Half a diesel engine? (Mechanical cylinder deactivation - 2 of 4 cylinders?)
 

Yet another half-baked idea.. :D

Could you take a 4 cylinder diesel engine, and only run it on 2 cylinders? Maybe remove 2 pistons/injectors.. would you run into trouble with th eengine management system?

No because the engine would be out of ballance.

Technically, it's possible. Removing pistons 1&2 or 3&4 would keep it the "most balanced", as they are not companion cylinders (don't rise and fall together). But you would somehow have to block the oil gallery in the crankshaft where the rods would be missing.

There is little point in attempting this on a diesel, as diesel engines are far more efficient at light loads than gas engines and there is not much efficiency to be gained through cylinder activation. If I understand correctly, cylinder deactivation adds efficiency through a reduction in pumping losses, which isn't an issue in diesel engines.

I believe that mechman600 is correct (at least for an inline 4). Generally 1 & 4 are positionally the same (but on different strokes--360 deg apart) as are 2 & 3 (180 deg off from 1 & 4. This is similar to an inline 6 where 1 & 6, 2 & 5, 3 &4 are together. I know an inline 6 can be run on either 1-3 or 4-6. Generally there's some additional vibration, but I believe it's tolerable.

So I believe it would technically be possible. That being said, it's going to be a ton of work, for not a huge amount of gain. To do it right, you should deactivate the valves and drill holes through the deactivated piston. And after you're done, you'll still have frictional losses pretty much the same as before, and the thing will probably take forever to warm up. Plus you'll still have all the weight of the original engine.

Also with connecting rods removed the crankshaft would have little to no oil pressure (depending on how the oil holes are drilled), which would be bad and the crank shaft would just be dumping oil out of the oiling system through the uncoverd rod journal hole so you would have very low over all oil pressure.

There are plenty of very light weight sub 2L diesels with less than 4 cylinders why do this to a big old 4cyl?

Say what?!? Remove inactive pistons!

Then you end up with the oil pressure problem already brought up and, more importantly, it keeps it more balanced to keep the mass in place.

Wow, I can't imagine considering keeping all that useless reciprocating mass going up and down... even with the ring packs (largest single contributors to internal engine drag) removed, holey pistons will create an awful, awful internal airflow/pumping loss mess.

It would be far easier to plug up a few oil galleys than it would be to perforate pistons.

Yes, the rings can be taken off and if the valves are closed off and the holes are large enough, there shouldn't be much added pumping loss.

I only metioned the hole drilling, because I've seen it done on research engines where somebody had a multicylinder engine and wanted to create a single-cylinder research engine. So I know it can be done. Whether there are better ways of doing it, I don't know.

Maybe decapitate the entire piston crown including ring lands leaving only skirt and boss for con rod. Even then it wouldn't flow air very nice. I would only do that if a con rod throw (on the crank) filler piece wasn't feasible.