The Great Debate: are diesel engines the future?

Cessna JT-A throttle

Is this the throttle quadrant of the future?

The struggle to find a long term replacement for leaded avgas, while confusing and sometimes frustrating, has been well documented (we’ve done our part too). What all the coverage makes clear is that 100LL will not be around forever. But in the absence of a viable replacement fuel, this news has caused a lot of concern amount aircraft owners. What is the future?

Trendy answers include small turbine engines and electric airplanes, but both look more like marketing dreams than engineering reality. The small turbine engine idea–say, a mini PT-6 that only put out 300 hp–is certainly appealing, with its high level of reliability and proven design. But most people who know turbine engines say that this idea simply isn’t possible, at least not at a price that anyone would pay. The technology cannot be scaled down enough.

Electric airplanes, on the other hand, are a new technology, one that will have to be scaled up dramatically to meet the needs of general aviation pilots. Right now it is mostly used on gliders and some under-development LSAs. The fact is, current battery technology simply can’t power any airplane for more than about an hour, and can’t power a four seat airplane at all. The dream of an electric airplane that you simply plug in to recharge is probably many years off (if ever).

With traditional piston engines fading, and small turbines and electric motors unable to pick up the slack, all eyes have fallen on the diesel engine (or “compression engine” in Cessna’s marketing speak). While these have been around for decades, diesels are earning renewed attention because of their relative fuel efficiency (30% better in some cases) and their ability to burn Jet-A. Cessna’s new Skylane JT-A is the most visible example of this diesel renaissance, although there are many others, from Diamond’s DA-42 to the DeltaHawk engine program. But diesels are usually heavier than traditional piston engines, and no one is yet delivering a high horsepower diesel that could power high performance airplanes.

What do you think? Are diesel engines the technology of the future for general aviation airplanes? Or is there a better option? Would you buy a diesel-powered airplane? Add your comment below.

34 Comments

  1. C. Frank Bales says:

    I like many things about diesel engines and the time for them to be a part of general aviation appears to be now. They are fuel efficient and develop useful power at low rpms which bode well for use in aircraft. It is my understanding that the problem has been with the inability of propellers to handle the vibration of the power impulses from the diesel engines. Thanks to new prop technology, that major obstacle appears to have been successfully overcome so bring on the diesels!

  2. sage says:

    Funny that as the EPA strangles diesels in the auto industry GA is trying to main stream them. I am a huge fan of diesel pickups due to the longevity, power, and economy. As the EPA has started meddling with diesels the fuel costs have surpassed gas prices, emissions have killed the fuel economy and the ecconomic benefits are gone. However I think they would be a great benefit to GA aircraft. The only question is how do we get Cummins involved?

    • Michael Rosing says:

      “as the EPA strangles diesels in the auto industry”

      Actually, diesel auto sales are increasing much faster than the overall car market: http://tinyurl.com/cly8qqy

      • David says:

        EPA? Diesels are more efficient than ever thanks to the trickle down of gas engine technology. Can you say electronic fuel injection? This probably would not have happened if not for clean air standards. Do you want to go back to carberators? Can you remember what LA and NYC air looked like 30 years ago? Could you get a car 30 years ago that got 30 mpg?

        • Buck says:

          Could you get a car 30 years ago that got 30 mpg?

          Actually much better – my 71 Honda AZ 600 averaged 44.6 mpg over the 4-1/2 years I owned it. That said, diesels have a lot to offer in today’s auto market.

      • Greg Johnson says:

        Perhaps it is just Michigan, but there are a growing number of diesel cars for sale on Craig’s list. The price of diesel fuel for cars is higher than gasoline. When gasoline was 29.9/gal diesel was a couple of pennies cheaper than that. Diesel engines are advertised as more fuel efficient but the cost of the fuel puts the cost per mile of a diesel parallel to a gasoline engine’s efficiency. If current car diesel is allowed to be used in airplane engines then there would be a significant savings.

  3. tom says:

    Our flying club (Paramus Flying Club) already has a diesel 182 (the SMA STC–the previous generation of the engine in the 182 JT-A). So our throttle quadrant has looked like that since 2009. What’s not to like? Per gallon cost is typically lower than 100LL, cruise fuel burn is significantly less (10 gph vs 15 gph properly leaned), and our MX costs have been lower than the stock Continental. Single lever FADEC operation takes the pilot out of the equation in terms of things like proper fuel mixture management, which contributes to consistent fuel economy even when flown by less experienced pilots. And greater fuel economy also gives you greater loading flexibility. If you plan to go 5 hours, you end up burning 350 lbs of Jet-A rather than 450 lbs of 100LL, so you save 100 lbs, even allowing for the fact that Jet-A is a pound heavier than 100LL.

