Compared to traditional hydrocarbon fuels, ammonia presents significant challenges as a fuel, including high ignition energy, low reactivity, slow flame propagation, and high NOx emissions, which hinder its use as a renewable fuel. Blending ammonia with fossil fuels like natural gas improves its combustion reactivity and helps mitigate CO2 emissions. However, there is still much to understand about the complex dynamics of ammonia and its blends with hydrocarbons. Key areas such as reaction kinetics mechanisms, ignition properties, flame propagation behaviors, and methods for controlling combustion performance under various conditions require further elucidation. This paper reviews recent advancements in experiments and numerical simulations aimed at developing stable, and low-emission combustors for ammonia-fired power generation. Recent burner and flame configurations, including non-swirling jets, single-stage swirl burners, two-stage burners, and newly developed double-swirl burners are analyzed for their flame stability and pollutant emission potential when firing ammonia and ammonia blends. Chemical kinetic modeling of ammonia and its blends plays a crucial role in understanding combustion behavior and pollutant emissions, particularly for NOx. However, there are challenges in predicting NOx emissions accurately, with significant disparities among different models. High-fidelity numerical simulations using detailed and skeletal mechanisms, direct numerical simulation, and large eddy simulation, have helped uncover crucial operational conditions affecting combustion and pollutant emissions, such as combustor pressure, air dilution, wall cooling, fuel/air mixing, and fuel blending. Nonetheless, the accuracy of chemical kinetic models and their integration into turbulent flow simulations remain critical limitations for numerical simulations of ammonia combustion.

This work explores the feasibility of pure methanol combustion in a light-duty diesel engine assisted by a glow plug (GP). The simulations represented a mild engine load with an indicated mean effective pressure of 7 bar. An extensive computational study was conducted, and the successful operation of the pure methanol compression ignition engine was demonstrated. The effects of the GP position, spray umbrella angle, the relative angle (RA) between the glow plug and jet trajectory, and the injection strategy on the engine performance were evaluated. The autoignition of methanol-air mixture was found to primarily occur at an equivalence ratio between 0.2 and 0.4. However, an even richer mixture accompanied the lower temperature due to intense heat absorption of evaporation, significantly prolonging the ignition delay. Therefore, to improve the ignition and combustion heat release processes, RA was optimized to adequately control the mixture distribution around the GP. At each position of the GP, the optimum RA differed due to the complex flow and air-fuel mixing within the combustion chamber, which became smaller (from 12.5 degrees to 5 degrees) when the GP was moved anticlockwise from the intake port to the exhaust port regions. Furthermore, a split injection strategy was proposed to ensure the successful ignition of the methanol jets. The engine performance exhibited a high sensitivity to the pilot and main injection timings. A small pilot mass fraction of no higher than 20% was recommended to mitigate fuel jet-GP interaction and fuel impingement in the squish region.

Ammonia is a carbon-free fuel that can be produced from renewable energy sources and has the potential to replace fossil fuels, exerting a significant impact on the decarbonization of power production and propulsion industries. However, the challenge lies in the high NOx x emissions, narrow flammability, and low flame speed of ammonia/air mixtures. In this paper, we study a novel concept of double-flame premixed co-combustion (DFPC) of ammonia and methane in a double-swirl premixed combustion burner, which results in low NOx x emissions and high flame stabilization. Large eddy simulations using a detailed chemical kinetic mechanism and planar laser-induced fluorescence imaging of OH and exhaust gas NO emission measurements are employed to investigate the fundamental mechanisms behind flame/flame interactions and NO emissions. The main findings are: (a) NO emissions can be reduced by 90% using the DFPC concept along with a significant broadening of flammability; (b) the outer methane/air flame stabilizes the inner ammonia flame in the shear layer of the two flames; (c) combustion products and excess oxygen leaked across the shear layer decrease the equivalence ratio of the inner ammonia/air mixture, reducing the NO formation of close-to-stoichiometric ammonia/air flame but increasing the NO formation in the fuel-rich ammonia/air flames; (d) mixing of the combustion products from the inner and outer flames reduces the NO emissions in the flue exhaust gas.

This work explores the methanol compression ignition combustion assisted with a glow plug. The effects of the glow plug position, spray umbrella angle, the relative angle between the glow plug and jet trajectory, and the injection strategy on the engine performance were evaluated. Of these parameters, the relative angle between the glow plug and jet was found to be the dominant factor affecting the ignition and combustion heat release. At each position of the glow plug, the optimum relative angle differed due to the complex flow and air-fuel mixing within the combustion chamber. Among the four representative regions, the east region yielded slightly higher thermal efficiency and lower combustion loss. Compared to the single injection strategy, the split injection strategy was more effective in promoting the ignition process. The engine performance exhibited a high sensitivity to the pilot and main injection timings. Furthermore, the slightly narrower spray umbrella angle yielded better fuel economy because less fuel was trapped within the squish region, leading to faster flame propagation and less unburned fuel.

