Direct piston displacement control of free-piston Stirling engines

[摘要] ENGLISH ABSTRACT: Control of beta-type free-piston Stirling engines has been the topic of research and developmentfor many years. In this dissertation, an alternative approach to free-piston Stirling enginecontrol, namely direct piston displacement control, is proposed.Direct piston displacement control entails the instantaneous and direct control of the pistondisplacement to control the engine according to preferred criteria, e.g. maximum powerconversion or efficiency. To control free-piston engines in this manner, it is necessary to independentlycontrol both the displacement of the displacer and the power piston in real time. Theprimary arrangement by which to achieve this is through external control of the instantaneousforces exerted by the linear electrical machines fixed to the pistons. The challenge of displacementcontrol is whether suitable linear machine technology exists or whether technology couldbe established that would adhere to the requirements of real time direct control.To answer the question whether direct piston displacement control is at all possible, a processwas followed to set specifications that linear machines should adhere to and to set designguidelines for linear machines and free-piston Stirling engines.The first step was to establish the ability to simulate free-piston Stirling engine dynamicsaccurately. This was done by adapting a second order formulation and to verify and improvethe accuracy thereof by comparing simulated results with experimental results of one of thebest documented Stirling engines, namely the GPU-3 engine. It was found that this secondorder formulation could simulate the GPU-3 engine to a fair degree of accuracy.Key indicators were defined and later refined with the view of setting specifications. A casestudy of the influence of a range of variations, including operational, dimensional and othervariations, on the dynamics of the GPU-3 was then undertaken. From the findings of this casestudy, specifications of the key indicators and design guidelines were established.A design optimisation approach was proposed to evaluate linear machine topologies. Thisapproach makes specific provision for the specifications that linear machines need to adhereto, as well as for representative dynamic responses of the forces exerted on the linear machineby the displacer or the power piston. These representative responses and the associated pistondisplacement were determined for the displacer, the power piston and the combination of thetwo from the study conducted to set specifications.An air-core, longitudinal flux linear machine with surface mounted permanent magnets(LFPM) was then evaluated to determine its suitability for direct piston displacement control.This linear machine topology was optimised for the traditional approach to establish a benchmarkwith which to compare subsequent optimisations. The LFPM linear machine not onlycompared well with other topologies for the traditional application in resonant free-piston Stirlingengines, but it was found also to be able to perform displacement control for both thedisplacer and the power piston. For both pistons, displacement should however be limited tosinusoidal displacement, and in the case of the displacer, an important qualification is that thelinear machine should be assisted by spring forces to reach practical design optimisations.Direct piston displacement control is shown to be possible. Future work should concentrateon the practical implementation thereof in free-piston Stirling engines.

[发布机构] Stellenbosch University