[t vago]

Originally Posted by drmiller100

when water turns to steam it expands.

You must first account for the fact that your water injection scheme must allow for your liquid water to rise in temperature to water's boiling point. This is something you are neglecting.

The specific heat of liquid water is about 4.18 J/gram-K. In other words, you need to add 4.18 joules thermal energy to a gram of liquid water in order to heat it up by 1 degree K.

The specific heat of steam is about 2 J/gram-K. In other words, you need to add about 2 joules of thermal energy to a gram of steam before it rises 1 degree K.

The specific heat of air is about 1 J/gram-K. In other words, you need to add about 1 joules of thermal energy to a gram of air before it rises 1 degree K.

Now, for purposes of this little "thought experiment," as you would call it, we can consider that exhaust gas (that stuff that we're going to inject with water, remember?) will have a specific heat of about 1.05 J/gram-K. Do the math yourself. So, taking 28.86 grams (or one mole) of exhaust gas at 600 C and 150 kPa, we then spray 1.8 grams (or 0.1 mole) of liquid water at 30 C into it.

Now, in order for a gram of water to rise in temperature by 1 degree K, it needs to take about 4.18 J from a gram of exhaust gas. That means that for every 1 degree K per gram rise in liquid water temperature, the exhaust gas must drop 4 degrees per gram. You do remember, of course, that in order for liquid water to become steam, it must first reach the temperature at which it would turn to steam.

So, to heat that 1.8 grams of liquid water from 30 C to 100 C, we need about 527 joules from the 28.86 grams of exhaust gas we have. That will cause the exhaust gas to cool off by about 499 K. So, now, instead of exhaust gas at 600 C, we now have exhaust gas at 101 C. Hm... that's about the same temperature as the water we just injected!

Applying the ideal gas law, we find that the pressure dropped from 150 kPa to 64 kPa, which was below atmospheric pressure last time I checked.

Oh, but wait! We need 2260 more joules per gram of water to transform it from a liquid to a gas (which I like to call "steam"). You read that right - we need approximately 541 times as much heat energy to turn the liquid water into steam, than we did just to heat up that water by 1 degree C! Whereever are we going to get that from the exhaust gas we just got done cooling off?

So, instead of coming up with some clever new way of pushing a piston that somehow eluded the best minds of the 20th century, we merely came up with a novel way of cooling off exhaust.

Originally Posted by drmiller100

it expands a fair amount.

You're about as ignorant of thermodynamics as you are of aerodynamics.

Originally Posted by drmiller100

I haven't done the math on the heat

This much is obvious.

Originally Posted by drmiller100

Sounds like we are both guessing.

My guesses are backed up by college-level thermodynamics, supplemented by Naval nuclear training. What are your guesses backed up by?