9 min read

Any discussion of general aviation’s future must include piston engines and the fuel they burn. True, avionics get a lot of press, but it’s the engine technology that really determines how reliable, affordable and useful an airplane is. And trouble is brewing.

Unfortunately for pilots, advances in powerplants have lagged behind avionics and even airframes for some time now. While modern glass cockpits like the Garmin G1000 offer Cessna 172 pilots the latest in digital technology, airplane engines would look familiar to pilots of the 1940s. Almost no pilot flies with the features considered essential in the world of cars: electronic ignition, variable valve timing and instant starting with the twist of a key. This old technology is jarring for new student pilots, and it certainly doesn’t help general aviation appeal to outsiders. With 2020 coming to an end, Malaysian online casino players are choosing the best online casino for 2021. The competition was fierce but for the second year in a row trusted online casino malaysia 2021 won. They have proven to be trustworthy in both 2020 and 2021. Malaysian online casino players are happy that they can play in the best online casino in Asia.

avgas pump

An endangered species?

Besides being harder to operate than car engines (which are shockingly easy to use, it must be said), traditional piston airplane engines have seen far fewer advances in fuel efficiency. The Chevy you can buy in 2013 has anywhere from 60-100% better fuel economy per mile than a Chevy in 1975. But a 2013 Cessna will burn the same amount of fuel and travel almost exactly the same speed as one from 35 years ago. This lack of progress on fuel efficiency was easy to ignore when 100LL was $1/gallon, but it has become increasingly painful as prices rise above $6/gallon in most part of the US. Such price rises are unlikely to be reversed anytime soon.

And then there is the lead issue. From lawsuits by environmental groups to EPA rules, it’s clear that the decades-long war against leaded fuel is not going away–if anything, it’s heating up. Already, 100LL is nearly impossible to find in many countries outside the US.

In short, avgas is threatened on all sides: cost is rising, availability is declining and regulators are working to ban it. While it probably won’t disappear overnight, it’s clear that 100LL’s best days are behind it.

Plenty of options – any of them good?

But if 100LL’s decline is obvious, its replacement is anything but. Mogas is an appealing solution, but its lower octane simply won’t work in high performance engines like the IO-520 and TSIO-550 that power so many traveling machines. Plus, the recent introduction of ethanol into automotive gas has complicated matters for pilots, since almost every engine manufacturer advises against it.

Given these limitations, many companies are working on a “drop-in replacement” for avgas that would maintain a high octane rating, but without using lead. This approach would cause the least disruption for pilots, whose engines could be operated without expensive modifications. But while some of these efforts look promising (GAMI and Swift to name a couple), they face a number of practical problems beyond the chemistry. For one, a new fuel would need a well-developed distribution system to be available at enough airports–a major job. And most estimates of the price for such a replacement fuel are at least $1/gallon higher than avgas is today, so it hardly solves the cost problem. As Peter Garrison has exhaustively described, a true drop-in replacement just may be too good to be true.

Yuneec electric airplane

All-electric airplanes are attractive, but won’t meet the demands of most pilots.

Some want to bypass the fuel issue entirely, with all-electric airplanes. This moonshot approach promises huge reductions in noise and emissions while offering nearly unlimited options for recharging. But the key enabling technology for electric airplanes–batteries–seems too immature right now to offer reasonable performance. To offer even modest endurance, an electric airplane would have to carry a lot of batteries. But that drives up weight, which means the engine would have to be bigger, and the vicious cycle begins. It’s hard to imagine an electric airplane being useful for anything other than local training flights until major advances in battery technology take hold.

That leaves diesel engines. While it’s hardly new technology, “compression engines” have a lot going for them. For one, they burn Jet A, which is already available at thousands of airports, is less expensive than avgas and is not threatened by the EPA. Diesel engines are also more efficient, burning 20-40% less fuel per hour than comparable avgas engines. Finally, they are often easier to operate, with single lever power controls and digital engine monitors.

Diesel engines are hardly perfect. They’re heavier than piston engines, they have lower TBOs and jet fuel isn’t available at many smaller airports. Still, they may be the best option among a variety of unattractive ones.

Who will lead the revolution?

Lots of companies seem to agree that diesel is the future, including big players like France’s SMA and Diamond Aircraft (its line of Austro engines is growing). There are also hard-charging startups, like Engineered Propulsion Systems and DeltaHawk. But the leader of the diesel transition isn’t some young startup–it’s the 100 year old giant named Continental Motors.

