50 Reasons Generators Fail, Which Generator Failures Impact You?

The Thing About Generator Fuel

Most generators are used for standby emergency power. The keyword there is “standby”, which means the generator’s normal state is off and not using fuel. Yes, most of them are set to automatically run once a week for 30 to 60 minutes to exercise, but relatively little fuel is used when they do. More importantly though, enough fuel must be stored for the generator to provide emergency power ranging from say 4 hours to 48 hours depending on the type of operation it supports. With dependable utility power, some generators go for months and even years without having to provide emergency power, so most of the fuel just sits.

Most generators run on diesel fuel because of its self lubricating feature. When diesel fuel just sits it will become contaminated within a short period of time. Once that happens it needs to be cleaned. Otherwise the contaminates will impact the fuel system and could harm the generator engine. The contaminates will also impact the tank, so in this article we will address that as well.

Fuel Contaminates

I group fuel contaminates into two categories. One is combustible and the other is non-combustible. It's important to understand the difference before clarifying fuel polishing and fuel filtering. First, non-combustible.

Non-Combustible Contaminants (Bad)

Non-combustible contaminates are any particulate matter that got into the fuel that was not there when the fuel came out of the manufacturing plant (refinery). I refer to them as ‘bad’ because only bad things can happen from having them in the fuel system. Non-combustible contaminants have been known to include such things as water, dust, sand, dirt, sediment, rust, hair, microbes, and bugs. I've heard of some pretty wild stories about other things being pulled from the bottom of fuel tanks, including rates and birds.

Water In Fuel

Before moving on to combustible contaminants I want to explain something about water contaminants. Water is definitely a contaminate when it’s present in fuel, but a lot of people ask how it gets there. Water gets into fuel tanks by water condensation. That’s when water vapor (air with water molecules) makes contact with the tank walls and condenses (changes) into water liquid. This reaction is accelerated when the tank walls are cold. As the water liquid builds up and gets heavy it’s surface tension gives way and it drips into the fuel. Most of the water then sinks to the bottom because water is heavier than fuel.

Combustible Contaminants (Good)

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Combustible contaminants are agglomerates that are derived from the fuel itself. They are fuel that has degraded/consolidated (clumped together into solids). They are what sludge is mostly made of. says that diesel fuel is “a blend of distilled crude oil and catalytically “cracked” crude oil. The catalytic hydro-cracking process basically takes long carbon-chain molecules and breaks them apart, leaving a mixture of relatively unstable, electrically charged molecules that want to clump (agglomerate) back together. From the moment the diesel fuel leaves the refinery, the fuel begins to degrade, and fuel that has been stored for a while will clog filters.”

I refer to fuel that has clumped back together as a contaminate because, while in the clumped/solid form it can plug fuel lines and prevent the engine from running. I’ll explain why it’s a 'good' contaminate shortly.

Fuel Cleaning

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Fuel Cleaning is the process of removing fuel from a tank, removing the 'bad' contaminates, processing (restructuring) the 'good' contaminants, and then putting it back. The term is not related to the basic fuel filter that every engine has to catch very small amounts of contaminants that inadvertently get into new fuel. However, when fuel cleaning is needed, but not done, that basic fuel filter can quickly become plugged as can be seen in the picture and prevent fuel from getting to the engine.

Fuel becomes contaminated in several ways, but we’ll get to that in a bit. What’s critical to recognize at this point is that you never want contaminated fuel going to your engine, clogging up fuel lines and injectors, gumming up valves and other moving parts, and causing who knows how much down time, maintenance, and repairs.

Knowing When Fuel Needs To Be Cleaned

Foster Fuels says that “fuel begins to degrade significantly after only 6 months . . .” However, that can vary depending on the type of fuel and the conditions in which it’s stored. For example, if diesel fuel sits for just a few months in a hot environment and water is present, a microbial population (colonized of bacteria, mold, and yeast) will begin to grow.

Samples and video can determine the extent of contamination in your tank. Most services today can take those for you. It may give you peace of mind to see the contamination before you go to the expense of cleaning.

Even if your generator runs from time to time for a few minutes, if you have hundreds or thousands of gallons of fuel that for the most part just sit, it will become contaminated. For that reason you should have it cleaned on a regular basis. So the next question is what is the right way and wrong way to clean fuel. Therein lies our topic; Fuel Polishing vs. Fuel Filtering.

Fuel Filtering

Filtering or filtration is generally defined as the process of separating suspended particles from a fluid by flowing both through a porous material in which the fluid can pass while the suspended particles are retained. The amount and type of suspended particles the porous material, sometimes called filter media (the material filters are made of), will retain depends on what it’s made of and how it’s designed. As it relates to fuel filtering, the liquid in the fuel tank is the fluid and the combustible and non-combustible contaminants are the suspended particles.

It's important to note that in some fuels, certain suspended particles at the molecular level are a vital part of the fuel's chemical make up and need to be there. Just keep that in mind as we move forward.

To competitively price their service, many fuel cleaning companies simply pump the fuel through a series of filters that removes the combustible and non-combustible contaminants and then back into the tank. They continue circulating the fuel through the filters until field tests show that all suspended particles are gone. That method is called “fuel filtering”.

Simply put, fuel filtering removes anything and everything from the fuel, whether it’s good or bad. Therein lies the problem. When fuel filtering is done to diesel fuel, combustible contaminants are removed. The components of combustible contaminants are a vital part of diesel fuel’s function. One of the most vital ones is the lubricating component that diesel engines depend on. More on that to follow.

