DIESEL FUEL ANALYSIS OVERVIEW

Diesel fuel analysis is based on legal requirements, recommended specifications from engine manufacturers and trouble-shooting a fuel related problem.  The following information describes test packages, followed by their purpose.

TESTS FOR DIESEL FUELS

ANALYSIS EXPLANATION AND RECOMMENDATIONS

CETANE NUMBER:  Identifies fuel ignition delay characteristics.  If there is a significant delay between the fuel injection and the beginning of combustion, a characteristic pre-detonation “knock” will occur, as well as vibration.  Cetane Number requires a single engine to perform the test.  Most commercial laboratories perform a ‘Calculated Cetane Index’ that includes a calculation based on the API Gravity and the mid-point boiling range of the Distillation.  Calculated cetane index, while a useful estimate, is not always representative of the actual cetane.

GRAVITY:  Both ‘Specific’ and ‘API’ Gravity are indications of the density of weight per unit volume of a fuel.  This is a critical property of marine fuels that require centrifuging.  It is part of the calculated cetane index.  It can also be an indicator (with distillation) of the type of base stock used; i.e., high specific gravity is associated with paraffinic hydrocarbons number.

 DISTILLATION:  Identifies volatility characteristics of fuel.  Fuels having too low volatility reduce power output and fuel economy through poor atomization.  Fuels having too high volatility reduce power output and fuel economy through vapor lock or inadequate droplet penetration from the nozzle.  The valves at different boiling points can identify whether the engine will be hard to start (warm-up time), whether excessive smoking (white smoke at startup), and/or odor are likely.  Low carbon residue and minimal fuel dilution. 

VISCOSITY:  A measure of a fluid’s resistance to flow.  In a diesel fuel viscosity affects the shape of the fuel spray characteristics.  With high viscosities the fuel jet tends to be a solid stream instead of a spray of small droplets.  As a result the fuel is not well distributed in the air required for burning.  This results in poor combustion, loss of power and fuel economy.  Low viscosity causes a spray that is too soft to penetrate the combustion chamber for good mixing.  Low viscosities are precursors to leaks, and fuel component wear due to reduced lubricity.

COLD TEMPERATURE PROPERTIES (Cloud, Pour and CFPP)

CLOUD POINT:  Identifies the temperature at which wax forms and first precipitates from solution.  This test determines the likelihood of fuel filters plugging during cold weather operations.  Additives may reduce pour point without having any effect on cloud point.

POUR POINT:  The lowest temperature at which the fuel can be pumped.  Satisfactory operation of an engine would be in a temperature range between the cloud and pour point.

  Position of fuel lines and filters play a role in successful cold weather operation.  IF lines are protected from external air, and the filter can pick up engine heat there is less likelihood of a filter-plugging problem.  

COLD FILTER PLUGGING POINT.  Cold Filter Plugging Point (CFPP) is the highest temperature, expressed in multiples of 1 °C, at which a given volume of fuel fails to pass through a standardized (45 micron) wire mesh filter.  This procedure is repeated, as the specimen continues to cool, for each 1 °C below the first test temperature.  Testing is continued until the amount of wax crystals which have separated out of solution is sufficient to stop or slow down the flow so that the time taken to fill the pipette exceeds 60 seconds or the fuel fails to return completely to the test jar before the fuel has cooled by a further 1 °C.  The indicated temperature at which the last filtration was commenced is recorded as the CFPP.

The significance and usefulness of this procedure is that the CFPP of a fuel is suitable for estimating the lowest temperature at which a fuel will give trouble-free flow in certain fuel systems.  In the case of diesel fuels used, results are usually close to the temperature of failure in service except when the fuel system contains, for example, a paper filter installed in a location exposed to the weather or if the filter plugging temperature is more than 12 °C below the cloud point value.  Domestic heating installations are usually less critical and often operate satisfactorily at temperatures somewhat lower than those indicated by the test results.  The difference in results obtained from the sample “as received” and after heat treatment at 45 °C for 30 minutes can be used to investigate complaints of unsatisfactory performance under low temperature conditions. 

In summary, this test informs you of cold flow problems that could not otherwise be determined by pour or cloud point. 

