Harley-Davidson looks to a form of liquid-cooling to bring its Big Twins up to future emissions standards. Forty-five-degree Big Twins are Harley’s claim to fame; the engines produce tons of torque, they’re narrow and they do sound great (loud pipes notwithstanding). And they’re a Harley tradition.
The dilemma the company faces is that it is becoming increasingly difficult to meet emissions and noise standards while producing reasonable amounts of power without melting the engine using an air-cooled design. A move to liquid-cooling would solve this, but eliminating cooling fins would also drastically alter the look of the engine, which serves a prominent styling function.
According to a patent request published on May 19th of this year, Harley seems to have a solution by producing a liquid-cooled cylinder head based on the existing air-cooled design.
The cylinder head is not a typical liquid-cooled design, but more of an air-liquid hybrid. The majority of the cooling will still be achieved by cooling fins, both on the cylinders and heads, but a passage enters each head on one side of the intake port, passes through the head and around the exhaust port before exiting on the other side of the intake port. This should significantly cool the cylinder head.
In the patent, the liquid-cooling system is shown on a Harley touring bike, with twin radiators and cooling fans hidden out of sight in the fairing lowers, while ducts direct hot air away from the rider. A water pump and thermostat housing are located low in the frame, just ahead of the engine. The heads use quick-connect fittings so the coolant hoses can be disconnected without tools for servicing the engine.
Externally, the bike should look no different than the current air-cooled FL models. One problem that will arise is where to hide the radiator on Harley’s non-touring bikes like the Softail or Dyna models, should the system be fit on those machines.
It’s not the first time an air-cooled engine was converted to liquid-cooling; Ducati produced the 906 Paso engine in the late 1980s. There’s no news from Harley when and if we’ll see this system on a production bike, but it’s likely coming soon.