It's easy to be embarrassed by the state of the general aviation technology when we have to admit to our non-pilot friends that our engines use such quaint items as magnetos and (gasp!) carburetors. Many pilots and aircraft owners wish they could, for instance, take their bleeding-edge-tech motorcycle engine and strap it into the airframe. Farfetched? AVweb's newest columnist, Marc Cook, has some thoughts about this kind of thing, and he'll share them each month in his Motor Head column.
July 18, 2004
This will be no surprise to anyone who knows me: I love engines. Just dig 'em. Internal combustion, jet, compression-ignition, Rolls-Royce Trent, Briggs & Stratton, big or small -- it really doesn't matter. If it turns chemical energy into mechanical work, I'm hot to know more about it. Airframes are nice, and useful I suppose as a life-support system for a glorious noise-spewing, twist-making mah-chine. Nothing beats a motor. (Yes, I know a motor is properly an electric device ... can we just go with the flow?)
And I'll stay up late reading about engines of just about any description. One night my wife caught me squinting at an artist's rendering of the massive Trent 900 due to go into the double-decker Airbus A380. There I was, nose against the page only to see her withering stare in the corner of my eye. "What?" She sighed, "Never mind ..."
It's like that sometimes, being a motor head.
In my professional life as a writer, I've managed to stick close to the subject, working first in motorcycles, then aircraft, and then, er ... motorcycles again and, here we go for some more air time. There's a bit of car and truck writing in there, too, but I'll confess this much: The engines, good as they are, are almost nowhere to be heard and often nowhere to be seen beneath great quantities of plastic sound-deadening material. Snooze. With motorcycles and aircraft, the engines are nearly out there in the open -- at least, it doesn't take much to pull the wraps and have a good gander at what's making all the noise.
It's Different Up Here
Having an interest in many facets of motoring -- and I think you can call flying motoring, with a nod to sailplane enthusiasts -- provides an interesting living illustration of the engine-tech spectrum. New and exciting on one side, dull and all-too-familiar on the other, right? Hardly. When it comes down to it, technology in general aviation does what it needs to do -- and what the market is permitted to do, based on an amazingly complicated, government-infused R&D process and limited potential market.
A motorcycle manufacturer, by comparison, can spend millions developing a new, high-performance model secure in the knowledge that it will sell at least 10,000 copies in the U.S. alone and probably 50,000 worldwide the first year. Then, when it's not the latest and greatest, it can move the technology to a second-tier model and continue to sell, say 8000 a year through the life of the production tooling. The downside is that the rate of development obsoletes these high-performance bikes every two or three years; used to be a four-year development cycle sufficed, but no more. Still, the market for the product is so strong it's economically viable to turn the product lines every couple of years, which in turn creates new demand, funding the cycle all over again.
Seeing It From the Other Side
"It's how big?" my friend from the motorcycle biz asked when I began telling him about the Lycoming IO-540 in the Bellanca Viking we were flying. Yep, 540 cubic inches, or 8.8 liters to my Canadian friend. "And it makes how much power?" He didn't seem terribly impressed that an engine so large could muster a mere 290 horsepower. "But it's turning only 2575 rpm," I told him, waiting for him to work out the math to arrive at the actual torque figure. (That would be 591 pound-feet.) "Hmmm," he said, not quite sure what to make of the whole deal, but I could imagine him starting to think about the tractor-like qualities of the big Lycoming. I didn't have the heart to tell him that it was running on fixed-timing magnetos and a mechanical fuel-injection system about as sophisticated as a lawn sprinkler.
We had leveled off for cruise and he watched as I reset power and tweaked the mixture, paying my usual devotion to the engine monitor.
"Man, that's a lot of fussing," he said. Yes, it's true that balancing the needs of the engine against fuel efficiency requires some extra effort, especially compared with the everything-automatic motorcycles he's used to.
"How often do you have to fix one of these?" I told him this particular engine had been doing its thing for 1900 hours over 34 years without major work. Maybe a change of spark plugs now and again. At a typical speed for the airplane, that's approximately 290,000 miles. I could see the wheels turning in his head, trying to decide if flying above the clouds with this creaky old engine was sane or not when I delivered the coup de grace: "Oh, and we run it near 70 percent of its maximum power for almost its entire life."
