An Investigation into the Surface Area-to-Volume Ratio of Wankel Rotary Engines and how it Affects Heat Transfer and Heat Losses within them

Wankel rotary engines are renowned for having lower efficiency than classic reciprocating engines. One of the factors affecting the efficiency is an unfavourable surface area-to-volume ratio given by the particular geometry of the engine, which increases the heat loss during the combustion phase. A novel and specific study on this aspect was carried out in this work by implementing a general parametric routine in Octave/Matlab. It was able to compute the surface area-to-volume ratio and execute a sensitivity analysis on specific engine geometrical parameters (e.g. housing width “b”) in order to determine the geometrical configuration with the minimum surface area-to-volume ratio for a given swept volume, compression ratio and K factor (i.e. the ratio between generating radius and eccentricity). The aforementioned procedure was then applied considering the geometry of the Advanced Innovative Engineering 225CS rotary engine. Three virtual geometrical configurations with the same displacement as the 225CS were generated using the aforementioned sensitivity procedure. Subsequently, the heat transfer was computed for the real engine and employing some assumptions for the virtual configurations by using experimental data previously collected on the 225CS. It was found that the minimum surface area-to-volume ratio is obtained with extremely narrow rotors but that, counterintuitively, the heat transfer and loss is considerably worse due to an increase of the Nusselt number.