Comparative performance and emission characteristics of peanut seed oil methyl ester (PSME) on a thermal isolated diesel engine

Energy 167 (2019) 260e268

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Comparative performance and emission characteristics of peanut seed oil methyl ester (PSME) on a thermal isolated diesel engine a, * €
ur Oztürk Ug , Hanbey Hazar a, Fikret Yılmaz b a b

Department of Automotive Engineering, Technology Faculty, Firat University, Elazig 23119, Turkey Department of Physics, Faculty of Art and Science, Gaziosmanpasa University, Tokat, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 March 2018 Received in revised form 20 October 2018 Accepted 31 October 2018

In this research, cylinder liners of a diesel engine were coated with a 150 mm thick Fe2B layer using boronizing method. The upper surface of the piston was coated with a 300 mm thick CoNiCrAlYttra þ NiCrBSi layer by using plasma spraying method. D-2, biodiesel (PSME-100) and blend (PSME-50) were used as test fuels. PSME fuel was produced through alkali catalyzed transesterification method. As-prepared fuels were used separately in coated (CE) and standard engines (SE) to compare performance and exhaust emission of both engines. In order to see the isolation effect of Fe2B layer, 3-D finite element transient-state analyses were carried out. According to the results, brake thermal efficiency (BTE) and exhaust gas temperature (EGT) of coated engine were considerably enhanced while brake specific fuel consumption (BSFC), CO, HC and smoke emission were decreased compared to those of standard engine. However, NOx emission of CE was higher than that of uncoated one, which was attributed to high combustion temperature and long combustion process in CE. These results were further confirmed with finite element simulation. The decreasing BSFC and increasing BTE for coated engine has been attributed to the elevated temperature of the combustion chamber by the effect of thermal insulation. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Boronizing Isolation Diesel engine Biodiesel Emission Performance

1. Introduction Concerns about the declining fossil fuel stocks, international agreements related to the environmental awareness and demands from the consumers lead automotive manufacturers to develop eco-friendly and less fuel consuming engine technologies. In this sense, low heat rejection (LHR) engines attract attention of researchers because of their low fuel consumption values and less harmful emissions. As known, only one-third of the energy generated as a result of burning of fuel in internal combustion engines is converted to a useful work but a part of the remaining energy is consumed for cooling and the other part is disposed from the exhaust [1e3]. The LHR engine concept aims to create a combustion zone with thermal insulation by coating the combustion chamber elements with low heat conductivity materials. By thermal insulation and prevention of heat transfer to the cooler, the heat reflects back to combustion chamber. In this way, the energy to be

* Corresponding author. Department of Automotive Engineering, Technology Faculty, Firat University, Elazig 23119, Turkey. € E-mail address: [email protected] (U. Oztürk). https://doi.org/10.1016/j.energy.2018.10.198 0360-5442/© 2018 Elsevier Ltd. All rights reserved.

consumed for cooling decreases, after-burning temperature and pressure increase. High temperature and pressure component increase the intensity of the thermal vortex in the combustion chamber. As a result of these intense vortexes, unburned HCs join to the combustion, so the thermal efficiency increases, while the fuel consumption and hazardous exhaust emissions decrease [4e6]. Differences between the combustion characteristics of LHR engines and the conventional engines can be listed as the increasing of gas temperature in-cylinder, reduced ignition delay period, shortening of premixed phase, beginning of the combustion before the sufficient air-fuel mixture, increased diffusive combustion phase and the increase in the total combustion process [7]. According to the second law of thermodynamics, if the heat loss can be reduced, thermal efficiency increases, so the thermal isolation of the combustion chamber can improve thermal efficiency [8]. Consequently, when the combustion chamber elements are coated with a layer which has a low heat transfer coefficient, energy losses can be reduced and surface deformations can be minimized. Many researchers have reported that the thermal insulation of the combustion chamber reduces the heat transfer to coolant, increases thermal efficiency and decreases the fuel consumption and

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Nomenclature and symbol ASTM BP BTE BSEC BSFC CE CO CV EGT FC