He thought about that for a second, turned to faced me with eyebrows raised. Then he squirmed a bit in his seat and changed the subject to the weather.
Among my many friends who have an interest in both flying and riding, this kind of cross-cultural engine comparison is nothing unusual. And it's always the motorcycle guys who puff up at the thought of their incredibly powerful, high-tech engines. Gee, they seem to think, if general-aviation had anywhere near that kind of technology, we'd all be flying faster and a whole lot less fuel.
The question of the moment is: Are they right?
Well, first of all, the "average" modern motorcycle engine is an amazing thing technically. And here I'm talking about the sporting motorcycles -- not Harleys. Although, truth be told, Harley has developed its line of engines to a very high degree of refinement and reliability within the constraints of what the average Hog buyer will accept. Recently, Harley introduced a new model called the V-Rod, which uses an engine developed in part by Porsche that is the embodiment of the state-of-the-art in motorcycle powerplants: four valves per cylinder, liquid cooling, high horsepower. But it's not a "traditional" Harley so it's been off to a slow sales start. Shame, it's a really good motorcycle.
But the real high tech is in the Japanese sportbikes, which with each new model year seem to reset the limits of clever design, low weight and high power. The current power king is a Kawasaki, the Ninja ZX-10R. It's 998cc (61 cubic inch) four-cylinder engine produces an astounding 160 horsepower at the motorcycle's rear wheel. Accounting for driveline losses of, on average, 13 percent from the crankshaft to the rear wheel, puts it at about 180 horsepower gross. That's an eye-watering amount of power in a lightweight motorcycle, but what really amazes me is that this power comes with a year-long warranty and full compliance with stringent noise and emissions regulations. Oh, and despite a 12.7:1 compression ratio, it'll run on 91-octane pump gas all day long. The engine's four valves in each combustion chamber are operated by dual overhead cams, of course, but feature tolerances so tight and materials so good that the valve-adjustment interval is every 20,000 miles. (On some modern sportbikes, it's even longer, making the interval longer than the predicted lifespan of the bike.)
Almost needless to say, the Kawasaki and motorcycles like it will run just about forever with only routine maintenance. In fact, the only recurring problem with this class of engine has to do with the transmission and clutch, and overwhelmingly as a result of rider abuse.
And they're dead cheap. You can buy this Kawasaki's engine, with the rest of the motorcycle attached, for $11,000.
What's This Got To Do With Airplanes?
Inevitably, my motorcycling friends ask why aircraft don't have high-specific-output engines. They're often surprised when I tell them I wouldn't want such a thing. For starters, motorcycle engines -- indeed, most high-output engines found outside of aviation -- are never intended to be used at high power for very long. I'd estimate the average duty cycle of a motorcycle engine around 5 percent power, maybe 10 percent on the low-horsepower versions. Outside of racing, there just aren't the opportunities to open up one of these beasts on public roads. After all, what is any sane person going to do with a sub-4-second 0-60-mph time or a top speed in excess of 170 mph?
I honestly can't imagine the internals of a 12,000-rpm motorcycle engine being happy at 75-percent power or more beyond short bursts. I've been to the Japanese factories, and have seen the brutal endurance testing carried out on new designs, but as a practical matter these engines are run at maximum power a for a small fraction of their lives.
Perhaps what's most amazing about this breed of motorcycle is the electronic integration. For example, the entire engine-control computer is in a box about twice the size of a pack of playing cards. It integrates the electronic fuel injection, which has separate maps for each of the four cylinders, with the ignition control, also a three-dimensional map that sets the spark event according to engine speed and load. On top of that, the computer is also controlling a butterfly valve in the exhaust system that varies system back-pressure to smooth out the torque curve. In addition, all the new fuel-injection systems have double butterfly valves, one controlled by the rider and the other by the computer. The goal is to smooth out throttle response and make the engine more predictable, a good thing when you have so much power in a package that weighs, including an average-sized rider, a little more than 600 pounds. That would be like having almost 1100 horsepower in your average sedan. Or 900 horsepower in your typical Bonanza. (Not that I'm saying it wouldn't be fun ... for awhile.)
