Shorter Combustion Duration Research Articles

Exploration of the reactivities of homemade binary pyrotechnics

Understanding the properties of explosives is the basis for investigating and analyzing explosion cases. To date, due to the strict legal control of standard explosives and initiators, homemade pyrotechnics composed of oxidizers and fuels have become popular explosive sources of improvised explosive devices (IEDs) threatening greatly social stability and personal safety. The reactivity of pyrotechnics strongly depends on their intrinsic characteristics and operating conditions, which determine the efficiencies of heat and mass transfer between the reaction zone and the unreacted zone. Herein, the tests of thermodynamics, pressurization characteristics, and combustion propagation behaviors are conducted to explore the effects of oxidizer species, particle size, and loading density on the reactivity of homemade binary aluminum-based pyrotechnics. The results show that the pyrotechnics with potassium chlorate (KClO3) have the strongest reactivity with the highest pressurization rate (dp/dt) and the shortest combustion duration. Compared with their counterparts based on aluminum microparticles(mAl), pyrotechnics consisting of Al nanoparticles (nAl) possess superior reactivity as expected, which results from the relatively short heat and mass transfer distances. The nAl-based pyrotechnics have a low reaction exothermic peak temperature, great heat release, great aluminothermic reaction completeness, and high produced peak pressure with several orders of magnitude higher pressurization rate. Increasing the loading density of the pyrotechnics over a certain value can change the dominant mode of heat transfer from convective to conduction, sharply decreasing the pressurization characteristics and combustion front propagation velocities (vp). The results of theoretical calculations using the NASA-CEA codes show that loading density can alter the reaction process of the pyrotechnics, leading to a decrease in the predicted pressure per unit mass for Al/KNO3 or Al/AP, and an increase for Al/KClO3. For nAl/potassium nitrate (KNO3), the density is between 1.0 and 1.25 g cm−3, across which dp/dt decreases by one order of magnitude from 0.148 to 0.014 MPa ms−1. In addition, vp decreases by three orders of magnitude from 0.040 to 0.078 m s−1. Distinct pressurization behaviors of nAl/AP are observed at a density of 1.5 g cm−3, while the variation in nAl/KClO3 reactivity fluctuates. These results are beneficial for the damage assessment of scenes caused by an explosion and for inversely calculating charge parameters.

Effect of pre-chamber volume and nozzle number on turbulent jet combustion characteristics of GDI engines

There are few studies on the effect of turbulent jets on the combustion characteristics of passive pre-chamber (PC) gasoline engines, and their mechanism needs to be elucidated. Therefore, a 3-D simulation was performed using CONVERGE software. The results reveal that a modest expansion in the PC volume contributes to combustion due to the increased jet energy, but when it is too large, the above effect is weakened. In this study, at a smaller 0.8 ml volume, the combustion is still similar to traditional spark plug ignition. At middle 1.6 ml volume, the mixture concentration in the PC increases and the uniformity is better, resulting in the highest jet velocity and shortest jet duration, thus achieving the fastest flame propagation and the highest heat release rate (HRR). However, at a larger 1.8–2.0 ml volume, the mixture concentration near the spark-plug is sharply reduced, thus the reduced combustion rate in the PC leads to insufficient jet energy in the subsequent jets and resulting deterioration in combustion. An appropriate increase in nozzle number also improves combustion by introducing more ignition sources. But more nozzles not always improving the combustion. In the eight-nozzle case, three nozzles are close to the exhaust valve so that the intake is inhibited, resulting in a worsening of the ignition. The combustion under six-nozzle shows the highest HRR and shortest combustion duration. However, the combustion deteriorates at eight-nozzle, since the increase in the ignition point is not enough to compensate for the negative impact of the reduced jet intensity on combustion.

