Are Diesels Really More Economical?

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Centurion 1.7

This week's news brought an emergency airworthiness directive for Thielert's new Centurion 2.0. The AD requires a bracket to prevent cracking in a high-pressure line between the engine's injection pump and the common rail injection manifold. A little over 500 airplanes are affected.

This AD reflects one of several daunting challenges with aerodiesel engines: although they're perceived as smooth running thanks to clever engine mounts, the engines themselves vibrate more intensely than most avgas engines do. The torsionals along the crankshaft are so intense, in fact, that no one has come up with a way to mount a metal prop on these engines and have it last long enough to satisfy even minimal service expectations. The Thielert engines currently have wood/composite props, which soak up vibes and deliver satisfactory service life.

Although I'd be perfectly happy with the composite props, some would-be owners aren't and they cite this as a reason for not being interested in diesels. But as avgas approaches a $5 average, will the economy of diesels sway the market? To answer this question, I spent the last couple of weeks exhaustively researching the diesel vs. gas economy question. (The final report will appear in the April issue of The Aviation Consumer.)

In a nutshell, what I found was this: Best case, the Thielert diesels are running a fuel efficiency of about .38 BSFC, versus .39 for the very best gasoline engines—the Continental IO-520s and IO-550s. Lycoming engines aren't quite as efficient, typically running at .40 to .42, but as much as .45 BSFC in some instances.

If you run the numbers through a TBO cycle, the diesels beat the Lycomings on a cost-per-hour basis, but they run closer to equal with the Continentals. Diesels are definitely hobbled by their high overall costs. The Lycoming IO-360 used in Diamond's Star costs about $25,000 to overhaul, while the four-cylinder Thielert costs $53,000 to replace. (This figure includes mandatory replacement of key parts at 600 hour intervals.) So even though the diesel costs twice as much to overhaul, its fuel efficiency makes up for it and it's cheaper to operate.

That equation falls apart when larger displacement engines are compared. Continental's IO-550 costs about $32,000 to overhaul, versus a staggering $97,200 plus for Thielert's 350-HP Centurion 4.0. But if that engine also runs at .38 against the Continental's .39, it doesn't save enough fuel to offset the higher overhaul costs.

Of course, the real driver for diesel technology is availability of fuel first, cost of fuel second. Nonetheless, U.S. buyers are looking at diesel in both pure efficiency terms and with the hope of at least holding the cost on flying. The smaller diesels—despite their maintenance issues—seem to deliver on that promise. But the large displacement diesels aren't there yet.

Comments (23)

I am surprised that Thielert diesels are so fuel inefficient. Many years ago, farm and construction machinery manufacturers switched 100% to diesels, and many marine manufacturers are now doing the same.

I think diesels are ultimately the way to go, but first someone will have to come up with an engine that is a whole lot cheaper to buy and operate

Posted by: RICHARD ROSS | March 17, 2008 7:00 AM    Report this comment


I am going on record to say diesels will never pass the long term test in current non-composite production aircraft. Metal airplanes were never designed or tested for the intense vibration of a diesel. We have 60 + years fo experience using gasoline engines so their vibrations are manageable.

The most available fuel in the world is unleaded autogas. the answer as you and I know is a system like GAMI's Prism that will allow high compression engines to run safely with lower octane rating fuel.

Posted by: Dick Merrill | March 17, 2008 8:02 AM    Report this comment

Not being an engineer but what ever happened to the rotary engine aka Wenkel. I believe that is very smooth. Maybe it wasnt fuel efficient?

Posted by: Eugene Haller | March 17, 2008 9:55 AM    Report this comment

I was wondering if they tried to use a cog belt delt drive to dampen the vibes to the prop and I'm sure they have to be some engine mounts that will make the diesel work without hurting the air frame.

Posted by: Herb bates | March 17, 2008 12:08 PM    Report this comment

The Continental TIOL-300, with 11.5 to 1 compression ratio had a bsfc of .35. It put out 170 HP at 2800 RPM climbing to 187 HP at 65,000 feet. Of course we used 145 octane fuel but what's a little lead among friends? The engine was used on the Boeing Condor UAV.

Posted by: JOHN DALE | March 17, 2008 9:49 PM    Report this comment

You need to take in the account that jet-A is 13.3% heavier than avgas, and fuel is priced in gallons, not pounds. Therefore if the BSFC of a diesel is same as an avgas engine and if gallon to gallon, avgas costs the same as Jet-A, a diesel engine would result in 13% less in fuel cost.

Additionally let's not forget the potential of a "highway diesel" STC for aero diesel engines certified for jet-A. Given the nature of diesel engine, such STC would be fairly easy to develop.

