High Auto-ignition Temperature Research Articles

Conversion of Post-Refining Waste MONG to Gaseous Fuel in a Rotary Gasifier

Biodiesel manufacturing frequently employs sustainable materials like soybeans, microorganisms, palm extract, jatropha plant, and recycled frying oils. The expansion of biodiesel manufacturing has escalated the volume of waste byproducts, encompassing glycerin and non-glycerin organic matter (MONG), jointly known as raw glycerin. MONG is characterized by a low calorific value, a high autoignition temperature, and significant viscosity at room temperature. As a waste product, it negatively affects the natural environment due to the lack of viable disposal methods. Hence, there is a need for its conversion into high-calorific gaseous fuel with significantly less environmental impact. One of the methods for converting MONG into gaseous fuel is the pyrolysis process. This study describes the pyrolytic conversion of MONG conducted on a test stand consisting of a rotating chamber with a shell filled with liquid lead as a heating medium. Based on the measurements and balance calculations, the amount of heat required to preserve the autothermal process was determined. The calorific value and composition of the pyrolytic gas were measured, revealing that 70% of the gas involves compounds characterized by a high calorific value. As a result, the calorific value of dry, purified gas equals 35.07 MJ/kg. A life cycle assessment has been conducted, in order to determine if the produced gaseous fuel matches sustainable development criteria. MONG-based gas is a sustainable replacement of, e.g., natural gas, lignite, or hard coal; however, it allows us to avoid 233–416 kg/h CO2 emissions per 1 MWt of heat.

Numerical and experimental investigation of performance and emission characteristics of hydrogen enrichment with prosopis juliflora biodiesel in CI engine using RSM and ANN optimization

IC engines need renewable fuel due to environmental degradation and fossil fuel depletion. Biodiesel was a popular alternative fuel. However, hydrogen has zero emissions and a higher auto ignition temperature. This study examines diesel engine operation with hydrogen-enriched juliflora biodiesel/diesel blend (B20). The 4-Stroke, direct injection diesel engine (KIRLOSKAR-AV1) produces 5.2 kW at 1500rpm. A B20 compound was injected into the combustion cylinder via the intake manifold during testing. Hydrogen was supplied through the intake manifold. Diesel (D0), B20, and B20 with hydrogen are tested on 6 lpm, 9 lpm, and 12 lpm respectively. Increase in hydrogen concentration with B20 blend will increased brake thermal efficiency. The mixture B20 at a flow rate of 12 L per minute increased the BTE by 27.51%, which is a 4.88% improvement compared to the original B20. Similarly, this concentration exhibit an 18.75% reduced Brake Specific Fuel Consumption (BSFC) compared to the B20 combination. CO and HC emissions decreases and increased in NOx emissions while increasing H2 flow. Further, to develop the RSM and ANN predicted model for optimization of CI engine characteristics responses. The RSM model analyzed the lower mean square error (MSE) and actual relative error (ARE) for the engine responses compared to the ANN model.

Experimental and computational (Chemical Kinetic + CFD) analyses of Self-Recuperative annular tubular porous burner for NH3/CH4 -air Non-Premixed combustion

Ammonia is now being explored as a potential gas turbine fuel because of its renewable and carbon-free nature. Ammonia combustion possesses various limitations, such as high NO emissions, low burning velocity, and high auto-ignition temperature. Moreover, dual-fuel combustion approaches can be used to mitigate these drawbacks. However, the limitations of available information about the resulting emissions are insufficient to encourage widespread adoption. In this work, an NH3/CH4-air fueled self-recuperative high aspect ratio annular tubular porous burner is developed for gas turbine power applications. In this burner, an annular zone surrounds the combustion chamber to preheat the incoming air, while two perforated discs are mounted between the combustor and annular zone for uniform distribution of air. Eight Zirconia foams (4 of 10 PPI and 4 of 20 PPI) are stacked at the upstream side of the combustor. Experimentally, air preheat temperature, combustor exit temperature, NO, and CO emissions are analyzed and measured for various ammonia addition (0–41 % by weight) percentages, equivalence ratio of 1.0, and heat inputs of 17 & 21 kW for 10 PPI and a combination of 10 and 20 PPI porous foams. A preheated air stream coming from the annular zone speeds up fuel dissociation and helps in stabilizing the flame on the upstream side of the combustor. Combustor exit temperatures are lower in sandwich cases of 10 + 20 PPI than the porous foams of 10 PPI. However, with porous foams of 10 PPI, the air preheat temperature is higher. The insertion of porous foams shows better recirculation of heat and stable combustion is observed for higher percentages of ammonia and higher thermal inputs. The OH radicals are distributed for larger areas inside the dumb combustor, while for porous combustion, it is distributed near the porous foams. The porous sandwich cases of 10 + 20 PPI shows higher combustion efficiency compared to 10 PPI and without porous cases. The chemical kinetic analysis shows that the HNO and HCO radicals act as the main intermediate channel for NO and CO production.

