Search

Homogenous lean combustion in a direct-injection spark-ignition (DISI) engine is a promising pathway to achieve significantly improved fuel economy, making already competitive petrol engines even more attractive as a future powertrain option. This study aims to enhance the fundamental understanding of flame growth occurring in a DISI engine with varied charge equivalence ratios of 1.0 to 0.6 while keeping a low compression ratio of 10.5, a typical side-mounted injector, and early injected homogenous charge conditions. A new flame front vector analysis is performed using the flame image velocimetry (FIV) method applied to 100 cycles of high-speed flame movies with trackable contrast variations and pattern changes in the flame boundary. A spatial filtering method is used to decompose the bulk flow component and high-frequency flow component with the latter being interpreted as turbulence. The flame front FIV analysis shows that excess air leads to slower flame front growth and lower turbulence causing an exponential decrease in the burning rate. Compared to the stochiometric charge condition, a leaner mixture with 0.6 equivalence ratio results in an up to 5 m/s decrease in the flame front growth and 3 m/s decrease in the flame front turbulence. Spatial variations increase up to 2.8 times in the flame front vector magnitude and up to 2.25 times in the turbulence, particularly in the early phase of the flame growth. The results suggest a new engine design for higher turbulence generation is required to extend the lean limit, and thus higher fuel economy is achieved in a DISI engine.

This study implements Flame Image Velocimetry (FIV), a diagnostic technique based on post-processing of high-speed soot luminosity images, to show the in-flame flow field development impacted by after-injection in a single-cylinder, small-bore optical diesel engine. Two after-injection cases with different dwell times between the main injection and after-injection, namely, close-coupled and long-dwell, as well as a main-injection-only case are compared regarding flow fields, flow vector magnitude, and turbulence intensity distribution. For each case, high-speed soot luminosity movies from 100 individual combustion cycles are recorded at a high frame rate of 45 kHz for FIV processing. The Reynolds decomposition using a spatial filtering method is applied to the obtained flow vectors so that bulk flow structures and turbulence intensity distributions can be discussed. The results show significant after-injection-induced flow structures as a group of vectors travelling back toward the center of the combustion chamber upon the jet impingement on the piston bowl wall and then jet-to-jet collision. Despite higher cyclic variations caused by the after-injection, increased bulk flow magnitude and turbulence intensity upon the after-injection event suggests locally enhanced mixing. Compared to the long-dwell after-injection, the close-coupled after-injection shows more significant turbulence enhancements.

This study applies Flame Image Velocimetry (FIV) to show the in-flame flow field development with an emphasis on the jet-jet interaction and jet-swirl interaction phenomena in a single-cylinder small-bore optically accessible diesel engine. Two-hole nozzle injectors with three different inter-jet spacing angles of 45°, 90° and 180° are prepared to cause different levels of jet-jet interaction. The engine has a swirl ratio of 1.7, which is used to evaluate jet-swirl interaction of the selected 180° inter-jet spacing nozzle. High-speed soot luminosity imaging was performed at a high frame rate of 45 kHz for the FIV processing. For each inter-jet spacing angle, a total of 100 individual combustion cycles were recorded to address the cyclic variations. The ensemble averaged flow fields are shown to illustrate detailed flow structures while the Reynolds decomposition using spatial filtering is applied to analyse turbulence intensity. The results showed reduced bulk flow magnitude and turbulence intensity at smaller inter-jet spacing, suggesting the two opposed wall-jet heads colliding immediately after the jet impingement on the wall can cause flow suppression effects. This raised a concern on the mixing as lower inter-jet spacing creates more fuel-rich mixtures in the jet-jet interaction region. Despite lower flow magnitude, the cyclic variation was also estimated higher for narrower inter-jet spacing, which is another drawback of the significant jet-jet interaction. Regarding the jet-swirl interaction, the wall-jet head penetrating on the up-swirl side showed lower bulk flow magnitude as the counter-flow arrangement suppressed the flow, similar with the narrower interact-jet spacing results. However, the turbulence intensity was measured higher on the up-swirl side, suggesting the relatively weaker swirl flow vectors opposed to the penetrating wall-jet head could in fact enhance the mixing.

