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  #31  
Old 15-09-2004, 05:30 PM
hotrod hotrod is offline
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Default work sheet

That is an interesting work sheet on the effects of WI, but you need to read the cautions about the model. I did a lot of playing with it and at reasonable amounts of injectant, it seems to give very good information.

It does not do any "bounds checking". If you put absurdly high amounts of water in, it while happily compute the cooling from total evaporation even though the amount of water is far beyond what is necessary to reach 100% humiditiy. Sometimes giving unrealistic air charge temps as a result.

So like any tool its no better than the base assumptions of the model. It does however, clearly indicate the basis for the assumption that post intercooler injection is the best "from the perspective of intake air cooling per gram of water injected".

It has no way of accounting for things like the change in timing of the pressure peak in the cylinder with different amounts of water and how that would effect the power output of a specific engine design.

The pre-turbo injection totally discounts any changes in the turbocharger compressor effeciency and any secondary feedbacks like, higher mass flow, lower turbo discharge temp, or lower exhaust gas back pressure that might result from that increase in effeciency.

Of course all those changes would be turbo type specific and would require a ton of code to cover all the popular turbochargers.

Larry
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  #32  
Old 15-09-2004, 09:47 PM
b_boy b_boy is offline
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I too have played with that calculator. And for me the best thing about it is getting a handle on how the different components of intake, IC, turbo, AFR, and WI cooperate to make hp. I agree with hotrod the model used in the calculator is too simple for our stuff and does not account for the saturation limit of dissolved water. The 100% "humidity" is an upper bound of WI, but both ambient air saturation and temperature affect this bound. Ideally we'd like to have a system that never reached the 100% saturation limit at any point in the air path (the bound is going to change, being highest just after compression, and probably the lowest while inside the intercooler". We dont' want precipitation at any point (e.g rain fall, fog formation), that will lead to water build up and all sort of "unintended" effects.

One calculation that I think would be worth performing is to figure the amount of heat produced at the highest operating RPM of your turbocharger and then calculate using the heat of evaporation for water +/- % methanol the amount of fluid needed to bring the compressor to isothermal compression. Assume that you are getting "explosive evaporation" as hotrod suggested.

Instead of using a compressor map with "predicted efficiencies" and a PR, if one were to actually measure pressure and temperature pre and post turbo, the energy going into heat should fall out of the ideal gas law. Now we divide this energy loss by the energy gain of evaporation, and we'll get a percentage of water/methanol injection. It's a ball park ideal figure, but it could be informative. Anyone have thermocouples in said places to take the measurements, and a boost gauge?
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  #33  
Old 16-09-2004, 08:51 PM
b_boy b_boy is offline
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Ok, I'm out on a limb here, but I did some thermodynamic calculations on water absorbtion of heat during compression at the turbo impeller. The calculation is in 4 parts and relates to my WRX STI 2.5 L turbo car.

Part 1 Assumptions and definitions

At 15-16 psi the engine is consuming ~250-275 g air/sec
Temperature of compression at 60% turbo efficiency, produces ~100 degrees C change at 7000 RPM
Heat of evaportation 540 calories/ gram of water
1 calorie = 1 degree C/ gram/sec at one atmosphere
1 calorie = 4.184 joules
Specific heat of nitrogen gas 0.25 calories/degree C (air is mostly nitrogen)


Part 2 Heat change under compression and amount of water to restore starting temperature


For 1 degree change in water temp we get 1 calorie*grams/sec.

For 100 C change (increase) of air (nitrogen) of 1 gram = 25 calories to heat 1g of air 1 degree C

540 calories/gram of water evaporated, heat of evaporation

Too cool 250 g air / sec heated 100 C costs 250*25 or 6250 calories / sec

6250 calories/ 540 calories/g water is 11.6 g water / sec = 11.6 ml water/ sec or 695 ml of water / min

Thus, 695 ml/min of water brings the compressor to isothermal compression with 100 C change in temp
during compression.


Part 3 Engine's consumption of air at 7000 RPM in a 2.5L engine


For my 2.5 L engine at one atmosphere and 7000 RPM, that's 3500 cylinder fillings per min,
or 2.5L * 3500, or 8750 L / min * 1000 ml/L =

8.8 million ml of air / min

At 15 psi of boost the amount of air is double or 17.5 million ml / min


695 ml/min cools 17.5 million ml/min air 100C or

0.004% (by volume) water to air ratio = isothermal compression with 100C temperature change


Part 4 Aquamist pre-turbo injection effects with my set up


100% stated "efficiency" of a turbo compressor is not isothermal, but adiabatic compression.

