Motor Head #2: Excerpts from the Oshkosh Notebook

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AVweb's new engine columnist, Marc Cook, found fresh tech everywhere he looked at Oshkosh, and some of it for certified airplanes, even.

Motor Head

Yes, I know that by now Oshkosh -- er, the EAA Air Venture 2004 -- is desperately old news, what with the kids back in school and some of us actually settling down to get some work done after the summer holidays. But I have to tell you that I'm still excited by what I saw at the show. And it amounts to this: There's a whole lot going on in new powerplant technology. Some of it may even end up in production aircraft.

Diesels Make a Splash

The drumbeat for aero diesels is growing stronger. On merit, diesels have a lot to recommend them. Last year, while in Europe, I drove a Ford Galaxy minivan with the 1.9-liter, direct-injection turbodiesel -- it's actually a borrowed VW engine -- and a six-speed manual transmission. It hauled five full-sized adults and a ton of camera gear without complaint, easily making 100 mph on the Autostrada and turning in mid-20s fuel economy -- a big deal when diesel is nearly $4 a gallon. It rarely hinted it was a diesel. Quiet and surprisingly revvy, this oil burner broke every preconception I had about diesels, while highlighting what I always thought would be the strengths: great torque and excellent fuel economy.

Diesels have inherent efficiency because of their very high compression ratios -- the very thing that makes the fuel/air charge light off without a spark plug -- giving a high expansion ratio as well. Diesels run without a throttle plate, so there is less pumping loss. (You could get the same deal with your avgas engine by always running at full throttle, controlling power output with engine rpm and mixture.) Also, Jet A is approximately 10-percent more dense than avgas -- and engines really burn fuel by the pound, gallons is just to make the math easy -- so you'll have a few more BTUs in your Skylane's fuel tanks. (Naturally, the downside of this higher density is that you'll lose some payload.) Finally, the very nature of diesel engines -- the high-strength components necessary to handle the combustion events and diesel fuel's inherently slower expansion rate -- makes them prone to sluggish responses (something the turbo helps a lot) and limits their maximum-rpm potential. Not ideal for a sports car (and probably dreadful in a motorcycle) but perfect for an airplane.

Both SMA -- the Renault, EADS, Snecma consortium -- and Thielert are making a lot of noise right now, even if there are few of these engines flying.

The Word From SMA

SMA SR305-230

At the show, SMA had an upbeat press conference in the middle of a downpour -- up to you to decide if this is a good omen or bad -- to announce ongoing work on the engine itself and the arrival of new retrofit programs. They repeated their somewhat-old news that the four-cylinder, 305-cubic-inch SR305 -- once rated at 230 hp for five minutes and 200 continuous -- had been approved for 230 continuous. Then SMA said it was at work on a 300-hp version of the same basic engine. I was told it would not be a new engine -- or even more displacement or additional cylinders -- but a development of the existing SR305. All the extra power comes from "revised fueling." Given that the 230-hp engine runs an eyebrow-raising 80 inches of manifold pressure, I can only imaging how hard they'll have to run it for 300 hp. The intercooler on the 230-hp version is already massive.

SMA makes a lot of big claims for the 230-hp engine, including an out-of-the-box 2000-hour TBO -- aiming for 3000 hours -- and almost unbelievable fuel economy. Supposedly, the engine will sip 6.6 gallons of fuel an hour at 65-percent power, or about 150 hp. Taking into account Jet A's greater density, this is a breathtaking 0.28 pounds per hour per horsepower. Large marine diesels are reputed to have efficiencies of 0.25 pph/hp. The best avgas engines can do is around 0.39 pph/hp -- that's running lean of peak EGT, where you'll find the most economy and the least heat -- but the majority of the fleet is out there sucking down 0.42 to 0.45 pph/hp in cruise and a lot more for the takeoff and climb.

A Cessna 182 retrofit with the 230-hp engine is approved and should start showing up late this year, with a PA-28 version to follow. I have to believe the company sees these projects as stopgaps, a chance to get some in-the-air development done as it waits for the original-equipment contracts to work out. Cirrus and Maule are most closely linked, but you know others are watching.

At the show, SMA said the Cessna 182 retrofit -- for 182P-and-later models -- will run $77,000, including a new engine mount, engine, and propeller, but less installation, estimated at 80 to 100 hours. For this outlay, you'll get an airplane with greater at-altitude performance thanks to the turbocharger and considerably improved fuel efficiency. (I will believe 6.6 gph at medium cruse when I see it, however.) Your trusty Skylane will lose payload from the heavier fuel and engine installation: All told, the four-cylinder SMA is some 70 pounds heavier than the six-cylinder engine that came out. You will get a single-lever power control and bragging rights. Finally, I feel compelled to point out, you'll get one of the ugliest cowlings ever put on an airplane. In Europe, diesels are put in sexy cars -- it can be done. The Skylane is far from sexy, but the new glass around the SR305 does it absolutely no favors.