Posted by: G 46R | March 17, 2008 10:32 PM    Report this comment

Regarding the Wankel rotary engine, yes they are very smooth but they are also very thirsty. The Mazda RX-8 automobile is a great example. It performs well, is very smooth, but has disappointing fuel economy for its size and performance.

Posted by: Tim McDaid | March 18, 2008 1:26 PM    Report this comment

Diesel fuel requires less refining than other fuels.
Operation is simpler requiring no mixture adjustments for altitude while also providing strong torque. Diesel developments will be made.
Wankel enignes have no time that the whole of the combustion force is applied at right angels to the power shaft. That improves smoothness But costs efficency. It is better to compare Wankel engines with thirstier turbines. How about developing a Wankel to run on A-1 ... Heavy fuel with low TV.

Posted by: Noel Loveys | March 19, 2008 12:14 PM    Report this comment

Seems like we're comparing a very established (avgas engine) situation to one with very new players (compression ignition aero engines aka diesels). I think time is on the side of the diesels. Although not yet certified, the Deltahawk 160/180/200 hp could be better with regard to vibration (2-stroke, twice the (smaller) power pulses) and much better with regard to cost (not made in Germany). Let's see what it looks like when/if there are a few more players with a few more approaches.

Posted by: TOM LUBBEN | March 19, 2008 1:19 PM    Report this comment

In the UK the situation is different. Jet A1 has no fuel tax and is therefore only one third of the price of avgas. (AVGAS 3$ per litre abut 14$ per US gallon) so it's o contest.

Posted by: Allan Denham | March 20, 2008 6:44 AM    Report this comment

The most available fuel in the world is diesel. Try finding unleaded auto fuel anywhere in the developing world. Diesel fuel removes that logistic barrier to operators outside of the Continental United States. Larger market, more units sold, lower unit cost.

Posted by: D Malone | March 20, 2008 10:09 AM    Report this comment

There was a WW2 German effort to fly diesels, I believe in one of their bomber twins. It was a failure. Might be interesting to explore why.

Posted by: FRANK NATOLI | March 20, 2008 4:59 PM    Report this comment

Don't compare a non-turbo to a turbo engine. The numbers for turbo engines are far worse. The same is true for the average consumption for a typical mission, because diesel are not cooled via fuel in any stage of the flight. Avgas will be gone soon or later due to lead. There is no future for LyCoSauros!

Posted by: Wolfgang | March 21, 2008 6:51 AM    Report this comment

Hi Paul,

All of us at DeltaHawk Engines, Inc. read with interest your article regarding diesel aircraft engine economies. You make some great points regarding the state of the art in currently certified diesel engine technology. We would like to point out, however, that not all diesel aircraft engines are the same.

In case you are not aware of us, DeltaHawk has developed a purpose-built, clean-sheet-of-paper diesel aircraft engine. This engine has gone through years of development and testing, and we are currently in the FAA certification process. There are numerous new aircraft that are being designed around our engine as well.

Our engine is different in many ways from the currently certified heavy fuel engines, the most notable being that we have chosen to use a V-4, two-stroke, direct-drive design. The two-stroke design has inherent benefits of smoother running, smaller torque pulses, and smaller displacement, along with reduced weight and size.

We would like to make a number of points regarding your analysis. We certainly agree that BSFC is a proper fuel consumption measure from a pure engineering perspective, but a comparison of BSFC with two different fuel types can lead to confusion. BSFC, of course, is pounds per HP per hour. But Jet-A weighs 12% more per gallon than gasoline. So if you took two hypothetical engines with the same BSFC, one burning gas and the other a diesel burning Jet-A, the diesel would actually burn 12% less GPH for the same power output. At the end of the day, pilots don't really care about BSFC; pilots buy fuel in gallons (or liters) so consumption data based on volume is what counts.and a 12% improvement in GPH is a heck-of-a-lot!

In addition, as DeltaHawk engines do not have efficiency losses from valve trains, high RPM operation, or prop-reduction units, our BSFC numbers are better than the diesel engine you quoted in your article.our BSFC is much nearer to .34. That excellent BSFC, along with the 12% weight vs. volume issue makes our total fuel economy in GPH approximately 23% better than the best-case numbers quoted for a big-bore Continental.

The mounting and propeller issues that were raised also do not apply to DeltaHawk engines. No special engine mounts are fact, our mounts are typical Lord or Barry mounts like those used in the traditional gasoline engine world. And as we developed our engine we have been working closely with two large US propeller companies. Based on mutual test data, we are very confident that our engine WILL be certified with metal propellers, both constant speed and fixed pitch.

