This work investigates the influence of direct fuel injection into oxygen-depleted environment or negative valve overlap interval on combustion characteristics of a gasoline direct injection homogeneous charge compression ignition (GDI-HCCI) engine. When the fuel is injected into the O2-depleted condition, it can be converted into smaller hydrocarbons due to thermal cracking, partial oxidation, and water-gas shift reaction. These reformed species are known to influence the ignition delay and main combustion timing simultaneously. 

In experiments, a single cylinder research engine based on a production one is boosted to reach IMEPn=0.55 MPa in which its thermal efficiency peaks at 40-41%. Experimentally, the main combustion phases are advanced under single-pulse fuel injection into the NVO compared with that of intake stroke. NVO peak in-cylinder pressure are lower than that of motoring which emphasize that endothermic reaction occurs during the recompression interval when the fuel is injected into trapped residuals. Insufficient oxygen concentration could play a role in this evaporative charge cooling effect. This limits oxidation reaction and the thermal effect is not pronounced. To further understand the recompression reaction phenomena, 0D simulation with three different chemical reaction mechanisms is studied to emphasize that the main combustion phase advancement is kinetically limited by the NVO reforming. The 0D simulated results show the same increasing tendencies of classical reformed species of rich-mixture combustion such as C3H6, C2H4, CH4, CO, and H2 as a function of injection timing. By adding these reformed species in to the main fuel-air mixture,predicted ignition delays are also shortened.

Effects of the reformed species on main combustion are confirmed by 3D-CFD with chemical kinetics and the results show that OH radical generation is advanced under NVO fuel injection compared with that of intake stroke conditions thus earlier heat release and cylinder pressure are noticeable. In addition, parametric studies on injection pressure and double pulse injection on engine combustion are also performed experimentally. Maximum pressure rise rate and NOx are significantly reduced and single-digit NOx emission is achieved under certain cases.


Experimental and Numerical Analysis on the Influence of Direct Fuel Injection Into O2-Depleted Environment of a GDI-HCCI Engine

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

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