old K and Kiwi. Experimentation is hard, but its all we have to progress with.

 

The story about Jabiru oil cooling comes to mind, where the initial probe position wrongly gave the message that an oil cooler was not necessary.

 

 

[facthunter]

 I'm not surprised that doesn't work well,  It's not in  a free air flow,  has an adverse position for slow flight and is acting against the natural  "hot air"convection effect.. People shouldn't have to do a lot of experimenting if more information was readily accessible. Illustrations and explanations of best practice would/should be available. (somewhere). Nev

 

 

[onetrack]

The louvres are facing the wrong way. If they were reversed, the airflow over the cowling would draw hot air out and upwards, utilising the venturi effect, aided by the convection effect,

 

 

[skippydiesel]

The louvres are facing the wrong way. If they were reversed, the airflow over the cowling would draw hot air out and upwards, utilising the venturi effect, aided by the convection effect,

You may get some "venturi effect"  but other than after engine shut down there will be little if any convection.

 

Most likely the cowling will be pressurised, to some degree, by prop wash/forward motion, forcing air through/over engine components & out of whatever exit apertures are available.

 

Movement  (speed/volume/direction) of air through & around the engine bay & engine components is difficult to predict without the aid of wind tunnels , pressure, speed & direction monitoring equipment.

 

Just making an entry/exit smaller/larger, adding another, facing forward/back can have surprisingly counter intuitive results.

 

Very well engineered cooling/air delivery systems are often surprisingly small (entry/exit) check out VH-SGS http://www.worldrecordplane.com/aircraft-development-story

 

Note: Despite many sales/manufactures claims to high speed Rotax 912 powered aircraft (often very sexy/speedy looking) few have been subject to third party review /assessment like SGS - view all other claims with extreme scepticism.

 

 

[Paul davenport]

Slightly off topic but can anyone tell me the difference between gen 2 heads and ge3 fin  wise . My mates gen 3 roller rocker runs cooler without any mods than mine did and I noticed his heads have more fins where the oil supply tube would be on the gen 2 this cavity is filled with fins

 

 

[Yenn]

I assume you have a Rotax engine with water cooled heads and air cooled cylinders. The heat under the cowl comes from the hot air which has cooled the cylinders. That air is then going through the carries. If you cowl the whole engine so that cooling air is ejected away from the carbide intake, then you should have better running. Alternatively draw air from on external high pressure area and again, better performance. That would probably be just in front of the windscreen in the top of the cowl, similar to a cars air intake location under the windscreen wipers.