Attempts to minimize nitrogen oxide emission from

Environ Sci Pollut Res DOI 10.1007/s11356-017-8573-9

RESEARCH ARTICLE

Attempts to minimize nitrogen oxide emission from diesel engine by using antioxidant-treated diesel-biodiesel blend Hasan Khondakar Rashedul 1 & Md Abdul Kalam 1 & Haji Hassan Masjuki 1 & Yew Heng Teoh 1,2 & Heoy Geok How 1,2 & Islam Mohammad Monirul 1 & Hassan Kazi Imdadul 1

Received: 6 June 2016 / Accepted: 5 February 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract The study represents a comprehensive analysis of engine exhaust emission variation from a compression ignition (CI) diesel engine fueled with diesel-biodiesel blends. Biodiesel used in this investigation was produced through transesterification procedure from Moringa oleifera oil. A single cylinder, four-stroke, water-cooled, naturally aspirated diesel engine was used for this purpose. The pollutants from the exhaust of the engine that are monitored in this study are nitrogen oxide (NO), carbon monoxide (CO), hydrocarbon (HC), and smoke opacity. Engine combustion and performance parameters are also measured together with exhaust emission data. Some researchers have reported that the reason for higher NO emission of biodiesel is higher prompt NO formation. The use of antioxidant-treated biodiesel in a diesel engine is a promising approach because antioxidants reduce the formation of free radicals, which are responsible for the formation of prompt NO during combustion. Two different antioxidant additives namely 2,6-di-tert-butyl-4methylphenol (BHT) and 2,2′-methylenebis(4-methyl-6-tertbutylphenol) (MBEBP) were individually dissolved at a concentration of 1% by volume in MB30 (30% moringa biodiesel with 70% diesel) fuel blend to investigate and compare NO as well as other emissions. The result shows that both antioxidants reduced NO emission significantly; however, HC, CO, Responsible editor: Philippe Garrigues * Hasan Khondakar Rashedul [email protected]

1

Centre for Energy Sciences, Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

2

School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal, 14300 Penang, Malaysia

and smoke were found slightly higher compared to pure biodiesel blends, but not more than the baseline fuel diesel. The result also shows that both antioxidants were quite effective in reducing peak heat release rate (HRR) and brake-specific fuel consumption (BSFC) as well as improving brake thermal efficiency (BTE) and oxidation stability. Based on this study, antioxidant-treated M. oleifera biodiesel blend (MB30) can be used as a very promising alternative source of fuel in diesel engine without any modifications. Keywords Moringa oleifera biodiesel . Antioxidant . NO emission . Combustion . Diesel engine

Introduction Biodiesel is a promising alternative diesel fuel which is renewable, less toxic, and biodegradable derived from many vegetable oil crops or animal fats (Carvalho et al. 2016; Ramkumar and Kirubakaran 2016). It has been proven for many years that the biodiesel-based fuel can fulfill the increasing demand of energy as the petroleum-based diesel is depleting day by day (Mofijur et al. 2015b; Thangaraja et al. 2016). Nowadays, it has become a very potential source of energy, which has significant environmental benefits. Numerous researches have been done on the emission characteristics of biodiesel fuel, and it has been proven that biodiesel-based fuels can minimize the HC, CO, and particulate matter (PM) emissions compared to diesel fuel (Monirul et al. 2016; Rashedul et al. 2014; Thangaraja et al. 2016). Biodiesel has a very good lubricity property and can lower the life cycle CO2 emission as well (Fernández-Tirado et al. 2016; Monirul et al. 2016). However, many researchers have reported that the combustion of biodiesel in diesel engine generally produces higher NOx emission (Prabu and Anand 2015; Rashed et al. 2016a; Rashedul