Experimental study on the optimization of operating conditions for hybrid powertrain gasoline engines

With the rapid expansion of the market for hybrid vehicles, the development of dedicated hybrid powertrain engines has become an important research direction. This study focuses on the techniques to improve the fuel economy of dedicated hybrid powertrain operating conditions of 2000 rpm, 110 Nm. Turbocharging can enhance the performance and fuel economy of gasoline engines while reducing emissions. However, intake boosting increases the pressure and temperature of the mixture in the cylinder at the compression end timing, leading to a higher risk of knock, which is not conducive to the safety and fuel consumption reduction of gasoline engines. Low-temperature combustion with mixture dilution can effectively reduce the occurrence of knock and NOx emissions. In this study, the effects of EGR and VVT strategies as different dilution methods on the combustion of turbocharged gasoline engines were investigated. The experimental results indicate that both EGR and VVT can achieve mixture dilution in the cylinder, reducing the combustion temperature and thus lowering NOx emissions. Furthermore, advancing the ignition timing results in CA50 trending towards the optimal fuel consumption point, shortening the combustion duration CA10–90, and reducing ISFC. Compared to VVT, EGR can better suppress knock, leading to an earlier ignition phase CA50, a shorter combustion duration, and ultimately less fuel consumption. The research findings provide technical support for the optimization of dedicated hybrid powertrain operating conditions.

A quasi-dimensional model of multiple-cycle fuel film evaporation in a port fuel injection spark-ignition methanol marine engine

For a port fuel injection (PFI) spark-ignition (SI) methanol marine engine, the proportion of residual methanol (Xf) depending on the fuel film evaporation (FFE) plays a crucial role in the fuel-air mixing process, combustion performance and emissions. Therefore, a quantitative assessment method on quasi-steady-state FFE was proposed and implemented to calculate the Xf, which was utilized to develop a novel FFE model aimed at improving the predictability of engine simulation. Then, the impact mechanism of injection timing on the engine's performance and emissions under different intake pressure conditions was revealed utilizing the Xf. Delaying the end of injection timing from −400°CA to −180°CA resulted in an early decrease of Xf, and then began to increase after the Xf reversal point. The shortest combustion duration (CA10-90) and the largest brake thermal efficiency (BTE) were achieved at higher Xf levels. Furthermore, optimizing injection timing could reduce HC and NOx emissions by up to 42.38 % and 48.54 %, respectively, though such adjustments need to be carefully tailored to different loads. Finally, a 1-D engine simulation incorporating the FFE model has been constructed and successfully replicated the effects of fuel film evaporation on engine combustion.

Experimental study on combustion and emissions of an ammonia/diesel dual-fuel engine using split-injection strategy

Blending high-reactivity fuels is recognized as an effective way to improve poor combustion performance of ammonia (NH3). In this work, pressure trajectories, flame evolutions, and emission characteristics of ammonia/n-dodecane dual fuels were studied in an optical engine, addressing split-injection strategy. Synchronization measurement of in-cylinder pressure and high-speed photography was performed for combustion evolutions, and FT-IR spectroscopy was adopted for nitrogen-based emissions. Results demonstrate that pre-injection timing and its energy ratio play a significant role in improving engine thermal efficiency and pollution emissions. Specifically, compared to the single-injection strategy, split-injection strategy leads to a reduced peak in-cylinder pressure, optimized combustion phasing, and shortened combustion duration, achieving an increase in the indicated mean effective pressure by 15.8 %. An early pre-injection timing with an energy ratio of 40 %∼60 % yields superior thermal efficiency compared to single injection at the same injection timing. For the delayed pre-injection timing, increasing pre-injection energy ratio is required to achieve optimal engine performance. Combustion visualizations show that the improved flame development under split-injection conditions is attributed to high-reactivity fuel stratification and enhanced premixing processes. Regarding nitrogen-based emissions, unburned NH3 and N2O emissions are insignificantly reduced, and NOx emissions correlate with the sensitivity of flame development to in-cylinder temperature. On the premise of ensuring stable combustion, all nitrogen-based emissions are reduced at a pre-injection energy ratio of 40 %, and an energy ratio of 60 % reduces N2O and unburned NH3 emissions while increasing NO and NO2 emissions. The current work can provide useful insights into the optimization and control of ammonia/diesel dual-fuel engines in terms of combustion and emissions.

