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Biodiesels: Oxidizing enhancers to improve CI engine performance and emission quality
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel 5 6
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Biodiesels: Oxidizing enhancers to improve CI engine performance and emission quality
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Vu H. Nguyen a, Phuong X. Pham b,⇑
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a b
Le Quy Don Technical University, Viet Nam School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia
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h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A biodiesel was derived from residues
of a palm cooking-oil production process. B0, B10 and B20 blends were successfully tested in a modern single-cylinder engine. Correlations between fuel oxygen content and engine performance were observed. Oxygen content suppresses particle formation and decreases HC emission. Influence of oxygen content on NOx formation is minimal.
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a r t i c l e 3 6 3 4 34 35 36 37 38 39 40 41 42 43 44 45
i n f o
Article history: Received 4 February 2015 Received in revised form 14 March 2015 Accepted 1 April 2015 Available online xxxx Keywords: Biodiesel Oxygen content Ignition delay Particle NOx
a b s t r a c t Biodiesels, renewable fuels derived from biomass, animal fat, algae, wastes or residues, are amongst potential candidates to improve energy security and to reduce greenhouse gas emissions. Different from fossil diesel which mainly consists of hydrocarbons, biodiesels are oxygenated fuels. Oxygen contained in biodiesels, as an oxidizing enhancer, plays a crucial role in improving the auto-ignition quality. The oxygen enrichment coming from biodiesels causes the fuel–air mixture in the auto-ignition zone where the mixture is quite rich to become leaner, consequently improving combustion and emission quality. As such, a quantitative analysis of the impact of oxygen content in biodiesel–diesel blends in engine combustion and emission concentrations is crucial in giving key clues to fuel manufacturing as well as to internal combustion engine modeling. This study aims to establish a link between the oxygen content and engine performance as well as emission concentrations when using biodiesel blends B0, B10 and B20 (0%, 10% and 20% of biodiesel by volume in biodiesel–diesel mixtures, respectively). The biodiesel was derived from residues of the manufacturing process of palm cooking oil using methanol transesterification with the aid of a high hardness solid ceramic metal catalyst. The engine used in this test is a modern common-rail, single-cylinder engine operating under a fixed injection condition (injection timing, pressure and duration) but with a wide range of engine speeds. The results show that the oxygen enrichment coming from the fuel blends tested in this study lowers the engine power which is attributable to lower heating values of the biodiesel blends compared with that of the fossil diesel. However, the oxygen contained in the blends suppresses particle formation and decreases hydrocarbon and carbon monoxide concentrations. Ó 2015 Published by Elsevier Ltd.
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⇑ Corresponding author. E-mail addresses: (P.X. Pham).
[email protected],
[email protected]
http://dx.doi.org/10.1016/j.fuel.2015.04.004 0016-2361/Ó 2015 Published by Elsevier Ltd.
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Abbreviations AFR OFR FO FAME NHRR IMEP DCA TDC BTDC
Air Fuel Ratio Oxygen Fuel Ratio Fuel Oxygen Content Fatty Acid Methyl Ester Net Heat Release Rate Indicated Mean of Effective Pressure Degree of Crank Angle Top Dead Centre Before Top Dead Centre
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1. Introduction
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A recent analysis done by the U.S Energy Information Administration (EIA) [1] shows that, for several decades to come, liquid fuels will remain the dominant energy source to fuel internal combustion engines (the principal means of transportation). It is estimated that consumption of liquid fuels will contribute up to 99.28% of the total amount of fuel consumed by means of transportation in 2040 [1], albeit it is believed crude oil will be depleted in about 65 years [2]. Biodiesels are regarded as environmentally friendly fuels comprised entirely of renewable materials which have the potential to reduce fossil diesel dependency and greenhouse gas emissions [3–5] if their benefit and risks are carefully examined and managed [6,7]. Biodiesels are mixtures of fatty acid methyl (or ethyl) esters derived from vegetable oils, animal fat, algae, or residues of food processing processes by transesterification with the aid of methanol (or ethanol) as a solvent to lower the viscosity and surface tension sufficiently to enable adequate atomisation of the sprays in compression ignition engines [8–15]. While there are always two oxygen atoms in one fatty acid methyl ester, the oxygen content by mass in biodiesels depends on the fatty acid ester profile, specifically carbon chain length and unsaturation level. As such, it could be useful to link those parameters (carbon chain length, unsaturation degree and oxygen content) with fuel properties and engine performance as well as emission characteristics when using biodiesels. This paper reports our initial effort to examine the influence of oxygen content in biodiesel blends on fuel properties, engine performance and emission concentrations. Although oxygen contained in fuels can enhance the combustion process, it results in a lower heating value [8–11,13–15]. At the same air–fuel equivalence ratio (k), mixtures of oxygenated fuel and air are always leaner compared to mixtures of hydrocarbon fuels and air [16,17]. This is extremely important in spray autoignition, similar to the one in a compression ignition (CI) engine, as the autoignition occurs at a zone with a rich fuel–air mixture (equivalent ratio is about 3–4) [18]. Moser et al. [19] compared blends of ultra low sulfur diesel with 20% of soy bean methyl esters and observed that oxygen in the fuel improves lubricity, increases kinematic viscosity, lowers sulfur content, and results in inferior oxidative stability. The influence of fuel oxygen content on engine emission concentrations has been reported in several studies for alcohol [20– 22], non-alcohol fuels [23], and some short carbon chain ethers [24,25]. The addition of ethanol and methanol into diesel fuel was found to effectively decrease particle emission [20,21]. A high blending ratio of alcohol, however, creates serious problems with the fuel supply system and engine performance which is due to the poor solubility and lubricity and lower ignitability of alcohols compared to fossil diesel. Non-alcohol additives including methyl
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SoI SoC ID CN HHV LHV ISPM ISNOx
Start of Injection Start of Combustion Ignition Delay Cetane Number Higher Heating Value Lower Heating Value Indicated Specific Particle Mass Indicated Specific NOx
t-butyl ether (MTBE) and dimethyl ether (DME) were investigated for improvement of engine combustion and emissions [23]. DME has advantages over MTBE and alcohol due to its high ignitability, however, it is not suitable to mix with diesel as the gaseous fuel is transported much faster compared to the liquid competitor. Some ethers with oxygen content from 12.5% to 35% have been tested in a direct injection (DI) CI engine [24] and it was found that the amount of soot emission reduces with an increase of the blending ratio of the oxygenated fuels with fossil diesel. Suppression of soot in a CI engine fueled with ethers and diesel has been observed by Westbrook [25] using a detailed kinetic model. The soot suppression has been described as a reduction of soot precursors and the suppression efficiency strongly depends on the molecular structures of the oxygenated fuels. To the authors’ knowledge, there is a limited number of studies on the correlation of oxygen content in biodiesels and engine performance as well as emission concentrations. In this work, we aim to fill this gap by reporting the influence of fuel oxygen content on combustion and emission characteristics of a single cylinder common-rail engine operating with blends of biodiesel derived from residues of the manufacturing process of palm cooking oil.
