The effects of DI fuel properties on the combustion and emissions characteristics of RCCI combustion

Abstract

An experimental investigation has been conducted to explore the effects of direct-injection (DI) fuel properties in dual-fuel reactivity-controlled compression ignition (RCCI) combustion on a single-cylinder, heavy-duty diesel engine. The combustion and emission characteristics of four DI fuels with cetane number (CN) ranging from 34 to 100 were compared under different combustion boundary conditions. The results showed that a higher CN DI fuel allows more advanced SOI timing, more EGR, and higher premixed ratio. However, although the operable DISOI timing and EGR regions can be extended, the operable Rp region is narrowed for higher CN fuels. In addition, there is a competing relationship between DI fuel CN and EGR/premixed ratio (Rp) in controlling CA50, PPRR, and HC/CO emissions. With late injection strategy, enlarging the reactivity gradient through increasing CN of DI fuel shows more promising effects on combustion rate and pressure rise rate reduction, which is desirable for high load extension. Comparing the highest indicated thermal efficiency (ITE) cases of different CN fuels, it’s illustrated that the approach of lower EGR (<45%) with high Rp (∼80%) is suitable for low CN fuel (i.e. CN34) with early injection strategy to obtain a lower PPRR and a better ITE simultaneously. However, at late injection condition, more EGR with lower Rp (<80%) is required to control NOx/soot emissions. For high CN fuel (CN ≥ 56), the approach of higher Rp (>80%) with high EGR (∼45%) is suitable with either early or late injection strategy to simultaneously improve the PPRR, ITE and NOx/soot emissions.