Investigation of Ash Formation, Accumulation, and Distribution in Diesel Particulate Filters Using Lubricant Additive Tracers

The accumulation of lubricant-derived ash in diesel particulate filters (DPF) adversely affects engine efficiency, and is the single most important factor limiting the filter’s useful service life. The location of the ash deposits in the DPF, whether accumulated in a layer along the channel walls or packed in a pug at the end of the channels, plays a crucial role in determining the extent to which the ash impacts filter performance. This work presents results of targeted experiments designed to carefully track the evolution of the ash deposit formation and accumulation processes. Specially-formulated lubricants containing only calcium, zinc, or magnesium additives were used as chemical tracers and applied to load the same DPF in carefully designed time-sequence variations. Subsequent filter post-mortem analysis utilized scanning electron microscopy in conjunction with energy dispersive x-ray analysis to identify the chemical tracers in the ash layer and channel end-plugs. The results provide a quantitative measure of ash build-up along the channel walls, and the subsequent transport and formation of ash plugs at the end of the DPF channels. Studies with these additive tracers also showed large differences in DPF pressure drop as a function of ash chemistry. In general, calcium- and magnesium-based ash resulted in the largest increase in filter pressure drop, while ash containing primarily zinc compounds exhibited little increase in pressure drop for the same ash level in the DPF. Furthermore, despite being formulated to the same 1% total sulfated ash level, differences in ash accumulation rates between each of the lubricants provide additional insight into the magnitude of individual additives’ impact on DPF performance. Although the ash problem presents a significant challenge to lubricant and additive formulators and engine and aftertreatment system manufacturers alike, these results enhance the fundamental understanding of how ash is accumulated and distributed in the DPF. Further, the results are useful to understand the manner in which the accumulated ash affects exhaust flow restriction and filter pressure drop, as well as catalyst performance. Eventually, means of controlling both the location and packing characteristics of the ash deposits may be developed to extend DPF service life and minimize the impact of the accumulated ash on filter performance.