Impact of antioxidant additives on the performance

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Jun 23, 2017 - free fuel, it results in complete combustion and increased emission of ... calorific value of biodiesel is less than that of diesel fuel, whereas ... added antioxidants can be used in diesel engines with no .... Methyl oleate. 18:1. 296.49. CH3(CH2)7CH=CH(CH2)7CO2CH3. 42.74 ... Molecular weight (g/mol).
Impact of antioxidant additives on the performance and emission characteristics of C.I engine fuelled with B20 blend of rice bran biodiesel Karthikeyan Alagu, Beemkumar Nagappan, Jayaprabakar Jayaraman & Anderson Arul GnanaDhas Environmental Science and Pollution Research ISSN 0944-1344 Environ Sci Pollut Res DOI 10.1007/s11356-018-1934-1

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Author's personal copy Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-1934-1

RESEARCH ARTICLE

Impact of antioxidant additives on the performance and emission characteristics of C.I engine fuelled with B20 blend of rice bran biodiesel Karthikeyan Alagu 1 & Beemkumar Nagappan 1

&

Jayaprabakar Jayaraman 1 & Anderson Arul GnanaDhas 1

Received: 30 December 2017 / Accepted: 3 April 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract This manuscript presents the impact of addition of antioxidant additives to rice bran biodiesel blend on the performance and emission characteristics of compression ignition (C.I) engine. Rice bran methyl ester (RBME) was produced from rice bran oil by transesterification using sodium hydroxide as catalyst. An experimental investigation was conducted on a single-cylinder fourstroke C.I engine to analyze the performance and emission characteristics of rice bran methyl ester (RBME) blended with diesel at 20% by volume (B20) with and without addition of 1000 ppm of two monophenolic antioxidant additives, butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT). The results showed that the BHA- and BHT-treated B20 blend decreased the brake specific fuel consumption (BSFC) by 2.1 and 1.2% and increased the brake thermal efficiency (BTE) by 1.04 and 0.5% compared to B20. The BHA- and BHT-treated B20 blend produced mean reductions in NOx emission of 12.2 and 9.6%, respectively, compared to B20. The carbon monoxide (CO) and hydrocarbon (HC) emissions of BHA- and BHT-treated B20 were increased by 14.8–16.6% and 10.6–11.2%, respectively, compared to B20. However the emission levels were lower than those of diesel. Keywords Antioxidant additives . Rice bran biodiesel . Emission . Performance . Butylated hydroxyanisole . Butylated hydroxytoluene

Nomenclature ASTM American society of testing and materials IV Iodine value BHA Butylated hydroxyanisole NOx Nitrogen oxides BHT Butylated hydroxytoluene PM Particulate matter BP Brake power RBME Rice bran methyl ester BSFC Brake specific fuel consumption RBO Rice bran oil BTE Brake thermal efficiency SN Saponification number C.I Compression ignition

Responsible editor: Philippe Garrigues * Beemkumar Nagappan [email protected] 1

School of Mechanical Engineering, Sathyabama Institute of Science and Technology, Chennai, India

B100 CO B20 EGT B20 + BHA HC B20 + BHT

Rice bran biodiesel/RBME Carbon monoxide 20%RBME+80%Diesel Exhaust gas temperature 20%RBME+80%Diesel+1000 ppm BHA Hydrocarbon 20%RBME+80%Diesel+1000 ppm BHT

Introduction Biodiesel is a renewable and clean burning alternative fuel for diesel engine. It can be produced from edible and non-edible vegetable oils, algae biomass, and animal fats [Hasan Khondakar Rashedul et al. (2017)]. These oils/fats are basically triglycerides which have higher viscosity, and this can be reduced by transesterification reaction, in which triglycerides are converted into esters. The esters contain biodiesel and glycerin and the later one is removed as by product [Fazal et al. (2011)]. Since biodiesel is an oxygenated and sulfurfree fuel, it results in complete combustion and increased emission of nitrogen oxides (NOx) compared to conventional

