Fuel Temp

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Colin
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Fuel Temp

Post by Colin »

When I have been at 15.5k for an hour and the outside temperature is 15F, and then I start the pump to dump my 26 gallons from the aux tanks into the main tanks, why doesn't the fuel temperature change? Shouldn't the aux fuel be really, really cold?

And does anyone in their Twinstar use Prist?
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Re: Fuel Temp

Post by CFIDave »

When you transfer fuel from the aux tanks to the mains, the rate of transfer is only about 1 gallon/minute. Most of the time when I do this, I'm down to about 17 gallons of fuel in the main tanks, minimizing the effect of dilution by colder aux tank fuel. Furthermore, hot excess fuel (after being compressed by the engine's high pressure fuel pump) continues to be re-circulated back into the main tanks, and the main tanks themselves are being subjected to the same 15F outside air/wing temperature as the aux tanks.

The result may be that all of these factors combined keeps the main fuel temperature from dropping.

Often when I purchase JetA I have no choice but to purchase it with Prist already added. That's OK, since Prist has no adverse effect on diesel engine operation.
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Colin
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Re: Fuel Temp

Post by Colin »

Got it. That all makes sense.
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Rich
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Re: Fuel Temp

Post by Rich »

Fuel may be heated by passing through the fuel system and back, but not by itself being compressed - it's a liquid and therefore won't be compressed at all. And while a gas will be heated by compression, when it's allowed to expand again, it cools back down.
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Re: Fuel Temp

Post by carym »

CFIDave wrote:
Often when I purchase JetA I have no choice but to purchase it with Prist already added. That's OK, since Prist has no adverse effect on diesel engine operation.
Dave, I know that you know this, but I think it best for JetA pilots to be aware (as there has been many accidents caused by the lack of this knowledge). Prist needs to be at the correct concentration to prevent water from condensing or turning t ice in the cold temps we fly at. If you use Prist, it is recommended that you always use Prist so that the concentration of Prist in the fuel is optimal. Otherwise, the risk of water precipitation and icing in the fuel system becomes a distinct possibility. I am unaware of this happening to any DA42, but it has certainly happened to more than one turboprop.
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Re: Fuel Temp

Post by CFIDave »

Unlike with jet or turboprop engines, Austro or Thielert/Continental diesel aircraft engines running on JetA don't need to use Prist when flying in very cold conditions -- because a substantial percentage of the fuel is automatically heated when it flows through the engine's high pressure common rail and then re-circulated back into the fuel tank.

However, if the fueler does add Prist to the JetA put in the tanks, it won't hurt anything on these diesel engines. So the bottom line is that DA40 NG/DA42/DA62 pilots really don't have to track the Prist concentration, if it exists in the fuel.
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Re: Fuel Temp

Post by carym »

CFIDave wrote:Unlike with jet or turboprop engines, Austro or Thielert/Continental diesel aircraft engines running on JetA don't need to use Prist when flying in very cold conditions -- because a substantial percentage of the fuel is automatically heated when it flows through the engine's high pressure common rail and then re-circulated back into the fuel tank.

However, if the fueler does add Prist to the JetA put in the tanks, it won't hurt anything on these diesel engines. So the bottom line is that DA40 NG/DA42/DA62 pilots really don't have to track the Prist concentration, if it exists in the fuel.
My concern has always been with the aux tanks, as the fuel in there is not heated. As you know, we had several problems with the check valve in these tanks when they were first delivered, and I got concerned that with inadequate Prist in the aux tanks there might be icing around that check valve that will prevent delivery of fuel from the aux tank to the main tank. Are you saying that this can not happen happen?
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Re: Fuel Temp

Post by CFIDave »

carym wrote:My concern has always been with the aux tanks, as the fuel in there is not heated. As you know, we had several problems with the check valve in these tanks when they were first delivered, and I got concerned that with inadequate Prist in the aux tanks there might be icing around that check valve that will prevent delivery of fuel from the aux tank to the main tank. Are you saying that this can not happen happen?
I've not heard of any problems with fuel transfer from the aux tanks due to the cold or fuel icing. (I did have an aux tank check valve issue that was fixed on my DA42, but it wasn't related to temperature.) There's nothing in the Austro or Diamond manuals recommending Prist be added for cold weather operation. But you're the one who flew your DA42 all around Alaska, so I'll defer to your experience. :)
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Re: Fuel Temp

Post by dgger »

carym wrote:My concern has always been with the aux tanks, as the fuel in there is not heated. As you know, we had several problems with the check valve in these tanks when they were first delivered, and I got concerned that with inadequate Prist in the aux tanks there might be icing around that check valve that will prevent delivery of fuel from the aux tank to the main tank. Are you saying that this can not happen happen?
I have a single data point with pre-mix, but one that I found rather extreme. Before starting to warm up the engines at -35C we took premixed fuel. It was probably still -32C when we started-up about 1:20h later. Needsless to say we had planned our leg to be able to return should the fuel transfer fail, but things went smoothly even in the cold.
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