Underloading Diesel Generator
What is underloading a generator?
Underloading is when a diesel generator is run with no load or only a very light load. In the industry, this is also known as wet-stacking.
This can happen for several reasons, including:
- Facility or site managers not wanting to risk interrupting operations by transferring the true load to the generator
- Data centres protecting UPS battery warranties
- A generator being over-specified when purchased
- A site or business reducing its electrical demand over time
Why is underloading a diesel generator a problem?
A diesel engine needs to operate under a substantial load to reach its optimum operating temperature. If it does not reach this temperature, some fuel may remain unburnt and soot can begin to build up.
Over time, this can lead to:
- Glazing of cylinder bores
- Sticking piston rings
- Reduced compression
- Carbon build-up on injectors
- More unburnt fuel passing into the exhaust system
Common signs of wet-stacking
The first sign is usually heavy smoke from the exhaust.
As the issue worsens, unburnt fuel can condense in the exhaust and mix with soot, creating a thick dark liquid that may look like engine oil. This liquid can seep from the exhaust and may appear to be an oil leak.
If it builds up in the exhaust silencer, there is also a risk of ignition when the generator is later placed under a higher load due to increased exhaust temperatures.
If a generator continues to operate with little or no load for long periods, permanent engine damage may occur.
How to avoid underloading
When specifying a new generator, ensure the unit is correctly sized for the required load without being significantly oversized.
If the generator has been future-proofed with additional capacity, or the existing installation is oversized, a load bank testing programme should be implemented.
A load bank creates an artificial demand, typically between 75% and 100% of prime load. In most cases, this helps burn off carbon deposits and can prolong the life of the generator.
As a general guide, the generator should typically be run at 75% of prime power for 2 hours in every 100 hours of use.
Need help or advice?
If you need further clarification about generator underloading, wet-stacking, or generator misuse, please contact our After Sales team.
