Particle number (PN) emissions from gasoline, diesel, LPG, CNG and hybrid-electric light-duty vehicles under real-world driving conditions

The current experimental study presents solid (non-volatile) particle number emissions with a size cut-off at 23 nm (SPN23, referred to as PN hereinafter) under real-world driving conditions for 8 passenger cars powered by diesel, gasoline, Liquefied Petroleum Gas (LPG), Compressed Natural Gas (CNG) and hybrid-electric powertrains. The objective was to identify the highest emitters and assess whether PN focus should be extended to engine and fuel types other than diesel and gasoline direct injection (GDI). Results (under the cold-start RDE-compliant route) show that a Euro 6b gasoline vehicle equipped with port fuel injection (PFI) engine, comprised the highest PN emitter (4.5 times higher than the Euro 6 limit of 6 x 10(11) p/km), while similar emission levels (up to 3.7 times higher than this limit) were detected in the older-technology (Euro 3 and 4) gasoline and (retrofitted) LPG PFI vehicles. This limit was also exceeded (by 1.7 times) by one gasoline direct injection (GDI) Euro 6b non-GPF vehicle. PN emissions from the hybrid-electric vehicle (equipped with gasoline PFI engine) were lower than the other Euro 6b PFI vehicle, but still high compared to typical PFI engines found in literature. The lowest emissions were detected in diesel and CNG engines (as low as 7.8 x 10(9) and 2.4 x 10(10) p/km respectively). Euro 6b LPG vehicle was found to be a low emitter (3.3 times lower than the 6 x 10(11) p/km limit), while similar emission levels were detected in the other Euro 6b non-GPF GDI vehicle. Among the different driving sections (urban, rural, motorway), urban and especially the cold phase had the highest contribution in most cases. Finally, a significant PN reduction potential was observed with LPG and CNG over gasoline (up to 93% and 90% respectively), although the opposite trend was detected in an old-technology retrofitted LPG vehicle. These results indicate that close attention should be focused on other engine and fuel types, apart from diesel and GDI. Regulatory initiatives towards air pollution reduction should include measures for the reduction of high-emission old-technology gasoline and LPG vehicles, while modem gasoline PFI engines should also be further scrutinized.