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diesel fuel. However, the particulate matter (PM), carbon monoxide (CO), and total hydrocarbon (HC) emissions are lesser than diesel [Balaji and Cheralathan (2015a, b)]. The calorific value of biodiesel is less than that of diesel fuel, whereas viscosity, density, pour point, flash point, and cetane number are higher than diesel [Syed Ameer Basha and Raja Gopal (2012), Rashedul et al. (2014), Yuvarajan Devarajan et al. (2017)]. The main limitation in the use of biodiesel blends is its poor oxidative stability [Rosen Dinkov et al. (2009)]. Biodiesels reacts with oxygen in the atmosphere to produce vaporizable compounds and caustic carboxylic acids which may cause damage to the engine components. To forbid the premature oxidation of unsaturated biodiesel esters, antioxidant additives are used with biodiesel, keeping them fresh and increasing their shell life [Misraa and Murthy (2011), Kivevele et al. (2011)]. Several studies show that by adding antioxidant additives, the oxidation and NOx levels are considerably reduced [Hasan Khondakar Rashedul et al. (2017), Yuvarajan et al. (2017a, b)]. Biodiesel oxidation is affected by a variety of factors, including the composition of the fuel itself and conditions of storage [Gerhard Knothe (2007), Senthil Ramalingam et al. (2018), Rashed et al. (2016a, b)]. Many researchers studied the impact of antioxidants on the performance and emission characteristics of diesel engine fuelled with biodiesel blends. Sathiyamoorthi and Sankaranarayanan [2016] studied the effect of addition of two antioxidants BHA and BHT at 2000 ppm with lemongrass oil-diesel blend (LGO25) on engine performance and emission characteristics and concluded that the antioxidant additives exhibited an increase in brake thermal efficiency (BTE) and decrease in exhaust gas temperature (EGT) and specific fuel consumption (BSFC) and reduction of NOx emission. BHA antioxidant additive exhibited a better stability than BHT and provided a maximum NOx reduction of 11% than LGO25 without any antioxidant additives. Thus, LGO25 with added antioxidants can be used in diesel engines with no modification. Rizwanul Fattah et al. [2014a, b, c] studied the effect of addition of two antioxidant additives BHA and BHT at 1000 ppm to palm biodiesel blend (B20) on engine performance and emission characteristics; this study reveals that the reduction of NOx emissions is by 12.6%. Varatharajan and Cheralathan [2013] conducted experimental study on the addition of p-phenylenediamine-derived aromatic amine antioxidants in a soybean biodiesel and experienced the significant reduction of NOx and slight increases in smoke, CO, and HC emissions. Ileri and Kocar [2013] concluded that the addition of 2-ethylhexyl nitrate (EHN) with B20 (20 vol.% canola oil methyl ester and 80 vol.% diesel fuel blend) produced the best mean oxides of nitrogen (NOx) reduction of 4.63% among the of four synthetic antioxidants, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertbutylhydroquinone (TBHQ), and 2-ethylhexyl nitrate (EHN). Balaji and Cheralathan [2015a, b] found that the

addition of antioxidant additive (A-tocopherol acetate) in various proportions (100 to 400 ppm) with methyl ester of neem oil was effective in increasing the oxidation stability and reducing the NOx emissions at the expense of slight increases in HC, CO, and smoke emissions. Velmurugan and Sathiyagnanam [2016] analyzed the experimental exploration of the three antioxidants DEA (diethylamine), PHC (pyridoxine hydrochloride), and TBHQ (tert-Butylhydroquinone) on emission and performance of a single-cylinder diesel engine fuelled with methyl ester of mango seed. The experiment was conducted with different antioxidant concentrations of mango seed methyl este r m ixtures (100, 250, 50 0, and 1000 ppm).The results exhibited that PHC is effective in controlling NOx emissions than TBHQ and DEA. Many researchers showed that the addition of antioxidant was a prospective solution for controlling NOx emission of biodieselfuelled diesel engines [Varatharajan et al. (2011), Jiafeng Sun et al. (2010), Rizwanul Fattah et al. (2014a, b, c)]. The objective of this work is to study the impact of antioxidant additives (BHA, BHT) on the performance and emission characteristics of a single-cylinder diesel engine fuelled with rice bran biodiesel blend. With reference to the literatures studied, no considerable experimental studies have been reported on the effect of addition of antioxidant additives to rice bran biodiesel blend. The antioxidant additives were used at a concentration of 1000 ppm, which provided high induction periods compared to other concentrations. The main reason for adding antioxidant to biodiesel is to further improve its oxidation stability and to reduce NOx emission. BHA and BHT were chosen as they are inexpensive and commercially available. The performance and emission characteristics were analyzed and compared with B20 without antioxidants and diesel.

