Performance and Emissions of an Ammonia-Fueled SI Engine with Hydrogen Enrichment

International audienceWhile the optimization of the internal combustion engine (ICE) remains a very important topic, alternative fuels are also expected to play a significant role in the reduction of CO2 emissions. High energy densities and handling ease are their main advantages amongst other energy carriers. Ammonia (NH3) additionally contains no carbon and has a worldwide existing transport and storage infrastructure. It could be produced directly from renewable electricity, water and air, and is thus currently considered as a smart energy carrier and combustion fuel. However, ammonia presents a low combustion intensity and the risk of elevated nitrogen-based emissions, thus rendering in-depth investigation of its suitability as an ICE fuel necessary.In the present study, a recent single-cylinder spark-ignition engine is fueled with gaseous ammonia/hydrogen/air mixtures at various hydrogen fractions, equivalence ratios and intake pressures. A small hydrogen fraction is used as combustion promoter and might be generated in-situ through NH3 catalytic or heat-assisted dissociation. The in-cylinder pressure and exhaust concentrations of selected species are recorded and analyzed. Results show that ammonia is a very suitable fuel for SI engine operation, since high power outputs could be achieved with indicated efficiencies higher than 37% by taking advantage of the promoting effects of supercharging and hydrogen enrichment around 10% by volume. High NOx and unburned NH3 exhaust concentrations were also observed under fuel-lean and fuel-rich conditions, respectively. While hydrogen enrichment promotes the NH3 combustion efficiency and helps reducing its exhaust concentration, it has a promoting effect on NOx formation, assumedly due to higher flame temperatures. Therefore, it is recommended to take advantage of the simultaneous presence of exhaust heat, NOx and NH3 in a dedicated after-treatment device to ensure the economic and environmental viability of future ammonia-fueled engine systems

EXPERIMENTAL STUDY ON NH3/H2/AIR COMBUSTION IN SPARK-IGNITION ENGINE CONDITIONS

International audienceThe mitigation of climate change implies the increasing use of variable renewable energy sources. Energy storage and transport solutions will contribute to ensure the stability, reliability and flexibility of the energy systems. Ammonia is a well-known chemical of formula NH3 and, amongst other electrofuels, a promising energy carrier and carbon-free combustible fuel. There-fore, it is of significant interest to study ammonia combustion systems. The present investiga-tion focusses on premixed ammonia/hydrogen/air combustion to assess stability ranges, perfor-mance and pollutant emissions by means of a systematic parametric study, in the purpose of optimization in the case of a current spark-ignition engine. Gaseous ammonia blends with a wide range of hydrogen fuel fractions and equivalence ratio were tested at two different loads. Results show a power output and indicated efficiency benefit of low H2 enrichment for slightly rich and slightly lean mixtures, respectively. Hydrogen is therefore mainly an ignition promoter, rather than a global combustion promoter assumedly due to high thermal losses. A small amount of H2, along with supercharged operation greatly improves the performances of the engine and its stability, thus rendering NH3 a very suitable fuel for SI-engines in case of in-situ H2 gener-ation. Hydrogen also mitigates unburned NH3 emissions, yet not sufficiently but those could be combined with the evenly elevated NOx emissions in dedicated selective catalytic reduction systems

Combustion Characteristics of Ammonia in a Modern Spark-Ignition Engine

International audienc

Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures

International audienc

Uncertainty in measuring laminar burning velocity from expanding methane-air flames at low pressures

International audienceThe experimental determination of laminar burning velocity remains essential to evaluate the combustion potential of any fuels but also to validate kinetic mechanisms. Recently, researchers are making great efforts to improve the accuracy of the different setups and techniques to determine this parameter. This work proposes an attempt to summarize the different factors contributing to the uncertainty of the expanding spherical flame method. In particular, the validity of two hypothesis (adiabatic flame propagation and thin flame front) is discussed in the case of stoichiometric methane-air flames in low-pressure environment (from 0.2 to 2 bar). Last, the effect of spark electrode diameters was also considered (0.2, 0.5 and 1 mm)

Experimental study of RCCI engine – ammonia combustion with diesel pilot injection

Ammonia is seen as one potential carbon-free fuel, especially for maritime applications. Since SI engines require a significant ignition energy for large cylinders, engine manufacturers are targeting the use of ammonia in Compressed Ignition (CI) engines. Because of ammonia’s high auto-ignition temperature, to ensure that the combustion occurs in a CI engine, a pilot injection of a higher reactivity fuel must be used, as in Reactivity Controlled Compression Ignition engines. In the present study, the objective was to provide first unique data about the efficiency and pollutant emissions for a single cylinder compression ignition engine with a diesel energy fraction as minimum as possible (down to less than 2%) at a constant 1000 rpm. Experiments cover the impact of a wide variation of equivalence ratios of NH3-air mixtures from ultra-lean to slightly rich conditions. CO2, CO, NH3, NOX, N2O, UHC values were measured with a Fourier Transform Infrared (FTIR) spectrometer. Results of CO2 and N2O are presented as CO2-Equivalent (CO2eq) impact. Combustion stability was achieved for most conditions but not for the leanest ones. Furthermore, under lean conditions for a similar ammonia content, the minimum CO2eq is reached with a slightly higher Diesel Energy Fraction than the minimum possible. Finally, both leanest and richest conditions present a higher level of CO2eq compared to the range of ammonia/air mixtures at stoichiometry or just below