Effect of diglyme on diesel engine combustion and un/regulated emission

ABSTRACT The influence of diglyme on diesel engine combustion and emission, including regulated and unregulated emissions, was carried out at various engine modes, in which ultra-low sulfur diesel and diglyme were employed, and diesel-diglyme blends were designed containing specific mass oxygen content varied from 2% to 20% corresponding to 5% to 53% diglyme blended by volume. With the addition of diglyme, actual air/fuel ratio decreased in general, combustion initiated earlier by 5.3% to 14.6% at tested modes but weighted center postponed with short ignition delay and combustion duration, meanwhile maximum in-cylinder combustion temperature reduced by 3 to 30°C, which affected emissions significantly.Consequently, the optimized PM-NOx trade-off was obtained with particulate and NOx decreased simultaneously, but formaldehyde and acetaldehyde increased by the varation from 3% to 190% with the addition of diglyme. Sulfur dioxide emission was much lower with ultra-low sulfur diesel compared that with other fuels including conventional diesel, about one order of magnitude smaller, and diglyme could bring further reduction by maximum of 62.5%. In addition, the oxygen in diglyme promoted CO oxidation and NO2 conversion from NO, leading to an increased NO2/CO ratio by even to 375.9%.

Experimental study on ammonia-diesel co-combustion in a dual-fuel compression ignition engine

Ammonia, as a carbon-free renewable fuel and a potential hydrogen carrier, is gaining increasing interest in addressing greenhouse gas emissions in industrial applications and transportation. This paper presents experimental studies on the co-combustion of ammonia with diesel fuel in a single-cylinder industrial compression ignition engine operating in dual-fuel mode. The energy share of ammonia ranged from 0 to 60%. Increasing the proportion of ammonia compared to diesel fuel in the compression ignition engine leads to a change in the combustion process character, increasing the kinetic combustion phase at the expense of diffusive combustion. However, it does not result in an increase in the peak pressure rise (PPR) beyond the permissible value of 1 MPa/deg for piston engines. It increases ignition delay and shortens the combustion duration, while also increasing thermal efficiency and decreasing specific energy consumption. An increase in the NH3 share results in a decrease in combustion efficiency, a reduction in unit emissions of NO, CO2, and soot, and an increase in CO emissions. Beyond 42% ammonia share, it reduces unit THC emissions. At a 42% ammonia share, the dual-fuel engine achieves the highest indicated thermal efficiency (ITE), lowest specific energy consumption (SEC), shortest combustion duration, and lowest unit soot emissions.

An Optical Study on the Ignition and Flame Development Initiated by a Combination of Passive Pre-Chamber and Spark Ignition

ABSTRACT Pre-chamber jet ignition combined with spark ignition in main chamber has advantages in adjusting the combustion phase and improving the burning rate for spark ignition engines. In this study, optical experiments based on a constant volume combustion chamber and high-speed schlieren imaging were conducted to investigate the ignition and flame development under the combined ignition mode with a passive pre-chamber and spark plug using methane fuel. Two modes of flame development were observed with different ignition time interval of pre-chamber ignition and spark ignition (Δt ign). At relatively short Δt ign, the “jet-disturbed flame” mode occurs with a three-stage combustion process: laminar flame; flame acceleration by jet-flame interaction and re-ignition at the jet root; flame deceleration. The maximum global flame speed can be increased from 2 m/s to 9 m/s under λ = 1 and from 1 m/s to 5.5 m/s under λ = 1.3. The ignition delay is relatively short and the heat release shows a slow and multi-stage feature. Moreover, the ignition delay and combustion duration in main chamber are relatively not sensitive to the variation of nozzle diameter. As the spark ignition becomes later (longer Δt ign), the “spark-initiated turbulent flame” mode occurs. In this mode, a rapidly developing turbulent flame is initiated by the spark plug, with a global flame speed up to 16 ~ 18 m/s under λ = 1 and 6 ~ 11 m/s under λ = 1.3. Compared with pre-chamber ignition, a shorter combustion duration is obtained at certain Δt ign and misfire can be avoided because of the synergy of the combined ignition mode.

