Intake Air Flow Research Articles

NUMERICAL AND EXPERIMENTAL RESEARCH ON THE DI-CI ENGINE PERFORMANCE CHARACTERISTICS UNDER LPG - DIESEL DUAL FUEL COMBUSTION MODE

Properties of Liquefied Petroleum Gas (LPG) are suitable as an alternative fuel for internal combustion engines. This work aims to combine both numerical optimization analysis of LPG injector placement angle and subsequent experiment with selected mounting injector angle to investigate the effects of using LPG as a parallel-partial substitute fuel with diesel fuel under dual-fuel combustion mode. An electronic injection fuel control system was applied to the modified intake manifold to generate the appropriate LPG injection. Three mounting angle injector positions including 450, 900 and 1350 in the upstream direction of intake air have been defined by Ansys Fluent to take into account efficient LPG-air mixing. Then, the experiment was conducted with LPG-diesel dual-fuel combustion mode (DFC mode) and compared to entirely diesel combustion mode as baseline data. Different load conditions ranging from idle to 4.0 kW were imposed at a constant engine speed of 1700rpm. The obtained results revealed that the injector’s mounting angle position by 45 opposite to the intake air flow showed the correlation with the calculated LPG-air ratio in dual fuel combustion reaction. In DFC mode, the brake thermal efficiency (BTE) decreased on average by 4.6% and the LPG substitution rate gradually decreased while brake-specific fuel consumption (BSFC) increased for all engine loads. The exhaust temperature in the dual-fuel combustion mode was found to be higher than that of full diesel fuel mode at low loads (less than 2.5 kW) and began to decrease at higher loads.

Study of indoor local thermal comfort for impinging jet ventilation combined with ductless personalized ventilation under different cooling loads

Excessive temperature stratification and draught are two prominent drawbacks for impinging jet ventilation (IJV). Using IJV together with ductless personalized ventilation (DPV) (i.e., IJV + DPV) has been proved to be an effective way to reduce temperature stratification, but the draught rate around occupant would be increased. In the current work, an in-depth study on the characteristics of indoor local thermal comfort for IJV + DPV operating with different scenarios is done. Predictive models for the local thermal comfort indicators are developed to optimize the operating parameters (including supply velocity of IJV (Vs) and intake airflow rate of DPV (Df)) with different room cooling loads (Q). The results show that the application of IJV can be extended to spaces with high cooling loads (e.g., larger than 60 W/m2) by using together with DPV, but the risk of thermal discomfort increases. To avoid local thermal discomfort, the combination relationship between Vs and Df should be properly matched and this relation becomes more stricter with the increase of Q. It also obtains that the maximum cooling load that IJV + DPV can eliminate is less than 170 W/m2. At last, to ensure the IJV + DPV create a good local thermal comfort environment, design chart about the feasible ranges of Vs and Df are established for different values of Q. On the whole, the current study not only can provide a basis for the application of IJV + DPV in spaces with high cooling load, but also can provide theoretical guidance for the operation control and optimization design of the IJV + DPV.

Increasing the Efficacy of an Air Conditioning Unit by Utilizing Phase Change Material With Cylindrical Configuration

ABSTRACTThe goal of the current study is to determine how the SST and the standard turbulence models prediction on PCM with cylindrical configuration affect AC performance and PCM discharging when coupled with an AC unit. For simulation, 308.15 K and 318.15 K, the inflow air temperature has been considered with a fixed 33.6 L/s intake air flow rate. The low outside temperature charges the PCMs during the night. During the daytime, heated ambient air is cooled by the PCM heat exchanger before passing over the unit condenser. The present outcomes show that using the standard model, the cylindrical PCM has the lowest time of complete melting. The temperature contours demonstrate that turbulence occurs, particularly at higher temperatures, in the PCM melting zone within the solid region. This implies that there is increased convection in this area. The maximum improved percentage in COP increases as the rising input air temperature for both turbulence models increases. The average power saving of AC at 308.15 K of an input air temperature for 83.33 min is predicted by both the standard and the SST to be 14.0905 W and 14.1089 W, respectively.