Materials and methods Biodiesel production Rice bran oil (RBO) was obtained from the local market, Chennai, Tamil Nadu, India. Since free fatty acid (FFA) of RBO is 3.4%, base catalyst transesterification is preferred. Methanol and the base catalyst NaOH pellets were used for the transesterification process. A batch reactor of 5 l capacity equipped with a condenser, a magnetic stirrer with an installed tachometer, a thermometer pocket with a thermocouple, and a stopper to remove samples was used to produce biodiesel from RBO. The reaction temperature was maintained within ± 0.1° by means of a constant temperature heating mantle. Recommended quantity of RBO was preheated in the batch reactor. The catalyst NaOH of 5 g quantity and the oil to methanol molar ratio of 1:7.42 were mixed together in a conical flask. The solution in the conical flask was transferred to

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the batch reactor containing preheated RBO after reaching the required reaction temperature of about 60 °C. The reaction took place for 90 min. After the reaction, the solution was poured into a separating funnel and allowed to settle for 12 h, and then the lower glycerol layer was removed. A rotary evaporator was used to remove the residual methanol. The obtained biodiesel crude was then washed several times with warm deionized water to bring the pH value of 7 which is equal to the pH value of distilled water.

Preparation of test fuels Three test fuels were prepared, rice bran biodiesel (B100) blended with diesel at 20% by volume (B20), B20 with BHA at concentration of 1000 ppm (B20 + BHA), and B20 with BHT at concentration of 1000 ppm (B20 + BHT) using an ultrasonicator at 2000 rpm for 30 min. Then, the physical properties of the fuels were determined as per the ASTM standards.

Properties of test fuels The properties of diesel, B100, B20, B20 + BHA, and B20 + BHT were measured and compared with ASTM biodiesel standards. The tested properties were found to be having an agreement with ASTM standards. Table 1 shows the properties of fuels and accuracy value of various equipments. The oxidation stability of B20 and B20 with antioxidants was determined using the Rancimat method (accelerated oxidation test). The saponification number (SN), iodine value (IV), and cetane number of biodiesel was calculated using Eqs. 1, 2, and 3, respectively [Rizwanul Fattah et al. (2014a, b, c)]:  SN ¼ ∑

Table 1

560*Ai MWi

 ð1Þ



 254*D*Ai IV ¼ ∑ MWi     5458 CN ¼ 46:3 þ −ð0:225  IV Þ SN

ð2Þ ð3Þ

where Ai is the percentage of each component, D is the number of double bonds, and MWi is the mass of each component. Infrared studies were used to study the presence of fatty acid methyl ester. Gas chromatography (GC) mass spectroscopy analysis was used to determine the fatty acid composition of rice bran methyl ester. From the GC, it was identified that there were various esters present in the mixture from 0 to 20 min of injection time. Table 2 illustrates the fatty acid esters of the tested RBME with their molecular mass and percentage of each component. The properties of BHA and BHT are presented in Table 3.

Experimental setup The experiment was carried out in the I.C engine laboratory of the Mechanical Engineering Department, Sathyabama Institute of Science and Technology, Chennai, India. The experimental setup consists of a single-cylinder, four-stroke, VCR (variable compression ratio) diesel engine connected to an eddy current type dynamometer for loading. Figure 1 shows the layout of the experimental setup. Setup is provided with a U-tube manometer with an orifice fitted with an air tank on the suction line that measures air consumption. The exhaust gas temperature was measured by a K type thermocouple. The fuel consumption was measured by means of a graduated glass burette connected with the fuel tank. The setup has the provision to interface airflow, fuel flow, temperatures, and load measurement. The setup was interfaced with a lab viewbased engine performance analysis software package BEngine soft^ for on line performance evaluation. The exhaust emissions were measured using AVL Digas 444 analyser. The

Properties of fuels

Properties

Diesel

B100

B20

B20 + BHA

B20 + BHT

ASTM standard

Test method

Accuracy

Calorific value (kJ/kg) Density at 15 °C (kg/m3) Flash point (°C) Cloud point (°C) Pour point (°C) Kinematic viscosity at 40 °C (cst) Saponification number Iodine value Oxidative stability (h) Cetane number Latent heat (kJ/kg)

