Diesel Fuel analysis – All you ever wanted to know and more…

Diesel Fuel Routine Testing.

Note – Fuel regulations are constantly changing and vary by country. The threshold values used in this document are based on latest information as of point of writing based on UK regulations, but if in doubt some limits may be outdated please contact your laboratory if you have any questions. Additionally, ask for if there have been any updates to this guide to reflect any new regulation limit changes.

Much of the testing is performed to confirm the fuel meets specification at the point of the refinery. There are often complex transportation and supplier chains before the product finally meets the client and the product can be held in bulk storage where contamination can occur over time. Hence, in addition to the standard refinery testing to specification that is performed there are additional places where fuel testing can be performed.

  • Specification testing – testing to full standard specification of fuel – commonly a mandatory requirement. Examples include:
    • Point of sale to client – confirmation product meets requirement at point of delivery.
    • Periodic spot checks – usually annual, especially on critical pieces of equipment, or where more than one fuel type is used on site to confirm no cross-contamination.
  • Condition Testing – confirmation fuel has not been contaminated during bulk storage. These are generally more condensed suites to look for contamination such as dirt, water and microbes. Examples include:
    • Bulk storage tank checks to confirm no water, dirt or microbial growth. Usually performed 3 monthly, or post tank cleaning. Bulk tank testing is usually performed in two locations:
      • Bottom of tank – To identify sediment, water and sludges that require drain off.
      • Centre of tank – to identify overall condition of the bulk of the tank.
  • Warranty Testing – confirmation correct fuel has been used in the machinery for warranty purposes. This is commonly requested by OEMs to confirm if a failed machine is covered by warranty and if the warranty has been invalidated by mis-fuelling. Examples include:
    • Any machine failures where suspected fuel faults.
    • Suspected mis-fuelling e.g. Petrol in diesel.

Elemental Analysis – Elements within the oil analysed by either using X-Ray diffraction/dispersion patterns or spectroscopic analysis of wavelengths of light emitted when sample injected into plasma flame.

Diagnostic significance: This is usually used to test for presence of lubricating oil contamination (calcium, Magnesium, Phosphorus or Zinc), as well as Iron corrosion in bulk storage tanks and the sulphur is within specification. In marine applications Vanadium and Nickel are also good indicators of cross contamination with heavy fuel oil products.

Elemental Analysis – Sulphur (30ml)

This is the most commonly monitored fuel property on diesel across all industries. This is because regulations have continuously lowered the level of allowable sulphur to be used owing to its environmental implications and its link to emission gases causing acid rain. Additionally, sulphur in the presence of water (produced in the combustion process) forms sulphuric acid – battery acid, which means more acidic by-products causing corrosion to engine parts (see earlier sections “Tests on LearnOilAnalysis report” – Acid number and Base number). Finally, sulphur in the form of active sulphur can lead to corrosion of injection system components (see copper corrosion). This has meant that, for example, some fuel regulations have reduced sulphur content from 1000ppm to as low as 10ppm over the space of a decade.

Sulphur maximum limits are also variable by industry and application with a fuel acceptable for off-highway standby generators not acceptable for combustion in a road operating vehicle. In addition, in the marine industry bunkering in one location with different regulations for fuel, can mean although the sulphur is acceptable in one location, it cannot be burnt in another location – hence the need to monitor every batch of fuel, even if the fuel met specification at point of sale. One of the problems in lowering the sulphur is that despite its obvious disadvantages, sulphur is an excellent lubricant and so the constant lowering of sulphur content causes headaches for both OEMs in operating more fuel efficient fuel injection rail systems with tighter tolerances as well as fuel suppliers in supplying fuel with sufficient lubricity properties to prevent injectors seizing.

Water Content (20ml)

Water is a poor lubricant, causes corrosion, promotes microbial growth and also is not combustible, so its content needs to be tightly monitored to ensure efficient operation of the machinery. It additionally is not taxable, so in transport chains its content needs to be monitored closely to determine if 50000L of bulk delivery fuel is indeed that or if 500L of it are water for instance. In a market where exceptionally large quantities of bulk fuel storage fuel are bought and sold each day, and profit margins in the industries buying the fuel are quite small, any additional expense that should not be incurred can have a big impact on the businesses involved.

