Low-pressure Injection Research Articles

Research on the Internal Flow and Cavitation Characteristics of Petal Bionic Nozzles Based on Methanol Low-Pressure Injection

This paper aims to discuss the internal flow and cavitation characteristics of petal bionic nozzle holes under different injection pressures to improve the atomization effect of methanol. The FLUENT (v2022 R1) software is used for simulation. The Schnerr-Sauer cavitation model in the Mixture multiphase flow model is adopted, considering the evaporation and condensation processes of methanol fuel to accurately simulate cavitation and internal flow performance. The new nozzle hole is compared with the ordinary circular nozzle hole for analysis to ensure research reliability. The results show that the cavitation of the petal bionic nozzle hole mainly occurs at the outlet, which can enhance the atomization effect. In terms of turbulent kinetic energy, the internal turbulent kinetic energy of the petal bionic nozzle hole is greater under the same pressure. At 1 MPa, its outlet turbulent kinetic energy is 38.37 m2/s2, which is about 2.3 times that of the ordinary circular nozzle hole. When the injection pressure is from 0.2 MPa to 1 MPa, the maximum temperature of the ordinary circular nozzle hole increases by about 33.4%, while that of the petal bionic nozzle hole only increases by 12.3%. The intensity of internal convection and vortex is significantly reduced. The outlet velocity and turbulent kinetic energy distribution of the petal bionic nozzle hole are more uniform. In general, the internal flow performance of the petal bionic nozzle hole is more stable, which is beneficial to the collision and fragmentation of droplets and has better uniformity of droplet distribution. It has a positive effect on improving the atomization effect of methanol injection in the intake port of methanol-diesel dual-fuel engines.

Virtual Development of a Single-Cylinder Hydrogen Opposed Piston Engine

A significant challenge in utilizing hydrogen in conventional internal combustion engines is achieving a balance between NOx emissions and brake power output. A lean premixed charge (Lambda ≈ 2.5) allows for efficient and stable combustion with minimal NOx emissions. However, this comes at the cost of reduced power density due to the higher air requirements of the thermodynamic process. While supercharging can mitigate this drawback, it introduces increased complexity, cost, and size. An intriguing alternative is the 2-stroke cycle, particularly in an opposed piston (OP) configuration. This study presents the virtual development of a single-cylinder 2-stroke OP engine with a total displacement of 0.95 L, designed to deliver 25 kW at 3000 rpm. Thanks to its compact size, high thermal efficiency, robustness, modularity, and low manufacturing cost, this engine is intended for use either as an industrial power unit or in combination with electric motors in hybrid vehicles. The overarching goal of this project is to demonstrate that internal combustion engines can offer a practical and cost-effective alternative to hydrogen fuel cells without significant penalties in terms of efficiency and pollutant emissions. The design of this novel engine started from scratch, and both 1D and 3D CFD simulations were employed, with particular focus on optimizing the cylinder’s geometry and developing an efficient low-pressure injection system. The numerical methodology was based on state-of-the-art commercial codes, in line with established engineering practices. The numerical results indicated that the optimized engine configuration slightly surpasses the target performance, achieving 29 kW at 3000 rpm, while maintaining near-zero NOx emissions (<20 ppm) and high brake thermal efficiency (~40%) over a wide power range. Additionally, the cost of this engine is projected to be lower than an equivalent 4-stroke engine, due to fewer components (e.g., no cylinder head, poppet valves, or camshafts) and a lighter construction.

Coupling of the best-estimate system code and containment analysis code and its application to TMLB’ accident

With the development of advanced pressurized water reactor technology, the thermal-hydraulic coupling effect between the containment and the primary system becomes increasingly tight. In order to meet the demand for integrated safety analysis between the containment and the primary system, this paper investigates a direct coupling method between the best-estimate system code Advanced Reactor Safety Analysis Code and the containment analysis program ATHROC (Analysis of Thermal Hydraulic Response Of Containment). The feasibility of this direct coupling method and the applicability of the coupled program for overall safety analysis are demonstrated using Marviken two-phase flow release experiments. The ATHROC/ARSAC coupled program is employed to analyze the impact of the pressure relief function of the CPR1000 nuclear power plant pressurizer on the behavior of the primary system and containment during the TMLB’ accident. The calculation results indicate that these measures can reduce the pressure of the primary system to the level acceptable by the low-pressure injection system, but at the same time, they cause the pressure in the containment to rise to nearly 0.4 MPa. Therefore, to ensure the structural integrity of the containment, it is necessary for the non-passive hydrogen recombiner to effectively reduce the hydrogen concentration, thereby avoiding additional pressure increase in the containment due to hydrogen deflagration, which could lead to overpressure failure. The findings of this study are of significant reference value for improving the safety performance of thermal-hydraulic systems in operational Gen-II and advanced Gen-III pressurized water reactor nuclear power plants.

