High service intervals in combination with outstanding fuel cleanliness are the current demands for fuel filter systems. Therefore, an alignment of different filter stages is an alternative to one single, high efficient filter cartridge. When balancing the size and increasing the efficiency by each stage, ultra-high filtration efficiencies can be realized by keeping a compact design.
For the validation of such a system, the key question is to determine the optimal combination of filter media and the comparability between lab test and the field. The different standards to determine efficiency and dust holding capacity use different test dusts and already lead to very different results regarding the dust holding capacity.
For a field relevant development, the boundary conditions for each stage need to be considered. A pre-filter is usually operated on the suction side of the low pressure fuel pump while the main filter is located on the pressure side. Therefore the final differential pressures of these stage are significantly different. The pre-filter is therefore optimized to not exceed e.g. 300 mbar differential pressure at end of life but the main filter is designed to make use of an allowed pressure level of e.g. 2 bar after the same mileage. It is possible to integrate multiple filter layers or stages within either the pre- and/or main filter. With the gradient structure of these stages, the overall size of the filter system can be reduced.
It is important to take special care about the choice of the kind of pre-filter media and amount regarding the dust holding capacity, because there is only little tolerance on the suction side for high differential pressure. On the pressure side, the fuel pump may compensate a higher loading by more mechanical work that is not possible on the suction side.
To analyze the influence of the suspension on the filter loading independently of the influence of different test methods the experts at MANN+HUMMEL developed a new test rig. The test enables the testing of up to four stages in line with using different test dusts such as the ISO 12103 A3, ISO 12103 A2, ISO 12103 M2, ISO 12103 M2 with soot and JIS Z8901 Class 11. Furthermore, different test liquids can be used. The main difference between the liquids is the viscosity.
The efficiency and dust holding capacity of fuel filters are characterized by test standards e.g. according to ISO 4020-6.4 and ISO 19438. The equipment for these test methods is available and well established in the fuel filter development. Usually pre- and main filters are characterized separately and designed based on test results for each stage. Up to now there is no established test method and test bench for a multi stage filter testing.
In contrast to most of the standard test methods this test bench is designed to use more field relevant low viscous test fluids and variable test contaminants. In addition, up to four different flat sheet samples connected in series can be tested simultaneously. This offers further variability for different test procedures. For each stage the differential pressure increase during particle loading can be determined. This enables an ideal design of different filter media layers and/or pre- and main filter combinations can be implemented to reach the optimum dust holding capacity. The filtration area of each flat sheet can be varied independently and allows a further degree of freedom in filter design and development. The multi stage flat sheet test bench is fully validated based on established test standards and fulfills the criteria regarding cleanliness, stability and reproducibility.
Modern filtration systems feature a multistage water separation. This concept contains the filtration step to remove particles (1st stage), a coalescer to increase the water droplet size (2nd stage) and a hydrophobic barrier (3rd stage). Between stage two and three the enlarged water droplets settle through the sedimentation gap towards the water collection section. The water separation can be integrated either in the pre-filter or in the main filter stage, depending on the requirements. The dust holding capacity of the system is not directly affected by the water separation stage, because the coalescer is not holding back any particles, since it is protected by the upstream filtration stage. This is important to ensure a high water separation at the end of life when the system has reached the change interval. Indirectly it is more challenging to meet a high dust holding capacity with a 3 stage water separation concept because it requires more space compared to an outdated 1-stage concept (water repellence on the dirt side of the filter bellow).
The design of multistage filter systems can and will improve the overall performance regarding dust holding capacity, filtration efficiency and robustness in comparison to the single stage approach, but a comprehensive testing respecting the high variability of field conditions is essential. Especially top-level filter media with a efficiency of greater than 99.9 % to 99.99 (beta 10000) acc. ISO 19438 @ ≥ 4 µm(c) will benefit from less efficient filter stages on the upstream side to keep the overall dust holding capacity high, although they would reach the same efficiency by only using one stage.
Printed on 2019-04-21