American Society for Testing and Materials Brake power Brake Thermal Efficiency Brake Specific Energy Consumption Brake Specific Fuel Consumption Coated Engine Carbon Monoxide Calorific Value of the Fuel Exhaust Gas Temperature Fuel Consumption

hazardous emissions (except NOx) [2,4e6,9]. On the other hand, some researchers have reported that thermal insulation reduces the volumetric efficiency of the engine. However, thanks to the turbocharger, volumetric efficiency can be improved [10,11]. In practical applications, thermal insulating materials are deposited by various methods such as plasma spraying, electron beam physical vapor deposition (EBPVD) and high velocity oxy fuel (HVOF) spraying. In these applications, the coating material does not diffuse into the substrate surface and the layer adheres only to the surface with a bonding strength. Unlike other coating processes, boronizing is a thermo-chemical surface hardening process which has a diffusion mechanism. It enriches the substrate surface by diffusion of boron atoms at elevated temperature. The characteristic feature of the boron layer is that it has a tooth-like structure. The interlocking structure of boronized layer with the base metal provides excellent layer adhesion. There is no possibility of peeling off the layer from the surface since it has a diffusion process. Besides, Fe2B layer also has superior mechanical properties such as high wear resistance, high corrosion resistance at elevated temperatures and high strength, high thermal durability and low thermal conductivity [12,13]. Taking these advantages into consideration, application of boronizing to an engine could provide many benefits beside the thermal isolation. Biodiesel, which is a biodegradable, non-toxic fuel type without sulfur, is named as mono alkyl esters of long chain fat acids produced from vegetable oils or animal fats. At the end of the chemical reactions, ester and glycerin are formed. While ester becomes a fuel, glycerin is used in many sectors as a valuable product. It can be used by mixing with a standard diesel fuel at certain rates [14,15]. Transesterification method, a process of converting an ester into another ester, is a process of re-esterification producing fatty acid esters and glycerin through the reaction of vegetable oils with a monohydric alcohol (methanol, ethanol) accompanied by a catalyst (acidic, basic catalysts or enzymes). Ester mentioned here is a hydrocarbon chain being capable of bonding with another molecule. Glycerin in vegetable oil causes the oil to be thick (high viscosity) and sticky. Glycerol is removed from the triglycerides by transesterification method and the radicals in the alcohol replace them. Due to this process, viscosity decreases, fuel atomization improves, cetane number and calorific value increase [16]. General equation for the transesterification reaction is as follows; RCOOR’ þ R^00 OH 4 RCOOR” þ R’OH (Ester) þ (Alcohol) (catalyst) (Ester) þ (Alcohol)

HC LHR NA NO NOx PSME PSO ppm rpm SE TBC

l

Hydrocarbon Low Heat Rejection Naturally Aspirated Nitric Oxide Nitrogen Oxides Peanut Seed Oil Methyl Ester Peanut Seed Oil Part Per Million Revolution Per Minute Standard Engine Thermal Barrier Coating Relative Air-Fuel Ratio

changes on the engine or by applying various thermal or chemical treatments on raw oils. In this study, effects of both transesterification method to produce biodiesel from raw peanut oil by using alkali-catalyzed and a modification in the diesel engine were studied. The obtained biodiesel was mixed with D-2 fuel at the rate of 50% (v/v). PSME-50, PSME-100 and ASTM D-2 fuels were used as test fuels. Performance and emission values were measured using test fuels in the coated and standard engines. Computer-aided finite element method was used to determine the temperature distribution of both Fe2B coated and uncoated cylinder liners. Additionally, with this study, a coating layer having high thermal resistance and low friction resistance like Fe2B was applied on combustion chamber of a four-cylinder engine for the first time and performance and emission values were examined. 2. Materials and methods 2.1. Coating The cylinder liners and valves of the test engine were coated by using box-boronizing method. Table 1 shows boronizing process parameters. As a result of boronizing process, Fe2B layer reached a thickness of 150 mm on the surface of cylinder liner as shown in a SEM image Fig. 1. The upper surface of the piston was coated using the atmospheric plasma spraying method. Table 2 shows plasma arc spray parameters applied in the coating process. Before the coating, 300 mm chipping was removed from the upper surface of the piston assuming that it would change the compression ratio of the engine. Table 3 shows the thermal and mechanical properties of the coating layers and GG-25 cast iron. Fig. 2 shows an image of all coated pieces. All coated pieces were integrated into the engine and brought to a working condition. 2.2. Biodiesel production For the biodiesel production, crude peanut oil harvested in Turkey was used. Fatty acid composition of peanut seed oil (PSO) is presented in Table 4. The oil/alcohol stoichiometric ratio was 6:1 (v/ v). 1.25 wt.% of the oil was determined as the weight of NaOH. Then,

Table 1 Boronizing process parameters. Treatment

In order for vegetable oils and animal fats to be used as fuel, high viscosity problem should be solved first. For this situation, high viscosity problem is tried to be overcome either by making some

261

Pre-heating Boronizing Cooling in furnace

Boronizing Agent ®

Ekabor 2 Ekabor®2 Ekabor®2

Deoxidant ®

Ekrit Ekrit® Ekrit®

Temperature 

0e950 C 950  C 950-25  C

Time 3h 4h 8h

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Fig. 1. SEM image of Fe2B coating layer of the cylinder liner.