As for overhaul costs, again, not all diesel engines are alike. Our engine is an extraordinarily simple design with 40% fewer parts than a comparable four-stroke design. We anticipate overhaul costs to run around ten thousand dollars.

To add a few more points, our critical altitude is in excess of 18,000 feet, we have redundant air, cooling, and fuel systems, and the total installed weight of our 200HP engine rivals an IO-360, with a smaller overall package size.

Paul, we are located in Racine, Wisconsin, between Milwaukee and Chicago. You certainly are welcome to visit us at any time. We also will be exhibiting at both Sun-n-Fun and Oshkosh, and we'd love to meet you and show you what DeltaHawk is all about. We can show you what makes our engine truly unique and why we believe it will be a major game-changer in the future of aviation. We have been a bit slower to market, but we will dramatically change the diesel aviation scene.

Best regards,

Dennis R. Webb
DeltaHawk Engines, Inc.

Posted by: Dennis Webb | March 24, 2008 3:06 PM    Report this comment

First, I don't know exactly how the BSFC calculations work, but your numbers fly in the face of my experience. My 172N's were burning 8 gph before, and with the 1.7L Thielerts we were running in the range of 4.3 gph. Granted that we use lower power settings than the average pilot might, but the settings are the same before and after. So, for the same operating regime I'm burning just over half the fuel. That's the result of about 1800 hours on two aircraft. One timed out and we just installed a 2.0L in its place, and that engine is burning slightly less - 4.1 gph or so - at the same power settings. All the higher math aside, this really shows up at the end of the week. We're talking 240 gallons of avgas as opposed to 129 or so of cheaper Jet A. I expect that the better torque curve on the diesels has something to do with this as well.

On the AD - This was the result of a line not being properly mounted, which was a design flaw. We just did the fix last week, which required replacement of the line and an additional mounting point. Vibration is and always will be an issue with diesels, but that's something that's been solved in the field by others. I think this was more an oversight on the part of Thielert rather than an indication of things to come.

I've had my fun with these engines at times to be sure, but overall I'm still convinced that they're a far better deal than the Lycomings.

Posted by: STAN FETTER | March 25, 2008 9:20 AM    Report this comment


Here's the math:

The Twin Star's 1.7s were burning 5.8 to 5.9 GPH (38.86 to 39.53 lbs) at 80 percent. That's 108 HP.

38.86 divided by 108 is .3598 or rounded to .36. At the higher number, it rounds to .37.

Pulled back to 50 percent, the fuel flow is down to about 3.7, which is still around .36 to .37.

Posted by: Paul Bertorelli | March 25, 2008 9:23 AM    Report this comment

I fly in the upper midwest, how do diesels compare to lycomings and continentals in in cold climates(temperatures below zero degrees F.)--especially with cold starts below 32 degrees F.?

Posted by: Unknown | March 25, 2008 12:22 PM    Report this comment

Recently Avweb reported on a lawsuit filed by a flight school on the excess downtime and high cost of maintenance when using the Thielert diesel. What came of that?

Posted by: Phillip Peterson | March 25, 2008 4:10 PM    Report this comment

Re the lawsuit mentioned in AVWeb, the caption is Millen Aviation v. Diamond Aircraft Industries, and the trial is scheduled to commence next month (June) in Austria. The suit involves two DA40 TDIs which were leased by a flight school in England and returned by the lessee as a result of recurring and allegedly unsolvable engineering issues related to the Thielert engines. The outcome will be closely-watched unless it is settled on a confidential basis. Considering the implications to Diamond of a negative outcome in light of the recent Thielert backruptcy, settlement of this case seems highly likely.

Posted by: nicholas budd | April 28, 2008 7:27 AM    Report this comment


Posted by: ABDELATI KABEEL | May 10, 2008 12:48 PM    Report this comment

All of you, Gentlemen, forget, that diesel engines are open throttle (or actually "no throttle") devices. The change in power is realized solely by changing the amount of fuel injected, not by throttle closing. It means that especially by lower power settings there are no losses for s.c. pumping effect of the engine. The fuel consumption comparisons stated above are by max. h.p. By 40-50% power setting the diesel engine is 40% more effective than any given gasoline engine.

BTW.: Thielert is bankrupt in the meantime, bad news for owners of these engines, but probably good news for DeltaHawk, Wilksch and others developing new constructions.

For me the aero-diesel engine is here to stay, in spite of all the turbulence...

Posted by: Paul Kowalski | May 21, 2008 6:23 AM    Report this comment


Posted by: ABDELATI KABEEL | May 21, 2008 10:49 AM    Report this comment

yeah.. As they say that diesel is economical. how will I know then?
pmp classes online PMP Certification online

Posted by: mary grace | April 2, 2011 10:01 PM    Report this comment

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