The free-piston Stirling engine ( F P S E ) is a heat driven mechanical oscillator from which power can be extracted. Linear dynamics is applied here in order to obtain: a stability criterion, a means for determining the oscillation frequency, relative amplitudes of the dynamic components, eflects of friction on starting and the locus of the roots of the system determinant. Three common configurations of these engines are investigated. NOTATION 2 LINEAR MODELReferring to Fig. 1, piston motion causes changes in working gas pressure, p , that excite motion of the dis-Dlacer which transfers working gas between the hot and The MS wus receioed on 26 June 1984 and was accepted for publication on 25 'Old 'paces thus changing p aid hence the force On March 1985. the piston. Oscillation occurs under proper conditions 91/85 0 IMechE 1985 0263-7138/85 $2.00 + .05

NASA seeks high-power conversion systems for space applications and their requirements emphasize high efficiency and low mass. This paper presents preliminary designs and specific power estimates of high efficiency, free-piston Stirling machines for 5 kW e , 10 kW e and 25 kW e outputs. In each layout, several engine configurations have been considered: single and dual opposed beta and three, four and six cylinder alphas. A critical factor in obtaining high specific power is related to the specific power optimization of the alternator. This is shown to be a strong function of the piston amplitude. Monocoque finned heads are found to be compromised by temperature gradients across the pressure-containing walls. These temperature gradients become significant at power levels above 2 kW. A stepped piston three-cylinder alpha arrangement is shown to have favorable dynamics for high efficiencies and good specific powers.

In order to improve the specific power of piston-cylinder type machinery, there is a point in capacity or power where an advantage accrues with increasing number of piston-cylinder assemblies. In the case of Stirling machinery where primary energy is transferred across the casing wall of the machine, this consideration is even more important. This is due primarily to the difference in scaling of basic power and the required heat transfer. Heat transfer is found to be progressively limited as the size of the machine increases. Multiple cylinder machines tend to preserve the surface area to volume ratio at more favorable levels. In addition, the spring effect of the working gas in the so-called alpha configuration is often sufficient to provide a high frequency resonance point that improves the specific power. There are a number of possible multiple cylinder configurations. The simplest is an opposed pair of piston-displacer machines (beta configuration). A three-cylinder machine requires stepped pistons to obtain proper volume phase relationships. Four to six cylinder configurations are also possible. A small demonstrator inline four cylinder alpha machine has been built to demonstrate both cooling operation and power generation. Data from this machine verifies theoretical expectations and is used to extrapolate the performance of future machines. Vibration levels are discussed and it is argued that some multiple cylinder machines have no linear component to the casing vibration but may have a nutating couple. Example applications are discussed ranging from general purpose coolers, computer cooling, exhaust heat power extraction and some high power engines.

Abstract:The environmental footprint of ultra-low temperature (ULT) freezers as used in bio-repositories, universities and other research organizations is investigated. These freezers, employing the cascade refrigeration system, use between 10 and 20 times the energy of an average household refrigerator/freezer. In addition, they often require high greenhouse gas potential (GWP) refrigerants. A new technology employing the Stirling cycle machine promises to reduce energy consumption of ULT freezers by 50% or more. The cascade and Stirling systems are compared for equivalent sized freezers in terms of embodied energy and equivalent CO 2 production from cradle to gate and use, including total equivalent warming impact (TEWI) estimations. End-of-life issues are discussed but not quantified. It is shown that Stirling technology is able to significantly reduce the environmental impact of ULT freezers.

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