Harley-Davidson water-cooled engines: the Twin-Cooled system

Let's discuss the innovative liquid cooling of American twin-cylinder engines: Liquid cooling has been speculated about for Harley-Davidson's Big Twin engines for many years, and it finally arrived in 2014, under the famous Rushmore project.
Its implementation is more a matter of emissions control (for which it is important to stabilize the engine temperature), whose regulations are becoming more and more stringent, forcing the brand's engineers to perform true technological feats in order to keep the classic V-Twin as faithful as possible to its origins.

History of development
An engine transforms chemical energy into mechanical energy, generating large amounts of heat in the process. For the engine to function properly, this heat must be controlled in some way to prevent damage to components. Traditionally, HD engines use air cooling, equipping the cylinders and cylinder heads (the parts of the engine where the most heat is generated) with large aluminum fins through which air flows while the vehicle is moving. This air, as it passes through, "steals" heat from the engine, lowering its temperature.

It's worth noting that the liquid cooling implemented by Harley-Davidson differs from traditional cooling, where the cylinders and heads are mounted inside "wet jackets" through which water circulates.

To preserve the traditional air-cooled look, a concept tested a few years ago with the XR1200 models is used, where the cylinder heads were fitted with an internal oil circuit connected to an external radiator (Precision Cooling) that managed to lower the operating temperature of the hottest parts around the exhaust valves.

“Invisible” solution
After successfully experimenting with this system in sports models, the Milwaukee factory simply had to replace the oil with water and they almost had a liquid cooling system.
But not everything is as simple as it seems. As with the ABS braking system, the challenge was to completely conceal the system so that it wouldn't affect the bike's classic aesthetic.
The solution is to divide the water radiator into two parts, which are hidden within the lower fairings.

A mixture of equal parts water and coolant circulates through the radiators, driven by a pressure pump located below the voltage regulator at the front of the chassis. If necessary, separate fans mounted behind each radiator will be activated.

The filler cap, which regulates the overall system pressure, along with the expansion tank, are located inside the lower right fairing.
The system features a temperature sensor and a thermostat that cuts off the flow of coolant to the radiators when the engine is cold, thereby reducing the time it takes to reach operating temperature. This is when coolant flows to the radiators to cool the circuit.

Models and applications
Along with the benefits of temperature control to meet increasingly stringent regulations, there are other areas where improvements can be achieved thanks to the liquid cooling system.
One of them is that the sensation of heat coming from the engine has been noticeably reduced for the driver, and together with the new location of the exhaust manifolds, passenger comfort is also improved.
The other advantage is that the cylinder compression ratio has been increased from 9.6 to 10:1, thereby increasing the engine's power.

The present and the future
Currently, models with "Twin Cooling" liquid cooling, as MoCo calls it, are limited to certain versions of the Touring range. However, it's very likely that the application will gradually expand to more models in the coming years...

Frank Burguera

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