Effect of excess air ratio and ignition timing on the combustion and emission characteristics of the ammonia-hydrogen Wankel rotary engine

As a hydrogen carrier, ammonia can suppress knock and enhance thermal efficiency of the hydrogen-fueled Wankel rotary engine (WRE), and achieve zero carbon emissions. This research established a three-dimensional fluid dynamics model coupled with detailed reaction kinetics of ammonia and hydrogen and verified it based on experiments. Incorporating 10% volume fraction of ammonia into the hydrogen-fueled WRE eliminates knock and decreases the excess air ratio (λ) from 1.8 to 1.4, effectively improving the indicated mean effective pressure (IMEP). The results indicate that when λ exceeds 1.4, flame propagation accelerates with higher concentration of the mixture. This enhances peak in-cylinder pressure and heat release rate, but it results higher NOx emissions. As λ varies from 1.8 to 1.4, NOx emission levels rise by 47.4%. At λ ≤ 1.2, the rapid flame propagation leads to short combustion duration, diminishing the power output. At this stage, the NO formation is dominated by H radicals, and the NOx production reaches its minimum value at λ of 1.0. In summary, the ammonia-hydrogen WRE achieves optimal performance at λ of 1.4 and ignition timing of −5 °CA after top dead center, the indicated thermal efficiency reaches 36.9% and the IMEP achieves 0.683 MPa.

Effect of multi-injection strategy on characteristics of methanol-fueled direct injection spark ignition engine

Present paper numerically investigates the effect of injection strategy and start of injection (SOI) timing on in-cylinder flow, air–fuel mixing, fuel distribution near spark plug, engine performance, and exhaust emissions for highly stratified methanol-fueled, multi-injection, direct injection spark ignition engine having high compression ratio. SOI is kept constant at −23° crank angle (CA) after top dead center (ATDC) with a spark timing (ST) of −2° crank angle (CA) ATDC. Mass of fuel is divided into pilot and main injection ports having pilot to main fuel injection mass ratio of 1:1, 1:2, and 1:3 at 0° and 2° dwell times between main and pilot injections. As the quantity of fuel in main injection increases, pressure rise rate increases, which results in higher in-cylinder pressure and higher rate of burning that gives higher apparent heat release. Due to higher peak pressure rise rate and faster burning in the case of 2° crank angle (CA) dwell time, shorter combustion duration is achieved compared to 0° crank angle (CA) dwell time. In the case of multi-injection, faster burning rate enhanced in-cylinder temperature; therefore, nitrogen oxides (NOx) emissions are higher. Pilot to main fuel mass ratio of 1:3 has resulted highest indicated thermal efficiency, lowest specific fuel consumption, lowest soot, and carbon monoxide (CO) emissions.

Effects of natural gas admission location and timing on performance and emissions characteristics of LPDF two-stroke engine at low load

Low-pressure gas-admission marine two-stroke engines have significant potential for reducing emissions in the shipping industry. However, they exhibit poor combustion quality under low-load conditions, leading to lower thermal efficiency and high levels of HC and CO emissions. This study numerically evaluated the impacts of the gas admission location and timing on engine performance and exhaust gas emissions under low load conditions. The study revealed that different gas admission locations form distinct NG-air stratifications. Lower gas admission locations resulted in longer ignition delays and shorter combustion durations. The unburned CH4 and CO emissions were significantly reduced by a maximum of 20.14% and 42.29%, respectively. A trade-off exists between NOx and N2O emissions owing to the different temperature conditions of their formation. The NOx emissions increased continuously as the gas admission location was lowered to a maximum of 3.4218 g/kWh; a value slightly above the Tier III emission standards. Advancing the timing of gas admission further decreased the ignition delay time, NOx, and unburned CH4 emissions. This study revealed that in-cylinder natural gas mixture stratification due to varying gas admission locations and timings can improve the combustion efficiency and exhaust gas emissions of a natural gas-diesel dual-fuel engine under low-load conditions.

Syngas-diesel dual-fuel engine performance using H2/CO top gases from the steel industry furnaces

