Flow-Field Analysis of Isobaric Combustion Using Multiple Injectors in an Optical Accessible Diesel Engine

Abstract

Isobaric combustion has shown the potential of improving engine efficiency by lowering the heat transfer losses. Previous studies have achieved isobaric combustion through multiple injections from a single central injector, controlling injection timing and duration of the injection. In this study, we employed three injectors, i.e. one centrally mounted (C) on the cylinder head and two side-injectors (S), slantmounted on cylinder head protruding their nozzle tip near piston-bowl to achieve the isobaric combustion. This study visualized the flame development of isobaric combustion, linking flow-field details to the observed trends in engine efficiency and soot emissions. The experiments were conducted in an optically accessible single-cylinder heavy-duty diesel engine using n-heptane as fuel. Isobaric combustion, with a 50 bar peak pressure, was achieved with three different injection strategies, i.e. (C+S), (S+C), and (S+S). Bottom-view high-speed soot luminosity images were recorded at a frame rate of 20 kHz for all cases, together with pressure traces. Flame image velocimetry (FIV) analysis was performed on the high-speed soot luminosity images to obtain a qualitative description of the flow-field obtained for the three injection strategies. Distinctive vortex structures were evident from the FIV analysis and that can be attributed to strong flame-wall and flameflame interactions. For the C+S and S+S injection strategies, the distinct large vortex structures were found near the bowl-wall while for the S+C case, vortex structures are less prominent. The large vortex structures close to the cylinder walls contribute to lower gross indicated efficiency and higher soot level intensity of the C+S and S+S cases, compared to the S+C configuration.

Optical Diagnostics of Isobaric and Conventional Diesel Combustion in a Heavy-Duty Diesel Engine

“…The peak motoring pressure (PMP) of CDC and isobaric combustion were fixed at 50 and 70 bar respectively with the peak cylinder pressure (PCP) of 70 bar selected for both cases. This is consistent with our previous studies [12][13][14]. The desired PMPs were attained using intake air pressure of ~1.7 bar for CDC and ~2.2 bar for isobaric combustion.…”

“…The piston bowl is 22 mm deep and the entire piston is made up of quartz. Compared to the ω-shaped piston used in our previous studies [14,23], a flat-bowl piston is preferred such that the distortion correction from the bottom-view imaging can be circumvented. One of the side quartz windows of the windows holder was utilized for background illumination during Mie scattering imaging, while the other windows were unused.…”

“…This was followed by a PIV time-series operation which includes the following steps: circular geometric mask set up for the region of interest, a sequential cross-correlation algorithm applied to consecutive image frames using multi-pass Discrete Fourier Transform (DFT), and a universal outlier detection median filter for removing the erroneous vectors. The detailed optimization procedure for FIV is based on our previous works conducted in the same engine [13,14].…”

“…Isobaric combustion has shown the potential of achieving high engine efficiency while improving the individual losses of heat transfer and exhaust when coupled with a heat recovery system such as split-cycle concepts. Isobaric combustion achieved using singleinjector-based multiple injections [11][12][13] or multiple injectors [14][15][16][17][18][19] can effectively control the heat release rate profile while keeping the peak cylinder pressure (PCP) and bulk gas temperature lower than conventional diesel combustion. Due to reduced flame temperature, isobaric combustion has the benefits of lower heat transfer losses with reduced thermal NO formation.…”

Influence of primary reference fuels on isobaric combustion in a heavy-duty optical diesel engine

“…Previous works on isobaric combustion employing multiple injectors revealed that using an optimized combination of side and central injectors resulted in higher gross indicated efficiency and lower soot emissions [22]. A recent numerical study [23] shows that by optimizing the side injectors' orientation and the number of holes in the nozzle, the vortical structures near the bowl-wall can be suppressed, leading to a reduction in the soot emissions.…”