Environ Sci Pollut Res

et al. 2015; Rizwanul Fattah et al. 2014a). According to the analysis of the impact of biodiesel on engine emission reported by Environmental Protection Agency (EPA) (2002), there is 10% increase of NOx emission for 100% pure biodiesel as well as more than 2% increase for B20 (20% biodiesel with 80% diesel) in diesel engines. NOx causes a wide variety of health and environmental impacts which includes acid rain, plant damage, global warming, water-quality deterioration, visibility impairment, lung damage, painful breathing, heart attacks, and ground-level ozone, and produces toxic chemicals which may cause biological mutations etc. (Fujii and Managi 2016; Mofijur et al. 2015a; Sanjid et al. 2016). Depending on the stoichiometric ratio, temperature range, and kind of nitrous species existent in the combustion zone, it is possible to differentiate flourishing parts of chemical reactions, which are called the mechanisms of nitrogen oxides formation (Bär et al. 2016; Rashedul et al. 2015). During combustion of diesel fuel, NOx is formed by three major mechanisms: thermal NOx formation (Zeldovich mechanism), prompt NOx formation (Fenimore mechanism), and fuel NOx formation (Palash et al. 2014; Varatharajan et al. 2011). Thermal NOx is formed at a very high temperature of 1700 K by the reaction of atmospheric oxygen (O2) and aromatic nitrogen (N2) (Palash et al. 2014; Zhang and Boehman 2007). Prompt NOx is produced when the hydrocarbon radicals react with atmospheric nitrogen. Fuel NOx is produced by reacting nitrogen and oxygen during combustion. Biodiesel fuels do not contain any nitrogen, for this fuel NOx formation from a biodiesel fuelled diesel engine is negligible (Rashed et al. 2016b; Rizwanul Fattah et al. 2014b). McCormick et al. (2001) reported that the higher NOx formation of biodieselfueled diesel engine could be the reason by the increase of NOx formation via prompt NOx formation instead of being driven by the thermal NOx formation. They also reported that the double bond of biodiesel might contribute to produce higher hydrocarbon radicals in the premixed combustion, which could result higher prompt NOx formation, supported by many researchers (Palash et al. 2014; Zhang and Boehman 2007). Fenimore (1971) also explained that during combustion of biodiesel molecular nitrogen reacts with hydrocarbon radicals (C, C2, CH, CH2, and C2H), which is the major contributor to form higher prompt NO emission. As the concentration of free radicals increases, the rate of production of N, HCN, and NO increases. Garner and Brezinsky (2011) also observed that rate of formation of hydrocarbon radicals is higher during combustion of biodiesel than that of diesel and found higher NO emission. In another study, McCormick et al. (2006) speculated that higher prompt NOx formation is one of the primary causes for higher formation of NOx emission of a biodiesel-fueled diesel engine. However, they also suggested that the addition of antioxidant to biodiesel is a very promising and effective approach to control the formation of prompt NOx. Numerous researches have been explored by

many researchers (Kivevele et al. 2011a; Varatharajan and Cheralathan 2013) (Fattah et al. 2014; Gan and Ng 2010; Kivevele et al. 2011b; Ryu 2010) about the effect of antioxidant on biodiesel NO emission, and all supported McCormick et al.’s (2006) investigation. Antioxidants have aromatic ring structures and are able to delocalize the unpaired electron. Antioxidants can neutralize free radicals by donating electron(s) to eliminate the unpaired condition of the radical. Antioxidants directly react with the reactive radicals and neutralized them. (Khurana and Agarwal 2011). The objective of this investigation is mainly to observe the impacts of two different antioxidants namely BHT and MBEBP (provided by Lanxess especially for biodiesel) on the NO emission of a single cylinder diesel engine fuelled with biodiesel produced from a nonedible vegetable oil called M. oleifera. No experimental investigations have been found based on the effect of antioxidant with M. oleifera-based biodiesel. A very few reports have been made on engine combustion characteristics of a diesel engine fueled with antioxidanttreated biodiesel. Hence, this paper also aims to investigate the impact of antioxidant additives on engine combustion characteristics of a biodiesel-fueled diesel engine.

