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Ceramic fibers with high specific surface area and adequate high-temperature strength are commercially available for filtration of diesel particulates and in-situ hot regeneration. The manufacturing of a deep bed filtration medium, using such brittle fibers, became possible after a special knitting technique was developed which forms the loops with minimum friction and pretension. Within this structure, the fibers are very little constrained and expose their active surface almost completely. Hence, high filtration efficiencies in the range of 95% could be demonstrated with favorable back-pressure characteristics. Blow-off phenomena were never observed. Endurance testing on engines, with full-flow burner regeneration, proved the high robustness to mechanical and thermo-mechanical loading. This is one of the particular advantages of the new concept.

Ceramic fibers, in a knitted structure, offer an elastic deep-filter medium having a very high specific surface. The robustness of this trap, and its invulnerability to thermo-shock, was demonstrated during a further year of development and tests. By using new manufacturing techniques, the filtration efficiency was further improved, pressure losses reduced, and the required volume diminished. New insight was obtained regarding the employment of the fiber medium for catalysis. The filter concept permits regeneration either electrically or by fuel-additives. The layout versatility facilitates deployment on vehicular and stationary engines, in the pre-turbo position, too.

The development of particulate-traps for big engines is more difficult than for automobile applications. The usual placement, after the turbocharger, necessitates complex solutions to challenges in size, flow distribution and regeneration. The placement of the particulate trap ahead of the turbocharger has technical and financial advantages, and has previously been extensively investigated, but did not prevail because of poor reliability of the monolithic traps. This paper investigates the knitted fiber trap, a mechanically and thermically dependable unit, developed for integration into the engine. A modular design makes the trap very compact. Filtration rate and pressure loss are satisfactory. The filter element has not shown any weakness. A typical deficiency of this application, that needs further investigation, is worsening of the engine's transient response by the thermal inertia of the filter material.

Knitted fiber particulate traps facilitate deep-bed structures. These have excellent filtration properties, particularly for ultra-fine particulates. They are also suitable as substrate for catalytic processes. The two characteristics are: high total surface area of the filaments, and good mass transfer. These are prerequisites for intense catalytic activity. The deposited soot is uniformly distributed. Therefore, temperature peaks are avoided during regeneration. The tested coatings lower the regeneration temperature by about 200°C to burn-off temperatures below 350°C. Further improvements seem attainable. Thus, a purely passive regeneration appears feasible for most applications. The system is autonomous and cost effective. However, in extreme low load situations, e.g. city bus services, the necessary exhaust temperatures are not attained. Hence, burners or electrical heating is necessary for trap regeneration.

Microfibers with high specific micro-surface can be knitted into two-dimensional structures with large internal porosity. Catalytically active metals can be deposited on the fibers with high dispersion by wet-impregnation, sol-gel or CVD, respectively. These microfiber knits may be used for exhaust gas treatment systems with a triple function: particle filtration, gas conversion and muffling. The total oxidation of propane on Pd and Pt coated fibers has been studied as a test reaction. Conversion temperature could be remarkably reduced compared to cellular structures. For a bimetallic (Pt-Pd) coating, the activity is independent of humidity or oxygen concentration. Thus a catalytic converter based on micro-fiber knits appears feasible. Its high mass and heat transfer prevent hot spots. And it functions as submicron filter for combustion aerosols. Integrated electric heating can also be provided in case of low gas temperatures. First tests on engines show promising results.