Particle emissions from a HD SI gas engine fueled with LPG and CNG

•Gaseous and particles emissions within the Euro VI homologation limit.•Bimodal particle size distribution function over the WHTC.•LPG and CNG soot emissions ascribable to idle phases.•PN 5–23 nm contributes about 20% to the total PM for both fuels.•Larger presence of oxygen functional groups in cold-start WHTC soot. An extensive experimental campaign was addressed to study the emissive behavior of a heavy-duty (HD) spark ignition (SI) gas engine fueled with LPG and CNG, mainly focusing on particle emissions analysis. The engine was tested in transient condition, along the World Harmonized Transient Cycle (WHTC), also analyzing the engine response in correspondence of cold- and hot-start WHTCs. Overall, for both CNG and LPG, regulated emissions are within the Euro VI homologation limits. The average particle size distribution function indicates that particles with diameters below 23 nm contribute to about 20% of total PN for both the fuels. The results reveal that the particles are emitted in the first part of tests, highlighting a correlation of soot and PN emissions with some specific phases of the test cycle, namely in correspondence to the passage from long engine idle periods to speed/load increments, in both cold- and hot-start WHTCs. No significant variation in the distribution of primary particles dimension is discernible in the particles sampled along the different WHTC versions, indicating that cold- and hot-start WHTCs negligible affect the formation of the nuclei cores at the early stage of the particles formation. The thermal reactivity of both LPG and CNG-derived soot is in line with typical engine particulate matter: the soot is completely burnt in the temperatures range between 550 and 680 °C and a moderately higher reactivity in case of soot from cold-start CNG tests was detected. Such difference was related to the larger presence of oxygen functional groups.