Materials and methods Test fuels Present study was carried out with a vegetable oil named M. oleifera. Biodiesel was produced through transesterification process from crude oil. In this study, 100% pure moringa biodiesel was blended with petroleum diesel at blending ratio of 30/70 (MB30: 30% biodiesel and 70% diesel) by volume. Two different antioxidant additives were used in this study such as BHT and MBEBP, and were collected from Lanxess chemical company, Germany. Each antioxidant was added 1% (v/v) to pure biodiesel blend MB30. Three biodiesel blends including MB30 and MB30 with two antioxidants were prepared and tested in this study and diesel used as baseline fuel. Table 1 shows the properties of diesel, biodiesel, and its modified blend with antioxidant additives, and details of antioxidants are listed in Table 2. Experimental setup and method The experiments were performed using a water cooled, naturally aspirated single-cylinder, four-stroke, DI (direct injection) diesel engine. The maximum output of the engine was 7.5 kW. The detail specifications of such engine are stated in Table 3. An electronic control unit (ECU) was used to control the engine. Figure 1 illustrates the schematic of the engine testing system. An AC synchronous dynamometer (ST-7.5) with 7.5 kW was used to maintain

Environ Sci Pollut Res Table 1

Physicochemical properties of test fuels

Properties

Standard methods

Diesel

Calorific value (MJ/kg)

ASTM D240

45.304

40.052

43.869

43.765

43.817

Viscosity at 40°C (cSt)

ASTM D445 ASTM D4052

3.233 818.7

4.895 857.10

3.816 836.40

3.994 836.62

3.892 836.55

EN14112 ASTM D93

58.0 68.5

5.38 150.5

10.0 93.5

27.52 95.0

23.39 95.5

ASTM D4737 ASTM D86

48.9

54.88

51.49

52.23

51.70

IBP

177.0

214.0

175.0

169.0

165.0

10% 50%

232.0 289.0

332.0 334.5

250.0 312.0

251.5 314.0

253.5 314.0

90% FBP

349.0 374.0

339.0 339.0

343.0 360.0

345.5 362.5

346.0 359.0

Density at 40°C (kg/m3) Oxidation stability (h) Flash point (°C) Cetane index Distillation temperature (°C)

MB100

MB30

MB30 + BHT

MB30 + MBEBP

IBP initial boiling point, FBP final boiling point

engine speed and provide loading to the engine. The airflow rate of the engine was measured with a turbine called airflow meter turbine, which can measure the airflow in the range 2–70 L every second. The fuel flow rate of the engine was measured with a positive displacement (DOMA05H) flow meter. A Kistler 6125B pressure transducer was used to measure cylinder pressure. Gaseous emission including NO, CO, and HC was measured using a BOSCH gas analyzer, while smoke opacity was measured by BOSCH RTM 430 opacimeter. The experiment was performed under a constant load of 20-Nm condition with varying speed ranging from 1000 to 1800 rpm with an interval of 200 rpm. Initially, the engine was started with baseline diesel fuel and allowed to run for more than 30 min to attain steady state condition. After this, it was changed to biodiesel blends and allowed to run for 10 min

Table 2

for each operating conditions. After completing all the experiments, fuel was drained out completely from the fuel tank and fuel line and again run with diesel fuel until biodiesel blends were fully ejected from the fuel line as well as from the injector.

Uncertainty analysis of equipment The percentage (%) uncertainties of different parameters, such as BTE, BSFC, NO, HC, CO, and smoke opacity, were calculated using the percentage (%) uncertainties of different measuring instruments used in the experiment. The measurement accuracy, range, and percentage uncertainties of all used instruments are listed in Table 4. The overall uncertainty of the experiments was ±4.1%, which was calculated as follows:

Details of antioxidants

Ingredient name CAS number Flash point Density Physical state Structure

2,6-Di-tert.-butyl-4methylphenol 128-37-0 Closed cup: 190°C 1.048 gm/cm3 Liquid

2,2’-methylenebis(4-methyl6-tert-butylphenol) 119-47-1 Closed cup: 180°C 1.04 gm/cm3 Liquid


h Overall uncertainty of the experiment ¼ √ ðuncertainty of rate of fuel flow; m f Þ2 þ ðuncertainty of BTEÞ2 þ ðuncertainty of BSFCÞ2 þ ðuncertainty of NOÞ2 þ ðuncertainty of HCÞ2 þ ðuncertainty of COÞ2 þ ðuncertainty of Smoke opacityÞ2

i

ð1Þ

þ ðuncertainty of Crank angle encoderÞ2 þ ðuncertainty of Pressure sensorÞ2 h i ¼ √ ð2Þ2 þ ð1:74Þ2 þ ð1:95Þ2 þ ð1:3Þ2 þ ð1Þ2 þ ð1Þ2 þ ð1Þ2 þ ð0:03Þ2 þ ð1Þ2 ¼ 4:1%