This study performs endoscopic high-speed imaging to enhance the fundamental knowledge of in-cylinder flow structure and flame development process in a selected high-tumble production engine. The endoscopic high-speed particle image velocimetry (eHS-PIV) was performed for varied engine speeds and intake valve closing (IVC) timings to evaluate their impact on the in-cylinder flow structure in a motored engine condition. On another endoscope engine sharing the same hardware, high-speed flame imaging was conducted to visualise spark stretch and flame propagation. The flow and flame measurements were repeated for over 100 cycles and the ensemble-averaged results are compared. The eHS-PIV showed that a strong tumble vortex is generated during the piston compression with the flow directed towards the exhaust side. As the piston reaches top dead centre (TDC), however, a complex flow breakup involving multiple flow components occurs. This is followed by lateral flow vectors travelling back towards the intake side, which is termed as the bounce-back flow. For a tested engine speed range of 1700–2700 revolutions per minute (rpm), 2500 rpm shows the most significant bounce-back flow as a result of competition between the remaining exhaust-ward tumble flow strength and the newly formed bounce-back flow strength. At a retarded IVC timing, the flow loss leads to a weakened tumble flow and subsequently no bounce-back flow formation to maintain the exhaust-ward TDC flow direction. From the comparison between the flow results and spark/flame high-speed images, a strong positive correlation is found between the TDC flow direction and spark plasma stretch, and subsequently the flame propagation direction. The findings indicate that the TDC flow direction should be considered as a key parameter in the engine design and operating condition settings.

Analyzing the heat transfer effectiveness of fire suppression systems at droplet level through experimentation is difficult and costly. To overcome this issue, traditional Eulerian–Lagrangian model...

This study shows the structure of soot particles sampled directly from wall wetting-induced pool fires formed on the piston top in a spark-ignition direct-injection (SIDI) engine. Of particular interest is its variation with injection pressure considering the current trend of high-pressure DI system development to reduce engine-out particulate emissions. Thermophoretic particle sampling was performed for transmission electron microscope (TEM) imaging, which was post-processed for statistical analysis of key morphology and internal structure parameters. These include the size distribution of soot aggregates and primary particles as well as carbon-layer fringe-to-fringe gap and concentricity. With the fixed engine speed and load conditions, in-flame soot particles are compared to the exhaust particles sampled simultaneously at selected 5 and 15 MPa injection pressures corresponding to low-speed/low-load and high-speed/high-load in practical engine operation. From the TEM images and statistical analysis, it was found that more concentrated and taller pool fire developed for 5-MPa injection leads to smaller soot aggregates composed of smaller soot primary particles due to suppressed soot growth in fuel-rich flames. However, the soot particles formed in 15-MPa injection-induced pool fires are at a more reactive status evidenced by less defined core-shell boundaries and higher fringe separation. The higher soot reactivity results in enhanced soot oxidation, which explains smaller soot aggregates and primary particles found for the 15-MPa injection in the exhaust sample.

Hydrogen-diesel dual direct-injection (H2DDI) is successfully implemented in a compression-ignition engine, which is developed to circumvent the pre-ignition and knocking limitations inherent to port fuel-injection hydrogen engines. An automotive-size single-cylinder common-rail diesel engine was modified to fit an additional high-pressure hydrogen injector in the cylinder head. The engine is operated at intermediate load with constant fuel-energy input using an energy-substitution principle – the diesel injection duration is decreased as the hydrogen amount is increased while adjusting the diesel injection timing to fix the combustion phasing. The results show that, at early hydrogen injection timings, the heat release rate and engine-out emissions show trends indicating premixed combustion whereas later injection timings exhibit hydrogen mixing-controlled combustion behaviour. At 50% hydrogen substitution ratio and optimised direct injection timing of 40 ⁰CA bTDC, the uncompromised indicated efficiency of 47% is achieved while the combustion-induced noise is decreased by 6 dB and the engine-out NOx emission is kept below 11 g/kWh.

