Potential of reactivity-controlled compression ignition with reverse reactivity stratification (R-RCCI) fueled with gasoline and polyoxymethylene dimethyl ethers (PODEn)

Abstract

To improve the trade-off between thermal efficiency and peak heat release rate (HRR) of partially premixed combustion (PPC) and the combustion efficiency of reactivity-controlled compression ignition (RCCI), the combustion mode with premixed high-reactivity fuel and direct-injection (DI) low-reactivity fuel, called RCCI with reverse reactivity stratification (R-RCCI), was explored at low loads in a light-duty diesel engine in this study. Compared with diesel, polyoxymethylene dimethyl ethers (PODEn) has better volatility, which is beneficial for the formation of premixed charge, so it was used as the premixed high-reactivity fuel for R-RCCI in this work. The gasoline and P20G80 (PODEn/gasoline blends with the volume fraction of 20%/80%) were respectively applied as the DI low-reactivity fuel. By investigating the combustion characteristics of R-RCCI, it is found that R-RCCI can break the trade-off between combustion efficiency and nitrogen oxides (NOx) emissions. This is because the combustion efficiency of R-RCCI is dominated by the spray location of the DI fuel rather than the 50% burn point (CA50). As the start of injection (SOI) timing is retarded, the fuel injected within the piston bowl increases, and combustion efficiency, as well as indicated thermal efficiency (ITE), is considerably promoted. Meanwhile, CA50 progressively retards with delayed SOI timing, which effectively reduces NOx emissions. The soot emissions of R-RCCI are also extremely low. The maximum ITE of PODEn/P20G80 R-RCCI is significantly higher than that of PODEn/gasoline R-RCCI. This occurs because the higher reactivity of P20G80 can reduce the sensitivity of CA50 to SOI timing and improve combustion stability, so a more delayed SOI timing is allowed to improve ITE. With the same engine configurations, R-RCCI can reduce peak pressure rise rate and improve combustion stability, while enhancing combustion efficiency and ITE compared with RCCI at the low-load conditions tested in this study.