water injection to gas engines 7

[PISTO]

Some time ago I was looking for options to inject water to a gasoline engine to improve power and milage and read several of the opinions. I got discouraged because the practicality of it was not there to compensate for the trouble, but I was surprised nobody could explain why the water injected into the combustion air would improve eficiency.

Opinions ranged from no benefit to confirmation that on wet days the engine seems to run better. The point is why?

In my opinion the explanation is that of adiahbatic cooling, that is cooling without heat transfer, the better known exaple of this is to cool air through humidification in hot dry climates.

If you inject water to combustion air the air will cool off thus reducing it volume and therefore increasing the oxhigen concentration, much the same way an afterccoler does.

Does it make sense?

Pisto

 

[GregLocock]

" nobody could explain why the water injected into the combustion air would improve eficiency"

You sure about that?

SAE paper 1999-01-0568 Water injection effects in a single-cylinder CFR engine

Author(s): Lanzafame, R.,

Cheers

Greg Locock

 

[PISTO]

To Greg Locock:

Greg, whay I am sure about is that if to dry hot air you add water the air cools significantly and following the law of gases, if you cool a gas it reduces its volume and if the oxygen content remains the same that means you actually put more oxygen per cubic feet of air to the engine.

I would say then that if you leave in a hot dry area (Las Vegas for instance) your engine would improve eficiency more by injecting water than if you live in Portland where the temperature drop would not be significant, so it depends on the intake air conditions also.

PISTO

 

[patprimmer]

PISTO

I think you are right about the latent heat of vaporisation af the water and volume and climate, however a running engine is a dynamic situation and it gets a bit more complex. ie if you reduce the volume after the air is in the cylinder, and the valves are closed, no more oxygen can enter the closed system. Iwould suspect that a significant amount of the water evaporates on the compression stroke and dureing combustion.

The main benifit of this is to prevent detonation by effectivly reducing peak cylinder pressures before and just after ignition.

This in itself will probably reduce power if no detonation would have occured without the water. It only improves power by allowing a higher compression ratio, or more boost in a forced induction engine.

I think this was covered in great detail in several previous threads

Regards
pat

 

[PISTO]

Pat:

I think the clue is not what happens inside the cylinder but on how much oxygen enters the system and I think more oxygen enters as a result of the reduced volume by cooling the air before entering the chamber. What happens afterwards, inside the cylinder is another system.

I think, for practiacl reasons, what counts is how much benefit you get under normal conditions and how much does it cost in money and troubles to have a system that adds moisture to the air, as for me I decided it is not worthwhile, even though I live in a dry area for eight months a year.

PISTO

 

[REDMANE]

Several different combustion variables are affected by water injection.

1st. charge cooling.

2nd. slowing of flame front speed, improving combustion completion.

3rd. additive effect expansion, when water changes state from a liquid to a gas in the heat of combustion. the ratio of expansion of water from a liquid to a gas(steam) is about 600 to 1.

While any one variable may not be significant; the sum of the three do have a positive impact on engine performance.

REDMANE

 

[PISTO]

REDMANE

Good point, I overlooked the expansion factor of the water in the combustion.

But still the issue is if it would be practical to install a water inyection system on passenger cars for intance or diesel truck engines. The volume of water would need to be controled in proportion to the volume of air either electronically or vacum which is proportional to the air volume in a way.

If somebody knows of a system that is simple, simple to install and inexpensive let me know.

PISTO

 

[patprimmer]

Spearco make an electronic and vacuume controlled unit

Regards
pat

 

[patprimmer]

Read previous threads for details

Regards
pat

 

[turboice]

This paper goes into some of your questions -

http://www.turboice.net/?documents/turbowhitepaper/title.htm

Ed.

 

[patprimmer]

If water evaporates in the manifold, it cools the aie, but also displaces a substantial part of it, so yuo end up with a denser charge containing a lot of water vapour and less oxygen.

To get the benifit of high expansion ratio, you need to turn the water to gas IN THE COMBUSTION CHAMBER.

If you ever had a chance to look at an engine running with a transparent manifold, I am told by those who have, you will be amazed at how much liquid fuel is thrashing around in there. Greg Locock has done this, and is my source of that information.

Water has a much slower evaporation rate than petrol. Just leave a cup full of each in a safe open area and see the difference

You will find some research papers on this site

http://naca.larc.nasa.gov/

It's part of NASA's site and is an archive of research into aircraft technology. Water injection was a topic of interest before and dureing WW11

Regards
pat

 

[turboice]

Correct me if I am wrong - but won't the water only evaporate to 100% RH and then the vapor and liquid water will stay in equilibrium? And as the charge cools the ability to maintain water in vapor becomes increasingly difficult resulting in much of it condensing back into liquid.

Experience and many papers have illustrated that the density of oxygen is not reduced by the injection of the levels of water normally used which are almost always less than 1.5% water to air and frequently less than 1%.

Additionally injection in the combustion chamber does not provide the cooling of the intake valves that provides improved VE.

You will lose the expansion ratio benefit to the extent of water that does vaporizes but only to 100% RH. The liquid and present water in the charge will remain effective.

