Diesel-like Conditions Research Articles

Numerical investigation of equivalence ratio effects on Flex-Fuel mixing controlled combustion enabled by prechamber ignition

There is interest in adapting renewable and low-carbon intensity fuels to heavy-duty engines to help displace criteria and greenhouse gas emissions associated with diesel combustion. Low-carbon fuels have inherently low cetane numbers and thus pose a significant challenge when considering direct substitution in diesel combustion systems. Use of an actively fueled prechamber as an ignition source to initiate mixing-controlled combustion (PC-MCC) of low-cetane fuels is becoming an attractive combustion mode to alleviate the identified reactivity deficit. This work focuses on the utilization of an active prechamber to facilitate diesel-like combustion of bioethanol-gasoline fuel blends in a heavy-duty engine. Recent results in this emerging technology suggest that ignition quality is uniquely coupled to prechamber equivalence ratio. In this numerical study, the fundamental implications of prechamber equivalence ratio on the ignition performance of direct injected fuels ranging from E10 to E100 are investigated using CONVERGE. Parametric studies of the prechamber operating strategy were assessed at diesel-like conditions to characterize the performance trends relative to a diesel baseline at the same boundary conditions. Simulation results indicate that PC-MCC is flex-fuel capable and achieves diesel-like ignition qualities and combustion processes for all fuels considered under stoichiometric and rich prechamber conditions. To characterize the equivalence ratio trends observed, a novel inflow boundary modeling technique was utilized to prescribe turbulent jets in place of the prechamber where selective speciation of the jet composition was conducted to isolate the ignition contributions of radical and reacting species in lean, stoichiometric, and rich jets. The inflow boundary modeling suggests that excess fuel and combustion intermediates present in jets produced from stratified and fuel-rich prechamber operation promotes higher jet temperatures and as such are superior ignition sources. Relative to a lean prechamber jet, the peak temperature of a reacting jet from a rich prechamber was up to 600 K hotter at fixed distances from the orifice exit. Radicals also demonstrated an influence on the ignition process, but the combustion mode was identified to be thermally dominant.

Experimental characterization of direct injection liquid ammonia sprays under non-reacting diesel-like conditions

Ammonia is seen as a promising fuel to replace fossil fuels internal combustion engines, given the advantages of zero carbon emission and extensive experience in its synthesis at large scale. The ignition of ammonia in direct injection engine is still a challenge due to its unfavorable combustion characteristics, but the investigation of ammonia spray characteristics is an important step to shed light on the optimum combustion strategy. In this paper, the evolution of direct injection ammonia sprays was measured at engine-like conditions generated in a constant pressure facility by means of diffused back-illumination imaging and schlieren. In a first step, ammonia spray behavior is compared against that of Diesel fuel. Experimental results show a slower penetration of ammonia mainly due to a lagging starting penetration phase, which cannot be later recovered. In terms of liquid length, initial stages show the same pattern as for ammonia, while a shorter stabilized liquid length can be observed. Parametric trends over different operating conditions show a similar behavior to Diesel against ambient density and temperature, as well as with injection pressure and nozzle diameter. Such results, backed up by the application of a 1D spray model developed for Diesel-like fuels, suggest that ammonia sprays under non-reacting conditions have a mixing-controlled evolution.

Experimental study on flame structure and soot formation of jet/wall impinging combustion under diesel-like conditions

Experimental study on flame structure and soot formation of jet/wall impinging combustion under diesel-like conditions

Experimental Investigation of Heat Transfer in a Flat-wall Impinging Diesel Spray Flame under Diesel-like Conditions

The heat loss through the walls of Internal Combustion Engines (ICEs) needs to be minimized and improved since it has a detrimental impact on the overall thermal efficiency of the engine. This study focuses on investigating heat transfer in a flat-wall impinging diesel spray flame, simulating diesel conditions, to optimize engine parameters. The effects of factors such as various injection pressures, nozzle hole diameters, impingement distances, and oxygen concentrations are analysed in combined and individually. Experimental techniques, such as high-speed imaging, heat flux sensors, and thermocouples, are used to visualize spray flame, measure heat flux profiles and temperature distributions, respectively. Preliminary results and their implications on heat transfer and heat loss are discussed. Regarding the parametric studies investigating the effect on wall heat loss under conditions similar to those of a small diesel engine, it was observed that the transferred heat on the wall was significant in certain conditions. These results emphasize the effectiveness of manipulating the injection rate profile as a viable step to suppress the heat transfer through the wall.