    The only real down side is the higher initial cost. But hopefully that will come down as the number of diesels grows.

  4. Tom Yarsley says:

    When Jet-A is all that we’re allowed to burn, diesels will look even more attractive than they do today. The only question in my mind is what percentage of the avgas-burning fleet will get turned into beer cans. My guess is that just about everything that’s under 260hp will become instantly worthless until buyers who are faced with three choices emerge:

    1. Buy a brand new diesel-powered vehicle (~ $600k – $1mil).
    2. Install an STC’d diesel in an existing airframe (~ $60k – $100k).
    3. Stop flying (free).

    Our flying club operates one of the nicest Tomahawks on the planet ( ! ) but I find it hard to believe that the members would sink 60 grand into a new engine for a PA-38. Faced with spending $200k for an LSA or locking the hangar door for good, I suspect they’ll choose the fond memories.

    In the small-single new-vehicle arena, I’m predicting that within 5 years, diesels are all that we’ll see in 4-6-place unpressurized vehicles that manage to remain in production.

    • No tin cans from Tecnam planes no or anytime in the next 40 years. They burn 87 octane auto fuel.

      LSA are not $200,000, they burn car gas. You can purchase a LSA for $95,000 that burns 3-4 gph, fun to fly and well constructed. Tecnam.net

      Just got in a P2008 with a turbo that flys 140 kts. high altitude. 36% more efficient on fuel for the same engine without tubo.

  5. Stephen Phoenix says:

    Diesels would be appealing to FBOs also; only having to carry one type of fuel instead of two. The one invention which is really needed for diesels (and turbines) is a fuel perfume which masks the nasty smell of Jet-A. Actually, if we ever could get to biodiesel the problem would take care of itself. The bio fuels don’t seem to smell so bad.

    Small turbines do not appear to be a great solution due to initial cost and particularily, fuel burn. I have seen evidence that the initial cost issue is solvable with new designs, modern computer manufacturing techniques and larger quantities, but the fuel burn issue seems tougher. A breakthrough there is about as likely as a big breakthrough in batteries for electric airplanes.

    On the other hand, I don’t think it is impossible that automotive type gasolines couldn’t work in airplanes. Just because current automotive fuels are not compatible with 1939 era fuel systems does not make it impossible.

    • Tom Yarsley says:

      It’s all about octane ratings. High-compression-ratio engines require detonation suppression. Tetra-ethyl lead is the additive of choice. These days, “lead” is a synonym for “death.” Without a fungible replacement for 100LL, most of the higher-performance piston engines out there will die of thirst. The data says that’s fewer than half of the engines, but about 80% of all of the flown hours. It’s an existential problem, and it’s not going away.

      • Stephen Phoenix says:

        Well, actually it is not all about octane ratings. There are a few other considerations like vapor pressure, percentage of volatiles and material compatibility. Octane rating has been handled in the auto world with knock sensors. That technology has been more difficult to incorporate in the airplane world due to high ambient vibration levels and the need for guaranteed power output for performance. But I believe that Lycoming has a handle on that and I believe Rotax also has a solution. It appears to me that a bigger problem is feeding the fuel to the engine. At high temperature and altitude a 1939 engine driven rubber diaphram pump, drawing fuel from an atmospherically vented tank is potentially a bigger problem and harder to retrofit into existing airplanes.

        • Tom Yarsley says:

          Stephen:

          I didn’t mean to leave you with an impression that the other considerations you mention in both of your posts aren’t meaningful. Like you, I believe that they likely can be overcome. FADEC and variable timing can prevent engine destruction, but when all other wizardry is exhausted, the FADEC has to reduce the power output of the engine, to avoid detonation. Ultimately, octane rating can’t be finessed (thus my comment). The consequent re-writing of the aircraft’s performance charts is a deal-breaker for most operators of those (high C-R) engines.

          If the only method of retaining 100% power is to replace the existing high-compression engine (because you can’t buy fuel for it anymore), you might as well apply that old adage: “when you go to the well, get a good drink.” In this case, that means buying a diesel – if for no other reason than to be assured of having a reliable source of fuel.

