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  #11  
Old 19-08-2004, 05:11 PM
JohnA JohnA is offline
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Default Re: spray point

Quote:
Originally Posted by hotrod
...In regard to the effects on the compressor mass flow, maximum results appear to occur with mist flow of about 3% - 10% of the air mass, ...
another 2 minor points:
1. a 2 litre engine at 1 bar boost and 5Krpm will be consuming around
10.000 litres of air per minute, or 10cubic metres of air, which is about 12.2 kg air/minute.
10% of that equates to 1.2kg water /min, that's quite a lot isn't it? With the aquamist pump and nozzles, it will have to be closer to 3% in practice.
Would this be enough?

2. Wouldn't the extreme centrifugal forces in the compressor force the tiny water droplets to become liquid again?
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  #12  
Old 20-08-2004, 08:20 AM
SaabTuner SaabTuner is offline
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Default Re: spray point

Quote:
Originally Posted by JohnA
Quote:
Originally Posted by hotrod
...In regard to the effects on the compressor mass flow, maximum results appear to occur with mist flow of about 3% - 10% of the air mass, ...
another 2 minor points:
1. a 2 litre engine at 1 bar boost and 5Krpm will be consuming around
10.000 litres of air per minute, or 10cubic metres of air, which is about 12.2 kg air/minute.
10% of that equates to 1.2kg water /min, that's quite a lot isn't it? With the aquamist pump and nozzles, it will have to be closer to 3% in practice.
Would this be enough?

2. Wouldn't the extreme centrifugal forces in the compressor force the tiny water droplets to become liquid again?
A 2 liter engine consumes 2 liters every TWO revolutions (720 degrees) so assuming 100%VE, 100% intercooling, it would consume 5,000 liters/minute.

Adrian~
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  #13  
Old 20-08-2004, 10:08 AM
JohnA JohnA is offline
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Default Re: spray point

Quote:
Originally Posted by SaabTuner
..A 2 liter engine consumes 2 liters every TWO revolutions (720 degrees) so assuming 100%VE, 100% intercooling, it would consume 5,000 liters/minute.

Adrian~
you think you got me there on the two revolutions thing? :twisted:
Well you didn't, because I mention 1 bar boost pressure, which multiplies it by two.

Nice try though. :wink:
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  #14  
Old 21-08-2004, 05:46 PM
hotrod hotrod is offline
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Default stuff

Four cycle engines only intake stroke every other revolution for each cylinder

JohnA -- actually its not all that much for two reasons. First your not on full boost for more than just a few seconds at a time unless your making a landspeed record run on the salt flats.

Also 1.2 kg/min is only about 20 cc/sec or about 2/3's of a shot glass of fluid, divided among the engines cylinders. On a 4 cylinder 2 liter engine with 500 cc/min injector running static will inject 2000 cc/min of fuel

The turbo and the air flow currents and heating through the rotor actually tend to shred and tear the dropplets apart and cause nearly explosive evaporation, very little liquid water exits the rotor.

If you want to have a base reference on how much water an engine can ingest during WWII Pratt and Whitney ran what they called flood tests on their aircraft engines where they turned up the water injection to the point that liquid water was pouring out the exhaust ports and the engines still ran with no problems. That level of injectant reduced the engine power from a max of 2000-2800 hp down to about 600 hp but there were no problems.

As far as the limits of the pump and injectors you are probably correct. That is why I am running a 100 psi Shurflo pump with a capacity of 1.4 gal/min ( 5.3 L/min) and a 5 Gal / hr nozzle ( 315 cc/min) on a rather small 13G turbocharger. The 13G only flows about 25 lb/min of air (11.45 kg/min) which comes out very close to 3 % of air flow for the spray flow.

I will probably be adding nozzles here soon.

