Carb Ice--Ultralights

Carb Ice-Ultralights

Any engine equipped with a carburetor is subject to carburetor icing. One purpose of the carburetor is to control the flow of air and fuel.  The other is to cause the atomization of the fuel and to mix it with the air.  The means used to move the fuel into the air is a pressure difference between the fuel supply and the air--this pressure difference is caused: 1) thru the use of a venturi that increases velocity, thereby reducing pressure; and 2) by the restriction caused by the throttle plate or slide. Fuel enters the air thru an orifice (i.e. "jet") that begins the atomization process which is then further developed as the fuel is broken into small droplets by the moving air. The amount of fuel is more or less "controlled" by change in air flow with change in throttle position somewhere in the range of 13-15 parts air to 1 part fuel.

When air is accelerated thru a venturi, the pressure is reduced as is it's temperature. When pressure is reduced by the restriction of the throttle plate, temperature is reduced. Hence air flowing thru the venturi that impacts the downstream throttle plate is colder than ambient and air downstream of the closed or partially closed throttle plate is even colder. 

While the air actually cools the atomized fuel, the relatively volatile gasoline will still begin to evaporate. Gasoline is a mixture of many different fluids that evaporate at different temps. Some of the compounds will evaporate at temps as low as 40F below zero. The evaporation of the more volatile gasoline components will cool the air and surfaces of the carburetor significantly. 

The amount of moisture in the incoming air will determine what happens when the air and carburetor surfaces are cooled. If the air is cooled below the "dew point" then water vapor will condense to liquid and wet the inside of the carb. If the surface/s of the carb are below freezing, then ice will form. IT IS THIS LIQUID WATER IMPINGING ON A CARBURETOR SURFACE THAT IS BELOW 32F THAT FORMS THE CARB ICE. 

So, first there must be enough water vapor in the air that a reduction in air temperature causes the water to condense. Second, the surface/s of the carb must be below 32F.

The chart above shows the various temperature and humidity levels and the corresponding risks of carb ice.  Note that carb ice is always more of an issue during "glide" with a closed or partially closed throttle. 

Note that the difference in the amount of air flowing thru the carburetor from 40% throttle to full throttle is only 10-15% but falls off below 40% throttle. Ice forming below 40% throttle is therefore much more due to fuel vaporization than due to reduced pressure and velocity in the venturi. The reduced pressure causes a higher rate of evaporation and more temperature drop than any temp drop due to the reduced air pressure itself.  A large part of the fuel in this "intermediate" and/or "idle" stage is from the low speed jet downstream from the throttle plate. 

The above assumes a careful pilot that avoids flying in clouds and conditions of very high humidity. 

The Tillotson HR197a Diaphragm Carburetor shown below and used on the MZ201 is a bit different than the Bing 54 Carburetor variable sliding venturi on a Rotax or other “slide” carbs like Mikuni’s on Kawasaki 440. The Bing and most “slide types” are much less susceptible to carb ice. On the Tillotson, carb ice is likely to occur downstream of the throttle plate butterfly. But, any carb where fuel is evaporated and not just atomized inside the carb is susceptible to carb ice under certain conditions.

(Butterfly throttle plate carburetors have different performance characteristics as compared to slide type carburetors. The butterfly types are thought to have better performance during the transition from 60% to full throttle. The butterfly throttle plate is less ideal with less than 50% throttle.)

To offset the heat lost from the carb surface and air to the evaporated fuel in order to raise the carb temperature there are five sources of heat input to prevent or melt ice:  1) Heat from an external source heating the air or the carb itself: 2) Heat added from air flow on the outside surfaces of the carb assuming ambient temps are above 32F; 3) Additional ambient temp air into the carb caused by opening the throttle assuming ambient temps are above 32F; 4) Heat conducted from the engine to the carb via the manifold; 5) Heat radiated from the nearby cylinder head cooling fins.

So, assuming ambient temps are warmer than 32F, increasing the load and RPM by opening the throttle will add heat and increase the carb temperature. Granted more fuel will be added, but the increased air flow at 13-15:1 ratio will overcome the heat lost due to evaporation. 

Remember, during conditions when carb ice is probable, carb temps will almost always be lower than ambient temps which often results in condensation inside and outside of the carburetor. Ice can only form on the inside of the carb if it’s inside surface is below 32F,

The existence of carb ice is difficult to “prove” or document as the “evidence” tends to disappear by melting before the carb can be expected. There are many anecdotal “stories” of “near death experiences” and engine out accidents blamed on carb ice.  Some of these issues may have different causes such as contaminated fuel. But no doubt, carb ice can form and can block the low and intermediate jets causing engine shutdown with a restart being impossible while in flight. 

It is also possible when ambient temps are low, in high humidity for ice on the outside of the carb without internal carb ice being present. 

