water injection = Higher Efficiency?

[drmiller100]

when water turns to steam it expands.

it expands a fair amount.

I haven't done the math on the heat, and it sounds like you haven't either.

Sounds like we are both guessing.

[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?

[ProDarwin]

Originally Posted by t vago

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.

Well, to be fair, you are cooling more than the exhaust. Both the Crower Six-Stroke (2006) and the Dyer Six-Stroke (1915) were able to run without a cooling system as well

In reference to your above argument, the energy that converts the water to steam comes from the piston/valves/chamber/etc, not just the exhaust air. The problem is that extracting the heat from there just increases the delta-T between the combustion and chamber walls (and therefore heat transfer) on the next power stroke, effectively weakening it.

There is probably a reason Crower never finished his patent application, and why Dyer's engine never went anywhere 100 years ago.

[t vago]

Originally Posted by ProDarwin

Well, to be fair, you are cooling more than the exhaust. Both the Crower Six-Stroke (2006) and the Dyer Six-Stroke (1915) were able to run without a cooling system as well

True that. I just wanted a simple enough presentation to show the absurdity of trying to recover waste heat by spraying water into a combustion chamber full of spent exhaust gas.

Originally Posted by ProDarwin

There is probably a reason Crower never finished his patent application, and why Dyer's engine never went anywhere 100 years ago.

It's much the same as with HHO or magnets. Despite hard empirical proof to the contrary, some true believers will continue to push their version of how things should be. Might as well harness the power of all them midiclorions in the cells of their bodies, for all the good it'll do.

[drmiller100]

how many grams of exhaust gas in the cylinder?

what is the expansion ratio of water to steam?

[drmiller100]

seems like if we are running "efficient" we are somewhere in the neighborhood of 14:1 air to fuel.

If we run twice as much water as fuel, then our final ratio will be something like 7:1 water to exhaust gas (rounding a bit).

one of the interesting things is our pumping losses go towards zero - if we get the water up close to 200 or so before we inject it, the "vacuum" of the intake will cause it to boil in the partial vacuum.

gotta love it when the engineers start spouting degrees and years of service to the guvment. fwiw, I'm pretty sure i've got you covered, but my schooling was more theoretical and less train driver driven.

[t vago]

Originally Posted by drmiller100

how many grams of exhaust gas in the cylinder?

what is the expansion ratio of water to steam?

What difference does it make? You completely missed the point.

Look, even if you injected 1% by mass of liquid water into the exhaust inside a combustion chamber when the piston is TDC, you're not going to get the result you think you're going to get, even if the water's near 100 C anyway.

That pesky latent heat of vaporization, that you keep neglecting, is going to cause the exhaust to cool off as the liquid water vaporizes. What's more, once water enters the vapor phase, it's a gas. It gets treated as a gas, and it behaves as a gas. It'll reach equilibrium temperature with the exhaust gas it's mixing with. The whole gas mixture will be at a lower pressure than would exist had the exhaust gas not been sprayed with liquid water, because the temperature dropped due to the water taking heat energy away from the exhaust gas in the process of becoming steam.

By the way, what's the pressure of steam at 100 C?

Originally Posted by drmiller100

seems like if we are running "efficient" we are somewhere in the neighborhood of 14:1 air to fuel.

Efficient part-throttle operation is actually around 17:1 with gasoline, which is why Lean Burn is so popular among those who are able to use it. The rest of us are not allowed to use it as a factory option because of the Clean Air Act of 1990. Try again.

Originally Posted by drmiller100

If we run twice as much water as fuel, then our final ratio will be something like 7:1 water to exhaust gas (rounding a bit).

Is this a guess, or is there something you can show? And you do realize that exhaust gas has water vapor in it, right?

Originally Posted by drmiller100

one of the interesting things is our pumping losses go towards zero - if we get the water up close to 200 or so before we inject it, the "vacuum" of the intake will cause it to boil in the partial vacuum.

Assuming that you mean to flood the intake manifold with steam, then yes, pumping losses do approach zero. However, that's because you're diluting the intake charge with an inert gas (water vapor), thereby forcing the throttle to open up to maintain the same oxygen intake, not because of any magical properties of water becoming steam inside a combustion chamber.

Originally Posted by drmiller100

my schooling was more theoretical and less train driver driven.

This much is also obvious. You keep on with your "thought experiments" and such, never bothering to once run actual experiments to see if your ideas might actually work. Philosophers run thought experiments. Engineers work with reality.

[oil pan 4]

Is anyone really going to reinvent the wheel and put this engine on the street?

[kevman]

Originally Posted by t vago

What difference does it make? You completely missed the point.

Engineers work with reality.

your posts are quite informative & you clearly Know your thermo, i get your arguement on latent heats required just to arrive at the point where water turns to steam.
but then again, your only considering the heat energy available/escaping with the exhaust gasses.
let me ask you this, theoretically speaking, an ideal engine will have no/minimum heat transfer to outside the working medium (pison/valves/cylinder ect) correct? that is why ceramic coated pistons/liners/sleeves ect are benificial as i understand it. which means more heat trapped inside to do work.

[serialk11r]

drmiller I really don't mean to be blatantly offensive, but you touting your "theoretical schooling" is basically just as bad if not worse, especially given how you tend to frequently forget about some important factors at play.

t vago, not all theory heavy people are like him :P

kevman, when the temperature is high in the combustion chamber the pressure is also high, and if the heat escapes then the gas loses energy and pressure, and you get less work out of it. I think the reason we haven't seen the "6 stroke" engine in reality is because most of that heat energy is going out the gases in the tailpipe rather than being retained in the metal, and so you don't have much energy to work with. If you had an engine with no friction and perfect insulation you'd have something like 40-50% (depending on a bunch of parameters of course) of the heat energy of the fuel leaving in the exhaust gas itself. A cooling system can be thought of as sapping heat energy from the combustion itself, but it also cools the exhaust ports so it's not too clear how the heat energy is distributed there.

At any rate something like 30% of the heat energy is leaving the tailpipe at temperatures around 400-700 (or higher, which is kinda bad but it happens) C. The cooling system runs at 90-100C, and the engine internals are kept well below 200C, and the flame front shouldn't be hitting the piston early when temperatures are highest on a good combustion chamber design. The higher the temperature of the heat source the easier it is to get power out, so you can see why people are turning to the exhaust for waste heat recovery.