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2. Fuel manufacturing
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Biodiesel used in this work was derived from residues of a palm cooking oil production process using a methanol transesterification process with the aid of a high hardness solid ceramic metal catalyst. It was found that the residues are still rich in fatty acid esters which have potential to manufacture biodiesel. The catalyst was invented by Yoo et al. [26] using a sintering process. A recent application to manufacture the biodiesel used in this work was reported in [27]. In this paper, three fuel blends including B0, B10 and B20 (corresponding with 0%, 10% and 20% by volume of biodiesel in biodiesel–diesel mixtures, respectively) are experimentally tested in an engine and therefore this section briefly reports the important physicochemical properties of these fuel blends in comparison with those of pure biodiesel (B100) and fossil diesel. Table 1 shows the fatty acid ester compositions and important properties of diesel (B0), blends B10 and B20 used in this paper. Physicochemical properties of the pure biodiesel (B100) are also listed in Table 1 for further information. Fatty acid ester profiles of the biodiesel were measured using a gas chromatography mass spectrometry (GCMS) analysis. The iodine value and saponification number of the biodiesel imply that this is a long carbon chain length and partially unsaturated biodiesel. Viscosity was measured using the Brookfield DV-III Rheometer and following the ASTM D445 standard. Higher heating values of the biodiesel blends were tested using a bomb calorimeter. Cetane numbers were tested using a cooperative fuel research (CFR) engine operating under ASTM D613 standard.
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Testing method
B100
B10
B20
D
Ester content Glycerin content Phosphorus content Sodium/potassium, combined Oxidation stability, at 110 °C
[wt%] [wt%] [wt%] mg/kg Hrs
EN 14103 ASTM D6584 ASTM D4951 EN14538 EN14112
98.91 0.0 0.0002 0.1 6.02
– – – – 111.9
– – – – 24.07
– – – – –
Palmitic, C16:0 Stearic, C18:0 Oleic, C18:1 Linoleic, C18:2
[wt%] [wt%] [wt%] [wt%]
– – – –
28.09 9.53 43.47 18.02
– – – –
– – – –
– – – –
Iodine value Saponification number Acid number Water content Flash point Kinematic viscosity, at 40 °C Rel. density, at 15 °C Higher heating values Cloud point Pour point Cetane number
gI/100 g mgKOH/g mgKOH/g % kl °C mm2/s [kg/m3] MJ/kg °C °C –
EN14111 ASTM D66404 ASTM D664 ASTM D95-05 ASTM D 93 ASTM D445 ASTM D 1298 – ASTM D 2500 ASTM D97 ASTM D613
48.0 177.3 0.06 0.20 152 4.1 0.869 – 18 – 66.9
– – – – – 3.25 0.841 43.86 – 3 53.7 –
– – – – – 3.38 0.845 43.11 – 3 54.5 –
– – – – – 3.14 0.836 45.19 – – 52.4
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3. Experimental setup
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Experiments were conducted using a single-cylinder research engine schematically shown in Fig. 1 and the engine specifications are described in Table 2. This is an AVL 5402 common-rail highspeed CI engine. The engine has a central fuel injector and four valves equipped with a dual overhead camshaft valve-train (DOCH). The engine is equipped with a Bosch common-rail injection system which is capable of injecting fuel at a maximum pressure of 135 MPa. Fuel consumption was measured using an AVL-FuelBance 733S. In-cylinder pressures were measured using a flush mounted water-cooled AVL QC34C quartz piezo-electric pressure transducer. CO and CO2 were measured using a non dispersive infrared detector (NDIF). NOx was detected using a chemiluminescence analyzer while total hydrocarbon (THC) levels were detected by a flammable ionization meter. Particle concentration was measured using an AVL Opacimeter 439. The injection duration remained constant at 800 ls. The engine was operated under a wide range of speeds (1400–2400 rpm), the injection timing varied between 16 and 24 degrees of crank angle (DCA) before top dead center (BTDC) while the injection pressure was kept constant at 400 bars. To examine the influence of oxygen content on engine performance, results reported here are only for one fixed injection timing, 20 DCA BTDC. The effect of injection timing variation will be reported in a separate work.
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4. Results and discussion
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4.1. Biodiesel heating value
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Heating value (HV), a measure of the energy content in a fuel, is dependent on the fuel’s compositions. There are two types of calorific value namely higher heating values (HHV) and lower heating values (LHV). The HHV of a fuel, also known as gross calorific value, is the heat energy released by a complete combustion of 1 kg of that fuel at constant volume at the standard conditions (101.3 kPa and 25 °C). The HHV takes into account the latent heat of vaporization of water. LHV, however, does not include the water vaporization heat and it is therefore defined as net calorific value. HHV is usually measured using a bomb calorimeter while LHV is usually estimated by subtracting the heat of vaporization of water
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vapor from HHV [5]. The water vaporization heat of a fuel can be computed using hydrogen content of the fuel. Higher heating values of six saturated mono fatty acid methyl esters – FAMEs (C8:0, C10:0. C12:0, C14:0, C16:0 and C18:0) and four unsaturated FAMEs (C16:1, C18:1, C18:2, and C18:3) are plotted versus fuel oxygen content by weight in Fig. 2a. Heating values of fatty acid esters used in this analysis are available in [5]. It is clear from Fig. 2a that the HHVs are almost proportional to the oxygen content of these FAMEs. An increase from 10% to 20% in the oxygen content leads to a reduction of approximately 12% in HHV. Since the proportional function of the fuel oxygen content has been observed for HHV of FAMEs, a question could be raised whether a similar trend is expected for LHV. As discussed in the first paragraph of this section the LHV of a fuel is usually estimated by subtracting the water vaporization heat from HHV which is usually measured using a calorimeter. The water vaporization heat is a function of hydrogen contained in this fuel. Hydrogen content of FAMEs having a similar number of double bonds is a linear function of oxygen contained in those FAMEs. For example, saturated FAMEs (Cn H2n O2 ) have hydrogen content FH = 2n⁄FO/32 = 0.0625n⁄FO; where n is the number of carbon atoms in the FAMEs’ molecule, and FH and FO are the hydrogen and oxygen content by mass, respectively. Combining this and the linear function observed for HHV as discussed, a linear trend could be developed for FAMEs’ LHV. An attempt has been made in Fig. 2b to develop a link between HHV of biodiesel blends used in this paper and their oxygen content. It is quite interesting in Fig. 2b that a similar trend as that shown in Fig. 2a is also observed. A penalty of 3% is shown in heating value when utilizing blend B20 with respect to fossil diesel. The outcome implies that heating values of biodiesel constituents as well as biodiesel blends could be computed as a function of oxygen fraction in the fuels.
