Hot Air, Cold Air, Delta-T
Around 1936, NACA (which would become NASA in 1957) measured the rise in temperature of fresh intake charge in a large radial aircraft engine as it ran. The engineers found that as the charge passed through the intake port, past the intake valve, and into the cylinder, its temperature rose 75 degrees Fahrenheit.
If that seems like a lot, consider that for decades motorcycle engine intakes drew in hot preheated air as it streamed back either from the cylinder-head fins or, on liquid-cooled bikes, from the coolant radiator.
In either case, engines lost power because heating causes air to expand, reducing its density and therefore its ability to contribute to power. This is the same as the loss of air pressure with increasing altitude—by the time we get to 13,000 feet, air density has fallen to half its sea-level value—and engine power along with it. Consider why hot-air balloons rise.
Although some minor effort had been made prior, it wasn’t until 1991 that 500 GP two-stroke racers began to breathe from sealed airboxes fed by forward-facing intakes. Cool air at last!
What to Cool First?
Now we come to another question: Which side of an engine’s cylinder head should receive the greater cooling? It was traditional to assume the hot exhaust side of an air-cooled four-stroke cylinder head should face the fresh cooling air, leaving the intake side to be cooled by air already heated by passing through exhaust-side fins.
This seems to make sense because heat transfer depends on “delta-T”—the temperature difference between the cool and hot materials. Directing fresh air to the exhaust side first seems right for this reason. But in 1940, when Pratt & Whitney began design work on the R-4360, its last large piston aircraft engine, it did it the other way around, cooling the intake side with fresh air and then the exhaust side with that heated air. Why?
The answer goes back to our 1936 NACA paper, which showed that power was being lost by heating the fresh charge as it entered the cylinder—heating that resulted from how hot the intake side of the head was. How about if we reverse the cooling, cooling the intake side with fresh cool air, and only then letting that heated air cool the exhaust side?
What they found was a power gain essentially for free. Because the exhaust side of the head is so hot, thanks to the exhaust valve and its port, the delta-T for its hot fins was not reduced much by routing air to cool the intake side first. And making the intake side of the head even cooler with cold air reduced the temperature of the fresh charge passing through it.
During the 1990s, World Superbike teams seeking every scrap of race-winning power began to do the same, routing the just-cooled water from the radiator to the intake side of the head first, then across to cool the exhaust side afterward.
In February of 2020 I found myself at Honda’s Collection Hall at Motegi Circuit, Japan. What did I see there but a late-model NSR500 two-stroke GP bike with its coolant routed to also cool the crankcase. Simple two-stroke motorcycle engines use the crankcase as a pump to refill the cylinder with fresh charge. The hotter the crankcase is in operation, the more intake air density and power is lost from heating the charge air passing through it.
German engineers in World War II cooled the heads of BMW’s 14-cylinder radial 801 in yet another way. They led cooling air down on top of the center of each head, some of it flowing one way through the hotter exhaust-side fins, and some in the opposite direction through fins on the intake side.
Maximizing Delta-T
During the 1930s it was discovered that, with proper cooling, an engine’s normal power could be doubled by using a compressor to stuff twice as much mixture into its cylinders: supercharging. Thus was born the “Hyper” concept of developing 2,000 hp engines so compact that they could fit entirely inside the wings, yet were more powerful than the largest conventional engines of the time. Because forcing cooling air through a radiator’s closely spaced passages requires power, they had an idea: “Let’s maximize delta-T in our radiator by pushing coolant temperature up really high—say, to 300 degrees Fahrenheit. That way, we can make the radiators smaller” (because each cubic foot of air passing through the rad would carry away a lot more heat than air passing through a rad that held conventional 190-degree engine coolant). Millions went into this program.
Call it Murphy’s Law, or recite the old phrase, “The best-laid plans of mice and men gang aft a-gley.” Not every R&D scheme comes up a winner. Cooling with 100 percent ethylene glycol, the researchers found it impossible to stop that tricky fluid from leaking everywhere. And while the “Hyper” team were laboring away, the wind-tunnel crowd had developed airfoils that were thinner, with attractively lower drag (one such airfoil enabled the P-51 to escort US bombers all the way to their distant targets in WWII). These developments made the idea of burying highly supercharged engines in an airplane’s wing impracticable. The program was dropped. Read the dramatic story in NACA tech reports and tech memoranda.
But years later, here is Ducati using the very same attractive idea: pushing up coolant temperature to increase delta-T so it can cool its engines with smaller radiators and hotter coolant, producing less aero drag.
One day a friend emailed me a promotional Chevrolet film from 1937, showing coolant circulation inside the head of the classic Chevy six. At each of the half-dozen exhaust ports was a small coolant sparger—a little nozzle that directed coolant from the radiator onto the hot region of the exhaust valve seat and port.
In the back pages of Jane’s Fighting Aircraft of World War I we can find quite a bit of detailed information on captured German aircraft engines. Among that data? Those engines too included nozzles that directed cooled water returning from the radiator, right onto the hot exhaust ports. Hurrah for delta-T.
Directed Cooling
Engineers are methodical, and when engines overheat they get to thinking. “Why,” they ask, “don’t we just put little windows in one of these heads so we can see what’s happening?” When they did, they saw steady formation of steam bubbles on those hot parts, enough steam to interfere with cooling. Then came the idea of putting little nozzles in the heads to direct speedy jets of coolest water onto those parts.
A related thought is the oil-cooling jets located down in engine crankcases and aimed up at the undersides of the piston crowns. Direct cooling! Today even touring-bike engines have such jets, but it started with tuner/cam grinder Tom Sifton including jets in the Harley race engines he built 60 years ago.
Once upon a time the snowmobile industry had a bit of a cooling revolution. The “old way” was to put a lot of coolant volume around the head and cylinder. Unfortunately, this works poorly, because the bigger the water passages, the lower the coolant velocity, and the easier it is for a hot layer to form next to metal surfaces and possibly fill the local volume with steam. It’s better by far to design much smaller passages that accelerate coolant velocity, making its flow turbulent. Turbulence greatly improves cooling because it continually brings cooler liquid from deep within the flow into direct contact with hot surfaces. Sled engines designed after this revolution have the modern “shrink-wrapped” cooling jackets rather than the tubby “jug of water” look of earlier designs.
Once, when I was visiting a professor at Rensselaer Polytechnic Institute, I spied some interesting pieces of tubing on a bookcase. The prof explained their odd shape, which was pinched in alternately, first on one side, then on the other. “Ah,” he said. “That’s a little invention I’m proud of. It’s for aircraft oil coolers used in cold weather. The problem is that oil below its pour point congeals, so hot oil passes only through the centers of the cooler tubes, surrounded by congealed oil that keeps it from being cooled. That shape causes the flowing oil to be constantly redirected, allowing it to melt through the congealed oil and be cooled by contact with the cold metal tube.”
Every successful machine is a thick book of ideas.
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