Injecting prior to turbo comp' impellers

[cheekychimp]

Theory:-

i) By using a turbo exhaust housing with a smaller AR we can help a large turbo spool faster, but the trade off is that the housing is unlikely to be able to flow enough air in the upper rpms to be able to realize the full potential of the compressor.

ii) By using pre-turbo water injection to move compression away from adiabatic towards isothermic we improve compressor efficiency enabling it to flow more air.

Question:-

If we reach a point whereby the exhaust housing is already creating a restriction to airflow, will pre-turbo injection

a) Have NO noticeable effect because the housing is unable to support any further increase in flow?; OR

b) Make the air denser thereby allowing more air to pass through the housing?

[cheekychimp]

I would also like to get hold of a table to give me an idea of what sort of volume of flow I can expect from running an air atomizer nozzle with 150 psi of liquid pressure and 400 psi of air/gas pressure.

I'm concerned that liquid flow will be dramatically reduced as a result of the very high pressure. I looked at the nozzles Darren posted and at 60 psi of liquid pressure, flow was reduced from 2.28 gph at 50 psi of air pressure to 1.11 gph at 70 psi.

[JohnA]

Precompressor injection sees no boost because it takes place always under mild vacuum (before the compressors obviously!)
So the pump not flowing enough is not really a problem.
Good atomisation and aiming at the eye of the compressor are more important considerations.

[cheekychimp]

Okay, so that might still work then. What about my post above. The consensus previously seemed to be that this was only really beneficial for a compressor that was being maxed out and was unable to flow any more air. What about the situation I described where the restriction is the size of the exhaust housing. Will making the air denser allow me to push more through a smaller aperture. In theory it seems logical that it would but I'm wondering if the end result will be as effective as on a compressor that is reaching the end of it's efficiency threshold.

The thing is that whilst I understand that one of the main reasons that this works on smaller compressors is because they start to generate so much heat when they move out of their efficiency island, it also seems highly likely that where the exhaust housing is 'too' small, similar problems of heat and friction are going to occur where the big turbo is forcing more air into the housing that can pass the restriction. I'm sure that cooling the charge air and making it denser would have an affect here, I'm just not sure how much.

If anyone can spread some info here I'd be very grateful. I'm intending to use a Holset turbo on my application because of the turbo's durability and it's reputation for spooling quickly. The main criticism that is levelled against the turbo with the smaller exhaust housing however is that it loses between 50-100hp of it's full potential at the upper rpm range as a trade off for the very fast spool up of such a large turbo.

The possibilty of having a 65 lb/min turbo reaching 22 psi by 3500 rpms or sooner and making 600 hp is pretty exciting.

[JohnA]

Originally Posted by cheekychimp

.. What about the situation I described where the restriction is the size of the exhaust housing. Will making the air denser allow me to push more through a smaller aperture. In theory it seems logical that it would but I'm wondering if the end result will be as effective as on a compressor that is reaching the end of it's efficiency threshold.

We'd need good experimental data for a this.
I've not come across any.

The possibilty of having a 65 lb/min turbo reaching 22 psi by 3500 rpms or sooner and making 600 hp is pretty exciting.

Indeed :smile:

[hotrod]

Based on my experience I would "guess" that the answer is it may help some.
It makes the compressor more effecient (it takes less work to reach a given flow)
That implies that there will be less resistance for the turbine to overcome to reach a given flow so it should spool a bit quicker.

I know in my setup when the pre-compressor injection was working the turbo seemed to hit very hard when it turned on.
Sooooo the seat of the pants dyno says it might work but only some before and after testing would give you solid data to work with.

If I remember the NACA studies correctly at 3% water to air flow by weight the compressor flow increased about 10% over its normal flow, so I would think it would not be unreasonable to think the compressor should come on boost 10% quicker. If you hit full boost at 3500 rpm without it then perhaps you would get to full boost at 3200 with the injection.

Only my gut reaction but it sounds reasonable to me.

Larry

[cheekychimp]

That's very interesting, but that brings two things to mind;

1) My initial reason for doing this was to see if I could gain any of the power back at the top end that this turbo allegedly loses from using a smaller exhaust housing. The basic problem is that whilst the turbo flows 65 lb/min at it's peak, the exhaust housing might flow lets say only 52-55 lb/min. So I'm just not sure if using water injection is going to bring the 55 lb/min threshold on earlier giving me less rpms in my powerband or if the air being denser means the smaller exhaust housing my be able to flow more than 55 lb/min if the air is denser and therefore more condensed/compressed. Surely it stands to reason that 55 lbs of heated air is going to take up more space than 55 lbs of denser, saturated, cooler air?