Results and discussion Combustion characteristics In this study, the in-cylinder pressure, start of combustion (SOC), and heat release rate (HRR) were used to investigate the combustion characteristics. The cylinder pressure data are acquired at a fixed interval of 0.125 °CA and average over 100 consecutive cycles through combustion analyzer. The apparent HRRs (Qnet) are estimated from the measured cylinder pressure data by using the First Laws of Thermodynamics applicable to the closed part of engine cycle expressed as (Imtenan et al. 2015; How et al. 2014; Teoh et al. 2014): dQnet γ dV 1 dP ¼ P þ V dθ γ−1 dθ γ−1 dθ

ð2Þ

In-cylinder pressure and heat release rate The cylinder pressure with respect to crank angle diagram for MB30 and its antioxidant-treated blends at 1800-rpm engine speed is shown in figure. All tested fuels showed the peak incylinder pressure within the range of 8.875 °CA to 9.125 °CA after top dead center (ATDC). From the figure, it can be seen that diesel (B0) fuel showed the highest peak in-cylinder pressure of 67.95 bar at 9.000 °CA ATDC, while MB30 blend showed the highest in-cylinder pressure of 68.51 bar at 8.875 °CA ATDC for the same operating condition. The peak cylinder pressure of a Table 3

Specifications of the engine

Engine type

Single (01) cylinder, four (04) stroke, naturally aspirated DI diesel engine

Rated power, kW Bore, mm Displacement, cm3 Compression ratio No. of injector nozzles Nominal injector nozzle diameter, mm Rated speed, rpm

7.8 92 638 17.7:1 5 0.134 2400

compression ignition engine depends on the mass of fuel burned in the premixed combustion phase (Imtenan et al. 2015). Relatively higher molecular weight fraction as well as cetane number of biodiesel (MB30) fuel compared to diesel led to higher the peak cylinder pressure. However, the addition of BHT to MB30 blend lowered the in-cylinder pressure to 68.31 bar at 9.125 °CA ATDC due to its higher cetane number compared to pure MB30. MBEBP with MB30 blend exhibited similar result. The peak cylinder pressure for MBEBP blend was 68.44 bar at 9 °CA ATDC. Heat release rate is the promising approach to understand the combustion phenomena. The HRR with respect to crank angle for all tested fuels is shown in the figure. MB30 and its modified blends with antioxidant fuels exhibited more or less similar HRRs as the baseline diesel. Firstly, during the compression stroke, work had been done by the cylinder piston to the compressed air, indicated by negative values (as seen in Fig. 2) of heat release rate. The heat release rate became positive, as the injected fuel started burning. The combustion process of a diesel engine may be imaginary parted into two combustion phases, namely premixed combustion phase and diffusion combustion phase (Qi et al. 2009). All test fuels showed rapid premixed combustion followed by a diffusion combustion period. The start of combustion of diesel fuel was −4.375 °CA ATDC followed by MB30– 4.500 °CA ATDC. The start of combustion of biodiesel blends occurred a bit earlier than that of baseline diesel. Earlier start of combustion may be attributed to the higher cetane number, which resulted shorter ignition delay as well as higher density of biodiesel compared to diesel (Imdadul et al. 2016). The reason for the lower peak pressure of diesel could be longer ID. However, the cylinder pressure during late combustion for diesel fuel is marginally higher than that of biodiesel blends. This is may be due to delay combustion, and some proportion of fuel cannot be burnt completely during the main combustion, and then, it continues to burn during the late combustion phase (Ileri 2016). Antioxidant-treated blends showed little bit advanced SOC of −4.750 °CA ATDC for MB30 + BHT and −4.625 °CA ATDC for MB30 + MBEBP due to their higher cetane index compared to pure MB30. The peak HRRs in the premixed combustion phase for diesel were 30.51 J/°CA occurred at −1.875 °CA ATDC, whereas for MB30, it was 33.78 J/°CA at −1.625 °CA ATDC. The lower peak HRR of diesel fuel was attributed to its relatively