RPV top and bottom break SBLOCA with passive emergency core cooling system using ATLAS test facility

A SBLOCA test, named as C2.1 in the frame of the OECD/NEA ATLAS3 project, using a passive emergency core cooling system (PECCS) was performed by Korea Atomic Energy Research Institute (KAERI). In the test, two simultaneous 2-inch equivalent breaks occurred at top and bottom nozzles of the reactor pressure vessel of ATLAS test facility. The main purpose of the test is to investigate thermal hydraulic transient behavior in a reactor coolant system (RCS) during a SBLOCA scenario and to evaluate the effectiveness of PECCS. From the observation on thermal hydraulic phenomena during the test, the test period was divided into three characteristic phases such as pressure plateau, safety injection, and clad temperature re-increase phases. Continuous inventory loss of the primary loop through two break nozzles led to a primary system pressure decrease and a clad temperature increase. Even though high-pressure safety injection tanks (HPSITs) were actuated in an early stage of the pressure plateau phase, safety injection from HPSITs, however, were not actually injected. Safety injection flows from the HPSITs and SITs were practically injected to the downcomer only after the opening of automatic depressurization valves (ADVs). With a continuous decrease of a primary system pressure, low-pressure (LP) injection was started. However, LP injection could not change the increasing trend of the clad temperatures. Post-test calculations were performed using MARS-KS 1.4, and calculation results were compared with those of the test.

Experimental investigation on rehabilitation effectiveness of ductile RC beams with various seismic damage levels using dynamic and static-cyclic loading protocols

With respect to the seismic damage of a ductile reinforced concrete (RC) beam in an RC building structure with seismic design, since the functionality of the building must be recovered, the maximum strength and ductility of the member must be restored by repair or retrofit. Therefore, this work focuses on the effectiveness of the selected repair and retrofit methods in rehabilitating a ductile RC beam with a specified damage level. For this purpose, eight RC beam specimens are designed based on the seismic design requirements in ACI 318-19 to investigate their mechanical properties using the test. In the experiment, loading protocol tests of two types - traditional static-cyclic loading protocol and earthquake-then-cyclic loading protocol—are conducted to quantify the effectiveness of rehabilitation of the RC beam specimens with the selected repair or retrofit methods. The repair and retrofit methods include surface treatment, epoxy low-pressure injection, epoxy resin mortar coating, rebar planting and steel plate retrofitting. Finally, the test results related to the development of strength, stiffness and cracking are used to quantify the effectiveness of rehabilitation of the ductile RC beams with various seismic damage levels using the selected repair or retrofit methods.

High-pressure injection or low-pressure injection for a direct injection hydrogen engine?

The study delves into the potential application of the direct injection (DI) hydrogen engine as a carbon-neutral power source, exploring mixing and combustion dynamics across varied injection pressures. This research identifies an optimal injection strategy based on a 3-cylinder 4-stroke cycle turbocharged DI hydrogen engine crucial for enhancing the combustion behavior, significantly influenced by mixture formation. Numerical simulation results for a single cylinder show that rapid flame propagation hinges on a relatively rich mixture surrounding the spark plug. Surprisingly, low-pressure injection demonstrates superior combustion performance under the late-injection strategy, emphasizing the importance of an adjusted nozzle diameter to compensate for reduced flow rates. Furthermore, increasing the nozzle diameter from 0.8 mm to 3.0 mm with an optimal injection timing approaches the same combustion performance under the same injection pressure of 15 MPa. Moreover, low-pressure injection not only augments nozzle sealing and durability but also exhibits promising combustion efficiency and power generation under specific conditions. The study underscores the need for forthcoming research in low-pressure DI hydrogen engines, stressing the significance of high-performance injectors, emission mitigation technologies, and evolved combustion theories.

Saline-assisted fascial exposure (SAFE) technique to improve nerve-sparing in robot-assisted laparoscopic radical prostatectomy.

