A comprehensive review of biodiesel as an alternative fuelfor compression ignition engine
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A comprehensive review of biodiesel as an alternative fuel for compression ignition engine Ambarish Datta n , Bijan Kumar Mandal Department of Mechanical Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India article info Article history: Received 11 December 2014 Received in revised form 14 December 2015 Accepted 17 December 2015 Available online 7 January 2016 Keywords: Biodiesel Alternative fuel Properties Production Performance Emission Review abstract In search of alternative fuels for CI engines, many experimental studies have been carried out and posted in the literature during the last few decades. This paper presents a comprehensive review on the pro- duction, performance and emissions from a compression ignition engine using biodiesel as alternate to fossil based diesel fuel. The properties of biodiesel produced from different sources and their fatty acid composition have also been described. The experimental set up used by different researchers for the investigations and their findings regarding performance and emissions with respect to mineral diesel have been presented in short for a large number of studies. For better illustration of the facts, results of a few experimental studies available in the literature have been presented in the form of different graphs for selective important performance and emission parameters as case studies. The overall impression is that the performance of the engine slightly deteriorates with the use of biodiesel partially or fully instead of diesel, but the environmental aspects are significantly improved. & 2015 Elsevier Ltd. All rights reserved. Contents 1. Introduction ........................................................................................................ 800 2. Production of biodiesel and its properties ................................................................................ 800 2.1. Production of biodiesel ......................................................................................... 800 2.2. Properties of biodiesel .......................................................................................... 802 3. Engine performance with biodiesel ..................................................................................... 803 3.1. Effect on brake thermal efficiency ................................................................................ 803 3.2. Effect on brake specific fuel consumption .......................................................................... 805 3.3. Effect on exhaust gas temperature ................................................................................ 807 3.4. Summary of performance analysis ................................................................................ 808 4. Effect of biodiesel on engine emissions .................................................................................. 808 4.1. Effect on CO emission .......................................................................................... 808 4.2. Effect on CO 2 emission ......................................................................................... 811 4.3. Effect on NO x emission ......................................................................................... 813 4.4. Effect on HC emission .......................................................................................... 814 4.5. Effect on smoke emission ....................................................................................... 816 4.6. Summary of emission analysis ................................................................................... 819 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2015.12.170 1364-0321/& 2015 Elsevier Ltd. All rights reserved. Abbreviations: ASTM, American Society for Testing and Materials; BSFC, Brake specific fuel consumption; bTDC, Before top dead centre; CI, Compression ignition; COME, Canola oil methyl ester; DI, Direct injection; EGR, Exhaust gas recirculation; EGT, Exhaust gas temperature; EN, European standards for products and services by European Committee for Standardization; ESG, Eruca sativa gars; FFA, Free fatty acid; GHG, Green house gas; GTL, Gas to liquid; H 2 SO 4 , Sulfuric acid; HOME, Honge oil methyl ester; IS, Indian Standard; JOME, Jatropha oil methyl ester; KOH, Potassium hydroxide; MEPS, Methyl ester of paradise oil; MOEE, Mahua oil ethyl ester; MOME, Mahua oil methyl ester; NaOH, Sodium hydroxide; NOME, Neem oil methyl ester; PBDF, Petroleum based diesel fuel; PKOME, Palm kernel oil methyl ester; PNOME, Peanut oil methyl ester; RME, Rapeseed methyl ester; SOME, Seasame oil methyl ester; SVO, Straight vegetable oil; THC, Total hydrocarbon; VOME, Vegetable oil methyl ester; WFO, Waste frying oil; WPOME, Waste palm oil methyl ester n Corresponding author. E-mail address: [email protected](A. Datta). Renewable and Sustainable Energy Reviews 57 (2016) 799–821
Transcript
Renewable and Sustainable Energy Reviews 57 (2016) 799–821
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
http://d1364-03
AbbreCanola oCommitIndian Sester; NRME, RaWPOME
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journal homepage: www.elsevier.com/locate/rser
A comprehensive review of biodiesel as an alternative fuelfor compression ignition engine
Ambarish Datta n, Bijan Kumar MandalDepartment of Mechanical Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India
a r t i c l e i n f o
Article history:Received 11 December 2014Received in revised form14 December 2015Accepted 17 December 2015Available online 7 January 2016
x.doi.org/10.1016/j.rser.2015.12.17021/& 2015 Elsevier Ltd. All rights reserved.
viations: ASTM, American Society for Testing ail methyl ester; DI, Direct injection; EGR, Exhtee for Standardization; ESG, Eruca sativa garstandard; JOME, Jatropha oil methyl ester; KOaOH, Sodium hydroxide; NOME, Neem oil mepeseed methyl ester; SOME, Seasame oil meth, Waste palm oil methyl esteresponding author.ail address: [email protected] (A.
a b s t r a c t
In search of alternative fuels for CI engines, many experimental studies have been carried out and postedin the literature during the last few decades. This paper presents a comprehensive review on the pro-duction, performance and emissions from a compression ignition engine using biodiesel as alternate tofossil based diesel fuel. The properties of biodiesel produced from different sources and their fatty acidcomposition have also been described. The experimental set up used by different researchers for theinvestigations and their findings regarding performance and emissions with respect to mineral dieselhave been presented in short for a large number of studies. For better illustration of the facts, results of afew experimental studies available in the literature have been presented in the form of different graphsfor selective important performance and emission parameters as case studies. The overall impression isthat the performance of the engine slightly deteriorates with the use of biodiesel partially or fully insteadof diesel, but the environmental aspects are significantly improved.
