Compared to conventional diesel combustion (CDC), isobaric combustion can achieve a similar or higher indicated efficiency, lower heat transfer losses, reduced nitrogen oxides (NOx) emissions; however, with a penalty of soot emissions. While the engine performance and exhaust emissions of isobaric combustion are well known, the overall flame development, in particular, the flow-field details within the flames are unclear. In this study, the performance analysis of CDC and two isobaric combustion cases was conducted, followed by high-speed imaging of Mie-scattering and soot luminosity in an optically accessible, single-cylinder heavy-duty diesel engine. From the soot luminosity imaging, qualitative flow-fields were obtained using flame image velocimetry (FIV). The peak motoring pressure (PMP) and peak cylinder pressure (PCP) of CDC are kept fixed at 50 and 70 bar, respectively. The two isobaric combustion cases, achieved using multiple injections, are maintained at the CDC PMP level of 50 bar for the low-pressure case (IsoL) and CDC PCP level of 70 bar for the high-pressure case (IsoH). For each operating condition, soot luminosity signals are captured at a frame rate of 20 kHz, and a semi-quantitative velocity flow-field is obtained from FIV postprocessing. Consistent with previous metal engine experiments, isobaric combustionin particular IsoH, resulted in similar gross indicated efficiency, lower heat losses but higher exhaust losses, compared to CDC. The soot luminosity images of CDC show initial signals originated close to the bowl-wall for certain jets while for the isobaric combustion, the flames corresponding to each jet are clearly distinguished during the earlier flame development process. The vector field distribution within the flames shows the transition of flame-wall impingement to flame-flame interaction regions between the neighboring jets for each combustion mode. Furthermore, higher flame-flame interaction regions and uniform distribution of signals around the combustion chamber for isobaric combustion, justifying higher soot formation and lower heat transfer losses, respectively, compared to CDC.

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.

Heavy-duty vehicles face increasing demands of emission regulations. Reduced carbon-dioxide (CO2) emission targets motivate decreased fuel consumption for fossil fuel engines. Increased engine efficiency contributes to lower fuel consumption and can be achieved by lower heat transfer, friction and exhaust losses. The double compression expansion engine (DCEE) concept achieves higher efficiency, as it utilizes a split-cycle approach to increase the in-cylinder pressure and recover the normally wasted exhaust energy. However, the DCEE concept suffers heat losses from the high-pressure approach. This study utilizes up to three injectors to reduce the wall-gas temperature gradient rendering lower convective heat losses. The injector configuration consists of a standard central injector and two side-injectors placed at the rim of the bowl. An increased distance from side-injector to the wall delivered lower heat losses by centralizing hot gasses in the combustion chamber. Computational fluid dynamics (CFD) simulations investigated two different piston bowls, in a heavy-duty diesel engine, to obtain in-cylinder conditions for one, two -and three-injector concepts. One-dimensional (1D) simulations then used the CFD data to obtain the complete efficiency analysis of the DCEE concept. The results showed that the three-injector case improved the brake thermal efficiency and reduced the heat transfer losses, compared to the two-injector and single-injector cases. In particular, the three-injector case resulted in a high indicated and brake thermal efficiency of 58.5 % and 54.2 %, respectively.

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Compared to conventional diesel combustion (CDC), isobaric combustion can achieve higher thermal efficiency while lowering heat transfer losses and nitrogen oxides (NOx). However, isobaric combustion suffers from higher soot emissions. While the aforementioned trends are well established, there is limited literature about the high-temperature reaction zones, the liquid-phase penetration distance, and the flame tip propagation velocity of isobaric combustion. In the present study, the line-of-sight integrated imaging of Mie-scattering, combustion luminosity, and CH* chemiluminescence were conducted in an optically accessible singlecylinder heavy-duty diesel engine. The engine was equipped with a flat-bowl-shaped optical piston to allow bottom-view imaging of the combustion chamber. The experiments were conducted using nheptane fuel for CDC and isobaric combustion modes. The peak cylinder pressure (PCP) and the fuel mean effective pressure (Fuel MEP) for both combustion modes are kept as 70 bar and 19 bar, respectively. For a given combustion mode, flame image velocimetry (FIV) analysis was performed on the consecutive combustion luminosity image pairs to obtain in-flame flow-field details, and the liquid-phase penetration distance and flame tip propagation velocity were also estimated. The combustion luminosity and FIV analysis show that compared to CDC, isobaric combustion has resulted in less prominent flame-flame interactions, which explained the lower heat release rate. Double-injection-based CDC has resulted in a shorter spray penetration distance compared to triple-injection-based isobaric combustion; however, shorter than the boundary of piston-bowl wall. In addition, it was found that CDC has a sudden increase in flame tip propagation velocity preceded by flame propagation due to flame-piston bowl impingement from the second injection and successive movement towards the squish region. However, isobaric combustion showed a trend of constant or slightly decreased flame tip propagation velocity for which the second injection flames remain within the piston bowl wall.