Natural gas is known as a relatively clean fossil fuel due to its low carbon to hydrogen ratio compared to other transportation fuels, which yields a reduction of carbon monoxide, carbon dioxide, and unburned hydrocarbons emissions. However, it has a low cetane number, which makes it a difficult fuel for use in compression ignition engines. A potential solution for this issue can be adding small amounts of argon, as a noble gas with a low specific heat to modify the intake conditions. In this numerical study, a commercial compression ignition engine has been modeled to evaluate the auto-ignition of natural gas with the modified intake conditions. Different amounts of argon added to the intake air are examined in order to attain the optimal operating conditions. A detailed chemistry solver is implemented on a 53-species chemical kinetics mechanism to calculate the rate constants. The results show that compression ignition of natural gas can be achieved by adding small amounts of argon to the intake air. It drastically increases the in-cylinder temperature and pressure near TDC, which enables the auto-ignition of the injected natural gas. Moreover, it leads to the reduction in ignition delay and heat release rate, and expands the combustion duration. Emissions analysis indicates that NOx and CO2 can be significantly diminished by increasing the amount of argon in the intake composition. This study introduces an efficient and clean compression ignition engine fueled with natural gas running in optimal operating conditions using argon addition to the intake. Keywords: Natural Gas, Noble Gases, Clean Combustion, Argon Power Cycle, Compression Ignition Engine 1. INTRODUCTION The use of natural gas in internal combustion engines produces less CO2, CO, and HC emissions compared to the other common fossil fuels due to the relatively low carbon to hydrogen ratio [1]. Moreover, it possesses a high octane number (~120 - 130), which enables the engine to run at a higher compression ratio and achieve a higher thermodynamic efficiency [2]. Natural gas has relatively wide flammability limits, which allows for operation at ultra-lean conditions [3, 4]. The significantly large proven reserves of natural gas (200 trillion cubic meters worldwide) combined with the capability to operate a cleaner and more efficient cycle make it an attractive alternative fuel [5]. The use of natural gas as a standalone fuel, or as a blend with diesel or hydrogen, has been extensively studied in traditional spark ignition engines [6-9] and more advanced cycles such as homogeneous charge compression ignition (HCCI) [10-14] and reactivity controlled compression ignition (RCCI) [15-18]. Compression ignition cycles generally have a higher thermodynamic efficiency due to the ability to operate at a higher compression ratio. However, investigations on natural gas compression ignition engines are quite limited due to its low cetane number [19]. It is practically impossible to auto-ignite natural gas in common compression ignition engines since high in-cylinder temperatures are required. A number of studies tried to introduce a solution by using a secondary fuel such as diesel [20-22], hydrogen [23], rapeseed methyl ester (RME) [24], dimethyl ether (DME) [25], and methyl propanoate (MP) [26]. An alternative approach is using noble gases as the working fluid doi: 10.1115/ICEF2019-7159 Pre-Print 2 to increase the in-cylinder temperature. Among the noble gases, argon has proven to be the leading choice due to the accessibility and a relatively lower cost. Argon possesses a lower specific heat compared to nitrogen. Thus, it theoretically yields higher temperatures at TDC in comparison to nitrogen, the primary component of air. Additionally, argon has a higher specific heat ratio (1.67) compared to nitrogen (1.40), which increases the ideal thermodynamic efficiency. A number of studies have assessed the use of argon in engines as the working fluid [27, 28]. Argon is shown to be capable of decreasing the ignition delay and increasing the in-cylinder temperature [29, 30]. Moreover, the theoretical thermal efficiency can be increased up to 20% when compared to the conventional engines using nitrogen as the working fluid [31]. The current study aims to evaluate the feasibility of using natural gas as a standalone fuel in a compression ignition engine with partial replacement of nitrogen by argon. Figure 1 shows a schematic diagram of the hypothetical argon assisted natural gas compression ignition engine. Nitrogen, argon, and oxygen are introduced to the engine through the inlet manifold, and natural gas is directly injected into the comparably high temperature air, and auto-ignites. Due to the extremely lean operating condition and the excess amount of oxygen, the products mainly include nitrogen, argon, oxygen, and slight amounts of water vapor and carbon dioxide. The water vapor can be readily condensed, and separated to hydrogen and oxygen through an electrolysis procedure. The oxygen can be recirculated to the inlet manifold to compensate the amount of consumed oxygen in the cylinder during the combustion process. The carbon dioxide in the products can be separated using a proper membrane, and the remaining gases including nitrogen, argon, and oxygen return to the inlet manifold for the next cycle. The feasibility, cost, energy efficiency, and system analysis of this hypothetical cycle should be investigated. However, the scope of this study as shown in Figure 1 is confined to the combustion process inside the engine. For this objective, numerical simulation is chosen as a rational first step to explore the numerous input and output parameters associated with the non-premixed combustion of natural gas in the specified engine. 