X-ray diagnostics of dodecane jet in spray A conditions using the new one shot engine (NOSE)

[EN] Quantifying liquid mass distribution data in the dense near nozzle area to develop and optimize diesel spray by optical diagnostic is challenging. Optical methods, while providing valuable information, have intrinsic limitations due to the strong scattering of visible light at gas-liquid boundaries. Because of the high density of the droplets near the nozzle, most optical methods are ineffective in this area and prevent the acquisition of reliable quantitative data. X-ray diagnostics offer a solution to this issue, since the main interaction between the fuel and the X-rays is absorption, rather than scattering, thus X-ray technique offers an appealing alternative to optical techniques for studying fuel sprays. Over the last decade, x-ray radiography experiments have demonstrated the ability to perform quantitative measurements in complex sprays. In the present work, an X-ray technique based on X-ray absorption has been conducted to perform measurements in dodecane fuel spray injected from a single-hole nozzle at high injection pressure and high temperature. The working fluid has been doped with DPX 9 containing a Cerium additive, which acts as a contrast agent. The first step of this work was to address the effect of this dopant, which increases the sensitivity of X-ray diagnostics due its strong photon absorption, on the behavior and the physical characteristics of n-dodecane spray. Comparisons of the diffused back illumination images acquired from ndodecane spray with and without DPX 9 under similar operating conditions show several significant differences. The current data show clearly that the liquid penetration length is different when DPX 9 is mixed with dodecane. To address this problem, the dodecane was doped with a several quantities of DPX containing 25% ± 0.5 of Cerium. Experiments show that 1.25% of Ce doesn’t affect the behaviour of spray. Radiography and density measurements at ambient pressure and 60 bars are presented. Spray cone angle around 5° is obtained. The obtained data shows that the result is a compromise between the concentration of dopant for which the physical characteristics of the spray do not change and the visualization of the jet by X-ray for this concentration.This work is supported by ANR Research National Agency (ECN-France project). I.C. is supported by ANR PLANEX ANR-11-EQPX-0036-01.The authors would like to thank Thierry Seguelong for DPX9 supply and Gilles Bruneaux for scientific discussions.Chiboud, I.; Arjouche, H.; Nilaphai, O.; Dozias, S.; Moreau, B.; Hespel, C.; Foucher, F.... (2017). X-ray diagnostics of dodecane jet in spray A conditions using the new one shot engine (NOSE). En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 755-762. https://doi.org/10.4995/ILASS2017.2017.4705OCS75576

A numerical study of ammonia combustion in spark-ignition and reactive-fuel pilot-ignition engines

Decarbonizing internal combustion engines (ICEs) requires the use of fuels produced from renewable energy, with easy storage and characterized by a combustion process with zero carbon dioxide (CO2) emissions. Ammonia (NH3) perfectly fits all these requirements. However, its use as fuel for ICEs calls into question many of the consolidated aspects related to ignition and flame propagation processes studied during the last decades. NH3 differs from conventional hydrocarbon fuels for a higher minimum ignition energy and auto-ignition temperature, as well as for a lower combustion speed and energy density. Experimental investigations carried out in both metal and optical engines proved the feasibility of NH3 operation as pure fuel in spark-ignition (SI) engines or in reactive-fuel pilot-ignition (RFPI) engines with a pilot injection of a high-reactivity fuel. In this work, computational fluid dynamics (CFD) methodologies consolidated with conventional fuels are applied to simulate a selection of operating points on such experiments. The flame area model (FAM) from Weller is employed for the SI operation, while the tabulated well-mixed (TWM) model is used for the RFPI mode. The effects from a NH3-air dilution, a spark-timing advance and an increase in injection duration are studied to identify the main challenges related to the NH3 combustion modelling in ICEs. The results show that numerical models capture the measured trend of spark-timing and injection duration variations at both stoichiometric and lean NH3-air mixtures. However, for the SI mode, aspects such as the laminar-to-turbulent transition stage and the heat release rate dependency on the ignition time require further modeling improvements. Similarly, for the RFPI mode, the auto-ignition delay of the dual-fuel mixture and the turbulent flame speed are numerically underestimated. Therefore, all these aspects represent challenges that need to be addressed in CFD models to improve NH3 ignition and combustion prediction

Numerical investigation of the effect of pressure on heat release rate in iso-octane premixed turbulent flames under conditions relevant to SI engines

A series of direct numerical simulations (DNS) of iso-octane/air turbulent premixed flames in the thin reaction zones regime have been performed in order to investigate the effect of pressure on heat release rate under conditions relevant to spark-ignition (SI) engines (up to 20 bar and 800 K in the unburnt gas). Chemistry is represented by a reduced kinetics mechanism containing 74 species and 976 reactions (reduced from CaltechMech). The effect of pressure has been isolated by fixing the Karlovitz numbers, the Lewis numbers, and the ratio of integral length scale to laminar flame thickness. On one hand, the results suggest that pressure has very limited effect on the mean heat release rate, such that turbulent burning velocity is proportional to the turbulent surface area. In addition, while the chemical pathways are strongly affected by pressure in laminar flames, the global effect of turbulence on these pathways is negligible, independent of pressure. On the other hand, the local distribution of heat release rate was found to be significantly affected by pressure through differential diffusion-chemistry effects