Most General Aviation planes with air cooled engines have a ductwork system that provides air heated by the exhaust pipes directed into the carburetor, controlled with a “carb heat” control cable. This is impractical for ultralights because of weight. Motorcycles and snowmobiles sometimes have an electric heating element attached to the carburetor body. This is possible and an elegant solution but again can be impractical due to the weight of the necessary large capacity battery.

The photo below from Jack B Hart shows a HR197a Tillotson Carb with a heating element installed. Jack does not share whether it was effective.

One necessary item that needs to be added to an ultralight is a carb temp gauge. I believe the GRT EIS had an option for this readout. The sensor that can be used is similar to those used for CHT. The carb temp gauge will alert the pilot to a cold carburetor where carb ice might form. Testing will determine the correlation of the carb temp gauge reading to the actual surface of the inside throttle bore just downstream of the throttle plate. In other words, a reading of 36F on the gauge may indicate 32F inside the carb.

I conducted a diligent research effort to find any scientific or technical research on the effect of using a typical two stroke cycle fuel/oil mix. I found none. But here is my hypothesis: THE OIL MIXED WITH THE FUEL SIGNIFICANTLY REDUCES THE AMOUNT OF FUEL EVAPORATION INSIDE THE CARB. FUEL EVAPORATION OCCURS DOWNSTREAM OF THE CARB INSIDE THE CRANKCASE WHERE THE MIXTURE IMPINGES ON THE CRANK AND ROD, AND IS VIOLENTLY AGITATED BY THE ROTATING PARTS BEFORE ENTERING THE CYLINDER.  IN ADDITION, THE OIL CREATES A FILM ON THE CARB SURFACES MAKING THEM LESS SUSCEPTIBLE TO ICE ACCUMULATION. HENCE, TWO STROKE ENGINES ARE LESS SUSCEPTIBLE THAN FOUR STROKES TO CARB ICE. 

Carb ice can form in a two stroke engine—ice will simply form more often and more easily in a four stroke. A four stroke needs a more aggressive system to prevent and overcome carb ice.

To prevent carb ice with a two stroke engine ultralight (particularly for engines having carb with throttle plate vs a slide):

1) Avoid flying when ambient temps are in the 30-65F range when the dew point is less than 15F lower than ambient. Keep in mind that temps and humidity are often different at higher altitudes. (Avoid the red zone in the chart) Avoid flying near clouds or in hazy high humidity inversion layers or areas with light ground fog. (Generally afternoon flying is safer than morning flights as the “dew point spread” generally is higher in the afternoon.) This “rule” applies to all engines-even those with slide controlled carbs like the Bing 54.

2) Avoid prolonged periods of operation with less than 50% throttle. Pressure downstream of the throttle is significantly lower with much higher fuel evaporation cooling at low throttle positions. Periodically apply full power to maintain CHT. Avoid sudden reduction in throttle. (Prop pitch is part of the equation—cruise level flight should only be possible with more than 50% throttle and ultralight should descend with 50% throttle. My preference for an ultralight is more a “climb” than a “cruise” prop. This limits top speed, but provides other benefits including faster climbs, less engine stress, and shorter takeoff rolls,

3) During “glide descent” for landings use forward slip to steepen descent so as to avoid prolonged periods with less than 50% throttle. Periodically apply full power to “clear” engine and maintain CHT.  Do not reduce power below 50% until you are sure you can safely land with a possible hypothetical engine out, prop stopped scenario.

4) Diligently monitor Carb Temp and EGT. Ice formation is often indicated by a drop in RPM and EGT.

To deal with impending ice or to melt ice formed: Raise carb temp by increasing load and RPM. In other words, full power and climb at Vx=40mph. This introduces a massive amount of warmer ambient air (if above 32F) and increases cylinder and engine temp. Move throttle to break any ice formed. Remember that the ice, if any is interfering with the low and intermediate fuel jets but not the high speed jet. Identify an emergency landing site just in case. Do not reduce throttle until a “safe” carb temp is reached, signs of ice are eliminated, and an emergency landing site is identified.  Return of the throttle to 50% throttle may cause an engine out if ice is still present.

Another consideration is to use fuel with a relatively lower initial volatility. In other words a low Reid Vapor Pressure and a relatively high 10% Evaporation Temp. Combine this with a 40:1 fuel oil mixture ratio and temperature drop in carb will be reduced significantly from pump gas and 50:1 mix. 

Fuel                             Reid VP   10% Evap

Sunoco 260 GTX         4.7            182 F

Sunoco Optima            7.0            140F

Pump Gas                   9-15           100-120F

Note: Remember that the formation of ice in the carb generally requires the water vapor to change to a liquid. In other words, the temperature of the air needs to fall below the dew point. When the dew point is below 20F, generally the vapor will change quickly to ice crystals and flow thru the carb with little adverse effect.