Scaling combustion recession after end of injection in diesel sprays

Abstract

While soot and NOx emissions tend to be lower for partially-premixed compression ignition (PPCI) strategies compared to more conventional conditions, a common theme amongst PPCI strategies are the excessive unburned hydrocarbons (UHC) that originate from overly lean mixtures near the nozzle due to end-of-injection entrainment. The focus of this work is on combustion recession, which governs the outcome of near-nozzle UHC and has been identified as a process that is strongly influenced by end-of-injection transients in addition to in-cylinder thermodynamic conditions and injection parameters. Combustion recession is the process whereby the initially lifted reaction zone retreats back towards the nozzle following end-of-injection thus consuming UHC that would otherwise remain near the nozzle and persist into the expansion stroke. In an effort to link end-of-injection combustion transients to ambient thermodynamic conditions and injector parameters, a scaling methodology to predict the likelihood of combustion recession in diesel sprays was developed. This methodology relates ignition timescales during steady injection to mixing timescales after end-of-injection. Reasonable agreement between the predicted scaling and measured combustion recession data is shown over a very wide range of ambient conditions, injector parameters, and end-of-injection transients. The success of this scaling suggests that combustion recession is autoignition dominated, heavily influenced by injector design, and that the mixing-limited vaporization assumption can be extended to the study of end-of-injection phenomena. This work also suggests that combustion recession is a more robust parameter for correlation to UHC than ignition dwell.