Jet-A for Diesel AC engines a no go?

ScottM

ScottM

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Aeronews is reporting today that Exxon/Mobile sent out a letter stating a whole list of concerns about the use of Jet-A in Diesel powered aircraft.

LINK

The technical reasons they cited are:
  • Ignition Quality- "The fact that the minimum cetane required to establish airworthiness has not been determined, in combination with the fact that cetane is not measured as part of the jet fuel specification, means that ExxonMobil cannot guarantee the ignition performance of the jet fuel it supplies and cannot know if the aircraft will be airworthy after fueling."
  • Freezing Point- "Unlike turbine powered aircraft, piston powered aircraft do not reach speeds that cause heating of the fuel in the wing due to friction caused by airflow. It is therefore possible that an aircraft powered by a diesel engine could reach altitudes where the fuel would begin to freeze in flight."
  • Lubricity- "Diesel engines rely on the fuel to lubricate key components of the fuel injection system. ExxonMobil Aviation cannot guarantee that the lubricity performance of the jet fuel it supplies will meet the requirements of aviation diesel engines."
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More bad news for the diesel aircraft side of the house. I have been watching the diesel issue for a while to see if I should upgrade the plane to a diesel power plant when the current piston/gas unit I have now needs to be overhauled. I hope they get this all figured out soon as I think it is going to get harder and harder to get 100LL
 
I have often wondered how kerosene would be a suitable replacement for diesel...

I think the military runs one type of fuel...JP-8 or soemthing like that, in most of their vehicles...Hmmmmm.
 
Um - the engines in question are certified to burn Jet-A. The fact that they are Diesel engines refers to the ignition process, not specifically the fuel type.
 
I was going to suggest that Exxon wanted to dodge any and all liability associated with G.A. "Your honor, Exxon did not approve or permit the use of our fuels in any diesel aircraft including the accident airplane, therefore the deceased pilot assumed all liability for his actions"

I did notice this at the bottom:

"Customers insisting upon purchasing jet fuel for their diesel engine aircraft must sign an indemnity agreement before fueling."
 
I think this is the case of a lawyer trying to cover some bases.

If the engine manufacturer and the FAA have approved the engine to operate using Jet-A, then as long as the Exxon Jet-A fuel meets specifications, they should be safe. But given that Thielert is out of business, and it's hard to sue the FAA, Exxon may be saying "don't come looking at us!".

I don't believe you can legally run standard auto diesel in these certified aircraft engines - does anyone know differently?
 
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Greg Bockelman said:
If your airplane is an experimental, could you run auto diesel then?
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I'm sure you could. The specs between Jet A, road diesel and even home heating oil aren't that different, mainly just sulpher content. The major difference is in the quality control -- to ensure the fuel meets specs, combat contaminants, etc.
 
The freezing point argument made me laugh.....is there really that much skin friction to combat the temps at fl450? This just sonds like some lawyer at EXXON got scared about liability concerns regarding GA airplanes.
 
vontresc said:
The freezing point argument made me laugh.....is there really that much skin friction to combat the temps at fl450? This just sonds like some lawyer at EXXON got scared about liability concerns regarding GA airplanes.
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Yeah, I thought that was pretty stupid too. If you're worried about ice in the fuel you could use Prist or another icing inhibitor. At many FBOs it's premixed anyway.
 
vontresc said:
The freezing point argument made me laugh.....is there really that much skin friction to combat the temps at fl450? This just sonds like some lawyer at EXXON got scared about liability concerns regarding GA airplanes.
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Actually, for jets, there is. They add things like PRIST to further lower the freezing temp, and some airplanes have tank heaters, but the temps on the wings at .7x mach is significant.
 
vontresc said:
The freezing point argument made me laugh.....is there really that much skin friction to combat the temps at fl450?
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Actually, Pete, there is. One of the things we have in our arsenal to fight low fuel temps is to increase speed. I don't recall exactly, but I think .01 mach increase in speed is worth something like half a degree, or one degree increase in fuel temp.

It isn't much, but it IS measurable.

Having said that, is a diesel powered recip capable of that kind of altitude? I don't think so.
Is a diesel powered recip capable of that kind of speed? I don't think so.

Bottom line here is Exxon is smoking something funny if they think those are issues applicable to the Diesel Recip.
 