In the Thielert Corner

Thielert Centurion 1.7

It's much the same story with the Thielert aero-diesel, a 1.7-liter, four-cylinder engine adapted from an automotive design. (I noticed that the engine on display in the Superior Air Parts booth had a fuel-injection part labeled "Mercedes-Benz.") The main difference here is that it's an OEM leading the charge, in the form of the Diamond Twin Star. In Europe, this engine is certified in the DA-40 four-seater and there are STCs for the Cessna 172 and Piper Warrior/Cadet, which operate at a slightly reduced max-gross weight. Thielert is also a bit more conservative than SMA in its fuel burn figures, claiming 0.35 pph/hp -- call it 5 gph in the engine's intended 97-horsepower cruise mode.

This 135-hp engine must use a gear-reduction drive to get prop rpm down to a workable value, but is otherwise pretty straightforward. A FADEC system with no mechanical reversion runs the show. With the prop reduction and a liquid cooling system adding to the otherwise necessarily beefy diesel architecture, the Thielert comes in at 295 pounds with accessories but not fluids. A Continental IO-240-B weighs 212 pounds dry. Some of that extra weight is in internal beef-up, some in systems, and a bit in the mounting scheme. I noticed that the rubber isolators on the display engine were very, very soft -- and will probably be a frequent replacement item -- no doubt because inline-fours vibrate intensely as do diesels. The combination must be impressive. Do you want fries with that shake?

One of Thielert's future engines -- a 4.0-liter V-8 -- ought to be inherently smoother. And, at 310 hp, quite powerful. But its listed weight of 667 pounds -- again, dry but with all accessories -- is more than 200 pounds greater than a Continental TSIO-550.

Diesel fanatics will point out that both the Thielert and the SMA engines are four-stroke designs, and that two-stroke diesels are lighter and more powerful. It's hard to know just where we'll stand on this subject until we get a two-stroke on the nose of an airplane. Actually, in the case of the V-four DeltaHawk diesel, that would be the tail of the airplane, as it's already flown on a Velocity homebuilt. The DeltaHawk is in testing now with certification projected for "sometime in 2005." FAA certification can be demanding -- and I think it's fair to say that DeltaHawk doesn't have the resources of SMA or even Thielert (whose association with Superior Air Parts is a smart move toward getting stateside support.) I would be very impressed if DeltaHawk got a certified engine ready in '05, but I will concede that it's staked out the right horsepower range -- 160 to 200 hp -- to hit the fat part of the fleet.

Not So Fast

Jet A-burning aero-diesels have still got tremendous hurtles to clear, especially in the U.S. For one, the aviation community is hugely conservative, and the engines introduce a lot of new technology at once. Second, the economics aren't there just yet. In Europe, the price difference between avgas and Jet A is much greater than here. (In fact, during a casual bit of Saturday-afternoon prowling, I found Jet A to be 20 to 30 cents a gallon cheaper in places, but in some cases as much or more than avgas. The average difference in Southern California? Ten cents a gallon, in Jet A's favor. There are also a lot of airports where you can't get Jet A at all.

Sorry, friends, that's nowhere near a strong argument for putting a $77,000 package on the nose of a Skylane. I think it'll be the OEM installations that will drive this market, just as it has for glass cockpits in GA.

Two New Gas Burners

BRP V-6

While at the show, I had a chance to poke around the flying test beds for the Bombardier (BRP) V-6 and the Mistral rotary. The BRP V-6 looks impressive -- and imposing hung way up there from the firewall of the Murphy Moose kitplane -- but the installation looks fairly complex. My colleague, Aviation Consumer Editor Paul Bertorelli, was supposed to fly this airplane in the last month but the flight was held up by mechanical problems. If I can get to Florida before he clears his schedule again, I'd love to fly it.

Same deal with the Swiss Mistral turbocharged rotary, currently flying in a Piper Arrow. Loosely based on the Mazda 13B two-rotor engine, the Mistral has many specialized pieces and fits relatively well in the nose of the Piper. It is, however, fuel injected, liquid cooled and turbocharged, so there's lots of plumbing around the beer-keg-sized engine. The turbo version is good for 230 hp, while the normally aspirated stablemate puts out 190 hp. These are good numbers but, if anything, the rotary is going to fight for efficiency. Rotaries, like two-stroke gasoline engines, struggle to achieve fuel specifics common to most current (read: traditional) aircraft engines.

These are two programs to watch closely, no doubt, but they appear to be much earlier in the process than the SMA or Thielert designs.

Whither Honda?

Honda Aircraft Engine

I spent a lot of time at Oshkosh hunting for the elusive 370-cubic-inch Honda aircraft engine only to be denied. Nothing at the Continental booth -- and no clues from the show personnel where it might be -- nor even a hint of it among the many experimental airframe manufacturers. Hmmm. Could it be dead in the water?

