Is Air-Cooling Better Than Liquid-Cooling?

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By Kevin Cameron, Cycle World

As pointed out by readers, there have been successful air-cooled auto engines such as the Volkswagen Beetle, various Porsches, the Chevrolet Corvair, the Czech Tatra V-8, etc. Some of them were quite powerful, including Porsche’s 917 racing car with its 9,000-rpm cooling fan. What they all share, which almost no air-cooled motorcycle has had, is some form of thermostatic control.

Unlike aircraft, which spend most of their lives at cruise power, autos must idle and cover a range of vehicle speeds from traffic creep to maximum. For this reason—and because it’s not easy to place a car’s cylinders and heads in free-stream air—air-cooled auto engines are given cooling fans with ducts and baffles to force the blower’s output to flow through the fin spaces of cylinders and heads. In addition, the flow of cooling air can be regulated to control engine temperature. This is especially important when cabin heat is taken from engine cooling airflow.

© Cycle World Archives   For the better part of a decade, Suzuki’s four-cylinder GSX-R750 racer-replica relied on a combination of air- and oil-cooling. Liquid-cooling was adapted in 1992—one year later for US buyers—and has been employed ever since.

Worst Cooling

Perhaps the least satisfactory air-cooling of all time was that of the rotary engines powering so many World War I fighting aircraft. A rotary’s crankshaft is bolted, immobile, to the aircraft’s firewall, and the crankcase with radially attached cylinders and heads, whirled around with the propeller. This was useful at that time of low flight speeds—typically 60 mph at the beginning of that war—because the whirling motion moved the hot parts of the engine rapidly through the air even when the airplane was sitting still. 

As the cylinders whirled, their leading sides cooled well but their trailing sides were hardly cooled at all; there was nothing to keep the surrounding air from just blowing around the cylinders as if they were so many warm fence posts on a windy hilltop. The resulting cylinder distortion could not be sealed by conventional piston rings, so a thin brass replica of the leather cup washer on the piston of an old bicycle pump had to be used instead. Such seals needed replacement every 10 to 25 hours of operation.

The power loss caused by the whirling cylinders limited rotaries to about 200 hp, so after WWI they were replaced by static radials in which the crankshaft whirled around but the cylinders stood still.

Need For Cooling Air Baffles

As air-cooled aircraft engines were made to give greater and greater power, just having the cylinders stick out in the breeze created similar distortion and a lot of drag. Eventually, heads and cylinders were enclosed in streamlined nacelles, then closely baffled so the air taken in at the front of the nacelle was forced to flow through fin space all the way around to the downwind side of every cylinder.

Smoking At Start-Up

If you watch videos of radial engines being started, you will see that when they fire a big cloud of oily smoke is ejected. This is oil that gravity has carried into the bottom cylinders during shutdown, now being vaporized by combustion. The same happened when BMW’s horizontal-cylinder K75 triple and K100 four were parked on their sidestands, allowing oil to run down the cylinders. At start-up, they smoked briefly in classic fashion. Normally, the oil systems of engines with downward-pointing cylinders scavenge waste oil from between the inward-projecting lower cylinder spigots. Oil cannot accumulate in the pistons themselves because of the speed of their movement. Many thousands of inverted V-12 engines were built by Daimler-Benz to power Germany’s wartime Bf 109 fighter aircraft.

Power Differences Between Air- And Water-Cooled Engines

Anyone who studies the aircraft piston engines of WWII will notice that horsepower per displacement is greater for liquid-cooled engines than for air-cooled. A typical water-cooled engine, the Rolls-Royce Merlin that powered the British Spitfire and American Mustang aircraft, was at the end of the war giving some 1,750 hp from a displacement of 1,650ci, while the turbocharged R-1820 air-cooled (static) radials on B-17s gave more like 1,200 hp from 1,820ci. In power-per-displacement form that is 1.06 hp/ci for the liquid-cooled and 0.66 hp/ci for the air-cooled.

Why this difference? Engine power is limited by the abnormal combustion called detonation or engine knock. The cooler the piston crown and interior surface temperature of the combustion chamber can be made, the more the engine’s supercharger boost can be safely increased before detonation begins. Because a coolant mixture of water and ethylene glycol is hundreds of times more dense than air, it not only cools engines better in general but can be pumped through small passages to cool specific hot spots that require it. Detroit now calls this practice “strategic cooling,” but it has existed for more than 100 years.