This study aims to utilize H2-CO-rich waste gases as an alternative energy source for power generation in a dual-fuel engine setup, where diesel is used as an ignition source. A Computational Fluid Dynamics (CFD) model is developed and validated against experimental data using synthesis gas compositions containing H2, CO, CO2, and N2. A chemical kinetics model is integrated to enhance combustion accuracy, combining the diesel_14 and Gri-mech 3.0 sets to compute reactions involving diesel and synthesis gas/flue gases. Following validation, the study analyzes the dual-fuel engine's performance using H2-CO-rich waste gases: Blast Furnace Gas (BFG), Blast Oxygen Furnace Gas (BOFG), and Oxygen Blast Furnace Gas (OBFG) under various inlet temperatures. BOFG with diesel combustion results in higher peak pressure and heat release due to its significant combustible content (75.4 %). When considering BFG with diesel, the SOC occurs at 1⁰ bTDC, whereas for the BOFG and OBFG, ignition commences at 0 TDC. Also, the BOFG's short combustion duration led to an IMEP of 0.88 MPa, enhancing engine capacity than the OBFG and BFG. The exit amount of CO and UHC is higher in the case of BFG compared to the other fuels. Also, BFG exhibits lower MPRR from 0.33 to 0.5 MPa/deg, attributed to its prolonged combustion duration. Higher combustion temperatures increase NOx emissions, notably for BOFG, due to elevated in-cylinder temperatures. In conclusion, this study demonstrates the potential of H2-CO-rich waste gases in dual-fuel engines for power generation, highlighting BOFG's performance advantages while addressing emissions concerns.

Experimental investigation of the influence of pre-chamber structural parameters on combustion and emission characteristics of a high compression ratio SI engine

The passive pre-chamber exhibits great potential for practical applications in the internal combustion engine due to its simple structure. However, it faces difficulties in scavenging process, making optimization of pre-chamber structure particularly important. In this study, the effects of pre-chamber ignition systems with different structures and original spark ignition were conducted on a high compression ratio direct-injection SI engine to investigate the combustion, performance, and emission characteristics. The results show that the increase in pre-chamber volume weakens the in-cylinder combustion process and engine performance, while increasing the pre-chamber orifice diameter can enhance the combustion stability at medium to high loads, but will weaken the engine performance at low load. Compared to spark ignition, pre-chamber ignition results in an earlier 50% combustion position, a shorter combustion duration and lower COVIMEP, with these effects becoming stronger as load increases. At high load, the pre-chamber ignition (PC1) advances the 50% combustion position by 5.6°CA and shortens the combustion duration by 16.7%. In addition, the indicated thermal efficiency of the pre-chamber ignition achieves an increase of 0.84% at the optimal point and 2.5% at high load. Overall, using the smaller orifice diameter and volume in the pre-chamber can improve in-cylinder combustion process and increase thermal efficiency across the load range. For emission analysis, the pre-chamber ignition leads to an increase in NOx and HC emissions due to the influence of in-cylinder combustion temperature and quenching effect, while the CO emissions are similar to spark ignition.

Optical Study on the Effects of Methane Equivalence Ratio and Diesel Injection Mass on Diesel-Ignited Methane Combustion Process

Pilot diesel ignition is an effective approach for achieving efficient and clean combustion of natural gas. In this study, a rapid compression and expansion machine (RCEM) was constructed for examining diesel-ignited premixed methane combustion. The effects of the methane equivalence ratio and pilot diesel mass on the combustion process of diesel-ignited premixed methane gas were investigated. The results show that the combustion process can be divided into two stages: diesel dominance and premixed methane combustion. An increase in the methane equivalence ratio inhibits diesel combustion, leading to delayed CA10 and OH radical generation. However, it enhances premixed methane flame propagation and improves the heat release rate, resulting in a shorter combustion duration. An increase in the pilot diesel mass contributes to a larger flame area and higher OH generation intensity in the ignition region; however, too large a diesel mass inhibits methane flame propagation towards the diesel nozzle due to an extended injection duration. In conclusion, a larger pilot diesel mass can achieve better overall combustion performance, but excessive amounts may be counterproductive.

Identification and Parametrization of Key Factors Affecting Levoglucosan Emission During Solid Fuel Burning.

