5. Combustion in Diesel Fuelled Partially Premixed Compression Ignition Engines

Global concerns about depleting fuel reserves, environment degradation, and sustainability have motivated researchers to explore advanced combustion strategies. In this regard, low-temperature combustion (LTC) has emerged as a promising combustion strategy that can simultaneously reduce soot and NOx emissions with the additional benefit of superior fuel economy. However, researchers must address its primary challenges, w.r.t. controlling ignition timing, combustion phasing, and high heat release rate (HRR), to practically implement it on a large scale. Partially premixed compression ignition (PPCI) is a type of LTC that can address these drawbacks. In PPCI, the injection is closer to ignition when compared with homogeneous charge compression ignition (HCCI), which provides superior control over the ignition. It achieves better mixing and low in-cylinder temperatures using medium to high exhaust gas recirculation (EGR) levels. Optical diagnostics are powerful tools used by researchers to understand the physical behaviour of these complex processes in engines or engine-like conditions. The main attempt of this chapter is to summarize the understandings obtained by researchers about the two-stage combustion occurring in PPCI conditions using optical diagnostics. In the build-up, a brief introduction is given to the conventional diesel combustion (CDC) model and the chemical kinetics involved in the LTC to provide an overall understanding.