Filtration: The heart of filtration (1/2004)

Oil filter cartridges

Whether oil, fuel or air – no matter what has to be filtered, the most important part of any filter is the filter material,the so-called filter element. Depending on specifications, it consists of specially finished paper, fleece or a combination of both.

Filters made from paper
The basis for MAHLE and Knecht paper filters are high-grade cellulose fibres impregnated with special resins.

However, it is not only the quality of the raw materials that determine the performance of a filter. At least as important is the processing. The special resins that are used to impregnate the paper make it resistant even to high temperatures, moisture and other chemical effects.

In addition to the impregnation, it is also the embossing that is decisive for the quality of the filter. After a pre-heating phase, the paper is pleated according to the specifications and provided with the embossing. To make the paper mechanically resilient and resistant to chemical and thermal effects, the impregnation is cured afterwards. This creates filters with stable pleat geometry that prevent the feared forming of blocks also under high load and ensure that air, oil or fuel can flow freely through the paper at all times. The fineness of the filters can reach 1 ìm depending on the requirements and offers safe conditions for highest dirt elimination.

Filters made from paper are used in air, fuel, oil and hydraulic filters.

Block forming for example in air filters – an expensive phenomenon

The impregnated cellulose fibres under the microscope.

Everybody who has exchanged a cheap air filter will be familiar with this phenomenon: the individual pleats of the filter stick together to form blocks, so that the airflow through these areas of the paper is restricted. With fatal consequences! This so-called block forming increases the flow resistance, the filtration deteriorates, the life span of the filter is reduced and the fuel consumption and pollutant emission increase significantly. The reason for block forming is usually instable pleating of the paper.

Our advice: always replace used up air filters with filters that are made with stabilised filter paper. In order to prevent loss of stability and shape, MAHLE and Knecht MICRO-STAR® filter papers are impregnated with special resins in a complex procedure.

Filters made from plastic (fleece)
The fleeces are made from plastic fibres. Fleeces are made up of several layers in order to reach efficiencies up to 99.9 % for 1 ìm particles and to achieve low flow resistance at the same time. In general it can be said: the finer the fibre, the better the dirt collection. Density and fineness of the fibres increase from the dirt to the clean side. This creates a funnel effect.

Fleeces are used in cabin filters and, in combination with a layer of filter paper, in fuel filters. For cabin air filters, MICROSTAR ® filter elements made from fleece are available.

Filters made from paper and plastic (multilayer filter media)


	When very high dirt collection capacities are required and when the efficiency can be in the range from 3 to 5 ìm, it is recommended to use a combination of fleece and paper. The additional layer of plastic fibres significantly improves the efficiency and increases the life span at the same time. For multilayer filter media from MAHLE and Knecht, the individual filter layers are combined in a complex production process. Multilayer filter media are used for modern diesel fuel injection systems (direct injection, common rail or pump injector).

Of equal importance: the accurate fit
The choice of the filter medium and its accurate processing are only part of optimum filter quality. Just as important is the precise fit. Unfortunately, this is not always guaranteed. It often happens in cheap filters that a so-called bypass effect occurs: unsealed spots can appear, because the filter and the housing may not fit together precisely and allow unfiltered air or oil to get into the system. You can feel safe with MAHLE and Knecht filters: whether paper or fleece, the filters are always perfectly matched to the particular assembly situation. For example, in cylindrical filters for commercial vehicles, the face ends of the pleat star are fitted with a foamed or plasticized end plate. For various filters, additional safety inserts are used. This protects the engines and cabins of your customers and saves you from unpleasant complaints.