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Old 12-07-2008, 03:13 AM   #1
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Default Could WI Improve Spool?

Having a few thoughts all hit me at the same time. I'm wondering if WI would help spool the turbo. I think it would. My thinking....

The turbine has to produce shaft work to drive the compressor. Of course it gets the power from the exhaust. So as the engine speeds up, it moves more and more energy through the turbine. The turbine can only take a % of this energy and convert it into shaft work, and this % also varies with the mass flow across the turbine, load, etc.

Generally speaking, work done on a turbine is the change in enthalpy across the turbine. Enthalpy, h, is a combination of properties. Namely, U+Pv, where U= internal energy, P=pressure, and V=volume. U = m*u. IE- Internal energy is a function of mass. So adding water injection would increase the mass flow across the turbine. The more enthalpy we move across the turbine, the more work it spits out, the sooner we get boost. Am I wrong?

So in theory WI should help spool. Seems like we should run as much as safely possible to get spool up. Tomorrow I'm'a do some mass flow calculations and try to figure out how much water we could inject per unit fuel, safely. Or maybe WI after combustion, pre turbine....

Just think about it. h = U + Pv. Putting a 3" down pipe drops the pressure on the outlet of the turbine. Reducing P_exit reduces the enthalpy on that side, which means a bigger delta h. V is a function of engine speed and volumetric efficiency. Bigger engines spool sooner, headwork helps spool, etc. Anything that helps increase V. So increasing U should do the same to help spool, no?

[/engineer-to-be]
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Old 12-07-2008, 03:19 AM   #2
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how would you inject water into the manifold, but before the turbine? are there nozzles that can take the heat? Seems like an easy concept to test out if you are injecting into the intake
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Old 12-07-2008, 03:23 AM   #3
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WI after combustion sounds a little dumb, why use it if you'll miss out on the cooling effect in the combustion chamber, you know?

But yes, the theory is sound that WI decreases spool time. The catch-22 is that before say, 4-5 psi "they" say it will foul your plugs (or something) so you have to wait until then to get the effect. That's a good case for the pre-turbo post-combustion setup but as ryerye pointed out there are logistical hurdles there at a minimum.
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Old 12-07-2008, 03:53 AM   #4
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The problem is that a lot of the spool energy comes from the heat. In order to move more air through the turbine, you must increase the volume of air either by increasing the actual volume or decreasing the temperature. Less heat = bad for spool.

Same reason retarding timing helps with spool. Less power = more heat = more spool.
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Old 12-07-2008, 11:25 AM   #5
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the mass is there if you inject before combustion. law of conservation of something or other.

you'd have to see if the mass flow component is greater than the heat energy component at the turbine inlet.
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Old 12-07-2008, 12:36 PM   #6
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Originally Posted by y8s View Post
the mass is there if you inject before combustion. law of conservation of something or other.

you'd have to see if the mass flow component is greater than the heat energy component at the turbine inlet.
Could you explain that?
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Old 12-07-2008, 12:38 PM   #7
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which part? if you add water to hot gas, you decrease the temperature because of the latent heat of vaporization. so your enthalpy goes down (I think?) but your mass from the added water goes up.
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Old 12-07-2008, 12:46 PM   #8
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Originally Posted by y8s View Post
which part? if you add water to hot gas, you decrease the temperature because of the latent heat of vaporization. so your enthalpy goes down (I think?) but your mass from the added water goes up.
Well, say the motor's taking in 10lb/min of air. That has some initial enthalpy and a final enthalpy after combustion. If we introduce another mass flow into the engine, say some WI, then that's more enthalpy going in, and more out, no? I see what you're saying about it reducing the temperature. But when water becomes a superheated vapor it will expand, increasing pressure. Not an expert, but I'm pretty sure the turbine is more efficient operating with a bigger delta P than a small delta P and a larger delta T. IE-steam power plant.

