Modelling and Control of a 96V Hybrid Urban Bus

Abstract

This paper describes the development and on-vehicle validation testing of next generation parallel hybrid electric powertrain technology for use in urban buses. A forward-facing MATLAB/Simulink powertrain model was used to develop a rulebased deterministic control system for a post-transmission parallel hybrid urban bus. The control strategy targeted areas where conventional powertrains are typically less efficient, focused on improving fuel economy and emissions without boosting vehicle performance. Stored electrical energy is deployed to assist the IC engine system leading to an overall reduction in fuel consumption while maintaining vehicle performance at a level comparable with baseline conventional IC engine operation. Regenerative braking is integrated with the existing braking systems on the vehicle, and the control system tailored to maximise the amount of energy recuperated during deceleration events and accelerator pedal lift off without adversely impacting on the normal behaviour of the vehicle. The control system was implemented on both prototype single (Streetlite) and double-deck (Streetdeck) vehicle configurations for real vehicle testing with partner Wrightbus. The hybridisation has reduced equivalent CO2 emissions by 34% (single-deck)/ 35% (double-deck) over the conventional Euro VI diesel vehicle on the Low Carbon Vehicle Partnership UK bus cycle (based on London Bus Route 159). These results compare favourably with alternative powertrain technologies currently available with similar certification. Moreover, the next generation hybrid urban bus has several distinct advantages as it is less restricted by infrastructure, range, or terrain issues, and has a comparatively lower purchase price point. Hybrid bus technologies offer the option of maintaining existing service levels without significant modifications to operations or budgets while achieving significant reductions in average fleet emissions. Introduction/Overview With demand for increasingly fuel-efficient vehicles growing, largely driven by ambitious targets to reduce (or eliminate) tailpipe emissions, there has been a surge in hybrid-electric and electric technology development within the automotive industry. Similar drivers within the bus industry have seen many of the same technologies entering into service. The earliest hybrid technologies relied on smart ancillary or ‘micro’ hybridisation of existing diesel powertrains, introducing combinations of start-stop technology and transferral of auxiliary systems to electric power sources, alongside schemes targeted at minimisation of weight. Later moves towards full hybridisation have resulted in a number of successful full series and parallel hybrid urban bus vehicles going into service globally. However, challenges with balancing achievable emission and fuel consumption reductions against increased total cost of ownership still remain. Estimates of total cost of ownership per $/km range from 2.85$/km to 2.98$/km for full hybrid vehicles, compared against 2.61$/km for an equivalent diesel vehicle [1]. This is largely due to the high purchase price point of the vehicle, with purchase cost (on average) 45% (parallel powertrain) to 59% (series powertrain) higher than an equivalent diesel vehicle [1]. While fully electric bus architectures are emerging into the global marketplace, hybrid technologies are likely to still be prominent in the bus sector through to 2035 (and potentially beyond) [2] as component weights, achievable ranges, lower component availability, reliability, cost, aggressive and varied duty cycles in the bus sector all continue to present barriers to mass adoption of fully electric technologies. To mitigate the increased cost of the full hybrid powertrain, alternative hybrid powertrain topologies could provide a cost-effective alternative within the bus industry. Improved integration of motor generator units (MGUs) into the powertrain can allow for quick firing of the engine, providing additional torque to improve fuel consumption performance during full load acceleration events and, in some configurations, transmitting torque pulses to the engine to reduce vibration. These power assist strategies can be particularly attractive for urban style driving cycles, as is typically encountered within bus operation. The current work presents a 96V parallel-hybrid bus control strategy which has been developed and implemented on both single and double deck next generation hybrid bus topologies. The system has been proven to deliver CO2e Well-to-Wheel emissions savings of up to 35% over an equivalent Euro VI configuration through vehicle testing and has been certified in the UK as a Low Emission Bus. The paper will first present the forward-facing modelling strategy which was developed to support the preliminary sizing of components in the vehicle architecture and testing of the initial rule based deterministic control strategy, before presenting the vehicle testing and certification results. Background to Hybridisation in the Bus Industry Local emissions legislation has had a significant impact on the technology choices within the bus sector, driven by the phased introduction of low and ultra-low emissions zones in many towns and cities [3,4]. These new zones have long term impact on the continued viability of many of the more traditional diesel-based bus architectures, particularly as regulations become increasingly more stringent. Various solutions including low-emission diesel, hybridisation, full electrification and alternative fuels have emerged but there are still outstanding issues with cost of acquisition,