Experimental Study On Combustion And Emission Characteristics Of Multi-cylinder HCCI Engine Based On Fuel Design

With the challenges of energy preservation and environment protection, engineemissions reduction and thermal efficiency improvement have become a key to solvethese problems. Homogeneous Charge Compression Ignition (HCCI) as a newcombustion mode with higher efficiency and lower emissions has great potential.Although a lot of technical means have been carried out to realize the application ofHCCI mode in engines, they have their own technological bottlenecks. The fueldesign technology as one of gasoline HCCI technologies has shown somesuperiorities as fast response, wide load range and relatively uncomplicated structurechange. Therefore, this project focuses on the combustion and emission characteristicsof a dual fuel HCCI.Based on principles of the optimized kinetic process strategy, modifications have beencarried out on a4-cylinders gasoline engine, such as adjusting the compression ratioto13, utilizing exhaust heat to the inlet air and so on. Further, an independentn-heptane injection system has been installed to realize dual fuel HCCI technologyand stable HCCI combustion is finally realized on the gasoline engine. In order tostudy combustion and emission characteristics of dual fuel HCCI in differentoperation conditions, the effects of EGR rate, inlet air temperature and spark assistedignition on dual fuel HCCI were discussed. The pressure and emissions data wasrecorded and averaged in each operation condition while the EGR rate varied from0to45%, the inlet air temperature varied from55℃to170℃and the spark advanceangle varied from0°CA to100°CA. The test results of EGR showed that, increasing the rate of gasoline leads to delayedcombustion timing, extended combustion duration, higher HC and CO emissions andlower EGR rate in optimal operating condition. As the rate of n-heptane increases,HCCI combustion becomes less sensitive to EGR rate. When dual fuel ratio is fixedand the EGR rate varies, CA10and CA50show a linear relationship, the slope ofwhich positively correlates with the rate of gasoline.The test results of the inlet air temperature showed that, the variation of inlet airtemperature mainly affects the low-temperature heat release process of n-heptane andthe density of inlet air, and these two factors both restrict the heat release process andtemperature atmosphere in the cylinder when inlet air temperature decreases.Increasing the rate of gasoline leads to more limited range of inlet air temperature, theaverage of which is higher. When dual fuel ratio is fixed and inlet air temperaturevaries, CA10and CA50show a linear relationship. As inlet air temperature decreases,HC and CO emissions increase, and the increasing rate gradually becomes higher.The test results of spark assisted ignition showed that, the main influence of sparkassistance is to bring forward the combustion phase, and the influence becomesincreasingly obvious as the rate of n-heptane decreases. When dual fuel ratio is fixedand the spark advance angle varies in a narrow zone from30°CA to40°CA, theinfluence upon combustion phase reaches the strongest. With less n-heptaneproportion in fuel, the spark assistance has a great influence on restraining the cyclicvariation.