Diesel vs. Mogas
Last month, on the drive from our hotel in Austria to the Aero exposition halls in Friedrichshafen, the route carried us over a stretch of one of those famous autobahns with no speed limit. Actually, it may have had a posted limit, but no one was paying any attention to it. I wound up our little Ford Ka rental to a blistering 162 Kph—about 100 MPH, a speed at which it's not terribly confidence inspiring.
As is common in Europe, I looked in the rearview mirror to see a pair of headlights closing at the speed of heat. It was a Mercedes turbodiesel that swished by doing probably 230. It was out of sight in 20 seconds. You see a lot of this in Germany because Germans like their high-performance cars and they like economy, too. It wouldn't surprise me if the Merc was making 30 MPG even at that speed.
In Europe, diesels have about 53 percent of the automotive market, so the fact that this technology migrated to airplanes, first by Thielert and now by Austro, was inevitable. Diamond didn't push diesel sales in the U.S. because it felt U.S. buyers don't get—or at least don't want—diesel engines in cars or airplanes. (Boats and trucks are another matter.) One comment on our Austro factory tour video summed it up: "408 pounds; 168 HP. No thanks."
This is a fairly typical diesel prejudice not entirely without merit, but also likely ignorant of careful analysis. On paper, both the Thielert and Austro diesels have several factors going against them. The big one for Austro is weight, but that's an issue with Thielert, too. Second, they have gearboxes. It's not like reduction gearboxes haven't been successful in airplanes and going way back, too. The Rolls Royce Merlin had a gearbox. So do many radial engines. It's just that for light aircraft, gear trains have had a checkered history and the truly bulletproof light GA engines have tended to be direct drive. Last, diesel engines generate strong torque pulses that are hard on props, requiring either a clutch (Thielert) or a torsional damper (Austro).
Thielert attacked the weight problem by recasting the basic Daimler-Benz four-cylinder diesel it used as a core in aluminum, a decision which brought its own problems. Austro, on the other hand, stuck with Daimler's original cast iron as a tradeoff for durability, leaving it up to the airframer to certify at a higher weight and reduce empty weight. (Diamond did both, cancelling out much of the Austro's higher weight for its diesel twins.)
In general, the diesels have been more expensive to buy, have shorter overhaul periods and have periodic component inspections that are a nuisance. Furthermore, Jet A is only a bit cheaper than avgas and in some places, it's more expensive. So you add all this up, and the diesel is doomed, right? Well, it would be save for one simple fact: Diesels are more economical than gasoline engines by a margin great enough to more than offset those disadvantages. I've interviewed a number of Thielert owners, reviewed their numbers and sure enough, even with all the troubles Thielert has been through, they still come out even or a bit ahead against a gasoline engine. As diesels mature, the trend line moves more in the diesel's favor.
Here's a simple example. A 180-HP parallel valve O-360 costs about $20,000 to overall, with a 2000-hour TBO and burning about 9 GPH, block to block. Using $6.04 avgas, the hourly operating cost is $54.36 for fuel and $10 for the engine reserve.
Compare that to the Austro, which costs about the same to overhaul—actually, at $19,400, a little less. (It's not clear to me if this is loss-leader pricing. I'm plugging in the numbers Austro gave me.) The engine burns about 6.5 GPH of $5.65 Jet A for an hourly of $36.72 for fuel and $16 for the engine reserve for a total of $52.72. That's $11.64 an hour less than the gasoline engine. Not impressed? Me, neither, because over the life of engine, that's about $14,000 in savings, a portion of which will be chewed up by the required inspections and component replacements. (To be fair, the Lycoming will have some of those expenses, too.)
But here's where the diesel turns a big corner. Diesels are traditionally high durability engines and Austro would like to get the TBO to 2700 hours. If they succeed, the engine reserve drops to $7 and the hourly goes to $43.90 against the gasoline engine's $64. Over the 2000-hour run of both engines, that's $40,000 less and that ain't chump change for a flight school, which is where many of these airplanes are used. The Thielert numbers aren't as favorable because at 1500 hours, it has to be replaced at a cost of $42,000, but the directionality is the same. At 2000 hours, the Austro reserve goes to $9.70, but it still comes out ahead of the gas engine.
