Is it the hydrogen or the oxygen or a combo effect?
Offline(Just thinking out loud)
Less Air density =Less Fuel = Less oxygen
More Air density = More Fuel = More oxygen
In a vehicle controlled by a computer that has sensors to measure air pressure, air density and flow to determine the proper fuel mixture.
The computer would be programmed with the amount of oxygen in the air so it could adjust the fuel to the proper ratio.
If you added just Hydrogen before the sensors, that should make the computer think there is more oxygen than there is. But the computer does not detect hydrogen so it would think it was air or since Hydrogen is less dense than air it may be doing the opposite. Causing the a leaning effect while still adding more fuel to compete for the oxygen. Need to test.
IF you added just Hydrogen after the sensors, that should add unknown fuel to the equation and reduce your need for gasoline. Need to test.
If you added just Oxygen before the sensors, that should make the computer think it's air as the computer does not check for oxygen on the intake side. But should help with the combustion of any excess gasoline. Causing a leaning effect on the mixture.
If you added just Oxygen after the sensors, that would increase the air density and would help with the combustion of any excess gasoline. Causing a leaning effect on the mixture.
Does this make sense?
In my humble opinion it is both. Specifically when you increase the ratio of oxygen in any combustion scene you increase the amount of hydrocarbons burned. However, the hydrogen itself has a much greater flame speed and according to pancea U. Hydroxy increasing flame speed 1.6 time normal gas rate. That entails much higher pressure at TDC, while the increase burn rate continues the thermal expansion to a more complete end. The hydrogen has a very small quench distance. ( can recall it but have it in my notes), which allows higher compression ratios to benefit from hydroxy.
Hope this helps. The book "Fire Dynamics" , Drysdale is the author if memeory serve me correct, deals with many type of flame types.
Peace,
Brad