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Experimental investigation of particulate matter structures under partially premixed combustion in a compression ignition engine
Fuel 259 (2020) 116286
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Full Length Article
Experimental investigation of particulate matter structures under partially premixed combustion in a compression ignition engine ⁎
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Hanyu Chena, , Minfei Wanga, Xi Wangb, , Deqiang Lia, Zhixiang Pana, Choongsik Baec,
T
⁎
a
School of Energy and Power Engineering, Wuhan University of Technology, Wuhan 430063, China School of Physical Education, Jianghan University, Wuhan 430056, China c Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Diesel and light hydrocarbon Partial premixed compression ignition Particulate matter Morphology TEM-EDS Particle size distribution
A study on particulate matter (PM) of a compression ignition engine under partial premixed combustion has been performed. Particularly, the microscopic analyses (SEM and TEM) of several samples at the exhaust were carried out to describe the morphology and ordering degree of particles produced by the combustion of diesel and 15% diesel blended with 85% light hydrocarbon (named DLH). Characterization parameters such as fractal dimension, fringe separation distance, fringe length and fringe tortuosity were analyzed to further study the particle structure. Besides, the particle size distributions at medium and high load were compared. The results demonstrated that the particle size distribution presents as quasi-monodisperse and the average diameter of DLH is smaller than that of diesel. The diesel particles move to a larger size range at higher load, while DLH particles show an obvious shift to smaller particles at higher load, which is related to the competition between surface growth and oxidation rates.
1. Introduction Due to high thermal efficiency, reliability and economy, diesel engines are widely used in vehicles, agricultural machineries and engineering equipment. However, diesel engine emissions are one of the main sources of environmental pollution [1]. Particulate matter (PM) emissions have caused haze formation to a great extent and is also harmful to human health [2]. Therefore, the formation mechanism and control of PM have become an increasingly important issue in recent years. Under the background of global energy shortage and increasingly stringent emission regulations of internal combustion engines, various alternative fuels were explored. And the combustion and emission performance of engines were comprehensively studied in real engines by changing the proportion of fuel components. Koder et al. [3] investigated the injection, ignition and combustion of jatropha oil, soybean oil and diesel oil in a 2.2L common-rail diesel engine with a twostage turbocharging concept and high pressure EGR. The size distribution for all test fuels at a low- and mid-load engine-operating point (EOP) were examined by applying different EGR rates. For oxygenated additives application, diesel engines adapted dual fuel mode more and more, and soot emissions decreased significantly with the increase of oxygenated alternative fuels, such as alcohols, natural gas, biodiesel
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and dimethyl ether (DME) [4]. Chen et al. [5] investigated the combustion and emission performance of a common rail diesel engine fueled with diesel and ethylene glycol. It was found that ethylene glycol reduce ultrafine particles (UFPs) and decrease the average diameter of UFPs particularly. In addition, some studies have been carried out on the emissions of hydrocarbon fuel surrogate components blended with diesel. Qian et al. [6,7] studied the effect of iso-alkanes structure and aromatics blended with diesel on combustion characteristics and regular emissions respectively, and demonstrated that the addition of isoalkanes reduced HC, NOx and soot emissions [6]. However, blending aromatics caused peak value of particle number/size distribution curve moving towards to the small-size sides, and the single-ring aromatics showed higher particle emissions than multi-ring aromatics [7]. Li et al. [8] chose n-dodecane, iso-dodecane, tetralin and decalin to mix with diesel in 10% and 20% volume fraction. The effect of fuel components on unregulated emissions such as formaldehyde, acetaldehyde, ethylene, propylene, methane, and particle number and particle size were analyzed in detail. In order to meet the requirements of fuel economy and emission characteristics, the new combustion modes have also been deeply studied. Homogeneous charge compression ignition (HCCI) combustion model is a typical representative. However, the high pressure rise rate, load extension and combustion phasing control need to be overcome in
Corresponding authors. E-mail addresses: [email protected] (H. Chen), [email protected] (X. Wang), [email protected] (C. Bae).
https://doi.org/10.1016/j.fuel.2019.116286 Received 1 July 2019; Received in revised form 21 September 2019; Accepted 24 September 2019 Available online 27 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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output torque were controlled automatically using an electrical dynamometer (AVL504/4.6 SL). Using a piezoelectric pressure sensor (Kistler 6052C) coupled with a combustion analyzer (Dewetron M0391E) to measure the combustion pressure in the cylinder. Fig. 1 shows the schematic diagram of the test engine. In the ideal condition of PPCI mode, the end point of fuel injection and the start point of combustion are separated, which makes the fuel and air have a better premixed state at the beginning of combustion. This not only avoids the formation of soot caused by the local mixture over-concentration, but also facilitates the expansion of operating load in both high and low directions. In this study, light hydrocarbons are premixed with air through port fuel injection (PFI), and then mixed with a small amount of diesel directly injected (DI) into cylinder to form the mixture for combustion. The agglomerated aluminum foil is plugged in the soot sampling port of the exhaust pipe, so that the soot particles can accumulate on the surface of it for sampling. Diesel and light hydrocarbon are used in the experiment to form the mixture fuel according to the mass ratio. Light hydrocarbon is a mixture with C5 and C6 as the main components and has a higher low heating value than diesel. High blending ratio of light hydrocarbon will make it difficult for diesel engine to start and work normally. Therefore, 85% light hydrocarbon mixed with 15% diesel and pure diesel fuel are used in the test scheme. The physical parameters of diesel and light hydrocarbon are shown in Table 2. Fig. 2 shows the in-cylinder pressure, heat release rate and pressure rise rate curves of the two test fuels at 100% engine load of 1000 r/min. As shown in the pressure curve, it can be observed that the maximum cylinder pressure of DLH is lower than that of diesel, and its appearance is also delayed 2°CA. Lower temperature and pressure lead to a delay in ignition time, which can be seen from the heat release rate curve. But the maximum heat release rate of DLH is higher than that of diesel. The full mixing of fuel-air improves the fuel burning rate and complete combustion. As can be observed from Fig. 2, the pressure rise rate curve and the heat release rate curve have almost the same trend, and the corresponding phase moves backwards. Soluble organic fractions (SOF) on the particle surfaces have viscosities that promote the easy sticking of particles together. Therefore, particles should be pretreated to avoid affecting the shooting effect. Aluminum foil is placed in ethanol and oscillated by ultrasonic. We have done a lot of experiments to determine the reasonable ultrasonic time and frequency according to the properties of the sample and the reference provided by the ultrasonic instrument. The ultrasonic time was finally determined to be 30 min and then centrifuged for 5 min. This process filters out some of the SOF components, reduces the particle adhesion and disperses the agglomerated particles. It is conducive for SEM analysis. SEM (JSM-7500F) with 1.0 nm resolution and 15 kV accelerating voltage is used. It mainly uses the high-energy electrons to bombard the surface of particle samples, and utilizes the secondary electron emission effect produced by the sample to obtain the high-resolution sample surface structure information. TEM (JEM-1400Plus) with 0.38 nm resolution is used to observe and analyze the micromorphology and structure of the particles characterized by the fractal dimension of diesel particulate matter. TEM with higher resolution (Talos F200S) with 0.25 nm resolution is used to observe the nanostructure of primary carbon particles, whose geometric structure irregularity and density can be characterized by fringe separation distance, fringe length and fringe tortuosity. ImageJ software is used to measure the particle size and 100 particles are randomly selected for measurement under typical running condition. In the test, the engine speed is set to 1000r/min, and two loads, 75% and 100%, are selected. PPCI is achieved by PFI of 85% light hydrocarbon and DI of 15% diesel. For each experimental condition, several analyses have been realized, according to the summary in Table 3. The combustion particle morphological characteristics of pure diesel and DLH under the same conditions are compared. Finally, by changing the
the actual implementation of HCCI strategy [9]. Partial premixed compression ignition (PPCI) in diesel engine cylinder is a combustion mode between direct injection compression ignition (DICI) and HCCI. Literature [10–12] investigated the gasoline PPCI concept by using gasoline-like fuels in common-rail compression ignition engines. Won et al. [13], Yu et al. [14] and Valentino et al. [15] conducted an experimental study of gasoline/diesel blended fuel in PPCI mode on a high-pressure common-rail direct injection compression combustion engine. It can be seen that PPCI mode has been applied in practice and has the potential to improve both NOx and soot emissions and thermal efficiency. Jaasim et al. [16] simulated soot particle formation and emissions from low octane gasoline-like naphtha fuel under PPCI condition. The results showed that PPCI presents typical stratified combustion characteristics, and under its operating conditions, most of the main soot conditions throughout the entire combustion are avoided. The most direct way to determine whether alternative fuels and new combustion modes can be applied in practice is to study and compare their economy and emission characteristics. Diameters distribution, composition and concentration, SEM-TEM analyses are the most commonly used methods to understand PM morphology [17,18]. Liu et al. [19] carried out SEM analysis on soot particles generated from catalytic diesel combustion. The result showed that the average particle size of catalytic diesel decreases apparently compared with diesel fuel. Potenza et al. [20] studied the PM compound morphology based on SEM-TEM by changing the injection parameters that could affect soot production on a turbocharged engine. The results demonstrated high influence of injection strategies on soot morphology-composition and on catalyst efficiency. However, previous researches [20,21] focused on the impact of injection strategy variation (fuel injection timing, injection pressure, pilot injection) on engine particulate emissions. What’s more, several studies [22,23] focused on pure diesel combustion, revealing the evolution of particle size and number under different conditions (operation parameters, EGR rate, injection parameters, combustion type). So far, little research has been done on the microscopic characteristics of light hydrocarbon particles under PPCI combustion mode. In this study, the particle emission of diesel and DLH was studied by changing the engine operating condition in PPCI mode. Comparing with pure diesel fuel, the microstructure and particle size distribution of DLH were analyzed in detail based on electronic microscopic images. In addition, the effects of light hydrocarbon blending and operating conditions on the particulate emission characteristics of dual fuel engines were revealed, which provides a theoretical basis for finding ways to reduce particulate emission in the future.