Gasoline compression ignition (GCI) engines have potential to improve fuel economy and reduce emissions harmful to health and the environment compared with conventional diesel combustion engines. T...

The evolution and combustion of H2 jets were investigated in an optically-accessible constant-volume chamber under simulated direct-injection (DI) compression-ignition (CI) engine conditions. The parameters varied include injection pressure (84–140 bar) and ambient temperature (1000–1140 K). A detailed characterization of the injector system and the ensuing jet penetration process is reported first. High-speed schlieren imaging, OH∗ chemiluminescence imaging and pressure trace measurements were subsequently used to investigate the auto-ignition and combustion of the H2 jets. The results show that the ignition delay of H2 jets under such conditions is sensitive to ambient temperature variations, but not to injection pressure. Optical imaging reveals that the combustion of H2 jets mostly initiated from a localized kernel, before spreading to engulf the whole jet volume downstream of ignition location. The imaging also indicates that after ignition, the flame recesses back towards the nozzle and appears to attach to the nozzle to form a diffusion flame structure.

This study measures in-flame flow fields in a single-cylinder small-bore optical diesel engine using Flame Image Velocimetry (FIV) applied to high-speed soot luminosity movies. Three injection pressures were tested for a two-hole nozzle injector to cause jet-wall interaction and a significant jet-jet interaction within 45° inter-jet spacing. The high-pressure fuel jets were also under the strong influence of a swirl flow. For each test condition, soot luminosity signals were recorded at a high framing rate of 45 kHz with which the time-resolved, two-dimensional FIV post-processing was performed based on the image contrast variations associated with flame structure evolution and internal pattern change. A total of 100 combustion events for each injection pressure were recorded and processed to address the inherent cyclic variations. The ensemble-averaged flow fields were used for detailed flow structure discussion, and Reynolds decomposition using a spatial filtering method was applied to obtain high-frequency fluctuations that were found to be primarily turbulence. The detailed analysis of flow fields suggested that increased injection pressure leads to enhanced jet flow travelling along the bowl wall and higher flow vectors penetrating back towards the nozzle upon the impingement on the wall. Within the jet-jet interaction region, the flow vectors tend to follow the swirl direction, which increases with increasing injection pressure. The FIV also captured a turbulent ring vortex formed in the wall-jet head, which becomes larger and clearer at higher injection pressure. A vortex generated in the centre of combustion chamber was due to the swirl flow with its position being shifted at higher injection pressure. The bulk flow magnitude indicated significant cyclic variations, which increases with injection pressure. The turbulence intensity is also enhanced due to higher injection pressure, which primarily occurs in the wall-jet head region and the jet-jet interaction region.

Great attention to the efficiency benefits of spark ignition direct injection engine has been averted due to its problematic particulate emissions. In the present study, the fundamental knowledge of wall-wetting-induced spark ignition direct injection soot particles is enhanced through direct particle sampling from pool fire on the piston top surface and cylinder liner as well as from the exhaust stream. The sampled soot particles are imaged using transmission electron microscope, and the image post-processing for statistical morphology and internal structure analysis is performed to better understand the soot formation and oxidation processes. The experiments were performed in a single-cylinder optical spark ignition direct injection engine where diffusion flame luminosity was recorded using a high-speed camera through the cylinder liner window, with which the overall sooting level was understood, and the pool fire location was identified. Given the in-flame soot sampling experiments in the spark ignition direct injection engine were new, error analysis was conducted in terms of the number of fuel injections and engine run-to-run variations. This sampling technique then was applied for various injection timings in the intake stroke. The data analysis and physical interpretation was focused on a piston-wetting condition at the most advanced injection timing of 320 °CA bTDC and a liner-wetting condition at the most retarded injection timing of 180 °CA bTDC in the present study. Between these two different wall-wetting conditions, it was found that the piston-wetting condition has larger soot primary particles and soot aggregates. The internal carbon-layer fringe shows longer length, less tortuosity and smaller gap, indicating more mature and carbonised soot. This was consistent with more significant and wider distributed pool fire and thus longer soot residence time within the flames. When the exhaust soot particles were analysed, however, it was found that the reduction in soot aggregate size was much higher and the carbonisation was more progressed for the piston-wetting condition than those of the liner-wetting condition. This suggested higher soot oxidation later in the expansion/exhaust stroke for the piston-wetting condition, which potentially can be better utilised for engine applications.

