Fuel-air Mixing Research Articles

Mixing and flame structure study of inverse swirl diffusion flames

Swirling flames appear in many practical applications to enhance flame stabilization. However, relatively few studies have investigated the inverse swirl diffusion flames (ISDFs) in detail. In ISDFs, a central swirling air jet is surrounded by an annular methane fuel jet. In the first part of this work, the stability map of ISDFs is studied. This map shows two distinctive stability regions: a swirling-jet-like region at low swirl and a compact swirling flame region at high swirl intensity. Regardless of the bulk air jet velocity (U), these two regions are separated by an optimum swirl intensity -leading to the highest flame stability. Then across these two regions, the development of the fuel–air mixing and the flame-flow interaction were investigated via acetone-PLIF and simultaneous PIV/OH-PLIF measurements. At the same U, increasing the swirl intensity leads to different mixing patterns. An annular-rich jet exists in the swirling-jet-like region versus a compact short mixing field in the compact flame stability region. Moreover, the development of the fuel–air mixing field downstream of the burner exit impacts the flame-flow interaction. The fast and early fuel–air mixing enhances the mutual interaction between the lower edge of the flame and the wrinkled flame, and the spots of the high heat released from the downstream regions - hence a highly stable flame is obtained. The lower flame edge is seen to track the instantaneous lower stagnation point of the swirling flow, while the increased heat release spots/flame wrinkling track the shear layer vortices. Beyond the optimum swirl intensity, increasing the swirl intensity retarding the fuel–air mixing and weakens the mixture ignitability, straining the flame in the vicinity of the flame edge or the vortices of the shear layer. These results suggest that suitable modifications of the mixing field close to the burner nozzle of ISDFs may increase flame stability.

Study on combustion performance of microgas turbine combustor with different fuels

AbstractA low‐pollution single‐tube microgas turbine combustor using different gas fuels was studied. The combustion performance of two kinds of landfill gas and natural gas was tested in a single‐head combustor. The effects of fuel nozzle position, and the fuel distribution ratio between pilot and main combustion stage on pollutant emission were investigated under rated load condition. The flow field, temperature field, and pollutant generation characteristics with different fuel nozzle diameters and nozzle positions were analyzed by numerical simulation. The results show that: the combustor can form an obvious central recirculation zone, which is convenient for ignition and stable flame propagation; in the case of limited premixed length, the influence of fuel nozzle position on fuel–air mixing uniformity is less obvious than that of fuel nozzle diameter. The fuel–air mixing uniformity plays a decisive role in the temperature field and the generation of pollutants in the combustor. The combustor has good pollutant emission characteristics, and the NOx emission can be reduced to about 10 ppmv (15% O2 concentration) under the appropriate combination of fuel nozzle diameter and position, which provides a reference for the further optimization of low‐emission combustor.

Comparison of performance and emission characteristics of homogeneous charge compression ignition with conventional compression ignition engine

The performance and emissions of homogeneous charge compression ignition (HCCI) diesel engines were analysed at varying intake air temperatures (Ti) and compared with conventional diesel engines. HCCI engines featured outside fuel–air mixing systems. Diesel was used as a fuel for the testing of port fuel injection pressure (two bars, four bars, and six bars), as well as intake temperature Ti (60 °C, 80 °C, 100 °C, 120 °C, and 140 °C). The outcomes of the experiments presented that both Ti and port fuel injection pressure had a substantial impact on the fuel–air mixture. Up to 120 °C, the blending of air and fuel had a proportionate effect on Ti. When heated above 120 °C, it caused significant knocking and emissions. The Brake thermal efficiency of HCCI is similar to that of diesel engine when HCCI engine operated at an intake air temperature of 120 °C and injection pressure of 4 bar where as in other modes it less than the efficiency of diesel engine. The nitrogen oxide emissions were significantly reduced in the all HCCI engine mode operations compared to diesel engine. The minimum emissions were engine when HCCI engine operated at an intake air temperature of 120 °C and injection pressure of 4 bar. The smoke opacity was significantly reduced in the all HCCI engine mode operations compared to diesel engine. The minimum emissions were engine when HCCI engine operated at an intake air temperature of 120 °C and injection pressure of 4 bar. The HC and CO emissions of HCCI engine at all conditions were slightly higher than the diesel engine. But, the minimum emissions were obtained when HCCI engine operated at an intake air temperature of 120 °C and injection pressure of 4 bar without knocking. Finally, it is concluded that HCCI engine operated at an intake air temperature of 120 °C and injection pressure of 4 bar will give the optimized perform ace and emission characteristics when compared to diesel engine.

