Aviation Is Overdue For Fresh
Approach
to Powerplant Design
This 2-part series first appeared in the
Nov-Dec 96
and Jan-Feb 97 issues of the TBO Advisor.
If you haven't read the rest of this issue of TBO Advisor, do it now. Save this article for last.
OK. Now you've read about sticking valves and worn guides and problems with cylinders and camshafts and spark plugs and turbochargers. The powerplant problems go on and on, ad infinitum, ad nauseum. One trend seems clear: Engines are not getting better. Prices aren't going down, reliability isn't going up, maintenance isn't getting easier, and it's becoming harder to find anything good to talk about.
We are marooned at the far end of a learning curve, and being stuck out here isn't any fun. The time span between the Wright brothers first flight and Lucky Lindy's Atlantic crossing was only 24 years. Think of it! In 24 years, mankind learned aerodynamics, flight structures, navigation, and most of all, we learned powerplants. Twenty-four years.
What have we accomplished in engine technology in the last 24 years? Not much. And there has been a fair amount of negative progress as well. All the big aviation money has moved into development of gas turbines, and there has been no incentive to spend increasingly larger sums to eke out smaller and smaller improvements in piston powerplants. That's what the end of a learning curve is all about.
One of the immutable laws of aviation is this: Every improvement in airframe design is preceded by an improvement in powerplant design. It was true for the Wright boys, it was true for a long line of military and commercial aircraft, and it is true today. The new Cessna 172 isn't improved from a 24 year old model (or one nearly twice that old) because the powerplant hasn't improved. Switching from brand C engines to brand L engines is no answer.
Personal aviation needs a new powerplant. We need to begin from scratch, on a clean sheet of paper, at the start of a fresh learning curve. We need the teething problems that accompany new technology. We need the exciting competition that comes from rapid and revolutionary advances in the state of the art. Most of all, we need this powerplant so the airframe designers can produce revolutionary new planes that will fly higher and faster and do lots of things we can't do now.
So, for a few minutes let's step back from the minutae of platinum electrodes and ash dispersants and lifter leakdown rates. Let's consider what the powerplant on that clean sheet of paper should look like.
Of course we would all like to see better fuel economy. Fewer gallons burned means fewer dollars spent, and more importantly it means more payload carried or more range in an existing airframe. In a new aircraft design, increased fuel economy translates into less weight, thus less induced drag, thus more airspeed.
But how much we will burn is preceded by the question of what we will burn. Look at the reality of the situation: As long as we fly to and from airports, we are limited to a fuel that's available at airports. Mostly 100LL and Jet A. It's a rare FBO that can get his insurance carrier to allow car gas to be pumped, even tho he's only pumping it into aircraft that already have full FAA blessing to use this non-aviation fuel. And it's a rare airport that will allow the FBO to operate uninsured. We can expect two things in the future. First, no new fuel is going to be approved and the insurance industry will continue to have the final say on what we can purchase at the airfield. Second, 100LL won't be outlawed, but the costs of handling it will skyrocket. Consider what will happen when OSHA, EPA, state regulators, et all, start paying attention to simple little things like fuel filters. First, the A&P will have to wear special protective gear when he changes your filter because it is contaminated with a toxic substance. Then he is stuck because he can't keep the old filter and he can't throw it away. Ditto for the protective gear. Whatever the A&P does, you will have to pick up the tab. Likewise for fuel cells, fuel pumps, mufflers, fuel lines and hoses. Even valves, pistons, cylinders, and spark plugs are contaminated by lead, and one of these days they will be treated accordingly. And if you don't believe it, ask your A&P what he is supposed to be doing with your used engine oil now.
Maybe tetraethyl lead is really nasty stuff, maybe it isn't. Doesn't matter, the enforcement is coming and we had better find something else to burn. Aviation is the last user of leaded fuel and it's time is short. The only remaining choice is Jet A.
So, in one corner of that clean sheet of paper, make a note that the fuel will be kerosene.
