What Causes Wet Stacking In Diesel Engines?

A diesel engine, like other internal combustion engines, must have the exact air-to-fuel ratio and be able to maintain its designated operational temperature for a complete fuel burn to run at maximum efficiency. When a diesel engine is run at low speeds, it will not reach the proper operating temperature.

Unburned fuel is exhausted and noted as wetness in the exhaust system when the diesel engine runs below its designed operating temperature for lengthy periods of time, hence the term “wet stacking.”

How do you prevent wet stacking?

Wet stacking is quite widespread in the field of diesel engines. If you don’t run your engine at recommended operating temperatures 100% of the time, you’re going to have some wet stacking. When diesel generators are not used at least 60% of the time, they run at suboptimal temperatures, which means the engine never achieves the temperature required to burn off the surplus fuel and carbon deposits, resulting in “wet stacking” in the exhaust system.

How Do I Know It’s Happening?

Black ooze surrounding the exhaust pipe or continuous black exhaust smoke are frequently the first signs of wet stacking. If you operate your generator below the recommended operating temperature, run it at less than 60% load, use the incorrect air-to-fuel ratio, leave it idle for lengthy periods of time, or run it with too much or too little fuel in the tank, you can presume it’s wet stacking.

How Can I Prevent It?

To avoid wet stacking, exercise your generator according to NFPA and manufacturer guidelines (at least once a week with at least a 60% load), run your engine at optimal temperatures, keep the fuel tank full, have a qualified technician maintain your generator at regular intervals, and make sure the internal temperature of your generator reaches manufacturer recommendations if you’re operating in cold conditions.

If you observe a buildup of gasoline and soot particles in your engine, the solution is sometimes as simple as running it at maximum power for a few hours to burn them off. Higher particle levels may need the use of a load bank to simulate a full load on your generator, as well as the use of a competent technician to complete the load banking procedure.

What Happens if I Don’t Address it?

Unburned fuel will begin to build up in your exhaust system if you do not address wet stacking at scheduled maintenance intervals. This can clog your injectors and reduce the performance of your generator. These gasoline deposits can also cause backpressure, degrade your engine’s surface, and reduce the overall system’s efficiency. These negative effects will dramatically reduce equipment life, resulting in higher repair expenses.

Wet stacking can also have an impact on the quality of your engine oil. The pistons do not meet the cylinder as they should since your engine is running at sub-optimal temperatures. Unburned fuel may leak into the oil pan, causing the oil to become diluted. This reduces the effectiveness of your oil to protect your engine and increases wear.

Wet stacking increases pollution, and most localities have laws prohibiting wet stacking-related smoke emissions. If the EPA discovers this, it can result in significant fines.

What is wet stacking and how can it be prevented?

You can avoid wet stacking later on by carefully arranging your energy system with a sufficient load and capacity during the initial setup stage, so that you don’t regularly run a diesel engine below its suggested load. However, many businesses expect to expand, thus constant load banking is required to keep the engine running smoothly. Load banking allows us to run an engine at a higher-than-recommended capacity (often up to 80%) with predictable rates and stages for a set period of time without disrupting your business’s normal operations. During this time, we may keep a careful eye on your generator’s performance and clean away any unburned fuel that could lead to wet stacking.

What causes generator wet stacking?

Let’s start by defining generator wet stacking. Wet stacking is a phrase that refers to a diesel engine dripping a thick, dark liquid from its exhaust pipes, or, as they’re more commonly known, “wet stacking.” “stacking” The issue is created by running the engine at low revs for long periods of time, allowing unburned gasoline and soot to enter the exhaust system. The word is now used to describe an engine that isn’t totally burning all of the fuel provided to its cylinders. This condition can drastically decrease engine performance if left unchecked for an extended length of time.

A diesel engine produces only enough power to drive its accessories and overcome internal friction when it is not under load. Spark plugs are not used in a diesel engine. It relies on the cylinder’s hot compressed air to evaporate and ignite the fuel. Conditions for combustion are less than optimal when the air is cooler than the design temperature. The fuel ignites and begins to burn, but it does not totally burn. Soot—small, hard particles of unburned carbon—remains as evaporated fuel. Fuel vapors condense in the exhaust system and combine with soot to generate a dark, thick liquid that resembles engine oil. It could be dripping from the exhaust ports or oozing from the turbocharger. The name “liquid on the exhaust stacks” comes from the sight of liquid on the exhaust stacks “Wet stacking,” says the author.

What is the best way to tell if my generator is Wet Stacking? If any, or all, of the following circumstances exist, your generator is most likely inefficient:

Expense – Excessive wet stacking will reduce engine life by several years and cause it to fail before it is scheduled to be replaced.