Numerical and boundary condition effects on the prediction of detonation engine behavior using detailed numerical simulations

High-fidelity numerical simulations of an experimental rotating detonation engine with discrete fuel/air injection were conducted. A series of configurations with different feed-plenum pressures but with constant equivalence ratio were studied. Detailed chemical kinetics for the hydrogen/air system is used. A resolution study for the full rotating detonation engine (RDE) system simulation is also conducted. Two kinds of boundary conditions, a total pressure boundary and a constant mass flow rate boundary, are used to assess the effects of the inlet boundary. As mass flow rate is increased, the total pressure boundary causes more error in the axial pressure distribution while the constant mass flow rate gives a better solution for all cases ran. The simulations confirm experimental findings, and reproduce qualitative as well as some of the quantitative trends. These results demonstrate that a) fuel-air mixing is highly non-uniform within the detonation chamber, leading to variations in local equivalence ratio, b) the fuel and oxidizer injectors experience significant backflow as the detonation wave passes over, but recover at different rates which further augments the inefficiencies in mixing, and c) parasitic combustion in the mixing region makes the detonation wave weak by extending the reaction zone across the wave.

Analysis of ducted fuel injection at high-pressure transcritical conditions using large-eddy simulations

Ducted fuel injection (DFI) is a proposed fuel injection concept for achieving substantial reductions in emissions. In this concept, the fuel is injected through a coannular duct, resulting in increased fuel-air mixing and minimized formation of soot and other unwanted combustion products. Apart from comprehensive experimental investigations on DFI, so far computational studies have been limited to single-point Reynolds-averaged Navier Stokes simulations. Therefore, the objective of this work is to complement these studies by performing large-eddy simulations using a diffuse-interface method to examine the physical mechanisms and combustion processes of DFI, specifically focusing on the mixing process and the effect of fuel-ducting on combustion and pollutant emissions. To this end, finite-rate chemistry simulations are performed of the DFI configuration corresponding to the Engine Combustion Network Spray A injector at transcritical conditions (n-dodecane fuel, 60 bar pressure and 1000 K temperature chamber conditions). A two-equation soot model is employed for the qualitative analysis of soot emissions. Direct comparisons of averaged and instantaneous flow field results with the Spray A configuration are performed to assess the effect of DFI on the first- and second-stage ignition and soot formation. Compared to the free-spray condition, the results show that the DFI case exhibits a combination of (i) increased mass flow rate and entrained air, (ii) larger pressure drop magnitude and flow velocity, and (iii) a closer-to-stoichiometric mixture composition (both globally and locally), each of which is conjectured to contribute toward reduced soot production.

The Influence of Heat Recirculation Segment of Meso-Scale Combustor o Gas and Liquid Fuel Inside Microscale Combustor

This study aims to determine the effect of heat recirculation segments configuration on the stability of the flame. Mesoscale combustor used is made from duralumin-quart glass tube, and Ø 3.5 mm diameter combustion chamber. The heat recirculation segments part is made of duralumin as a segment of evaporation and air-fuel mixing. Combustor A has heat recirculation segments length of 7 mm, while combustor B has a heat recirculation segments length of 10 mm. The fuel used is liquid hexane and butane gas fuel. The results showed, the stability of hexane flame in combustor A between the equivalent ratio of ɸ 0.73-1.28 temperature can reach 803.8oC. Combustor B is stable at an equivalent ratio of ɸ 0.76-0.98 to a temperature of 580.9oC. Furthermore, for butane flames; combustor A, stable in the range of ɸ 0.68-1.31 temperature to 931.3oC. Combustor B has a stability range of ɸ 0.79-1.41 temperature reaches 857.2oC. Combustor A with a 7 mm heat recirculation segments length is proven to produce wider flame stability, using both liquid fuel and gas. So, combustor A can be recommended to be applied to micro power generators. It should be noted that butane flames appear to have more flame stability than hexane flames. This is because gaseous fuels are more homogeneous (gaseous) so they are more quickly mixed with air.

