Already been done. . . .

... Chamy... ure an idiot... u should just go jump off a cliff... no 1 cares about ure fuel economy ideas... if u want 2 save fuel dont floor the accelerator at every light

Cadillac (American company) had a v8 that could turn into a 6 or a 4. Didn't work, so they stopped making them. Maybe an Asian or German company could do it.

The Cadillac system worked, just poorly. What your talking about is called Cylinder Deactivation and as you will see, Chrystler, Mercedes, GM and Honda all have working prototypes of this idea.

Turns out that the amount of fuel required to move a car is more directly related to the sum of the forces the engine has to overcome. Wind resistance, wheel friction --and even piston friction--remain the same whether three or eight cylinders are running.

And there's an easier way to decrease the amount of fuel an engine uses--just close the throttle. I'm not teasing, I really mean it. No matter how many cylinders a car has, engine output still has to equal the power required for that speed or acceleration. An because these engines still turn the same number of cylinders (just with the valves closed and no fuel injection), the friction doesn't change, hence the efficiency (power supplied/fuel used) really doesn't change either. Actually, the efficiency is slightly worse, because they're turning the pistons that aren't doing anything.

Maybe if you could design a crankshaft that could disengage half of it, you'd gain efficiency and this plan would be worthwhile. It would be hard to keep it smooth, though.

As for your plan of starting with a 4-cylinder and turning cylinders off from there, in my commuter car, the rpm's are nearly twice a V8 because it needs every piston-explosion it can get. I don't think I could use less than four unless I was okay with 45mph.