The availability of adequate amount of conventional fossil fuelfor internal combustion engines and the associated effects ofglobal warming and other environmental issues arising due to thecombustion of fossil fuels are the two most threatening problemsof our present day civilization. The rapid industrialization andurbanization are also making our planet unsafe for us and for thegenerations to come. People are now all well aware of the lethaleffects of environmental pollution created by the random use offossil fuels. China tops the list of green house gas emitters andIndia is not far behind. In fact, India is already the fifth largestgreenhouse gas emitter of the world and is expected to becomethe third largest GHG emitter by the year 2015. Transport sectorcontributes significant amount of GHG emission [1–3] particularlyin the developing and developed countries. The maximum amountof green house gases added to the atmosphere are from electricityand transportation sectors and the corresponding values are 34%and 27% [3]. Also the vehicle population throughout the world isincreasing rapidly and in India the growth rate of automotiveindustry is one of the largest in the world. It has been anticipatedquite clearly that the problem cannot be solved with the con-ventional fossil fuels as their reserves are limited and also theemission norms are expected to be more stringent in future [4].This situation can be handled by using biofuels as fuels for com-pression ignition (CI) engines wherever possible.
Another concern is the peak oil theory, which predicts a risingcost of oil derived fuels caused by severe shortages of oil during anera of growing energy consumption. According to the “peak oiltheory” [5], the demand for oil will exceed supply and this gap willcontinue to grow, which may cause a growing energy crisisstarting between 2010 and 2020. According to Demirbas [6], apeak in global oil production may occur between 2015 and 2030.After that the production process will highly decelerate. India isthe world’s fourth largest petroleum consumer after United States,China and Japan [7] which makes India dependent upon the oilexporting countries for meeting its own energy demand.
Diesel engine is the most fuel efficient combustion engineamong the available ones and the transport sector mainly usesdiesel fuel due to its better fuel economy and more effectivepower. Several countries such as USA, Germany, France, Italy,Brazil, and Indonesia are using biodiesel blended with diesel.Malaysia normally uses palm biodiesel as an alternative to dieselin their country despite the use of palm oil as edible oil also [8].The soyabean and the rapeseed biodiesels are generally used inUSA and Europe respectively [9]. In a country like India, biodieselalso can be used as alternative automotive fuel and also in othersectors as CI engine fuels. Biodiesel is a fastest growing alternativefuel. India has huge potential for biodiesel, but it is not yetexplored properly to replace at least some percentage of mineraldiesel with biodiesel. It will be more effective and sustainable, ifbiodiesel is produced from non-edible type oil seeds, like karanja(Pongamia Pinnata) and ratanjyot (Jatropha Curcus) [10–13]. Theabove oil seeds can be cultivated in the wastelands available inIndia. Another advantage of biodiesel is that it can be used ininternal combustion engines in a similar fashion as petro dieselwithout any modification of engine geometry.
Rudolf Diesel, the father of diesel engine, demonstrated thefirst use of vegetable oil in compression ignition engine. He usedpeanut oil as fuel for his experimental engine. With the availability
of cheap petroleum and appropriate methods for the refinement ofcrude oil to obtain petro-diesel, diesel engine started evolving.Later after 1940, vegetable oils were used again as fuel in emer-gency situations, during the period of Second World War.
Because of the increase in the crude oil prices, limited reserveof fossil fuels and also for the environmental concern, researchersshowed renewed interest on vegetable oils for producing suitablealternate to the diesel fuel. Researchers from different corners ofthe world are making sincere attempts to find out the suitablealternative to diesel fuel which does not require major enginemodifications. The literature is already rich with many experi-mental findings, but the observations are not always unidirec-tional. Thus, there is a need of summarizing most of the workscarried out on biodiesel in the last few decades. Motivated by this,the authors have attempted to review the important works onbiodiesel to get the state of the art of biodiesel production pro-cesses, its performance and emission characteristics as CI enginefuels. The authors have also presented some of the experimentalresults from the literature to supplement the summarizationprocess.
2. Production of biodiesel and its properties
Biodiesel are produced from feedstocks which are renewable innature. Since biodiesel is thought to be the alternative fuels forcompression ignition engines which use diesel as the fuel, theproperties of biodiesel should match with the fuel properties asspecified by ASTM and/or EN as well as IS standard in India.
2.1. Production of biodiesel
For the commercialization of biodiesel as CI engine fuelthroughout the world, different production processes of it shouldbe identified and made available to the people working at grassroot level. The raw materials needed for its production may varyfrom country to country. Keeping this in mind researchers aretrying to find several ways to produce biodiesel from locallyavailable different feedstocks such as vegetable oil – both edibleand non-edible, animal tallow, waste cooking oil and algae. As theviscosity of the oils and fat derived from the above mentionedfeedstocks is much higher and unsuitable for using in unmodifiedCI engines, the first step is to reduce its viscosity. This is donethrough a chemical reaction called transesterification. In thisprocess, the triglyceride present in the oil or fat reacts with alcohol(methanol or ethanol) in the presence of a catalyst which isalkalime in nature. A catalyst such as sodium or potassiumhydroxide is required. Glycerol (also called glycerin) is produced asa byproduct. The overall reaction of the transesterification processhas been shown in Fig. 1 following Saka and Kusdiana [14].
