The characteristics that make diesels so effective at high loads also pose a potential reliability and performance issue when run lightly loaded. Running at low load can produce engine operating temperatures below designed levels. When that occurs, fuel combustion is incomplete. The products of incomplete combustion include carbon substances that deposit onto internal engine parts such as pistons, piston rings, and exhaust valves. In addition, wet, unburned fuel residues can collect downstream in the exhaust system, a condition termed “wet stacking”.

The problem with wet-stacking is that the sooty sludge from incomplete combustion accumulates on surfaces throughout the exhaust system. This causes several issues. Where deposits are heavy, they can restrict exhaust flow, causing excess back pressure that can reduce performance or even cause shutdown. They can also contaminate pollution control devices, which increases pollutant emissions. Under some conditions, the residues can also present a fire hazard.

Solutions for Wet Stacking

The most basic solution for addressing wet stacking is to run engines “harder”, that is, with greater amounts of load, thus generating more heat. On this point, some manufacturers state that, following several hours of low-load operation, a best practice is to run engines for a prescribed period of time at high load to increase engine exhaust temperatures and burn off accumulated sludge. However, this solution does not address the reason a genset was running at low-load to start with. If there was insufficient load during a generator run, where will additional load be found to correct the resulting condition? The reason for running low load must be evaluated first.