2. Experimental setup and method The test engine is Z6170ZLCZ-1 diesel engine produced by Zibo Diesel Company in China. Table 1 lists the main technical parameters of the test diesel engine. The whole test system mainly includes engine, dynamometer, fuel supply system, intake and exhaust system, electronic control system and sampling device. The engine speed and Table 1 Specifications of test engine. Parameters
Details
Combustion system Number of cylinders Displacement/bore/stroke Compression ratio Rated speed/rated power Brake specific fuel consumption Injection/injection pressure
4-Valve PPCI 6 27 L/170 mm/200 mm 14.5 1000 r/min/330 kW 200 g/kW.h Direct injection/up to 35 MPa PFI injection/0.45 MPa 50°CA BTDC/40°CA ABDC 65°CA BBDC/50°CA ATDC
Intake valve opening/closing Exhaust valve opening/closing
2
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Fig. 1. Scheme of the experimental setup.
Table 2 Fuel Properties.
Table 3 List of experimental conditions and analyses.
Properties
Diesel
Light hydrocarbon
Test conditions
Fuel
Speed (r/min)
Load (%)
SEM
TEM
EDS
Molecular formula Liquid density (kg/L) Low heating value(MJ/kg) Cetane number Octane number Boiling point(℃)
C15-C23 0.82–0.88 42.5 52.5 – 170–350
C5,C6 0.63–0.68 48.1 – 70 36
1 2 3 4
Diesel
1000 1000 1000 1000
75 100 75 100
– – – √
√ √ √ √
√ √ √ √
DLH
used to study morphology and dimension of soot and particles, combined with the calculation of fractal dimension. In Fig. 3, several SEM images at different magnifications, related to the test of DLH, at 1000r/min speed and 100% load, are illustrated. From the results of SEM image, the particles are packed by thousands of quasi-spherical carbon particles with different diameters. These elementary particles accumulate in the presence of electrostatic force, liquid bridging force, Van der Waals force and other anchoring strengths, forming clusters of particles with different densities [24]. Most of them are chain-like or flocculent and the research of Liu et al. [19] showed
engine load, the particle size distribution of pure diesel fuel and DLH are showed to facilitate the law analysis of the load effects on them.
3. Result and discussion 3.1. Microscopic morphology of particles The results of SEM and TEM analyses are presented. The images are
Fig. 2. Combustion characteristic curves of different test fuels. 3
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Fig. 3. SEM images at different magnifications.
Fig. 4. TEM images of particles.
dimension. The calculation formula for it can be expressed as follows:
the same morphologies. TEM images of pure diesel and DLH particles at 1000r/min, different loads are shown in Fig. 4. It can be seen that the particles are composed of a certain number of spherical primary carbon particles. They are gathered together by collision. In the TEM image, there are regions with different color shades, mainly due to the accumulation of primary carbon particles. At medium load, the particles appear spherical and cluster, while at high load, most of them appear as chains. Under the same load, the number of DLH particles is smaller, so is the particle size. Moreover, the arrangement between particles is sparser and the agglomeration state is lower. On the one hand, the light hydrocarbon fuel mainly contains light components, and the combustion is more efficient. In detail, the combustion of light hydrocarbon produces fewer polycyclic aromatic hydrocarbons (PAHs) and other gas phase composition that help particles growth. PAHs is the precursor of particles, and less PAHs means less particles produced by DLH. Moreover, the adsorption and condensation of PAHs and gas phase composition will lead to the surface growth of particles. Less PAHs also means that the smaller particle size of DLH. On the other hand, particles are agglomerated by Brownian motion or turbulent diffusion motion [25], including collision, adhesion, fracture and grafting. The particle size and environmental factors determine the coagulation morphology between primary carbon particles. The differences in the agglomeration states are evident from Fig. 4. From the perspective of fractal theory, these agglomeration processes are fractal growth of particles. The complex structure of particles displayed under TEM is actually a fractal structure. In this study, the fractal dimension represents the overlap degree between primary carbon particles and can quantitatively clarify the density of particle structure, contributing to study the average particle size and distribution characteristics of primary carbon particles. Zhu et al. [26] and Kholghy et al. [27] calculated the fractal dimension to help analyze the morphological characteristics of particles. And the larger the fractal dimension is, the higher the overlap degree between the primary carbon particles is. In this paper, the fractal characteristics of particles at the same magnification factor are studied by box counting
DB = −lim r→0
logN (r ) logr
(1)
where DB is box counting dimension, r is the side length of the box, and N(r) is the number of boxes needed for the whole area. TEM image is binarized by the software ImageJ, and the data obtained by fractal calculation is fitted to a straight line. Taking the diesel particles at 75% load as an example, the fitting line of fractal dimension is shown in the Fig. 5. The fractal dimension can be obtained by calculating opposite number of the slope. The same method is used to calculate the fractal dimension of particles under other conditions, and the results are shown in Table 4. It can be seen that the fractal dimension is between 1.6 and 1.8. Kellerer et al. [28] considered that the fractal dimension of typical soot ranged from 1.6 to 1.9. On the whole, the fractal dimension of diesel particles is
Fig. 5. Fitting line of fractal dimension for diesel at 75% load. 4
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schematic diagram is shown in Fig. 8. These parameters can not only directly reflect the physical structure of primary carbon particles at nanoscale, but also closely related to the chemical characteristics such as oxidation rate and surface functional groups of particles. At the same time, the formation of the microstructure of primary carbon particles has a strong correlation with the formation conditions (time, temperature, fuel molecular structure and composition, etc.) [30]. This helps to understand the formation process of particulate matter and provides a theoretical basis for the exploration of emission reduction. Fig. 9 shows the calculation results of fringe separation distance. The distribution is in the range of 0.33–0.4 nm and that of diesel is more concentrated, mainly centered on the range of 0.35–0.39 nm. The mean separation distance of diesel at 75% and 100% load are 0.372 and 0.367 nm and DLH are 0.361 and 0.364 nm, respectively. It can be concluded that the separation distance of diesel is slightly larger than that of DLH. The calculation results of fringe length are shown in Fig. 10. The distribution presents a unimodal characteristic for all running conditions and the peak value fall within 0.6–1.0 nm, accounting for about 25%. The mean length of diesel at 75% and 100% load are1.000, 0.900 nm and DLH are 1.014 and 1.060 nm, respectively. It can be seen that the fringe length of diesel oil is smaller than that of DLH. Fringe tortuosity is defined as the ratio of the actual length of the microcrystal to the linear distance between the endpoints of the microcrystal in this study. The computed results are shown in Fig. 11. The distribution is mainly centered on the range of 1.1–1.8, and the mean tortuosity of diesel at 75% and 100% load are 1.420 and 1.420, and DLH are 1.389 and 1.360, respectively. The curvature degree of primary carbon particles of diesel is obviously larger than that of DLH. It can be found that there is a great correlation between fringe separation distance, fringe length and fringe tortuosity of primary carbon particles. In the whole process of evolution, oxidation plays an obvious role. The long and flat carbon layers are considered to be more graphitized, with fewer active sites and less oxidation than short and curly carbon layers. The oxidation activity of the edge carbon layer is greater than that of the base layer. The larger fringe separation distance indicates that the internal structure of the primary carbon particles is looser, as shown in Fig. 6. The fringe length of DLH is larger, indicating that shorter microcrystals are oxidized and the carbon layer is graphitized, with less boundary deficiency. The influence of fuel composition on fringe separation distance and fringe tortuosity is the same in trend, but the influence on fringe length is opposite to that of the first two nano-parameters. This is because that the increase of fringe tortuosity leads to overlap of electron orbitals in microcrystals, which can cause mutual repulsion between electrons, leading to repulsion between adjacent microcrystalline layers. Under the action of such repulsion, fringe length decreases and fringe separation distance increases, finally making the microcrystalline layer reach a stable state [31,32]. To deepen the metallic nanoparticles composition, an EDS analysis is performed. The ordinate of the energy spectrum represents the intensity, and higher strength means higher concentration of elements. Fig. 12 (a) shows EDS spectrum of pure diesel particles under 75% load. The spectrogram is a spectrum from a limited area. Fig. 12 (b) and Fig. 13 are all spectra from summed. It is shown in spectra that the main
Table 4 Fractal dimension under different tests. Test conditions
Fitting line
DB
Diesel 75%load Diesel 100%load DLH 75%load DLH 100%load
y = −1.7658x + 14.1699 y = −1.7918x + 14.3628 y = −1.7470x + 13.6089 y = −1.6397x + 13.5897
1.7658 1.7918 1.7470 1.6397
larger than that of DLH, indicating the higher degree of clustering between particles. This may be due to the higher temperature in the pure diesel combustion environment, which increases the collision rate between soot particles and easily forms agglomerated particles. On the other hand, DLH particles mainly present the chain-like distribution, which enlarged the radius of gyration, resulting in a smaller fractal dimension. This quantitatively confirmed the conclusion above that the arrangement between DLH particles is relatively sparse. 3.2. Structural analysis of primary carbon particles The structure, morphology and cluster degree of the particles can be reflected by SEM and TEM images, but the microscopic structure of the primary carbon particles cannot be fully shown. In this study, high resolution transmission electron microscopy is used to magnify the particles locally and obtain the micro-morphology of the primary carbon particles. The obtained images were processed and calculated to analyze the parameters representing the microstructure characteristics of particles, such as fringe separation distance, fringe length and fringe tortuosity. The microscopic images of primary carbon particles under different test conditions are shown in Fig. 6. It can be found that these primary carbon particles show a spherical carbon layer structure similar to fingerprint. They are usually divided into core and shell, as shown in the Fig. 7. The core is amorphous carbon microcrystalline, showing the curved and irregular structure, while the shell is usually composed of long microcrystalline, showing the regular and clear carbon layer structure. This kind of structure is the equilibrium configuration formed by the polycyclic aromatic hydrocarbons (PAHs) lamellae under the reflection of high temperature [29]. It can also be observed that the primary carbon particles contain a swirling multi-nucleus structure, as shown in Fig. 6 (a). This kind of structure is formed by the initial soot produced by the cracking of several particles, which gathered together and then be wrapped up by rapid surface growth. The primary carbon particles reorganize and microcrystalline grow under high temperature combustion, gradually tend to graphitize and transform to ordered structure. The ordering degree of primary carbon particles can be characterized by fringe separation distance, fringe length and fringe tortuosity. Fringe separation distance refers to the distance between two adjacent ordered microcrystalline layers in the nanostructure of primary carbon particles. Fringe length refers to the physical length of the microcrystalline layer. Fringe tortuosity is defined as the ratio of the fringe length to the distance between two endpoints of the microcrystal. The
Fig. 6. Microscopic image of primary carbon particle. 5
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Fig. 7. “Core-shell” structure of primary carbon particles.
