speedracer1407
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Diesels And General Aviation; Why So Rare?

Fri Mar 09, 2007 10:59 am

I was cruising around the Diamond Star websight a while back and was reminded of some questions i've been harboring about the rarity of diesel-powered aircraft. Just about everything that's designed to run at constant speed and powered by a piston engine is powered by a diesel. Modern turbodiesels are now vastly more powerful for their size than they were decades ago. The advantages seem obvious for aviation:

-High torque at low RPM
-Greater fuel efficiency
-Ability to run on cheaper, more widely available Jet-A

The only disadvantages I can think of is wieght, since the internal parts of a diesel must be somewhat sturdier than that of a gasoline engine. But at over 5 liters of displacement, the ubiquitous Lycoming O-320 is, well huge for it's power output, especialy compared to the 2 liter Centurion diesel of similar power used in the Diamond star.

So what gives? Is this just another example of General Aviation moving far slower than technology allows? Or are there real downsides to diesel engines in GA aircraft that I'm not considering?

O
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RichPhitzwell
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RE: Diesels And General Aviation; Why So Rare?

Fri Mar 09, 2007 11:07 am

Off the top of my head, older diesels trucks had a hard time at altitude, plus they are as you stated heavy.
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NKP S2
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RE: Diesels And General Aviation; Why So Rare?

Fri Mar 09, 2007 11:21 am

Traditionally poor power vs weight ratio.
 
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SEPilot
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RE: Diesels And General Aviation; Why So Rare?

Fri Mar 09, 2007 8:39 pm

There are several problems with diesel engines on airplanes; weight is the most obvious, as has been stated the power/weight ratio is generally worse than a comparable technology gasoline engine. But actually a bigger one is vibration; the power strokes on a diesel are much more violent than on a gasoline engine, and pose significant problems for the propeller and the airframe structure. I believe Thielert has minimized this by using a geared engine that turns much faster, but that has problems as well. They have not been around long enough to see if there will be airframe or propeller problems after extended use, and we don't really know how well the gearbox will stand up. The venerable flat four or six turning at under 3000 RPM has proven to be very reliable, and it is not an accident that it has stayed around as long as it has. Another problem, nowhere near as severe, is fuel cleanliness. Fuel for a diesel must be 100% filtered, as a very small particle can clog an injection nozzle, and if the filters are not maintained they can clog and choke off the fuel supply. Gasoline engines (even fuel injected ones) are much more tolerant on this score.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
BAE146QT
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RE: Diesels And General Aviation; Why So Rare?

Fri Mar 09, 2007 9:28 pm

Diesels are also rather more efficient than gasoline engines in terms of waste heat. Many Diesel cars have an additional "oil burner" on board to provide cabin heat because the engine cooling system doesn't provide enough itself.

I imagine this might be an issue for deicing (and heating requirements in general) for an aircraft. The additional burner would be additional weight over and above an already heavy donk.
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N231YE
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RE: Diesels And General Aviation; Why So Rare?

Fri Mar 09, 2007 11:47 pm

Isn't there a modification that enables engines burning 100LL to be run on diesel fuel?

If I remember correctly, the modification includes removing the ignition system, and replacing the original cylinder heads with ones that allow for the installation of the fuel injectors, in addition to wider cooling intakes, fuel rail system, etc.

-------------------------------

While on the topic, may I ask what diesel for airplanes really is; actual diesel fuel or jet fuel?
 
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SEPilot
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 12:26 am

Quoting N231YE (Reply 5):
Isn't there a modification that enables engines burning 100LL to be run on diesel fuel?

Absolutely not; diesel fuel can not be ignited by spark at starting temperatures; it is ignited by heat from compression. This requires compression ratios of at least 16:1, as opposed to 7:1 to 8.5:1 for most aircraft (100LL) engines. Also, as mentioned earlier, the components of a diesel engine have to be much heavier than gasoline engines because of the much stronger power pulses. Remember what happened in the early 1980's when Oldsmobile tried to take a gasoline V-8 and convert it to diesel? The result was a disaster (the engines self-destructed in about 50,000 miles) and turned American car buyers against diesels for a generation at least. I think what you are thinking about is that aircraft engines designed to run on 80 octane avgas can be approved to run on autogas; no modifications are required. A few engines that are designed for 100LL can be approved to run on 93 octane autogas, but for most of them the only approved fuel is 100LL, and no modifications are available to change that.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
N231YE
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 12:55 am

Quoting SEPilot (Reply 6):

I might be thinking of a conversion for model airplanes...because I know something similar exists.

