Date of Award
Master of Engineering (ME)
Dr. R. Sowerby
The thesis commences by tracing the underlying research work that established the feasibility and technically justified the current programmes for the development of the implantable artificial heart, more particularly those programmes based upon the use of a heat engine as the power unit of the artificial heart.
The provision of a heat source for the heat engine, whether it be a continuously generating source, such as a mass of a suitable radioisotope, or a thermally rechargeable material, adds to the burden of weight and bulk that the user must carry. This inconvenience could be reduced if the thermal efficiency of the engine can be increased. Thus, in the light of a heavy and probably an unending and continuously increasing demand for a practicable artificial heart the author considers that further work is justified if the engine efficiency can be raised, albeit by only a small amount.
The author's main endeavour is to identify an ideal engine cycle that appears to promise the most likelihood of high efficiency. It seems remarkable that there is little record of similar investigations readily available in the literature, and therefore, the present author has taken the approach to develop and evaluate parameters for a number of ideal cycles: Carnot, Stirling, Ericsson and the regenerative Otto cycle. The parameters for these cycles are compared, together with the published results for the Thermocompressor, the Schmidt isothermal and the Schmidt adiabatic cycles.
Working fluid mass distribution between the hot and cold spaces of the Schmidt isothermal engine is investigated, revealing the low utilization of the working fluid when large dead volumes occur in the engine. The hot volume versus cold volume relationships of the Schmidt and rhombic drive engines are analyzed and compared with the relationship for the true Stirling cycle.
Illustrations of the arrangement principles of historical and current "Stirling" engines, as well as the constructions of contemporary artificial heart engines are presented to indicate how problems of arrangement have been answered.
The main findings are:
(a) Of the isothermal cycles considered, i.e. cycles of potential Carnot efficiency, the true Stirling cycle is significantly the best and better than the adiabatic cycles.
(b) The ideal adiabatic cycles achieve efficiencies tending to the Carnot level as the compression ratio tends to unity. Working fluid utilization is least when the ideal efficiency is maximum, that is at unity compression ratio. Inevitable losses in the practical engine indicate that the maximum efficiency will be attained at a compression ratio significantly greater than unity.
(c) The effect of regenerator inefficiency in the Stirling engine can be partly compensated by increasing the compression ratio.
(d) The hot space-cold space relationships of the Schmidt and rhombic drive engines deviate significantly from those of the ideal Stirling cycle.
It is concluded that an engine built to run as closely as possible to the true Stirling cycle is the most promising route to the best attainable efficiency, also that in practice the compression ratio is likely to strongly influence the achievable efficiency.
The author recommends building an initial research engine with its hot-cold spatial relationships cam controlled to the requirements of the true Stirling cycle, also to be adjustable for comrpession ratio. The major purpose of this initial engine would be to prove the veracity or otherwise of the cycle selection and to determine how the efficiency varies in practice with compression ratio. For comparison, cams would also be made to stimulate other cycles, e.g. the Schmidt cycle. Suggestions are given for features that would facilitate the operation and performance measurement of the engine.
In presenting this work the author also considers that he has compiled a useful bibliography which contains the more pertinent references in the field.
Pettingill, Tom K.O., "Thermodynamics of Stirling Type Engines for the Artificial Heart" (1977). Open Access Dissertations and Theses. Paper 407.