3D-CFD Modelling of Gas Exchange and Combustion Inside the Expander of a Recuperated Split-Cycle Engine

The demand of game-changing technologies to improve efficiency and abate emissions of heavy-duty trucks and off-road vehicles promoted the development of novel engine concepts. The Recuperated Split-Cycle (R-SC) engine allows to recover the exhaust gases energy into the air intake by separating the compression and combustion stages into two different but connected cylinders: the compressor and expander, respectively. The result is a potential increase of the engine thermal efficiency. Accordingly, the 3D-computational fluid dynamics (CFD) modelling of the gas exchange process and the combustion evolution inside the expander becomes essential to control and optimize the R-SC engine concept. This work aims to address the most challenging numerical aspects encountered in a 3D numerical simulation of an R-SC engine. First, the impact of both the mesh features and the boundary conditions is evaluated on the transonic conditions of the air, which enters the expander under a wide range of expansion ratios (1.1÷30). Simulations in a 2D axi-symmetric domain allowed to identify the suitable mesh size required for a correct prediction of the engine volumetric efficiency. Afterwards, simulations of fuel-air mixing and combustion clarified the peculiarities of combustion in R-SC engines, where ignition and combustion are strongly affected by the nonconventional in-cylinder charge motions and temperature stratification.