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Old 03-05-2008, 07:44 PM   #21
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I would expect that the pre-turbo injection's effectiveness would have more to do with exactly what you are saying about the temp change (how'd you do a delta symbol? Nice.) in the cylinders - as air is being compressed above atmospheric by the turbo just as it is in the cylinder head.

I agree it would not make sense to expect a significant enough heat transfer just from the water traveling through the tubes - which is why injection is usually done at the throttle body, or even with some set-ups I've seen, in the individual runners just before they meet w/ the head.

But perhaps the water vaporizes in the turbo as the air is compressed, basically doing exactly what it does in the cylinder head, combating the rise in temperature from compression. If so, I would expect you would get a substantially cooler mixture exiting the turbo? But hey, I'm dealing all in theory - I really don't know what I'm talking about.

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Old 03-05-2008, 08:26 PM   #22
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The theory is sound, and in fact some supercharger owners have done just this. But of course the lobes in the supercharger are turning much, much more slowly than the wheel in a turbo.

When visualizing the tendency of compression to generate heat and thus promote the vaporization of water, consider the pressure ratios (and thus the heat ratios) involved. The pressure ratio of a totally bad-*** turbocharger setup is maybe 2:1, and that's sufficient to generate >200F temperatures. The compression ratio of a typical Miata engine is, what, 9:1 to 9.5:1? Lot more heat generation going on inside the chamber than in the turbo. I wonder what the actual temperature is inside the average N/A combustion chamber just prior to ignition? I tried figuring it out with Gay-Lussac's Law, and for an input temperature of 100F and a pressure ratio of 9:1 I keep coming up with 2700K, which is 4400F. That can't be right...

Is there a chemist in the audience?
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