Effect of Synthetic Antioxidants on Emission Characteristics of a

Coconut Biodiesel Powered Diesel Engine

I. M. Rizwanul Fattah, H. H. Masjuki, M. A. Kalam and B. M. Masum

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

Abstract. Biodiesel is a green fuel produced from renewable resources. It is a clean-burning alternative

fuel which drawn attention of the energy researchers for last two decades. Coconut biodiesel (CME) is one of

the promising biodiesels in South East Asian region. This paper presents experimental investigation to

determine the ability of antioxidant added coconut biodiesel blends to improve engine exhaust emissions

characteristics of a diesel engine. Antioxidants butylated hydroxyanisole (BHA) and butylated

hydroxytoluene (BHT) were added at a concentration of 2000 ppm to 20% CME (B20). A 55 kW 2.5L four-

cylinder diesel engine was used to carry out tests at constant speed of 3500 rpm at half and full load. BHA

added B20 produced 1-1.1% lower NOx, 8.3-13.9% lower carbon monoxide (CO) but higher hydrocarbon

(HC) emissions at the operating condition compared to B20.

Keywords: Antioxidant, Biodiesel, Coconut biodiesel, Emission

1. Introduction

Biodiesel refers to mono-alkyl esters of long-chain fatty acids (FAs) prepared from plant oils, animal fats

or other lipids, designated B100. Increasing fossil oil prices, limited reserve of fossil fuel and environmental

concerns have boost up the research on biodiesel fuels. Moreover, global carbon dioxide (CO2) emissions

from fossil-fuel combustion are increasing every year that intensifies air pollution and magnifies global

warming problems caused by CO2. As biodiesel is a carbon neutral fuel it helps in reduction of overall CO2

emission. Advantages of biodiesel over petroleum diesel fuel include derivation from renewable feed stocks,

superior lubricity and biodegradability, lower toxicity, no sulphur and aromatics content, higher flash point,

and reduced emissions of carbon monoxide (CO), total hydrocarbon (THC) and particulate matter (PM) [1],

[2]. Disadvantages include limited feed stock availability, higher production cost, inferior oxidative

consumption, and higher nitrogen oxides exhaust emissions [3], [4].

Vegetable oils like soybean, rapeseed, palm, sunflower etc. are promising feedstocks for biodiesel

production. Triglyceride molecules that are main constituents of these oils are transesterified with an alcohol

such as methanol, in presence of a catalyst to form FAAE [5]. However, they differ in properties based on

their fatty acid composition. In this study, coconut oil methyl ester (CME) is used. It has about 14 wt.%

oxygen which provides similar performance and lower exhaust emissions compared to biodiesel from other

feedstocks [6]. CME contains majority of short chain fatty acid esters, which also gives rise to superior cold

flow properties. How et al. [7] studied the effect of 10%, 30% and 50% blends of coconut biodiesel on

performance and criteria regulated emissions along with polycyclic aromatic hydrocarbons (PAHs) in a multi

cylinder diesel engine. They observed 0.4-20% higher BSFC, 8.5%-42.4% lower smoke, max. 37.6% lower

HC and 40.1% lower PAH emission compared to diesel fuel at different throttle settings. Kinoshita et al. [6]

reported shorter ignition delay, similar thermal efficiency, lower HC and CO emission, 8% NOx reduction

and 50% smoke reduction compared to gas oil.

Corresponding author. Tel.: +603 79674448; fax: +603 79675317.

E-mail address: rizwanul.buet@gmail.com.

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2014 4th International Conference on Future Environment and Energy

IPCBEE vol.61 (2014) © (2014) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2014. V61. 17

The major drawback of biodiesel is its inferior oxidative and storage stability. Coconut biodiesel contains

small percentage of unsaturation, which also makes it susceptible to oxidative degradation. İleri and Koçar [8]

studied the effect of four antioxidant addition at various concentration to 20% canola biodiesel blend on

engine performance and emission characteristics of a turbocharged direct injection diesel engine. They found

2-ethylhexyl nitrate (EHN) diminishes NOx emission effectively with a mean 4.63% reduction. However,