Comparative Study of ANN and SVM Model Network Performance for Predicting Brake Power in SI Engines Using E15 Fuel

Currently, artificial neural networks (ANNs) and support vector machines (SVMs) are the most common applications of machine learning approaches. In this study, a comparative study of ANN and SVM is presented to evaluate the performance of each model in predicting the brake power (BP) of GX35-OHC 4-stroke, air-cooled, single cylinder gasoline engine with E15 (15% ethanol + 85% gasoline) fuel. Two models are compared based on experimental dataset that has single output (BP) and five inputs, engine speed (S), engine torque (T), intake air temperature (Tair), intake air flow (Qair), and fuel consumption (ṁ). The samples were split into three sets: Training set (70%), Validation set (15%), and the Test set (15%) based on 60 samples. The results are compared through different graphs such as target vs actual values, regression plots, histograms of prediction errors, residual plots, learning curves, and error distributions. The results showed that SVM model is indicated to have lower RMSE (0.0044) and higher EVS (0.9953), while ANN is shown to have lower value of MAPE (1.51%). These results have significant implications for the use of ANN and SVM models in real-world applications that need gradual comprehensibility and model generalization. In addition, work done with the models outlined above should try and test them in other engines and operating conditions to demonstrate the model’s and performance.

Modelling the evaporative cooling effect from methanol injection in the intake of internal combustion engines

Renewable methanol is one of the most promising alternative fuels for internal combustion engines. However, its much higher latent heat of vaporization compared to traditional fossil-based hydrocarbon fuels poses new challenges. On both spark-ignition (SI) engines and dual-fuel (DF) engines, methanol can be introduced through injectors installed in the intake path, with its evaporation then causing a cooling effect to the intake air flow. While this is beneficial in mitigating knock with both SI and DF operations, it could potentially lead to cold-starting issues in SI engines and incomplete combustion in DF engines. To properly model the in-cylinder behaviour, the mixture temperature after methanol injection needs to be accurately predicted. A simple yet effective methanol evaporation model based on the liquid droplet and film evaporation mechanisms is thus proposed here to quantify the evaporative cooling effect from methanol injection. The model first treats the methanol spray as a group of droplets with identical size, and after reaching the wall the model assumes that the remaining methanol forms a thin liquid film. The evaporation rates and the consequent temperature drops of these two modes are calculated separately. The calculation results indicate that only a negligible amount of methanol evaporates as droplets, with the predicted temperature drops agreeing well with the validation datasets when taking only the film evaporation into account. The key factor for this good agreement is that the balance between the heat and mass transfer needs to considered when evaluating the surface temperature of a liquid methanol film. The proposed model also suggests that the droplet evaporation can be greatly improved with an injection angle nearly parallel to the air flow, or a finer initial droplet size. Both measures are more effective than increasing the air temperature before injection.

Study on the active regeneration characteristics of DPF in diesel-methanol dual-fuel engine under different exhaust oxygen concentrations

This study proposes a method to control exhaust oxygen (O2) concentration by adjusting the intake throttle valve, achieving intake throttling, and reducing intake airflow. The impact of exhaust O2 concentration at the inlet of the diesel oxidation catalyst (DOC) the diesel particulate filter (DPF) performance during active regeneration in diesel engines is investigated. Experiments are conducted under methanol substitution rates (MSR) of 0% and 20% to investigate how different exhaust O2 concentrations affect the performance of the DOC and DPF during active regeneration. A comparative analysis of fuel consumption and cumulative CO2 emissions during DPF active regeneration under different exhaust O2 concentrations is performed. Research results indicate that decreasing exhaust O2 concentration reduces exhaust flow, raising DOC inlet temperature. This enhancement improves catalytic conversion efficiency, accelerating the rise in DPF inlet temperature. The average conversion efficiency of DOC to unburned methanol exceeds 98.07%. After initiating DPF active regeneration, O2 concentration sharply drops, leading to elevated CO2 emissions. As soot combustion advances, the required O2 diminishes, resulting in a gradual rise and eventual stabilization of O2 emissions. Economically and environmentally, irrespective of MSR (20% or 0%), there exists an optimal exhaust O2 concentration that minimizes effective regeneration time during DPF active regeneration, achieving minimal fuel consumption and the lowest CO2 mass emissions. This study offers theoretical support for optimizing energy-saving and emission-reduction processes during DPF active regeneration.

