[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

if the oxygen has nothing to burn with it becomes inert like recirclated exhaust gases

Surplus oxygen ends up reacting with the nitrogen, which by the way is at a higher volume than oxygen on the atmosphere (and at the intake tract of an engine).

I wonder if CO2 also has a higher thermal capacity since it has about the same specific heat ratio as water vapor of around 1.3.

Remember CO² is already being used for refrigeration purposes, and I'm not considering only the so-called dry ice. Even some newer HVAC setups resort to CO² instead of those synthetic gases.

I haven't had much luck in seeing how the phase change effect efficiency. All I know is that both water and water vapor have a lower specific heat ratio. I'm guessing that means even with the phase change there's a lower specific heat ratio. In other words, unless you increase the compression ratio the energy extracted from the resulting steam will be lower than the energy that can be extracted from heated nitrogen.

In other words you add a specific amount of fuel and oxygen, you burn it, that creates a specific amount of heat energy. If that heat energy heats a working fluid in an enclosed cylinder the working fluid expands causing pressure. But if the working fluid is water the resulting pressure will be lower than with nitrogen resulting in less energy that can be extracted. In theory, the way you'd extract more energy from the water would be to have a higher compression ratio.

The increased moisture content absorbs a higher amount of thermal energy from the intake air stream and the aerodynamic heating inherent to the compression stroke, releasing that heat at a much more convenient and homogeneous way through the entire combustion chamber, leading to a more complete combustion.

Back on a positive note, in diesel engines water mixed with the fuel and injected can lower CO, PM and NOx emissions. It's one of the fuel ways of lowering both PM and NOx at the same time since most (if not all) other methods lower one but increase the other.

Water and fuel are injected always separately on a Diesel engine.