Bmw's Valvetrain Voodoo

BMW'S VALVETRAIN VOODOO

WHILE MOST MANUFACturers are still arguing whether double overhead camshafts are indeed superior to the single-cam variety, BMW recently unveiled technology that does away with the traditional valvetrain entirely. That means the demise of the camshaft, cam bucket, rocker arm, even the camchain and its attendant tensioning blades.

The traditional camshaft has one severe limitation: The timing of the opening and closing of the exhaust valves is fixed, and there fore optimized for a rela tively narrow powerband. Although variable valve timing is now common on cars (and has been tried on some Japanese domesticmodel 400s), it's only a tem porary solution. Typically, current systems can only alter intake timing by about 40 de grees, so volumetric efficiency is optimized for just a few thou sand revs and compromised everywhere else.

Radical even compared with the pneumatic valves exclusive to Formula One cars, BMW'S new electromagnetic system uses magnets and a computer to open both intake and exhaust valves with continuously variable lift and duration. Tested on one of BMW'S big inline-Six car en gines, the new system controls each valve individually via its own electromagnet. Monitored by the same computer that me ters fuel injection, the valves can be opened and closed for any pe riod desired. At idle, valve tim ing is very short with the intake valve open just a few degrees of duration. At peak rpm, the valves can be kept open for much longer than with a conven tional camshaft.

BMW's motivation for this wizardry is to eliminate the throttle butterfly and control engine output by continuously modifying the lift and duration of the intake valve. Reportedly, the aerodynamic disruption of

the partially closed throttle but terfly causes significant fuel economy losses-as much as 18 percent at idle. Eliminating the obstruction allows a clear path for the incoming mixture from the airbox to the intake valve. That, along with the more rapid opening and closing of the valves, also generates 5 percent more power.

In terms of motorcycling, large-displacement Singles and Twins are currently the only possible benefactors of this technology since the electromagnets controlling the valves are redlined at 7000 rpm. As well, these engines' large inlet tracks and throttle plates would benefit the most from the improved intake efficiency.

In the meantime, BMW still has to overcome such

obstacles as the heavy power demand of the electromagnets, as well as packaging their heft and bulk. Though production versions of electromagnetic valves are not expected before the beginning of the new mil lennium, the company has working models, so it may not be long before the magic of grinding cams is a lost art.

David Booth