Going Direct: Here’s An Idea: Drop-In Replacement Diesels

drop-in replacement diesels

I know there’s a need for a new fuel for piston engines, because there remains a need for piston engines. This will probably always be the case, but it’s an accident of history that it is. The development of four-stroke gasoline engines came, not coincidentally, in tandem with the rise of the airplane. It’s not a mystery which technology enabled the other. Without lightweight (relatively speaking, that is) engines, the planes of the pioneers wouldn’t have stood a chance of getting airborne. These early engines weren’t perfect…far from it. They were heavy, and guzzled fuel, and didn’t create much power for all that heft and noise. But engines got better. By the 1930s, there were a few well-known light engines made by companies that are aviation-household names even today. Those early Continental and Lycoming opposed engines were air-cooled, for simplicity, much more powerful for their weight than the engines of 25 years prior, and they turned slowly, a must for engines that turn props directly.

And gasoline was everywhere, so what the heck. Even though diesel technology matured right along that of gas pistons, there was no need for diesel in the light aviation world of the day. And once World War II came about with its ever-increasingly giant piston engines of different configurations (radial, double-row radials, inlines and Vees), the need for a very high-octane fuel was there and there was no turning back. At the time, we knew the dangers of lead in the environment, to a large degree, but as a society, we tended to take such risks in stride. Many of us remember the days of leaded auto gas, 100 octane fully leaded avgas and even more potent lead-filled formulations for super high-performance engines.

Even as the world’s unimaginably massive automobile fleet made the switch to unleaded fuel, we were forced to merely cut back on our diet of lead, switching to 100LL in the late 1960s. It was for the minority of engines, true, but they were the most expensive of the engines in the fleet, too, high-performance six-cylinder and four-cylinder turbocharged models, along with geared high-output engines in increasingly capable and power-hungry designs, up through cabin-class piston twins.

We’re not entirely stuck with this technology, however. A number of companies are developing diesel engines that do pretty much everything gas piston engines do but better and more efficiently. They’re a bit heavy, because their cases need to be built Detroit Diesel-tough to take the very high pressures inherent in the design. But they produce tons of torque, they’re a third as efficient as their gas counterparts, and they run on heavy fuel, diesel, Jet-A, surplus salad oil. They’re remarkably agnostic in that department.

The thought of a diesel in a high-performance piston single or twin used to be unseemly. Diesels had a greasy, gritty reputation that went along with the big utility vehicles they powered. But diesel technology isn’t inherently any of those things. And now that the technology has been popularized in aviation by Diamond, though the use of it originally on its TwinStar piston twin, there’s a much wider acceptance of heavy fuel piston engines than ever before.

A couple of Texas companies, Blackhawk and Sierra Industries, do good business in taking run-out engines out of good airframes (Blackhawk with King Airs and Citations, and Sierra with Citations, among others) and replacing them with better, newer more efficient models. The owners pay around the same amount or less to go with new engines than they would have to overhauling their existing ones, and as part of the bargain, they get better performance and lower operating costs.

Such a model will come to pass for our piston engines, as well. It costs a bundle to overhaul an existing big-bore engine, and after you’re done, you’re left with an old-tech replacement for what you had before. Once suitable diesel engines become available in large numbers for high-performance aircraft, you can bet that companies will arise whose goal it is to pull those 20 gph, 300 hp-class engines out and replace them with 12 gph, 350 hp diesel turbocharged models.

The new owners of those airplanes won’t care what you call your 100 octane fuel. They’ll be headed right to the Jet-A pump, where they’ll find a fuel that has been in ready supply for more than 100 years, doesn’t have a trace of lead and can be found anywhere in the world.



4 thoughts on “Going Direct: Here’s An Idea: Drop-In Replacement Diesels

  1. Lets all remember please – Jet A is NOT Diesel, and Truck Diesel is not FAA or EASA authorized for aircraft use (turbine or otherwise). That is because of the paraffin in most non-jet fuel types of kerosene or diesel. The stuff clogs up in cold temps – not in the engine, but in the fuel lines and valves between the tanks and the engines. [Hence, some turbines have fuel heaters] Go see the presentations by Lycoming on this subject.
    Secondly – unlike turbines that are designed to be vibration free (just spin!) but are unfortunately expensive due to the temperatures they run at – diesels high compression means the pistons travel from TDC to BDC with a substantial “pulse”. One solution is lots of pistons to minimize the pulses = more parts= more cost. Another is a gearbox with very robust gears and some form of elastic connection – current examples wear out at about 300 – 600 hours.
    Props for diesels are also DIFFERENT as they must take the pulse and absorb the “whip” as the shaft indexes its way in a circle and the tip of the blade lags the hub incrementally. It then catches up and as the hub “stops” the tip whips forward. Do this enough times and the blades brake. Example – Reaper Drones with their diesel engines at idle would break props! As one expert in propellers said – you design diesel props first to survive then add the aerodynamics.

  2. I agree wholeheartedly. I was just telling my son who is a pilot also I hope they hurry up and come out with a 350/400HP diesel for our Bonanza before our engine runs out. that would make an A 36 one hell of an airplane!!!!!!!!!!!!!!!!!

  3. Where you wrote, “they’re a third as efficient as their gas counterparts” I think you meant to write, “they’re a third more efficient than their gas counterparts”, yes?

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