Quote:
Originally Posted by mpgmike
Water vapor expands at 12X the rate of nitrogen with the same amount of BTU heat input. Carbon dioxide even more so. EGR is actually a superior expansion medium to air.
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That is incorrect. The specific heat ratio is worse with water and CO2. Water does expand a lot when it vaporizes, but it takes a whole lot more heat.
Nitrogen is around 1.4, water 1.3 and CO2 1.28.
Plug those numbers into thermal expanssion equasion and you get less kinetic energy from heat energy (smaller numbers are worse). You can make up the difference if you increase the compression ratio though.
This is one of the reasons water injection tends to hurt fuel consumption in gasoline engines even though it's anti-knock properties should do the opposite unless you increase the compression along with the water injection.