2. MATERIALS AND METHODS A numerical simulation is carried out in order to examine the capability of natural gas as a fuel in compression ignition cycles, and to understand its combustion characteristics in argon-nitrogen-oxygen mixtures. The Y ANMAR NF19SK engine is selected with the parameters given in Table 1. Previously, experimental studies on non-premixed combustion of hydrogen in argon-oxygen mixtures were conducted in this engine [32], and the experimental data are available to validate the current numerical modeling [33]. Most of the operating conditions such as initial temperature, initial pressure, and injection pressure are kept the same. However, the engine speed is changed from 600 rpm to 1200 rpm, which is within the normal speed range of natural gas engines. The injection duration is increased from 5 °CA to 10 °CA for optimization of the combustion process. The overall equivalence ratio is calculated as 0.09, which means the engine runs at an extremely lean operating condition. This provides an excess amount of oxygen in products, which can be Figure 1. Argon assisted natural gas compression ignition engine. The scope of this study is specified by the dashed box. Table 1. Engine Specifications Table 2. Composition of natural gas doi: 10.1115/ICEF2019-7159 Pre-Print 3 recirculated to the inlet manifold through EGR. Natural gas is introduced in the modeling using five of its main components, methane, ethane, nitrogen, carbon dioxide, and propane with the mole fractions indicated in Table 2. Different amounts of argon replacement from 10% to 80% by volume are investigated, and the results are compared against no argon replacement, when nitrogen is the only working fluid. All concentrations including argon replacement written in this paper are based on volume. 2.1. Numerical Simulation This study is conducted in CONVERGE CFD software, a powerful tool designed specifically for internal combustion engine applications. It automatically generates orthogonal grids, which is a huge advantage in complex and detailed geometries such as engines. The geometry of the current study is created in a 3D CAD software, and imported into CONVERGE. Figure 2 shows the geometry and the grid configuration of the model. The simulation is based on a closed cycle, meaning the duration between intake valve close (IVC) and exhaust valve open (EVO). The domain consists of two different regions: (1) injector, which involves natural gas at 80 bar and 400 K, and (2) cylinder containing oxygen-nitrogen-argon mixture at initial pressure and temperature of 1.14 bar and 400 K. The simulation begins when the piston is at BDC, and the two regions are disconnected until -15 °CA aTDC when the injection starts, and the natural gas enters the cylinder due to the pressure gradient. At -5 °CA aTDC, the injection ends by detaching the two regions. To increase the computational efficiency, a relatively coarse base grid (5 mm) is selected, and mesh refinement is implemented using a fixed embedding on the nozzle and near nozzle areas, and adaptive mesh refinement (AMR) on velocity and temperature. AMR automatically splits a cell if gradients exceed a certain user-defined value (1 m/s and 2.5 K here). Figure (d) shows how the grid configuration changes during the injection and combustion where large gradients of velocity and temperature appear in the solution. A sensitivity analysis for the grid resolution is carried out, and the results are presented in Table 3. A base grid size of 5 mm with a permanent embedding scale of 2 (1.25 mm), and a sequential embedding scale of 2 (1.25 mm) for injector, and 3 (0.625 mm) for near nozzle areas during the injection event provide an optimized and reliable grid configuration. Higher levels of embedding result in the same mass of injected gas, and increase the computational cost. For gas simulation, a Redlich-Kwong equation of state is used since it provides a more realistic result compared to the ideal gas law. The SAGE detailed chemistry solver is employed, which simulates the combustion by calculation of reaction rates of each elementary reaction using well-known Arrhenius equation: 𝑘=𝐴𝑇%𝑒(()*+,-⁄) (1) where A (pre-exponential factor), b (temperature exponent), and Ea (activation energy) are empirical parameters. For chemical kinetics, a CHEMKIN format GRI-Mech 3.0 mechanism Figure 2. (a) Geometry of the model, (b) Grid configuration on a slice of the model, (c) Fixed embedding on the nozzle and near nozzle areas during injection, (d) Adaptive mesh refinement during combustion and flame propagation. Table 3. Grid embedding sensitivity analysis. A base grid size of 5 mm and embedding levels of 2 and 3 (underlined) for the injector and near nozzle areas are chosen to