DIRT IN DIESEL

As engine technology advances, so too are more demands being placed on fuel manufacturers to produce a cleaner and purer quality of fuel. Vehicle manufacturers contend that to operate a high quality diesel engine, it is essential that high quality operating agents are used. Engine oil, coolant and fuel have to match the standard of technical quality of the engine. From the operator's side, the increasing cost of fuel, of replacement engines and parts and of maintenance and downtime has also made fuel quality an increasingly important issue. Users continue to demand better than the national standard specification (SABS 342) - one such specification being particulate content in diesel or diesel cleanliness. This article is intended to shed some light on the not-so-well-recognised fuel cleanliness property and it's impact on fuel handling and engine utilisation.

The Problem
Diesel fuel properties have varying effects after production at the refinery - in its transportation, in storage prior to use and ultimately in the vehicle fuel tank and engine. The main problem is that particulates or sediment entering the fuel system alters the volumetric accuracy of the injector systems hence negatively affecting the performance of the engines. To this end, users look to the fuel suppliers for fuel cleanliness specification guidance information during storage and handling and ultimately, during use in the vehicle's fuel tank and engine.

It is no longer just enough to issue a certificate of quality on the fuel leaving the refinery gate. Users are now demanding for fuel suppliers to assist with test methods to measure and monitor these fuel properties at the time of receipt at their site. Shell has listened to it's customers and has investigated the problem with the objective of setting up some guidelines on fuel cleanliness. 

The cost of NOT using clean diesel 
Costs related to the effects of fuel cleanliness are less quantitative, mainly because of a lack of awareness. Investigation has shown that the larger percentage of vehicles on our roads today use standard filters ranging from about 15 to 25 microns. More recently, high efficiency fuel filters - about 2 to 5 microns - have been introduced which are designed to cope with the higher pressures and smaller orifices (<0.005mm) of injector nozzles in modern engines. This places more demand on the need for a cleaner fuel to prevent premature blockage of filters which, in turn, will lead to filter replacement at frequencies shorter than the scheduled maintenance. This downtime can be costly and disruptive to operations which require 24 hour service from their vehicles.

In addition, as the orifice size of injector nozzles reduces, so too will the user's demand for clean diesel monitoring at the point of receipt increase - a request which should not be viewed as unreasonable.

There will, however, always remain the argument as to what costs more - the cost of filter replacement versus the predicted cost of engine life, injector life, downtime and maintenance. My money is on the latter. The cost of cleaning up fuel does have significant cost penalties that would have to be absorbed into the 'food' chain.

The nature of particulates found in diesel fuel 
Essentially, eight types of materials can be found as particulates in diesel fuel. These types are shown in Table 1 together with some examples.

It is also important to understand the nature of the sediment one is dealing with in order to treat the root cause. Fungus, yeast and bacteria are normally treated with good house-keeping practices and in severe cases, with anti-bacterial agents. It's the hard abrasive metallic and silica type particles that cause injector wear damage.

How do we measure dirt we cannot see?
Gravimetric methods are specified to measure sediment (particulates) in diesel. A known volume of fuel is filtered through a pre-weighed test membrane filter (0.8 micron) and the increase in the membrane filter mass is determined after washing and drying. The change in the mass of the control membrane filter, located immediately below the test filter, is also determined. The particulate content is determined from the increase in mass of the test membrane filter relative to the control membrane filter. The result is reported in mg/L, which can be converted to mg/kg or ppm by dividing by the density.

Table 2 shows the different gravimetric methods used for monitoring contamination in diesel. Note the wide range that exists in the maximum specifications between, for example, SABS 342 through to the US Federal specification.

Some users incorrectly make use of the 'oil cleanliness' method ISO 4406 to monitor cleanliness in diesel. ISO 4406 is used to describe particulate contamination in lubricants. It is neither technically equivalent to nor is it in alignment with OEM requirements for diesel cleanliness. Shell's experience is that ISO 4406 not only lacks correlation with the gravimetric methods but is also inapplicable. Lubricants and fuels have functionally different roles in an engine and the culprit particulates in each case need to be measured and monitored differently.

To evaluate actual filter loading ability, the Society for Automotive Engineering has developed the SAE J905 test method. The results obtained by this method can be correlated with IP 415 and ASTM D2276 values to establish tolerance and condemning limits.

Correlation between clean diesel and injector wear and performance 
User requirements for diesel cleanliness will vary depending on the level of contamination that can be tolerated by specific end-user systems. Studies done by the Society for Automotive Engineering (SAE) together with Donaldson Filters have shown that in going from normal fuel filters of 20 microns to using high efficiency fuel filtration of 3 microns, fuel injector wear can be reduced by at least a factor of two in both on- and off-road vehicles. In general, fuel filter ratings of less than 5 micron (beta=75) contributed to significant wear reduction.

In another study, the SAE correlated the data above with the change of injector performance in actual case studies. Significant shifts were recorded for the volume of fuel injected under dynamic and static conditions and the tip leakage rate. The fuel injector is a component of very high-precision designed to meter fuel to a high degree of accuracy. The correct behaviour of the engine depends on the injector doing it's job properly, otherwise there will be repercussions in terms of noise, smoke, soot, wear and emissions. 

Shell Filterability Method
Because of the unique characteristics of different types of particulates (see Table 1) in diesel fuel and their ability to plug filters and also filter porosity ranging from 3 microns for high efficiency filters through to normal fuel filters of 10-20 microns, the above tests may not be most appropriate. Shell, on the cutting edge, has understood this and has developed an in-house filterability method.

The Rapid Filterability Rig (Shell's own method) can be used to monitor a fuel's filterability in markets where fuel can be easily contaminated. It is a rapid laboratory test procedure and is used to measure the tendency of diesel fuels to block filters. It is unique and more realistic to methods mentioned above in that it uses the actual filtering medium of a specific filter used in a specific application.

Conclusion
Use of clean diesel and finer filtration does reduce abrasive wear rates and has a significant influence on performance shifts of injectors. Significant changes in injector performance due to poor filtration and use of unclean fuel affects vehicle start-up, driveability and emissions.

The use of cleaner fuel will reduce injector wear and maximise injector performance that in turn will improve long term vehicle durability and reliability. Future fuel system designs are trending toward finer filtration down to 5 micron and below. The selection of finer levels of filtration involves trade-offs in terms of filter life, capacity, flow restriction and other factors. These factors must be balanced to provide optimum system performance and at a reasonable cost. 

Fuels supplied by major oil companies today are sold to SABS 342:1998 specification. The question is, will this continue to be adequate to deliver the increasing performance demands placed on modern diesel engines.

Finally, injector wear due to particulates (dirt) in diesel can be significantly minimised if customers become aware of the problem and enforce good housekeeping practices. 

Shell Technical Advisors will be available to assist customers with fuel filtration and injector wear advice on request. Please call our toll free number 0800 021 for assistance.

Sources:

• Total Filtration - paper by SAE
• Correlation of Fuel Filtration Levels to Fuel Injector Wear and Performance Shifts - paper by SAE
• Particulate Contamination in Middle Distillate Fuels - ASTM D 2276 / IP 415
• User Needs for Diesel Stability and Cleanliness Specifications - by L.L. Stavinoha
• Rapid Rig Method for Determining the Filterability of Diesel Fuels - Shell Research Center, UK
• Fuel Filter Test Method - SAE J905