With Aquamist and a 0.5 mm jet I can inject about 125 ml / min, that is about 18% of 695 ml / min.

The turbo is operating at ~60% adiabatic efficiency at 7000 RPM, the 695 ml/sec water to brings it
to isothermal compression which corresponds to about to 118% efficiency at a pressure ratio of 2,
a difference of 58%.

18% (my 125 ml/min) of 58% is 10.5%

60% efficiency of turbo + 10.5% cooling from water = 70.5% efficiency of compression adiabatic.

250 ml / min water injection gives twice that or 81% compressor efficiency.

So with just 125 ml/min WI, compressor efficiency is near 70% at redline, quite a feat!

It's like having a 20% larger turbo.

I don't know the actual change in temperature during compression at 7000 RPM on my '04 Subaru STI with
the stock turbo. 100C is a reasonble guess, but it could be as high as 150C if the compressor efficiency
is lower than the predicted 60%. Thus, these numbers are only a rough estimate. While isothermal
compression would be nice to achieve, I can be happy with just extending the efficiency of the turbo
beyond it's normal bounds, and make up for the adiabatic heat by injecting more water after
the intercooler. Under this scenario maximum total water injection is 15% of fuel mass, very reasonable.
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  #34  
Old 16-09-2004, 09:56 PM
hotrod hotrod is offline
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That fits well with my experience.

The cliffs notes version is:

As you make the compression more isothermal, the true operating point of the compressor moves to the left on the compressor map. Since most performance applications push the compressor off the compressor map on the right side as they try to squeeze the last bit of performance out of the turbo this is in most cases a good thing.

Here at high altitude, the already small TD04L-13G which comes stock on the WRX is not just operating off the compressor map, it is on the next page.

When I started with pre-compressor WI, it made the turbo much more responsive in the mid range, and improved the max airflow a small but noticable amount. The mid range rpm onset of boost became so agressive I had to modify my boost controller settings to keep the throttle from becoming an ON-OFF switch.

Larry
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  #35  
Old 17-09-2004, 07:03 PM
b_boy b_boy is offline
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Thanks Larry for the "real world" experience.

It's just as I would like, move the compressor effectiveness back (left) into a more efficient island of the compressor map. I'll compress not necessarily more air, but compress it more easily with less intoduction of heat.

What effect the water will have on impeller speed questionable. While theoretically more viscous, the water mist may evaporate so quickly at the impeller to essentially behave as a gas, albeit a more dense gas. I believe it will slow the impeller to some degree, another benefit that I think Saabtuner mentioned.

I'm going to try to get WI volume up to 20% of fuel, such that when shunting 1/3 to the turbo we sending a bit more water/methanol through the turbo, around 0.7% by air mass. Or I'll shunt a larger percentage than 1/3. I'll have to see what works best.

We'll see how it works around mid-Oct.
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  #36  
Old 18-09-2004, 08:43 AM
JohnA JohnA is offline
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Quote:
Originally Posted by b_boy
...With Aquamist and a 0.5 mm jet I can inject about 125 ml / min, that is about 18% of 695 ml / min.....
I've measured this jet at well over 200cc/min, almost 240cc actually with the engine running.
That was static, with no boost (or vacuum) in front of the nozzle.
If it's pre-compressor, then maybe the flow could be even higher (I'm not sure about this without testing further)
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John

www.max-boost.co.uk
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  #37  
Old 18-09-2004, 10:11 AM
Richard L Richard L is offline
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I wish I can contribute more but it is getting beyond my level.
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aquamist technical support
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  #38  
Old 18-09-2004, 10:52 AM
SaabTuner SaabTuner is offline
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Quote:
Originally Posted by b_boy
What effect the water will have on impeller speed questionable. While theoretically more viscous, the water mist may evaporate so quickly at the impeller to essentially behave as a gas, albeit a more dense gas. I believe it will slow the impeller to some degree, another benefit that I think Saabtuner mentioned.
It might slow the impeller slightly ... or it might speed it up. The reason I think either is possible is that whether it slows or speeds up will depend on exactly where the water is atomized. If atomized near the tips of the impeller blades (as I suspect it would be) the over-all pressure drop across the turbo would be reduced at a given mass-airflow, and that reduction could allow the compressor to speed up much more quickly. I think Larry said something about it turning the throttle into an "on/off switch", which would go along with those lines perhaps?

