Experimenter

MAR 2015

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

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32 Vol.4 No.3 / March 2015 UNDER THE COWL rip itself to pieces, starting and stopping its own consid- erable mass. If the crown and piston are lighter, stresses are reduced and heat transfer is enhanced. But less mate- rial alone also makes the piston weaker. The ACE solu- tion significantly reduces mass in modern, heat-tolerant extruded billet A2618 alloy and also—another technology from car racing—cools the piston crown by squirting oil on the underside of the piston crown. This oil's duty is to absorb heat and transfer it through the oil cooler to the air. The thinner piston crown gives up its heat quicker than a heavy one. Modern lubrication allows improved piston skirt design, keeping the piston from welding itself to the cylinder bore. A smaller skirt is lighter, but it also has less surface. That's where modern oils help; offset piston pins help a lot; and the skirt's surface finish itself keeps oil between the piston and the cylinder wall. Just the right type and amount of roughness is necessary. Beyond materials and lubrication are fundamental design considerations. The top land is noticeably narrow; this reduces "crevice volume," reducing the onset of deto- nation. ACE also employs "scuff bands" for a conformal fit; they are designed to roll over and deform, matching the shape of the bore. Higgs explained, "If there were no scuff bands, the side of the piston would simply wear. With a big top land clearance, wear is not an issue, but we optimize the piston and reduce clearances. Big clearances used to be the only way to make things fit and work; we have better materials and technology today." Further reducing top-end reciprocating weight, the piston pins are shorter, have a smaller diameter, and are held in place with simple clips. They are made of C350 steel and are diamondlike carbon (DLC) coated to operate in regimes of low lubricity. Rings are thinner, reducing inertia and its ugly cousin, flutter. ACE uses a three-piece oil ring to better "hug " the cylinder wall. ACE also has two "flavors" of connecting rods. The stroker 409 rod uses a reverse four-bolt design that allows clearance to the camshaft and allows more even clamping of the bearing. Rods for the standard crank are aerospace 4340 steel with a bronze small end bushing that incorporates an "oil eye" to capture oil. The bottom end sports two bolts that thread directly into the cap, reducing weight. The counterweighted, "round" 409 crankshaft is the largest crank that can fit inside the standard engine case. Higgs said, "We sketched the design and did a fatigue analysis. We have a longer stroke, a smaller crankpin—and less stress than a standard 360 crank! The heavy weights gave us mass to help absorb some of the nastiest pro- peller-induced vibrations. We also kept rotating inertia close to the crank centerline, reducing stress." The round The specialized rod bolts demonstrate the attention paid to designing each component for its task. The intricate fi nning on the monolithic cylinder adds to cooling. The piston employs modern design and materials to yield a strong and light component. Photography courtesy of ACE

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