The diesel engine has long been a mainstay in industrial power. Heavy equipment like tractors, excavators, earth movers and other utilitarian machines rely on compression ignition power for many reasons. But, what about the opposite end of the spectrum? The very light, delicate, and graceful forms common to the general aviation community are embracing this versatile powerplant. The application of diesel in aircraft is nothing new, in fact it has been in use for nearly 80 years. When considering the flying machine has only been with us for about 112 years, diesel has been a neglected fuel of aviation from the beginning.

However, diesel fuel is making a comeback in modern aircraft. The advantages are becoming more and more obvious as fuels requiring increased refining grow in price, and scarcity. At Diesel Army we are interested in diesel culture as a growing community, and aviation is one of the fastest growing markets. We hope to give you a retrospective on the history of diesel in airplanes, and introduce some of the new powerplants breaking into the market. The next time you look overhead you just may hear the familiar sound of a diesel engine.

Advantages Of Diesel

The primary reasons for diesel’s explosion into the aviation market are the availability and affordability. Light aircraft operating in remote regions of the world are often challenged by a lack of availability when it comes to fuel. Traditionally, piston-engine powered aircraft run on Avgas, a gasoline-based fuel engineered to perform in the specific application of aviation.

Avgas is generally a tetraethyl lead enhanced fuel — like race gasoline. Most general aviation engines are air-cooled to reduce weight, and therefore run higher cylinder head temperatures. The availability of Avgas is variable in countries with less infrastructure. Aide, or humanitarian flights in and out of the continents of South America, Africa, and Australia are often dictated by fuel planning. This limits the reach of said operations.

Diesel fuel is widely available, and if aircraft can be retrofitted to operate on it, the radius of range will increase such that almost anywhere is reachable. Diesel engines are known for their robust design and reliable operation. The number-one paramount of aviation engines is reliability. If your car stops running you pull over and get out, but this is not usually an option in aircraft. Two-stroke diesel engines eliminate the need for a valve-train. This simplification alone cuts the number of moving parts down to around half.

Drawbacks Of Diesel

Diesel fuel is a petroleum byproduct, however it does not have the same properties as gasoline. Specialty gasolines like Avgas are blended for the specific needs of aviation in that they are intended for engines running at higher altitudes. Conditions like vapor-lock are combatted by formulations that prevent liquid fuels from vaporizing in lines, and then causing a fuel starvation. Diesel fuel suffers from gelling, icing, and microbial growth issues under adverse conditions.

Aircraft operating in the freezing wastes of the polar regions, or the humidity of tropical or jungle environments may face problems avgas does not experience. In the end, the decision to convert to diesel is an individual application issue. Where are you flying, what are the fuel options, and what performance do you require out of your airframe?

A Little History Lesson

The first successful diesel-powered aircraft was the Junkers JU-86. Junkers was a German manufacturer of aircraft in the mid-1900s. Primarily producing aircraft for the Luftwaffe, it was responsible for many well-known airframes, and the first use of a diesel powerplant. The JU-86 was a large, twin-engine bomber and transport aircraft. First designed in the early 1930s this genesis of diesel and aviation combination featured strange powerplants that foreshadowed the brilliant of simplicity of two-stroke diesel.

Many aviation engines are arranged in a horizontally-opposed configuration to present the least possible frontal area, and therefore create the least drag. The JU-86 was powered by twin-Jumo 205 engines. These diesel monsters were vertically opposed, six-cylinder, twelve-piston, liquid-cooled, supercharged engines, and displaced a whopping 16.63 liters each. The output was around 868 horsepower each, meaning the JU-86 had the power to climb at a rate of 900 feet per minute, and carry a maximum load of around 11,000 pounds.

Diesel fuel has not always been used on diesel engines. Jet aircraft operate on fuels known as Jet-A JP-8, and others. All of these fuels are basically kerosine, the most basic petroleum distillate fuel. Diesel fuel has been substituted for jet fuel in a pinch, and is a point for debate among military scholars and NATO (North Atlantic Treaty Organization) politicians. A single fuel concept for all land and air vehicles is a goal of many forces worldwide.

Modern Aircraft

Modern diesel aircraft are sometimes the products of retrofits, and others are purpose-designed to employ these non-conformist engines. When we think of diesel, grace and flowing form are not the usual knee-jerk associations. Companies like Diamond Aircraft Industries has been working to reverse that stereotype. The DA42 Twin Star is a diesel-powered aircraft that looks just as deserving as any other high-end private aircraft when it comes to aesthetics.

This twin-engined, four-place aircraft features a modern composite construction and a fully electronic “glass” cockpit from Garmin. With touches like these, one expects the powerplant to be something equally exotic.

Powering the DA42 are a choice of engines. Replacing the conventional Lycoming IO-360 gas engines are either a pair of Thielert Centurion 2.0-liter diesels, or in 2007 and up models, the Austro AE-300 engines. The advantages of these engines over their gas contemporaries are hard to argue against. Incredible fuel efficiency is possible at 3.2 gallons per hour, and the comfort of a quiet ride will make the interior that much more luxurious for the few who can afford the $600,000 price tag.

