Filtration and coagulation efficiency of sub-10 nm combustion-generated particles

Filter efficiency with regard to the different particle sizes is evaluated by measuring particle size distributions before and after a diesel particulate filter. Results show that sub-10 nm particles are not sufficiently removed by the filter. Filter capture is almost complete for particles with sizes greater than 10 nm but the collection efficiency decreases to values of the order of 40-50% for sub-10 nm particles. This is of concern particularly when using fuels or operating conditions that produce a huge number of sub-10 nm particles not removed by the filter and hence emitted from the engine. The objective of the study is not to measure the filtration efficiency of a specific after-treatment system but to give a general warning on the capability of current particulate filters in removing sub-10 nm particles.