To ensure reliable starting under cold weather conditions (< 0 oC ambient), gasoline engines use fuel enrichment, leading to higher soot formation and greater tailpipe particle number (PN) emissions. In gasoline direct injection (GDI) engines, PN emissions are higher due to liquid fuel impingement on cold surfaces of the combustion chamber and piston. This study characterizes solid (mostly elemental carbon) and semi-volatile (organic) particle number, mass, and size distributions during cold-cold engine start-up from light duty vehicles. Particle emissions were sampled from vehicles upon engine start-up after an overnight soak, with an average ambient temperature of -8 ± 7 oC. The average PN emitted during 180 seconds by GDI and PFI vehicles were 3.09E+13 and 2.12E+13 particles respectively. Comparing to 2017 Euro 6 New European Driving Cycle (NEDC) limit on cumulative particles emitted over the entire test cycle, most PFI and GDI vehicles exceeded this limit in 6-12 seconds after a cold-cold start. In addition, EPA Tier 3 particle mass requirements are exceeded for tested GDI vehicles due to their characteristically high concentration of accumulation mode particles. In comparison, diesel vehicles with DPF’s were the cleanest, with particle concentrations close to background levels. Cold-cold starts were followed by a five-minute soak and a second engine start. GDI vehicles’ total emitted PN was almost two to three times that of PFI vehicles during restart. This study highlights the need for better particle control strategies to reduce particle emissions during engine start-up in cold ambient temperatures. Our work also confirms that under cold-cold start conditions, most of the total exhaust particles are solid soot and only a small fraction are semi-volatile.
