From Military Aviation to Peacetime Motorcycling

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Modern cylinders are coated with hard plating to improve piston cooling. (KTM/)

Pull a cylinder off a late-model motorcycle engine and you will likely see the very same piston-cooling oil jets that greatly contributed to the reliability of post-World War II aircraft piston engines, those pioneers of worldwide commercial aviation. Without such oil jets, the only way pistons can rid themselves of the combustion heat entering their crowns is by conducting that heat to the cooler cylinder walls.

Piston-cooling oil jets had first been used on diesel locomotive engines, but were subsequently adopted by US aircraft engine makers to end scoring or seizure from high piston temperatures. After the war, innovative engine builder Tom Sifton applied such jets to the Harley dirt-track engines he developed for the great Joe Leonard.

In a fascinating book I’m reading, Calum E. Douglas’ The Secret Horsepower Race, I’ve found many examples of technologies developed to boost aircraft piston-engine power in WWII. And in the intervening 70-plus years, many of those technologies have been adopted in modern automotive or motorcycle engines.

Losing the Liner

Look at almost any late-model motorcycle engine’s cylinder bore and you will no longer see the traditional (and heavy) iron cylinder liner. German engine-development engineers racked their brains for ways to boost power, equaling or exceeding the performance of Allied fighters. Their testing confirmed that by eliminating the iron liner, which conducts heat poorly, and applying hard, porous chromium plate directly onto an aluminum cylinder, they could cool hard-working pistons faster.

In the early 1960s, when Yamaha was confronted with high two-stroke piston temperatures causing outright seizure or piston-ring sticking from oil gumming, the Tuning Fork folks tried the same thing. First they tried anodizing the aluminum cylinder wall, turning it into hard ceramic aluminum oxide. When that proved unreliable they adopted hard but porous chrome plating, and by 1967 they had a durable solution. Current production motorcycle engines employ even more durable hard plating such as Nikasil to improve piston cooling and shed the 6 pounds that iron liners can add.

Oiling Issues and Oil Frothing

Fighter engines of WWII, operating at high supercharger boost, were prone to crankshaft plain-bearing failures caused by inadequate oiling or by oil containing froth. Engineers abandoned the normal automotive oiling method of pushing oil from the main bearings radially into drillings in the crankshaft in order to lube the con-rods. Instead, they pumped oil into one end of the crankshaft. This “end-feed oiling” succeeded because it no longer had to push oil radially inward against “centrifugal force” as normal oiling did.

When Honda had persistent con-rod big-end-bearing streaking in the RC30 and RC45 Superbike race engines, it tried higher oil pressure. When that failed it adopted end-feed oiling in 1997, and were able to banish the streaking problem with only 12 psi of oil pressure. Honda’s current 1,000cc Fireblade production sportbike and Superbike engine have end-feed oiling from both ends of the crankshaft.

Froth in engine oil destroys bearings because 1) air is not oil, and 2) air under oil-pump pressure can sneeze the oil out of bearings, allowing overheating and failure. This was a severe problem in Superbike racing engines until they were provided with very deep sumps that could guarantee the presence of froth-free oil at the pump pickup. Older readers will recall that early Honda production bikes had rotating centrifugal air/oil separators driven by a small chain, duplicating wartime German aviation practice. With such systems the heavier oil is thrown to the outside, while entrained air is forced to the inside, from where it is vented.

In the 1960s, Ed “The Camfather” Iskenderian began to call valve overlap “the fifth cycle.” In supercharged drag race engines of the time, combustion-chamber conditions were similar to those in high-boost fighter engines. The Germans had found value in using very long valve overlap to allow the supercharger to blow through the combustion chambers, removing any remaining hot exhaust gas and cooling both intake and exhaust valves. To vary this effect with altitude, Daimler-Benz developed hydraulic variable valve timing (VVT). Such VVT systems have become commonplace on auto and motorcycle engines today. By eliminating valve overlap at lower rpm, a modern VVT system prevents loss of unburned fuel out the exhaust, but boosts torque by altering valve timing to allow overlap at higher revs.

Gasoline Direct Injection

The Germans adopted direct cylinder fuel injection for their WWII aircraft engines because 1) they believed their current carburetor technology inadequate, and 2) they thought it would be easier to make injection accurately controllable. Such direct injection, now called GDI (Gasoline Direct Injection) by the auto industry, boosts power by feeding an engine 100 percent air rather than a mixture of part air and part fuel vapor. The Motus V-4 motorcycle (sadly no longer produced) originally planned to have a GDI fuel system.

Wright Aero originally gave the big radials it built for the B-29 a carburetor fuel system. Chronic problems with unequal mixture distribution led to lean backfiring and induction-manifold fires. This forced the development of German-style cylinder injection. Of some 32,000 B-29 engines built, roughly the last 7,000 were equipped with direct injection, making induction fires impossible. Since the 1990s, GDI has become commonplace on automotive engines.

During WWII the US had the advantage of large alkylation plants to boost the antiknock rating (octane number) of aircraft fuels. Yet, to maximize the volume of fuel produced, both Germany and the US added heavy aromatics to av-gas as octane boosters. When I was at European motorcycle GPs in 1981, factory teams were still carrying “just-in-case” quantities of toluene, an aromatic—added as needed to keep race engines operating knock-free on suspect “official track gasoline.”

Surface-gap spark plugs looked rad in 1971, but Bosch built and tested them in the 1940s.

While the arrival of operational jet-propelled warplanes late in WWII killed government-funded development of large aircraft piston engines, technologies developed in that era continue to find new applications in this century, and on our bikes.

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