FLASH POINT:  The temperature to which a fuel must be heated to produce an ignitable vapor-air mixture above the liquid fuel when exposed to an open flame.  Flash point is a requirement for determining handling and transport of fuel more than a quality test for the fuel itself.  There is no evidence that variations in flash point have any effect on auto-ignition temperature, or combustion performance. 

SULFUR:  Sulfur causes engine damage primarily as a result of its combustion by products, ultimately leading to acid corrosion of components from sulfuric acid.  The two methods in use to meet the legal requirements for analysis of low sulfur are:  ASTM D-2622 and ASTM D-4294.  At this time only ASTM D-2622 can provide a legal defense.  ASTM D-2622 is the method in use by CTC Analytical.

WATER & SEDIMENT:  The presence of water and sediment causes engine wear, filter plugging, corrosion of the injection system and possible failure of the engine.  The presence of water and sediment in fuel can almost always be traced to storage and handling of fuels.  Damage to engine components can be prevented by pre-filtering fuel.

A)     Marine residual fuels may have severe problems related to sediment.  It may become necessary to identify the sediment.  ASTM D-4484 was developed to accomplish this task.

B)     When water/sediment is detected in fuel, it is good practice to determine if any bacteria have made the fuel their home. 

CARBON RESIDUE:  This test measures the carbonaceous material remaining in the fuel after all the volatile components are vaporized in the absence of air.  Conradson carbon residue was used as a specification test for all fuels.  The value of the procedure is not questioned when run on residual fuels.  The value is questionable on 2-D. 

MICROORGANISMS:  When water is present there is always the possibility of microorganism growth.  This could be yeast, mold, aerobic and anaerobic bacteria growth.  At best they can plug filters and cause a terrible odor, at worst they are capable of corrosion and pitting of metals. 

FUEL STABILITY:  Fuel will inevitably come into contact with air and water.  If the fuel contains unstable compounds, storage in the presence of air can cause formation of gums and sediments.  These in turn can cause filter plugging, combustion chamber deposits, and gumming or lacquering of injection system components. 

COLOR:  Diesel fuel should be:  Clear (low sulfur over the road 2-D), Blue (non-taxable off-road 2-D). There is no widely available quantitative method for identifying the dyes.  The ASTM procedure for color can identify ‘clear’ but blue is identified visually.  If the color were not clearly defined that would be so stated on the report.  The legal responsibility for dying the fuel is at the refinery.  The purchaser has the legal responsibility to purchase the fuel appropriate for the application. 

APPEARANCE & ODOR:  The method used is a ‘clear/bright pass/fail’ method.  If the fuel appears ‘hazy’ there is the assumption that water is present.  If there is an odor it can either be mercaptans or microorganisms.  Mercaptans are analogous to alcohols and phenols but contain sulfur instead of water.  Some anaerobic bacteria produce an odor like hydrogen sulfide: like ‘rotten eggs’. 

ASH:  A small sample of fuel is burned in a weighted container until all of the combustible matter has been consumed.  The unburnable residue is the ash content, reported as % by weight of the fuel.  Fuel system clearances are close fit.  Ash contributes to wear in the fuel system and plugging of the filters and nozzles.  It can also increase the deposit level in the engine, which contributes to overall wear increases.

CORROSION:  A total acid number and/or a copper strip corrosion test can be performed on fuel.  Either test may be more cost effective as a trouble-shooting test than a regularly performed test.

HEAT OF COMBUSTION:  A calculation, reported as BTU content of fuel, using gravity and mid-boiling range of distillation for calculation.  An additional method provides the gross and net heat of combustion (calorific value) based on gravity, ash content, water and sulfur.

DIESEL FUEL SAMPLING & SHIPPING

CTC Analytical has certified sample cans for shipping diesel fuel.  It is preferable to ship diesel fuel samples by ground transport.  Fuel samples are classified as Class 3 hazardous materials.  They must be properly labeled and entered in the shipper’s certification for hazardous materials, for air shipments.

DIESEL FUEL SPECIFICATIONS*

     *  Dyed diesel can be red (tax exempt) or blue (off road high sulfur diesel).  No technical details will be shown on these fuels.

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