Optical Diagnostics On The Mechanism Of The New Combustion Modes Of IC Engine

Efficient and clean combustion technologies are the key to meet increasingly stringent regulations of exhausting emissions and fuel consumption（CO₂）.Many new combustion modes are proposed,among which low-reactivity fuel partially premixed combustion（PPC）and dual-fuel highly premixed charge combustion（HPCC）modes gain great attentions,because of their ultra-low NO_x/soot emissions and high engine efficiency.However,knowledge about the in-cylinder process of PPC and HPCC is still quite limited.The present study focuses on exploring the in-cylinder combustion mechanism of PPC and HPCC by multiple optical diagnostics on an optical engine,using primary reference fuel of 30_v%（volume fraction）n-heptane and 70_v%iso-octane.The in-cylinder fuel/air mixing,ignition and flame development processes are analyzed to reveal the principal combustion characteristics of PPC and HPCC.This work aims to provide theoretical foundation for the development of the efficient and clean combustion technologies.An integrated laser diagnostics system for in-cylinder process was established,including the units of optical engine,lasers,image acquisition as well as signal control and synchronization.A fast intake heating system,an easy-assembled mirror unit and a piston ring installation device used for moveable cylinder liner were designed based on the original optical engine.Several laser diagnostic techniques were realized on the optical engine system.Fuel-tracer laser-induced fluorescence（PLIF）was used to quantify the fuel concentration and reactivity stratifications.Using only one YAG laser and one dye laser,non-simultaneous and simultaneous measurement of formaldehyde（CH₂O）and OH were realized qualitatively by PLIF technique.The soot concentration during engine combustion was quantitatively measured by two-color planar laser-induced incandescence（PLII）technique.Using the above multiple laser diagnostic techniques,as well as traditional methods of natural luminosity imaging and spectrometry,PPC at mid-low load was studied first.Results indicated that ignition kernels in PPC first appeared in the regions of high fuel concentration（equivalence ratio about 0.7¹.2）,and then flames developed into the low-concentration regions.In the initial stage of PPC after the formation of ignition kernels,distinct flame front propagation process was shown.And the flame speed of this flame front propagation（10⁶0 m/s）was much higher than that in spark-ignition engines（slower than 10 m/s）.With advancing direct-injection（DI）timings,the flame front propagation process in PPC was less pronounced due to lower fuel stratification degree,and the overall flame development of PPC was dominated by the sequential auto-ignition.Early-injection timings had significant impact on PPC at low engine load in that there was a DI“timing window”（about-30°^-60°CA ATDC）.When the fuel was delivered at this“timing window”,the“fuel-trapping effect”of squish region and piston crevice would prevent the fuel from entering the combustion chamber after hitting on the piston top and cylinder wall,resulting lower combustion efficiency.When the PPC combustion efficiency drop due to spray-wall impingement took place,there was certain residual CH₂O residing in the low-concentration region of the combustion chamber,which would become a source of UHC.When misfire happened,the combustion chamber would be filled with CH₂O,because the fuel/air charge only underwent low-temperature heat release（LTHR）phase,but missed the high-temperature heat release（HTHR）phase.Double-injection strategy could reduce the impact of spray-wall impingement under early fuel-injection condition.The first DI timing should be earlier than the“timing window”and the ratio between the first and second fuel-injection should approach to one.Then,HPCC was studied at a relatively higher engine load of about 7 bar gross IMEP.Results indicated that the fuel stratification degree got higher with retarding DI timing,and high-reactivity region（PRF number about 30⁶0）moved to the combustion chamber wall,forming a low-reactivity region（PRF number about 60⁹0）near the center part of the combustion chamber.Ignition kernels first formed in the high-reactivity region,and then flames developed into the low-reactivity region.With higher fuel stratification degree,the consumption of CH₂O and formation OH in HPCC got slower,and chemiluminescence intensity from band spectra of OH,CH（431.4 nm）and C₂（516.5 nm）increased gradually and successively.In the middle-late stage of combustion,the natural luminosity was dominated by soot radiation in the high-reactivity region.Compared with the low-load diesel LTC conceptual model proposed by Musculus et al.in 2013,a major difference in HPCC was that the HTHR phase indicated by OH radical could extend to the central part of combustion chamber.Late-injection HPCC presented a two-stage combustion process:an auto-ignition first happening in the high-reactivity region and then the combustion in the low-reactivity region.These two combustion processes were separated in both time and space,and could result in lower combustion pressure-rise rate in HPCC.Compared with PPC which showed flame front propagation in initial phase of combustion,early-injection HPCC was dominated by sequential auto-ignition.This was mainly due to the difference in fuel reactivity stratification degrees.There was no fuel reactivity stratification for PPC,resulting in longer ignition delay and less tendency to auto-ignition.No mixing-controlled combustion was shown in HPCC at middle-low load,which led to low soot formation in combustion.Consequently,it was difficult for PLII technique to get useful signals in this combustion mode.