Levoglucosan (LG) is a pyrolysis product of cellulose and hemicellulose at low combustion temperatures. However, LG release cannot be determined only by considering the contents of cellulose and hemicellulose exclusively due to the complexity of combustion processes and the physical-chemical properties of the fuel. This study detected the emission factors (EFs) of LG from 22 different solid fuel samples (including coal and biomass) by considering 18 different fuel properties and five combustion parameters. The average LGEFs during solid fuel burning varied in a range of 0.03-136 mg kg-1, with a magnitude difference of 1-4 orders. While the variations in cellulose (59.5-368 mg g-1) and hemicellulose (73.5-165 mg g-1) contents of fuel samples were only one- to 6-fold. A short combustion duration (<150 min) and a medium combustion temperature (200-400 °C) influenced by volatile and ash contents are crucial for the generation and accumulation of LG. A random forest coupled with the Akaike information criterion stepwise regression model successfully explained 96% of the total LG emission variation using three variables (ash content, cellulose content, and modified combustion efficiency). The ash content promoted coke formation and LG chain cracking by increasing the pyrolysis temperature and is considered the most important factor. The alkali metal in ash can reduce the energy barrier of intramolecular ring contraction reactions and inhibit the dehydration reactions, which led to additional heat being utilized by the competitive pathways of LG formation. This study provided a method to address the parametrization and release mechanisms of combustion source emissions.

Study of single and split injection strategies on combustion and emissions of hydrogen DISI engine

Hydrogen energy shows excellent potential as an alternative fuel for internal combustion engines (ICE) in terms of cleanliness and efficiency. However, the specificity of hydrogen density and diffusion rate leads to further optimization of the working process of the hydrogen direct injection spark ignition (DISI) engines. In this study, the effects of single injection and split injection strategies on the combustion and emissions of a hydrogen DISI engine were investigated at medium/low load. The specific results showed that premature hydrogen injection could trigger pre-ignition events, and it was appropriate to set the earliest hydrogen injection timing to 180° CA BTDC after the intake valve closing. For the single injection strategy, the brake thermal efficiency (BTE) increased first and then decreased with the delay of the start of injection (SOI). When SOI = 100° CA BTDC, the optimal BTE (36.1 %) could be obtained, and nitrogen oxide (NOx) emissions could still be limited to less than 0.1 g/(kW·h). For the split injection strategy, Delaying the secondary end of injection (SEOI) beyond 30° CA BTDC rapidly worsened NOx emissions to over 1.5 g/(kW·h). Delaying SEOI achieved higher efficiency but slightly worse the coefficient of variation COVIMEP than the single injection strategy. In addition, increasing the secondary injection mass fraction (SIMF) to 30 % could achieve an 18 % higher peak heat release rate than a single injection and shorter combustion duration. However, over 20 % SIMF resulted in an increase of NOx emissions to over 4 g/(kW·h). The energy distribution indicated that adjusting the injection strategy to increase BTE was mainly achieved by reducing power of heat loss (PHL).

Investigation of ammonia cracking combined with lean-burn operation for zero-carbon combustion and NO/N2O/NO2 improvements

Ammonia has received significant attention as a carbon-free fuel and a promising carrier of hydrogen. Ammonia cracking presents an effective method for on-line hydrogen production, eliminating the need for hydrogen transportation and storage. This paper aims to study the comprehensive impacts of ammonia cracking ratio (CR) and equivalence ratio (ER) on combustion performance and NO/N2O/NO2 behaviors under lean-burn operation mode. The ammonia/hydrogen mechanism was updated and simplified to enhance the calculation efficiency in 3D simulation. Visualization measurements for flame propagation and NO and 3D simulations were simultaneously performed in a constant volume combustion chamber (CVCC). The results showed that ammonia blending with the cracked gas of H2/N2 for CR can significantly enhance the combustion rate under lean-burn operation at an ER of 0.5. Nevertheless, this enhancement leads to an increase in NOx due to elevated OH and HNO formation, regardless of N2 separation. In the context of varying ER, ranging from 0.5 to 0.3, the study revealed that increasing CR combined by decreasing ER at a certain point can sustain a higher combustion rate while reducing NOx emissions. For instance, CR45ER0.4 yields a much shorter combustion duration and lower NOx compared to pure ammonia at an ER of 1.0. However, a further increase in CR, such as CR80ER0.3, results in higher NOx emissions again. Moreover, complete cracking or separation from the non-cracked ammonia proves favorable for NOx elimination due to the absence of fuel-NO. The current study provides insights on high-efficiency and clean zero-carbon combustion technology.