Larry
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  #15  
Old 21-08-2004, 06:52 PM
JohnA JohnA is offline
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Default Re: stuff

Quote:
Originally Posted by hotrod
Four cycle engines only intake stroke every other revolution for each cylinder
...and 4-cylinder, 4-stroke engines (inline) have one cylinder in intake stroke every half revolution...the end result is the same: every 2 rpm 2litres of air have been inhaled (assuming 100% VE of course)

But I'd like to believe that the level of discussion here is a bit higher

Quote:
JohnA -- actually its not all that much for two reasons. First your not on full boost for more than just a few seconds at a time unless your making a landspeed record run on the salt flats.
Yeah, but when calculating (roughly ofcourse) the max water flow, we have to assume sustained full throttle.
And Volumetric Efficiency won't be 100% of course, especially at full-power revs.

Quote:
The turbo and the air flow currents and heating through the rotor actually tend to shred and tear the dropplets apart and cause nearly explosive evaporation, very little liquid water exits the rotor.
explosive evaporation ... interesting. Didn't know it even exists! :lol:

It's OK for people with traditional superchargers to say that injecting all this water before the blower is fine. But I had reservations about centrifugal compressors. If the centrifugal forces don't mess up the mist, then that's one less headache.

So such a setup wouldn't be boost-driven like std water injection, would it?
To realise the gains of near iso-thermal compression we'd want to inject water from zero boost, yes?

Do you think that's safe then?
Unfortunately my aquamist setup is not boost-sensitive, it's rather on/off.
:cry:
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  #16  
Old 21-08-2004, 11:17 PM
hotrod hotrod is offline
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Default details

Quote:
If the centrifugal forces don't mess up the mist, then that's one less headache.
Very little of the mist survives the path through the compressor in liquid form. In the discharge you have much cooler air at very high relative humidity but almost no water droplets.

Literally 10's of thousands of military aircraft engines in the period 1939 - through the early 1950's used before the compressor injection with no problems at all. It is also widely used in the tractor pulls, and on diesel trucks, CART race cars I understand, used a pre-compressor injection of some of their fuel for the same purpose, a handful of production cars and trucks have also used the system.

All you have to do is avoid large water droplets or a solid stream of water from impacting the impeller.

Quote:
So such a setup wouldn't be boost-driven like std water injection, would it?
To realise the gains of near iso-thermal compression we'd want to inject water from zero boost, yes?
Depends on how you are defining boost driven --- some systems use a pressurized water reservoir so that boost pressure provides the motive force for injection. These systems are best for trucks pulling heavy loads up mountain passes where they stay on boost for extended periods of time. A presurize reservoir system will not react quick enough in my opinion for a high performance application.

Other systems use a pump supplied pressurized system with a valve of some sort that opens to allow spray at a certain boost pressure.

You generally do not want a continous on system (except for an rv or truck as mentioned above). On my system I have a pressure switch which activates at about 8 psi to turn on the spray. It is a simple on/off WI system. It will spray anytime boost is over the 8 psi threshold I have currently set.

As far as computing max water flow, your correct to establish a limiting case you would figure at max air flow which would typically be at max power rpm and a true engine VE of about .85-.88 for most engines.

However for detonation control your most critical flow rate is in the midrange max torque rpm range, say 3000-5500 rpm. That is where the engine has max VE and by definition you have max torque because you have max cylinder pressures. This is the RPM range where a sudden loss of WI can be fatal to the engine.

If you look at a manifold pressure log of a typical turbocharged engine when the turbo begins to make serious boost you go from near normal atmospheric pressure to a significant boost pressure >6 psi for example in just a fraction of a second ( ie only a 1000 rpm change in engine speed at WOT).

On my WRX on a 3rd gear WOT blast, I go from -1.2 psig manifold pressure at cruise, to 13.5 psig manifold pressure in 0.922 seconds when I just stab the throttle wide open, on the shift to 4th I go from 10.4 psig to -8.4 psig manifold pressure in 0.234 seconds as I close the throttle for the shift, and then jump to +5.9 psig .235 seconds later as I go back to WOT and another 235 milliseconds later (the time resolution of this log), I am backup to a manifold pressure of 11.9 psig. (this is all at 5800 ft altitude).

