What pilots in Europe, Africa, South America and Asia already know about aviation diesels — or, to use the more technically precise term, compression-ignition engines — we’re slowly learning for ourselves here in America. And the more we learn, the more we’re starting to think we could get used to the smell of jet fuel powering our piston airplanes.
As you may know, diesels offer some compelling economic advantages over gasoline engines. An important one is better specific fuel consumption — the measure of power output for each pound of fuel consumed. Another benefit of diesels, which run perfectly well on jet-A, centers on the rising price and uncertain future of avgas. In some parts of the world, 100 low lead can cost more than $20 a gallon — if it’s even available. Here in America, the Environmental Protection Agency has set a goal of outlawing leaded aviation gasoline before the decade is out. The FAA is preparing for tests of replacement fuels, but what it will cost to fill up our tanks is a big unknown. “More than today” is our best guess.
So has the aero diesel’s time finally arrived? Outside North America, turbocharged diesel aircraft engines are in high demand, and that trend will only accelerate. Here in the United States, there is a growing fascination with diesels, but it’s too soon to say we’ve embraced them as the answer to our woes. After all, diesels are heavier, they usually require liquid cooling, they’re often limited by certification in how high they can operate, and they cost more to build. The success of diesels in America, if it happens, will be rooted in the cost-benefit analysis — and right now the math doesn’t favor diesels over a good-old Lycoming or Continental piston gasoline engine.
Cessna is betting big on a four-cylinder engine from SMA for the new 182 JT-A.
But that could change in a hurry depending on what happens with avgas. One thing is certain: The days of 100LL aviation gasoline powering the general aviation fleet are numbered. Maybe that’s OK. If you’ve been following diesel developments lately, you know today’s diesel-cycle engines are really nothing like the dirty and noisy ones many of us remember from diesel cars of the 1970s and 1980s. They no longer spew toxic soot or rattle at idle like a coffee can full of marbles. They also deliver impressive power, and as experience is gained with these newer technology engines, we’re seeing time between overhalls (TBOs) stretch to 1,500 or 2,000 hours and beyond. Eventually, TBOs for diesels could be longer than gasoline engines.
An Image Problem
Still, no matter how good the technology becomes, diesels won’t supplant gasoline engines in America anytime soon. One reason is the sheer number of gasoline piston engines out there, flying in everything from aging Cessna 150s to brand new Cirrus SR22s — around 225,000 of them in all. Another is the continuing dominance of gasoline engines in nearly every production piston single and twin sold today. And many smaller general aviation airports throughout the United States sell only 100LL avgas, limiting where you can fly a jet-A-burning airplane. Also, because they lack an ignition source to keep their fires lit, diesels are restricted in how high they can fly, even with twin turbochargers and hot glow plugs — although this too could be a temporary issue that goes away as diesel experience is gained.
The latest Diamond DA42-VI with its twin Austro diesels and supplemental oxygen is a prime example. It has a service ceiling of just 18,000 feet — which also happens to be the single-engine ceiling for the
airplane. Diamond says it has flown the DA42’s Mercedes-Benz-produced Austro engine to 30,000 feet without ill effect, but for now they are limited by certification from venturing into the rarefied air of the flight levels. Diesels can be prone to flaming out at high altitude where the air is less dense — if it’s cold enough and the air is thin enough, a successful restart could be in doubt.