Frost on Carb of gasser

Could someone please explain the science of frost on air intake & carb while running a gas tractor during the summer....and winter?

Yesterday used 65 HP gasser w/ loader to move snow, started running rough, frost was on carb. and air intake above carb. Shut down tractor, frost melted, restarted after 30 mins. ran fine no frost.

What causes the frost?...also have this in the summer with a bush hog...
Thanks in advance, this site has a wealth of information thanks to it's followers.

Air flow through the venturi drops the air temperature and them you have frost.
Brian

Where I used to work we had supplied air respirators in the paint booth and just down line from the hood we had a cylinder that was full of "venturies" and air redirectors to speed up the air flow which in turn cooled it and in the hot summer it was just like having air conditioning inside those hoods. Same principal. You can produce ice in 80 degree weather just by moving air!

The frost forms on the outside AND inside. The inside frost is why the engine runs rough, you shut it off and the residual heat melts the ice. Piston driven airplanes have manual carburetor heat that can be applied. Icing is not an issue with fuel injection because there is no venturi to ice up.

I would like to add that the evaporation of the fuel also adds to the cooling effect.

It is due to the venture in the carb. The vacuum on the one side of the venture is where the air fuel mixture is pulled in thru a small opening and exits thru a larger opening thus causing the air to expand helps the fuel air mixture to atomize.

Let the air out of a large tractor trailer tire and watch the frost build up on the end of the valve stem. The escaping air causes a vacuum around the outer tip of the valve stem as the air pressure escapes and expands to a lower pressure which causes a rapid drop in temperature.

An air conditioning system works on the same principal. As the Freon goes from high pressure to low pressure thru an orfice. At the exit of the orfice the pressure turns to vacuum causing the freon to vaporize letting it take on heat from inside the cab of a vehicle. As it absorbs heat and warms the Freon it turns back to a liquid.

Any liquid that turns to a gas requires energy. The science term is called heat of vaporization.

Blow on your hand as hard as you can. Then blow real soft on your hand. Just like a air=conditioner works.

Thanks guys,
what can be done to prevent the frost?
Is the carb out of adjustment?
LR

Like the others say, the venturi effect combined with the temp drop caused by the evaporation of the gasoline. Seems to be more of a problem as the temp gets within 10 degrees of freezing temp on a humid day.
Using Gasoline with 10% Ethanol will pretty much eliminate the internal carb frosting. Another thing you can do is duct hot air from the muffler or exhaust manifold to heat the air going into the aircleaner..

Well if you remember the older car and trucks had a simple air tube going into the air cleaner off the exhaust and what it did was warm to carb so frost would not be a problem. If you simply rig up a side curtain or other such thing to hold in the engine heat so it warms the carb that helps a lot. Other way is simply fire it up and let it warm up real good before you use it but if real cold and humid may not works. Just a simple fact of life and the way a carb works

Same as windchill I know some people say that's balony but my Sonett would freeze up on cold days at the top of the carb before the Venturi and then clean up once the manifold came up to heat.
Real bummer when you are in a hurry and have to stay below 40 to run.
Walt
Ps put a flash pan around the manifold and heat the carb with it.

Have a Farmall B that will do that. I have taken a piece of stove pipe just a little larger in diameter than muffler and sliped it over the muffler and make a tube from stove pipe to the stack of air cleaner. With this on you are pulling warm air from around muffler and sucking it through the carb. With this on tractor performs like its running in the summer.

Evaporation of the fuel and the effect of the venturi are part of the reason for the temperature drop but the major cause is the pressure drop caused by the throttle plate being mostly closed. The closed throttle plate also causes a venturi affect.

At low rpms the vacuum is greatest in the manifold and the volume of both fuel and the air going through the venturi at their lowest. It is also when carburetor icing is most likely to occur. Under heavier loads the throttle plate is more fully open and the manifold vacuum drops off. At this point there is more fuel being evaporated and there is much more of a venturi effect at the venturi but carburetor icing is less likely to occur.

