Originally founded as a silencer and spark arrester company in the 1920s, Huss Group introduced one of the first diesel particulate filter (DPF) systems with electrical regeneration in 1985. In 2005, the company began work on a project with Fraunhofer IKTS in Dresden, Germany, that resulted in a patented DPF made of a liquid-phased sintered silicon carbide (LPS-SiC).
SiC is traditionally known as an abrasive grinding material, though recently its use in other applications, such as DPFs, has expanded due in part to its heat- and chemical-resistance characteristics. Another advantage of SiC is that, compared to cordierite, it can take a soot load of 8-10 g/L without cracking during regeneration.
According to Huss, "the secret" to its DPF is in the material, the product itself, and its ceramic manufacturing process. The sintering process is one of the most critical and costly parts of the production process because, ultimately, the stability of the filter is dependent on the way the SiC grains are connected.
The liquid-phase sintering process in combination with a DPF application is quite new. The biggest advantage is that it takes place at a much lower temperature than R-SiC (R stands for re-crystallized) sintering. Despite this, the LPS-SiC filter shows the same characteristics as competitive R-SiC filters, including pore sizes of 10 micron and a narrow pore size distribution.
Huss chose a 200 cpsi (cells per square inch) cell density, which it says is the optimum balance between stability and filtration volume. The wall thickness of 0.369 mm (0.015 in) is enough to provide good mechanical stability. Additionally, the high filtration volume offers improved backpressure performance. Huss chose wall-flow filtration to achieve the 99.9% soot collection of all its other products.
Instead of using the traditional squared channels, Huss designed triangular channels, which increases the filtration area by 14%. The single filter elements have a trapezoid outer shape, allowing for the configuring of different outer shapes when they are later glued together. Nearly round or nearly oval shapes can be achieved without facing the time-consuming and costly production step of cutting the filter to the desired shape and fettling the filter later.
“This is not done because we are not able to extrude a filter that consists of one element," said Alan Pittel, Huss' OEM Sales Manager for the U.S. "We do this purposefully. Due to high and irregular soot loads, the filter often heats up to 1000°C during regeneration. Soot burns at 550°C, but the regeneration process is exothermal and therefore creates additional heat. The seams between the filter elements play an important role in equalizing the different heat gradients and prevent the filter from being cracked.”
Customers in heavy-duty on- and off-highway applications often require large filters for their engines. Currently, the biggest filter from Huss is the 350 series with a 12 in (305 mm) length and 25 L filtration volume. Competitive 12- x 12-in (305- x 305-mm) filters have a filtration volume of around 19 L, 25% less than the Huss filter.
Huss' production process is semi-automated, and most of the production equipment was developed in-house. Process engineering is therefore one of the leading elements in the production process design, which has led to good results. The filter has already been verified by VERT (Swiss standard for particulate filters), TÜV (the technical control board in Germany), and KBA (the federal motor transport authority in Germany).
Huss uses CAD techniques including CFD and FEM analysis as well as in-house engine test benches in its R&D department. Those technologies allow it to serve as the link between the engine and the vehicle manufacturer, which also leads to good results: the company has grown 45% annually since 2003.
Last year, Huss subsidiary CleanDieselCeramics opened a new factory dedicated to the production of LPS-SiC filters, which won the 2009 Fraunhofer Award this past June. The facility has the capability to produce 10,000 filters per year for both on- and off-highway applications.
Thomas Rahn, CEO, Huss Group, wrote this article for SAE Off-Highway Engineering.