Fig. 8. A schematic of nano-structural characteristic parameters for particles.
Fig. 11. Distribution of fringe tortuosity.
component of pure diesel nanoparticles is C. Aluminum is compatible with combustion chamber alloys. The existence of Cu is compatible with piston material, suggesting the presence of the piston wear phenomenon. The same conclusion was reached in reference [20]. The spectrum of DLH is presented in Fig. 13. It can be seen that similar metal components are detected in DHL particles and the main component of the nanoparticles is C. Compared with Figs. 12 (b) and 13 (b), it can be found that C and O elements increase significantly. The increase of O means more oxidation of soot particles in DLH. Finally, Zn and P traces are present probably due to the lube oil [33]. The composition of emissions is highly dependent on the lubricating oil and fuel type [34], and may also be affected by other factors. Higher load and temperature may enhance metal content in ultrafine exhaust particles [35,36].
Fig. 9. Distribution of fringe separation distance.
3.3. Size distribution of particles Particulate matters are generally divided into nucleation mode (Dp < 50 nm), accumulation mode (50 nm < Dp < 1000 nm) and coarse mode (Dp > 1000 nm) by particle diameter. Nucleation mode particles are comprised primarily of incompletely burned carbon nuclei, volatile hydrocarbons formed during dilution cooling, sulfur compounds and some metal compounds. Accumulation mode particles are mainly formed by the collision and aggregation of carbon particles and the adsorption and condensation of volatile substances on the surface. Coarse mode particles are caused by deterioration of spray quality or abnormal injection and present in extremely small amounts. The running condition of diesel engine and the composition of fuel will have an impact on the carbon particles size. The formation of PM usually starts from the part of uncompleted combustion in fuel-rich region. Therefore, more complete combustion, such as PPCI, can effectively reduce PM emissions in theory. In this paper, the calculation method for the mean diameter of primary carbon particles is as follows: take several TEM pictures at each
Fig. 10. Distribution of fringe length.
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(a) Diesel at 75% load (a) DLH at 75% load
(b) Diesel at 100% load (b) DLH at 100% load
Fig. 12. Metallic nanoparticles EDS spectrum of diesel.
Fig. 13. Metallic nanoparticles EDS spectrum of DLH.
working conditions, and select five of them. A total of 100 representative primary carbon particles with clear edges were randomly selected to calculate the diameter and then the average value was obtained. The size distributions of diesel particles are shown in Fig. 14. Although there are some data scatter due to the limited number of particles considered for each condition, the normal distribution provides an approximate fit to the entire dataset. From the standard deviations and the mean diameters, the size distributions of the primary particle diameters can be considered as quasi-monodisperse. It can be seen that the particle size distributes in the range of 15–60 nm and the mean diameter is around 30 nm. Lapuerta et al. [37] found that the size distribution of primary carbon particles in diesel engine fueled with ordinary petrochemical diesel is between 10 and 60 nm, and Neer et al. [38] considered that the average particle size of primary carbon particles is between 19 and 35 nm. Their results are very similar to those of this paper. It is observed that the number of particles moves towards a larger size range at higher load, which was consistent with the results reported by Lee et al. [39]. At medium load, the air-fuel ratio in cylinder is relatively higher, and there is sufficient air to provide a more uniform environment for combustion reaction. With the increase of load, the air-
fuel ratio decreases, which aggravates the uneven degree of fuel-air mixing and combustion status. The probability of further growth of aggregated particles increases, resulting in the increase of the average particle size. Fig. 15 shows the size distribution of DLH particles. It generally be considered as quasi-monodisperse, the same as that of diesel. But the peak value presents in a smaller particle size range. The particle size is mainly distributed in the range of 10–55 nm and the mean diameters are 29.39081 nm and 25.96712 nm at 75% and 100% loads, respectively. Compared with diesel, the mean diameter of DLH is smaller, and the nucleation mode particles account for a larger proportion. Soot formation has been found to be strongly dependent on air entrainment in the lifted portion of the jet as well as by oxygen in the fuel and to a lesser extent the composition and structure of hydrocarbons in the fuel [40]. The final properties of particulates emitted from an engine mainly depend on the relative contributions of carbon nucleation, growth, oxidation, and aggregation processes within the cylinders. From the perspective of particle formation, Light hydrocarbons contain short carbon chains and most of them are straight chains, and combustion produces fewer polycyclic aromatic 7
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(a) DLH at 75% load
(a) Diesel at 75% load
(b) Diesel at 100% load
(b) DLH at 100% load
Fig. 14. Size distribution of diesel particles.
Fig. 15. Size distribution of DLH particles.
hydrocarbons and other gaseous components that help the growth of particles, which means fewer nucleating particles. Moreover, the adsorption and coagulation of PAHs and vapor components on the surface of the particles and the collision and coagulation between the primary carbon particles will make the particles grow. Therefore, fewer PAHs also mean smaller particle size of DLH. Contrary to diesel, it is observed that there is an obvious shift to smaller particles at higher load. The particle size of basic carbon particles mainly depends on the competition between oxidation of soot particles and surface growth [38]. From the beginning of soot particle growth, it is accompanied by the oxidation reaction of nucleating particles, soot particles and soot aggregates. The rate of oxidation and surface growth is also affected by combustion environment, including gas phase material, residence time, flow characteristics and temperature. For diesel fuel, the increase of combustion temperature is beneficial to the oxidation of accumulated particles and the decrease of its concentration. However, with the increase of load, the fuel injection increases, and the short mixing time causes uneven mixing of fuel-air, resulting in deterioration of combustion quality. In the soot formation area of diffusion flame, the soot formation rate is quite high, but oxidant is scarce. When the surface growth rate exceeds the oxidation rate, the tendency of fuel surface growth increases by adsorbing gaseous substances such as PAHs and coagulating at high temperature. In the PPCI combustion mode, light hydrocarbon has higher volatility and the ignition time is later. The fuel droplets have sufficient time to mix with air, which is conducive to the formation of homogeneous mixture. Soot diffuses to the rich air area after formation, and the number density of soot does not increase. The growth of soot particles changes from
surface growth to condensation and grafting. Oxygenation becomes prominent because of the sufficient oxidant. With the increase of load, the combustion temperature in cylinder increases gradually, which increases the oxidation probability of HC and other substances produced by combustion in cylinder. When the oxidation rate exceeds the surface growth rate, the tendency of carbon particles to form larger ones is weakened. 4. Conclusion In this study, experimental study has been conducted to investigate the microstructure and particle size distribution of diesel and DLH fuel at medium and high load under PPCI mode, respectively. On the basis of the results and analysis presented above, the main conclusions can be drawn as follows: (1) In the images of SEM and TEM, the particles appear spherical, cluster and chains, and the cluster state of DLH particles is lower than that of diesel. The fractal dimension of DLH is smaller, quantitatively indicating that the particle structure of DLH is sparser than that of diesel. (2) The primary carbon particles present a “core-shell” structure similar to fingerprint under high resolution electron microscopy. The fringe length of diesel are smaller than those of DLH, and the fringe separation distance and fringe tortuosity is larger, which demonstrates that the ordering degree of DLH particle structure is higher. (3) The distributions of the primary particle size can be considered as quasi-monodisperse and the peak value of DLH presents in a smaller 8
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particle size range. The average diameter of DLH is smaller than that of diesel, and they have the opposite trend with the increase of load. The surface growth rate of diesel particles exceeds the oxidation rate because of the deterioration of combustion quality, which makes particles move to a larger size range at higher load. The full combustion environment increases the oxidation probability of HC and other substances produced by in-cylinder combustion, leading to an obvious shift to smaller particles at higher load. (4) Light hydrocarbons have good prospects in the application of engines. In terms of PM emissions, DLH is cleaner than diesel. The reduction of particulate emissions is of great significance to environmental protection.