create the optimized grid configuration. doi: 10.1115/ICEF2019-7159 Pre-Print 4 including 53 species and 325 reactions is introduced to the modeling. A varied time step with a minimum of 1×10-8 s, a maximum of 1×10-4 s, and a maximum convective CFL (Courant-Friedrichs-Lewy) limit of 1.0 is used to regulate the time step when required. For turbulence, a RNG k-ε (Re-normalization group) RANS (Reynolds-averaged Navier-Stokes) model is used to calculate the turbulent kinetic energy and dissipation rate at a relatively moderate computational cost. 2.2. Model Validation In order to validate the numerical results, an experiment on the same engine involving 21% oxygen and 79% argon, and operating at 600 rpm with a compression ratio of 10 is simulated. The pressure trace is then compared against the experimental data [32]. Three different base grid sizes of 4, 5, and 6 mm are evaluated, and the results are shown in Figure 3. The error in prediction of the peak pressure is 4.4%, 4.6%, and 5.1% respectively. Based on this result, the base grid size of 5 mm is selected for the entire study since it possesses a good level of agreement with the experimental data. 3. RESULTS AND DISCUSSION Nine different intake gas compositions ranging from 0% argon replacement (air) to 80% argon replacement are assumed for comparison of the combustion characteristics of natural gas. The oxygen concentration is kept constant (21%) in all cases. The effect of argon addition on pressure, temperature, heat release rate (HRR), ignition delay, and emissions is investigated, and the results are extensively discussed. 3.1. Combustion Characteristics The resulted pressure and heat release rate curves are shown in Figure 4. The case in which nitrogen (0% argon addition) was the working fluid does not ignite, as expected. However, all other cases yield ignition of the natural gas and combustion inside the engine. However, the cases with at least 30 percent argon addition indicate more practical curves compared to 10 and 20 percent. Increasing the amount of argon in the intake Figure 4. Pressure and HRR at different argon addition conditions. Figure 5. Maximum and average temperature at different argon addition conditions. Figure 3. Comparison of the modeling and experimental results. dx shows the base grid size. doi: 10.1115/ICEF2019-7159 Pre-Print 5 composition leads to an earlier combustion and a higher peak pressure. Additionally, it extends the combustion duration. The heat release rates however show a more complex behavior with respect to the argon addition. The lowest peak heat release rate belongs to the 80 percent argon addition (~16 J/°CA), and the highest peak heat release rate occurs when 30 percent of nitrogen is replaced by argon (~41 J/°CA). Increasing the amount of argon above 30 percent reduces the peak heat release rate, and accelerates the onset of combustion. For 10 and 20 percent argon addition, the pressure and heat release rates show abnormal combustion with a low and extremely late heat release. The average and maximum temperature in the domain are shown in Figure 5, which are completely in agreement with the pressure and heat release rate data. As seen, the average temperature increases with addition of argon in the inlet gas composition. The maximum temperature as an appropriate representative of ignition indicates that decreasing the amount of argon delays the onset of combustion due to the lower in-cylinder temperature before TDC when the natural gas is injected. The first jump in the maximum temperature curve is due to the opening of the nozzle and start of injection at -15 CA aTDC. When the nozzle opens, a high pressure stream of natural gas enters into the cylinder and creates a high speed flow. Therefore, the temperature of some near nozzle areas slightly increases due to the high speed flow of the injected gas and the mixing process. Fluid mechanics and gas dynamics cause this jump in the maximum temperature curve. Thus, it does not affect the HRR curve. Due to the extremely lean operating condition (0.09), the average temperature is not severely high. This aids to the reduction of NOx, which is discussed in the emissions section of this paper. In order to have an explicit understanding of the effect of argon addition on the auto-ignition of natural gas, the in-cylinder pressure and temperature at start of injection (SOI) are recorded and plotted as shown in Figure 6. At -15 °CA aTDC when the injection begins, the pressure is 29.3 bar if only nitrogen and oxygen exist in the cylinder. Replacing 50 percent of nitrogen by argon increases the pressure at SOI to 34.4 bar (~17%). This pressure can further advance to 39.4 bar (~34%) if 80 percent of nitrogen replaces by argon. More importantly, the average in-cylinder temperature at SOI, which is a decisive parameter in auto-ignition, boosts from 948 K in case of no argon to 1110 K with 50 percent and 1271 K with 80 percent argon addition. This is due to the lower specific heat of argon compared to nitrogen, which enables the mixture to attain a higher temperature and facilitates the auto-ignition of natural gas. Both temperature and pressure graphs are concave upward, which implies that the effect of argon addition on in-cylinder temperature