Effects of piston bowl geometry and mixture formation strategy in a cng/diesel dual-fuel compression-ignition engine

Abstract

Dual-fuel (DF) combustion using natural gass(NG) and diesel has been recently receiving attentions in the freight transportation areas owing to its potential use in achieving better fuel economy and cleaner combustion. In this work, a full-metal engine test and optical engine test were conducted to characterize the combustion process of NG/diesel DF combustion in a 1.0 L optically-accessible single-cylinder engine. The optical engine test utilized high-speed visualization of natural-luminosity flame and OH*/HCHO* chemiluminescence. The diesel injection timing and NG substitution ratio (NGSR) were varied to implement diverse in-cylinder blending conditions. A novel flame regime separation method based on color image segmentation in a hue-saturation-value (HSV) color space was used to quantitatively compare the spatial distributions of premixed and diffusion flame regimes. Because NG has a lower carbon content and higher auto-ignition resistance compared with diesel, the natural luminosity images for larger NGSRs revealed a significant reduction in the diffusion flame regime accompanied by retarded flame development. An earlier injection of the liquid diesel shifted the location of the early flame growth toward the piston bowl wall and created a rapid influx of propagating flame, while effectively suppressing the formation of intense soot radiation through longer ignition delay. The calculation of reaction zone growth speed from the chemiluminescence imaging confirmed that the flame kernels in DF combustion with larger premixed grade initially develop by means of sequential auto-ignition, followed by flame propagation towards ignitable background mixture. In the performance engine test, these observations were verified by the exhaust emissions measured using the full-metal version of the engine and the same fuel supply parameters, specifically regarding the compliance of the smoke and nitrogen oxide emissions with the Euro VI regulation. The NG/diesel RCCI operation successfully demonstrated a cleaner and more efficient combustion compared to the conventional diesel engine. However, the reduction in NOx emission was realized at the cost of combustion loss penalty and combustion phasing delay. The modified piston bowl geometry in bathtub shape could reduce heat transfer loss and suppress the unburned hydrocarbon emission at the same time, leading to improve the NOx-efficiency trade-off relationship in the design of the DF engine.