Engine Enigma

October 23, 2013 by Bruce Landsberg
320px-Porsche_PFM_3200_aircraft_engine

Porsche PFM 3200 aircraft engine Technik Museum Speyer, Germany

Last week’s blog generated a number of excellent comments largely to the point that we shouldn’t be paying for all the government we’re getting. Brandon responded that $50K for a Lycoming IO540 was too much and that he could get a similar car engine for about $10K installed. Reading between the lines, perhaps he’s concerned about the FAA’s certification burden on the engine manufacturers. I completely agree with the sentiment, but there are some areas where we really do need bulletproof equipment. There may be some debate whether FAA’s engine directorate is providing that function—I can’t answer that.

But let’s look at the differences between car engines and aircraft powerplants (specifically piston)—they’re not equal. There have been some great automotive conversion experiments but none, to my knowledge, have fully lived up to the promise of being commercially viable.

Car engines typically run at 20 to 30 percent of rated power and almost never hit 100 percent unless you still have teen drag-racing fantasies. Race cars are another place where engines are routinely ridden hard—hold that thought. Aircraft engines operate at 100 percent on every takeoff and then spend most of their lives at 65 to 75 percent.

In the experimental world, VW and Corvair air-cooled engines have been modified with varying degrees of success. Some large block V-8s have also been tinkered with, and I flew an experimental Cessna 172 on a really hot Kansas afternoon with a Ford Escort 4-banger. It had a belt-driven gearbox because the rpm/torque ranges of car engines just won’t work with propellers. You can’t just bolt a prop onto the front. A really stout reduction-gearing system is needed, which impacts cost (significantly) and weight and balance (significantly). Let’s just say that the Escort-powered Cessna’s performance was lackluster, and let it go at that.

Mooney and Porsche conducted perhaps the best commercial experiment in the mid 80′s with a 210-horsepower modified Porsche engine. The engine had a racing heritage and thus was theoretically capable of operating in those high percentage ranges. Apparently a thermodynamic barrier wasn’t factored in. If you ran the car at 100 miles per hour the fuel burn would probably be in the 5 to 6 gph range (racers help me out). The airplane needed about 11 gph and it was designed to go 2,000 hours TBO. Most race car engines might last for a couple of races and then be replaced—not the typical aviation profile.

Too many of Mooney’s Porsche engines were shelling out at 400 to 600 hours. It was a marketing disaster, and Mooney PFMs were retrofitted with big bore Continentals at the factory’s expense.

Now to Brandon’s point, the Porsche engine was certified but not especially reliable. Our experience with old technology engines is generally good. The manufacturers have some other economic realities that have nothing to do with the FAA: product liability and low volume. As I’ve said all along, the aviation cost challenges are multi-faceted, and if they were easy to address they would have been. That said—we shouldn’t stop trying. I don’t like the alternative.

Bruce Landsberg
President, AOPA Foundation

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12 Responses to “Engine Enigma”

  1. Bent Esbensen Says:

    When considering the success of commercially viable auto engine conversions, please do not forget the Centurion diesel, which is a Mercedes Benz A-Class engine.
    One might argue that the gearbox has had its share of snags, but the road towards commercial sustainability might be cleared now that Continental owns it.

  2. Chris Cuneo Says:

    Also, remember the BSFC (brake specific fuel consumption) of well tuned Lycomings and Continentals is better, in cruise flight, than any gasoline engine automotive conversion, and can be on par with diesels. The engineers that designed the air cooled engines we love to hate, really understood what they were doing. Large cylinders, low RPM, direct drive and air cooling have distinct advantages. From thermodynamic advantages, to cooling drag advantages, and simple frictional advantages. The conventional aircraft engine, with it’s advantages, results in aircraft with a practical useful load, good range and excellent reliability.
    Another issue to consider. Jet fuel weighs more and is more energy dense. However, aircraft are weight sensitive. Fuel cost issues and gallons consumed aside, well tuned aircraft engines can match or come very close to diesel cruise efficiency by weight of fuel consumed.

