MAHLE air filters—we think dirty air requires innovative solutions

Clean air intake is one of the most important conditions for optimal engine power, high torque, low fuel consumption and minimized pollutant emissions. With MAHLE air filters, up to 99.9 percent of dust, soot and tire wear particles are filtered out. At the same time, an optimal air/fuel mixture is assured. The high particle uptake capacity ensures long service life under extreme conditions such as heat, cold or chemical contaminants. Our quality filters prevent premature wear of valves, cylinder working surfaces, piston rings, bearings and other engine components. To ensure optimum filter performance, all filters should be replaced within the service intervals stipulated by the car manufacturers.

Unfiltered air in the intake passage can contaminate the air flow mass sensor and distort the measuring results, cause malfunctions of the fuel injection and wear of the engine components. To prevent bypass air, our filters are form-fitted and the seals have been carefully selected.

PUR seals made from specially developed polyurethane (PU) foam ensure continuous sealing between dirty and clean sides. These seals are resistant to aging, chemical effects and are temperature stable. Their flexibility is designed to exactly fit the geometry of the sealing area.

In the passenger car sector, round and panel elements are fitted in air filter housings mounted to the engine or chassis and are stabilized with glue beads at the dirty side and support grids at the clean side, depending on the specifications of the car manufacturer. For dust rich areas, an additional pre-cleaner in form of a foam mat is placed at the dirty side. For higher surface loads, metal or plastic support studs are used.

In commercial vehicles, robust and at the same time low-weight air filter systems made from recyclable plastic are used that can also reduce air intake noises. In order to achieve a highly stable large filter surface, the filters are mostly cylindrical. To improve efficiency, radial sealing and axial supports are used. Additional safety inserts in the form of special non-woven cylinders protect the clean side of the filter element during maintenance and replacement.

The first two decades of the automobile were marked by permanent engine failures—in particular, due to the high dust concentration on the unpaved roads in those days. The dirt particles reached the combustion chambers, where they caused severe abrasions of the piston rings, pistons and cylinder walls. This resulted in reduced engine power or even the dreaded piston seizures. Only when the so-called air cleaners were invented in the mid-30s, repair and service intervals of up to 4,000 km could be reached. The success story of the internal combustion engine is therefore hard to imagine without air filters. However, it has been a long journey from the first oil bath air filter of the 30s to the modern air intake modules of today.

The beginning: a mesh of wires
In the first air filters, the filter element was a wire mesh enclosed by a metal housing. The working principle of the so-called “oil bath air filter“ was based on physical flows. The deflection of the air stream in the metal mesh produced a sieving effect that was utilized to separate the dirt. In order to collect the dirt the steel mesh was covered with engine oil. These filter elements had to be dismantled regularly, cleaned with cleaner's solvent and finally covered in oil again.

As engines became more and more efficient and fuel consumption decreased at the same time—the air requirement increased many times over. Filter elements made from wire mesh had now reached their limits, even when they were later combined with textile inserts.

In addition, filtration had to become finer and finer. There was also the demand to save weight and reduce size—and to cut back on servicing requirements in several respects. An oil filter needing extensive cleaning and problematic disposal did not meet the demands any longer: a new type of air filter system had to be developed.

An important innovation: Filters made of paper
When filter elements made of paper came on the market in 1953, they soon superseded the earlier metal mesh elements thanks to their far superior filter performance.

Only a short time later, in 1957, the Knecht Filterwerke (today: MAHLE Filtersysteme) developed a special kind of folding system for the filter paper and took out a patent under the trademark “MICRO-STAR“ for this system. This pleat system is still the standard in filter technology to this day.

The design of air filters has been adapted to meet the changing demands all the time. Initially, circular filter elements were used: a circular filter element is made from filter paper that is fixed with PUR foam. In order to separate dirt side and clean side at the pleat ends, these were sealed by a PUR end plate with sealing rib. This system of circular air filters is still used to this day.

The present time: Variable-length intake manifolds and complex intake systems
Already in the 90s, MAHLE developed so called variable-length intake manifolds in close cooperation with the automotive industry. On the one hand, this allows the engine to generate high torques even at low engine speeds and high peak power at high engine speeds on the other. A tumble flow that is generated deliberately in the combustion chamber (see text to the right) improves mixture formation and the combustion process. This results in reduced fuel consumption and exhaust emission.

Apart from air filtration, the complete intake systems used in modern engines with fuel injection systems have a number of additional tasks. Today's air filter housings often contain an airflow mass meter, a blow-by gas recirculation intake (see text to the right) as well as a service indicator and a heat shield that protects from direct thermal stress. Accordingly high are the demands on MAHLE as development partner and system supplier of the international automotive industry.

A glimpse ahead: The three-stage resonance system
Another milestone in air intake module technology is the threestage resonance system developed by MAHLE, which can be controlled via two flaps integrated into the intake manifold. Resonance charging is effective over a wide speed range in three engine speed stages up to 7,000 rpm. The resonance system, which has been optimized and virtually verified with the aid of charge calculations, has already been used successfully in practice. MAHLE offers a range of products and developments that goes also here far beyond air filtration and includes control units for the resonance system and a crankcase ventilation system with oil mist separator, pressure control and feed line into the air intake module.