Generally, methanol is used to produce biodiesel because of itsavailability and lower reaction time, and the final product (bio-diesel) is called as methyl ester of the raw oil used. Sometimes,ethyl alcohol is also used for the production of biodiesel, and it iscalled as the ethyl esters of the corresponding oil. Ideally, trans-esterification is potentially a less expensive way of transformingthe large, branched molecular structure of the bio-oils into smaller,straight chain molecules of the type required in regular dieselcombustion engines. The approximate proportions of different
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reactants and products in the transesterification reaction are:
100kgs of oilþ10kgs of methanol-100kgs of biodieselþ10kgs of glycerol
A biodiesel blend is pure biodiesel mixed with petro-diesel inappropriate proportions. Biodiesel blends are referred to as Bxx.The ‘xx’ indicates the amount of biodiesel by volume in the blendof biodiesel and petro-diesel [6]. For example, B80 refers to a blendof 80% biodiesel and 20% petro-diesel by volume. The biodieselproduction techniques from various raw feedstocks as reported bydifferent researchers are presented in the next part of this section.
Sharma and Singh [15] produced biodiesel from karanja andmahua oils as well as the mixture of the two non-edible oils in thesame ratio on volume basis. The higher fatty acid content of theabove said two straight vegetable oils forced them to use two stepreactions. The first one was acid esterification for lowering fattyacid content to a desired limit. The second step was alkalinetransesterification for the conversion of the already treated oil tofatty acid methyl ester or biodiesel. H2SO4 and KOH were added ascatalysts with methanol for esterification and transesterificationprocesses respectively. They preferred methanol over ethanolbecause methanol was less costly and the reaction was also faster.Ilkılıç et al. [16] produced biodiesel from safflower oil by transes-terification process using NaOH as a catalyst. After separation ofglycerol from product, H2SO4 was added as a depolarizer and thebiodiesel was then washed adding equal amount of water toseparate catalyst and the remaining portion of alcohol.
Saka and Kusdiana [14] employed a method to produce bio-diesel through transesterification reaction of rapeseed oil withoutusing a catalyst. The above said method was termed as a super-critical methanol biomass conversion method. The pressure andtemperature in this process were quite elevated compared to thenormal pressure and temperature of transesterification reaction.Venkanna and Reddy [17] produced biodiesel from honne oilthrough a three stage transesterification process with methanolwhich comprised of acid esterification, alkali transesterificationand post treatment. H2SO4 was used as catalyst in acid esterifica-tion and KOH was used in alkali transesterification. The posttreatment method consisted of gentle water wash thrice usingdistilled water. Biodiesel was produced from eruca sativa gars(ESG) vegetable oil by Li et al. [18] on lab scale through transes-terification process with methanol. They used a heteropoly acidsalt as catalyst during the transesterification process of ESG oil.
Production of biodiesel from rubber seed oil through a twostage method of transesterification with methanol, which followedan alkali esterification using H2SO4 as a catalyst and transester-ification with methanol using NaOH as a catalyst was studied byRamadhas et al. [19]. Biodiesel production from non-edible animaltallow was studied by Oner and Altun [20]. Biodiesel was preparedthrough transesterification of tallow with methanol in the pre-sence of NaOH as catalyst. Ghadge and Raheman [21] producedbiodiesel from mahua oil having high free fatty acid in it. Firstly,the fatty acid content was determined by a standard titrimetrymethod and after that a pretreatment method was involved forlowering the higher acid value. Finally, the transesterificationreaction was carried out with methanol using KOH as an alkalinecatalyst. Biodiesels from different straight vegetable oils havinghigh phosphorous content and having either low or high acid
values were studied by Mendow et al. [22]. Due to low acidity ofsoyabean oil, direct transesterification with methanol as an alcoholwas used to obtain biodiesel using NaOH as a catalyst. Due tohigher acid value of crude coconut oil, they used a two stagemethod of transesterification with methanol. It was consisted ofalkali esterification using H2SO4 as a catalyst followed by trans-esterification using NaOH as a catalyst to obtain biodiesel.
Charpe and Rathod [23] treated waste sunflower frying oil withmethanol as an alcohol in the presence of P. fluorescens enzyme ascatalyst for the production of biodiesel. Due to low cost theresearchers used waste sunflower frying oil as raw material and P.fluorescens as catalyst because of its higher conversion rate. Soy-bean oil was used to produce biodiesel by Lin and Lin [24] throughtransesterification process with methanol using NaOH as a catalystaccompanied by peroxidation to improve the fuel properties of thebiodiesel. Water wash and distillation process were used toremove un-reacted methanol, water and other impurities. Nabiet al. [25] and Srivastava and Verma [26] produced biodiesel fromkaranja oil. Nabi and the co-workers first removed water and othercontaminants from karanja oil and then reduced the fatty acidconcentration of the oil by acid esterification process using H2SO4
as catalyst. Thereafter, they followed the normal transesterificationprocess using methanol in the presence of NaOH as a catalyst.
On the other hand, Srivastava and Verma [26] did not employany pretreatment (like acid esterification) of raw karanja oil, butthey used some after treatment of the biodiesel produced by thetransesterification process. The after treatment method employedwas bubble wash method with the aid of 10% phosphoric acidsolution in warm water. For getting the final quality biodiesel, itwas purified by passing air through aquarium stone for at least24 h. The whole process was repeated three times to get the finalproduct in the form of karanja biodiesel. They have tested bothkaranja oil and its biodiesel and the biodiesel yield was found to be84% which was lower than that obtained (97%) by Nabi et al. [25].Sharon et al. [27] produced biodiesel from used palm oil bytransesterification process in laboratory scale setup with theaddition of methanol in a proportion of 6:1 molar ratio, usingNaOH as a catalyst. After completion of the reaction process, gly-cerol was separated using separating funnel. Patil and Deng [28]prepared biodiesel from raw jatropha and karanja oils in two stepsnamely, acid esterification and alkali transesterification. But, incase of corn and canola oils only the alkali transesterification stepwas needed as the fatty acid content of them were lower than thatof jatropha and karanja. In another study, Ghadge and Raheman[29] reported the production of biodiesel from crude mahua oil bytransesterification reaction with methanol in the molar ratio of 6:1by using KOH as an alkaline catalyst followed by a pretreatmentmethod consisting of determination of ph value by titrimetry andesterification of crude mahua oil with methanol by using H2SO4 asa catalyst. Wang et al. [30] also followed two step esterificationprocesses consisting of acid esterification and alkali transester-ification for the production of biodiesel from non-edible oils. Acidesterification was done by anhydrous sulfuric acid as an acid cat-alyst and alkali transesterification was done by KOH as an alkalicatalyst with methanol. Lu et al. [31] reported a pre-esterificationof crude jatrohpa oil, using sulfuric acid as a catalyst during pro-duction of biodiesel. After pre-esterification process the researchertransesterified crude jatropha with methanol using KOH as acatalyst.
