Multiscale characterization of exhaust and crankcase soot extracted from heavy-duty diesel engine and implications for DPF ash

Abstract

Engine-dynamometer bench tests were performed to evaluate the severity of engine operation resulting in oil degradation, set limits for drain intervals and provide recommendations for oil formulators to develop an additive package offering enhanced engine hardware protection. Engine oil was sampled at specified intervals to monitor depletion of additives, changes in viscosity, soot agglomerates and wear elements. A myriad of characterization techniques such as temperature resolved XRD, TEM, EDS, BET, Raman, DLS, TGA and ICP-OES are employed to develop a holistic understanding of crankcase soot interaction with lubricant additives influencing soot-induced wear, and oxidative properties of exhaust soot trapped in the DPF. XRD phase analysis of residue left behind after crankcase soot oxidation showed the presence of crystalline compounds embedded in the soot structure (CaSO4, Ca3(PO4)2, and Zn3(PO4)2); exhaust soot showed no crystalline compounds and had 85 wt% less residue after oxidation compared to crankcase soot. The residue inevitably forms ash precursors that are trapped in the DPF (Diesel Particulate Filter) affecting the filtration performance and fuel efficiency of the vehicle in the longer run. The turbostratic structure of both soot samples remains the same prior to oxidation; however, the embedded crystalline and amorphous species in the crankcase soot structure are different compared to exhaust soot. Ex-situ experiments were performed on soot-ash mixture with heated microscopy stage to allow for better visualization of ash formation, sintering and transport properties in the DPF channels. Findings in the study help in developing a comprehensive insight into ‘soot ecosystem’ in diesel engine vehicles.