Quoting SEPilot (Reply 6):
I think what you are thinking about is that aircraft engines designed to run on 80 octane avgas can be approved to run on autogas; no modifications are required.

There are a few small modifications that need to be done when running a low compression (80 octane) engine on mogas; the EAA offers kits to overcome some of these limitations.

Keep in mind one has to test the fuel for ethyl alcohol, a common oxidizing agent for autos, but incompatible for aviation, mainly due to separation and icing.
 
Dougloid
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 12:59 am

The only people who ever pulled this stunt off and made it a paying proposition was Junkers with their excellent series of opposed piston diesel engines that they built.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn
 
PolymerPlane
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 1:19 am

Quoting SEPilot (Reply 6):
Absolutely not; diesel fuel can not be ignited by spark at starting temperatures; it is ignited by heat from compression. This requires compression ratios of at least 16:1, as opposed to 7:1 to 8.5:1 for most aircraft (100LL) engines

Actually this is not true. Diesel fuel can be ignited by spark plugs (you can try igniting a bucket of diesel fuel with an electric spark Big grin). However, since the octane rating is lower than gasoline, if you put diesel fuel into gasoline engine, the diesel fuel will auto ignite before the piston reaches the combustion point. In diesel engine, this problem is countered by injecting the fuel just before the ignition point. The compression ratio of 16:1 is required to have complete auto ignition. That's why the older diesel engine had a lot of problems with the fuel injector.

I am not sure however, if one can reduce the compression ratio of a gasoline engine low enough before the auto ignition point of diesel fuel, thus allowing the engine to run on diesel fuel.

Cheers,
PP
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A342
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 4:13 am

Quoting RichPhitzwell (Reply 1):



Quoting NKP S2 (Reply 2):



Quoting SEPilot (Reply 3):

The problems you mention belong to the past. Modern diesels have both a good high-altitude performance and power-to-displacement ratio, thanks to turbochargers. Some state-of-the-art diesels are made from aluminium, basically eliminating the weight problem. But I admit, that technology is quite expensive. However, as the displacement is much lower compared to traditional Lycoming / Continental flat gas engines, the weight of aviation diesel engines isn't much higher, if at all. Vibrations are greatly decreased if balance shafts are used.

Quoting N231YE (Reply 5):
While on the topic, may I ask what diesel for airplanes really is; actual diesel fuel or jet fuel?

The Thielert 1.7 and 2.0 as well as the SMA305-230 can be run on both. The Thielert 4.0 can only use Jet fuel, I assume the same will be true for the new Thielert 3.2
Exceptions confirm the rule.
 
ShyFlyer
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 4:19 am

Quoting N231YE (Reply 5):
While on the topic, may I ask what diesel for airplanes really is; actual diesel fuel or jet fuel?

In the US, the FAA will only permit Jet fuel to be used.
I lift things up and put them down.
 
lapa_saab340
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 4:41 am

I think the turbodiesels will end up replacing the dinosaur Lycoming/Continentals eventually. The cost of conversion right now is still steep, but that can change if the demand grows enough. The weight of the engines is comparable, and although the weight of jet fuel is a bit higher than avgas, the reduction in fuel consumption more than makes up for it.

The day will probably come when 100LL is no longer produced. The EPA has pushed to have lead eliminated from avgas already. MOGAS STCs are available only for a number of the Lycomings/Continentals, but with ethanol finding its way into auto gasoline that road might close up as well. Any further increase in cost of fuel will make the economy of the turbodiesels more attractive. FADEC is another improvement; it's ridiculous that in this day and age we are still fiddling with prop/mixture controls on recip engines.

IIRC, the SMA conversion for the 182 has a TBO on the engine of 2000 hrs, which they want to try to increase to 3000. Thielert has a time between replacements instead of overhaul. I don't recall the hours but I think it was similar to SMA.
 