Modern Engines

Diesel powerplants for aircraft range in design and layout just as much as gasoline engines. Manufacturers of conventional Avgas engines are joining in on the diesel revolution. Both Lycoming and Continental have presented certified options for the general aviation consumer. Unique engines like the Superior Gemini, DeltaHawk, and Centurion are blazing new trails for retrofit, and homebuilt applications. In the next section we will address a few of the individual engines that are most noteworthy.

Gemini

The Gemini Diesel is a small horizontally-opposed engine meant as a replacement for the conventional 360 cubic-inch avgas powerplants so prolifically common on the skies. Borrowing design from the historic Jumo 205, the Gemini is a six-cylinder, 12-piston, two-stroke, supercharged or turbocharged engine. These small engines produce between 100 and 125 horsepower, perfect for the light sport aircraft (LSA) community. For those unfamiliar with an engine that has more pistons than cylinders — the Gemini employs two crankshafts on the outer extremes and drive pistons towards each other in the sleeves.

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The covering and uncovering of the ports in the sleeves allows for the intake and exhaust of gases. Forced induction has been an optional standard feature on Avgas engines for as long as we have wanted to climb to high altitudes. As the air thins out it needs to be compressed to maintain performance. Forced induction on a two-stroke diesel is not a performance enhancer, it is a necessity. Compression-ignition engines rely on cylinder pressure to ignite diesel fuel, in a two-stroke the compression stroke is joined with the exhaust stroke for considerable overlap.

DeltaHawk

DeltaHawk Diesel engines are some of the most exotic and forward thinking of the powerplants we will examine. Arranged in a “V” configuration these two-strokes are available as four-cylinder engines to replace the flat 360, but according to Diane Doers, CEO of DelatHawk, “We are going to be doing a six-cylinder, and eight-cylinder to take us to the 250 horsepower, [and] 420 horsepower.”

These liquid-cooled diesels feature redundant forced-induction systems. A turbocharger will feed a supercharger for extra security if a system failure were to happen. The oiling system of the DeltaHawk engines is a particularly interesting feature. Two-stroke diesels do not employ oil-fuel mixtures to lubricate the internal components the way gasoline engine do, this engine features a drysump system that allows mounting of the engine as a “V” or “A.” That’s right, you can mount this engine upside down if your packaging needs are restrictive.

DeltaHawk has thought beyond the needs of the general aviation consumer. The engines it produces have been developed for use in UAV (unmanned arial vehicles), ground power units (GPU), and other applications.

Centurion

The Thielert Centurion line of diesel engines may be the most relatable and recognizable to the mainstream diesel enthusiast. If you spend more time looking under the hood of a car or truck than the cowel of an airplane, these are the engines for you.

Based on Mercedes-Benz automotive engines, these powerplants feature turbocharging, and electronic FADEC (Full Authority Digital Engine Control) management, a feature not commonly found on propeller-driven aircraft, in favor of more simple mechanical fuel injection.

Some of the Mercedes-Benz donor engines used to create Thielert engines include the OM668 and OM629, the later of which is the groundwork for the Centurion 4.0, the most powerful of their offerings. Producing 350 horsepower, thanks to a large turbocharger, this FADEC controlled V8 represents the future of technology in aviation diesel systems.

Lycoming And Continental

Lycoming and Continental are the big two. Like Chevrolet and Ford are to cars, they are the top engine manufacturers in general aviation. Both are widely known for their four, six, and eight-cylinder, horizontally-oppposed, air-cooled, Avgas engines. Of course aviation nuts are nostalgic, so these engines have changed very little in the past 100 years, but certainly aware of the market, Lycoming and Continental have released aviation diesel engines for their loyal enthusiasts.

The Lycoming DEL120 is a 120 cubic-inch engine that was originally designed as an industrial powerplant. Lycoming general manager Michael Kraft explains how this engine spawned it’s aviation wings out of a supply contract with General Atomics for its UAV project, but is still relevant to general aviation users, “The engine is not specifically designed for the General Atomics Gray Eagle, it actually is a 200 horsepower engine that would fit nicely into everything from a Piper Archer to a [Cessna] 172, to a variety of aircraft models, and the full intent of it is to bring to to a solo certification.”

Continental, on the other hand, has released its TD300, a four-cylinder, turbocharged diesel. From the exterior it looks very similar to the air-cooled Avgas models with which most aircraft owners are well acquainted. Mike Gifford of Continental Motors said, “We’ve completed certification of this Continental Motors engine, and we’re going to enter into the market in several different areas. We’re going to enter into the new aircraft production — of existing airframes, converting them a conventional gasoline engine over to a diesel. We’re also going to be concentrating on new airframes — newer technology airframes for a lot of emerging markets globally.”

Unlike many of the engines we have covered, the TD300 is a four-stroke, turbocharged engine, making it a little less intimidating because it resembles technology that is already present in thousands of aircraft.

Conclusions

If you are a homebuilt aircraft enthusiast looking to stand out from the crowd, or the type of flying you do takes you into remote places, diesel may be a consideration. The technology is not new, but in terms of mass accessibility, it is only just becoming available. When it comes to annuals, and rebuilds, your local A and P (airframe and powerplant technician) may be in over his head. The added start-up cost may not be offset by the increase in engine life and fuel economy for many years. As the grasp of the EPA tightens, and Avgas is threatened, we may find diesel a palatable alternative, but until then it won’t see wide spread-use. As a private pilot myself, and an enthusiast of exotic engines the potential of diesel is an exciting prospect for the general aviation community.