Coupling effect of thermal conducting and low-temperature reaction process on ignition characteristics under diesel-like conditions

The typical behavior of misfiring after low-temperature ignition (LTI) in the cold-start of a diesel engine is mainly caused by thermo-kinetic imbalance and is related to the coupling effect of thermal conducting and low-temperature chemistry. This paper abstractly constructed a two-zone model to characterize such a diesel-like operating condition and investigated the coupling effect of thermal conducting and low-temperature chemistry on ignition characteristics mechanistically. The results show that at relatively low temperature/equivalence ratio environments, thermal conducting has a stronger effect on the ignition process and acts mainly on the chemical reaction rate rather than the reaction pathway. At lower temperature environments (600 K–700 K), the two zones' temperature trajectories almost overlap before LTI. At a higher temperature (700 K–800 K), thermal conducting and low-temperature reactions together affect the ignition process, causing the lower-temperature zone to earlier translate from LTI to high-temperature ignition (HTI), in contrast to the situation without thermal conducting. Thermal conducting induced by fuel-concentration gradient has an effect after the LTI of richer zones, which causes the HTI to delay, but for the leaner zone, it increases the low-temperature reaction rates sharply and advances the ignition. This study provides theoretical support to overcome the unstable ignition problem of cold-start.

Robust two-colour pyrometry uncertainty analysis to acquire spatially-resolved measurements

Two-colour pyrometry (2CP) has been used over several decades to study engine-relevant combustion processes, but results are generally regarded as qualitative or semi-quantitative. In many current 2CP systems, large measurement errors are introduced by parallax because the two measured wavelengths are not from the same line of sight. This work presents a spatially-resolved 2CP system with the objective of quantifying and reducing measurement uncertainty. An optical setup that eliminates parallax in 2CP is used together with pixel-by-pixel calibration of the camera sensor to increase measurement accuracy. Primary uncertainty terms are identified, and an error propagation analysis is performed to compute uncertainties in the final results of soot temperature, soot concentration parameter, KL, and soot mass. These methodologies are applied to investigate an auto-igniting fuel spray in a constant pressure flow rig at diesel-like conditions of high ambient pressure and temperature. Results show bias uncertainty of around 200 K (≈10%) for temperature and about 40%–60% for KL. High uncertainty was found to occur on the diffusion flame front where both optical thickness and soot concentrations are small. However, these uncertain measurement zones with relatively low soot concentrations contribute minimally to the total soot mass present in the reacting jet during the temporal evolution of the flame.

Experimental study of soot particles characteristics for hydrous ethanol diesel emulsified fuel under diesel-like conditions

The emulsified fuel of the commercial diesel fuel and 20 wt% water-containing ethanol with 30% volume fraction of hydrous ethanol was burned in a constant-volume combustion chamber (CVCC). At varying sampling locations along the axis of jet flame, soot particles are thermophoretically sampled and analyzed using high resolution transmission electron microscopy (HRTEM). Many small inception particles bulging from the surface of large “liquid-like” particles are observed at the upstream of jet flames. As these particles move toward the downstream, many growing immature primary particles gradually appear on those large “liquid-like” particles' surfaces, then carbonized particles fuse together and form larger soot aggregates, the large soot aggregates finally crack to generate many smaller particles. During this process, the diameters and the radius of gyration of soot particles decrease steadily, while the fractal dimensions have the opposite trend. The chaotic inner structure of primary particles progressively becomes an ordered structure. These results possibly indicate a new formation mechanism of soot particles; Compared to diesel fuel, soot particles derived from emulsified fuel are much immature with higher oxidation reactivity. The two-color images indicate the potential of reducing soot emissions by using hydrous ethanol diesel emulsified fuel increases the flexibility in controlling engine emissions.