200 ml base catalyst methoxide solution (methyl alcohol þ NaOH) was prepared and added into the reaction. The temperature was  stayed at 55 C and the reaction lasted at 400 rpm for 75 min. Fig. 3 shows separation of biodiesel and waste product glycerin at the reaction-end phase. Biodiesel produced was subjected to washing and vaporization process. So it was completely purified from remaining trace amount of water and methanol. Finally, filtration process was performed using filter paper and ASTM D-2 and PSME fuels were stored by mixing them at a ratio of 50-50% (v/v). Table 5 shows the physicochemical properties of the obtained fuels. 2.3. Performance measurements Experiments were basically classified as standard and coated engine tests. Throughout the tests, an AKSA brand, A4CRX18 model 4-cylinder, in-line type, naturally aspirated, 15 kW/HP, 1500 rpm constant speed engine was used. Fig. 4 shows an image of the experimental set-up, and Tables 6 and 7 present the technical specifications of the test engine and dynamometer, respectively. During the test of the standard and coated engines, ASTM-D-2, PSME-50 and PSME-100 were used as test fuels. Before the measurements, the engine was operated for 10 min to make it stable and the measurements were performed after reaching normal working conditions. The tests were carried out under 4 breaking loads including idling of the engine. 2.3 kW, 4.6 kW and 6.9 kW loads were used as brake powers (BP). All tests were repeated three times. Exhaust gas temperatures were measured using K-type thermocouple. Formula expressions used in the experiments are given in the following equations: BTE ¼BP/[FC*CV]

(1)

BSFC ¼ FC/BP (Kg/kW.h)

(2)

BSEC ¼ BSFC x CV (kJ/kW.h)

(3)

Fig. 2. Image of all coated parts. Table 4 Fatty acid composition of peanut seed oil (PSO). Carbon number

Molecular weight

%

C C C C C C C

256.42 284.48 282.46 280.45 312.53 310.50 340.58

10.48 3.18 51.56 30.39 1.2 1.9 0.95

16:00 18:00 18:01 18:02 20:00 20:01 22:00

Fig. 3. Reaction-end phase separation, biodiesel and glycerin.

Table 2 Plasma spraying parameters. Plasma gun

Sulzer-Metco 3 MB

Current (Ampere) Voltage (Volt) Spray distance (mm) Gas pressure (psi) (Ar/H2) Applied coating thickness (mm)

500 62 70 80/15 250 mm (NiCrBSi)þ50 mm (CoNiCrAlYttra) ¼ 300 mm

Table 3 Thermal and mechanical properties of the parts. Materials

Thermal conductivity (W/mK)

Density (kg/m3)

Specific heat (J/kgK)

GG-25 Fe2B AlSi12CuNi NiCrBSi

45 25 155 15

7200 7430 2700 8000

510 1386 960 452

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Table 5 Physicochemical properties of fuels and their blends. Properties

Peanut seed oil

PSME-50

PSME-100

Diesel (ASTM-D:2)

Standard

Density (kg/m3)  Kinematic viscosity at 40 C (cst) Calorific value (kJ/kg)  Flash point ( C) Cetane index

895 22.75 39200 198.4 e

861 3.758 41725 109.6 52,20

885 5.166 39879 186 53,40

850 3.05 42800 56 49e55

ASTM D 1798 ASTM 445 ASTM D 240 ASTM D 93 ASTM-D 4737

2.4. Emission measurements Emission and smoke measurements were performed using a Bosch BEA-350 model exhaust emission device. Table 8 shows the measurement range, the resolution of the measurements and the uncertainty of the computed results of the tested parameters. The uncertainties in the measurements of the parameters are evaluated using the standard procedure described in references and ASME PTC 19.1e2005 [17,18]. The overall experimental uncertainty was computed according to the principle of propagation of errors as ± 3.06% and is presented as follows: Overall experimental uncertainty ¼ square root of [(uncertainty of BP)2 þ (uncertainty of BSFC)2 þ (uncertainty of BSEC)2 þ (uncertainty of BTE)2 þ (uncertainty of 2 2 CO) þ (uncertainty of HC) þ (uncertainty of NO)2 þ (uncertainty of Smoke Opacity)2 ] ¼ 3.06%.

Fig. 4. Image of the experimental set-up.

Table 6 Technical specifications of the test engine.

2.5. Numerical analysis of transient-state temperature distribution field

Manufacturer

AKSA

Fuel type Model Number of cylinders and build Aspiration and cooling Maximum standby power (kW (1500 rpm)) Total displacement (L) Bore and stroke (mm) Compression ratio Rated speed (rpm) Governor Oil capacity (L) Coolant capacity (L) Intake air flow (m3/min) Radiator cooling air (m3/min) Exhaust gas flow (m3/min1) Start system (D.C.) Injection type and system Injection timing

Diesel A4CRX18 4 Cylinders, In Line Naturally aspirated 15 1.808 80  90 18.0:1 1500 Mechanical 5.00 9.00 2.00 120.00 5.80 12 V Direct 17 bTDC

Table 7 Technical specifications of the dynamometer. Manufacturer

AKSA

Alternator Made and Model Frequency (Hz) Power (kVA) Design Voltage (V) Phase A.V.R Voltage Regulation (±) Insulation System Rated Power Factor Cooling Air (m3/min)

AK 113 50 16.3 Brushless 400 3 SX460 1.5% H 0.8 4.26

Computer-aided numerical simulations of thermal analyses are very useful and effective ways to determine the temperature distribution of materials under varying conditions. In order to see the effect of the coating layer on the heat transfer and temperature distribution, 3-D finite element transient-state, isotropic linear elastic model was used for analyses on both Fe2B coated and standard cylinder liners. In the model, surface to surface contact elements were defined between Fe2B layer and cylinder liner. Analyses were performed using Solidworks® premium 2016 simulation package produced by Dassault systemes. 3. Results and discussion 3.1. Transient-state temperature distribution analysis In the working media of combustion chamber in a diesel engine, the average temperature of burned gas is 600e800  C and con vection coefficient is 700e800 W/m2 C [19]. In this study, thermal   boundary conditions of cylinder liner were 700 C, 700 W/m2 C,  initial and ambience temperature was 25 C and total time period of analysis was 90 s. The heat transfer in the oil film was neglected. Table 9 shows the meshing data and Fig. 5 shows the finite element mesh of the cylinder model used in the analysis. The coefficient of thermal conductivity is a very important factor to estimate the temperature distributions and heat flows on the materials [20]. Besides, the thickness of material has an influence on the heat transfer. As shown in Table 3, the thermal conductivity coefficient of the Fe2B layer was about twice lower than that of GG25 gray cast iron. Also, the thickness of the Fe2B layer was 0.15 mm and cylinder liner was 1.85 mm. According to the transient-state analysis results, at the end of the 90 s time period, the maximum and minimum surface temperatures of standard cylinder was

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Table 8 Resolution of the measurements and the uncertainty of the computed results. Component

Measurement Range

Resolution

% Uncertainty

CO CO2 HC O2 Lambda NO Smoke Opacity (Degree of opacity) Computed BP BSFC BSEC BTE

0.00e10.00% vol. 0.00e18.00% vol. 0e9999 ppm 0.00e22.00% vol. 0.500e9.999 0e5000 ppm 0e100%

0.001% vol. 0.01% vol. 1 ppm 0.01% vol. 0.001 1 ppm 0.1%

±0.01 ±0.05 ±0.01 ±0.04 ±0.0001 ±0.02 ±0.1

e e e e

±0.106 kW ±0.006 kg/kWh ±270.77 kJ/kWh ±0.003

±1.54 ±1.57 ±1.58 ±1.43

Table 9 Meshing data of cylinder model. Mesh type

Solid Mesh

Jakoben points Element size Tolerance Total node Total element Maximum aspect ratio

4 4.66242 mm 0.233121 mm 17165 50811 34.391

Fig. 6. The temperature distributions and time-temperature diagrams of standard and coated cylinders.

Fig. 5. Finite element mesh of the cylinder model.

697.843 and 687.405  C, respectively. The maximum temperature of the Fe2B coated cylinder was 695.120 and 680.983  C, respectively. The temperature distributions and time-temperature diagrams on the standard and Fe2B coated cylinders are shown in Fig. 6. Maximum temperature points were in the upper regions of the cylinders while the minimum temperatures were at the skirts of the cylinders. These temperature differences were caused by the wall thicknesses of the cylinders. Due to the increasing of wall thickness, the temperature distribution rate and heat transfer capacity reduced. It appeared that the standard cylinder started to heat up quickly (Fig. 6). At 26th seconds, the temperature over the  coated cylinder reached at 500 C, while the standard cylinder had a  temperature of 560 C. A temperature difference of 11% was observed between the coated and standard surfaces. The same pattern was observed up to 42 s. After the 58th second, almost stability was reached over the surface temperatures. Even at 90th second, there was no exact equilibrium. Thermal isolation properties of materials are proportional to the thermal diffusivity. As the thermal conductivity coefficient of the materials decreases, the heat transfer capacities become weaker [20,21]. 3.2. Brake specific fuel consumption (BSFC) Fig. 7 illustrates the variation in BSFC of SE and CE at 2.3 kW,

4.6 kW and 6.9 kW brake power when fuelled with D-2 and PSME blends. It was observed that CE had a decrease in BSFC for all fuels. CE operated with D-2 showed 9, 11, 2.5% reduction in BSFC with respect to the load, respectively. On the other hand, reduction percentages of BSFC were 16, 8 and 2.5% based on the loads for PSME-50 and 5, 10 and 30% based on the loads for PSME-100, respectively. Fuel consumption in both engines tended to decrease for second load of 4.6 kW. This situation was due to the proper air-fuel mixture and adequate time for combustion. PSME-50 and PSME-100 fuels exhibited much more BSFC than that of D-2 fuel. It can be result of weak physicochemical properties of blends such as low calorific value, high density and viscosity [22]. Beside, CE showed a decrease in BSFC for all fuels as compared to SE. Ignition delay period is an important factor affecting fuel consumption and it is directly related to temperature in-cylinder. In our case, the insulation of the combustion chamber elements, such as cylinder liner, piston surface and valves prevented the heat transfer to the coolant and the heat was reflected back to the combustion chamber. The trapped heat energy resulted an increasing in gas temperature in-cylinder. An increase in temperature accelerated the combustion process of the atomized fuel-air mixture, led to a shorter combustion reaction time and improves energy conversion rate. Thus, lower amount of fuel was needed to produce the same output power, resulting in a decrease in BSFC of coated engine [2,10,23]. On the other hand, combined effect of elevated temperature and intense vortices caused more unburned HC into reaction. The reduction in HC and CO emissions is an indicator of the increase in combustion efficiency. The HC and CO emission results of our study were in good agreement with the

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proportional to fuel consumption as given in Eq. (3). As mentioned above, reduced heat transfer by the thermal insulation resulted in decrease in fuel consumption and improved energy conversion rate. Thus, BSEC values decreased in coated engine for all fuels. This difference was more apparent in blended fuels. It could be due to the fact that the rich oxygen content of the fuels improved the combustion [10,27,28]. 3.4. Brake thermal efficiency (BTE)

Fig. 7. BSFC values of CE and SE for D-2, PSME-50 and PSME-100 fuels.

decrease in BSFC values. Many studies have reported that elevated temperature in-cylinder results in decline in BSFC values and increase in combustion efficiency [24,25].

3.3. Brake specific energy consumption (BSEC) Fig. 8 shows the variation of BSEC of SE and CE for D-2 and PSME blends with respect to the brake powers. The coated engine operated with D-2 alone showed 8, 10 and 3% reduction in BSEC with respect to the brake powers, respectively. On the other hand, BSEC reduced by 15, 8 and 6% for PSME-50 fuel and 6, 11 and 29% for PSME-100 fuel, respectively. PSME-50 and PSME-100 fuels had high BSEC values compared to that of D-2 fuel in both engines. Calorific value of a fuel is an important parameter for calculation of BSEC. As can be seen in Table 5, PSME and blends had lower calories than D-2 fuel. To achieve the same power ratio at the output shaft, the engine needs to consume much more biodiesel fuel because of its low calorific value and other poor characteristics such as high density and viscosity. So, biodiesel fuels exhibited high BSEC values compared to D-2 [26]. As seen in Fig. 8, BSEC values of CE were lower than those of SE. In terms of comparing the engines with respect to BSEC values, the calorific value and brake power were constant data, whereas fuel consumption value was variable parameter in computing. It means that BSEC was directly

Fig. 8. BSEC variations in CE and SE for D-2, PSME-50, and PSME-100 fuels.

Fig. 9 presents the increase of BTE with increase in load for test fuels in both coated and standard engine. It is remarkable point that coated engine exhibited higher BTE than that of standard engine at all conditions. In comparison with the standard engine, the coated engine operated with D-2, PSME-50 and PSME-100 showed 10, 10 and 14% increase in BTE at 6.9 kW of brake power, respectively. The purpose in LHR engines is to improve the thermal efficiency by preventing the heat that will go to the coolant. The combination of calorific value and weight (mass) flow rate is the determinant factors for the energy amount entering the engine. In addition, the viscosity difference between the fuels also affects the efficiency with its effect on fuel atomization. In coated engine, the heat transfer was reduced to the coolant and thus, temperature distribution and heat release were controlled due to the thermal resistance on the surfaces. Many researchers have reported an improvement in BTE due to reduced coolant heat losses [11,29]. In addition, the result of transient-state temperature distribution analysis that we performed supported this discussion. Prevention of the heat which went to the coolant increased production of waste heat in the combustion chamber of the engine and thus elevated the temperature. The elevated temperature shortened the ignition delay and facilitated the vaporization of fuel droplets and resulted in an increase in engine power [5]. Beside, BTE of PSME blends was lower than that of D-2 fuel under 1st and 2nd load values. As can be seen in Table 5, the results may be related to lower calorific value and higher viscosity value of PSME blends compared to D-2. Fuel properties such as high viscosity negatively affect the air-fuel mixture and cause poor atomization during combustion, reducing the quality of the burning. Thus, combustion and thermal efficiency could decrease [30,31]. 3.5. Exhaust gas temperature (EGT) EGT measurements are a significant parameter that helps us to

Fig. 9. BTE variations of D-2, PSME-50, and PSME-100 fuels in CE and SE.

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have information about the after-burning temperature and fuel characteristics. Fig. 10 shows the variations of EGT values for both engines and fuels. As seen in Fig. 10, EGT values increased in parallel with increasing load for both fuels and engines. This situation can be explained by the fact that the injection pressure increased with increasing load developed fuel atomization and combustion efficiency increased with increasing air fuel ratio [32]. EGT value of D-2 fuel in CE increased at the rate of 4% under the 1st load, 13% under the 2nd load and 6% under the 3rd load compared to SE. The average rate of this increase was 7.5%. This increase rates for PSME50 and PSME-100 fuels were 7 and 11% under the 1st load, 18 and 17% under the 2nd load and 11 and 9% under the 3rd load, the average being 12 and 13%, respectively. The adiabatic medium formed by thermal barrier coating decreased the heat transfer to the cooler, reflected the heat back to the combustion chamber and thus increased EGT values. Low heat conduction of the TBC material increased the heat distribution in the combustion chamber [33]. Another remarkable result was that PSME blend fuels had higher EGT value compared to D-2. This can be associated with the fact that extra oxygen content of biodiesel mixture fuel increased the combustion efficiency and thus increased EGT values [10,34]. 3.6. CO emission

Fig. 11. CO variations of D-2, PSME-50, and PSME-100 fuels in CE and SE.

temperature of the combustion chamber retained the combustion process of the fuel in the exhaust phase. Therefore, CO emission decreased for all fuels compared to SE [5,33]. In other words, CO decreased due to the high combustion efficiency in LHR engine. Since high oxygen content improved the combustion, CO emission was low in biodiesel blended fuels compared to SE [35].

CO emission is a result of incomplete combustion during the combustion of petroleum fuels. Air-fuel ratio, in-cylinder temperature, the compression and the amount of oxygen are important factors affecting the CO emission volume of the combustion reaction [5,35]. Fig. 11 shows the variations of CO emissions of D-2 and PSME-50 and PSME-100 fuels in CE and SE. CO emission tended to increase in both engines and fuels along with increasing load and this increase was caused by the fact that fuel cannot find sufficient time for combustion in the cycle. When the engines were compared in terms of fuels, PSME blend fuels produced lower CO emission in both engines. It is reported that CO emission is a result of incomplete combustion and lack of oxygen in the combustion chamber increases CO emission. A 10% O2 (by mass) in the structures of biofuels makes the combustion environment oxygen-enriched [36,37]. Therefore, the addition of PSME fuel into D-2 fuel supported and improved the complete combustion. Additionally, as shown in Fig. 11, the CO emission in CE showed a significant decrease compared to SE. There was a reduction of 13% for D-2 fuel under full load value. While a decrease of 28 and 25% occurred for PSME-50, PSME-100 fuels respectively, under full load. Due to the thermal insulation in CE, post-compression temperature increased, blend front level decreased and then high temperature accelerated CO oxidation in the late combustion phase. High

HC emission is an organic compound formed as a result of incomplete combustion. Unburned HC amount depends on the operation conditions and fuel properties of the engine. Fig. 12 shows variations in HC emissions of D-2, PME-50 and PSME-100 fuels in CE and SE. HC emission values increased in both engines along with increasing load. However, HC emission values of the biodiesel blended fuels were lower in both engines compared to D2 fuel. This finding was associated with the fact that presence of the oxygen increased the combustion efficiency. Also, the change in the fuel stoichiometry and high cetane index of biodiesel fuel shortened the ignition delay [9]. Many researchers have reported lower HC emission for LHR engine [27]. The decrease rate in HC emission for D-2 fuel in CE was 53% under maximum load. These rates were 52 and 45% for PSME-

Fig. 10. EGT values of CE and SE for D-2, PSME-50 and PSME-100 fuels.

Fig. 12. HC variations of D-2, PSME-50, and PSME-100 fuels in CE and SE.

3.7. HC emission

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50 and PSME-100 fuels, respectively. As reported in literature, extinction period shortens and combustibility limits increases in LHR engines. Also, excessive amount of oxygen in biodiesel fuel provides a complete combustion [16,38]. In our case, high temperature of the both in-cylinder and combustion wall accelerated the oxidation reactions and allowed the combustion process to end in a shorter time, resulting in a decrease of HC emission. 3.8. NOx emission NOx is a hazardous emission resulting from the oxidation of the atmospheric nitrogen entering into chain reactions when the after burning combustion temperature exceeds 1600 C. NOx formation mechanisms are named as Zeldovich kinetics [5,39]. The most basic factor in the formation of NOx is the high temperature. However, engine load, combustion chamber material, combustion chamber homogeneity and mixture density are also important parameters [40]. Fig. 13 shows variations in NOx emissions of both engines and fuels. As can be seen in Fig. 13, NOx emission increased in parallel with the increasing load for both engines and all three fuels. The NOx increase in CE was 11, 31 and 31% under maximum load for D2, PSME-50, PSME-100 fuels, respectively. NOx emission in CE was higher under all loads and for all fuels compared to SE. An increase of NOx emission in CE due to increasing after-burning temperature is an expected outcome. The increases in the exhaust gas temperature in the present study also confirmed this situation. The thermal isolation of the combustion chamber caused the heat reflecting back to the combustion chamber, resulting in an increase of after-burning temperature. Thus, higher combustion temperature and longer combustion process in CE increased the NOx emission. Engine load, speed, combustion chamber content, homogeneity and mixture density are important parameters in the formation of NOx. When incylinder combustion temperature extends 1600  C, heat distribution rate improves, the ignition delay shortens and N molecules begin to take part in the reaction. If N molecules can find sufficient reaction time, NOx compounds are formed [40]. The increase observed in the PSME-50 and PSME-100 fuels was caused by the fact that there was extra oxygen in the biodiesel and this oxygen increased the amount and rate of oxidation with nitrogen [5,27,29]. 3.9. Smoke density Even though the actual air-fuel ratio (A/F) in internal combustion engines is higher than the theoretical complete combustion

Fig. 14. Smoke variations of D-2, PSME-50 and PSME-100 fuels in CE and SE.

value, the fuel droplets in the cylinder cannot find enough air around them. This causes the formation of smoke (carbon particles) as a product of incomplete combustion. The structure of smoke varies chemically and physically according to the position in the flame. Fig. 14 shows the variations in smoke density of both engines. Smoke emission increased along with load in both engines and both fuels. When the amount of incoming fuel increased along with the increased load, sufficient air could not be provided and thus the amount of smoke formed as a result of rich mixture also increased proportionally. The decrease rate in smoke emission in CE was 15% for D-2 fuel under maximum load. Compared to SE, decrease rates in smoke emission were 30 and 49% for PSME-50 and PSME-100 fuels under maximum load, respectively. As can be seen in Fig. 14, CE produced lower smoke emission with all fuel types. In addition, PSME-50 and PSME-100 fuels had lower smoke emission compared to D-2 fuel. Biodiesel fuels and their mixtures had very important positive effect on smoke emission due to their better combustion properties and the extra oxygen content in their structures. In LHR engines, biodiesel fuels exhibit lower smoke emissions due to the better atomization and vaporization at higher temperatures [29]. However, the intense turbulence occurring due to the effect of the high temperature and pressure caused more C to enter into reaction, increasing smoke oxidation. On the other hand, poor atomization and vaporization of the biodiesel improved due to the high temperature of LHR and consequently smoke emission decreased, which is in agreement with other studies [27,29].

4. Conclusions In the present study, for the first time, the boronizing was used as a coating method to insulate the engine parts. A 150 mm thick Fe2B layer was deposited on the surface of cylinders and valves. In order to see the effect of the boronized layer (Fe2B) on heat transfer and temperature distribution, 3-D finite element transient-state, isotropic linear elastic model was used for analyses on cylinder model. In addition, performance and emission analyzes were carried out on coated and standard engine using D-2, PSME-50 and PSME-100 fuels and the results were compared. The conclusions are summarized as follows:

Fig. 13. NOx variations of D-2, PSME-50 and PSME-100 fuels in CE and SE.

 SEM image showed that boron was successfully diffused to the substrate to form a 150 mm thick Fe2B layer.

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 Application of the boronizing on the metallic surfaces provided superior thermal resistance. 3-D finite element analyses indicated that Fe2B layer had a good thermal isolation property compared to standard GG-25 cylinder.  The coated engine showed better results in performance parameters such as BTE, BSEC, BSFC compared to the standard engine. Poor properties of biodiesel could be tolerated using coated engine.  Compared to standard engine, BTE at 6.9 kW for D-2, PSME-50 and PSME-100 was 10, 10 and 14% higher in coated engine, respectively.  The coated engine reduced the BSFC by 2.5, 2.5 and 30% at full load for D-2, PSME-50 and PSME-100 fuels, respectively.  The coated engine reduced the hazardous emissions (except NOx) for all fuels.  For D-2 fuel, emissions of CO and HC and smoke density were 13, 53 and 15% lower, respectively, whereas NOx was 11% higher in coated engine.  Decreases in CO and HC emissions and smoke density in coated engine were 28, 52 and 30% for PSME-50 fuel, and 25, 45 and 49% for PSME-100 fuel, whereas NOx emission increase was 31% for both fuels. Scope for future work. Based on the results of the present study, thermal insulation of a diesel engine by boronizing could be a promising method in automotive industry to improve engine performance and decrease emission levels. Application of boronizing to an engine could provide many benefits such as long term service life, higher thermal durability and higher oxidation resistance at elevated temperatures as well as thermal isolation. So, boronizing as a coating method in automotive industry merits further studies. Especially, investigations evaluating the volumetric efficiency, heat release variation, cylinder pressure characteristics of direct injection diesel engine with test fuels are needed. Acknowledgements The authors would like to thank the Firat University for the financial support of investigations. This work has been sponsored by Firat University Scientific Research Projects Management Unit under the Grant numbers: TEKF.13.03. References [1] Li B, Li Y, Liu H, Liu F, Wang Z, Wang J. Combustion and emission characteristics of diesel engine fueled with biodiesel/PODE blends. Appl Energy 2017;206:425e31. [2] Musthafa MM. Development of performance and emission characteristics on coated diesel engine fuelled by biodiesel with cetane number enhancing additive. Energy 2017;134:234e9. [3] Dhinesh B, Raj YMA, Kalaiselvan C, KrishnaMoorthy R. A numerical and experimental assessment of a coated diesel engine powered by highperformance nano biofuel. Energy Convers Manag 2018;171:815e24. [4] Hejwowski T. Comparative study of thermal barrier coatings for internal combustion engine. Vacuum 2010;85(5):610e6. [5] Hazar H. Characterization and effect of using cotton methyl ester as fuel in a LHR diesel engine. Energy Convers Manag 2011;52(1):258e63. [6] Garud V, Bhoite S, Patil S, Ghadage S, Gaikwad N, Kute D, et al. Performance and Combustion characteristics of thermal barrier coated (YSZ) low heat rejection diesel engine. Mater Today: Proceedings 2017;4(2):188e94. [7] Sun X, Wang W, Bata R, Gao X. Performance evaluation of low heat rejection engines. J Eng Gas Turbines Power 1994;116(4):758e64. [8] Shrirao PN, Pawar AN, Borade A. An overview on thermal barrier coating(TBC) materials and its effect on engine performance and emission. Int Rev Mech Eng 2011;5(5):973e8. [9] Aydin H. Combined effects of thermal barrier coating and blending with diesel fuel on usability of vegetable oils in diesel engines. Appl Therm Eng 2013;51(1e2):623e9.

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