how does WI affect spool?

[rallimike]

As I understand it, WI lowers combustion temperatures, and, I assume, exhaust gas temperatures. Forgive my ignorance, but wouldn't that mean that the turbo will spool more slowly? Wouldn't both the velocity and volume of the exhaust gases be decreased? Conversly, would WI increase the efficiency of a turbo with a smallish hotside?

[hotrod]

Yes and Yes (sorta).

Yes spraying too early will slow down the spool up. That is why most folks do not trigger the WI until they get up near 70% or so of max boost.

As far as helping a turbo with a small hotside, it will probably help some but not a lot. If you cool the exhaust gas too much, although the gas volume drops, so does the speed of sound in the exhaust flow, and that changes the maximum speed that gasses can pass through the throat of the turbine. Cooler gasses reach sonic flow at lower speeds so you won't gain as much as you might hope.

Larry

[Richard L]

As I understand it, WI lowers combustion temperatures, and, I assume, exhaust gas temperatures. Forgive my ignorance, but wouldn't that mean that the turbo will spool more slowly? Wouldn't both the velocity and volume of the exhaust gases be decreased? Conversly, would WI increase the efficiency of a turbo with a smallish hotside?

I firmly believe that water injection should only be activated when the combustion temperature becomes destructive. Injecting water when your engine is running at an acceptable combustion temperature (refected by the egt ), will loose power and slow down spooling time for sure.

If there are excess heat available (high combustion temperature), the gaseous water will accelerating the spool up time for sure. I am really having problem explaining this.



Ideally you want plenty of hot gas out of the exhaust without the extreme temperature to make up the volume.

[rallimike]

Hi Richard
For the year 2005, there sure seems to be a lot of 'black art', as opposed to science, in WI. Maybe you need to design a 2E, where WI is triggered by EGT.

[JohnA]

....I firmly believe that water injection should only be activated when the combustion temperature becomes destructive. Injecting water when your engine is running at an acceptable combustion temperature (refected by the egt ), will loose power and slow down spooling time for sure..

How very true.

Below is an extract from the excellent book Internal Combustion Engine in Theory and Practice, Vol 1, Second Edition Revised.
I've copied/pasted this from my website.
http://www.max-boost.co.uk/max-boost/intake/WI.htm
Please note that I've only kept the 'core' of the calculations, to make the point clear.

(I have combined and simplified examples 12-3 and 12-4)

Effect of Humidity:

Suppose we have an 8cyl engine producing 300bhp at 4500rpm, 60F and dry air.

Power at humidity 80% and temp 120F (very hot and humid): 230bhp (dry air: 268bhp)

Power at humidity 50% and temp 20F (very cold and humid): 318bhp (dry air: 324bhp)

Note how power is always down when humidity is present.

Another effect of humidity, example 12-5:

Engine producing 300hp, at 100F, humidity zero.

Same engine only produces 243hp, still at 100F but in saturated air.

Here just the effect of reduced air density lost us 57hp, yikes.

[fastedd]

firmly believe that water injection should only be activated when the combustion temperature becomes destructive. Injecting water when your engine is running at an acceptable combustion temperature (refected by the egt ), will loose power and slow down spooling time for sure..

Johns post indicate that humidity reduces an engines power. Fair enough. However we don't just inject water to lower EGT or combustion temps, I thought we injected water on turbo cars to lower charge temps as well.

If your able to lower the charge temps with water injection, doesn't that increase the density of the charge and therefore power? I guess you have to limit water injection to the points when you need to cool the charge and/or lower combustion temperatures?

So if humidity decreases power but WI increases air charge density i guess it would also be beneficial to find the crossover point when one dominates the other. Assuming that boost pressure has a relationship with charge temperature, I guess this means not setting the pressure switch which activates boost at too low a value?

I guess this also implies that an intercooler cooling a charge vs water injection would be more effective (assuming the same drop in temps) because the intercooler does not increase humidity?

[JohnA]

I used to believe that WI can be used as an 'extra' layer of safety, just like most people do.
However, experience has shown to me that this is not the case.
I currently see two options in real life:

1. You prepare and map everything without WI --- and then use WI as an extra antidet protection, accepting the cost of a few dozen horsepower (oh yes, it can easily be more than that!)

2. You prepare and map everything for operation with WI: well into DetLand, by conventional tuning standards. Ludicrous advance can (and SHOULD) be dialled in while WI is running, and ludicrously lean mixtures can (and SHOULD) be dialled in under boost.
Not for the faint of heart :lol:

A friend was mapping a 2lt turbo yesterday, with one of the most prominent Autronic tuners. He said how surprised the tuner was with the advance angles he could safely dial in, he commented that he had never seen this before. Then he mentioned that he had WI activated at 1 bar, lol...
No wonder, eh, the 0.5mm nozzle on a 2lt engine running 17psi throws quite a few water molecules in between the fuel molecules. Combustion slows down big time.

WI is the equivalent of retarding the ignition by several degrees. That's why a lot more advance has to be dialed in, to compensate for this. Or else lots of power is lost and spool is slower than before.