Experimental Study on the Effects of Spray–Wall Interaction on Partially Premixed Combustion and Engine Emissions

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Pre-chamber combustion (PCC) is a promising engine combustion concept capable of extending the lean limit at part load. The engine experiments in the literature showed that the PCC could achieve higher engine thermal efficiency and much lower NOx emission than the spark-ignition engine. Improved understanding of the detailed flow and combustion physics of PCC is important for optimizing the PCC combustion. In this study, we investigated the gas exchange and flame jet from a narrow throat pre-chamber (PC) by only fueling the PC with methane in an optical engine. Simultaneous negative acetone planar laser-induced fluorescence (PLIF) imaging and OH* chemiluminescence imaging were applied to visualize the PC jet and flame jet from the PC, respectively. Results indicate a delay of the PC gas exchange relative to the built-up of the pressure difference (△P) between PC and the main chamber (MC). This should be due to the gas inertia inside the PC and the resistance of the PC nozzle. The PC jet can be either continuous or intermittent depending on the △P and pressure fluctuation amplitude. Distinct PC jet with low speed is witnessed after 15° CA ATDC, which could account for the postcombustion of the PCC engine in the literature. The probability distribution analysis of the PLIF and OH* images presents a much longer penetration length of the PC jet than that of the flame jet. This means that the flame jet resides in an atmosphere of the unburned gas mixture from the PC when it appears in the MC. The flame jet and PC jet show longer penetration length and become more stable with the enriching of the prechamber charge from lean to stoichiometric. However, the overall PC jet characteristics regarding the penetration length and probability distribution become less sensitive to the PC global excess air ratio (λ) when the PC charge is close to stoichiometry.

Section: Experimental Setupsmentioning

confidence: 68%

Simultaneous Negative PLIF and OH* Chemiluminescence Imaging of the Gas Exchange and Flame Jet from a Narrow Throat Pre-Chamber

Tang¹,

Sampath²,

Marquez³

et al. 2020

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“…Thus, these two parts are corrected separately based on the grid-mapping using MATLAB image processing toolbox. A similar correction procedure can be found in our previous work [14] and the works of Wang et al [15] and Miles et al [16]. Finally, the two parts of the distortions-corrected images were combined and a mask with the piston top and bowl boundary was applied to the image to show the spatial relationship between the fuel jet and the piston bowl.…”

Section: Fuel-tracer Plif Imagingmentioning

confidence: 96%

Optical Study on the Fuel Spray Characteristics of the Four-Consecutive-Injections Strategy Used in High-Pressure Isobaric Combustion

Tang

Sampath

Sharma

et al. 2020

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High-pressure isobaric combustion used in the double compression expansion engine (DCEE) concept was proposed to obtain higher engine brake thermal efficiency than the conventional diesel engine. Experiments on the metal engines showed that four consecutive injections delivered by a single injector can achieve isobaric combustion. Improved understanding of the detailed fuel-air mixing with multiple consecutive injections is needed to optimize the isobaric combustion and reduce engine emissions. In this study, we explored the fuel spray characteristics of the four-consecutiveinjections strategy using high-speed imaging with background illumination and fuel-tracer planar laser-induced fluorescence (PLIF) imaging in a heavy-duty optical engine under non-reactive conditions. Toluene of 2% by volume was added to the n-heptane and served as the tracer. The fourth harmonic of a 10 Hz Nd:YAG laser was applied for the excitation of toluene. The PLIF image distortion caused by the side window curvature and the optical piston was mitigated using a correction lens and corrected with a grid mapping technique. The effects of hydraulic delay and injection dwell on the in-cylinder liquid-phase fuel penetration and vapor-phase fuel distribution were evaluated under different combinations of the four direct injections. The high-speed imaging of the liquid-phase spray shows that a short injection dwell reduces the hydraulic delay of the injector, resulting in an increase in both the peak liquid-phase penetration length and the injection duration. The fuel-tracer PLIF imaging clarifies the spatial fuel distribution of the four consecutive injections involved with the interaction between the vapor-phase spray and the piston bowl wall and the squish region. The intensity distribution in the PLIF images confirms that a longer injector hydraulic delay leads to a shorter peak vapor-phase spray penetration length and a reduced flow rate.

“…C+S, S+C and S+S are shown in the first, second, and third row of Figure 5, respectively. A detailed procedure about the bottom view distortion correction method applied to a standard ω-shape piston in the same engine is explained in the previous study [37]. The selected images shown in Figure 5 are from the 25th fired cycle of skip-firing mode, which is a closer representative cycle to the averaged pressure traces of isobaric combustion.…”

Section: High-speed Soot Luminosity Imagesmentioning

confidence: 99%

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

Panthi¹,

Goyal²,

Houidi³

et al. 2021

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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.