Environ Sci Pollut Res Fig. 1 Schematic diagram of engine setup

lower maximum in-cylinder pressure compared to MB30. It can be seen that the slope of HRR during premixed combustion phase for MB30 is steeper compared to diesel fuel and the area under the premixed combustion phase (first peak) of MB30 is wider than diesel due to more premixed charge during premixed combustion. The peak HRRs during premixed combustion of BHT and MBEBP blends were 33.09 J/°CA and 33.39 J/°CA; both occurred at −1.75 °CA ATDC. This may be due to the higher cetane number, which resulted shorter ignition delay, which in turn lowered the peak HRRs during premixed combustion (Imtenan Table 4

et al. 2015). In diffusion control phase, diesel fuel showed a little bit higher HRR compared to biodiesel blends. Engine performance and emissions Brake thermal efficiency Figure 3 shows the BTE of diesel, diesel-biodiesel (MB30) blend, and its antioxidant-treated blends with respect to engine speed (1000–1800 rpm). It was observed that the maximum

List of measurement accuracy, range, and percentage uncertainties

Measurement

Measurement range

Accuracy

Measurement techniques

% Uncertainty

Load Speed Time Fuel flow measurement CO NO HC Pressure sensor Crank angle encoder Computed BSFC BTE

±120 Nm 60–10,000 rpm – 0.5–36 L/h 0–10% by vol. 0–5000 ppm 0–20,000 ppm 0–25,000 kPa 0–12,000 rpm

±0.1 Nm ±1 rpm ±0.1 s ±0.01 L/h ±0.001% ±1 ppm ±1 ppm ±12.5 kPa ±0.125°

Strain gauge-type load cell Magnetic pick up type – Positive displacement gear wheel flow meter Nondispersive infrared Electrochemical Nondispersive infrared Piezoelectric crystal type Incremental optical encoder

±1 ±0.1 ±0.2 ±2 ±1 ±1.3 ±1 ±1 ±0.03

– –

±7.8 g/kW/h ±0.5%

– –

±1.95 ±1.74


TDC

80

60

B0 MB30 MB30+BHT MB30+MBEBP

Cylinder pressure (bar)

70 60

50 40

50 30 40 20 30 10

20

Heat release rate (J/ CA)

Fig. 2 In-cylinder pressure and HRR variation with crank angle

0

10 0 -10

0

10

20

30

40

-10

Cranke angle ( CA)

average BTE for diesel was 28.37%, whereas the lowest average BTE for MB30 was 27.79%. The result between B0 and MB30 was found very statistically significant at a level of p = 0.0053. It is seen from the figure that the BTE values decreased simultaneously from 1000 to 1600 rpm and again increased at 1800 rpm. The average reductions of BTE for MB30, MB30 with BHT, and MB30 with MBEBP were 2.04, 1.52, and 1.34%, respectively, compared to diesel. This may be due to the lower calorific value as well as the higher mass flow rate of biodiesel fuels. The atomization and vaporization of biodiesel fuels are lower due to their higher viscosity which may also be the reason for lower BTE of biodiesel blends compared to diesel (Imdadul et al. 2016; Rashed et al. 2016b). Adding antioxidants BHT and MBEBP increased the average BTE by 0.57 and 0.76%, respectively. Shorter combustion duration and relatively higher heat release rate during diffusion control phase of antioxidant-treated blends may be the reason for higher BTE compared to pure MB30 (Velmurugan and Sathiyagnanam 2016; Garner et al. 2009).

