Performance prediction of a Martini type of Stirling engine

In this study, a Martini type free piston Stirling engine concept is proposed and its performance analysis is conducted. The Martini engine consists of a heating-cooling unit and an expansion unit for working gas. The heating and cooling of the working gas is accomplished via displacing the gas between a hot and a cold volumes taking place in a displacer cylinder. The displacer is kinematically driven by a crank mechanism. The expansion of the working gas is accomplished via a power piston working in a separate cylinder. The power piston has no kinematic driving mechanism but one of its ends is adjusted to a mechanical spring while the other end is exposed to the gas pressure. The power piston bears an I type magnet at the center of itself. The expansion cylinder is equipped with a linear alternator to produce electricity. While the piston and the equipped magnet is performing a reciprocal motion in the expansion cylinder, the conductive wire coil wound around the expansion cylinder generates an electric current via using the work generated by the piston. The analysis conducted in this study involves the prediction of thermodynamic performance parameters of the engine as well as dynamic characteristics. The displacer is assumed to be driven by a crank mechanism turning with a constant speed and the motion of the displacer is defined kinematically. The motion of the power piston is defined with a differential equation obtained from the Newton law of motion. The flow friction of working fluid is calculated with Darcy equation. Variation of temperatures in nodal volumes are calculated with the first law of the thermodynamics given for unsteady open systems. Input parameters of the analysis were optimized for 2500 rpm engine speed. By introducing 1000 K hot end temperature, 400 K cold end temperature and very realistic values for the other inputs, the thermal efficiency of the Martini engine has been predicted as 20.6%. When the flow friction in the regenerator is ignored, the thermal efficiency increases from 20.6% to 21.5% and the cyclic work generation increases from 236.5 J to 254.8 J. For inputs optimized for 2500 rpm engine speed, the power output of the engine was predicted as about 10 kJ. When the regenerator heat transfer area was increased from 0.69 m(2) to 1.38 m(2), the thermal efficiency increased from 17.5% to 34.4%. However, further increase of regenerator heat transfer area was restricted by collusion between piston top and cylinder top. The Martini engine was found to be a useful supplementary device to couple with Internal Combustion Engines to increase the thermal efficiency of them. The thermal efficiency of the modified Internal Combustion Engine is expected to be 5% more than the non-modified engine.