A LUCKY SOLUTION

NEWS FROM MAHLE DEVELOPMENT: INNOVATIVE HORSESHOE-SHAPED AIR FILTER
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 – WHAT IS THAT?
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 socalled 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.

WHAT DOES THIS MEAN FOR THE AIR FILTER DESIGNERS?
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

NOT ROUND, NOT FLAT – BUT HORSESHOE-SHAPED
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.

COMPACT AND RESILIENT
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.

DURABLE
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.

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

HIGH TECH ALSO IN THE PRODUCTION
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.

NOT ONLY CLEAN, BUT PURE
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.

MAINTENANCE FRIENDLY AND RELIABLE FIT
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.