A CABIN AIR FILTER HAS A LOT OF WORK. AND IT SOMETIMES NEEDS A LITTLE HELP: FROM A COARSE OR PRE-FILTER.
Car manufacturers have been using cabin air filters for many years now to make sure that the passengers get enough clean air to breathe. Whether passenger cars, trucks, buses or agricultural and construction machinery: almost every vehicle today has a “cleaner” – either as a simple particulate filter or combination filter with activated carbon layer. Particulate filters filter solids out of the air we breathe and protect us against fine particulate, pollen, industrial dusts and other microscopic, respirable particles. Activated carbon filters also absorb smells and toxic gases such as nitrogen oxides, petrol vapours or fertilizers and break down up to 99 % of ozone.
Additional coarse filters are often used in vehicles with a long service life that are operated in heavily polluted areas. This type of filter mat can often be seen in traffic as roof-top structures on buses. Others are fitted in the air intake system before the actual cabin air filter and installed as pre-filters in the heater and air-conditioning casing. These coarse filters are used to ensure the maximum filter service life for the complete filter surface area of the cabin air filter: if a mass of coarse particles hits the fanfold surface of the filter this can lead to a premature clogging of the inlet side of the air filter – even to the dreaded pleat bellows clogging that prevents the passage of any air at all. Upstream coarse filters can reduce one-sided clogging of the surface of the cabin air filter on the inlet side, thus helping to ensure that the entire filter capacity is used before the cabin air filter is changed during routine inspections.
CABIN AIR PRE-FILTER (LAP): FOAM OR FLEECE
Pre-filters are normally made of polyester foams or fleece mats. The choice of material depends on the specifications of the vehicle manufacturer.
FOAMED: A HOMOGENEOUS STRUCTURE
Polyester foams consist of a homogeneous block with uniform pores produced by a special thermophysical process, reticulation. After cooling the foam is cut into slices and shaped by cutting dies.
 | | Reticulation – what’s that?
Reticulation is the term used to describe the subsequent removal of foam membranes in a pressure vessel charged with gas. This process produces a uniform, open pore lattice structure with a minimum flow resistance for gases and fluids. |
The filter effect of the reticulated material depends on the pore size PPI (pores per inch, in other words the number of open linear pores per inch = 25.4 mm). The higher the PPI number the smaller the particles that can be filtered. Common foams have between PPI 10 and PPI 80. MAHLE Original and Knecht LAP pre-filters are made using foams with a PPI of 30 since extensive laboratory tests have proven the optimum relationship between service life and coarse particle filtration for this pore size.
Polyester foam is resistant to ozone and partly resistant to hydrolysis, it withstands ageing and temperatures up to 90 °C and it is free from plasticizers and CFCs. The material can be recycled and poses no problems for thermal disposal.
FLEECE MATS: PROGRESSIVE LAYERS
These fleece mats are based on approx. 10 and 20 mm thick polyester fibre mats that are made up of various, progressive layers so that they become denser and denser on the clean air side. This structure combines a maximum filter performance with optimum dust retention properties.
Filter mats are classified in filter classes according to DIN EN 779.
| Filter class | | Filtration efficiency |
| G2 | | up to 80 % |
| G3 | | up to 90 % |
| G4 | | > 90 % |
Weights of between 100 g/m² and 200 g/m² are used depending on the requirements. For example, a G2 filter mat with a weight of 100 g/m² will primarily stop coarse particles whereas a G3 filter mat of the same thickness and a weight of 200 g/m² can filter out much finer particles.
Polyester fleece is hardly flammable (classification of inflammability F1), resistant to hydrolysis, withstands ageing and temperatures up to 100 °C. The material is free from silicon, plasticizers and CFCs, can be recycled and is suitable for thermal disposal.
 | | The surface of the cabin air filter can become clogged with coarse dirt particles – right through to pleat bellows clogging (see left) – whereas the lower depths of the pleats remain relatively clean (see right). A pre-filter protects against this premature wear (LAP). |
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REPLACING CABIN AIR FILTER MATS (LAP)
Since the coarse filters are often located in front of the actual cabin air filters they can usually be replaced with no great effort by the driver. Only a few steps are needed and these are described in detail in the MAHLE fitting instructions (see illustration example).
LAP filters are either used as pre-filters in addition to particulate filters (LA) or as combi-filters (LAK). Older commercial vehicles only filter the outside air for the driver’s cab through filter mats. MAHLE Original and Knecht cabin air filter mats (LAP) are also available for a number of these commercial vehicles.
MAHLE is one of the top suppliers of cabin air filter mats on the free aftermarket. Following a successful market launch the range of products is constantly expanded, with a focus on commercial vehicle applications and older passenger cars as of model year 1980. Popular examples include the Renault Truck Midlum or Volvo 760. Cabin air filter mats will also be available as of October 2010 for commercial vehicles from DAF, MAN, Mercedes, Neoplan, Saab, Webasto or cars such as the Mercedes-Benz S-Class. The future LAP program will appear in the online filter catalogue in autumn.