What's the next powerplant characteristic you would like to improve? How about noise and vibration? Yes, I know there are some macho pilots who equate noise with power and potency. That was true in the day of Waldo Pepper and it's great nostalgia. It's also fun to remember our teenage years when we roared around in hopped up cars with muffler cutouts. As individuals we've outgrown that foolishness, but our airplanes haven't. Like it or not, personal aviation will either coexist with society or personal aviation will be finished. Our numbers are too small to have any political clout, each pilot vote is competing with 250 non-pilot votes. Waldo Pepper is half a century gone and if we don't quiet our flying machines society will do it for us. Or do it to us. In point of fact, society has been doing it already. Look at the airfields that have been closed or faced with curfew. Look at the corporations that will not permit employees to travel in light aircraft, and the insurance policies that don't cover you during Part 91 flying. Look at the losses we've experienced recently in airspace over National parks and such, or the unsafe practices forced on us in the name of noise abatement, or the negative television coverage devoted to aircraft incidents. General aviation is considered to be a dangerous menace, and society is giving us a warning. As long as we keep sounding like Hell's Angels, we're going to be treated like Hell's Angels.
So make another note. The new engine is going to be silent. And without vibration.
Performance. What is wrong with performance today? Mainly, we have this thing called ceiling. We have been limited by powerplant ceiling since the beginnings of aviation and we consider it as natural as night and day. But an aircraft doesn't have to be altitude-limited by it's engine. Wouldn't you like to fly a powerplant that doesn't lose power at altitude? If a given plane could hold constant power, it would cruise twice as fast at 40,000 ft as at sea level, due to reduced drag alone.
Eliminating ceiling, as we know it, will have a profound impact on aircraft designs, and on how planes are used. But how do we do that? First, we have to abandon those familiar four cycles: Suck-Squish-Pop-Ptui. We are flying the exact same four cycles of the Wright brothers, and of Glen Curtiss and Charles Lindbergh, the same four cycles Bob Hoover flew in his P-51 over Europe fifty years ago and is flying in his Shrike today. Suck-Squish-Pop-Ptui.
Of course airplanes will continue to have a ceiling. There will always be some altitude above which we cannot go. After all, the air isn't very thick around this planet and airfoils need air. So there is a limit. Perhaps a more immediate limit will be life support, we must address the question of how to get down alive when things go wrong. Or perhaps something else will limit how high we climb. But it need not be the engine. The capability to cruise above the weather will fundamentally change the way light aircraft are used. When we have the ability to fly in the low drag of the upper atmosphere we will enjoy greatly increased airspeed and range.
Don't get me wrong. I'm not suggesting that we should re-engine an Archer or a Baron and go flying at FL410. And that is precisely the point. Totally new airframe designs will be needed to match the potential of our new powerplant. These aircraft will offer so many advantages that every one of us will want to trade the old bird in for a new one. More importantly, the advantages will attract many who are sitting on the aviation sidelines now. This can lead to a renaissance in general aviation, and carry us into the new century with exciting things to look forward to rather than what we have now: a glorious past and a bleak future.
So, on that clean sheet of paper, add the requirement that the powerplant should not lose power at altitude.
There are lots of other attributes we could wish for. Reliability, maintainability, affordability, light weight, and so on. But for now let's stick with the basic three: kerosene fueled, silent, and no loss of power at altitude. As we develop the new engine, we'll try to keep the other characteristics similar to existing piston powerplants. Remember, we're starting at the beginning of a new learning curve so we can expect continual improvements rather than more and more of the status quo.
Does this powerplant exist? Yes it does. And it is time to adapt it to the unique needs of aviation.