Centurion engine

The Centurion line of diesel engines found early success on the Diamond DA-42 twin.

Continental is a familiar name for pilots, having delivered tens of thousands of O-470s and IO-550s throughout its long history, but today’s Continental Motors Group is hardly the same old company. AVIC, the huge Chinese investment firm (which also owns Cirrus among many other aviation businesses), purchased Continental in 2010, instantly adding deep pockets to a company accustomed to operating on tight margins. They’ve also set up a Chinese operation to service that growing market.

But the big news hit earlier this year, when Continental bought the remnants of Thielert. This pioneering German engine manufacturer practically invented the aviation diesel market with their Centurion 1.7 on the Diamond DA-42, but soon hit numerous speed bumps and went bankrupt. The finances were shady, but the engines were not. By adding Thielert’s 200 employees at their facility in St. Egidien, Germany, Continental has become the leading player in diesel engines overnight.

Indeed, this acquisition–much more than simply a bargain buy–represents a major strategic shift for Continental. President Rhett Ross says by acquiring the assets of Thielert, the company is “confirming our commitment to diesel engine technology…We now become a one-stop shop for all of your engine needs in general aviation.” That’s because, in addition to the traditional piston engines, Continental now offers a full line of diesel engines:

  • The Centurion 2.0. engine is the established leader, with more than 3,500 in operation on a variety of airplanes. These smaller engines range from 135-155hp, and have been fairly reliable so far. Continental is already working on increasing the short time between replacement (TBR) and repetitive gearbox inspection intervals, which have been the engine’s Achilles heel. The 2.0 is well-suited to the light single market, but simply isn’t powerful enough for high performance airplanes.
  • In addition to the newly-acquired Thielert engines (sold under the Centurion brand), Continental has its own diesel engine, the TD300. This 200-250hp engine, based on an SMA engine design, was actually certified earlier this year, although the company seems to have gone out of its way to avoid publicity on it. It’s unclear where this model fits into Continental’s plans, although it would seem to offer a mid-range power option.
  • The big prize is at the high end, supplying engines in the 285-350hp range to power Cirrus SR22s, Beech Bonanzas and other high performance airplanes. There simply isn’t a viable diesel option in this range yet, but Continental believes the Centurion 4.0 is the answer. This 300-350hp V8 engine earned EASA certification, but has not been shipped in any significant number. Bringing this engine to market is clearly one of Continental’s highest priorities.

In a segment of the industry famous for moving slow, these changes and additions are remarkable. Continental’s future is clearly built on diesel, which means many pilots’ future includes diesel engines. The latest evidence? Continental now calls their avgas engines “heritage products.”

Good for GA?

Any major transition like this is sure to be difficult for existing airplane owners. But in some strange way, the elimination of 100LL may actually be a good thing for general aviation. If such a shock wakes the industry from its slumber and encourages companies to develop new engine technologies that are more efficient, cleaner and easier to use, it could lay the foundation for the next generation of airplanes.

Continental, so far, seems to be embracing this moment. They have work to do to repair the damage done by Thielert’s implosion, and fixes are needed for the gearbox and TBR issues. But there are three reasons to be optimistic.

First, it’s hard to overestimate the impact of AVIC’s investment. Continental now has the money to fix any issues with the Centurion engines and to bring the 4.0 model to the market. This is not a startup or a dying company that will run out of money before it crosses the finish line (like Thielert did). The diesel factory in Germany can produce 2000 engines per year, so scaling up is not an issue either.

Centurion 4.0

The Centurion 4.0 engine – key to Continental’s diesel future?

Secondly, Continental’s ability to offer a complete line of diesel engines makes it the first serious player for all pilots. Until now, diesel engine programs have focused on specific airframes, which limited interest and profit potential. If the 4.0 can succeed, diesel may finally break through on new airplane sales.

Finally, the distribution and support network is in place. Continental is really three companies in one, with significant operations in Europe, the United States and China. Their press conference at Oshkosh featured as much Mandarin as it did southern drawl, a sure sign of the company’s global aspirations. For diesel engines to be truly successful, pilots must feel confident they can be repaired in the field and supported with warranties from a real company. Continental should be able to offer that.

