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Evaluation of Performance and Emission Characteristics of Biodiesel Fuel Produced from Rapeseed Oil

The objective of the present study is to examine and compare the performance and emission characteristic of two biodiesel fuels produced from rapeseed oil via transesterification method. Tested biodiesel fuels (ROME (Rapeseed Oil Methyl Ester) and ROEE (Rapeseed Oil Ethyl Ester)) were selected based on their properties obtained from an optimization of transesterification conditions. A Yanmar diesel engine has led to evaluating their performance parameters such as fuel consumption rate, exhaust gas temperature and emission characteristic corresponding to nitrogen oxides (NOx), carbone monoxide (CO) and carbon dioxide (CO2). A comparative analysis was carried out using normal diesel fuel tested in same experimental conditions. Fuel consumption rate was measured by observing the volumetric rate from the fuel tank of the engine supported by stopwatch. The exhaust gas temperature and emission characteristic were measured simultaneously by using a testo 350 flue gas analyzer. According to the results, biodiesel fuels showed a higher fuel consumption rate and exhaust gas temperature under an increase of engine speed. They also exhibited lower NOx emission with a slight rise in CO and CO2 emission compared to mineral diesel fuel. ROME exhibited low emission gas compared to ROEE and mineral diesel. It can be evaluated as a promising alternative fuel for diesel engine.


Introduction 
Diesel engine constitutes the most cost effective internal combustion engine used in engineering machinery for power generation [1]. A vast majority of the most feasible options thanks to its renewability and compliance with fuel standard [7,8]. It can be obtained from biological sources such as vegetable oil, animal fat, algae and recycling cooking oil. These raw materials contain triglycerides molecules which are converted into fatty acid alkyl ester (biodiesel) by using a specific reaction called transesterification. This is a chemical process involving triglycerides with alcohol in the presence of a catalyst to yield biodiesel and glycerol as by-product [9][10][11][12][13]. The properties of the biodiesel depend strongly on this reaction and are strictly checked before its use in diesel engine. Nevertheless, the incomplete combustion of biodiesel in diesel engine releases harmful gas through the atmosphere. Among them, CO, CO 2 , and NOx are widely emitted. These emission gases are generally analyzed and compared to those from normal diesel in order to evaluate the viability of the synthesized biodiesel. In this line of investigation, many researches considered that CO emissions are reduced when diesel engine is fueled by pure biodiesel [14][15][16]. Experiments was carried out by Kalligeros et al. [17] and Usta et al. [18] to estimate the emission characteristic of biodiesel fuels produced from sunflower oil and tobacco seed oil respectively. Their results reveal that a low CO emission was noticed from produced fuels [17,18]. Many alcohol types are used in biodiesel production (methanol, ethanol, etc.). Their nature can influence the CO emission. Particularly, biodiesel based methyl esters emitted less CO compared to those based on ethyl esters [19]. On the other hand, biodiesel fuels are considered to emit low CO 2 compared to mineral diesel with 50%-80% reduction in CO 2 emissions [20]. In fact, biodiesels are lower-carbon fuels and have lower elemental carbon-to-hydrogen ratio than diesel [21,22]. However, in some case, CO 2 emissions from them are found more important compared to normal diesel fuels. This can be explained by the high combustion efficiency of diesel fuels [23,24]. As for NO x emission, some investigations found out an increase of this gas when using biodiesel to fuel diesel engine.
Nevertheless, Utlu and Kocak [25] and Hansen et al. [26] have noticed a decreasing trend of this gas from their researches. Some reasons are given to explain the complexity of NO x emission. Many researchers believe that the oxygen content in biodiesel is responsible for this tendency of high NO x emission. However, Lapuerta et al. [27] found that the oxygen content in biodiesel has no influence on NO x emission. Recent studies still show an effective reduction of these harmful gases when using biodiesel as an alternative to diesel fuels [28][29][30].
Therefore, consideration must be taken regarding fuels quality of biodiesel for better estimation of these emission gases. This quality characteristic is expressed by the fuels' properties of biodiesel which can be controlled by the transesterification process. Thus, the objective of this present study is to investigate the engine performance and emission characteristics of two biodiesel samples (ROME (Rapeseed Oil Methyl Ester) and ROEE (Rapeseed Oil Ethyl Ester)) obtained from rapeseed oil by transesterification reaction. Data obtained are analyzed to evaluate their quality as alternative for regular diesel and to confirm the efficiency of transesterification strategy used.