This work numerically investigated the feasibility of methanol compression ignition combustion for light-duty diesel engine applications by using a glow plug (GP) to promote ignition. A comprehensive parametric study was conducted to assess the combustion characteristics depending on the GP position, the relative angle between the GP and injector, and other initial conditions. Optimal design parameters were identified. It was demonstrated that GP can enable successful ignition and combustion of methanol at the operating conditions under study. Among the many parameters considered, the relative angle between the GP and injector was found to be one of the most critical parameters in controlling the ignition and complete combustion. Increasing intake temperature promoted combustion speed and engine performance, but excessively high intake temperatures led to higher wall heat transfer loss and lower ITE. An appropriate level of the pilot injection mass was found to increase ITE, with the minimum loss of combustion efficiency attained at a pilot mass fraction of 10%. Increasing the intake pressure further improved the engine performance, primarily owing to the reduced wall heat transfer loss. In contrast, the combustion was not significantly affected by the change in injection pressure, although slightly higher ITE was obtained at the lower injection pressure. It is expected that higher thermal efficiency is achievable with further optimization of design parameters.

To understand key practical aspects of ammonia as a fuel for internal combustion engines, three-dimensional computational fluid dynamics (CFD) simulations were performed using CONVERGETM. A light-duty single-cylinder research engine with a geometrical compression ratio of 11.5 and a conventional pentroof combustion chamber was experimentally operated at stoichiometry. The fumigated ammonia was introduced at the intake plenum. Upon model validation, additional sensitivity analysis was performed. The combustion was modeled using a detailed chemistry solver (SAGE), and the ammonia oxidation was computed from a 38-specie and 262-reaction chemical reaction mechanism. Three different piston shapes were assessed, and it was found that the near-spark flow field associated with the piston design in combination with the tumble motion promotes faster combustion and yields enhanced engine performance. The simulation results suggest that operating an engine with ammonia requires substantial spark advancement because its combustion duration is significantly longer relative to conventional hydrocarbon fuels as a result of its low laminar burning velocity. Tradeoffs between combustion efficiency and NOx, and thermal and combustion efficiencies were observed. Moreover, as the engine speed was increased, further spark advancement was needed as the physical time for combustion development is shorter. Ultimately, it was demonstrated that simultaneous optimization of operating conditions and piston design can provide appreciable gains in combustion and thermal efficiencies.

In this study, the piston bowl geometry and injector design of a light-duty GCI engine were co-optimized using computational fluid dynamics (CFD) and advanced machine learning (ML) techniques to maximize the capability of a GCI technology. This study was performed at low-(6 bar), mid-(11 bar) and high-load (22 bar) indicated mean effective pressure (IMEP) conditions. The 3-D CFD setup was first validated against experimental data. Then, the injector and the piston bowl were simultaneously co-optimized. In total, 13 (ten piston-and three injector-related) design parameters were considered. At each load condition, 128 DoE cases were generated, and the features and performance of the top three designs were analyzed. The best DoE solution was selected by defining weighted merit values to provide one best design across all load conditions. The simulated dataset was used for further optimization using advanced ML techniques. It was found that a flatter design with low center height, wide and shallow bowl, and the low lip was preferable in the low-and high-loads. At the low-load partially premixed compression ignition (PPCI) mode, spray targeting the upper lip and divided into the bowl and squish zones for enhanced mixing is preferred. At the high-load mixing-controlled diffusion combustion mode, where injection occurs near the top dead center (TDC) and diffusion burn is the dominant combustion mode, targeting the lower lip is favorable. At the mid-load with a high premixed ratio, the combustion was close to homogeneous charge compression ignition (HCCI), and the piston bowl design had limited effect. Regarding the optimum injector parameters, larger number nozzles with smaller diameters were favorable at low-load to control partially premixed charge. At high-load, larger and fewer number nozzles are recommended. Lastly, the optimum ML design provided similar performance at mid-load but a 3.8-4.5 % reduction in fuel consumption compared to the baseline cases at low-and high-load conditions.

