Wet Stacking (Unburned Fuel Accumulation)
This is the accumulation of unburned fuel on the exhaust side as a result of a diesel engine running for long periods at low load or at idle. Because the cylinder cannot reach sufficient temperature, combustion is not completed. Its consequences include an increase in exhaust back pressure, soot on injectors and valves, thinning of the oil with fuel, and loss of power.
A diesel engine burns efficiently with high compression and high cylinder temperature. At idle the cylinder temperature stays below this threshold; part of the injected fuel passes through unburned. Over time a dark, oily deposit forms in the exhaust manifold and on the turbo side — that is where the name comes from.
The chain of consequences is as follows:
- Glazing on the cylinder wall and the related increase in oil consumption.
- Thinning of the oil with fuel and a drop in lubrication quality.
- Soot on injectors and valves, rise in exhaust back pressure.
- Loss of power and efficiency.
The practical measure is to run under load. Stopping the engine rather than leaving it at idle during long waits, running it regularly at sufficient load and avoiding unnecessarily long warm-up periods is enough. Speed control helps indirectly here: dropping quickly to idle during short pauses and returning to the work speed when work resumes shortens the engine's total idling time.
Context in which this term is used: Electronic speed and throttle control on diesel engines