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SEPilot
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 4:53 am

Quoting PolymerPlane (Reply 9):
Diesel fuel can be ignited by spark plugs (you can try igniting a bucket of diesel fuel with an electric spark

I will question this; what I said is that diesel fuel will not ignite at STARTING temperatures by spark. I used to have an old all-fuel John Deere tractor; it would run on gasoline, kerosene, or diesel fuel. But in order to start it you had to run it on gasoline until it warmed up; otherwise it would not run. Trying to start it cold on kerosene (I never did try diesel) would mean draining the carburetor and cleaning the plugs. I'm sure that an aircraft engine would have the same problem. The other problem, as you note, is the octane rating. The John Deere had a compression ratio of 4:1, and it was also extremely heavy. Running on kerosene did cause significant knocking, but it could take it.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
Pope
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 5:03 am

Compare the number of airports where Jet-A or diesel is available to the number of airports where 100LL is available.
Hypocrisy. It's the new black for liberals.
 
N231YE
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 7:05 am

Quoting A342 (Reply 10):
The Thielert 1.7 and 2.0 as well as the SMA305-230 can be run on both. The Thielert 4.0 can only use Jet fuel, I assume the same will be true for the new Thielert 3.2



Quoting ShyFlyer (Reply 11):
In the US, the FAA will only permit Jet fuel to be used.

Thanks for answering my question...I was a little confused about the terminology used with diesel, ie, some companies referred to diesel as actual diesel, while others used the term for jet fuel.

Quoting Lapa_saab340 (Reply 12):
IIRC, the SMA conversion for the 182 has a TBO on the engine of 2000 hrs, which they want to try to increase to 3000.

Isn't already approved for 3000TBO? If so, the savings from the extra 1000 hrs over the Lycomings and Continentals along with the fuel's cheaper costs does not seem bad at all...if I owned an C182 and flew it frequently, I might be inclined to pay for this modification.
 
jetstar
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 11:21 am

Quoting SEPilot (Reply 6):
Remember what happened in the early 1980's when Oldsmobile tried to take a gasoline V-8 and convert it to diesel? The result was a disaster (the engines self-destructed in about 50,000 miles)

Actually they were self destructing at around 5000 to 10,000 miles. Even though almost all of the test engines were failing on the dymo tester, GM management insisted they be
sold to the public to raise their average corporate fuel economy and avoid the financial penalties the US government would impose on GM if they failed to meet the average corporate fuel economy for all their GM products.

A friend of mine had a Diesel Olds in the early 1980’s and GM had to replace his engine and also extended the warranty.

Quoting BAe146QT (Reply 4):
Diesels are also rather more efficient than gasoline engines in terms of waste heat. Many Diesel cars have an additional "oil burner" on board to provide cabin heat because the engine cooling system doesn't provide enough itself.

I had a 1980 VW diesel rabbit and the heater was very effective, even in Vermont in the middle of the winter.
 
speedracer1407
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 2:45 pm

Quoting A342 (Reply 10):
The problems you mention belong to the past. Modern diesels have both a good high-altitude performance and power-to-displacement ratio, thanks to turbochargers. Some state-of-the-art diesels are made from aluminium, basically eliminating the weight problem. But I admit, that technology is quite expensive. However, as the displacement is much lower compared to traditional Lycoming / Continental flat gas engines, the weight of aviation diesel engines isn't much higher, if at all. Vibrations are greatly decreased if balance shafts are used.

These are exactly my, admittedly non-expert thoughts. If automotive diesels are anything to go by, which I know it usually isn't, modern turbodiesels are both efficient and powerful for their size and weight. Then again, maybe the automotive thing isn't as removed as one might think. Thielert's aviation diesels are based on Mercedes automotive diesels.

And even if we forget the advanced state of automotive turbodiesel, Theilert's 2 liter turbodiesel wieghs 330 lb inlcuding "all accessories," while the O-320 weights roughly the same at 320 lb "total installed" wieght.

Quoting SEPilot (Reply 3):
But actually a bigger one is vibration; the power strokes on a diesel are much more violent than on a gasoline engine, and pose significant problems for the propeller and the airframe structure. I believe Thielert has minimized this by using a geared engine that turns much faster, but that has problems as well.