Investigation on suitable swirl ratio and spray angle of a large-bore marine diesel engine using genetic algorithm

The swirl flow and spray angle are crucial to the organization of the combustion process in marine engines. In this work, to explore the suitable swirl ratio and corresponding spray angle for large-bore marine diesel engines, the artificial neural network (ANN) coupled with the genetic algorithm (GA) was used. At the same time, to increase the swirl ratio without affecting the scavenging efficiency, the cylinder gas recirculation (CGR) strategy was adopted in this manuscript. Compared with the original design, results indicate that the optimal swirl ratio is about 10 for the large-bore marine diesel engine when applying the CGR strategy. Due to the improved fuel–air mixing process and combustion process, the thermal efficiency and the output power of the optimal case increased by 4.86% and 5.2%, respectively. The present results are helpful for the design and optimization of the fuel–air mixing and combustion processes for large-bore marine diesel engines.

Experimental study on flame combustion characteristics of large-bore marine diesel engine based on endoscopic technology

Large-bore marine diesel engines have the characteristics of poor ignition performance and insufficient combustion in the cylinder. This work revealed the combustion and emission performance of large-bore marine diesel engines based on endoscopic visualization technology. The flame temperature and soot distribution were analyzed in radial and axial directions. Results show that the large-bore diesel engine has a poor combustion effect because of the large fuel injection quantity and insufficient fuel-air mixing effect in the cylinder. The temperature in the cylinder rises twice in the late stage of combustion. The average temperature rises by 3.8%, caused by the secondary ignition of part of the unburned diesel. In addition, the large-bore engine produces a large amount of soot due to an insufficient mixing effect. It can be observed from the radial flame visualization images that the propagation speed of the flame is slow. The time required for the flame to propagate to the wall at 50% load is reduced by 31% compared with 25% load. The downward movement of the piston causes the flame tumble flow in the cylinder, resulting in an apparent asymmetric structure of the flame development.

Effect of In-Cylinder Low-Pressure Direct Injection Strategy on the Atomization and Combustion Process of a Small-Scaled Gasoline Wankel Rotary Engine

To solve the problem of poor atomization and combustion in a small-scaled gasoline Wankel rotary engine (WRE) with low-pressure port injection mode, the numerical simulation was used to optimize the injection strategy. Firstly, the effects of in-cylinder temperature and pressure on gasoline atomization characteristics were studied, and the optimal injection timing was determined. Based on this, the influence of injection position, injection angle, and installation direction on the atomization, fuel-air mixing, and combustion processes of low-pressure direct injection (DI) small-scaled gasoline WRE was investigated. The results show that the injection angle is a key factor in determining the gasoline atomization characteristics. Injecting along the direction of rotor rotation causes the impingement between fuel bundle and combustion chamber pocket, resulting in the smaller Sauter mean diameter (SMD) and liquid penetration length (LPL). The installation direction of nozzle plays an important role in the airflow movement. When the nozzle is vertical-installed, the airflow repeatedly crosses to form multiple eddies, making the fuel to move more easily towards the front of combustion chamber. When the nozzle is parallel-installed at the lower edge of the installing zone and injecting along the direction of rotor rotation, the peak in-cylinder pressure is the largest and increased by 21% compared to the original port injection. By this injection strategy, the problem of incomplete combustion for the studied small-scaled gasoline WRE could be almost completely solved.

Experimental Study of Effect of Injection Timing on Port Fuel Injection Gasoline, Port Fuel Injection Compressed Natural Gas, and Direct Injection Compressed Natural Gas Engine Performance, Combustion, and Emissions Characteristics