32

B0

MB30

MB30+BHT

NO emission There are many factors that affect the formation of NO in the engine cylinder such as higher cylinder temperature, oxygen percentage in the fuel, air-fuel ratio, and residual time (Palash et al. 2013; Rizwanul Fattah et al. 2014a; Ryu 2010). Figure 4 shows the NO emission variation of four tested fuels with engine speed. The average NO emission produced by diesel fuel was 3.612 g/kWh, whereas for MB30, it was 4.112 g/ kWh which means that the NO emission produced by MB30 was 13.84% higher than that of diesel and this result was found extremely statistically significant at a level of p = 0.0001. This may be attributed to the higher fuel bond oxygen and atomic weight of MB30 compared to diesel. Higher oxygen content (10–15% higher than diesel) of biodiesel as well as poor atomization which caused delayed combustion resulted higher flame temperature which in turn resulted higher NO emission (Sanjid et al. 2016; Varatharajan and Cheralathan 2013). The formation of CH radicals during B0

MB30+MBEBP

MB30

MB30+BHT

MB30+MBEBP

10.5

31

9.5

30 8.5

NO (g/kWh)

BTE (%)

29 28 27 26

7.5 6.5 5.5

25 4.5

24 23

3.5

1000

1200 1400 1600 Engine speed (rpm)

Fig. 3 BTE characteristic with varying engine speed

1800

1000

1200

1400

1600

Engine speed (rpm) Fig. 4 NO emission characteristic with varying engine speed

1800


CO emission Carbon monoxide of biodiesel fuelled engine is usually lower compared to diesel-fueled engine (Sanjid et al. 2014). Figure 5 shows the CO emission variation for all tested fuels against the engine speed with constant engine load of 20 Nm. From the figure, it is seen that, as the speed of the engine increased, the CO values decreased. At lower engine speed, poor atomization and unequal distribution of little portions of burning fuel across the combustion chamber together with lower gas temperature may lead to lack of local oxygen and incomplete combustion, which resulted B0

MB30

MB30+BHT

0.08 0.06 0.04 0.02 0 1000

1200

1400

1600

1800

Speed (rpm) Fig. 6 HC emission characteristic with varying engine speed

higher CO emission compared to higher engine speed. Results showed that the average reduction in CO for pure MB30 (1.198 g/kWh) was 21.02% compared to diesel (1.517 g/kWh), and this result was found extremely statistically significant at a level of p = 0.0001. This could be attributed to some reasons reported by many researchers (Ashraful et al. 2014; Sajjad et al. 2015; Sanjid et al. 2014): (a) presence of more oxygen content in biodiesel fuel, which helps complete combustion of fuel, and (b) higher cetane number helps to minimize the probability of the formation of fuel-rich zones. Adding antioxidant to biodiesel enhances the production of CO. Average production of CO for MB30 with BHT was 1.639 g/kWh and, for MB30 with MBEBP, was 1.442 g/kWh, which is higher than pure MB30 due to incomplete combustion caused by antioxidants addition. Antioxidants reduce carbon oxidation by scavenging the hydroxyl (·OH) radicals (Palash et al. 2014). B0

10 Smoke opacity (%)

3

1

MB30+MBEBP

0.1

12

1.5

MB30+BHT

0.12

MB30+MBEBP

2

MB30

0.14

3.5

2.5

CO (g/kWh)

B0 0.16

HC (g/kWh)

combustion of biodiesel is higher than diesel combustion, and the higher formation of free radicals is the primary cause of higher NO emission (Palash et al. 2014; Rizwanul Fattah et al. 2014a). This could be another reason for such higher (13.84%) NO emission for MB30. From the figure, it can be seen that addition of both antioxidants reduced the NO emission. On average, MB30 with BHT produced NO emission of 3.933 g/kWh, and for MB30 with MBEBP, it was 3.938 g/ kWh. Both values are statistically significant at a level of p = 0.0019 for BHT blend and p = 0.0011 for MBEBP blend. The mean reductions of NO emission for BHT with MB30 and MBEBP with MB30 were 4.35% and 4.23%, respectively, compared to neat MB30. This may be due to the addition of antioxidants to biodiesel that reduce the free radicals formation resulting lower premixed HRRs during combustion (Rashedul et al. 2015; Hess et al. 2005). Lower HRRs in the premixed combustion phase resulted in lower cylinder temperatures and resulted in lower NOx emissions. The observed trend with antioxidants BHT and MBEBP is in agreement with other studies (Palash et al. 2013; Rashed et al. 2016a; Rizwanul Fattah et al. 2014b; Varatharajan and Cheralathan 2013).

