Large amounts of testing is being performed to optimise the efficiency of oil / water separators in diesel engines. The filters remove any solid components (rust, paint chips) that may cause wear on the engine as well as undissolved water (that will be present in the diesel). Large water droplets are reasonably easy to remove, but increasingly the water droplets are present in the form of a fine emulsion. 

The recent addition of various fuel additives (especially in low sulphur diesels) act as surfactants and have the unwanted side effect of reducing the emulsion drop size (we now have a stable emulsion with a high Zeta potential (charge) so need to remove it). As part of filter design, filters must be tested to examine how they will behave in field. Currently new test methods are being written to standardise testing conditions between laboratories. A new version of ISO16332 will address this. The equivalent US standard is SAE J1488.

The principal use is to validate the effectiveness of the test rig, and how potential droplet recirculation during testing could influence the particle size distribution. Laser diffraction has long been used as a testing mechanism for these filters. This has ranged from traditional laser diffraction measurements (such as a Mastersizer) to high concentration laser diffraction measurements (Spraytec or Insitec). Traditional laser diffraction will require dilution, and as it is known that droplet size depends on the surfactant concentration reducing this (by diluting the sample) can have the unwitting effect of changing what is being studied, so a high concentration measurement makes more sense. 

The high concentration laser diffraction measurements are also real time, so how rapidly filtration occurs can also be studied. The test rigs are set up to measure at realistic flow rates to the “real world” so the ability to measure at much higher flow rates than a traditional laser diffraction measurement is also required. There is also the advantage of the measurements being real time, rather than having to extract samples at appropriate times for later laboratory analysis, this means the whole process can be easily followed.

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