Abstract Compressed natural gas (CNG) has gained popularity due to its wide availability, higher efficiency, and lower emissions compared to gasoline. However, the lower flame speed characteristics of CNG with conventional port injection reduce the CNG engine volumetric efficiency and power output. CNG's lower gas jet momentum during a low load operation creates a non-uniform air-fuel mixture that affects ignition and combustion quality. Direct injection of CNG with optimum injection timing is expected to improve volumetric efficiency, ignition quality, and combustion process. In this study, a comparative study on the effect of end-of-injection (EOI) timing on volumetric efficiency, thermal efficiency, combustion duration, and emissions was carried out in a single-cylinder port fuel injection (PFI) spark-ignition engine using gasoline and CNG, and direct injection (DI) spark ignition engine using CNG. The experiments were performed at two-part load operations of 20% and 40% throttle at 900 and 1500 rpm. Experimental results indicate that the PFI CNG engine is more influential in EOI timing than gasoline engines. The performance of the PFI CNG engine is improved when the injection occurs during the intake valve open period compared to the closed valve period with higher thermal efficiency, volumetric efficiency, and indicated mean effective power (IMEP). A shorter flame development angle and combustion duration were observed when EOI timing was in the open intake valve condition. DI CNG improved volumetric efficiency at advanced EOI timing compared to the PFI CNG engine. However, the combustion process is critically dependent on injection timing and air-fuel mixing duration. A three-dimensional computational fluid dynamics simulation was conducted to evaluate the effect of advanced and retarded EOI timing on DI CNG engine's in-cylinder turbulent kinetic energy development and in-cylinder equivalence ratio near the ignition point. An excess advanced EOI timing resulted in stratified rich and retarded EOI timing results in loss of turbulence energy, leading to a slightly rich and lean mixture for advanced and retarded EOI timing, respectively. Hence, an optimum EOI timing provides a conducive environment to initiate the combustion and flame front propagation. Further, advanced EOI timing was required at higher throttle opening and engine speed. The emissions in DI CNG were also greatly affected by EOI timing.

The characteristics of waste-cooking palm biodiesel-fueled CRDI diesel engines: Effect hydrogen enrichment and nanoparticle addition

The influence of iron nanoparticle (INP) addition (75 ppm) and hydrogen enrichment (10 lpm) with waste cooking palm biodiesel blend (WCB) on a CRDI diesel engine is evaluated. A blend of 20% WCB and 80% diesel is used, and the dosing level of INP has been kept at 75 ppm, which has been decided based on the oxygen content of biodiesel. Results indicate that the combination of H2 enrichment and INP addition improves the BTE and BSFC of biodiesel blends as that of diesel. A maximum improvement of BTE of 7.1% than that of diesel is obtained at 90% loading. The combined impact of better hydrogen combustion characteristics and improved air-fuel mixing with nanoparticles reduces CO and HC emissions by 37.5% and 41.8%, respectively, for the WCB fuel sample. However, NOX emission shows an elevation of 27.4% compared to diesel. Combustion parameters, namely ICP (80.1 bar) and HRR (89.5 J/˚CA) indicate an improvement of 5.3% and 6.7% compared to diesel for WCB + INP + H2. This is owing to the combination of hydrogen's rapid flame speed and INP-added biodiesel's increased thermal conductivity.

Experimental investigation of the effect of duct structure geometry on macroscopic spray characteristics and air entrainment of duct fuel spray

The fuel–air mixing process can be improved via straight (ST) ducted fuel injection, but there is a risk of greater heat transfer loss due to prolonged spray tip penetration (STP) and spray impingement. In this study, we propose a convergent-divergent (CD) duct spray to produce an acceptable STP to improve the spray air entrainment (SAE) and spray cone angle (SCA). Three CD and two ST ducts were designed and installed in a constant-volume chamber. An optical Schlieren visualization system with a high-speed camera was used to examine the effects of duct geometry on the macroscopic spray parameters of SCA, STP, spray area (SA), SAE, and spray morphology. Compared with the free spray, the longest STP was noted for the ST2 duct spray, whereas the shortest STP was observed for the CD4.5 duct spray; the STP decreased by 14.5 %, whereas the SCA increased by at least two times for CD4.5 and CD6 duct sprays. A typical trade-off relationship between SCA and STP was confirmed for larger diameter CD duct sprays. When the injection pressure increased from 120 to 140 to 160 MPa, the STP of the ST2 duct spray increased by 1.7 % and 2.4 %, and that of the CD4.5 duct spray increased by 3.1 % and 7.0 %, respectively. At the late spray stage, the CD4.5 and CD6 duct spray had the better SAE ability, followed by the ST3 and CD3 duct sprays, ST2 duct spray exhibited slightly lower SAE values than the free spray. A typical mushroom-shaped spray head was observed near the outlet of the ST2 and ST3 duct sprays, while a cotton-shaped spray was observed for the CD duct sprays during the initial early spray stage. The development of the CD duct spray showed a left–right swing phenomenon with an S-shaped track, particularly for CD4.5 and CD6 duct sprays, which can promote the fuel–air mixing process with a larger SCA. The CD4.5 duct spray demonstrated considerable potential for improving the fuel–air mixture with a stronger SAE and larger SCA, as well as the ability to reduce the heat transfer loss with a shorter STP and weaker spray impingement.