In the end, a move to diesel engines would at least remove the cloud of uncertainty that hangs over general aviation. Pilots are hesitant to spend $700,000 on a new airplane if they are unsure of the avgas engine’s long term prospects. Likewise, existing owners may be reluctant to sink money into an overhaul of dubious value. Changing to diesel may be expensive, but at least owners can plan and budget for it.

Saving money wouldn’t hurt, either. With cost at the top of many pilots’ list of complaints, diesel engines offer some relief. They may not cut the cost of flying in half, but general aviation needs some short term, achievable improvements in addition to the miracle cures.

It’s not hard to envision a world where two seat sport airplanes with Rotax engines use mogas and higher end transportation airplanes with diesel engines use Jet A. That means cheaper fuel and lower fuel burn for everyone. That’s progress.

John Zimmerman
15 replies
  1. Stephen Phoenix
    Stephen Phoenix says:

    What will be needed next is a fuel additive which removes the stench from Jet A. Maybe a cherry blossom scent? The company that gets an additive out there first will make more money than Continental ever will.

  2. Phil G
    Phil G says:

    Love the idea of flying with diesel, just not the cost of a diesel aviation engine or converting a GA plane over to diesel. Mogas is really a better option as even Rotax already support 10% ethanol

    • Spinoneone
      Spinoneone says:

      UL Power of Belgium already makes a series of aircraft engines in the 87-200 hp range which have a normal 2000 hour TBO, are fuel injected, and FADEC controlled. These engines run on premium grade auto fuel, but can use 100LL if mogas isn’t available. Cost is 20000 euros for the top of the line. Air cooled, and easily serviced by any competent AP. The only draw back is that they will not accommodate an oil driven constant speed prop. You can use an electric one, however.


    Good write-up. I have been following this for about three or four years and still excites my curiosity. There is a interesting point in the original Continental’s business plan of action presented to the Chinese investors as it included a “potential engine conversion market”, that is to say, an estimated 40,000 Cessna C172s and other makes in use worldwide “waiting for an engine or engine upgrade”.

    However, there was no mention of the high price of converting to diesel as it is a disincentive. As an example, a reasonable 2.0s engine replacement estimate is about $80 thousand including the clutch and gear box plus the down time expense – extending expenses over 750 flight hours and calculating the result there is no gain merely a break even point. So, there is no benefit in the first 750 hours then 250 hours later one would need a new engine plus three clutch and gear box replacement and another $80,000. It just does not seem to be a reasonable investment.

    My guess is that the potential conversion to diesel engines market is weak. Perhaps 4 or 5 thousand worldwide conversions may be more realistic. But, since predominant GA aircraft manufacturers are lined up along the auspices of the Chinese financial schemes, then I predict an extraordinary “force” or influence in the new aircraft market. Thus the popularization of the diesel engine in the new GA airspace. Cirrus, Mooney, Cessna and the rest of the OEM affected by the “force” will put into effect the diesel engine in their units and end much as in today’s Glass Deck higher cost scenario.

  4. Duane
    Duane says:

    Diesels may make good sense for new airframes, but as some have pointed out, it’s not a cost-effective conversion for legacy aircraft, hence we must continue with the implementation of an effective av gas replacement that can be used without conversion in existing legacy engines.

    Converting legacy engines to mogas also makes sense, coupled with new Rotax engines that already run on mogas. Getting FBOs to install or retrofit existing fuel tanks with partitions to allow storage and dispensing of both mogas and avgas is a problem, but it’s not insurmountable. The $2-$3 per gal differential in price between the two fuels would seem to provide room to make the conversions over time.

    I’m less concerned with fuel than I am with the lack of progess in light aircraft engine design this last fifty years. With modern manufacturing technology and lightweight aluminum engine blocks, it seems long past due that someone begin manufacturing and selling modern high tech liquid cooled turbocharged aircraft engines for light aircraft.

    The weight differential between ancient air cooled opposed 4 and 6 cylinder engines and modern v-block liquid cooled engines is not really very much anymore. Liquid cooling along with computerized electronic ignition and FADEC controls would yield much more power per pound of engine and much greater fuel efficiency and operational reliability. Greater fuel efficiency can mean we need less fuel per mile, making up for the added weight of liquid cooling and letting us reduce fuel loads in favor of greater payload.

