Analysis of a newly developed hybrid pneumatic powertrain configuration for transit bus applications

Abstract

In the current study, a new hybrid compressed natural gas, liquid nitrogen-based powertrain configuration is developed and analyzed for more environmentally-benign transit buses. The conceptual design extracts power from high-pressure nitrogen via three-stage expansion air motors. When the high-pressure LN2 tank level is critically low, a hydraulic pump powered via a CNG-based internal combustion engine is utilized to repressurize the liquid nitrogen. At the end of the expansion process, low-pressure nitrogen is condensed via a helium-based cryocooler system and stored in a low-pressure tank. The performance of the present conceptual system is evaluated via energy and exergy approaches. The power requirements, fuel consumption ratios and CO2 emission ratios are presented with respect to driving modes. The utilization of different fuel types as a secondary energy source for the powertrain is comparatively evaluated. The effects of various design and operational parameters on the system performance are investigated through parametric studies. The proposed system can operate with energy and exergy efficiencies of 46.6% and 67.5%, respectively while emitting 0.0768 kg CO2 eq per kilometer in city driving at a speed of 40 km/h (11.11 m/s).