Study of the lubrication system of a Wankel rotary engine to be applied as range extender

The aim of this thesis is to investigate the application of a small Wankel rotary engine (Rotron RT300, 300 cm3) as range extender, studying which are its main limits, understanding which aspects can be modified or optimized in order to understand if it is worth to use an engine with limited automotive diffusion for such an application. The small number of production cars using this engine is due to the fact that Wankel has several problems linked to fuel consumption, oil consumption and pollutant emissions. Part of these problems are strictly connected to the lubrication system: this project focuses on the study of the main components that are lubricated, to understand how and if it is possible to substitute the lubricant oil in order to reduce the friction losses and the seals wear, and to find out which is the best one in term of pollutant formation and emission according to the current regulations. To achieve these results, the project starts from the acquisition of the curves, provided by the manufacturer, of torque, power and specific fuel consumption at full load, from 3000 to 8000 rpm: using analytical formulas, the brake mean effective pressure (BMEP) has been calculated and, combining it with the engine geometrical characteristics, it has been possible to reconstruct the ideal combustion cycle on a pressure-volume diagram. At this point, three different lubricant oils have been chosen, in order to understand how their chemical and physical characteristics can influence the lubrication of the seals and the performance of the support bearings: SAE 0W30, SAE 10W40 and a synthetic oil for 2 stroke applications. The combination of the pressure previously obtained with the rotational speed, gave the possibility to compute the force exchanged between apex seals and engine housing: it was then used, in a lubricated contact model based on Stribeck’s formula, to compute the power losses for rubbing friction and to compute the apex seals wear, according to the Archard’s model. Next, an analytical model has been applied to support bearings and to the shaft-rotor bearing in order to compute the total power losses, so the mechanical efficiency: the results have been compared to the ones obtained in the case in which the roller bearings are substituted by bushings, in the same operating conditions and with the same lubricant oil. The final step was to understand how all these aspects influence the fuel consumption and, since oil and fuel are injected in a blend at 2% in volume, the oil consumption. Finally, the complete mathematical model, obtained for each lubricant oil, has been used to test the possible application of this rotary engine as range extender on both NEDC and WLTC, to understand what could be the increment of drivable range and what could be the duration of the apex seals after which a substitution is needed, compared to a common reciprocating engine used as range extender.