Boyles Law related to Carb Icing

[slick1537]

Hello all, I am having trouble understanding something perhaps you can all help with. The basic rule as I understand it is that as air cools, it becomes more dense, and as air warms it becomes less dense. When you talk about carborator icing in the r22, the pressure inside the manifold is being decreased, but the temperature is decreasing aswell. Can anyone explain why this happens?

[Copterpilot213]

I don't have time to give you a full answer but the carburator acts like a venturi tube. The air/fuel mixture speeds up while going through the small area near the butterfly valve, causing it to cool down.

 

I'll try to give a more complete reply later when I have time, if someone hasn't beat me to it.

[Collective Down!]

Hello all, I am having trouble understanding something perhaps you can all help with. The basic rule as I understand it is that as air cools, it becomes more dense, and as air warms it becomes less dense. When you talk about carborator icing in the r22, the pressure inside the manifold is being decreased, but the temperature is decreasing aswell. Can anyone explain why this happens?

 

The carburetor's job is to turn liquid fuel into gaseous fuel. The carburetor's venturi is used to create a suction to draw the fuel into the carburetor and the resultant low pressure also helps the fuel evaporate into a gas. When you change states (from solid to liquid, or liquid to gas, etc.) there is a latent heat exchange. When ice melts into water, it actually absorbes latent heat from its surroundings.

 

Back the the carburetor... When the fuel changes from liquid to vapor, it must absorb latent heat from the air coming through the carburetor. This causes the fuel/air mixture to be slightly cooler than the original air temperature. Now if that incoming air was really moist, and now it's cooled down during this process, the water will precipitate out in the form of ice if that resultant temperature is below freezing (ie carb ice). You can have icing form in the throttle venturi due to the evaporate cooling of the fuel, or immediately down stream of the throttle valve due to the sharp drop in pressure and temperature.

[lelebebbel]

When you pressurize a gas, it gets hot. When you reduce the pressure, it cools. This is a basic thermodynamic principle.

 

Buy a compressed air can at a hardware store and open it - the can will get very cold, and the nozzle will eventually freeze over when the air streams out.

 

Adding the fuel vapors provides additional cooling inside the carburator, but the main cooling effect comes from the reduced pressure.

 

 

When the throttle (and thus, the choke valve) is almost closed, the air has to go through a very narrow opening. This creates a very strong venturi effect right at the choke valve, very low pressure at that point due to the high speed of the air, and a high risk of ice buildup. This is the reason why the POH for the R22 says: "Full Carbheat below 18in"

Edited October 9, 2007 by lelebebbel

[Goldy]

This causes the fuel/air mixture to be slightly cooler than the original air temperature.

 

Slightly cooler could mean as much as -40F...which is why you are in danger of carb ice with temps below 75F.....

[Sparker]

off topic: I love this stuff.

[Helihead]

some one already said it, increasing velocity decreases pressure, so the temp drops as well.

 

But how is boyles law related?

 

I don't know what boyles law has to do with it I am sure it has something to do with it maybe i guess. i am looking and boyles law says for a given mass, @ constant temp, pressure times the volume is constant. (for an ideal gas)

 

So Boyles law

 

Is this what would happen,

the pressure could decrease and volume could increase and the TEMP WOULD NOT CHANGE.

 

SO for carb icing SINCE THE PRESSURE DECREASES and the MASS STAYS THE SAME that is why the TEMP DECREASES TOO?? but what happens to the volume?

 

(so what is gasoline after it is atomized?it is not an ideal gas, so does boyles law still apply?)

(would its voulume or mass change as it changes states from liquid to gas?)

 

I guess what I am getting to is IS IT BOYLES LAW THAT APPLIES and if not what law does apply. or is it charles and gay lussacs law or not either one?

 

We know what conditions we are more likely to get carb icing, and that it does happen, as long as we get the basics, we are ok, i was just curious

Edited October 9, 2007 by Helihead

[kodoz]

Is this what would happen,

the pressure could decrease and volume could increase and the TEMP WOULD NOT CHANGE.

 

SO for carb icing SINCE THE PRESSURE DECREASES and the MASS STAYS THE SAME that is why the TEMP DECREASES TOO?? but what happens to the volume?

 

(so what is gasoline after it is atomized?it is not an ideal gas, so does boyles law still apply?)

(would its voulume or mass change as it changes states from liquid to gas?)

 

I guess what I am getting to is IS IT BOYLES LAW THAT APPLIES and if not what law does apply. or is it charles and gay lussacs law or not either one?

 

Ugh, there are many gas laws and several different effects in the carburetor contribute to the temperature drop. Also, the effect of the air entering the carburetor is separate from the effect evaporation of the fuel has, even tho both contribute to cooling.

 

Pressure decrease + volume increase = cooling of the air as it moves through the carburetor (other's have already clearly explained this; Bernoulli, Boyle, Charles, and Gay-Lussac all apply). Bernoulli decreases the pressure, Boyle tells us that a change in volume accompanies a change in pressure, and Charles and Gay-Lussac tell us the relationship between those changes and the temperature changes that have to occur too. Unless you're taking a class or building a better system, knowing what dead guy's equation to use is probably more involved than you need.

 

Evaporation of the fuel-->the change in state involves heat exchange. Boyle et al don't apply to liquids (altho there are probably similar hydrodynamic equations), but the principle is the same: the "pressure" of a liquid is greater than that of a gas. As it's volume changes, its pressure changes and heat is absorbed (ie, it absorbs heat). The analogy of fuel evaporating off your hand works--your hand cools because the atomic bonds in the fuel absorb heat from your body when they evaporate. In the carburetor, atomized fuel is still liquid, but it then evaporates and causes the cooling effect.

 

Forget ideal gases--There are no ideal gases. This is a theoretical principle that describes how molecules in a gas should interact to make Boyle & company's laws work out.

 

Mass changes don't happen except in fission and fusion reactions (remember conservation of mass? Mass ~ weight, and in the carburetor, mass doesn't come into play). Pressure and volume tell you how much mass you're shoving into a space, temperature tells you the energy contained by that mass.

 

One thing that's always screwed with me conceptually is how compressing a gas (eg, in a turbine) causes its temperature to increase. I understand the equations but not the physical principles.

[Helihead]

Thanks, for the info.

[betr_thn_Icarus]

Seems to me that the volume is the same. I look at it like this: I want to transport 1000 gallons of water down 2 seperate tubes in the same amount of time. One tube is 4 inches and one is 12 inches. I want all of the water to travel into the new location at the same time. The water passing through the 4 inch tube is going to have to travel a lot faster for a given time period, but the end is still 1000 gallons so the volume stays the same. Your pressure is less multiplied by a constant volume. The only thing left to change is the temperature. This is similar to the d=r/t type formula. When you change one variable the opposite one gets the effect.

 

Favorite quote from this thread is by lelebebbel.... "This is a basic thermodynamic principle."

 

Not insulting you, I know what you really mean but it's a funny comment.

 

 

 

-SOK

[slick1537]

I do believe I got it after talking to my friend who is an engineer. Basically the volume inside the manifold is a constant. When you decrease pressure, there are less molecules inside the manifold to bounce around and hit each other, thus creating less heat. However outside of the vacuum the gas law does not apply.

[beckwith]

One thing that's always screwed with me conceptually is how compressing a gas (eg, in a turbine) causes its temperature to increase. I understand the equations but not the physical principles.

 

I don't understand the equation at all but I remember one of my teachers in high school had sort of a silly analogy to explain it. it was: if you take a bunch of people and pack them into a room they keep bumping into eachother and tempers flare. so in in compresed air the molocules all run into each other creating heat, from friction I think.