TMetzinger said:
Actually, for jets, there is. They add things like PRIST to further lower the freezing temp, and some airplanes have tank heaters, but the temps on the wings at .7x mach is significant.
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I'd sure like to know how much. It's not like the wings glow in the dark.:rofl:
Prist is required in the Learjet , but that's operating at minus 54 degrees or so. I can't imagine a piston diesel getting high enough to see those kinds of temps.
 
Thanks Greg, my guess is that some lawyer looked at the accident statistics of the 100ll market vs the traditional Jet A market, and they didn't like the potential liability exposure.
 
Lance F said:
I'd sure like to know how much. It's not like the wings glow in the dark.:rofl:
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It's not like going fast stops ice from forming either if you <cough> forget to turn on the bleed air in advance. Also, one problem with coming down from altitude into a humid environment is that you get frost on the wings even if the surface temperature is above freezing because the fuel in the wings is cold.
 
I agree that the altitudes the piston airplanes operate on generally won't cause the fuel to freeze (pilot and pax would freeze first!).

The other issues should have already been addressed in the certification of the engines on Jet-A.

Frankly, I wonder if the guy behind this consulted with corporate counsel (not just his department lawyer). I wonder if there's a law that precludes this indemnification or renders it moot? Otherwise, they should do the same thing with 100LL.
 
Lance F said:
I'd sure like to know how much. It's not like the wings glow in the dark.:rofl:
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Well, glowing in the dark is kind of an over statement.:rofl: Yeah, I noticed the smiley.

It isn't that there is tremendous amount of heat created, it is just that there is enough that the wing is X degrees above the ambient air temp. It may not be much, but it is measurable. And increasing speed MAY make enough difference in fuel temp to keep it from gelling.

And it seems the British Air 777 pilots are going from heroes to goats because they ignored low fuel temps enroute. Seems other flights in the area were changing altitudes to get out of extreme cold air to keep fuel temps up. Why the fuel would wait until on final approach, in much warmer air, to gel is beyond my feeble understanding, but it seems the investigation is pointing in that direction.
 
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Greg Bockelman said:
It isn't that there is tremendous amount of heat created, it is just that there is enough that the wing is X degrees above the ambient air temp. It may not be much, but it is measurable. And increasing speed MAY make enough difference in fuel temp to keep it from gelling.
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That's interesting, because in the Lear 35, which as Lance said, requires Prist, if you get a fuel filter light at altitude which is probably ice, they tell you to try transferring fuel from the fuselage tank into the wings. The thought behind that is that has been more insulated from the cold temperatures than the fuel in the wings.
 
Everskyward said:
That's interesting, because in the Lear 35, which as Lance said, requires Prist, if you get a fuel filter light at altitude which is probably ice, they tell you to try transferring fuel from the fuselage tank into the wings. The thought behind that is that has been more insulated from the cold temperatures than the fuel in the wings.
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I honestly don't know what kind of additives are in the fuel we use. I do know that if there is any it is added before it gets to the airplane. IOW somewhere at or before the fuel farm.

As far as transferring from fuselage to wings, we don't have that option. Fuselage fuel is used first, THEN the wing fuel. The wing tanks remain full until the fuselage tank is empty.
 
Greg Bockelman said:
I honestly don't know what kind of additives are in the fuel we use. I do know that if there is any it is added before it gets to the airplane. IOW somewhere at or before the fuel farm.
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The 35 is a "Legacy" (old) design that doesn't have fuel heaters. I'll bet most modern airplanes don't require Prist.

Greg Bockelman said:
As far as transferring from fuselage to wings, we don't have that option. Fuselage fuel is used first, THEN the wing fuel. The wing tanks remain full until the fuselage tank is empty.
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Yeah, that's a completely different system. In the Lear the fuel feeds from the wings to the engine so you need to empty the wings somewhat before you can even transfer fuel from the fuselage tank or the wings will overfill and the fuel will be vented overboard.
 
Ok, this is what our manual says about low fuel temps:

"TAT increases approximately 0.5 degrees C to 0.7 degrees C for each .01 Mach increase in airspeed. In extreme conditions it may be necessary to descend as low as FL 250"

That is Boeing research.
 