[14] Yu C, Wang JX, Wang Z, et al. Comparative study on gasoline homogeneous charge induced ignition (HCII) by diesel and gasoline/diesel blend fuels (GDBF) combustion. Fuel 2013;106:470–7. [15] Valentino G, Corcione FE, Iannuzzi SE. Effects of gasoline-diesel and n-butanoldiesel blends on performance and emissions of an automotive direct-injection diesel engine. Int J Engine Res 2012;13:199–215. [16] An Yanzhao, Jaasim Mohammed, Vallinayagam R, et al. Numerical simulation of combustion and soot under partially premixed combustion of low-octane gasoline. Fuel 2018;211:420–31. [17] Wei Y, Wang K, Wang W, et al. Comparison study on the emission characteristics of diesel- and dimethyl ether-originated particulate matters. Appl Energy 2014;130:357–69. [18] Ma Y, Zhu M, Zhang D. Effect of a homogeneous combustion catalyst on the characteristics of diesel soot emitted from a compression ignition engine. Appl Energy 2014;113:751–7. [19] Liu J, Yang J, Sun P, et al. Experimental investigation of in-cylinder soot distribution and exhaust particle oxidation characteristics of a diesel engine with nanoCeO2 catalytic fuel. Energy 2018;161:17–27. [20] Potenza Marco, Milanese Marco, de Risi Arturo. Effect of injection strategies on particulate matter structures of a turbocharged GDI engine. Fuel 2019;237:413–28. [21] Liu B, Cheng XB, Liu JL, Pu H. Experimental investigation of injection strategies on particle emission characteristics of Partially-premixed low temperature combustion mode. Appl Therm Eng 2018;141:90–100. [22] Ni PY, Bai L, Wang XL, Li RN. Characteristics of evolution of in-cylinder soot particle size and number density in a diesel engine. Fuel 2018;228:215–25. [23] Wu BY, Zhan Q, Zhang SK, et al. Effect of heavy-duty diesel engine operating parameters on particle number and size distribution at low speed condition. Int J Auto Tech-Kor 2018;19:623–33. [24] Zhou YF, Shi Q, Huang ZL, Wang JD, Yang YR. Particle agglomeration and control of gas-solid fluidized bed reactor with liquid bridge and solid bridge coupling actions. Chem Eng J 2017;330:840–51. [25] Martos FJ, Martín-González G, Herreros JM. Semi-empirical model for indirect measurement of soot size distributions in compression ignition engines. Measurement 2018;124:32–9. [26] Zhu Jinyu, Lee Kyeong Ook. Ahmet Yozgatligil, Mun Young Choi. Effects of engine operating conditions on morphology, microstructure, and fractal geometry of lightduty diesel engine particulates. P Combust Inst 2005;30:2781–9. [27] Kholghy Mohammad Reza, Afarin Yashar, Sediako Anton D, et al. Comparison of multiple diagnostic techniques to study soot formation and morphology in a diffusion flame. Combust Flame 2017;176:567–83. [28] Kellerer H, Koch R, Wittig S. Measurements of the growth and coagulation of soot particles in a high-pressure shock tube. Combust Flame 2000;120:188–99. [29] Hurt RH, Crawford GP, Shim HS. Equilibrium nanostructure of primary soot particles. P Combust Inst 2000;28:2539–46. [30] Vander Wal Randy L, Tomasek Aaron J. Soot oxidation: dependence upon initial nanostructure. Combust Flame 2003;134:1–9. [31] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes. Academic Press; 1996. p. 5–29. [32] Yehliu K, Vander Wal RL, Boehman AL. Development of an HRTEM image analysis method to quantify carbon nanostructure. Combust Flame 2011;158:1837–51. [33] Choi S, Seong H. Lube oil-dependent ash chemistry on soot oxidation reactivity in a gasoline direct-injection engine. Combust Flame 2016;174:68–76. [34] Alves Célia, Barbosa Cátia, et al. Elements and polycyclic aromatic hydrocarbons in exhaust particles emitted by light-duty vehicles. Environ Sci Pollut Res 2015;22:11526–42. [35] Lim L, Lim C, Yu LE. Composition and size distribution of metals in diesel exhaust particulates. J Environ Monit 2009;11:1614–21. [36] André JM, Joumard R. Modelling of cold start excess emissions for passenger cars. INRETS Report 2005. No. LTE 0509. [37] Lapuerta M, Martos FJ, Herreros JM. Effect of engine operating conditions on the size of primary particles composing diesel soot agglomerates. J Aerosol Sci 2007;38:455–66. [38] Neer A, Koylu UO. Effect of operating conditions on the size, morphology, and concentration of submicrometer particulates emitted from a diesel engine. Combust Flame 2006;146:142–54. [39] Lee J, Sung NW, Huh KY. Prediction of soot particle size distribution for turbulent reacting flow in a diesel engine. Int J Engine Res 2011;12:181–9. [40] Tree Dale R, Svensson Kenth I. Soot processes in compression ignition engines. Prog Energy Combust Sci 2007;33:272–309.
Acknowledgments This work is sponsored by “State Key Laboratory of Engines at Tianjin University (K2018-09)”, “the Key Laboratory of Marine Power Engineering & Technology, Ministry of Transport (KLMPET 2016-01)” and “research fund of Center for Materials Research and Analysis, WHUT (2018KFJJ07)”. The authors also thank Associate Professor Kai Xu and Associate Professor Jiangjun Wei for pictures processing. References [1] Moon Gunfeel, Lee Yonggyu, Choi Kyonam, Jeong Dongsoo. Emission characteristics of diesel, gas to liquid, and biodiesel-blended fuels in a diesel engine for passenger cars. Fuel 2010;89:3840–6. [2] Agarwal AK, Gupta T, Kothari A. Particulate emissions from biodiesel vs diesel fuelled compression ignition engine. Renew Sustain Energy Rev 2011;15:3278–300. [3] Koder Alexander, Schwanzer Peter, Zacherl Florian, et al. Combustion and emission characteristics of a 2.2L common-rail diesel engine fueled with jatropha oil, soybean oil, and diesel fuel at various EGR rates. Fuel 2018;228:23–9. [4] Geng Peng, Cao Erming, Tan Qinming, Wei Lijiang. Effects of alternative fuels on the combustion characteristics and emission products from diesel engines: a review. Renew Sustain Energy Rev 2017;71:523–34. [5] Chen H, Zhang P, Liu YK. Investigation on combustion and emission performance of a common rail diesel engine fueled with diesel-ethylene glycol dual fuel. Appl Therm Eng 2018;142:43–55. [6] Yong Qian, Yahui Zhang, Liang Yu, Zhen Huang, Xing-Cai Lu. Effects of Iso-Alkanes as surrogate components blending in diesel fuel on the combustion process and emission characters. SAE Technical Paper 2016-01-2181. [7] Qian Yong, Qiu Yue, Zhang Yahui, Xingcai Lu. Effects of different aromatics blended with diesel on combustion and emission characteristics with a common rail diesel engine. Appl Therm Eng 2017;125:1530–8. [8] Li Zilong, Liu Guibin, Cui Xianfeng, et al. Effects of the variation in diesel fuel components on the particulate matter and unregulated gaseous emissions from a common rail diesel engine. Fuel 2018;232:279–89. [9] Velji A, Yeom K, Wagner U, Spicher U, et al. Investigations of the formation and oxidation of soot inside a direct injection spark ignition engine using advanced laser-techniques. SAE Technical Paper 2010-01-0352. [10] Pastor JV, Garcia-Oliver JM, Garcia A, Mico C, Durrett R. A spectroscopy study of gasoline partially premixed compression ignition spark assisted combustion. Appl Energ 2013;104:568–75. [11] Saxena S, Bedoya ID. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog Energy Combust Sci 2013;39:457–88. [12] Ciatti S, Subramanian SN. An experimental investigation of low-octane gasoline in diesel engines. J Eng Gas Turbines Power 2011;133:092802. [13] Won H-W, Peters N, Tait N, Kalghatgi G. Sufficiently premixed compression ignition of a gasoline-like fuel using three different nozzles in a diesel engine. P I Mech Eng D-J Aut 2012;226:698–708.