CO emission increased for all antioxidants. Varatrajan and Cheralathan [9] the effect of two aromatic amine

antioxidants (N,Nˊ-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD))

added soybean biodiesel on engine criteria emissions in a single cylinder diesel engine. They found 9.35%

reduction of NO with a penalty of 9.09% increase in CO and 10.52% increase in HC for DPPD added 20%

soybean biodiesel. In another study, Varatharajan et al. [10] investigated effect of antioxidants on NOx

emission of jatropha biodiesel fuel containing 0.025%-m of additives, p-phenylenediamine, ethylenediamine ,

L-ascorbic acid, α-Tocopherol acetate, and BHT from a single cylinder diesel engine. Test results show that

p-phenylenediamine produced a mean reduction of 43.55% of NOx compared to neat biodiesel. However,

antioxidants addition increased HC and CO emission compared to neat biodiesel as well as blends. Kivevele

et al. [11] reported antioxidant PY dosed biodiesel showed lower BSFC compared to undosed biodiesel,

while having few effect on CO, HC and NOx emission. However, at full load condition, stabilized biodiesel

showed similar heat release as of diesel. As addition of antioxidant is preferred for higher oxidation and

greater storage stability, hence its effect on engine performance and emission needs proper investigation. The

present study studies the use of BHA and BHT added CME blends on engine emission characteristics of an

indirect injection diesel engine.

2. Materials and Methodology

2.1. Materials

In this study, crude coconut oil was purchased from local market. The antioxidants Butylated

hydroxyanisole (BHA) and 3, 5-di-tert-butyl-4-hydroxytoluene (BHT) were purchased from Sigma-Aldrich

(M) Sdn. Bhd. Other chemicals such as methanol, potassium hydroxide (KOH) and anhydrous sodium

sulfate (Na2SO4) were obtained from Merck Chemicals (Malaysia). Chemolab Supplies Sdn. Bhd. supplied

qualitative filter papers.

2.2. Methodology

In this study, biodiesel (CME) was produced using alkali catalyzed transesterification method. In this

process, 1.5 L crude coconut oil was placed with 25% v/v (of oil) methanol and 1% w/w (of oil) of

potassium hydroxide (KOH) in a 2.2 L jacketed reactor. Temperature was maintained at 60°C using

circulating water bath for 2 h and the mixture was stirred at 1100 rpm using motor stirrer. Afterwards, a

separation time of 12h was given to this mixture to separate out glycerin from methyl ester. The lower layer

containing impurities and glycerin was discarded. Then, the methyl ester was washed with distilled water to

remove the entrained impurities and glycerin. In this process, 50% (v/v) of distilled water at 60°C was

sprayed over the esters and shaken gently. The opaque lower layer containing water and impurities were

taken out. Then, methyl ester was distilled under vacuum distillation at 65°C for 1 h using rotary evaporator

to remove water and methanol. Finally, methyl ester was dried using anhydrous Na2SO4 for 3 h and filtered

using qualitative filter papers. Cetane Number (CN) of the produced biodiesel were calculated using

equations described in Ref. [12].

The test fuels were fossil diesel, 20% biodiesel in diesel (B20). To determine the effect of antioxidant

2000 ppm of BHA and BHT was added to B20 (B20 BHA and B20 BHT). Test fuels were blended using a

homogenizer device at a speed of 3000 rpm for ten minutes. Table 1 show the physicochemical properties of

biodiesel as well as tested fuels.

The tests were carried out at the Engine Laboratory of Mechanical Engineering Department, University

of Malaya on a four-cylinder diesel engine. The detail of the engine is described in Table 6. The test engine

coupled to Froude Hofman AG250 eddy current dynamometer. Ambient conditions during test was 30°C and

53% relative humidity. To carry out tests using biodiesel blends in this engine, it was first run with diesel for

few minutes to get a steady operating condition. Then fuel was changed to biodiesel blend. After

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consumption of 0.5 l of blend, the data acquisition was started to ensure the removal of residual diesel in the

fuel line. After each test engine was again run with diesel to drain out all the blends in the fuel line. This

procedure was followed for all the blends.

The engine was operated at rated load speed of 3500 rpm with 50% and 100% load. Fuel flow was

measured using KOBOLD ZOD positive-displacement type flow meter. REO-dCA Data Acquisition System

collects the data automatically. The exhaust emissions were measured using BOSCH BEA-350 exhaust gas

analyser. In this equipment, the CO measuring instrument use the non-dispersive infrared detectors, the NO

analyser uses the heated chemiluminescence detector (CLD), and the HC analyser uses the heated flame

ionization detector (FID). The accuracies of CO, HC and NO are 0.001 %vol., 1 ppm vol. and ≤ 1 ppm vol.

respectively. All the measurements were triplicated and the performance and emission measurements during

each test were highly repeatable within that test series.

