Particle nucleation-accumulation mode trade-off: A second diesel dilemma?

Historically, regulators of diesel emissions have formulated their legislation in gravimetric terms (i.e., in g/km or g/kWh). This quantitative approach, while arguably valid for gaseous emissions such as NOx, does not appreciate the complexity of particulate matter. More recently, much research, along with new legislation, views particulate matter in a more qualitative way and focuses on such metric as the total particle count and size distribution. While the trade-off in gravimetric particular matter and NOx is well known as the “diesel dilemma”, this study, for the first time, demonstrates that there is also a trade-off in particulate size modes (accumulation-nucleation). To this end, a wide range of fuel properties and engine operating conditions are tested on a dedicated heavy-duty test rig. The main message that can be drawn from the results is that the optima for both the classical and newly presented accumulation-nucleation trade-off appear to be concurrent in nature. This implies that when the position on the curve of any given data point is known for one trade-off, the associated location on the other can be qualitatively predicted. Given the breadth of the operation conditions and fuel properties tested in this study, it would appear that this concurrence, which could help guide engine calibrators and aftertreatment specialists, is quite robust indeed.