Quote:
Originally Posted by Air-Hybrid
Harlan:
"Also injecting steam allows you to recoup exhausted heat and preheat your intake at the same time."
Surely this flies in the face of the principles underpinning a Heat-Engine (Carnot).
The only true efficiency gains of HAIs is that improved evaporation of the fuel in warmer air allows for a cleaner burn.
Dumping heat from the exhaust into the intake has (by Carnot) to lower the thermal efficiency of the cycle.
So only in the circumstances where the gains arising from improved fuel evap. outweighs the efficiency losses caused by lowering thermal efficiency should WAI be used. And then the system should only ever be driven by coolant heat - because disposing of the heat in the coolant has it's own efficiency wins (reduced rad opening/aero and coolant pumping), whereas exhaust heat is actually best serving the engine if remaining as hot as possible.
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You are exactly right.
Lets say we inject some small droplets of water into the intake. At atmospheric pressure, under most conditions, we would expect some of the water to "evaporate" and cool the intake charge. At part throttle, this effect is negligible, especially at part throttle under cruise conditions.
Once the intake valve is closed, the piston starts coming up, and pressure start rising. A quick glance at vapor pressure charts shows us as we increase the pressure, water vapor will condense into liquid - it "rains" inside the combustion chamber. Another example of this is the water in the bottom of our air compressor tanks - compress the air, and the water condenses.
After the ignition event, temperatures skyrocket. Back to the vapor pressure charts, and we see some of the liquid turns to vapor, keeping the humidity level at 100 percent as the piston goes down. This keeps pressure high, giving more thrust, and actually lowering the temperature of the gasses.
So, in this case, the exhaust gas temperature is actually lower, but the pressure was higher.
The link given shows ethanol and methanol makes more horsepower then gasoline at WOT.