Optimizing air inlet designs for enhanced natural ventilation in indoor substations: A numerical modelling and CFD simulation study

This paper investigates the ventilation and heat dissipation performance of a 110 kV indoor substation under natural ventilation conditions using computational fluid dynamics (CFD). The objectives are to evaluate the influences of air inlet design parameters including location and size, and transformer load, on the airflow distribution, temperature field, and cooling efficiency. The study finds that staggered opposite inlets optimize cooling uniformity without airflow attenuation. Compared to a single inlet, the maximum transformer temperature is reduced by 1.3 °C and energy utilization increases by 9.1 % with staggered inlets. Increasing the inlet length ratio initially improves cooling until an optimal point (The length ratio is 1.10), while reducing the inlet height ratio decreases airflow and efficiency. With load increasing, the intake airflow rises but at a reduced rate, and the temperature difference can exceed 15 °C under high loads. In summary, optimizing inlet design enhances natural ventilation performance in indoor substations, but limitations exist at high loads, indicating supplemental mechanical ventilation may be required.

A study of swirl flow measurements in a cylinder under an intake flow similar to an engine operating condition using a rotating slit disk valve

A new swirl measurement system was developed by installing bevel gears and a slit rotary valve into an impulse- type swirl measurement system of the type traditionally used to measure the swirl ratio and flow coefficient of engine intake ports. When operating this new swirl measurement system, it was confirmed that the characteristics of the intake air flow rate to the cylinders were similar to those of a typical operating engine condition. When the valve lift was limited to the cam angle range of 160 to 200 (the valve lift is maximized at a cam angle of 180°), the flow coefficient Cf increased as the cam angle increased under a constant camshaft rotation speed. Moreover, as the rotation speed of the camshaft increased, the Cf value decreased slightly. The swirl ratio NR in the cam angle range of 160 to 200 showed a nearly constant value with an increase of the cam angle at a constant camshaft rotation speed. There were also no significant changes in Cf with an increase in the camshaft rotation speed. NR measurement results while changing the camshaft rotation speed cannot be obtained by the traditional impulse swirl measurement method. NR measurement results with the new swirl measurement system can be used as basic data when calculating spray dispersion characteristics considering the engine rotation speed.

Simulation and experimental research on the optimization of airflow organization and energy saving in data centers using air deflectors

The airflow organization of the data center directly affects the temperature control performance and the energy efficiency of the cooling equipment. The servers at the bottom of the rack usually suffer from insufficient airflow rate and poor cooling effect. This is because of the limited distance between the bottom servers and the perforated floor, and the small horizontal velocity of the supply air flow. This study aims to improve the uniformity of the cooling airflow in the vertical direction of the rack by the air deflectors, thereby further improving the overall airflow organization in the data center. The size and installation of the deflectors in the data center were optimized according to both the experiment and numerical simulation results. From the results, it is recommended to install the deflector with a width of 100 mm at an angle of 45° under the perforated floor for the rack with the single-side airflow supply. For the rack with the double-side airflow supply, the width of the deflector should be 100 mm and installed at an angle of 30° to the perforated floor to achieve the best airflow distribution. Consequently, the intake airflow rate for the bottom servers significantly increased, and the occurrence of the local hot spots was effectively reduced. The numerical simulation of the airflow organization with and without the deflector was conducted by ANSYS. The results show that, the installation of the deflectors increased the inlet airflow rate for the rack by 16.98% and improved the energy efficiency of the dater center air conditioners by 1.98%.

Enhancing airflow dynamics in airjet spinning: A machine learning approach to optimize nozzle design

This research delves into using machine learning techniques to enhance the airflow dynamics in Airjet spinning. It focuses on understanding the factors that affect airflow by examining components of a spinning nozzle including the fiber inlet element, injector nozzle and spinning spindle. A prototype Airjet spinning nozzle was developed to evaluate the Intake Airflow and Airflow Rate, which serve as the basis for a simulation model. A total of 501 data points were empirically gathered, and machine learning methods were applied to uncover patterns and make predictions. The study combines conventional measurement techniques with data analysis tools, like linear regression, decision trees, random forest and support vector regression. After developing a applicable machine learning tool, the hyperparameters are optimized with the goal to improve the model’s reliability. Following this optimization process it was found that the CatBoost model outperformed ML models in terms of all performance metrics. Furthermore insights, into how nozzle features impact airflow dynamics were obtained through sHapley Additive exPlanations (SHAP) analysis. The results indicate that specific nozzle design parameters play a significant role in improving airflow dynamics. Integrating machine learning techniques into the design process marks a departure from conventional empirical methods. This scientific method allows for a more rapid and precise Airjet nozzle design process, fostering innovation in textile manufacturing technology.