MB30

MB30+BHT

MB30+MBEBP

8 6 4 2

0.5

0

0 1000

1200

1400

1600

Engine speed (rpm) Fig. 5 CO emission characteristic with varying engine speed

1800

1000

1200 1400 1600 Engine speed (rpm)

1800

Fig. 7 Smoke opacity characteristic with varying engine speed


HC emission The main reasons of the formation of hydrocarbon (HC) are flame quenching, fuel-rich zone, misfiring, and desorption of lubricating oil (How et al. 2014; Teoh et al. 2014). Figure 6 shows the variation of hydrocarbon (HC) emission for diesel (B0), MB30, and its two antioxidant-treated blend with engine speed. From the results, it can be seen that the average decrease of HC emission for pure MB30 was 17.44% compared to diesel due to the fuel-bound oxygen, which enhances the combustion process as well as decreases the fuel-rich zone. The difference between the result of B0 and MB30 was found extremely statistically significant at a level of p = 0.0008. BHT and MBEBP gave the mean increase in HC emission by 14.88 and 11.92%, respectively, while blended with MB30 compared to pure MB30. This can be attributed to the reduction of free radicals formation by antioxidants. Peroxyl (HO2) and hydrogen peroxide (H2O2) radicals are formed during oxidation, and it is converted into hydroxyl (OH) radicalsby absorbing heat (Eqs.(3) and (4)). H 2 O2 →2OH

ð3Þ

HO2 →OH þ O

ð4Þ

The antioxidant reduces the concentration of peroxyl and hydrogen peroxide radicals. The increase in HC emission is due to the reduction in free radicals and has a significant effect on the formation of OH radicals and the oxidation of CO and HC [23]. Smoke opacity Smoke is the indicative of soot emission which is the major element of particulate matter (PM) emission. The higher oxygen content, the higher reduction of smoke (Masum et al. 2014; Teoh et al. 2014). Figure 7 shows the variation of smoke opacity with varying speed. All biodiesel blend showed lower smoke opacity compared to diesel. MB30 produced about 30.39% lower smoke opacity than diesel, and the difference between the result of B0 and MB30 was found statistically significant at a level of p = 0.0109. The lower smoke produced by biodiesel fuel blends can be explained due to the presence of more oxygen, which reduce the probability of fuel-rich zone formation (How et al. 2014). Adding BHT and MBEBP to pure MB30 increased the smoke values of 13.33 and 12.62%, respectively, compared to pure MB30. This happened may be due to antioxidant that reduces the oxygen availability from the blend.

Conclusion The main aim of the study was to identify the impacts of two different antioxidants on the NO emission of M. oleifera biodiesel-fueled diesel engine as well as investigate the

combustion, engine performance, and other exhaust emission characteristics. From the obtained results, it can be concluded that both antioxidants, BHT and MBEBP, were quite effective to minimize NO emissions. Pure MB30 blend produced 13.84% higher NO emission compared to B0. Adding BHT and MBEBP to MB30 reduced the NO emission by 4.35 and 4.23%, respectively, compared to pure MB30. However, they do have significantly more CO, HC, and smoke opacity. It should be noted, however, that these emission values are well below the diesel emission levels. Adding antioxidants to MB30 reduced peak HRR in the premixed combustion phase compared to pure MB30. The addition of antioxidants to MB30 also improved engine performance parameters significantly as well as some key properties like oxidation stability, cetane number, and flash point without changing other properties much. Acknowledgements The authors would like to appreciate the University of Malaya for financial support through research grant FP051-2014B and RP016-2012E.

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