BMW Adopts Variable Valve Timing On 2019 R1250GS Adventure Bike

Leaked manufacturer-produced video reveals “Shift Cam” system in operation

A dramatic new video shows how BMW's Shift Cam variable-valve-timing system—expected to debut on the 2019 R1250GS—operates using a concept originally devised in 1902 for submarine engines.

Why would variable valve timing be useful on a big on/off-road motorcycle? Because such a machine must have two otherwise mutually exclusive natures: 1) The authoritative top-end power required to propel such a substantial 600-pound machine at high speeds; this “GSX-R factor” requires later intake valve closing and more exhaust/intake-valve overlap to fill engine cylinders at high load. 2) The unfailing throttle response and controllable bottom-end and midrange torque to get the most from limited off-road grip and constantly changing surfaces; this “tractor factor” requires valve timings more like those of a Harley-Davidson, with intake valves beginning to open near top center and closing very soon after bottom center.

Because so much of an engine’s character is determined by its valve timing, the only way to combine these two necessary natures is by having two sets of camshaft lobes. That is exactly what BMW has given its new R1250GS, a system that can quickly shift from one set of lobes to the other.

As you watch this video, you will see two sets of lobes on the upper (intake) camshaft. The forward end of that cam carries what looks like a transmission shift drum with wavy slots milled into its outer surface. Below that drum is a black electrical solenoid controlling the movements of two pins. When the forward pin is extended, engaging a slot in the drum, the intake cam is pulled forward, putting the high-load lobes in action against the Formula 1-inspired pivoted finger followers. When that pin is retracted and the rearward pin is extended, the cam is shifted to the rear, causing the part-load cam profiles to drive the valves. In the video, you will see that part-load lift is only 50 to 60 percent of the lift on the high-load profiles.

The concept of shifting the whole camshaft axially to engage a different set of lobes dates back at least to 1902 when Swedish engineer Anton Carlsund applied it to the problem of reversing a submarine’s diesel engines. In those days, cam shifting required several men, shouted orders, and a precise series of motions. BMW’s system automates that process.

Variable valve timing in itself is not new, but there is much more to what BMW has accomplished with its Shift Cam system than combining two different engine characters in one. On high load, throttles are nearly open and rpm is rising. At higher revs, intake velocity is also high, carrying lots of kinetic energy into the cylinders to produce the strong turbulence that accelerates flame speed, producing quick, efficient combustion. But at middle and lower revs, intake velocity falls because the engine’s pistons move more slowly. This causes flame speed to slow, increasing heat loss and reducing torque. To prevent this, BMW’s Shift Cam system gives its part-load cam profiles less lift, recovering much of the intake velocity that would otherwise be lost at lower loads.

Yet even that is not the full extent of what BMW is seeking with Shift Cam. Notice in the video that on the high-load cam profiles both intake valves move together, but on the part-load profiles one valve opens ahead of the other. This staggered opening lets the first valve to open generate old-time axial swirl (charge motion rotating around the cylinder axis). This may address certain running problems that can specifically afflict engines with large cylinders—poor idle stability (caused by incomplete charge mixing, creating zones in which mixtures with leftover exhaust gas ignite and burn late) and even carbon deposition in some part of the cylinder. The lower valve lift of the low-load cams speeds up the charge entering the cylinder, giving it the energy to more fully mix with the inevitable exhaust residuals there. And the initial axial swirl produced by staggered valve opening directs fresh charge flow around the combustion chamber, preventing residual exhaust from lingering unmixed.

I suspect the result of these features is a more stable idle, more rapid initial throttle response, and steadier, stronger running on part load. In general, whatever improves mixing and combustion can also be useful in meeting ever-tougher emissions standards.

Why not leap forward to one of the completely flexible variable-valve-timing-and-lift systems we’ve heard so much about? The obvious reason is that such systems have not yet achieved the combination of reliability and cost required for use on motorcycles.

Why not drop the internal-combustion engine altogether and adopt electric drive? The battery required to drive the Energica Ego Corsa all-electric models in the 2019 MotoE World Cup would add 200 pounds to the weight of a current combustion-powered MotoGP bike. That 200 extra pounds would take the weight of the new R1250GS to nearly 800 pounds. Are you ready for that?

How quickly does Shift Cam change from low-load cam profiles to high load? As you can see from the video, it takes just the time between valve closure and valve opening (you can’t slide the camshaft while valves are lifted), so the shift takes of the order of one engine revolution. At 3,000 rpm, this is 0.020 of a second. Chains drive a gear in each cylinder head, by which both intake and exhaust cams are actuated. That gear is double width so the intake cam’s gear can remain engaged at all times as it shifts.

This is a further example of the refinement that is steadily being achieved with the 130-year-old internal-combustion engine to combine its best qualities in ways that were previously impossible.