Engine Development As Permanent Crisis

Reliability should be the last thing modern riders have on their minds.

Boeing B-29 engine
A hard-knock life: Interim measures for the 18-cylinder radials that powered the Boeing B-29 included cuffs placed on propeller blades to push a greater flow of cooling air into the nacelle, where sheet-metal “blast tubes” directed some of it onto deep fins surrounding the exhaust-valve seats.Harley-Davidson

Because corporations understandably prefer to appear infallible, their public image has no place for the setbacks and crises that inevitably occur in new product development. With such events safely sealed in company archives, the image of engineering becomes that of a smooth, inevitable progress, from good to excellent to perfect.

Because this is so, I have been fascinated since 1966 by the information on aircraft piston-engine development on public record as NACA’s Reports, Technical Notes, and Technical Memoranda. NACA, which became NASA in 1957, was the National Advisory Commission for Aeronautics, a development group that was kept very busy through World War II saving US airframe and engine manufacturers from goofs like B-24 and P-47 tails coming off in flight.

Pistons had a hard life in air-cooled aircraft engines because, until the post-WWII coming of piston-cooling oil jets, their only means of cooling were the fins on the cylinder barrels plus accidental internal oil splash thrown off the whirling crankshaft.

In WWI, the cylinders and crankcases of rotary engines whirled around and the engine’s crankshaft was bolted solidly to the firewall. Cylinders and their finely pitched cooling fins were machined from steel forgings. Cooling was workable because the engine, whirling at 1,200 rpm or so, was pushing its barrels and heads through the air at more than 200 mph.

Efforts to push rotaries beyond 250 hp were stymied by fast-increasing windage losses so the future belonged to static radials whose crankshafts rotated while their crankcase and cylinders did not.

When Wright Aeronautical Corporation in Wood-Ridge, New Jersey, began development of the R-3350 18-cylinder radial engine for the B-29 bomber (3,350 was the engine’s displacement in cubic inches) cylinder finning was machined into the outer diameter of each heavy Nitralloy steel barrel forging. Shaped sheet-metal baffles were used to direct cooling air all the way around each cylinder’s fins to the rear.

On WWI rotaries, without such baffles, only the leading face of each cylinder and head ran cool, while the trailing side ran hot, deforming cylinders out of round and requiring overhaul each 10 to 25 hours of operation. WWI was a permanent emergency for aircraft technologies. As the 3350 was made to develop more power, its pistons, rings, and cylinder bores began to score frequently—a symptom of marginal lubrication. Wright developed a system of using down-passing oil-scraper rings at the top of each piston and an up-passing scraper below the wrist pin, the idea being to trap enough oil between those taper-faced scrapers, which were about 3 inches apart, to maintain full-film lubrication. The idea has appeal, but it didn’t work; the scoring continued.

Harley-Davidson XL1200CX
Producers of large-displacement air-cooled engines use local liquid-cooling to keep exhaust valves and seats well-sealed and healthy. Harley-Davidson has long circulated oil around the exhaust-valve seats of the Sportster. Versions of its Big Twin do the same with antifreeze.Harley-Davidson

Now engineers decided that the hard conditions of initial break-in, when showers of wear particles are generated, was producing cylinder wall and ring surfaces too rough to lubricate. For a time, prelapped rings seemed to help, but at continuous high power nothing was working. What if the cylinder walls are so hot that oil vaporizes right off of them?

With no doubt a heavy sigh, engineers sought to improve barrel cooling by replacing the original integrally machined steel fins with a shrunk-on aluminum “muff” with deeper fins machined into it, baffled to make cooling airflow nearly encircle each barrel. This was logical because the heat conductivity of aluminum is four to five times greater than that of steel.

Why not just make the whole barrel out of aluminum? In England, internal-combustion engines pioneer Harry Ricardo had seen the result: broken aluminum cylinders that were fired off of test engines, out through the roof of the test cell, and a hundred feet into the air. Barrels had to be steel. Forged steel.

With war came sudden crisis: Piston rings were wearing to half their original installed radial thickness in 30 hours!

Then how about deeper fins? Making steel or iron fins deeper becomes useless after a point because iron’s low conductivity transmitted little heat to their tips. But aluminum’s greater heat conductivity could keep 2-1/2-inch-deep fins hot all the way to their tips.

As soon as WWII began, every maker of aircraft engines worldwide caught “the piston-ring disease.” In peacetime, aircrews fly infrequently and are careful with the throttles—governments eager to limit spending vote little money for replacement engines! But with war came sudden crisis: Piston rings were wearing to half their original installed radial thickness in 30 hours! Oil was pouring through engines at rates that made long flights impossible. Why? Crew training was accelerating—by 1944 the USAAF was trying to fly its oil-streaked TB-29 training aircraft 20 hours a day—and throttles were “balls to the wall,” pushed all the way forward, much of the time.

Suddenly single-cylinder test engines were running up test hours around the clock as aircraft engine makers struggled to master the secrets of hard chrome plating top compression rings. A few years ago, I spoke with the late John Minnich, who ran Wright’s chrome ring crash program. Just a skim plating wouldn’t do; they didn’t have a 600-hour runner until they got to 0.007 inch of hard chrome on the top ring’s wear face.

They weren’t out of the woods yet. The chromium layer, being much, much harder than the normally long-wearing cast iron ring surface it replaced, broke in very slowly, if at all. The new rings were breaking in only in a few shiny spots of contact, with blowby occurring between them.

By the war’s end, five different designs of cylinder and head, plus a series of five sets of differently designed cooling baffles had been tried.

Back to the drawing board. Longer engine break-in was impractical because it used up too much aviation gasoline, so rings were now lapped to a smooth, true cylindrical shape in special rigs. Even prelapped chrome rings weren’t a solution because barrel temperature remained high enough to keep lubrication marginal. Barrel temperature had to be reduced. Again.

In all, by the war’s end, five different designs of cylinder and head, plus a series of five sets of differently designed cooling baffles had been tried (two of those baffle types were installed by the hundreds in hectic field-update programs half a world away). And continuing lack of engine durability was a strong factor in a March 1945 change of B-29 tactics, bringing the aircraft down from their originally designed bombing altitude of 30,000 feet to just 5,000 to 8,000 feet at night. Why? Low air density above 20,000 feet resulted in chronically poor cooling, even with the advanced “W” fin barrel-cooling system, which crowded 54 fins onto each cylinder. Instead of being machined from solid, W fins were pressed from thin aluminum sheet as 180-degree elements which were then swedged by special tooling into shallow grooves machined into barrel outer diameters. As the war ended in August, a program to fan cool the R-3350 engine was dropped.

When you ride your new motorcycle away from a dealership with the warranty in your inside jacket pocket, there is no trace of the hard work (and possible desperate crises) of development. Engine operation is normal, and there are a dozen sensors reporting to your ECU to keep it that way.