Pollution – The amount of smoke emitted by wet stacking is restricted in many urban areas.

Power – Deposits reduce maximum power even before an engine is damaged. An engine that has been prematurely worn will produce less maximum power than it was planned to produce.

Maintenance – A moist stacking engine will require significantly more maintenance than one that is appropriately loaded.

A few hours of operation at a load of about 75% to 100% of the generator’s nameplate rating, raising the exhaust temperature high enough to evaporate the unburned fuel in the exhaust system and blow away the soot, is usually enough to cure wet stacking. If wet stacking has not yet reached the point when carbon buildup can only be eliminated by a significant engine overhaul, built-up fuel deposits and carbon can be removed by running the diesel engine at the required operational temperature for many hours. However, under that load, the exhaust temperature is much over the auto-ignition temperature for diesel fuel, and fuel and soot can ignite within the exhaust system on rare occasions. It’s critical to have a competent generator maintenance specialist oversee the load testing method if a unit has a history of extended operation at low load, or if there’s no documentation indicating it’s been exercised recently at acceptable load.

Additional Load Bank Testing Information: Generator Brochure for Load Bank Testing

Selecting the Right Generator Maintenance Plan (for more information on generator maintenance).

Is wet stacking normal?

Wet stacking has a number of negative impacts on a diesel engine generator, and if left unchecked for a long time, it can lead to lower engine performance or permanent engine damage, necessitating a costly significant engine overhaul.

How long can a diesel engine run continuously?

Your car’s gasoline engine should last roughly 200,000 miles before it requires a major maintenance or you need to purchase a new vehicle. Diesel engines, on the other hand, may run for 1,000,000-1,500,000 miles without having any serious maintenance. In fact, a well-maintained diesel engine can last for 30 years or more on the road.

According to Capital Reman Exchange, there are three key factors for a diesel engine’s lifetime, endurance, and reliability:

A diesel engine is gear-driven in design. Gears, unlike other parts that might be broken or damaged, are easy to repair and never lose their timing. Gear-driven water and oil pumps are available on most diesel automobiles. Parts and components are less likely to fail as a result of this.

Diesel-powered vehicles are typically built with heavy-duty components that can withstand the vehicle’s power, resulting in less wear and tear on all parts of the engine.

Diesel engines are also fantastic since they are self-cooling, which means they have a far lower possibility of overheating. There are multiple sensors and thermostats in use, which means that if one fails, the engine will not overheat. A steady supply of coolant flows freely through the engine thanks to many piston-cooling nozzles.

Compression ignition is used by a diesel engine to use its fuel to power itself. This happens when diesel fuel and air are squeezed to the point that heat is generated, resulting in spontaneous combustion. This spontaneous combustion, according to Digital Trends, is significantly more favourable for a long-lasting engine.

Can a diesel engine get wet?

Wet stacking, if overlooked, can cause serious harm to your diesel engine over time. Unburned fuel will begin to accumulate in your engine, blocking injectors and reducing performance. Deposits can also cause backpressure and limit the performance of the turbo system of the engine. Worse, they’ll degrade engine surfaces over time, reducing the product’s lifespan.

Engine oil is also affected by wet stacking. Because the engine isn’t as hot, the pistons don’t expand as much as they should to contact the cylinder wall. Gases and unburned fuel enter the oil pan beneath the cylinder as a result, diluting the oil. This reduces the oil’s ability to protect your engine and causes it to wear out faster.

Higher pollution and emissions, reduced power, and increased maintenance are some of the other negative consequences.


Backup and prime power solutions for every institution, from mission critical data centers to neighborhood grocery stores, rely on system health and reliability. A generator set is an important component of the power system, and its appropriate operation and maintenance are critical for long-term system reliability and uptime.

While the operation, application, and load profile of power systems vary depending on the purpose and complexity, all power systems are designed with the same goals in mind: to provide reliable electricity while also maximizing system efficiency. It is critical to understand system operation, load profiles and schemes, and required maintenance in order to meet these design goals. The purpose of this study is to look at how generator sets operate under low-load settings and what can happen if they are used outside of these parameters.


To begin, it’s crucial to understand that generator sets are built to run, and more specifically, to run with a load. This may seem insignificant, but properly loading a generator set is critical for availability, engine health, and extended engine life.

Depending on the application and rating, each generator set’s ideal operation targets will vary. In general, standby and prime-rated diesel generator sets are intended to run at 50 to 85 percent of full load, whereas continuous-rated diesel generator sets are geared to run at 70 to 100 percent load. Natural gas and biogas generator sets are designed to operate between 70 and 100 percent of their nameplate rating, regardless of application or rating.

