Why Bigger Air-Cooled Engines Become Harder To Cool | Cycle World

Why Bigger Air-Cooled Engines Become Harder To Cool

Engineers must do whatever it takes to make powerplants reliable

We’ve seen steady growth in displacement of traditional air-cooled engines from makers such as BMW and Harley-Davidson. With that growth has come a need for more intensive cooling than can be provided by just covering every surface of cylinders and heads with fins. That more intensive cooling has taken two forms—oil-cooling and local liquid-cooling of exhaust-valve seat areas—both of which have aroused resistance from traditionalists.

We know that the great radial aircraft piston engines of WWII derived at least 30 percent of their cooling from the use of oil coolers and increased circulation of oil. But when one of Harley-Davidson’s engine redesigns clearly needed an oil cooler, it was vetoed by the styling department. Why? Because like a thick wallet in a jeans hip pocket, “It spoils the line.” To get around this, engineering sent less oil to the engine’s cylinder heads, causing their temperature to rise enough to dissipate the heat that would otherwise have gone to the oil cooler. Higher cylinder-head temperature is the enemy of durability, such that today’s Harley CVO models carry large coolant radiators and pump that coolant through passages around the hot regions around their exhaust-valve seats. Engineers must do whatever it takes to make powerplants reliable, whether styling and the custom mags like it or not.

Harley-Davidson, Milwaukee-Eight, V-twin

2018 115th Anniversary Harley-Davidson Fat Boy 114


Why should a 107 or 114ci engine need coolant radiators when smaller previous engines did okay with just cooling fins? Back in the 1930s, the aircraft engine people hit this very same problem. Every year the budding airlines and the military expected more and more power, so aircraft engines were being bored and stroked to increase their displacement. In 1930 Pratt & Whitney stroked its nine-cylinder R-1690 Hornet by 3/8 of an inch to make the R-1860 and ran straight into reliability problems.

I wondered what was going on here. Why would the 1690 cool okay and the 1860 be a problem? Then I remembered that about half the heat that goes into an engine comes from its exhaust ports. Making the stroke a bit longer doesn’t create much extra area onto which to put extra cooling fins, but the bigger cylinder does blow 10 percent more super-hot exhaust gas through its exhaust port.

That is where the trouble starts. The bigger you make the displacement, the greater the blow-torch volume of hot gas that blasts through the exhaust ports, resulting in higher head and cylinder temperatures, unless something is done to efficiently remove that heat.

And that, ladies and gentlemen, is why today’s big-inch air-cooled engines are being given supplemental liquid-cooling.