Total time under load in Low gear is about 1.5 seconds, 2rd gear was 3.828 seconds, 3rd gear 2.359 seconds, and 4th gear only lasts a couple of seconds before I'm over 100 mph.

So in a highway acceleration type situation, you would seldom see more than 7 - 8 seconds of flow, with brief interruptions of about .5 second every 2 - 3 seconds, so your actual duty cycle for the spray activation will be something like 80% or so. In my case I have an accumulator in line with the solenoid so the system pressure changes very little over this short interval.

If you are looking for detonation supression you need it most in the midrange rpms near the engines torque peak. That is when the engine is most likely to detonate under high engine load. You want the WI to kick in just below the rpm range and boost pressure that you can first experience detonation under heavy load. If you turn it on early you cool the exhaust gasses just as the turbo needs hot EGT's to help it spool up. So in that case you delay the WI spray as long as you safely can.

If your looking for maximum efficiency of the compressor, you want the spray to come on at a boost pressure when the compressor is just moving out of its maximum effeciency island on the compressor map. What you are doing is artificially stretching the compressor map to the right so the compressor has a wider island of efficiency.

In my case I will eventually be using a compound system with some spray pre-turbo for the purpose of maximising the compressors efficiency, and if I need it, a secondary jet spraying pre-throttle body to control any detonation that is not supressed by the pre-turbo injection.

Larry
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  #17  
Old 22-08-2004, 12:33 AM
SaabTuner SaabTuner is offline
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Default

On advantage of injecting pre-compressor is that, should the WI fail, it would take several seconds for all the water to evaporate off the walls of all the tubing before the engine. This was noted by the folks at Linkoping University when doing experiments with WI and an Ionization Gap Sensor feedback ignition system.

If you have a very very good ECU on the engine a WI failure would be not much worse than running into a bad tank of fuel and the ECU should eliminate the knocking within one or two engine rotations. Most engines are designed to be able to handle rapid changes in octane even if they like higher better.

EGT might be another problem, but if you feel the car suddenly getting slower it doesn't take a rocket scientist to know something might be amiss and worth investigating.

Adrian~
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  #18  
Old 22-08-2004, 07:55 AM
JohnA JohnA is offline
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Default Re: details

Quote:
Originally Posted by hotrod
..In my case I will eventually be using a compound system with some spray pre-turbo for the purpose of maximising the compressors efficiency, and if I need it, a secondary jet spraying pre-throttle body to control any detonation that is not supressed by the pre-turbo injection.
That's easier to implement actually, I'll probably give it a try. Just Tee the existing WI line to another nozzle just before the compressor. The aquamist pump should keep atomisation at decent levels, eh?
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  #19  
Old 22-08-2004, 08:58 AM
SaabTuner SaabTuner is offline
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Default Re: spray point

Quote:
Originally Posted by JohnA
Quote:
Originally Posted by SaabTuner
..A 2 liter engine consumes 2 liters every TWO revolutions (720 degrees) so assuming 100%VE, 100% intercooling, it would consume 5,000 liters/minute.

Adrian~
you think you got me there on the two revolutions thing? :twisted:
Well you didn't, because I mention 1 bar boost pressure, which multiplies it by two.

Nice try though. :wink:
Technically the engine consumes 5,000 L/min but at 29.4 psi instead of 14.7. :wink: But fair enough, I didn't notice the 1 bar bit.

Adrian~
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  #20  
Old 22-08-2004, 09:06 AM
JohnA JohnA is offline
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Default Re: spray point

Quote:
Originally Posted by SaabTuner
..Technically the engine consumes 5,000 L/min but at 29.4 psi instead of 14.7. :wink:
The exact psi figure depends on atmospheric pressure conditions, doesn't it? And assuming sea-level is a bit presumptious. :smile:

Tell you what - why don't we end this futile excercise in bandwidth wastage? :wink:
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