Also I mentioned earlier that it would cool the impeller wheel slightly, which when boosting past 2 bar can be quite beneficial.

Adrian~
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  #39  
Old 22-09-2004, 03:55 AM
b_boy b_boy is offline
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It's interesting that you would say that Saabtuner. I've reading about air density and humidity. Surprisingly humid air is often stated as less dense. Let's take a different look at this, not from a throttle/turbo response view, but an efficiency point of view.

At the same temperature, water+air should be more dense than dry air alone--a simple more molecules/cubic area argument. I think the reason for the humid air being seen as less less dense than dry air is a misunderstanding of temperature, hot air being significantly less dense than cold air--hence the hot air ballon--and hot air holding more water--higher saturation point (the mid-west is summer).

If we picture a droplet entering the center of the impeller, it is immediately whisked centrifugally outward and "backward," evaporating along the way, perhaps explosively. Locally, liquid water is being transformed into gas, along with the air being compressed. On a purely local level there are more molecules in a gaseous state at the end of the compression, but since the liquid evaporated heat was absorbed.

If we imagine a model were droplet size reduces exponentially as it traverses the impeller to expeller length, at the beginning the density of the air with or without the droplet is nearly the same, however, at the end of the traverse (notably the point of most leverage) the air + droplet gas will be much more dense since the gaseous contribution of water will be huge.

I think what we will see by injecting water is that there is a slowing of the impeller wheel that results in the same compression of air at that slower speed plus the added benefit of reduced temperature of that compressed air. It my case, turning water at 5000 RPM, we may shift the impeller speed back to where it was at 4000 (and 70% efficiency), as we increase RPM on up to 6000, 7000 while increasing the water injection correponding, we can try to stay at the same 4000 RPM, 70% efficiency range.

This is one way to look at: view it as creating a static impeller speed of high adiabatic efficiency (e.g. the 4000 RPM equivalent impeller speed, say 120,000 impeller RPM) by injecting more water to slow the impeller down. But we don't want to do that, we want the impeller to continue it's upward climb in speed so that it can compress more air. The reality is most likely in middle, slowing the impeller some (while cooling the air that is compressed), but allowing for more compression with increasing speed (but still cooling the air compression). There is no way density of water + air is going to completely overcome the torque of the exhaust side.

Now, coming back to the rapid throttle/turbo response view. The quickness of the throttle is going to be mainly a function of the rate of air and fuel entering the engine. Early in the turbo's speed range, slowing the turbo's ascent in speed will be a disadvantage, prior to peak compression, in fact the impeller is an impediment to air flow if the wheel is not turning at all. Once a decent efficiency is reached though, any further cooling of the air being compressed will result in more air in the cylinder for every turn of the impeller, thus the rate of air will be faster at that instant, and the rate will increase with increasing water injection linearly. I think that is where you could see an increase in throttle response over no WI pre-turbo, but it would depend on maintaining the temperature difference as the compressor continues to spool.

I can't believe we stumped Richarl Lamb with this stuff. To his credit, it is more turbo theory than WI theory. We are all learning something I guess.

Thanks to John A for the measurement. I'll have three 0.5 mm jets, so I don't think I'll achieve his volume as it is distibuted between the jets unless I add another pump or greatly increase the pressure.

In the end, it's going to come down to testing it out. :wink:
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  #40  
Old 22-09-2004, 11:12 PM
SaabTuner SaabTuner is offline
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I think there may also be some mixing here of the term "pressure". Static pressure and flow pressure (or total pressure) are somewhat different things.

"evaporating along the way, perhaps explosively. Locally, liquid water is being transformed into gas, along with the air being compressed."

When thinking of this problem remember that the turbocharger's compressor wheel only accellerates the gas. The compression takes place in the diffuser scroll in the housing. Most of the evaporation/boiling should take place towards the tips of the impeller as that is where the static pressure should be lowest (though velocity pressure quite high). Thank the Bernoulli principle for that one.

While I'm not sure if the water's evaporation would increase or decrease the density of the gas, I think it's fairly safe to say that the evaporation of water in the compressor would significantly change the flow through the compressor.

The reason I say that is that on the forward facing edges of the blades there is much higher static pressure than on the backwards facing edges. Since more water will evaporate in lower pressure, the low pressure regions may either get MUCH lower, or get much LESS lower, depending on what effect the water has on static pressure when it evaporates. In either case, it would dramatically change the flow field through the impeller.

Adrian~
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