The carburetor supplies the engine with the correct air-fuel mixture for all conditions of operation.
- It atomizes the fuel and mixes it with air.
- It controls the delivery of this correct mixture to the engine.
Carburetors come in different designs.
This is a basic downdraft carburetor, with a fixed-size venturi. Fuel is pumped to the float bowl where a float and a needle valve control its level. The float bowl is open to the atmosphere. The barrel of the carburetor has an air horn and a venturi. A throttle valve below the venturi is linked to the accelerator pedal. Its action allows airflow into the carburetor barrel.
The shape of the venturi constricts air-flow and lowers pressure there. The pressure difference between the bowl and the venturi causes fuel to spray out of the fuel discharge nozzle. It mixes with the air, and the mixture passes the throttle valve and enters the intake manifold, and the engine.
Depressing the accelerator increases air speed through the carburetor. This lowers air pressure at the nozzle. Atmospheric pressure on fuel in the float bowl stays constant, so more fuel is forced into the venturi to mix with the increased air. This keeps the air-fuel ratio roughly constant for a range of throttle openings.
The throttle valve also controls flow of mixture into the engine. Opening it lets more mixture be delivered, which increases engine power and speed. Closing it has the opposite effect.
Mechanical fuel pumps
Fuel pumps on carbureted systems can be electrical or mechanical.
The mechanical fuel pump for the carburetor system is usually mounted on the cylinder head or the engine block.
It has a flexible diaphragm. That’s a flexible piece of neoprene rubber separating 2 chambers. This diaphragm is operated by an eccentric on the camshaft. It rotates, making the rocker arm move. This movement is transferred to the diaphragm, pulling it down. That draws fuel into the pumping chamber, above the diaphragm. The diaphragm spring moves the diaphragm up, and this forces fuel from the pumping chamber, out of the pump and into the carburetor.
When the engine needs more fuel, the diaphragm moves through a long stroke to pump a lot of fuel.
When less fuel is needed, pressure builds up in the fuel line to the carburetor, and in the pumping chamber above the diaphragm. The diaphragm spring can’t push the diaphragm up so far, and the pumping stroke is reduced.
Some pumps also have a return line to send excess fuel back to the tank. As the fuel circulates, it cools the fuel pump and lines, and reduces the chance of vapour locks.
Electric fuel pumps
Carburetor systems can also use electric fuel pumps.
Most of these pumps are located outside the tank, though some are inside the tank.
One widely-used pump is the diaphragm type. It has an electrical section, and a mechanical section.
When the ignition is switched on, current magnetizes the solenoid. The magnetic field energizes the armature, which pulls down the diaphragm. This creates an area of low-pressure that draws fuel into the pump.
Pulling down the diaphragm breaks the circuit and stops the current. The solenoid is no longer an electromagnet. The armature is released, and the diaphragm spring forces up the diaphragm, which forces fuel out of the pump and on its way to the carburetor. This action continues, delivering fuel each time.
When the engine needs less fuel, pressure builds up in the fuel lines to the carburetor, and reduces how much fuel is delivered.
Some carburetor-equipped vehicles with electric pumps have a safety switch that prevents the pump continuing to run if the engine stops.
Tanks & lines
Where the tank is mounted depends on where the engine is, and on space and styling. Safety demands that it is positioned well away from heated components, and outside the passenger compartment.
Most tanks are made of tinned sheet steel that has been pressed into shape.
Some passenger car tanks are made of non-metallic materials. Aluminium or steel is used on commercial vehicles.
The tank is usually in 2 parts, joined by a continuous weld around the flanges where the parts fit together. Baffles make the tank more rigid.
They also stop surging of fuel and ensure fuel is available at the pickup-tube.
Fuel expands and contracts as temperature rises and falls. So fuel tanks are vented to let them breathe.
Modern emission controls prevent tanks being vented directly to the atmosphere. They must use evaporative control systems.
Vapor from the fuel tank is trapped in a charcoal canister, and stored there, until it is burned in the engine. A vapor or vent line with a check valve connects the space above the liquid fuel with the canister. This valve opens when pressure starts to rise, and lets vapor through, but not liquid.
Liquid fuel closes the check valve and blocks the line, stopping liquid fuel reaching the charcoal.
Some systems have a small container, called a liquid-vapor separator, above the fuel tank. It also prevents liquid fuel reaching the charcoal.
This gasoline tank has a small separator tank and a number of vents.
They’re from different parts of the tank so that for as many vehicle positions as possible, at least one is always above the level of gasoline in the tank.
Fuel lines
Fuel lines are usually made of metal tubing or synthetic materials.
A fuel supply line carries fuel from the tank to the engine. A return line may also be provided to allow excess fuel to return to the tank. This helps prevent the formation of vapor that can occur in the fuel supply during hot conditions.
The fuel filler is where fuel enters the system. The filler neck is a pipe that extends above the fuel tank.
On unleaded gasoline vehicles with catalytic converters, the filler neck is designed to prevent leaded fuel being added. Its diameter is smaller than those on leaded vehicles, and a trapdoor inside the filler can only be opened by the nozzle of the unleaded gasoline spout.
The filler cap seals the filler neck to keep out water and foreign objects. Water can corrode internal passages of fuel pumps and carburetors.
The cap on the gasoline tank also stops gasoline vapor escaping and polluting the atmosphere. This is important for gasoline which is very volatile and vaporizes easily, especially in warmer climates.
Some gasoline caps have a low pressure valve built-in. It keeps a balance between the pressure in the tank and the outside atmospheric pressure.
As gasoline in the tank is used, the air space above the fuel increases. This causes a fall in pressure, compared to outside atmospheric pressure. The valve then opens and lets more air into the tank.
A fall in temperature can cause a fall in the pressure in the tank. The valve opens to admit more air till inside and outside pressures equalize again.
The fuel gauge shows how much fuel is still in the tank. It has a gauge unit on the dash, and a sender unit in the tank. This unit indicates the level of fuel in the tank, and transmits this information to the gauge in the dash-panel.
Charcoal canister
The charcoal canister is used as an emission control device and consists of a sealed canister of activated charcoal with connections on the top of the canister. It’s used as a temporary storage device for fuel vapor from the fuel tank.
When the engine is stopped, vapor from the tank passes through the systems vent pipe into the canister where it is absorbed by the charcoal.
When the engine is running, the lower pressure in the inlet manifold draws in the gasoline vapor out of the charcoal, into the intake manifold.
The process is controlled so it doesn’t upset the normal operating air-fuel mixture
Carburetor filters
Fuel must be kept clean, or the jets in the carburetor could be blocked, or corrosion can occur. So fuel systems always contain filters.
In a carbureted system, a fine filter of nylon or gauze is fitted to the pick-up pipe in this gasoline tank. A large surface area is provided to allow adequate flow rate, and better filtering.
An in-line filter, which is usually made of a clear or opaque plastic, is normally fitted either between the tank and the pump, or between the pump and carburetor.
This filter traps dirt, and water. The amount of contamination can usually be seen through the plastic.