A new multi-location soot sampling method is used to enhance the knowledge about the structural evolution of in-flame particles in a light-duty optical diesel engine. Through thermophoresis-based particle sampling performed at multiple in-bowl locations, the soot structures are shown for both early formation stage and later stage from the same combustion event. Three different jet-spacing angles of 45°, 90° and 180° were studied to analyse how different levels of jet–jet interaction impact the soot particle morphology and internal structure. One selected jet–jet interaction condition was further analysed to show differences in soot structures between the up-swirl side and down-swirl side of the wall jets. From transmission electron microscopes (TEM) images of the sampled soot particles and their statistical size analysis, it was found soot particles initially formed within 45∘ separated jet–jet interaction region have un-solidified premature aggregates due to limited carbonisation in the locally fuel-rich mixtures. When these soot particles travelled on the down-swirl side of the jets, they became solidified and carbonised while the oxidation was evident from the smaller soot primary particle and longer carbon-layer fringe and lower tortuosity. The higher mixing on the up-swirl side of the jets further enhanced the soot oxidation, resulting in even smaller soot primary particle, fragmentation of large soot aggregates, and even longer and less curved carbon-layer fringes. Regarding jet–jet interaction, the 180° jet spacing angle created no jet–jet interaction condition on the soot sampler locations. For smaller jet-spacing angles, the increase in jet–jet interaction promoted the soot formation as evidenced by larger and more complex soot aggregates formed due to more active soot aggregation and agglomeration. The soot oxidation became limited at higher jet–jet interaction conditions, which led to more amorphous soot internal structures.

The use of color-ratio pyrometry (CRP) methods, with variable or prescribed soot content (KL) to image flame–wall interactions was examined, with results compared with that obtained using the more mature two-color pyrometry (TCP) technique. The CRP and TCP methods were applied to flame–wall impingement images recorded in a optically-accessible constant volume combustion chamber (CVCC) under compression-ignition (CI) engine conditions. Good correlation in the result trends were observed for the CRP method with fixed KL output and that generated using TCP. Slight discrepancies in the predicted absolute temperature values were observed, which were linked to the difference in the KL value prescribed to the CRP method, and the KL value predicted using TCP. No useful output was obtained with CRP method with variable soot output because of channel noise. A simplified flame transparency modeling was performed to assess the inherent errors associated with the pyrometry methods. The results indicated that the uncertainties arising from the fixing of the KL output appeared acceptable.

This study shows the in-cylinder soot reduction mechanism associated with injection timing variation in a small-bore optical diesel engine. For the three selected injection timings, three optical-/laser-based imaging diagnostics were performed to show the development of high-temperature reaction and soot within the cylinder, which include OH* chemiluminescence, planar laser–induced fluorescence of hydroxyl and planar laser–induced incandescence. In addition, detailed soot morphology analysis was conducted using thermophoresis-based soot particle sampling from two locations within the piston bowl, and the subsequent analysis of transmission electron microscope (TEM) images of the sampled soot aggregates was also conducted. The results suggest that when fuel injection timing is varied, ambient gas temperature makes a predominant effect on soot formation and oxidation. This is primarily combustion phasing effect as the advanced fuel injection moved the start of combustion closer to the top dead centre, and therefore, soot formation and oxidation occurred at elevated ambient gas temperature. There was an overall development pattern of in-cylinder soot consistently found for three injection timings of this study. The planar laser–induced incandescence images showed that a few small soot pockets first appear around the jet axis, which promptly grow into large soot regions behind the head of the flame marked planar laser–induced fluorescence of hydroxyl. The soot signals disappear due to significant oxidation induced by surrounding OH radicals. When the injection timing is advanced, the soot formation becomes higher as indicated by higher total laser–induced incandescence coverage, increased sampled particle counts and larger and more stretched soot aggregate structures. However, soot oxidation is also enhanced under this elevated ambient temperature environment. At the most advanced injection timing of this study, the enhanced soot oxidation outperformed the increased soot formation with both peak laser–induced incandescence signal coverage and late-cycle coverage showing lower values than those of more retarded injection timings.

