CONTROL LINE SPEED
Glenn Lee, 819 Mandrake Drive, Batavia IL 60510
FUTURE DEVELOPMENTS
A few columns back I wrote that I would comment about coming technology and future improvements in our model engines. This is a good time to make a few predictions about materials that I expect to see in coming years.
Our model engines have been "top dog" in the world of internal-combustion prime movers. We get more horsepower per cubic inch of displacement than any automotive, aircraft, snowmobile, motorcycle, or other engine that I am familiar with. Big engine manufacturers haven't even tried to design for matched thermal expansion as we have in our ABC and AAC material combinations. So I'll stick my neck out and make a few predictions about possible new stuff. Maybe this technology has already been used—I'll explain later.
Our pistons in the ABC or AAC engines are made out of a cast aluminum alloy containing a high percentage of silicon. The silicon reduces the coefficient of thermal expansion of the alloy to approximately that of cast iron and almost matches the expansion of the sleeve when it gets warm during running. If the coefficient of expansion is any greater, the piston will seize in the cylinder (like 2024 or 6061 alloys would do, if you tried them). If you build it loose enough to eliminate the seizing, the engine won't start unless you use a piston ring as many engines do.
For minimum friction and maximum performance we want a lapped piston. The silicon also reduces wear and friction between the piston and the chrome-plated sleeve. Aluminum can't run against bare steel, brass, or other aluminum without high friction and galling. Cast iron makes a pretty good piston, but it gets too heavy in any engine larger than .35.
We have pretty well learned what kind of shapes and fits we need for the piston and the bore, so any more improvements must come primarily from the reduction of friction. Maybe new coatings and/or alloys can do that.
It's difficult to develop alloys that have to be melted for fabrication. The molten elements will form various crystals at different temperatures during cool-down, which may precipitate before the bulk of the material has cooled. The result is a non-homogeneous mixture. It took many years to create the high-silicon alloys that we have, and the silicon percentage is limited to 19–22%. The alloys also have special rare-earth elements that reduce the tendency of the silicon to form large, hard, brittle crystals during cooldown.
Various companies and individuals have been experimenting with nanometer-sized crystals and metals. Alan Thomas and John Parker, of Nanophase Technologies Corp., wrote an interesting article for Materials Magazine a few years ago. A micron is one millionth of a meter, and a nanometer is one thousand times smaller than a micron. These tiny particles are called nanophase materials, and they can be compacted into metals and ceramics that are stronger and tougher than those made from larger crystals or particles. Ceramics made from these tiny particles can act like metals, and metals made from nanophase powder can act like ceramics. Some nanophase metals can exhibit a fivefold increase in tensile strength and hardness. For comparison, a nanophase grain is to a conventional grain as the tip of a ball point pen is to a basketball.
These nanophase metal powders are synthesized by gas-phase condensation. You could think of it as making "metal snow" by the rapid condensation of metal vapor. The process is accomplished in a vacuum chamber that contains a resistively heated refractory boat or an ion-bombardment sputtering apparatus or a cold surface to collect the vapors. A metal placed in the boat melts and evaporates or metal vapors are sputtered by being bombarded by high-energy electrons. The metal vapor rapidly condenses into metal crystals that do not have time to grow larger than a few nanometers in diameter.
The vacuum chamber is necessary to eliminate oxygen and other impurities that might contaminate the metal, and it can be back-filled with an inert gas or with a reactive gas that you need to create specific alloys or coatings. The metal crystals drift to a liquid-nitrogen-cooled cold finger, where they collect loosely. These powders can then be scraped from the cold finger and consolidated in molds under pressure and heat or by an extrusion process.
If you want to make ceramics, you can evaporate a metal in an oxygen atmosphere. The ceramics formed from these nanophase crystals are remarkably dense and tough and sometimes have physical properties superior to conventional ceramics. If this technology can be scaled up economically, there may be some interesting new possibilities for pistons, sleeves, and coatings. These ceramics are easier to fabricate, are less brittle than conventional ceramics, and have exceptional strength. Any material that can be converted to a vapor phase can be formed into nanocrystals by gas-phase condensation. The ceramics can be hot-pressed to almost 100% density with binders and the ultrafine microstructure allows a high degree of dimensional detail, so parts can be formed to finished dimensions in the mold. These ceramics are ductile, and can deform without failing or cracking.
So what do these nanophase materials have to do with model engines or motoring? What possible use could they be to us?
How about a lighter, stronger, more wear-resistant, perfectly matched thermal-expansion alloy piston? We can tailor the alloying ingredients to anything we want or need. Start with aluminum; add silicon to lower the expansion coefficient; add ceramic powders for wear resistance and less friction; possibly some hard nitride particles, also for wear and friction; and how about the possibility of beryllium for less weight?
Of course, you have to be aware of the toxic effects of some metal powders and beryllium, but many of these powders may be dangerous. Proper precautions would have to be maintained at all times, but this is standard procedure anyway.
Maybe a ceramic piston is possible, although it might be a little heavy; but a ceramic sleeve is a definite possibility. Ceramic ball bearings are already available, but they can't stand the pounding of two-stroke engines. Maybe nanophase ceramic balls would stand the stress better than the presently used silicon-carbide balls.
It would also be possible to create a nanophase material cored with self-lubricating materials and/or wear-resisting materials concentrated in the end bearings. Nanophase steel-alloy crankshafts would also be possible, with built-in friction-reducing surfaces on the crankpin. We could even think about a crankcase without a sleeve, ceramic heads, and who knows what else?
Is this material already here? Possibly! The Russians and eastern Europeans have pretty well dominated FAI Speed in recent years. Their piston material is an extruded aluminum alloy that seems to perform better than anything else that we have been using.
I managed to obtain a piece large enough for the piston in one of my Norvels. It is an extruded alloy; it machines very well, and seems to be quite strong and wear resistant. I have not analyzed the material, but have heard rumors that it contains a larger percentage of silicon than our 19–22% alloy that we have been using. It's possible that the Russian alloy could be made from nanophase or similar techniques. Maybe we will find out sometime.
Transcribed from original scans by AI. Minor OCR errors may remain.
