Electrical system for a hybrid heavy commercial vehicle

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Electrical system for a hybrid heavy commercial vehicle

Walker, A. / Lamperth, M.

Heavy Goods Vehicles are the primary method for transport of goods in the UK. Studies have shown that a hybrid-electric powertrain for a 7.5 tonne commercial vehicle, operating on a delivery duty cycle, can offer reduced polluting emissions and improved efficiency. The urban / suburban nature of the duty cycle can be satisfied by electric-only driving in urban areas, and conventional driving with electric-assist elsewhere. As the powertrain unit cost is predominantly due to the Energy Storage Device (ESD) price, the cycle demand and electrical system must be designed with highest efficiency use of the ESD as a primary consideration. The paper discusses an explicit design analysis of the electrical system and the requirements for highest efficiency (cost effectiveness). The set of relations used are provided for the reader's use. For the urban delivery application, a Permanent Magnet motor-generator for electric-only and electric-assist driving regimes, of 30 kW continuous / 40 kW peak, has been designed, which operates up to 2700 rpm. This is also used to start the diesel engine. A V-source inverter is considered, using Space-Vector Pulse Width Modulation. An ESD capacity of 26 kWh has been specified to provide the required electric-only driving range, and, in this case, a NiMH pack is employed. This requires cycling management to achieve maximum life. The implication for the common and maximum charging and discharging currents, according to the power demand, is considered. These current values are used to assess the main system losses, and overall efficiency is computed for the system, from battery through to motor-generator. An effort has been made to minimize these losses in motoring operation during electric-only driving.