FSAEracer03

That's right. I wasn't trying to imply that the intake charge is cooled in the intake ports or runners but in the chamber through latent heat vaporization and through a lower A/F. As A/F increases, combustion temps rise. Because the intake temps of a turbo motor will almost always be greater than an N/A, the A/F can compensate to bring combustion temperature down to where it likes to be for peak power.

Same here. I'm wondering if we're talking about the same study.

Here's the website:
http://www.tvu.com/CombCompleteweb.htm

And here's a sample series of photos:

Bengt Sweden

I don't think this is correct. I was taught that the water interferes and slows down the combustion process. Thereby you do not get as high peak pressure/temperature. On the other hand, more energy remains in the gas and gives higher exhaust temperatures. Therefore, it can actually be potentially dangerous to use water injection without advancing the ignition.

Bengt

FSAEracer03

Chris was dead on with that. The benefit is that you can run higher boost levels and therefore increase bmep and torque. How? Increased efficiency. The water injected in has a low temperature compared to the chamber. Trying to come into equilibrium with the system, the water droplets heat up and vaporize. The enthalpy of the process increases in the case of the water, and decreases in the case of the combustion chamber. When enthalpy goes down in a system, all things equal, the temperature drops. Lower cylinder temperature means a higher knock limit. Not only does the shear increase in combustion efficiency give you a little extra power (and lowers your bsfc and emissions), it allows you to run higher boost pressures.

Bengt Sweden

Did you pass?

Of course temperature drops due to the waters heat absorbtion. I just wanted to point at that it may not be the major contributor to lowering peak temperatures. This of course depends on the amount of water. The more water the more important the heat absorbtion part.

Bengt

FSAEracer03

Hahaha, yeah I passed. Clever... you're quick on wits, Bengt

Actually, I think if anything, you have that reversed. If you have too much water spray, the heat absorbsion per droplet goes down. At a certain point, and I'm not sure how much water this would take, the water droplets won't vaporize and then you still have water droplets in your combustion chamber about to be ignited with the air-fuel charge.

I would go so far to say, in fact, that a properly operating water injection system lowers temperatures solely by latent heat vaporization. It's such a fast and entire process that the amount of temperature drop from the water is encompassed by it. Furthermore, you don't want to slow down the burning process. You want to speed it up and get the most turbulent airflow in the chamber as possible when the valves are closed and compression is taking place. One of the problems of current I.C.E. combustion chambers is that the laminar flame speed is only about 1 m/s... crunch some numbers and figure out how many degrees at 6000rpm that takes. We want to minimize the degrees of combustion.

If no one knows already, we try to get laminar airflow from air filter to valves. Past that point we want the most turbulent flow possible to get the best flame response to ignition. Better response equals more flame front travel. More flame front travel equals faster combustion... and that, my friend, is what we need. Tweak the timing and spark to get the most complete burn ending exactly at exhaust valve opening. Tweak the fuel delivery with piston and head geometry to make compression flow turbulent to get the most efficient and fastest burn. You do that and you waste nothing and get the most power out of your fuel.

Does it always happen like that? Hell no. But that's the theory behind it all.

Bengt Sweden

You are missing the point.

The question I raised was, which of the physical and the chemical reaction is the main contributor to the drop in peak temperatures.

Are you claiming that there is no chemical interference at all? How do you know?

You are correct that you don't want to slow down the combustion process, that's why water injection seldom is a good idea.

There is no efficiency gain, there is an efficiency loss with water injection.

Bengt

Bengt Sweden

Think of it as dillution.
HC and O2, trying to find each other, will bounce into inert H2O delaying a reaction.

Chris White

True – the purpose of water injection is to allow for higher boost levels than possible with out injection. So technically it does not increase efficiency and I would even agree that there is a slight loss if all other parameters being equal.

On the other hand….it is fun to tell folks that you have a steam engine….

Chris White

macnewma

Or it allows you to run the same boost with a higher margin of error. There is something to be said for being conservative.

Max

Bengt Sweden

Unfortunately the risk of error increases when adding water injection. So many things can go wrong.

macnewma

Bengt, what things are you referring to? If you are not reliant upon the WI to run obscene boost, the worst thing that can happen is that it can stop working, right?

Max

Bengt Sweden

...or that you fill up with a lot of water.
Also you need to increase ignition advance to not get too hot exhaust gas.
Then you must have a control system that retards if you run out of water to not induce knock.
You don't want it to drip after shut down either for risk of corrosion.

It can work well if well designed, but risks increases.

IMHO there are better ways to optimise an engine.

macnewma

Understandable. I think you can significantly minimize those risks though.

About the timing advance, is this necessary if, all things being equal including boost, you switch from no WI to using WI? Why would the EGTs get higher?

Max

Bengt Sweden

The EGT get's higher because of the slower combustion. This results in that the heat energy isn't properly transformed to mechanical power and you can actually have combustion still happening when the exhaust valve opens, which will fry the valve.
The turbine is also at risk. I am not sure about the 951 with such a distance to the turbine but other turbo cars can easily damage the turbine.
If it gets very hot you fry it directly. If it is just on the borderline, the wings will creep and deform reducing efficiency. This is what limits the life cycle of a gas turbine.
It depends of course on how hard you drive the car if it is at risk.

I have run a Volvo Turbo with water injection in a dyno for 5 hours a day for several months at full power and could actually see the temperature difference when changing amount of water just by looking at the colour of the glow of the turbine. We had 900 C as safety limit.

Bengt

FSAEracer03

I guess I did misunderstand you... because that's not what I was saying whatsoever.


I would argue the opposite side on that one. Dyno tests done in the development of the Heibar water injection system show that emissions were drastically reduced with water injection showing a more complete burn. NOx were down, hydrocarbons down, CO down... if it's making the burn better and more complete, how is that not more efficient?

Also, the whole point to water injection is dropping temperatures, correct? If that's so, density is an inverse function of temperature, so with water injection temp drops comes an increased volumetric efficiency.