V-8-6-4

[applepi66]

GM uses cylinder deactivation in all of their new V8 engines (excluding the LS3)

They call it Active Fuel Management (AFM) and it seems to work pretty well. I can't speak on how the vibration is under constant load, but it's been in production for a while now.

[Slim Jim]

I'd say barely perceptible. I drove a Chevy Impala SS with the LS4 a few years ago that had displacement on demand. The only way I knew I was driving on 4 cylinders was when a little green light lit up on the dash, and the exhaust note changed. No noticeable vibration at all.

[vmo]

Would it be possible for players to make full size V8s and then cut off certain cylinders so as to have better gas mileage much like Cadillac did in the early '80s (and failed) or like Chrysler does today with the 300c. I think I read somewhere that GM still makes some engines like this (maybe?).

I think this variable displacement would be fun to get a lot of power out of an engine with the ability to change into a more fuel efficient car while cruising.

The new VAG cars some models had this system, called COD (Cylinder Of Demand).

[WizzyThaMan]

Just when I thought the engine designer was hard enough already as it is right now

[maffc]

Interesting concepts, but wouldn't the inactive cylinders cause"drag" making the 4 cylinder mode inefficient compared to an actual inline 4?
Wouldn't there be better efficiencies to be had by still using the full set

[Killrob]

The trade-off is that you have more friction, yes, but also you can run at a higher throttle setting, which again makes the engine more efficient. Efficiency rapidly decreases with low throttle settings for powerful engines, so it does make sense. At all times, of course a real 4 cylinder engine would be more efficient.

[maffc]

Ah I see, so rather than let the engine slow to 2k rpm drop half the cylinders and keep running at 4k and retain more of the efficiency

[bundyaxl]

I was gonna say, several companies are doing the cylinder de-activation. Mercedes did this recently as well. Probably still doing it.

It also helps with idling. Big engines have more trouble sucking in air when there is no flow from driving, so the cylinder shutoff is supposed to help with this air restriction as only half the engine really needs air, depending on how they do it. From what I've seen I think the valves stay closed in some methods anyway.

I also drove a 300 with the cylinder shut off and didn't notice any difference in vibration in 4 cylinder mode. Barely notice any difference between the two. Of course the 8 cylinder mode was mainly in use during acceleration so hard to tell a difference in most cases.

[Killrob]

Ah I see, so rather than let the engine slow to 2k rpm drop half the cylinders and keep running at 4k and retain more of the efficiency

Nah, rather it is like this (simplified example):

V8 mode: Friction = F; Max Power @ Cruising RPM = 200kW; Throttle to maintain cruising speed in highest gear: 10%; Efficiency @ Cruising RPM @ 10% Throttle = 700 g/kWh
V4 mode: Friction = F; Max Power @ Cruising RPM = 100kW; Throttle to maintain cruising speed in highest gear: 20%; Efficiency @ Cruising RPM @ 20% Throttle = 400 g/kWh

So by having all cylinders running the engine potentially produces so much power at a certain RPM, that throttle has to be in a really inefficient regime. Friction is the same in both instances, so you can as well drop a few cylinders to run the others more efficiently.

[maffc]

Thank you for that "layman" explanation. I like to understand new things