Effects of low-carbon high-reactivity fuels on combustion and emission characteristics in a part-load condition of a DICI engine

Diesel engines face a challenge on environment pollutions, especially nitrogen oxides (NOx) and particle emissions. In recent years, advanced combustion modes were developed and fuels for different physical and chemical performances were investigated. Many researches showed that naphtha fuels well suited for compression ignition engines. In this paper, two reformates from naphtha fuels, named as High Reactivity Gasoline (HRG) and High Reactivity Gasoline blended with ethanol (HRE), and diesel were used to learn the effects on combustion and emissions of a single-cylinder compression-ignition (CI) engine. Experiments were conducted at a low load operating point, 1055 r/min-7.2 bar IMEP (gross indicated mean effective pressure) which could significantly show the effects of different fuels' characteristics. A dual split fuel injection strategy was employed. In the experimental condition, fuel injection pressure, intake pressure, and exhaust gas recirculation (EGR) rate were maintained the same for all test fuels. Horiba MEXA-7200 and DMS500 were used to measure gas emissions and particle number (PN) emissions respectively. Results showed that HRG and HRE realized a stable combustion on diesel engine by using a two-stage injection strategy. HRE had the longest ignition delay leading to a single-stage heat release rate which was different from that with the other fuels. As a result, the trends of combustion and emission characteristics were unique. For HRE, the second injection timing (SOI2) dominated the maximum cylinder pressure rise rate (PRMAX) while it showed little effect on PRMAX using the other fuels. Fuel with low chemical reactivity could reduce the NOx emission and led to high CO and THC emissions. However, HRE had an overlong ignition delay led to an overlap of two combustion stages. The combustion was fast and the NOx emission became worse. In the experimental condition, the nucleation mode particles dominate the PN emissions. HRE and diesel had huge nucleation mode particle emissions due to the short combustion duration and the enhanced diffusion combustion, respectively. In conclusion, HRG had advantages on emissions which showed the lowest PRMAX, NOx and PN emissions and HRE had the advantage on engine working efficiency which could reach 47.8% in this study.