Table 1: Characteristics of biodiesel blends and fossil diesel

Property Diesel B100 B20 B20 BHA B20 BHT

Calorific value (MJ/kg) 44.395 37.026 42.813 42.781 42.776

Kinematic Viscosity at 40°C (mm2/s) 4.0482 3.0741 3.6738 3.7007 3.6930

Dynamic Viscosity at 40°C (mPas) 3.3968 2.6439 3.0941 3.1181 3.1111

Density at 15°C (kg/m3) 851.2 876.3 860.3 860.7 860.5

Oxidation Stability (h) - 11.25 25.45 48.65 45.79

Flash point (°C) 82.5 122.5 93.5 94.5 94.5

Cetane Number 48 64.7 - - -

Cloud Point (°C) 8 -5 - - -

Pour Point (°C) 7 -4 - - -

3. Results and Discussion

Fig. 1(a) illustrates the effect of antioxidant addition on NO emissions. Since NO is the major component

of NOx, NO will be treated as NOx here. The thermal (Zeldovich), prompt (Fenimore), N2O pathway, fuel-

bound nitrogen and the NNH mechanism are the most common mechanisms for NOx formation in diesel

combustion. Among them thermal and prompt are the dominant mechanisms of NOx formation in biodiesel

combustion [13]. Due to presence of oxygen in the fuel, B20 produces 0.55%-3.25% higher NOx compared

to diesel as it helps in better combustion and provides high local peak temperatures [14], [15]. Antioxidant

addition to the blend has shown positive impact in reducing NO emission. Addition of BHA to B20 reduced

1.1% and 0.53% NOx emission at 50% and 100% load respectively relative to diesel. In addition, it also

provides 1.6% and 3.6% reduction of NOx compared to B20. Phenolic hydroxyl groups present in BHA

interfere the prompt NOx mechanism [16]. However, BHT addition provided about 2.2% and 3.7% increase

in NOx at 50% and 100% load respectively compared to diesel.

CO is formed during combustion, whenever charge is burned with an insufficient air supply with low

flame temperature [17]. The variation of CO emission with engine speed is shown in Figure 1(b). At 50%

load B20, “B20 BHA” and “B20 BHT” produced 20.8%, 8.3% and 12.5% lower CO emission compared to

diesel. At full load, they produced 44.6%, 40.9% and 42.9% lower CO emission compared to diesel. Higher

CN exhibits shorter ignition delay and permits for better combustion. Then oxygen content of biodiesel

comes into play which enhances the combustion process. High oxygen content ensures higher in-cylinder

combustion temperature promoting more complete combustion [18]. These results are in agreement with

previously reported trends [19]. Addition of BHA and BHT to B20 produced 6.5% and 13.02% higher CO

emission at full load that is primarily due to incomplete combustion resulting from antioxidant addition [20].

Fig. 2 shows that B20 produces about 30% and 50% lower HC emission both at half and full load

compared to diesel. This can be attributed to higher CN and oxygen content of the fuel [21]. Higher CN

reduces combustion delay, which in turn reduces HC emission. It is clear from the figure that addition of

antioxidants led to 14.3-28.5% increase in HC emission compared to B20 which may be attributed to

reduction of oxidative free radical formation.

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Fig. 1: (a) NO emission (b) CO emission with varying load at 3500 rpm

Fig. 2: HC emission with varying load at 3500 rpm

4. Conclusion

Since coconut biodiesel is slightly susceptible to oxidation, it needs antioxidant treatment. B20 produces

higher NO but much lower CO and HC emission due to its high oxygen content. BHA provides emission

benefits like reduction of 0.53-1.1% NO, 8.3-40.9% CO and 20-35% HC compared to diesel depending on

load. At full load effect of addition of BHA is more prominent. BHT, on the other hand, produced 2.2-3.7%

higher NO. However, CO and HC emission was 12.5-42.9% and 10-35.7% lower compared to diesel. The

effect on PM remains to be determined.

5. Acknowledgements

The authors would like to thank University of Malaya for financial support through High Impact

Research grant titled: Clean Diesel Technology for Military and Civilian Transport Vehicles having grant

number UM.C/HIR/MOHE/ENG/07.

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