May 16, 2012

Effects of Poor Quality Fuel on Diesel Engine Reliability
by David Doyle,CLS, OMA I & II
Vice President & Operations Manager

Advances in engine technology over the past years have created engine designs which demand more performance for their size with tighter tolerances and higher pressure fuel system injectors and pumps. The quality of today’s diesel fuel has not kept up with engine technology. Poor fuel quality and the use of contaminated fuel can result in wear, premature engine failure or decreased performance.

Tight tolerances of today’s fuel injection systems and high-pressure fuel pumps are susceptible to impurities in the fuel. Poor quality fuels will wear out injectors and pumps prematurely and reduce overall equipment reliability. Impurities and contaminants can include environmental particulate, asphaltenes from unstable fuel and water.  

The most significant quality issue in the field is contamination, mainly from dirt and water. Particulate contamination, prior to fuel system filtration, should be less than 50 milligrams per gallon (15 milligrams per liter). Much of the source of dirt and water originates from onsite storage tanks, conveyance to equipment fuel tanks, and equipment fuel tanks themselves. Injector life is reduced by about half with a water content of 0.05% (500 ppm) compared to 0.02% (200 ppm) ppm. Water content should optimally stay under 0.02%. When fuel contains up to 0.05% it will begin to show a haze. Water content at or above 0 .05% will cause damage to the fuel injection system. Moisture will also adversely affect the lubricating properties of diesel fuel by displacing the fuel. When moisture reduces fuel combustibility and becomes heated in the combustion chamber water immediately turns to steam and damage injector tips. Continuous water contamination will corrode fuel systems as well as promote bacterial growth, which results in plugged fuel filtration systems. Excessive moisture at low ambient temperatures can plug fuel systems due to ice formation, especially when wax formation grows around the ice crystals.

Fuel stability is an ongoing concern, especially with recirculate fuel in the fuel system. Unstable fuels form insoluble gums and asphaltenes. Not only will these plug filters and injectors but they can also affect the combustion efficiency of the engine, resulting in a loss of power. Improper fuel droplet size due to damaged injectors combined with asphaltene particulates requires higher temperatures and compression to combust fully. This is not always attainable during the normal combustion cycle. Fuel that is degraded or poor quality can also become acidic, which will cause corrosion in engine and fuel system components. With the inclusion of biodiesel into the market there has been a rise in the need to measure acid content.

Poor physical properties of diesel fuel can also affect the reliability and service life of diesel engines. Diesel fuel is intended to cool and lubricate fuel injection systems. Poor lubricity will result in excessive wear and premature failure of injectors and fuel pump. Poor lubricity can be the result of using too light of a distillate cut or inherent poor lubricating properties of the manufactured fuel. Severe processing used to reduce sulfur and aromatics in today’s diesel fuels reduce the amount of surface protecting agents in the fuel, which reduces lubricity. Proper additives are required in the manufacturing process to maintain lubricity properties. Heavier fuel cuts can create combustion chamber deposits which will reduce the life of cylinder rings and liners. A low cetane number can also result in engine damage over time. If the cetane number of the fuel is low, engine knock and starting trouble can result from delayed ignition. Diesel knock during idling, poor warm up and white smoking after startup will result from too of a low cetane number.

When filter elements and injectors are routinely replaced at premature time periods for service life this is an indication of poor quality fuel that should be investigated. Injectors should generally last 15,000 hours and filter elements a 1,000 hours or more.

Though fuel usually is not tested with the same frequency as in-service lubricants, it is advisable to have it inspected when engine reliability issues arise that may be the result of poor fuel quality or when product integrity is suspected. Standard testing can look for proper physical properties such as density, distillation range, cetane index, viscosity, flashpoint, and acid number. Testing for contaminants can report water and particulate content, microbial growth and undesirable metals. Fuel degradation, asphaltenes, and storage stability can be looked at with the appropriate tests. Lubricity assessment can be performed when other tests do not indicate sources for cause of failed pumps and injectors. Testing of your fuel not only provides a resource for trouble-shooting, it can also serve as a periodic inspection of storage tank cleanliness and integrity. This is particularly useful for standby generator units and demanding service environments.





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