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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]

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]

Due to the common operating characteristics of medium duty (MD) gasoline engines, achieving higher load at the expense of fuel efficiency is not an acceptable tradeoff. EGR dilution-stoichiometric combustion can suppress the knocking and improve the thermal efficiency, but has problems such as slow combustion rate and poor combustion stability, which have limitations in optimizing combustion for MD gasoline engines with strong swirl. The experimental investigations in this study focus on the interaction of charge motion and EGR on anti-knock performance and thermal efficiency under different load conditions, and verifies the potential to improve the thermal efficiency of inclined swirl combustion system. The results show that at low-medium load, the tumble-dominated inclined swirl scheme can enhance the effectiveness of EGR in knock suppression, thus reducing the introduction of EGR to avoid its adverse effects on the combustion duration. At medium-high load, although the above benefit wears off, it can enhance the anti-knock performance and thus advance the spark timing by increasing the EGR tolerance. The EGR ratio corresponding to the highest thermal efficiency increases with higher load. The difference in the in-cylinder flow field will affect the combustion rate in a high EGR dilution and high load condition. The scheme adopting the compound intake ports and inverse cuneiform-shaped combustion chamber can improve both EGR tolerance and initial combustion rate under high EGR dilution, with the most significant improvement in thermal efficiency, and the improvement range will increase along with the increase of load. [ABSTRACT FROM AUTHOR]

The effects of additives tetrahydrofuran (THF) and ethanol on combustion, performance and emissions were researched on a six-cylinder turbocharged diesel engine with varying injection strategies. Four fuels were tested, i.e. pure diesel, T15E0, T5E10, and T5E20. Under low load, the maximum cylinder pressure of blended fuel decreased and that of T15E0 decreased with the maximum decrease of 3.35%, from 6.56 MPa to 6.34 MPa. The maximum cylinder pressure was also reduced by 4.24% at most under high load, from 9.90 MPa to 9.48 MPa. With the addition of tetrahydrofuran, the brake specific fuel consumption (BSFC) increased, and the brake thermal efficiency (BTE) decreased. In comparison to pure diesel, the addition of 20% ethanol increased BTE from 35.4% to 35.9%, which was a 1.34% rise. For various injection strategies, the optimized soot emissions of T5E20 decreased by 66.94%, from 0.00289 g/kW·h to 0.00096 g/kW·h, while the maximum increase ratio of NOx emissions was 9.67%, from 9.88 g/kW·h to 10.84 g/kW·h. T5E20 has the potential to increase BTE while decreasing NOx and soot emissions concurrently. The results indicate that tetrahydrofuran is a good cosolvent for diesel-ethanol blended fuel, and ethanol exhibits good performance as the principal component of the blended fuel. [ABSTRACT FROM AUTHOR]

Compressed air energy storage is one of the green energy storage and conversion systems. In this work, the compressed air was directly supplied to a stationary diesel engine to investigate the effects of compressed air characteristics on the process of thermal-work conversation in diesel engines. Then, the thermal efficiency of supplying compressed air to a diesel engine is compared against a simple hybrid system that consists of a compressed air engine and a baseline diesel engine. The results indicate that the compressed air affects the thermal work conversion of diesel engine. The gross indicated mean effective pressure is being used to quantify the in-cylinder pressure work that transferred to the piston motion only for the combustion and expansion strokes, which increases as the pressure of compressed air increases and decreases as the temperature of the compressed air increases. The gross indicated thermal efficiency that represents the ability of thermal-work conversion is maximum at intake pressure of 300 kPa and intake temperature of 303 K with a value of 44.3%, which is 19.7% higher than that of the baseline diesel engine with 100 kPa intake pressure and 303 K intake temperature. The diesel engine using compressed air shows 6.64% higher overall efficiency than the simple hybrid system. The soot emissions increase with increasing intake pressure, and decrease with increasing intake temperature. As the intake temperature increases, NOx emissions increase. As the intake pressure increases, NOx emissions first decrease and then increase. [ABSTRACT FROM AUTHOR]