**1. Introduction**

In recent years, the demand for improved transportation systems has been on the rise owing to the enhancements in the quality of individual living. Public transportation plays an increasingly significant role in urban centers worldwide in the terms of its economic, environmental and sustainable capacity [1]. As part of the expansion of urban public transport networks, bus transit systems make up a growing proportion of urban passenger transport. There were 608,600 public buses in China by 2017. At this time, the passenger volume for the bus transit system was approximately 74.5 billion annually, accounting for 58% of the total urban passenger transport volume, which was far more than the rail transport volume [2].

However, concerns regarding air pollution triggered by the emission of traffic exhausts are unavoidable. Consequently, the appeal for the amelioration of air quality to guarantee a certain standard of living led considerable attention to the electrification of urban transportation systems [3]. It is noteworthy that the use of electric buses is not a new concept. The use of battery-powered electric buses can be traced back to the early 2000s, which matured gradually during the mid-2000s, followed by dramatic developments during the past decade [3].

Energy efficient vehicles are being promoted worldwide as an acceptable approach to address vehicle-based air pollution. Some developed countries, like the United States and Sweden, are making efforts to improve energy efficiency and reduce the use of fossil-fuel buses in mass transit systems [4]. Nevertheless, China has been increasing its number of fossil-fuel vehicles rapidly—vehicles with comparatively high emissions and excessive energy usage, which has resulted in severe health, energy and congestion problems [5]. In order to address this dilemma, and keep the pace of the trend of global cities, China has been embracing the use of pure electric buses owing to their characteristics of zero emissions, low noise, superior driving stability and good economic efficiency [6].

The Chinese government has promulgated policies to support this transition, such as financial subsidies for bus-related industries and companies to vigorously promote the use of electric buses [7]. In 2015, Green development has become a national strategy [8], and national ministries have enacted some approaches to managing a mass transit infrastructure in relation to electric vehicles [9].

Vehicle performance indicators are fundamental factors for selecting the proper bus type. However, certain weaknesses of pure electric buses, such as a relatively short endurance mileage and a long charging time, are pending obstacles to be overcome. Therefore, the selection process for pure electric buses is more complicated than it is for common buses.

There are three generally accepted methods used to carry out dynamic behavior testing of new electric vehicles—computer simulation test, test bench simulation, and road testing [10].

Compared with the first two methods, which have shorter test cycles and lower costs and are generally used by manufacturers, road tests are more intuitive to reflect the vehicle properties, as the consequence of their superior capacity to provide accurate results in a setting close to real operational conditions, thus increasing the reliability for bus companies to test vehicle performance.

In regard to the selection of electric vehicles, Zhang et al. concluded that the evaluation and selection of new electric vehicles from certain developed countries like United States, Germany and Japan, relies on relevant research institutions and is based on the advanced vehicle testing activity and fuel cell technology [11], however, it is not the case in China. Although Chinese government has enacted several standards related to pure electric vehicles in terms of basic safety and power systems [12], the selection processes based only on the minimum criteria set out in these standards is inadequate and may not provide sufficient information relevant to actual operation.

In spite of the aforementioned policy deficiency in the standard setting of the pure electric vehicle selection, some research has been conducted in light of the assessment of the pure electric buses, defining some critical performance indicators of the pure electric buses, which can serve as a reference for the selection, e.g., Wang and Zheng used energy consumption per kilometer to evaluate the economic efficiency of pure electric buses [13]. An analysis by Wan et al. argued that that vehicle purchase cost, maintenance cost and energy consumption should be taken into account as well [14].

More recently, Li stated that, in Urumqi, when comparing two different buses and Bus Rapid Transit (BRT) vehicles from different manufacturers, energy consumption and service life should be used [15]. Zhang et al. compared the speed and power consumption of several kinds of electric buses in Kunming and several other cities using road tests [11].

However, there are two main limitations lying in the existing research: (1) The assessment for the pure electric buses is not systematic enough, and thus couldn't provide a clear guidance. (2) The current studies focus on the definition and establishment of theoretical indicators but lack practical validation and specific implementation of the current assessment evaluation model. (3) The environmental impact of electric buses should be emphasized more in the literature, which has been paid increasing public attention these years.

Therefore, the purpose of this paper is to present how the Nanjing Bus Company selects suitable pure electric buses based on a comprehensive selection model. In addition, this paper aims to provide a suitable example of a framework and a method of electric bus selection for other public transportation companies in terms of the environmental indicators.
