Fuel Lines (Automobile)

9.3.

Fuel Lines

The components of the fuel system are connected by fuel and vapour lines and hoses. These allow supplying fuel to the carburettor, to return excess fuel to the tank, and to carry fuel vapour. Fuel lines must be routed so as to remain as cool as possible. If any part of the line is accessible to much heat, the gasoline passing through it vapourises more rapidly than the fuel pump can create suction. Lower pressure or partial vaccum in the fuel pump also causes fuel to vaporise. This condition produces a vapour lock due to which the fuel pump delivers only vapour to the carburettor and also the vapour goes out the bowl vent without the engine receiving any gasoline.
The vapour-return line normally runs from the fuel pump or the fuel filter to the fuel tank. This vapour-return line is connected to a special outlet in the fuel pump. Any vapour that forms in the fuel pump returns to the fuel tank through this line. The vapour-return line also permits excess fuel being pumped by the fuel pump to return to the tank. This excess fuel, due to continuous circulation, helps keep the fuel pump cool.
Some vapour-return lines have an in-line check valve, which prevents fuel from feeding back to the carburettor from the fuel tank through the vapour-return line. In normal operation, the pressure of vapour from the fuel pump unseats the check ball and allows the fuel vapour to flow to the fuel tank. If, however, fuel tries to feed back to the carburettor, the pressure of the fuel forces the check ball to sit, blocking the line.
In some fuel system a vapour separator (Fig. 9.8) is connected between the fuel pump and the carburettor. The separator consists of a sealed can, a filter screen, an inlet and outlet fitting, and a metered orifice, or outlet connecting to the fuel tank. The bubbles of vapour, which reach the separator alongwith fuel, rise to the top of the vapour separator. The vapour is then forced by fuel pump pressure to pass through the outlet pipe to the fuel tank where it condenses into liquid.


Rigid Lines.

All fuel lines which are fastened to the body, frame, or engine are of seamless steel tubing. Steel springs are also wound around the tubing at certain points to protect against damage. Wherever fuel line is replaced, only steel tubing should be used. Copper and aluminium tubing should not be substituted for steel tubing. These materials do not withstand normal vehicle vibration and may also react with gasoline chemically.
In some vehicles, rigid fuel lines are secured along the frame from the tank to a point close to the fuel pump. The gap between frame and pump is then bridged by a short flexible hose, which absorbs engine vibrations. In other vehicles a rigid line is directly run from the tank to pump.

Flexible Lines.

Synthetic rubber hoses are used in most fuel systems where flexibility is needed. Connections between steel fuel lines and other components of the system are often made with short hose sections. The inside diameter of fuel delivery hose is generally larger (8 to 10 mm) and of the fuel return hose is smaller (6 mm).
Materials of vapours vent lines must be capable of resisting attack by fuel vapours. A metal or plastic restrictor is mostly used in vent lines to control the rate of vapour flow. These are located either in the end of the vent pipe, or in the vapour-vent hose itself (Fig. 9.9). When used in the hose instead of the vent pipe, the restrictor is required to be removed from the old hose

Vapour separatorRestriction in the fuel vent line.
Fig. 9.8. Vapour separator. Fig. 9.9. Restriction in the fuel vent line.

Fuel Line Mounting.

Fuel supply lines from the tank to the carburettor are rounded to follow the frame along the under-body of the car (Fig. 9.10). Vapour and the return lines are usually on the frame rail opposite to supply line, but may also be routed with the fuel supply lines. All rigid lines are fastened to the frame rail or under-body by screws and clamps or clips. Hoses are fastened to steel lines by hose clamps.
Fuel line routing.
Fig. 9.10. Fuel line routing.

Fittings and Clamps.

Brass fittings used in fuel lines are either of the flared type or the compression type (Fig. 9.11). Flared fittings are more common. During replacement of tubing,
a double flare should be used to prevent the flare from cracking and to ensure a good seal. Compression fittings have a separate sleeve, a tapered sleeve, or a half sleeve nut to make a good connection. Various types of clamps are used to secure fuel hoses.
Fuel line fittings.
Fig. 9.11. Fuel line fittings.

Next post:

Previous post: