Experimental research on effects of biodiesel fuel combustion flame temperature on NOX formation based on endoscope high-speed photography

Journal Pre-proof Experimental Research on Effects of Biodiesel Fuel Combustion Flame Temperature on NOX Formation Based on Endoscope High-speed Photography Gongping Mao, Kaikai Shi, Cheng Zhang, Shian Chen, Ping Wang PII:

S1743-9671(20)30002-7

DOI:

https://doi.org/10.1016/j.joei.2020.01.002

Reference:

JOEI 679

To appear in:

Journal of the Energy Institute

Received Date: 16 July 2019 Revised Date:

6 January 2020

Accepted Date: 6 January 2020

Please cite this article as: G. Mao, K. Shi, C. Zhang, S. Chen, P. Wang, Experimental Research on Effects of Biodiesel Fuel Combustion Flame Temperature on NOX Formation Based on Endoscope High-speed Photography, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2020.01.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd on behalf of Energy Institute.

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Experimental Research on Effects of Biodiesel Fuel Combustion Flame

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Temperature on NOX Formation Based on Endoscope High-speed Photography

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Gongping Mao a,*, Kaikai Shi a, Cheng Zhang a, Shian Chen a, Ping Wang b

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a

School of Automotive and Traffic Engineering, Jiangsu University, Zhenjiang 212013, China b

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Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China

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Abstract: Biodiesel has the potential to be used as an alternative to diesel fuel, while NOX emissions from

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biodiesel engine are higher than those of diesel engine. According to the Zeldovich mechanism, thermal NO is the

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main pathway for NOX formation. Therefore, studying the temperature of biodiesel combustion flame is of great

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significance for understanding the formation mechanism of NOX. Hence, this paper is devoted to the investigation

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of effects of biodiesel combustion flame temperature on NOX formation by using Endoscope high-speed

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photography. The experimental work has been carried out on a diesel engine fueled with three different

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blending-ratios of biodiesel/diesel fuel (B0, B50, B100). The image of the engine combustion process was captured

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by high-speed endoscopic photography, and the flame temperature field distribution was calculated based on the

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brightness of the in-cylinder combustion flame image. The parameters of in-cylinder flame temperature and their

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changing laws with engine speed and blending-ratios were also analyzed by using six indicators, such as NOX

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M ), flame formation flame area ( SNO ), flame area change velocity ( VNO ), mean flame area change velocity ( V NO

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area duration ( ϕ NO ), flame appearance efficiency ( ηNO ) and adiabatic flame temperature ( TP ). Results showed

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that with the increase of biodiesel fuel proportion, the positions of SNO appearance and disappearance were

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M advanced, the peak values of SNO , VNO , V NO ,ηNO became greater, and ϕ NO became shorter, which indicated that

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more beneficial environment to NOX formation occurs as biodiesel fuel proportion increases.

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Keyword: Biodiesel; Flame temperature; NOX emissions; Endoscope high-speed photography; Diesel engine

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1. Introduction * Corresponding author. E-mail address: [email protected]

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Increasing energy demand and environmental degradation are two major factors driving the search for

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alternative fuels to replace traditional petroleum fuels [1]. Biodiesel is a renewable fuel that can be produced from

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biological materials [2]. It is also a clean fuel because it has the potential to reduce greenhouse gas emissions and

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effectively reduce the formation of particulates in exhaust gases [3]. Compared with diesel, biodiesel has the

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advantages of oxygen content and high cetane number [4]. Moreover, biodiesel contains no sulfur and aromatic

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compounds, greatly reducing its toxicity [5]. When using biodiesel as a fuel, only minor modifications to the diesel

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engine are required [6].

30

However, in recent years, biodiesel emissions have become increasingly serious. In order to reduce the

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emissions of different proportions of biodiesel and diesel fuel blends, scholars have conducted extensive research

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on different types of biodiesel. Can et al. [7] studied the exhaust emissions of different ratios of canola biodiesel

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and diesel blends. The results showed that as the ratio of canola biodiesel increases, NOX emissions increase, but

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CO, THC and smoke emissions were low. When Malik et al. [8] used coconut methyl ester (CME) biodiesel, Sen et

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al [9] used broiler biodiesel (BRFB) and Asokan et al [10] used 1:1 ratio of watermelon and papaya seed (WP)

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biodiesel, they also reached the same conclusions as Can et al. [7]. In general, fuels blended with biodiesel increase

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fuel consumption, reduce emissions of HC, CO, smoke and particulate matter, and slightly increase NOX emissions.

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However, Zehni et al. [11] studied the effect of biodiesel addition ratio and EGR rate on diesel engine emission

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characteristics, and obtained the result of NOX emission reduction with the increase of biodiesel ratio for the

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specified EGR rate.

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Many technologies for reducing NOX emissions from biodiesel can be broadly classified into pre-combustion

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treatment and post-combustion treatment techniques [12]. Common pre-combustion treatment techniques include

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different additives, exhaust gas recirculation (EGR), water injection and injection timing delays. Rashedul et al. [13]

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utilized two antioxidants, Vallapudi et al. [14] used a 10% EGR rate to reduce NOX emissions from biodiesel.

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Common post-treatment techniques include nitrogen oxide adsorbent catalysts, selective catalytic reduction (SCR)

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and selective non-catalytic reduction (SNCR). Bhattacharyya et al. [15] first pretreated NOX in exhaust gas used a

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discharge plasma, then used industrial waste Red Mud as an adsorbent/catalyst to reprocess NOX to enhance a NOX

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removal efficiency of 90%. Later, Madhukar et al. [16] used a plasma/ozone injection method to increase the NOX

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reduction capacity in the exhaust gas to replace the NOX treatment of the adsorbent/catalyst, thereby reducing NOX

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emissions by up to 95%. Haridass et al. [17] and Vedharaj et al. [18] reduced NOX emissions through SCR, SNCR

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systems installed in exhaust pipes, respectively.

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Unfortunately, most research on the use of biodiesel has focused on assessing the compatibility of biodiesel

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with conventional diesel engines, as well as an understanding of combustion and emissions characteristics. Little

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research has focused on any real-time surveys in the combustion chamber of engine. The understanding of the

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combustion process can be achieved with simple or complex optical diagnostic techniques, in particular the

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application of optical studies to the evaluation of combustion processes, such as flame temperature analysis. Some

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optical diagnostic techniques for testing the performance of combustion processes have emerged with the needs of

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research and applied to the understanding of actual combustion processes, such as laser Doppler velocimetry (LDV),

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particle image velocimetry (PIV), laser-induced fluorescence (LIF), spectroscopy and two-color methods. Optical

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endoscopes were first used in medical examinations of the human body and later entered the mechanical field for

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inspection of complex parts. Applying optical technology to an engine, such as applying an endoscope system to a

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combustion process of a cylinder, enables real-time and visual understanding of the fuel injection process and the

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development of the combustion flame.

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Many researchers have applied endoscope systems to study engine intake flow, fuel injection and combustion

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characteristics. Sementa et al. [19] studied the spray and combustion processes of gasoline and ethanol fuels

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through optical imaging of endoscopic systems. Ramírez et al. [20] , Mistri et al. [21] and Li et al. [22] studied the

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properties, combustion and emission characteristics of biodiesel blends used an endoscopic system. Abd Aziz et al.

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[23] used an endoscopic ICCD camera to capture and process flame images to investigate early flame development

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characteristics in homogeneous and stratified combustion situations. Schuck et al. [24] used a high-speed video

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endoscope to establish an optical path connected the inlet and the combustion chamber to analyze the mechanism of

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particle formation by observing air movement and fuel spray. Gessenhardt et al. [25] developed a large-caliber

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UV-transparent endoscope system that imaged the gas phase temperature in an engine by two-color toluene

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fluorescence. Liu et al. [26] developed an endoscopic tomography system to reduce the high cost of observing high

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turbulent combustion flames in engines. Doll et al. [27] and Schroll et al. [28] applied endoscopic observation of

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the limited optical path in the burner to the temperature distribution, velocity distribution and combustion

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fluctuations at the high energy interface between the burner and the turbine, which achieved low NOX emission

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technology.

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In addition to the literature that applies endoscopes to optical studies of combustion processes, real-time

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understanding of spray and pollutant formation is obtained. Some scholars used the endoscope to study the above

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problems while using the flame temperature analysis method to process the flame image. Jiotode et al. [29] studied

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the in-cylinder combustion of jatropha vegetable oil-diesel, used MATLAB to qualitatively analyze the spatial soot

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and flame temperature, and used the CCT method developed by Hernandez-Andres et al. [30]. The radiation

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calculated the flame temperature and soot distribution within the engine cylinder. They [31] also studied the

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in-cylinder combustion of diesel-alcohol blend fuels and found that the mixture burned and discharged better. In

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addition, they [32] also used the above flame image analysis method to evaluate the effect of the percentage of

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biodiesel blending and engine load on the flame temperature distribution and in-cylinder soot. The results showed

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that the space flame temperature distribution and soot formation of biodiesel both were lower than diesel. Agarwal

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et al. [33, 34] also used the same analysis and treatment methods to study the biodiesel combustion start time, flame

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temperature and soot distribution. Iorio et al. [35] experimental results showed that lower flame temperature and

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soot concentration lead to reduce NOX and particulate emissions. Xu et al. [36] studied the effect of oxygen

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concentration on the flame temperature and soot distribution in the cylinder under low load. The flame temperature

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was calculated from the heat radiation intensity emitted by the soot particles in the flame image. The results showed

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that the lower the oxygen concentration, the brightness of the flame in the image was also darker, the flame

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temperature was lower, and the distribution area of the soot was reduced.

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NOX emissions in diesel engine exhaust are NO and a little amount of NO2 [37, 38]. NO mechanism

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formations are thermal NO, prompt NO, N2O and fuel NO mechanism; among them, the thermal NO is the main

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way to NO formation [39]. Hence, the NOX of the diesel engine is mainly thermal NOX. A favorable factor for the

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formation of thermal NOX is an environment with high temperature and high oxygen content. Biodiesel has a

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higher cetane number than diesel and theoretically produces a lower flame temperature, which reduces NOx

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emissions. However, the oxygen free radicals of biodiesel will be more reactive in the influence of in-cylinder

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vortex motion. These free radicals accelerate the combustion speed and increase the diffusion combustion

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temperature, which is beneficial to the formation of NOx [40]. Therefore, studying the combustion temperature of

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biodiesel in the cylinder, especially the temperature of the combustion flame, is of great significance for

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understanding the formation mechanism of NOX.

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In order to provide more further understanding of NOX formation in the combustion chamber, this study

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provides a method for analyzing NOX formation based on the temperature of biodiesel combustion flame.

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According to the theory of thermal NO formation in diesel engines, six evaluating indicators for combustion flames

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in engine cylinders are innovatively proposed. First, the endoscopic high-speed photography technology is used to

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obtain images of the combustion process of diesel and biodiesel hybrid fuel engines with different mixing ratios.

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Then, the flame temperature field distribution is calculated based on the brightness of the combustion flame image

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in the cylinder. Finally, the variation characteristics of the in-cylinder flame characteristic parameters and the

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mixing ratio of biodiesel, and the relationship between the combustion flame temperature and the generation of

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NOX are analyzed.

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2. Methodology

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The methodology part of this study is arranged as follows: The equipment and instruments required for the

116

experiment are introduced in Section 2.1. In Section 2.2, the biodiesel fuel is configured and its characteristics are

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compared. The methods flame image acquisition and preprocessing are described in Section 2.3. The calculation

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method of flame temperature is established in Section 2.4. Finally, six new evaluating indicators for temperature

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characteristics is developed in Section 2.5.

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2.1 Experimental setup

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The experimental setup is shown in Fig. 1. The engine is a four-cylinder (straight, water-cooled, four-stroke,

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naturally aspirated, in-cylinder direct injection) diesel engine. During the experiment, the engine speed was kept

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constant at 1500 r/min and 2200 r/min in no-load running state. The specifications of the engine are shown in Table

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1, and the fuel injector parameters are displayed in Table 2.

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A type Kistler 6117BFD17 pressure sensor is adopted to measure in-cylinder pressure. A DEWE-800-SE

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combustion analyzer is used to obtain the pressure indicator diagram. A Horiba mexa7200 is utilized to detect NOX

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concentration.

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In this experiment, the Endoscope high-speed photography is AVL 513 Engine Video System, it uses a

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diameter of 4 mm endoscope for image shooting. The position of the endoscope on the cylinder head is shown in

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Fig. 1. It needs to open two holes with the same size as the endoscope in the cylinder. Unlike other high-speed

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photography devices need to arrange the light path in the cylinder, this method greatly reduces the influence of the

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high-speed photography on the working process of the diesel engine.

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Visioscope is a visualization system for the combustion and injection process of diesel engine. It can be used

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to study the fuel injection process and the development process of the combustion flame. The system can

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automatically complete synchronization, lighting and recording. The main technical parameters are listed in Table

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3.

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A tiny visualization system was added to the cylinder head during the test. In addition, no other engine

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modification was performed, in order to minimize the impact of other factors on the experimental results. The

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measurement results of all parameters were the average value of the engine test repeated three times under the same

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test conditions. Uncertainties of the parameters can reflect the confidence and accuracy of the measurement.

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Therefore, uncertainties of all measurement parameters were also evaluated in this study. The results showed that

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uncertainties (95% confidence) of the experimental parameters were less than 5%, and it could be considered that

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the experimental conditions and test results were reasonable and met the requirements for use.

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2.2 Fuel characteristics

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Biodiesel fuel was prepared by rapeseed oil (RME, produced by Arowana) through a transesterification

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process, the diesel fuel in this paper was 0# diesel (from China National Petroleum Corporation). Agilent GC/MS

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6890GC/5973NMSD was developed to detect the ester composition of biodiesel fuel, the result was shown in Table

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4. According to the measurement results of the components proportion of biodiesel fuel, the molecular formula and

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oxygen content of the biodiesel fuel used in this experiment could be calculated, the molecular formula is C19H36O2.

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Table 5 compared the physical and chemical properties of biodiesel with international similar standards (EN

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14214:2013 [41], ASTM D6751-15c [42] and GB 25199-2017 [43]). It can be seen from the Table 5 that the

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oxygen content and kinematic viscosity of biodiesel are higher than those of diesel, but the low calorific value is

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lower than that of diesel. Various indicators of biodiesel basically conform to the international standards of the

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same kind.

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2.3 Flame image acquiring and preprocessing

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In the case of the original injection pressure and fuel injection advance angle, the combustion process of the

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diesel engine, fueled with three kinds of diesel and biodiesel blends (B0, B50, B100 which refered to the volume

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ratio of biodiesel was 0, 50%, 100%) for different conditions, were captured by Endoscope high-speed photography,

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then the flame image were collected. Fig. 2 shows the collected combustion engine image of the diesel engine.

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Affected by the fuel combustion characteristics and the environment of image acquisition, there are problems such

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as noise interference and low contrast on the flame image. Therefore, when studying the flame characteristics, the

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flame image should be preprocessed.

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According to different purpose of denoising, flame image denoising methods can be divided into mean

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filtering, adaptive wiener filtering, median filtering, morphological noise filtering and wavelet denoising. Median

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filtering can perform local smoothing on two-dimensional images, which can overcome the problems of image blur

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caused by linear filtering. So this paper used the median filtering method. The basic principle is to use a

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neighborhood containing odd points as the moving window, and replace the gray value of the middle pixel of the

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window with the median value of the gray value of the pixel in the moving window. The specific operation process

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is depicted in Fig. 3.

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Due to the low contrast of the in-cylinder combustion flame image, the flame image is locally too bright or too

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dark. In this paper, the histogram equalization is utilized as an enhancement method for the combustion flame

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image in the diesel engine. The object of the histogram equalization process is a gray image, that is, the obtained

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histogram is a gray histogram. The gray histogram is a statistical table that reflects the gray distribution of the

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image. The abscissa indicates the gray scale from 0 to 255, and the ordinate indicates the number of pixels

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corresponding to each gray scale. The gray histogram directly reflects the brightness and clarity of the image. The

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dim flame image has a very uneven histogram distribution. The number of pixels in some gray levels is extremely

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high, while the number of pixels in some gray levels is extremely low, which makes the overall contrast of the

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image low. Histogram equalization is to equalize the excessive and too few gray levels on the original image, so

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that the histogram of the original image becomes uniform, thereby enhancing the contrast of the whole image and

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enhancing the flame image.

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2.4 Calculation method of flame temperature

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According to the principle of radiation temperature measurement [44], the brightness (gray level) of flame

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image was used to calculate the temperature (Equations (2) - (4)). The endoscope high-speed photograph device

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was employed to gain the image of in-cylinder combustion flame and extract the gray level of each point in the

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high-speed photograph of the combustion process. After preprocessing the flame image, the temperature of each

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point could be calculated according to the relationship between gray level and temperature. Then, the temperature

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field distribution of the image was obtained. Specific steps are as follows:

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MATLAB is used to extract the three primary color luminance values R(Red), G(Green) and B(Blue) of the

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pixels on the flame image, and the grayscale N is calculated according to the relationship between the luminance

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values of the three primary colors and the grayscale N [45]. The gray level N can be calculated by equation (1):

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(1)

N=0.299R +0.587G+0.114B

Where, R, G, B represent the red, green and blue brightness levels of the temperature points in image, respectively.

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The relationship between the gray level and the temperature of the flame image is expressed in equation (2):

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1  N 4 TL =TH  L  F ( N )  4  NH 

1

(2)

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Where, TH and TL denote the highest temperature and the lowest temperature of the flame image respectively.

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NH and NL refer to the corresponding gray level of the highest temperature and the lowest temperature in image

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respectively.

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The correction factor F(N) can be calculated by equation (3): N  1.11 （ H = 4）  N  L F (N ) =  N −N N  H H < 4） 1 + 0.11 × （  N −N N  H L L

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Where, N is the corresponding gray level of the required temperature point.

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Select the flame image at a certain crank angle, use MATLAB to extract the maximum gray level on the

(3)

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image and estimate the maximum temperature. Several sets of data (T, N) are calculated from equations (1), (2),

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and (3). Based on these data, the fit obtains the relationship between the gray level and the temperature of the flame

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image, as shown in equation (4):

T= a × Nb

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(4)

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Where, a and b are constants.

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The temperature distribution of the photo and the area of each temperature region are calculated by using

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MATLAB. The average temperature (Tm) can be calculated by equation (5):

Tm =

∑T S ∑S i

i

i

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Where, Si is the area of temperature region, Ti is the temperature of temperature region.

(5)

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According to the pressure indicator diagram measured by the combustion analyzer, the mean temperature

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variation law of the cylinder combustion with crank angle can be computed, and the mean combustion temperature

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Tmc at a certain crank angle can be obtained.

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The mean temperature Tm at a certain crank angle is calculated by equation (5), and Tm is compared with Tmc

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(the same crank angle as Tm). if |T − T | ≥ 20K, then adjust the previous maximum temperature and recalculate

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Tm until |T − T | < 20 , The cycle program realizes the whole compute process.

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2.5. Evaluating indicators of temperature characteristics

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Analysis of the combustion process image obtained by high-speed photography is an effective way to explore

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the details of the combustion process in the flame area. It can be used to evaluate the characteristics of the engine

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combustion flame. The mechanism of NO formation recognized in diesel engine requires a comprehensive analysis

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of the NOX formation mechanism. There are three recognized sources contributing to NOX emissions namely

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thermal, prompt and N2O mechanisms [46]. Fennimorey [47] proposed prompt NO in 1975, where the NO

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temperature was below 750°C. Pandey et al. [48] presented N2O mechanisms in 2012. This mechanism dominated

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the combustion process at high pressure and lean air-fuel ratio conditions. Zeldovich [49] firstly proposed thermal

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NO. Because biodiesel does not contain N, thermal NO is an important source of total NOX emissions in engines

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based on biodiesel. Thermal NO is produced by a high temperature dependent chemical reaction. The NOX in the

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biodiesel combustion process are mainly derived from N in the air, and the N in the air exists in the form of

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inorganic substances. Nitrogen bond N2 chemical bond breaking into atom N requires high activation energy, and

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in-cylinder combustion provides this high-energy environment. The high combustion temperature (1700K) breaks

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the strong chemical triple bond of the molecule N and forms a highly active atomic N, which reacts with oxygen

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and produces thermal NO. A important factor affecting the rate of formation of thermal NO is the combustion

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temperature. A study [50] found that thermal NO had a strong exponential relationship with temperature. When the

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temperature was lower than 1640K, the effect of thermal NO on total nitric oxide production was small, but when

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the temperature was higher than 1670K, the effect of thermal NO on total nitric oxide production became

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significant.

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According to different temperatures, the combustion flame is divided into two regions.

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H: high-temperature region, T > 1700K;

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L: low-temperature region, T ≤ 1700K.

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This work focuses on temperature in the area above 1700K, records this temperature region as TNO and puts

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forward the following new evaluating indicators.

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2.5.1 NOX formation flame area

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In general, different flame temperatures correspond to different gray levels. The gray level enhances as the

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flame temperature increases. The color flame image is converted into a grayscale image, and the high temperature

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region is defined as a NOX forming flame region having a temperature greater than 1700K, denoted as SNO . i S NO = ∑ S NO

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(6)

Where, ϕi is the crank angle. S Ni O is the flame region for NOX corresponding to each crank angle, this index is developed to investigate the temperature field transient distribution of the in-cylinder thermal NO. The temperature in the region above 1700K and the variation laws of this temperature region with the crank

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angle, speed and RME addition ratio of diesel engines was studied.

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2.5.2 Flame area change velocity

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During the combustion process of the diesel engine, the fuel vapor in the premixed part is mixed with air and

253

burned quickly, then the combustion is diffused. Most NOX is produced during diffusion combustion. The local

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diffusion time scale is not always much larger than the combustion reaction characteristic time of reaction rate in

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cylinder turbulence. When they are in the same order of magnitude, local extinctions can occur. From the image of

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high-speed photography, the light and shade degree of the flame is not in a uniform rising or decreasing trend, and

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the evaluation of non-premixed flames diffusion rate in turbulent flow is a difficult problem in the combustion field

258

of internal combustion engine.

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In this paper, the change rate of the same temperature region and the intensity of the non-premixed combustion

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are considered to have the correspondence. Flame area in same temperature region with the change rate of crank

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angle is adopted to analyze the combustion. The concept of flame area change velocity ( VNO ) is proposed, which

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corresponds to the change speed of flame area per unit crank angle.

VNO =

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2 S 1NO − S NO ϕ1 − ϕ 2

(7)

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Where, S 1N O and S N2 O are respectively the corresponding area of adjacent crank angle in which the temperature

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in image is above 1700K. This paper only considers the condition when flame area is enlarged, that is the condition

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2 of S 1NO＞ S NO . ϕ1 and ϕ2 are the corresponding crank angle of flame area.

267 268

The diffusion rate of the flame is changing with the crank angle. Therefore, mean flame area change velocity M ( V NO ) is introduced.

M VNO =

269

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1 n i ∑VNO n i−1

(8)

i Where, V NO is the rate of change of the flame area corresponding to different crank angles, n is the number of

271

the flame area change velocity.

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2.5.3 Flame area duration

273

The residence time of N2 and O2 in high temperature is one of the important factors, which affect the

274

formation of NOX. In this paper, the corresponding combustion duration is determined according to the change of

275

flame temperature area, and the concept of the duration of the flame area ϕ NO is proposed to characterize the

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existence time of the temperature environment.

ϕ NO = ϕend − ϕ start

277 278

(9)

Where, ϕ end and ϕ start are respectively the corresponding crank angle of the SNO disappearance and emergence

279

position.

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2.5.4 Flame appearance efficiency

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The ratio of the total area of the combustion process to the duration of the temperature region is defined as

282

flame appearance efficiency of NO flame area. This parameter is developed to characterize the efficiency of the

283

engine in a cycle, which appears to be beneficial to the NO environment. It can be calculated by equation (10):

η NO = ∑ NO ϕ NO S

284

285 286 287

2.5.5 Adiabatic flame temperature According to Fernando et al. [51] analysis method of combustion flame temperature, adiabatic flame temperature can be calculated by equation (11):

TP = TO +

288

289 290 291

(10)

HU (1 + α grav, s)Cp

(11)

Where, T0 is the ambient temperature, HU and C p are respectively the low calorific value of the fuel and the mean specific heat of combustion products, α grav , s is the mass equivalent air-fuel ratio. Based on interpolation, the mean specific heat C p is determined.

Cp =

292

∑W C i

pi

(12)

293

Where, Wi and C pi are respectively the mass fraction and specific heat of component i.

294

If the general molecular equation for biodiesel is C x H 2 y O2 z , then the theoretical chemical equation for the

295 296

combustion of biodiesel is as follows:

CxH 2 yO 2 z + a[O 2 + 3.76 N 2]  → xCO 2 + yH 2O + 3.76aN 2

(13)

y −z 2

(14)

2x + y − z ) 6x + y +16z

(15)

297

a = x+

298

α grav, s = 34.32(

299

In short, the six evaluation indexes of temperature characteristics are closely related. SNO is the high

300

temperature transient NOX formation flame area in the cylinder. In the same temperature region, VNO is the flame

301

area change velocity per unit crank angle when the flame region is enlarged. The difference between the total SNO

302

disappearance and the appearance position is the flame area duration ϕ NO , and the average value of the VNO within

303

M the ϕ NO is the mean flame area change velocity V NO . The ratio of the total SNO to the ϕ NO is the flame appearance

304

efficiencyηNO . The adiabatic flame temperature TP is an intrinsic parameter that affects the other five variables. The

305

higher the flame temperature, the larger the area SNO that affects the high-temperature transient NOX formation in

306

the cylinder.

307

3. Results and discussion

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3.1 Flame image preprocessing analysis

309

Fig. 4 is an effect diagram of the combustion flame image of the diesel engine cylinder after median

310

filtering and noise decreasing. It can be seen that the flame image noise removal after median filtering is obvious.

311

The local area on the flame image is well smoothed, and the pixels on the image that are different from the

312

surrounding brightness values are filtered out, so that the flame image is clearer.

313

Fig. 5 (a) shows the original flame image and its gray histogram, Fig. 5 (b) depicts the enhanced flame image

314

and its gray histogram. It can be seen that the number of pixel points of each gray level in the gray histogram of the

315

original flame image is very uneven. The number of pixels with gray level of 0-50 is obviously higher, the number

316

of pixels with gray level of 51-255 is significantly smaller, and the overall contrast of the image is lower. In the

317

gray histogram of the flame image, the distribution of the number of pixels of each gray level after the histogram

318

equalization is more balanced. The number of pixels with gray level of 51-255 increases. The range of gray level is

319

about 0-255, greater than the range of the original image. The contrast is increased and the flame brightness is

320

brighter.

321

3.2 Flame temperature calculation result

322

Select a flame image for a crank angle, use image processing software to estimate the maximum temperature,

323

and determine the gray level of the temperature is 232. Combine them with each of the red, green and blue gray

324

levels. Point to the image and get a set of data (T, N). Based on the data, the relationship between the gray level and

325

the temperature of the flame image is obtained. From the calculation of a set (T, N) obtained by equation (4), a is

326

604.2107 and b is 0.2592. According to the relationship between T and N, compute the temperature of the

327

remaining points and obtain the distribution of temperature. At the same time, MATLAB fits the area of each

328

temperature region.

329

Verify the accuracy of the calculation. According to the equation (5) of Tm, the Tm in image is 1846K when

330

the crank angle is 7.9 °CA. According to the pressure indicator diagram measured by the combustion analyzer, the

331

Tmc is 1798K when the crank angle is 7.9 °CA. Obviously, |T − T | ≥ 20K. Then adjust the previous

332

maximum temperature to 2210K and recalculate Tm, until|T − T | < 20 . Finally, the relationship between the

333

gray value and the temperature value is calculated and fitted, as shown in equation (16).

T = 605.4107 × N 0.2292

334

(16)

335

Fig. 6 is a partial effect diagram of the flame picture (A) and the temperature field distribution (B) at part of

336

the crank angle when the engine speed is 1500 r/min and the fuel is B0. From the Fig. 6, the high temperature

337

region (H) and the low temperature region (L) in the combustion flame can be easily distinguished.

338

With this processing, analysis and calculation method, when the engine speed is 1500 r/min and 2200 r/min,

339

and the fuel is B0, B50 and B100, the pre-processed flame image can be processed, and the temperature can be

340

calculated. The temperature values corresponding to different gray values at different crank angles are obtained in

341

turn. By fitting the size of the temperature region corresponding to the crank angle, the area of the high temperature

342

region is obtained, that is, the NOX formation flame area, and other evaluating indicators are obtained.

343

3.3 Evaluating indicators results and analysis

344

3.3.1 NOX formation flame area

345

When the engine speed is 1500 r/min and 2200 r/min, the in-cylinder flame area of NOX at various crank

346

angle are given in Fig. 7. It can be seen that when the speed is 1500 r/min, as the proportion of biodiesel increases,

347

SNO expands. However, at 2200 r/min, it is reduced. This is because at low speeds, the inherent oxygen content of

348

biodiesel can accelerate the burning rate, increase the maximum in-cylinder temperature, and create an environment

349

conducive to NOX formation, resulting in an increase in flame area [52]. As the engine speed increases, the duration

350

of fuel mixing and combustion is reduced, the fuel cannot be completely burned. The residual exhaust gas in the

351

combustion chamber and the cylinder is increased, so that the maximum in-cylinder temperature is lowered, the

352

formation of NOX is suppressed to some extent, and the flame area is reduced [53]. At the same blending ratio,

353

when the speed is increased, the turbulent motion in the cylinder is strengthened, the area of contact between the

354

unburned mixture and the flame surface is expanded, and the combustion reaction in the cylinder is more intense,

355

which increases the flame area [54].

356

Table 6 and Table 7 list the occurrence and disappearance positions of the high temperature region at different

357

rotational speeds, respectively. It can be seen that at the same speed, with the increase of the proportion of biodiesel,

358

the occurrence and disappearance position of SNO are advanced. On one hand, due to the high oxygen content in

359

biodiesel, the combustion condition is improved and the combustion become sufficient. On the other hand, larger

360

elastic modulus of biodiesel lead to a more obvious trend of advanced fuel injection [55]. At the same blending

361

ratio, compared to 1500 r/min, if the speed is increased to 2200 r/min, the position of occurrence and disappearance

362

of SNO is delayed. This is due to a more favorable environment for NOX formation at low speeds.

363

3.3.2 Flame area change velocity and mean flame area change velocity

364

The rate of change of the difference between the areas of SNO corresponding to adjacent crank angles in Fig.

365

7, the flame area change velocity can be obtained. Fig. 8 shows the flame area change velocity in the SNO region at

366

different speeds. There are two peaks in the curve, they are just the obvious peaks in the stages of premixed

367

combustion and diffusion combustion, which are located in the early and the later stage of combustion. The two

368

durations are the occurrence and disappearance of the SNO , which depict that the change of flame velocity become

369

more severe. The peak value and the mean value of VNO are analyzed. At the speed of 1500 r/min, the first

370

VNO peak values of the B0, B50 and B100 are 4.85 cm2/°CA, 5.16 cm2/°CA and 6.91 cm2/°CA respectively. The

371

corresponding positions are -1.15 °CA, -1.45 °CA and -2.45 °CA respectively. The second VNO peak and it is

372

corresponding position are 6.29 cm2/°CA, 7.21 cm2/°CA, 8.53 cm2/°CA and 28.40 °CA, 25.60 °CA, 23.50 °CA

373

respectively. At the speed of 2200 r/min, the first VNO peak value of the B0, B50 and B100 are 7.82 cm2/°CA,

374

9.27 cm2/°CA and 11.68 cm2/°CA respectively. The corresponding positions are -2.80 °CA, -2.85 °CA and

375

-3.14 °CA respectively. The second VNO peak and its corresponding positions are 9.48 cm2/°CA, 12.09 cm2/°CA,

376

15.89 cm2/CA and 28.95 °CA, 23.45 °CA, 21.70 °CA respectively. It can be seen that at the speed of 1500 r/min, as

377

the proportion of biodiesel fuel increases, the peak position advances and the peak value enhances. This is because

378

as more biodiesel participates in the combustion, the increase in oxygen concentration accelerates the combustion

379

rate and the flame area changes at a higher peak speed and burns more severely [56]. In addition, the inherent

380

oxygen content of the biodiesel and the higher cetane number shorten the ignition delay [57]. And the speed change

381

curve moves toward the stop point, causing the peak to advance. However, at a speed of 2200 r/min, the peak

382

position is delayed. This is because as the speed enhances, the flow resistance of the in-cylinder mixture increases,

383

and the fuel mixing effect is degraded compared to the low rotation speed, so that the combustion quality is

384

degraded.

385

By analyzing the average value of the flame area change velocity in the SNO region in Fig. 8, the

386

mean flame area change velocity can be obtained. Fig. 9 compares the mean flame area change velocity

387

M at different speeds. At the speed of 1500 r/min and 2200 r/min, the V NO of the B0, B50, B100 are 2.21

388

cm 2 /°CA, 2.42 cm2 /°CA, 4.21 cm 2 /°CA and 2.22 cm 2 /°CA, 3.48 cm2 /°CA, 4.85 cm 2 /°CA respectively. It

389

can be seen that when the speed is constant, as the proportion of biodiesel fuel in the mixed fuel

390

M increases, the V NO enhances. This is because the oxygen inherent in biodiesel accelerates the burning

391

M M rate while also increasing the V NO . In addition, when the blending ratio of biodiesel is constant, V NO

392

enhances with the increase of the speed. This is because although the VNO is reduced, the total

393

M combustion duration in Fig. 8 is extended, so that the V NO is enhanced.

394

3.3.3 Flame area duration

395

From the crank angle corresponding to the occurrence and disappearance of the SNO region in Fig. 7, the

396

duration of the SNO in cylinder can be obtained. As can be seen from Fig. 10, at the speed of 1500 r/min and 2200

397

r/min, the ϕ NO of the B0, B50, B100 are 36.1 °CA, 35.0 °CA, 34.4 °CA and 35.8 °CA, 34.5 °CA, 33.5 °CA

398

respectively. The duration of the SNO decrease with the increase of the proportion of biodiesel fuel in blends. As

399

mentioned above, the residence time of oxygen is the important factor, which affect the formation of NOX. The

400

oxygen in biodiesel, which accelerate the burning rate and the disappearance position of SNO . The decrease

401

reduce the ignition delay and inhibit the increase of NOX, but the oxygen in biodiesel enhance the NOX. Therefore,

402

the oxygen in biodiesel is the main reason for the increase of NOX.

403

3.3.4 Flame appearance efficiency

ϕ NO

404

The ratio of the SNO region area in Fig. 7 to the flame area duration in Fig. 10 can be used to

405

obtain the flame appearance efficiency. Fig. 11 shows the comparison of the NO flame appearance

406

efficiency. At the speed of 1500 r/min and 2200 r/min, the

407

10.0 cm 2 /°CA, 11.2 cm2 /°CA and 11.1 cm2 /°CA, 11.9 cm 2 /°CA, 12.3 cm2 /°CA respectively. It depicts

408

that

409

the greater the

410

at that time. The calorific value of biodiesel fuel is lower than the diesel fuel, but the higher oxygen

411

content in biodiesel fuel increase the burning rate, which improve the NO appearance efficiency.

412

3.3.5 Adiabatic flame temperature

ηNO of the B0, B50, B100 are 9.0 cm2 /°CA,

ηNO enhance with the increase of the proportion of biodiesel fuel in blends. The higher the speed, ηNO when fuel with the same fuel. Because it had the poor cooling effect in the cylinder

413

According to the results of 0# diesel by gas chromatography mass spectrometry, the molecular

414

equation of diesel was obtained. The equation (13) and equation (14) were developed to calculate the

415

proportion of CO 2 and H 2 O in the combustion products. The equation (11) was used to calculate TP of

416

the fuel, and the change of adiabatic flame temperature with the proportion of biodiesel is presented in

417

Fig. 12. It can be seen that TP decrease with the increase of the proportion of biodiesel. Since the TP

418

depend on the initial temperature, pressure and reactant property. The initial temperature and pressure

419

are invariant in experiment, so the property of the reactants is the main influence factor. The low

420

calorific value of biodiesel is lower than diesel, so the low calorific value decrease with the increase of

421

biodiesel proportion. The TP present the trend of Fig. 12, which has the same trend with the relative

422

temperature research of Jha et al. [51].

423

A bus connecting the above six indicators can be obtained. at a speed of 1500 r/min, with the

424

increase of the proportion of biodiesel fuel, the inherent oxygen content in biodiesel can accelerate the

425

combustion speed, enhance the maximum in-cylinder temperature, form an environment favorable for

426

NO X formation, and expand the SNO area. The oxygen content can improve the combustion conditions,

427

the combustion becomes sufficient, and the occurrence and disappearance of SNO is advanced. Higher

428

oxygen content and faster combustion speeds increase the peak value of VNO and advance accordingly.

429

M increase, At the same time, the acceleration of combustion speed makes V NO

430

ηNO increases. The initial temperature and pressure remain unchanged. The low calorific value of

431

biodiesel is lower than that of diesel. As the proportion of biodiesel increases, TP decreases. At 2200

432

r/min, the difference is that as the proportion of biodiesel fuel increases, the speed affects more than the

433

oxygen content. The duration of fuel mixing and combustion is shortened, the combustion is incomplete,

434

the residual exhaust gas in the cylinder increases. The maximum combustion temperature in the cylinder

435

decrease，which is not conducive to the formation of NO X . The reduction of SNO , and the delay of

436

M VNO peaks. However, the duration of SNO made V NO increase overall.

437

4. Conclusions

ϕ NO decreases and

438

(1) Based on the theory of thermal NO formation of diesel engines, evaluating indicators of

439

M combustion flame of biodiesel engine are proposed: SNO , VNO , V NO , ϕ NO , ηNO and TP . Based on the

440

principle of the brightness measurement method, the evaluating indicators are calculated and evaluated.

441

(2) As the proportion of biodiesel fuel increases, the occurrence and disappearance of SNO is

ϕ NO decreases, ηNO increases and TP decreases. At 1500 r/min, as the

442

M advanced, V NO enhances,

443

proportion of biodiesel fuel increases, SNO increases and the peak value of VNO enhances and

444

advances. At 2200 r/min, as the proportion of biodiesel fuel increases, SNO decreases and the VNO

445

peak is delayed.

446

(3) According to the research methods and results of this paper, NO X can be predicted. Due to the

447

higher oxygen content in biodiesel, the NO X formation environment is better than diesel in the

448

combustion process. At the action of biodiesel or diesel and biodiesel blended fuel, the NO X content

449

increases. This provides an idea for developing a new method for predicting NO X based on flame

450

diagnostics.

451

Acknowledgements

452

This work was financially supported by Postdoctoral Science Foundation of China (Grant NO.

453

2017M621642), National Natural Science Foundation of China (Grant NO. 91741117), and A Project

454

Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions

455

(PAPD).

456

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Table 1 Parameters of diesel engine Engine parameters

Specification

type

Straight line, water cooled, four stroke

Intake mode

aspirated

Number of cylinders

4

Bore/(mm)

102

Stroke/(mm)

118

Combustion chamber type

ω

Rated power/(kW)

70.6

Rated speed/(r/min)

3200

Maximum torque/(N·m)

245

Maximum torque speed/(r/min)

2200

Table 2 Injector Parameters Parameter

Unit

Value

Orifice number

-

5

Orifice diameter

mm

0.22

Valve diameter

mm

5

Spray angle

deg

150

Initial pressure

MPa

23

Installation angle

deg

13

Needle valve lift

mm

0.3

Nozzle length

mm

2.5

Table 3 Parameters of Visioscope Parameter

Unit

Value

The maximum exposure rate

t/s

60000

The image resolution

pixel

640 × 480

Dynamic range

bit

12

The minimum exposure time

ms

10

Flash time

ms

30

The maximum flash energy

J

16

Storage capacity

null

30000

Table 4 Components of biodiesel fuel detected by GC/MS Retention time/s

Proportion /%

Component

Reference Library

Semblance /%

9.5 10.4 10.5 10.8 11.5 11.7 13.1 13.3

9.9 72.1 6.2 3.1 1.9 0.8 5.3 0.7

Methyl palm Methyl ester of oleic acid Stearic acid methyl ester Linoleic acid methyl ester Twenty carbon methyl ester Methyl eicosanoate Erucic Acid Methyl Ester Twenty-two acid methyl ester

115419 13870 115468 113735 13835 115427 13822 115472

97 99 98 99 93 97 90 99

Table 5 Comparison of physical and chemical properties of biodiesel with international advanced standards Properties

Biodiesel

0# Diesel

ENa 14214: 2013

ASTMb D6751-15c

GBc 25199-2017

Density at 15°C (kg/m3) Kinematic viscosity at 40°C (mm2/s) Flash point (closed) (°C) Sulphate ash content (%) Moisture and sediment content (%) Copper corrosion level (50°C,3h) Cetane number Acid value (in KOH) (mg/g) Oxidation stability at 110°C (h) Low calorific value (MJ/kg) Oxygen content (%) Sulfur content (%)

885 4.86 123 0.010 0.02 1 51 0.45 3.05 38 10.8 0.0007

810~845 3.0~8.0 ≥60 ≤0.01 ≤1 ≥51 ≤7 ≤2.5 42.5 0 ≤0.2

860~900 3.5~5.0 ≥101 ≤0.02 ≤1 ≥51 ≤0.5 ≥8.0 ≤0.0010

1.9~6.0 ≥93 ≤0.020 ≤0.05 ≤3 ≥47 ≤0.5 >3 ≤0.05/0.0015

820~900 1.9~6.0 ≥130 ≤0.020 ≤1 ≥51/49 ≤0.5 ≥6.0 ≤0.0010/0.0050

a

European Standards (EN); American Society for Testing Materials (ASTM); c Chinese National Standards (GB). b

Table 6 The occurrence position of high-temperature region Speed

Area

B0

B50

B100

1500 r/min

H

-0.4°CA

-2.7°CA

-3.6°CA

2200 r/min

H

-0.4°CA

-2.1°CA

-3.1°CA

Table 7 The disappearance position of high-temperature region Speed

Area

B0

B50

B100

1500 r/min

H

44.5°CA

35.8°CA

32.7°CA

2200 r/min

H

42.1°CA

38.1°CA

35.8°CA

7.9 A

11.0 A

14.2 A

(a)Original grayscale image (b) Decrease noise after flame image

（a）Original flame image and its gray histogram

（b）Enhanced flame image and its gray histogram

A

B -1.9 A

-0.4 A

1.6 A

4.5 A

7.9 A

11.0 A

14.2 A

17.7 A

20.7 A

23.3 A

26.8 A

29.4 A

34.3 A

37.3 A

A

B

Fig. 1. Experimental setups Fig. 2. Combustion flame images Fig. 3. Flow chart of median filter operation Fig. 4. Median filter processing of combustion flame image Fig. 5. Flame image histogram equalization Fig. 6. Flame image and temperature field distribution Fig. 7. NOX formation flame area Fig. 8. Flame area change velocity Fig. 9. Mean flame area change velocity Fig. 10. Flame area duration Fig. 11. Flame appearance efficiency Fig. 12. Adiabatic flame temperature

Highlights • Endscope high-speed photography is used to capture the engine combustion process. • It provides a new method to calculate the in-cylider flame temperature. •Six evaluating indicators are proposed to analyze the flame temperature characteristic. • It is found that the NOX formation environment of biodiesel is better than diesel.