Diesel Generators – Fuel and The Fuel System

Diesel generators burn red diesel fuel.

This fuel is generally recommended for diesel generator sets.

Red diesel is the term for tax free fuel commonly used in all diesel generators, unless biodiesel is used.

A diesel generator’s fuel system must supply the diesel engine with a continuous and clean supply of fuel. Bulk fuel is usually stored in a large tank, and the fuel transferred to a smaller day tank near the engine by means of an electric motor-driven pump. The diesel generator’s system must be located and constructed in accordance with good safety practices and local codes. Any flexible non-metallic lines used to route the fuel inside the building should meet Fire Regulations. Check for any possibility of trapped fuel lines or sharp protrusions that could over time puncture the fuel lines. Any air entering the fuel line can cause the diesel generator to fail.

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Bulk Storage – Diesel Generators

The quantity of fuel stored must also be regulated, as fuel stored for long periods will sediment and this applies to all diesel generators. A regular check of the BS standards of your fuel supplier should be carried out to ensure that your fuel is always of the correct standard and grade

A large capacity storage tank is desirable to encourage bulk purchases and minimise dirt contamination.  Maintaining a diesel generator’s tank and keeping the tank full minimises condensation, particularly if the fuel is seldom used.  It may be located either above or below ground level, but the high fuel level in any part of the system must not exceed the height of the injectors in the engine.  This prevents any possibility of fuel leaking by the injectors into the cylinder and thus causing the diesel generator to fail.

The storage tank fill tube should be located for convenience and safety of the filling operation.  A vent must be provided to relieve air pressure created by filling and prevent a vacuum forming as fuel is consumed.

Silt and water builds up over time in diesel generator fuel storage tanks

A means of periodically drawing water and sediment from the tank of the diesel generator must be provided, as over time bacteria, silt and water vapour contaminants build up in the red diesel generator tank. In every case diesel generator tanks must be round-bottomed and tilted about 2° to allow complete removal of these contaminants.

Ground settling must also be taken into account when installing the tank to ensure that the drain cock remains at the lowest level.  In underground tanks, water may be removed by pumping through a tube placed down the fill pipe.  Avoid seasonal settling by burying the tank below the frost line and check local winter weather conditions to ensure the depth is safe and correct.

If a day tank situated in the belly of the diesel generator is not used, then a separate bulk tank must be located near to the diesel generator so as to provide a ready fuel supply to the engine-mounted transfer pump.

Copper bearing steel tanks are preferable but black iron tanks and fittings are satisfactory.  Galvanised fittings or tanks should be avoided, because of possible reactions with fuel impurities clogging the fuel filter. In any case all tanks for diesel generators must be double skinned or bunded or both to allow any leakage of fuel to be retained within the bunded area.

Diesel Generators -Fuel Information

Fuel Specification Requirements
Cetane Number

(Pre-combustion Chamber Engines): 35 minimum
Viscosity: 100 SUS at 100° F maximum
Pour Point: 10° F (6° C) below ambient temperature
Cloud Point: Not higher than ambient temperature
Sulphur: Adjust oil change period for high sulphur fuel
Water and Sediment: 0.1% maximum Some fuel specifications that meet the above requirements:-
ASTM D396 No. 1 and No. 2 Fuels (Burner Fuels)
ASTM D975 No. 1-D and No. 2-D Diesel Fuel Oil
BS2869 Class A1, A2, B1 and B2 Engine Fuels
BS2869 Class C, C1 and C2 and Class D Burner Fuel
DIN51601 Diesel Fuel
DIN51603 EL Heating Oil

Diesel Generators – Fuel Delivery to the Engine

The delivery line for carrying the fuel to the diesel generator’s engine-mounted fuel transfer pump and the return line for carrying excess fuel back to the tank should be no smaller in size than the fittings on the engine.  If the distance from the diesel generator’s fuel tank to the engine exceeds 30’ (9.15 m), or if ambient temperature is extremely low, larger fuel supply and return lines should be used to ensure adequate flow.

Beware of Pinch Points!

Every diesel generator has fuel lines, and these should be regularly checked to ensure they are not kinked, twisted or trapped, as any restriction can cause the generator to hunt and/or fail. Also, these can cause wear and reduce the life of the diesel generator fuel line. Check and check again.

Fuel Overflow

The fuel line may be constructed from steel, black iron pipe, copper tubing, galvanised pipe or SWA fuel hose pipe, but as a cautionary note we do not advise any zinc bearing alloy be used.  The overflow line from the day tank (or, if no day tank is used, the fuel return line from the engine) should be of the same material and one size larger to allow free flow.

Fuel Return Line

The return fuel line from the diesel generator tank should enter the top of the tank and contain no shut-off valves.  Avoid dips in this line so that air may pass freely and prevent any vacuum in the fuel system.  The fuel suction line should be positioned to remove fuel from a point about 2” (5.1 cm) above the bottom and, if possible, at the opposite end of the tank to the return line.  If the fuel line enters the top of the diesel generator’s tank, a pipe should be provided inside the tank to extend the line to the proper distance from the bottom.