Greg Bockelman said:
"TAT increases approximately 0.5 degrees C to 0.7 degrees C for each .01 Mach increase in airspeed. In extreme conditions it may be necessary to descend as low as FL 250"
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Interesting, I know that TAT rises with speed but it never would have dawned on me to use it as a way to prevent fuel icing. But in a practical sense, taking the Lear for example, we normally cruise at .77 and wouldn't really try to cruise faster than .80 so that would only give about a 2C temperature difference.
 
Everskyward said:
But in a practical sense, taking the Lear for example, we normally cruise at .77 and wouldn't really try to cruise faster than .80 so that would only give about a 2C temperature difference.
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When the chips are down, that is enough. :smile:
 
Greg Bockelman said:
Ok, this is what our manual says about low fuel temps:

"TAT increases approximately 0.5 degrees C to 0.7 degrees C for each .01 Mach increase in airspeed. In extreme conditions it may be necessary to descend as low as FL 250"

That is Boeing research.
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Those two sentences make no sense together. Temperature, airspeed, altitude...which is it?
 
Let's say about 1200# (one wing's worth) of Jet A got so cold as to crystallize some of it. There's no way a 2deg difference on a portion of the wing surface is going to make any worthwhile difference. Transfer fuel from the truck tank if you've got any, turn on the Standby Pumps, hope it gets warmer.
 
Everskyward said:
Interesting, I know that TAT rises with speed but it never would have dawned on me to use it as a way to prevent fuel icing. But in a practical sense, taking the Lear for example, we normally cruise at .77 and wouldn't really try to cruise faster than .80 so that would only give about a 2C temperature difference.
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By my calculation, there would be about a 35C difference between TAT and SAT at .8 Mach if the SAT was 0C. The increase is less as the SAT goes below 0C.
 
lancefisher said:
By my calculation, there would be about a 35C difference between TAT and SAT at .8 Mach if the SAT was 0C. The increase is less as the SAT goes below 0C.
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I wasn't talking about the difference between TAT and SAT. I was talking about the difference between flying at .77 and .80 which is only .03 mach, which would only make a difference of approximately 2C.
 
Jet A not for Pistons?

Exxon is apparently asking FBOs whom they supply with Jet A to send them an indemnification for fueling Piston AC with Jet A. They say their prodcut is not designed for that and there may be some issues. Diamond dosent' know what the heck they are talking about. Here we go with another obstacle to making GA more affordable.
 
Re: Jet A not for Pistons?

AdamZ said:
Here we go with another obstacle to making GA more affordable.
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Maybe in the future, but operating a GA diesel is anything but affordable at THIS point in time.

But I get your point.
 
Re: Jet A not for Pistons?

Yea Greg, your right I should have said obstacles to having other choices in GA. Until recently Jet A seemed to be an answer to ever skyrocketing 100LL prices
 
Re: Jet A not for Pistons?

AdamZ said:
Yea Greg, your right I should have said obstacles to having other choices in GA. Until recently Jet A seemed to be an answer to ever skyrocketing 100LL prices
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Yeah, but it goes beyond that. As far as I know, there is only one certified diesel aircraft engine. Maybe WAS. Acquisition costs were significantly higher and there was no provisions for overhaul. One needed to purchase a NEW engine to replace it at OH.

Don't get me wrong. I think Jet A is the long term solution to the questionable viability of 100LL. It is just that there is no economical Jet A substitute at THIS point in time.
 
Re: Jet A not for Pistons?

Greg Bockelman said:
Yeah, but it goes beyond that. As far as I know, there is only one certified diesel aircraft engine. Maybe WAS. .
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Besides the Diamond Star didn't Cessan make a couple of 172 Diesels or were they all after market STCs
 
Re: Jet A not for Pistons?

I think that Cessna's diesel project was shelved when Theilert started having problems. There are one or two aftermarket ones out there, I think.
 
Re: Jet A not for Pistons?

AdamZ said:
Besides the Diamond Star didn't Cessan make a couple of 172 Diesels or were they all after market STCs
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There is, or was, a dealer in Dallas that does STC'd diesel changes to new airplanes. It's been a while since I've seen the info.

I want to say it was Van Bortel but I'm not certain. There's nothing on their web site about the diesels. Whoever it was, they advertised in Controller for some time.
 
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