IMO, it is really exxpected to have the graphs this way, pure water has:
1. higher heat capacity - volume per volume H2o hold a lot more heat. A smaller capacity cooling sytem with pure water can absorb more heat faster..
2. And_ it has also higher thermal conductivity - Pure water gives up more heat with lesser temperature gradient. In a relatively cooler radiator environment pure water will release more heat faster.
3. Plus_ Pure water has lower viscosity - it will move faster in the cooling system.
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SO PURE WATER HAS A HIGHER HEAT TRANSFER ABILITY....
But is pure water the best coolant for engines, lets take the major CONS:
1. Pure water has a lower freezing point(0C or 32F) -
Freezing is not a problem in the Philippines.
2. Pure water has a lower boiling point(100C or 212F). - Most cars today has a normal operating range between 195F to 212F with the Thermovalve fully opening at around 195F. Using a radiator cap with at least 0.9 rating (that is 0.9 x 1 bar or about 14.7 PSI) will raise the boiling point of pure water to 250F or 120C.
There is 3F increase in boiling pt. per one PSI pressure applied. Mixing it with 'coolants also raises its BP. But as said, you really don't want your coolant temperature go beyond 212F coz' the engine metal temperature is much higher than the coolant temperature. The risk for a TOASTED ENGINE is looming.
3. Pure water has a higher surface tension - this translates to a lesser percent coverage per surface area of the cooling system. Redline's water wetter and other SURFACTANTS claim to solve this.
4. With Pure water corrosion is high. Ethylene glycol in pure form is also highly corrosive.
Redline has enlightening technical info and figures, it is good to read their primer on their water wetter product.
http://www.redlineoil.com/whitePaper/17.pdf
Anti-freeze, anti-boil, and anti-corossion properties need not be sourced solely from Glycol products, they can be bought separatety according to your needs and fancies.
The older vehicles operate in a lower operating range, but with the attempt to produce more power per engine displacement, very high fuel economy, and very strict emmision control the results are the high heat producing engines in our time. Diesel engine manufacturers in the US, as diesel are gaining popularity, are predicting a 30% hotter diesel engines in order to comply with the government's regulations on pollution control.
Solution, 50/50 glycol coolant mix
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