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4.2. Biodiesel cetane number
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Cetane number (CN) is a dimensionless indicator of the ignition quality of auto-ignition fuels. CN is usually linked to the ignition delay times of the fuels. A lower CN fuel usually has a longer ignition delay time with respect to higher CN fuels. However, too low or too high a CN can cause engine operational problems [5]. If a CN
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Fig. 1. Schematic of experimental engine.
Table 2 Single-cylinder research engine specifications.
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Manufacturer Serry
AVL CI engine – 5402
Number of cylinders Valves Injection type Nozzle type Max. rail pressure [MPa] Number of hole/diameter [mm] Spray included angle [°] Bore Stroke [mm] Connection rod [mm] Swept volume [l] Swirl ratio Rated power [kW] Compression ratio Dynamometer type
1 4 DOHC Bosch high-pressure common-rail Valve covered orifice 135 5/0.17 142 85 90 138 0.51 1.78 9 (at 3200 rpm) 17.3:1 APA100
is too high, combustion can occur before the fuel and air are properly mixed, resulting in incomplete combustion and smoke. If a CN is too low, engine roughness, misfiring, higher air temperatures, slower engine warm-up, and also incomplete combustion occur. Consequently, most of the US engine manufacturers suggest a range of CN between 40 and 50 for diesel fuels [5]. Biodiesels usually have a higher CN compared to fossil diesel. Minimum CN values of biodiesels are specified in both ASTM D6751 (min. CN = 47) and EN14214 (min. CN = 51). Cetane numbers of diesel fuel tested in this work and mono FAMEs are plotted in Fig. 3a versus fuel oxygen content by mass. In this figure, the black and solid arrow represents an increasing trend in the carbon chain length of saturated FAMEs while an increase in the unsaturation degree is illustrated by the brown dashed arrow. It is evident from the figure that diesel’s CN is close to that of methyl decanoate, C10:0 [28], and an increase in the
carbon chain length (see the black solid arrow) leads to a decrease in the fuel oxygen content and a substantial increase in cetane number. The cetane number is almost doubled when the carbon chain length increases from C10:0 to C18:0. An increase in unsaturation degree (see the brown dashed arrow), on the other hand, significantly decreases the cetane number of the fuels. With the same carbon chain length, CN of C18:3 is only one-fourth that of C18:0. A link between oxygen content and cetane values of the biodiesel blends used in this work are also observed in Fig. 3b. In this figure, CN of fossil diesel (or B0), biodiesel blends (B10, B20, B40, B60 and B80) and pure biodiesel (B100) are plotted versus fuel oxygen content. Only B0, B10 and B20 were used in the engine tests however the cetane number of other biodiesel blends were also tested targeting for a study of reactivity for these fuel blends. As can be seen from this figure, biodiesel blending enhances CN of the fuel mixture. An approximate 40% improvement in the cetane value occurs when the fuel oxygen content increases from 0% to 12%. The oxygen content in biodiesels may cause the fuel–air mixture to be leaner at the auto-ignition zone of the fuel spray which consequently improves the reactivity of the biodiesel blends.
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4.3. Biodiesel injection timing
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It is found in this study that fuel injection timing is similar regardless of the fuel type and this is expected with a common-rail injection system. In a common-rail system, the lines connecting the fuel pump and injectors are being highly-pressurized. If the energizing time is electronically fixed by the engine control unit, the injection process (and more specifically the start of injection) is expected to be almost independent of fuel properties and engine working conditions (engine speed and torque) [29]. In mechanical systems, however, the higher compressibility (or bulk modulus) of biodiesels compared to fossil diesel leads to an earlier injection
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(a) Mono FAMEs
5
(b) Tested biodiesel blends and fossil diesel
Fig. 2. Correlation of fuel oxygen content and higher heating value (HHV): (a) mono-fatty acid methyl esters; (b) biodiesel blends and fossil diesel.
(a) Mono FAMEs and biodiesels used in[30,31]
(b) Biodiesel blends and fossil diesel
Fig. 3. Correlation of fuel oxygen content and CN for: (a) mono-fatty acid methyl esters including saturated FAMEs (C10:0, C12:0. C14:0, C16:0 and C18:0) and unsaturated FAMEs (C18:1, C18:2 and C18:3); (b) biodiesel blends and fossil diesel used in this paper.
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time [5]. The findings in this study are in good agreement with previous works [30,31].
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4.4. Engine performance and exhaust emission concentrations
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Fig. 4a shows the p h data for biodiesel blends (B0, B10 and B20). Similarly, the net heat release rate (NHRR) is shown in Fig. 4b. In order to examine the role of the oxygen content, data
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(a) In-cylinder pressure
shown in Fig. 4a and b as well as throughout this section is only for one fixed engine condition, 1800 rpm of engine speed and 20 DCA of injection timing. It is clear from Fig. 4a and b that the auto-ignition times of biodiesel blends B10 and B20 occur earlier compared with that of fossil diesel (B0). Adding biodiesel into the fuel mixture enhances its reactivity, as discussed in Section 4.2, consequently reducing the ignition delay times. A difference between ignition delay times of fossil diesel and these two
(b) Net heat release rate
Fig. 4. In-cylinder pressure (a) and net heat release rate (NHRR) (b) of B0, B10 and B20 at 1800 rpm, 20 DCA of injection timing, 400 bars of injection pressure.