It's called the Stirling cycle engine, and it has been around since the year 1816. Rather than suck- squish-pop-ptui, the Stirling cycle consists of heat-cool. That's it. It's a sealed system, similar to a refrigerator in the sense that the same working fluid goes round and round without being consumed. Stirlings are external combustion engines, and can burn whatever fuel is available. That takes care of requirement number one. Steady continuous combustion doesn't make any appreciable noise, that satisfies requirement number two. In 180 degrees rotation of a four cylinder Lyc or Continental, torque varies from a negative 100% to a positive 350%! The extreme torque reversals place severe constraints on propeller design. On the other hand, the same size four cylinder Stirling has torque that varies smoothly between 95% and 105%. No torque reversals, no need for a prop that must also serve as a flywheel, and silky idling at very slow speeds. Smooth power can save weight in engine mounts and isolators, too.
Heat and cool. Heat and cool. The same chunk of gas is alternately heated and cooled. Heating a trapped body of gas raises it's pressure, and the pressure pushes a piston which does work. Then the gas is cooled, and the piston returns. The Stirling cycle.
A similar thing is happening in our engines today. Combusting the mixture of fuel and oxygen creates heat, which mostly heats the nitrogen which makes up 78% of air. Increased nitrogen pressure is main thing that pushes the piston down in a conventional I.C. engine. Stirling engines prefer a lower-molecular-weight gas, because it takes less heat energy (i.e., less fuel) to produce a given pressure change in a small molecule. Air has a molecular weight of about 28.9, while Helium is only 4. So air takes about seven times more energy to produce a given force. This is somewhat comparing apples and oranges, and I don't want to get mired down in the thermodynamics here. The Stirling is certainly more fuel efficient than the powerplants we're flying now, but not seven times as good. Stirlings aren't even twice as good. (But again, the learning curve is ahead.) It's very doubtful that SFC will ever be twice as good as today's quoted numbers, but I think it's possible that real world fuel consumption could improve by a factor of two.
Doesn't gentle heat-cool beat suck-squish-pop-ptui?
Requirement number three, constant power at altitude, is where the Stirling really excels. Like any other heat engine, the cycle operates between two temperatures. Energy (heat) is input at high temperature, the heat "quality" is converted to shaft horsepower, and the energy is rejected at a lower temperature. The bigger the temperature spread, the more output. At altitude, with cold ambient temperatures, the Stirling will actually achieve better efficiency and greater shaft output than it did at SL. So instead of going twice as fast at 40,000 ft, we'll really be going more like 2.3 times as fast! Since the engine is a sealed system, it has no knowledge of ambient air pressure or aircraft altitude. All it knows are the hot and cool temperatures presented to it. And if you doubt that a fire can be maintained at that altitude, please don't share your concerns with a Lear driver who rides around at FL510 propelled by a couple of fires. Or the SR71, or......you get the idea.
But do real Stirling engines exist? You bet they do. There are Stirling-powered submarines, major automakers all over the world have designed and operated Stirlings, and RC models have flown under Stirling power. NASA has displayed their Stirling-powered blue Dodge pickup truck at Oshkosh a number of times. You may have seen it carrying pilots up and down the flightline during the afternoon airshow. My favorite Stirling quote is found in Joe Walker's book Stirling Engines, where he is telling about witnessing tests of an 800 HP General Motors Electromotive Division engine at La Grange, IL in 1967. "It was said at the time that the engine could be reversed in less than a revolution (but only when the stress office engineers were not present!)."
One of the more spectacular (and least successful) Stirling engines was installed in the 2200 ton ship Ericsson in 1853. It had four cylinders, each with a diameter of 14 feet and stroke of six feet, direct-coupled to a 32 foot paddle wheel. Engine layout was such that the pressure was applied to the bottom side of the pistons, and the tops were exposed. During a demonstration trip, reporters were invited to step onto the pistons and ride up and down. (Try that with your TSIO sumthin-er-other!)
