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The present study investigated the characteristics of particulate matter (PM) emissions in a single-dome, Rich-Quench-Lean, model combustor under different operating conditions typical for aero-engines, with special emphasis on identifying the individual effects of pressure, temperature, and equivalence ratio on PM emissions in aero-combustors. Results showed that the number-based particle size distribution was shifted from the nucleation mode ( 60 nm for the different pressure cases investigated. Regarding the influence of combustion temperature, a non-monotonic response of particle geometric mean diameter with increasing primary zone average temperature was found in that particle size increased and decreased in the temperature range below and above 2270 K respectively. This non-monotonic response with temperature variation is likely caused by the competition between soot growth rate and oxidation rate. It is postulated that the increasing trend of particle size is a result of more carbonaceous particles formed under elevated temperatures providing surface areas available for the condensation of organic compounds, while the soot oxidation rate eventually overtakes at higher temperatures leading to the observed decreasing trend.

This paper reports the experimental study on lean ignition (LI) and lean blow-off (LB) behaviour of butyl butyrate-based biofuels in a gas turbine combustor. The butyl butyrate-based biofuels were formulated (butyl butyrate–ethanol blends with the volume percentage of ethanol 0, 10%, 30%, 50% respectively, named BE-0, BE-10, BE-30, BE-50). The aviation kerosene RP-3 was also tested as a reference fuel. A combustor of an aero-engine was fabricated to conduct experiments on these fuels. The statistic method Design of Experiments (DoE) was employed to correlate LI and LB with fuel properties and operating conditions, and then analyse the significance of these experimental variables. The results indicated that all test biofuels had lower equivalence ratio of LI than RP-3, but the LB between RP-3 and the biofuels of high ethanol fraction (30% and 50%) had no appreciable difference at low air flow rate. The results also demonstrated that fuels with high ethanol fractions tended to ignite and blow off the flame at higher equivalence ratios. Meanwhile, the equivalence ratio of both LI and LB decreased at high inlet air flow rate for all the test fuels. RP-3 could combust under a larger range of air conditions yet its stability was more sensitive to air flow rate than test biofuels. Two predictive equations of LI and LB were obtained via Design of Experiments (DoE) and demonstrated that the lower heating value (LHV) of fuels, air pressure drop in the combustor, fuel pressure and inlet air pressure of the combustor were the main factors influencing LI and LB.

Spray collapse has been widely observed under both flash-boiling and non-flash-boiling conditions when multi-hole gasoline direct injection (GDI) injectors with compact nozzle configurations were used. The main objective of this study is to further understand the collapse mechanisms by using liquids with appreciably different properties (propane, n-hexane and iso-octane). The tests were carried out in a constant volume vessel with ambient pressures ranging from 0.6 bar to 11.0 bar and fuel temperatures from 30 °C to 110 °C. Flashing propane sprays presented a non-collapse feature under elevated-ambient-pressures but flash-boiling conditions. By close-up examination of flashing propane sprays over a wide range of liquid temperatures and ambient pressures, it was found that there should be an ambient pressure threshold between 1.0 and 3.0 bar between the collapsed and the non-collapse sprays for propane. The spray collapse occurred as the ambient pressure is below the threshold. The non-collapse feature for flashing propane sprays under the ambient pressures beyond the threshold was attributed to the prohibition of nucleation and bubble growth under elevated ambient pressures.

Semi-volatile organic compounds (SVOCs) from diesel engine emissions have detrimental impact on air quality and human health. This study aims at characterizing the compositional SVOC emissions of a commercial diesel engine in both gaseous and particulate phases concurrently by using a two-dimensional Gas-Chromatography Time-of-Flight Mass-Spectrometry (GC × GC-ToFMS). 52 SVOCs including n-alkanes, n-alkyl-cyclohexanes and PAHs have been identified and quantified, and the contributions of diesel fuel and lubricating oil to the SVOC emissions have been evaluated under different engine loads by using the Positive Matrix Factorization (PMF) method. The distributional patterns of the gaseous SVOC emissions were found to be similar to those of the diesel fuel, while those of the particulate SVOC emissions were strongly associated with the lubricating oil. The PMF analysis illustrated that diesel fuel contributed to the SVOC emissions by nearly 70% at low load (442 ± 38 ug/kg·fuel), while lubricating oil dominated the SVOC emissions by over 50% at high load (595 ± 93 ug/kg·fuel). Additionally, we found that combustion by-products had nontrivial contributions to the SVOC emissions, increasing from 13% to 38% (from 83 ± 10 ug/kg·fuel to 443 ± 63 ug/kg·fuel) as load increased. The results obtained in this study could facilitate a better understanding of the SVOC formation mechanisms, and hence provide guidance for engine optimization towards SVOCs mitigation.