Preparation of microstructure controllable Al/WO3/F2603 MICs by droplet microfluidic technology to improve combustion performance

The microstructure of metastable intermolecular composites (MICs) is crucial for their energy release and combustion performance. In this paper, Al/WO3/F2603 composites with controllable microstructure were prepared by droplet microfluidic technology. The microstructure control method of composite materials based on droplet microfluidic technology (DMT) was studied. Particles with controllable size, shape, composition, structure and function were prepared by droplet template, and the formation mechanism of particle microstructure was introduced. The Al/W/F-2 composites with high-quality spherical encapsulation have excellent flow properties and wettability. The ignition delay, combustion time and pressure output of the composites were monitored by a combustion and pressure test system. Compared with the physically mixed samples, the Al/W/F-x composites prepared by DMT have shorter ignition delay, shorter combustion duration, and higher pressurization rate. The analysis of combustion products showed that the encapsulated Al/W/F-x composites reduced the sintering and agglomeration of reactants and promoted the combustion efficiency. The results show that the characteristic structure design of the composite material can establish the interface synergy, enhance the thermal mass transfer between the components, and thus reduce the ignition delay and improve the combustion performance. DMT can realize the controllable preparation of composite microstructure, which provides a reference for the design and preparation of other composite materials.

Effects of hydrogen and EGR on energy efficiency improvement with ultra low emissions in a common rail direct injection compression ignition engine fueled with dimethyl ether (DME) under HCCI mode

This study deals with enhancing the energy efficiency with ultra-low emissions and extending the operating range (maximum load limit) of dimethyl ether (DME) fueled homogeneous charge compression ignition (HCCI) engine using hydrogen and exhaust gas recirculation (EGR). In the neat DME mode, the possible maximum load limit with controlled auto-ignition (CAI) was found as 24% (BMEP of 1.67 bar). Beyond 24% load, the knocking was observed with an advanced combustion phase, and shortened combustion duration resulted in uncontrolled auto-ignition (UAI) combustion. The addition of EGR extended the maximum load limit to 52% (BMEP of 3.62 bar) (60% EGR). Near-zero levels of NOx, zero smoke, and marginal CO and HC emissions reduction were observed with the optimized EGR rate (35%). The hydrogen fuel addition along with DME at an optimized EGR rate resulted in combustion phase retardation with the elimination of the low-temperature reaction (LTR) region. The energy efficiency (EE) and indicated thermal efficiency (ITE) increased along with zero smoke emission, ultra-low NOx, CO, and HC, and lower CO2 emissions till 12% of hydrogen energy share (HES). Thus, the combined strategy of EGR and hydrogen could effectively extend the operating range with ultra-low emissions levels.

Effects of intake parameters and compression ratio on ammonia combustion and emissions in SI engines

Ammonia is an attractive hydrogen carrier and zero-carbon fuel for achieving de-carbonization targets. However, realistic engines with pure ammonia often suffer from serious issues like ignition instabilities, combustion variations, and heavy emissions. It has shown that ammonia combustion is highly sensitive to combustion boundary conditions. However, comprehensive parametric studies on combustion and emissions of ammonia engines are still lacking currently. In this work, ammonia combustion and emissions were numerically investigated in a spark-ignition engine under various operating conditions, emphasis on pressure characteristics, combustion flow field, and NOx and unburned NH3 emissions. The results show that ammonia combustion can be promoted significantly at elevated intake temperatures and pressures and compression ratios, manifesting increased peak pressure, accelerated flame propagation, and shortened combustion duration. Indicated power increases at elevated compression ratios and intake pressures while decreasing at elevated intake temperatures. Equivalence ratios affect ammonia combustion nonmonotonically, and the highest peak pressure and thermal efficiency are achieved at Φ = 1.2 and 1.1, respectively. Besides, equivalence ratios have large impacts on the shortening of late-stage combustion duration (i.e., CA50-CA90). Regarding emission characteristics, it shows that fuel-NO is dominant while thermal-NO is minimal. Equivalence ratios can result in pronounced NO emissions under lean-burning conditions. Unburned NH3 decreases while NO2 and N2O remain low levels at an elevated compression ratio, intake temperature and pressure and equivalence ratio of around 1.1. This work demonstrates the significance of active thermo-atmosphere in the optimization of ammonia engine performance.