Why Water-Cool Parts of Air-Cooled Engines?

Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

There are good reasons why makers of large air-cooled motorcycle engines are applying local liquid cooling around their exhaust valve seats.

Motorcycle manufacturers closely monitor warranty claims because they know that if two machines out of the thousands made in a year have exhaust valve failure, that could mean one of two things;

  1. That the two failures are statistical accidents or,

  2. That some problem exists, such that next year there could be ten such failures, with 100 following the year after. In a word, disaster.

Autos set the standard for what people expect, so today consumers expect perfection from expensive motorcycles as well. No more good-natured joking about “The Prince of Darkness” if the electrical system quits on a dark night.

Therefore every manufacturer has testing programs to expose potential problems in private (like out on that 8-mile “hot circle” test track down in Texas), rather than risk failures in public, where they can quickly compromise a machine’s reputation.

During WW II four-engined B-29 bombers initially operated from India and China. As soon as they hit hot weather there were engine failures, crashes, and B-29s were grounded pending investigation. The center of the problem turned out to be heat-warping of the cylinder heads of their large air-cooled engines, causing exhaust valve leakage, overheating, and failure.

Detailed inspection showed two types of warpage;

  1. The exhaust valve guide was no longer perpendicular to the hard seat insert in the head, resulting in one-point contact between valve and seat. Leakage of hot combustion gas rapidly led to valve breakage, often causing an engine wreck.

  2. The exhaust valve seat distorted into an oval rather than round shape, resulting in two-point contact between valve and seat (with the same result as in (1) above).

Exhaust valve cooling takes place mainly through full contact between the hot valve and the cooler valve seat. In the case of the valves in B-29 engines, a secondary cooling path was via sloshing of liquid sodium partly filling the hollow exhaust valve, carrying heat from the valve head (hot) to the cooler valve stem and valve guide.

Service teams carrying redesigned cooling baffles and other parts were flown out to update aircraft in the field, but the problem of cylinder head heat distortion was never definitively solved.

Air-cooled aircraft engine cylinders are wrapped in sheet-metal baffles which force cooling air to pass only through fin spaces. It had been learned in the 1920s that engine cylinders just sticking out into the wind cooled unevenly, resulting in out-of-round cylinders and leakage of both combustion gas and oil. The fins facing the wind were cool, but the back sides of cylinders and heads ran very hot. Cooling baffles, by forcing cooling air to pass through fin space all the way to an exit slot at the back of each cylinder brought this distortion problem under some control.

Why was this not necessary for air-cooled motorcycle cylinders and heads? It was a matter of duty cycle. B-29 bombers in India and China faced this mission profile;

a. 1 1/2 hours full throttle and rich mixture for take-off and climb

b. 4 - 4 1/2 hours at 80% power and rich mixture

c. 2 hours 70% power, normal mixture

d. 4 hours at 50-60% power, normal mixture.

Here on the ground, such sustained use of high power never happens. We accelerate through the gears, dumping combustion heat into heads and cylinders, and then comes a corner or a yellow light or a policeman. While we are off-throttle, the cooling process catches up and parts temperatures drop. Normally this averages to acceptable temperatures for all components.

But as motorcycles have become more capable, riders have routinely expected more and more from them. Tour bike GVW has risen toward 1350-lb and the 55-mph speed limits of the “OPEC era” have been relaxed.

And what about the few bikes whose owners take full, all-day advantage of those 85-mph speed limits in certain western states? While towing a trailer plus maybe a substantial passenger and full luggage? With a headwind? Up the Rockies? High power, hot day, heavy load. Sounds more and more like “B-29 territory”, engine-wise.

Do we issue a bulletin telling the world our bikes can’t take it, so please go slow? Nope - corporate suicide.

Do we wrap our beautiful, traditional finned cylinders and heads in stamped tin aircraft-style cooling baffles? No, can’t do that either, because the traditional appearance of our engines is a huge part of their sales appeal. People love those cooling fins and their highlighted tips.

Air-cooled motorcycle engines used to be jetted comfortably rich as part of what kept exhaust valves alive – just like B-29 engines thrashing across the Himalayas at 80% throttle in “auto-rich” carb setting. But that leaves behind a stream of unburned hydrocarbons that is no longer acceptable. When EPA just said no to rich jetting, all builders of air-cooled motorcycle engines had to scramble to improve cooling.

How about just making the fins bigger? Gotta be careful here – Styling really hates to change anything that the public loves. Get those focus groups in here and let ‘em look at cylinders with extra fin area. Are they frowning? What are they saying? Nobody leaves without filling in the questionnaire!

When large aircraft piston engines were nearing the end of their era, desperate measures were taken. One was to flood the rocker-boxes with oil, thereby hoping to cool not only the hot valve stems but by conduction, the entire head. There was no valve stem seal in existence at that time which could survive, so the result was heavy smoke and high oil consumption. Sorry – not practical for EPA-legal production motorcycles.

The bigger the engine, the less hard it has to work, right? So let’s throw some extra displacement at the problem. This is a strong temptation for makers of air-cooled touring engines, for as highway speeds and vehicle GVWs rise, more oomph is required to maintain acceptable performance. But there’s a bite. It seems attractive to add 5% or so to an engine’s displacement, but that means that the same exhaust valve, port, and surrounding cooling structure of fins now has to pass 5% more hot exhaust gas each cycle. If nothing increases the engine’s cooling capacity, the only way it can rid itself of the extra heat is to become hotter. And when it does, the cylinder heads are pushed that much closer to heat deformation and problems with exhaust valve sealing.

With all these constraints, the practical solution is to liquid-cool the exhaust valve seats. Looking at the engine, this is invisible, so traditional appearance is retained. Yes, some kind of coolant radiator(s) has to be tacked onto the bike but Customer Relations will handle that (When Pierce-Arrow used a worm-gear rear axle to give their cars a lower floor, Engineering couldn’t get rid of its noise, so PR billed the model as “The Car with the Aristocratic Hum.”).

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