To provide a summary of our initial experience and assess the impact of the Saline-Assisted Fascial Exposure (SAFE) technique on erectile function (EF), urinary continence, and oncological outcomes after Robot-Assisted Laparoscopic Radical Prostatectomy (RALP). From January 2021 to July 2022, we included patients with a baseline Sexual Health Inventory for Men (SHIM) score of ≥17 and a high probability of extracapsular extension (ECE), ranging from 21% to 73%, as per the Martini etal. nomogram. A propensity score matching was carried out at a ratio of 1:2 between patients who underwent RALP + SAFE (33) and RALP alone (66). The descriptive statistical analysis is presented. The SAFE technique was performed using two approaches, transrectal guided by micro-ultrasound or transperitoneal. Its principle entails a low-pressure injection of saline solution in the periprostatic fascia to achieve an atraumatic dissection of the neural hammock. Potency was defined as a SHIM score of ≥17 and continence as no pads per day. At follow-up intervals of 6, 13, 26, and 52 weeks, the SHIM score differed significantly between the two groups, favouring the RALP + SAFE (P = 0.01, P < 0.001, P < 0.001, and P = 0.01, respectively). These results remained significant when the mean SHIM score was assessed. As shown by the cumulative incidence curve, EF rates were higher in the RALP + SAFE compared to the RALP alone group (log-rank P < 0.001). The baseline SHIM and use of the SAFE technique were independent predictors of EF recovery. The use of the SAFE technique led to better SHIM scores at 6, 13, 26, and 52 weeks after RALP in patients at high risk of ECE who underwent a partial NS procedure.

Performance Assessment of Pile Models Chemically Grouted by Low-Pressure Injection Laboratory Device for Improving Loose Sand

The complexity and partially defined nature of jet grouting make it hard to predict the performance of grouted piles. So the trials of cement injection at a location with similar soil properties as the erecting site are necessary to assess the performance of the grouted piles. Nevertheless, instead of executing trial-injected piles at the pilot site, which wastes money, time, and effort, the laboratory cement injection devices are essential alternatives for evaluating soil injection ability. This study assesses the performance of a low-pressure laboratory grouting device by improving loose sandy soil injected using binders formed of Silica Fume (SF) as a chemical admixture (10% of Ordinary Portland Cement OPC mass) to different (W/C) water/cement ratios (by mass materials) mixes. Trial grouting processes were executed to optimize the practical ranges of the operating factors of the laboratory device to obtain consistent grouted model pile samples. The paper examined the relations of the binders' W/C ratios with the densities, elasticity modulus (E), and Uniaxial Compression Stress (UCS) of the grouted piles. The investigation results show that as the binder W/C ratio rises, the grouted pile samples' dry density, E, and UCS values decrease. For the binder injected with a W/C ratio of one and 10% SF additive by weight of cement mass, the highest values of the grouted pile for density, E, and UCS were about 2.32 g/cm3, 23 MPa, and 2000 MPa, respectively. The UCS of the grouted pile proved that the binders' W/C ratios and the SF addition have an evident effect on the investigated factors of the grouted piles.

Fuel reactivity stratification assisted jet ignition for low-speed two-stroke ammonia marine engine

Ammonia (NH3) has gained much attention as a carbon-free fuel due to its great potential to reduce carbon emissions in the shipping industry. However, ammonia is difficult to use directly in existing marine engines due to its low flame propagation speed and high auto-ignition temperature. In this work, a new combustion mode for low-pressure injection low-speed two-stroke ammonia marine engines: Reactivity Stratification Assisted Jet Ignition (RSAJI) is proposed and explored. The results of the study show that the RSAJI mode is very promising. Especially at low and medium loads, the peak pressure can be increased by 27.3%, and NH3 and N2O emissions in the exhaust gas can be reduced by 87.1% and 99.7%. Two key parameters: direct injection timing and ammonia substitution rate (ASR) determine the reactivity stratification in the main-chamber, which directly affects the jet ignition effect and turbulent flame speed. A thorough analysis of engine performance shows that the injection timing should be appropriately far from the Top Dead Center (75°CA BTDC & 86.4°CA BTDC). Further increases in ammonia substitution rates (ASR = 90%, 95%) will result in a reduction in power performance gains but will improve the indicated thermal efficiency (>50%).