The above review shows that biodiesel can be produced fromvarious raw materials under different conditions using different cata-lysts depending on acid values of raw feedstocks. The biodiesel yielddepends not only on the type of feedstocks used, but also depends onmolar ratio of alcohol to oil, the catalyst type and its amount andreaction conditions such as temperature, duration and sometimespressure. The information related to different production processes as
Table 1Biodiesel production using various types of feedstocks with reaction variables.
Name of Researchers Feedstock Alcohol type Molar ratio ofalcohol to oil
Catalyst used Catalyst amount Reaction condition Biodiesel yield (%)
Saka and Kusdiana[14]
Rapeseed oil Methanol 42:1 NaOH – 350–400 °C at 45–65 MPa
95%
Sharma and Singh[15]
Karanja oil Methanol 6:1 H2SO4 1 ml 50 °C for 1 h 98.6%Mahua oil 8:1 KOH 1 wt% 95.71% 94.0%
(hybrid)Mixture of twoIlkılıç et al. [16] Safflower oil Methanol – NaOH 0.4% 55–65 °C for 2 h –
Li et al. [18] Eruca sativa gars oil Methanol 6:1 heteropoly acid 0.04 m mol 65 °C for 12 h 98.1%Ramadhas et al. [19] Rubber seed oil Methanol 6:1 H2SO4 0.5% 40–50 °C for 2 h -
9:1 NaOH 5 gOner and Altun [20] Animal tallow Methanol 6:1 NaOH 2 g 60 °C for 3 h –
Ghadge and Rahe-man [21]
Mahua oil Methanol – H2SO4 1% 60 °C for 30 min 98%6:1 KOH 0.7%
Mendow et al. [22] Crude soybean oil Methanol – NaOH 0.35% 60 °C for 2 h –
Coconut oil H2SO4 60 °C for 1.5 hCharpe and Rathod[23]
Waste sunflower frying oil Methanol 3:1 P. fluorescensenzyme
5% 45 °C 63.84%
Lin and Lin [24] Soybean oil Methanol 6:1 NaOH – 60 °C –
Nabi et al. [25] Karanja oil Methanol – H2SO4 1% 50 °C for 1 h 97%NaOH 1%
Srivastava and Verma[26]
Karanja oil Methanol – NaOH 28.5 g 70 °C for 1 h 84%
Sharon et al. [27] Palm oil Methanol 6:1 NaOH 0.6 wt% 65 °C for 3 h 87%Patil and Deng [28] Karanja, jatropha, canola and
corn oilMethanol 9:1 H2SO4 0.5% 60 °C for 2 h 80%
KOH 2% 55 °C for 1 h 90–95%KOH 0.45 g
Ghadge and Rahe-man [29]
Mahua oil Methanol 6:1 KOH 0.7% w/v 60 °C for 30 min 98%
Wang et al. [30] Euphorbia lathyris, Sapiumsebiferum, Jatropha curcas oil
Methanol 8:1 H2SO4 0.2–1 wt% 60 °C for 30 min 86.2%6:1 KOH 0.6–1.2 wt% 88.3% 86.2%
Lu et al. [31] Jatropha curcas oil Methanol 6:1 metatitanic acid 1.3% 90 °C for 2 h 98%KOH
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followed by the researchers have been summarized and presented inTable 1 to get an overall idea at a glance.
Minute look at the above said table reveals that the biodieselyield has an average value of more than 85%. The analysis of theresults shows that maximum yield of 99% is obtained with ESG oil.Mahua, jatropha, karanja and rapeseed oil have also high yield inthe range of above 90%. The yield fromwaste frying oil is normallylow and it is only 63.84% with sunflower based waste frying oil.
Another interesting point can be noted that the yield is notalways the same for a particular oil. Depending upon the fatty acidcontents and its distribution, acid esterification process may beneeded before the alkaline esterification to lower the acid value ofthe oil. This process will produce more biodiesel than if onlyalkaline esterification is used in case of oil having higher freefatty acid.
More recently, production of biodiesel has been started fromdifferent types of algae and it was found that the oil yield of algaebased biodiesel is significantly higher than the oils and animal fatsdescribed earlier in this section. These biodiesels are generallyreferred in the literature as third generation biodiesels and thiscould be a potential alternative due to its much lower gestationperiod [32] and huge availability [33]. A comprehensive review onit has been presented by Mata et al. [32]. Khan et al. [33] has alsopresented a critical evaluation on the prospects of biodiesel pro-duction from microalgae. They have emphasized the need toexplore the possibilities of producing biodiesel from microalgae, asit will not raise the ‘fuel vs. food’ debate. This is due to the fact thatthe production of microalgae does not require the normal landwhere cereal crops are being produced. Demirbas et al. [34] alsoproduced biodiesel from algal oil by using transesterification
method. Deng et al. [35] also stated that microalgae have thepotential to become the viable alternative for production ofbiodiesel.
