Achieving Ultra-Lean Combustion Using a Pre-Chamber Spark Ignition System in a Rapid Compression-Expansion Machine

Combustion processes operating under fuel lean conditions are a promising technology for internal combustion engines, achieving low emissions and very high efficiency. In traditional spark ignition engines, the charge dilution affects the flame propagation speed, leading to a combustion instability. A way to overcome these limits consists on the replacement of the spark with a pre-chamber ignition system. The combustion starts in the pre-chamber and develops in the main chamber through multiple and distributed ignition points, ensuring fast burn rate and stability. This paper focuses on the performance evaluation of a pre-chamber spark ignition system operating under ultra-lean conditions. An experimental campaign is carried out using a rapid compression-expansion machine fueled with liquid iso-octane as a surrogate fuel for gasoline. The initial layout of the machine is modified to accommodate two injectors in order to have an independent control of the fuel/air equivalence ratio on each chamber. The pre-chamber injector provides the necessary fuel to obtain a rich mixture which facilitate the early stage of the combustion. The concept is studied in boosted conditions at low equivalent engine speed (around 1500 rpm), which are prone to knock on standard SI combustion. The results show a low cycle-to-cycle dispersion in terms of ignition time and pressure peak for all the boundary conditions. The heat release rate and the combustion duration are assessed through the study of the pressure evolution in both chambers. Furthermore, the in-cylinder pressure trace allows to identify knock occurrence in certain conditions, analyzed in terms of MAPO (Maximum Amplitude of Pressure Oscillations) index. Finally, an intensified camera enables combustion visualization tests with OH* chemiluminescence technique, which help to qualitatively describe the event.