water injection = Higher Efficiency?

[kevman]

Originally Posted by oil pan 4

I'm adding your single post to my WMI wiki.

im not sure if thats constructive critiscism or sarcasm.

either way, i figure this was the forum to throw this idea out. the science behind it, especially the thermodynamics side of it agrees.

i do however admit i didnt do a forum search before i posted.

i still think a High Compression + Lean Burn + Advanced timing + Water Injection setup should create a noticeable increase in FE over just a Lean Burn + Advanced Timing setup.

[drmiller100]

I could see water injection helping in 3 ways.
1. Reducing pumping losses. Add enough hot water/steam, and vacuum should be less, leading directly to less pumping losses.
2. More power. Add enough water, and the water turning to steam increases cylinder pressures after combustion, giving more power for a given amount of fuel.
3. You could add more compression, directly leading to more efficiency.

[t vago]

#2 is factually incorrect.

Any time you add a liquid into the combustion chamber, the act of vaporizing that liquid will reduce the peak temperature of the mixture captured inside the combustion chamber. Due to Boyle's law, this will also reduce the peak pressure, and cause the process of extraction of mechanical energy to become less efficient.

And if you add enough water to the incoming mixture, you'll just quench whatever combustion does occur.

The entire idea behind water injection is to cool the charge mixture enough to prevent detonation and/or preignition. This makes it possible to safely force in more charge mixture than would otherwise be prudent. Yes, it also means that water injection also allows a higher compression of the existing charge mixture than would otherwise be prudent.

[drmiller100]

how does an old school steam engine work in a locomotive????

[t vago]

Originally Posted by drmiller100

how does an old school steam engine work in a locomotive????

Well, the steam engine is an example of the thermodynamic process known as the Rankine cycle. Basically, you have a coal fueled fire that heats up a pressure vessel that contains water. The water boils to become steam, which is then further heated so as to put the steam into superheat. This is necessary, because the steam has to be above atmospheric pressure to effectively push the locomotive forward.

The superheated steam is then admitted to one side of the piston via a reciprocating valve mechanism. As the steam enters the piston, the steam pressure causes the steam to expand and push against the piston. Since the reciprocating valve is still open, we can consider the steam pressure to be relatively constant, so we can model the steam expansion as an isobaric process.

The reciprocating valve then closes, while the steam further pushes against the piston. At this point, the steam pressure inside the cylinder drops as the piston travels. Due to Boyle's law, the steam temperature also drops. This can be modeled as an adiabatic process. The piston travels to its distant end.

The reciprocating valve then opens up in the opposing direction, and steam is admitted to the other end of the piston, beginning the process described above to push the piston back to its starting point.

It is important to note that the Rankine cycle is substantially different than the Otto cycle, in terms of execution.

[Frank Lee]

Seems this topic morphed or vascillated from adding water as a means of enhancing the expansion cycle to using water as a facilitator (detonation suppressant) in order to enjoy the benefits of higher compression and/or leaner mixtures. Personally I think the latter is more likely to be useful.

[t vago]

Here are some good questions for those seeking to use water in an attempt to recover waste heat:

What is the total heat energy of one mole of exhaust gas at 600 C and 150 kPa? You may assume, for purposes of this question, that exhaust gas is 80% diatomic nitrogen, 11% carbon dioxide, and 9% water.

What is the total heat energy of a 1/10th of a mole of liquid water at 30 C?

Mix the two together. What is the final temperature of the resulting mixture? What percentage is liquid? What is the final pressure? Is this pressure higher or lower than the pressure of the original exhaust gas?

[drmiller100]

imagine the combustion chamber at 600C full of hot gasses at a pressure of 500 psi.

Add excess liquid water. My GUESS is the pressure will go up slightly, the temperature will go down significantly.

Now expand the volume by letting the piston go down. As the piston goes down, the pressure normally would go down, but the liquid water is rapidly turning to steam.

Overall, on average, the resultant temperature WILL be significantly lower over the entire stroke of the piston as the latent heat of vaporization for water is very high.

Will the average pressure be higher or lower with water?

Thought experiment two. Imagine a 4 or 2 stroke engine, except it has superheated piston, and we have direct injection of liquid water. We inject the water at TDC.

Will this engine "run"????

[ProDarwin]

I think some of the confusion in this thread comes from two different topics being discussed:

A) What is traditionally referred to as water injection - injecting water with the air/fuel mixture on a 4 stroke engine to help cope with high compression or high boost.

B) Add 2 strokes to a 4 stroke engine. At TDC during the 4th stroke (exhaust) inject a small amount of water, which will immediately vaporize due to the heat of the cylinder and piston, creating a second power stroke. This is the concept of the Crower 6 stroke engine. As a result of excess heat being carried out of the combustion chamber during the 5th/6th stroke, more aggressive timing/boost/compression can be used during the traditional 4-stroke cycle.

[t vago]

Originally Posted by drmiller100

imagine the combustion chamber at 600C full of hot gasses at a pressure of 500 psi.

Add excess liquid water. My GUESS is the pressure will go up slightly, the temperature will go down significantly.

Points are not awarded for guessing. If you add even a small amount of low temperature water (low temperature meaning that the water temperature is such that it's still a liquid) to a set amount of high temperature gas, the temperature of the gas will drop. Because the temperature of the gas drops, so does its pressure. Since water needs much more heat energy to raise its temperature by 1 degree C, as compared to the gas in question, it's going to lower the gas temperature that much more. Finally, that water has to absorb even more heat energy from the gas if it's going to vaporize (latent heat of vaporization, right?), which is going to further drop the temperature of the gas. The gas temperature will drop significantly. You're not going to get the results you've envisioned.

You apparently are making the mistake that exhaust gas has the same specific heat capacity as water. This is not true.

Originally Posted by drmiller100

Now expand the volume by letting the piston go down. As the piston goes down, the pressure normally would go down, but the liquid water is rapidly turning to steam.

You really need to work the math out yourself. This statement is factually incorrect.

Originally Posted by drmiller100

Will the average pressure be higher or lower with water?

With water? Lower. Much lower.

Originally Posted by drmiller100

Thought experiment two. Imagine a 4 or 2 stroke engine, except it has superheated piston, and we have direct injection of liquid water. We inject the water at TDC.

Will this engine "run"????

Not nearly as well as the standalone engine without direct water injection, if it runs at all.

I'll construct a graph later on, that graphically shows what happens when water is added to exhaust gas.