Today cabin filters have become very efficient at protecting car drivers against particulates, pollen, tyre abrasion, allergens, unpleasant odours and corrosive gases. But that was not always the case.
By 1985 there were already more than 30 million cars registered in Germany, which certainly contributed to air pollution. Tests on inner city routes with heavy traffic volumes showed the air directly over the road surface to be between three and six times and in extreme cases between five and ten times more polluted by particles and corrosive gases than the air at the side of the road.
The contaminated ambient air flowed through the ventilation system and entered the passenger compartment without a filtration process and therefore in the worst case could even accumulate and be even harmful to the occupants of the vehicle through increased particle levels, corrosive gas concentrations, unpleasant odours and trigger possible consequences such as loss of concentration, development of fatigue and headaches. The cabin filter provided a solution and soon the field was recognised by MANN+HUMMEL as a new application area.
Start of the particulate filter and extended functions of the combination filter
As a result, the company developed special filter media able to prevent dust, gas and aerosol contaminants from penetrating the vehicle interior. The first step was to separate particles such as pollen, street and industrial dust and wear particles derived from brakes, tyres or the clutch with the aid of cellulose media (Figure 1) and later also synthetic fibres.
Figure 1: Filter element with cellulose medium
In the early days the main priority was the separation of pollen. This could be efficiently separated using filter media developed by MANN+HUMMEL which in turn led to the installation of a new production line for cabin filters in the mid-nineties at MANN+HUMMEL and the go-ahead for the series production of particle cabin filters. In a particle filter the pleated filter medium is made from a high performance synthetic non-woven material (Figure 2).
Figure 2: Filter element with synthetic non-woven material
The separation is made according to the known principles of depth filtration, which are the inertial effect, the barrier effect and the diffusion effect. Particles in the air flow through the filter and are separated on the fine fibres of the filter media. After the millennium, the discussion intensified regarding the increase in fine dust pollution.
In order to meet the requirements for higher separation efficiency and a simultaneously lower pressure drop, synthetic filter media are usually electrostatically charged. This leads to charges on the surface of the fibres which in turn leads to electrostatic forces. This additional effect serves to increase separation efficiency in comparison to uncharged media. This so‑called electret media is able to filter nearly 100% of particles with a size of 1 µm from air.
In addition to the requirements relating to separation efficiency and pressure drop, a further important characteristic which determines performance is the so-called dust holding capacity. Filter media with a gradient design are usually used to enable the highest retention of dust in the filter element, see Figure 3. These are filter media which are characterised by a structure which is oriented to the depth of the media, whereby in the direction of the air flow a relatively open layer with course fibres serves to take on the function of a pre-separator which separates coarse particles. This is followed by a layer with ultra-fine fibres for the separation of finer particles.
Figure 3: REM scan of cross-section of filter medium with gradient structure
Particles only describe a part of the contaminants contained in the air, whereby in the next development stage of cabin filters the function to adsorp gases was added. In those days the subject of the adsorption of gases was a completely new field for MANN+HUMMEL (Figure 4).
Figure 4: Filling activated carbon
After a short time, further functions were added to include the combination of the separation of particles and the simultaneous adsorptive reduction of corrosive gases and odours and the filter we know today as the combination filter went into series production. In this process a small layer of activated carbon is applied. The high porosity of the activated carbon specially developed for the purpose results in a highly specified surface which can adsorb corrosive gases and odours.
In the last ten years the challenges facing cabin filters have further increased with regard to the fine filtration of particulates, the ever tighter installation space and higher performance requirements and cost pressure. In the second part of my blog contribution, I will report on the latest developments. Stay tuned!