In this study, computational fluid dynamics (CFD) and machine learning (ML) were used to investigate the effects of and optimize the injector design parameters for light-duty gasoline compression ignition (GCI) engine. This study was performed at part-(6 bar) and high-load (22 bar) indicated mean effective pressure (IMEP) conditions. The effects of number of nozzles (nNoz), spray angle (SA) and plume angle (PA) while maintaining total nozzle area (TNA) and start of injection (SOI) were first investigated. The increased nNoz and PA enhanced fuel/air mixing, especially in the bowl region, by even spray distribution at part-load condition, but the effects are negligible at high-load mixing-driven combustion due to high in-cylinder temperature and pressure at the time of main injection. On the other hand, SA had significant effect on the air utilization and hence engine performance and emissions at both part-and high-load conditions. The best design from this manual parametric study pro-duced a balanced air utilization between piston bowl and squish region resulting from the fuel spray targeting the upper lip of the bowl. The second phase of this study focused on the optimization of the injector and injection parameters including nNoz, SA, nozzle hole diameters (dNoz) and SOI using design of experiment (DoE) approach. 32 DoE cases were generated and best design was selected at each load point. It was found that the SA and SOI are the most influential injection parameters. Specifically, two optimum SOI regimes at part-load conditions have been identified. These are a) early injection resulting in retarded combustion and low pres-sure rise rate and b) late injection yielding high combustion efficiency and low hydrocarbon emission. At high-load condition, SOI right before top dead center (TDC) is most preferable and early injection should be avoided to minimize pressure rise rate. The narrow SA (90 degrees (deg) in this study) as well as wide SA were found to produce optimum performance. Subsequently, ML algorithm was used to further optimize the injector and in-jection parameters. As a result of this optimization study, 10.7% reduction of fuel consumption at part-load and 7.5% reduction at high-load were achieved.

Gasoline Compression Ignition (GCI) is an engine-fuel technology which incorporates a combustion process using gasoline fuels in a compression ignition (CI) mode. GCI has the potential to offer high fuel efficiency while achieving ultra-low emissions. The objectives of the GCI engine development are to improve the fuel economy, meet the peak torque capabilities and comply with light duty emission regulations. The other key objective is to provide cost effective GCI engine design solution to automakers by minimizing the hardware complexity and maximizing compatibility with existing engine components. While GCI is under development for several years, there are several key technology risks (KTR’s) that need to be addressed. During cold start, the criteria pollutant emission is a challenge when the catalyst is not effectively warmed up. At low loads, misfires and partial burns lead to excessive HC and CO emissions. The increased pressure rise rate and particulate emissions are challenges at high load. The control challenges include transient Exhaust Gas Recirculation (EGR) control and combustion robustness control due to cycle to cycle stability needs. Furthermore, the fuel injection system durability and boosting system capabilities are required to be demonstrated. This book chapter presents strategies to address those KTR’s at different engine operating conditions. The engine is based on a gasoline engine architecture with a spark plug and high pressure gasoline fuel injection system, termed as spark assisted gasoline compression ignition (SAGCI) engine. The results reported in this chapter are specific this particular hardware configuration of GCI engine fueled by RON 91 gasoline. During cold start, a split fuel injection strategy with extremely retarded spark timing is proposed to rapidly warm up the catalyst and minimize the criteria emissions. At low loads, typical spark ignited (SI) combustion and spark assisted GCI strategies are preferred. During GCI operation at low loads, spark is enabled as a supplemental means for robust combustion control. GCI based on partially premixed compression ignition (PPCI) strategy is preferred at medium loads to meet the emission targets. Cold EGR, re-breathing and different fuel injection strategies are used as control variables to ensure better combustion control. The high load GCI is based on diffusion combustion with optimum fuel injection strategy that targets to reduce the pressure rise rate and soot emissions.

A coupled Eulerian-Lagrangian approach was employed to Engine Combustion Network (ECN) Spray-G simulations. The Eulerian in-nozzle flow simulation was conducted with a small plenum attached to the nozzles, and the results were fed to the Lagrangian spray simulation. For Eulerian simulation, the homogeneous relaxation model (HRM) coupled with the volume of fluid (VOF) method was used. HRM proved to be good at predicting the phase change phenomena due to vaporization mechanisms, that is, both cavitation and flash boiling. As a one-way coupling, quantities such as rate of injection (ROI), mass injected through each hole, discharge coefficient, spray plume angle and half cone angle predicted from the Eulerian simulations were used as the initial and boundary conditions for the subsequent Lagrangian spray simulations using the blob injection model. Non-flashing (Spray-G1) and flashing (Spray-G2) spray was simulated, and the results were validated quantitatively against the published data in terms of the liquid and vapor penetration lengths, and good agreements were obtained. Furthermore, the simulation predicted the liquid and gas axial velocity and sauter mean diameter for Spray-G1 condition in agreement with the droplet size and particle image velocimetry (PIV) measurements from literature.