Manufacturer service intervals and component life projections are based on operating in these ranges in order to provide the best combination of product performance, power density, and long-term operational life. As a result, the design process is crucial in ensuring that the power generation system is sized to function within the manufacturer’s recommended load levels while still satisfying the facility’s requirements. Underloading generator sets for long runs has a negative influence on product health, operation, and uptime, as well as increasing the risk of unexpected occurrences and shutdowns.


Operating a diesel generator at less than 30% of rated output for long periods of time has a negative influence on the unit. Engine exhaust slobber, also called as exhaust manifold slobber or wet stacking, is the most common symptom. Engine slobber is a black, oily liquid that can leak from exhaust manifold joints when the engine is running at low or no load for a lengthy period of time. When the engine is running at high idle with little or no load, the heat in the cylinder is reduced, allowing unburned fuel and oil deposits to seep via the exhaust slip joints.

Visible slobber does not always indicate an engine problem, although it can suggest underloading, low ambient temperatures, or a low jacket water temperature. Engine slobber, while ugly, is unlikely to harm an engine in most circumstances. Slobber, on the other hand, is a sign of underloading and could indicate further underloading effects. Long durations of light loading can result in deposit build-up behind the piston rings, deposits forming inside the cylinders, and cylinder liner polishing in extreme circumstances. These situations can cause power losses, poor performance, and faster component wear, resulting in higher maintenance costs and unanticipated downtime or failure.


Gas generator sets with outputs greater than 1000 kW are often employed in prime power and non-emergency standby applications with a constant load profile and higher load levels. For gas generator sets, optimal operating conditions might range from 50% to 100% of the rated load. Caterpillar recommends that natural gas generator sets not be loaded below 50% of their rated load for any period of time, and that the best range for operation is 70% and above.

Although gas engines do not typically slobber, low-load operations have significant consequences. Gas engines have insufficient cylinder pressure to retain oil control in the cylinder at low loads. As a result, the oil can pass through the rings and into the combustion chambers, resulting in ash deposits. The compression ratio is altered by these deposits, which reduces the detonation margin. Detonation can occur if the detonation margin is decreased sufficiently. Detonation shortens the engine’s life, damages components, and causes unscheduled shutdowns or breakdowns.

The extended use of gas generator sets at low loads, similar to diesel generator sets, can lead to deposit build-up on the valves, spark plugs, and behind the piston rings. Deposits in the cylinder might form in extreme circumstances, resulting in cylinder liner polishing.

Furthermore, natural gas engines operate rich at low loads in order to maintain combustion and avoid misfiring. The engine will diverge from the intended emissions levels if the air-to-fuel ratio is too high, potentially resulting in non-compliance with required emissions laws. A high air-to-fuel ratio also raises temperatures and speeds up component wear.

All of these factors, as with diesel generator sets, can lead to power losses, poor performance, and accelerated component wear, resulting in higher maintenance costs and unplanned downtime or failure.


Diesel oxidation catalysts (DOC), selective catalytic reduction (SCR) components, and diesel particulate filters (DPF) are all popular aftertreatment components in various locations and applications, and they are all affected by low-load operation. Low-load operation, if not designed and planned properly, will have an impact on all aftertreatment components, causing emissions targets to be missed and, eventually, engine shutdown.

Back pressure limitations can approach critical levels in a short amount of time if a DOC or DPF is operating below the minimum exhaust temperature, resulting in generator set shutdown. This problem is exacerbated in dispersed or modular systems when no paralleling capacity exists to distribute load among numerous units and ensure that a generator set does not operate at low loads for long periods of time.

In applications with an SCR system, maintaining the minimal temperature is especially crucial. If the SCR system does not achieve the required operating temperature, it will not start dosing diesel exhaust fluid (DEF) into the exhaust stream, resulting in higher-than-expected emissions and potentially jeopardizing federal or local site licenses.

To help fulfill minimum exhaust temperature standards, some SRC systems may require an extra exhaust heater. While this may help maintain temperature requirements, it also adds to the system’s complexity, expense, and maintenance requirements, and it ignores the engine’s influence from underloading. For enhanced long-term system dependability and durability, ensuring that each generator set fulfills its minimum load targets is a more effective strategy.


Diesel and gas generator sets, when properly maintained, can run at low loads for long periods of time with no negative consequences. Each impacted generator set should operate at a higher load level after operating at low load levels to boost the cylinder temperature and pressure, which cleans the deposits from the combustion chamber. Furthermore, if low load operation is projected to occur on a frequent basis, a more aggressive maintenance schedule will help to ensure that there is no excessive component wear and that the risk of unplanned downtime is reduced.

