Ongoing concerns about the adverse health effects of carbon particulates in diesel engine exhausts continue to drive the quest for improved performance from Diesel Particulate Filter (DPF) systems. Two of the main areas in which improvements are being sought are in enhanced filtration efficiency of very fine particles (<50 nm), particularly immediately after regeneration (the periodic removal of accumulated particulate via combustion), and in improved thermal shock resistance. To contribute to these aims, this work focuses on creating novel composite materials through the introduction of fine ceramic fibres.

Filtration efficiency is considered by experimental measurement of porosity and permeability. Porosity of the material is shown to increase with fibre content, providing a larger volume for soot collection. The permeability is shown to slightly decrease due to the fine nature of the fibres. However, these small fibre diameters, in comparison to the larger particles, offer an increased surface area and scope for an improved soot trapping capability.

A simulation, modelling the particles and fibres as simplified spheres and cylinders, allows an understanding into the effect of porosity, fibre proportion, and the sizes of the particles and fibres on the permeability. An empirical equation linking these factors is suggested. A model of this type, on a hybrid particle-fibre-void composite material, has not been studied in the literature, and so offers a novel insight into the complex behaviour of such materials, practicable not only to DPFs, but to any other applications of such composites systems. The model is shown to have good agreement with the experimentally derived data.

Finally, the effect of fibres on the thermal shock resistance is investigated. The fibres show improved fracture toughness, and variability in the stiffness which suggests, via a derived figure of merit, that thermal shock resistance improves with increased fibre proportion. Thermal shock is induced experimentally using a vacuum plasma sprayer, and the experimental results, measured as a decrease in stiffness as a consequence of micro-cracking, are comparable to those of the merit index.

The study concludes that there appears to be a significant benefit to adding fine ceramic fibres into current DPF materials. The increased porosity, whilst also significantly increasing the specific surface area, suggests the filter could offer a higher soot loading capacity, thus lengthening the time between regenerations. The inclusion of small diameter fibres predicts that the period of initially low filtration efficiency of a clean filter could be shortened, with little impact on the permeability. Fibres also offer significant advantages for the thermal shock resistance. Fibres are well-known for their toughening ability and this remains true for these hybrid particle and fibre composites.