Experimental Study On Stratified Charge Compression Ignition Combustion With Two-stage Fuel Supplying

A new concept of Stratified Charge Compression Ignition (SCCI) was developed to combine the advantages of Homogenous Charge Compression Ignition (HCCI) and Direct Injection Compression Ignition (DICI). SCCI process can achieve efficient and clean combustion at full load ranges.An experimental study of SCCI cumbustion was conducted on a single-cylinder engine, applying two-stage fuel supplying method. That is port injection (PI) and in-cylinder direct injection (DI). A portion of the fuel was delivered to the intake manifold to create the homogeneous charge during the intake and compression strokes, and the additional fuel was directly injected into the cylinder before TDC, resulting in the stratification of mixture composition, concentration and cylinder temperature. The stratified compound combustion of fuels with high cetane number usually presented a three-stage heat release characteristic. The LTR and HTR processes of premixed combustion resulted in Active Thermo Atmosphere Environment (ATAE), which triggered and controlled diffusive combustion of DI fuel.Based on the fuel design concept, two sets of experiments were conducted. One is n-heptane/diesel dual-fuel SCCI combustion research, and the other one is diesel/gasoline blended fuel SCCI combustion research.Dual-fuel compound combustion was investigated, applying two-stage fuel supplying of PI n-heptane and DI diesel. It is easy for n-heptane to creat homogeneous charge, and its self-ignition could occur before TDC. Then DI diesel was used to realize diffusive combustion, resulting in the stratification of mixture composition, concentration and cylinder temperature. The focus of this effort is to discuss the potential of SCCI concept to expand the operating range and achieve clean combustion, through the adjustment of premixed ratio (rP), equivalence ratio(Φ), and EGR rate from 0% to 40%. SCCI combustion characteristics of diesel/gasoline blended fuel were evaluated, employing PI and DI two-stage fuel supplying. Considering high volatility, easy vaporization and mixture formation of gasoline and high ignitability of diesel fuel, a blend of the two different fuels with opposite but complementary properties was expected to reach an excellent compromise. This part of experiment tested three kinds of blended fuels D70, D60 and D50 (diesel fuel volume 70%, 60% and 50% respectively), examining the effects of fuel properties, rP andΦon combustion characteristics, emissions and load range.The main results are summarized as follows:1. SCCI combustion enabled higher power output than neat HCCI combustion, and the presence of diffusive combustion of DI fuel eliminated the misfire constraint at low loads.2. CA10 (10% heat release) of SCCI combustion advanced obviously to the crank angle before TDC. And the start of HTR was found to be advanced as rP increases. Due to the ATAE of PI fuel, the ignition delay of the DI fuel was shortened, and the onset of diffusive combustion advanced as well. Accordingly, SCCI combustion could effectively control ignition timing.3. The values of cylinder temperature and pressure rise rate were found to be reduced by SCCI mode. And the curves of cylinder temperature and pressure rise rate rose earlier with the increase of rP. Therefore, SCCI concept had obvious effects on combustion speed.4. SCCI combustion could lower CO and HC emissions through the further oxidation of DI fuel. And the staged combustion of SCCI could reduce NOX emission significantly under lean and homogeneous condition, while maintaining acceptable smoke emission.5. Since introducing exhaust gases could smooth combustion reactions by a dilution effect, EGR made it possible to suppress the excessively advanced start of combustion by low temperature reaction. Addtionally, CA10 retarded and the combustion duration extended with the rise of EGR rate. Consequently, EGR could actually affected SCCI combustion timing.In conclusion, SCCI concept is an excellent compromise of neat HCCI and conventional DICI combustion modes. The effects of different fuel design methods, premixed ratios and EGR rates on combustion characteristics, emissions and load range were investigated. Then rudimentary optimal control methods were concluded, which would lay a good foundation for the further research of SCCI concept.