Experimental study on the transient-state performance of diesel engines at different speeds using the synergistic regulation of VGT and VVT

Heavy-duty diesel engines, the heat engines with the highest thermal efficiency, usually operate under transient conditions. Thus, it is important to study the transient performance of heavy-duty diesel engines. This paper aims to solve the problems of combustion deterioration, poor response, and emission deterioration caused by the mismatch between the air intake response and the fuel system under the strong transient condition of sudden loading of 1 s under constant speed. In this paper, an experimental study on the transient performance at different speeds under the coordinated regulation of a variable geometry turbocharger (VGT) and variable valve timing (VVT) is conducted. The study found that the transient torque response is affected by combustion in the cylinder and pump work. During the low-speed transient process, due to reduced airflow and pumping losses, the VVT is switched off while the VGT delay is increased to improve air response. Consequently, the mixture in the cylinder is fully burned, and improved transient performance is obtained. In the high-speed transient process, the engine air intake flow is improved. Through the VVT is ON at the appropriate time and the VGT hysteresis control, the pumping loss can be effectively reduced, and excellent transient performance can be achieved to ensure the fast response of the in-cylinder charge. Given sudden loading from 10% to 100% within 1 s under a high speed of 1600 r/min, the VVT switches on with a 0.15 s delay, and the VGT is controlled with a 0.4 s delay. A torque response of 0.82 s can be achieved, and the soot peak value is reduced by 66.26%, and the accumulated value of soot is reduced by 46.91%. At a low speed of 1000 r/min, given sudden loading from 10% to 100% within 1 s, the 0.6 s VGT delay can reduce the accumulated value of soot by 78.57% and 56.22% compared with delays of 0.2 and 0.4 s.

Determination of the influence of the incoming flow on the vortex zones at the entrance to the suction sockets. Part 1. The Axisymmetric problem

The features of the construction of a computational algorithm for calculating circular suction sockets in the conditions of an incoming flow by the method of discrete vortex rings are considered. Flow patterns are given at different ratios of the incoming and intake air flow velocities. The dependences of the characteristic sizes of vortex zones arising at the inlet to the suction-sockets on the velocity of the incoming flow, the length and angle of inclination of the socket are determined. Expressions are obtained for the critical velocity of the incoming flow, at which the flow separation mode for the first vortex zone changes from separation inside the bell to separation outside it. The obtained results can be useful for the construction of local suction systems, profiled according to the found outlines of vortex zones.

The Study of Velocity Field in Front Opening Unified Pod by CAE

The front opening unified pod (FOUP) is a packing box for contamination control for semiconductor wafer transport. As the wafer fabrication process developes towards nanoor atom level, the semiconductor wafer storage device should advance from the particle prevention function into the airborne molecular contamination (AMC) removal function. Therefore, it is necessary to design/redesign a function for removing AMC or moisture inside the FOUP. This study used the design of leading diffuser tubes in the FOUP and pores in the surfaces of diffuser tubes to generate gas diffusion. This is to achieve a uniform distribution of the wafer surface velocity field and a uniform dehumidification function of the wafer surface. Based on the analysis results, when circular diffuser tubes are introduced in the FOUP and the intake air flow was set at 0.2-0.3 m3/hr, the interlayer wafer surface in the FOUP could achieve uniform distribution of velocity field. As a result, the humidity difference among various zones of wafer surface could be reduced, and the yield and quality of the wafer cutting process could be controlled.

Study on the adsorption mechanism and properties of silver-loaded zeolite for radioactive iodine

