At an event, the power demand varies a lot: high during the show or opening hours, low at night or during the build. A diesel generator copes badly with these long hours almost unloaded.
What happens in the engine
Lightly loaded, the engine stays too cold: the fuel does not burn completely. Unburnt fuel and soot build up in the exhaust, then seep, black and oily, from the joints: this is what manufacturers call “wet stacking”. The deposits also foul the valves, pistons, turbocharger and injectors, fuel dilutes the oil and oil consumption increases. In extreme cases, the cylinder liners become polished.
It is not an immediate failure, but a warning sign: in the long run, the engine wears faster.
From what load?
There is no single threshold. The figure most often given by manufacturers is 30% of the generator's rating: below it, the set should not run for long. Some go down to 20%; others require more than 60% for small portable generators. One manufacturer states that its standby and prime generator sets are designed to operate between 50 and 85% of their rating.
Example: a 160 kW generator delivering only 24 kW at night runs at 15% of its rating, half the 30% most often recommended.
On recent (Stage V) generators fitted with a diesel particulate filter, it matters even more: without a hot exhaust, the filter does not regenerate and can clog until the generator shuts down.
The cures
- Correct sizing: a generator chosen for the peak, or “just in case”, spends most of its time very lightly loaded. Hence the value of a realistic power assessment, with the diversity factor.
- Several generators in parallel, or twin generators, switched off according to the load.
- A battery that takes the low load, at night for example, while the generator is off.
- A load bank, which adds load when consumption is too low.
- Switching the generator off rather than leaving it running unloaded for long periods.
After a long period at light load, the load is increased gradually, then the generator is run well loaded for a while to burn off the deposits, following the manufacturer's instructions.
A generator too big for its load does not wear less: it fouls up.
Right-sizing generators is part of our responsible energy approach.