Kestabilan Pencucuhan Hidrogen dalam Persekitaran Gas Adi dalam Enjin Pencucuhan Mampatan

Hydrogen fuel promises a high engine efficiency and the ability to eliminate harmful emissions in an internal combustion engine. However, the ignition stability of hydrogen in compression ignition engine is unstable due to the high-auto ignition temperature of hydrogen. Noble gases such as argon, krypton and xenon have a larger molecular weight compared to nitrogen. Therefore, replacing nitrogen with noble gases appears to be a promising option due to their high specific heat ratio. This paper aims to investigate the ignition stability in a heavy molecular noble gases atmosphere in a CI engine. In this study, Converge CFD software simulates a singlecylinder compression ignition engine model based on the Yanmar NF19SK engine parameters. Hydrogen combustion in a noble gas atmosphere resulted in low ignition stability when operated at low intake temperature. Based on this study, we found that increasing intake temperature improves the ignition stability. As a result, argon is the most preferable among other noble gases. Nevertheless, future research should investigate the other noble gases’ capability under different parameters to improve the engine's thermal efficiency.

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

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

Investigation of intake air temperature effect on co-combustion characteristics of NH3/gasoline in naturally aspirated high compression ratio engine with sub-chamber

Recently, ammonia (NH3), which has a higher energy density than hydrogen, has gained attention for zero-carbon emission goals in the transportation sector. However, in a conventional internal combustion engine (ICE), NH3 combustion mechanism is still under investigation. In this paper, to further expand the knowledge on the adoption of NH3 in ICEs, authors conducted NH3/gasoline co-combustion experiments in a modified, 17.7:1 compression ratio, naturally aspirated spark-assisted CI engine with sub-chamber. The sub-chamber was chosen in order to enhance the combustion speed of NH3. In addition, the sub-chamber was equipped with glow and spark plugs to overcome the high auto-ignition temperature of NH3. Engine performance and NOX emissions were studied under three different intake air temperatures. During the experiments, NH3 content was increased gradually where the engine was run under lean conditions. Although higher NH3 content was achieved compared to our previous work, increasing the intake air temperature resulted in decreased charging efficiency. In addition, corrosion was found on the piston ring after 120 h of operation, negatively affecting the engine performance. Furthermore, NH3/gasoline co-combustion duration was shortened drastically with the influence of the sub-chamber, where the longest combustion duration under the present conditions was found to be 17°CA.

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. DOI: https://doi.org/10.18573/jae.6

Numerical study of heat transfer of hydrogen combustion in noble gases atmosphere in compression ignition engine

The high energy content of hydrogen and zero carbon emission from hydrogen combustion is very important for compression ignition engine development. Hydrogen requires a very high auto-ignition temperature, which encourages replacing nitrogen with noble gases with higher specific heat ratio during compression process. In noble gases-hydrogen combustion, higher combustion temperature potentially leading to a higher heat loss. This paper aims to investigate the effect of hydrogen combustion in various noble gases on heat distribution and heat transfer on the cylinder wall. Converge CFD software was used to simulate a Yanmar NF19SK direct injection compression ignition engine. The local heat flux was measured at different locations of cylinder wall and piston head. The heat transfer of hydrogen combustion in various noble gases at different intake temperatures was studied using the numerical approach. As a result, hydrogen combustion in light noble gases such as helium produces faster combustion progress and higher heat temperature. The hydrogen combustion that experienced detonation, which happened in neon at 340 K and argon at 380 K, recorded a very high local heat flux at the cylinder head and piston due to the rapid combustion, which should be avoided in the engine operation. At a higher intake temperature, the rate of heat transfer on the cylinder wall is increased. In conclusion, helium was found as the best working gas for controlling combustion and heat transfer. Overall, the heat transfer data gained in this paper can be used to construct the future engine hydrogen in noble gases.

