Injecting prior to turbo comp' impellers

[Richard L]

I am reading and learning. Love to have some practicle examples and results.

[hotrod]

Just thought I would try and post some images of the turbo I just pulled out. It took a while to figure out how to get descent resolution images.

Richard I will send them to you by email if you could post them up as I have no place to host them.

Keep in mind a couple things:
1. On casual examination the impeller looks just fine, its only when you look close that you see a bit of errosion at the outer impeller tips.

2. I suspect I was over spraying (3%) mass air flow, and I also experimented with spraying at high idle speeds (very low throttle setting). I discovered that this would act to cool the intercooler prior to drag racing.

(note to self, very good idea for post turbocharger pre-intercooler injection point . If you could manually trigger this pre-intercooler spray prior to drag racing, or Autox event all the evaporation takes place in the intercooler and quickly cools the structure of the intercooler. Due to the engines air flow you completely eliminate any need for external fans to cause cooling of a heat soaked intercooler while the car is stationary)

I suspect during these experiments I may have had some liquid water streaming down the walls of the turbo inlet pipe. I believe, this could be a possible cause of the erosion at the compressor impeller tips.

IF that proves out to be the case, it might be useful to put a trap of some sort in the inlet tube that would re-suspend any surface water in the air flow. This could be in the form of a ridge that would stop the droplets that tend to run down the surface of the inlet tube and cause them to be ripped apart by the high speed airflow as they try to climb the lip.

3. Last but not least, it is important to note that I also made about 6 drag strip runs with this turbo using no air filter of any kind, to determine the intake air restriction the filter creates, and its effect on turbo boost. This was through a cold air intake system that had the intake in front of the right front tire and directly behind the front bumper. At the strip I run at there is a section of gravel/dirt you must drive over as you pull off the strip. I know I sucked up some dirt on at least one of those events. Shortly after that experiment, I pulled the discharge off the turbocharger and found small amounts of aluminum dust in the turbo to intercooler tube and some very fine dust. I wiped this off and it did not return after that. If you look closely at one of my pictures you will see a clear "meteor crater" impact about a quarter mm in dia. on the leading edge of one impeller blade from a piece of grit.

My personal judgement is most of the errosion is due to these no air filter tests, and the effects of the water were minimal but I cannot prove that case. After I run my current turbo for a comparable period of time I will pull the intake tube and check for similar wear without having ever made any filter free passes.


Larry

[Richard L]

Here are the pictures of larry's turbo. Picture restored - lost and found.

[SaabTuner]

Old Saab99 Turbos had a pre-turbo injection system factory stock. It was common after 100K miles or so to see some pitting in the compressor blade. But at that point, the turbo was going to be replaced anyway. I've never heard of it causing any problems.

Adrian~

[hotrod]

Thanks Richard!

Saab tuner
It was also used on 10's of thousands of military aircraft without serious problems.

There is however an urban legend sort of belief in the performance community accompanied with horror stories of "some kid" etc. etc. that ruined his turbo in a matter of minutes after trying a pre-turbo setup.

The main thing I wanted to demonstrate is that it does not cause immediate catastrophic damage. In my case I considered this turbo to be expendable as I already had its replacment on the floor beside my desk. After seeing the condition of the compressor and considering what I put this puppy through, I had not qualms about using pre-turbo WI on my new setup.

Larry

[SaabTuner]

Like in those NACA reports.

I just wanted to point out that it was used on automobiles, which are usually at least perceived to have longer service intervals. (Whether they actually do or not.) Of course, it was used on Saabs, which are usually perceived as having short service intervals and low reliability.

http://www.saabcentral.com/trivia/mileage.php

Oh well. :P

Pre-turbo injection is certainly much much cheaper. When I work out a WI system, it's going to be multi-port, and I'm going to do my best to get it CARB Exempted. Otherwise I'll fail emissions. (Some people pass, but not if you end up at the notorous "test only" stations. Then you're toast. I have quite a bit of experience with that ... dammit.)