Fuel Properties
Biodiesels used in this study were obtained from rapeseed oil by transesterification process. The reaction involved methanol and ethanol to produce ROME and ROEE, respectively. The transesterification conditions were optimized based on the variation of the molar ratio of alcohol to rapeseed oil and alcohol type while other reaction parameters were kept constant. Each fuel produced was tested for compliance with ASTM (American Society for Testing and Materials) standards for density, viscosity and high heating value then compared to those of mineral diesels. From this optimization process, the optimal transesterification reaction conditions which give the best fuel properties were found to be an alcohol/oil molar ratio of 18:1, 1% of potassium hydroxide as catalyst, 30 min of reaction time, 60 °C of reaction temperature and stirring speed of 650 rpm. ROME and ROEE produced with these conditions present good fuel properties where ROME gives the best results. Both of them were used in the engine test. Mineral diesel was also tested in same condition to provide comparative data. Properties of these tested fuels are listed in Table 1, which are taken from the previous study [31].

Engine Test
Engine test was performed by using a Yanmar diesel engine TF70V-E model as shown in Fig. 1. Its specifications are given in Table 2.
Test was performed outdoors at ambient temperature of 32 °C and 29% of humidity. During each test, the engine was fully warmed up for 30 min prior to the beginning of data collection. The engine power was adjusted by the engine speed varied for 1,000, 1,500 and 2,000 rpm. A digital tachometer HT-5500 was used to measure the engine speed. Between each test, the fuel tank was completely drained and rinsed several times with the fuel for the next test. Each experiment was repeated three times to calculate the mean values. All data are obtained after the engine has reached a steady operation. Fuel consumption rate was measured by observing the volumetric rate from the fuel tank supported by stopwatch. The exhaust gas temperature and emission characteristic from the biodiesel were analyzed simultaneously by using a testo 350 Flue gas analyzer made by TESTO, Inc (from Germany). The gas analyzer was connected to the end of engine's chimney as shown in the experimental set up (Fig. 2) and its specifications are given in Table 3.

Effect of Biodiesel on Fuel Consumption Rate
Fuel consumption rate is a parameter used to estimate the long-term viability of fuel during engine operation. Fig. 3 shows the evolution of fuel consumption rate of fuels with the variation of engine speed. It can be seen that for any tested fuel, the fuel consumption rate increases with the increase of engine speed. In fact, when the engine speed is increased the friction horsepower increased according to the drop in the mechanical efficiency to maintain a fixed torque output thus leading to more fuel consumption [32]. By using biodiesel fuels, the engine fuel consumption rate is higher compared to when using mineral diesel. This difference can be explained by the low energy content of biodiesel compared to normal diesel (Table 1) [33,34].
As the result, a larger amount of biodiesel fuel must be supplied to the engine to attain the same engine power output as that of conventional diesel. At low and medium engine speed (1,000 and 1,500 rpm), the fuel consumption rate of ROEE is slightly higher than that of ROME because of the slight differences in their fuels properties. However, at high engine speed this difference disappeared as shown in Fig. 3.

Effect of Biodiesel on FGT (Flue Gas Temperature)
The FGT or exhaust gas temperature gives an indication about the combustion temperature. Its variation can influence the amount of exhausted pollutants from the burning fuel [35]. From Fig. 4, there is an increase in FGT with increase of engine speed for all fuels. In fact, diesel engine usually consumes more fuels to maintain a constant torque output when its speed is increased. Consequently, this releases more heat and hence increases the FGT [36]. Most of works of the literature stated that higher FGT is obtained when using biodiesel compared to other diesel fuel [37]. However, lower values are recorded for ROME during the experiment (Fig. 4). This can be attributed to the fact that, compared to mineral diesel, ROME exhibits higher latent heat of vaporization and lower heating value. Thus, more heat is needed for ROME vaporization, while the energy released by this biodiesel is lower than from the same mass of mineral diesel. As the result, the low FGT values for ROME can be considered as acceptable [38].

Acknowledgment
This work was supported by the JICA (Japan International Cooperation Agency) through the program of ABE Initiative (African Business Education Initiative for Youth) and the collaboration with Kumamoto University. We are so grateful to Thermal Engineering Laboratory for the use of their facilities.