9
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Full Length Article
Experimental investigation of particulate matter structures under partially premixed combustion in a compression ignition engine ⁎
⁎
Hanyu Chena, , Minfei Wanga, Xi Wangb, , Deqiang Lia, Zhixiang Pana, Choongsik Baec,
T
⁎
a
School of Energy and Power Engineering, Wuhan University of Technology, Wuhan 430063, China School of Physical Education, Jianghan University, Wuhan 430056, China c Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Diesel and light hydrocarbon Partial premixed compression ignition Particulate matter Morphology TEM-EDS Particle size distribution
A study on particulate matter (PM) of a compression ignition engine under partial premixed combustion has been performed. Particularly, the microscopic analyses (SEM and TEM) of several samples at the exhaust were carried out to describe the morphology and ordering degree of particles produced by the combustion of diesel and 15% diesel blended with 85% light hydrocarbon (named DLH). Characterization parameters such as fractal dimension, fringe separation distance, fringe length and fringe tortuosity were analyzed to further study the particle structure. Besides, the particle size distributions at medium and high load were compared. The results demonstrated that the particle size distribution presents as quasi-monodisperse and the average diameter of DLH is smaller than that of diesel. The diesel particles move to a larger size range at higher load, while DLH particles show an obvious shift to smaller particles at higher load, which is related to the competition between surface growth and oxidation rates.
1. Introduction Due to high thermal efficiency, reliability and economy, diesel engines are widely used in vehicles, agricultural machineries and engineering equipment. However, diesel engine emissions are one of the main sources of environmental pollution [1]. Particulate matter (PM) emissions have caused haze formation to a great extent and is also harmful to human health [2]. Therefore, the formation mechanism and control of PM have become an increasingly important issue in recent years. Under the background of global energy shortage and increasingly stringent emission regulations of internal combustion engines, various alternative fuels were explored. And the combustion and emission performance of engines were comprehensively studied in real engines by changing the proportion of fuel components. Koder et al. [3] investigated the injection, ignition and combustion of jatropha oil, soybean oil and diesel oil in a 2.2L common-rail diesel engine with a twostage turbocharging concept and high pressure EGR. The size distribution for all test fuels at a low- and mid-load engine-operating point (EOP) were examined by applying different EGR rates. For oxygenated additives application, diesel engines adapted dual fuel mode more and more, and soot emissions decreased significantly with the increase of oxygenated alternative fuels, such as alcohols, natural gas, biodiesel
⁎
and dimethyl ether (DME) [4]. Chen et al. [5] investigated the combustion and emission performance of a common rail diesel engine fueled with diesel and ethylene glycol. It was found that ethylene glycol reduce ultrafine particles (UFPs) and decrease the average diameter of UFPs particularly. In addition, some studies have been carried out on the emissions of hydrocarbon fuel surrogate components blended with diesel. Qian et al. [6,7] studied the effect of iso-alkanes structure and aromatics blended with diesel on combustion characteristics and regular emissions respectively, and demonstrated that the addition of isoalkanes reduced HC, NOx and soot emissions [6]. However, blending aromatics caused peak value of particle number/size distribution curve moving towards to the small-size sides, and the single-ring aromatics showed higher particle emissions than multi-ring aromatics [7]. Li et al. [8] chose n-dodecane, iso-dodecane, tetralin and decalin to mix with diesel in 10% and 20% volume fraction. The effect of fuel components on unregulated emissions such as formaldehyde, acetaldehyde, ethylene, propylene, methane, and particle number and particle size were analyzed in detail. In order to meet the requirements of fuel economy and emission characteristics, the new combustion modes have also been deeply studied. Homogeneous charge compression ignition (HCCI) combustion model is a typical representative. However, the high pressure rise rate, load extension and combustion phasing control need to be overcome in
Corresponding authors. E-mail addresses: [email protected] (H. Chen), [email protected] (X. Wang), [email protected] (C. Bae).
https://doi.org/10.1016/j.fuel.2019.116286 Received 1 July 2019; Received in revised form 21 September 2019; Accepted 24 September 2019 Available online 27 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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output torque were controlled automatically using an electrical dynamometer (AVL504/4.6 SL). Using a piezoelectric pressure sensor (Kistler 6052C) coupled with a combustion analyzer (Dewetron M0391E) to measure the combustion pressure in the cylinder. Fig. 1 shows the schematic diagram of the test engine. In the ideal condition of PPCI mode, the end point of fuel injection and the start point of combustion are separated, which makes the fuel and air have a better premixed state at the beginning of combustion. This not only avoids the formation of soot caused by the local mixture over-concentration, but also facilitates the expansion of operating load in both high and low directions. In this study, light hydrocarbons are premixed with air through port fuel injection (PFI), and then mixed with a small amount of diesel directly injected (DI) into cylinder to form the mixture for combustion. The agglomerated aluminum foil is plugged in the soot sampling port of the exhaust pipe, so that the soot particles can accumulate on the surface of it for sampling. Diesel and light hydrocarbon are used in the experiment to form the mixture fuel according to the mass ratio. Light hydrocarbon is a mixture with C5 and C6 as the main components and has a higher low heating value than diesel. High blending ratio of light hydrocarbon will make it difficult for diesel engine to start and work normally. Therefore, 85% light hydrocarbon mixed with 15% diesel and pure diesel fuel are used in the test scheme. The physical parameters of diesel and light hydrocarbon are shown in Table 2. Fig. 2 shows the in-cylinder pressure, heat release rate and pressure rise rate curves of the two test fuels at 100% engine load of 1000 r/min. As shown in the pressure curve, it can be observed that the maximum cylinder pressure of DLH is lower than that of diesel, and its appearance is also delayed 2°CA. Lower temperature and pressure lead to a delay in ignition time, which can be seen from the heat release rate curve. But the maximum heat release rate of DLH is higher than that of diesel. The full mixing of fuel-air improves the fuel burning rate and complete combustion. As can be observed from Fig. 2, the pressure rise rate curve and the heat release rate curve have almost the same trend, and the corresponding phase moves backwards. Soluble organic fractions (SOF) on the particle surfaces have viscosities that promote the easy sticking of particles together. Therefore, particles should be pretreated to avoid affecting the shooting effect. Aluminum foil is placed in ethanol and oscillated by ultrasonic. We have done a lot of experiments to determine the reasonable ultrasonic time and frequency according to the properties of the sample and the reference provided by the ultrasonic instrument. The ultrasonic time was finally determined to be 30 min and then centrifuged for 5 min. This process filters out some of the SOF components, reduces the particle adhesion and disperses the agglomerated particles. It is conducive for SEM analysis. SEM (JSM-7500F) with 1.0 nm resolution and 15 kV accelerating voltage is used. It mainly uses the high-energy electrons to bombard the surface of particle samples, and utilizes the secondary electron emission effect produced by the sample to obtain the high-resolution sample surface structure information. TEM (JEM-1400Plus) with 0.38 nm resolution is used to observe and analyze the micromorphology and structure of the particles characterized by the fractal dimension of diesel particulate matter. TEM with higher resolution (Talos F200S) with 0.25 nm resolution is used to observe the nanostructure of primary carbon particles, whose geometric structure irregularity and density can be characterized by fringe separation distance, fringe length and fringe tortuosity. ImageJ software is used to measure the particle size and 100 particles are randomly selected for measurement under typical running condition. In the test, the engine speed is set to 1000r/min, and two loads, 75% and 100%, are selected. PPCI is achieved by PFI of 85% light hydrocarbon and DI of 15% diesel. For each experimental condition, several analyses have been realized, according to the summary in Table 3. The combustion particle morphological characteristics of pure diesel and DLH under the same conditions are compared. Finally, by changing the
the actual implementation of HCCI strategy [9]. Partial premixed compression ignition (PPCI) in diesel engine cylinder is a combustion mode between direct injection compression ignition (DICI) and HCCI. Literature [10–12] investigated the gasoline PPCI concept by using gasoline-like fuels in common-rail compression ignition engines. Won et al. [13], Yu et al. [14] and Valentino et al. [15] conducted an experimental study of gasoline/diesel blended fuel in PPCI mode on a high-pressure common-rail direct injection compression combustion engine. It can be seen that PPCI mode has been applied in practice and has the potential to improve both NOx and soot emissions and thermal efficiency. Jaasim et al. [16] simulated soot particle formation and emissions from low octane gasoline-like naphtha fuel under PPCI condition. The results showed that PPCI presents typical stratified combustion characteristics, and under its operating conditions, most of the main soot conditions throughout the entire combustion are avoided. The most direct way to determine whether alternative fuels and new combustion modes can be applied in practice is to study and compare their economy and emission characteristics. Diameters distribution, composition and concentration, SEM-TEM analyses are the most commonly used methods to understand PM morphology [17,18]. Liu et al. [19] carried out SEM analysis on soot particles generated from catalytic diesel combustion. The result showed that the average particle size of catalytic diesel decreases apparently compared with diesel fuel. Potenza et al. [20] studied the PM compound morphology based on SEM-TEM by changing the injection parameters that could affect soot production on a turbocharged engine. The results demonstrated high influence of injection strategies on soot morphology-composition and on catalyst efficiency. However, previous researches [20,21] focused on the impact of injection strategy variation (fuel injection timing, injection pressure, pilot injection) on engine particulate emissions. What’s more, several studies [22,23] focused on pure diesel combustion, revealing the evolution of particle size and number under different conditions (operation parameters, EGR rate, injection parameters, combustion type). So far, little research has been done on the microscopic characteristics of light hydrocarbon particles under PPCI combustion mode. In this study, the particle emission of diesel and DLH was studied by changing the engine operating condition in PPCI mode. Comparing with pure diesel fuel, the microstructure and particle size distribution of DLH were analyzed in detail based on electronic microscopic images. In addition, the effects of light hydrocarbon blending and operating conditions on the particulate emission characteristics of dual fuel engines were revealed, which provides a theoretical basis for finding ways to reduce particulate emission in the future.