43,350 828 68 8 7 3.25 – – 58 48 250

37,054 885 162 3.3 2.5 4.54 192 68 9 58 272

41,776 839 88 3 2.2 3.92 – – 20 52 266

41,676 841 90 3 2.2 3.93 – – 35 54 257

41,646 841 92 3 2.2 3.94 – – 28 53 260

– 860–900 130 minimum – – 1.9–6 – 120 3 minimum 47 minimum –

ASTM D240 ASTM D1298 ASTM D93 ASTM D97 ASTM D97 ASTM D445 – – – – –

± 0.1% ± 0.1 kg/m3 ± 0.1 °C ± 0.1 °C ± 0.1 °C ± 0.35% – – – – –

Author's personal copy Environ Sci Pollut Res Table 2 Fatty acid composition of RBME

Fatty acid ester

Structure

Molecular mass (Mi)

Formula

% (Ai)

Methyl myristate

14:0

242.4

Methyl palmitate Methyl palmitoleate Methyl stearate Methyl oleate Methyl linoleate Methyl linolenate Methyl arachidate Methyl eicosenoate Methyl behenate

16:0 16:1 18:0 18:1 18:2 18:3 20:0 20.1 22.0

270.45 268.43 298.5 296.49 294.47 292.46 326.56 324.54 354.61

CH3(CH2)12COOCH3 CH3(CH2)14CO2CH3 CH3(CH2)5CH=CH(CH2)7 COOCH3 CH3(CH2)16CO2CH3 CH3(CH2)7CH=CH(CH2)7CO2CH3 CH3(CH2)3(CH2CH=CH)2(CH2)7CO2CH3 CH3(CH2CH=CH)3(CH2)7COOCH3 CH3(CH2)18COOCH3 CH3(CH2)7HC=CH(CH2)9CO2CH3 CH3(CH2)20COOCH3

0.36 16.37 0.30 2.20 42.74 34.19 1.39 0.73 0.64 0.29

Methyl lignocerate

24.0

382.66

CH3(CH2)22COOCH3

Methyl hexacosanoate

26.0

410.72

CH3(CH2)24COOCH3

engine used to test is a single-cylinder four stroke engine working under an ambient temperature of 27 °C. Engine specifications are given in Table 4.

Uncertainty analysis An uncertainty analysis was performed on each measurement technique, in order to prove the accuracy of the experiments. The uncertainty limits for BP, TFC, and BTE were computed using the principle of the root mean square method given by Holman [2001] as follows: Let BR^ be the computed result function of the independent measured variables x1, x2, x3, ..................... xn, as per the relation. R ¼ f ðx1 ; x2 ; :………………xn Þ

ð4Þ

Let error limits for the measured variables or parameters be x1 ± Δn1, x2 ± Δn2, ......................., xa ± Δxa and the error limits for the computed result be R ± ΔR: " ΔR ¼

∂R ΔX 1 ∂X 1

2

 þ

∂R ΔX 2 ∂X 2

2

 þ ……………… þ

∂R ΔX n ∂X n

2 #1=2

ð5Þ Using Eq. (5), the uncertainty in the computed values was estimated. The uncertainty details of measured quantities are given in Table 5. Table 3 Properties of BHA and BHT

0.49 0.29

Engine tests To carry out the experimental study, the engine was first started with diesel and allowed to run in no-load condition for 15 min to get warmed-up. The performance parameters, emissions, and exhaust temperature were measured at four different engine loads (25, 50,75, and 100%) at a constant speed of 1500 rpm. The readings were taken at steady state conditions. The engine was then made to run on three test fuels (B20, B20 + BHA, and B20 + BHT) and readings were taken. To get optimum results, the engine tests were performed for three times and the average readings are considered for the performance calculations. The performance parameters of the engine were analyzed in terms of brake thermal efficiency, brake specific fuel consumption, and exhaust gas temperature, and emission parameters were analyzed in terms of CO, HC, and NOx emissions.

