Have you ever had a gasoline-powered engine in a car, truck, or tractor cold start fine, but a few minutes later, the rpm is dropping, the engine chugging and blowing black smoke from the tailpipe? Eventually, it stalls. By the time you can restart it, the problem has miraculously healed itself. Then you have experienced the ice age that engineers were fighting. It is called carburetor icing and is a real devil to eradicate.
What the battle is about
To understand how ice can form in a carburetor, it needs to be recognized that it has a venturi. By nature of design, the venturi is wide and then tapers and eventually widens to its original dimension. The purpose of the venturi is to speed up the airflow and cause a depression, or in other words, a low-pressure region. The low pressure working in conjunction with the atmospheric pressure on the fuel in the float bowl allows a carburetor to fuel the engine. A side effect of the air going through the venturi is that the temperature drops substantially.
The best example of how the air temperature drops while going through a venturi was something, we all did as a kid. We would blow up a balloon and then, while creating a restriction, let some of the air slowly out. We would notice that not only would the air cause a whistling sound, but it would be much cooler.
That is precisely what is happening inside a carburetor venturi. Now add cold and damp air into the mix, and the throat of a carburetor becomes an ice-making machine.
The phenomena can be called either carburetor or intake system icing. The restriction of the throttle plate causes a cooling effect. If the air is moisture-laden, then it has the opportunity to create ice.
The conditions required for carburetor icing are cool, moist air with a temperature as warm as 55 degrees F and as low as 13 degrees F, under the proper conditions.
In most instances, once the ambient air temperature gets down in the low teens, the propensity to ice is significantly reduced and is almost nonexistent. The confusing part for many is that a carburetor will suffer the most dramatic icing with an ambient temperature between 45- and 50-degrees F, well above freezing.
At lower temperatures and humidity levels, there is not enough water in the air to cause a problem. At higher ambient conditions, the temperature drop caused by the evaporating gasoline is not enough to freeze any condensed moisture.
Thus, carburetor icing is the result of both the venturi’s cooling effect and the transformation of the liquid fuel to a vapor, which by the tenet of the latent heat of vaporization, drops the temperature.
The engine’s objection to idle, which first shows itself by slowing down and running rich, is due to the ice forming on the throttle plate(s) and effectively making it larger and choking off the air. Once the ice build-up gets large enough, the throttle plate’s opening created by the idle speed screw is negated, and the engine stalls.
In truly short order (30 seconds to a minute), the ice melts from the ambient heat, and the engine starts and runs fine as if nothing happened. Usually, by now, it is warm enough under the hood for the condition to not reoccur. You then drive the tractor off, not giving it much concern and thinking it was an anomaly – that is until the weather conditions are right again.
This event is known in the industry as idle icing.
Though carburetor icing at idle and low engine speeds is the most prominent, it can also occur at high speeds, long after the engine has been running. This is an incredibly challenging problem to diagnose. It is identified as cruise icing.
When cruise icing takes place, it does not attack the throttle plate. If it did, it would be moot since it is opened a good deal under load and cannot choke off the airflow.
When cruise icing happens, the engine begins to lose power but will idle fine. This is due to the ice building up in the venturi and blocking the main metering system’s passage. The engine is running on only the idle system even though the throttle plate is open. The drop-in power can become so substantial that the tractor can come to a complete halt, idle fine, and the engine revs up with the transmission in neutral and no load.
Historically, you look under the hood to search for the extreme lack of power, allowing the carburetor to heat, soak, and melt the ice. The tractor is then driven away with no apparent problem. Cruise icing is a severe and complex problem to identify. It can happen at the same high ambient temperature that will evoke idle icing.
Attacking the problem
The battle to eradicate both idle and cruise icing was fought on two fronts: supplying heat to the carburetor and with the help of the oil companies by modifying the gasoline to include anti-icing agents.
The most effective means of combating icing, though, is by its archenemy, heat. The problem was that both gasoline and engines do not want heat all the time. Heating the intake manifold and carburetor is both a friend and a foe.
Gasoline has only a 50% vaporization rate when the ambient temperature is as warm as 60 degrees F. By introducing heat near the carburetor, you can significantly improve the fuel’s vaporization when the engine is cold and during the intermediate stage between full cold and operating temperature. But this comes at a cost once the full operating temperature is met.
Heating of the charge (fuel and air mixed together) displaces oxygen and reduces the engine’s power. A general rule is that for every ten-degree F change in charge temperature, power is impacted by one percent. When the charge is cooler, power goes up, and when it is hotter, power goes down.
Thus, the heat would need to be introduced to combat icing when the engine was cold and all the way through as it approaches fully warm. This worked very well to control idle icing, but cruise icing could happen an hour or two in the tractor’s fieldwork if the conditions were correct.
The methods to control icing were centered on using exhaust heat from the cylinder head exhaust port. They had it near the intake manifold and the carburetor mounting pad. The heat from the exhaust gas would then migrate up into the carburetor and not only help vaporize the fuel on a cold day but increase the temperature in the venturi and around the throttle plates to not allow icing to occur.
A component of the icing was also the volatility of the gasoline. As time went on, the composition of fuel was altered as a byproduct of evolution. The needs of the engine and environmental concerns changed. The volatility of the fuel has an impact on its propensity to allow icing.
The oil companies did their part to help fight icing and used two different approaches – cryoscopic and surfactant.
Cryoscopic anti-icers are used to lower the freezing point of water and are predominately alcohols or glycols. It was common to use isopropyl alcohol, hexylene glycol, di propylene glycol (known as DPG), and other water-soluble oxygenated compounds. The amount of these additives varied by geographic region and gasoline manufacturer and was not mandated by any government standard.
Thus, by the nature of their chemical compounds or the lack of them, some fuels may be more prone to carburetor icing.
Some oil companies used to advertise that they offered a “winter blend” of gasoline and touted their fuel’s ability to provide proper cold-weather performance without ever mentioning carburetor icing.
Another approach with gasoline was one based on a surfactant. With this theory, the moisture in the atmosphere and the venturi’s cooling effect, and the phase change of the fuel were not addressed. In contrast, the gasoline’s surfactant would coat the carburetor and throttle plates’ throats to not allow ice crystals to attach to them. Even though the icing was still occurring, it could not stick to the carburetor parts, so the engine was benign to the event.
As with most things, there were other variations on the mechanical approach for controlling ice formation. Some manufacturers ran engine coolant through a spacer fitted between the carburetor and the intake manifold mounting pad.
This was not a standalone approach, though, since the coolant would be slow to build heat on a cold day while the exhaust gas was hot upon the first combustion event. It was a means though to further control both the vaporization of the fuel and fight ice formation.
The interesting thing about carburetor icing was that though every company took design steps to limit and eradicate it, for some reason, there were engines and carburetor styles that seemed to be more prone to the phenomena than others.
For example, when I was a kid, the first tractor I recall that we had was a Hart-Parr Oliver Row Crop 70 tricycle. That engine was prone to carburetor idle icing while our Ford 9N was not, but my 1968 Dodge Dart Slant-Six was.
The more you are around engine, the more you respect the fact that they are like people or animals. They have a distinct nature to “catch” specific ailments while others do not.
That is what makes them so wonderful and a subject of our affection!