Combustion enhancement and soot reduction of kerosene-based fuel under engine-relevant conditions enabled by fuel reactivity modulation

Liquid fuels incorporating biomass components (e.g., bio-alcohols) offer a great potential for improving combustion performance and energy efficiency. However, the insufficient reactivity and Leidenfrost effect of these fuels are two long-standing challenges for its stable combustion under wide engine-relevant conditions. Herein, the effect of methoxydiethylborane (MDEB) on the spray combustion characteristics promotion of kerosene and binary kerosene/ethanol-based fuels is reported, where the suppression of Leidenfrost effect under air conditions is demonstrated. The synergistic effect of MDEB on oxidation heat release and active radical formation not only enable the resultant kerosene/ethanol-based suspended droplets to present high evaporation rate (∼44 % reduction in evaporation time), excellent ignition performance, e.g., elevating ignition reliability and ignition delay time dropping to 1051 ms (∼67 % reduction) at an ambient temperature of 820 °C, and soot reduction property (few residual black particles displayed in suspension unit) compared to neat fuel droplet, but also make single kerosene-based hybrid fuels exhibit improved atomization and jet combustion characteristics. Notably, the negative growth behavior of evaporation time associated with the Leidenfrost effect that is dominated by low thermal conductivity air and local fuel vapor layer can be eliminated by introducing MDEB. Further, the jet dynamic reflects the improvement of gas phase diffusion and fuel–air mixing in jet and radial directions, and the increased flame propagation rate and heat transfer efficiency, which highlights the applicability of fuel reactivity modulation strategy for the enhancement of combustion stability under wider engine-relevant conditions. The findings thus provide a promising way to engineering scalable liquid fuels for advanced aviation engines, and realizing pollutants control for efficient energy conversion and management.

Macroscopic and Microscopic Spray Characteristics of Dimethyl Ether in a Constant Volume Spray Chamber Using a Mechanical Fuel Injection System for Automotive Applications

Abstract Spray investigations are critical for understanding internal combustion engine combustion. Optimised spray atomisation helps improve engine output/performance and reduce tailpipe emissions. The spray from the injector nozzle depends on nozzle hole diameter, fuel injection pressure, ambient density, pressure and temperature in the spray chamber, and test fuel properties. This study evaluated macroscopic and microscopic spray characteristics of dimethyl ether (DME) and baseline diesel under atmospheric conditions (1.013 bar pressure at 298 K temperature). It correlated the spray parameters with distinctive physicochemical properties of diesel and DME using dimensionless numbers, namely Reynolds number, Weber number, and Ohnesorge number. The fuel injection system consisted of a high-pressure mechanical injection pump and mechanical fuel injectors having an original equipment manufacturer fixed nozzle opening pressure in the constant volume spray chamber. The microscopic spray investigations were performed using a phase Doppler interferometer along the spray direction at three axial distances (50, 70, and 90 mm) from the nozzle. The three orthogonal spray droplet velocities of diesel and DME were compared. The droplet number-size distributions for baseline diesel and DME were compared. Macroscopic spray characteristics were evaluated using high-speed imaging. Reynolds number was higher for DME, leading to more turbulence in the spray and accelerating the spray breakup phenomenon. Weber number of DME was also much higher than baseline diesel due to its lower surface tension. The higher Weber and lower Ohnesorge numbers justified the finer droplets of DME sprays. DME showed superior spray atomization characteristics than baseline diesel, leading to superior fuel–air mixing and efficient and sootless combustion.