Due to the continuous development of nuclear energy, a large number of radioactive elements will be produced during the treatment of nuclear waste and spent fuel, and the typical representative is radioactive iodine. Adopt to adsorb the radioactive iodine in the waste gas, the self-made silver-loaded zeolite was used to dynamically adsorb the radioactive iodine containing waste gas in this study. The adsorption effect of silver-loaded zeolite under different adsorption conditions, such as different linear velocity, different adsorption temperature, different pressure drops, different filler layer depth and other factors were studied. By exploring the experimental results, it can be seen that changing the different linear velocity in the adsorption column has little effect on the saturated adsorption competence of zeolite, and will affect the saturated adsorption time; Changing the filling depth of zeolite in the adsorption column will change the resistance in the adsorption process, affect the length of saturated adsorption time, and lead to the increase of its adsorption competence with the increase of the depth; The adsorption temperature of zeolite was controlled by controlling the ambient temperature of the adsorption column. It can be seen that the optimal adsorption temperature of zeolite is 150 ℃; Change the intake air flow to control its pressure drop, it can be seen that the pressure drops increases, the adsorption effect increases, and the adsorption time is shortened. The results of zeolite adsorption show that its adsorption competence is 52.05 ∼ 220.71 mg/g. Compared with other adsorption materials, zeolite has stable adsorption effect and low cost. It is a promising adsorbent with good industrial application prospects.

Rotor Cascade Assessment at Off-Design Condition: An Aerodynamic Investigation on Platform Cooling

Off-design condition of a rotor blade cascade with and without platform cooling was experimentally investigated. The ability of the gas turbine to operate down to 50% to 20% of its nominal intake air flow rate has an important consequence in the change in the inlet incidence angle, which varied from nominal to −20°. Platform cooling through an upstream slot simulating the stator-to-rotor interface gap was considered. The impact of rotation on purge flow injection was simulated by installing fins inside the slot to give the coolant flow a tangential direction. Aerodynamic measurements to quantify the cascade aerodynamic loss and secondary flow structures were performed at Ma2is = 0.55, varying the coolant to main flow mass flow ratio (MFR%) and the incidence angle. The results show that losses strongly increase with MFR. A negative incidence allows a reduction in the overall loss even when coolant is injected with a high MFR. The more negative the incidence, the greater the loss reduction.

Influences of a variable nozzle turbocharger on the combustion and emissions of a light-duty diesel engine at different altitudes

With social and economic developments, more light-duty vehicles are operating at high altitudes, but the reduced intake air flow in plateau areas, leads to poorer engine combustion and greater emissions. An effective means of improving plateau adaptability are light-duty diesel engines employing variable nozzle turbochargers (VNTs), but the research to date on how VNTs influence the combustion and emissions of diesel engines at high altitudes has lacked both depth and a systematic approach. Therefore, in the present research, experiments were conducted to investigate the influences of a VNT on the combustion and emissions of a light-duty diesel engine at different altitudes (0 m, 1000 m, and 2000 m) by using a plateau-environment simulation device. Also, the influences of altitude and VNT nozzle opening on the in-cylinder working process and pollutant generation were studied by means of three-dimensional numerical simulations. The results show that the VNT had almost the same ability to enhance the intake air flow at different altitudes within the same range of VNT nozzle opening, and the effect of increasing altitude on diesel engine performance was similar to that of increasing VNT nozzle opening. With increasing altitude or VNT nozzle opening, the spray penetration length increased, the in-cylinder equivalence-ratio distribution became less even, the start of combustion was delayed slightly, the combustion duration was extended, and the in-cylinder mean temperature increased. With increasing altitude or VNT nozzle opening, the emission of NOx decreased gradually, while those of CO, HC, and soot increased. Compared with the case at sea level, the influence of VNT nozzle opening on CO, HC, and soot emissions was more significant in plateau areas.

Mixed Convection in a Horizontal Channel–Cavity Arrangement with Different Heat Source Locations

Several researchers are very interested in mixed convection heat transfer because of how widely it is used, particularly for solar thermal collectors, cooling electronic equipment, and chemical process instruments. Using COMSOL-Multiphysics, this article establishes laminar coupled mixed convection heat transfer characteristics across a horizontal channel–cavity architecture. Investigations are conducted into the effect of heat source location on isotherms, velocity distribution, pressure, temperature, average and local Nusselt numbers, and air density. The intake airflow Reynolds number is assumed constant on 2.8814. The enclosure with an isothermally heated right wall in the shape of a “<” as a heat source in three configurations (heat source in the base (1st case), in the upper step (2nd case), and the below step (3rd case). The obtained numerical results present that the higher heat transfer is performed in case two because the heat source is near the contact surface between the channel and the cavity. With the hot sources’ locations being altered, the velocity distribution seems to be unchanged. The increase in the positive y axis has no impact on the pressure distribution throughout the channel. Changing the position of the heated source does not seem to have any impact on the pressure distribution. Air density profiles start to diverge across cases around y = 0.035 m; the third example has a larger value than the second case, and the latter case has a larger value in the density distribution than the former. The contact between the enclosure and the channel (y = 0), where the greatest Nusselt number also occurs, exhibits the highest heat transfer. The maximal Nusselt number falls as y’s absolute value rises.

