Quote:
Originally Posted by ConnClark
I figured I would chime in on this. The boiling point of water is not always 100C. It can be less or more depending on the pressure its under. under ten atmospheres of pressure its boiling point goes up to about 180C. Also at that pressure the steam expansion ratio is less than 175 times its volume as water.
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For purposes of illustration, it's enough to show that the water will cool off the exhaust gas due to its sucking out the exhaust gas's heat energy to satisfy its latent heat of vaporization requirement. I'm not about to go into overly complicated details for somebody who does not know what water's critical point is (can't find the pressure of steam at 400 C, indeed).
Quote:
Originally Posted by ConnClark
In my research into water injection on a gasoline engine it is most useful as an internal coolant. The next most useful thing it does is act as a knock suppressant allowing more boost or higher compression ratios. Lastly it can be used as a intake charge coolant to cram about 0.5 to 1.5% more air/fuel mixture into the cylinder. None of these things is extremely useful for hypermileing .
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Agreed. I have done some study into water injection, too. At one point, I was going to do a binary staged water injection system that would deliver approximately the same amount of water per cylinder, regardless of engine speed, for purposes of turbocharging. I even worked out the math using Excel and found out that the water charge would vaporize completely while the cylinder was on its compression stroke, while providing the cooling mentioned above.
BTW, this is why extra gasoline is used at WOT for WOT enrichment, as well as why tuners enrich the charge mixture for forced induction applications. Gasoline also provides cooling by evaporation on the compression stroke, but nowhere near as efficiently as water does. IIRC, gasoline has a latent heat of vaporization of about 2 J/g-K.