So if Stirlings are so good, why don't we have them already? Good question, with many answers. For one thing, we do have them already. One of the interesting characteristics is the bilateral nature of the Stirling. If you input temperatures, the engine puts out shaft horsepower. But if you input horsepower, the engine outputs temperature difference. (Can you imagine producing gasoline that way?) Stirlings are uniquely suited to producing very cold temperatures, and are used to make liquid nitrogen, oxygen, etc. Some of those Stirlings are as big as a bus. At the other extreme, tiny Stirlings are used in satellites to cool the image sensors. The next time you look at the weather satellite pictures, remember that you have a Stirling to thank.
There are other reasons Stirlings aren't all around us, of course. In the early days, they competed with steam engines which were more suited to the material of that period, cast iron. If Bessemer had come along 30 years sooner it's possible that we would have enjoyed the age of Stirling rather than the age of steam. Today we have a variety of high temperature alloys, so the materials problem has been solved. Another Stirling problem, in all honesty, is that oddball engines sometimes attract oddball engineers. (By the same token, stodgy old engines may attract engineers that are lacking in vision!)
Most of the recent Stirling R&D has been directed to automotive applications, and Stirlings don't make good car engines. It's like the old saying in the printing trade: "Price, quality, speed, pick any two". In the case of Stirling engines, it's fuel economy, rapid control response, and simplicity. You can pick any two. If a new technology is going to displace existing automotive engines, however, it must have all three. NASA and the automakers have learned that the Stirling doesn't have sufficient advantages to overcome the huge inertia of doing engines the way they're presently done.
But as we've seen so often, car engines and aircraft powerplants are very different animals, even though they both suck-squish-pop-ptui. Cars are doing rather well with their present engines, airplanes are not.
In the next installment I'll include some Stirling drawings, and get into the nitty-gritty of adapting the Stirling cycle to aircraft propulsion. Stay tuned.
Harnessing the
Stirling Engine's
Potential
Why aren't new aircraft offering better performance than the old ones? The answer is simple: powerplants haven't changed. The well known four cycles, suck-squish-pop-ptui, have gone about as far as they can go. We are stuck at the far end of a learning curve that dates back to Orville and Wilbur. Historically, every advance in aircraft design has been preceded by an advance in powerplant design, and piston engines stopped advancing half a century ago.
In the first part of this series, we discussed the potential advantages of the Stirling cycle aircraft powerplant. Silent power, no vibration, the ability to burn Jet A (or almost anything else), and perhaps best of all, power that does not decrease at altitude. In fact, the power will actually increase as the ambient temperature decreases, allowing us to fly fast and high above the weather.
To understand the Stirling, it helps to sit back and put aside everything you know about existing powerplants. Clear your mind of cams and lifters and valve seats and magneto timing and piston rings and a zillion other little bits and pieces. Stirlings are truly different.
Consider a closed container filled with air, or perhaps filled with a gas such as helium (Figure 1). If the gas is heated, it's pressure rises. Doubling the absolute temperature doubles the pressure. This change in pressure could be used to push a piston and do work. When the gas is cooled the pressure drops, and the piston returns. Of course this would be awfully slow and inefficient because the container would have to be heated and cooled along with the gas.
Figure 2 shows the addition of a device called a displacer. True to it's name, it is not a piston but is used to "displace" the gas. One end of the cylinder is kept hot and the other end is kept cool, and stroking the displacer is all that's needed to move the gas from hot space to cold.
(When I'm explaining the Stirling to kids, I ask them to imagine a bathtub. One end of the tub is hot, the other is cold. In the bathtub is a fat lady, and when she moves to the hot end, all the water goes to the... by this point their eyes are twinkling, the kids are 'way ahead of me, they completely understand the function of the displacer!)
Figure 3 adds the power piston, and we have the basics of a Stirling engine. Note that there are no valves, no cam, no intake, no exhaust. The same gas is alternately heated and cooled, heated and cooled. Gas molecules never wear out. The motion of the displacer leads the power piston by 90 degrees. First the displacer moves to the cold end, pushing the gas to the hot end, thus increasing it's pressure. Next, the increased pressure forces the power piston down. Third, the displacer moves to the hot end (and the gas to the cold end), thus dropping the pressure. Last, the power piston returns. And it all begins again.
