Some Tidbits on Engine Cooling

[Admin]

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/)

Perhaps you like to start your day with a fresh cup of hot coffee. This morning I watched water starting to boil in a plug-in glass kettle. The water in such kettles reaches its temperature rapidly because instead of a circular heating element at the bottom there are only 270 degrees of heat. That intentionally creates a tumble motion which, by constantly circulating the coldest water over the heat, boils water more quickly than the stove.

As I watched, little steam bubbles began to form over the heating element, but they quickly collapsed as they were swept away by the cooler circulating water. That made me think of what happened in two-stroke snowmobile engines a few years ago, when their horsepower shot up after the engine builders adopted the lessons of GP bike porting. Along with the increased power came cooling problems that, at first, nobody was solving.

Early Systems

Early liquid-cooling in bikes had taken a similar direction: Just enclose the cylinder and head in a big tub of water, as the old 1920s’ agricultural engines did, and hope for the best. It worked well on those farm engines, because they were making maybe 5 hp per liter. The system Bultaco used on its 1960s TSS roadracers, which produced 200 hp per liter, had no pump, relying instead on something called “thermosiphon cooling.” As water heats up, it expands and becomes slightly lighter than cooler water. With extra-large-diameter hoses connecting the cylinder and head to a radiator, the water would slowly circulate by gravity. More than one maker, after a few seizures, abandoned trying to water-cool the cylinder; they went back to giving it fins and just water-cooled the head. Why? The really hot area around the two-stroke cylinder’s exhaust port generated steam bubbles that compromised local cooling, and because water circulation was very slow (large coolant passages, no pump) the steam bubbles were not swept away by rapid flow. Steam is a gas, and gases are excellent heat insulators; that’s how a down jacket works.

Steam-bubble formation was a serious problem until people realized that intuition is not infallible. The really big water jackets of early liquid-cooled motorcycle engines looked right because they held a lot of water, but worked badly because the water in them barely moved and wasn’t fast enough to scour away those steam bubbles.

Suddenly the whole power-sled industry “got it” as their engineers realized that tighter water jackets with smaller-section water passages and plenty of fast-moving pumped coolant motion resulted in the high turbulence that constantly brings cooler water from inside the flow into contact with hot engine surfaces. Cooling problems came under good control. Ever notice how late-model liquid-cooled sportbike engines tend to look more and more like the black monolith from 2001: A Space Odyssey? That’s because water jackets have shrunk and now fit closely around cylinders and combustion chambers so that water moves through them fast enough to get the job done.

When you cool the cylinder with water you may also have to change to a piston alloy of lower heat-expansion rate.  The piston, being very light, heats up after cold-start much faster than the cylinder, which weighs nine times what the piston does yet receives only two or three times as much combustion heat. Lots of trouble when a piston outgrows its cylinder.

Four-Stroke Applications

Some will say yeah, that’s OK for the ring-dings, but they’re history now. What about the real action: four-strokes? As long ago as World War I, engineers realized the steam-bubble problem was causing substandard cooling around hot places like exhaust-valve seats and exhaust-valve-guide bosses. To combat that they tried using an internal manifold that brought cooled water from the radiator to little jets directed right onto the hot spots; fast-moving coolant, with the largest possible temperature difference between hot surface and cooler coolant. Today, special attention for hot spots is called “strategic cooling” as if it were something new, maybe from the latest TED Talks. In fact it is more than 100 years old.

Air-cooled engines can still be successful, but at a lower level of specific power (horsepower per liter). My favorite example is Rob Muzzy’s best Z1-based air-cooled Kawasaki Superbike engine of 1982, making 152 hp from 1 liter’s displacement. The liquid-cooled engines of the same displacement now running in World Superbike are making 240 hp per liter—almost 100 hp more.

Or look at Reno-style air racing, whose aircraft are still powered by engines (all radically modified) that originated in WWII: liquid-cooled Rolls-Royce Merlin V-12s making 4,000-odd horsepower from 1,650ci (155 hp per liter), versus air-cooled Wright R-3350 18-cylinder radials, also making 4,000-odd horsepower but from 3,352ci (77 hp per liter). In this example, the air-cooled engine needs almost exactly twice the displacement to equal the power of the liquid-cooled engine. The teams running the big radials know they can turn up their supercharger boost and easily make lots more power, but they also know their efforts won’t last even a minute at that level. Parts quickly over-temperature, detonation begins, and it’s time to put on some more coffee and build another engine.

Air-cooling, properly engineered, can work well, but liquid-cooling becomes necessary when very high rates of heat removal are needed. That’s why so many beloved classic air-cooled motorcycles engines (think Harley-Davidson) have recently had to have their cooling augmented by circulating radiator-cooled oil or water around their hottest parts: the exhaust-valve seats.

Ever Heard of Wind Chill?

One more thing—I can’t leave out my favorite heat-transmission story. We’ve all met the guy who says, “See, she’s overheating bad because the water’s moving through the engine too fast to pick up the heat.” Check that concept out next time you’re in the cozy ranger cabin atop some tall winter mountain while the outside temperature is 80 below zero and the wind is gusting to 100 mph. Care to take off all your clothes and run outside for a quick lap around the cabin? Think you’ll make it because that fast-moving wind won’t have time to pick up any heat from your pink skin? Or is there such a thing as wind chill factor? The faster the wind blows, the faster it takes away heat from warm objects such as yourself. Works the same way when we try to cool engines, whether what’s blowing is air or liquid coolant.

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[Guest]

 

 

"This morning I watched water starting to boil in a plug-in glass kettle"

 

[bonio]

@goat You got further than I did

[Mr Fro]

I'm disappointed he didn't specify the type of glass in the kettle.

 

Sloppy journalism. 

[Bender]

14 minutes ago, Mr Fro said:

I'm disappointed he didn't specify the type of glass in the kettle.

 

Sloppy journalism. 

Let alone the water, I mean could have been anything, evian, perrier,  tap.... 

[Mr Fro]

2 hours ago, Bender said:

Let alone the water, I mean could have been anything, evian, perrier,  tap.... 

Can't be tap water, it would be all furred up.

 

I'm guessing llama piss.

[Bender]

41 minutes ago, Mr Fro said:

Can't be tap water, it would be all furred up.

 

I'm guessing llama piss.

That's Southern water, it's fine oop North. 

[Mr Fro]

9 hours ago, Bender said:

That's Southern water, it's fine oop North. 

Still going with llama piss.

[Bender]

10 minutes ago, Mr Fro said:

Still going with llama piss.

 

Must be a southern delicacy for making Tea doon sooth

[Mr Fro]

18 minutes ago, Bender said:

 

Must be a southern delicacy for making Tea doon sooth

Got to get rid of the chalky taste mate.

[fastbob]

Kevin must be so busy typing that he never leaves the house . 

[Bender]

Just now, fastbob said:

Kevin must be so busy typing that he never leaves the house . 

That could be the next installment 

 

Kevin leaves the house. 

[fastbob]

Just now, Bender said:

That could be the next installment 

 

Kevin leaves the house. 

There doesn't seem to be a lot anyone can do with Kevin's knowledge other than become knowledgeable . I've got a liquid cooled bike and an air cooled bike and there's not a lot I do about that . 

[Mississippi Bullfrog]

2 hours ago, fastbob said:

There doesn't seem to be a lot anyone can do with Kevin's knowledge other than become knowledgeable . I've got a liquid cooled bike and an air cooled bike and there's not a lot I do about that . 

You can use a kettle to make your cup of tea rather than a pan on the stove. Greta will be pleased.