2. Experimental setup and method The test engine is Z6170ZLCZ-1 diesel engine produced by Zibo Diesel Company in China. Table 1 lists the main technical parameters of the test diesel engine. The whole test system mainly includes engine, dynamometer, fuel supply system, intake and exhaust system, electronic control system and sampling device. The engine speed and Table 1 Specifications of test engine. Parameters
Details
Combustion system Number of cylinders Displacement/bore/stroke Compression ratio Rated speed/rated power Brake specific fuel consumption Injection/injection pressure
4-Valve PPCI 6 27 L/170 mm/200 mm 14.5 1000 r/min/330 kW 200 g/kW.h Direct injection/up to 35 MPa PFI injection/0.45 MPa 50°CA BTDC/40°CA ABDC 65°CA BBDC/50°CA ATDC
Intake valve opening/closing Exhaust valve opening/closing
2
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Fig. 1. Scheme of the experimental setup.
Table 2 Fuel Properties.
Table 3 List of experimental conditions and analyses.
Properties
Diesel
Light hydrocarbon
Test conditions
Fuel
Speed (r/min)
Load (%)
SEM
TEM
EDS
Molecular formula Liquid density (kg/L) Low heating value(MJ/kg) Cetane number Octane number Boiling point(℃)
C15-C23 0.82–0.88 42.5 52.5 – 170–350
C5,C6 0.63–0.68 48.1 – 70 36
1 2 3 4
Diesel
1000 1000 1000 1000
75 100 75 100
– – – √
√ √ √ √
√ √ √ √
DLH
used to study morphology and dimension of soot and particles, combined with the calculation of fractal dimension. In Fig. 3, several SEM images at different magnifications, related to the test of DLH, at 1000r/min speed and 100% load, are illustrated. From the results of SEM image, the particles are packed by thousands of quasi-spherical carbon particles with different diameters. These elementary particles accumulate in the presence of electrostatic force, liquid bridging force, Van der Waals force and other anchoring strengths, forming clusters of particles with different densities [24]. Most of them are chain-like or flocculent and the research of Liu et al. [19] showed
engine load, the particle size distribution of pure diesel fuel and DLH are showed to facilitate the law analysis of the load effects on them.
3. Result and discussion 3.1. Microscopic morphology of particles The results of SEM and TEM analyses are presented. The images are
Fig. 2. Combustion characteristic curves of different test fuels. 3
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Fig. 3. SEM images at different magnifications.
Fig. 4. TEM images of particles.
dimension. The calculation formula for it can be expressed as follows:
the same morphologies. TEM images of pure diesel and DLH particles at 1000r/min, different loads are shown in Fig. 4. It can be seen that the particles are composed of a certain number of spherical primary carbon particles. They are gathered together by collision. In the TEM image, there are regions with different color shades, mainly due to the accumulation of primary carbon particles. At medium load, the particles appear spherical and cluster, while at high load, most of them appear as chains. Under the same load, the number of DLH particles is smaller, so is the particle size. Moreover, the arrangement between particles is sparser and the agglomeration state is lower. On the one hand, the light hydrocarbon fuel mainly contains light components, and the combustion is more efficient. In detail, the combustion of light hydrocarbon produces fewer polycyclic aromatic hydrocarbons (PAHs) and other gas phase composition that help particles growth. PAHs is the precursor of particles, and less PAHs means less particles produced by DLH. Moreover, the adsorption and condensation of PAHs and gas phase composition will lead to the surface growth of particles. Less PAHs also means that the smaller particle size of DLH. On the other hand, particles are agglomerated by Brownian motion or turbulent diffusion motion [25], including collision, adhesion, fracture and grafting. The particle size and environmental factors determine the coagulation morphology between primary carbon particles. The differences in the agglomeration states are evident from Fig. 4. From the perspective of fractal theory, these agglomeration processes are fractal growth of particles. The complex structure of particles displayed under TEM is actually a fractal structure. In this study, the fractal dimension represents the overlap degree between primary carbon particles and can quantitatively clarify the density of particle structure, contributing to study the average particle size and distribution characteristics of primary carbon particles. Zhu et al. [26] and Kholghy et al. [27] calculated the fractal dimension to help analyze the morphological characteristics of particles. And the larger the fractal dimension is, the higher the overlap degree between the primary carbon particles is. In this paper, the fractal characteristics of particles at the same magnification factor are studied by box counting
DB = −lim r→0
logN (r ) logr
(1)
where DB is box counting dimension, r is the side length of the box, and N(r) is the number of boxes needed for the whole area. TEM image is binarized by the software ImageJ, and the data obtained by fractal calculation is fitted to a straight line. Taking the diesel particles at 75% load as an example, the fitting line of fractal dimension is shown in the Fig. 5. The fractal dimension can be obtained by calculating opposite number of the slope. The same method is used to calculate the fractal dimension of particles under other conditions, and the results are shown in Table 4. It can be seen that the fractal dimension is between 1.6 and 1.8. Kellerer et al. [28] considered that the fractal dimension of typical soot ranged from 1.6 to 1.9. On the whole, the fractal dimension of diesel particles is
Fig. 5. Fitting line of fractal dimension for diesel at 75% load. 4
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schematic diagram is shown in Fig. 8. These parameters can not only directly reflect the physical structure of primary carbon particles at nanoscale, but also closely related to the chemical characteristics such as oxidation rate and surface functional groups of particles. At the same time, the formation of the microstructure of primary carbon particles has a strong correlation with the formation conditions (time, temperature, fuel molecular structure and composition, etc.) [30]. This helps to understand the formation process of particulate matter and provides a theoretical basis for the exploration of emission reduction. Fig. 9 shows the calculation results of fringe separation distance. The distribution is in the range of 0.33–0.4 nm and that of diesel is more concentrated, mainly centered on the range of 0.35–0.39 nm. The mean separation distance of diesel at 75% and 100% load are 0.372 and 0.367 nm and DLH are 0.361 and 0.364 nm, respectively. It can be concluded that the separation distance of diesel is slightly larger than that of DLH. The calculation results of fringe length are shown in Fig. 10. The distribution presents a unimodal characteristic for all running conditions and the peak value fall within 0.6–1.0 nm, accounting for about 25%. The mean length of diesel at 75% and 100% load are1.000, 0.900 nm and DLH are 1.014 and 1.060 nm, respectively. It can be seen that the fringe length of diesel oil is smaller than that of DLH. Fringe tortuosity is defined as the ratio of the actual length of the microcrystal to the linear distance between the endpoints of the microcrystal in this study. The computed results are shown in Fig. 11. The distribution is mainly centered on the range of 1.1–1.8, and the mean tortuosity of diesel at 75% and 100% load are 1.420 and 1.420, and DLH are 1.389 and 1.360, respectively. The curvature degree of primary carbon particles of diesel is obviously larger than that of DLH. It can be found that there is a great correlation between fringe separation distance, fringe length and fringe tortuosity of primary carbon particles. In the whole process of evolution, oxidation plays an obvious role. The long and flat carbon layers are considered to be more graphitized, with fewer active sites and less oxidation than short and curly carbon layers. The oxidation activity of the edge carbon layer is greater than that of the base layer. The larger fringe separation distance indicates that the internal structure of the primary carbon particles is looser, as shown in Fig. 6. The fringe length of DLH is larger, indicating that shorter microcrystals are oxidized and the carbon layer is graphitized, with less boundary deficiency. The influence of fuel composition on fringe separation distance and fringe tortuosity is the same in trend, but the influence on fringe length is opposite to that of the first two nano-parameters. This is because that the increase of fringe tortuosity leads to overlap of electron orbitals in microcrystals, which can cause mutual repulsion between electrons, leading to repulsion between adjacent microcrystalline layers. Under the action of such repulsion, fringe length decreases and fringe separation distance increases, finally making the microcrystalline layer reach a stable state [31,32]. To deepen the metallic nanoparticles composition, an EDS analysis is performed. The ordinate of the energy spectrum represents the intensity, and higher strength means higher concentration of elements. Fig. 12 (a) shows EDS spectrum of pure diesel particles under 75% load. The spectrogram is a spectrum from a limited area. Fig. 12 (b) and Fig. 13 are all spectra from summed. It is shown in spectra that the main
Table 4 Fractal dimension under different tests. Test conditions
Fitting line
DB
Diesel 75%load Diesel 100%load DLH 75%load DLH 100%load
y = −1.7658x + 14.1699 y = −1.7918x + 14.3628 y = −1.7470x + 13.6089 y = −1.6397x + 13.5897
1.7658 1.7918 1.7470 1.6397
larger than that of DLH, indicating the higher degree of clustering between particles. This may be due to the higher temperature in the pure diesel combustion environment, which increases the collision rate between soot particles and easily forms agglomerated particles. On the other hand, DLH particles mainly present the chain-like distribution, which enlarged the radius of gyration, resulting in a smaller fractal dimension. This quantitatively confirmed the conclusion above that the arrangement between DLH particles is relatively sparse. 3.2. Structural analysis of primary carbon particles The structure, morphology and cluster degree of the particles can be reflected by SEM and TEM images, but the microscopic structure of the primary carbon particles cannot be fully shown. In this study, high resolution transmission electron microscopy is used to magnify the particles locally and obtain the micro-morphology of the primary carbon particles. The obtained images were processed and calculated to analyze the parameters representing the microstructure characteristics of particles, such as fringe separation distance, fringe length and fringe tortuosity. The microscopic images of primary carbon particles under different test conditions are shown in Fig. 6. It can be found that these primary carbon particles show a spherical carbon layer structure similar to fingerprint. They are usually divided into core and shell, as shown in the Fig. 7. The core is amorphous carbon microcrystalline, showing the curved and irregular structure, while the shell is usually composed of long microcrystalline, showing the regular and clear carbon layer structure. This kind of structure is the equilibrium configuration formed by the polycyclic aromatic hydrocarbons (PAHs) lamellae under the reflection of high temperature [29]. It can also be observed that the primary carbon particles contain a swirling multi-nucleus structure, as shown in Fig. 6 (a). This kind of structure is formed by the initial soot produced by the cracking of several particles, which gathered together and then be wrapped up by rapid surface growth. The primary carbon particles reorganize and microcrystalline grow under high temperature combustion, gradually tend to graphitize and transform to ordered structure. The ordering degree of primary carbon particles can be characterized by fringe separation distance, fringe length and fringe tortuosity. Fringe separation distance refers to the distance between two adjacent ordered microcrystalline layers in the nanostructure of primary carbon particles. Fringe length refers to the physical length of the microcrystalline layer. Fringe tortuosity is defined as the ratio of the fringe length to the distance between two endpoints of the microcrystal. The
Fig. 6. Microscopic image of primary carbon particle. 5
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Fig. 7. “Core-shell” structure of primary carbon particles.
