How it works
Jamie Turner is Professor of Engines and Energy Systems at Bath University. Here’s what he thinks of BMW’S new variable valve system…
‘It bounds off up the road with a muffled farty exhaust note beloved by GS owners the world over’
HOW CUNNING?
It’s a clever use of a technology that really suits the architecture. It wouldn’t surprise me to discover that BMW had always intended to apply it right from the beginning of the life of this version of the boxer.
NOTHING’S NEW
Shiftcam is similar to Audi’s Valvelift system. Honda (cars) and Porsche use different valve mechanisms to achieve rotational swirl at part load.
ACTUATOR
This only switches between the two cams at revs when both make the same torque. This makes the switch imperceptible. Fuel injection settings, throttle angle and spark advance will change simultaneously. The switching process for each cylinder will be 360 degrees apart, because of the basic firing intervals of the boxer.
TWO CAM PROFILES
Mechanically, this is one of two places where the magic happens. The two cam profiles for each valve share the same ‘base circle’ – the circular portion where the valve is shut – and profile switching occurs by sliding the cam axially during this part of the rotation.
SHIFT GATE
The other magical place. The shift gate is a Y-shaped track cut into the camshaft, with an electronically-actuated pin above the two legs of the Y. When the ECU demands a cam change, one of the pins in the actuator fires into its corresponding leg at precisely the right time. As the camshaft rotates, the pin causes it to slide axially, switching the cam profiles. Then the pin withdraws. This happens fast – at 3000 rpm, one engine rotation takes 0.02 seconds.
YET MORE CLEVERNESS
At part load the system needs less time to get the mixture in, so the valves have different profiles to the high load ones which give a degree of ‘Miller cycle’ operation – the darling of the automotive world at the moment – and improves fuel economy. But at low flow rates this reduces turbulence in the air/fuel charge, potentially reducing smoothness. BMW’S solution? Make the inlet valves open at different times, which causes rotational intake swirl, giving faster combustion, smoothness and fuel economy.