Balancing the GHG emissions and operational costs for a mixed fleet of electric buses and diesel buses

Making the urban buses electric is regarded as a major strategy to reduce greenhouse gas (GHG) emissions and environmental impacts of fossil fuels. In practice, not all diesel buses (DBs) are replaced by electric buses (EBs) because of budget constraint. This paper investigates the balance of the deployment problem for a mixed fleet with DBs and EBs in the sense of total GHG emissions and operational costs by incorporating the effect of the spatial-temporal passenger flows. The balance strategy of fleet deployment is defined the Pareto optimal allocation of EBs among bus lines to minimize simultaneously the total operational cost and GHG emissions. A real-world urban bus system of Liuzhou City in China is conducted. We find that the bus lines located in the downtown with higher passenger loading would prefer to adopt EBs at the peak hours, and most DBs are allocated to the bus lines with long travel distance at off-peak hours in the suburb. Therefore, the reduced emission by adopting EBs mainly concentrates on the center of the city, and more produced emissions of DBs are distributed far away from the downtown. When all DBs replaced by EBs, the upper bound of the carbon emission reduction ratio is 77.04%, which reduces from 207.15 tons to 47.56 tons per day.