Results and discussion Effect of antioxidants on fuel properties The addition of antioxidants BHA and BHT at 1000 ppm to B20 resulted in 0.25 and 0.5% increase in kinematic viscosity. It is observed that there was no significant change in cloud

Antioxidant additives

BHA

BHT

Chemical structure

(CH3)3CC6H3(OCH3)

[(CH3)3C]2C6H2(CH3)

Molecular weight (g/mol) CASS number Assay Auto ignition point (°C)

OH 180.24 25013-16-5 ≥ 98.5% 315

OH 220.35 128-37-0 ≥ 99% 417

Author's personal copy Environ Sci Pollut Res Fig. 1 Layout of experimental setup

point and pour point with the addition of antioxidant additives. The addition of antioxidants BHA and BHT at 1000 ppm to B20 resulted in 0.24 and 0.31% decrease in calorific value. The addition of antioxidants increased the oxidation stability of the B20 fuel. The antioxidant BHA exhibited better oxidation stability than BHT. The addition of BHA and BHT to B20 at 1000 ppm provided the induction periods of 35 and 28 h, respectively. The antioxidants delay or interrupt the oxidation reaction by donating a hydrogen atom to a free radical (especially peroxyl radical ROO*). The hydrogen is released from the active OH groups of BHA and BHT and then donated to the free radicals [Sathiyamoorthi and Sankaranarayanan (2016), Rashed et al. (2016a, b)]. Table 4

Engine specifications

Make/model

Kirloskar TV1

Brake power (kW) Speed (rpm) Cylinder bore (mm) Compression ratio Injection timing Injection pressure Cooling type Stroke length (mm) Connecting rod length (mm) No. of cylinders No. of strokes Swept volume Dynamometer Fuel tank capacity Overall dimensions Pump

3.5 1500 87.50 17.5:1 23 o bTDC 210 bar Water cooled 110 234 1 4 661.45 (cc) Eddy current type 15 lit with glass metering column W 2000 × D 2500 × H 1500 mm Monoblock type

Performance characteristics Brake specific fuel consumption Brake specific fuel consumption (BSFC) is defined as the mass of fuel consumed by the engine per unit brake power output. A low value of BSFC is desirable since for a given power less fuel is consumed. Figure 2 shows the variation of BSFC with BP for all the test fuels. It was observed that diesel has the lowest BSFC of about 5.26% (on an average over the load range) lower than that of B20. The increase in BSFC of B20 was due to the lower energy content and high viscosity of B20 compared to diesel fuel [Ileri and Kocar (2013), Rizwanul Fattah et al. (2014a, b, c), Palash et al. (2014), Yuvarajan Devarajan et al. (2017)]. The mean values of BSFC for diesel, B20, B20 + BHA, and B20 + BHT were 0.38, 0.42, 0.40, and 0.41 kg/kWh, respectively. The addition of antioxidants BHA and BHT at 1000 ppm to B20 resulted in mean reductions in BSFC as 2.1 and 1.2% (on an average over the load range) in comparison with B20. The reduction in BSFC may be due to the friction reduction

Table 5 Uncertainty details of measured quantities

Measured quantity

Uncertainty

BP BSFC TFC BTE EGT CO HC NOX

± 0.08 kW ± 0.0035 kg/kWh ± 0.012 kg/h ± 0.48% ± 1 °C ± 0.01% ± 0.15% ± 0.15%

Author's personal copy Environ Sci Pollut Res Fig. 2 Variation of BSFC with BP for different fuels

properties of antioxidants which may result in high power output. This result is in agreement with previous studies [Sathiyamoorthi and Sankaranarayanan (2016), Rizwanul Fattah et al. (2014a, b, c)]. Brake thermal efficiency (BTE) Brake thermal efficiency (brake thermal efficiency) is defined as the ratio of brake power (BP) to energy in the fuel burned to Fig. 3 Variation of BTE with BP for different fuels

produce this power. Energy supplied is the product of the mass flow rate of fuel and calorific value of the fuel. The variation of BTE with BP for different fuels is shown in Figure 3. The BTE was found to increase with increase in percentage of load for all the test fuels. The BTE increases with engine load because the ratio of friction to brake power goes down. This may result in reduction in heat loss and increase in power [Kivevele et al. (2011)]. BTE decreased for B20 without antioxidants by 2.9% (on an average over the load range) than

