Characteristics of instantaneous particle number (PN) emissions from hybrid electric vehicles under the real-world driving conditions

Abstract

The control strategy of hybrid vehicles is mainly optimized for better fuel efficiency and hybrid electric vehicles were reported to have higher particle emissions than conventional vehicles. To assess the high instantaneous particle number (PN) emission characteristics, four hybrid electric vehicles with different technical parameters were tested under real-world driving conditions. The high instantaneous PN emission was clustered as high emission sets (HES) and HES were evaluated with the vehicle driving conditions and engine operating conditions. The results revealed that 20% of the accumulated particles are emitted within 2% of the driving time and 80% of the accumulated particles are emitted within 20% of the driving time. Most of the HES are observed at the speed lower than 60 km/h (urban speed limit), with relatively higher engine speed. HES have higher mean positive acceleration values than their RDE counterparts. But the largest acceleration doesn’t represent high particle emission, because, with the electric motor, acceleration is decoupled with the engine variation. The HES could be observed at rich/lean fuel conditions, high/low state of charge, and before/after the engine is fully warmed-up depending on the vehicle’s technical characteristics. Regeneration could increase the PN emission by a factor of five. Fuel-efficiency centered strategy should be combined with the pollutant-control centered strategy to achieve cleaner hybrid technology. These results could also be useful in future policies and PN emission models.