Impact of a split injection strategy on mixing, ignition and combustion behavior in Premixed Charge Compression Ignition combustion

TL;DR

This study investigates how a split injection strategy affects mixing, ignition, and combustion in PCCI engines using optical diagnostics, revealing enhanced mixing and complex ignition dynamics influenced by operating parameters.

Contribution

It demonstrates the impact of split injection on mixing and ignition behavior in PCCI, highlighting the role of low temperature reactions and local mixture conditions, which was not previously detailed.

Findings

Split injection increases mixing rate of subsequent fuel spray.

Low temperature oxidation contributes over 20% to heat release.

Ignition dynamics are strongly affected by local mixture and operating strategy.

Abstract

Mixing, ignition and combustion behavior in a rapid compression and expansion machine operated under Premixed Charge Compression Ignition (PCCI) relevant conditions are investigated by combined passive optical and laser-optical high-speed diagnostics. The PCCI concept is realized using a split injection schedule consisting of a long base load injection and two closely separated short injections near top dead center. Previous studies of close-coupled double injections under constant ambient conditions showed an increased penetration rate of the subsequent fuel spray. However, the aerodynamic gain from the preceding injection is counteracted by the density rise during the compression stroke under transient engine conditions. The study confirms that the rate of mixing of the subsequent fuel spray is significantly increased. Regarding combustion behavior, the thermodynamic analysis exhibits contributions of low temperature oxidation reactions of more than 20 % to the total heat release, with a notable amount of unburnt fuel mass varying from 25 to 61 %. The analysis of the optical data reveals the multi-dimensional impact of changes in operating parameters on the local mixture field and ignition dynamics. The onset of low temperature reactivity of the first short injection is found to be dominated by the operating strategy, while the location is strongly related to the local mixing state. Low temperature ignition of the consecutive fuel spray is significantly promoted, when upstream low temperature reactivity of the preceding injection is sustained. Likewise, it is shown that high temperature ignition is accelerated by the entrainment of persistent upstream low temperature reactivity.