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the diesel fuel. The lower BTE of B20 may be due to the combined effect of lower calorific value and high viscosity [Sathiyamoorthi and Sankaranarayanan (2016), Kivevele et al. (2011)]. The values of BTE at full load for diesel, B20, B20 + BHA, and B20 + BHT were 30.68, 27.78, 28.82, and 28.28%, respectively. The antioxidants BHA and BHT treated B20 resulted in increase in efficiency as 1.04 and 0.5% (on an average over the load range) in comparison with B20. The lower BSFC and higher power output are the reasons for higher thermal efficiency [Sathiyamoorthi and Sankaranarayanan (2016), Rashed et al. (2016a, b)].

in useful heat energy loss which in turn decreases the thermal efficiency. The antioxidants BHA and BHT treated B20 reduced the EGT by 5.25 and 3.74% (on an average over the load range), respectively. The reductions in EGT may be due to the presence of antioxidant additives which hinder the fuel conversion slightly. Similar results were noted by other published investigations [Rizwanul Fattah et al. (2014a, b, c), Senthil Ramalingam et al. (2016)].

Emission characteristics NOx emissions

Exhaust gas temperature Exhaust gas temperature plays a vital role in the analysis of exhaust emissions especially NOx emissions [Sathiyamoorthi and Sankaranarayanan (2016), Rizwanul Fattah et al. (2014a, b, c)]. Figure 4 shows the variation of EGT with BP for all the test fuels. It was observed that the engine operation with diesel results in an EGT of about 6.57% lower than B20 fuel (on an average over the load range). The higher EGT of B20 may be attributed to early start of injection and reduction of the premixed combustion phase as a result of shorter ignition delay compared to diesel. Higher CN and poor atomization resulting from higher viscosity of biodiesel blend causes the presence of unburnt fuel particles in the premixed combustion phase [Rizwanul Fattah et al. (2014a, b, c)]. These unburnt fuel portions continue to burn in the later stage of the diffusion combustion phase resulting in higher exhaust gas temperature [Sathiyamoorthi and Sankaranarayanan (2016), Rizwanul Fattah et al. (2014a, b, c)]. This phenomenon results Fig. 4 Variation of EGT with BP for different fuels

NOx is the most dangerous pollutant at the combustion stage. The main reasons for NOx formation include higher combustion temperature, longer combustion duration, and high oxygen concentration of fuel [Velmurugan and Sathiyagnanam (2016), Ileri and Kocar (2014), Senthil Ramalingam et al. (2016)]. The average NOx emission of B20 was increased by 22.2% in comparison with that of diesel fuel as shown in Figure 5. The increased NOx emission of biodiesel is due to various mechanisms not only the fuel properties. There are different theories to explain the effect of biodiesel on NOx emission [Mueller et al. (2009)]. The pressure waves in the fuel line of the engine move faster due to the higher bulk modulus of compressibility of biodiesel. Because of this phenomenon, the fuel injector open earlier in the engine cycle resulting in earlier combustion and high combustion temperatures, hence increasing the formation of thermal NOx [Palash et al. (2013), Mueller et al. (2009)]. The combustion of biodiesel blends start earlier due to shorter ignition delay

Author's personal copy Environ Sci Pollut Res Fig. 5 Variation of Nox emission with BP for different fuels

compared to diesel. The long combustion duration and ample concentration of oxygen in the biodiesel improve the combustion efficiency and hence form higher NOx compared to diesel fuel [Sathiyamoorthi and Sankaranarayanan (2016), Rizwanul Fattah et al. (2014a, b, c)]. The antioxidants BHA and BHT treated B20 produced average reductions in NOx emission of 12.2 and 9.6% compared to B20. The –OH group of BHA and BHT scavenge reactive radicals such as peroxyl radicals (RO2) and result in phenoxyl radical (R-O) which is poorly reactive and limits oxidative reaction. Reductions in NOx emission with addition of antioxidants are in agreement with other researches [Varatharajan et al. (2011), Balaji and Cheralathan (2016), Ileri and Kocar (2013)].

conversion of HC [Rashed et al. (2016a, b)]. The combined effect of early start of injection due to shorter ignition delay and oxygen content of biodiesel prolong the combustion process and enhance the oxidation of unburnt HC, thus decreasing the HC emission. The conditions such as post-flame oxidation, higher flame speed also support the oxidation process. The antioxidants BHA and BHT treated B20 resulted in mean increases in HC emission of 10.6 and 11.2% compared to B20. Higher HC emission for antioxidant treated biodiesel blends may be attributed to the reduction in oxidative capability of HC. However, the level of HC emission is still lower than that of diesel fuel. Similar result was published by other investigators [Varatharajan and Cheralathan (2013), Kivevele et al. (2011)].