A number of major studies have demonstrated that the SVOC (Semi-volatile organic compounds) within engine emissions derive predominantly from unburned fuel and lubricants, and are a major contributor to primary atmospheric aerosol containing thousands of organic compounds. The GC × GC-ToF-MS (2 dimensional Gas Chromatography – Time of Flight – Mass Spectrometry) comprehensive analytical technique was utilized in this study, to resolve the complex mixtures and characterize the SVOC content in eight different commercial lubricants, including 5 W30 synthetic and semi-synthetic, mineral and base oil. In order to quantify the aliphatic isomers, which comprise the largest component of the lubricants, a TIC-M.Q./Mass (Total ion current ratio to the molar quantity/mass) method has been developed. The TIC intensity was observed to be proportional to the molar quantity of n-alkanes for carbon number C25. Additionally, the TIC intensity of the alkyl-cyclohexanes under the identical retention indices were found to have an equivalent response to those of the n-alkanes, showing that the quantitative calibrations derived for the n-alkane series could be applied to estimate the concentrations of isomeric aliphatic compounds with similar molecular weight. Furthermore, a mesh method was introduced to group the alkane species (n-alkanes, branched alkanes and cyclic alkanes etc.), combining with the use of a soft EI (electron impact) ionization (14 eV) to retain the distinct identity of the isomers with less fragmentation, which allowed the TIC-M.Q./Mass methodology to integrate all the constitutional isomers present in the lubricating oil samples. By utilizing this methodology, compositions from different samples were comprehensively compared, leading to the following conclusions: 1) the synthetic and semi-synthetic oils contained a larger abundance of HMW (high molecular weight) aliphatic compounds (carbon number C24–C29), while those in the LMW (low molecular weight range, carbon number C18–C25) were predominant in the mineral and base oil; 2) cycloalkanes were predominant in the synthetic and semi-synthetic oils; whereas the branched alkanes were more prominent in the mineral and base oils; 3) for lubricants used for a short period, a slight increase of LMW compounds was observed, while the HMW compounds underwent a decrease, whereas, there was an overall mass reduction for all the aliphatic compounds detected in the oil samples used for six-months.

This study proposed a new method for measuring planar Sauter Mean Diameter (SMD) of multi-component fuel spray using a SETScan optical patternator, a particle imaging velocimetry (PIV) and a previously validated evaporation model. Malvern experiments were conducted to obtain line-of-sight SMD, which could be considered as a reference value to compare with the planar SMD due to spray symmetry. The SETScan patternator provided the planar local surface area densities at two axial cross-sections of the spray (250 mm and 400 mm below the nozzle), and the total surface area was calculated by integrating local surface area density over the concerned cross-section area. The PIV was used to measure droplet velocities at the same cross-sections, so that the time for the droplets passing through the laser sheet of the patternator (concerned cross-sections) could be determined. The total droplet volume captured within the laser sheet was then calculated by multiplying the time and the initial flow rate. The planar SMD at the cross-section is the quotient of the total volume divided by the total surface area. However, there existed a noticeable discrepancy between the Malvern SMD and planar SMD at the lower cross-section due to the evaporation being ignored. Therefore, an evaporation model was adopted to correct the total droplet volume at the lower cross-section. In particular, the evaporation model based on the droplet heat and mass transfer incorporates the non-ideality consideration for multi-component fuel evaporation by determining the activity coefficients of fuel components. It turned out that the Malvern SMD was close to the proposed planar SMD at the upper cross-section (250 mm) for all the test fuels without evaporation consideration. For lower cross-section (400 mm), the proposed planar SMD modified by incorporating an evaporation model, fit the line-of-sight SMD value better than the planar SMD without evaporation correction. The planar SMD method provides relatively complete information on a planar basis and can be used for characterizing non-symmetry sprays as well.

A dual-species sensor based on scanned-wavelength-modulation spectroscopy (scanned-WMS) with the 2f/1f detection is developed for calibration-free CO and NH3 detection in high-temperature environments. A 2.3 μm distributed feedback (DFB) diode laser was used to exploit the CO and NH3 lines centred at 4300.7 cm−1 and 4301.13 cm−1, respectively. The DFB laser was scanned across the target absorption lines at 100 Hz with fast sinusoidal modulation at 5 kHz. The spectral fitting strategy was employed to least-square fit the simulated and measured WMS-2f/1f spectra. The integrated absorbance of target transitions inferred from the scanned-WMS-2f/1f spectral fitting was used to determine the species concentration. The performance of the developed sensor was validated by simultaneously measuring species concentrations under various temperatures (300–700 K) and at 1 atm in a high-temperature gas cell. The noise equivalent absorption (NEA) coefficient of 3.5 × 10-6 cm−1 for CO and 4.9 × 10-7 cm−1 for NH3 was achieved at 700 K with an absorption path length of 20 cm. The developed dual-species sensor has the potential to be applied in the exhaust of practical combustion systems such as NH3 monitoring for NOx abatement in selective catalytic reduction (SCR) aftertreatment and CO sensing for optimizing combustor efficiency.

