What Does the Carburetor Do?

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http://abbysenior.com/mechanics/fuel.htm

Disclaimer: This article is only meant to give you a general idea of what a carburetor does. It does not necessarily apply in all circumstances to the carb used on karting engines.

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The purpose of the fuel system is to supply the right amount of fuel for the engine to burn, and control the speed of the engine.

What does the Carburetor Do?

2 Jobs:

Mixes air and fuel together in the correct proportion under all conditions.

Regulates the speed of the engine.

The carburetor must mix the air and fuel together in the correct proportions under all conditions, and those conditions change depending upon whether the engine is cold or hot, idling or at high RPM, accelerating, decelerating, or staying the same speed.
To deal with those changing conditions, the carb has different "circuits".
Air fuel ratio will need to be anywhere from 8:1 to 15:1 by weight. We are not used to thinking about air as having weight, so if it was described by volume one gallon of fuel would need approximately 15,000 gallons of air to burn it.
To help us understand how a carb works, we are going to "build" one.
We'll start with a tube or pipe for the air to go through, called an "air horn". We'll mount it on an intake manifold sp one carb will feed all our cylinders, otherwise, we'd have to use a separate carb for each cylinder.





We'll put a mounting flange on the bottom, and where the manifold meets the head, and put gaskets in between the flanges, so that the only way air can get into the cylinder, is through the air horn.
Install a small reservoir called a float bowl (6),off the side of the air horn (1), to hold the gas before it goes into the air horn. We will maintain the level of fuel in the float bowl with a float which rides up and down on the gas (7).





The float controls a needle (4) and seat (3) which shuts off the gas when float level is reached. A vent (2) allows atmospheric pressure to act on the fuel in the float bowl so we don't get a vacuum created and starve the engine for gas. The float can be made of hollow brass, or foam.
Somehow, we have to get the gas from the float bowl into the air horn, this is done with a main discharge tube.





Notice how the one end of the main discharge tube is in the bottom of the float bowl, and the other is in the air horn, with the nozzle just above float level.
We also need the amount of gas to increase with the amount of air.

Bernoille's principle states that: "when the speed of an air stream increases, its pressure decreases".

What this means is that if you force a stream of air to speed up, a vacuum is created in it. This is the same principle that allows airplanes to fly. The top of the wing is a curved surface, while the bottom is flat. As the wing moves through the air, the air moving over the wing speeds up, while the air moving under the wing stays the same speed. This creates a vacuum over the wing (lift). When the lift is greater than the weight of the aircraft, it will fly.
In a carburetor, we put a smaller version of an airplane's wing around the main discharge tube, just above float level. We call this restriction, a "venturi". The faster the air moves through the venturi, the more vacuum is created in it.





There is a pressure differential between the float bowl and the venturi, and so the fuel flows out the main discharge tube and into the air stream. The fuel is drawn up from float level, and out into the air stream where it forms tiny droplets and vapourizes. The faster the air flows, the more fuel comes out. If a smaller venturi is added inside the main venturi, venturi vacuum is increased.




A venturi is simply a restriction in the air horn that forces the air to speed up when it goes through it. The faster the air goes through, the more fuel comes out. This is called "venturi vacuum" and is greatest at fastest air flow.



An "air bleed" causes the fuel to break up into smaller droplets to vapourize better. the smaller the droplets we can break the gas into, the faster it will vapourize.

Remember : An engine runs on gasoline vapour, not liquid. Droplets are still a liquid.



A "main jet " is added to the bottom end of the main discharge tube to limit the amount of fuel. The main jet is just a screw in brass plug with a hole drilled in it. The larger the hole is, the more gas will pass through it, so the richer the air - fuel ratio will be. If you want a leaner air - fuel ratio, put in a smaller main jet.

We need to control engine speed.




This is done by restricting the flow of air into the engine by putting a shaft through the body of the carb, and placing a disc on the shaft. This is the "throttle plate". When it is placed across the air horn, it restricts the air flow and therefore, engine speed. When the plate is moved parallel to the air flow, there is no restriction, and therefore, the engine speeds up.
Any restriction in the air flow creates a vacuum in behind the restriction. Remember the venturi; at high airflow, the venturi was a restriction, and therefore venturi vacuum was created in the venturi. At low airflow the venturi was not a restriction, the air passed through it easily. At closed throttle, the throttle plate restricts the amount of air getting into the engine, and therefore a vacuum is created downstream from the throttle plate. This is called "manifold vacuum". Manifold vacuum is greatest at closed throttle, and is least at wide open throttle. Manifold vacuum and venturi vacuum work opposite each other. When manifold vacuum is high, venturi vacuum is low, and vice versa.

Our carburetor would now work. If we took our drawing down to the metalwork shop, built our carb, and bolted it on an engine, the engine would run. It wouldn't start very well, if at all,....it wouldn't idle,.......it wouldn't run at low speed,....... it wouldn't produce much power,........it wouldn't accelerate......, but if all we wanted to do was drive down the road at 40 MPH it would run! Obviously our carb still needs some work.
We will go through it and solve its drivability problems one by one.

First Problem:

- the engine is difficult to start especially when it is cold, and doesn't run well until it is warm.

Why?
- an engine runs on fuel vapour. Liquids don't burn. The problem with gasoline is that it is a liquid. Before we can get it to burn, we must mix it with air. Gasoline vapourizes very well at high temperatures, but not at low temperatures.

Solution : The Choke System

Because only a 50% of the fuel vapourizes, we must add twice as much to make sure there is enough to burn.
A butterfly valve is added above the venturi, so that when the choke valve is shut, engine vacuum acts on the main discharge tube to draw a huge amount of air into the air stream
Engine cranking speed is from 150 to 300 RPM, so at that speed, the engine's fuel requirement is low. The main circuit is capable of supplying enough gas to allow the engine to run at 2500RPM, so it can easily supply enough fuel for a rich mixture on start. When the engine is cranking over cold, the choke plate should be shut tight, and a huge amount of fuel enters the air stream.

This mixture is the richest mixture of all, and is around 8 : 1. In the drawing above, the choke shaft is centered on the butterfly valve. This is, in fact, incorrect. This would mean that that choke plate would not open when the engine starts, it would remain shut, and the engine would starve for air. The shaft is really not on center, but off towards one side, so the choke plate opens slightly when the engine starts. this allows a leaner air - fuel ratio of around 10:1 when the engine starts.

Because of the extremely rich mixture on choke, it is very easy to "flood" the engine. This is when there is so much liquid fuel in the combustion chamber that there is not enough air to support the combustion and the engine won't run. A choke "unloader" opens the choke plate when the throttle is depressed all the way. This allows the maximum amount of air through the engine to clear out all the extra gas, and allow the engine to start.

Choke Operation:

Manual - a cable runs to the choke so when the driver pulls on a knob on the dash, the choke plate shuts. The worst thing about a manual choke, is that the driver forgets to turn it off.

Automatic - A bi-metallic or thermostatic coil, which is sensitive to heat closes the choke plate when the engine is cold. When the coil heats up, it allows the off - center shaft, and a vacuum device called a "vacuum break", to open the choke. As the engine heats up, the choke plate automatically opens the choke up, leaning out the mixture. The bi - metallic coil has to be heated somehow. It can be heated by heat from the exhaust pipe, electrically, or by water from the cooling system.

Fast Idle

Because of the poor vapourization of fuel, poor fitting engine parts, thick oil, and extra friction inside the engine when cold, the engine would stall at idle if it was left the same as it was when hot. A "fast idle cam" opens the throttle slightly when the choke is on to prevent stalling.