This paper will review the field of diesel emission control with the intent of highlighting representative studies that illustrate the state-of-the-art. First, the author reviews general technology approaches for heavy and light duty applications. Given the emerging significance of ultrafines to health, and to emission control technologies, an overview of the significant developments in ultrafine particulate science is provided, followed by an assessment of filter technology. Regarding NOx control, SCR (selective catalytic reduction) and LNT (lean NOx traps) progress is described. Finally, system integration examples are provided.
In general, progress is impressive and studies demonstrate that high-efficiency systems are within reach in all highway vehicle sectors. Engines are making impressive gains, and will increase the options for emission control. Regarding ultrafines, studies are emerging providing chemical detail as a function of size fraction, and more is known about the structure and reactivity of soot. Filter technology is focusing on optimization, with work being done on better ways to regenerate the filter and improve system back pressure and durability. Mixed mode engine operation provides significant flexibility in this regard, and better control of hydrocarbon species going into the system is a major advancement. SCR NOx control is focusing on low-temperature performance and system control. LNTs are continuing the rapid evolution of past years, with much better sulfur management and new configurations. System integration of filters and NOx control is in road testing for heavy duty applications using SCR. DPF+LNTs help prototype light-duty diesels comfortably attain US Tier 2 Bin 5 standards, but without aging.