How Carburetors Works in Old Classic Cars
The automotive carburetor is an intricate device which must perform automatically over a wide range of engine operating conditions, such as varying engine speeds, load and operating temperature.
Its major function is to meter, atomize and distribute an explosive mixture of fuel and air to each combustion chamber. The amount of mixture delivered is regulated by the throttle.
To be specific, the carburetion system must supply the engine combustion chambers a mixture of gasoline and air in such a state that it will explode when a spark is put to it.
Liquid gasoline doesn’t explode, but because gasoline is very volatile, it vaporizes very easily, and when enough vaporizes, any slight spark will set it off. It is the vaporized mixture that explodes, not the liquid gasoline.
If you let gasoline sit in a glass for a little while to evaporate, just bring a lighted match, or a lit cigarette near it it will explode.
After the explosion, it will continue burning rather slowly.
What are we getting at here; why does it explode if it is allowed to evaporate ?
And how does a carburetor “evaporate” enough gasoline into the air quickly enough to cause up to 20,000 explosions per minute.
This is what happens in some of today’s high power engines. This is what the carburetor provides for. Does it sound like it might be a complicated device? It’s not. Especially if we understand what makes it work.
So before we begin to describe the operation of the carburetor, let’s take a look at some basic principles to help understand its operation.
Atomization means breaking the liquid fuel into very small particles, so that it can be rapidly vaporized and mixed with air. In the carburetor, this is accomplished by spraying the liquid into a swiftly moving air stream which dries the liquid fuel (evaporates it) and mixes it thoroughly with the air.
This is what was mentioned earlier. It is quite difficult to get liquid gasoline to burn. It is necessary to vaporize or evaporate it. That is why when we left the glass of gasoline sit for a while, we could have had a minor explosion.
Therefore, we must find a means of quickly evaporating a fair amount of gasoline.
How about vaporizing it?
Vaporizing the gasoline is to create a fine spray which consists of many fine droplets which can quickly evaporate.
To aid vaporization, an air bleed can be provided. By means of an air bleed, some air is admitted into the fuel before it is sprayed out of the nozzle.
This helps to break the liquid into a fine mist as it leaves the nozzle. This whole process is referred to as atomization.
Complete combustion demands a correct mixture ratio between fuel and air commonly called the air / fuel ratio. Too much fuel results in what is called a rich mixture, while too little fuel results in a lean mixture.
To obtain all possible energy by combustion, the right amount of fuel must be mixed with a given amount of air.
The metering job of the carburetor is to furnish the proper air/fuel ratio for all operating conditions, so that engine operation will neither be too lean for power requirements or too rich for economy.
For good combustion and smooth engine operation, the air and fuel must be thoroughly and uniformly mixed and delivered in equal quantities to each cylinder and evenly distributed within the combustion chamber.
Good distribution requires good vaporization. A gaseous mixture will travel much more easily around corners in the manifold and engine. If the fuel is not atomized, the liquid particles being relatively heavy, will try to continue in one direction and will hit the wall of the manifold or travel to another cylinder.
To compensate for these problems, manifolds are tailored to the engines to minimize the sharp corners and thus provide as smooth a flow as possible.
The carburetor’s principal job in distribution is to break up the fuel as finely as possible and furnish a uniformly vaporized mixture in the manifold.
We know what we’ve got to do with the fuel –
1) atomize it (make it burnable)
2) meter it (provide the proper amount of fuel/air mixture)
3) distribute it evenly throughout the incoming air.
Of course this all occurs within the carburetor,
but how ? How do we squirt the gasoline into the incoming air stream to vaporize it?
Certainly we know the fuel pump doesn’t do this, it simply keeps the float bowl chamber full. But yet this same fuel must be squirted or drawn into the air stream.
Let’s look at how it might be done.
When you drink through a straw, what actually happens? The liquid in the glass is under atmospheric pressure.
Place the straw into it and some liquid comes into the straw but not higher than the liquid level in the glass and still under atmospheric pressure.
Now put the straw into your mouth and begin to draw on it. What happens is, you lower the pressure in the straw. That is, you create a partial vacuum.
There is now a pressure differential between the liquid in the straw and that in the glass.
The liquid in the straw has less pressure on it from the air in the straw than does the liquid in the glass which is affected by atmospheric pressure.
So what happens ? It can’t just sit there like this.
You know you do drink through a straw.The greater pressure on the liquid in the glass forces that liquid in the straw up to your mouth.
This is the same thing that happened in the fuel pump. When the diaphragm was raised by the rocker arm, it lowered the pressure on that end of the fuel line and the air pressure in the tank forced the fuel into the pump chamber.
Let’s give up drinking for a while and get back to mixing fuel with the air flowing through the carburetor.
All we’ve got to do is connect the float chamber to the air passage, create a vacuum in the air flowing through and we’ll be able to draw up fuel into the aiestream.
Can we create a vacuum in air which is at atmospheric pressure?
Remember that this atmospheric air is not stagnant; it’s flowing. Consider air flowing through a tube.
The pressure exerted on all sides of the tube is the same, both inside and out. But if we build a constriction at some point in the tube, the air flowing past this constriction is forced to speed up as it passes through.
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.