I added some stuff to the oil that's supposed to increase the seal between the rings and the cylinder wall and raise the compression.
You'll find out one way or the other soon enough, but dropping a cylinder is very obvious on mine. Having a 50% misfire I think would be quite unpleasant. But once you have your system working, it's entirely under your control whether you run the 'steam cycle' every other cycle or every third cycle or whatever.Since you're injection into the chamber at TDC you'll have a lot of pressure working against you. Have you worked out how you're generate enough water pressureIt sounds as if you have got the petrol injection cut-off sorted out but I'm still not clear how you're going to time the water injection. I get that you can trigger it from the spark, but you need to know which of the pair of cylinders is in the compression stroke. I suppose you could monitor one of the fuel injection pulses and work the engine phase out from that, if you know where the fuel injection pulse occurs in the engine cycle.I assume you would enable this whole water injection system when the load and revs corresponded to cruise conditions and leave the engine to run normally the rest of the time.Do you have any sort of cat system or EGO feedback system on this car? One thing EGO sensors don't like is having fluid dumped on them, and the missed firing cycle would throw any EGO sensing out of wack. Suppressing the fault codes is one thing, but if your ECU relies on an accurate EGO signal then it won't be happy.Given your experience working on engines, I'm sure you must be tempted to try the full 6-stroke conversion.
#4, unless this engine is very old and worn, everything you do ( and I mean everything) will not improve and may worsen your losses. There are very few lubricants that can beat oil. You can use a synthetic oil, look for independent tests to see which ones actually make an improvement by reducing friction losses in a real motor. #3, the injectors fire while the air is moving through the port, and it doesn't get on the manifold wall to any signficant degree. The reduction in efficiency... Or perhaps more appropriately, the increase in consumption between a cold cylinder and a hot one varies signficantly, but generally we're talking losses of anywhere from 10 to 40%. Google for "cold engine vs hot engine fuel consumption" or something like that, to find information on it. Injecting water before the compression stroke means that you will have to use fuel to heat the water, push the piston up against TDC and then it loses energy again as the piston drops and the water may or may recondense, but the heat is lost to the cylinder walls and combustion chamber, piston, etc. As to the idea of an 8 stroke motor... the friction and pumping losses for the 'non power' extra revolutions could not possibly be made up by anything you do. Pumping losses - that is the effort required to pull a vacuum on the closed throttle plate and to try to shove the exhaust or air out the exhaust system are the single largest factor in poor fuel economy at cruise. When you close the throttle, the amount of fuel consumed per horsepower absolutely skyrockets. Look up BSFC for throttled engines, and note how efficiency is improved when you open the throttle and produce more power. This is why tall gears at cruise work so well, because you have to open the throttle a lot, reducing pumping losses by reducing intake vacuum (raising manifold absolute pressure) while moving the car at highway speeds. As for engine controllers, look at MegaSquirt, which is a wholly programmable generic engine controller. I believe it may be open source, too, not sure. It's not cheap, but it's also not expensive.
It takes heat to turn water into steam. If you inject cold water at the top of a compression stroke, some may turn into steam, but the process of turning cold water into steam absorbs the heat from the compressed air, and the pressure will DROP, not rise. It will consume mechanical power to do this. As the piston starts down, the cooled air cools even more as the pressure drops, which can cause water to condense, further reducing the pressure against the piston going down. It will take far more power to push the piston up, inject the water, than will be recovered on the way down. There is nothing to gain here. NOTHING. It will consume water, fuel, and time, and you'll have a severe loss in fuel economy. You could, in theory, inject superheated water under high pressure, but that consumes large amoutns of fuel to heat the water and keep it compressed (not flash to steam). There is no efficiency to be gained here. None.