A comparison between low- and high-pressure injection dual-fuel modes of diesel-pilot-ignition ammonia combustion engines

Ammonia, as a carbon-free fuel, is drawing more and more attention due to its potential of decarbonization for marine engines. The dual-fuel mode that can compensate for the poor combustion properties of ammonia has exhibited some prospects for practical application, but more relevant researches are urgent to promote the carbon-free fuel, especially for the high-pressure injection dual-fuel (HPDF) mode. In this paper, the ammonia spray experiments under the diesel-like conditions are conducted using the high-speed photography to calibrate the spray sub-models. Then, the combustion and emission characteristics of the low- and high-pressure injection dual-fuel modes of diesel pilot-ignition ammonia combustion engines are numerically studied and compared. Particularly, the equivalent CO2 considering the global warming potential of CO2 and N2O are evaluated (i.e. CO2 + 300 × N2O). The results show that the maximum ammonia ratio about 80% by energy is recommended for the low-pressure injection dual-fuel (LPDF) mode and further increase the diesel replacement ratio will increase the probability of misfire, while the HPDF mode has the potential to achieve a 97% diesel replacement ratio. Compared with the pure diesel mode, the HPDF mode has similar indicated thermal efficiency, cooling and exhaust loss, while it can significantly reduce the equivalent CO2 with little increase in the NH3 emissions. Owing to the reduced heat transfer loss, the LPDF mode has the potential to achieve a higher indicated thermal efficiency than the pure-diesel mode, but its ability to reduce the equivalent CO2 would be relatively limited because the significant increase of N2O emissions if exhaust after treatments were not employed. The comparison between the LPDF mode and HPDF mode shows that the former case has the potential to achieve a higher indicated thermal efficiency, while the latter case can significantly reduce the engine-out NH3, NOx and greenhouse gas emissions.

High-pressure Spray and Combustion Characteristics of Ammonia Jets under Diesel-Like Conditions

High-pressure Spray and Combustion Characteristics of Ammonia Jets under Diesel-Like Conditions

Effect of swirl ratio on charge convection, temperature stratification, and combustion in gasoline compression ignition engine

Various low-temperature combustion strategies (namely, homogeneous charge compression ignition, reactivity controlled compression ignition, and partially premixed charge compression ignition) have shown the potential to comply with upcoming and prevailing stringent emissions legislations. Low octane gasoline has emerged as an ideal fuel candidate for premixed charge combustion under diesel-like conditions in gasoline compression ignition (GCI) engines. GCI is an excellent technology to rectify future global energy demand imbalance, because it aims to replace diesel (which is in short supply) with low octane fractions/naphtha (which is in surplus supply) in compression ignition engines. However, this novel combustion concept requires modifications in the conventional design of diesel engines. The combustion chamber shape and in-cylinder flows play a crucial role in charge distribution and temperature stratification. Therefore, understanding the combined effect of combustion chamber geometry and in-cylinder flows is essential for future engine designs. GCI combustion engine simulations for varying swirl ratios (SRs) were performed in CONVERGE CFD software to understand the effect of in-cylinder air motion on the mixture stratification and combustion. A 1/7th sector geometry for a conventional re-entry piston bowl was modeled and then simulated. Two different mechanisms were used for model validation. The results indicated that the large-scale flow structures govern the fuel distribution in the combustion chamber. The charge convection because of increased swirl has a substantial effect on the combustion characteristics of the engine. A distinguished ignition kernel was observed for all test cases. An interfacial region with counter-rotating vortices formed a lean mixture zone, hindering flame propagation and combustion. A lower SR, shallow depth piston, and modifications to avoid flame quenching in the squish zone need to be further investigated to optimize the engine performance.

