A Future For Air Cooling?

A FUTURE FOR AIR COOLING?

IGNITION

TDC

THE VIRTUE OF SIMPLICITY

KEVIN CAMERON

During the first half of the 20th century, the motorcycle established itself as the simplest of useful vehicles: for transportation, for sport, or for military applications. Motorcycles had air-cooled engines, generally of one or two cylinders, that could be easily kickstarted. They were basic, providing not a lot more than two wheels, an engine, and a place to sit. That very simplicity was at the center of the motorcycle’s appeal. Just hop on, start the engine, and give your life a whole new direction, wherever you wanted to go.

Now a cornerstone of that simplicityair cooling—is coming under siege, with some pundits predicting that the next round of European emissions standards cannot be met with air cooling.

Large air-cooled engines of traditional design—Harley’s 1200 Sportster, some Big Twins, and certain BMW flat twins— are being given “strategic cooling,” the auto industry’s name for intensive cooling of problem areas. The 1200 Sportster circulates engine oil through passages in its heads, encircling its exhaust valve seats, and Harley’s Rushmore suite of enhancements to its biggest engines does the same but with water/glycol engine coolant instead of oil.

This strategic cooling is not motivated by upcoming emissions changes but by problems inherent in how modern motorcycles are used. As highway speeds have risen and the weights of large motorcycles have grown, more power is needed. Harley’s Big Twin started life with 6ici in 1937 but is now at 103d and lioci.

Where’s the problem? More power equals more heat. Talk to anyone who rides a big-displacement bike and you’ll hear about the heat that pours off its engine. But a harder problem is inside, as exhaust gas rushes past the open exhaust valves and their seats, heating them. Every time we increase engine displacement, we push that much more hot exhaust gas past valves and seats. But the outside of the engine—the surface on which we can put more cooling fins—is quite limited. If we space the cooling fins closer together, we increase their resistance to air flowing between them. Ditto if we make the fins deeper.

If we enclose the cylinders and heads in sheet metal shell baffles to force air to flow through fin spaces all the way around to the backs of the cylinders, we can’t see the fins that so many riders love. And all the while, artists in the styling department are telling us any change in appearance is certain sales suicide.

Big air-cooled engines can get very hot in stop-and-go traffic. Cylinders distort, rings can no longer seal, and the engine may begin to smoke. As hot as it is, it may also begin to knock. Harley has provided its “parade mode” to counter this, turning off the fuel injection to the rear cylinder under specific conditions.

Meanwhile, finless flat-black liquidcooled engines have radiator fans that click on robotically to handle such situations by blowing away the excess heat. But that in itself offends those who find charm in the motorcycle’s simplicity—as simple as a favorite pair of boots. Yes, water pumps, fans, thermostats, and radiators are now very reliable and provide excellent function, but what if you just don’t particularly like them? What if you feel they are car parts that don’t belong on your motorcycle? Cars have doors and windows; motorcycles have sky and wind.

If air cooling is to have a future, cylinder thermal distortion has to be reduced to something piston rings can seal, keeping engine oil from entering combustion chambers, burning, and being detected as exhaust emissions. Metal surface temperatures in the combustion chambers must be held below values that either lead to combustion knock (higher internal temperature is why air-cooled engines have lower compression ratios than liquidcooled ones) or to slow metal creep—the “ovalizing” of exhaust valve seats, leading to leakage that in turn causes even more heating, leading to failure.

BY THE NUMBERS

1998 LAST YEAR OFTHE AIR-COOLED PORSCHE 911, WHICH WAS KNOWN INTERNALLY AS THE 993

4.9 OIL CAPACITY, IN LITERS, OF THE AIR-AND OIL-COOLED HONDA CB1100.

TEN MILLIONS OF AIR-COOLED ENGINES PRODUCED EACH YEAR BY BRIGGS & STRATTON FOR OUTDOOR POWER EQUIPMENT, SUCH AS LAWN MOWERS.

Thus, at present, complexity is being called upon to preserve simplicity. Exhaust valve seats are being liquid-cooled whether styling likes it or not. Pistons, whose hot crowns might otherwise heat incoming mixture enough to provoke detonation, are being cooled by oil jets from below.

Air-cooled Porsches had cooling fans to force air through their engines’ fin spaces, but motorcyclists see anything with a fan as either a scooter or a golf-cart engine. Nasty!

Bottom line: With all the pushing and shoving of interested parties, the design space remaining into which an air-cooled engine can fit is not large. Styling hated oil coolers and they didn’t want more or bigger fins, so in the resulting contest with the laws of physics, they got a little banged up and had to accept two coolant radiators that resemble bathroom space heaters. Honda put a lot of effort into engineering its aircooled CB1100 but even so had to oil-cool the area around its spark plugs (translation: cool the hot combustion chambers). The effectiveness of air cooling decreases as engine size increases, so if there’s to be a future for it, we’ll see more liquid-assisted cooling, possibly via hidden oil coolers with attached fans.

Many fondly remember the big air-cooled Japanese engines such as Honda’s CB750, Kawasaki’s Zi, and Suzuki’s GS series. Despite their reputations from 40 years ago, in modern terms they hardly produced any power. To be as successful as they were, they were intentionally made heavy, especially in the cylinder head. That extra metal absorbed and stored heat during bursts of acceleration and dissipated it to the merry little breezes when the throttles closed again. They were not asked, as modern touring engines definitely are, to cruise across Death Valley in August, two-up, towing a trailer. At 85 mph.

AIR-COOLED PORSCHES HAD COOLING FANS TO FORGE AIR THROUGH THEIR ENGINES’ FIN SPACES, BUT MOTORCYCLISTS SEE ANYTHING WITH A FANAS EITHER A SCOOTER ORA GOLF-CART ENGINE.

Highways speeds, despite Mr. Gore’s movie, are up, and therefore so is displacement. That puts big air-cooled engines behind the eight ball, as above. The bigger you make it, the harder it is to cool.

Maybe we have to be satisfied with that old air-cooled enduro up at the lake cabin, the bike all the kids learned on, which probably still has the same oil in its crankcase that the dealer put there in 1987. It always starts and runs. It has the virtue of simplicity. EUi