HC emissions CO emissions The variation in HC emissions with brake power is shown in Figure 6 for all the test fuels. The parameters such as fuel type, engine adjustment, operating conditions, design, and characterization of fuel spray are responsible for HC emission [Rizwanul Fattah et al. (2014a, b, c)]. HC emission mainly consists of unburnt fuel particles as a result of incomplete combustion [Sathiyamoorthi and Sankaranarayanan (2016)]. The fuel mixed to leaner than the lean combustion limit during the delay period, and undermixing of fuels which leave from nozzle sac volume and nozzle holes late in the combustion process are the two major causes for HC emission [Sathiyamoorthi and Sankaranarayanan (2016), Rizwanul Fattah et al. (2014a, b, c)]. The average HC emission of B20 was decreased by 21% in comparison with that of diesel fuel. High CN and oxygen concentration of B20 result in better

The main reason for CO emission in exhaust gases may be attributed to incomplete combustion associated with rich fuelair ratio and low oxygen concentration. The variation of CO emissions with brake power is shown in Fig. 7 for all the test fuels. The average CO emission value of B20 without antioxidant additives was decreased by 28.33% in comparison with that of diesel fuel. Low CO emission is due to the combined effect of oxygen concentration in the chemical structure of biodiesel fuels and high cetane number [Rizwanul Fattah et al. (2014a, b, c), Ileri and Kocar (2013), Rashed et al. (2016a, b)]. Shorter ignition delay caused by high CN of biodiesel blend allows longer combustion duration. The high oxygen content is crucial for proper combustion. Now the oxygen content of biodiesel comes in to act and enhances the

Author's personal copy Environ Sci Pollut Res Fig. 6 Variation of HC emission with BP for different fuels

combustion process. This results in more complete combustion with higher in-cylinder combustion temperature, thereby promoting greater conversion of CO to CO2 than for diesel fuel [Rizwanul Fattah et al. (2014a, b, c)]. The antioxidants BHA and BHT treated B20 produced average increases in CO emission of 14.8 and 16.6% compared to B20. The delay caused by antioxidants to convert CO to CO2 resulted in higher CO emission for antioxidant-treated biodiesel blends

Fig. 7 Variation of CO emission with BP for different fuels

[Rizwanul Fattah et al. (2014a, b, c)]. The peroxyl (HO2) and hydrogen peroxide (H2O2) radicals formed during oxidation are further converted into hydroxyl radicals (OH) by extracting heat from the combustion chamber. The conversion of CO to CO2 is mostly affected by these OH radicals [Palash et al. (2014), Rashed et al. (2016a, b)]. The antioxidants BHA and BHT treated B20 fuel decreased the concentration of peroxyl and hydrogen peroxide radicals, which hindered the

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CO conversion process significantly. This result is in agreement with previous studies [B. Ashok et al. (2017), Varatharajan and Cheralathan (2013)].

Conclusions The objective of this experimental study was to investigate the impact of antioxidants on the reduction of NOx emission and the performance characteristics of diesel engine fuelled with rice bran oil biodiesel blend. The following conclusions were drawn based on the experimental results; & &

&

& &

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Blending of 20% of RBME with diesel has good agreement with ASTM standards. The antioxidant BHA exhibited better oxidation stability than BHT. The addition of BHA and BHT to B20 at 1000 ppm provided the induction periods of 35 and 28 h, respectively. The addition of antioxidants BHA and BHT at 1000 ppm to B20 resulted in average reductions in BSFC as 2.1 and 1.2% and the average increase in efficiency as 1.04 and 0.5% in comparison with B20, respectively. The antioxidants BHA and BHT treated B20 reduced the EGT by 5.25 and 3.74%, respectively. The antioxidants BHA and BHT treated B20 produced average increases in CO emission of 14.8 and 16.6% and mean increases in HC emission of 10.6 and 11.2% compared to B20. The antioxidants BHA and BHT treated B20 produced average reductions in NOx emission of 12.2 and 9.6% compared toB20. The addition of BHA shows better performance compared to BHT.

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