Diagnostic significance: The cause of water is usually due to fuel storage conditions in that tanks ‘breathe’ – i.e. the air temperature in the day and night changes meaning there is a constant circulation of air – usually through breathers – into the tank. At night this cool air condenses on tank walls and over time there can be a large build-up of water content in the storage tanks. Additionally, tanks tend to be drained or pumped from the bottom meaning if the tank has not been disturbed for a long time water sinks to the bottom of the tank because water is denser than fuel. This means that even low water contents overall, are found in high concentrations when drawing from the tank bottom.

Cleanliness – Sediment or ISO cleanliness (30ml)

The most common failure type of a fuel sample when testing to fuel specification is due to environmental contamination such as dirt. Dirt is usually removed by the fuel filtration systems, but the tolerances and pressures of injector rails means that it is not practical to have filtration at sizes smaller than the tolerances of the injectors meaning very small particulate matter may pass through and cause injector seizing or scoring. For this reason it is important to keep the fuel exceptionally clean owing to the fine tolerances of the parts in use.

Cleanliness can be measured by ISO cleanliness code routinely (see “tests on LearnOilAnalysis report – contamination section to explain more about ISO codes”). However, as part of specification testing it is usually measured by gravimetric filtration where a filter patch (usually 0.8µm pore size depending on method used) is weighed before and after filtering a set volume of fuel to determine the weight of the insoluble contamination.

Microbial Growth (10ml)

Microbial growth is usually only present when water is also present in the system as all known cellular life requires liquid water to thrive. Hence, minimising water content is an excellent way of reducing microbial growth. The reason microbial growth is so important is that they tend to get filtered out by the filtration system in addition to water and the filters act as an excellent breeding ground for microbial growth. This growth can completely block filters meaning insufficient fuel reaches the injector rail and the engine stops.

There are two main ways of detecting microbial growth. One is to measure as a culture, where the microbes are incubated for sometimes several days or even weeks to identify the number that grow per volume, termed Colony Forming units (CFU/ml). This is useful for determining the microbe type e.g. bacteria/fungi or yeast/mould so that a suitable biocide may be used. However, please note local regulations vary on the use of biocides in fuel systems – so please check with your local authority if you can use them. The use of tank cleaning services where the tanks are flushed and cleaned is becoming more common – in these cases the particular organism is not as significant and the speed of testing becomes more relevant. In these situations, fast turnaround test kits are quite popular. These can produce results in just a few hours of microbial activity by biochemistry as opposed to microbiology. cATP looks at the presence of life by life-presence confirming biochemical reactions as opposed to culturing and growth. The testing is usually more expensive than performing standard culture methods, but does give a faster result, which in critical equipment is usually something that is worth the extra cost.

Specialist Diesel fuel specification testing

Ash (20ml)In method IP4 (ASTM D482) a measured weight of sample is burned in a crucible until ash and carbon are the only remaining products. The residue is then heated in a high temperature furnace at 775OC to remove the carbon, leaving only the ash. Upon cooling the ash is weighed to give the percentage ash of the total product.

Diagnostic significance: The ash can come from lubricating oil, metallic deposits / cat fines or solids such as dirt and rust. This measures the amount of ash forming material to identify if combustion will produce unacceptable deposits affecting both cylinder wear rates (as these are abrasive) as well as particulate emissions. The Abrasive ash content contribute to fuel pump, piston, ring and injector wear and can ultimately lead to an upper cylinder and engine failure if severe.

Calculated Cetane Index (150ml)This is used as an alternative to expensive actual engine test-bed testing on a single cylinder comparing performance with blends of n-hexadecane (straight chains) and alpha-methylnapthalene / heptamethylnonane (non-straight chains of carbons). The method looks at Density/API Gravity and the mid-boiling point of the fuel to give an estimated Cetane Value, termed the cetane index.