Flash boiling and plume interaction are common phenomena occurring in gasoline direct injection (GDI) spray at throttling and low load engine conditions. Combined with optical engines and low-pressure vessels, several optical techniques, such as backlight imaging, Mie-scattering, and laser sheet imaging have been employed to study the flash boiling morphology. However, in the 2D images resulting from these techniques (projection views or planar imaging), the 3D information is lost. Those methods are then incapable of providing satisfactory information, especially for the study of multi-plume interaction in flash boiling spray, since multi-plume interaction is not a 2D event. This paper reports the implementation of a 4D tomographic reconstruction method from multi-view diffused back illumination (DBI) images, used for the first time in spray characterization. This cost-effective and time-saving method with a simple experimental setup clarifies the 3D spray structure and fuel trajectory change from non-flashing conditions to flare flash conditions, and quantifies the 3D characteristics of individual plumes in non-flash conditions.

In gasoline engines, including conventional gasoline direct injection (GDI) engines and newly developed gasoline compression ignition (GCI) engines, flash boiling of the spray occurs during throttling or low load operations. Superheated fuel that is injected into the cylinder, where the gas pressure is lower than the fuel’s saturation vapor pressure, experiences a fast phase change. Plume interaction and spray collapse can occur as a consequence of flash boiling. The structure of flashing spray has not been well elucidated experimentally because of strong multiple-scattering effects in conventional laser sheet imaging due to illumination of out-of-laser-plane droplets. Here, the structured laser illumination planar imaging (SLIPI) is implemented for the first time to study flash boiling sprays. Both front-view and side-view cross-sections are examined to reveal spray behaviors during collapsing events. A comparison of the reconstructed 3D spray volume by SLIPI and conventional laser sheet imaging clearly shows the advantage of SLIPI in resolving the inner structure of the collapsed spray. The near-nozzle region on the injector axis is found to be hollow, indicating that spray collapsing occurs a bit downstream of the nozzle instead of immediately at the nozzle. This observation could not be obtained by conventional laser sheet imaging nor by diffused back illumination (DBI) techniques. In this work, the central tip observed in the 2D DBI image at Rp = 0.1 case has been proven to be not a ’central jet on injector axis’ formed due to radial collapse, but a longer projection on the image caused by stronger adjacent plume circumferential interactions.

The effects of charge gas temperature, gas pressure, and fuel type on liquid penetration length and cone angle of a hollow-cone spray from an outwardly opening piezoelectric injector was investigated. High-speed diffused back-illumination extinction imaging (DBIEI) was used to perform the measurements in a constant pressure vessel. The conditions studied in this work are relevant to gasoline direct injection (GDI) and gasoline compression ignition (GCI) engine combustion: injection pressure was kept at 120 bar, in-chamber pressure and temperature were varied from 1 bar to 10 bar, and 21 degrees C to 200 degrees C, respectively. Experiments were performed with two high-reactivity gasoline fuels, namely high-reactivity gasoline 1 (HRG1) and high-reactivity gasoline 2 (HRG2), which have similar auto-ignition characteristics but substantially different distillation and physical properties. These fuels are candidates to work with future engine designs with a better performance. The motivation here was to understand what physical properties are important in air fuel mixture preparation. Liquid penetration reduces with increasing gas temperature and pressure for both fuels whereas cone angle increases only with increasing gas temperature. At high temperatures, HRG2 yields up to 25% longer liquid penetration length compared to HRG1. For a detailed understanding of effects of fuel properties on liquid penetration, a detailed numerical study was conducted using CONVERGE. By changing a single fuel property at a time, the effects of each fuel physical property was isolated. It was found that density, specific heat, surface tension and heat of vaporization are the most influential physical properties on spray penetration.

This work was sponsored by Saudi Aramco under the FUELCOM program and King Abdullah University of Science and Technology. We would like to thank Convergent Science for providing the CONVERGE software.

The research reported in this publication was supported by the Fuel Technology Division at Saudi Aramco R&DC and by the King Abdullah University of Science and Technology (KAUST).

In this study, the spray and combustion characteristics of high reactivity gasoline (HRG) fuel of RON 77 were tested and compared with E10 certification fuel under the gasoline compression ignition (GCI) engine conditions using a high-pressure multi-hole GCI engine injector. A comprehensive characterization in terms of the rate of injection, spray morphology under flash boiling conditions, penetration lengths under both nonevaporative and evaporative conditions, and ignition delay at reactive conditions was performed. It was found that both the high reactivity gasoline and E10 certification fuel exhibit very similar characteristics. The ignition delay times were found to be very similar between both the fuels tested under ambient temperatures higher than 800 K. This work further serves as an extensive database to validate and calibrate the spray models, combustion models and reaction mechanisms for computational fluid dynamics (CFD) driven development of GCI engines.