Wet stacking occurs when unburned diesel fuel and carbon deposits accumulate in the exhaust system because the generator operates at too low a load for extended periods. When combustion temperature and cylinder pressure are insufficient, injected fuel does not burn completely and residue begins to build up inside the engine and exhaust path.
In my experience supplying standby and industrial units for export projects, wet stacking is almost always related to load percentage and sizing decisions rather than engine quality.
In simple terms, wet stacking happens when a diesel generator runs too lightly loaded for too long.
What Wet Stacking Actually Means
Wet stacking is not simply visible black smoke. It is a combustion imbalance.
A diesel engine is designed to operate within a specific load band. When the generator carries sufficient load, cylinder pressure rises and combustion temperature becomes high enough to fully burn the injected fuel.
When the load percentage drops too low:
- Combustion temperature decreases
- Cylinder pressure drops
- Fuel atomization becomes incomplete
- Unburned fuel and soot enter the exhaust system
Over time, this leads to oily exhaust residue, carbon buildup on injectors and valves, and reduced engine efficiency.
I have inspected standby units in office buildings and small industrial facilities where the generator was oversized for safety. In reality, the unit rarely exceeded 20 to 25 percent load during testing. After a year of light-load operation, carbon accumulation inside the exhaust elbow was clearly visible.
Why Low Load Causes Wet Stacking
Wet stacking is directly tied to load percentage.
Most diesel generator manufacturers recommend operating above 30 to 40 percent of rated capacity for healthy combustion. Below that range, the engine does not reach optimal thermal conditions.
The mechanism develops in a clear operational sequence:
- Low load reduces fuel demand
- Lower fuel demand reduces combustion temperature
- Reduced temperature lowers cylinder pressure
- Fuel does not burn completely
- Residue accumulates in the exhaust system
This is not a manufacturing defect. It is an operational behavior issue.
One common sizing mistake I see in export projects is selecting a generator far larger than the actual running load. The intention is to allow future expansion, but the result is chronic low-load operation, especially in telecom sites, backup-only installations, and small factories.
Proper load planning often matters more than simply selecting a larger generator for safety margins.
Real Project Example
A telecom backup project once specified a 100kVA generator for a load that rarely exceeded 25kVA. On paper, this appeared conservative. In practice, weekly exercise tests ran at only 20 to 25 percent load.
Within several months:
- Exhaust soot increased
- Fuel consumption per kWh rose
- Injector cleaning intervals shortened
The issue was not engine brand or manufacturing quality. It was load mismatch.
We eventually recommended either resizing the unit or introducing periodic load bank testing to increase operating load temporarily.
How to Prevent Wet Stacking
From practical field experience, prevention comes down to three engineering decisions.
1. Correct Generator Sizing
Ensure that normal operating load stays above 30 percent of rated capacity. Occasional light load is acceptable. Continuous low-load operation is not.
2. Periodic Load Bank Testing
Applying artificial load using a load bank raises combustion temperature and helps burn off accumulated carbon. For standby systems that rarely reach sufficient operating load, a properly sized load bank becomes a practical preventive tool.
3. Avoid Extended No-Load Running
Short no-load startup periods are normal and often recommended before applying load. However, extended no-load or ultra-light-load operation should be avoided.
I explain in detail when no-load starting is acceptable and when it becomes harmful in our full analysis here:
https://waltpower.com/is-it-good-to-start-a-diesel-generator-at-no-load/
Understanding the difference between brief startup procedures and prolonged low-load running is critical to long-term engine reliability.
Is Wet Stacking Dangerous?
Wet stacking is not immediately catastrophic, but long-term neglect can lead to serious maintenance problems.
Potential consequences include:
- Injector fouling
- Turbocharger contamination
- Increased fuel consumption
- Reduced rated output over time
In severe cases, persistent carbon buildup can shorten engine life.
The key issue is not whether a generator can run at low load. It can. The real question is how often and for how long.
Conclusion
Wet stacking is a direct result of prolonged low-load operation that lowers combustion temperature and allows unburned fuel to accumulate in the exhaust system.
At its core, wet stacking is a load management issue rather than an inherent engine defect.
In most projects I have handled, correcting load percentage or introducing controlled load bank testing resolved the issue without major mechanical intervention.
If you are evaluating generator sizing or standby operation strategy, understanding wet stacking is essential. It connects directly to broader decisions about no-load operation, load management, and long-term engine durability.