  3. Thomas Boyle Says:

    The “20% of rated power” argument has been thoroughly addressed elsewhere. Modern auto engines are re-used as marine engines, for example, where they are run at 75% power continuously; they are perfectly capable of doing it. (Honda offers its Fit engine as the BF90 outboard motor, for example.) Yes, race engines expire quickly, but they run well north of 10,000 RPM (and hot); auto engines are redlined at around 5,500.

    People used to believe you couldn’t put a gearbox on an aircraft engine. They used to believe you couldn’t run an aircraft engine at 5,500 RPM. They used to believe that liquid cooling was impractical and unreliable. To all of that, one word: Rotax. (And, by the way, the 1,500hp of a P51 is delivered through a gearbox.)

    All that said, you’re right, an airplane engine has to be engineered for use in an airplane. Home conversions of auto engines tend to have fatal flaws, but handling the impulses that run from the crankshaft through the gearbox to the tip of the propeller – and back! – is not handiman engineering; it takes real expertise, computers, and a lot of expensive trial and error and reliability testing. If Honda did that, and mated it to its mass-produced engines, and made 10,000 or so, it might deliver an aircraft engine for $15k.

    But, Honda won’t offer (or knowingly let someone offer) their engines for aircraft use. We have a few small-volume manufacturers of engines for aircraft use. And, low volumes are expensive in the engine business.

    Then, on top of that, add certification.

  4. Mark Leuzinger Says:

    I sold my Mooney 201 and bought a Porsche Mooney (PFM) in 1988. There were many Cessnas and Robins flying around Europe using the Porsche engine but Mooney was the only one here. Product liability and slow Mooney sales killed the project off in this country. The Porsche factory rep flew around in a Porsche powered Cessna 182. I flew it a couple of times and the difference between the Porsche and Lycoming powered plane was astounding. The 201 would do 160 knots on 10.6 gph while the PFM did 158 knots on the same fuel burn.

    That plane was years ahead of its time starting with the overhead cam engine. The reciprocating parts were balanced to within a gram. The heads were cross flow with hemispherical combustion chambers. It had balanced intake and equal length exhaust headers, fan cooling so you couldn’t shock cool the engine, all electric systems with dual electronic ignitions, dual batteries, two main busses, and no vacuum pump. Everything worked off one power lever so you didn’t have a mixture control, prop control, cowl flaps or a manifold pressure gauge. The Bosch fuel injection did all the work for altitude and air density. That really cut the workload for single pilot IFR flights.

    It was about half way to TBO when Porsche bought the plane back. Yes, I loved that plane and hated to see it go.

  5. Duane Says:

    Ditto Mr. Boyle’s comments. Marine engines – both inboard, automobile-style V8 and V6 engines, as well as both 2 cycle and 4 cycle outboards – are regularly run for thousands of hours at RPMs in the range of 4,000 to 5,500 rpm, in about the same operating range of power settings (55% to 75%) that aircraft engines are run. No big deal.

    Reduction gearing is not that big of an added expense – after all, all automobiles have transmissions and/or transfer boxes (if 4wd), plus expensive running gear (multiple drive shafts, reduction gears, wheels and brakes, and suspension systems. Rotax engines that are the dominant powerplant for LSAs are geared engines operating at 5,500 rpm – and they work great.

    The concept is not for the average “backyard mechanic” to buy an automotive crate engine and make a conversion for aircraft use. The idea is for a manufacturer to take advantage of the very large number of really excellent automotive engines, whose development costs were spread over hundreds of thousands or even millions of units – and make some fairly simple factory mods to adapt them to aircraft use. The holdback is FAA certification and the relatively low production volume for aircraft engines.

    Modern liquid-cooled auto engines with computer-controlled electronic ignition produce much more power per pound, operate much more fuel-efficiently, and are much more reliable than equivalent power opposed air cooled aircraft enginees. And they last longer too and cost much less to buy and maintain. Liquid cooling results in smaller operating temp ranges, allowing tighter tolerances which produces greater efficiency and longer life. Many of today’s pilots don’t realize that many of the earliest aircraft, up through World War Two fighters like the P-51 mustang, used liquid cooling to achieve maximum performance.