As the end of this section, the flow chart showing differentprocesses along with their reactants and products for biodieselproduction from vegetable oils has also been shown in Fig. 2 fromthe work of Sharma et al. [36]. The flow chart for biodiesel pro-duction frommicroalgae has also been presented in Fig. 3 based onthe work of Najafi et al. [37].
2.2. Properties of biodiesel
The performance, combustion and emission characteristics ofany biodiesel fueled engine depend on the thermo-physicalproperties of biodiesel. The viscosity, density, cetane number,calorific value, flash and fire points, cloud and pour points are themajor properties of biodiesel which are to be considered. Severalresearchers have reported that the properties of biodiesel dependupon their fatty acid contents and chemical compositions. There-fore, before using the biodiesel in a compression ignition engine itis mandatory to measure its properties as specified by ASTMD6751 and EN 14214 standards, which are the most popular andwell known standards for biodiesel. In India, IS 15607 standard isfollowed for using biodiesel as fuel in an automotive engine. Dif-ferent properties of biodiesel produced from various oils and thoseof petro-diesel are compiled from the previous works of severalresearchers and are presented in Table 2 for comparison.
Generally, the properties of biodiesel are similar to that ofpetro-diesel. Among all the parameter, viscosity is the mostimportant as it is directly linked with the injection system of the
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engine. The higher viscosity affects the fluidity as well as theatomization during combustion which may also cause incompletecombustion and carbon deposits at the injectors [6,52,53]. Flashpoint is the measure of flammability of the fuel. Flash point is alsoinfluenced by the chemical properties of biodiesel, such as pre-sence of double bonds, number of carbon atoms [52]. Cetanenumber is the measure of combustion quality and the highercetane number implies a shorter ignition delay. Since, biodieselgenerally have longer fatty acid carbon chain, its cetane number ishigher than diesel. This also enhances the ignition quality byshortening the delay period, which finally increases the combus-tion duration [6,52]. Calorific value is the measure of energycontent of a fuel [54]. Since biodiesel contains 11% oxygen by
Vegetable Oils Recycled
Residue
Dilute acid esterification
Transesterification
RefiningGlycerin Refining
Methanol Recovery
Crude BiodieselCrude Glycerin
Methanol + KOHSulphur + Methanol
Glycerin
Fig. 2. Typical flow chart of biodiesel production [36].
Fig. 3. Flow chart of biodiesel production from microalgae [37].
Table 2Properties of diesel and biodiesel produced from different feedstocks.
weight [6]; the higher heating values are relatively lower thanpetro-diesel [6]. For the above said reason, the lower calorificvalue is less than that of diesel [52]. Qi et al. [55] observed that dueto the higher density and lower heating value of biodiesel thepower output and the torque are lower than those of diesel.Moreover, biodiesel contains very small amount of phosphorousand sulfur and hence the emission of oxides of sulfur (SOx) isalmost negligible. In addition, the higher flash point (more than100 °C) of biodiesel makes the storage and transportation issuesless important. The properties of biodiesel at low temperatures arepoorer than those of diesel oil. The pour point is generally higherthan that of diesel and this may create some complications for theoperation in cold weather. Biodiesel has good lubricant propertieswith respect to diesel oil, in particular, diesel with a very smallamount of sulfur. This is very important to reduce wear in theengine and the injection system. Table 3 shows average fatty acidprofile for different feedstocks for biodiesel fuel. Although the fattyacid distributions are not the same in different feedstocks, but apattern is noticed among the different feedstocks. These variationswill affect the cetane number of biodiesels produced from variousfeedstocks.
3. Engine performance with biodiesel
Alternative fuels or supplementary fuels used in engines arenormally evaluated on the basis of both engine performances andtheir environmental impacts. The most important performanceparameters considered by the researchers in the field of internalcombustion engines are power output and exhaust gas tempera-ture, specific fuel consumption and brake thermal efficiency. Thissection presents and discusses the results of different studiesavailable in the literature related to the above said parameters ofcompression ignition engine using biodiesel and blends of bio-diesel and petro-diesel as fuels.
3.1. Effect on brake thermal efficiency
The ratio of the brake power developed by the engine and theenergy released per unit time due to complete combustion of fuelis called brake thermal efficiency of the engine. From the energeticpoint of view, it is the most important parameter for the evalua-tion of performance of an engine. Some of the results on brakethermal efficiency of CI engine reported in the literature by dif-ferent researchers have been presented in the following section.
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Gumus [56] observed a reduction in brake thermal efficiency ofa four stroke single cylinder diesel engine using hazelnut kernel oilmethyl ester (biodiesel) as fuel. This type of behavior was attrib-uted to the lower heating value and higher viscosity of biodieselthan that of diesel, which resulted higher brake specific fuel con-sumption and led to a decrease in brake thermal efficiency. He alsoreported from his experimental investigation that with theadvancement of injection timing and increase in compression ratioand injection pressure, brake thermal efficiency increased as thefuel consumption decreased under the above mentioned condi-tions. Puhan et al. [57] noted a slight decrease in brake thermalefficiency with linseed biodiesel compared to mineral diesel.Ozsezen et al. [58] used waste palm oil methyl ester (WPOME),canola oil methyl ester (COME) and petroleum based diesel fuel(PBDF) as fuels to run an inline six cylinder water cooled, directinjection, naturally aspirated four stroke 6.0 L Ford Cargo CI enginehaving compression ratio 15.9:1. They observed that the use ofWPOME and COME with respect to the use of PBDF resulted inmarginal reductions of brake thermal efficiency by 1.42% and 0.12%respectively at full load condition and at a constant speed of1500 rpm. It can be concluded that the chemical energy of theabove said test fuels have been converted into mechanical energyalmost in the same manner.
