Every generator enquiry comes to the same question on the fuel side: where does the fuel sit? Under the set, in a belly tank, or beside it, in a day tank fed from bulk storage? The answer is rarely a matter of taste. Four things decide it, and they are usually known before the set is even chosen.
How many hours
The first is autonomy: how long the set must run without a fuel delivery. A standby set covering a few hours of outage can carry that fuel under itself. A prime power set, or a standby set that a design standard obliges to run for a day or more at full load, cannot - the volume does not fit in a frame that is still serviceable. Then the belly tank keeps a working reserve and a bulk tank, with a transfer system, provides the autonomy.
Where the set stands
Outdoors or in a container, the belly tank is the natural choice: no fuel room, no separate foundation, no pipework between tank and engine. The set arrives as one package and is filled where it stands. In a plant room the picture changes. Fire codes in many countries limit the fuel that may be stored inside a building, and the limit is often below what a belly tank would hold. A small day tank near the engine and the bulk tank outside is then the arrangement the authority will accept.
How the fuel arrives
A set that is refuelled by tanker at the machine needs a filling point at the set and a tank that takes a full delivery without overflowing - the belly tank, with its high-level switch and vent, does that. A set that is fed from a central storage needs transfer pumps, a return line and a controller that keeps the tank between two levels. Day tanks are built around that logic; belly tanks can be fed the same way, and in multi-set plants often are.
Several sets
With one set, one tank. With four or eight sets, the question becomes whether each set has its own reserve or all share one. Individual belly tanks keep every set independent and simplify the fire separation; a shared day tank system saves tanks but adds pumps, valves and the control that keeps the fuel moving to the right engine. Data center designs with redundancy requirements usually want each set to be able to run on its own tank while the shared system is being serviced.
What goes wrong
The mistakes we see are the same in both arrangements. A capacity quoted as nominal and read as usable, so that the set stops hours before the autonomy ends. A tank that stands out of level and reads a wrong fuel level. A filling point placed where the tanker cannot reach. Fuel that has stood for years without a drain being opened or a polishing system fitted, so that the first real outage starts with a blocked filter. None of these is about the type of tank; all of them are about the specification.
AXON Technic supplies belly tanks, daily fuel tanks and fuel polishing systems for generator sets of any make, as part of a container package or on their own. The fuel systems guide lists what to send with an enquiry.