[Diesel_Dave]

You have several issues.

First, if the valves are still active on the de-actived cylinders, then you're going to have pretty much the same friction & pumping work to overcome as you did when firing on all cylinders. You might gain a little efficiency by running the one cylinder at a high load factor, but probably not much.

Second, if you didn't deactivate the valves, it's going to wreak havoc on the AFR control because the O2 sensor will see all the fresh air from the non-firing cylinders and massively over-fuel the firing cylinders.

Third is the mechanical balancing issue. Like Frank said, a single cylinder engine needs a massive flywheel to have enough retained momentum to complete the compression stroke.