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biodiesel blends is approximately 2–3 DCA as shown in Fig. 4a and b. However, ignition delay times of biodiesel blends B10 and B20 are quite similar as shown in these figures. Allen et al. [32] investigated auto-ignition characteristics of fossil diesel and canola-derived biodiesel using a rapid compression machine at a temperature range of 676–816 K and found that biodiesel ignites faster and increases the temperature sensitivity with respect to fossil diesel. It can also be seen that both premixed and diffusion combustion are visible in Fig. 4b. For example, premixed combustion of the biodiesel blends occurs in approximately 5 DCA duration and generates a pretty high heat release rate with a peak at approximately 5 DCA before top dead center (BTDC). The diffusion combustion occurs with much lower heat release rate and without an observable peak. This is in contrast with findings in Pham et al. [30,31] that the heat release rate is generated mainly in diffusion combustion process in their engine. It is notable that the fuel injection timing used in [30,31] is at around top dead center (TDC) and therefore the premixed combustion duration is quite short compared to that in this current study. Indicated mean of effective pressure (IMEP) values of biodiesel blends at three engine speed conditions (1400, 1800 and 2400 rpm, respectively) are plotted versus fuel oxygen content in Fig. 5. It should be noted that IMEP is a linear function of indicated power and is commonly used to describe characteristics of engine performance. It is noted from Fig. 5 that IMEP values reduce slightly when fuel oxygen (FO) content increases from 0% to 1.2%. When FO exceeds 1.2, however, IMEP reduces more rapidly. The trends are true for all of the engine speed conditions tested here. These results imply that the reduction in IMEP (which is due to lower calorific value of biodiesel blends) could be almost eliminated when using low blending ratios of biodiesel. This can be attributed to an improvement in lubricant quality when adding biodiesel into fossil diesel [5]. Low biodiesel blending ratios such as B2 to B5 have been reported to increase engine power and thermal efficiency [5]. At a high blending ratio, however, the advantage gained from lubricant enhancer is minimal with respect to the penalty in engine power which is due to calorific value reduction. It should be noted that the maximum mass fraction of oxygen contained in the biodiesel blends tested in this study is about 2.5% (corresponding to B20) and therefore the results observed here could be thought to be biased. However, our recent experiment campaign in a 6-cylinder common-rail engine operating with different biodiesel blends (B0, B20, B50 and B100) was conducted and the link between biodiesel oxygen content and combustion as well as emission was also observed. In the campaign, different biodiesels with different molecular structures were examined
Fig. 5. IMEP of biodiesel blends and fossil diesel versus fuel oxygen content at three engine speed conditions (1400, 1800 and 2400 rpm respectively), 20 DCA of injection timing, 400 bars of injection pressure.
and some results were reported in [33,30,31,34]. Primary results of the oxygen role have been reported in Pham’s PhD thesis [33] and further work on this topic is in progress and will be reported in the future. Relative indicated specific emission concentrations are plotted versus the fuel oxygen content in Fig. 6a–e for indicated specific particle mass (ISPM), indicated specific NOx (ISNOx), indicated specific carbon monoxide (ISCO), indicated specific carbon dioxide (ISCO2), and indicated specific hydrocarbon (ISHC), respectively. These are normalized values using corresponding indicated specific emission levels of diesel. Relative ISPM of B10, for example, is the ratio of ISPM concentration of B10 to that of fossil diesel. It is evident from Fig. 6a that the relative ISPM is proportional to the FO and somewhat independent of the engine speed conditions. An increase in the fuel oxygen content from 0% to 2.5% results in a decrease of approximately 30% in the ISPM concentration. The oxygen in biodiesel constituents (FAMEs) significantly suppresses the particle formation and enhances the particle oxidation which leads to a reduction in particle concentration. The oxygen atoms could cause suppression of soot formation by effectively removing carbon from reaction pathways that lead to soot precursors [35]. Similarly, ISNOx is shown in Fig. 6b. It can be seen that the relative ISNOx varies in a narrow range from 0.95 to 1.05. Compared to NOx level of fossil diesel, a slight decrease is observed for B10 while a slight increase is shown for B20. NOx mechanisms are very complex and encompass many chemical processes. A comprehensive review on this topic is available in [36]. NOx formation can be attributed to thermal NOx (where the nitrogen from inlet air is oxidized at high temperature in the combustion chamber, >1500 °C), prompt NOx (formed by high speed reactions at the flame front), and fuel NOx (produced from nitrogen-bearing fuels) [37]. A link between NOx formation and ignition delay time was found in previous studies [38,39]. The difference in the cylinder temperature could have certain effects on thermal NOx mechanism [24]. In this paper, B10 and B20 have quite similar ignition delay time as shown in Fig. 4a and b, however, their ISNOx levels are about 10% different. This implies that ignition delay is not a sole factor affecting NOx formation. More work is required to understand NOx mechanism of the biodiesel blends. Relative ISCO and ISCO2 levels are shown in Fig. 6c and d, respectively. In general, an increase in fuel oxygen content leads to a decrease in ISCO while an increase in ISCO2 level is observed. A small decrease in the CO2 concentration is shown when FO increases from 0% to 1.2% (Fig. 6d) and this could be attributable to the only slight reduction in engine power when using B10 as discussed earlier in this section. The opposite trend in CO and CO2 levels is usually expected because the extra oxygen from oxygenates enhances the chemical process to convert CO into CO2. Fig. 6e plots relative ISHC concentrations versus fuel oxygen content. When FO increases, similarly to the ISPM trend observed in Fig. 6a, a significant reduction is also obtained for ISHC levels. This is acceptable as ISPM and ISHC are two measures of unburnt fuel proportions. It is notable that discussions related to biodiesel blends in this paper relates only to a specific biodiesel manufactured from residues of a palm cooking-oil production process. Influences of variations in fatty acid ester profiles of biodiesel produced from different feedstocks on biodiesel properties [5], biodiesel atomization, combustion and emission characteristics [33] are significant. Some results related to the role of oxygen in biodiesel properties, biodiesel atomization, combustion and emission characteristics have recently been reported in [40,41,34,42,43] for a wide range of biodiesels with different molecular structures. It has also been revealed that a critical key to reducing the total particle mass, particle size, particle number, and black carbon concentration is to increase the FO. However, an increase in the FO leads to a
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(a) ISPM
(c) ISCO
(b) ISNOx
(d) ISCO2
(e) ISHC Fig. 6. Relative indicated specific emission concentrations of biodiesel blends versus fuel oxygen content at 20 DCA of fuel injection timing, 400 bars of injection pressure, and three different engine speed conditions (1400, 1800 and 2400 respectively).
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substantial increase in the total particle number per unit of particle mass, the amount of black carbon per unit of particle mass, and the reactive oxygen species concentration [30,31].
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5. Conclusions
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A biodiesel was manufactured using methanol transesterification of waste palm oil (residues of cooking oil production processes) and then successfully tested in a modern common-rail CI engine. Compared to fossil diesel, the biofuel is an oxygenated fuel
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which has a higher cetane number but a lower heating value. Other properties of this biofuel are comparable with those of fossil diesel. The effect of oxygen enhancement (obtained by adding the biodiesel into fossil diesel) is examined in a single-cylinder research engine. Biodiesel blends (B0, B10 and B20) were tested which supply up to 2.5% of the fuel oxygen content by mass. The engine was operated under a broad range of engine speeds (1400, 1800, and 2400 rpm) and fuel injection timing (16–24 DCA). Engine combustion characteristics and exhaust emission concentrations were reported. An increase in fuel oxygen content leads to a reduction
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in fuel heating value and consequently a reduction in engine power while the biodiesel enhances the cetane number which leads to shorter ignition delay times. The FO is a key to reducing particle concentrations, HC, and CO while the effect of oxygen content on ISNOx is minimal.
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Acknowledgments
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The authors are grateful for the support of the Directorate of programs on ‘‘Biofuels development until 2015 and vision for 2025’’, The Ministry of Industry and Trade – Vietnam.
