Sylvie Willis
The black discolouration of diesel fuel is a result of microbial contamination. This occurs when water present in the diesel separates into a distinct layer below the fuel, creating an environment for bacteria and fungi to thrive. The microbes consume the hydrocarbons in the diesel fuel, forming a visible biomass that floats between the water and diesel fuel. This process is similar to how milk turns into cottage cheese. Additionally, the increased presence of heavy end asphaltene in today's diesel fuel contributes to the discolouration. Asphaltene requires higher temperatures to combust, resulting in incomplete combustion and the release of black smoke.
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What You'll Learn
Microbial contamination
Water can enter diesel fuel through various means, such as condensation, rainwater, or even absorption from the air. This water accumulation leads to a process known as "phase separation," where the water forms a distinct layer below the fuel. The interface between the diesel and water creates favourable conditions for microbes, including bacteria and fungi, to thrive. These microbes consume the hydrocarbons in the diesel fuel, leading to the formation of a visible biomass layer, also known as a "rag layer," between the water and diesel.
The growth of microbes in diesel fuel can cause the fuel to turn black or appear dark and cloudy, which is a significant indication of severe contamination. This microbial contamination can have detrimental effects on diesel engines. The microbes can plug filters, affect the pH of the fuel, and lead to tank corrosion. As a result, the risk of system failure and engine shutdown increases.
To prevent microbial contamination, it is crucial to control water buildup in the fuel tank. This can be achieved through the use of products specifically designed to manage water content, such as Bell Performance Dee-Zol and DFS Plus. Additionally, keeping tanks topped up can help minimise water condensation by reducing the empty space in the tank. Regular microbial testing of fuel samples is also recommended to detect and address contamination issues promptly.
In cases of severe microbial contamination, a mobile fuel polishing service or system may be required to remove the sludge and contaminants from the fuel. However, it is important to note that preventing microbial growth is a continuous process, and a single fuel polishing service may not provide a permanent solution. Automated fuel maintenance systems, such as those available for Tier-4 engines, are more effective in maintaining fuel quality and preventing microbial growth by regularly filtering out contaminants and removing water.
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Incomplete combustion
There are several factors that can cause incomplete combustion in diesel engines, leading to black smoke:
Dirty or Faulty Fuel Injectors
Fuel injectors play a critical role in delivering the right amount of fuel to the engine at the right time. If the fuel injectors are dirty, clogged, or faulty, they may deliver too much fuel or not enough, resulting in an imbalance in the fuel-to-air ratio. This imbalance leads to incomplete combustion and the production of black smoke. Regular maintenance and cleaning of fuel injectors are important to prevent this issue.
Incorrect Fuel Timing
Injecting fuel into the engine at the wrong time can also cause incomplete combustion. This can be due to issues with the fuel injectors or other components of the engine. Observing the smoke and using diagnostic tools can help identify these issues. Adjusting the timing and seeking professional assistance are important steps to ensure proper combustion.
Excess Carbon Build-up
Over time, carbon can accumulate on the walls of the combustion chamber. This build-up can interfere with the combustion process, preventing the fuel from burning completely. Regular cleaning of the combustion chamber by a professional is necessary to address this issue.
Improper Fuel Mixture
Using the wrong type of fuel or an improper fuel mixture can also lead to incomplete combustion. The fuel mixture needs to be carefully balanced for the engine to run correctly. A mixture that is too rich or too lean will not combust completely, resulting in black smoke.
Fuel Pump Malfunction
A fuel pump that is not working correctly can contribute to incomplete combustion. This may affect the delivery of fuel to the engine, impacting the combustion process and resulting in black smoke.
Addressing these issues through regular maintenance, cleaning, and replacement of parts is crucial to prevent incomplete combustion and reduce the negative impacts on engine performance, fuel efficiency, and environmental pollution caused by black smoke emissions.
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Microbial growth
The microbes that contaminate diesel fuel include bacteria such as Clostridium, Desulfotomaculum, Desulfovibrio, Flavobacterium, Acidovorax facilis, Pseudomonas, and Sarcina, as well as fungi such as Aspergillus, Candida keroseneae, Fusarium, and Hormoconis resinae. These microbes find fuel to be an excellent source of food, and they can produce biomass, slime growths, and acidic byproducts that cause corrosion and clog filters, leading to costly repairs and even engine shutdown.
To prevent and control microbial growth in diesel fuel, it is crucial to minimise water buildup in tanks. Procedures such as fuel recycling through water separation and regularly discharging water from the bottom of tanks can help. Keeping tanks topped up can also reduce the chances of water condensation forming. Additionally, fuel tank insulation and stabilising fuel temperature are important measures to inhibit microbial growth. In cases of severe contamination, sludge removal, tank cleaning, and the use of diesel fuel biocides or microbial growth removers are necessary.
