The fuel system on a diesel engine is a highly specialized set of components which…
Fuel Storage Systems
Bulk fuel is usually stored in large main storage tanks and transferred to smaller auxiliary tanks (service
tanks or day tanks) near engines by electric motor-driven pumps as shown in Figure 3.
If auxiliary tanks are not necessary, the main fuel tank must provide a ready fuel supply to the engine-mounted transfer pump.
Main Fuel Tank
The main fuel tank represents the primary fuel reservoir in all applications, and must have adequate capacity for the intended application. Rule of thumb for tank size is to find the fuel consumption rate at 100% load factor (depending
on application: Prime, stand-by etc.) and multiply it with the number of hours between refills. Fuel consumption rates are shown on the Engine Technical Data Sheets for the specific engine. Additionally, 10% should be added to the result; 5% for expansion at the top of the tank, and 5% for sediment settlements at the bottom.
A power plant with one (1) 3516B diesel generator set, rated for 1145 bkW (1560 bhp) at 100% load. The fuel rate for the engine is 284 L/hr (75 G/hr) as found in TMI.
The time between tank refills is based on weekly fuel tanker truck deliveries, so refill time is 168 hours.
Tank vol. = 284 x 168 x 1.1 = 52,583 L
Tank vol. = 75 x 168 x 1.1 = 12,600 gal
Auxiliary Fuel Tanks
Note: Additional clarification is needed for C175. Reference TMI and the special instructions REHS4726.
Auxiliary fuel tanks, service tanks and day tanks are secondary fuel tanks located between the main fuel
tank and the engine. These tanks are required in the following situations.
– The main fuel tank is located on the same level but more than 15 m (50 ft) away.
– The main fuel tank is located 3.7 m (12 ft) or more below the engine.
– The main fuel tank is located above the engine fuel injectors.
Any of the above conditions can cause unsatisfactory engine starting and operation. The purpose of an auxiliary tank is to relieve the fuel pressure “head” from the fuel transfer pump and injection equipment for efficient fuel flow.
A manual fuel priming pump, offered as an attachment, or an electric motor-driver boost pump may allow operation under conditions more severe than those previously described; but where starting dependability is required, Caterpillar recommends the use of an auxiliary fuel tank.
Auxiliary tanks offer convenient and ready fuel storage while providing a settling reservoir for water, sediment and sludge. An example of an auxiliary fuel tank is shown in Figure 4.
Fuel Service Tank or Day Tank
Auxiliary tanks such as fuel service tanks or day tanks can be quite simple. It usually consists of a small metal tank, either floor or wall mounted, in the immediate vicinity of the engine. The tank is usually sized to hold approximately eight hours of fuel, based on the engine’s fuel consumption rate at full load.
Refilling can be accomplished by gravity, a hand pump, or with a motor-drive pump. Motor-drive
pumps can be either manually or automatically controlled. For convenience and safety, automatic control is usually employed using a float-actuated, electric motor-drive pump.
The refilling pump can be positioned either at the auxiliary tank or at the main tank outlet. The performance capability of the pump must be considered during placement.
Features of the auxiliary tank, as shown in Figure 5, should include the following.
- Fill line – Located above the high fuel level, with outlet baffled to prevent agitation of sediment in the tank.
- Delivery line – Located near the bottom but not so low as to pick up collected sediment or condensation.
- Return line – To carry excess fuel back to the auxiliary tank. Should have its outlet baffled for the reason described above.
- Overflow line – Allows excess fuel to return to the main tank in event of overfilling of the auxiliary tank.
- Vent line – Allows air pressure to equalize as tank is drained or filled (vent cap should be located away from open flame or sparks).
- Drain valve – Allows removal of condensate and sediment.
- Sight glass or float-type gauge – Provides a positive check on fuel level.
To prevent damage to the fuel filter housings, the return line should have no valves or restrictions to allow dangerous pressure buildups.
Flexible rubber hoses, used as fuel return lines, should be supported to prevent closing off over time due to
weight of the hose and fuel. Hard fuel lines prevent this problem, but a flexible connection is still required to isolate vibration between the line and the tank.
A nonflammable tank mounting will maximize fire protection.
The overflow line should be at least two pipe sizes larger than the fill line. To simplify engine maintenance, a shut-off valve in the supply line is useful.
The delivery line, carrying the fuel to the engine-mounted fuel transfer pump, and the return line, carrying excess fuel back to the tank, should be no smaller in size than the respective fittings on the engine.
Larger fuel supply and return lines ensure adequate flow if the fuel tank supplies multiple engines over 9 m (30 ft.) away from the tank or when temperatures are low. Consult general dimension drawings for the sizes for each model.
It is important that the fuel return line is sloped down to the tank with no traps or obstructions in the line.
If this is not done, the fuel system is prone to air-lock with consequent hard-starting.
The auxiliary tank should be located so that should be close enough the level of the fuel when the tank is full is no higher than the injection valves. On electronic unit injector fuel systems, static pressure on the fuel system will cause fuel to leak from the injectors leading to excessive fuel dilution of the engine oil. Static pressure would allow fuel
to leak into the combustion chambers in the event of injection valve leakage. The tank to the engine so that the total suction lift to the transfer pump with the fuel at low level, plus the line loss of the supply line, is less than the fuel pump’s maximum suction lift capability. This figure should be minimized for better starting. A float valve or solenoid valve in this type of day tank regulates the fuel level to keep it below the level of the injectors.
Note: For C175 installations that are set up such that excess fuel from the engine returns to the main tank and also require the fuel supply day tank to be located higher than the main tank, a check valve may be required to be installed in the return line to prevent fuel drainage and loss of prime. Reference REHS4726.
Fuel Head Limiting Tank
If overhead mounting is unavoidable, include an open/close solenoid shut off valve in the supply line and a 3.45 kPa (0.5psi) check valve in the return line. Be sure return restriction does not exceed 350kPa (51psi) at speed and load.
Base Mounted Tanks
Base mounted day tanks are sometimes used to provide a convenient and close source of fuel with adequate capacity for four to eight hours of operation. While minimizing the floor space needed for fuel storage, the height of the engine will increase significantly with this option designed to ease maintenance.
Fuel returning to the main tank may, because of its volume, aid with cooling, but returning to the day tank is permissible.