Fig. 8. A schematic of nano-structural characteristic parameters for particles.
Fig. 11. Distribution of fringe tortuosity.
component of pure diesel nanoparticles is C. Aluminum is compatible with combustion chamber alloys. The existence of Cu is compatible with piston material, suggesting the presence of the piston wear phenomenon. The same conclusion was reached in reference [20]. The spectrum of DLH is presented in Fig. 13. It can be seen that similar metal components are detected in DHL particles and the main component of the nanoparticles is C. Compared with Figs. 12 (b) and 13 (b), it can be found that C and O elements increase significantly. The increase of O means more oxidation of soot particles in DLH. Finally, Zn and P traces are present probably due to the lube oil [33]. The composition of emissions is highly dependent on the lubricating oil and fuel type [34], and may also be affected by other factors. Higher load and temperature may enhance metal content in ultrafine exhaust particles [35,36].
Fig. 9. Distribution of fringe separation distance.
3.3. Size distribution of particles Particulate matters are generally divided into nucleation mode (Dp < 50 nm), accumulation mode (50 nm < Dp < 1000 nm) and coarse mode (Dp > 1000 nm) by particle diameter. Nucleation mode particles are comprised primarily of incompletely burned carbon nuclei, volatile hydrocarbons formed during dilution cooling, sulfur compounds and some metal compounds. Accumulation mode particles are mainly formed by the collision and aggregation of carbon particles and the adsorption and condensation of volatile substances on the surface. Coarse mode particles are caused by deterioration of spray quality or abnormal injection and present in extremely small amounts. The running condition of diesel engine and the composition of fuel will have an impact on the carbon particles size. The formation of PM usually starts from the part of uncompleted combustion in fuel-rich region. Therefore, more complete combustion, such as PPCI, can effectively reduce PM emissions in theory. In this paper, the calculation method for the mean diameter of primary carbon particles is as follows: take several TEM pictures at each
Fig. 10. Distribution of fringe length.
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(a) Diesel at 75% load (a) DLH at 75% load
(b) Diesel at 100% load (b) DLH at 100% load
Fig. 12. Metallic nanoparticles EDS spectrum of diesel.
Fig. 13. Metallic nanoparticles EDS spectrum of DLH.
working conditions, and select five of them. A total of 100 representative primary carbon particles with clear edges were randomly selected to calculate the diameter and then the average value was obtained. The size distributions of diesel particles are shown in Fig. 14. Although there are some data scatter due to the limited number of particles considered for each condition, the normal distribution provides an approximate fit to the entire dataset. From the standard deviations and the mean diameters, the size distributions of the primary particle diameters can be considered as quasi-monodisperse. It can be seen that the particle size distributes in the range of 15–60 nm and the mean diameter is around 30 nm. Lapuerta et al. [37] found that the size distribution of primary carbon particles in diesel engine fueled with ordinary petrochemical diesel is between 10 and 60 nm, and Neer et al. [38] considered that the average particle size of primary carbon particles is between 19 and 35 nm. Their results are very similar to those of this paper. It is observed that the number of particles moves towards a larger size range at higher load, which was consistent with the results reported by Lee et al. [39]. At medium load, the air-fuel ratio in cylinder is relatively higher, and there is sufficient air to provide a more uniform environment for combustion reaction. With the increase of load, the air-
fuel ratio decreases, which aggravates the uneven degree of fuel-air mixing and combustion status. The probability of further growth of aggregated particles increases, resulting in the increase of the average particle size. Fig. 15 shows the size distribution of DLH particles. It generally be considered as quasi-monodisperse, the same as that of diesel. But the peak value presents in a smaller particle size range. The particle size is mainly distributed in the range of 10–55 nm and the mean diameters are 29.39081 nm and 25.96712 nm at 75% and 100% loads, respectively. Compared with diesel, the mean diameter of DLH is smaller, and the nucleation mode particles account for a larger proportion. Soot formation has been found to be strongly dependent on air entrainment in the lifted portion of the jet as well as by oxygen in the fuel and to a lesser extent the composition and structure of hydrocarbons in the fuel [40]. The final properties of particulates emitted from an engine mainly depend on the relative contributions of carbon nucleation, growth, oxidation, and aggregation processes within the cylinders. From the perspective of particle formation, Light hydrocarbons contain short carbon chains and most of them are straight chains, and combustion produces fewer polycyclic aromatic 7
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(a) DLH at 75% load
(a) Diesel at 75% load
(b) Diesel at 100% load
(b) DLH at 100% load
Fig. 14. Size distribution of diesel particles.
Fig. 15. Size distribution of DLH particles.
hydrocarbons and other gaseous components that help the growth of particles, which means fewer nucleating particles. Moreover, the adsorption and coagulation of PAHs and vapor components on the surface of the particles and the collision and coagulation between the primary carbon particles will make the particles grow. Therefore, fewer PAHs also mean smaller particle size of DLH. Contrary to diesel, it is observed that there is an obvious shift to smaller particles at higher load. The particle size of basic carbon particles mainly depends on the competition between oxidation of soot particles and surface growth [38]. From the beginning of soot particle growth, it is accompanied by the oxidation reaction of nucleating particles, soot particles and soot aggregates. The rate of oxidation and surface growth is also affected by combustion environment, including gas phase material, residence time, flow characteristics and temperature. For diesel fuel, the increase of combustion temperature is beneficial to the oxidation of accumulated particles and the decrease of its concentration. However, with the increase of load, the fuel injection increases, and the short mixing time causes uneven mixing of fuel-air, resulting in deterioration of combustion quality. In the soot formation area of diffusion flame, the soot formation rate is quite high, but oxidant is scarce. When the surface growth rate exceeds the oxidation rate, the tendency of fuel surface growth increases by adsorbing gaseous substances such as PAHs and coagulating at high temperature. In the PPCI combustion mode, light hydrocarbon has higher volatility and the ignition time is later. The fuel droplets have sufficient time to mix with air, which is conducive to the formation of homogeneous mixture. Soot diffuses to the rich air area after formation, and the number density of soot does not increase. The growth of soot particles changes from
surface growth to condensation and grafting. Oxygenation becomes prominent because of the sufficient oxidant. With the increase of load, the combustion temperature in cylinder increases gradually, which increases the oxidation probability of HC and other substances produced by combustion in cylinder. When the oxidation rate exceeds the surface growth rate, the tendency of carbon particles to form larger ones is weakened. 4. Conclusion In this study, experimental study has been conducted to investigate the microstructure and particle size distribution of diesel and DLH fuel at medium and high load under PPCI mode, respectively. On the basis of the results and analysis presented above, the main conclusions can be drawn as follows: (1) In the images of SEM and TEM, the particles appear spherical, cluster and chains, and the cluster state of DLH particles is lower than that of diesel. The fractal dimension of DLH is smaller, quantitatively indicating that the particle structure of DLH is sparser than that of diesel. (2) The primary carbon particles present a “core-shell” structure similar to fingerprint under high resolution electron microscopy. The fringe length of diesel are smaller than those of DLH, and the fringe separation distance and fringe tortuosity is larger, which demonstrates that the ordering degree of DLH particle structure is higher. (3) The distributions of the primary particle size can be considered as quasi-monodisperse and the peak value of DLH presents in a smaller 8
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particle size range. The average diameter of DLH is smaller than that of diesel, and they have the opposite trend with the increase of load. The surface growth rate of diesel particles exceeds the oxidation rate because of the deterioration of combustion quality, which makes particles move to a larger size range at higher load. The full combustion environment increases the oxidation probability of HC and other substances produced by in-cylinder combustion, leading to an obvious shift to smaller particles at higher load. (4) Light hydrocarbons have good prospects in the application of engines. In terms of PM emissions, DLH is cleaner than diesel. The reduction of particulate emissions is of great significance to environmental protection.