Reduced installation space, increased demands on pedestrian protection—this are only two examples from the extensive specification book of the newly developed Mercedes-Benz engine OM 651. The four-cylinder diesel, which is equipped with 2 turbo chargers in the more powerful versions, is currently used in the C-Class (W204, models 200 CDI, 220 CDI and 250 CDI), the E-Class (W212, models 220 CDI and 250 CDI) as well as in the GLK (125 and 150 kW).

The challenge for the MAHLE engineers was the development of an air filter that would fit into an engine compartment space severely limited due to a multitude of other equipment and that would also comply with stipulations regarding pedestrian protection.

Pedestrian protection requirements are regulations contained in the manufacturer independent crash test programme Euro-NCAP (European New Car Assessment Programme). This association of European ministries of transport, automobile associations and insurance companies has developed the so-called NCAP crash test, which consists of three parts: the protection of adult passengers, the protection of children sitting in the rear seats of the car—and pedestrian protection.

Since 2005, new vehicles in Europe have to comply with certain standards for the protection of pedestrians in the event of impact accidents. This is achieved on part of the manufacturers with a variety of measures—for instance, with a softer design of the vehicle front. The degree of pedestrian protection is evaluated by NCAP and classified according to a 0 to 4 star rating.

For the new development of the OM 651, a minimum distance between the bonnet and the highest point of the air filter housing was specified to comply with the demanded pedestrian protection. The design required that the engine management system was placed on the air filter housing, which presented an additional challenge—the filter housing needed therefore not only to be very flat but also designed to carry the weight of the engine management system. Another important matter was the acoustic behaviour of the air filter.

This required the air filter designers to deal with the following issues:

  • Overall height as low as possible
  • Accommodation of the filter surface that is required for reaching the demanded values of dirt pick-up capacity.
  • Clean air outlet and intake of unclean air in parallel at the same side according to customer specifications (not as usual at opposite sides).
  • High inherent stability and high static load bearing capability.
  • Maximum robustness and stiffness in order to withstand the high thermal and mechanical operational loads.
  • Reduction in noise emissions in order to reach the customer demands in terms of acoustic and noise emission values.
  • Long service life

It was not possible to comply with the given specifications by applying the usual round filter or plate element designs. The solution lay therefore in a completely new design principle: the horseshoe shape, which allowed the implementation of all demanded characteristics into the design.

With clever air guiding geometry under maximum utilisation of the available installation space and appropriate ribbing at the housing, a high inherent stability and resilience was achieved despite the low overall height. A horseshoe-shaped pleat bellow geometry was used to provide a large filter surface within minimum space.

The design played special attention to an even load over the filter element during operation in order to achieve a long service life. This homogeneity was achieved thanks to specially designed air guiding in the filter housing. The MAHLE development team created theoretical mathematical models to verify the design in the early development stage of the filter.

To reduce noise emissions, a cross brace—a so-called tie rod—was placed centrally in the air filter.

The simulation of the flow pattern shows what routes the air takes trough the filter. To achieve maximum capacity, an even air intake over the surface into the filter medium has to be assured.

Novel design concepts require often also innovations in production methods. The new horseshoe-shaped air filter presented several challenges to the MAHLE filter experts and their production know-how.

For example, the paper element is plasticized. This means that the base plastic plate is heated until it becomes paste-like, allowing the paper element to be pressed into the plastic plate and produce a mechanically firm and air tight bond after solidifying. The actual plastic material must withstand the pressure and is not permitted to deform any further. The injection moulded parts must be extremely even to allow the paper elements to be pressed reliably into the upper and lower cover plate of the polypropylene filter element.

New production processes had to be developed for the adhesive bonding of the paper element with the locating plate for the air flow mass meter. These have been filed for patent in the meantime and are now exclusively used by MAHLE.

It is necessary to hold very close tolerances in order to keep the connection between the air flow mass meter and the seating hole tight for the entire service life. The flow characteristics of the plastic during injection moulding and the subsequent crystallisation have been computed extensively during the development phase and are the basis for making the series tools that now ensure safe and stable production.

Even smallest dust particles that get into the filter module during manufacturing and are then sucked in during operation, can cause engine malfunction. High demands on the residue-free cleanness of the filter module are therefore made in the specifications: The filter must meet high-purity demands to protect the fuel injection system with its extremely close tolerances and minimal nozzle openings as well as the turbo charger. In order to assure this, every filter is produced under cleanroom conditions and cleaned using a specially developed process with extraction systems.

When the filter is exchanged, it needs to be pulled off from the air flow mass meter and the new element has to be slip over again. To facilitate fitting and removing, the O-ring is provided with a special slide layer.

The CAE (Computer Aided Engineering) simulation of the pressure distribution Delta P illustrates the pressure gradient from inlet (red) to the outlet of the cleaned air (blue). It is important to provide even pressure distribution at the outside (before entering the filter medium) in order to utilise the maximum capacity of the filter medium.