Combustion, performance and emission analyses of a CI engine operating with renewable diesel fuels (HVO/FARNESANE) under dual-fuel mode through hydrogen port injection

The use of hydrogen in internal combustion engines is pointed out as an alternative to reduce greenhouse gas emissions. In applications that require high levels of torque and low engine speeds, compression ignition (CI) engines are more appropriate. However, because of the high auto-ignition temperature of hydrogen, its use in these engine types is more suitable when the dual-fuel concept is applied. This study comprehensively investigates, through experimental techniques, the use of hydrogen port-injection in a four-stroke single-cylinder CI engine operating with the renewable diesel-like fuels hydrotreated vegetable oil (HVO) and farnesane, in comparison to fossil diesel dual-fuel operation. In this sense, the present work aims to fill a gap in the literature by performing a novel analysis of dual-fuel operation with hydrogen, considering different substitution fractions, and using groundbreaking biofuels, such as HVO and farnesane. The results showed that in-cylinder pressure and temperature were increased with H2 enrichment for every pilot fuel, but green diesel fuels presented lower values than those for diesel operation. Furthermore, hydrogen port injection slightly delayed the start of combustion and increased the ignition delay, but a reduction in both premixed and diffusion combustion duration was observed. Reductions in PM, CO, and CO2 emissions were reported during H2 addition for every pilot fuel, while increased NOx was observed. Despite this increase, both HVO and farnesane decreased the emissions of this pollutant in single and dual-fuel operations, compared with fossil diesel. In addition, both renewable diesel fuels presented higher BTE than diesel for every studied H2 mass flow.

Effects of fuel injection strategy and ammonia energy ratio on combustion and emissions of ammonia-diesel dual-fuel engine

As the greenhouse effect continues to intensify, ammonia, a carbon-free fuel, is gradually gaining attention. The ammonia-diesel dual-fuel (ADDF) engine can effectively overcome the disadvantages of high auto-ignition temperature and slow flame speed of ammonia, which is an effective way to reduce greenhouse gas (GHG). Aiming at achieving high thermal efficiency and low emissions of ADDF engine, the effects of ammonia energy ratio (AER) and injection strategy on combustion and emission characteristics were studied by combining experimental and simulation results. It was found that as the AER increased from 0 % to 90 %, the gross indicated thermal efficiency (ITEg) decreased continuously. Due to the slow flame speed of ammonia and the inhibition of the dehydrogenation process during the low temperature reaction of diesel, the emission of unburned ammonia increased significantly, reaching 31 g/kwh under the condition of 50 % AER. Improving the combustion efficiency of ammonia and reducing the emission of unburned ammonia were the keys to improving the ITEg of ADDF engine. Amino groups have a denitrification effect, and nitric oxide (NO) can be reduced to more stable nitrogen (N2). Therefore, NO emissions decreased when the AER increased. The production and consumption of nitrous oxide (N2O) were influenced by temperature, and its production region was mainly the low temperature region where the reaction of ammonia occurred, while N2O was reduced to N2 at high temperature conditions. As the AER increased, the deterioration of N2O emissions was not conducive to reducing GHG. The double injection strategy can significantly improve the activity of the mixture, accelerate combustion, reduce unburned ammonia emissions and incomplete combustion losses. By optimizing the injection strategy and regulating the combustion process, an ITEg of 49.18 % can be achieved when the AER was 50 %, and GHG can be reduced by 14.2 %.

Effect of methane supplementation on the performance, vibration and emissions characteristics of methane-diesel dual fuel engine

The increasing energy demands, especially in transportation sector, and the challenges of excess pollution and environmental degradation caused due to the conventional fuels, as well as their limited availability has highlighted the need to look for alternative fuels to sustain future needs. Methane is capable of catering to these demands due to its wide availability, both in renewable and non-renewable energy sources. The present work explores the effect of methane supplementation on the performance and emission characteristics as well as the vibrations in internal combustion engines. A four-stroke compression ignition engine is modified to run as a methane-diesel dual fuel engine where methane is inducted through intake manifold and diesel is directly injected into cylinder. Tests are performed by varying engine load and methane energy levels up to 75%. Our study shows that the participation of methane at lower load conditions is weak due to its higher auto ignition temperature and higher calorific value. The emissions, particularly CO and NO, are observably higher at 75% load conditions due to the efficient combustion and higher temperature at higher load conditions. The vibration studies on the dual fuel combustion indicates that the introduction of methane also suppresses the frequency spectrum of combustion noise and reduces the ringing intensity level of vibration for complete spectrum of engine loads, with the effect being prominent at higher loads. Overall, our results suggests that combustion of methane in dual fuel diesel engine shows distinct characteristics at contrasting load conditions.

