How they actually do displacement on demand in cars (as far as I know) is they not only shut off the fuel injectors and spark, but they also disconnect the valves from the camshaft such that the cylinders which are no longer firing do not have to act as air pumps. In an IC engine there is a lot of loss, known as 'pumping losses', which occurs when the engine has to pull air in through the filter, intake, throttle body and valves and then push the exhause out of the exhaust pipe. So by disconnecting the valves and leaving them all closed when the cylinders are not firing, the engine gains a little more efficiency.
The truck definately is super slow on 4 cylinders. It wont do more than 50mph on the highway on flat ground, but if I am cruising down a hill at 65 I can switch it to 4 cylinders to save a little gas and maintain my speed. Also in traffic at slow speeds I am saving a little gas too. I would like to make it run on 4, 6 or 8 cylinders eventually if possible.
For those of you interested in more details, read further. For others who just want to see a pic of my lovely new range rover named Clifford, here ya go!
As for the timing, you actually dont have to touch it. The timing all runs off the timing belt, and everything in the engine is still functioning as it should (valves opening on time, pistons moving correctly, spark firing as it should) the only difference is that when the engine computer sends the signal to the fuel injectors, they dont open, and dont let any fuel in the cylinder. The engine doesnt even know the difference!
I think you guys are right in that if I didnt just switch off a whole bank, it might be able to run smoother, but I did one bank for 2 reasons: 1) it was easy since the injectors on each side have common grounds, and 2) there is an oxygen sensor on each bank of cylinders, so if I switch off only a few cylinders on one side of the engine, the air/fuel ration (which is ideally 14.7lbs of air for 1lb of fuel) will be wrong in that bank. This is because the engine expects all four cylinders on that side to be firing. So I suspect that the engine controls each set of four cylinders seperately from its respective O2 sensor. Therefore if 2 cylinders were pumping air and two were buring fuel the O2 sensor on that side would think that only half of the fuel that should be burned was being burned (and it would be correct, only 2 out of 4 are buring fuel). But the engine would assume that instead of 2 out of four cylinders burning fuel, all four woudl eb burning half the fuel they were supposed to, and register a 'lean' mixture condition. This means that there is too much air and not enough fuel for proper combustion. It would then try to put more fuel into those cylinders, which in reality would just make the two that were firing correctly have twice the fuel they needed, know as a 'rich' condition. Spark plug fouling among other problems would then occur.
Another advantage of the four cylinders is that when I am driving, I have my foot down on the gas pedal waaay further than I do with 8 cylinders. This initially sounds like it might just counteract the fact that I am running four cylinders, but in actuality it makes the engine as a whole more efficient. Again, it is because the 'pumpin losses' that occur in an engine. The more opent he path is from the outside air into the engine (i.e. and open throttle), the more efficient it will be since the engine does not have to use a lot of power to pull the air into itself. So it helps doubly!
Sorry for the long explaination, but I actually have put a lot of thought into this project! I definately want to figure out how to do 4 6 or 8 cylinders and have different amount of power for when I need it. I guess I could call it 'Power on demand' or 'Power Now'.