The numbers above reflect U.S. fuel prices. In Europe, the equation favors diesel more strongly because avgas is so expensive. Jet A is about 18 percent cheaper than avgas in Europe. In the U.S. the Delta is about 6 percent.
Mogas is a field leveler both in Europe, where it's more common, and in the U.S., if it can gain a meaningful market foothold. In the U.S., according to AirNav's surveys, mogas averages $4.62 when dispensed at an airport. That's about $1.40 less than avgas. Plugged into the model above, the 2000-hour life cycle savings for mogas against avgas would amount to $25,600—basically the cost of an engine overhaul and then some. Not bad, albeit not quite as efficient as the diesel.
On the other hand, if the 180-HP power band is framed as mogas against diesel, you could argue the two are about equal in the U.S. Although the diesel still enjoys a slightly lower life-cycle fuel cost, its inspections and component replacements can eat that up. In Europe, mogas ranges from $7 to $9, while Jet A is $7 to $10.50 Where the prices are the same, the diesel enjoys a huge advantage in overall cost. If the spread is $3 in favor of mogas, the mogas engine wins, but by a smaller margin because it burns more.
So where does this leave the aerodiesel market? It's a good question that no one can really answer. Thus far, diesels have gained a foothold—just. Thielert has manufactured about 2600 engines and Austro about 450. Diamond is the only OEM to bet big on diesel, with the Austro investment—both because of economy and fuel availability. Looking eastward into Russia and Asia, Diamond is looking for a world fuel. Jet A is definitely it; mogas might be; avgas definitely isn't. Diamond doesn't think mogas will be a player on airports in Russia and eastward.
Continental is covering the board with a couple of new engines approved for mogas, but also an emerging diesel project. Lycoming is bearish on diesel and has aggressively pursued low-octane approvals for many of its engines, albeit not exactly mogas, but a lower-octane aviation fuel. With its new 912 iS, Rotax remains firmly in the gasoline camp, with engines approved for mogas up to E10. The 912 iS is fuel-efficient enough on gasoline that I doubt anyone will bother with a small horsepower diesel.
The major inflection point will come in the high-horsepower market—the 300-HP engines and higher used in airplanes such as the Cirrus and Cessna's heavier models. It seems unlikely that mogas will be a player here and when I say mogas, I'm excluding low-octane alkylate-based offerings like Total's new 91UL. So the visible choices may be the elusive replacement for 100LL or Jet A/diesel. Since I think the likelihood of an affordable, economical turbine is about zero, that leaves a Jet A burning piston with 300 or more horsepower.
Austro is working on this very thing in conjunction with Eurocopter. The AE440, a 400-HP plus V-8. Austro's timeline on this is about a decade, at which point it sees a market gap opening. Besides the large lump of capital required, the biggest challenge will be weight. The under 200-HP diesel represents a power-to-weight sweet spot; as diesels get more powerful, they also get heavier and at some point, they're just too heavy to work no matter what their power output. (Pipistrel, of course, took the tack of developing a low-drag airframe that requires a small displacement gasoline engine that can operate on mogas. But the airplane is some distance from having market impact.)
Where this is going is anyone's guess. The diesel numbers hold up to short-term scrutiny, but the technology just doesn't have enough field experience to prove itself long term. Austro has inarguably proved that the engines aren't too heavy to perform well. Mogas suffers from some of the same ill-informed prejudices that diesel does, but neither it nor diesel are igniting a wild-eyed buying frenzy. As for avgas, well, who the hell knows? All we get are bland and vague assurances that a replacement will be there. Not exactly the stuff of confidence building.
My guess is that a fragmented mix will emerge, dominated by avgas engines for another five to 10 years, with diesel and mogas powerplants grabbing a slice of the market. If you believe in the economy of scale holding prices down, that may not bode well for either diesel or mogas making a dent in the universe quickly, as Steve Jobs used to say. That's especially true if you're an airport operator trying to maintain three fuel systems in a declining market.