Conventional wisdom might suggest such a duck and run maneuver means Honda has shelved the idea, but I think I know better. What you're seeing is a very common tactic among what are forward-thinking yet conservative companies such as Honda. (I would put Honda's motorcycle peers in the same boat, along with the Toyota juggernaut.) It goes like this: Float a trial balloon and watch public reaction. Will pilots accept an aircraft engine from Honda? Then gauge the response from airframe manufacturers: In a world populated by new-think diesels and new, smaller turbines, would a well-conceived yet surprisingly conventional, opposed-four engine make sense to them? I obviously am not privy to the conversations at Honda but my educated guess is that there was plenty of interest from both camps. If Honda could build an aircraft engine with the kind of care and technical prowess it puts into the glovebox hinge in an Accord, the world would wear out the lobby carpet in no time.

What's next? Again, without solid information -- none of my long-serving contacts within Honda are in any way involved or even know who is, it's that large a company -- I'd say the project has gone back into "the machine" for further development. I expect to see an airplane flying sometime in 2005 with that engine. Quite possibly it will be in an airframe of Honda's own design, but just as likely in some familiar mule, just to reduce the number of variables. It will be done with little fanfare, and by the time it's general knowledge, the engine's future will be well mapped-out, with growth versions in early development and serious talks with the airframers for specific applications well along.

Ironically, the aircraft producer most likely to benefit from the Honda engine is the one least likely to use it: Cessna. C'mon, Textron; give a little.

Rolling and Rocking

Superior XP-360

Owners of certain Lycoming engines know all too well about camshaft and lifter distress. But a potential cure for it -- one more workable than flipping the engine on its head to put the cam underneath, of course -- was shown at Oshkosh. Almost simultaneously, Lycoming and Superior Air Parts announced work on hydraulic roller lifters. Lycoming is calling it an ongoing product improvement, while Superior is touting its relationship with Thielert's racing arm. Either way, the roller tappets will likely show up in experimental engines first, then for production engines. Superior's XP-360-Plus experimental engine will lead the way and I suspect that the company, as is its wont, will likely lead the retrofit end of this deal.

So what's the big deal? Roller lifters replace the conventional flat face of a tappet or follower -- they're basically interchangeable terms for the device that fits between the camshaft and the pushrod -- with a ball-bearing-supported wheel. Because the wheel is free to roll, there's no longer a sliding motion as the follower traces the profile of the cam lobe. This technology also promises less valvetrain friction -- and friction removed is free horsepower -- along with the ability to use considerably different cam profiles, one of the benefits touted by Superior for its design.

Lycoming owners know that all but a few of their engines use what's called "mushroom tappets," where the face that contacts the cam is larger than the bore the lifter slides in, so removal requires disassembly of the engine. Continentals and the so-called "76 series" Lycomings -- the O-320-H2AD is the most common of them -- use lifters that can be removed much more easily, leading a lot of savvy owners to perform periodic lifter removal, inspection and replacement before corrosion takes hold. Sorry to say that for design reasons the new roller tappets must be installed from the cam side of the engine -- no removing them from the lifter bores from the outside. I'm all for having wear items in easy reach, but it's possible the roller technology will make routine removal unnecessary anyway.

Finally, Prism

Prism System

At Oshkosh, I managed to pigeonhole GAMI chief engineer George Braly on the status of PRISM, the company's adaptive-timing ignition system. I've been watching this idea for a long time, since it was first run on a simple, two-cylinder stationary engine circa 1999 and on an aircraft engine in 2000. It has progressed significantly in the time since, thanks in part to GAMI's Carl Goulet Memorial engine test facility, almost surely the most advanced in the country dedicated to GA engines. (Goulet was a forward-thinking engineer for Continental.) This spring, I got to see the mighty Lycoming TIO-540-J2BD run on the stand, inhaling 100LL as well as auto-fuel swill, using both fixed timing and the adaptive PRISM system.

Put simply, it does what it's supposed to do, at least on the stand. With the autogas in the system, PRISM merely retarded the ignition timing until the Lycoming was happy. With 100LL in the pipe, PRISM simply set the ignition timing to place the peak of the combustion pressure where it will do the most good for torque. In this sense, it's a no-compromise setup: Hit the spark where it makes most sense and let the system's ability to actively and continuously monitor combustion pressure act as the buffer to destructive detonation. (Actually, I confess to using the terms "merely" and "simply" recklessly here, as PRISM's processing power runs a lot deeper.)

For the moment, I'd like to set the discussion of PRISM aside and revisit it here when GAMI has an airplane flying on the system. I'm told it'll be in an experimental airplane soon, with certification to follow. For the naysayers who point out that PRISM has been a long time coming, I'll just say this: GAMI must, with this program, certify some bleeding-edge technology in a flight-critical system on a budget a fraction the size of Continental's. If you think the guys from Ada, Okla., should have been done by now, I suggest you try it yourself.

Next time: A look at the experimental engine offerings from Oshkosh, for all my friends crazy enough to build their own airplanes. Been there, done that ...


Got motors on your mind? Check out the rest of Marc's columns.