Injecting prior to turbo comp' impellers

[masterp2]

Originally Posted by hotrod

Second question, is your CAC an air to air system postioned infront of the radiator? If so then the heat transfer to the radiator would be due to the CAC pre-heating the air going through the radiator. Yes, exactly

Two options come to mind, first under the heading of keeping it simple, you could also look at direct water spray on the outside of the CAC or on the radiator or both, rather than injection in the air stream between the turbo and the CAC. But requires too much water and not as much fun

The other way to take heat load off the cooling system is to add an oil cooler. The oil moves quite a bit of engine heat. On the WRX in stock configuration it uses an oil cooler that passes heat to the engine coolant. Folks have had good luck adding an air cooled oil cooler and passing that heat directly to the air rather than the radiator through the coolant.


When you refer to "gas" are you talking about propane injection? No, gasoline vs diesel

The pre-compressor injection will only increase the max air flow by a few percent, so your 5% gain seems about right in that regard.

As I understand the effects on diesels you need to add some fuel to make full use of the air that you are already flowing so if you upped your fuel flow and then held the EGT temps in check with WI you should get more power than 5%

Larry

Correct, a diesel is just a big air pump, meter in some fuel (fuel is subject to timing vs a spark) and compress until it bangs. At first glance, detonation appears to not be the issue it is in gassers.

I guess I am not clear why water-meth solution added pre turbo, vs post IC, results in less power added.

[hotrod]

Pre-compressor injection is a special case situation.

In a perfect world the ideal place to inject WI water/methanol mix would be after the CAC and as far as practical before the intake manifold. That gives you time for the spray to mix well with the intake, and you get the additional cooling of the air charge from evaporation . That additional reduction in charge air temp, increases the VE of the engine (ie you actually get more air in on each intake stroke for the same manifold pressure)

On the other hand, pre-compressor injection is a trade off. By injecting ahead of the compressor it increases the mass flow through the compressor so it is very helpful on engines that have undersized compressors. The down side of pre-compressor injection, is that it reduces the effeciency of the CAC slightly.

An intercoolers heat dissipation rate is controlled by two things, the mass flow of cooling air through the intercooler, and the average temperature differential between the charge air and the cooling air. Cooling varies as a log function of the temp difference.

If you inject the WI mist prior to the compressor you end up with a cooler discharge from the turbocharger for a given boost condition. That means a smaller temperature differential between the charge air and the cooling air flow --- result, your manifold air temp can actually be slightly higher with pre-compressor injection only, than it would be if you injected the same amount of fluid in the post CAC location.

If you have an adequate sized turbocharger compressor using pre-compressor injection is not as productive as the same injection rate post CAC.

On a car with a significantly undersized turbocharger, the increase in mass flow through the turbocharger out weighs the slight reduction in cooling from the intercooler.

Hope that makes sense?

That is why with my new turbo I will be limiting the pre-compressor injection only to high temp, high flow situations, as at moderate boost and engine rpm the turbo has plenty of flow.

Larry

[masterp2]

I completely understand your explanation, nicely done. As a Chemical Engineer, I have a good working knowledge of the thermo dynamic.

Now the balancing act for my unique application: How to design one WI system that serves 2 purposes. Cooling enhancement for hi-load towing focusing on lowering CAC and egt temps, plus a second (stage 2)performance aspect (has to be fun afterall :lol: ).

Question: Can you make a guess at how much flow to dedicate to pre-CAC, post turbo misting to cool that charge 100 degrees (350 to 250) on a 30% RH day? I assume it would be the same flow, whether pre-turbo or post-turbo? It's a 6.6 l motor (diesel). Diesel posts seem to suggest 400 cfm moving at full tilt, but not sure on this.

Using your rationalle, I don't want to overfeed this stage, just kill the fire a bit. Save the big water for the 2nd performance stage. Unless you have another idea.

[hotrod]

Well I used a 3 and a 4 gal/hr nozzle on my pre-compressor WI setup on a turbo that was rated with a max flow of 360 CFM. The 4 gal/hr was a bit too much, and the 3 seemed to be about right. Since I was pushing that turbo very hard a lot of that water went to cool the compressor discharge to sane levels.

In your case you have:
6.6 L displacement @ 30 psi, lets assume 4000 rpm --- engine needs about --

6.6 L = 403 CID x 2000/1728 = 466 CFM, at 30 psi equals a pressure ratio of `approx 2:1 . Your engine should need about 932 CFM at 4000 rpm or 64.3 lb/min of air flow. (this is air flow into the compressor inlet at atmospheric pressure).

Not sure what the VE for a diesel engine is so this is at 100% VE, for a gasonline engine you'd mulitply by about .85 to get a real world air flow.

If we do a quick on the napkin computation of the specific heat of that air flow we get:

64.3 lb/min x .024 btu/lb x 100 deg F delta T = 154.3 BTU/min deg F

Latent heat of water = 970 BTU/lb deg F at atmospheric pressure. (it will be a little lower at high pressure but this is close enough for a ball park computation).

To absorbe that heat flow you, would need to evaporate about 0.16 lb of water /min or 73 ml/min of water. That would be just over a 1 gal/hr water flow, or about the same as an Aquamist 0.4 mm nozzle at 40 psi line pressure.

Not trying to be condescending by working it out, but just wanted to show where the numbers were coming from, so everyone could follow the reasoning.

Larry

[TurboGTi]

I;m not sure if this was disccussed but can i run straight nethanol and still inject Pre-turbo?

Will it have any effect on the turbo or the inlet temperature?

[hotrod]

Yes you could inject pure methanol pre-compressor, but you would get more cooling effect with a water methanol mix.

Larry

[masterp2]

Larry, excellent work on explaining the math. How about another?

How much water does it take to bring 900cfm from 10% humidity to 100% humidity? What is the resultant temp if ambient is 120 degrees, assuming a 70% efficient, 30 psi turbo.

[hotrod]

I can't give you an exact number as I lost a reference book I used to have on compressed air systems. If you need an exact answer you might try calling a company that specializes in industrial compressed air systems. They have to work out the water weight in the compressed air to design, drying systems so the user does not end up getting wet compressed air at their tools.

I found one reference that gives some ball park values on a chart of lb water in lb air at different temps and pressures.

Airs ability to hold water vapor changes significantly at higher pressures so it is not a simple relationship.

As you can see below as pressure goes up, the amount of water vapor the air can carry at a given temp goes down. Also as the air temp goes down so does the water capacity.

Bottom line if you saturate the air going into the compressor, as it leaves it will be quite dry, while it is still hot, but after being cooled in the CAC it will become super-saturated as the temp approaches the compressor inlet temp. In other words if you had a transparent intercooler you'd probably see a dense fog going through the throttle body.

A secondary conclusion from the above fog formation on cooling just came to me ! The temperature where your charge air becomes supersaturated and forms a water fog will be a hard limit to the minimum temperature the intercooler can reduce the charge air temp to. Once water fog begins to form, the latent heat of condensation of the water vapor released as the charge cools will exactly balance the heat taken out by the intercooler. This is likely the reason pre-compressor injection can appear to reduce the effeciency of the intercooler.

900 CFM would be about 63 lbs / min air flow at 1 atm


From the chart in the Chemicals Engineers handbook (Perry and Chilton)
5th editon, Fig 3-4

Air at:
1 atmosphere and 100 deg F holds 0.04 lb water / lb air
2 atmosphere and 100 deg F holds 0.03 lb water / lb air
3 atmosphere and 100 deg F holds 0.015 lb water / lb air

1 atmosphere and 125 deg F = 0.09 lb water /lb air
2 atmosphere and 125 deg F = 0.045 lb water /lb air
3 atmosphere and 125 deg F = 0.03 lb water /lb air


1 atmosphere and 200 deg F holds > 1 lb water / lb air
2 atmosphere and 200 deg F holds 0.43 lb water / lb air
3 atmoshpere and 200 deg F holds 0.23 lb water / lb air

Assuming your intake air is completely dry:

If you take a small turbo like I have on the WRX which has a max flow of 35 lb / min, and the inlet temp is 100 deg F, and the exit temp from the intercooler is 125 deg F (at 2 bar boost = 3 bar absolute pressure), that 35 lb of air could hold a max of about (.03 x 35) or 1.05 lb or water per minute. That is just over 475 cc/min of liquid water completely evaporated.

If your intercooler is heat soaked, then that same air flow at 200 deg F and 3 bar absolute would hold almost 8 lbs of water per minute or nearly 3654 cc / min of liquid water completely evaporated.


We know that the pre- compressor injection drastically cools the intake charge, so if the inlet air is cooled to 50 deg F by the water evaporation before it hits the compressor, that cool air can only hold a max of about 0.008 lb water / lb of air, or .28 lb of water or 127 cc/min will be able to evaporate before it arrives at the compressor. (that is equal to about .8% of mass air flow).

If the air leaves the compressor at 200 deg F and 3 bar absolute pressure then on exit it could hold a max of 0.23 lb water / lb air. That would equal 8.05 lb water / lb air (3655 cc/min).

If your injecting at 3% of air flow then you injected only 1.05 lb water, so the relative humidity of the air leaving the compressor will be less than 13%.

Now you cool it down to 125 deg as it goes through the intercooler and you have that same 1.05 lb water but it is traveling in an air stream that is now is only capable of holding about 0.03 lb water / lb air, or 1.05 lb water --- surprise surprise your at 100% humidity as the air leaves the intercooler.

( I didn't plan this to come out at exactly saturation but I'll take the lucky break )

For your example of 900 CFM you have 63 lbs/min air flow in. If you get 30 psi boost ( 3 bar absolute) and it exits the CAC (intercooler) at 125 deg F then at 100 % RH it could hold 0.03 x 63 = 1.89 lb water / min or 858 cc/min at saturation.


Someone check my numbers but I think that is all correct!
Larry

[masterp2]

This is so funny, I was browsing threads (after posting earlier today) and came across this. Said to myself, this is the response to the question I posed earlier. It took 5 minutes to figure out that this was the same thread!

Now I have to go back and read it better. Thanks Larry.

[masterp2]

Yes, the hot air really holds a LOT more water. Then it stands to reason that, if misted efficiently, more total water can be evaporated post-IC only, vs pre-turbo plus post-IC??

Does the turbo performance enhancement of pre-turbo misting make it better performing anyway?