New Techniques for Damage Assessment of Diesel Particulate Filters
The maximum soot load capacity for ceramic Diesel Particulate Filters (DPFs) is sometimes limited by a thermal crack failure mechanism associated with high temperature gradients which can occur during regeneration of highly loaded parts—particularly at low exhaust flow rates. The filter material and construction can be optimised for resistance to thermal cracking, however, the precise conditions which give rise to thermal failure of DPFs can be difficult to establish accurately and repeatably. For instance, thermal failure of DPFs may occur at the onset of the heating due to the exotherm of trapped soot, or during cooling (for instance at the fuel cut during deceleration or start of idle). The time of occurrence of thermal failure can help to establish the worst conditions for filters. Sectioning parts post-test is often conducted to establish the nature and location of any damage. However non-destructive testing allows for the possibility of progressive testing of single parts—allowing determination of the ‘Soot Mass Limit’. Post-test scanning techniques have been demonstrated (e.g. X-Ray/CT scanning). These allow non-destructive testing, but are generally expensive, and require the DPF to be removed from the can. This paper describes important considerations for application of two existing post-test evaluations as follows. (1) Radial and axial ultrasound ‘Time-of-flight’ measurement. (2) Internal imaging of the DPF with a small borescope. Also presented are two novel non-destructive techniques for assessing damage to DPFs as follows. (1) An in situ technique capable of measuring filter vibration events during DPF operation which may be associated with thermal crack damage. A surface microphone coupled directly to the filter substrate through a hole in the can and intumescent matting measures brick vibration, while a background detector measures exhaust pipe and canning vibration events in order to discriminate metallic thermal expansion. Vibration and internal thermocouple data is presented from exothermic regenerations for several different filters loaded with soot on a commercial Diesel Particulate Generator with standard Diesel fuel and fuel treated with a catalytic additive. The extension of the technique to testing on a vehicle is demonstrated. (2) A relatively simple, post-test evaluation which involves reverse aspiration of DPF test parts with a cold Diesel soot aerosol generated with compressed air. The technique can locate DPF cells where the soot aerosol is not filtered though the substrate between the inlet and outlet channels. The deposition of soot on the substrate is shown to be an indicator of internal damage and, together with simple optical microscopy, can help to identify failure mechanisms. The paper presents examples of the above techniques to examine thermal damage to Silicon Carbide and Aluminium Titanate DPFs which have been subject to ‘worst case’ regenerations.
KeywordsDPF Crack SML DPG Diesel
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