    The best modern auto engines, like the Nissan/Infiniti 3.7L V6, produce upwards of 350 shaft horsepower in an engine that weighs only about 350 pounds – about 80 pounds less than a 300-hp Lycoming IO-550 or Continental IO 540. The cooling system will not add that much weight either given the high airflow speeds available in aircraft. A turbo charger would add maybe three or four thousand more on top of a base cost of around $10K.

    The only real constraint here is the vast expense of FAA certification that could be spread over only a few thousand, or perhaps tens of thousands of units per year. The numbers just won’t work under the present regulatory scheme.

  6. Tom Says:

    I doubt regulatory costs, per se, have much to do with it. It’s just a small part of the overall fixed cost of developing, producing and selling engines (including other fixed costs like capital, tooling, overhead and R&D). The problem is the diminishingly small volumes of the aviation market. Toyota sells something like 4.5 million passenger cars a year. They probably have maybe a dozen basic engine designs at any given moment shared among these cars—so on average something like 400,000 cars per engine design (in round numbers). And each engine design might go without major changes for a few years, so you are probably spreading all the fixed cost of developing, producing and selling the engine over 1-2 million units, maybe more. (Even before you account for the R&D economies of scale where technologies from one engine get shared with others.)

    That means that if you spend an additional $20 million dollars more on R&D, or some other fixed cost, it increases the cost of an engine by $10.

    In the U.S., there are currently something like 150,000 piston aircraft. Add the rest of the world, and maybe the total is 200,000? Based on that, you are probably talking about a market that has maybe 10,000 new units a year? Add the replacement market, and you are still probably talking about at most 20,000 units a year? Spread across two different engine manufacturers? No wonder we are still using 1960s technology.

    Like any problem of fixed costs, the only really effective way to address the problem of R&D (including regulatory costs associated with new designs or technology) is through volume. So we are going to continue to be plagued with much higher prices for everything, until we start to significantly increase volume. And that probably won’t happen until we solve the economics. Talk about a vicious cycle. You can blame the government all you want (and they certainly deserve some of it), but high costs are fundamentally baked into the economics of the industry.

  7. Emil Dular Says:

    The problem is very much one of scale. Back in 1995 the newspaper was full of little factoid fillers for the fiftieth anniversary of World War Two. One morning I read one that the United States produced more aircraft than the next five major combatants combined; some 300,000 airplanes! Sounds impressive until you realize that was everything from Waco CGA-4 gliders to Boeing B-29′s. The same paper had another article that said the Chrysler and Ford have revised their estimate of that year’s North American new car market down to 5.5 MILLION units!

    I doubt there have been 5.5 million man carrying airplanes built since Wilbur and Orville! The Ford truck plant in Louisville was building a quarter million F-150′s a year. One model of truck, in one plant, just about equaled the total unit aircraft production for 4 years of WW2. If your name was Champion would you rather sell a week’s worth of sparkplugs to Ford or a year’s worth to Lycoming?

    There’s nothing cheap about airplanes except the owner.

  8. Alan Rose Says:

    Engine HP, torque, and redline RPM are all functions of camshaft design. An automotive engine, (that will run on unleaded fuel) with a properly designed camshaft producing max HP and torque at 2500-2700 RPM’s would be a great aircraft engine. No gearbox needed, proven fuel efficiency, and readily available parts. Automotive engines repeatedly run for 200,000 miles or more, which, at an average of 60mph, equates to a TBO of over 3,000 hours. If you could get the cost of an aircraft engine down to 12-15K, maybe more people would be able to own an airplane, (or build one), more units would be sold, price goes down more, etc., etc. The only problem has already been mentioned: THE GOVERNMENT!

  9. Bruce Landsberg Says:

    Speaking to a merchandizing friend, who’s very successful, and the point has been brought out by many of you – volume seems to be one of our major downfalls. I’m not too optimistic about the demand for aircraft engines spiking anytime soon.

    I had forgotten about the Rotax, high RPM and gearbox setup. It does work – wonder if they could do something in the 250 hp to 300hp range?

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