EVERY carb/engine does this in cold temps/high RH conditions. If it offends you, figure out away to get around the laws of physics, or move to a warmer area!

1. Add side panels along side the engine to direct warm air from the radiator to blow across the outside of the carburator to keep it warmer. Plywood, canvas or cardboard will work.

2. Some engines have heat risers to send heat from the exhaust manifold to the intake manifold. Check your owners manual.

3. As others have suggested, build a stove onto the air intake to draw air across the exhaust manifold or muffler to pre-heat the air. Many 1970's and 1980's cars and pickups had this feature before fuel injection became popular.

Carb icing is worst under light loads where the throttle is closed and there is a high vacuum inside the intake manifold.

One of the few formulas I remember from high school physics:

PV=NRT

P (pressure) times V (volume) = N (the amount of gas) times R (a constant depending on the gas) times T (temperature)

What this tells you is that when the pressure drops (because of the venturi) something on the other side of the equation also has to drop to keep both sides equal. The only thing that can change is T - the temperature.

Not telling.....it's magic. :>)

Allan

1. expansion is a cooling process.
2. Evaporation is a cooling process.

You get both thru the venturi section of a carburetor.

Cooling thru the carburetor has been proven to be as much as 60 degrees F.

So, you can experience carburetor ice on a 90 degree day. On both the inside and outside of a carburetor. That's why reciprocating aircraft engines with carburetors have a carb heat system.

Bernoulli's equations relates pressure to velocity:
p/rho + 1/2 v^2=constant. Using the gas law, p/rho= RT gives us RT + 1/2 V^2 =Constant.

So as velocity increases, T decreases. The other situation that is occuring is that gasoline droplets volatilize in carb venturi as the pressure drops and that further decreases the temperature as the change of phase from a liquid to a gas takes energy and that energy has to come from the air so it cools the air even further. The net result is that the surface area around the venturi gets very cold and frost will form on the carb at ambient temperatures as high as 50°F. External frost won'tharm performance as long as the likage is not effected.

If the air is very humid, ice will form on the fuel nozzle and in the venturi, throttle plate, etc internally and cause really ratty running. There is nothing that can be done save heating the inlet air to prevent this type of internal icing. Generally allowing the engine to run ratty will eventually allow the exhaust air to heat the intake manifold on these old tractors and that allows the internal icing to clear up.
Many of you remember when the first EPA emmission regs were applied to cars that heated intake air was used because of the leaned out mixtures used on carbs of that era were particularly susceptable to internal icing. Recip engines used on aircraft use carb heat to preclude internal icing also.

Hope that helps you understand this phenomena.

I had a case backhoe gas that did the same thing. You need to stop the cold air blowing on / into the carb . I could have bought a winter kit from the case dealer but instead sold the machine and bought a diesel.

and on piston planes carb heat is turned on as part of the landing checklist. It's usually only used during landing. Why? You're at altitude where the air around and being pulled through the carb is colder, but most importantly the engine has been throttled back to idle for decent. Idle produces the most vacuum thus the most cooling/icing potential as noted by earlier posters. Icing might not happen at all, but if it did--well, flying is one place having an engine fail to give full power right on cue can be life ending.

Turning on carb heat is usually part of the emergency steps taken if the engine starts to quit on you in flight. Right along with setting the mixture to rich and giving it full throttle etc. Carb icing can cause rough running and lack of full power due to restriction and turbulence of the air through the iced up carb and if it's bad enough ice can freeze up the linkage or butterfly valve preventing you from operating the throttle.

Carb heat pulls intake air from a shroud over the exhaust manifold. Interestingly the switch point for the intake air source is made aft the air filter thus eliminating a problems due to a clogged air filter when using carb heat as well. Given the plane was in the air and the limited use of carb heat, dirty air was not a concern.

You could try this! Theory is it preheats the intake air.