A Quasi-Dimensional Two-System Burn Rate Model for Pre-Chamber-Initiated SACI Combustion

State-of-the-art spark-ignition engines mainly rely on the quasi-hemispherical flame propagation combustion method. Despite significant development efforts to obtain high energy conversion efficiencies while avoiding knock phenomena, achieved indicated efficiencies remain around 35 - 40 %. Further optimizations are enabled by significant excess air dilution or increased combustion speed. However, flammability limits and decreasing flame speeds with increasing air dilution prevent substantial improvements. Pre-Chamber (PC) initiated jet ignition combustion systems improve flame stability and shift flammability limits towards higher dilution levels due to increased turbulence and a larger flame area in the early Main-Chamber (MC) combustion stages. Simultaneously, the much-increased combustion speed reduces knock tendency, allowing the implementation of an innovative combustion method: PC-initiated jet ignition coupled with Spark-Assisted Compression Ignition (SACI). The jets penetrating the MC establish a flame propagation combustion that – with appropriate boundary conditions – triggers a controlled volume reaction in the remaining charge. The resulting ultra-fast combustion process converges to the ideal thermodynamic constant-volume cycle leading to indicated efficiencies of >45%. However, implementing this combustion method requires precisely adjusted boundary conditions and a suitable geometrical design (e.g., compression ratio). This paper addresses the development of a fast-running quasi-dimensional burn rate model for PC-initiated SACI combustion to conduct robust design studies and complement existing testing methodologies (3D-CFD, experimental). The modeling approach considers two thermodynamic systems (PC and MC) connected through orifices. Both systems use the two-zone entrainment model for flame propagation combustion. Furthermore, the eventual MC volume reaction is modeled by a multi-pseudo-zone approach based on a distributed auto-ignition integral. The models are integrated into the so-called cylinder module developed at the Institute of Automotive Engineering Stuttgart and validated using measurement data of two single-cylinder research engines using different fuels (E100, RON95E10), loads (IMEP = 6 − 15 bar), excess air dilutions (λ = 1 − 2.8) and compression ratios (12.6 – 16.4), showing a satisfactory prediction of the burn rates and pressure curves.