Combustion Process of the Compound Supply CNG Engine

Objective: In order to study the lean combustion process of a natural gas engine by separating the combustor, a spark ignition natural gas engine with separated combustors was retrofitted from a S195 single-cylinder diesel engine. Methods: The electronic control system controlled the gas supply and the spark plug ignition. A low pressure injection valve was set in the inlet pipe to form a lean mixture while a high pressure injection valve was placed in the subsidiary chamber to create a rich mixture, which was then ignited and injected into the main combustor, where the lean mixture was subsequently ignited again to achieve stratified combustion. Results: The test results showed that steady ignition is feasible in the system and verified the impact of the shape of the main combustor on HC, the impact of channel diameter on NOX production, and the impact of the ratios of high-pressure gas and low-pressure gas on HC and NOX. The combustion conditions of high-pressure gas and low-pressure gas in the engine combustor vary greatly. Our results signify that the shape of the main combustor has a great impact on the performance of the engine, that is, a shorter propagation distance can reduce the generation of HC. Conclusion: The best ignition advance angle under different conditions was determined using a spark ignition natural gas engine. The ratios of high-pressure gas and low-pressure gas greatly impact the performance and emission of the engine. The reduced diameter of the channels between the main and subsidiary combustors can enhance the stratification and facilitate the secondary ignition.

Investigation of Mold Filling Simulation, Segregation, and Rheological Properties in Low Pressure Injection Molding of Alumina Parts

Investigation of Mold Filling Simulation, Segregation, and Rheological Properties in Low Pressure Injection Molding of Alumina Parts

Evaluation of Trunk Injection Techniques for Systemic Delivery of Huanglongbing Therapies in Citrus

The bacterial pathogen associated with citrus huanglongbing (HLB) resides in the phloem of affected trees. The widespread abundance of the vector in Florida, the Asian citrus psyllid (Diaphorina citri), and the location of the pathogen in the tree vascular tissue limits the efficacy of foliar-applied therapies. Trunk injection is a crop protection strategy that applies therapeutic compounds directly into the tree vascular system, enabling their systemic distribution within the tree. However, limited information is available on the most effective methodology for implementing trunk injection at the commercial scale and the extent of damage inflicted by the injection. In this study, 5-year-old HLB-affected ‘Midsweet’ sweet orange (Citrus sinensis) trees were injected with the insecticide imidacloprid, the antibacterial oxytetracycline, or water. Injections occurred in Jun and Oct 2020 using three trunk injection techniques. Trees were monitored for external wounding and internal damage associated with injection, as well as tree health, bacterial titers, and yield for two production seasons. Low-pressure injection caused the least damage; however, it was less effective at delivering the tested compounds than medium- or high-pressure injection. Despite causing the greatest extent of external and internal damage at the injection site, injection of oxytetracycline significantly improved tree health, reduced bacterial titers, and increased yield in the two seasons of this study. Imidacloprid injection caused less wound damage but did not result in any lasting benefits to the trees. These results suggest that trunk injection of oxytetracycline could be an effective strategy for managing HLB and that the damage inflicted by this crop protection strategy can be reduced by selecting a suitable injection technique.

CFD investigation the combustion characteristic of ammonia in low-speed marine engine under different combustion modes

As a carbon-free hydrogen-carrying fuel, ammonia can effectively solve the problem of greenhouse gas emissions and has great application potential in marine engines. At present, low pressure and high pressure injection are the two main combustion modes of low-speed marine engine, and the former is closer to the Otto cycle, while the latter is closer to the Diesel cycle. In this study, a low pressure direct injection natural gas/diesel dual fuel engine was converted to an ammonia/diesel engine (LP). High pressure direct injection ammonia/diesel engine was converted by a diesel-only engine (HP). The application of ammonia was compared in above two combustion modes. Results show that using ammonia in HP mode can easier get high power, but LP’s IMEP, efficiency performance is better. However, LP mode has a large risk of ammonia escape (2.79%) and unburning (5.49%), and N2O in the emission is also worthy of attention and further research. In addition, due to the difference in N2O, the greenhouse effect of LP mode is also greater. For ignition, LP mode is not only affected by diesel injection, but also in-cylinder swirl and equivalent ratio due to the influence of premix mode. Currently, the HP mode has more potential for marine engine.

