Does humidity have the same effect as water injection?

Or am I totally off?

 

I doubt it. It's the phase change that causes the heat to be sucked out. So, since it's already in vapor phase, you don't get the benefit of "the latent heat of vaporization" converting the mist (liquid) to vapor.

 

So the vapor in the air wouldn't have any type of cooling effect in the combustion chamber? Not speaking of IAT, rather the effect of lowered EGT.

 

Google tells me that the effects of humidity on the combustion process are negligible, compared to water injection, which is an important process in certain high compression engines at WOT.

 

My first inclination is, no. I know that humid air transfers heat a lot better, but we are talking about extreme heat. If you think about it, water is actually produced in the combustion process and then it "steals" some of the heat generated to vaporize. Pretty sure that's why Lower Heating Value (LHV) is the term people care about because it accounts for that theft so you know that portion of heat can't be converted to work. If you're dealing with so much effing heat that it's easily vaporizing a bunch of water, sending saturated humid air vs completely dry air in shouldn't make a significant difference, I don't think.
If you Google latent vs sensible heat, it should help to understand where I'm coming from.

 

Hey, don't let me be the guy to put this to bed!
I'm sure there are others with some insight into this. I read a tiny bit and I got the impression that it depends on the lean or rich state of combustion, too.

 

interesting i never really though about this i think its something that would need testing however. i believe with high humidity the water isnt really in a "true" vapor form, as evidence by flash rust when welding in a high humidity environment, if it wasnt at least partially liquid form still then it wouldnt be able to vaporize on the metal and cause rust.. i think that this could potentially reduce egt temps some, but not going to be nearly as effective as water injection, more from volume then from the state the water is in. i do believe(and i could be wrong) that humidity isnt really vapor or liquid form completely it is still in a phase change process.

 

The reason I opened this thread is because I lose power in humid weather. But I can make up for it by increasing boost 1-2psi (in extreme cases). So I was wondering if the extra 'water' in the air would have any affect on preventing detonation.

I can understand that water injection is totally different -- water is being added to rather than displacing oxygen. So for power adding (via timing advance) it makes perfect sense.

But I'm trying to compensate for humid days at the track running the same timing, not add power.

 

A google search provides:

Probably the vast majority of people on the street if you asked them would say that humid air is heavier than dry air. After all, humid air has more moisture in it and therefore should weigh more than air that contains less moisture. But, in actuality dry air weighs more than humid air (Figure 1). This is because moist air contains more water molecules which tend to displace oxygen and nitrogen molecules (actual oxygen levels in air are not significantly affected by humidity). Since a water molecule (H20)weighs approximately 36% and 44% less than those of Nitrogen (N2) or Oxygen (O2) respectively, the air becomes lighter as the percentage of water molecules contained in the air increases. Though the density of air does decrease as humidity increases, the effect is actually quite small. For instance, the density of 40oF air decreases less than one percent as the relative humidity increases from 20% to 100% . Though it is true that the density of warmer air changes more with humidity (due to the fact warm air can hold more moisture than drier air), the differences are still very minimal (less than 1%) at temperatures typically seen.

So it would seem that since the oxygen levels are not significantly affected by humidity, the power levels should not be significantly lower..

 

And you're sure that it's a humidity thing, not temp, right? You've directly compared 2 days at the same temp with different humidity levels?

 

As jpc quoted, humid air is less dense than dry air, equating to lower power. Water injection could displace oxygen during intake I suppose if it was in gas phase though I suspect most of it remains as atomized droplets.

 

He quoted that it was less dense by less than 1%. If you're losing power on humid days it's probably more so from heat than humidity seeing as how, according to jpc, actual oxygen levels in air are not significantly affected by humidity.

 

Sorry, I scanned his post too quickly. He started out correctly (in that it is the molecular weight difference of lighter water vapor molecules displacing the heavier O2 molecules that is the basis for both the decreased density and the loss in power) but he came to the wrong conclusion. The oxygen in water vapor is not utilized in the combustion process, so while overall oxygen content may not be significantly affected, the mass of oxygen molecules available for combustion decreases. I would submit that temperature is equalized in most of our intercooler set ups and AIT is about the same at the manifold. The true difference in power is related to air density changes given that temperature and volume are relatively stable in a typical turbo system. Density is mass/volume - it is the mass air (free O2) change that accounts for the power loss. Humid air is going to be less dense with water vapor displacing the available oxygen gas. I haven't done the calculations or looked up references, nor do I have the inclination at the moment, but I believe the logic is accurate.

EDIT: If anyone wants to actually perform the calculations, please note that I believe jpc is misled by data associated with relative humidity, which changes drastically with both temperature and pressure. It is much more representative to use absolute humidity (actual mass of water vapor per unit volume of air) and calculate using reasonable intake chamber temperature and pressure values (100 deg F at least, 1 bar pressure above atmosphere).