Something else to keep in mind when comparing such disparate powerplants is that motorcycles get crummy mileage. That is, they may have a lot of horsepower per cubic centimeter but they're not even remotely fuel efficient. My own bike, an Aprilia Tuono, powered by a Rotax-built 998cc twin-cylinder engine, struggles to get more than 35 mpg. Considering that it takes all of 15 horsepower to maintain highway speed, you can see the problem.
Clearly motorcycle and aircraft powerplants are at the opposite ends of the power and efficiency spectra.
Honda Takes a Stab
|Honda Aircraft Engine
Last year at Oshkosh, tucked away in a corner of the TCM booth, was a display engine of a design floated by Honda for a 370-cubic-inch, four-cylinder, direct-drive engine. Think of it as a fairly sophisticated IO-360 with liquid cooling and a version of the Aerosance/TCM FADEC system. I think it's critically important to understand the significance of Honda's design strategy. This is a fiercely competitive company, as proud of its engineering prowess as any company I know. When given the chance to out-tech its competition -- particularly in motorcycles -- Honda will take up the opportunity with vigor.
That the Honda design is a slow-turning, large-displacement engine with pushrod valve actuation says everything to me about the inherent rightness of our existing aircraft-engine designs. Believe me, if Honda thought it could do a better job with a double-overhead-cam, high-revving -- and, most crucially--high-tech engine, it would have. Remember, this is a company that prides itself on arriving at the cleverest, least-obvious solutions to everyday engineering challenges. I can only imagine the soul searching involved for Honda's engineers to crank out such a seemingly low-tech design.
Although the Honda engine's level of technology seems to pay homage to conventional aircraft engines, I have no doubt that its quality would be amazingly good. Back to bikes for a second: Several manufacturers have taken to a new way of casting aluminum using a vacuum die-casting method. The resulting parts are light -- because they don't have to be over-engineered to accommodate the normal variability in cast materials -- and astoundingly beautiful in their as-cast form. Moreover, the typical motorcycle is fabulously well built. The engine castings are nearly perfect; the frame welds are often a work of art (where you can see them, anyway, as the hidden welds are done by the much less artful robot); and the precision is nothing short of amazing. Everything fits every time. Lightweight aluminum engines do not break. Alloy motorcycle frames made from a combination of pressure-cast, die-cast, sand-cast and extruded members are immensely strong and measure within millimeters from one example to the other across their most distant points.
Here's where I'm hoping Honda's influence will come to bear on general aviation. I am among those who believe the basic aircraft engine -- even acknowledging that it is a design more than 40 years old -- is an incredible thing. No other engine I'm familiar with combines the power and weight with efficiency. For what it needs to do, the typical O-320 or IO-550 is a terrific design. If Honda produces a "modern" flat engine with the typically jewel-like quality of a motorcycle engine, everyone will sit up and take notice.
Now Let's Move Forward
What general-aviation engines need amounts to this: Better manufacturing and a dose of new technology overlay. The last first: Continental's FADEC system is a good groundbreaker, much like the first fuel injection systems that replaced carburetors in motorcycles not too many years ago. GAMI's Prism system, an adaptive ignition system, will take the technology to the next level. I've been watching this system for years, and have seen it in action on a variety of real, live engines; it holds incredible promise but will not be an easy thing to certify. (This is not my bright idea; just ask the GAMI staff.)
For the engine manufacturers proper, the focus needs to be quality control. It astounds me that in designs so mature, so supposedly well-known by the production department and everyone else with a wrench in his pocket, still have premature failures because of things like out-of-round cylinder barrels or valve guides not placed exactly in the center of the valve seats. This is basic stuff.
And while I appreciate that the world of supplier manufacturing -- where you don't build every part in-house -- is often harsh, particularly for a small-volume concern, I believe the issues can be resolved. For motorcycles, there are multiple suppliers for most every part, but the secondary or tertiary suppliers are seldom used. They don't have to be.
In any event, I'm off to Oshkosh in the next few days, and I'm eager to see what's out there. Maybe Honda will show us something special. I'm ready.