Wet stacking is a condition in diesel engines in which not all the fuel is burned and passes on into the exhaust system. The word "stacking" comes from the term "stack" for exhaust pipe or chimney stack. The oily exhaust pipe is therefore a "wet stack".
This condition can have several causes. The most common cause is idling the engine for long intervals, which does not generate enough heat in the cylinder for a complete burn. "Idling" may be running at full rated operating speed, but with very little load applied. Another is excessive fueling. That may be caused by weak or leaky injectors, fuel settings turned up too high or overfueling for the given rpms. Cold weather running or other causes that prevent the engine from reaching proper operating temperature can cause a buildup of fuel due to incomplete burn that can result in 'wetstacking'.
In diesel generators, it is usually because the diesel engine is running at only a small percentage of its rated output. For efficient combustion, a diesel should be run under at least 60 per cent of its rated power output.
It is detectable by the presence of a black ooze around the exhaust manifold, piping and turbocharger, if fitted. It can be mistaken for lubricating oil in some cases, but it consists of the "heavy ends" of the diesel fuel which do not burn when combustion temperature is too low. The heavier, more oily components of diesel fuel contain more stored energy than a comparable quantity of, say, gasoline, but diesel requires an adequate loading of the engine in order to keep combustion temperature high enough to make use of it. Often, one can hear a slight miss in the engine due to fuel buildup. When the engine is first placed under a load after long periods of idling and wetstacking it may blow some black exhaust out as it burns that excess fuel off. Continuous black exhaust from the stack when under a constant load is also an indication that all the fuel is not being burned.
Below are a list of excerpts from published literature regarding the condition:
Espen Dalsøren Tufte, Norwegian University of Science and Technology
Publication: June 2014
“Low load operations of diesel engines are defined by DNV GL [2] as engine operations at loads below 40% of maximum continuous rating. Engine loads below 25% are defined as extreme low loads. Engine loads in the range of 40– 80% is defined as regular generator operation load…. From an engine designer’s point of view, short periods of low load operations are acceptable given that the engine is brought to full load on regular basis.”
Burl Donaldson, Patent Assigned to New Mexico State University Technology
Publication Date: Feb 1, 2005
Reference Patent: US6848419 B1
“In order to avoid failures from either type of wetstacking, the conventional advice is that the engine should never be operated for extended periods of time at less than half-load. In order to assure that the engine operates at a load of at least 50%, artificial loads are sometimes applied... Wetstacking also occurs when solid carbon or nonvolatile liquids accumulate along the cylinder wall or in the piston ring grooves and inhibit expansion and sealing of the ring against the cylinder wall. The cylinder can become glazed which further contributes to poor charge air compression heating and poor combustion of fuel. The performance of the engine continues to deteriorate until charge air compression heating is no longer sufficient for the engine to operate.”
The Northern Engineer, The Geophysical Institute, University of Alaska-Fairbanks
Publication: Volume 18, Number 1, Spring 1986
“Furthermore, since most high-speed diesel engines cannot be operated for long periods at much less than 40 or 60 percent load without eventually suffering from mechanical failures, the problem of how to deal with part-, load operation becomes a major concern· of operators and system designers alike.”
Amit Kumar Jain and V. T. Ranganathan, Dept. of Electrical Engineering, Indian Institute of Science, Bengaluru
K. Vijay Kumar and G. Guruswamy, IE Power Technologies Pvt Ltd, Peenya Industrial Area, Bengaluru
Publication: NPEC 2010
“In diesel driven alternators, the engine is always required to run at a fixed speed, irrespective of the load. In such systems, the diesel engine has high fuel consumption when operating at light load and constant speeds. Moreover, during such a run, not all the fuel is burned by the engine and wet-stacking is produced.”
Tawanda Hove, Dept. of Mechanical Engineering, University of Zimbabwe, Zimbabwe
Henerica Tazvinga, Dept. of Electrical, Electronic and Computer Engineering, University of Pretoria, South Africa
Publication: Journal of Energy in Southern Africa • Vol 23 No 4 • November 2012
“It is already well known that when the diesel engine is operated for long periods at a partial load, a condition known as ‘wet-stacking’ occurs (Donaldson, 2005). This is mainly attributed to incomplete combustion of fuel when the engine runs at low operating temperature. This results in reduced fuel efficiency and, simultaneously, shortening of engine operating life, and the time interval between routine maintenance calls.”
George Fell, Patent Assigned to Multiquip, Inc.
Publication Date: August 26, 2014
Reference Patent: US8816651 B1
“Electric generators, particularly portable engine-generator units, are often operated under variable load conditions. Such portable units are also often left unattended, which further causes them to be operated in sub-optimal conditions. It is well known that operating engine-generators under fluctuating load conditions can present a number of problems. As explained in U.S. Pat. No. 3,530,300 to Gunther, et al., diesel-fueled engine-generators are especially susceptible to problems associated with light-load conditions. In particular, a lightly loaded, diesel-fueled engine-generator may cause the engine portion of the machine to operate such that fuel within the engine remains unburned. This unburned fuel forms tar or carbon deposits, or both, which can collect within various parts of the engine, including exhaust pipes. This condition has historically been referred to as “wet stacking.””
Volt Master America, Engine Driven Generator Set Owner’s Manual
Publication: 2593-CON, Revision 05/2009
“Operating the engine without an electrical load for prolonged periods of time (one hour or more) will eventually cause damage to the diesel engine (sometimes called wet stacking which is NOT covered by the engine manufacturer’s warranty) or damage to the rings, fuel oil entering the lubricating oil and excessive vibration. Breaking in the diesel engine can require 5 to 10 hours of operation. It is extremely important to have electrical loads of at least 25% to 50% of the rated capacity of the generator’s nameplate rating to have the valves properly seat.”
National Technical Systems
Publication: Report No. 556-1193-18, September 1984
“Any diesel engine will exhibit wet stacking symptoms if the intake air is cold enough and if the fuel required for power is insufficient to maintain an appropriate combustion chamber temperature. Carried to an extreme, this condition can result in one or more cylinders ceasing to fire…. The self-cleaning oven typically requires four hours at 800 degrees F to remove carbonaceous material. Ignition points of engine-formed hydrocarbons range from approximately 500 to 900 degrees F, and the ignition point of carbon is approximately 925 degrees F. A normally operating diesel engine ingests 15% or more excess air at maximum power.”