Biodiesel is a promising alternative fuel for diesel engines. Polyoxymethylene Dimethyl Ethers (PODE), which features high cetane number, low viscosity and high oxygen content, is often used as a fuel additive. In this study, PODE is blended with biodiesel as a major components of the fuel to improve the low-temperature flow properties and the viscosity of biodiesel, aiming to achieve superior engine emission characteristics and efficiency compared with the baseline operation working with neat diesel. The experiments are conducted using diesel (D100), biodiesel (B100) and biodiesel/PODE blend fuel (B85P15) in a direct-injection diesel engine operating at different loads. Combustion data, fuel consumptions and gas emissions are measured. The results show that the engine operates smoothly with B100 and B85P15, at high load B85P15 has obviously higher diffusion combustion heat release rate, and the total combustion duration of B85P15 is 5 CAD shorter than that of D100. B85P15 produces the lowest soot emissions (

As PAHs (Polycyclic Aromatic Hydrocarbons) are the main precursor of soot formation during the combustion, the investigation of PAHs formation is essential for the understanding of the soot formation and soot reduction in combustion. In this study, a specially designed burner and the corresponding fueling system was used to stabilize a laminar diffusion flame of n-heptane up to 3.0 bar before it becomes unstable. Using the combination of LII (Laser Induced Incandescence) and LIF (Laser Induced Fluorescence) techniques, the PAHs and soot formation and their distributions in the studied flames were obtained and explained. The results showed that PAHs were almost surrounded by soot and were present in the lower part of the flame. Moreover, the integral soot and PAH intensities exhibited a power law dependence on the pressure, being proportional to pn with n of 1.38 ± 0.32 and 1.49 ± 0.25 respectively under the pressure range of 1.0–3.0 bar.

Currently, general aviation aircrafts have growing demand for internal combustion engines burning heavy fuels (i.e. diesel or kerosene) due to the concerns on the safety, costs and availability of aviation gasoline (AVGAS). The application of heavy fuels requires the change of combustion mode from pre-mixed mode to diffusion mode, which will inevitably increase the particulate matter (PM) emissions as incomplete combustion products. In this work, the size-resolved number concentrations of the PM emissions emitted from an internal compression ignition engine burning diesel, RP3 and Fischer-Tropsch (FT) kerosene were studied by a Scanning Mobility Particle Sizer Spectrometer (SMPS). An opacimeter was utilized to measure the opacity of the soot emissions (linearly related to the soot mass), which was in consistent with the SMPS data. Results demonstrated that the FT fuel produced the lowest PM emissions due to absence of sulfur and aromatic contents. Diesel turned out to have the greatest ‘sooting’ tendency and produced more accumulation mode PM in number than FT fuel by a factor of four, and more PM in mass by approximately three times. Moreover, the effects of fuel types and engine operational parameters were quantified in a systematic manner by adopting the Response Surface Method (RSM) in Design of Experiments (DoE). According to the ANOVA (Analysis of Variance), the DoE derived model was statistically significant and demonstrated that the engine load was the dominant factor for soot generation, followed by injection pressure and fuel types. Relevant combustion parameters and their link with PM emissions were further discussed, illustrating that atomization process had great impact on the ignition delay and thus affected soot generation.

Ultrafine particle emissions from Gasoline Direct Injection (GDI) and Port Fuel Injection (PFI) vehicles under the New European Driving Cycle (NEDC) were measured by using the Differential Mobility Spectrometer 500 (DMS 500) for size distribution information and Transmission Electronic Microscopy (TEM) for morphological analysis. The first phase of the ECE 15 cycle (representing the urban driving condition) within the NEDC was examined to evaluate the cold start effect on particulate matter (PM) emissions. Even though the ECE 15 cycle1 only occupies 1/6th of the time of the NEDC, the cold start emissions accounted for more than 50% of the total particle number for GDI vehicles and approximately 70% for PFI vehicles. Further analysis in terms of the Count Medium Diameter (CMD) of the PM emissions is also discussed. A linear regression method was adopted to analyze the correlation between PM emissions and vehicle parameters. For GDI vehicles, acceleration demonstrated a strong correlation with PM emissions, particularly under warm start conditions. The effects of the after treatment device Three Way Catalyst (TWC) on PM emissions was also investigated.

