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Author Topic: For and against water injection  (Read 19648 times)
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Blue
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« on: June 28, 2008, 08:39:25 PM »

OK, I appreciate that there is a lot of history behind water injection making power and keeping motors from blowing up at high power densities.

That said, what the water really does is cool the incoming charge, retard the burn rate in the combustion chamber to reduce pre-ignition, and provide an additional quantity of working gas for the engine to expand and produce pressure.  In any installation and technique listed here, we need to separate these three effects.

I could go into a long explanation of the physics and my own experience with large bore engines, but it would be better to have you guys talk about your own experience within these three categories. 

Specifically, cooling the intake charge without affecting anything else has advantages and effects equivalent to running on a cooler day.  Retarding burn rate to reduce pre-ignition (as opposed to detonation) is a separate effect that requires a higher amount of water.  Reducing pre-ignition through TBC's, smoothing sharp edges, or other chamber effects has to be compared to water injection without the penalties of injection.  Last, the best use of water is to absorb excess heat where we know we simply have too much heat in the combustion chamber and need to stuff some more inert mass in there (beyond the nitrogen in the atmosphere) to absorb it and push the piston down instead of melting it.

All of this has to be understood independent of the addition of methanol (a fuel) and all of its issues.

LSR has lots of good information, and lots of mythology on water injection.  To understand when and how much water to inject, we have to separate all these effects and turn the mythology into fact.
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« Reply #1 on: June 28, 2008, 11:41:36 PM »

OK, I appreciate that there is a lot of history behind water injection making power and keeping motors from blowing up at high power densities.

That said, what the water really does is cool the incoming charge, retard the burn rate in the combustion chamber to reduce pre-ignition, and provide an additional quantity of working gas for the engine to expand and produce pressure.  In any installation and technique listed here, we need to separate these three effects.

I could go into a long explanation of the physics and my own experience with large bore engines, but it would be better to have you guys talk about your own experience within these three categories. 

Specifically, cooling the intake charge without affecting anything else has advantages and effects equivalent to running on a cooler day.  Retarding burn rate to reduce pre-ignition (as opposed to detonation) is a separate effect that requires a higher amount of water.  Reducing pre-ignition through TBC's, smoothing sharp edges, or other chamber effects has to be compared to water injection without the penalties of injection.  Last, the best use of water is to absorb excess heat where we know we simply have too much heat in the combustion chamber and need to stuff some more inert mass in there (beyond the nitrogen in the atmosphere) to absorb it and push the piston down instead of melting it.

All of this has to be understood independent of the addition of methanol (a fuel) and all of its issues.

LSR has lots of good information, and lots of mythology on water injection.  To understand when and how much water to inject, we have to separate all these effects and turn the mythology into fact.


Eric,

I and many others find your knowledge and information sharing to be among the best, I find that often when I cannot understand what a person posts the first time I read it is due to one 1 of 3 reasons.

1. Words and meanings are often misspelled so badly that you cannot take the writer seriously.
2. The writer used big words/nomenclature to try to "impress" the readers with their command of the English language.
3. The writer is a very knowledgeable person and is willing to assist/dumb it down to a level that is more understandable to the masses and I thank you for that as I do not desire spending time to look up meanings/terminology to decipher what the writer is trying to convey.

Thank you again,

John
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« Reply #2 on: June 28, 2008, 11:58:05 PM »

Noonan, I'm definitley for "dumbing it down".
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« Reply #3 on: June 29, 2008, 12:03:21 AM »

Noonan, I'm definitley for "dumbing it down".

Amen.  grin
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« Reply #4 on: June 30, 2008, 02:31:22 AM »

   Blue & Others
 We have used water injection in the 222 car for years. As i have posted for before [ and am sure you know] if it was't for water injection in ww2 aero engines, we would be speaking German. From information in my aero books i learned they were injecting about 40% water-methanol to the fuel rate. I have since read up to 100% in some cases. We used 20% because their water is 5o%methanol. Janes aircraft books 0n ww2 engines has specs.on most engines, one spec is were they picked up 8% hp with water injection alone. Water injection or anti detonation injection [adi] allowed them to increase boost pressures and make more power.Most racers don't know the history of these great engines if they picked up one these books they would be impressed and humbled.
   I have a graph that shows how Ricardo picked up brake mean effective power by increasing water injection and boost from the maximum of 237 bmep at the riches setting max power level, to 290 bmep which was the limit of the dyno.
 Hope to have bvillrcr take a photo and post it here.
 
                                  JL222                                 
                                       
   PS  Ricardo also was able to lean the engine back to less than the fuel rate at full rich and maximum cylinder pressures fell and the gross heat flow to the water system fell to the same level at 170 bmep without water injection.
 
« Last Edit: July 01, 2008, 12:20:16 AM by jl222 » Logged
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« Reply #5 on: June 06, 2010, 06:45:37 AM »

Gee...this is an old one, but interesting never the less.

So let me see if i understand:

Water injection has 3 ways to help

    1 cooling the charge to the cylinder -  making it denser therefore allowing better filling of the combustion chamber.

    2  retarding the burn rate - which seems to be the same as increasing the octane rating of the fuel.

    3  absorb excess heat - ?

I understand number 1. As water is vaporized it absorbs heat in order to change from liquid to gas (swamp cooler effect) . this cools the charge making it denser.

