Underloading Diesel Generator

What Is It?

The running of any diesel engine with no load or very light loads. In the industry this is also known as "wet-stacking".

This can be due to facility or site managers not wanting to risk interrupting daily business by transferring the true load to the generator, or data centres wanting to protect their UPS battery warranty.

It could also be due to a generator being over specified when purchased, or a company that has downsized their electrical requirements.

Why Is It a Problem?

A diesel engine needs to operate under substantial load, to reach its optimum operating temperature. If it fails to reach this temperature then a percentage of the fuel will be unburnt and a build-up of soot can occur.

Within the engine soot can cause internal glazing of the cylinder bores, sticking of the piston rings, reducing compression and the injectors are likely to have a build-up of carbon causing more unburnt fuel to pass through the combustion chamber into the exhaust.

The first sign of this problem is heavy smoke from the exhaust, eventually, the unburnt fuel will condense in the exhaust and mix with the soot, to create a thick dark liquid that looks like engine oil. This liquid will seep from the exhaust, and will appear to be an oil leak. This liquid can build up in the exhaust silencer and there is the risk of it igniting from the increased heat of the exhaust fumes, if the generator is subsequently put under full load.

If a generator continues to be operated with no load or very light load there is a possibility that permanent engine damage can occur.

How to Avoid It

When specifying a new generator, ensure that the unit is adequate for the requirement, without being too large.

If the machine is future proofed with additional capacity, or the existing installation is over capacity, a load bank testing programme must be implemented.

The load bank should be used to create an artificial demand for between 75% and 100% of prime load and this, in most circumstances, will burn off carbon deposits, prolonging the life of the generator.

Typically the generator should be run at 75% of prime power for 2 hours in every 100 hours use.

Please call our Technical Sales team, if you need any further clarification concerning generator underloading or misuse.