Transient Performance of Diesel Particulate Filters as Measured by an Engine Exhaust Particle Size Spectrometer

The performance of diesel particulate filters (DPF) has historically been evaluated by gravimetric efficiency, which measures the mass of particulate matter (PM) trapped in the filters. This method does not measure the filtration efficiency at different PM size ranges and, therefore, cannot provide information on fractional performance of DPFs. This fact becomes significant because the adverse effects of diesel PM emissions on human health and the environment are size dependent. A previous study investigated the fractional performance of DPFs under steady state conditions using a scanning mobility particle sizer (SMPS). In the real world, however, nearly all engines operate under transient conditions. DPFs also have to perform under such conditions, so any measurement of performance must be able to react to quickly changing DPF conditions.

This study investigates the PM emission pattern of a diesel engine and the performances of a diesel oxidation catalyst (DOC) and various wall-flow DPFs under transient conditions. The emission levels were measured with and without the aftertreatment devices in place utilizing an engine exhaust particle size spectrometer (EEPS), along with other classifying instruments. The fractional performances of the DOC and DPFs were characterized at certain points throughout the transient cycle. It was observed that the size distribution of engine-out PM emissions could be either mono- or bi-modal with the peak able to switch quickly between the nuclei and accumulation modes during a transient operation. A PM penetration pattern was found to be closely associated with engine-out PM emissions, especially for accumulation mode particles, though efficiency of the DPF was greater than 99% after loading. Therefore, if an engine's PM output cannot be reduced, the best method to further decrease total PM mass emissions is to improve the removal efficiency of accumulation mode particles by aftertreatment devices.