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

Particle size distributions are measured at the exhaust of a passenger-car diesel engine burning a Sulphur-free diesel oil. Different operating conditions of loads and engine speed, representative of low-loads are analyzed. Particles with sizes ranging from few nanometers up 1 mu m are generated during diesel combustion. Operating conditions strongly affect the size distribution of the particles but overall they maintain a bimodality with a first mode, identified as nucleation mode, in the form of sub-10 nm particles, and a second mode in the form of soot particles and agglomerates. The lower is the engine load, the lower the emission of mass concentration of particulate matter but the higher the emission of particle numbers. Measurements performed in not-firing conditions confirmed that particles are generated during combustion more than by lube oil or mechanical friction. 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.