and pressure is increasing. Figure 7 shows the ignition delay versus average temperature for varied argon addition cases. Ignition delay can be calculated using the extrapolation of the maximum slope of the pressure, temperature, or heat release rate back to the start of injection condition. In this study, heat release rate was used for calculation of the ignition delay. For 10 and 20 percent argon addition when the temperature is less than 1100 K, a quite long ignition delay is observed (more than 1.5 ms or 10 °CA). Increasing the amount of argon in the intake composition reduces the ignition delay. For 40 percent argon addition, the ignition delay is slightly more than 1 ms (7 °CA), and for 60 percent and above argon addition, the ignition delay becomes significantly Figure 6. In-cylinder pressure and average temperature at start of injection (-15 °CA aTDC) at different argon addition conditions. Figure 7. Ignition delay versus average temperature. The percentages on data points represent nitrogen replacement by argon. doi: 10.1115/ICEF2019-7159 Pre-Print 6 shorter (less than 4 °CA). This is due to the relatively higher temperature (above 1200 K) for these cases. As seen, the desired ignition delay can be readily regulated by changing the concentration of argon in the intake gas composition. 3.2. Species and emissions Numerical simulations can enable the investigation on the history of species during ignition and combustion development without requiring intricate and costly optical measurement methods. Implementation of a reliable chemical kinetics mechanism such as GRI-Mech 3.0 on the CFD modeling combined with the adaptive mesh refinement and an established chemistry solver such as SAGE are sufficient for acquiring informative data on species production and consumption. Figure 8 shows the history of crucial species and radicals along with the recorded average temperature for the 50 percent argon addition case. The injection duration is highlighted for a better understanding of the species development. The average temperature at SOI (-15 °CA aTDC) is slightly above 1100 K, and it reaches a maximum of 1368 K. For intermediate temperature ignition applications such as compression ignition, homogeneous charge compression ignition (HCCI), rapid compression machines (RCM), and engine knock studies, key reactions are as following [34]: 𝐻+𝑂3+𝑀→𝐻𝑂3+𝑀 (2) 𝐶𝐻7+𝐻𝑂3→𝐶𝐻8+𝐻3𝑂3 (3) 𝐻3𝑂3+𝑀→𝑂𝐻+𝑂𝐻+𝑀 (4) 𝐶𝐻7+𝑂𝐻→𝐶𝐻8+𝐻3𝑂 (5) where M is a third body, CH4 is a representative for alkanes, and CH3 is an example of an alkyl radical. After start of injection, HO2 and H2O2 radicals are produced through reactions 2 and 3. These two radicals are associated with the low temperature heat release. They maintain growing with the temperature increase due to the compression in the cylinder. At about 1150 K when the crank angle is roughly -8 aTDC, H2O2 breaks rapidly, and generates OH radicals (reaction 4). This is believed to be the key reaction for ignition since two OH radicals are produced in a quite short time. Figure 8 indicates that the change of slope in the temperature curve is completely synced with the H2O2 decomposition, which is in agreement with the literature [35]. With production of OH, the fuel which is mainly methane reacts with OH and generates an alkyl (CH3) and water vapor (reaction 5). The results indicate that reactions 4 and 5 are decisive for prediction of ignition and onset of combustion. Since this engine is operated at an extremely lean condition, near zero unburned hydrocarbons are observed in the products, and Figure 8. History of species mole fractions during combustion stage at 50% argon addition condition. Figure 9. Comparison of emissions at different argon addition conditions. Figure 10. Mole fractions of the products at 50% argon addition condition. doi: 10.1115/ICEF2019-7159 Pre-Print 7 the only emissions produced are CO2 and NOx, which are plotted versus crank angle in Figure 9. In order to prevent confusion caused by redundant crossing curves, only four cases involving 10, 30, 50, and 80 percent argon addition are presented to understand the generic effect of nitrogen replacement by argon on the production of emissions. Both CO2 and NOx emerge earliest at 80 percent condition followed by 50, 30, and 10 percent, which is in agreement with the start of combustion for these cases. After ignition, CO2 and NOx concentrations in the cylinder increase immediately. The most rapid growth of CO2 and NOx is observed at 30 percent argon addition, which possesses the highest peak of heat release as seen in Figure 4. The production of NOx and CO2 continues at a slower speed with the drop of heat release rate, and ends when the heat release rate becomes zero. The final mass of the emissions has a reverse correlation with the amount of argon added in the inlet composition. This means that the 10 percent case produces the largest amounts of emissions followed by 30, 50, and 80 percent argon addition cases. The result indicates that the replacement of nitrogen by argon does not exclusively reduce the NOx emissions due to the decrease in the number of nitrogen atoms, it additionally diminishes the CO2 emission in products. The final products of the 50 percent argon addition case