Also the expansion of the small amounts of water we are talking about are not as significant as the role water plays in the completion of the C0 -> CO2 oxidation which is where substantial amounts of energy is released and which can only effectively be achieved by the presence of water - the more water there is in the mixture the faster this powerful part of the combustion process will occur. BMEP is increased by water injection primarily increasing the rate of this otherwise slow step in combustion.

 

[patprimmer]

That's pretty well my point

Earlier in the thread, it was stated that the power gains from water injection were due to the cooling effect increasing the VE. I stated that most of the improvement came what happened inside the combustion chamber.

This wasn't accepted, so I let it go.

This deal has been covered in great detail in earlier threads.

I can't see a lot of point in repeating it all again.

The real quantifiable benifit of water injection is that it stops detonation in engines where the fuel is inadequate for their compression ratio

The real fringe benifit is that it decokes the engine.

The real dissadvantage is that you might destroy an engine if the water injection system fails

You can also destroy an engine by hydraulic lock if the water ever syphons back into the inlet when the car is parked on a steep slope.

I have never seen any evidence to suggest that a well tuned car leaves a lot of CO after the power stroke is complete, unless the car is deliberately rich to cool the charge and help avoid detonation. Water is certainly the better solution to avoiding detonation, when compared to extremely rich mixture.

You are absolutely correct re the relative humidity thing and temperatures. Little water will evaporate until the temperature is raised substantially, like dureing the later part of the compression stroke, and even more so at the point in the power stroke where the flame really gets going.

Regards
pat

 

[bvicknair]

Remain open to possibilities.

The Graneau-Graneau exploding water experiments show a sudden reduction in H2O-H2O cohesion. No steam is generated.

How many labs are currently studying this phenomena? I know of at least four.

Could an internal combustion engine act as a catalyst for the process?

 

[funnelguy]

Very interesting thread for me. Short anecdote. I was present for a dyno tuning session on a 1.3L 2V race engine. CR was over 13:1. Weber carbs on an IR setup. We had made quite a few pulls when the engine builder running the dyno suddenly shut it down. We asked why and his response was that the engine had suddenly started "making too much power".

We pulled plugs and looked at crowns. #2 cyl was far too clean. Pressurised the coolant and found a trickle in the #2 intake runner. I've been interested in water injection ever since.

 

[CanadaTech]

Correct me if I'm wrong, but I believe that general combustion theory states that the power an engine can produce is proportional to mass flow. Increasing the density of your combustion mixture by adding water should do this.

One may also want to check the expansion rates of H2O vapor vs. air.

Previously discussed theories should also prove true.


Also, I live in a particularily dry climate. Driving in the rain shows marked improvements in both milage (up to 20%) and power.

 

[patprimmer]

How can the density in the cylinder change due to flow, after the inlet valve closes.

The mass flow only normally applies to air, and the oxygen it contains. It may also apply to other oxidising agents such as nitrous oxide, but that is a whole different set of circumstances.

If the water evaporates in the manifold, it cools the air, but also displaces it with water vapour, having a net reduction in air density, consequently also reducing the mass of oxygen in the charge in the manifold.

Actually, very little water evaporates in the inlet tract, or in the cylinder before the valve closes.

Re your fuel economy gains, you probably slow down a little in the rain, even if it feels faster because the car is slideing about and spinning wheels.

The auto company engineers do know about water injection. It was used by engineers in aircraft dureing WW11, and if I remember corectly, there are research papers dating back to WW1. I really can't be bothered repeating the research again, so you will need to do it yourself if you really want to know.

I have already given links to sources of research papers, as has Greglocock and Turboice, if anyone wants to read real data.

This subject has been covered extensively in previous threads.

I for one will not further waste my time trying to explain simple facts again and again to those who do not look up the previous posts or threads, or links.



Regards
pat

 

[EricFahlgren]

Pat wrote: If the water evaporates in the manifold, it cools the air, but also displaces it with water vapour, having a net reduction in air density...

Well, I always wondered if that were true so I did the math. It turns out that the net effect is an increase in O2 density, albeit quite small for NA charge temperatures (and, of course, taking into account various assumptions). Here's a link to my results:

http://not2fast.wryday.com/thermo/water_injection/opt_mass.shtml

 

[SBBlue]

Another consideration:

It requires less work to compress cool air than hot air.

I did the calculations comparing the amount of work it takes to do a 10:1 compression on air at 70 deg F and 110 deg F.

At 70 deg F: 88.7 BTU
At 110 deg F: 94.9 BTU

For an engine running 20 pounds of air a minute (roughly the amount of air a 5.9 liter "Ram Diesel" with a turbocharged engine runs at 1800 rpm), the horsepower the engine uses compressing air during the compression stroke is as follows:

At 70 deg F: 88.7 hp
At 110 deg F: 94.9 hp

Using some engine simulation software I compared the engine brake cylinder efficiency at the two temperatures. At 70 deg F the efficiency was 43.27%; at 110 deg F it was 42.65%. For a diesel engine flowing at 20 lbs/min this would work out to about 2.5 hp.

It appears to me that another effect of water injection may be to decrease the charge temperature and therefore decrease the amount of work the engine has to do during the compression stroke.

 

[turboice]

SBBlue - that is an excellent insight - I am going to need to add that to my write up on WI.