As the major vehicle markets transitioned to Euro VI-equivalent, soot-free vehicle emission standards, most progressively limited the sulfur content in diesel fuel to 10 parts per million (ppm) or less, also known as ultralow-sulfur diesel. But this fuel transition is not automatic, and requires both policy and planning. Indeed, Euro VI vehicles are sometimes fueled with higher sulfur content fuels during the transition, or because of regional fuel quality differences. This paper examines what this could mean for real-world emissions performance.
Through a review of recent literature, the authors find that using fuels containing greater than 10 ppm sulfur can be problematic. The main risk of using 50 ppm sulfur fuel comes if there is long-term exposure; this creates a cascading effect that begins with impaired operation of the diesel oxidation catalyst (DOC) and efficiency degradation of catalyst sites in diesel particulate filters (DPFs) and zeolite-based selective catalytic reduction (SCR). DPF regeneration is also affected by higher sulfur and it could lead to performance loss in passive DPF systems. A vehicle that has an aftertreatment system that is not designed for 50 ppm operation—i.e., a post-Euro IV/4 vehicle—would be at high risk of DPF and SCR failure due to thermal degradation. As such, nations adopting soot-free vehicle emission standards that wish to achieve real-world emissions reductions would do well to set a limit of 10 ppm sulfur for diesel fuel imports and plan to move to 10 ppm domestic production in the near future.