Plane & Pilot
Tuesday, March 25, 2014

What’s Up With Diesel?

Diesel power was once touted as the savior from the threatened avgas shortage. Is that still true?

Diesel aircraft engines do have several advantages over avgas mills. The two most important are fuel availability—jet fuel is rapidly becoming available practically everywhere—and efficiency. Diesels are far more efficient than avgas mills. Spontaneous combustion engines enjoy a specific fuel consumption of about .32 lbs./hp/hr., whereas avgas engines rarely score better than .42 lbs./hp/hr. That's 31% less fuel burn to generate the same power. Additionally, jet fuel is considerably denser than avgas, though it takes up the same space volumetrically. Diesel weighs 6.7 pounds/gallon compared to 6.0 pounds/gallon for avgas. While that may cut into payload, it also translates to an additional 12% efficiency.

In other words, diesel engines start off about 43% more efficient than avgas mills. In real terms on comparable 200 hp engines running at 75% power, an avgas power plant could be expected to burn 10.5 gph, roughly the book burn in a Piper Arrow, Mooney 201 or Cardinal RG (all of which used the popular Lycoming IO-360). A diesel using the same percentage of power will consume more like 6.0 gph to produce the same 150 hp.

That has a major impact on range, enough that the Diamond Twin Star was the first diesel-powered, fixed-wing aircraft to cross the North Atlantic nonstop. The Diamond made the trip in 12.5 hours, burning a mere 2.87 gallons per hour per engine.

Most diesels are turbocharged for smooth, quiet operation up high, and they're often computer controlled with FADEC (Fully Automatic Digital Engine Control). That means you can dispense with the prop and mixture controls, and there's no longer any need to worry about hot or cold starts.

The question still remains whether there's a sufficient market for diesels that transcends the need for a fuel to replace avgas. On one hand, it's hard to imagine that any 100LL replacement could be developed, certified and in place within the next five years. On the other, the road to certification can take nearly that long.

Diesels (which, ironically, aren't certified to run on diesel fuel) can burn a fuel that's here and now. We have only to wait and see if the major American manufacturers will adjust their thinking to embrace the new generation of an old technology.


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