The aim of this study is to investigate the impact of walls on soot processes of a post-injection strategy at different dwell times. The experiments were performed in an optically accessible consta...

This study shows how ethanol addition impacts in-flame and exhaust soot in spark-ignition direct injection (SIDI) engines through direct and simultaneous sampling of particles from the flames and exhaust stream. The thermophoresis-based soot sampling and transmission electron microscope (TEM) imaging was performed for various ethanol blending ratios ranging from 0 to 60%. A quantitative analysis is performed in the images obtained from a standard TEM by yielding the in-flame and exhaust soot number counts, projection area, as well as morphological parameters such as soot primary particle diameter, aggregate radius of gyration, and fractal dimension. In addition, the internal structures and reactivity status of in-flame and exhaust soot particles are unveiled using a high-resolution TEM with carbon fringe length, tortuosity and fringe-to-fringe separation extracted from processed carbon layer fringe images. The results show that both the number counts and projection areas of exhaust soot are much lower than those of the in-flame soot at any fixed ethanol blending ratios. The exhaust soot is smaller in size for both the primary particles and aggregates with more compact aggregate structures, longer and straighter fringe, and lower fringe separation, all indicating significant soot oxidation occurred inside the cylinder before the particles exit through the exhaust. With increasing ethanol blending ratio, almost linear reduction in number counts and projection area is found for both the in-flame and exhaust soot particles, while both soot aggregates and primary particles become smaller with more stretched aggregate structures and shorter, more curved, and narrower gap fringe structures, indicating soot particles with higher reactivity. The significantly lower soot emissions achieved with higher ethanol blending ratio is found to be due primarily to the suppression of soot formation within the flame as evidenced by higher reduction rate of number counts, projection area, and primary particle and aggregate sizes for the in-flame soot than that of the exhaust soot.

The composition and carbon nanostructure of soot are important parameters influencing health and climate effects, and the efficacy of soot mitigation technologies. We used laser-vaporization, electron-ionization aerosol mass spectrometry (or SP-AMS) to systematically investigate relationships between aerosol mass spectra, carbon nanostructure (HRTEM), and composition (thermal-optical carbon analysis) for soot with varying physicochemical properties. SP-AMS refractory black carbon concentrations (based on clusters) were correlated to elemental carbon (r = 0.98, p −8) and equivalent black carbon (aethalometer) concentrations. The SP-AMS large carbon (C+≥6, midcarbons and fullerene carbons) fraction was inversely correlated to fringe length (r = −0.97, p = 0.028) and linearly correlated to the fraction of refractory organic carbon that partially pyrolize during heating (r = 0.89, p −4). This refractory organic carbon material was incompletely detected with conventional aerosol mass spectrometry (flash vaporization at 600 °C). This suggests that (SP-AMS) refractory carbon cluster analysis provides insight to chemical bonding and nanostructures in refractory carbon materials, lowcarbons (C+≥5) indicate mature soot and large carbons indicate refractory organic carbon and amorphous nanostructures related to C5-components. These results have implications for assessments of soot particle mixing state and brown carbon absorption in the atmosphere and enable novel, on-line analysis of engineered carbon nanomaterials and soot characteristics relevant for climate and health.

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.