Fuel Polishing

Fuel polishing is NOT "just another word for filtration" as some will have you believe. It's the careful removal of non-combustible contaminates below levels stated in ASTM D975 (Standard Specification for Diesel Fuel Oils) while re-suspending (keeping) combustible contaminates to maintain ASTM standards for BTU value, lubricity, and cetane.

Photo curtesy of King Tanktechnic (UK)
Fuel polishing uses a combination of centrifuges, coalescers, and filtration (filters) to remove only the non-combustible contaminates. You can see an animated illustration of the fuel polishing process at Fuel Purification Systems.

Because water is heavier than fuel, the centrifuges and coalescers remove most of the water in the fuel without removing the needed combustible contaminates. Filters that block water, but not combustible contaminates, are used to remove the last bit of water that is suspended or entrained in the fuel.

How are combustible contaminates (the stuff sludge is mostly made of) converted back to there manufactured/refined form? In short, the process equipment does it! Combustible contaminates (fuel clumps/solids) are broken down and re-suspended in the fuel by the process pumps, centrifuges, and coalescers when they go through them. Then special filters are used that allow them to pass through while preventing the non-combustibles from passing through.

Primary Difference

The primary difference between fuel polishing and fuel filtering is that fuel polishing acknowledges that fuel itself can degrade/consolidate (clump together into solids) and that it needs to be kept, where as fuel filtering does not. Fuel polishing recognizes that combustible contaminates should be changed back to their original form and re-suspended in the fuel so the fuel stays within industry specifications and functions as it’s intended to. Fuel filtering ignores that and simply pulls all contaminates out without regard for their value to the fuel and the engine.

Ouch! Is it really that bad? Well, you decide.


The clumped/solid combustible contaminates that formed from the fuel itself are made from the fuel components that give it lubricity and BTU value. This is particularly true for diesel, kerosene, home heating oil, and some JP (jet propulsion) fuel. If you simply cycle the fuel through filters over and over again and remove all of those clumped/solid combustible contaminates without putting them back to their original form and returning them to the fuel, you’ll knock the fuel out of spec. Additionally the fuel won’t have the power or lubricity that your diesel generator engine needs to run properly and properly lubricate itself! Will that impact the overall operation of the engine? YES! Does that mean the engine won’t last as long? YES! Could that result in major repairs? YES AGAIN!

Additionally, if your engine is engineered to Tier-1 requirements or greater, then it’s going to be really sensitive about the fuel it consumes. Tier ratings are related to emissions standards and the engineering required to meet those standards. The higher the Tier, the tighter the standard is, which means engineering relies more and more on precise fuel specifications. You take your fuel out of spec, you’re going to have engine problems.

On top of all that, if the engine fails for whatever reason and the manufacturer discovers that the fuel being used is out of spec, there is a strong possibility that the warranty will become void!

Fuel Cleaning Options

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There are several options to make sure your fuel stays free of contaminates and meets spec. 1) Have a fuel cleaning service clean your fuel and tank on a regular basis. 2) For bulk fuel storage, install permanent fuel polishing equipment that automatically cleans the fuel in pre-determined cycles. 3) Use a cleaning service or similar permanent type equipment for tanks that are part of mobile assemblies. Remember, keeping the fuel clean will keep the tank clean.

Fuel Cleaning Cost

Actual prices can’t be provided here because of the range of methods used (fuel polishing vs. fuel filtering, service vs. permanent equipment, etc.), types of fuel, amount of fuel, accessibility, and so on. However, if you have enough fuel stored, having it cleaned can cost significantly less than replacing it. Especially if you factor in tank cleaning. After you interview a few fuel/tank cleaning companies do the math. It will probably make the decision for you.

Fuel Tank Cleaning

Refuel Environmental Services says that “If fuel tanks are contaminated with water, sludge or bacteria, simply filtering the fuel without cleaning fuel tanks can quickly lead to the fuel becoming re-contaminated.” Unless you clean your fuel so often that the tank does not become contaminated, it’s strongly recommended that you always clean the tank when you clean the fuel and vice versa.

The tank cleaning process generally goes as follows
  1. Fuel and contaminates are pumped from the tank to fuel cleaning equipment.
  2. Fuel is cleaned (hopefully using the fuel polishing method) and then pumped into a clean temporary holding tank.
  3. Materials remaining at the bottom of the tank that the pump did not get are vacuum extracted.
  4. The interior of the tank is thoroughly cleaned using high-pressure wash & rinse heads and approved, fuel compatible cleaning solutions. That includes the tank walls and all devices that penetrate the tank (such as fuel loading drop tube, overfill prevention devices, supply line drop tubes, mechanical fuel level sensors, etc.). All tank cleaning waste should be pumped and/or drained into mobile waste holding tanks for removal to a certified dumping facility.
  5. Cleaned fuel is pumped from holding tank into cleaned tank.

Tank Cleaning Cost

The cost to clean your tank is going to very based on the following factors:
  1. Size of tank.
  2. Access to tank.
  3. Condition of tank
  4. How much fuel is in tank.
  5. Are you also cleaning the fuel as well.

What Should You Do?

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It’s hard to prevent fuel degradation in standby generator diesel fuel tanks. However, Foster Fuels says that “fuel polishing can reverse that degradation and save you from expensive damage to your equipment and downtime.” We strongly recommend that you 1) make fuel and tank cleaning part of your generator maintenance plan and 2) make sure your fuel cleaner uses the fuel polishing method, not fuel filtering.

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