          As an alternative (for low-power engines), you could purpose-design a powerplant that would run happily on E-15 motor fuel – if you accept the operating limitations that would accompany that choice (altitude being chief among them). Rotax appears to be taking that road, although it appears doubtful that we’ll see any powerplants in the 200hp – 300hp range that utilize their approach.

          It’s ironic that the discontinuance of unleaded avgas could cause a bifurcation that sees diesels dominate the 300hp + category, and E-15-compatibles dominate the sub-150hp category. Curiously, that would leave nothing attractive in the “sweet spot” of yore (O-320/360/540s). That’s a sure recipe for orphaning an enormous proportion of the existing lightplane fleet – certainly the vast majority of the 4-seaters, because you’re not going to see many practical (payload/range) 4-seat single-engine vehicles that rely on 125hp engines. (Maybe some 4-seat twins?)

          I really believe that this coming avgas crisis is the direst element of an existential threat to GA as we’ve known it. At the risk of abusing the phrase “perfect storm,” we’re witnessing the confluence of several circumstances:
          1. The practical obsolescence of three quarters of the existing fleet of GA airplanes. Avgas is part of it; looming (2020) equipment requirements are the rest of it.
          2. The retirement (from flying) of the baby boom generation of pilots who themselves were an “echo” generation – the spawn of the post-WW2 burst of GA pilots who created the post-war and mid-1970s GA market booms.
          3. Regulation-based hassles and an Internet-based world in which face-to-face contacts are of decreasing importance, are dissuading subsequent generations of risk-averse yuppies from participating in GA in large numbers.

          The good news is, the arrival of autonomous-aircraft technology will open up GA to a whole new population of non-pilot aviators. The bad ( ? ) news is, the whole thing amounts to an approaching asteroid for GA as we’ve known it. What will emerge on the other side probably will be a GA world that is bifurcated into hand-flown gasoline-powered 2-place pleasure craft, and highly-automated / autonomous 4-8-place vehicles fueled by kerosene (diesel or turbine). It’s a fundamental re-boot of the entire industry.

          The coming Part 23 re-write could help – a lot, if it goes in the right direction (another topic). But the Obama administration’s constant demonizing of GA is a telling sign of what the Powerful think about those of us who ‘cling to our private airplanes.’

  6. Bruce Ziegler says:

    Diesel power seems to be making some real progress on the mid to higher HP engines. The Cessna 182 installation is particularly attractive. No matter what application, diesels always cost more, and may not be scalable to smaller planes. They may make sense for new aircraft, and the higher HP planes that are making an alternative to 100LL difficult to find. I think for the small, sport flyers some version of autogas will be the only cost effective, viable solution.

  7. Karl says:

    Is no one concerned about the emissions of a diesel engine, they may be more efficient but it releases more harmful chemicals than the traditional engine. Economically the diesel engine is suitable but environmentally it is not.

    • tom says:

      Let’s be clear about a few things. First of all, when we are talking about aviation “diesel” engines, we are talking about *diesel cycle* engines that burn Jet-A (close to kerosene), not diesel fuel. All the information I could find on emissions of jet fuel had to do with burning it in jet engines and turbines, not in internal combustion engines. Is burning Jet-A cleaner than burning diesel fuel? That I don’t know.

      The other thing is that the whole impetus for finding an alternative to 100LL is that 100LL has a lot of TEL in it. Over 2 grams per gallon, compared to 0.1 grams per gallon that used to be contained in leaded auto fuel–so over 20 times the lead concentration. So we aren’t replacing a clean fuel with a dirty one. We are trying to find an alternative aviation fuel that doesn’t contain lead in it.

      The final point is that diesel cycle pistons engines will run fine on bio-Jet-A, which are significantly cleaner. We know this because one of our members just tried this in our plane. Right now, bio-Jet-A is about $60 a gallon. But perhaps in the not too distant future, production will go up, and prices will come down to make it a realistic choice.

    • G. says:

      Again, the automotive industry is lightyears ahead here. Diesel aircraft engines can be made to release far fewer emissions, but sadly it appears much of the industry has little concern for such matters.

      Hopefully we’ll see a broader range of powerplants and setups in the near future that can begin to rectify this.