A skeletal chemical kinetic mechanism for ammonia/n-heptane combustion

Progressively stricter pollutant emission targets in international agreements have shifted the focus of combustion research to low carbon fuels. Ammonia is recognized as one of the promising energy vectors for next-generation power production. Due to the low flame speed and high auto-ignition temperature, ammonia is often burned with a high reactivity pilot fuel (e.g. diesel). However, chemical kinetic mechanisms describing the combustion of ammonia and large hydrocarbon fuels (such as n-heptane, a surrogate of diesel) are less developed. In this work, a skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed using a joint decoupling methodology and optimization algorithm. A sensitivity analysis of the ignition delay times of the ammonia/n-heptane mixture is performed to identify the dominant reactions. A genetic algorithm is used to optimize the mechanism further. The final skeletal mechanism is made up of 69 species and 389 reactions. The skeletal ammonia/n-heptane mechanism is validated against the experimental data for combustion of pure ammonia, ammonia/hydrogen and ammonia/n-heptane mixtures, including the global combustion characteristic parameters such as ignition delay times measured in shock tubes or rapid compression machines, laminar burning velocities measured in heat flux burners or spherical flame vessels, and species data measured in jet-stirred reactors. Comparing the results from the skeletal mechanism with those from other mechanisms from the literature is conducted to evaluate the mechanism further. The present skeletal mechanism can well predict the combustion processes for a wide range of conditions, and the mechanism is computationally efficient, showing good potential to model ammonia/n-heptane combustion with good accuracy and efficiency.

Ammonia/Methane combustion: Stability and NOx emissions

In the framework of the energy shift toward Renewable Energy Sources, ammonia is considered a valuable energy vector, due to its very high hydrogen-density and well-established production processes. Despite the many potential advantages, its combustion features (narrow flammability limits, high auto-ignition temperature, potential high fuel-NOx emissions) may hinder its wide utilization. To overcome these issues in conventional combustion, “fuel enhancers” are also used. Differently from conventional systems, MILD Combustion already proved to be very effective in oxidizing ammonia in terms of stability and NOx emission. Nevertheless, a “fuel enhancer” can be also useful in MILD Combustion conditions to further improve the process characteristics. The present study focuses on the ammonia/methane combustion characteristics under MILD Combustion conditions in a lab-scale burner. Gaseous pollutant emissions (NOx, NH3, H2, CO) and process stability limits were analysed as a function of the equivalence ratio and NH3/CH4 fuel composition. Results showed that the use of NH3/CH4 blends extends the stable operational range of the system, in terms of both working temperatures and equivalence ratios, with respect to pure NH3. On the other hand, blends produce higher NOx emissions, with respect to both the pure NH3 and CH4 cases. Experimental data were compared with chemical kinetics modelling results. Chemical pathways and rate of production of main intermediate products highlighted that oxidation pathways of carbon and nitrogen-based species are essentially decoupled as experiments suggested. In contrast, the interplay of the methane and ammonia main chemical pathways affects the DeNOx channel in correspondence of the observed NOx emission increase.

Study of the Effect of Research Octane Number on the Auto-Ignition of Lubricant Oil Surrogates (n-Hexadecane).

Engine oil is considered one of the sources for pre-ignition in downsized boosted direct injection spark-ignited engines. When interacting with fuel sprayed in the combustion chamber, engine oil forms an ignitable mixture and can cause an ignition event before firing the spark plug. Because high research octane number (RON) fuels are difficult to auto-ignite and tend to suppress the knock in an internal combustion engine, studying their interaction with engine oil is essential. Hence, in the current study, a suitable lubricant oil surrogate, namely, n-hexadecane, is mixed with iso-octane and n-heptane at different concentrations to investigate the auto-ignition behavior at elevated pressures. Five sets of fuels (PRF0, PRF20, PRF50, PRF80, and PRF100) were prepared to get a wide range of RONs and blended with n-hexadecane at 15, 25, 35, and 45% mixture concentrations (vol %). These experiments were conducted in a constant volume combustion chamber, keeping the initial temperature constant at 300 °C. A single droplet of the mixture was suspended on a thermocouple bead to record the droplet’s lifetime temperature. It was observed that hexadecane mixed with PRF0, PRF20, PRF50, and PRF80 showed similar auto-ignition behaviors. The time of ignition (TI) for these mixtures initially increased until 25% concentration of the fuel in n-hexadecane, and further addition of fuels to 35% and higher concentrations showed a gradual decrease in TI. Ignition of mixtures with 35% and 45% fuel concentrations is attributed to n-heptane, as its low temperature chemistry is the dominant factor in its high reactivity compared to iso-octane. TI increased with the increasing concentration of PRF100 mixtures in hexadecane, unlike other PRF fuels tested in this study. This is because iso-octane is a high RON fuel with a higher auto-ignition temperature, making it challenging to auto-ignite.

