Does humidity affect engine temp?


23 replies to this topic
  • kree123420

Posted May 31, 2007 - 09:43 AM

#1

So if the temperature outside is 90 degrees with humidity is the engine temperature the same on a 90 degree day with no humidity? My guess would be no, it would be less but I also think the heat created from the engine could maybe offset my logic?

  • Crazyced

Posted May 31, 2007 - 09:47 AM

#2

So if the temperature outside is 90 degrees with humidity is the engine temperature the same on a 90 degree day with no humidity? My guess would be yes, but I also think the heat created from the engine could maybe offset my logic?

If it doesn't change the temp on the thermometer, why would it for your engine? :thumbsup: I may be wrong but that's how I see it.

It seems hotter for us because of biological reasons I think.

  • kree123420

Posted May 31, 2007 - 10:41 AM

#3

Now sadly I never finished high school but I think I remember humid air being denser, so I think the engine would be under slightly more stress in humid air , thus creating a slightly higher temp right? My god am I bored today, this is really splitting hairs

  • Honda125 358

Posted May 31, 2007 - 10:43 AM

#4

I never got into learning how humidity affects the carb, but I think it does affect it.

  • 2strok4fun

Posted May 31, 2007 - 12:38 PM

#5

humid air has less room for oxygen, so there is a slight shift in jetting to the rich side, that could make it cooler, in theory.

  • trailnewcomr

Posted May 31, 2007 - 12:40 PM

#6

i think i will agree with 2strok4fun on this one

  • DGXR

Posted May 31, 2007 - 12:49 PM

#7

Man this topic is really stretching it... Since humidity is basically water vapor in the air, humid air is indeed denser and it makes the air temperature feel a little more "strong" (for lack of a better word). In other words, if it is 90 degrees with humidity, that heat will be transferred to our skin/bodies more strongly and it will feel hotter than 90 degrees dry heat. I doubt it would make any difference in cooling an air cooled engine, but if it did, it would be a very minor effect. Not the gospel truth, just my $0.02. And I think 2strok4fun is right about the jetting. :thumbsup:

  • cleonard

Posted May 31, 2007 - 01:00 PM

#8

It is a minor effect, no more than a few percent, but humid air can hold more heat than dry air. Because of this it will carry away heat a little better. As previously stated it will also slightly richen the mixture also lowering the heat. Both are small effects. Perhaps a few degrees at most.

  • HawkGT

Posted May 31, 2007 - 02:30 PM

#9

...Since humidity is basically water vapor in the air, humid air is indeed denser....


Seems like that would be the case. Turns out it's not [wink]. Water vapor molecules are actually lighter than either the nitrogen or oxygen molecules they replace. Replace is the key word there. As 2strok4fun already mentioned, the water vapor molecules displace oxygen (and nitrogen) molucules. Humid air is less dense AND has fewer oxygen molecules per unit volume.

molecular weights of the molecules involved:
H2O (2 hydrogen atoms and 1 oxygen atom) = 18
O2 (a pair of oxygen atoms) = 32
N2 (a pair of nitrogen atoms) = 28

Seems counterintuitive based on what humid air feels like when it surounds us. But humid air is in fact lighter weight than dry air. Water in it's liquid form is certainly more dense than gaseous oxygen and nitrogen--but the water found in humid air is a gas not a liquid.

What humidity does to the amount of oxygen in the air is likely to have the biggest impact on an engine in most cases. But humidity levels do have other impacts. High intake air humidity (generally) slows deflagration rates (flame speed). I "know" that because of what's been published in SAE papers although I'm not familiar with the precise mechanisms by which this happens. High intake air humidity also lowers an engine's minimum required octane rating by cooling the combustion chamber. This is similar to why some WWII aircraft used water injection systems. Injecting liquid water into the engine has a strong enough impact on deto suppression they were able to run higher levels of supercharger boost for longer periods before engine heat force a dial-back of power. Guess I'm getting of the subject a bit... :thumbsup:

  • RideRaceLive123

Posted May 31, 2007 - 04:46 PM

#10

Seems like that would be the case. Turns out it's not [wink]. Water vapor molecules are actually lighter than either the nitrogen or oxygen molecules they replace. Replace is the key word there. As 2strok4fun already mentioned, the water vapor molecules displace oxygen (and nitrogen) molucules. Humid air is less dense AND has fewer oxygen molecules per unit volume.

molecular weights of the molecules involved:
H2O (2 hydrogen atoms and 1 oxygen atom) = 18
O2 (a pair of oxygen atoms) = 32
N2 (a pair of nitrogen atoms) = 28

Seems counterintuitive based on what humid air feels like when it surounds us. But humid air is in fact lighter weight than dry air. Water in it's liquid form is certainly more dense than gaseous oxygen and nitrogen--but the water found in humid air is a gas not a liquid.

