Knock Assessment in Wankel Rotary Engine Adopting Low Octane Rating Fuels

Series hybrid vehicles with internal combustion range extenders are a promising solution for sustainable transportation. In this application, net zero carbon emissions can be achieved using renewable fuels. Fischer-Tropsch-derived e-gasolines/naptha allow for high energy density and safe liquid fuels. However, Fischer-Tropsch naptha fuel derivatives must undergo several processing stages to reach current engine-grade octane ratings, negatively affecting the synthesis's profitability and energy efficiency. Gasoline engine technologies capable of operating with low-octane fuels could allow the adoption of unprocessed Fischer-Tropsch gasoline. The rotary Wankel engine design suits range extenders thanks to its high power-to-size ratio. In this study, the knocking tendency of homogenous charge spark-ignition rotary Wankel engines is numerically assessed through Chemkin-Pro spark-ignition engine zonal model for knock assessment. Rotary Wankel engines are modeled by providing the corresponding time-dependent profiles of volume, wall surface area, wall global heat transfer coefficient, and burnt gas fraction retrieved from previous experimental work. Different engine load conditions have been investigated spanning engine shaft rotational speeds from 3000RPM to 6000RPM and brake mean effective pressures from 3 bar to 7 bar. Detailed kinetic modeling of normal-heptane/iso-octane primary reference fuels is adopted to describe the autoignition tendency of different octane-rating blends. Simulations show a low knocking tendency under the investigated conditions, therefore suggesting the suitability of rotary engines for the adoption of unprocessed Fischer-Tropsch fuels.