Fuel & Ambient Temp

[RichTee]

We are flying the Savannah S with the 912 ULS engine. We are operating in high ambient temperatures....at times in excess of 45-47deg C. This can clearly result in a high air-box temp.

 

In these conditions, using MowGas, we have noted that fuel vaporization is a serious risk. This is particularly the case when the plane has been standing on the runway between the morning & afternoon session...so the fuel has warmed up sitting in the wing.

 

We have seen abnormally high fuel flow readings and low fuel pressures even when the AUX fuel-pump is on.

 

I have a few questions:-

 

1. What is MAX air-box temp we should accept ?

 

2. Would it be better to use AvGas ?

 

Any other comments / suggestions would be welcomed.

 

Thanks

 

 

[erd72]

Hi

 

To my opinion air box température should not be a problem.

 

Just you Will havé less power in this conditions in comparison with 20 celcius.

 

Savannah with the fuel return si not sensitive To the Vapor lock, but 100 ll help also in this Way.

 

But leaded petrol is acceptable but not good for the rotax 912.

 

My pressure is close to 5 psi what is your ?

 

Électric pump is used Only in case of issue with the mecanical one.

 

 

[facthunter]

If the carburetter body gets too hot the fuel in the bowl will get bubbles in it and the float buoyancy will be much less. the float may sink and the fuel overflow as a result. This may be causing your high fuel flow readings. It's not a desirable situation and has been known to happen in cars . When they had carburetters. Nev

 

 

[erd72]

In the case you should see a petrol leakage. you can Check engins off , électricité pump on. Rotax applied a BS beginning of this year, a pair of floats havé To be less That 7 g. Mine were concerned.

 

With the fuel excedent return to the left tank, you should not havé bubbles or vapor lock.

 

But Facthunter is right it can appear be an highest temps of the carburator body (usuelle few minutes alter stopping the Warm engine)

 

To prevent this i open the oil flap to help To cool the area upper the exhaust. It is also good for the cdi (They do not accept high temp )

 

 

[old man emu]

Just did a rough calculation for Density Altitude using this data: Airport Altitude 5000, QNH 1024 OAT 45C Rel Hum 10% and this gives a Density Altitude of about 9000 ft. Can this be a cause of poor performance, or are you noticing vapour locks?

 

OME

 

 

[facthunter]

I did work on this problem with BP's chief Fuel technologist some 20 years ago. They were most helpful. Mogas is not just ONE thing . It's a mixture and some of the elements evaporate at quite low ambients. This forms bubbles in the fuel like when you take the cap off a beer. Once that happens the density of the fuel is much reduced and the float may sink allowing the inflow of an unregulated amount of fuel, which has to go somewhere. Would avgas be better?. I would think so, as it's known to resist vapourising better than mogas, and it's controlled product of some consistency. It's probably the worst when the heat soaks from the engine after use. If you can find some shade and have a small fan (with hose) to cool it will help. Improvise a home made evaporative cooler a possiblity? Nev

 

 

[old man emu]

Nev,

 

How about adapting the concept of a Coolgardie Safe?

 

 

https://en.wikipedia.org/wiki/Coolgardie_safe

 

OME

 

 

[old man emu]

I've written elsewhere about the Partial Pressure of a volatile liquid and the effect of temperature on it and therefore its rate of evaportation. The problem that RichTee faces is due to the decrease in Partial Pressure as a result of heating.

 

Evaporative emissions depend mainly on ambient temperature, fuel volatility and fuel system design [8]. As far as the fuel is concerned, Vapour Pressure (Dry Vapour Pressure Equivalent - DVPE) is a key parameter: higher values of DVPE, which is measured at 37.8 °C, mean that the fuel is more volatile or, in other words, that more fuel will evaporate at that temperature. The range of vapour pressure of Avgas is 40 - 50 kPa while that of Mogas is 45 - 100 kPa. This brochure from BP provides a bit of easy-to-read information: http://www.bp.com/content/dam/bp-air/en/Files/Fuel/Air%20BP%20Avgas%20leaflet.pdf

 

When blended with gasoline, ethanol is known to cause an increase of the ethanol/gasoline blend DVPE compared to the base gasoline, up to a point, then decrease it.

 

Here are two research papers that deal with petrol/alcohol mixes.

 

http://publications.jrc.ec.europa.eu/repository/bitstream/JRC36839/6839%20-%20EVAP_Final_Report_final_isbn.pdf

 

OME

 

OJN_203_ Vapor Pressures of Alcohol-Gasoline Blends.pdf

 

OJN_203_ Vapor Pressures of Alcohol-Gasoline Blends.pdf

 

OJN_203_ Vapor Pressures of Alcohol-Gasoline Blends.pdf

[aj_richo]

So what you're saying is, don't fly when the ambient temperature is 45-47C, especially after the plane has been standing on the runway between the morning & afternoon for who knows how long....?

 

 

[old man emu]

No. I'm saying be aware that OAT not only affects the amount of Lift generated by a wing, and the power generated by an engine, but it also affects the performance of a carburetor and the whole fuel system.

 

Also, it's an argument against adding alcohols to fuels used for aviation, and support of avgas (being a much more constant product)

 

OME

 

 

[RichTee]

HiTo my opinion air box température should not be a problem.

Just you Will havé less power in this conditions in comparison with 20 celcius.

 

Savannah with the fuel return si not sensitive To the Vapor lock, but 100 ll help also in this Way.

 

But leaded petrol is acceptable but not good for the rotax 912.

 

My pressure is close to 5 psi what is your ?

 

Électric pump is used Only in case of issue with the mecanical one.

Hi Erd72,

 

Thanks for your response... I'm very alive to the reduced performance due to increased density altitude (less oxygen for the engine, less air density for the prop to bite, and less air density for the wings) but my main concern here is the possibility of vapor issues due to v hot ambient conditions. I normally don't fly with the aux pump on however when doing low work I recon its best to have both on. We don't fly in the heat of the day (10am to 4pm) but even at 4pm the ambient can be v high.

 

Thanks,

 

Rich

 

 

[RichTee]

Just did a rough calculation for Density Altitude using this data: Airport Altitude 5000, QNH 1024 OAT 45C Rel Hum 10% and this gives a Density Altitude of about 9000 ft. Can this be a cause of poor performance, or are you noticing vapour locks?OME

Hi Old Man Emu,

 

Thanks for the info....will download those PDFs and have a read.

 

My main concern (in this thread) is the fuel system with high ambient temps. The performance was just fine (operating from a low strip at 1,500 agl...so density alt was fine)....but a vapor issue at 200-300 ft agl would not be fun !!!

 

I like your Coolgardie Safe....was discussing this am...and we recon we will fly the morning session...leave the airie with low fuel in her...and only fill the tanks again for the afternoon session just before flying...and ensure its from a cool source. That should help.

 

We are also looking to switch to AvGas during the hotter months.

 

Cheers,

 

Rich

 

 

[Ultralights]

High temps wont worry the Savannah with Mogas, as they have a constant circulating fuel system, vapour lock is unheard of. i have flown my Savannah at 44deg Max, no issues with fuel, but at those temps, density height plays havoc with all aspects of performance.. Best air box temps for most effective fuel vapourization is 21DegC.