Diagnostic significance: The test is used to identify the level of contamination with non-straight chain hydrocarbons and aromatic products, with the higher the value indicating lower aromatics / double bonds, meaning less energy will be required to break these bonds, a shorter ignition delay a more efficient combustion process. This is an excellent test to use on diesel fuel and is used similarly to octane rating on petrol fuels as a diagnostic tool for ignition delay problems. Ignition delay is where there is a prolonged delay between time of injection and combustion, leading to a rapid rise in pressure due to the accumulation of fuel before combustion. This rise in pressure can cause an audible knocking sound, and caused increased stress on bearings and liners from this vibration. This is a test that is very unlikely to fail routinely as the refining processes in manufacturing diesel are usually excellent. Hence it is usually contamination with another product that is the cause of low cetane indicies.

Carbon / Micro-Carbon residue (10% bottom) (10ml)This method looks at the tendency of the fuel without additives to form carbon deposits.

Diagnostic significance: This shows the tendency of the fuel to cause ‘coking’ of the engine. The deposits left behind may form heat insulated hot spots and high stress cracking. Additionally, the hot spots can lead to deposit formation in injectors (blocking / scoring the injectors) and potential pre-ignition.

Copper corrosion (35ml)See “Specialist Lubricant Tests” for details of this test, which are also performed on lube oils.

Diagnostic significance: This is only likely to be abnormal if contaminated with another product with high sulphur. Its importance is on its effects and tendency to corrode copper parts of the fuel system.

Cloud Point, Pour Point and CFPP (45ml) – See “Specialist Lubricant Tests” for details of this test, which are also performed on lube oils. One particular issue with fuels is when the fuel gets to low temperatures the system does not normally heat the fluid above atmospheric temperature (as in a lube system). This means wax crystals may not dissolve after forming even when temperatures return to above the cloud point are reached as the crystals require additional energy to re-dissolve above the cloud point. Wax is difficult to dissolve as it is a good insulator as well.

Diagnostic significance: Their significance is really in its ability to be pumped and block the fuel filters and exposed fuel lines at cold temperatures due to precipitation of wax causing restrictions in flow. Pour point is particularly useful to identify the lowest temperature the fuel can be pumped between bunkers and tankers etc. Pour point can also be used to determine the effectiveness of pour point improver additives, which help inhibit wax precipitation.

Distillation Testing (120ml)The distillation heats up and boils the fuel to determine the volatility and different fractions within the fuel mix. This is important as volatility effects fuel economy, soot formation / engine deposits, exhaust emissions and overall power output.

Diagnostic significance: This is only likely to be abnormal if contaminated with another product. A low initial boiling point (IBP) suggests contamination with a higher volatile fluid. A high 10% recovered temperature can cause difficulties with starting the engine as the fuel is not volatile enough to ignite or the time taken to achieve operating temperature may be extended, meaning longer warm-up time. A high 50% recovered temperature can lead to smoking issues as excessive incomplete combustion. A high 90% recovered temperature increases the risk of engine deposits and fuel dilution as the fuel is not reaching the desired temperatures to completely combust.

Density (15ml)The density of the fuel is an important physical property of the oil like viscosity.

Diagnostic significance: The causes of a high viscosity can be contamination with denser products such as biofuel (FAME), whilst low density can be contamination with less dense solvents or products such as petrol.

Flashpoint (110ml)- See “Specialist Lubricant Tests” for details of this test, which are also performed on lube oils.

Diagnostic significance: The significance is in the safety for transportation (closed cup/mini-flash) where the vapour collects above the sample in the headspace of the container and can flash to form a fire. Maximum storage temperatures are usually a minimum 10OC less than the flashpoint.

FAME (total) – Fatty Acid Methyl Ester – Biofuel (25ml) – As diesel is not a renewable fuel source, renewable sources such as rapeseed or palm oil derived fuels are now being introduced into diesel fuel blends to give a renewable source to the fuel. The amount that can be used differs by country and OEM ability to run the fuel type, but UK regulation is max 7% for EN590 specification, but parts of Europe use 20%, 80% or even 100% biofuel blends.

