Keeping Engines Cool, Part 2

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Editor’s note: This is Part Two of a three-part series on the topic. The final installment will appear soon. Enjoy!

How Are Pistons Actively Cooled?

How to cool pistons? There are three possible heat-outflows paths:

1. From the hot piston to the cooler cylinder wall
2. From the hot piston to the random oil splattering everything in the crankcase
3. From the hot piston to the air moved about the crankcase by the rising and falling pistons

The first method in our list used to be pretty much the whole story, though aircraft engine designers did notice some reduction of piston temperature when they increased the oil circulation rate to the crank and rod bearings. Much piston heat passes to the cylinder wall through the piston rings, which are separated from the wall only by a micron or so of oil film. Crankcase oil splatter has long been a secondary source of piston cooling.

So how about  No. 3—heat transfer to crankcase air? This is ineffective: Since aluminum is more than 2,000 times denser than air, it would take a lot more air than there is in an engine’s crankcase to have much effect on piston temperature.

Related: Keeping Engines Cool, Part 1

Aluminum Replaces Iron in Heads and Cylinders

As engine power per cubic inch of displacement increased through the 1920s and ‘30s, the low thermal conductivity of iron cylinders (excellent wear surfaces though they were) allowed pistons to run hot enough to make powerful engines detonate on the desired compression ratio. Nobody wanted to drop compression, as that’s what makes torque. As a rule of thumb, peak combustion pressure is 100 times the compression ratio. OK, then; if we can’t drop the compression, we have to drop those iron cylinders and put higher-conductivity aluminum cylinders in their place.

How much higher? Picking through the Handbook of Chemistry and Physics reveals a figure of 0.109 (metric measure) for iron and from 0.5 to 0.6 for aluminum, a ratio of 5-to-1 in favor of aluminum. How does that work? Metals, having many easily lost electrons in their atomic orbitals, are in effect filled with a “gas” of extremely mobile energy carriers—those same electrons. This also applies to electrical conductivity—those free electrons are easily pushed this way or that by magnetic fields, making metals excellent electrical conductors. Even better than aluminum in this way are copper and silver. But 104 years ago, A.H. Gibson, wrestling with the problems of air-cooling at the Royal Aircraft Factory as Britain fought World War I, made measurements showing that to conduct away a given rate of heat flow, a head made of aluminum would be the lightest. Yes, a copper head would conduct heat better than aluminum, but copper is three times heavier! That’s precisely why Yamaha made just the 2-inch cylinder cap of its GP bike heads out of copper in the late ‘90s—the rest of the casting was light aluminum. And, legend tells us, when AJS was initially designing its famed “Porcupine” 500 GP twin of the late 1940s, it went as far pricing out how much it would cost to make the engine’s heads out of silver. (Les Graham was 500 champion on the bike—with aluminum heads—in 1949.)

Water-Cooling Replaces Air-Cooling

Everything became much easier when circulating water replaced air as the principal cooling medium. Water is wonderful stuff for many reasons (it’s why we can exist on this planet), but among its attributes is a very high specific heat, a measure of how much heat it takes to raise the temperature of 1 gram of the material by 1 degree Celsius. Again, the good book tells us that a gram of water can absorb 4.5 times more heat per degree Celsius than can a gram of aluminum, and between eight and nine times as much as a gram of iron. That’s why liquid-cooling is the choice when the most intensive heat removal is required. Stuart Shenton, long-serving tech chief of Suzuki’s 500 GP team, once noted that as a bike pounds around the Daytona banking, its heat output is stored in its coolant, whose temperature rises steadily. When the bike reached the infield, heat flow dropped and the radiator could pull the water temperature back down.

When Harley’s famous racing manager Dick O’Brien and design draftsman Piet Zylstra were laying out the then-future-classic XR V-twin in 1971, O’Brien stated that he wanted a full inch of aluminum on top of the combustion chamber. He said that because he knew it takes time for heat to flow from the chamber surface to the actual cooling fins themselves, and from them into the flowing air. Too much time! That being so, he knew the engine had to store that heat very near where it was being released, so that heat could flow away to the fins later, when the rider had entered a turn and was on closed or part throttle. That’s why he wanted that inch of solid aluminum: as a heat sink, somewhere to store the heat until there was time to deal with it. Part of Zylstra’s job was to keep things in balance, so they compromised at 3/4 of an inch.

In modern liquid-cooled engines, very slim water passages filled with fast-moving pumped coolant greatly speeds up the heat-removal process, and effectively scours away any steam bubbles that form. The result is cooler combustion-chamber surfaces, which in turn allow detonation-free operation with sky-high compression ratios. Just a few years ago, 13-to-1 seemed impossible, for ragged-edge racing use only. Now plenty of fairly ordinary production bikes use compression ratios that high and run reliably and well with them. Fast-moving water is ace at carrying away heat.

Parts Three will address cylinder-liner materials, spark plugs, bearing heat, and more.

 

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