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How clean is your diesel?

Diesel engine and fuel system technology continues to evolve at a rapid rate, driven by the desire to improve performance, efficiency and reliability and to reduce harmful emissions. Global pressure on fuel supply, coupled with the search for environmentally friendly fuel sources means that biodiesel is becoming more common.

Diesel engines are now especially sensitive to even the slightest fuel contamination, with the latest specifications of diesel fuel more prone to spoilage and contamination. Both factors work together to make storing and using diesel more difficult.

Understanding some of the issues associated with modern-day diesel fuel, its storage and use, will help to alleviate the issues we see all too often.

To meet today’s demanding performance, emissions and fuel economy requirements, the latest generation of diesel engines use advanced electronic fuel injection systems with injection pressures in excess of 35000 psi. Ten years ago, injection pressures rarely exceeded 3000 psi. With injector nozzle openings as small as 2-3 µm, they are designed to deliver a precise quantity of fuel, in a well-defined spray pattern with an injection cycle lasting around .005 seconds. Product reliability expectations are also high and diesel fuel injectors are required to operate for up to 10,000 hours. To achieve this clean, dry fuel is vital.

Biodiesel is nothing new, indeed Rudolf Diesel, the inventor of the diesel engine, tested peanut oil fuels in his engines and vegetable oils were widely used during World War Two as a diesel substitute.

The most common form of biodiesel is fatty acid methyl ester and it is this that is now added to diesel to form the blends throughout the world. Unfortunately, a downside of biofuel is that it is hygroscopic, which means it readily absorbs moisture from the atmosphere. Biodiesel does have superior lubricity to that of diesel and its lubricating properties are one of the reasons it is added to low sulphur diesel.

Even microscopic particles that are present in fuel can cause accelerated wear to fuel system components. Particle contamination of diesel fuel will lead to abrasive wear of the injectors and fuel pump components which results in pressure leakages, poor fuel delivery and engine performance problems. The effects of water in diesel fuel can be severe; corroding metal components, concentrating acids that accelerate corrosion, it can react with certain additives, reducing fuel lubricity (which can cause seizing of injectors and pumps) can cause explosive damage to fuel injector tips as the water becomes super-heated during the combustion process. Where water is present in storage tanks it can promote the growth of what is known as diesel bug, and can also accelerate fuel oxidation.

Not only can it cause fuel system and engine damage, it also promotes microbial growth in fuel systems. Being heavier than diesel, water tends to fall to the bottom of a fuel tank. Where enough water gathers, it will form a layer at the tank bottom between the stored fuel and the tank base and this interface is one area where the diesel bug growth can occur. It will also form colonies in other areas of the tank, pipework and fuel filters.

*Diesel bug - microbial growth within a fuel tank or system is commonly known by the generic term ‘diesel bug’. Diesel bug isn’t a single type of organism, there are up to 100 different types of bacteria, moulds and yeasts that have been found inhabiting fuel systems.

The level of microbial contamination in a fuel system is hard to predict as every system is different and offers a unique environment. Where serious contamination occurs, it is highly likely that a biofilm will have formed within the fuel tank.

Biofilm is a complex structure of microbes in which cells adhere to the walls and floor of the fuel tank and to each other. It begins to form when free-floating microbes land on a surface and attach themselves to it. These microbes then start to divide and attract other microbes to join the colony.

Periodically, often following fuel tank turbulence, chunks of the biofilm will slough off, this gives the microbes the opportunity to colonise other areas of the fuel system, and this can be particularly problematic as large pieces of biomass will block fuel filters. Live microbes settling on a filter also start forming biofilms, especially a problem where the filter is only used intermittently such as standby generators.

Another problem caused by biofilm formation is the excretion acid, this acid will erode a metal fuel tank, pipework and filters quickly and will result in fuel spillages. If this acid remains in the fuel pump elements and injectors the results can be catastrophic.

Asphaltenes, gums and organic contaminants as well as other particles such as road dust, grit, rust and engine wear can also be carried back in the fuel spill return line. All particles, regardless of source have the potential to cause wear or damage to the fuel injection system or engine.

Due to its aggressive solvent properties, when biodiesel is used in a fuel system for the first time it can have the effect of loosening deposits from fuel tanks and lines. This can result in clogged filters soon after a change to biodiesel. This same solvent effect can also cause degradation of rubber seals and gaskets in the fuel system.

Prevention is better than cure. Good fuel housekeeping is a combination of effective processes, a little technology and some good old-fashioned common sense. There are several preventative measures that help, including purchasing diesel from reliable sources; ensuring tanks are in good condition, leak-free and well maintained; conducting regular inspections for contamination and introducing an annual test programme; keeping fuel cool to minimise water absorption; ensure fuel tanks are as full as possible; using tank drains and routine fuel polishing to remove water and other contamination, and ensuring maintenance staff are appropriately trained.

Protecting your equipment against bad fuel is a good place to start. It’s not always possible to ensure that the diesel going into your equipment is to the correct specification or of good quality. You are to some degree at the mercy of the fuel suppliers although in the UK fuel delivery hygiene is of a high standard. However, you could be storing fuel which is old and degrading/oxidising all the time. This is where fuel sampling, monitoring and polishing provides peace of mind that your equipment is fully protected.

In conclusion, due to global emissions legislation, the drive for improved performance and efficiency and the increase in the use of biodiesel, we must all take a very different approach in understanding that fuel is a vital component of the engine that needs to be kept in optimum condition. Modern engines are particularly sensitive to fuel contamination and good housekeeping is essential to maintaining your fuel quality. The main consideration when it comes to your fuel housekeeping is ensuring that water is eliminated entirely from the fuel system; this prevents fuel system damage and will prevent fuel spoiling. Drawing on the advice of the major fuel companies and engine manufactures, we recommend, as a minimum, the use of fuel polishing and additives for stored fuel and the introduction of robust fuel management procedures including a fuel testing regime.

To discuss your fuel and any other generator maintenance query, talk to the experts.


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