Research On Low Temperature Combustion And Combustion Paths In A Heavy Duty Diesel Engine

Low temperature combustion has the potential to achieve clean and highefficiency combustion at low and medium loads, and has attracted more and moreattention in recent years. Therefore, achieving sufficiently rapid fuel-air mixing ratesthroughout the whole combustion history is the key technology to achieve newconcept combustion. In-cylinder temperature control is crucial to realize LTC. Basedon the in-cylinder charge condition （temperature, density and oxygen concentration）control and fuel injection strategy control, aiming at clean and high efficiencycombustion process, the factors influencing mixing rate and chemical reaction rate innew concept combustion conditions are explored using simulation and experimentalstudies. The main conclusions are reached as follows:With increasing of charge density, the overall fuel-oxygen equivalence ratio isdecreased, the fuel-oxygen mixing rate is enhanced, the chemical reaction rate isincreased. Especially, high charge density promotes the fuel-oxygen mixing rate in thelater phase of engine combustion, quickens the oxidation of the unburned fuel and theconversion rate of the CO into CO₂; the combustion duration is shortened, whichleads to higher indicated thermal efficiency. The charge density as a comprehensivefactor of temperature, pressure, mixture compositions and so on, increases the totalheat capacity as the exhaust gases recirculation （EGR） does, decreases the averagein-cylinder temperature, which is beneficial to deceasing NO_xemissions anddecreases the reliance on high EGR rate.Retarding the intake valve closing timing（IVCT） decreases the effective compression rate, which allows further increasingcharge density within the in-cylinder pressure limit. Retarding IVCT decreased thecompression temperature and the combustion temperature, which is beneficial todeceasing NO_xemissions.However, the high oxygen quantity and the quickenedcombustion rate caused by high charge density, leads to high local combustiontemperature and higher NO_xemissions. Therefore, proper EGR rate is used todecrease oxygen concentration. The factor that affects NO_xemissions most is stilloxygen concentration.Based on high charge density, proper oxygen concentration andretarded IVCT, the high density-low temperature combustion （H Density-LTC） andthe combustion paths control strategies are proposed. CFD simulation and engineexperimental results show the higher thermal efficiency and very low emissions are obtained with applying the H Density-LTC concept.The typical LTC uses the EGR rate higher than60%and the oxygenconcentration lower than15%, which decreases the soot and NO_xemissionssimultaneously. However, the high CO and UHC emissions are obtained; thecombustion efficiency and the thermal efficiency are decreased at low and mediumloads. High EGR rate inhibits the combustion temperature and the soot formation,which is the mechanism to decrease the soot emission in typical LTC. At high and fullloads, high charge density enhances the fuel-oxygen mixing rate in the wholecombustion process, which decreases the formation rate of soot and increases theoxidation rate of the soot; consequently, the net soot emission is decreased.To fully utilize the bump combustion chamber, single injection in thecompression stroke nearer to TDC is used. Combined with proper injection timing ofthe single injection and EGR rate, the ignition delay, the mixture formation beforeignition, and the subsequent combustion process can be controlled. At higher load,comparing with single injection, the optimized EGR rate and injection timing ofmulti-injections are used to retard ignition timing, optimize mixture formation andreduce the combustion speed to obtain low soot, NO_xand CO emissions.