Joint cement which may be affected by fuel should not be used in any part of the system.  All connections should also be made without any dependence on gaskets.  A length of flexible fuel line should be installed between the pipe from the fuel source (bulk storage or day tank) and the engine fuel inlet and return to prevent vibration damage to the pipes and fittings.

A diesel generator’s fuel tank should be installed and tested correctly at the time of commissioning, and then you can be sure of decades of trouble-free use. Remember, a diesel generator isn’t just for Christmas – it’s for life.

Diesel Generator Auxiliary Tanks

Diesel generator auxiliary or “day tanks” are desirable if the main fuel tanks are located more than 50’ (15.25 m) from the engine, or located above the engine, or are more than 12’ (3.65 m) below the engine.  Total suction head should not exceed 12’ (3.65 m).  Although they will not aid the engine in fast starting, they do offer a convenient and ready storage of additional fuel.  Diesel generator day tanks also provide a settling reservoir so water and sediment can separate from the fuel.

The auxiliary tank is located so that the level of the fuel is no higher than the fuel injection valves on the engine.  The tank should be close enough to the diesel generator’s engine to minimise total suction lift.

Tank Filters

Clean fuel filters ensure maximum engine life and dependability.  Anything else will endanger these characteristics.  The engine filter protects the fuel injection pumps and nozzles.  The incoming fuel must never bypass these filters.

Primary filters with 0.012” screens are available to extend the life of the engine filter and the transfer pump.  Water and sediment traps and filters can also be included upstream of the transfer pump, but pump performance must not be restricted.

In warm climates with large bulk storage, the diesel fuel may require full filtering every six to twelve months.  Every two years the fuel supply should be renewed to remove water, scale and bacterial growth.

Diesel Generator Fuel Selection and Engine Requirements

The fuel normally recommended for diesel generator sets is termed RED DIESEL.
When this fuel is also used for heating the building, a common storage tank for both the heating plant and the generator set is practical.  In addition to reducing installation costs, this arrangement may reduce fuel costs, as a consequence of quantity purchasing, and also eliminate fuel deterioration concerns.

Diesel engines have the capacity to burn a wide variety of fuels.  In general, the engine can use the lowest priced distillate fuel which meets the following requirements (fuel condition as delivered to the engine fuel filters), but you need clarification from each engine manufacturer.

Diesel Generator – Fuel Characteristics

The following additional information describes certain characteristics and their relation to engine performance.

A. Cetane Number:  This index ignition quality is determined in a special engine test by comparison with fuels used as standards for high and low cetane numbers.
B. Sulphur:  Since the advent of high detergent oils, sulphur content has become less critical.  A limit of 0.4% maximum is used for engines, without reducing oil change periods.  Oil change periods are reduced with higher sulphur fuel.
C. Gravity:  The measurement is an index of the weight of a measured volume of fuel.  Lower API ratings indicate heavier fuel which has a higher calorific value.
D. Viscosity:  This factor is a time measure:  flow resistance of a fuel.  Some low viscosity fuels are lubricants; a viscosity which is too high makes for poor fuel atomisation, thereby decreasing combustion efficiency.
E. Distillation:  This involves the heating of crude oil to relatively high temperatures.  The vapour which results is drawn off at various temperature ranges, producing fuels of different types.  The lighter fuels such as gasoline come off first, and the heavier fuels last.
F. Flash Point :  The lowest temperature at which the fuel will give off sufficient vapour to ignite momentarily when a flame is applied.
G. Pour Point:  This denotes the lowest temperature at which fuel will flow or pour.
H. Water and Sediment:  The percentage by volume of water and foreign material which may be removed from the fuel by centrifuging.  No more than a trace should be present.
I. Carbon Residue:  Percentage by weight of dry carbon remaining when fuel is ignited and allowed to burn until no liquid remains.
J. Ash:  Percentage by weight of dirt, dust, sand, and other foreign matter remaining after combustion.
K. Corrosion:  To determine corrosion, a polished copper strip is immersed in the fuel for three hours at 122° F (50° C).  Any fuel imparting more than a slight discoloration should be rejected.

Remember to order as heavy a fuel as your diesel engine and temperature conditions permit. But check the manufacturer’s recommendations before ordering your diesel fuel.
Fuel costs can represent approximately 80% of total operating costs for an engine.

NOTE: Don’t be caught out!

It is good economics to look closely at the largest cost first and remember that deliveries of fuel for a diesel generator, when less than 500 litres, are prone to a surcharge. Better to have a larger capacity fuel tank that avoids small deliveries of less than 500 litres each delivery. A regularly maintained generator will give up to 30 years of standby life, so think ahead and get a larger sized tank when you make your decision to install your power protection system.

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