Semi-volatile organic compounds (SVOCs) in gaseous and particulate states from a commercial light-duty compression ignition (CI) engine burning diesel fuel have been investigated by adopting two dimensional Gas-Chromatography Time-of-Flight Mass-Spectrometry (GC × GC-ToF-MS) coupled with a thermal desorption technique. Owing to the enhanced separation capability of the multidimensional gas chromatography, the SVOCs including 21n-alkanes (C12-C32), 16n-alkylcyclohexanes (C11-C25 and C33) and 13 polycyclic aromatic hydrocarbons (PAHs) (2–5 rings) have been identified and quantified under different steady engine loads. The experimental results demonstrated that at high engine load (8 bar BMEP @ 1800 rpm), SVOC emissions were higher than those at low engine load (2 bar BMEP @ 1800 rpm) by factors of 2.74 and 2.02 for gaseous and particulate states respectively. The impact of the diesel oxidation catalyst (DOC) on engine-out SVOCs was also analyzed. The results demonstrated that SVOCs did not reduce significantly (less than 16% for different compounds) at low engine load but decrease more dramatically at high engine load (by 37%–55%) when the DOC was above its light-off temperature. At high engine load, it turned out to be that the DOC mitigating effect was more remarkably on the gaseous SVOCs (38%–62%) than on the particulate SVOCs (26%–46%). Limited SVOC reductive capability of the DOC spurs the need of catalyst improvement, especially for capturing high molecular weight (HMW) organic compounds. Therefore, it would be desirable to further investigate the relationship between SVOC emissions and different types of aftertreatment techniques (i.e. DPF) in a more specific way.

In this work, a reduced toluene-decalin reaction mechanism containing 108 species and 566 reactions was proposed for computational fluid dynamics (CFD) simulations and polycyclic aromatic hydrocarbon (PAH) formation predictions in combustion engines. The present mechanism was validated with the available literature data including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCM), species mole fractions measured in premixed flames and jet stirred reactors (JSR), and laminar flame speeds. Moreover, a sensitivity analysis was performed on toluene and decalin flames to further investigate the main formation pathways of four representative PAHs: benzene (A1), naphthalene (A2), phenanthrene (A3) and pyrene (A4). In general, the simulation results exhibited a reasonable agreement with the experimental data. The negative temperature coefficient (NTC) behaviors of decalin IDTs were accurately reproduced by the proposed mechanism. The PAH species concentrations were also well captured for ethylene and benzene flames. The sensitivity analysis results indicated that the main formation reactions for PAHs have a strong link with ring structures. The decompositions of toluene and decalin primarily contributed to the formation of A1. For toluene, A2 was formed by the reactions A1- + C4H3 = A2 and IC4H5 + A1 = A2 + H2 + H, while for decalin, the self-combination reaction of C5H5 became the main pathway. In addition, the reactions of the aromatic molecules and radicals significantly promoted the formation of A3 and A4 for both toluene and decalin.

Herein we describe the application of historical data design under response surface methodology to formulate the correlation of the fuel spray SMD (Sauter Mean Diameter) with factors (viscosity, fuel injection pressure, air-blast pressure). The Design Expert software was used to analyze the experimental data to obtain the equation between the factors and the response. The range of the values of variables used in this experiment were viscosity of the fuels 9.71 × 10−4 Pa–1.207 × 10−3 Pa, the fuel injection pressure 0.1–1.0 MPa, the air-blast pressure from 0 Pa to 2000 Pa and a total of 120 test points were conducted. The data were converted into logarithmic scale in order to utilize the linear fitting function of the DoE (Design of Experiments). According to the ANOVA (analysis of variance), the most influential factor to the fuel spray SMD is fuel injection pressure, followed by fuel viscosity and air-blast pressure. R2 value of 0.9339 demonstrated that the regression equation obtained by historical data design fits the experimental results consistently.

The effect of gasoline/ethanol blends in different blending proportions (E0, E10, E20, E50, E70, E85 as #% by volume) on the characteristics of size-resolved particulate number and mass concentrations was investigated in a single-cylinder optical access engine using a differential mobility spectrometer (DMS500) under cold and warm conditions (20 °C, 80 °C) at a stoichiometric condition (1500 rpm, 0.5 bar manifold absolute pressure). The effects on the fuel spray characteristics were also investigated by capturing the injection images and by post-processing to compare the temporal development of the spray with the different fuels. In-cylinder fFID (fast Flame Ionisation Detector) measurements were undertaken to assess the mixture in-homogeneity in the region of the spark plug. As the ethanol volumetric percentage increases, both the total Pn (Particulate number) and Pm (Particulate mass) increase by a maximum of 16 and 11 for cold conditions and 7 and 8 for warm conditions. This is in agreement with the natural flame chemiluminescence images which showed that fuels with high ethanol proportions generated more soot than fuels with small blending percentages of ethanol. Spray images and combustion analysis showed that ethanol addition results in protracted injection and combustion periods. The effect on spray characteristics is more profound under cold conditions than under warm conditions. The mixture in-homogeneity increases as the ethanol content increases for E50, E70, and E85 and this would explain the increased level of particulate emissions, although the trend is less clear for the lower ethanol content fuels.