Liquid-Cooled Boxer Spied!

Liquid-Cooled Boxer Spied!

ROUNDUP

BMW's next-generation R series

BLAKE CONNER

BMW'S ICONIC AIR-COOLED BOXER Twin—that began life in the 1923 R32ߞhas finally reached a performance plateau, beyond which liquid cooling is the only practical means of big gains in efficiency and power output while meeting strict emissions require ments. The proof is this spy photograph snapped during development testing of what we believe is a liquid-cooled followup to the air/oil-cooled R1200GS.

BMW North America will not acknowledge that the bike even exists, but this photo clearly shows that it does.

Over the course of its many variations, the Boxer layout has served BMW well. “The designer of the original BMW flat-Twin, Max Friz, was an aircraft engine designer,” says CW Technical Editor Kevin Cameron. “At that time, the practice of cowling aircooled aircraft engines to force cooling air through their fin spaces had not yet emerged, so Friz did the obvious thing:

Next-generation GS can easily be identified by changes visible in the photo at the left. Cylinder heads feature intakes at 12 o'clock instead of at their former 9, which also means that the exhausts now exit from the bottom, at 6 o'clock, instead of out the front. Another obvious change is that the Paralever swingarm/sha ft-drive has been moved to the opposite side, while the exhaust silencer has been moved from left to right. A new Telelever fork has accommodations for radial-mount brakes. A cooling fan and large radiator are hidden behind the camou flaged but clearly bulkier bodywork. What we don't know is if BMW has taken advantage of the additional cooling capacity and increased the engine's displacement.

He stuck the cylinders out in the airstream. Because he was a sensible man, Friz made a horizontally opposed Twin for its fairly good natural balance.”

So, what factors have finally made BMW consider liquid cooling on its Boxer-Twins?

“A common cooling problem arises when engine displacement is enlarged,” says Cameron. “As bore and stroke are increased [for example, as BMW’s 500cc flat-Twin became a 600, a 750, etc., on its way to its present 1170cc], the displacement increases as the cube of dimensional change while the surface area available for cooling fins increases only as the square. As heat produced is proportional to the displacement, it is thus possible for an engine to outgrow its cooling capabilities.”

This is of particular concern on air-cooled engines with four valves per cylinder, which, adds Cameron, “always brings the potential for heat warpage of exhaust valve seats (leading to poor seating and leakage) or even to head cracking between the exhaust seats or from seat to sparkplug hole.”

Supplemental oil cooling is a good fix that reduces heat between the exhaust valves. But oil’s cooling capacity is only about 40 percent that of water.

To continue progress of the design, BMW has apparently been forced to make the switch to liquid cooling, much like Porsche did in 1998 with its flat-Six.

“Well-executed water cooling does a much better job of keeping valve seats dimensionally stable,” says Cameron, “and it also has potential for power increases

because: a) air entering a cooler cylinder expands less and thus loses less density, and b) a well-cooled head and piston can operate knock-free at a significantly higher compression ratio. Note that today’s liquidcooled motorcycle engines typically have compression ratios as high as 13:1, while air-cooled engines must generally remain in the vicinity of 10.5:1. Higher compression raises torque and improves fuel efficiency, both of which are important to the touring or sport-touring riders who make up a large percentage of BMW’s customers.”

Cooling has been an issue since the dawn of internal combustion. “These are not new problems,” says Cameron. “During World War I, German aircraft engines were mainly large inline watercooled Sixes, usually with four valves per cylinder. Problems with exhaustvalve-seat distortion and cracking led engineers of the time [95 years ago!] to adopt what modern automobile engineers claim as an innovation, under the name ‘strategic cooling’—the placement of small tubes inside the water jackets of the cylinder heads to direct cooled water at high speed onto the backsides of exhaust valve seats.” Although liquid cooling is far from new, the air-cooled Boxer has enjoyed a long and effective run without its help. But even traditionalists, like BMW and Porsche, have to eventually give in to the technical demands created by consumers’ desire for more power and better efficiency. So, it appears that the R-Series Boxer-Twin has been gifted a new lease on life and will hopefully continue its legacy long into the future.