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The fuel injection rate (ROI) is a crucial factor that affects the combustion and emissions of diesel engines. This study focuses on the injection pressure in a common rail system, which is divided into a high-pressure section and a low-pressure section. A control-oriented ROI shaping (ROIs) model is developed based on the switching strategy between high and low injection pressure. Three types of ROI were generated, namely ROIB (conventional ROI), boot-ROI (low followed by high injection pressure), and anti-boot-ROI (high followed by low injection pressure) respectively. The 1-D and 3-D numerical simulations are conducted to analyze the impact of the shaped ROI on combustion and emissions for steady condition and transient condition. In terms of overall results, boot-ROI shows significant advantage among the three types of ROI. For the steady condition, the boot-ROI was able to increase the IMEP (indicated mean effective pressure) (1.57 bar) at high load conditions with almost unchanged NOx emission. For low load conditions with delayed SOI (start of injection), the exhaust temperature is close to that of the ROIB with a reduction of 0.51 g/kW·h in NOx emissions. For transient condition, the boot-ROI also shows its advantage. It was found to improve the BSFC (brake specific fuel consumption) with almost unchanged NOx emission during load-down process. And in load-up process, the BSFC and soot emission also could be improved with slightly increase in NOx emission through advance of SOI when boot-ROI was adopted. The one-dimensional model using boot-ROI reduces fuel consumption by 2 g/kW·h in experiment with WHTC cycles, with slightly higher soot emission and similar NOx emission. [ABSTRACT FROM AUTHOR]

Methanol, as a carbon-neutral resource, exhibits significant potential in the automotive sector. The objectives of this study are to investigate the fuel economy, emission characteristics, and combustion properties of methanol in compression-ignition engines. The research findings indicated that the combustion duration of methanol significantly extended with increasing speed and load. In comparison to diesel, methanol exhibited a relatively lower peak heat release rate under various operating conditions. With an increase in injection pressure or an advancement in injection timing, the brake specific fuel consumption (BSFC) decreased initially and then increased. The optimal injection pressure for methanol was lower than that for diesel, while the optimal injection timing was advanced. As the EGR rate increased, BSFC gradually decreased at each speed. When comparing optimized methanol with diesel, the brake thermal efficiency (BTE) of methanol was 37.54%, slightly lower than that of diesel. Methanol's NOx, soot, and CO emissions were significantly lower than diesel, while methanol and formaldehyde emissions were higher, especially with a methanol emission of 0.83 g/kW.h. Overall, applying pure methanol as fuel in compression-ignition engines can significantly reduce emissions while maintaining the BTE as compared with that of diesel fuel, but the methanol emissions in the exhaust increase significantly. [ABSTRACT FROM AUTHOR]

Liquid ammonia is a promising clean fuel, but it has a greater tendency to flash-boil than conventional fuels. This study confirms the reliability of utilizing the Eulerian framework and homogeneous relaxation model for simulating flash-boiling liquid ammonia sprays by comparing the simulation results with experimental data. The characteristics of single-hole superheated liquid ammonia sprays are analyzed for different orifice aspect ratios and superheat degrees. The results indicate that superheat degree and orifice aspect ratio affect the external spray morphology by affecting the two-phase flow inside the orifice. The strong phase transition due to flash-boiling drives the spray expansion process and increases the velocity. Moreover, the extensive bubble breakup in flash-boiling sprays promotes faster atomization. However, flash-boiling also results in a more pronounced temperature drop in the spray zone. This research can provide valuable insights for future simulations and engineering practices involving flash-boiling ammonia sprays. [ABSTRACT FROM AUTHOR]

This work investigates the ignition and flame development processes of low reactivity fuel combustion under compression ignition conditions based on the large eddy simulation approach. The chemical explosive mode analysis (CEMA) is employed to characterize the local combustion features, including gas-liquid fuel zone, auto-ignition, diffusion-assisted, extinction, cool flame and post-ignition zone, among which auto-ignition and post-ignition are found to play a key role in the overall heat release process. The local flame propagation modes in gasoline compression ignition (GCI) are determined by quantifying the relative magnitude of diffusion/chemistry at a representative progress variable in the pre-ignition zone. The results show that autoignition fronts and deflagration waves exist simultaneously in the ignition and intense high temperature heat release (HTHR) stages, but autoignition fronts dominate. In addition, the chemical kinetic processes of four heat release periods are analyzed. The heat release during the ignition period is found to be dominated by the reactions CH3+ H (+M) <=> CH4 (+M) and CH3CHO + H <=> CH2CHO + H2. The reaction CH2OH + OH <=> CH2O + H2O always plays an important role in the heat releases during the other three combustion stages including intense HTHR, moderate HTHR and post-combustion. [ABSTRACT FROM AUTHOR]