Exhaust emissions and energy conversion of hybrid and conventional CNG buses

The real-driving emission (RDE) test was employed involving a conventional compressed natural gas (CNG) bus and its hybrid counterpart powered by the same engine type along the same route. By calculating the energy balance of engine and vehicle, hybrid bus saves energy in three ways: engine efficiency increase (30 %), regenerative braking (21 %) and without idling (7 %). But the higher curb weight offset 32 % of the benefits, resulting in 24 % improvement in comprehensive efficiency. The NOx and particulate number (PN) emissions of the hybrid bus could be reduced by 46 % and 39 %, respectively. The emission causes of all pollutants were further classified into: cold-start, restart and hot running. Restart accounts for 52 % of CO emission and hot running for 55 % of NOx emission. Frequent engine restart and short engine running durations (<50 s) impair efficiency and emissions. The findings can help policymakers assess the environmental impact of transport electrification.