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References
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel 5 6
4
Biodiesels: Oxidizing enhancers to improve CI engine performance and emission quality
7
Vu H. Nguyen a, Phuong X. Pham b,⇑
3
8 9
a b
Le Quy Don Technical University, Viet Nam School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia
11 10 12 1 4 215 9 16 17 18 19 20 21 22 23 24 25 26 27 28
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A biodiesel was derived from residues
of a palm cooking-oil production process. B0, B10 and B20 blends were successfully tested in a modern single-cylinder engine. Correlations between fuel oxygen content and engine performance were observed. Oxygen content suppresses particle formation and decreases HC emission. Influence of oxygen content on NOx formation is minimal.
31
a r t i c l e 3 6 3 4 34 35 36 37 38 39 40 41 42 43 44 45
i n f o
Article history: Received 4 February 2015 Received in revised form 14 March 2015 Accepted 1 April 2015 Available online xxxx Keywords: Biodiesel Oxygen content Ignition delay Particle NOx
a b s t r a c t Biodiesels, renewable fuels derived from biomass, animal fat, algae, wastes or residues, are amongst potential candidates to improve energy security and to reduce greenhouse gas emissions. Different from fossil diesel which mainly consists of hydrocarbons, biodiesels are oxygenated fuels. Oxygen contained in biodiesels, as an oxidizing enhancer, plays a crucial role in improving the auto-ignition quality. The oxygen enrichment coming from biodiesels causes the fuel–air mixture in the auto-ignition zone where the mixture is quite rich to become leaner, consequently improving combustion and emission quality. As such, a quantitative analysis of the impact of oxygen content in biodiesel–diesel blends in engine combustion and emission concentrations is crucial in giving key clues to fuel manufacturing as well as to internal combustion engine modeling. This study aims to establish a link between the oxygen content and engine performance as well as emission concentrations when using biodiesel blends B0, B10 and B20 (0%, 10% and 20% of biodiesel by volume in biodiesel–diesel mixtures, respectively). The biodiesel was derived from residues of the manufacturing process of palm cooking oil using methanol transesterification with the aid of a high hardness solid ceramic metal catalyst. The engine used in this test is a modern common-rail, single-cylinder engine operating under a fixed injection condition (injection timing, pressure and duration) but with a wide range of engine speeds. The results show that the oxygen enrichment coming from the fuel blends tested in this study lowers the engine power which is attributable to lower heating values of the biodiesel blends compared with that of the fossil diesel. However, the oxygen contained in the blends suppresses particle formation and decreases hydrocarbon and carbon monoxide concentrations. Ó 2015 Published by Elsevier Ltd.
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⇑ Corresponding author. E-mail addresses: (P.X. Pham).
[email protected],
[email protected]
http://dx.doi.org/10.1016/j.fuel.2015.04.004 0016-2361/Ó 2015 Published by Elsevier Ltd.
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Abbreviations AFR OFR FO FAME NHRR IMEP DCA TDC BTDC
Air Fuel Ratio Oxygen Fuel Ratio Fuel Oxygen Content Fatty Acid Methyl Ester Net Heat Release Rate Indicated Mean of Effective Pressure Degree of Crank Angle Top Dead Centre Before Top Dead Centre
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1. Introduction
71
A recent analysis done by the U.S Energy Information Administration (EIA) [1] shows that, for several decades to come, liquid fuels will remain the dominant energy source to fuel internal combustion engines (the principal means of transportation). It is estimated that consumption of liquid fuels will contribute up to 99.28% of the total amount of fuel consumed by means of transportation in 2040 [1], albeit it is believed crude oil will be depleted in about 65 years [2]. Biodiesels are regarded as environmentally friendly fuels comprised entirely of renewable materials which have the potential to reduce fossil diesel dependency and greenhouse gas emissions [3–5] if their benefit and risks are carefully examined and managed [6,7]. Biodiesels are mixtures of fatty acid methyl (or ethyl) esters derived from vegetable oils, animal fat, algae, or residues of food processing processes by transesterification with the aid of methanol (or ethanol) as a solvent to lower the viscosity and surface tension sufficiently to enable adequate atomisation of the sprays in compression ignition engines [8–15]. While there are always two oxygen atoms in one fatty acid methyl ester, the oxygen content by mass in biodiesels depends on the fatty acid ester profile, specifically carbon chain length and unsaturation level. As such, it could be useful to link those parameters (carbon chain length, unsaturation degree and oxygen content) with fuel properties and engine performance as well as emission characteristics when using biodiesels. This paper reports our initial effort to examine the influence of oxygen content in biodiesel blends on fuel properties, engine performance and emission concentrations. Although oxygen contained in fuels can enhance the combustion process, it results in a lower heating value [8–11,13–15]. At the same air–fuel equivalence ratio (k), mixtures of oxygenated fuel and air are always leaner compared to mixtures of hydrocarbon fuels and air [16,17]. This is extremely important in spray autoignition, similar to the one in a compression ignition (CI) engine, as the autoignition occurs at a zone with a rich fuel–air mixture (equivalent ratio is about 3–4) [18]. Moser et al. [19] compared blends of ultra low sulfur diesel with 20% of soy bean methyl esters and observed that oxygen in the fuel improves lubricity, increases kinematic viscosity, lowers sulfur content, and results in inferior oxidative stability. The influence of fuel oxygen content on engine emission concentrations has been reported in several studies for alcohol [20– 22], non-alcohol fuels [23], and some short carbon chain ethers [24,25]. The addition of ethanol and methanol into diesel fuel was found to effectively decrease particle emission [20,21]. A high blending ratio of alcohol, however, creates serious problems with the fuel supply system and engine performance which is due to the poor solubility and lubricity and lower ignitability of alcohols compared to fossil diesel. Non-alcohol additives including methyl
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SoI SoC ID CN HHV LHV ISPM ISNOx
Start of Injection Start of Combustion Ignition Delay Cetane Number Higher Heating Value Lower Heating Value Indicated Specific Particle Mass Indicated Specific NOx
t-butyl ether (MTBE) and dimethyl ether (DME) were investigated for improvement of engine combustion and emissions [23]. DME has advantages over MTBE and alcohol due to its high ignitability, however, it is not suitable to mix with diesel as the gaseous fuel is transported much faster compared to the liquid competitor. Some ethers with oxygen content from 12.5% to 35% have been tested in a direct injection (DI) CI engine [24] and it was found that the amount of soot emission reduces with an increase of the blending ratio of the oxygenated fuels with fossil diesel. Suppression of soot in a CI engine fueled with ethers and diesel has been observed by Westbrook [25] using a detailed kinetic model. The soot suppression has been described as a reduction of soot precursors and the suppression efficiency strongly depends on the molecular structures of the oxygenated fuels. To the authors’ knowledge, there is a limited number of studies on the correlation of oxygen content in biodiesels and engine performance as well as emission concentrations. In this work, we aim to fill this gap by reporting the influence of fuel oxygen content on combustion and emission characteristics of a single cylinder common-rail engine operating with blends of biodiesel derived from residues of the manufacturing process of palm cooking oil.
