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High Flyers

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

In the 1920s, as air cooling was fighting for a role in aviation, a US Navy man wrote that liquid-cooling aircraft engines made as little sense as air-cooled submarine powerplants. In either case the logical choice was to use the medium in which the vehicle operated as a coolant. Soon Admiral Moffett would show that the air-cooled engines his service favored were faster climbing, lighter, and more reliable than the liquid-cooled types they were replacing.

On October 29th, reader G_k expressed interest in the different needs of motorcycle and aircraft engines, while a day later Kieselguhr Kid later noted that aircraft engines directly coupled to their props are limited to the rpm at which the propeller tips near the speed of sound; any faster results in reduced efficiency and extra noise. Why not put reduction gearing between prop and engine, allowing the engine to turn faster and make more power? A gearbox adds weight, cost, and complexity. When the US produced the “Harvard” training airplane, powered by an air-cooled Pratt & Whitney R-1340 radial, they decided to accept the noise of sonic tip speed at takeoff as an acceptable trade for omitting reduction gearing. On the turboprop-powered XF-84H  of the mid-1950s a supersonic prop is said to have made ground personnel ill from its awful noise (its tip speed was Mach 1.18).

Perhaps the strongest influence on US general aviation has been liability law, which often stops innovation cold. As a result, the engines that have survived years of legal attack are conservative and basic in design.

As a boy I saw, in the “mechanix magazines,” notice of the Mooney “Mite,” a tiny postwar design (1947) powered by an overhead cam liquid-cooled Crosley auto engine coupled to a multiple V-belt reducer. Since then, many attempts have been made to modernize general aviation engines. Seven “Mites” were built with the Crosley engine before it was replaced by a tried-and-true direct-drive air-cooled Lycoming four.

Takeoff power

Aircraft engines must reliably endure comparatively long periods at takeoff power. Motorcycle engines, on the other hand, are constrained in their use of high power by obstacles such as buildings and policemen. Thus, air-cooled motorcycle engines have the luxury of enduring such head-warping power levels for only a few seconds at a time. Air-cooled bike engines are therefore designed with quite a lot of metal in their cylinder heads, metal into which heat can flow during such short bursts as accelerating out of the Daytona infield, onto the banking, and around to brake for the chicane.

Large commercial or military air-cooled radials were guaranteed for five full minutes of operation at takeoff power. Why such a limit? Because everything on airplanes must be light enough to fly yet just heavy enough to survive. Not for them the luxury of extra heat-sink metal on a big radial whose 18 cylinders alone weighed 1134 pounds. Such engines had to avoid getting their heads hot enough that the exhaust-valve seats would creep out of round or off center.

That is exactly what happened on WWII B-29 bomber engines. As soon as an exhaust valve lost full contact with its seat, the valve’s temperature rose out of the normal range. Soon, weakened by heat, the valve head would separate from the stem and a violent argument between the piston and the cylinder head would ensue. Aircrew called this “swallowing a valve,” and it happened all too often.

Cylinder-Head Heat and Deformation

Don’t imagine that such things are all behind us. Many a production motorcycle engine, operated beyond its intended design conditions, has behaved in this same way; the aluminum of the cylinder head slowly yields to the stresses on it, at a temperature far below its melting point. This is creep, the slow deformation of hot metal under stress. A familiar modern example is the gradual elongation of a jet engine’s turbine blades. Takeoffs are carefully arranged to minimize the large, multi-million-dollar commercial fan engines’ time-at-temperature exposure. An engine on the wing makes money; an engine in the Heavy Maintenance rebuild bay is a loss.

Imagine that you are a very peculiar person who goes to air museums and measures the cooling-fin pitch and depth on every engine you can get close to. With that information you can very clearly see the steady growth in cooling-surface area that was necessary as engine power grew steadily to meet the insatiable demands of commercial and military users.

Fin Pitch and Area

With aircraft engines it was possible to keep adding more and more fin area because their high speeds provided the energy to push cooling air through more and more closely spaced fins. But motorcycles can’t count on cruising at 200 mph, so their fins can be no closer than 1/4 inch apart. If the power you want can be cooled by that small amount of finning, then air cooling can do the job. If you want more power than the finning can handle, but use it only in short bursts (as in our Daytona example above) then you can add metal to the cylinder head. The extra material will soak up that heat in the short term, so the head never gets hot enough to cause detonating combustion. Dragster engines are a case in point—during each short run a lot of heat flows into engine structure, but cooling takes place back in the pit area, with multiple fans blowing on the engine.

If you need to make a lot of power continuously, you may need more intensive cooling than the free air stream can provide. Porsche adopted fan cooling as a solution—something that was considered in 1945-’46 for a follow-on version of the Boeing B-29. Beyond that lies liquid cooling, as water is 600 times more dense than air.

The density and high specific heat of water make it ideal for areas where extreme heating occurs and high-volume airflow cannot reach. Examples are the hot “bridge” between paired four-stroke exhaust valves and the cylinder-distorting hot exhaust ports of two-stroke engines.

Air Density and Altitude

There is another problem: altitude. Aircraft prefer to fly high, avoiding the greater drag and fuel consumption that comes with operating in the denser air nearer sea level. To prevent loss of power in thin high-altitude air, piston engines were supercharging to maintain performance. Eventually it proved impossible to properly cool the B-36 bomber’s six 28-cylinder engines, even with fan cooling; the thin air above 40,000 feet just didn’t have the ability to carry away that much heat. In March 1945, B-29 operations in the Pacific were brought down from 30,000 feet to between 8,000 and 10,000 feet, bringing a much-needed improvement in engine cooling and reliability.

Jet engines solve this problem by flowing about four times more air than they actually burn. This excess air, its density increased by passing through the engine’s compressor, acts as an effective internal coolant.

As a footnote, the need for ever-increasing power in air racing could not be met by providing more cooling-surface area; the engines they use have been out of production since the 1950s. The extra cooling capacity is provided by carrying several hundred pounds of water. During the race, this is sprayed onto cylinder heads and oil coolers. Every gram of water evaporated in this way carries away a wonderful 540 calories of heat.

When I was first involved with motorbikes it was a universal belief that liquid cooling was too heavy and complex for use with our favorite vehicles. But as motorcycle engines produced more and more power, that belief gradually melted away before examples to the contrary. When I weighed the heads of two four-valve 750s, one air-cooled and the other liquid-cooled, the air-cooled head proved significantly heavier. Why? Because of the extra metal required to keep head temperature from causing detonation.

After fiddling with half-measures such as pumpless “thermosiphon” water cooling, or water-cooling only the head, the industry embraced normal automotive pumped water cooling as its standard solution.

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