The four actions overlap, when the displacer is at the end of it's stroke the piston is at it's center, and vice versa. In a well designed engine it's possible to achieve 100% static and dynamic balance, thus no vibration.
Figure 4adds the concept of the regenerator. In his 1816 patent, Robert Stirling called it an "economizer". Regenerators today are made of stacked wire screens, or stainless steel wool, or foamed vitreous material. As the hot gas passes through the regenerator, it heats the regenerative matrix. That is, it leaves it's thermal energy in the matrix. When the gas reaches the cold end of the engine it is already most of the way cool, and the cooler only has to finish the job. An instant later the gas passes back thru the regenerator, picking the same energy back up. When it arrives at the hot end of the engine, the gas is already most of the way hot, and the heater only needs to top it off. On each stroke, the thermal energy is traded back and forth between the regenerator and the gas. This greatly reduces the amount of fuel needed, and also reduces the amount of energy thrown away in the coolant.
Compare this with today's powerplants. Suck (use crankshaft energy to pull new air and new fuel in), Squish (use crankshaft energy to compress the mixture), Pop (burn the fuel, raising the pressure to push the piston down and impart energy to the crank), ptui (use crankshaft energy to force the burned mixture out). Note that three cycles out of four require energy, only one provides it. No wonder today's powerplants vibrate so much. And what does the engine do next? It begins again with brand new air and brand new fuel, saving nothing from the cycles that have gone before.
Worse yet, today's most "advanced" powerplants have intercoolers. What is an intercooler? It's an energy disposal, similar to a garbage disposal. It is a device with only one function, to throw energy away. To get rid of it. This is energy that was resting in the fuel tank just a few seconds earlier. Why do we buy fuel (and pay taxes on it), haul it aloft, burn it, and throw the energy away? Because our powerplant technology is so far out on the end of the learning curve that we're getting desperate, that's why. We are willing to accept penalties in cost and weight and efficiency to squeeze a little more performance out of the tired old Otto cycle.
It's becoming clearer why the Stirling is more efficient.
The two common questions are: Can the Stirling run fast enough, and can it produce enough power? The answer to both is yes. To fully appreciate the speed of a Stirling you need to watch one run. (I always have one or two model Stirlings at the AirSport Altitude Alerter booth at Sun 'n Fun, Oshkosh, and AOPA convention; you are invited to stop by. It's fun to run an engine indoors while the crowd passes by, completely unaware! Isn't that the way airplanes should sound?)
Heating and cooling happen fast in a Stirling, but things happen fast in today's engines too. Think how little time is available to open an intake valve, suck in mixture, and close the valve again. At 2,400 rpm, the crank rotates 40 times each second. A single stroke occupies 1/80th of a second, or just 12.5 milliseconds. The valve has to move from closed to open in 3 or 4 thousandths of a second, and close just as rapidly, and it has to do it 72,000 times each hour. If ignition timing holds within one degree, that event amounts to 0.000035 sec (35 millionths of a second). Can you imagine mechanical contacts in a magneto holding that sort of precision? The point is, most of us don't have the imagination to grasp how fast things are happening in our engines. We know it works, and we accept it.
Then a Stirling engine comes along with no valves and no ignition, and it's hard to imagine!
Modern Stirlings are pressurized, sealed systems. Usually the working gas is helium or hydrogen because the lightest molecules need the least heat energy for a given change in temperature. (This is exactly the opposite of refrigeration or air conditioning, where the object is to move as much heat as possible and a heavy molecule such as Freon is needed.) Pressurization is used to increase the specific power of the engine. If the absolute temperature of a gas at ambient pressure (say 15 psi) is doubled, the pressure goes to 30, for a net "push on the piston" of 15 psi. But if the engine is pressurized, say, to 300 psi, then the same temperature difference raises the pressure to 600 psi, for a delta of 300 psi on the piston. For the same displacement, this amount of pressurization nets 20 times as much power. It's sort of like turbocharging to 600 inches of manifold pressure!
