The measurement of particle number emissions from diesel engines - a fuel perspective

A series of studies has been performed by Shell on diesel engine particle number emissions using a scanning mobility particle sizer (SMPS). The parameters investigated include fuel type, engine type, operating condition and sampling system. Light-duty (LD) vehicle studies have shown the particle number distribution to be very vehicle dependent: i.e. some vehicles show bi-modal particle size distributions at Idle and 15 km/h. The effect of fuels and operating condition on the particle number distribution differs between the vehicles investigated. The application of a particulate trap to a LD vehicle significantly reduced the particulate mass and particle number emissions. Initial heavy-duty (HD) studies, sampling from conventional particulate mass measurement systems, have shown only small differences in the size distribution of particle emissions which are attributable to fuel effects (1, 2 and 3). The one exception is RME (rape-seed methyl ester), which produces more smaller particles than conventional diesel fuel. More recent work has compared this conventional sampling system with a new high-dilution system; using the latter system, the particle size distribution was found to be insensitive to fuel sulphur levels, even in the nucleation mode, for the selected sampling conditions. This is in contrast to other studies which have indicated a sulphur effect on the nucleation mode particle numbers. The recent study also examined the stabilisation time of nucleation mode particle distributions at steady-state conditions after a step speed/load change. Although progress is being made in the understanding of many of the issues relating to reducing particle number emissions, it is hampered by the lack of a clear consensus of the precise particle size range which is critical to the health effects and the most appropriate sampling regime to be used. In the meantime, researchers must consider the full measurement range of particles, the steps required to achieve repeatable and reproducible measurements, and try to make a balanced judgement on the effects seen.