How Do Cylinder Size And Engine RPM Affect Heat Loss?

Exploring the effect of differences in surface-area-to-volume ratio.

Andrew Stroud at Daytona
Adequate airflow, low power cost: The late John Britten located a small cooler under the rider’s seat and forced air through it by taking in air at the highest-pressure zone, in the front of the fairing, and ducting it back out into the lowest pressure zone, the wake region behind the bike. This is Andrew Stroud at Daytona in 1994.Gold & Goose

During the “happy time” of AMA Superbike racing, it was always striking to see the difference in radiator area between the Ducati V-twins and the Yamaha inline-fours. The Ducatis’ radiators fit easily in the space available, but the Yamahas’ $9,000 radiators were of the maximum possible size, making that bike’s engine bay look like a full-rigged ship.

Twins were allowed 1,000cc, fours 750cc. But when you do the arithmetic, you find that total cylinder-wall area plus combustion chamber and piston-crown area is 10 percent greater for the Yamaha OW01 than for the 926cc Ducati of that time. In terms of cylinder surface area to displacement, the difference is even greater, 36 percent.

What does this mean? It means that the bigger the cylinder, the more of its volume never gets to touch a metal surface and lose heat to it. This is the effect of differences in surface-area-to-volume ratio.

In the case of a large marine diesel, the effect is even more striking. That Wärtsilä of 3-foot bore by 10-foot stroke has in the region of 20 times more displacement per unit of surface area than do the two motorcycle examples. This gives good and effective protection against excessive heat loss to coolant.

I was able to see this effect while reproducing a classic physics experiment to measure the ratio of specific heats of a gas. The idea was to send a measured amount of energy into a wire filament suspended in the center of a sealed volume of air and measure the resulting pressure rise for which I’d built a tiny pressure gauge. Observing the fluid level in this gauge through a microscope, I would discharge a precision capacitor across the filament. The gauge fluid would quickly rise, hold constant for a second or so, and then drop back. What was happening was that, as the filament heated the air in contact with it, the pressure would rise as the heated air expanded. It would stay constant as that small volume of heated air rose like a helium-filled balloon by convection toward the top of the sealed volume, touching nothing but the air around it. When that heated air contacted the cold surface (in this case, a gallon jug in a water bath), it lost heat to it and the pressure dropped.

Laguna Seca
AMA and World Superbike stars crest turn 1 at Laguna Seca in 1997, the Japanese inline-fours—Kawasaki, Yamaha, and Suzuki—visibly wider than the twin-cylinder Ducatis of that era raced by Pierfrancesco Chili (7), Carl Fogarty (4), and Neil Hodgson (9). American John Kocinski (3) won both legs en route to the world title.Brian J. Nelson

That’s only part of the story for the grand old man of the internal-combustion engine, Harry Ricardo, who did experiments in the 1920s that revealed about one-half of the heat rejected to coolant is lost through the walls of the exhaust port.

Why? Because conditions for rapid heat transfer are best there; the temperature difference between the hot gas and the metal inner surface of the port is very great, and the high-speed flow (the early flow is sonic) is extremely turbulent, constantly bringing fresh hot gas from the middle of the flow into contact with the walls.

For this reason, wise engine designers make their exhaust ports as short and straight as they can. This was an important feature of Harley-Davidson’s Evolution engine, and years later the late Jim Feuling made a good living getting Detroit designs out of thermal trouble by making exhaust valves and ports smaller; he was able to preserve or increase the original amount of flow because it was so easy to improve on Detroit’s “best design practice.”

This reveals another reason why that 750cc Yamaha needed a bigger radiator than did the equally fast 926cc Ducati: because the Yamaha had four exhaust ports to the Ducati’s two. Making that even worse was the fact that the exhaust ports of inline-fours would naturally—if Feuling would’ve had anything to do with it—have pointed up and forward. But, in order to fit the four exhaust header pipes behind the radiator, the ports had to face forward and down. The extra port length required to make this happen provided extra surface area for heat loss.

I therefore cringe when I see so many engines whose exhaust ports are “talking out of the sides of their mouths” from being bent around to clear frame members. Ouch! A particularly bad example is the rear cylinder head on the much-beloved Honda Hawk GT.

In racing, the bigger the radiator, the greater the cooling drag, and the greater the loss of top speed caused by it.

When I ask engineers about this, they shrug and say, “Yes, it’s not ideal but we have to get along with those guys in styling.” They know they can handle the heat by just adding an extra row or two to the radiator or by speeding up the water pump, as Yamaha did with the 1974 TZ750A.

On the street, there’s no reason to worry about needing a bigger radiator, but in racing, the bigger the radiator, the greater the cooling drag, and the greater the loss of top speed caused by it.

During World War II, engineers were thinking intensively about every detail, and it became clear that a coolant radiator could be designed as a low-grade jet engine. After all, a jet engine is just a duct that takes in air, heats it so that it expands, and exits the rear of the duct at a higher speed than it had entered. It is said that in this way the coolers on the de Havilland Mosquito were ducted to produce enough thrust to cancel cooling drag. Some have referred to this as the “Meredith effect.”

Providing the smooth ducting for such a thing on a bike is hard to imagine—John Britten tried! Ducati’s approach to cooling drag in racing was to increase coolant temperature, allowing the Italian bikes to use a smaller radiator and so take in a reduced volume of cooling air.