16-Wheelers In The Sky: Why Not Diesel Aircraft Engines?


Diesel aircraft engines.
Can an engine maker spell diesel without the 'die'?

Why Haven’t I Heard More About Diesel Aircraft Engines?

To kick off the content on Flyopia, I want to start with a subject that’s basically the heart of aviation: engines.  The Wright brothers had to develop their own aluminum engine to make the’ flyer’ glider work sans Kitty Hawk.  But since then the drive for cheaper, more reliable, and now more fuel efficient, engines (excluding the jet of course) has been mostly evolutionary and linear as far as I can tell.  So when I came across this article on AVWeb from 2008, I was intrigued.  While this particular article highlights the failure of the Thielert AG/Centurion diesel company , the mention of that engine made me pause and think, Hey, diesel?  Yeah, why not?’   Apparently, in the past few years there have been quite a few developments with diesels in general aviation beyond the dirigibles of yesteryear. For Thielert it didn’t pan out,  but perhaps there’s a future?

My TDI is Torqued

My car.

Another thing that stoked my interest about diesel aircraft engines was my appreciation of my current car.  Last year, I bought a 2012 Volkswagen Jetta Sportwagen TDI.  The TDI stands for ‘Turbocharged Direct Injection’, or diesel with a turbocharger.  Being the first diesel I’ve ever owned, I have been incredibly impressed with it.  It has much more power (and by that I mean torque) than I ever expected in such a relatively small engine (2.0L, 140 HP / 236 lbs-ft torque).  The gas mileage has also been great (over 35mpg in mixed driving mostly).  Plus, diesels have known reliability in long-haul trucks, and now with the new clean diesel technology, you no longer leave a soot trail visible from space.

So Why Not?

High torque with fewer mechanical parts and better fuel economy, what’s not to love?  I found another company that seems to still be in business selling diesels called DeltaHawk.  They advertise a lot of advantages in addition to the air mileage and aforementioned positives, and include benefits such as ‘single lever operation’ requiring no mixture control.  But does diesel in general have a future in aviation?  An old paper from a group called the Aircraft Engine Historical Society has a six-part discussion on the subject.  It was written in 1940 so it’s a bit dated ($0.05/gal fuel anyone?) but here’s the last part discussing advantages, of which they are numerous and seem like they would still hold true today.

Of course the diesel option, at least for my car, was quite a bit more expensive, and perhaps the same would be true of aircraft engines?  I would imagine that the low torque is only an advantage up to a point because you can only pitch a prop so much before you’re just moving air away.  A more complex gearbox is probably necessary, and perhaps that makes the price too exorbitant.  But with rising fuel prices, I would hope that in the long run maintaining and operating a diesel would outweigh the costs.  Just because one company couldn’t do it, does that mean the technology isn’t worth pursuing by others.  I’ll keep an eye out.