Adrian~

[Greenv8s]

Originally Posted by bill Shurvinton

Quickly. Normal compression is adiabatic, which to a first order means that as the gas is compressed, it gets hot. Now this heat is one of the biggest problems with forced induction, for 2 reasons. Firstly you have to get rid of the heat, and secondly you need to take power out the turbine shaft to perform the heating.( heat is work and work is heat).

Now with the right level of water injection, the heat is removed before it builds up, pushing the compression closer to isothermal (not all the way, but closer). In round terms this is about 30% more efficient (less exhaust gas required for the same boost, or more boost at the same exhaust flow).

Still trying to get my head round the thermodynamics, but its a bit of a puzzle. The evaporative cooling takes heat out of the air and avoids the air gaining temperature and pressure. At first glance it is obvious that this means there is less energy being put into the air, but isn't it just going into the water instead? Or have I missed the point? I guess that the outlet pressure would be similar whether the charge was cooled during compression or later in an intercooler, so reduced temperature implies reduced volume, speed and KE. Is this where the gains come from?

[b_boy]

The heat energy (kinetic) is being converted into entropic energy (liquid to gas transition--the heat of evaporation (heat needed to disassociate H2O molecules)).

Ideally at the moment the adiabatic heat is generated, it is absorbed by the water evaporation. Compression occurs with less heat imparted to the compressed air.

If liquid water traverses the impeller wheel without evaporating, we are no better off injecting before the turbo as after.

Now the water droplet will spend very little time traversing the impeller wheel. We what it to completely evaporate during its journey. Hence, the dialogue about droplet size. We can calculate this time frame with some precision, but the time to evaporate a droplet is a difficult one. Pressure, temperture, velocity, increasing surface to volume ratio, and methanol mixture, will make it a multivariable calculation, one best left to empircal determination.

Aquamist at 147 psi is our best bet for small droplet size. Smaller droplets can be made, but the pressure and nozzle sophistication to make them is untenable in automobiles with current technology. I know of no sensor that could easily be rigged to detect droplet exit from the compressor wheel (a spectrophotometer could work, but we are talking at least a $1000).

Hotrod has kindly displayed his turbo impeller after 7000 miles at 2.7% water to air mass. It shows some notable erosion at the edge (with extenuating circumstance). The erosion can be viewed as a cost of implementing the impeller injection, or we can wait for creation of a nozzle that expels smaller droplets. By injecting less water than hotrod, and only at high RPM and boost, I hope to reduce the erosion substantially (using a solenoid to inject only under the chosen parameters). None the less I expect some erosion to occur. If a new turbo is needed every 20,000 mi, that is a cost of running such a system. It remains to be seen if the benefit outweighs the cost. If a 10% increase in airflow can be achieved at high RPM, it would be worth about 40 hp; that is worth it to me.

[JohnA]

Originally Posted by hotrod

... it is important to note that I also made about 6 drag strip runs with this turbo using no air filter of any kind, to determine the intake air restriction the filter creates, and its effect on turbo boost. ...

When I was careless (sorry, experimental!) enough to run my turbo (bike) without airfilter, that's the sort of wear I saw on the compressor blades.
Didn't take long to happen either... :roll:

[hotrod]

Thanks, that backs up my circumstantial evidence and theory that that damage was from the "dirt experiment".

Really T'd me off at the time, I forgot there was a segment of dirt and gravel at the end of the strip where you turn off to pick up your timing slip. Lots of folks cut the corner short and thow dirt and sand up on the paved portion of the turn out. I took the turn out at about 30 mph and was in the crud before I realized it. My air intake sits right behind the front air dam and I figured the air dam passing close over the surface of the sandy pavement suspended a lot of crap in the air.

Like you said it only takes a single event. After that first pass, I took the turn off road very wide and slow and saw no more evidence of sucking dirt.

Interesting side note is that the damage is confined to the extreme outer blade tips, probably a combination of the swirl that develops ahead of the compressor inlet centrifuging the dust to the outside of the tube and the higher relative velocity of the blade tips vs the portion of the blades closer to the hub.

There is one guy that posts on another form that says he's run a pre-compressor injection for years with no sign of compressor damage.

Larry