The same applies to water versus air in motorcycle engines. Kawasaki’s air-cooled, 903cc Z1, built to the limit for racing by Rob Muzzy in 1982, gave a best of 152 hp at 10,250 rpm. But today’s ZX-10R as developed for World Superbike gives more like 230 hp at 14,500 rpm. Just as in the case of air versus water in aircraft engines, the air-cooled package gave roughly two-thirds as much power as a water-cooled of the same displacement.

Then why were air-cooled radials so widely used? The important variable was not horsepower per displacement, but horsepower per weight, and in this latter aspect radials were fully competitive.

Overcoming Heat Distortion

Manufacturers of beloved air-cooled designs such as BMW’s flat-twins and Harley-Davidson’s V-twins have supplemented their air-cooling with strategic liquid-cooling to reduce the temperatures of specific hot spots. The exhaust “bridges” of four-valve engines, the very hot material between the seats of paired exhaust valves, are almost impossible to cool with air. Maintaining the roundness and centered position of exhaust-valve seats in hard-working air-cooled engines is also difficult. Make metal hot enough and it “creeps” or deforms slowly away from any applied stress.

In WWII, the 18-cylinder Wright R-3350 radials that powered B-29 bombers suffered badly from exhaust-valve seat distortion caused by excessive cylinder-head temperatures. Once a valve began to leak, its temperature shot up—most of the valve’s cooling normally comes from full contact with the seat—and in a few hours the overheated valve would break, likely causing an engine wreck and/or fire. Flight engineers monitored engine temperature constantly, adjusting it by slightly opening or closing the cowl flaps, which regulated airflow through the engine’s fin space. But opening the cowl flaps rapidly increased drag, causing that aircraft to consume more than the planned amount of fuel and slow down.

Fan-Cooling

Because cooling above 20,000 feet became less effective due to reduced air density, plans were made to equip future B-29s with large cooling fans to push more of that altitude-thinned air through their engines’ fin space.

Those plans were not carried out, but the much larger and later B-36 bomber was given cooling fans that used 10 percent of the power of its six R-4360 engines to make operation at 40,000 feet possible. Another famous aircraft, the German Focke-Wulf 190 fighter with its BMW 14-cylinder 801-series radial engine, was also fan-cooled.

Look at any Vespa, mower, or golf-cart engine and you will see just such cooling fans. Boulevardiers and golfers apparently don’t feel their gender identity is threatened by cooling fans, but just try putting one on a motorcycle. Fan-equipped air-cooled engines can have their fins more closely spaced than those of a vehicle that depends for its cooling airflow only on the speeds possible on street or highway.

Because the great piston-powered aircraft typically cruised at 200 mph or more, they had plenty of ram pressure to push sea-level air through closely spaced fins, roughly seven times more pressure than does a motorcycle rolling on the interstate at 75 mph. But design a blower cooling system for that bike, with an impeller tip speed of 300 feet per second, and suddenly you have the same cooling air pressure as that 200-mph airplane.

Temporarily Storing The Heat

Designers of air-cooled motorcycle engines dealt with periodic bursts of speed by adding extra metal to their cylinder heads to act as a heat storage system. Head temperature rises as you open the throttle and continues to rise until you roll off (because of understandable fear of hitting something or encountering a policeman who has sworn to protect and serve). That extra material added to the head slows the rate of temperature rise, in most cases by enough to prevent provoking detonation. Once the rider rolls off, the heads and cylinders can shed the heat they’ve stored. When the late Dick O’Brien stood by his detail designer as the cylinder head of Harley-Davidson’s XR750 dirt-track engine was being dimensioned back in 1970–’71, he said, “I want an inch of metal on top of that combustion chamber!” They compromised at 3/4 of an inch. This is why the cylinder heads of air-cooled bike engines tend to be quite heavy.

Oil-Cooling

Another reader proposes oil-cooling as an alternative. Suzuki engineers, knowing that approximately 30 percent of a radial piston engine’s cooling occurred in its oil cooler, decided to make the early series GSX-R sportbike engines as hybrid air-/oil-cooled. While this simplified engine construction, it did incur some loss as water has a much higher heat capacity than does oil. Yoshimura Superbikes at the end of their air-/oil-cooled era bristled with add-on radiators. But as a cooling system for engines of moderate power, it was satisfactory. As development increased the power of GSX-R engines, oil-cooling that had become marginal was replaced in 1992 by more intensive water/glycol-cooling.

It’s not that one form of cooling is better or worse. It’s a matter of what’s most appropriate to the task at hand.