If the building load is insufficient, or if the client does not want to use critical loads for generator set maintenance, the first key consideration in managing low load is how to increase load to a system. This problem can be overcome by having access to pre-installed system load banks or by using a quick connect system that allows load banks to be easily connected to the power system for testing or repair. Taking these requirements into account during the design phase enables for smooth integration into the system, which can be more cost effective than retrofitting a site after it has been built and installed.

For diesel and natural gas generator sets, Caterpillar proposes a testing procedure. For every four hours of light load operation, Caterpillar recommends filling the diesel generator set to a minimum of 30% load for roughly 30 minutes. To ensure that the recommended exhaust temperatures are met during operation, temperature measurements should be made at the exhaust manifold prior to the turbo or in the exhaust stack right after the turbo.

Natural gas generator sets have slightly different needs. To begin, Caterpillar advises working hard to avoid underloading natural gas generator sets. Time limits on low load operation for natural gas engines are listed in Table 1 below. After the lower load operating time limit has expired, the engine shall be run for at least two hours at a load factor of at least 70%. Following these instructions will reduce engine maintenance and increase product health and longevity in the long run.

How do pilot injections reduce combustion noise?

With the widespread usage of common-rail fuel injection systems in diesel engines, the pilot injection approach has received more attention for reducing pollutants emissions and combustion noise. Pilot injection tactics result in a leaner and more homogeneous mixture in the combustion process, which partially fulfills Premixed Charge Compression Ignition (PCCI). As a result, partial PCCI can be applied to the combustion process of diesel engines using a pilot injection technique (PPCI). Pilot injection raises the in-cylinder temperature before main injection, which minimizes the ignition delay of the main spray and, as a result, the combustion noise, allowing the PPCI combustion model to be extended to high-load operation. However, because the mechanism of pilot injection impacts on combustion noise is not thoroughly known, it is difficult to determine the lower combustion noise among various pilot injection settings, making correct selection of pilot injection parameters problematic. Experiments were carried out on a single-cylinder DI-diesel engine with pilot and main injection under high load operating circumstances for this research. To investigate the impacts of pilot injection on combustion noise, a unique approach of synthetic in-cylinder pressure levels (CPLs) in various frequency ranges was developed. The findings show that the high frequency combustion noise is mostly influenced by pilot spray combustion, and that the later the pilot injection timing, the higher the combustion noise. When the time between pilot and main injection is short, increasing the pilot injection quantity increases the high-frequency combustion noise. In the meantime, because the pilot injection quantity has increased, the main injection quantity has decreased, resulting in lesser combustion noise in the middle frequency band.

What is wet stacking on a tractor?

Unburned gasoline travels through the exhaust system, causing wet stacking in diesel engines. The term “stacking” is derived from the word “stack,” which refers to an exhaust pipe or chimney stack. As a result, the greasy exhaust pipe is referred to as a “wet stack.”

This syndrome can be caused by a variety of factors. Idling the engine for long periods of time is the most prevalent reason, as it does not generate enough heat in the cylinder for a full burn. “Idling” refers to a machine that is running at full speed but with very little load applied. Excessive fuelling is another issue. This could be due to faulty or leaky injectors, excessively high fuel settings, or overfueling for the given rpms. Running the engine in cold weather or for other reasons that prevent it from reaching the proper operating temperature might result in a buildup of fuel due to incomplete combustion, resulting in ‘wet stacking.’ It’s usually because the diesel engine is only producing a small proportion of its rated output in diesel generators. A diesel engine should not be operated at less than 60% of its rated power output for efficient combustion.

The presence of a black ooze around the exhaust manifold, pipework, and turbocharger, if fitted, indicates wet stacking. It’s sometimes mistaken for lubricating oil, but it’s actually the “heavy ends” of diesel fuel that don’t burn when the combustion temperature is too low. The heavier, oilier components of diesel fuel have more energy stored in them than, example, gasoline, but diesel requires a sufficient load on the engine to sustain combustion temperature high enough to use it. Due to gasoline accumulation, one can often hear a minor miss in the engine. When the engine is first put under load after long periods of idling and wet stacking, it may emit black exhaust when the surplus gasoline is burned off. When the stack is under constant load, continuous black exhaust indicates that some of the fuel is not being burned. Furthermore, due to the low temperature in the engine, wet stacking can result in a build-up of diesel fuel in the engine that does not combust. As a result, the fuel economy suffers. This fuel leaks into the cylinders, causing the engine oil to become diluted. This diluted oil, if not changed on a regular basis, can cause increased cylinder wear and premature engine failure. Wet stacking can cause stack fires in extreme circumstances.

How do you fix a wet stacked generator?

If you catch wet stacking in your unit early enough, you can remedy it by running your generator at 75 percent load at its proper operating temperature.