WHITE PAPER
Benefits of No-Load Exercising for Diesel Generators
Justin Loritz Product Manager rehlko
Diesel generator operators are all too familiar with the problem of engine slobber or wet stacking, a buildup of unburned fuel in the exhaust system that can lead to decreased engine performance and premature failure. Wet stacking typically occurs when generators frequently run with little or no load because the generator is improperly sized or because adequate load is not available during the exercising period. Often times applications require added time and cost to connect external load banks to apply sufficient load to complete an exercise, which results in higher emissions and extra fuel costs. When generators run at less than 30% of capacity for extended periods of time, their engines are unable to sustain the optimal operating temperatures needed to burn fuel completely.Pressure inside the combustion chamber falls below crankcase pressure, and piston rings can’t expand enough to seal the space between the pistons and cylinder walls. WHAT WET STACKING CAN LEAD TO As depicted in Figure 1 , wet stacking fouls fuel injectors, causes excessive valve guide wear, and can lead to damaged pistons, piston liners, and rings. EMISSION CONSIDERATIONS In addition to shortened engine life and higher maintenance costs, operators may also run afoul of emissions regulations, which have grown tighter in recent years. EXPLANATION OF WET STACKING EXPLANATION OF WET STACKING
What Wet Stacking Can Lead To As depicted in Figure 1, wet stacking fouls fuel injectors, causes excessive valve guide wear, and can lead to damaged pistons, piston liners, and rings. Emission Considerations In addition to shortened engine life and higher maintenance costs, operators may also run afoul of emissions regulations, which have grown tighter in recent years. Wet stacking has become a more critical issue in recent years as diesel generator manufacturers have incorporated new emissions control technologies into their products. These include diesel oxidation catalysts (DOC), selective catalytic reduction (SCR) components, and diesel particulate filters (DPF). See Figure 2. Such aftertreatment technologies are all negatively affected by low-load operation, which can cause emissions targets to be missed. SCR and DPF components, in particular, must operate within a specific The solution for wet stacking for years has been to exercise the generators at 30% of the rated capacity once per month to burn off unused fuel or prevent buildup. However, advances in technology are making the need for this costly procedure less demanding. This paper asserts that some of today’s diesel generators can be safely run at 30% of the rated capacity or higher as little as once per year to maintain optimal performance and stay within emissions guidelines. Why It’s Necessary to Address Wet Stacking Wet Stacking: A Critical Issue Impact of New Emissions Technology exhaust temperature range. Addressing Wet Stacking Wet stacking doesn’t damage the engine in the short term, but over time it can lead to poor performance and reduced engine life. In extreme cases the exhaust stream can combust under rapid application of the high exhaust temperatures.
Explanation of Wet Stacking When generators run at less than 30% of capacity for extended periods of time, their engines are unable to sustain the optimal operating temperatures needed to burn fuel completely.Pressure inside the combustion chamber falls below crankcase pressure, and piston rings can’t expand enough to seal the space between the pistons and cylinder walls. Figure 1
Carbon deposits
Fuel drip
Figure 1
44 | Summer 2026 POWERLINE
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