Quote:
Originally Posted by Isaac Zachary
One problem with water is it's specific heat to expansion ratio is lower, about 1.3 instead of about 1.4 like air. So unless you change things like increase the compression ratio or advance the ignition timing you'll actually get worse fuel mileage.
|
So I used to think the heat capacity ratio was important, but air's heat capacity ratio falls with temperature, and EGR has a low heat capacity ratio too, so water isn't really different. You can think of water like ultra-cold EGR.
You're right though, if you lower the combustion temperature and slow down combustion, you could easily lose efficiency instead of gaining efficiency from cooler temps. Running water injection with an ultra high compression ratio is certainly a valid way to get more efficiency (though I don't think most people want to bother with filling a water tank). For people with easy access to E85, running more ethanol in the tank an easier way to accomplish the same thing.