Gas Chromatography / Mass Spectroscopy (GCMS) (20ml)Diesel, mineral oil, petrol or heavy fuel oil are not a single molecule throughout and are actually a mixture of various lengths of carbon chains to give an overall blended product. Hence no two samples of diesel will be exactly the same and there can be slight variations depending on crude oil used. Gas Chromatography/Mass Spectroscopy methods use different column types to separate a mixture of chemicals such as diesel into their individual components based on properties such as mass or charge of the molecules. Similar to filter paper chromatography that most people will be familiar with, the individual components are separated out so that comparisons can be made.

Diagnostic significance: There is no specification testing that requires GCMS. However, it is an excellent tool in identifying problems otherwise not detected by specification testing, or identifying the root cause of a specification failure. For instance a low flash point, density and viscosity on the specification may point to a low flash point product contaminating the fuel, but GCMS is required to identify what that contaminant may be. A classic example of mis-fuelling with petrol into a diesel tank is shown below.

GCMS Use in identifying Petrol in diesel. Bottom Left shows typical road diesel purchased from local fuel station. Top left shows road diesel with 5% petrol contamination. The right hand trace shows the overlay of both traces. In each case the ringed area is the point of interest on this trace when identifying the petrol contamination.

Lubricity (20ml)The lubricity of a diesel fuel can affect the engine components such as fuel injection pumps. The most common method used to determine lubricity is the high-frequency reciprocating rig (HFFR) method. In this method a vibrating non-rotating steel ball loaded with a specified mass, is lowered onto a disk submerged in the test fuel at a set temperature and time period. At the end of the test the wear scars on the ball are measured x100 magnification to determine the length of the scar in microns (µm), with <250µm being good, >600µm being bad and the target being set usually at a maximum of 460µm.

Lubricity Wear scar images (both at same magnification). Left the sample had a scar size of 394µm (pass), whilst the scar on the right was 685µm (fail).

Diagnostic significance: A high result suggests the fuel not only fails specification, but injectors are more likely to seize and overall component life is expected to decrease.

Oxidation Stability (Accelerated) – (600ml)Traditional oxidation stability testing involves testing over methods up to 24 weeks, which, apart from research purposes, is not practical for the time constraints and turnaround times required for specification confirmation and condition monitoring. Hence, most labs use accelerated methods for determining oxidation stability e.g. ASTM D2274 in which a fixed volume of fuel is bubbled with oxygen at a specific rate and temperature. The total filterable and adhesive insolubles formed during the test as measured in kg/m3.

Diagnostic significance: Even if, at point of testing, your fuel passed specification, over time the fuel ages and becomes contaminated. The level of which is determined by the several variables including age of fuel, storage conditions and local environment. If tanks are not fully drained, contaminated or aged fuel can accelerate the oxidation of fresh fuel when topped up. Causes of high oxidation stability can be because the sample has been stored in a tank with high sediment and water, which can lead to fuel darkening, blocked filters and lacquering. Once the contamination is removed, the fuel may not be fully stable, in which fuel anti-oxidant additives may be used to improve the oxidation stability of the fuel.

Viscosity (10ml) (see “tests on a LearnOilAnalysis report – Fluid condition” for more explanation on viscosity testing principles.) The viscosity is important to ensure effective flow of the fluid through injection nozzles, fuel lines and orifices.

Diagnostic significance: The viscosity is an important physical property. Causes for high viscosity include: too high biofuel, lubricating oil or heavy fuel oil contamination, whilst too low viscosity can be caused by solvent or petrol contamination.

Diesel Fuel Specification limits

Limits based on EN590: 2009 for Road vehicles / coastal vessels, BS2869: 2010 for off-road highway vehicles and stationary applications, and ISO 8217 for marine applications (none-costal). Regulations change by country, so consult your local authorities to for regional differences.

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