[racprops]

Here is a very good and deep article:


Rudolf Diesel invented the compression ignition engine in 1897. Shortly thereafter, advances in the refining industry produced gasoline--but not in its modern form. Back then, gasoline was basically a derivative of kerosene, a waste byproduct of early oil refining. It was considered a nuisance and was disposed of by being dumped onto the ground or into rivers.


An exhaust stove (arrow) pulled heat--but not exhaust gas--from the manifold and fed it to the carburetor.
Soon, however, inventors recognized that gasoline's high energy potential made it an ideal fuel, something that could advance engine development. There was a problem, though: Gasoline does not burn in liquid form. It needs to be atomized, emulsified and vaporized to ignite. This means it must be broken down into small particles, be mixed with air, and undergo a phase change through heat. The carburetor's job is to accomplish the first two steps, while a process called the latent heat of vaporization takes credit for the last step.


Vaporizing the fuel:

Early inventors were puzzled by the characteristics of gasoline; it took some time and innovation for scientists to fully understand its properties. We don't know who first discovered that gasoline's vapors are what burns, not its liquid form. One theory is that the discovery was triggered by lighting strikes after the unwanted fuel was discarded--dumped liquid gasoline leeched into the soil, but its vapors in the air above could still catch fire.


One property of gasoline is that the more heat it is exposed to, the more volatile its vapors become. In order to use gasoline as a fuel for an internal combustion engine, something had to heat it, forcing it into a vapor phase. That something is the carburetor.


Changing gasoline from a liquid to a vapor requires energy. The liquid molecule needs to break its bond and become a vapor in much the same way water evaporates from your skin on a hot day. The water gains the needed energy by taking a tiny amount of heat from your skin; that is what helps keep you cool. The heat does not disappear; rather, it is converted into the vapor molecule's added energy. As the vapor molecule moves away from your body, it takes this energy, called latent heat, with it. When the vapor molecule condenses back into a liquid, it releases latent heat, warming its surroundings.


Gasoline's phase change occurs in the intake manifold and requires the fuel to be fully atomized.


A poor atomization rate will result in only a small percentage turning to a vapor. Therefore, it's critical that the carburetor be efficient at breaking down the liquid gasoline particles and mixing them with air.


Just like water evaporates from skin, gasoline evaporates, changing phase to a gas, while traveling to the cylinder head intake runner. This process requires the addition of heat, and vaporization ends up cooling the intake manifold: The latent heat is transferred to the vaporized fuel and leaves the surrounding area at a lower temperature. The more thorough the rate of vaporization, the greater the amount of work the engine performs for the fuel consumed. If the rate is poor, the amount of potential energy (measured in BTU) obtained during the conversion process is correspondingly low.
Friend and foe

To the engine designer, heating the induction path has both a positive and negative effect.


Heat is necessary for gasoline to vaporize. A general rule that still applies to modern gasoline is that, at a temperature of 60 degrees Fahrenheit, only 50 percent of the liquid gasoline converts to vapor. At near freezing temperatures, that percentage declines to approximately 20 percent. At -45 degrees, there is no chance of vaporization and the gasoline engine will stop running. For the engine to start at this temperature, an external heat source is required.


These temperatures are not the ambient air temperature; rather, what has to be considered is the temperature in the intake manifold where the phase change will occur. Thus, it's possible to run a gasoline engine at -45 degrees or colder when the underhood temperature is above the ambient air temperature.


Due to the poor rate of vaporization of gasoline, carburetors require a device called a choke. The choke enriches the air/fuel ratio so there is a sufficient amount of vaporized fuel to allow the engine to start. Most early engines would crank at a mixture strength of 2:1 (two parts air to one part fuel) and warm up at a rate of 7:1. The mixture would gradually lean out to the desired 14.7:1 ratio, which is the stoichiometric value of gasoline. "Stoichiometric value" describes the exact recipe of air and fuel that will allow for the most complete release of chemical energy from the mass of fuel consumed.

Though heat is a friend to a cold engine, it is a foe once the engine is fully warmed up.


As air is heated, it becomes less dense; because oxygen is required to support combustion, the volumetric efficiency of the engine suffers. For approximately every 10 degrees the air is heated, the engine's power drops by 1 percent.


Additionally, heated air makes an engine more prone to abnormal combustion, better known as detonation. The inverse also applies: On a very cold day, an engine seems more powerful, because the air being ingested is packed with more oxygen molecules.


Thus, auto engineers needed to find a way to heat the incoming air when the engine was cold, to aid start-up and driveability during the intermediate temperature phase, while not limiting volumetric efficiency during normal operation.