MODIFICATION OF AMULTIHOLE INJECTOR TO A SINGLE-HOLE INJECTOR AND SPRAY CHARACTERISTICS OF THE MODIFIED INJECTOR BY TWO DIFFERENT IMAGING TECHNIQUES

In the present days, almost all the diesel engines are fitted with multihole injectors. Spray characterization of the injectors by different optical measurement techniques is very important to under-stand the air-fuel mixing and combustion procedure inside the cylinder. Instead of characterization of the dense sprays from a real multihole diesel injector, often a single plume is used to study the spray parameters. In the present work, spray characteristics are studied for a multihole injector with all nozzle holes open and with a single hole open conditions. Experiments have been conducted in a constant-volume, high-pressure, and high-temperature chamber with various injection pressures and at various ambient conditions. A high-pressure common rail injector with six holes is considered for the study. The penetration and cone angle of the spray from each hole are measured, and the hole-to-hole variation is studied. It has been observed that hole-to-hole variations with respect to spray penetration are prominent in low chamber pressures and injection pressures at the initial stage of injection, though they are minimal at later stages of injection. After that, five holes of the injector are blocked by microwelding, and the remaining single plume was studied and the results compared with the original multiplume spray. The study shows that the penetration length and cone angle of the single plume match well with that of the multiplumes at all chamber pressures and injection pressures. Later, the spray from the modified single-hole injector was characterized by two different imaging techniques, Mie scattering and diffused backlit imaging (DBI), to compare the penetration length and cone angle measured by the two imaging techniques. It is seen that the differences in penetration lengths by two imaging techniques are negligible, whereas the cone angles are greater in the case of backlit shadow imaging, particularly at higher temperatures.

STUDY ON THE EFFECT OF THE PRESENCE OF FUEL VAPOR IN THE SURROUNDING GAS ON THE MIXING CONTROLLED VAPORIZATION OF FUEL SPRAY

In different combustion devices, liquid fuel is injected inside the combustion chamber. The liquid fuel spray atomizes, vaporizes, and mixes with the surrounding gas before combustion. The vaporization rate of the injected fuel is very important because it controls the air-fuel mixing and finally the combustion rate. Similar to droplet vaporization, the concentration of fuel vapor in the surrounding gas influences the spray vaporization process. It changes the maximum liquid phase penetration or liquid length (LL) of a vaporizing spray. Hence the liquid length model should consider the ambient fuel concentration effect, but has been ignored up until now. The present work studies the liquid lengths of vaporizing sprays at different fuel vapor concentrations in the surrounding gas. The effects of ambient fuel concentration on heating and vaporization of the spray are analyzed in the context of a mixing controlled vaporization model. The results shows that the fuel concentration in the surrounding gas changes the maximum liquid phase fuel temperature and the specific energy required for vaporization. It is observed that as the ambient fuel concentration effect is ignored in the existing mixing controlled model, it fails to predict liquid lengths when the surrounding gas is mixed with fuel vapor. A modified model is presented where the mass fraction of fuel vapor in the surrounding gas is included in the model. The modified model is able to predict the LL values more accurately when the surrounding gas is mixed with fuel vapor.

Investigation of supercritical transition and evaporation process of a hydrocarbon droplet under diesel engine-relevant conditions

Fuel spray supercritical transition in the diesel engine has been concerned widely, especially supercharged ones, however, whether the transition can significantly affect the fuel–air mixing under diesel engine-relevant conditions is an issue that still needs evaluating carefully. Thus, the transition and evaporation of individual n-heptane droplets in a nitrogen environment under various ambient conditions were investigated through a high-pressure evaporation model. The influence of ambient pressures on droplet lifetime (τlife) and evaporation rate constant (K) varies with ambient temperature. When Tr < 0.98, with increasing pressure, τlife first increases and then decreases, and K decreases. When 0.98 < Tr < 1.28, as pressure increases, τlife would increase even if K increases, because the increment of initial heat-up periods exceeds the decrease in quasi-steady evaporation periods. When Tr > 1.28, with increasing pressure, τlife decreases, and K increases. As the droplet heats up, the droplet interface is where the binary mixture would reach the critical mixing state if ambient conditions are high enough. Increasing ambient density decreases the minimum ambient temperature (Tm) required for the transition to occur. Above the Tm, transitions occur earlier with the increasing energy density of ambient gas. Increasing ambient density decreases the minimum ambient temperature required for diffusive fuel-gas mixing modes to dominate the mixing process. For droplets large enough to ignore gas–liquid interfacial thickness, the Tm required for the attainment of critical mixing states is almost independent of droplet diameter.