Study of utilization of hydrogen as fuel in internal combustion engine

The heightened depletion rate of fossil fuels and growing degradation of environment has energized the various researchers to work prominently in search of an alternative, abundant and environment friendly fuels. Renewable and clean energy plays key role for the sustainable development of the society. This paper examines using hydrogen (H2) as the primary fuel in internal combustion (IC) engines. H2 seems to be a great contender among newly emerging renewable resources of energy and it is considered as the future of the fuels by researchers that may replace the hydrocarbons fuels. High auto ignition temperature for cranking engine, high flame velocity and low quenching distance are the main challenges when H2 is burnt in IC engines. In this paper, the effect of Exhaust Gas Recirculation (EGR), water injection, blending biodiesel, and ignition delay on performance parameters has been discussed to tackle the problems of soot, and high engine temperature while using H2 as fuel.

Performance and emission characteristics of a methane fuelled HCCI engine at various injection location and operating speed

Methane is a promising alternative fuel for stationary IC engines due to its ease of production and superior combustion properties. However, methane cannot be used independently in Homogeneous Charge Compression Ignition (HCCI) engines due to its high auto-ignition temperature. Another contributing factor is its lesser cetane number compared to its alternatives. A high reactivity fuel such as Di-Ethyl Ether (DEE) has to be used as an ignition improver by virtue of its high cetane number. In this study, methane and DEE are used as primary and secondary fuels respectively in HCCI mode. A mixture of methane and air along with the ignition improver DEE is injected in two locations – the first location being behind the inlet port and the second location being the manifold that is situated 10 cm away from the inlet port during suction stroke. The performance and emission of the engine are compared for both modes of DEE injection with torque varying from 10% to 90% of rated load and speeds ranging between 1800 rpm and 1900 rpm. A constant methane flow rate of 12 lpm is used. Port injection provides better thermal efficiency at 1800 rpm while manifold injection is observed to be superior at 1900 rpm. HC and CO emissions are lower with manifold injection at 1800 rpm whereas NOx emissions are comparable for both modes. Smoke and NOx emissions are lower with manifold injection at 1900 rpm.

Low-temperature auto-ignition characteristics of NH3/diesel binary fuel: Ignition delay time measurement and kinetic analysis

Ammonia (NH3) is gaining increasing interest as a carbon-free alternative fuel in engine systems. Co-firing NH3 with diesel can overcome the high auto-ignition temperature and narrow flammability limits of pure NH3, while exploit its advantage. This study presents the auto-ignition properties of NH3/diesel binary fuel, at various NH3 blending ratios (10%, 30% and 50%) in a rapid compression machine. Ignition delay times (IDTs) were measured spanning a temperature range of 670–910 K, pressures of 10–20 bar, and equivalence ratios of 0.5–1.5. Typical two-stage ignition and negative temperature coefficient (NTC) response were identified for the blends. Both the first-stage and the total IDTs increase with the increasing NH3 blending ratio, and there exists a non-linear inhibiting effect of NH3 fraction on IDT. A blending mechanism was then constructed based on the existing diesel mechanism and NH3 mechanism. The mechanism can predict the inhibiting effect of NH3 addition, but fails to well capture the IDTs over the whole temperature range, especially for the first-stage IDT and the NTC response. Further kinetic analysis, including species mole fraction history, brute force sensitivity analysis and reaction pathway analysis, were conducted to gain deeper insight into the auto-ignition chemistry of the blending fuel. These analyses suggest the rate parameters of reactions NH3 + OH ⇒ NH2 + H2O and NH2 + NO ⇒ N2 + H2O are critical to accurately predict IDTs, and the competition role of NH3 for OH radical inhibits the diesel low-T chain-branching sequence, which eventually leads to restraining the reactivity of the whole reaction system.

