GM’s New Sliding Camshaft: How Does it Work?

With the CT5, Cadillac announced that the 2.0L engine would offer cylinder deactivation. Not exactly a common feature on four-cylinder engines, but what caught our ear was how they were planning to do it. “A three-step sliding camshaft.” What? A camshaft that slides? How did we miss that same feature when it launched on the same engine in the XT4? More importantly, how on earth does that work? Let’s find out.

First of all, is the camshaft really sliding? Like is it moving around in the engine? The answer is yes. If you’re looking down the length of the engine, the camshaft can move toward and away from you. But a surprisingly large amount.

Patent filing shows cam solenoid (40) and cam slider groove (32,34,36). From GM Patent Filing

GM’s regular (we’d hate to call something that new and that cool old) Dynamic Fuel Management works by basically bypassing the lifters for a cylinder it wants to shut down. That takes a load of solenoids and a handful of moving parts. We went further into it last spring.

This system is different. With what looks like fewer moving parts. And simple is quite often better.

Actuators (dark grey boxes) and sliding cam elements

Each camshaft has three lobes for every valve. Where a normal cam would have one. There’s one lobe for high lift. Like when you want to make maximum power. There’s a second one for low lift, for when you want to improve fuel economy. Then there’s a third lobe. Which is really no lobe at all on cylinders two and four. It’s the one that closes those cylinders and deactivates them.

That part’s all quite simple. Multiple camshaft lobes have been used since Honda’s VTEC system came about. It’s how GM handles switching the cam lobes that’s actually fascinating.

See, most systems like this have multiple lifters. Or a way to shift the lifters or rocker arms. GM moves the whole cam instead.

The camshaft drive is splined. So the shaft containing the lobes is also splined and it fits onto the drive. Like putting a little straw into a bigger straw. So if you’re looking at the camshaft from the side, you can actually slide it left and right on the splines. It can only rotate with the drive, but it can slide back and forth. Not only that, but each cam is divided into two parts. They can slide independently, though only a small distance in either direction.

Sliding Cam Diagram. Splines just visible at left end and two-piece camshaft gap visible near the center

So what makes them slide? A groove in the cam and camshaft actuators.

The groove starts in a straight line on the cam, then it gradually moves to the right before straightening out again. It’s a guide. Like a track for your camshaft. Throw a switch and it goes left or right.

Actuators on the cylinder head have two prongs. To shift the cam, one prong is pushed down into the guide. That slides the cam to the desired lobe. To shift it back, the other prong is engaged.

So using a couple of sliding shafts and some solenoids, GM is able to make a variable lift cam that also has cylinder deactivation. With very few moving parts. And if for some reason the camshaft can’t slide? Well, you might lose a bit of fuel economy, but you’d get full power all the time. And that’s not so bad.

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