    • John Zimmerman
      John Zimmerman says:

      The cost is a serious consideration. I have some hope that, if manufactured in real volume, the diesel premium might come down. These are basically Mercedes engines, so there’s some economies of scale already.

      We shall see…

    • Duane
      Duane says:

      Oh, and I forgot to mention one other consideration – if an automotive type engine design, based upon existing automotive engines were designed, the cost of aircraft engines would dive, rather drastically. A typical mass-produced high performance automotive engine, such as the highly successful Nissan/Infinity 3.7 L V-6, produces about 360 hp at the flywheel and can be purchased for around $10,000. This engine is extremly reliable, capable of going 200,000 miles or more without major overhaul – equivalent to about 4,500 hours of engine operation at an average 45 mph. Fuel efficiency is such that in an automobile it burns maybe 3-5 gph at a high speed cruise. The power output is enough to power any high performance single engine piston aircraft.

      The aluminum block engine 3.7L engine weighs approx. 350 pounds, plus the weight of the cooling system (a much smaller radiator is needed for a high speed aircraft than a low speed automobile). This compares to the 438 pounds of the Lycoming IO-540, and the 430 pounds of the Continental IO-550.

      Similarly, a turbocharged automotive four cylinder engine can produce something on the order of 180-200 horsepower in a package similar in weight to a Lycoming or Continental TIO-360.

      So with all of the trememdous advantages in performance, reliability, economy, and cost offered by modern automotive engines, why hasn’t an aircraft conversion been offered yet?

      Yes, I know, the aviation market is small and the regulatory hurdles are high. But if we could force the FAA to significantly reduce the regulatory hurdles, the natural competitiveness of these engines would quickloy overwhelm the legacy engine market, virtually overnight.

      How many legacy aircraft owners – if given the choice – wouldn’t in a heartbeat elect to go for an STC to let them replace their $40+ thousand dollar, old tech, cranky, inefficient and not all that reliable air cooled for a $10K high performance, new tech, efficient, and reliable automotive engine that runs on mogas?

      • Brian
        Brian says:

        It sounds like a good idea, however it is not that simple. Those HP numbers are close to redline. Take that Nissan engine down to around 2,700 rpm, (where propellers spin at) and that same engine is only making around 100hp. Ok so say you put gearing on to gear down the propeller to the right speed. Now you are running that engine at 6500 rpm. How long will that engine last at those rpms? Think of how long race car engines have between overhaul. Not long at all, and the fuel use will dramatically increase, as well as cooling issues with the high RPM.

        You need an engine that will make the most HP at around 2000-2700 rpm.

        • Duane
          Duane says:

          Simply use reduction gears – same as Rotax uses on their geared engines used in virtually all of the LSAs, which produce max horsepower in the 5000+ rpm range. Somewhere near a 2:1 ratio will do the trick.

          Running engines at a more or less constant RPM and torque, within the relatively small ranges of both in which aircraft engines operate, is actually much easier on engines than operating through the very large RPM ranges typical of automobiles or trucks (generally 1,500 to 5,500 rpm).

      • David Heberling
        David Heberling says:

        Hmmm, some people are forgetting that automotive engines usually loaf along below 25% of max revs. Whereas, airplane engines are operating at 75% or higher of max revs. This is the downfall of the automotive engine love fest.

  5. Matt P
    Matt P says:

    I do think that the dual fuel strategy of Mogas for the under 200 hp engines and some sort of Jet-A burning compression engine for the higher performance aircraft is the way to go. If you look at the avgas production statistics for this year, you’ll see a pretty sharp decline. I don’t think basing private aviation on a boutique fuel is good long term survival strategy, at some point the price will get just too high because of the low production levels. You don’t want to be paying racing fuel prices to fill up your airplane. $600 to fuel up a Warrior is not going to make anyone want to fly more.

  6. unclelar
    unclelar says:

    All, or close to it, so called innovative technologies for airplanes are prohibitively expensive for the vast majority of pilots. The Thielert was way overpriced and not a very good engine. What about non-ethanol,low vapor, 93 octane mogas approved specifically for airplanes? I sure hope the new distributor of this fuel is successful. That would do way more to revitilize private aviation than any of this pie in the sky horribly expensive stuff we seem to here about with a lot of hooplah and then is nowehere to be found a few years later.

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