• Energy Research

Abstract The combination of distinctive physicochemical properties of biofuels and recent engine technologies offer benefits in terms of efficiency improvement and emissions reduction. The recent development in butanol bio-based production pathways encourage researchers to study their combustion characteristics. This paper experimentally evaluates the combustion and emissions of a gasoline direct injection (GDI) engine fueled with the butanol isomer/gasoline surrogate blends, in which primary reference fuel (PRF) and toluene primary reference fuel (TPRF) are selected as the gasoline surrogates, respectively. The flame propagation behaviors, in-cylinder pressure, apparent heat release rate, along with PN emissions from this optical GDI engine are discussed. First, butanol addition to the gasoline surrogates is found to slow down flame propagation, reduce peak cylinder pressure and heat release rate, and extend ignition delay and combustion duration. Further, among the four butanol isomers, n-butanol and tert-butanol are the most and least reactive fuels, respectively, whereas iso-butanol and sec-butanol show reactivities in between, as supported by the measured flame propagation and pressure traces, calculated heat release rates, as well as time scales describing the combustion progress, e.g. ignition delay and combustion duration. Finally, butanol addition reduces the PN emissions from the GDI engine, and the PN emissions reduction capacity of the four butanol isomers ranks as sec-butanol > iso-butanol > n-butanol > tert-butanol. Also, compared to the PRF/butanol blends, the PN emissions are reduced to a less extent when butanol is blended with TPRF.

Abstract The continuing advancement of high speed, combustion diagnostics calls for mathematical techniques that can extract key information from large datasets. This paper therefore describes a case study to compare the characterization of combustion dynamics behind a V-gutter flame holder using three mathematical methods: Fourier analysis, principal component analysis, (PCA), and wavelet analysis (WA). The comparison focuses on the analysis of the characteristic frequencies of flow–flame interactions, with a particular emphasis on the analysis of transient and unsteady combustion procedures, such as lean blow off. Experimental data obtained under a range of conditions were analyzed using all three methods, and several observations were made. When applied to the analysis of stable combustion processes, all three methods reported frequency characteristics that were similar both quantitatively and qualitatively. Under unstable and transient combustion conditions, the WA method is capable of revealing the dynamics of the frequency components in the measurements, while traditional Fourier and PCA methods encounter application restrictions. Lastly, these applications also demonstrated WA’s suitability for practical combustion measurements beyond chemiluminescence, such as its applicability to discrete signals, insensitivity to the choice of wavelet basis, and insensitivity to the target signal extracted from the raw measurements.

Abstract Flash boiling sprays have been widely studied recently because of its potential in generating fuel sprays with improved atomization efficiencies. Due to the strong multiphase influence, the exact spray atomization mechanisms of flash boiling sprays are not well understood yet, especially in the near-field where the optical depth is usually high for optical measurements. This investigation utilized a two-hole gasoline direction injection fuel injector to analyze the near-field atomization performance of a single spray plume under flash boiling conditions with a lower injection pressure to avoid the interference from dense sprays under higher injection pressures. High-speed backlit and Schlieren techniques were used to capture the liquid phase and gas phase distributions in the near-field and a microscope was incorporated to obtain detailed structure information of the fuel spray near the nozzle exit. Boundary conditions including fuel temperatures and ambient pressures were varied to study fuel atomization under different superheat indices and Cavitation numbers. Plume characteristics at different stages of the injection were investigated and significant multiphase influence can be found within the transitional flash boiling regime, including a two-zone atomization behavior at the beginning of the injection, a liquid skirt structure during the fuel injection, and a fishnet ligament pattern at the end of injection, respectively. This work also examined the impact of Cavitation number, superheat index, and ambient pressure on the properties of the external spray and such influence was represented by the near-field cone angle defined from near-field measurements.

Abstract Direct injection spark ignition (DISI) engines have been widely used in passenger cars due to their lower fuel consumption, better controllability, and high efficiency. However, DISI engines are suffering from wall wetting, imperfect mixture formation, excess soot emissions, and cyclic variations. Applying a new fuel atomization technique and using biofuels with their distinctive properties can potentially aid in improving DISI engines. In this research, the effects of isobutanol and 2-butanol and their blends with Toluene Primary Reference Fuel (TPRF) on spray characteristics, DISI engine combustion, and particle number (PN) emissions are investigated for conditions with and without flash boiling of the injected fuel. Spray characteristics are investigated using a constant volume chamber. Then, the combustion, flame propagation, and PN emissions are examined using an optical DISI engine. The fuel temperature is set to 298 K and 453 K for liquid injection and flash boiling injection, respectively. The tested blending ratio is 30 vol% butanol isomers and 70 vol% TPRF. The results of the spray test reveal that liquid fuel plumes are distinctly observed, and butanol blends show a slightly wider spray angle with lower penetration length compared to TPRF. However, under flash boiling injection, the sprays collapse towards the injector axis, forming a more extended single central vapor jet due to the plumes' interaction. Meanwhile, butanol blends yield a narrow spray angle with more extended penetration compared to TPRF. The flame visualization test shows that the flash boiling injection reduced yellow flames compared to liquid fuel injection, reflecting the improvements in mixture formation. Thus, improvements were noted in the heat release and PN emissions. Butanol addition reduced the PN emissions by 43% under regular liquid injection. Flash boiling injection provided an additional 25% reduction in PN emissions.

Abstract Flash boiling can enhance fuel breakup and atomization via the quick growth and eruption of bubbles when the high temperature fuel exiting the nozzle. For multi-hole fuel injectors, a high level of flash boiling might cause the plumes to merge into a single plume. Such phenomenon is known as spray collapse, which will change the preset spray targeting in real engines, and is not desirable for practical application due to a longer penetration it might cause. Although extensive works have been done on the collapse of flash-boiling, no consensus has been reached for its core mechanisms yet. This work used a two-hole injector to study the flash-boiling plume-to-plume interaction, which is considered to be a key inducement of spray collapse. Optical measurement approaches were used to examine the influence of fuel temperature, ambient pressure, and injection pressure on the morphology of the flash boiling spray. It was found that a secondary plume was generated by two primary plumes side-collision in the spray central region under some flash-boiling conditions, and its behavior had a clear relation with the level of flash-boiling and other boundary conditions. Additionally, elevating injection pressure and increasing ambient pressure are two doable options for controlling the flash-boiling targeting in real engines, through which non-collapse flash-boiling sprays can be obtained. Finally, the relation between the secondary plume and the spray collapse was clearly demonstrated, and possible collapse mechanism was discussed in details.