For use with low grade thermal reservoirs of small capacity and low temperature ({dollar}<{dollar}150 deg. C), such as are available from solar energy and waste heat from industrial exhausts, and with a suitable working fluid, an idealized piston engine is proposed for a variety of basic needs. This engine could have great acceptance within the solar industry as well as in the heating, ventilating and air conditioning (HVAC) industry. Hence, this study is concerned with the operational analysis and improvement of this engine to enhance its design parameters for economical manufacture and to find optimum operating conditions to increase the engine efficiency.; The study is divided into three main parts to analyze the engine theoretically and thermodynamically. In Part 1, a mathematical model is developed to select the best working fluids from among the many available by assigning priorities to the properties of the working fluids. In Part 2 of this study, a power process for the engine is proposed and work analyses are conducted to find the maximum shaft work per mass of working fluid, to find the best cutoff volume of the working fluid to get the optimum work per mass of fluid, and to find the optimum compression ratios within the geometric factors of the engine concerned. Mathematical relationships are derived to determine the optimum values for engine physical characteristics. Comparison of the work produced by the different working fluids selected in Part 1 is conducted. Also, the engine specific power for different engine speeds (rpm) is analyzed with variable compression ratios. In Part 3 of this study, internal irreversibilities are identified as 'lost-work' in the rolling diaphragm. For the least 'lost-work' per work-out of the engine, effects of variation in piston-cylinder clearances and in variation of fluid specific heat ratios are discussed.
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