A hybrid compressed natural gas-pneumatic system as a powering option for buses: A comparative assessment

Highlights: • A newly hybrid compressed natural gas (CNG)-pneumatic bus is designed. • Thermodynamic, economic and environmental performances of this design are comparatively assessed. • The operational reliability of the hybrid CNG-pneumatic bus is evaluated as the highest. • A potential transition from diesel to hybrid CNG-pneumatic buses can halve the CO₂ emissions from urban transportation. In response to environmental, energy, and efficiency problems of conventional vehicles, research and innovation on potential electrical, hydrogen, pneumatic, and hybrid solutions have recently been carried out by numerous researchers. The current study develops and evaluates a hybrid type compressed natural gas (CNG)-pneumatic powertrain concept for common buses to provide a cleaner solution for public transportation. In terms of tank-to-wheel efficiency, well-to-wheel efficiency, energy consumption, carbon dioxide (CO₂) emission, capital cost and driving range, the proposed system is comparatively evaluated with various powering options. The buses considered in the study are named based on powering fuel type: diesel, compressed natural gas (CNG), electricity, hybrid diesel-electric, hydrogen fuel cell and hybrid CNG-pneumatic. Moreover, an operational reliability concept is developed as an additional assessment criterion to define the most reliable powering choice. The operating conditions and related parameters of the proposed concept are given, and the obtained performance values of the concept are discussed through the comparative assessment study. The current results show that the hybrid CNG-pneumatic bus emits 0.80 kg CO₂ per km, making it the second nature-friendly option after the hydrogen fuel cell bus (0.312 kg CO₂/km). Furthermore, in terms of operational reliability, the results reveal that the hybrid CNG-pneumatic bus is the most reliable option, with values of up to 66% amongst the evaluated options.