This study examines the effects of biodiesel carbon chain length on the development of high-temperature flame and soot in single-cylinder, light-duty, optical diesel engine. Planar laser-induced fluorescence of OH (OH-PLIF) and laser-induced incandescence (soot-PLII) are performed separately using petroleum diesel and two biodiesel surrogate fuels. The selected surrogate fuels have a saponification number (SN) of 330 and 233 for shorter and longer carbon chain length, respectively, while the iodine value (IV) characterising the degree of unsaturation is kept similar. The laser-based images are shown together with the chemiluminescence images of naturally occurring cool-flame signals and electronically excited OH (OH∗). The start of high-temperature reaction timing was matched for the tested fuels by adjusting the injection timing so that the in-cylinder ambient conditions at the start of combustion were consistent. The results show that the longer carbon chain biodiesel has shorter ignition delay time, which makes a significant impact on the spatial and temporal development of high-temperature reaction zones. The OH signals for the longer carbon chain length biodiesel are observed close to the piston-bowl wall whereas they appear later in the penetrating front of the wall-interacting jet for the shorter carbon chain length biodiesel. Both fuels show soot formation occurring in the wall-jet head region while the longer carbon chain length biodiesel has higher soot concentration and larger area than that of the shorter carbon chain biodiesel. This is due to lower pre-combustion mixing, reduced fuel oxygen content, and possibly less significant OH-induced soot oxidation.

The engine combustion network (ECN) Spray A is modelled using the Reynolds-averaged Navier–Stokes-transported probability density function (RANS-TPDF) approach to validate the application of a new multiple mapping conditioning (MMC) mixing model to multiphase reactive flows. The composition TPDF equations are solved using a Lagrangian stochastic approach and the spray is modelled with a discrete particle approach. The model is first validated under non-reacting conditions (at 900 K) using experimental mixture-fraction data. Reactive simulations are then performed for three different ambient temperatures (800, 900, 1100 K) and oxygen concentrations (13, 15, 21%) at an ambient density of 22.8 kg/m3. The MMC mixing model is compared with the interaction by exchange with the mean (IEM) mixing model. The ignition delay predictions are not sensitive to the mixing model and are predicted well by both the mixing models under all the tested ambient conditions. The IEM model overpredicts the flame lift-off length (FLOL) at high temperature and high oxygen conditions with a mixing constant C ϕ = 2 . The MMC model with C ϕ = 2 and a target correlation coefficient r t = 0.935 between the mixture fraction and a reference variable used to condition mixing predicts good FLOL under all the conditions except 800 K. It is demonstrated that the lift-off length is controllable by changing the target correlation coefficient, while Cϕ and therefore the mixing fields are held fixed. In comparison to the MMC model, the IEM model predicts a higher variance of temperature conditioned on mixture fraction near the flame base owing to its lacking the property of localness. The mixing distance between the notional TPDF particles in the composition space is also higher with the IEM model and it is demonstrated that by changing rt, different levels of mixing locality can be achieved.

This study shows how the structure of soot particles within the flame changes due to the relative direction of the swirl flow in a small-bore diesel engine in which significant flame–wall interactions cause about half of the flame travelling against the swirl flow while the other half penetrating in the same direction. The thermophoresis-based particle sampling method was used to collect soot from three different in-flame locations including the flame–wall impingement point near the jet axis and the two 60° off-axis locations on the up-swirl and down-swirl side of the wall-interacting jet. The sampled soot particle images were obtained using transmission electron microscopes and the image post-processing was conducted for statistical analysis of size distribution of soot primary particles and aggregates, fractal dimension, and sub-nanoscale parameters such as the carbon layer fringe length, tortuosity, and spacing. The results show that the jet-wall impingement region is dominated by many small immature particles with amorphous internal structure, which is very different to large, fractal-like soot aggregates sampled from 60° downstream location on the down-swirl side. This structure variation suggests that the small immature particles underwent surface growth, coagulation and aggregation as they travelled along the piston-bowl wall. During this soot growth, the particle internal structure exhibits the transformation from amorphous carbon segments to a typical core–shell structure. Compared to those on the down-swirl side, the soot particles sampled on the up-swirl side show much lower number counts and more compact aggregates composed of highly concentrated primary particles. This soot aggregate structure, together with much narrower carbon layer gap, indicates higher level of soot oxidation on the up-swirl side of the jet.