What humidity does to the amount of oxygen in the air is likely to have the biggest impact on an engine in most cases. But humidity levels do have other impacts. High intake air humidity (generally) slows deflagration rates (flame speed). I "know" that because of what's been published in SAE papers although I'm not familiar with the precise mechanisms by which this happens. High intake air humidity also lowers an engine's minimum required octane rating by cooling the combustion chamber. This is similar to why some WWII aircraft used water injection systems. Injecting liquid water into the engine has a strong enough impact on deto suppression they were able to run higher levels of supercharger boost for longer periods before engine heat force a dial-back of power. Guess I'm getting of the subject a bit... :)


Crap....dude whats your job? A rocket scientist? You are :ride: smart.:thumbsup:

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  • trailnewcomr

Posted May 31, 2007 - 06:09 PM

#11

hawkgt ur explanation, although a bit jumbled and inconcise, gives an indirect answer of, humidity should technically, lower engine temperature, of course correct me if im wrong

  • HawkGT

Posted May 31, 2007 - 11:41 PM

#12

hawkgt ur explanation, although a bit jumbled and inconcise, gives an indirect answer of, humidity should technically, lower engine temperature, of course correct me if im wrong


Short answer is----> Yes, there are valid reasons why increased humidity should technically reduce the amount of heat produced by the engine. All by itself, typical changes in humidity probably don't add up to much of a difference though. For any given situation I'd guess there are a couple dozen factors of equal or greater importance.




You're certainly not the first person to accuse my of being inconcise! The longer answer is----> Well, I guess I'd have to divide the question into two parts:

1) How does humidity affect the amount of heat produced by the engine? (I already touched on this but will try and address it more completely)
2) How does humidity affect the engines ability to cool itself?

-Increases in humidity will affect the heat produced by an engine in a couple of ways. I reviewed some research documents (all very well referenced) and understand some of this a little better now. Water vapor acts as a diluent for the combustion charge. This reduces the flame temperature because of the specific heat of the water vapor. "Specific heat" refers to the amount of energy needed to heat up the water vapor to the same temperature as the burning components of the combustion charge. The effect is that the total tempurature of the burning mixture winds up lower than if the water vapor wasn't there in the first place.

-The diluent (the water vapor) also acts to slow the fuel burning rate. I'm pretty sure this is a function of the lower flame temperature, but I can't put my hands on the research data that may explicitly state that. But whatever the precise mechanism, the slower burn rate moves the average combustion later in the cycle which further lowers combustion temps because of expansion cooling.

-Since the water vapor in air displaces an amount of oxygen, a/f ratios get pushed rich (unless of course jetting is changed to compensate for the humidity). Rich a/f mixtures lower combustion chamber tempuratures. Specific heat, enthalpy of vaporization, and lower HP levels all contribute to produce this rich = cooler phenomenon.

-And then there's the reduced octane requirement created by increased humidity. IF an engine is running a fuel that is boreline adaquate in the octane rating department, it may experience occasional knocking/pinging (i.e. detonation). Deto increases engine temperatures by reducing (or even completley eliminating) the very thin boundry layer of insulating gas that prevents the hot combustion gases from actually touching the engine components. Combustion gas tempuratures are hot enough (several times over) to melt the aluminum of a piston. But they never actually come into contact because of the boundry layer. Deto blasts that protective layer away. That phenomenon is a primary mechanism by which heavy deto destroys engines. So IF an engine was experiencing a tiny little bit of deto that was contributing to high engine temps, a bunch of extra humidity in the intake air might relieve that burdon.

So for all those reasons, high humidity will lower the amount of heat the engine produces.


-As for the second part of the question, I'm really not sure how high humidity affects radiator or cooling fin efficiency. Cleonard suggested humid air has a greater capacity to carry away heat. On the other hand, dry air is more dense and I could imagine dense air having a greater capacity to absorb heat. Either way, as Cleanard mentioned--they difference must be small. I would like to know exactly what the answer to that part is though.

  • ohiomotoxer

Posted June 01, 2007 - 06:24 AM

#13

HUMANS feel humidity and wind chill, engine temperature is not in the picture.

Tuning on humid vs non humid days does change AFR.
I drage race and live by grains of water in the atmosphere.
Having a weather station lets me evaluate weather conditions so that I can make adjustments to the ignition timing and carb settings and to determine any adjustment in my predicted elapsed time.

At times, when we have eliminations we sometimes come right back to the line to run again in a few minutes.
My engine temperature is then higher than normal by 20~30 degrees, the ET does not change much because of the higher temp, so the engine being a few degrees hotter does not make much of a difference in the machines ability to run the same numbers.
Now if for some reason say a cold front would move in, THEN my dial would change if the ambient conditions changed.

I learned allot from drag racing. Racing dirt bikes leaves allot of room for tuning errors because there is no way to determine how the machine runs lap after lap.
On the strip you have incremental timers that give you numbers of how you do (aside from rider error or traction loss).
Not to mention racepack data loggers that show how the machine is doing on the run.



So to answer the original posters question:
Does humidity affect engine TEMPERATURE?
NO. Humidity DOES change AFR which in relation WILL change how the engine runs.