are illustrated as a pie chart in Figure 10. The major products include argon (39.1%), nitrogen (39.1%), and oxygen (18.7%). This indicates that the hypothesis of recirculating the excess oxygen to the inlet manifold due to the extremely lean condition is practical. Water vapor and carbon dioxide are the following products with 2 and 1 percent mole fraction, and the other products including NOx are quite negligible (0.04%). The composition of the products implies the feasibility of a relatively clean compression ignition engine using natural gas as the fuel and argon as the addition to the working fluid. Although the results of this study indicate a clean and efficient compression ignition of natural gas, more investigations are required to optimize the combustion characteristics and emission analysis. The calculated indicated mean effective pressure (IMEP) for different argon addition cases vary between 1.7 to 2.2 bar, which is relatively low. However, this modeling is at quite lean operating condition (about 0.1 equivalence ratio), and such an IMEP range is expected. An enhanced IMEP can be achieved by increasing the overall equivalence ratio using a multi-hole injector. On the other hand, increasing the injection pressure can cause a greater mass of fuel, which leads to a richer operating condition and consequently a higher IMEP . There are numerous parameters which can significantly affect the combustion process including running the engine at different compression ratios, varied intake temperatures, and boosted intake conditions. Furthermore, a specific injector design for natural gas can expand the range of the overall equivalence ratio. The lower density of natural gas compared to common liquid fuels used in compression ignition engines such as diesel yields smaller amount of the injected fuel. Optimization of the injector such as the number of nozzles, size of nozzles, and injection pressure can overcome this issue. Finally, a comprehensive system analysis is essential to assess the performance and efficiency of the hypothetical argon assisted natural gas compression ignition cycle. 4. CONCLUSIONS The feasibility of operating a compression ignition natural gas engine using partial replacement of nitrogen with argon as the working fluid was numerically evaluated, and the following represent the major outcomes of this study: • In the current engine with the specified operating conditions, the auto-ignition of natural gas does not occur when air is the working fluid. However, partial replacement of nitrogen with argon results in auto-ignition and combustion. Although abnormal combustion and quite long ignition delays are observed for 10 and 20 percent replacement, a more practical engine cycle is achievable when more than 30 percent argon is added to the intake. • Increasing the amount of argon in the intake gas composition leads to the reduction in ignition delay and heat release rate. Additionally, it increases the peak pressure, in-cylinder temperature, and combustion duration. The main reason for the abovementioned effects is the lower specific heat of argon compared to nitrogen, which enables the mixture to reach higher temperatures during compression. • H2O2, HO2, and OH radicals play an important role in ignition and onset of combustion. The investigation of species history indicates that the increase in temperature leads to the decomposition of H2O2, and production of OH radicals, which initiates the flame. This follows the fundamental chemical kinetics argument in intermediate temperature ignition applications. • The composition of the products shows that argon, nitrogen, and oxygen are the main products of this cycle (more than 96%), and water vapor and carbon dioxide are the minor products which can be separated through condenser and membrane. The remaining argon, nitrogen, and oxygen can be recirculated to the inlet manifold for the next cycle. The excess amount of oxygen in the products is due to the extremely lean operating conditions of this engine (overall equivalence ratio of 0.09). NOMENCLATURE A Pre-exponential factor AMR Adaptive mesh refinement aTDC After top dead center b Temperature exponent BDC Bottom dead center CA Crank angle CFD Computational fluid dynamics CFL Courant-Friedrichs-Lewy DME Dimethyl ether Ea Activation energy doi: 10.1115/ICEF2019-7159 Pre-Print 8 EGR Exhaust gas recirculation EVO Exhaust valve open HC Unburned hydrocarbons HCCI Homogeneous charge compression ignition HRR Heat release rate IMEP Indicated mean effective pressure IVC Intake valve close k Reaction coefficient M Third body MP Methyl propanoate NG Natural gas NOx Oxides of nitrogen Ru Universal gas constant RANS Reynolds-averaged Navier-Stokes RCCI Reactivity controlled compression ignition RCM Rapid compression machine RME Rapeseed methyl ester RNG Re-normalization group SOI Start of injection TDC Top dead center ACKNOWLEDGEMENTS This material is based upon work supported by Columbia Gas of Massachusetts under award #29797 with the University of Massachusetts Lowell. The authors would like to acknowledge Dr. Mohd Radzi Abu Mansor for his guidance on the numerical simulation. The authors would like to appreciate CONVERGE CFD software licensing team for their support.