[14] Yu C, Wang JX, Wang Z, et al. Comparative study on gasoline homogeneous charge induced ignition (HCII) by diesel and gasoline/diesel blend fuels (GDBF) combustion. Fuel 2013;106:470–7. [15] Valentino G, Corcione FE, Iannuzzi SE. Effects of gasoline-diesel and n-butanoldiesel blends on performance and emissions of an automotive direct-injection diesel engine. Int J Engine Res 2012;13:199–215. [16] An Yanzhao, Jaasim Mohammed, Vallinayagam R, et al. Numerical simulation of combustion and soot under partially premixed combustion of low-octane gasoline. Fuel 2018;211:420–31. [17] Wei Y, Wang K, Wang W, et al. Comparison study on the emission characteristics of diesel- and dimethyl ether-originated particulate matters. Appl Energy 2014;130:357–69. [18] Ma Y, Zhu M, Zhang D. Effect of a homogeneous combustion catalyst on the characteristics of diesel soot emitted from a compression ignition engine. Appl Energy 2014;113:751–7. [19] Liu J, Yang J, Sun P, et al. Experimental investigation of in-cylinder soot distribution and exhaust particle oxidation characteristics of a diesel engine with nanoCeO2 catalytic fuel. Energy 2018;161:17–27. [20] Potenza Marco, Milanese Marco, de Risi Arturo. Effect of injection strategies on particulate matter structures of a turbocharged GDI engine. Fuel 2019;237:413–28. [21] Liu B, Cheng XB, Liu JL, Pu H. Experimental investigation of injection strategies on particle emission characteristics of Partially-premixed low temperature combustion mode. Appl Therm Eng 2018;141:90–100. [22] Ni PY, Bai L, Wang XL, Li RN. Characteristics of evolution of in-cylinder soot particle size and number density in a diesel engine. Fuel 2018;228:215–25. [23] Wu BY, Zhan Q, Zhang SK, et al. Effect of heavy-duty diesel engine operating parameters on particle number and size distribution at low speed condition. Int J Auto Tech-Kor 2018;19:623–33. [24] Zhou YF, Shi Q, Huang ZL, Wang JD, Yang YR. Particle agglomeration and control of gas-solid fluidized bed reactor with liquid bridge and solid bridge coupling actions. Chem Eng J 2017;330:840–51. [25] Martos FJ, Martín-González G, Herreros JM. Semi-empirical model for indirect measurement of soot size distributions in compression ignition engines. Measurement 2018;124:32–9. [26] Zhu Jinyu, Lee Kyeong Ook. Ahmet Yozgatligil, Mun Young Choi. Effects of engine operating conditions on morphology, microstructure, and fractal geometry of lightduty diesel engine particulates. P Combust Inst 2005;30:2781–9. [27] Kholghy Mohammad Reza, Afarin Yashar, Sediako Anton D, et al. Comparison of multiple diagnostic techniques to study soot formation and morphology in a diffusion flame. Combust Flame 2017;176:567–83. [28] Kellerer H, Koch R, Wittig S. Measurements of the growth and coagulation of soot particles in a high-pressure shock tube. Combust Flame 2000;120:188–99. [29] Hurt RH, Crawford GP, Shim HS. Equilibrium nanostructure of primary soot particles. P Combust Inst 2000;28:2539–46. [30] Vander Wal Randy L, Tomasek Aaron J. Soot oxidation: dependence upon initial nanostructure. Combust Flame 2003;134:1–9. [31] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes. Academic Press; 1996. p. 5–29. [32] Yehliu K, Vander Wal RL, Boehman AL. Development of an HRTEM image analysis method to quantify carbon nanostructure. Combust Flame 2011;158:1837–51. [33] Choi S, Seong H. Lube oil-dependent ash chemistry on soot oxidation reactivity in a gasoline direct-injection engine. Combust Flame 2016;174:68–76. [34] Alves Célia, Barbosa Cátia, et al. Elements and polycyclic aromatic hydrocarbons in exhaust particles emitted by light-duty vehicles. Environ Sci Pollut Res 2015;22:11526–42. [35] Lim L, Lim C, Yu LE. Composition and size distribution of metals in diesel exhaust particulates. J Environ Monit 2009;11:1614–21. [36] André JM, Joumard R. Modelling of cold start excess emissions for passenger cars. INRETS Report 2005. No. LTE 0509. [37] Lapuerta M, Martos FJ, Herreros JM. Effect of engine operating conditions on the size of primary particles composing diesel soot agglomerates. J Aerosol Sci 2007;38:455–66. [38] Neer A, Koylu UO. Effect of operating conditions on the size, morphology, and concentration of submicrometer particulates emitted from a diesel engine. Combust Flame 2006;146:142–54. [39] Lee J, Sung NW, Huh KY. Prediction of soot particle size distribution for turbulent reacting flow in a diesel engine. Int J Engine Res 2011;12:181–9. [40] Tree Dale R, Svensson Kenth I. Soot processes in compression ignition engines. Prog Energy Combust Sci 2007;33:272–309.
Acknowledgments This work is sponsored by “State Key Laboratory of Engines at Tianjin University (K2018-09)”, “the Key Laboratory of Marine Power Engineering & Technology, Ministry of Transport (KLMPET 2016-01)” and “research fund of Center for Materials Research and Analysis, WHUT (2018KFJJ07)”. The authors also thank Associate Professor Kai Xu and Associate Professor Jiangjun Wei for pictures processing. References [1] Moon Gunfeel, Lee Yonggyu, Choi Kyonam, Jeong Dongsoo. Emission characteristics of diesel, gas to liquid, and biodiesel-blended fuels in a diesel engine for passenger cars. Fuel 2010;89:3840–6. [2] Agarwal AK, Gupta T, Kothari A. Particulate emissions from biodiesel vs diesel fuelled compression ignition engine. Renew Sustain Energy Rev 2011;15:3278–300. [3] Koder Alexander, Schwanzer Peter, Zacherl Florian, et al. Combustion and emission characteristics of a 2.2L common-rail diesel engine fueled with jatropha oil, soybean oil, and diesel fuel at various EGR rates. Fuel 2018;228:23–9. [4] Geng Peng, Cao Erming, Tan Qinming, Wei Lijiang. Effects of alternative fuels on the combustion characteristics and emission products from diesel engines: a review. Renew Sustain Energy Rev 2017;71:523–34. [5] Chen H, Zhang P, Liu YK. Investigation on combustion and emission performance of a common rail diesel engine fueled with diesel-ethylene glycol dual fuel. Appl Therm Eng 2018;142:43–55. [6] Yong Qian, Yahui Zhang, Liang Yu, Zhen Huang, Xing-Cai Lu. Effects of Iso-Alkanes as surrogate components blending in diesel fuel on the combustion process and emission characters. SAE Technical Paper 2016-01-2181. [7] Qian Yong, Qiu Yue, Zhang Yahui, Xingcai Lu. Effects of different aromatics blended with diesel on combustion and emission characteristics with a common rail diesel engine. Appl Therm Eng 2017;125:1530–8. [8] Li Zilong, Liu Guibin, Cui Xianfeng, et al. Effects of the variation in diesel fuel components on the particulate matter and unregulated gaseous emissions from a common rail diesel engine. Fuel 2018;232:279–89. [9] Velji A, Yeom K, Wagner U, Spicher U, et al. Investigations of the formation and oxidation of soot inside a direct injection spark ignition engine using advanced laser-techniques. SAE Technical Paper 2010-01-0352. [10] Pastor JV, Garcia-Oliver JM, Garcia A, Mico C, Durrett R. A spectroscopy study of gasoline partially premixed compression ignition spark assisted combustion. Appl Energ 2013;104:568–75. [11] Saxena S, Bedoya ID. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog Energy Combust Sci 2013;39:457–88. [12] Ciatti S, Subramanian SN. An experimental investigation of low-octane gasoline in diesel engines. J Eng Gas Turbines Power 2011;133:092802. [13] Won H-W, Peters N, Tait N, Kalghatgi G. Sufficiently premixed compression ignition of a gasoline-like fuel using three different nozzles in a diesel engine. P I Mech Eng D-J Aut 2012;226:698–708.
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