R U A LS1

Ok me and my friend are in this argument basically because he says a hot day doesnt cause detonation. (if it helps its another stang owner <img src="gr_images/icons/wink.gif" border="0"> )
Ok, i kept telling him that if its a 90* outside air temp. then the air is 90*. He came to a conclusion that moving air (like it is when it blows against your hand) somehow meraculously cools itself. I told him that it feels that way because the moving air is carrying the heat your skin is emmiting away from your body.
My friend was implying that heat (like on a hot day) wouldn't cause detonation because when you are moving at say 65 mph the air becomes cooler. How??? I dont know.
Please tell me im not the idiot here. Because from what he is saying if you were going 180 mph then you would have some 20 more horsepower due to the "fast moving air" cooling down.

[ February 24, 2002: Message edited by: R U A LS1 ]</p>

jmX

Ok, Its no real suprise he bought a mustang. <img src="gr_grin.gif" border="0">

R U A LS1

come now guys i want tons of opinions, or facts i should say. FORGET I EVEN SAID HE HAD A MUSTANG i was just pissed at the time of typeing <img src="gr_stretch.gif" border="0"> Pretend he had a vette, f-bod, viper, etc. whatever.

LS1BZNU

The reason hot air blowing against your skin is because of moisture (sweat, rain, ice, etc). The reason you sweat is to cool the body down. When air is blowing against it, you cool down.

Plan B

I’m not sure either, but doesn’t gas refrigeration work on the principle of gas velocity through expansion after compression. This should work with air as well. (Just not as efficient)

BTW a friend of mine, installed a sheet metal manifold on his 500+ NA rwhp car. Last weekend he started it up, on a 90 deg day with a very high humidity level. I noticed all the intake runners where icing up. At idle.

Engines, do seem to ping more on a hot day. It would be interesting to find out why.

Colonel

Yep, hot air is condusive to detonation and no, just because it's moving air doesn't make it cooler. Like explained above, it only feels cooler because it is moving the heat away from your body faster. This seems to be confusing your friend.

All gases get hotter when compressed and colder when decompressed. I don't see this pertaining to the car moving since the velocity (and therefore pressure) of the air entering the cylinders would be roughly the same whether the car is at rest or at speed.

Of course your intake air temperatures do tend to run higher when the car is slow moving or at rest due to heat buildup under the hood.

Bottom line, heat will make your engine more prone to detonation and pre-ignition.

[ February 25, 2002: Message edited by: Colonel ]</p>

R U A LS1

i was pretty darn sure that i was right about the sweat thing hence me being able to support my "theory" on detonation on a hot day.

R U A LS1

[quote]Originally posted by Colonel:
<strong>All gases get hotter when compressed and colder when decompressed.

Bottom line, heat will make your engine more prone to detonation and pre-ignition.

[ February 25, 2002: Message edited by: Colonel ]</strong><hr></blockquote>
Colonel, i agree with you except this only because of this incident. I know when you decompress a gas is gets colder like a co2 tank, but when you fill one, like when i played paintball a few years ago, the tank would get cool also not hot. You are compressing the air there arent you?

<img src="images/icons/confused.gif" border="0">

Pro Stock John

iTrader: (34)

Basically...

Detonation, or pre-ignition, is when it gets so hot in the cylinder that the mixture ignites before the cylinder has reached the top of the stroke.

I would think that moving air cools more than standing air, but then again what if the air is really hot, I'm not 100% sure.

schultzsj

iTrader: (1)

Moving air cools better than standing air because, as stated, it keeps cooler air in contact with the subject surface. Without air movement a hot surface heats up the air around it. Since heat transfer is proportional to the difference in temperature the heated air surrounding heated surface air slows heat transfer out of that surface and therefore slows cooling.

In addition to this, our skin feels (and is) cooler in a wind because it speeds evaporation of our sweat (evaporation cools). This is by a similar method to the convective cooling discussed above in that in still air the air surrounding our skin becomes more moist from evaporation of our sweat. When air is moist it can no longer support rapid evaporation and cooling therefore slows.

All that said (moving air cools faster than still air), moving air is not any cooler in itself than still air. A thermometer permitted to become steady state will read the same in outside air standing still as it will held out the window of you car in the same air while going 100 mph.

It is true that knock/ping/detonation is ignition of the air/fuel charge in the cylinder before the spark plug is fired. It is also true that compression heats a gas and that the air/fuel mixture in our cylinders become significantly hotter during the compression stroke. It should be apparent that it will be hotter on a 100 degree day where the air being pulled into the cyclinders is basically at 100 degrees that on a 40 degree day. The hotter the air/fuel mixture is, the higher it's energy level is and the more likely it is that it will self-ignite.

Steve

R U A LS1

Everything you guys have said is true, but the fact is, that when air suddenly starts to move whether a southernly breeze or from a fan, the fact that the air is moving isnt the cause of why the air is cool or feels cool on your skin.

My friend just thinks that air in motion (moving air)=the air mass cools itself (cools down) <img src="graemlins/fluffy.gif" border="0" alt="[Fluffy]" />

[ February 25, 2002: Message edited by: R U A LS1 ]</p>

Biochem

[quote]Originally posted by R U A LS1:
<strong>Everything you guys have said is true, but the fact is, that when air suddenly starts to move whether a southernly breeze or from a fan, the fact that the air is moving isnt the cause of why the air is cool or feels cool on your skin.