    • Sage says:

      I have a diesel pickup that gets 20 MPG and has for the last 600,000 miles, I promise it won’t pass CA emissions. The same truck but of the current model year with all the emission garbage is lucky to get 12 mpg and the reliability is gone. So how is burning more fuel better for the environment? If we look at the big picture and think of all the extra diesel burned with the oil drilling, refinement and shipping of nearly double the fuel to do essentially the same job, the carbon footprint has to be huge.

      So why don’t we focus on efficiency as the pinnacle of your environmental movement? Just as tom mentioned “If you plan to go 5 hours, you end up burning 350 lbs of Jet-A rather than 450 lbs of 100LL, so you save 100 lbs, even allowing for the fact that Jet-A is a pound heavier than 100LL.” So efficiency per gallon is even more of a contrast.

      If a diesel airplane had to have an EGR system a particulate filter, a catalytic converter and a urea tank like automobiles do in addition to the diesel engine they would be so heavy that they would never fly.

  8. ken says:

    My understanding is that the heavy environmental regulations with burning diesel fuel in a diesel cycle engine is related to keeping the resulting particulate and H2S out of the air. I don’t think this is an issue burning kerosene (jet a) in a diesel cycle engine. If it were, the amount of pollution coming from GA aircraft with diesel cycle engines is going to be orders of magnitude less than that which come from the many commercial jets and turboprops.

  9. Robert E. West says:

    I believe the Germans tried diesel powered aircraft during WWII and although I don’t know the outcome, I believe if they were viable we would be using now.

    • Emanuel Hedanek says:

      we fited Walter diesel engines to Siebel 105 in Prague,two of this planes make return plight delivery to gen. Romel in Afrika,speeking to pilots at Kbely airport after return the main comlains was only very low altitude,unposible fly over 500 metres,so Argus engines were fited back again.

  10. Melvin says:

    Well according to what I`ve reed the diesels failed at the same rate as did the gasoline engines. Reliability 50/50. Heavy offset by less fuel load.
    I would like to think that there is a good substitute for the gasoline engines. The Wankle engine with old school technology “turbo compounding”. Light weight and fuel consumption as good or better than the best recipts. The rotary engines don`t care if the fuel is the lowest octane. An other benefit is long TBO.
    My two cents.

  11. Matt P says:

    At some point, leaded avgas has got to go. Even if the EPA weren’t trying to get rid of lead, the oil companies will, as it’s the only mass market product that still uses TEL, and the handling of leaded fuels is problematic in an unleaded world. The problem in finding a replacement is that the avgas market is small and declining. Look at the US EIA’s avgas supplied figures: http://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=pet&s=mgaupus1&f=a Avgas usage is three quarters of what it was 10 years ago, and it appears to have decreased by seven percent last year. It’s still a good chunk of change , the wholesale market is probably still three quarter of a billion dollars at the wholesale level, but the rate of usage decline will probably chase a few refiners away, and if it becomes a boutique fuel, the price will jump a few dollars.

    91 octane unleaded 100% mogas is still available from distributors, many marinas sell it. It’s good enough for most engines from an 0-360 and smaller. Rather than try to come up with a replacement for 100LL for those bigger and higher compression engines, it probably makes more sense to switch to a compression ignition engine that burns jet fuel. I suspect any 100 LL replacement will be too expensive, and it will still be a tiny sliver of the market, and subject to all the problems that go with being a limited volume product. Turbine engines are too thirsty at lower altitudes and are expensive to build.

    I realize that this will probably ground a lot of older cabin class twins and complex singles, but no one expects transportation equipment to last forever. The newer ones could be converted, but I don’t expect the older ones will be worth keeping around.

    • tom says:

      I agree. Except to say that 100LL is already a boutique fuel. Three quarters of a billion dollars is nothing for the major oil companies. Split it three our four ways, and you are talking about couple of hundred million of AvGas revenue going to any one oil company. By contrast, the smallest of the five major oil companies, Chevron, had revenues of nearly $250 billion in 2011. The largest two, ExxonMobil and Shell, each had revenues of about $480 billion. So a couple of hundred million is hardly worth it to these guys, especially if it requires the upkeep of an infrastructure dedicated to handling lead. And of course AvGas sales is declining even faster in the rest of the world, and I can’t imagine that the emerging economies that might have a strong GA sector (like China) would invest in leaded-fuel infrastructure going forward.

