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.
Highlights
The need both to reduce CO2 emissions as well as to meet aggravated pollutant emission targets poses significant challenges to the mobility sector
Other than in fuel mixing controlled conven tional diesel or spark-ignited gasoline combustion, the Homogeneous Charge Compression Ignition (HCCI) rate limiting process is reaction kinetics, which are influenced by a complex interplay of mixture preparation and temperature control of ignition timing through intake air heating, exhaust gas recirculation (EGR) or exhaust valve closing timing [1,3,4,5,6]
The current study aims at providing detailed insight into mixing and combustion behavior within an optically accessible rapid compression and expansion machine (RCEM) operated on a split injection Pre mixed Charge Compression Ignition (PCCI) strategy by means of passive optical and laseroptical high-speed diagnostics
Summary
The need both to reduce CO2 emissions as well as to meet aggravated pollutant emission targets poses significant challenges to the mobility sector. Other than in fuel mixing controlled conven tional diesel or spark-ignited gasoline combustion, the HCCI rate limiting process is reaction kinetics, which are influenced by a complex interplay of mixture preparation and temperature control of ignition timing through intake air heating, exhaust gas recirculation (EGR) or exhaust valve closing timing [1,3,4,5,6] In this context, hybrid combustion concepts such as Premixed Charge Compression Ignition (PCCI) offer a huge potential to combine high efficiency/low emission characteristics of HCCI with the superior operability of conventional diesel combustion [5]. The analysis of optical data is complemented by a comparison of the jet metrics with literature correlations as well as a comprehensive thermodynamic analysis of global heat release rates and fuel consumption, linking the observed findings to engine oper ation and efficiency
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