Mahanta et al. [59] used karanja oil methyl esters for theirexperimental work on diesel engines. They found a 27% decreasein brake thermal efficiency of the engines when neat biodiesel wasused instead of neat diesel. Agarwal and Dhar [60] used neatkaranja oil and preheated karanja oil to run a four stroke dieselengine. The decrease in efficiency with this straight vegetable oilwas found to be even more. It was also reported that when SVOwas preheated, the efficiency increased but still remained muchlower than diesel fuel. The reductions in brake thermal efficiencywere found to be 45% and 25% with non-preheated and preheatedstraight vegetable oils respectively. The main reasons behind thesereductions in thermal efficiency were reported to be poor volatilityand higher viscosity of both the above said fuels. Rao et al. [61]reported a slight decrease in brake thermal efficiency of a singlecylinder direct injection air cooled gen-set diesel engine whenjatropha biodiesel was used instead of petro-diesel. According tothem, the decrease in efficiency of the engine with jatropha oilmethyl ester and its blends was due to the early start of com-bustion of biodiesel resulting an increase in compression work andheat loss.
Banapurmath et al. [62–63] experimented with biodieselsproduced from marotti oil (non-edible obtained from a medicinalplant in India) and honge oil on a single cylinder water cooled fourstroke diesel engine. They found a decrease in thermal efficiencywith the addition of marotti oil methyl ester (biodiesel) to diesel.
The highest brake thermal efficiency with neat marotti oil methylester was obtained at 80% load as 28.38%, whereas at the sameloading condition the value with diesel fuel was 31.25%. B20 blendof marotti oil methyl ester and diesel showed better thermalefficiency compared to other blended fuels. The increase in theefficiency with B20 compared to B10 was attributed to morecomplete combustion and the additional lubricity of biodieselwhich reduced the frictional power losses. Further increase ofbiodiesel percentage in the blends decreased the effective calorificvalue of the fuel and thus the efficiency was reduced. In case ofhonge oil methyl ester, brake thermal efficiency was lower thanthat of diesel. At 80% load condition, the authors found the max-imum efficiency. In case of diesel, it was 31.25%, whereas withhonge oil methyl ester it was 29.51%.
On the contrary, some reverse trends in the variation of brakethermal efficiency were also observed by several researchers. In anexperimental study conducted by Laforgia and Ardito [64] on anindirect injection diesel engine, it was observed that brake thermalefficiency increased with biodiesel by about 10% over mineraldiesel as fuel. Raheman and Ghadge [65] conducted an experi-mental study on indirect injection Ricardo E6 diesel engine withpre-combustion chamber. They found an increase in brake thermalefficiency with the increase in compression ratio because ofimprovement of combustion characteristics. They also observedthat the efficiency increased when advance ignition was provided.This was mainly due to more time available for injection and thusreduction in rapid combustion leading to an undesirable effectcalled knocking. However, the effect of increase of compressionratio and advance injection timing were noted to be more pro-minent in case of blends having higher percentages of biodieselcompared to diesel. An experimental work was carried out byDeore and Jahagirdar [66] on a single cylinder, four stroke, watercooled diesel engine of small capacity (3.5 kW) using biodieselsfrom jatropha and karanja and mineral diesel as fuels. Theexperiment was carried out keeping the compression ratio fixed at18. At higher load, it was observed that the brake thermal effi-ciency of the engine using jatropha biodiesel was higher thanusing diesel and karanja biodiesel by 9.29% and 2.76% respectively.
Song and Zhang [67] observed slightly higher thermal effi-ciency for soybean oil methyl ester compared to mineral diesel asfuel. They explained this increase in thermal efficiency from thefact that biodiesel contained small amount of oxygen which actedas combustion promoter and led towards complete combustion ofbiodiesel blended fuels. Ceviz et al. [68] observed that with the useof hazelnut oil methyl ester as fuel in a four stroke direct injectiondiesel engine, the effective efficiency increased by about 12% forB20 compared to mineral diesel. While experimenting on singlecylinder air cooled (using radial fan) diesel engine, Rath et al. [69]
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observed slight improvement in brake thermal efficiency usingneat karanja oil methyl ester instead of mineral diesel. At the sameoperating condition, the brake thermal efficiency with diesel was28.25%, whereas the thermal efficiency with neat karanja biodieselwas found to be 32.5%. The authors explained that the higherviscosity of karanja oil methyl ester increased the mechanicalefficiency of the engine and that resulted to the improvement ofbrake thermal efficiency.
From another experimental study by Raheman and Ghadge [70]on the indirect injection Ricardo E6 diesel engine with pre-combustion chamber, it was reported that at a compression ratioof 18:1 and injection timing of 40° bTDC, brake thermal efficiencyobtained using mahua biodiesel was comparable to that with neatdiesel fuel. Also, it was observed by Canakci [71] and Zhu et al. [72]that there was no such significant change in brake thermal effi-ciency of biodiesel in comparison to petro diesel when used in anunmodified diesel engine. Kong and Kimber [73] used neem bio-diesel in large diesel engines having capacity in the range of MW.They observed a slight decrease in brake thermal efficiency athigher load while using neem biodiesel as fuel. Finally, they con-cluded that on an average the thermal efficiency with neem-dieselblend was comparable to that of diesel. An et al. [74] experimentedwith waste cooking oil biodiesel on a four cylinder, four-stroke,turbocharged, direct injection Euro IV diesel engine at differentspeeds and two load conditions. At full load condition, biodieseland biodiesel–diesel blended fuels gave better brake thermalefficiency than that of diesel. However, at part load (25% of fullload) condition the result was found to be quite different and areverse trend was observed. It was attributed to low fuel/airequivalence ratios of biodiesel at part load, which could not turnits oxygenated nature to be an advantage at that stage. Also it wasreported that higher kinematic viscosity of biodiesel pre-dominated the atomization process as well as mixing with air andled to a poorer combustion, thus thermal efficiency was reduced.
