Filter & fuel servo (FI) icing and heated alternate air

[glider4]

I'm installing one of the AFP fuel injection systems in my IO-360B2B powered RV-8A. I will be based in damp Southern Oregon where we do have frequent icing potential during the winter and early spring months.

I'm using Van's Filtered Airbox (FAB) to direct and filter air before it reaches the Airflow Performance fuel servo. I know I'm are not suppose to get icing with fuel injection system and therefore do not need alternate heated air (a.k.a. carb heat) before the filter. But the AFP fuel servo has a large butterfly valve serving to control mixture. Isn't the lower pressure/higher velocity around the butterfly valve one of the two causes of icing in carbs? Obviously there's no cooling from fuel evaporating in the AFP fuel servo so that doesn't contribute to an icing potential. I am also thinking that an alternate air source of heated air would reduce the build up of ice on the air filter itself.

Is there any benefit to having an alternate, slightly heated, air source for use with the AFP fuel servo? Or would there be any harm, other than slightly reduced engine performance, in having a slightly heated alternate air source? I already have installed the unfiltered alternate air source "gate" in the FAB for emergencies.

I would appreciate thoughts on the wisdom of adding a "carb heat" alternate air source for use in this setup.

Albert Thomas
N880AT
RV-8A QB
(FWF & wiring)

[asav8tor]

I read your post and I'm not sure if you are asking either or both. You already installed the alt air after the filter. Are you thinking of switching it or adding a second alt source (heated)?

[andrew phillips]

As I understand carb ice, it's cause is the drop in temperature caused by the expansion of the fuel as it moves through the venturi in a carb. This lower than ambient air temp allows the moisture present in the air to freeze in this location. With fuel injection the fuel is usually delivered right behind the intake valve where there is little or no potential for freezing. The air moving through the rest of the system should not experience any real temperature drop so icing should not be an issue unless you are flying through freezing rain in which case you have gigger things to worry about!

[glider4]

As I understand it, carb ice is primarily caused by two factors. The biggest factor is the cooling caused by liquid fuel droplets evaporating into a gas state. The second factor is the cooling caused by a decrease in pressure as the air flows past a restriction - in this case the big butterfly valve in my fuel servo.

I already have installed the air filter bypass in the FAB and am considering whether or not in install the "carb heat" bypass flap in the FAB. If I would never need it, I'd rather not install it.

Here are my questions:
1. Can you get "carb ice" from just the cooling caused by air going past the large butterfly valve in my fuel servo unit?
2. What's the possibility of an air filter collecting ice to the point where air can't get past the filter?

Al Thomas

[SvingenB]

Carb icing is primarily caused by evaporation of fuel on the bytterfly valve itself, cooling it - and the air - down to the point where ice crystals forms directly on the valve, not after. On throttle bodies like the Ellison, you will not have any carb ice, but you can still in some severe circumstances get ice on the manifold due to the evaporation of liquid fuel running down the manifold.