Analysis of age and residual lifetime of air for blowing and intake airflow of air cleaner using CFD

Analysis of age and residual lifetime of air for blowing and intake airflow of air cleaner using CFD

Computational Prediction of COVID-19 Transmission in Internal Air-Conditioned Environments

COVID-19 has had destructive consequences for health, economy and has altered every aspect of everyday human activity. The outbreak was first identified in December 2019 in Wuhan, China. The declaration of the disease as a “Public Health Emergency of International Concern” for the World Health Organization took place on January 30, 2020. Furthermore, 105,5 million cases have been reported until 06 February 2021. Public distancing in internal environments has been applied as a safety measure to prevent transmission. A controversial topic is the safe distance from person to person. The social distancing regulation, for internal public places, has been arbitrarily defined ignoring the potential aerodynamics effects of inlets, such as air-conditioning units, windows and doors. The velocity of the intake airflow has the potential to transfer a droplet from the nose or the mouth of a patient in greater than the indicated distance. The present study focuses on a model of a supermarket that includes a ventilation system and open doors. For the transmission of COVID-19 in an air-conditioned internal space, two different designs were implemented and studied. Internal shelving, furnishing and human models are also being considered. The numerical results obtained are compared with those obtained by two well-known empirical models related to the effective velocity of incoming air and the virus concentration. It is concluded that the computational results obtained in the present study are in acceptable agreement with those obtained by simple empirical models, especially when the standard k-ε model of turbulence is used. Thus, for the cases of coughing and sneezing patients, where we studied the largest particles that sediment onto the floor, the 6-foot (≈1,82 m) rule applies well. However, pathogen-laced particles, coming for example from asymptomatic patients, travel through the air indoors when people breathe and talk. Therefore, there is not much benefit to the 6-foot rule because the air a person is breathing tends to rise and comes down elsewhere, so the person is more exposed to the average background than to a person at a distance. Future research should concentrate rather on the amount of time spent inside rather than distances. As the COVID-19 pandemic is progressing, the present study is flexible and can be applied generally in crowded places. Furthermore, the general outcome is that individuals should maintain the distance of 1,65 meters and it should be applied as guidelines to help reduce the infection risk.

Effect of Coolant Temperature on Performance and Emissions of a Compression Ignition Engine Running on Conventional Diesel and Hydrotreated Vegetable Oil (HVO)

To meet future goals of energy sustainability and carbon neutrality, disruptive changes to the current energy mix will be required, and it is expected that renewable fuels, such as hydrotreated vegetable oil (HVO), will play a significant role. To determine how these fuels can transition from pilot scale to the commercial marketplace, extensive research remains needed within the transportation sector. It is well-known that cold engine thermal states, which represent an inevitable portion of a vehicle journey, have significant drawbacks, such as increased incomplete combustion emissions and higher fuel consumption. In view of a more widespread HVO utilization, it is crucial to evaluate its performance under these conditions. In the literature, detailed studies upon these topics are rarely found, especially when HVO is dealt with. Consequently, the aim of this study is to investigate performance and exhaust pollutant emissions of a compression ignition engine running on either regular (petroleum-derived) diesel or HVO at different engine thermal states. This study shows the outcomes of warm-up/cool-down ramps (from cold starts), carried out on two engine operating points (low and high loads) without modifying the original baseline diesel-oriented calibration. Results of calibration parameter sweeps are also shown (on the same engine operating points), with the engine maintained at either high or low coolant temperature while combustion phasing, fuel injection pressure, and intake air flow rate are varied one-factor at a time, to highlight their individual effect on exhaust emissions and engine performance. HVO proved to produce less engine-out incomplete combustion species and soot under all examined conditions and to exhibit greater tolerance of calibration parameter changes compared to diesel, with benefits over conventional fuel intensifying at low coolant temperatures. This would potentially make room for engine recalibration to exploit higher exhaust gas recirculation, delayed injection timings, and/or lower fuel injection pressures to further optimize nitrogen oxides/thermal efficiency trade-off.