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Old 05-01-2021, 07:44 AM   #13 (permalink)
Ecky
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Gaptooth (retired) - '00 Honda Insight
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Quote:
Originally Posted by Isaac Zachary View Post
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.
Does this take into account the phase change, or is it just for water itself?

Generally speaking, anything that slows combustion is bad for efficiency. Fast combustion is the holy grail in highly efficient combustion. And, high temperature is only useful so far as it increases expansion or pressure. If you can get the same expansion with lower temperature, you have less energy loss.

In the case of my particular engine, which was fairly high compression (10.5:1) for an engine of its type in its era, knock is only really an issue above 80% load and below 2500rpm. If I run 93 octane fuel, I can advance timing to MBT and there would be no advantage to cooling combustion, but for any less than 93 I need to pull timing. So, water/meth could allow me to run cheaper fuels.

As far as practical limitations, I live in an area where temperatures can be below freezing as much as half of the year. Water just didn't inject well in those conditions.
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