Numerical analysis of combustion process and pressure oscillation phenomena in low-pressure injection natural gas/diesel dual fuel low speed marine engine

Dual fuel strategy is a promising way to achieve low-carbon emission for marine engines, but engine knock restricts the compression ratio and engine thermal efficiency by adopting premixed combustion mode. Engine knock of natural gas/diesel dual fuel low speed marine engine is not well understood under high compression ratio. The characteristics of combustion process and pressure oscillation phenomena were investigated by varying compression ratio, pre-chamber number, energy proportion of pilot diesel with a single-cylinder engine model with numerical simulation. The results show that, with increase in pre-chamber number using the same energy proportion of pilot diesel under compression ratio of 16, the maximum amplitude of pressure oscillation can be significantly reduced from 0.62 MPa to 0.23 MPa in main chamber, while both resonance modes follow (1,0) mode. With double pre-chamber, when the energy proportion of pilot diesel increases from 0.6% to 1.2%, due to the stronger intensity of jet flame, the heat release rate is accelerated as well as the accumulation of H2O2 radical become less before high speed spontaneous combustion, resulting in lower amplitude of pressure oscillation. When the energy proportion of pilot diesel increases from 1.2% to 3.0%, excessive amount of pilot diesel cannot strengthen the jet flame obviously while the amplitude of pressure oscillation increases slightly. These results suggest that about 1.0% in energy proportion of pilot diesel should be optimal by considering the thermal efficiency, pressure oscillation and NOx emission together.

Three-Dimensional-Printed Molds from Water-Soluble Sulfate Ceramics for Biocomposite Formation through Low-Pressure Injection Molding.

Powder mixtures of MgSO4 with 5-20 mol.% Na2SO4 or K2SO4 were used as precursors for making water-soluble ceramic molds to create thermoplastic polymer/calcium phosphate composites by low pressure injection molding. To increase the strength of the ceramic molds, 5 wt.% of tetragonal ZrO2 (Y2O3-stabilized) was added to the precursor powders. A uniform distribution of ZrO2 particles was obtained. The average grain size for Na-containing ceramics ranged from 3.5 ± 0.8 µm for MgSO4/Na2SO4 = 91/9% to 4.8 ± 1.1 µm for MgSO4/Na2SO4 = 83/17%. For K-containing ceramics, the values were 3.5 ± 0.8 µm for all of the samples. The addition of ZrO2 made a significant contribution to the strength of ceramics: for the MgSO4/Na2SO4 = 83/17% sample, the compressive strength increased by 49% (up to 6.7 ± 1.3 MPa), and for the stronger MgSO4/K2SO4 = 83/17% by 39% (up to 8.4 ± 0.6 MPa). The average dissolution time of the ceramic molds in water did not exceed 25 min.

Stiffness Characteristics of Pile Models for Cement Improving Sandy Soil by Low-Pressure Injection Laboratory Setup

Soil improvement has developed as a realistic solution for enhancing soil properties so that structures can be constructed to meet project engineering requirements due to the limited availability of construction land in urban centers. The jet grouting method for soil improvement is a novel geotechnical alternative for problematic soils for which conventional foundation designs cannot provide acceptable and lasting solutions. The paper's methodology was based on constructing pile models using a low-pressure injection laboratory setup built and made locally to simulate the operation of field equipment. The setup design was based on previous research that systematically conducted unconfined compression testing (U.C.Ts.). The soil improvement techniques were investigated by injecting a low-pressure mixture of water and ordinary Portland cement (O.P.C.) with (0.8, 1, and 1.3) W/C ratios. The study revealed the relationship between pile model samples (U.C.Ts.) and W/C ratios. It also showed that the pile model samples' (U.C.Ts.) result decreased from 14 to 12 to 10 MPa, respectively, with an increase in W/C ratios from 0.8 to 1 and 1.3, respectively. Furthermore, the stiffness characteristics of a jet grouting column were calculated based on Mohr's Circles theory, and numerous theoretical approaches obtained the consequences of tensile strength.

Comparison between cement and chemically improved sandy soil by column models using low-pressure injection laboratory setup

Abstract The jet grouting method for soil improvement represents an innovative geotechnical alternative for problematic soils when the classic foundations’ designs cannot be appropriate, sustainable solutions for these soils. This study’s methodology was based on producing column models using a low-pressure injection laboratory setup designed and locally manufactured to approximate the field-equipment operation. The setup design was inspired by the works of previous researchers, where its functioning was validated by systematically performing unconfined compression tests (UCTs). Two soil improvement techniques were investigated, one by low-pressure injection of a mixture of water and ordinary Portland cement (OPC) with 0.8, 1, and 1.3 W/C ratios. The other type uses silica fume (SF) as a chemical additive with 10% of the cement weight added to the water and cement mix with 1, 1.3, and 1.6 W/C ratios. The study revealed that the UCT results of SF column model samples were higher than those of OPC with an equal W/C ratio. For each binder type, the UCT sample results increase with a decrease in the W/C ratio.

