Investigation Of Combustion Process And EGR Stratification On Automotive Diesel Engine

Diesel has been changed not only on the appearance but also the combustionmode. It is never looked as heavy, noise and always with black smoke, instead,effective and reliable, save oil and clean are used to describe it, and widely used inautomobile since the first diesel came out. However, ultralow emissions and energycrisis challenge the diesel for combustion control to reduce emission on its generation,cycle life is a new topic for the requirement of energy efficiency, which meansafter-treatments and alternative fuel engines are limited compared with diesel. Tosolve the trade-off between NO_xand soot emissions, this work have analyzed steadyand transient operation of diesel combustion process, diesel combustion and emissionsof EGR stratification set artificially in cylinder, and how to realize in-cylinderstratification through intake process based on heavy and medium duty turbocharginghigh pressure common rail diesel engines by experiments and simulations.In this work steady and transient combustion process had been analyzed based onHD （heavy duty） diesel with common rail fuel injection system and turbocharger.Tested bed and simulation model were first established, and fuel injection parameterswere adjusted to obtain the relationship of CA50（crank angle when50%of fuelburned） and emissions with fuel injection parameters under steady condition.3Dcombustion and spray simulation on DI High pressure common rail HD diesel engineunder transient operations had also been conducted. Experimental results of constantspeeding increasing torque transient process rated at1650rpm, and torque increasedfrom10%to90%under10s, had been separated into discrete points based on cycle.NO_xemissions mechanism had been analyzed by volume and duration of in-cylinderhigh temperature area and φ-T diagram. As analysis shown, the simulation results ofseparated transient loads agreed with that of corresponding experimental data. NO_xemissions dropped under increasing torque process while soot emissions rosesubstantially as explained in φ-T diagram that in-cylinder equivalence ratio andtemperature march a lot to soot area and retreat to NO_xarea as loads increased.Uniformity of cylinder mixture was analyzed by Lorenz curve which is a graphicalrepresentation of the cumulative distribution function of the empirical probabilitydistribution of wealth. Under transient conditions, soot emission deterioration isattributable to more heterogeneous of in-cylinder fuel-air mixture compared to steadycondition. Simulation results and experimental analysises have reached an agreementthat NO_xemissions are dependent on duration of in-cylinder high temperature.Timing-sequential regionalized diesel combustion proposed in previous investigation is realized by post injection and proper in-cylinder chemical atmospheretransport to keep post injection fuel injecting to high oxygen concentration region.NO_xand soot emissions decrease simulanously, however, mass of post injection fueland post injection timimg are sensitive for threshold of emissions, consequently, EGRstratification is proposed based on the difference of NO_xand soot generation area andtiming, which means high EGR concentration at the region of spray while highoxygen at the other region. NO_xgeneration is depressed by high EGR during maininjection fuel combustion, and soot oxidation is enhanced by high oxygenconcentration realized by in-cylinder chemical atmosphere migration during postinjection fuel combustion. As a result, mass of EGR introduced in cylinder decreases,cooler lose of EGR decreases and exhaust energy flowing to turbocharger increasesmore importantly, in-cylinder mean oxygen concentration is higher than uniform EGRwhat will make combustion efficiency increasing.EGR stratification first discussed numerically using STAR-CD3.26by settingin-cylinder EGR distribution using subroutines. Combustion results of EGRstratification have similar heat release rate character compare to uniform EGR casewhile lower soot and NO_xemissions with the same mass of EGR introduced tocylinder. Axial and radical EGR stratification that defined artificially in cylinder underthe condition of1650r/min and50%load have been numerically analysed. It isindicated that both axial and radical distribution of EGR stratification make NO_xandsoot emissions decreasing simultaneously, however, radical EGR distribution worksbetter under low local EGR rate, axial EGR stratification degrades more in NO_xandsoot emissions. EGR stratification coupled with timing-sequential regionalized dieselcombustion has the potential to resovle the trade-off between NO_xand soot emissionswhile keep high indicated heat efficiency.EGR introduction strategies were discussed for diesel EGR stratification usingComputational Fluid Dynamics （CFD） and experimental test bed. And in-cylinderdistribution before the start of fuel injection for different stratification strategies wasanalyzed. In-cylinder gas heterogeneous distribution during intake and compressionstroke were realized by means of introducing EGR gas to tangential and spiral intakeport respectively and kept EGR away from fresh air using separated runner extendedto each intake valve. In this work, CO₂gas was selected to substitute for EGR.In-cylinder pressure of simulation results agreed with that of experiments. Twomethods were proposed to evaluate in-cylinder stratification, one is in-cylinderinhomogeneity and oxygen mass concentration field contour for local distribustioncharacter, the other is Lorenz curve and the Gini coefficient which is a graphicalrepresentation of the cumulative distribution function of the empirical probabilitydistribution of wealth in economics and the Gini coefficient is calculated based onLorenz curve and used to quantify in-cylinder inhomogeneity of different simulation cases for global distribustion character. Tested bed is reformed from CA6DL dieselengine for the intake system of first cylinder, two pipes were fitted separately totangential and spiral intake port of first cylinder, and exhaust sampling system wasinstalled to first cylinder for exhaust analysis. The problem of the CO₂injectionsystem is the leakage of CO₂to the other intake port, the maximum error is10%high,so the experimental results are just used to qualitative analysis. The results indicatethat CO₂injection strategies have the potential to meet the Euro4emisions stardardwhen in-cylinder mean oxygen mass concentration is0.21, as oxygen massconcentration lower further, NO_xand THC emissions reached the Euro5standardwith a little higher emission of CO and soot and lower indicated heat efficiency.In-cylinder distribustion simulation results suggest that in-cylinder stratification isobtained, however global inhomogeinty is very low by injecting CO₂to intake port.Timing intake has been proposed to obtain in-cylinder stratification for higherglobal inhomogeinty. Compare to CO₂injection strategy for different CO₂flow path,timing intake strategy obtain higher global inhomogienty. Further investigations onin-cylinder distribustin and combustion are conducted by splitting intake process fortwo phases, during first phase air flows into the cylinder and mainly distribute at thebottom of combustion charmber, and second phase for CO₂gas mainly distribute atthe top of cylinder, the in-cylinder inhomogeneity is higher and notable as oxygenmass concentration gradient is7.38%and the Gini coefficient is higher than0.2.Compared with uniform intake, NO_xand soot reduced substantially with the samemass of CO₂in-cylinder. NO_xdecreased51.2%and soot decreased13.4%. However,ignition delay lags as higher CO₂concentration compare to uniform intake atcombustion area, the peak of heat release rate is higher, while the cylinder pressure islower. Four characters have been used to evaluate in-cylinder distribution of intakecharge, mass ratio of O₂/CO₂distribution; it is the maximum value of CO₂and O₂distribution inhomogeneity. Axial distribution is gas stratification on piston movementdirection. And radical distribution is gas stratification on radical distance withcylinder axis, and spherical distribution is on fuel injection direction.