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2. Fuel manufacturing
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Biodiesel used in this work was derived from residues of a palm cooking oil production process using a methanol transesterification process with the aid of a high hardness solid ceramic metal catalyst. It was found that the residues are still rich in fatty acid esters which have potential to manufacture biodiesel. The catalyst was invented by Yoo et al. [26] using a sintering process. A recent application to manufacture the biodiesel used in this work was reported in [27]. In this paper, three fuel blends including B0, B10 and B20 (corresponding with 0%, 10% and 20% by volume of biodiesel in biodiesel–diesel mixtures, respectively) are experimentally tested in an engine and therefore this section briefly reports the important physicochemical properties of these fuel blends in comparison with those of pure biodiesel (B100) and fossil diesel. Table 1 shows the fatty acid ester compositions and important properties of diesel (B0), blends B10 and B20 used in this paper. Physicochemical properties of the pure biodiesel (B100) are also listed in Table 1 for further information. Fatty acid ester profiles of the biodiesel were measured using a gas chromatography mass spectrometry (GCMS) analysis. The iodine value and saponification number of the biodiesel imply that this is a long carbon chain length and partially unsaturated biodiesel. Viscosity was measured using the Brookfield DV-III Rheometer and following the ASTM D445 standard. Higher heating values of the biodiesel blends were tested using a bomb calorimeter. Cetane numbers were tested using a cooperative fuel research (CFR) engine operating under ASTM D613 standard.
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Testing method
B100
B10
B20
D
Ester content Glycerin content Phosphorus content Sodium/potassium, combined Oxidation stability, at 110 °C
[wt%] [wt%] [wt%] mg/kg Hrs
EN 14103 ASTM D6584 ASTM D4951 EN14538 EN14112
98.91 0.0 0.0002 0.1 6.02
– – – – 111.9
– – – – 24.07
– – – – –
Palmitic, C16:0 Stearic, C18:0 Oleic, C18:1 Linoleic, C18:2
[wt%] [wt%] [wt%] [wt%]
– – – –
28.09 9.53 43.47 18.02
– – – –
– – – –
– – – –
Iodine value Saponification number Acid number Water content Flash point Kinematic viscosity, at 40 °C Rel. density, at 15 °C Higher heating values Cloud point Pour point Cetane number
gI/100 g mgKOH/g mgKOH/g % kl °C mm2/s [kg/m3] MJ/kg °C °C –
EN14111 ASTM D66404 ASTM D664 ASTM D95-05 ASTM D 93 ASTM D445 ASTM D 1298 – ASTM D 2500 ASTM D97 ASTM D613
48.0 177.3 0.06 0.20 152 4.1 0.869 – 18 – 66.9
– – – – – 3.25 0.841 43.86 – 3 53.7 –
– – – – – 3.38 0.845 43.11 – 3 54.5 –
– – – – – 3.14 0.836 45.19 – – 52.4
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3. Experimental setup
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Experiments were conducted using a single-cylinder research engine schematically shown in Fig. 1 and the engine specifications are described in Table 2. This is an AVL 5402 common-rail highspeed CI engine. The engine has a central fuel injector and four valves equipped with a dual overhead camshaft valve-train (DOCH). The engine is equipped with a Bosch common-rail injection system which is capable of injecting fuel at a maximum pressure of 135 MPa. Fuel consumption was measured using an AVL-FuelBance 733S. In-cylinder pressures were measured using a flush mounted water-cooled AVL QC34C quartz piezo-electric pressure transducer. CO and CO2 were measured using a non dispersive infrared detector (NDIF). NOx was detected using a chemiluminescence analyzer while total hydrocarbon (THC) levels were detected by a flammable ionization meter. Particle concentration was measured using an AVL Opacimeter 439. The injection duration remained constant at 800 ls. The engine was operated under a wide range of speeds (1400–2400 rpm), the injection timing varied between 16 and 24 degrees of crank angle (DCA) before top dead center (BTDC) while the injection pressure was kept constant at 400 bars. To examine the influence of oxygen content on engine performance, results reported here are only for one fixed injection timing, 20 DCA BTDC. The effect of injection timing variation will be reported in a separate work.
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4. Results and discussion
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4.1. Biodiesel heating value
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Heating value (HV), a measure of the energy content in a fuel, is dependent on the fuel’s compositions. There are two types of calorific value namely higher heating values (HHV) and lower heating values (LHV). The HHV of a fuel, also known as gross calorific value, is the heat energy released by a complete combustion of 1 kg of that fuel at constant volume at the standard conditions (101.3 kPa and 25 °C). The HHV takes into account the latent heat of vaporization of water. LHV, however, does not include the water vaporization heat and it is therefore defined as net calorific value. HHV is usually measured using a bomb calorimeter while LHV is usually estimated by subtracting the heat of vaporization of water
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vapor from HHV [5]. The water vaporization heat of a fuel can be computed using hydrogen content of the fuel. Higher heating values of six saturated mono fatty acid methyl esters – FAMEs (C8:0, C10:0. C12:0, C14:0, C16:0 and C18:0) and four unsaturated FAMEs (C16:1, C18:1, C18:2, and C18:3) are plotted versus fuel oxygen content by weight in Fig. 2a. Heating values of fatty acid esters used in this analysis are available in [5]. It is clear from Fig. 2a that the HHVs are almost proportional to the oxygen content of these FAMEs. An increase from 10% to 20% in the oxygen content leads to a reduction of approximately 12% in HHV. Since the proportional function of the fuel oxygen content has been observed for HHV of FAMEs, a question could be raised whether a similar trend is expected for LHV. As discussed in the first paragraph of this section the LHV of a fuel is usually estimated by subtracting the water vaporization heat from HHV which is usually measured using a calorimeter. The water vaporization heat is a function of hydrogen contained in this fuel. Hydrogen content of FAMEs having a similar number of double bonds is a linear function of oxygen contained in those FAMEs. For example, saturated FAMEs (Cn H2n O2 ) have hydrogen content FH = 2n⁄FO/32 = 0.0625n⁄FO; where n is the number of carbon atoms in the FAMEs’ molecule, and FH and FO are the hydrogen and oxygen content by mass, respectively. Combining this and the linear function observed for HHV as discussed, a linear trend could be developed for FAMEs’ LHV. An attempt has been made in Fig. 2b to develop a link between HHV of biodiesel blends used in this paper and their oxygen content. It is quite interesting in Fig. 2b that a similar trend as that shown in Fig. 2a is also observed. A penalty of 3% is shown in heating value when utilizing blend B20 with respect to fossil diesel. The outcome implies that heating values of biodiesel constituents as well as biodiesel blends could be computed as a function of oxygen fraction in the fuels.