The present study shows the distinctive role of first injection proportion and fuel ignition quality in controlling the first-injection mixture homogeneity and the second-injection charge premixing, respectively, in a single-cylinder automotive-size gasoline compression ignition (GCI) engine running on a double injection mode. The engine was operated at 2000 rpm and the indicated mean effective pressure (IMEP) of about 940 kPa. From many previous studies on homogeneous charge compression ignition (HCCI) engines, it is well known that the increased mixture homogeneity can reduce engine-out emissions of particulates and nitrogen oxides (NO x ) while increased unburnt hydrocarbon (uHC) and carbon monoxide (CO) emissions become an issue. Also, many diesel combustion studies showed that the increased charge premixing causes the higher initial rate of heat release and thereby increasing engine efficiency at the expense of NO x emissions. In partially premixed charge GCI combustion engines implementing a near-BDC first injection and a near-TDC second injection, however, it is unclear how the overall mixture homogeneity and premixedness impact engine efficiency and engine-out emissions. In this study, both the first injection proportion and fuel ignition quality were varied considering their expected influence on premixed charge preparation. Specifically, conventional gasoline fuels with 91, 95, and 98 research octane number (RON) were tested for GCI combustion for various first injection proportions of 20–50%. For fixed combustion phasing conditions realised by the variations of the second injection timing, the results show that the increased first injection proportion causes a more stretched profile of the apparent heat release rate (aHRR) with a lower peak value due to the increased mixture homogeneity of the first-injection charge. This leads to increased engine efficiency as the heat release occurs for a longer period of time while the more homogeneous mixture causes reduced NO x emissions and combustion-induced noise. The increased mixture homogeneity, however, leads to increased uHC and CO emissions. In comparison, the decreased fuel ignition quality (or increased RON) at fixed mixture homogeneity of the first-injection charge results in higher aHRR and thereby increasing the engine efficiency, which is due to the increased charge premixedness of the second-injection fuel. Consequently, the NO x emissions and combustion-induced noise increase while the uHC/CO/smoke emissions become lower. Due to these compounding effects of first-injection mixture homogeneity and second-injection charge premixedness, it was found that the optimised GCI engine efficiency and emissions, for selected operating conditions and given engine geometry, are achieved at 40% first injection proportion and 95 RON fuel.

Constraining the climate impact of particulate brown carbon (BrC) will require identification of formation mechanisms and isolation of its different components to allow for source apportionment. For fresh combustion aerosols, the light absorption characteristics and the Absorption Ångstrom Exponent (AAE) are principally controlled by the combustion conditions in which the particles formed and evolved. We investigated the influence of combustion temperatures on the BrC or black carbon (BC) emission characteristics for a miniCAST soot generator (propane fuel) and a modern heavy-duty diesel engine (petroleum diesel and two renewable diesel fuels). Changes in the AAE, mass spectral signatures, and thermal-optical characteristics were studied. We show that changing operating parameters to gradually reduce the combustion temperatures in these two fundamentally different combustion devices result in a regression from BC dominated to BrC dominated particle emissions. The regression toward BrC was associated with: (1) an increasing mass fraction of particulate non-refractory polycyclic aromatic hydrocarbons (PAHs), (2) an increasing fraction of refractory organic carbon, (3) more curved soot nanostructures and shorter fringe lengths, and (4) increased signal from (refractory) large carbon fragments in IR laser-vaporization aerosol mass spectra. Based on these results we argue that fresh BrC dominated combustion aerosols are attributed to primary emissions from low temperature combustion, highlighting the influence of refractory constituents and soot nanostructure. Higher temperatures favor the growth of conjugated polyaromatic structures in the soot, a progression hypothesized to control the evolution from BrC to BC character of the emitted aerosols.