Experimental Research on the Disruptive Evaporation and the Motion Characteristics of Secondary Droplets for Emulsified Biodiesel with a Suspended Droplet Configuration.

The secondary atomization of droplets is one of the means to improve the efficiency of diesel fuel injection atomization. As a promising biomass fuel, emulsified biodiesel showed a good prospect in improving the atomization effect of diesel engines. In this study, a high-temperature and pressure-resistant evaporator was designed to simulate diesel-like conditions, and the evaporation and combustion experiments of emulsified biodiesel droplets were carried out. The morphological changes in the droplets were dynamically captured using a high-speed camera. According to the collected images, the evaporation characteristic parameters, the dynamic parameters of droplet motion, and the correlation between the original and secondary droplets were quantitatively analyzed. The gain effect of secondary atomization for droplets on the diesel engine spray was evaluated. The results showed that the emulsified biodiesel underwent nucleation, agglomeration, puffing, and explosion during the evaporation process, while the classic d2 law only meets a few cases of this fuel. The effect of high temperature was reflected in reducing the normalization time of droplet agglomeration and explosion, while the higher pressure inhibited the expansion of the droplets, thus slowing down the expansion rate and restricting the droplet volume. Driven by the water content, the time of droplet explosion was closer to the droplet lifetime. The evaporation process of the secondary droplet was similar to that of the original droplet over a reduced scale of 1–2 orders of magnitude. (Dd/Dd0)2 ≈ 1 was a necessary condition for the secondary droplets to be produced in large quantities. The average equivalent diameter of the droplets was distributed in the range of 80–140 μm, and the secondary atomization caused expansion of the spray range by 20–40%. Expansion of the range of secondary droplets was beneficial to shortening the ignition delay and increasing the combustion rate.

Effects of Ambient Oxygen and Density on Primary Soot Size under Diesel-Like Conditions Using a Lagrangian Soot Tracking Model

Effects of Ambient Oxygen and Density on Primary Soot Size under Diesel-Like Conditions Using a Lagrangian Soot Tracking Model

Morphology and nano-structure analysis of soot particles sampled within high-pressure diesel reacting jet flames under diesel-like conditions

Morphology and nano-structure analysis of soot particles sampled within high-pressure diesel reacting jet flames under diesel-like conditions

Experimental study of influence of Liquefied Petroleum Gas addition in Hydrotreated Vegetable Oil fuel on ignition delay, flame lift off length and soot emission under diesel-like conditions

The fundamental behaviour on ignition and combustion characteristics of blends of Hydrotreated Vegetable Oil and Liquid Petroleum Gas was investigated in a constant high pressure, high temperature combustion chamber, using a prototype lab-scale injection system adapted from a conventional common-rail system to conduct the injection events, ensuring that fuel was liquid at any point of the injection system and avoiding the formation of fuel vapour bubbles that could alter the injected fuel behaviour.The ignition delay, flame lift-off length and the soot formation were studied by means of high-speed imaging techniques, for different operating conditions. The aim of the work is to characterize the effect of Hydrotreated Vegetable Oil-Liquid Petroleum Gas blend ratios on the previously mentioned parameters.Experimental results show that the behaviour of the fuel blends follow the expected trends of conventional diesel type fuels when varying ambient temperature, density and injection pressure. Hydrotreated Vegetable Oil, being the highest reactivity fraction, controls auto ignition of the blend. However, Liquid Petroleum Gas acts as combustion inhibitor increasing both ignition delay and lift-off length as its ratio in the blend increases. As a consequence, the differences observed in terms of flame radiation suggest that increasing Liquid Petroleum Gas fraction reduces soot formation as it promotes a higher air/mixture.

