BMWS’ camprofile solution

Where the Ducati system has one camshaft profile that they vary the position of, BMW’S Shift Cam approach for their 1250 boxer engine uses two distinct cam profiles on the inlet camshaft. That allows them to make an engine with big power, but which still passes emissions and has driveability. There are part-load and full-load camshafts. Below 5000rpm and on part throttle, the part-load cam opens later than the high lift cam to shut the overlap down at part load and lower revs. It also has lower lift, isn’t open for as long and can’t flow as much air; as load increases it runs out of puff and the engine is effectively throttled by the valves. Make a large torque request – open the throttle – and the full-load cam slips into action, swapping profiles at the point where torque is identical so you don’t notice the change. On a wide throttle at 2000rpm you’re on the full cam, not the part load one. Over 5000rpm the engine is always on the full-load cam profile.

What it also has – and what makes it more interesting than the alternative system that BMW use on their S1000RR – is differential valve lift. For full load the cams for the two inlet valves in each cylinder have exactly the same profile, but when it switches to the part load/low lift setting one of the intake valves has a higher peak and longer duration. They have the same opening point, but one doesn’t open as far and also closes before the other.

What this does is give much more swirl in the cylinder at part load. This motion increases the turbulent kinetic energy of the mixture. At the same operating point on the full-load cam there is less turbulence, but on the part-load cam there’s three and a half times more motion. So when you ignite the

‘Big power, which still passes emissions’

mixture it burns faster because it’s moving more, giving faster combustion and a cleaner burn.

The part-load cams also close way before BDC (bottom dead centre – piston at the bottom of its travel). This means you can run with a more open throttle as you haven’t got as much time to get the air in. The terror of the four-stroke cycle as far as fuel economy is concerned is throttling loss – energy wasted by the engine just drawing the air past the throttle (it’s why it’s called a throttle – it literally throttles the engine to stop it breathing, and so ‘full throttle’ should actually mean fully shut!). If you can shorten the event and reduce the ‘angle area’ it doesn’t pull as much of a depression in the intake, which means you have the throttle more open for a given load thus reducing throttling loss and improving economy. As soon as I saw the new 1250’s system I thought it was neat. A system like it was first used by Audi, about ten years ago, and BMW possibly had it in mind when they designed the current version of the boxer. The cam drive is quite involved, they’re driving an idler gear with a chain, and the two cams are driven off the idler. To switch profiles they effectively pull and push the intake cam axially, and its gear stays in contact with the idler.

‘Where the Ducati system has one camshaft profile BMW’S Shift Cam uses two…’