technical problems with supercooled diesel fuel in C172 Thielert?
 

Hello, yet in this winter I plan to fly with a Thielert Engine C172 Diesel (IFR). I do not yet have a POH available from the charter company. But I have heard rumours saying that the A/C - when being equipped with the diesel engine - has a minimum fuel system temperature? How big is this number? I heard ... something around 32°F? This is still a quite "warm" temperature! I mean ... in the winter ... in FL 100, which is a totally normal GA - FLT cruising level ... it can easy cool down to -20°F etc.. Is a C172 with Diesel engine equipped in a manner that allows whilst flying with max. endurance speed to maintain a fuel temperature in excess of 32°F? E.g. by tank cell insulation? Or how did the Thielert engineers solve this problem?

Is there a fuel system temperature gauge and / or -indication in the cockpit?

What happens to diesel fuel when being cooled below 32°F? Does it flocculate or / and does the engine stall / cease its service? Any practical experiences / incident / accident reports are welcome. Yours ... lusthansa

Diesel fuel "Waxing" is a well known problem with road vehicles in low temperatures. Normally the fuel suppliers provide "summer" & "winter" grades, to help reduce the effects. Fuel tank heaters and insulated fuel lines are often used in cold climates. When the engine is running the fuel from the injector "spill back" lines is usually warm enough to keep the tank temperature up. I know JET A1 is very similar to road diesel, but quite what Theilert have done to get round this I'm afraid I don't know.

Diesel is a technology, not a fuel
 

The Thielert engines burn Jet fuel using the combustion cycle invented by Mr Diesel. Jet fuel will not "freeze" (actually, thicken) in any environment where operating a C172 is practical, even at the dizzy heights of FL100 (a touch high, incidently, for use as a normal cruising altitude for a 172).

Why is FL100 too high to be used as a "normal" cruising altitude in a C172? I regularly go to 10,000 feet (not just in 172s). While speed is life, altitude is life insurance...

Well it takes ages to acheive FL100 in a C172. Assuming reasonable cruise power settings, TAS is maximized in the 5-6000 ft range, if I recall correctly. I will allow that it gets mighty quiet up that high.

Looking at the original question ..

(a) diesel and avtur are very similar fractions .. as I understand it, avtur functionally is diesel with a bit less oil (plus a lot more attention to refining QA)

(b) with reasonable fuel handling QA avtur shouldn't have too much trouble with water freezing, although that can and has been observed

(c) the fuel is a complex mix of hydrocarbons each with its own freezing point

(d) as the fuel temperature decreases, there comes a point where the highest component hydrocarbon starts to freeze (start of wax formation).

(e) for determining the freezing point in the lab, when (d) occurs, the fuel sample is allowed to warm up until there is no evidence of phase transition .. the temperature at which this occurs is the declared freezing point. Typical test procedures may be reviewed at ASTM D2386/D5901/5972.

(f) different fuels will have different declared freezing points (and will/should be declared in the specification data) but a typical ballpark is around -45C (for one spec with which I am familiar) although individual test samples may well show low freezing temperatures (eg looking at several test sample reports for this spec, the sample freezing temperatures were around -55C).

(g) for later design standard aircraft, the AFM should have some guidance regarding fuel temperature problems.

I have an Industry paper with gives a lot of background if you would like to read it .. around 2Mb file

I am not familiar with the particular STC but I would be VERY wary of using diesel .. in Australia, for instance, I am told that there is no definitive standard for automotive diesel ... I have been caught out a couple of times in my vehicle with "bad" batches of diesel. Using avtur, though, should avoid that sort of problem.

The concern raised by lusthansa ought not to be frowned upon ... just that problem can and has occured with VERY raised eyebrows the resulting pilot response ....

"Something around" -32°F; more like -37°C.

Basic Jet A, which you will use in your engine, has a "freezing" point of around -40°C (or F -- doesn't matter). You are required to keep a fuel system temp of 3°C higher than that to prevent freezing. Jets that fly often at higher altitudes use fuel with icing inhibitors that have freezing points of as low as -50°C. They also have fuel heaters that double as oil coolers -- the warm lube oil warms the fuel and is cooled at the same time.

Intruder

... now, that's interesting .. recently I was researching temperature deltas for this topic without too much success ...

If I may ask ... whence comes your reference to 3 degrees ?

http://www.smartcockpit.com/pdf/plan.../systems/0019/

Scoll down to nearly the bottom of the fuel page for the 737NG.

It says "Minimum inflight fuel tank temperature: 3°C above the freezing point of the fuel being used or -43°C whichever is higher."

I have seen this limitation on other aircraft.

