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Old 13-07-2004, 02:09 AM
hotrod hotrod is offline
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Join Date: Oct 2003
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Default Yes

That info is consistant with the NACA studies.

The boiling point of water drops significantly with changes in pressure. This is due to the waters vapor pressure getting progressively closer to the atmospheric pressure.

With a vacuum pump, you can actually boil water at room temperature.

With water injection the water dropplets in the air stream that get heated during their passage through the intake port would certainly evaporate faster when they see the sudden pressure drop across the intake valve. Even if the air was saturated (could hold no more water vapor) in the intake port, after passing the intake valve and warming up its relative humidity would drop significantly as warmer air at a lower pressure can hold more water vapor.


The remaining micro dropplets would then be evaporated during the compression stroke, this will reduce the work necessary to accomplish compression and that more isothermal compression is one of the sources of the extra power and torque from water injection.

In those same NACA studies they broke down the relative sources of heat gain of the intake air charge, when they discovered that the final pressure in the cylinder as the intake valve closed, is very close to the manifold pressure. This means that most of the loss in VE in an engine is due to heating of the intake charge during the intake stroke.

I don't recall the exact proportions at the moment but the two dominant sources of heating of the intake fuel air charge is heat picked up from the hot intake valve head (which would be significantly reduced due the presense of water dropplets in WI ), and the thermal effects of the near sonic flow past the valve ie. waste heat from all the turbulance and frictional heating of the high velocity air flow.

Again with a higher water content in the intake air charge of a WI engine it would take significantly more heat energy to increase the air temp due to the latent heat of the water present.


[edit] --
These are two of the reports:
http://naca.larc.nasa.gov/reports/19...aca-tn-675.pdf

http://naca.larc.nasa.gov/reports/19...aca-tn-839.pdf
summary:
35 % of air charge heat gain is due to heat transfered from the intake valve and seat to the air charge.
30% of the air charge heat gain was due to inlet valve flow resistance and this effect rose quickly with rpm. Intake air temp went up in these tests 26 deg F at 1500 rpm and 12 deg F at 1000 rpm.

Lowering the intake valve and seat temps by 45 deg F gave the same VE improvement as a reduction in the temperature of the intake air charge of 5 deg F.

During these tests typical intake valve temperatures at 1000 rpm ranged from 620 deg F - 393 deg F.

It is intuitively obvious to me, that cooling of the intake valve/seat and reduction in heat gain during the intake process is likely a significant factor effecting improved MEP on WI engines.

It makes one wonder what would be accomplished by a very small spray nozzle in each port runner aimed at the valve head. I understand that in CART type race engines they forgo the normal fine spray injectors and spray a jet of methanol directly on the back of the valve head --- (I don't recall where I saw that tid bit).

This report discusses a very similar event when they were studying intake air flow, they saw a dense fog form in the cylinder even during very low humidity conditions when the piston moved down the bore on the "expansion stroke" (what would have been the combustion stroke on a running engine). This was due to the pressure drop and the heat lost to the cylinder walls during the compression stroke.

http://naca.larc.nasa.gov/reports/19...report-653.pdf


Larry
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