They work by shutting off certain injectors to save fuel, right? Wouldn't this cause an excessivly lean condition in the exhaust, causing the EGT to skyrocket? I couldn't imagine that being too good.

 

Not exactly.... It only cuts off fuel and spark in the cylinders that isn't firing. Fuel richens and cools the air that is compressed, and sure compressed air gets hot, but no where near the heat of ignited fuel, this isn't a diesel. Pumping losses is inherant with the butterfly valve throttle plate. We use it to meter air flow, but when it is almost shut (like light hwy cruising) the piston now has to pull hard to get the air, robbing tq from the oposing combusting pistons. So when a v8 shuts down 4 cylinders, now can open the throttle that much more reducing that pumping loss. Though a diesel has no throttle plate, so it doesn't have any pumping losses.



Don't read too much into it and think that WOT driving is more fuel efficient, sure your pumping losses are reduces, but generally speaking WOT you are running more rich, but also you are accelerating like a mofo, and the faster you accelerate, the more power is required. But if you can go WOT with fewer cylinders so that you aren't accelerating, it is much more efficient, This is one benefit.



Another is cylinder pressure, if you didn't suck much air in, even though you worked hard to suck in little air, you can't compress it that much, so you get very little power out of this little bit of air you worked so hard for. With a WOT one or two cylinders, you are gettting a ton of air in those cylinders, and the compress a whole bunch more giving a much higher cylinder pressure resulting in an increased efficiency and better mpgs.



IMO, it is pretty simple to implement and saved good mpg, I am surprised it isn't used more. I am sure there is more to it than that, there has to be. Or it would be a simple tuning fix, really no addition hardware is required.

 

One thing that has always puzzled me with variable displacement motors is the balancing of the engine.



Surely the firing order and crank alignment would be wrong? What works for a V8 is not going to work for an L4 or V4.

 

Unless you've got balance shafts or a huge flywheel and/or you aren't killing off one cylinder per revolution, dropping a cylinder on an inline four is asking for excessive NVH.

 

On some of the engines that deactivate cylinders, they also usually open on intake valve and one exhaust valve to reduce pumping losses. They just don't fire those injectors and don't fire those spark plugs...