Why aren’t diesels in airplanes? (Or are they?)

[mncruiser]

Why aren’t diesels in airplanes? Or are they?

Having some coffee this morning and this fine fella was crop-dusting. I’m having my own debate on diesel vs. gas in my boat search, and I was wondering why planes are not diesel, for many of the same reasons we all seem to mention here. Is it the weight? Fuel issues?

 

[kchace]

They are, there are several manufacturers including Continental

 

[rslifkin]

Small airplane diesels aren't as common as the very old avgas burning designs most use, but they do exist. Diamond makes a few planes with diesels, for example.

 

[mncruiser]

Any idea in general what the mix is in recreational aircraft? 75% gas, 25% diesel?

Outside of a jet turbine, does it change for commercial aircraft?

Yes, I’m trying to link boats and planes. Maybe it’s a poor comparison. A buddy of mine is just getting into flying and we’ve been having a friendly safety debate and it’s all very interesting.

It seems for planes, this would be much more important for reliability and safety.

If my boat quits, I can don the speedo and grab the Daffy Duck tube and float it out. Possibilities in planes exist - parachutes for the plane or person, glide it in.....but it seems like the person in the air has a lot more to deal with.

 

[Nomad Willy]

Weight and vibration.

 

[GoneFarrell]

Continental Diesel - Home

Note: interchangeable with gas motors, same weight. Never seen one stateside, may be a Euro thing.

 

[PPandE]

In airplanes, it's all about the power to weight ratio. Diesel engines are necessarily more heavily built to deal with the higher pressures involved. By the time an engineer solves those strength problems, the cost has approached or exceeded the manufacturing cost of a turbine engine. (cost measured in dollars per hours of engine life)

Piston aircraft engines rarely manage to put out one horsepower per two pounds of engine weight. Some recent diesel designs have reached that milestone but are still cost prohibitive for the general fleet. Compare that to the venerable Pratt and Whitney PT-6 which can be tweaked to over 1800hp and weighs in at under 400lb.

Then there's the regulatory approval process. Our FAA blindly follows tradition of design. The cost of approval for a new type of engine is way out of reason.

As for the reliability of the current generation of piston engines, they've been around so long that most of the problems have been designed or regulated away. For example, most of the naturally aspirated general aviation piston engines have a recommended life of only 2000 hours between overhauls. I've seen engines in service with more than twice that but by rebuilding early, failures are rare.

 

[Northern Spy]

TF member with one:

https://www.trawlerforum.com/forums/s3/ultimate-long-range-passagemaker-two-50864.html

 

[tiltrider1]

Most of my aviation experience is with turbines but I did fly a Diesel Longeze once. I’m not claiming to be an expert on this subject but I believe torque demands of the media air vs water play into the decision, probably second to HP vs weight.

 

[Ski in NC]

Weight. Very high cost to certify (stupid in my mind). Small market. Small benefit when all costs are considered.

 

[Sabre602]

Any idea in general what the mix is in recreational aircraft? 75% gas, 25% diesel?

Outside of a jet turbine, does it change for commercial aircraft?

In the United States, GA aircraft are probably 95% avgas, 4.9% Jet A, 0.1% other. Commercial are 99% kerosene.

As was stated above, there are indeed some diesel burners out there, most notably Diamond, with the rest experimentals. Most GA pilots in the US have never flown, or perhaps even seen one.

 

[Lepke]

At the end of WWII, Germany had a large flying boat that was diesel powered. Diesels have higher compression ratios than gas, as much as double, so the engines have to be more substantial - heavier.

 

[AdkChris]

Very high cost to certify.

I agree with Ski in NC.

I am amazed how much stuff in aviation and medical devices is based on old certifications. Both fields seem be be an interesting mix of new technologies and old certifications. Diesel would only be a replacement for gas piston engines and they are building even fewer new piston airplanes then trawlers so not much market for the few diesel airplane engines out there.

But from what I can see modern turbo diesels can be made to work fine in airplanes. There is the benefit of availability of Jet fuel, which is what aviation diesels burn, compared to AV gas which is very expensive and not as available outside the US lower 48.

 

[DDW]

There is another issue with diesels in aircraft: altitude. Compression ignition engines require high cylinder pressures to fire, at 18,000 feet ambient pressure is about 1/2 sea level, so the turbocharger needs to work twice as hard to achieve the same cylinder pressure. That increases charge temperature which limits power. Some of the same issues exist in gas engines but not to the same extent. The Diamond diesel aircraft have a fairly low service ceiling as a result.