The evolution of soot morphology and nanostructure along axial direction in diesel spray jet flames

This study aims at providing new insights into the soot formation and oxidation processes in diesel spray jet flames under the diesel-like conditions. Soot particles are sampled within a reacting diesel jet spray flame in a constant-volume combustion chamber under diesel-like high-temperature and pressure ambient conditions. The experiments are conducted with an injector hole of 0.28 mm and injection mass of 30 mg per shot. Soot particles at different locations along the axis of the diesel jet flame are sampled by a thermophoretic probe, and the variation of soot morphology and nanostructure of the sampled soot particles along the axial location in the flame is analyzed by using the transmission electron microscopy (TEM). At upstream of the jet flame, large “liquid-like” particles are observed in the TEM images. As these particles are delivered to the downstream, (1) there are many immature primary particles formed on the surface of large “liquid-like” particles, and (2) they are carbonized into the larger soot aggregates with the matured structure, and then at last, (3) the larger soot aggregates gradually break up into the smaller particles with relatively compact structure. Both the diameter of primary particles and radius of gyration of the soot aggregates decrease as the distance between the injector nozzle tip and the center of TEM grid is increased, while the opposite trend is found in the behavior of the fractal dimensions of soot aggregates. The high resolution TEM images show that the soot primary particles exhibit the turbostratic state at the upstream of the jet flame, while more matured structures with the graphitic crystallite in outer shell and several fine particles in the inner core are observed as the sampling distance from orifice is increased. The average fringe length increases while the average tortuosity and separation decrease with the distance from the nozzle. These results suggest that the mechanism of mature soot aggregates generated directly from the large “liquid-like” particles should be one of the ways of soot particle formation along the axis of diesel jet flames.

Superheat limit and micro-explosion in droplets of hydrous ethanol-diesel emulsions at atmospheric pressure and diesel-like conditions

Micro-explosion of emulsified fuels has been studied on promoting the secondary break-up and improving fuel-air mixing. However, most studies are limited to atmospheric conditions, and experimental observation of micro-explosion under diesel-like conditions is rarely reported. The predictive model for the superheat limit of hydrous ethanol-diesel emulsion needs to be re-examined. In this study, firstly, with the molecular kinetics theory a mathematical model for predicting the superheat limit of hydrous ethanol diesel emulsions are established. Then, the model is verified by the experimental results of micro-explosion in the emulsion droplets. The superheat limit is independent of volume fraction of hydrous ethanol in the emulsions. The intensity of micro-explosion obeys the parabolic rule when changing volume fraction of the hydrous ethanol. The intensity of micro-explosion with the homogeneous nucleation is higher than with the heterogeneous nucleation. The emulsions of 30% and 40% hydrous ethanol exhibit the higher probability for the homogeneous nucleation than the other emulsions. Finally, the droplet behaviors of the emulsions under the diesel-like high-pressure conditions are experimentally observed. While the micro-explosions can be clearly observed for some droplets, neither the micro-explosion nor puffing are observable for most other droplets. The mechanism behind the phenomena is discussed.

Morphology and nano-structure analysis of soot particles sampled from high pressure diesel jet flames under diesel-like conditions

Soot particles emitted from diesel engines have a significant impact on the atmospheric environment. Detailed understanding of soot formation and oxidation processes is helpful for reducing the pollution of soot particles, which requires information such as the size and nano-structure parameters of the soot primary particles sampled in a high-temperature and high-pressure diesel jet flame. Based on the thermophoretic principle, a novel sampling probe minimally disturbing the diesel jet flame in a constant volume combustion vessel is developed for analysing soot particles. The injected quantity of diesel fuel is less than 10 mg, and the soot particles sampled by carriers with a transmission electron microscope (TEM) grid and lacey TEM grid can be used to analyse the morphologies of soot aggregates and the nano-structure of the soot primary particles, respectively. When the quantity of diesel fuel is more than 10 mg, in order to avoid burning-off of the carriers in higher temperature and pressure conditions, single-crystal silicon chips are employed. Ultrasonic oscillations and alcohol extraction are then implemented to obtain high quality soot samples for observation using a high-resolution transmission electron microscope. An in-house Matlab-based code is developed to extract the nano-structure parameters of the soot particles. A complete sampling and analysis procedure of the soot particles is provided to study the formation and oxidation mechanism of soot.

