Filtration technology specialist Mann+Hummel announced at the 2009 Frankfurt Motor Show in September that it has developed and patented a new way of manufacturing monolithic particulate filters for diesel and gasoline engine applications. In contrast to existing production methods such as linear extrusion, the new process, called ceramic coated paper process (CCPP), offers additional levels of design freedom.
CCPP is based on a technology transfer from air filtration. The supplier uses an amended type of filter paper as a carrier medium to make the particulate filters. Very much resembling the manufacturing of regular air filters, the paper is firstly embossed to create a rippled filter structure. Then a ceramic adhesive is applied; in this step, the channels of the now corrugated paper are closed at alternate ends to force the exhaust gas flow through the filter channel walls. After that, the two-layer coil is rolled into a homogeneous structure basically looking like any coil made from corrugated paper.
This filter coil is then immersed in a suspension containing aluminum-titanate (ATi) particles. The carrier medium absorbs the particles until the paper fibers are completely surrounded by them. After drying, the filter coil is placed in an oven and the paper gets burnt completely. The particulate filter resulting from this step has two main properties that differentiate it from other manufacturing processes.
First, the paper carrier fibers result in two levels of porosity: micro-porosity between the sintered ATi particles and additional porosity resulting from the negative image of the paper fibers after cauterization. "Together, both levels of porosity make the filter highly efficient," said Andreas Franz, head of the supplier’s diesel particulate filter unit.
Second, embossing offers the option to give the individual filter channels a conical geometry, which helps to deposit particulates more evenly. "As a consequence, the filter can retain and hold more soot particulates than other filters," said Franz. "Wider openings of conical channels increase the inflow at the exhaust-in side. This, in turn, is a way to reduce the backpressure."
"By modifying the channel shape and pore size, filter properties can be adjusted to the needs of the individual application, which can be diesel or gasoline engines," Franz said. "Also, the position of the channel seals can be varied, and an additional catalytic coating can be applied. Using this option, a catalytic converter can be placed upstream or downstream within the same filter monolith."
Though the typical shape of the rolled filter coil will be round to make canning easier, it is also possible to manufacture oval or triangle-shaped geometries. "The geometric boundaries are defined by the wrapping process only," said Franz.
"Currently, we are setting up a pilot line that can manufacture 60,000 units per year to verify a reliable and stable process for mass manufacturing. This line will go into production at the end of 2009," Franz said.