To have a better idea about the variation of brake thermalefficiencies of CI engines fueled with different biodiesels, some ofthe results reported in the literature have been partially repro-duced and presented in Fig. 4(a)–(c) as case studies.
It has been observed that Puhan et al. [75], Banapurmath et al.[76] and Sharon et al. [27] carried out the investigations using verysimilar type of experimental setup. The corresponding resultsusing methyl esters of mahua, jatropha and used palm oil as fuelshave been shown in Fig. 4(a)–(c) respectively. The figures showthat the efficiencies with the above said three biodiesels are lowerthan that of diesel fuels. It can further be noted from figures thatthe efficiency increases with load upto certain value in all thecases for all fuels including neat diesel. But, the rate of increase of
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efficiency with load is lower with biodiesel than with diesel.However, the difference in efficiency with mahua biodiesel anddiesel remains almost same at all load conditions. The maximumdecreases in brake thermal efficiency for mahua, jatropha andused palm biodiesels are noted to be 13%, 7.2% and 7.26% respec-tively with respect to neat diesel. So, it can be concluded fromthese case studies that the brake thermal efficiency decreases ingeneral when biodiesels are used instead of diesel under the sameoperating conditions.
3.2. Effect on brake specific fuel consumption
Brake specific fuel consumption is one of the most importantparameters to describe the performance of an engine and isdefined as the fuel consumption rate to produce unit brake power.Generally, the specific fuel consumption of the blended fuel ismore because of the lower heating value of biodiesel than con-ventional diesel. The heating value of biodiesel is less than that ofdiesel due to around 11% oxygen content in the fuel which doesnot contribute to heat generation during combustion inside thecylinder [25]. The variations in specific fuel consumption rateusing biodiesel as fuel for various operating conditions of enginesas adopted by different researchers in the fields of alternative fuelsduring the past few decades have been reviewed and presented inthis section.
Gumus [56] observed that the brake specific fuel consumptionincreased when hazelnut kernel oil methyl ester was used as thecomplete replacement of mineral diesel as CI engine fuel. It wasbecause of its low heat content and higher viscous nature thatincreased the fuel consumption rate. About 22.66% increase inBSFC was observed throughout the load range. It was also reportedthat the brake specific fuel consumption decreased with theadvancement of injection timing, increase in compression ratioand injection pressure. Laforgia and Ardito [64] also reported thesame kind of behavior with biodiesel and the reasons behind thiskind of behavior were also same as explained by Gumus [56].Utlua and Kocak [77] conducted an experimental study on a fourcylinder, direct injection, turbocharged, intercooled diesel engineusing waste frying oil methyl ester and reported that the brakespecific fuel consumption with waste frying oil methyl ester was14.34% higher than that of diesel fuel. They concluded that the lowheating value and the higher density of waste frying oil methylester were responsible for the increased fuel consumption rate.
Raheman and Ghadge [65] investigated the effect of compres-sion ratio (varied from 18:1 to 20:1) and ignition timing (variedfrom 35 to 45° bTDC) on the performance of a Ricardo E6 engine.They found that lower compression ratio and retarded ignition
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timing caused a poorer combustion of biodiesel and it led to anincrease of specific fuel consumption by 38.3% for neat biodieselthan that of diesel. At higher compression ratio, the incrementreduced to 22.4% only. It was attributed to the lowering of viscosityand increase in volatility of biodiesel which yielded relativelybetter performance at higher compression ratio. The advancementof injection timing showed that mean BSFC was reduced by 15.8%for 10° advancement of fuel injection and the major portion(almost 11%) of this was recorded for 5° advancement from 35°bTDC to 40° bTDC. In another study by Raheman and Ghadge [70]on the same test setup and same test fuels, at fixed compressionratio of 18:1 and static injection timing of 40° bTDC, brake specificfuel consumption was found to be 41.4% higher for 100% mahuabiodiesel than that of diesel because of 12% low calorific value and4% higher viscosity of mahua biodiesel compared to those of dieselfuel. Kaplan et al. [78] experimented on a Peugeot make mediumduty diesel engine with sunflower oil methyl ester as fuel. Theyfound that the fuel consumption with sunflower oil methyl esterwas higher than mineral diesel. The 10% lower calorific value ofsunflower oil methyl ester with respect to diesel was the mainreason behind this kind of behavior, as pointed out by them.