Effects of the turbulence model and the spray model on predictions of the n-heptane jet fuel–air mixing and the ignition characteristics with a reduced chemistry mechanism

Reynolds-averaged Navier–Stokes simulations with an improved spray model and a realistic chemistry mechanism are performed for turbulent spray flames under diesel-like conditions in a constant-volume chamber. Comprehensive numerical analyses including two turbulence models (the renormalisation group k– ε model and the standard two-equation k– ε model) with different model coefficients are made. The distribution of the fuel mixture fractions is a very important factor affecting the combustion process. In this study, we also use the entrainment gas-jet model, modifications of the the spray model coefficient and two turbulence models to investigate extensively the influence of the gas-jet theory model on the fuel–air mixture process. First, a non-reacting case is validated by comparing the liquid-phase penetration and the vapour-phase penetration and also the mixture fractions at different axis positions. Second, approriate methods are confirmed according to accurate mixture fraction distributions to validate the combustion process. Because of the large number of species and reactions, the calculation of chemically reacting flows is unaffordable, particularly for three-dimensional simulations. Hence, the dynamic adaptive chemistry method for efficient chemistry calculations is extended in this work to reduce the computational cost of the spray combustion process when a reduced chemistry mechanism is used. The results show that, in the evaporation case, the gas-jet theory model can be used to obtain a relatively accurate fuel vapour penetration length with different influential factors and that improved numerical methods can effectively reduce the mesh dependence for the spray evaporation process. It is demonstrated that the Schmidt number Sc and the turbulence models significantly influence the mixture fraction distribution. Very good agreement with available experimental data is found concerning the ignition delay time and the flame lift-off length for different oxygen concentrations owing to the accurate fuel mixture fraction.

Modelling of spray and combustion processes by using the Eulerian multiphase approach and detailed chemical kinetics

This research deals with computational modelling of non-reactive and reactive turbulent spray processes. The spray process is modelled using the Euler Eulerian multiphase approach together with a size-of-classes model where the discrete phase is considered as continuum and divided into sub-classes. The combustion process is modelled by taking into account chemical kinetics and solving homogeneous gas phase reactions. The combustion model is implemented into a commercial computational fluid dynamics code, and used in combination with previously validated spray sub-models. Several non-reactive cases are modelled by comparing the fuel spatial and temporal development to the available experimental data. The modelled results show excellent agreement for fuel penetration and mixture distributions. Furthermore, the developed method is validated by modelling reactive spray processes within constant volume vessel, and by comparing results to the Engine combustion network experimental data. The vessel conditions correspond well to diesel-like conditions in terms of gas residuals, pressure and temperature. Finally, the given results show a good agreement for the lift-off length and the ignition delay trends compared to the experimental data, but a slight discrepancy in the combustion process occurrence is observed.

Assessment of the Cavitation Models Implemented in OpenFOAM® Under DI-like Conditions

Direct injection engine performance is strictly correlated to the fluid dynamic characteristics of the injection system. Actual DI engines, both Diesel and gasoline, employ injector characterized by high injection pressure that, associated to micro-orifice design, result in cavitation flow conditions inside injector holes. The cavitation has a beneficial effect on the atomization process and a negative one on the physical erosion generated by the vapor bubble collapse.In order to quantify both effects with a numerical approach, the reduced dimension and the complex flow structures reduce the efficacy of an experimental approach, thus the cavitation model used is of primary importance.The present work addresses the validation of the mixture model-based cavitation models that are implemented in OpenFOAM®, with particular focus on the Schnerr and Sauer model, using the experimental results, available in literature, for a two-phase flow in an optically accessible nozzle under diesel-like conditions.