#21
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You guys make my head hurt!
Observations from a mechanic that drag races are that injecting prior to the turbo produces positive effects in every instance. The ones I've seen that say injecting prior to the turbo creats pitting, are also the ones that run their cars with volocity stacks and NO air filter. Kent
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wrx wrencher |
#22
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I'm into trying this out. I've now read what I can on the pre-compressor experiments and it looks like if the injection is not continuous and droplet size is as small as possible, you'll have minimal wear on the impeller wheel.
I'll have the Element Hyday in my STI in about a week or two. I can program it to open a valve at 5000 or 6000 RPM at 15 psi of boost and let the Aquamist take over from there. From my reading on "wet compression" or "fogging" as decribed in the turbine engine lingo it looks like 2-3% water(alcohol etc):air is about all you need or want to shift toward isothermal compression. Any thoughts out there on how much to inject? I'm primarily interested in try to "stretch" out the compressor map at high RPM/boost when my stock turbo is starting to wheeze. How about droplet size from an Aquamist jet? Hotrod mentioned 10 uM. That is pretty small on the absolute edge of visual perception of a single dot. The smallest cells are 10 uM and you can only see them if you know what to look for. Some sort of atomizer like is used in "cold" humidifiers might be a better solution. They use a small apeture combined with some sort of high frequency osillation to break up the water. Or maybe an ultra sonic tip of some kind. |
#23
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drop size
The 10 micron drop size would be ideal, but appears to not really be necessary in a practical real world system.
Most common spray mist nozzles can achieve sprays with drop sizes down near 50 microns, plus you have evaporation that takes place as the mist moves down the induction path, which reduces the maximum drop size. I just finished a turbocharger swap, and I took a good look at the compressor impeller on my old turbo which I had pre-compressor injection on for several months at about the 3% rate. (4 GPH nozzle max rated air flow for the compressor at sea level is about 25 lbs/min. Here at altitude of 5800 ft it is probably about 20 lbs/min.) If the nozzle was spraying at its rated 4 GPH that would be about 252 cc/min or about .55 lb/min which works out to 2.7% of air mass at max flow. My turn on point is 8 psi so at the turn-on air flow, the mass fraction would be higher, probably near 5%. On casual examination you could see no evidence of compressor blade errosion, just the normal discoloration you see after the turbo has been in use for some 37 thousand miles. On very careful examination under high magnification (about 10x), you can just see a small bit of roughness on the outermost 1 - 2 mm of the compressor impeller blades at the very leading edge. I will need to do a similar examination of another turbo of the same design that was never exposed to water injection to determine if this is normal wear. This engine (like most Subaru WRX engines) has a crankcase breather inlet in the inlet tract a few inches ahead of the turbo inlet. Under high boost, these engines can blow noticiable amounts of oil mist into the intake. This over time results in a build up of a very thin layer of oil and dust "crud" on the inside of the inlet pipe and I'm sure that from time to time bits of this crud, and small oil dropplets gets suspended into the intake air stream under high air flow conditions. I also on 2 occasions ran this engine with no air filter for a couple of drag strip passes to determine how much influence air filter resistance had on performance, so there was most likely a small amount of dust ingestion from those experiments. Lastly when I first fabricated the system I assembled the prototype with a 35 psi pump which would not have given best misting behavior by the nozzles. The last couple months was with the system running at 100 psi max pump pressure which should have given a much finer mist. Larry |
#24
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Thanks for the data.
I'm thinking I will inject about 6 inches after the MAF sensor, the water will travel about another foot to the turbo. I'll look into getting a couple of thermocouples to measure temp post turbo and post intercooler. I'll have to rely on data logging to look at pressure change rates and increases +/- pre-turbo WI. I shoot for 2-3% injection rate and see how it goes. I'll have the pre throttle body injection going too. By the end of month I should have some experience and data. |
#25
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Excellent thread, the most compelling read on a forum ive had in ages.
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#26
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This is my pre-turbo injection nozzle, half way between air mass meter and compressor.
One thing I can say for certain is that the (aquamist) pump is louder now that it runs both nozzles. |
#27
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Aquamist 2d installed on 2 ltr engine tuned to 250 - 260 bhp @ flywheel.
Two questions, Ive decided to inject at the throttle body and before the turbo, what size jets will i need ? Intercooler becomes interheater ! Ive had the stock intercooler replaced with a larger one, im a bit nervous of having no intercooler, what about when pressure is below the injection trigger level (9 psi) or if the water injection fails ? I still have the stock intercooler, which i believe has a lower pressure drop. |
#28
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I would not worry about the intercooler becoming a heater just yet. Prove it's heating and then worry about it.
2-3% of air mass injection pre-turbo is just an educated guess. I read some turbine engine papers that indicated 2% was best and hotrod figured he was injecting 2.7%. Turbine engines are not the best model for our little turbos, much higher compression, much larger surface area. I'd love more input on what percentage to inject and droplet size if anyone has these data. Maybe Richard knows some mavericks. 2-3% is alot to inject. Generally WI injection is expressed as percentage of fuel injection 2.5% of air mass would correspond to 25% at 10:1 AFR, or 33% at an AFR of 12.5:1. That is a lot of water to move through the turbo, more than my pump and reservoir can flow. I will start with something more modest. I have two 0.5 mm jets at the throttle body, shunting to a third 0.5, 0.4, or 0.3 mm will give me a range of about 125 cc/min, 90 cc/min, and 60 cc/min, respectively. At its highest, that corresponds to about 0.5% water:air mass with 1% water:air mass left over for injection at the throttle body. So, I would be injecting only about 20% of the amount that hotrod injects. That may be too little, but I'm not sure anyone knows. I can monitor pressure and temperature post-turbo and see if temp continues to drop and pressure increase with added water. Commentary on this set up is most welcome. |
#29
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Another up-shot to injecting prior to the compressor is for those people who run greater than 30 psi of boost. Above Pressure Ratios of 3:1 the air-charge is heated enough at the aluminum compressor begins to weaken from the heat.
Holset has been working on their cast Titanium compressors to solve that problem, but water injection might also help to keep the compressor wheel sufficiently cool. Just a thought. It's probably already been discussed anyway. Here's Holset's page on "high pressure ratio compressors": http://www.holset.co.uk/files/2_5_1_...compressor.php Adrian~ |
#30
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Inlet tract Simulator
I found this link to a simulator from JohnA's web site, it has some features that simulate water injection before the TC or IC and after the IC.
I thought it would be interesting if i used the data i had from a RR to see the effects of WI. I entered the variables to the best of my ability, the models behavour is quite realistic. RR 230 bhp / 222 lbft @ 5371 on a cold March morning. I can only guess at VE. The link below has all the data saved to it, which means you can see what i mean when you change the WI settings: [admin - have to make the link's font smaller to enable the page wrap to function] http://not2fast.wryday.com/turbo/glo...orDutyCycle=85 Is interesting to note that a the model show that injecting 400cc before the TC, which is about 3.5% of the air liquid ratio, shows that the IC now only cools the charge by 1 C. The IC is at the threshold of becoming an Inter heater. Injecting 320 cc, which is about 2.9% of the air liquid ratio, (and the limit of the pump with 1 injector) shows the IC still being useful cooling from 31 C to 19 C. Probably shouldnt read to much in to this, but it would indicate that to inject at the manifold, a seperate pump would be desirable, in which case will the FIA 2 run two pumps and HSV's ? I remember that the MF2 can ! Your views.... |
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