Can anyone explain 2 and 3.
What is the mechanism that is slowing the burn rate? 
3 seems to be suggesting that by replacing some of the combustion charge in the cylinder with inert gas the heat will be reduced. But won't power be reduced as well? (this is what an EGR valve dose)
 
The reason I ask is I am planing my assault with my 1980 Chevy monza, which at the moment  runs a B&M 144 roots type blower on a small block 350ci engine. It is a street car and uses pump gas @ 6lbs of boost. I am going to convert it to a race only car and want to up the boost to the max possible (of course).
« Last Edit: June 06, 2010, 07:05:46 AM by Fheckro » Logged

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« Reply #6 on: June 06, 2010, 01:19:14 PM »


  Fheckro...seems like #1 is acomplishing #2. As far as reducing flame speed I'm not 100% sure but I've read that the higher
the temp the faster the reaction [ this was not about engines ] but it seems it would apply.
 Ricardo tested the results of water injection and steam seperatly and there were benefits to both.
 We just installed a new water injection system on the 222 car [Alcohol Injection Systems] and while talking to the owner [salesman] he mentioned a turbo car that was streching the valves at Bville, same problem we were having and hope to solve from water injection + get all the other benefits
 Their also installing a system on a pro-modified nitros car to slow down the flame speed.
 Water injection would be a good way to cool the intake on your roots blower. My charts show 189 deg. at 6lbs at 45% efficiency, 153 deg at 65% ...286 deg and 220 deg at 12 lbs these temps are on a 70 deg day.

                 JL222

                         
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« Reply #7 on: June 06, 2010, 03:58:15 PM »

I've asked in another thread if water injection is helpful for a normally aspirated engine, but got no response.  Anybody care to chime in, and if so, what would be a target ratio of air/fuel : water?

I've read where water injection can cost power because the cylinder volume has less air/fuel mixture.
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« Reply #8 on: June 06, 2010, 06:15:13 PM »

JL,
"My charts show 189 deg. at 6lbs at 45% efficiency, 153 deg at 65% ...286 deg and 220 deg at 12 lbs these temps are on a 70 deg day."

Can I get a copy of those charts?

I have no idea what max boost will be but I'm thinking it would be somewhere around 20. 

Thanks

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« Reply #9 on: June 06, 2010, 10:34:33 PM »

If you use Gulf Coast water, you will probably be too rich.

Will
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« Reply #10 on: June 06, 2010, 11:23:29 PM »

JL,
"My charts show 189 deg. at 6lbs at 45% efficiency, 153 deg at 65% ...286 deg and 220 deg at 12 lbs these temps are on a 70 deg day."

Can I get a copy of those charts?

Thanks



This might help you.

Tom G.

http://www.stealth316.com/2-adiabat1.htm

http://www.stealth316.com/2-turbotemp.htm
« Last Edit: June 06, 2010, 11:26:55 PM by desotoman » Logged

Asking questions is one's only way of getting answers. As a young boy I was always taught that there is no such thing as a stupid question. It suggests that the quest for knowledge includes failure, and that just because one person may know less than others they should not be afraid to ask rather than pretend they already know. In many cases multiple people may not know but are too afraid to ask the "stupid question"; the one who asks the question may in fact be doing a service to those around them.
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« Reply #11 on: June 07, 2010, 01:15:31 AM »

JL,
"My charts show 189 deg. at 6lbs at 45% efficiency, 153 deg at 65% ...286 deg and 220 deg at 12 lbs these temps are on a 70 deg day."

Can I get a copy of those charts?

I have no idea what max boost will be but I'm thinking it would be somewhere around 20. 

Thanks



  Sent personal message. Looks like your a little new to this so look for [my messages] that's you in the blue bar were
[ home is], to sent a message to me, click on the pm icon above ignore on my post, or reply through pm.

  20lbs is 394 deg at 45% efficiency 294 deg at 65% and 264 deg at 75% and all the numbers are figured at 14.7 atm.

      JL222

     
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« Reply #12 on: June 09, 2010, 03:31:45 PM »

got it , thanks

Tom,
"Compressors usually operate in the 55-75% efficiency range" <----in context with paragraph about turbo chargers.
do you know what the efficiency range of roots style blowers is?

Fred
« Last Edit: June 09, 2010, 03:52:33 PM by Fheckro » Logged

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« Reply #13 on: June 09, 2010, 10:33:32 PM »

Fred,

Don't take this to the bank, but as I recall standard GMC type 6-71 blowers are very efficient at low blower speeds. It is when you start to speed them up to make more boost, say over 15 lbs. their efficiency starts to rapidly drop, and they start creating lots of heat.

It has been years since I stayed up to date on GMC style superchargers. Nowadays you can get Hi Helix rotors installed in them, different configuration openings in the case, both top and bottom, etc. I am sure the latest super trick Roots style blower is much more efficient, than the old style GMC takeoffs.

If someone else has better information feel free to jump in and correct me.

Tom G.   
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« Reply #14 on: June 09, 2010, 11:32:13 PM »

I think some/most of the inefficiency of a low helix roots blowers is the air rushing in as the lobes separate at the top and rushig out as the lobes mesh at the bottom . Roots and screw blowers work on the same principle . High helix is a compromise between low helix inefficiency and screw blowers size and need for high rpm .
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