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4.2. Biodiesel cetane number
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Cetane number (CN) is a dimensionless indicator of the ignition quality of auto-ignition fuels. CN is usually linked to the ignition delay times of the fuels. A lower CN fuel usually has a longer ignition delay time with respect to higher CN fuels. However, too low or too high a CN can cause engine operational problems [5]. If a CN
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Fig. 1. Schematic of experimental engine.
Table 2 Single-cylinder research engine specifications.
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Manufacturer Serry
AVL CI engine – 5402
Number of cylinders Valves Injection type Nozzle type Max. rail pressure [MPa] Number of hole/diameter [mm] Spray included angle [°] Bore Stroke [mm] Connection rod [mm] Swept volume [l] Swirl ratio Rated power [kW] Compression ratio Dynamometer type
1 4 DOHC Bosch high-pressure common-rail Valve covered orifice 135 5/0.17 142 85 90 138 0.51 1.78 9 (at 3200 rpm) 17.3:1 APA100
is too high, combustion can occur before the fuel and air are properly mixed, resulting in incomplete combustion and smoke. If a CN is too low, engine roughness, misfiring, higher air temperatures, slower engine warm-up, and also incomplete combustion occur. Consequently, most of the US engine manufacturers suggest a range of CN between 40 and 50 for diesel fuels [5]. Biodiesels usually have a higher CN compared to fossil diesel. Minimum CN values of biodiesels are specified in both ASTM D6751 (min. CN = 47) and EN14214 (min. CN = 51). Cetane numbers of diesel fuel tested in this work and mono FAMEs are plotted in Fig. 3a versus fuel oxygen content by mass. In this figure, the black and solid arrow represents an increasing trend in the carbon chain length of saturated FAMEs while an increase in the unsaturation degree is illustrated by the brown dashed arrow. It is evident from the figure that diesel’s CN is close to that of methyl decanoate, C10:0 [28], and an increase in the
carbon chain length (see the black solid arrow) leads to a decrease in the fuel oxygen content and a substantial increase in cetane number. The cetane number is almost doubled when the carbon chain length increases from C10:0 to C18:0. An increase in unsaturation degree (see the brown dashed arrow), on the other hand, significantly decreases the cetane number of the fuels. With the same carbon chain length, CN of C18:3 is only one-fourth that of C18:0. A link between oxygen content and cetane values of the biodiesel blends used in this work are also observed in Fig. 3b. In this figure, CN of fossil diesel (or B0), biodiesel blends (B10, B20, B40, B60 and B80) and pure biodiesel (B100) are plotted versus fuel oxygen content. Only B0, B10 and B20 were used in the engine tests however the cetane number of other biodiesel blends were also tested targeting for a study of reactivity for these fuel blends. As can be seen from this figure, biodiesel blending enhances CN of the fuel mixture. An approximate 40% improvement in the cetane value occurs when the fuel oxygen content increases from 0% to 12%. The oxygen content in biodiesels may cause the fuel–air mixture to be leaner at the auto-ignition zone of the fuel spray which consequently improves the reactivity of the biodiesel blends.
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4.3. Biodiesel injection timing
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It is found in this study that fuel injection timing is similar regardless of the fuel type and this is expected with a common-rail injection system. In a common-rail system, the lines connecting the fuel pump and injectors are being highly-pressurized. If the energizing time is electronically fixed by the engine control unit, the injection process (and more specifically the start of injection) is expected to be almost independent of fuel properties and engine working conditions (engine speed and torque) [29]. In mechanical systems, however, the higher compressibility (or bulk modulus) of biodiesels compared to fossil diesel leads to an earlier injection
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(a) Mono FAMEs
5
(b) Tested biodiesel blends and fossil diesel
Fig. 2. Correlation of fuel oxygen content and higher heating value (HHV): (a) mono-fatty acid methyl esters; (b) biodiesel blends and fossil diesel.
(a) Mono FAMEs and biodiesels used in[30,31]
(b) Biodiesel blends and fossil diesel
Fig. 3. Correlation of fuel oxygen content and CN for: (a) mono-fatty acid methyl esters including saturated FAMEs (C10:0, C12:0. C14:0, C16:0 and C18:0) and unsaturated FAMEs (C18:1, C18:2 and C18:3); (b) biodiesel blends and fossil diesel used in this paper.
293
time [5]. The findings in this study are in good agreement with previous works [30,31].
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4.4. Engine performance and exhaust emission concentrations
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Fig. 4a shows the p h data for biodiesel blends (B0, B10 and B20). Similarly, the net heat release rate (NHRR) is shown in Fig. 4b. In order to examine the role of the oxygen content, data
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(a) In-cylinder pressure
shown in Fig. 4a and b as well as throughout this section is only for one fixed engine condition, 1800 rpm of engine speed and 20 DCA of injection timing. It is clear from Fig. 4a and b that the auto-ignition times of biodiesel blends B10 and B20 occur earlier compared with that of fossil diesel (B0). Adding biodiesel into the fuel mixture enhances its reactivity, as discussed in Section 4.2, consequently reducing the ignition delay times. A difference between ignition delay times of fossil diesel and these two
(b) Net heat release rate
Fig. 4. In-cylinder pressure (a) and net heat release rate (NHRR) (b) of B0, B10 and B20 at 1800 rpm, 20 DCA of injection timing, 400 bars of injection pressure.
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biodiesel blends is approximately 2–3 DCA as shown in Fig. 4a and b. However, ignition delay times of biodiesel blends B10 and B20 are quite similar as shown in these figures. Allen et al. [32] investigated auto-ignition characteristics of fossil diesel and canola-derived biodiesel using a rapid compression machine at a temperature range of 676–816 K and found that biodiesel ignites faster and increases the temperature sensitivity with respect to fossil diesel. It can also be seen that both premixed and diffusion combustion are visible in Fig. 4b. For example, premixed combustion of the biodiesel blends occurs in approximately 5 DCA duration and generates a pretty high heat release rate with a peak at approximately 5 DCA before top dead center (BTDC). The diffusion combustion occurs with much lower heat release rate and without an observable peak. This is in contrast with findings in Pham et al. [30,31] that the heat release rate is generated mainly in diffusion combustion process in their engine. It is notable that the fuel injection timing used in [30,31] is at around top dead center (TDC) and therefore the premixed combustion duration is quite short compared to that in this current study. Indicated mean of effective pressure (IMEP) values of biodiesel blends at three engine speed conditions (1400, 1800 and 2400 rpm, respectively) are plotted versus fuel oxygen content in Fig. 5. It should be noted that IMEP is a linear function of indicated power and is commonly used to describe characteristics of engine performance. It is noted from Fig. 5 that IMEP values reduce slightly when fuel oxygen (FO) content increases from 0% to 1.2%. When FO exceeds 1.2, however, IMEP reduces more rapidly. The trends are true for all of the engine speed conditions tested here. These results imply that the reduction in IMEP (which is due to lower calorific value of biodiesel blends) could be almost eliminated when using low blending ratios of biodiesel. This can be attributed to an improvement in lubricant quality when adding biodiesel into fossil diesel [5]. Low biodiesel blending ratios such as B2 to B5 have been reported to increase engine power and thermal efficiency [5]. At a high blending ratio, however, the advantage gained from lubricant enhancer is minimal with respect to the penalty in engine power which is due to calorific value reduction. It should be noted that the maximum mass fraction of oxygen contained in the biodiesel blends tested in this study is about 2.5% (corresponding to B20) and therefore the results observed here could be thought to be biased. However, our recent experiment campaign in a 6-cylinder common-rail engine operating with different biodiesel blends (B0, B20, B50 and B100) was conducted and the link between biodiesel oxygen content and combustion as well as emission was also observed. In the campaign, different biodiesels with different molecular structures were examined
Fig. 5. IMEP of biodiesel blends and fossil diesel versus fuel oxygen content at three engine speed conditions (1400, 1800 and 2400 rpm respectively), 20 DCA of injection timing, 400 bars of injection pressure.