EGR and Emulsified Fuel Combination Effects on the Combustion, Performance, and NOx Emissions in Marine Diesel Engines

Techniques such as exhaust gas recirculation (EGR) and water-in-fuel emulsions (WFEs) can significantly decrease NOx emissions in diesel engines. As a disadvantage of adopting EGR, the afterburning period lengthens owing to a shortage of oxygen, lowering thermal efficiency. Meanwhile, WFEs can slightly reduce NOx emissions and reduce the afterburning phase without severely compromising thermal efficiency. Therefore, the EGR–WFE combination was modeled utilizing the KIVA-3V code along with GT power and experimental results. The findings indicated that combining EGR with WFEs is an efficient technique to reduce afterburning and enhance thermal efficiency. Under the EGR state, the NO product was evenly lowered. In the WFE, a considerable NO amount was created near the front edge of the combustion flame. Additionally, squish flow from the piston’s up–down movement improved fuel–air mixing, and NO production was increased as a result, particularly at high injection pressure. Using WFEs with EGR at a low oxygen concentration significantly reduced NO emissions while increasing thermal efficiency. For instance, using 16% of the oxygen concentration and a 40% water emulsion, a 94% drop in NO and a 4% improvement in the Indicated Mean Effective Pressure were obtained concurrently. This research proposes using the EGR–WFE combination to minimize NO emissions while maintaining thermal efficiency.

A numerical study on knock combustion suppression using targeted fuel injection in an SI engine

A novel concept to suppress knock combustion using the targeted fuel injection in a spark-ignition (SI) engine is proposed, which utilizes a customized injector to inject a small portion of the total fuel toward the end-gas region in the late compression stroke so that charge cooling effect by fuel vaporization is imposed on the local mixtures. CFD studies were carried out to evaluate the effects of various targeted fuel injection strategies on the processes of charge cooling, air-fuel mixing and combustion in a 1.2-L gasoline SI engine with a high compression ratio. A knock prediction model adopting a level set G-equation model coupled with a transported Livengood-Wu integral correlation was applied to predict the knock event by capturing the occurrence of end-gas auto-ignition. It was found that the local gas temperatures in the sprayed region decreased and knock combustion tendency was suppressed indeed, and the degree of the reduction in local gas temperature and knock tendency was affected by the selected targeted injection strategy. However, the results also indicated that excessive fuels injected could result in worsen air-fuel mixing that led to incomplete combustion. Hence, the injection strategy needs to be optimized by balancing the knock tendency and combustion efficiency. The study further shown that the spark timing could be advanced by 3.3° crank angle in a particular injection strategy case by comparing with the non-targeted-injection case, which demonstrated the effectiveness of the proposed concept.

Comparative study on the spray and combustion characteristics between diesel and kerosene underlow-temperature combustion (LTC)mode conditions

Kerosene, an intermediate component of wide distillation fuel, has drawn increasing attention for low-temperature combustion mode due to its compromise of evaporation and ignition characteristics between diesel and gasoline. This study focuses on the comparison of spray and combustion characteristics between diesel and kerosene under low-temperature combustion mode conditions based on visualization tests in a constant volume chamber. Different injection pressures, ambient temperatures, and oxygen concentrations were tested at a constant ambient density of 16.67 kg/m3. The image processing results reveal that the liquid penetration distance of kerosene shows a maximum reduction of 33.5% compared to diesel with a smaller cone angle and spray area at 160 MPa and 800 K. At the same time, the ignition delay of kerosene is 0.9 ms longer than that of diesel, and the injection and combustion processes of kerosene are separated, making it much more similar to the premixed combustion. An increase in injection pressure improves the spray penetration distance but exerts little influence on the flame lift-off length and ignition delay for both fuels. Kerosene presents a lower spatially integrating natural luminosity, especially at low injection pressure. The liquid spray penetration distance and the ignition delay of kerosene are more sensitive to the increase in ambient temperature. However, the higher ambient temperature presents a greater effect on the SINL of diesel. With the decrease of ambient oxygen concentration, the ignition delay, flame lift-off length, spatially integrating natural luminosity, and soot oxidation rate of both fuels decrease obviously. Especially at 15 vol.% ambient oxygen, kerosene presents a much weaker luminosity. These results imply that kerosene exhibits a very similar trend to diesel in terms of spray and combustion characteristics. Moreover, kerosene has a stronger potential to reduce soot formation than diesel with a better fuel-air mixing process under low-temperature combustion mode conditions.