External Heat Engine Research Articles

Study of piston trajectory parameters on the performance of a kinetic Stirling engine

The Stirling engine is a type of external heat engine that is capable of extracting thermal energy from differential temperature sources and converting the energy into boundary work. Since the Stirling cycle is realized through four polytropic processes, the shape of the pressure-volume trajectory of the working medium fundamentally governs the output performance of the engine. Traditional α-, β-, γ-type engines have fixed piston trajectories, bounded by mechanical linkages, usually following approximately sinusoidal curves. Free-piston variants are capable of tailored piston trajectories if active displacer regulation is added, but still constrained by the spring-mass-damper dynamics. Currently, either kinetic or free-piston engine results in polytropic processes of the gas. During these processes, the boundary work may hinder heat transfer, potentially reducing the engine thermodynamic performance. To guide syntheses of more efficient engines, the key trajectory parameters need to be studied. This paper starts from the thermal-pressure-structural modeling of the engine, evaluates the 4 key processes during an ideal Stirling cycle, and proposes the trajectory parameters group, which finely tunes the gas curve as well as pistons’ trajectories, allowing for a detailed look into the relationship among the work medium the interactions between the pistons and the engine performance. Through establishment of a prototype engine, the models are validated through an inhouse designed and assembled test bed. Results show that manipulation of the gas volume curve is key to achieve isochoric heat addition or rejection, and the pistons trajectories determine the throttling loss performance. By applying isochoric heat transfer and reduced throttling condition, the net output work of the engine can be improved by nearly 100% against the sinusoidal baseline.

INVALIDITY OF ISOTHERMAL ANALYSIS IN ESTIMATING THE THERMAL PERFORMANCE OF STIRLING ENGINE AND CRYOCOOLER

The Stirling engine is an external heat engine, which is considered as the best option for extracting work from concentrated solar power applications. The most prominent characteristics of the engine are low noise, vibration, and emissions besides reflexivity of usage with any kind of heat source such as solar, biomass, industrial heat, etc. In the present paper, the STE-1008 gamma-type Stirling engine had been analyzed by using an isothermal model to demonstrate the failure of the model in analyzing the STE-1008 considering it firstly as an engine and secondly as a cryocooler. The energy equation had been used to demonstrate the disability of the isothermal model in achieving a successful thermal analysis for engine performance. In addition, a MATLAB code had been developed to check the credibility of the isothermal model in the estimation of the engine thermal parameters. The findings of the isothermal analysis revealed that the heat exchangers are unnecessary. But, in reality; all the necessary heat transfer occur within the heat exchangers rather than in the working space boundaries. Therefore, that is invalid conclusion. However, Schmidt's theory is capable of capturing the essential engine features superbly. In particular, it is capable of capturing the fundamental interplay between the mechanically restricted movement of the engine components as well as the thermodynamic cycle which is obtained from this theory.

Optimised Liquid Flooded Gas Cycle for Heat Pump and External Heat Engine Applications

Gas cycles, in their ideal form, promise the highest efficiency heat pump and external heat engine systems. The Ericsson and Stirling cycle promise Carnot efficiency, yet their seeming simplicity present significant engineering challenges in implementation. The primary challenge of both cycles is isothermal compression and expansion where finite time and the mechanical system’s insufficient surface area prevent ideal performance. Liquid flooding of the Ericsson cycle has been explored previously. Flooding with a high heat capacity liquid in effect increases the surface area for heat exchange, yet the experimental mechanical system uncounted significant losses, preventing suitable performance. This paper models a novel gas cycle system that combines aspects of the reverse Brayton air cycle and the Liquid Flooded Ericsson cycle. The enthalpy model in EES (Engineering Equation Solver) allows optimization of possible cycle arrangements and yields an optimal arrangement that constitutes a new gas cycle utilizing liquid flooding to achieve superior performance in a simpler cycle that previously envisioned. Possible applications of the new cycle include heat pump and external heat engines.

Waste Heat Recovery Applied to a Tractor Engine

Due to the environmental impact of pollution, the depletion and increasing price of fossil fuel resources, current research focuses on reducing vehicle fuel consumption. The modern agricultural sector is highly dependent on petrol, as no exploitation can work without tractors or agricultural machinery. One solution to reduce the dependence on petrol is to install waste heat recovery systems on engine exhaust gases. Tractors are good candidates for waste heat recovery since they are used at high load over long periods, which are ideal operating conditions for waste heat recovery systems. Several technologies can be used to achieve this aim, such as an external heat engine, thermoelectricity or thermoacoustics. The present study considers the external heat engine, in particular the Rankine-Hirn cycle which is a phase change fluid engine. This well-proven technology is the one most widely used in industry to recover lost heat. Different heat loss recovery techniques are presented in this paper, in which we detail our model of the Rankine-Hirn engine used to dimension our future test bench. We focus on the selection of fluid, pressure and temperature as a function of the hot source temperature. Lastly, we present an initial evaluation of the increase in efficiency depending on the variation in the heat source.