In early 2010, Harley-Davidson applied for a patent on a limited form of liquid cooling for cylinder heads of the type on its Big Twin. We know that a similar system, but using oil rather than water, has been incorporated into the XR1200R. Why limited? Why not just switch over to full liquid cooling? Harley is careful not to change too much. Owners make it clear that they like the look of an air-cooled engine, so Harley’s patent covers only solving a particular problem, not destroying the admired character of the Big Twin.
What problem? Over the years, many kinds of air-cooled engines have suffered heat-driven head cracking and exhaust-valve seat distortion leading to poor sealing. A rough-and-ready solution is to run a little rich—what you might call “fuel cooling.” This reduces combustion-flame temperature by a useful amount. But in our emissions-controlled era, fuel-air mixture cannot err in the direction of even a little richness.
A possible answer is to encircle the exhaust-valve seat with an unseen liquid-cooling passage and to circulate cooled oil or water through it. Oil is easy because all engines have oil pumps, but oil in very hot places can coke, blocking the flow. BMW “oil-head” four-valve engines have such a liquid-cooling passage running between each pair of exhaust valves arranged so that at engine shutdown, hot oil will drain from the passage before heat soak back from the hot exhaust-valve seat can bake it into a solid.
Water not only cools better than oil, it also cannot coke. Hence, the patent, which calls for small radiators and an electric water pump.
Aircrew of WWII B-29 bombers would have loved such a system installed on their large, 18-cylinder air-cooled radial piston engines. A major problem for the B-29, especially with lean cruising air-fuel mixture, was gradual exhaust-valve seat distortion, valve overheating and valve breakage. Even beautifully cast cooling fins 2½ inches deep failed to prevent this.
The purpose of engineering is not to seek some abstract “best-possible design” but to give us what we want. If Harley riders want traditional air cooling with modern reliability, this is one way to deliver it.