In a small-bore diesel engine, soot particles are sampled directly from the flames by placing a transmission electron microscope (TEM) grid inside the cylinder. The TEM images are taken from multiple on-grid locations to ensure enough number of soot particles are post-processed for statistically meaningful data of morphology parameters. This study presents variations in the soot TEM images and sizes of soot aggregates and primary particles from 30 different on-grid locations for each of jet-wall impingement and jet-jet interaction regions of the diesel flames. The TEM images show significant variations in soot aggregate size and structures for the two different sampling regions with overall larger and more complex soot for the jet-jet interaction region. The statistical results show that the difference in soot primary particle diameter is measurable after five images were processed. For soot aggregate radius of gyration, the two samples show no apparent variations until 25 images were processed, and the mean values levelled only when more than 27 images were processed. As the enough number of TEM images were processed, the larger sizes in both soot aggregates and primary particles were confirmed for the jet-jet interaction region.

The direct injection spark ignition (DISI) engines, while showing the extended controllability in operation conditions over port-injection engines, have a cyclic variation issue. For the past decade, many studies have been conducted to identify the causing factors of the cyclic variation and mitigate its negative effects. However, the fundamental knowledge of cyclic variation in DISI engines is still lacking. This experimental study measures cyclic variations in the in-cylinder pressure, early-stage burn duration and flame growth rate in a single-cylinder optical DISI engine and evaluates their correlations. While the in-cylinder pressure was recorded by the pressure sensor mounted in the spark plug, a high-speed camera was used to capture the flame propagation for over 100 individual cycles. The results show significant cyclic variations in both pressure derived data and flame image derived data; however, a clear positive correlation is observed between the burn duration and flame growth rate. Further analysis of the flames indicates higher level of flame front wrinkling measured in some individual cycles is a cause of the higher flame growth rate.

This study shows the in-flame flow-field development under strong flame-swirl interaction in a single-cylinder small-bore optical diesel engine. The experiment was conducted using a two-hole common-rail injector nozzle with a 180° inter-jet spacing angle for the detailed study of wall-interacting diesel jets. Through high-speed imaging of the natural combustion luminosity, Flame Image Velocimetry (FIV) – an innovative flow diagnostic technique, was applied to derive the in-flame flow fields. To address the inevitable cyclic variation, a total of 100 individual combustion cycles were measured, from which the ensemble-averaged flow-field development was calculated. In addition, detailed statistical analysis of the bulk-flow velocity and turbulence intensity was also conducted in both up- and down-swirl direction of the flame path through Reynolds decomposition of obtained flow field by a spatial filtering approach. The results show that the strong jet-wall interaction causes a radial jet flow along the piston-bowl wall. A lower bulk flow velocity was found in the up-swirl direction given the counter-acting swirl flow with respect to the flame development. However, this counter-acting flame-swirl interaction elevates the local turbulence level, as indicated by the higher calculated turbulence intensity, suggesting enhanced mixing.

With the increasing demand for energy capacity and power density in battery systems, the thermal safety of lithium-ion batteries has become a major challenge for the upcoming decade. The heat transfer during the battery thermal runaway provides insight into thermal propagation. A better understanding of the heat exchange process improves a safer design and enhances battery thermal management performance. This work proposes a three-dimensional thermal model for the battery pack simulation by applying an in-house model to study the internal battery thermal propagation effect under the computational fluid dynamics (CFD) simulation framework. The simulation results were validated with the experimental data. The detailed temperature distribution and heat transfer behaviour were simulated and analyzed. The thermal behaviour and cooling performance were compared by changing the abnormal heat generation locations inside the battery pack. The results indicated that various abnormal heat locations disperse heat to the surrounding coolant and other cells. According to the current battery pack setups, the maximum temperature of Row 2 cases can be increased by 2.93%, and the temperature difference was also increased. Overall, a new analytical approach has been demonstrated to investigate several stipulating battery thermal propagation scenarios for enhancing battery thermal performances. Australian Research Council (ARC Industrial Transformation Training Centre IC170100032) and the Australian Government Research Training Program Scholarship.