Experimental Study On The Fuel Design And Management Concept-Based HCCI/DI Stratified Compound Combustion

Homogeneous Charge Compression Ignition (HCCI), as a promising combustion technology with high efficiency and low emissions, has been widely investigated over the world in recent years. However, since the ignition of HCCI is dominated by the chemical kinetics, the control of ignition timing and burning rate is difficult to be solved, resulting in limited operating range of HCCI engine. In this study, Active Fuel Design and Management (AFDM) concept-based HCCI/DI stratified compound combustion is proposed. By timely and spatially controlling the physical and chemical characteristics of the fuel at both port injection stage and in-cylinder direct injection stage, HCCI/DI stratified compound combustion is developed to combine the advantages of HCCI and conventional DICI on cycle-to-cycle basis and achieve clean and efficient combustion at full load ranges without misfiring and knocking.Firstly, three types of dual fuel including n-heptane/diesel, iso-octane/diesel and ethanol/bio-diesel are utilized to investigate the combustion and emission characteristics of HCCI/DI stratified compound combustion. The experimental results indicate that n-heptane/diesel HCCI/DI combustion presents a three-stage heat release consisting of HCCI low temperature heat release, high temperature heat release and diesel diffusive combustion. The combustion characteristics of iso-octane/diesel and ethanol/bio-diesel both show a two-stage heat release without HCCI low temperature reactions. Compared with the prototype diesel engine, the NOx emission of dual fuel HCCI/DI stratified compound combustion is obviously reduced and also dramatically decrease of smoke is observed for iso-octane/diesel and ethanol/bio-diesel dual fuel configurations. In addition, the HC and CO emissions level of dual fuel HCCI/DI are between HCCI and DICI. The indicated thermal efficiency remains on the same level of the prototype engine. The investigation illustrates that the fuel characteristics determine the combustion characteristics of HCCI/DI, and the ratio of premixed combustion and diffusive combustion which timely and spatially affects the heat release distribution is determined by the premixed ratio. Moreover, the direct injection timing determines the phasing of diffusive combustion.Based on the concept of fuel design and management, using PRF with variable RON for port injection and n-heptane for direction injection, the effects of port injected fuel with variable RON and the premixed ratio as well as direct injection timing on the combustion phasing and heat release distribution of HCCI/DI is systemically studied. And the effects on engine efficiency and exhaust emissions are also investigated. It is found that the RON of port injected fuel determines the heat release phasing and distribution of the first and second stages of HCCI/DI, and consequently has influence on the third stage combustion. PRF fuel of RON50 is the optimal port injection fuel for HCCI/DI with the best thermal efficiency and lowest NOx emission. The premixed ratio is proved to be a significant factor of HCCI/DI stratified compound combustion. With optimized premixed ratio, the HCCI/DI combustion can avoid knock and keep very low NOx emission all over the load range. Accordingly, the optimization strategy is established. Optimal premixed ratio should decrease as the load increases, with small premixed ratio at high loads. The in-cylinder direct injection timing mainly affects the third diffusive combustion of HCCI/DI. By retarding the direct injection timing, the in-cylinder peak pressure and temperature as well as the peak heat release rate of the third stage diffusive combustion decreases and the overall combustion duration increases. Small fuel delivery advance angle is helpful to reduce NOx but would cause the deterioration of fuel economy and other emissions. Considering both the efficiency and emission, the fuel delivery advance angle of 18 ~0CABTDC is validated to be the best under the condition of the experiments. By optimizing the fuel characteristics, premixed ratio and direct injection timing, the combustion phasing and the heat release distribution of HCCI/DI are well controlled. Optimization of the three factors also makes the HCCI/DI engine achieve 100% load while keeping the NOx within 40ppm. In addition, the HC and CO emissions are much lower than the emissions of HCCI and the indicated thermal efficiency of HCCI/DI is higher than the prototype diesel engine.Furthermore, without spark ignition and hot EGR, the HCCI/DI combustion of in-cylinder directly injected iso-octane is achieved with the induction of thermal and chemical active atmosphere generated by the HCCI combustion of port injected n-heptane. The effects of port injected n-heptane equivalence ratio and iso-octane direct injection timing are investigated experimentally and numerically. The results indicate that the combustion mode of iso-octane is dependant on the iso-octane direct injection timing corresponding to the stage of n-heptane HCCI combustion. In case of direct injection before the low temperature reaction of n-heptane, premixed combustion of the iso-octane occurs. In case of direct injection at the NTC period or the initial high temperature reaction stage of n-heptane, the iso-octane mainly undergoes premixed combustion. In case of direct injection at the latter stage of n-heptane high temperature reaction or nearly after the high temperature reaction, diffusive combustion of iso-octane mainly occurs. The results show that in the case of iso-octane direct injection at the latter stage of n-heptane high temperature heat release, the iso-octane mainly undergoes high temperature diffusive combustion with stable ignition and insensitiveness to the port injected n-heptane equivalence ratio. In this condition, the indicated thermal efficiency of HCCI/DI combustion is higher than the prototype engine while the NOx emission can be maintained within 40ppm over the full load range.