Carburetor icing

The problem of carburetor icing has been around as long as the carburetor itself. The vaporization of the fuel removes heat from the intake manifold and carburetor venturi, which can cause ice deposits to occur when moist intake air condenses in the carburetor body or base plate assembly.


This normally happens at ambient temperatures that are above freezing, and can occur even up into the middle 50-degree range. Icing is most common, though, with air around 25 to 45 degrees that has a relative humidity of 80 percent or greater. At lower temperatures and humidity levels, there is not enough water in the air to create an icing problem; at higher ambient temperatures, ice won't form.


Ice deposits can be identified by the region of the carburetor on which they form and ultimately restrict the flow of both the air and mixture. Where the formations occur varies with carburetor design and engine application and use, and the location of the ice determines how the driveability is impacted.


If ice forms on the throttle plates, it will have a significant effect on the air/fuel ratio only at very low throttle openings or when the throttle is closed completely, such as when the engine is required to idle. This type of icing is known as idle icing and is most common in city driving during the engine warm-up period. During idle icing, the throttle plates become covered with ice and choke off the air, causing the engine to stall.


Ice can also form in the venturi of the carburetor during cruise conditions; this is called cruise icing. These deposits tend to cause a power loss during cruise conditions, because the deposits restrict air flow and can restrict the signal to the main metering circuit. The drop in power can become so great that the vehicle may come to a complete halt. This problem is hard to diagnose, because it takes only a few minutes for the ice to melt and the power to return if the engine is shut off, thanks to the heat soak that warms the carburetor when the engine has stalled or has been turned off. Oil companies work hard to develop additives to limit icing, but the laws of Mother Nature still prevail.


Some like it hot
During the early days of the automobile, many different methods of heating the incoming air to aid in fuel vaporization were tried. Detroit's various approaches can be broken down into seven categories:


All of these methods had their benefits and disadvantages. Using the engine coolant as a heating method did not do much to help the fuel vaporize right after startup. It took a few minutes for the coolant to gain any heat from the running engine; even then, the temperature built slowly. The slow rate of heat transfer into the coolant and then into either the carburetor or intake manifold was beneficial during warmup, but not at the initial start of the engine.


Exhaust gas proved to be much more effective; as soon as the engine fired, the exhaust temperature immediately reached a few hundred degrees and could warm the area where vaporization occurs. Placing the intake and exhaust manifolds in close proximity was an inexpensive and simple method to heat the fuel, but severely limited engine performance when the engine was fully warmed.


Packard's "fuelizer" was another solution, one that required a small combustion chamber in the intake manifold to create a flame and heat the charge. Though very effective, the fuelizer faced almost the same ignition temperature quandary that the manifold solution did. Its benefit was the combustion area was very small and could be ignited easier than the cylinder bore. Gasoline was supplied by a bypass pipe from the carburetor and the fuelizer employed its own dedicated spark plug.


Eventually, hot exhaust gas was chosen as the most practical and effective way to aid the vaporization rate. The system evolved into the modern heat riser valve, which directs spent gases from the exhaust manifold through the cylinder head and under the carburetor pad of the intake manifold on the floor of the plenum.


With any type of external heat source, a thermostatic control was needed to monitor the engine temperature and cut off heating when necessary to improve volumetric efficiency and octane tolerance.


The battle to vaporize fuel is still being fought today. Electronic fuel injection is designed to have the nozzle spray against the intake valve. With the valve face exposed to the combustion chamber, it quickly builds temperature and allows the conversion rate to improve greatly on a cold engine, when compared to a carburetor application.


But one of the greatest uses of fuel vaporization has been forgotten by most Americans. During World War II and the battle between Rommel and Patton in the African theater of operations, fuel was in very short supply, especially for the Germans.


Patton recognized this and some historians argue that a young G.I. named Pogue came up with an idea of placing an electric heating coil in the venturi of the carburetor used in the tank engine. Though it limited volumetric efficiency, the heated air greatly improved fuel efficiency. Patton then formulated a battle plan that got the German commander to chase him around the desert until Rommel ran out of fuel. Still mobile, thanks to Pogue's efficient engine adaptations, Patton then went in for the kill.


It may be that recognizing the latent heat of vaporization helped America win the war.


A personal note is how may cars now have a cooler plastic intake manifold, where this make more HP I suspect it also lowers possible good MPG…it is an interesting point that Power can help MPG but more power also lower MPG…I wonder at what point these two thing pass each another?