Nitrous?
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Joined: Nov 2004
Posts: 627
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From: Fallston, MD
Originally Posted by big_rizzlah
i wasn't counting on any phase change when i made the suggestion ( which like i said when i made it, i was very unsure of because i didn't know the specific heat capacity of nitrous )
people injecting water pre-turbo don't count on a phase change at all. . .compressing air doesn't generate that much heat. . .think about it. . .what is your average AIT pre intercooler? it's nothing near the boiling point of water, and air has a substantially lower specific heat, meaning that if it absorbed as much energy as water, it would be a lot warmer. . .
the aim of injecting anything pre-turbo is to make the compression more isothermal, where as it is largely adiabatic without any sort of injectant. . . i will be the first to say that the outcome will not be as dramatic as that produced by it actually burning in the motor, but it will still do that as well with this technique. this has the added advantage ( possibly ) of making the turbo have to do less work, resulting in the same boost levels at lower RPMs.
ryan
people injecting water pre-turbo don't count on a phase change at all. . .compressing air doesn't generate that much heat. . .think about it. . .what is your average AIT pre intercooler? it's nothing near the boiling point of water, and air has a substantially lower specific heat, meaning that if it absorbed as much energy as water, it would be a lot warmer. . .
the aim of injecting anything pre-turbo is to make the compression more isothermal, where as it is largely adiabatic without any sort of injectant. . . i will be the first to say that the outcome will not be as dramatic as that produced by it actually burning in the motor, but it will still do that as well with this technique. this has the added advantage ( possibly ) of making the turbo have to do less work, resulting in the same boost levels at lower RPMs.
ryan
First of all, there's a LOT of heat generated by compressing the air. That's the whole reason why the compression is adiabatic in the first place. The ONLY thing that pre-compressor injection is doing is creating quasi-isothermal by making the temperature rise almost nothing.
Secondly, as I stated before, it takes very little heat to bring water up to the boiling point. When you spray pre-compressor, you're misting the droplets as finely as possible (and they're probably further atomized by the compressor blades). These tiny droplets have much more surface area per volume than big droplets and thus they vaporize very quickly. You don't have to spray as much water to get the same cooling if your atomization is better.
There will likely be some liquid still in the intake tract, but a majority of what you're spraying will vaporize, if you're doing it correctly. To say that pre-compressor injection isn't relying on a phase change for quasi-isothermal compression is wrong. If you shoot a gas into the compressor, you're not going to get anywhere near the cooling.
Otherwise, everyone would stop worrying about blade impingement and just start spraying propane.
after researching some more. . .i agree with you almost completely rarson
the vaporization (of water) is relied upon, and in fact, fully utilized (assuming good atomisation). . .so much so that due to the maximum humidity air can sustain, not enough water can be put into the intake air stream to make the compression completely isothermal in lots of applications.
for this reason, people have begun experimenting with adding methanol, acetone, and likely propane (although i didn't find any examples using propane, i'm sure they're out there) to the pre-comp water injectant because the additional fluids/gases can absorb even more heat (the law of partial pressures says the air is saturated with water, but can still become saturated with several other gases as well) making it possible to cool intake charges below ambient in theory.
i wish i was smarter
in case anyone cares . . . (all units are kJ/kgK)
cv N2O = 0.69
cv C3H8 = 1.43
cv H2O = 4.19 (and that's not including the latent heat of vaporization)
the vaporization (of water) is relied upon, and in fact, fully utilized (assuming good atomisation). . .so much so that due to the maximum humidity air can sustain, not enough water can be put into the intake air stream to make the compression completely isothermal in lots of applications.
for this reason, people have begun experimenting with adding methanol, acetone, and likely propane (although i didn't find any examples using propane, i'm sure they're out there) to the pre-comp water injectant because the additional fluids/gases can absorb even more heat (the law of partial pressures says the air is saturated with water, but can still become saturated with several other gases as well) making it possible to cool intake charges below ambient in theory.
i wish i was smarter
in case anyone cares . . . (all units are kJ/kgK)
cv N2O = 0.69
cv C3H8 = 1.43
cv H2O = 4.19 (and that's not including the latent heat of vaporization)
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