 

[mncruiser]

Thanks for all the comments. You all are much more interesting than google!

 

[AdkChris]

There is another issue with diesels in aircraft: altitude. Compression ignition engines require high cylinder pressures to fire, at 18,000 feet ambient pressure is about 1/2 sea level, so the turbocharger needs to work twice as hard to achieve the same cylinder pressure. That increases charge temperature which limits power. Some of the same issues exist in gas engines but not to the same extent. The Diamond diesel aircraft have a fairly low service ceiling as a result.

Every aviation diesel I have heard of used a turbocharger. By their nature turbos work better a higher altitude. They used to make altitude compensated engines which was just a turbo sized to only maintain power output not increase power output.

The Diamond DA42 goes up to 18,000 feet which is well into supplemental oxygen levels.
https://www.diamondaircraft.com/en/private-pilots/aircraft/da42/tech-specs/

As for increasing the charge temperatures, intercoolers are cheap and light. The intercooler for my 100 HP Jetta was I think about $100 and weighed a couple of pounds. The standard lapse rate is 2 degrees C per 1000 feet so as the turbo does more compressing the ambient temperature is decreasing.

Would be interesting to look into I it but I think the definition of the standard lapse rate means that the decrease in temperature at altitude will exactly compensate for the increase in temperature from compressing the air to sea level density.

 

[pjtemplin]

Note that 'diesel' in an airplane likely has a different meaning than you think. If you do find an airplane with 1+ 'diesel' engines, it's likely burning Jet-A inside an internal combustion engine that leverages the Diesel cycle rather than the Otto cycle (which is the typical cycle inside a gasoline-burning engine).


Case in point: the DA42 linked above burns Jet A-1, Jet A, TS-1 (Russia, Ukraine), RT (Russia, Ukraine), No. 3 Jet Fuel (China), JP-8. (Yes, we're perhaps splitting hairs by talking jet fuel vs. diesel - my point is just that you won't see diesel pumps for said airplanes)

 

[AdkChris]

Note that 'diesel' in an airplane likely has a different meaning than you think. If you do find an airplane with 1+ 'diesel' engines, it's likely burning Jet-A inside an internal combustion engine that leverages the Diesel cycle rather than the Otto cycle (which is the typical cycle inside a gasoline-burning engine).


Case in point: the DA42 linked above burns Jet A-1, Jet A, TS-1 (Russia, Ukraine), RT (Russia, Ukraine), No. 3 Jet Fuel (China), JP-8. (Yes, we're perhaps splitting hairs by talking jet fuel vs. diesel - my point is just that you won't see diesel pumps for said airplanes)

True

But the difference between #2 heating oil, kerosene, diesel, Jet A thru JP-8 is pretty small.
A friend was servicing an oil burner and the fuel smelled different and the air volume setting was a bit different. Turns out the customer was a jet mechanic and for years when ever they had to dispose of questionable jet fuel he took it home and dumped it in his oil tank.
On the other end I am told all the diesels generators and heavy equipment at the antarctic South Pole Station run on JP-8 that was sucked out of the main tanks on the military turboprops that fly in supplies and personnel.

 

[Datenight]

True
Turns out the customer was a jet mechanic and for years when ever they had to dispose of questionable jet fuel he took it home and dumped it in his oil tank.

My Brother in law, a corporate pilot did that for years. Also ran his diesel VW on it.

Rob

 

[FF]

Aircraft operators select engines by the lifetime cost of every hour.

Turbines , so far are the choice for folks that need lots of hours.

 

[AdkChris]

Aircraft operators select engines by the lifetime cost of every hour.

Turbines , so far are the choice for folks that need lots of hours.

True enough if you are looking for more power and spending a long time cruising at altitude.

I'm not sure where the cut off numbers are for today's technology in are but in general:
Piston engines are best for less than maybe 400 HP per engine, operating below maybe 12,000 feet and short hops less than 250 miles(???). Piston engines are also much cheaper to buy.

Then turbo props do best between maybe 500 to 2,500 HP and fairly short hops of a few hundred miles. The turboprop is more efficient than a jet below maybe 30,000 feet so if you will be spending a significant percent of the flight getting up and coming down turboprops do well.

