Particulate emissions from aromatic containing fuels
Paper in proceedings, 2016
PM emissions are increasingly important, especially for marine applications. As the legislations for fuel sulphur content is introduced in the SECA region, the focus was turned into the effects of aromatics. Aromatic fuel is known to increase PM emissions, but preliminary tests of drop-in aromatics in a marine diesel engine resulted in lower PM emissions. The objective in this presentation is to further investigate the interplay between fuel properties and the engine operations in order to try to explain the lowered PM emissions.
By using ignition improver, the RoHR analysis showed very similar combustion (cylinder global phenomena) for the different fuels containing varying amounts of aromatics.
o This indicates that differences are due to local phenomena in the flame front, i.e. the difference is in the soot formation (rather than differences in soot oxidation)
o The PM emissions were lower for aromatic fuels in contrast to “common belief”. The reasons for better pre-mixing for aromatic fuels can be different and future investigation will try to pin-point the most plausible explanation, (see Figure 5). Possible explanations include:
Lower molecular weight of aromatics (compared to diesel fuel) will make use of less oxygen in the air, hence lower equivalence ratio and less soot formation
Physical properties such as density, viscosity and surface tension may also influence the spray break-up and contribute to a lower equivalence ratio. Furthermore, the influence of cavitation inside the nozzle may vary depending on fuel properties.
• The trend of lower PM emissions for aromatic fuel could be “broken” by a different injection strategy (here: rail pressure):
o When Pinj is sufficiently high, the effect of efficient pre-mixing (and hence shorter residence time at soot formation conditions) is dominating over increased soot formation due to presence of aromatic compounds (pre-cursors in soot formation).
o When Pinj is lower (toward more traditional injection pressures found in older diesel engines), the increased soot formation due to aromatics dominates over increased pre-mixing.
• By changing the operating conditions to exclude EGR, the comparison was improved and furthermore, the existence of nucleation mode particles could further help understand the fate of the soot formation