You WILL feel differences in the AFR if ambient conditions change during the ride time. THIS is what you will feel as far as performance loss/gain.

  • 2strok4fun

Posted June 01, 2007 - 06:46 AM

#14

High intake air humidity (generally) slows deflagration rates (flame speed). I "know" that because of what's been published in SAE papers although I'm not familiar with the precise mechanisms by which this happens. High intake air humidity also lowers an engine's minimum required octane rating by cooling the combustion chamber. This is similar to why some WWII aircraft used water injection systems. Injecting liquid water into the engine has a strong enough impact on deto suppression they were able to run higher levels of supercharger boost for longer periods before engine heat force a dial-back of power. Guess I'm getting of the subject a bit... :)


I could be talking out my a$$ on this, but I think humidity in the combustion chamber slows delflagration because the water vapor requires more energy (heat) to raise its temperature, and sinse deflagration combustion is the local heating along the flame front to a temperature that the fuel ignites, the water vapor slows combustion. But on the other hand, deflagration transfers the heat by radiation and convection, water vapor would suggest it could transfer heat by convection more easily, maybe its just slower to transfer heat due to the energy requirement.:thumbsup:

As far as water injection, I think that becomes a different animal than water vapor. Because intake charge temp is one of several factors contributing to detonation, liquid water injection reduces the octane requirement of a boosted engine by cooling the intake charge. Water vapor by itself dosent cool like liquid water because it is the phase change, evaporation, that removes the heat energy form the surrounding air. The displacement factor of the water injected is more than replaced by the cooler charge being denser so there is not a power loss.

  • 2strok4fun

Posted June 01, 2007 - 06:59 AM

#15

:thumbsup: :) :ride:
Guess I should of read thru the rest of the replies before I responded, because what I was *trying* to get at was about the specific heat or latent heat of evaporation (enthalpy).

  • HawkGT

Posted June 01, 2007 - 08:19 AM

#16

....I drage race and live by grains of water in the atmosphere....


I appreciate that input. Theory is great and all, but for the end-user the practical impact of it all is what really matters.




....As far as water injection, I think that becomes a different animal than water vapor. Because intake charge temp is one of several factors contributing to detonation, liquid water injection reduces the octane requirement of a boosted engine by cooling the intake charge. Water vapor by itself dosent cool like liquid water because it is the phase change, evaporation, that removes the heat energy form the surrounding air. The displacement factor of the water injected is more than replaced by the cooler charge being denser so there is not a power loss.


I figured I was stretching that comparison a bit. Liquid water injection sure does suck up a lot of heat in a way that water vapor ingestion doesn't.

Here's an interesting tid-bit. It's from a patent for an HCCI engine design. I like it because to me it suggests that sometimes we figure out that something works without really knowing everything about why it works. Also, it backs up what's already been said about specific heat.

....the ambient humidity of air drawn into the engine during the intake stroke affects combustion temperature via dilution of the charge with material that cannot be oxidized and because the specific heat of water is higher than air. Thus, as humidity increases, to keep autoignition timing constant, requires a higher initial charge temperature. However, the inhibition of hydrocarbon autoignition by water addition is much greater than can be explained by the decrease in system temperature. Some researchers in the field theorize that water addition exerts chemical inhibition on hydrocarbon autoignition kinetics....


I figure the really serious research on the subject died out when piston engined aircraft engines took a back seat to turbines. But maybe the newer research on HCCI combustion will bring some of it back....



:) :ride: :ride:
Guess I should of read thru the rest of the replies before I responded, because what I was *trying* to get at was about the specific heat or latent heat of evaporation (enthalpy).


:ride: :thumbsup:

  • Crazyced

Posted June 01, 2007 - 09:35 AM

#17

On the strip you have incremental timers that give you numbers of how you do (aside from rider error or traction loss).

I'm curious, how consistent can a drag racer be that you can confidently blame the time variation on engine performance?

  • ohiomotoxer

Posted June 01, 2007 - 09:40 AM

#18

HawkGT
I appreciate that input. Theory is great and all, but for the end-user the practical impact of it all is what really matters.


Hmmmmmmm I better tell every NHRA/IHRA or anyone else that counts water grains you said so

.........holy crap you've been doing this all wrong.....Professor no WAY you can beat Anderson by knowing water grains......Angelle....don't even bother to unload.......we can't count water grains anymore because its all "THEORY"........


I sO love the internet...............

  • ohiomotoxer

Posted June 01, 2007 - 09:41 AM

#19

I'm curious, how consistent can a drag racer be that you can confidently blame the time variation on engine performance?


Youre kidding right....
Extremely accurate.......

  • Crazyced

Posted June 01, 2007 - 09:50 AM

#20

Youre kidding right....
Extremely accurate.......

I see you edited your post there. :ride: :thumbsup: No I'm not kidding. I'm not familiar with drag racing and I may seems stupid but when you play drag racing video games, it can though to make a really consistent time so I guessed that the real thing would be tougher. :)

That's why I asked also.





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