Repost of "Is this possible or probably" mistakenl

[Forum Admin]

I will get the hang of forum administration eventually:

Richard L post:

I have read a few SAE papers on this topic - no details but passing comments, can anyone make some sense on the following claims:

During the induction stroke, when water-mist/air is induced and passed the inlet valves, there is a progressive pressure drop as the piston is moving downward. Two things are happening concurrently - Further evapoarion of water droplets will accelerate due to pressure drop inside the chamber as well as the heated surroundings prior to the exhaust stroke.

Water inejction will have a positive effect on the volumetric efficiency.

It would be great with someone can say if that is possible or probable? I have no view on this claim.

My response previously:

Add to that the improvement in VE as a result of absorbing the heat risidual in the valve on the way into the cylinder, plus the heat of friction move past the valve (maintains charge density).

Please add feedback.

[cheekychimp]

You might want to ask "hotrod" specifically as he is more knowledgeable than me. If however I am to understand correctly what he has told me before, I would sum up this theory in the following manner:-

All liquids have relevant freezing and boiling points. They also have points of vapourization dependant upon both temperature and pressure. By this I mean water in the cooling system remains liquid under pressure despite the fact that it's temperature is above 100 degrees celsius.

In the example above, water below 100 degrees would remain as a liquid. Unquestionably the excessive temperatures inside the combustion chamber would vapourise the water, but from a physics point of view, I would not have thought that the pressure drop would be a significant factor in the process of vapourization once the inlet valves are open because air is being drawn into the chamber replacing the 'vacuum' or low pressure area that would other wise exist.

The Volumetric Efficiency issue is more complicated. An increase of Volumetric Efficiency caused by evaporation inside the combustion chamber will probably occur but would be minimal at this point. Far mor important from a VE point of view I would have thought is the cooling of the air intake tract creating denser air 'ready' to be sucked or forced into the combustion chamber on the intake stroke.

What I am saying here is that, if air enters the combustion chamber at 70 degrees celsius (theoretical example) far more will get in than if that air was at 140 degrees celsius.

WI has been proven as an alternative to intercooling, but I would suggest on the basis of what I have read that it is only effective in suppressing detonation by reducing heat in the combustion chamber and NOT in increasing VE by reducing heat in the intake tract.

But I would welcome other views.

Paul.

[hotrod]

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

[hotrod]

I finally found the reference I mentioned in the post above regarding valve head cooling from fuel injectors.

It is in this post:
http://www.supras.com/~riemer/sonict...rs/RCtech.html


Here's the excerpt:

At 8000 RPM the intake valve is opening and closing at 66 times a sec. and is only open for an average of 9 Mil/Sec. At this cyclic rate the transient time to complete the delivery of fuel, from injector to valve, is critical. This is why; Indy car injectors are very precisely targeted and timed to provide a solid stream of fuel with non-existent atomization, LBDS, "Laser Beam Delivery System". In these engines the injectors can discharge fuel, at a "just prior to valve -open position", and get it all down the hole. As the fuel impinges the hot intake valve it virtually vaporizes and mixes quite well with the incoming air forming a very homogenous charge. This is one of the most extreme situations but it's a real interesting one. As an added benefit, the latent heat of fuel vaporized in the chamber also provides charge cooling that makes the mixture denser. A denser, heavier mixture (cold and thick) will produce more power then a thin (hot and light) charge.

...

It's a known fact that you can't burn fuel until it's atomized. It's also known that you can't burn fuel without air. The most important, of all known facts is that you can't burn anything, if it's not in the combustion chamber. The secret is to provide adequately atomized fuel with as much air as possible. Adequately atomized is the Secret Word of the day. Fuel does not have to be completely atomized at the injector tip (SMD of 10um - 20um) but it does have to get past the valve to do us any good. The more condensed the fuel delivery is the faster it will travel, (regulated by discharge area and pressure) and the more accurately it can be targeted. Resent (S.A.E.) "Injector Atomizing and Targeting" studies have provided us with one of the most prominent advances in High Performance Engine Management. These test programs have concluded that "accurate impingement onto the center of the valve head is vital for good vaporization" and "The targeting orientation of the injected fuel spray is a critical parameter in fuel evaporation" also that " Fuel injected directly onto the intake valve yields a significantly better engine response" (SAE950506) What all this means is, different engine designs require a different type of injector to operate efficiently and that 100% atomization is not always required or desired. In racing situations we usually have to do the best we can with what we have, or what's available. The goal, is of course, is to do the best in all cases, and in all situations. The best injector for your engine is the one that will yield an optimal fuel-air mixture and provide the required power output consistent with smooth and reliable operation. This is our goal, and all things considered, we feel that we provide an excellent service in this very specialized field.

The whole article is a very interesting read in its own right as well.

Larry