Optimizing RCCI combustion for improved engine performance under low load conditions: Impact of low-reactivity fuel and direct injection timing

Abstract

The objective of this study is to explore the challenges associated with the RCCI (Reactivity Controlled Compression Ignition) concept under low loads, where there may be insufficient combustion or misfires caused by a lean fuel mixture in the combustion chamber. To address this issue, optical diagnostics and post-processing techniques are utilized to investigate the effects of low-reactivity fuel and direct injection timing on combustion and flame characteristics in a dual-fuel engine under such conditions. The study tested three different low-reactivity fuels and five different direct injection (DI) timings while maintaining hydrogenated catalytic biodiesel (HCB) as a high-reactivity fuel. The research findings reveal that an increase in ethanol content results in longer ignition delay and a displacement of the ignition kernels towards the downstream of the fuel stream. Consequently, a more homogeneous combustion process and slower flame propagation occur, ultimately reducing the combustion intensity, cylinder pressure, and peak apparent heat release rate. Premixing E10 allows for a higher indicated mean efficient pressure (IMEP) of 0.20 MPa, exceeding E0 by 0.02 MPa and E100 by 0.05 MPa. The research also highlights that delaying the DI timing of HCB reduces ignition delay time and creates a higher concentration gradient of the combustible mixture, resulting in a higher apparent heat release rate and shorter combustion duration. Additionally, natural combustion luminosity images indicate that delaying the DI timing substantially impacts the characteristics of flame kernels generation and propagation. The ignition kernels formed shifted gradually from the near-wall region to upstream region of the spray, as the blue flame area and front propagation speed showed a significant increase. Retarded DI timing can achieve more favorable flame propagation and a higher IMEP under low loads conditions with a higher premixed energy ratio, while effectively reducing the impact of spray-wall impingement and pool burning phenomena. These findings provide insights into optimizing RCCI combustion for improved engine performance under low load conditions.