The Thielert engine produces its maximum power at 2300 rpm--I'm not sure what the gearbox is for--perhaps to increase the prop RPM? I can't imagine any diesel of any kind running at a significantly higher RPM than competing gasoline engines. As for vibration, balance shafts, as mentioned above, may add a bit of weight, but they certainly do wonders for smoothing things out.

The point of all this is that it seems to me like the fundamental disadvantages of diesel engines, at least the ones that we've all touched upon so far, are either no longer relevent because of the resources dumped into diesel technology in the past 15 years, or are easily solvable with a bit of development.

Again, perhaps there other, less obvious reasons for avoiding diesels in general aviation. For example, It's my understanding that those air-cooled flat 4 and 6 cylender gas engines are very modular; you can change out cylenders as needed. I can't think of any current air-cooled diesels, and I don't know if it's even practical to do so.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
pilotpip
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 4:19 pm

Certification is a very, very expensive proposition. This is the main reason that we're still using air cooled engines that are essentially unchanged since 1950.
DMI
 
rwessel
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 5:43 pm

Quoting Pope (Reply 14):
Compare the number of airports where Jet-A or diesel is available to the number of airports where 100LL is available.

I'm not sure which way you meant this. 100LL is commonly available only in North America, Australia, Brazil and South Africa. It is available, but very expensive, and often hard to find, in Europe, and very rare in much of the rest of the world (IIRC, a few years ago someone noted that there were *three* airports with 100LL available in Africa outside of South Africa).

Total global avgas production is about 2.2M gallons/day, a bit more than a third of that is produced in the U.S (about 760K gallons). And not all of that is 100LL. For comparison, jet fuel production (counting both Jet-A/A1/B and the military JP-x versions), is about 68M gallons/day in the U.S. and about 370M gallons of autogas are produced each day in the U.S.

What astonishing is that 100LL is actually available at the price it is, considering the miniscule production and the need for a bunch of separate equipment and facilities because of lead contamination (for example you can't store autogas in a tank that's been used to store 100LL without decontaminating it first).

Jet-A, on the other hand is almost universally available, again small GA fields, especially in the U.S. are the exception.
 
A342
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RE: Diesels And General Aviation; Why So Rare?

Sat Mar 10, 2007 8:56 pm

Quoting ShyFlyer (Reply 11):
In the US, the FAA will only permit Jet fuel to be used.

That's new to me. Here's quote from a FAA document posted on www.dieselair.com :

Quoting Dieselair:
Recognizing that interest in diesel engines within the general aviation community has grown appreciably in the past several years, the Federal Aviation Administration has issued a draft memorandum to clarify the certification requirements for 14 CFR part 33, type certification of diesel reciprocating engines.
"Interest in diesel aircraft engines has recently been renewed in response to the demand for engines that do not require leaded fuel for operation," the FAA states. (...) The FAA anticipates that in most cases, operators of planes with CI engines will use either Jet-A, or automotive diesel fuel in their engines.

BTW, I've read two flight reports of the Thielert 1.7. Both state that the engines run exceptionally smooth, almost turbine-like.
Exceptions confirm the rule.
 
ShyFlyer
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RE: Diesels And General Aviation; Why So Rare?

Sun Mar 11, 2007 4:09 am

Quoting A342 (Reply 20):
That's new to me.

The information I used came from FLYING Magazine's report on the DA42 (July 2006). It stated that, "Though the Centurion engine is derived from an automotive diesel, only Jet-A fuel is approved by the FAA. In Europe the engines are approved for either diesel fuel or Jet-A, but U.S. standards for diesel fuel are not as consistent as in Europe."

I've read the link you provided, and it looks as if the information you cite was posted mid-December 2006, obviously more current. Thanks for pointing that out!  Big grin
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A342
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RE: Diesels And General Aviation; Why So Rare?

Sun Mar 11, 2007 5:59 am

Quoting ShyFlyer (Reply 21):
I've read the link you provided, and it looks as if the information you cite was posted mid-December 2006, obviously more current. Thanks for pointing that out!

No problem ! BTW, with the new ultra-low-sulphur diesel, things should get better in the USA.
Exceptions confirm the rule.

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