My friend just thinks that air in motion (moving air)=the air mass cools itself (cools down) <img src="graemlins/fluffy.gif" border="0" alt="[Fluffy]" />

</strong><hr></blockquote>

Your friend isn't a graduate of such fine educational institutions like Spanish River HS or PBCC by any chance... <img src="gr_images/icons/wink.gif" border="0"> <img src="graemlins/gr_judge.gif" border="0" alt="[judgement]" />

As stated, the whole reason your body sweats (and dogs pant) is to cool itself through evaporation. The wind carries off the moisture. Try not to confuse him more by getting into the whole physics involved with high vs. low temp air and its ability to hold moisture though <img src="gr_images/icons/wink.gif" border="0"> That gets into barometric pressure and humidity indexes <img src="graemlins/gr_zzz.gif" border="0" alt="[boring]" />

R U A LS1

how did you know about Spanish River? <img src="images/icons/confused.gif" border="0"> I don't go there. He did though, but only for a year or so and graduated somewhere else. <img src="images/icons/rolleyes.gif" border="0">

[ February 25, 2002: Message edited by: R U A LS1 ]</p>

Scalpel

iTrader: (4)

I have a thorough write-up on this:
http://myhost.virtualgiant.com/killerc5/combustion.html
Have your buddy read it. Maybe it'll click for him but from the info you're giving I doubt it <img src="gr_sad.gif" border="0">

[ February 25, 2002: Message edited by: NastyC5 ]</p>

Visceral

iTrader: (1)

[quote]Originally posted by schultzsj:
<strong>
In addition to this, our skin feels (and is) cooler in a wind because it speeds evaporation of our sweat (evaporation cools). This is by a similar method to the convective cooling discussed above in that in still air the air surrounding our skin becomes more moist from evaporation of our sweat. When air is moist it can no longer support rapid evaporation and cooling therefore slows.</strong><hr></blockquote>

One more reasons a wet surface in a dry environment will cool, even when the surrounding temperature is higher than the object attempting to cool is called the "Latent Heat of Vaporization"

What this says is that when a substance moves between states (water to vapor, or gas), not only does it require the energy to raise the temperature of the liquid, but it requires a little bit more energy on top of that in order to vaporize the liquid. That little bit of energy is deducted as heat, and hence you have a cooler body. This is why water injection is such a popular cooling method for hot moving air.

\chris

Red02LS1

If the air outside is hotter than your body temp, ie 98.6 degrees, then wind will have an adverse effect. It will make your body warmer. So tell your friend to put his hand out the window on the highway when its 105 degrees out, and the wind will warm his hand up.

Freak

[quote]Originally posted by R U A LS1:
<strong>
Colonel, i agree with you except this only because of this incident. I know when you decompress a gas is gets colder like a co2 tank, but when you fill one, like when i played paintball a few years ago, the tank would get cool also not hot. You are compressing the air there arent you?

<img src="images/icons/confused.gif" border="0"> </strong><hr></blockquote>

Did this CO2 come from a tank also? if so it is compressd in that tank, and when you open the valve to the smaller tank, which has little or no pressure in it, that gas will lose some pressure. in order for the gas to move from one tank to the other it has to be at higher pressure in the tank it is coming from than the one it is going to.

Colonel is 100% correct. Gas, when compressed, gets warmer, and when it expands, gets cooler. This principal is the exact same thing that EVERY A/C system runs off of, regardless if it is in a house or a car or whatever.

schultzsj

iTrader: (1)

[quote]Originally posted by Red02LS1:
<strong>If the air outside is hotter than your body temp, ie 98.6 degrees, then wind will have an adverse effect. It will make your body warmer. So tell your friend to put his hand out the window on the highway when its 105 degrees out, and the wind will warm his hand up.</strong><hr></blockquote>

This is only true in regards to convective cooling (or heating in the hotter-than-body-temp example) - it is not true regarding evaporation of sweat. Temperatures above body temperature speed evaporation and therefore cooling if you sweat glands can keep up. The question in this case is, with a wind, does the increased evaporation caused by the higher temperatures (>99 degrees) offset the convective heating effect of blowing hot air.

The answer - only you can tell (it is dependent on your sweat glands and the amount of exposed skin). With me, here in Phoenix where it is very hot and dry, it needs to be in the 110-115 degree range before moving air no longer feels cooler than still air.

Steve

R U A LS1

friend still thinks the same.

Dean98TA

A little more insight to cooling...

It's a physical fact that evaporative cooling is much more efficient than simple convective cooling, but relative humidity of the air plays a HUGE part in determining how efficient evaporative cooling is. RH does not play a part in how efficient convective cooling is.

Say you have a temperature of 95 degrees F, with 85% RH (a typical Houston summer day), you will think it feels much hotter than if you were in Arizona on a 95 degree F day with only 20% RH.

The air temperature hasn't changed. The only thing that has changed is how fast the sweat on your skin can evaporate. Dry air can absorb (evaporate) your sweat faster than humid air. The faster sweat evaporates off your skin, the cooler you feel.

But that is definitely not because the air is getting cooler. Your friend is dead wrong on that one. <img src="gr_stretch.gif" border="0">