RP 321 - Fuel Crossover Line Protection And Configuration Guidelines: General

The fuel system of diesel engine powered trucks and tractors consists of fuel tank(s), filters, fuel pump(s), injectors, connecting hoses/tubes (lines) and sometimes fuel heaters and water separators. The fuel tanks are generally mounted on the frame rails of the powered vehicle. The filters and separator are generally in the engine compartment, sometimes on the engine. The fuel pumps are generally mounted on the engine and the injectors are located at the engine cylinders.

Fuel injected engines require supply and return fuel lines connecting the fuel tanks to the rest of the fuel system. The return line is needed because more fuel is provided to the injectors than they can use and the excess must be returned to the fuel tanks for cooling and storage.

In order to reduce the number of fueling stops and the cost of fuel, long haul trucks and tractors are generally equipped with two fuel tanks, one placed on each side of the vehicle. These two tanks are usually made to perform as one big tank through the use of a common interconnecting hose called the "crossover line."

Large crossover lines are sometimes provided to facilitate "one-side" fueling. Many jurisdictions require that a fueling nozzle be attended while filling a fuel tank. Vehicles not equipped with crossover lines and operating in these jurisdictions require two attendants to fill both tanks simultaneously.

The crossover line is usually connected to the bottom of each fuel tank to permit the free flow of fuel between both tanks. Connecting the crossover line to the top of tanks, with internal pipes to the bottom requires siphoning action between the two tanks. If air should get into a top-mounted hose for any reason, the vehicle will lose the use of one tank until the air is purged from the line.

There are other fuel line configurations that are being used on vehicles which do not utilize crossover lines. However, these have other design and operational considerations.