Design and performance investigation of flux-concentrated tubular linear generator for an external combustion free piston engine

Abstract

The increasing global desire for highly fuel efficient power systems and the need for environmentally friendly energy sources is driving much present research in electrical power. A linear power system, where a linear machine is driven directly by a free piston engine, offers scalability and a wide range applicability. Standalone power units, hybridised power systems and range extenders in electrified vehicles are all potential applications for this technology. This thesis explores the application of a Linear Joule Engine driving a Permanent Magnet Linear Machine for electrical power generation. Whereas most Joule cycle engines have a rotary compressor and expander, at smaller scale this configuration suffers from leakage around the blades. The linear engine uses a double acting free piston configuration running on the external combustion Joule-cycle, overcoming the low efficiency inherent in small scale gas turbines. The key element for electrical power generation, and the main focus of this thesis, is the development of a linear machine operating as a generator, the design of which is heavily constrained by the geometrical and the operational characteristics of the engine. Using specific constraints for an 5kW engine and by using two dimensional finite element analysis, a novel design methodology of tubular PM linear machine with modular armature winding and feasible arrangements of magnets on the translator member is outlined. The effect of core material, pole number and power conversion system on the machine design are investigated, highlighting the effect of the interconnected design variables on the resulting performance and material use, all satisfying design objectives. A Flux – Concentrated PM configuration is selected for further development. vi In order to accomplish an overall system performance investigation tool, at first the development of a general novel linear machine model is introduced and tested in a feedforward manner with accounts for all machine interacting electromagnetic forces. Then, a novel dynamic model incorporating both the linear machine model driven by the linear Joule engine model, coupled together in a closed loop form, is realized. The coupled model bridges mechanical and electrical parts of the engine-generator, and provides a solid dynamic performance prediction of the system focusing on identifying the effect of cogging force on system performance and the resultant electrical power loss and electrical efficiency. Compared with the reported cogging force reduction techniques, a novel structural technique and a selection criteria are presented with two dimensional axisymmetric finite element analysis verification showing the effectiveness of the proposed technique. Finally, a machine prototype of the selected design model is manufactured and tested on a bespoke test rig to validate the design model findings. Manufacturing recommendations and future achievable steps are reported for future development of the existing work.