Ammonia as Effective Hydrogen Storage: A Review on Production, Storage and Utilization

Ammonia is considered to be a potential medium for hydrogen storage, facilitating CO2-free energy systems in the future. Its high volumetric hydrogen density, low storage pressure and stability for long-term storage are among the beneficial characteristics of ammonia for hydrogen storage. Furthermore, ammonia is also considered safe due to its high auto ignition temperature, low condensation pressure and lower gas density than air. Ammonia can be produced from many different types of primary energy sources, including renewables, fossil fuels and surplus energy (especially surplus electricity from the grid). In the utilization site, the energy from ammonia can be harvested directly as fuel or initially decomposed to hydrogen for many options of hydrogen utilization. This review describes several potential technologies, in current conditions and in the future, for ammonia production, storage and utilization. Ammonia production includes the currently adopted Haber–Bosch, electrochemical and thermochemical cycle processes. Furthermore, in this study, the utilization of ammonia is focused mainly on the possible direct utilization of ammonia due to its higher total energy efficiency, covering the internal combustion engine, combustion for gas turbines and the direct ammonia fuel cell. Ammonia decomposition is also described, in order to give a glance at its progress and problems. Finally, challenges and recommendations are also given toward the further development of the utilization of ammonia for hydrogen storage.

The effect of ammonia addition on the low-temperature autoignition of n-heptane: An experimental and modeling study

Ammonia (NH3) is receiving increasing attention as an alternative engine fuel due to its carbon-free nature. However, its fundamental combustion characteristics, such as higher autoignition temperature and lower burning velocity compared to conventional hydrocarbons, limit its direct use in traditional engines. To confront this dilemma, hydrocarbon/NH3 blending fuels are seen as one of the most appropriate ways to exploit its advantage and offset its weaknesses. Using n-heptane as a representative of hydrocarbons, this study investigated the effect of NH3 addition on the low-temperature autoignition of n-heptane. The ignition delay times of five n-heptane/NH3 mixtures with NH3 fractions of 0%, 20%, and 40% were measured in a rapid compression machine at temperatures of 635–945 K, pressures of 10 and 15 bar, and equivalence ratios of 1.0 and 2.0. Experimental results show that the n-heptane/NH3 blending fuels exhibit pronounced low-temperature reactivity, and both the total and the first-stage ignition delay times increase with the increase of NH3 fraction. A blending mechanism of n-heptane/NH3 was compiled based on the existing n-heptane mechanism and NH3 mechanism. It is found that the blending mechanism is capable to predict the inhibition effect of NH3 addition on n-heptane autoignition and qualitatively capture the dependence of ignition delay time on equivalence ratio and oxygen mole fraction. Nevertheless, there are significant discrepancies between the experiments and the simulation. Furthermore, kinetic analyses, including species evolution, rate of production and sensitivity of OH radical were conducted sequentially to reveal the autoignition kinetics of NH3/n-heptane blends and the interaction between n-heptane and NH3. Suggestions are provided for the further development of the blending mechanism.

Combustion characteristics of direct injection hydrogen in noble gases atmosphere

Hydrogen combustion in the internal combustion engine (ICE) has currently taken part in the transportation industry due to its environmentally friendly and high engine thermal efficiency. Hydrogen direct injection introduction in the internal combustion engine eliminates the harmful emission from carbon and improves engine stability. The substitution from nitrogen to noble gases for the intake will solve the high auto-ignition temperature of hydrogen in compression ignition (CI) engine problem. This paper intended to study the combustion characteristics of direct injection hydrogen in oxygen-noble gas atmosphere such as argon and helium to find the suitable engine and injection parameters for the optimum engine efficiency. The simulation conducted using Converge CFD V2.4 software based on Yanmar NF19K direct injection CI engine. The simulation was done for different oxygen-noble gases such as argon, neon, and helium, while the injection timing and ambient temperature were modified to optimize the hydrogen combustion characteristics. The study found that noble gases help to increase the in-cylinder temperature and ignite the hydrogen. The study also found that early injection timing results in the highest in-cylinder pressure. Besides, increasing ambient temperature for intake decreases the in-cylinder pressure and lengthen the ignition delay. Therefore, the most suitable injection parameter for the most optimum hydrogen combustion characteristics is 380 K ambient temperature, early injection timing at -10°CA, and higher injection pressure. Helium produced higher in-cylinder peak pressure compared to argon due to its high specific heat ratio. Hence, further consideration of helium and good injection strategies should be applied for better engine thermal efficiency.