A study by researchers at Eindhoven University of Technology has found that the “persistent diesel dogma” of “the higher the cetane number (CN) the better” relative to the soot-NOx trade-off is valid in neither conventional or low temperature combustion operation. The open-access study, published in the journal Fuel also reported that a second piece of conventional wisdom—“the lower the aromaticity the better”— is valid in both combustion modes.
The researchers also devised a new, dimensionless parameter—Π—that holds distinct values for the various combustion modes. This can predict either a positive, neutral or negative impact of high CN and low aromaticity on the soot-NOx trade-off based on a given set of engine operating conditions.
When the investment decision was made to construct gas-to-liquid plants (GTL) plants, the conventional wisdom held at the time was the higher the cetane number (CN) the better. For most of the compression ignition’s (CI) history, this has proven to be correct. The underlying causality, however, is a bit more complex. As refineries invested heavily in cutting aromatic fractions, soot emissions dropped and engines ran smoother. These phenomena coincided with a rise in CN, a collateral result of removing the intrinsically low reactive aromatics from the cut. Given that this positive correlation between CN and overall engine performance was long accepted as a causality in the combustion community, it is not surprising that later GTL plants—as is frequently emphasized in associated marketing campaigns—were designed to produce as high CN as possible, limited only by a maximum paraffinic chain length owing to cold flow (e.g., wax forming) considerations.—Reijnders et al.
The Eindhoven team reviewed the extensive literature regarding the virtues of high CN, and tallied contradictory results, which they summarized, and from which they drew the following conclusions:
Irrespective of combustion regime and given a constant CN, a higher aromaticity in all cases has a negative impact on aforementioned trade-off.
While beneficial under HTC operating conditions, high CN yields diminishing and even negative emissions returns as combustion temperatures fall. Importantly, this trend appears to hold regardless of compression ratio (CR), engine size or fuel matrix.
For their study, the team conducted engine tests for a fuel matrix spanning a wide range of CN and aromaticity under both LTC, HTC and transitional regime operating conditions on a modified DAF heavy-duty CI engine (12.6-liter, in-line 6). Cylinder 1 was a dedicated test cylinder, the second and third cylinder had no function, and the remaining 3 cylinders were used to rotate the crankshaft to the desired RPM.
The fuels were a standard diesel (CN 51.6), low-cetane Fischer-Tropsch GTL diesel (CN 52.1) and high-cetane FT (CN 71.8).
They drew two main conclusions:
At equal aromaticity, there is no discernible benefit of a high CN with respect to the soot-NOx trade-off in the HTC mode. When operating in the LTC regime, a high CN even results in a penalty in the trade-off.
At equal CN, increased aromatic content always has a negative impact on the soot-NOx trade-off, irrespective of combustion mode.
Their new parameter Π effectively evaluated if aromaticity and CN impact the soot-NOx trade-off in a manner characteristic of either diffusion (HTC→Π>1) or premixed flames (LTC→Π<1).
Jos Reijnders, Michael Boot, Philip de Goey (2016) “Impact of aromaticity and cetane number on the soot-NOx trade-off in conventional and low temperature combustion,” Fuel Volume 186 Pages 24-34 doi: 10.1016/j.fuel.2016.08.009