Fuel Connections
A common problem with fuel systems is poor connections for pressure fittings. Another common problem is overtightening compression fittings. Overtightening the compression fittings will create a leak because the fittings will no longer seal.
Cleanliness during assembly is especially important. Ensure that the fuel tanks are cleaned out before installation. Ensure that the fuel tanks are cleaned out before filling with fuel.
The connection for the fuel supply line on the engine is a 1/2 inch UNS thread. The connection for the fuel return line on the engine is a 7/16 inch UNS thread.
Fuel Tank
Fuel tanks should have the following features:
- The filler neck should be raised so that water will not enter the fuel tank.
- The filler cap should be sealed so that water will not enter the fuel tank.
- A vent tube must be installed on the fuel tank. Ensure that water will not enter the vent.
- A drain will need to be installed on the fuel tank so that water and sediment can be removed.
- Internal baffles may be necessary to prevent the fuel from surging.
- The fuel tank should have a removable panel so that the inside of the tank can be cleaned.
- The piping for the fuel tank should be as simple as possible. Use the minimum number of valves and connections. This should lower the amount of problems with feeding the fuel to the fuel transfer pump.
- A water separator is required between the fuel tank and the fuel transfer pump. To avoid problems with venting the air after draining the water separator, the water separator should be located below the normal minimum level of fuel in the tank.
- The fuel tank needs at least two connections. A fuel supply line and a fuel return line connection are needed. A fuel tank should only supply fuel to one engine. However, if two engines are using one fuel tank, then each engine must have a fuel supply line and a fuel return line.
Typical Fuel Systems
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| Illustration 1 | g00745802 |
Typical arrangement of a simple fuel system (1) Fuel supply line (2) Water separator (3) Fuel valve (4) Fuel return line (5) Fuel tank | |
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| Illustration 2 | g00745803 |
Typical arrangement of a simple fuel system with connections at the top of the fuel tank (1) Fuel supply line (2) Water separator (4) Fuel return line (5) Fuel tank | |
A fuel system that is simple is the best choice. Refer to Illustration 1 for a typical arrangement. Some applications may require fuel to be drawn from the top of the tank and returned to the top of the tank. Refer to Illustration 2 for a typical arrangement.
Fuel tanks can be made of steel, aluminum, or fiberglass. Rubber fuel tank bags may also be used. The fuel supply line should be located at the rear of the fuel tank. This will ensure that all of the fuel is available when the vessel is moving. The fuel return should be located near the bottom in order to prevent air locks when the engine is stopped.
From the fuel tank, the fuel supply line goes to a water separator. The fuel lines may be made of copper or steel tubing. Compression fittings or soldered nipples may be used. Rubber fuel line may be used to connect the fuel lines to the fuel transfer pump. This will help prevent breakage when the engine moves on the flexible engine mounts.
The simple fuel system is satisfactory when one or more engines are drawing fuel from a single fuel tank. The system may also be used when one engine uses two fuel tanks. In this case, a balance line between the tanks should be installed. A shutoff valve should be installed on the balance line.
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| Illustration 3 | g00791472 |
(2) Water separator (5) Fuel tanks (6) Vent line (7) Fuel supply tank (8) Fuel returns (9) Fuel supply | |
Some applications may require multiple fuel tanks to achieve a certain operating range. Refer to Illustration 3 for a typical arrangement of multiple fuel tanks.
One fuel tank should be the main fuel tank when multiple fuel tanks are used. The other fuel tanks should be arranged so that the fuel drains into the main tank via gravity. The fuel system that is shown in Illustration 3 should be used if gravity can not be used. A supply tank is fed from the fuel storage tanks. The fuel supply line and the fuel return line is connected to the fuel supply tank. A vent line should be installed between the fuel supply tank and any one of the fuel storage tanks. The supply tank should be located below the fuel storage tanks.
An ideal arrangement would be one fuel tank to each engine. With the one fuel tank for each engine arrangement, only one engine will be affected if water or sediment is in the fuel tank. A simple fuel system will require the minimum number of valves and fittings. Again, this should ensure better performance during service.