I would think that the added mass flow of water would increase enthalpy, and the phase change of water would cause a drop in T and an increase in P pre turbine.
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Old 12-07-2008, 01:04 PM   #9
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Maybe if the resulting cooler charge at the intake mani drops the pressure a bit, so it improves the engine's VE ... ? Improving VE at a given RPM will improve spoolup at that RPM.
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Old 12-07-2008, 01:24 PM   #10
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Quote:
Originally Posted by patsmx5 View Post
Well, say the motor's taking in 10lb/min of air. That has some initial enthalpy and a final enthalpy after combustion. If we introduce another mass flow into the engine, say some WI, then that's more enthalpy going in, and more out, no? I see what you're saying about it reducing the temperature. But when water becomes a superheated vapor it will expand, increasing pressure. Not an expert, but I'm pretty sure the turbine is more efficient operating with a bigger delta P than a small delta P and a larger delta T. IE-steam power plant.

I would think that the added mass flow of water would increase enthalpy, and the phase change of water would cause a drop in T and an increase in P pre turbine.
So you ar basically saying that the water added to the intake charge, when going through the engine for combustion, will vaporize adding more heavyness to the air, enabling the heavier air to spin the turbine better.

That all makes sence, but the heat in the turbine is the questionable part. We need a test to determine how much power is lost due to heat loss. Like Y8 said.

Mabe a wraped dp would help inclose the heat and revers some of the cooling effects. kinda help channel the heat.
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Old 12-07-2008, 01:40 PM   #11
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Quote:
Originally Posted by Toddcod View Post
So you ar basically saying that the water added to the intake charge, when going through the engine for combustion, will vaporize adding more heavyness to the air, enabling the heavier air to spin the turbine better.

That all makes sence, but the heat in the turbine is the questionable part. We need a test.
So does the benefit of the added "enthalpy" from the water overcome the loss due to the drop in temperature?

I agree, a real world test is the only way to find out if this is something to try. Someone mentioned problems with water injection at low boost pressures and into vacuum, but I don't really buy that. A quick google search revealed that there are people that use water injection on their naturally aspirated engines without any real issues, you just have to inject less. In low boost/vacuum, there is just less air going into the engine and Im sure there is a maximum ratio before problems start to happen with WI.
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Old 12-07-2008, 04:33 PM   #12
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In the real world, if you're injecting starting at 4-5 psi, your turbo is already well on its way to peak boost, so the effect is slight, at least in my experience.
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Old 12-07-2008, 04:46 PM   #13
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Well, I'm gonna be running a larger-than-average turbo, so spool is gonna suck. If I did this, I would like to have it inject X% water per unit fuel. That % would be as much as possible if it helped spool. I think it would.

Here's my reason as to why lowering the temperature would help, not hurt, spool. We learned in thermodynamics that the most efficient process approach reversibility. The most efficient processes occur at low delta T's. So having a larger change in T is actually bad, not good, from an efficiency standpoint. My idea with the WI is two fold. 1) WI added an enthalpy flow, which increases total enthalpy in and out of the turbine. 2) the phase change will convert some of the heat into pressure, so that reduces the delta T.

Wish we had more engineer-types to chime in...

Last edited by patsmx5; 12-07-2008 at 04:57 PM.
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Old 12-07-2008, 04:50 PM   #14
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Then in your case it might be more beneficial than in mine, the Greddy turbo is already like a lightswitch (esp. with better DP and EBC...) so the WI only exacerbates the traction issue in low gears. I can't keep up with the engineer-speak, but it sounds like you'll definitely be in the market for a progressive controller.
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Old 12-07-2008, 08:14 PM   #15
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Why not just do pre turbo water injection? Wouldnt that have the same affect of a faster spool since you are improving the efficiency range of the compressor?
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Old 12-10-2008, 03:43 AM   #16
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Quote:
Originally Posted by Saml01 View Post
Why not just do pre turbo water injection? Wouldn't that have the same affect of a faster spool since you are improving the efficiency range of the compressor?
Sounds like a fine idea, but i dont think there are any nozzles that would work on an exhaust manifold
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Old 12-10-2008, 10:50 AM   #17
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Sounds like a fine idea, but i dont think there are any nozzles that would work on an exhaust manifold
I meant the pre-compressor, not pre-turbine.
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Old 12-10-2008, 11:00 AM   #18
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I have played with WI on my diesel truck. You cannot introduce it without some boost already present. Also, WI only works properly/effectively on a vehicle that is slightly underfueled or right on the edge. If you are running rich at all, you will see no benefit. I know it's not exactly apples to apples, but my experience is that it will increase power and effeciency, but not in the low rpms helping with spool. It is a mid and top end power gain. JMO.
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