As a side note, I believe that aircraft that use fuel icing inhibitors are typically ones that don't have oil to fuel heat exchange systems such as some bizjets.

My 747 flight handbooks:


It also depends on the place that sells the fuel. For example, if you go to Anchorage, AK, you will get Jet A50, with the -50C freezing point.

I've found that Jet A+ is more often sold outside the US.

Thanks, guys .. that will relate to the design standards publications requirements ... I'll run some more enquiries as there will be some actual or defacto standard around the traps which leads to the 3 degrees.

At this time of year S76 helicopters flying on the North Sea have anti icing chemicals added to the fuel during refuelling (prist or similar). If this was not a problem why do they bother?

IIRC Boeings almost universally have fuel heaters using bleed air. Although that doesn't address tank soakdown temperature, it does ensure fuel reaching the engine pump is warm enough.

A C-172 fuel tank would cool down much faster than a 747 (surface-to-volume ratio and all that...) If fuel temp is such a concern, I'd think some heater (either in-line, or in the tank, such as "spillback" or pump excess flow) would be necessary.

BTW, the pireps on the Thielert 172 discuss much improved altitude performance over the standard ship, mostly due to the constant-speed prop. :D

AFAIK, Boeings use the fuel-oil heat exchanger exclusively for fuel warming. That is the case with 747s, at any rate.

A C-172 is unlikely to be flying at a temp of -37C, so even cold soaked, the fuel would not be at risk of freezing. At that temp (or even 20 deg warmer), the pilot would be more at risk!

Its all in the POH that you'll get to read before you fly hopefully.

The Thielert Engined C172 is approved to use diesel (DIN EN 590), Jet A, Jet A-1 and various other fuels in any mixture ratio.

Each different fuel has its own minimum inflight temperature, if you have a certain ammount of diesel mixed with jet A-1 (ratio in POH) then you treat the fuel as though it is diesel. I don't have access to the POH today, but if you use Jet fuel then the min temp. is very low.

Jet fuel is our primary fuel - its cheaper, no temp issues, quality control should be better, has the anti fugals to stop the black death and is very slighly less smelly and slimy - you'll still end up smelling like a pole cat though. Diesel is a useful "get you home fuel", its available everywhere (off airfields though), and does not require another STC/AN98 compliance/public transport limitations etc.

Another thing to bear in mind is the fuel in the tanks is heated by the returned fuel from the high pressure pump system. The engine draws down a huge amount of fuel, pressurises it to several gazillion psi (heating it up), uses a tiny fraction and returns the rest to the tanks. They are noticeabley warmer after flying. Each tank has a digital temperature gage with a colour coded bar graph on the combined engine instrument gauge.

I've never had a fuel temp issue, I think you'd die from hypothermia first. The heater is rubbish - it uses a heat exchanger from the coolant instead of the exhaust muffler. You won't die of CO poisoning any more, the cold will get you instead.

Cold JetA temps . . .
 

I do not know about the Diesel 172, but I do know there is widespread use of Prist on small-medium turbine-powered aircraft when in cold temps.

The reason - as I understand it - is the JetA like almost all liquids will have some amount of absorbed water. This could even come from humidity.

When the tanks and their contents cool, the suspended water might seperate, and might freeze in some part of the fuel plumbing causing a blockage.

All modern Jet Airliners use Oil/Fuel heat exchangers. There are some
smaller Biz-Jets not having them (e.g. Beech Jet, they already had some
problems with flame-outs due to freezing fuel).
The heat exchangers allow to cool the oil while warming up the fuel.

The main problem with freezing fuel is not the fuel freezing in the tanks
but small amounts of water in the fuel freezing either in fuel lines or
in fuel filters. Therefore the heat exchanger is one of the first components
in the fuel system just before the fuel enters the engine / fuel pump.

In the Air Transat Glider incident this heat exchanger delivered the first
hint of the problem: As the fuel flow through the heat exchanger was
a lot higher than normal due to the fuel leak the oil temperature decreased
significantly on the problem engine ...

The US Type Certificate only allows the use of Jet A-1, Jet, Jet Fuel No. 3, JP-8 or JP-8+100. Diesel is specifically prohibited. See http://www.airweb.faa.gov/Regulatory...E/E00069EN.pdf
Granite City Flyer - Is Diesel a recent approval? Must be if its in the POH.

Brian - Diesel has been approved in europeshire for the 172 since I've been flying/spannering on them, about the last 2 years. All info in the POH.

These service bulletins also:
172
http://www.tae-service.de/doc-downlo...Rev_0_Fuel.pdf

pa28
http://www.tae-service.de/doc-downlo...200%20Fuel.pdf

GCF - Be interested in your comments on the engine both from a pilots view point and an engineer/maintainer.