and some results were reported in [33,30,31,34]. Primary results of the oxygen role have been reported in Pham’s PhD thesis [33] and further work on this topic is in progress and will be reported in the future. Relative indicated specific emission concentrations are plotted versus the fuel oxygen content in Fig. 6a–e for indicated specific particle mass (ISPM), indicated specific NOx (ISNOx), indicated specific carbon monoxide (ISCO), indicated specific carbon dioxide (ISCO2), and indicated specific hydrocarbon (ISHC), respectively. These are normalized values using corresponding indicated specific emission levels of diesel. Relative ISPM of B10, for example, is the ratio of ISPM concentration of B10 to that of fossil diesel. It is evident from Fig. 6a that the relative ISPM is proportional to the FO and somewhat independent of the engine speed conditions. An increase in the fuel oxygen content from 0% to 2.5% results in a decrease of approximately 30% in the ISPM concentration. The oxygen in biodiesel constituents (FAMEs) significantly suppresses the particle formation and enhances the particle oxidation which leads to a reduction in particle concentration. The oxygen atoms could cause suppression of soot formation by effectively removing carbon from reaction pathways that lead to soot precursors [35]. Similarly, ISNOx is shown in Fig. 6b. It can be seen that the relative ISNOx varies in a narrow range from 0.95 to 1.05. Compared to NOx level of fossil diesel, a slight decrease is observed for B10 while a slight increase is shown for B20. NOx mechanisms are very complex and encompass many chemical processes. A comprehensive review on this topic is available in [36]. NOx formation can be attributed to thermal NOx (where the nitrogen from inlet air is oxidized at high temperature in the combustion chamber, >1500 °C), prompt NOx (formed by high speed reactions at the flame front), and fuel NOx (produced from nitrogen-bearing fuels) [37]. A link between NOx formation and ignition delay time was found in previous studies [38,39]. The difference in the cylinder temperature could have certain effects on thermal NOx mechanism [24]. In this paper, B10 and B20 have quite similar ignition delay time as shown in Fig. 4a and b, however, their ISNOx levels are about 10% different. This implies that ignition delay is not a sole factor affecting NOx formation. More work is required to understand NOx mechanism of the biodiesel blends. Relative ISCO and ISCO2 levels are shown in Fig. 6c and d, respectively. In general, an increase in fuel oxygen content leads to a decrease in ISCO while an increase in ISCO2 level is observed. A small decrease in the CO2 concentration is shown when FO increases from 0% to 1.2% (Fig. 6d) and this could be attributable to the only slight reduction in engine power when using B10 as discussed earlier in this section. The opposite trend in CO and CO2 levels is usually expected because the extra oxygen from oxygenates enhances the chemical process to convert CO into CO2. Fig. 6e plots relative ISHC concentrations versus fuel oxygen content. When FO increases, similarly to the ISPM trend observed in Fig. 6a, a significant reduction is also obtained for ISHC levels. This is acceptable as ISPM and ISHC are two measures of unburnt fuel proportions. It is notable that discussions related to biodiesel blends in this paper relates only to a specific biodiesel manufactured from residues of a palm cooking-oil production process. Influences of variations in fatty acid ester profiles of biodiesel produced from different feedstocks on biodiesel properties [5], biodiesel atomization, combustion and emission characteristics [33] are significant. Some results related to the role of oxygen in biodiesel properties, biodiesel atomization, combustion and emission characteristics have recently been reported in [40,41,34,42,43] for a wide range of biodiesels with different molecular structures. It has also been revealed that a critical key to reducing the total particle mass, particle size, particle number, and black carbon concentration is to increase the FO. However, an increase in the FO leads to a
Please cite this article in press as: Nguyen VH, Pham PX. Biodiesels: Oxidizing enhancers to improve CI engine performance and emission quality. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.04.004
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(a) ISPM
(c) ISCO
(b) ISNOx
(d) ISCO2
(e) ISHC Fig. 6. Relative indicated specific emission concentrations of biodiesel blends versus fuel oxygen content at 20 DCA of fuel injection timing, 400 bars of injection pressure, and three different engine speed conditions (1400, 1800 and 2400 respectively).
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substantial increase in the total particle number per unit of particle mass, the amount of black carbon per unit of particle mass, and the reactive oxygen species concentration [30,31].
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5. Conclusions
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A biodiesel was manufactured using methanol transesterification of waste palm oil (residues of cooking oil production processes) and then successfully tested in a modern common-rail CI engine. Compared to fossil diesel, the biofuel is an oxygenated fuel
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which has a higher cetane number but a lower heating value. Other properties of this biofuel are comparable with those of fossil diesel. The effect of oxygen enhancement (obtained by adding the biodiesel into fossil diesel) is examined in a single-cylinder research engine. Biodiesel blends (B0, B10 and B20) were tested which supply up to 2.5% of the fuel oxygen content by mass. The engine was operated under a broad range of engine speeds (1400, 1800, and 2400 rpm) and fuel injection timing (16–24 DCA). Engine combustion characteristics and exhaust emission concentrations were reported. An increase in fuel oxygen content leads to a reduction
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in fuel heating value and consequently a reduction in engine power while the biodiesel enhances the cetane number which leads to shorter ignition delay times. The FO is a key to reducing particle concentrations, HC, and CO while the effect of oxygen content on ISNOx is minimal.
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Acknowledgments
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The authors are grateful for the support of the Directorate of programs on ‘‘Biofuels development until 2015 and vision for 2025’’, The Ministry of Industry and Trade – Vietnam.
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References
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