Likewise, if the absolute temperature ratio is 3:1, then the pressure ratio is 3:1 and more crankshaft power is produced. So it's important to operate the burner at as high a temperature as possible, and keep the cool end as cold as possible. More temperature spread, more output.
This is the characteristic that makes the Stirling so promising for aviation. The higher we fly, the colder it gets, the more power is produced. Voila, no more ceiling!
Wonderful changes will occur in aviation when we're not limited by a powerplant ceiling. Internal combustion engines-both piston and turbine-have always been limited by ceiling. In the future, there will still be an altitude above which we cannot go, but the limit will be the wing or something else; it won't be the engine.
One thing is certain: the way we design airframes will change completely. The way we fly will change too, with the newfound ability to cruise over the weather rather than through it. And for the first time in decades, there will be a good reason to swap the old bird for a new one.
Figure 5 offers a hint of what a Stirling aircraft powerplant will look like. A radial layout is pictured, but inverted inline or opposed configurations are practical too. The regenerative displacers are operated by gas pressure rather than a mechanical linkage. The "pistons" are thin metallic membranes separating the gas from hydraulic fluid. This fluid acts as the connecting rod, and at the same time serves to conduct away the reject (low temperature) heat energy, and as lubricant. The crankshaft (not shown) lies at the center, surrounded by a mechanism that is the subject of a current patent application and is best kept proprietary for now.
How is progress coming? Slowly. In any field there is pressure to maintain the status quo, and nowhere is this more true than aviation. For instance, consider the FAA's "research specialist program", where our tax money is being spent in aviation research and development. Specialties include advanced avionics, airframe icing, advanced control systems, propellers, manufacturing quality assurance technology, crash dynamics, electromagnetic interference, software quality assurance, advanced composite materials, human factors, metallic structural materials and processes, aeronautical communications, flight management, and propulsion control systems. Notice anything missing? The thing that is missing is the single item that has been the key to aeronautical progress from day one: The powerplant. FAA isn't doing anything to advance the one component that aviation needs most. They're too busy working on all the things that we don't need.
And it's not just the FAA. Look at NASA's AGATE program, which has the goal of leading the way to the GA aircraft of the next century. AGATE is interested in powerplant controls, but not powerplants. Same as FAA. Does anyone besides the government think the future of General Aviation really depends on single-lever power control?
The best place to keep up with Stirling progress is via Internet. Here at AirSport, we're in the process of setting up our web presence at http: //www.airsport-corp.com. Perhaps by the time this gets into print we will have our Stirling page, with lots of information and links to other Stirling sites worldwide. Other sources of Stirling information include: Stirling Machine World, 1823 Hummingbird Court, West Richland, WA 99353, phone (509) 967-5032 (subscriptions are $40 for 4 issues a year, and they sell many Stirling books); and Bailey Craftsman Supply, phone (573) 642-5998 (they sell books, videos, and engine kits).
I hope this glimpse into the future of aircraft propulsion has sparked your interest. Stirlings offer a lot of promise, but there are other possibilities too. ONRL is doing some very interesting work with electric propulsion, and there are other interesting things on the horizon. Aviation doesn't have to be forever constrained by suck, squish, pop, ptui.
****************
Footnote:
Darryl Phillips was the 1994 recipient of the Wolf Aviation Fund annual award for his work in promoting the Stirling engine for general aviation. He has been pursuing the goal of silent powered flight for nearly 20 years, and currently has a number of Stirling proposals before NASA. He is President of AirSport Corporation, 1100 W. Cherokee, Sallisaw, OK 74955 which makes Altitude Alerters and Transponder Monitors for general aviation. Darryl's email address is darryl@airsport-corp.com, or fax (918) 775-4000.