So, what's going on when water aids combustion?
It does not aid combustion in any way shape or form.
It simply acts as a steam engine and robs some heat from the flame front
The amount of energy that it takes away from the combustion is substantially less than the energy available from the expanding steam.
Thus the engine runs a lot cooler because you are actually using some of the 70 to 80 % of the energy from combustion that would normally go out the exhause as waste heat.
The auto industry played with it for years but gave up for exactly the same reason you old Kero tractor got replaced.
You have to start up on petrol to heat the engine.
When it is up to operation temperature you then have to get the water into the cylinder plus a little oil to stop scouring on the rings .
You also need to run the engine a bit leaner so you have to change the air fuel ratio.
When you shut down you have to finish back on fuel so as to leace oil on the cylinder wall or the engine & valves will rust between uses.
I knocked around with a bloke who set up an old Holden 138 Cu" grey engine with a deisel fuel injected head and two carburettors.
HE started the car on petrol then switched over to water and the second carb jetted very very lean.
The car got around 120 MPG on a run and had tons of torque, enough to strip all the teeth off several gears on more than one occasion.
Back in the 60's it was a difficult car to drive as all this had to be done manually and if you switched carbs too earlly you got detonation & a holed piston and if you switched too late you got water wet plugs.
So it was a set up that never would have worked with Joe Public.
However now days with computer controlled engines it will be a doddle you would just need 2 injectors per cylinder and when you get home the engine would go through the run down proceedure after you switched it off, A problem for people with small garages but I can see it making a come back as car makers get stretched to compete with "clean" electric vehicles.
After all I have an LPG vehicle that does the first bit quite well, you just need to program a run down into the computer.