Thermodynamic Modeling of an Epitrochoidal Engine Cycle

A thermodynamic performance model has been developed for a new four-stroke piston engine design in which the crankshaft path is epitrochoidal, or non-circular. The model is based on an Otto air-standard cycle with non-ideal effects of friction, valve and spark timing, heat transfer, volumetric efficiency, and fuel burn timing then added. The same modeling approach was then used in developing a standard circular path engine model for comparison: the result being two discrete models varying only in their crankshaft paths, and thus piston path timing. The thermodynamic modeling was one phase of a larger senior design project in which senior engineering students were tasked with answering the question of whether the epitrochoidal crankshaft path engine will produce more power than a standard circular path engine of the same size and compression ratio. The starting point for the effort was the epitrochoidal crankshaft design description given in the patent, followed by major challenges of design, fabrication, modeling, and testing of a prototype engine. A Briggs and Stratton model 20 was employed as the standard circular crankshaft path comparison engine, and upon which the epitrochoidal prototype engine geometry was based. The result was two test engines of the same size and compression ratio, but differing in crankshaft path. Thermodynamic modeling, however, is the primary focus of the paper presented herein. Details of the design/fabrication/testing phases of the larger project are included in another paper, currently in preparation. The modeling description starts with the development of the mathematical equations describing the piston movement within the standard engine and the epitrochoidal engine, followed by the specifics of the thermodynamic modeling and inclusion of non-ideal effects. The model calibration to measured data is developed next, and finally a prediction of the epitrochoidal engine performance is shown to agree with measured data for the prototype engine. The testing did in fact show an increase in power in the epitrochoidal prototype engine, and the modeling was used to explain the improvements attributable to the epitrochoidal design. These results also affirmed the comparative modeling approach that was used.