2) Bringing spool on quicker is something I hadn't considered but if I were to use this method, I had not initially intended having the water injection come on so early. If it was boost activated the activation point would have to be very low, perhaps 5-7 psi or perhaps rpm based. Making it rpm based would however not take into account different loads and airflow in different gears. I wonder if an airflow based activation point run off the MAS/MAF would be worthwhile?

I also intended to run the water in a mixer nozzle with propane injection. I'm not sure if injecting propane at such low rpms is a good idea in which case I'd need to consider a liquid only system or maybe an air based pump.

[hotrod]

For top end mass flow through the hot side your best bet is to try to keep the exhaust gas as hot as possible to bump up the choke flow point as high as you can. That might mean leaning things out a bit, or playing with ignition timing.

It's really hard to speculate without any hard data. I triggered my pre-compressor injection strictly off of manifold pressure.
On my second generation setup I was going to use both an RPM window switch and the MAP to do it so I was sure mixture speed in the inlet tract was maxed out before I started the pre-compressor injection.

If you have a simple means to do it, a mass air flow sensor input would probably be ideal as it would self adjust for rpm and boost and all those other variables.

Larry

[cheekychimp]

Alright, I'm going to need some help here. On the face of it it looks as if I can get a 60 psi compressor running off 12V DC (draw on the electrical system will have to be evaluated) which I can use to do this. What is really doing my head in here is trying to calculate flow. What really messes things up is that in air atomization nozzles (which seem to produce the smallest water droplets) increasing air pressure will reduce the amount of water flow (presumably because of the liquid that the air displaces).

Now given that the tables for these nozzles suggest water pressure of around 50-70 psi and a Shurflo pump will produce around 100-150 psi we should be able to increase liquid flow, but would this then require a higher air pressure to successfully atomize the increased volume of water? Is 60 psi of air enough or should I be looking at more? I have located a 100 psi air pump but it flows less volume of air. I still can't get my head around whether pressure or volume is most critical for atomization.

There are probably too many variables here for anyone to give me a straight answer, but can someone point me in the right direction. Someone said that good atomization rather than flow is the key, but a certain amount of flow will presumably be necessary to pull sufficient heat out to make the compression isothermic (or as close to isothermic as possible).

So lets just look at flow for a moment. I can bench test how much water the system flows in a min, by experimenting with air and water pressures.

I have a 2.3 litre engine and a turbo rated at approximately 65 lbs/min (however that may have been reduced by the fitment of the smaller exhaust housing). How much water do I need to do what I am trying to do and would the volume of water injected need to be different if I set the injection point much lower in order to assist in spool up.

I have a lot of this stuff figured out and can do it, but I really need help working out the flow calculations.

My other concerns are: -

(a) Will injecting too much water cause it to puddle in the intercooler creating problems?
(b) Will it be necessary to inject more water post intercooler for knock suppression?

[hotrod]

If you are injecting pre-compressor at 2%-3% rate compared to air flow you will have about all the WI you need.

Suppose you spray at 2% air flow rate, and you have a 11.5:1 fuel air ratio at red line.

If you are flowing 65 lb/min, that is 29510 grams of air per min.
At 2% flow that means you would be injecting 590 grams / min of water.

If you have an 11.5:1 air fuel ratio than your fuel is 29510/11.5 = 2566 grams / min
Since most gasoline has a density of about .78 then that is 3289 cc/min
10% water to fuel would be 329 cc/min
15% water to fuel would be 493 cc/min


You would be spraying about 18% per min to fuel if you sprayed 2% of air by weight with a max power AFR of 11.5:1.

The beauty of using the air flow is is self corrects as you lean out the fuel.
the higher your AFR (say 12.5:1) the higher your percentage of water/fuel at a fixed water to air ratio.

Since many engines will report mass air flow directly in grams /sec it is trivial to find 2% of your max air flow in grams per min, which directly converts to water flow in cc/min.

If you delay WI turn on until 10 psi Manifold pressure or so the turbo will be well spooled before the water comes on.

Bottom line:
(a) Will injecting too much water cause it to puddle in the intercooler creating problems? --- possibly but that would quickly evaporate as the intercooler cools down as long as it was not a huge amount of over spray.


(b) Will it be necessary to inject more water post intercooler for knock suppression?
Probably not in my opinion.

On the atomizing nozzles I've looked up they most only need to have about 30 psi air supply pressure to work properly.
Not sure why you would want to run such high pressure unless you tapping a pre-existing high pressure supply.
Larry