For longer flights where most of the flight is at altitude the higher speeds and power of jet engines do well. Jet engines are expensive to buy but I think the maintenance cost per HP hour is lower. (Yes I know Jet engines are not rated in HP but if you know the difference get over it. If you don't know the difference HP is my best explanation)

Then of course at stupid high altitudes and speeds rockets are the way to go. I have not seen any jets engines on ICBMs.

So dollars definitely count. But you always get the most value for your dollar if you chose the right technology for the job.

 

[DDW]

Every aviation diesel I have heard of used a turbocharger. By their nature turbos work better a higher altitude. They used to make altitude compensated engines which was just a turbo sized to only maintain power output not increase power output.

The Diamond DA42 goes up to 18,000 feet which is well into supplemental oxygen levels.
https://www.diamondaircraft.com/en/private-pilots/aircraft/da42/tech-specs/

As for increasing the charge temperatures, intercoolers are cheap and light. The intercooler for my 100 HP Jetta was I think about $100 and weighed a couple of pounds. The standard lapse rate is 2 degrees C per 1000 feet so as the turbo does more compressing the ambient temperature is decreasing.

Would be interesting to look into I it but I think the definition of the standard lapse rate means that the decrease in temperature at altitude will exactly compensate for the increase in temperature from compressing the air to sea level density.

You assume that turbocharger efficiency is 100% which it is distinctly not. When Diamond's first diesels had difficulties most were replaced with non-turbo'd gas Lycomings with the same service ceiling. Many contemporary turbocharged gas airplanes can operate at 25,000 and higher if permitted by RVSM. Turbocharged gas warbirds operated at 45,000. You'd have a very hard time getting enough pressure to fire a diesel at that altitude. But sure, at lower altitudes Diamond (with help from Mercedes) is proving that diesels are practical.

 

[Nickair]

In airplanes, it's all about the power to weight ratio. Diesel engines are necessarily more heavily built to deal with the higher pressures involved. By the time an engineer solves those strength problems, the cost has approached or exceeded the manufacturing cost of a turbine engine. (cost measured in dollars per hours of engine life)

Piston aircraft engines rarely manage to put out one horsepower per two pounds of engine weight. Some recent diesel designs have reached that milestone but are still cost prohibitive for the general fleet. Compare that to the venerable Pratt and Whitney PT-6 which can be tweaked to over 1800hp and weighs in at under 400lb.

Then there's the regulatory approval process. Our FAA blindly follows tradition of design. The cost of approval for a new type of engine is way out of reason.

As for the reliability of the current generation of piston engines, they've been around so long that most of the problems have been designed or regulated away. For example, most of the naturally aspirated general aviation piston engines have a recommended life of only 2000 hours between overhauls. I've seen engines in service with more than twice that but by rebuilding early, failures are rare.

I second everything said here. Great explanation.

.

 

[Mischief Managed]

Prior to WWII, Germany developed an interesting an inline, 6 cylinder, two-stroke, diesel aircraft engine with two crankshafts on the top and bottom of the engine and 12 pistons. Each cylinder had two pistons that opposed each other and came together in the middle to generate enough compression heat to trigger ignition. In one interesting airplane they used three of these engines, two wing engines driving props and one in the belly of the plane driving a huge supercharger that fed high pressure air to the two wing engines when at altitude. I don't think they were particularly successful, but they sure were fascinating.

 

[Ski in NC]

That was the Junkers Jumo opposed piston engine. Design was evolved into the Farbanks-Morse OP and the Napier-Deltic designs. FM engines are still in service, saw one on a dredge pump a couple years ago. Sound it made was something else.

When the Junkers came out, they really had not refined turbocharging enough to be practical. Now with modern turbocharging tech (variable vane, etc) you can take a diesel up to quite high altitude.

But do not shut it off up there!! All the turbo tech on the planet won't help a diesel start in that cold thin air. If it shuts down, you need to come WAY down in altitude to get a restart. Not convenient when crossing high mountains.

Cummins is developing an OP engine for military service right now. Probably can't be used commercially due to emissions, but we shall see.

Weight of a diesel plant is a bigger bugger than the fuel savings in the aero world.

1800hp out of a 400lb gas turbine? Awesome. The diesel would be like 4000lb if not more.

 

[FF]

I have been following LiquidPiston for a number of years. With DARPA funding I had hoped they would be further along by now

Claim multi their fuel engine is 30% more efficient than current engines.

Very light weight 1 LB per HP 5 or 6 years ago.

Very interesting web site with lots of diesel aircraft engine info.