Testing for microbial contamination is essential to diagnose and address the issue effectively. The ASTM D7463-08 test detects the presence of Adenosine Triphosphate (ATP), an energy-bearing molecule found in living cells, to confirm microbial presence and estimate their concentration. In-field tests like ATP-By-Filtration can also provide valuable insights into the extent of contamination and the success of remediation efforts.
To summarise, microbial growth in diesel fuel is a significant concern that can lead to costly issues and engine problems. Preventative measures, regular inspections, and effective treatments are crucial to minimise the impact of microbial contamination and maintain the reliability of diesel fuel systems.
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Fuel degradation
Diesel fuel degradation is a common issue that can lead to significant problems for engines and fuel filters. The degradation process is often the result of microbial contamination, which occurs when water present in the diesel separates into a distinct layer below the fuel, known as "phase separation". This creates an ideal environment for bacteria and fungi to thrive, as they feed on hydrocarbons from the diesel fuel above. Over time, the accumulation of microbes forms a visible biomass, or "rag layer", between the water and diesel fuel, which can turn the fuel black. Additionally, the higher asphaltene concentrations in today's diesel fuel can also contribute to degradation, as they require higher temperatures to combust. Incomplete combustion leads to dark, degraded fuel that negatively impacts engine performance.
The signs of diesel fuel degradation include excessively clogged filters, black smoke, and lower-than-normal RPMs at full throttle. While it is challenging to reverse the degradation process once it becomes noticeable, prevention is critical to avoid costly repairs. Treating fuel with stabilizers, removing free water from tank bottoms, and using biocides to address early microbial contamination are essential measures to maintain fuel quality. Automated fuel maintenance systems are particularly effective for Tier-4 engines, as they work on a programmed schedule to filter out contaminants and prevent microbial growth.
Microbial growth in diesel fuel is challenging to eradicate due to the difficulty of completely sterilising diesel fuel. As a result, surviving microbes develop resistant traits that render biocides ineffective over time. When microbial growth reaches a certain level, it can lead to the formation of sludge in the fuel tank, which requires the use of a mobile fuel polishing service or system to remove. Fuel polishing systems circulate and filter the fuel, removing contaminants and preventing future microbial contamination by eliminating water. However, a single fuel polishing service may not be sufficient to prevent microbial growth indefinitely.
Contaminant build-up from microbial activity can result in fuel filter plugging, impacting the performance of diesel engines. This build-up can include waste products from bacteria, fungi, yeast, and mould growth within the fuel. Additionally, the introduction of biocides to address microbial contamination can sometimes aggravate the situation, as they can turn organic materials into grit that settles at the bottom of the tank. This grit can then be stirred up during refuelling, finding its way into the fuel line. Therefore, it is crucial to address diesel fuel degradation proactively through proper fuel maintenance and storage practices.
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Fuel maintenance systems
Stored diesel fuel can start to deteriorate within 28 days of refining, increasing the chances of engine failure or damage. Diesel fuel can be contaminated by microbes, which can lead to a dark layer forming in the fuel, often referred to as the "diesel bug". This microbial contamination occurs when water in the diesel separates into a distinct layer below the fuel, creating an environment for bacteria and fungi to thrive.
Fuel maintenance services include fuel sampling, fuel polishing, fuel additive programs, and the de-watering of storage tanks. Mobile fuel polishing systems can be employed to remove sludge from fuel by circulating the fuel out of the tank for filtration. These systems can remove both large and small contaminants and help prevent future microbial contamination.
There are various providers of fuel maintenance systems, offering custom-tailored solutions for different applications. Fuel Technologies International (FTI) manufactures automated stored diesel fuel maintenance systems that provide preventative maintenance programs to fight stored diesel fuel degradation. FTI's systems are FM Approved and NFPA compliant, with automatic fuel filtration running on an optimum schedule. Other providers offer fuel management systems for private fleet usage, such as the Piusi MC Box, which is easy to install and ensures fast and precise monitoring of fuel usage.
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Frequently asked questions
The black colour of diesel fuel is due to the degradation of the fuel. This can be caused by microbial contamination, which occurs when water present in the diesel separates into a distinct layer below the fuel, creating an environment for bacteria and fungi to thrive. Additionally, today's diesel fuel contains higher concentrations of heavy end asphaltene, which can result in incomplete combustion, turning the fuel dark or almost black.
To prevent diesel fuel degradation, it is recommended to treat the fuel with stabilizers, remove free water phases from the tank bottom, and use biocides to kill any microbial contamination early on. Automated fuel maintenance systems are also effective in preventing microbial growth by regularly pulling fuel from the tank to filter out contaminants.
If your diesel fuel has turned black, it is recommended to use a mobile fuel polishing service or system to remove the sludge and contaminants from the fuel. Additionally, you can use fuel additives to speed up the cleaning process. However, it is important to note that once microbial growth has progressed significantly, it may be challenging to completely restore the fuel.