      • Matt P says:

        You’re right, I should have said that 100LL will become more of a boutique fuel. At least for now, it’s produced by the major oil companies. I’m not sure if this is common knowledge, but VP Race Fuels still produces 115/145 octane avgas. They don’t give a price on their web site, but an equivalent racing fuel sells for around $17 a gallon plus freight and taxes. If the majors give up producing avgas, that may be indicative of how much it will then cost

  12. tom says:

    The other angle on all this is how long the production of TEL is going to last. There is currently just one company that produces TEL (Innospec), and they produce it in just one plant. And they admit that this is a declining business for them. TEL accounts for about 10% of their business, but has been steadily declining over the last decade or so.

    In fact, they seem to be focusing their TEL business increasingly on remediation (clean up of no-longer-used lead infrastructure) rather than the actual sales of TEL itself. I think there are less than a dozen countries in which it is still legal to sell leaded auto gas.

    When that goes away, AvGas would be the only customer left. And it’s only a matter of time before this company decides it’s not worth it for them.

  13. I have been following this somewhat confusing debate about aircraft fuels.It’s clear to me that the issue is not octane rating (low octane
    rated a/c engines have always been available)but rather vapor pressure
    rating and quality control.Also fuel system sensitivity to the type of fuel,I.E.aluminum fuel components generally do not like alcohol in the fuel. On the subject of engine types you leave out gasoline direct injected engines.These engines with digital control can,and do,run with 11:1 C.R.on regular gasoline producing one hp/ci or better.Converting
    existing aircraft engines to G.D.I. would not be impossible especially at the manufacturing level.Perhaps someone should check with Lycoming/Continantal,Ill bet they have it figured out-just waiting for legal and market conditions to make it worthwile

  14. Jeff Sloan says:

    It would seem alot of folks have jumped on the diesel/jet fuel bandwagon. I fly mostly small airports on the west coast. Amazingly enough, not one of these small airports offers jet fuel and the folks that run said airports have NO intention of selling diesel/jet fuel.Just food for thought.

  15. Dan says:

    Diesel Fuel is not an option.
    Take a good look at other countries that operate diesel powered vehicles. China, South Korea and the Philippines for example. You can not breathe the air. The ppm air quality index is always around 100-300 or higher. The World Health Organization has declared diesel smoke as a leading cause of cancer. It really is.

    I know first hand how ugly this is. I’m stationed in South Korea where 80% of all vehicles are diesel powered. Ugly. Many of my team members, myself included, suffer from asthma like symptoms, even though we are in shape.

    This can never be allowed to happen in our beautiful country.

    All eyes are on Ethenol, and there is already an STC for this clean, high octane fuel on a Lycoming engine.

    We must choose wisely. It is not about us, or the present. It is about our future, and our childrens future.

    Respectfully
    Dan
    ATP, A&P IA, MS

    • tom says:

      Not to repeat myself. But we are talking about *diesel cycle* engines burning jet fuel (kerosene), not diesel fuel. I would imagine different fuel, different emissions.

    • James Feeny says:

      A matter of physics and chemistry: For every 1 BTU of energy that ethanol can produce, it currently takes 1.1 BTU to produce it. Where is the operating economy in this boondoggle? The farmers and the distillers make out and the taxpayers get it up the *** again

  16. Matt P says:

    I’m looking at the avgas productions stats for the first four months of the year, and it appears that it’s down by an astounding 11 percent. Is anyone doing any flying, or did a lot of pilots switch to mogas? I’ve always heard that the largest bulk of avgas consumption is done by the complex singles and twins, I wonder if the users of these aircraft are finding another way to do business.

  17. Paul H says:

    The issue that I see with the Cessna JT-A is the published critical altitude of 12,000 feet. Yes Cessna notes a service ceiling of 20,000 but with the critical altitude as a performance consideration factor the JT-A does not appear to be a replacement for the Cessna T182T. Compression engines do not appear to be a replacement for turbocharged pistons until they can get them, compression/diesel engines, to perform at the same altitudes as do turbocharged pistons. Yes it looks like the JT-A might work okay in the normally aspirated environment. Many small airports though currently carry only Avgas and not Jet A. So until Avgas goes away for good we have a dilemma and when it does what about the very large installed base of gas burners? Lots of questions and what ifs.

  18. James Feeny says:

    If you guys would care to investigate the Smithsonian website, you might discover that Packard Aviation once developed a diesel aircraft engine. As I remember it, the engine was an air cooled radial, seven or nine cylinder engine. Go look it up! It flew!

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