Simulation of Branch Pipes Fracture Condition in Low-Pressure Safety Injection System of PWR

In the low-pressure injection system (LHSI) of the PWR unit, two branch pipes of the recirculation pipeline are vulnerable to fracture during minimal-flow operation. Flowmaster was used to create a hydraulic model for the LHSI minimal-flow operation and to simulate the branch double-end shear fracture condition in order to predict the leakage flow volume and the water level drop rate of the refueling water storage tank (RWST) in order to analyze the impact of branch pipe fracture on the water leakage of the RWST. According to the study results, there is very little deviation between the operating condition of the system as it actually is and the Flowmaster model, and the leakage volume following branch pipe fracture is considerably less than that of the RWST. Leakage volume per minute is around 0.025% of the RWST’s overall volume, which has negligible influence on the unit’s safety. Based on the results of the simulation, this study suggests LHSI system operation recommendations.

Low Subcritical CO2 Adsorption-Desorption Behavior of Intact Bituminous Coal Cores Extracted from a Shallow Coal Seam.

This study focuses on improving fundamental understanding of low, subcritical CO2 adsorption-desorption behavior of bituminous coals with the aim to evaluate the utility of shallow-depth coal seams for safe and effective CO2 storage. Comprehensive data and a detailed description of coal-CO2 interactions, e.g., adsorption, desorption, and hysteresis behavior of intact bituminous coals at CO2 pressures <0.5 MPa, are limited. Manometric sorption experiments were performed on coal cores (50 mm dia. and 30- or 60-mm length) obtained from a 30 m deep coal seam located at the Upper Silesian Basin in Poland. Experimental results revealed that the adsorption capacities were correlated to void volume and equilibrium time under low-pressure injection (0.5 MPa). The positive deviation, observed in the hysteresis of adsorption-desorption isotherm patterns, and the increased sample mass at the end of the tests suggested CO2 pore diffusion and condensation. This behavior is vital for assessing low-pressure CO2 injection and storage capabilities of shallow coal seams where confining pressure is much lower than that of the deeper seams. Overall, CO2 adsorption depicts a type II adsorption isotherm and a type H3 hysteresis pattern of the IUPAC classification. Experimental results fitted better to the Brunauer-Emmett-Teller model than the Langmuir isotherm model. CO2 adsorption behavior of intact cores was also evaluated by characteristic curves. It was found that Curve I favored physical forces, i.e., the presence of van der Waals/London dispersion forces to describe the coal-CO2 interactions. However, analysis of Curve II indicated that the changing pressure-volume behavior of CO2 in the adsorbed phase, under low equilibrium pressures, cannot be ignored.

Experimental study of thermal efficiency and NOx emission of turbocharged direct injection hydrogen engine based on a high injection pressure

The direct injection (DI) hydrogen-fueled internal combustion engine (H2-ICE), as a promising carbon-neutral power device, has attracted more and more attention. However, the effects of the hydrogen injection strategy on the performance of H2-ICE lack in-depth analysis, even under the high injection pressure. Therefore, the present work investigated the thermal efficiency and NOx emissions with a single-hole injector over a wide range of high injection pressure and injection timing in a 3-cylinder turbocharged DI H2-ICE. The results show the strong dependence of high thermal efficiency on turbocharging and lean-burn strategies. The late-injection strategy can achieve higher thermal efficiency than the early-injection strategy, which can be applied at a wide range of loads. The effect of injection strategy on the brake thermal efficiency (BTE) mainly depends on the wall heat loss. Hydrogen late injection can reduce the wall heat loss by about 2% through strong mixture stratification. Furthermore, hydrogen late injection is always accompanied by a sharp increase in NOx emissions, even by orders of magnitude. This trade-off relationship between thermal efficiency and NOx emissions can be well optimized by a moderate late-injection strategy with an end-of-injection (EOI) of about 40 °CA. At the given load and start of injection (SOI), increasing the injection pressure from 5 MPa to 15 MPa can reduce the hydrogen injection duration by about 3 times. Under the higher injection pressure, the BTE performs a strong sensitivity to SOI. The BTE is improved by about 0.5% by optimizing SOI at PInj = 5 MPa mainly due to the reduced compression loss caused by further delay to the SOI allowed by higher injection pressure, while this improvement increases to about 2.3% at PInj = 15 MPa. However, at a given SOI, the highest BTE at PInj = 5 MPa is nearly 1.5% higher than that at PInj = 15 MPa, which implies the potential of low-pressure injection coupled with large-flow injectors. Finally, the DI H2-ICE performs well in terms of efficiency with a maximum BTE of 44.08%.