Hasimoglu et al. [79] conducted an experimental study withMercedes Benz, four cylinder, turbocharged, direct injection dieselengine using refined sunflower oil methyl ester (biodiesel) as fuel.They observed a 13% increase in the specific fuel consumptionwith biodiesel. This type of behavior was due to the combinedeffect of lower heating value and higher density as reported byHasimoglu and the co-authors. Qi et al. [80] studied the effect ofsoybean oil methyl ester on the performance, emission and com-bustion of a single cylinder, naturally aspirated, four stroke, watercooled, direct injection, high speed diesel engine. It was reportedby them that the 10.2% lower heating value of soybean oil methylester resulted about 11% higher biodiesel consumption rate thandiesel for a desired amount of power output. An experimentalinvestigation was carried out by Canakci [71] on a John Deere, fourcylinder, four stroke, turbocharged, direct injection diesel engineusing diesel and soybean biodiesel. He noted a 13.8% higher valueof brake specific fuel consumption in case of biodiesel with respectto mineral diesel. Zhu et al. [72] conducted an experimental studyin an Isuzu naturally aspirated, water cooled, four cylinder, directinjection diesel engine using waste cooking oil biodiesel and itsblends with ethanol and diesel. It was found that BSFC increasedapproximately by 13% with biodiesel mainly due to the lowercalorific value of biodiesel compared to Euro V diesel fuel.
Puhan et al. [57] used linseed oil methyl ester in a singlecylinder, four stroke, constant speed, vertical, air cooled, directinjection diesel engine. They observed approximately about 9%increase in specific fuel consumption with linseed oil methyl esterthroughout the load range compared to that of diesel under thesame operating conditions. Higher viscosity and low calorific valueof biodiesel were identified to be the possible reasons for theincrease of fuel consumption rate. Anand et al. [81] observed thatBSFC of waste cooking oil biodiesel and diesel blended fuel was17% higher compared to mineral diesel due to the combined effectof higher viscosity and lower calorific value of waste cooking oilbiodiesel, while using in a single cylinder, four cylinder, naturallyaspirated diesel engine. Ozsezen et al. [58] experimented withwaste palm oil methyl ester and canola oil methyl ester in a FordCargo, six cylinder, naturally aspirated, direct injection dieselengine and found that specific fuel consumption rates of the testedbiodiesels were higher than that of petroleum based diesel fuel by7.45% and 6.18% respectively. According to them, that kind ofbehavior was due to the higher density, which resulted in higheramount (mass basis) of biodiesel injection in the combustionchamber for the production of same amount of power output.
Also, poorer atomization slowed down the fuel–air mixture for-mation rate due to the higher kinematic viscosity of biodiesel.
Agarwal and Dhar [60] used neat karanja oil and preheatedkaranja oil for their experimental study. It was reported by themthat higher viscosity of karanja oil caused poorer atomization andincreased the fuel consumption compared to diesel. It was alsoreported that the viscosity of the oil reduced due to preheating ofthe karanja oil, which resulted better atomization and combustion,thus the fuel consumption rate was found to be less than that ofnormal karanja oil. The increments of fuel consumption rate werefound to be approximately 39% and 17% respectively for non-preheated straight vegetable oil of karanja and preheated karanjaoil. Song and Zhang [67] experimented on a four cylinder, fourstroke, supercharged, direct injection diesel engine with soybeanoil methyl ester. It was observed that lower calorific value andhigher density of soybean oil methyl ester caused a higher amountof biodiesel supply (by weight) to the combustion chamber due tohigher discharge of fuel injection pump and resulted in higherBSFC value than mineral diesel.
McCarthy et al. [82] conducted an experimental study on avertical, liquid cooled diesel engine and found an increase in BSFCof about 7% and 10% compared to that of diesel respectively for themixture of animal tallow (80%)-canola oil methyl ester (20%) andchicken tallow (70%)-waste cooking oil methyl ester (30%). Thiswas attributed to the lower calorific value or energy content ofbiodiesel. Ceviz et al. [68] experimented with hazelnut oil methylester (biodiesel) as CI engine fuel and reported that brake specificfuel consumption increased for B100 blend approximately by 12%and decreased for B20 blend by 8.2% compared to diesel. This kindof behavior was thought to be due to better combustion with B20.But calorific value of the fuel decreased by a large amount whenB100 was used and this was not compensated even by theimproved combustion of the fuel. Aksoy [83] experimented on asingle cylinder, four stroke, air cooled diesel engine with a pre-combustion chamber using waste frying oil methyl ester andsoybean oil methyl ester. Due to the lower calorific value of bio-diesels produced from raw soybean and waste frying oil the spe-cific fuel consumption increased by an average of 18.5% and 14.2%respectively compared to mineral diesel.
Rao et al. [61] also found the similar trend with jatropha bio-diesel. The increment in BSFC was noted to be marginal in case ofjatropha biodiesel. Yücesu and İlkiliç [84] experimented on a sin-gle cylinder, four stroke, direct injection, air cooled Lombardiniengine using cotton seed oil methyl ester and found about 8%higher fuel consumption of biodiesel compared to diesel. This wasmainly because of the higher mass and lower calorific value of theester which resulted in higher volume of fuel accumulation duringcombustion of biodiesel than that of diesel. Yadav and Singh [85]also reported an increment of brake specific fuel consumptionduring experimentation on a single cylinder, four stroke dieselengine with jatropha, karanja and neem biodiesels because oftheir lower energy content approximately by 10% than diesel. Incase of a lower amount blend of karanja oil methyl ester (B15 andB20), a reverse trend was observed by Mahanta et al. [59] duringan experimental study on a water cooled, direct injection, fourstroke diesel engine. It was reported by them that due to 10%higher oxygen content of biodiesel it exhibited better combustioncharacteristics and hence the fuel consumption with B15 and B20blended fuels was approximately 14% lower at full load conditioncompared to diesel.
Deore and Jahagirdar [66] observed that with jatropha biodie-sel at compression ratio 18 and lower load, the specific fuel con-sumption was lower than diesel by 9.61% and at higher load it washigher than diesel by 3.57%. It was also observed that specific fuelconsumption was lower than diesel by 6% at lower load and higherthan diesel by 5% at higher load corresponding to a compression
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ratio of 16. On the other hand, the increase in BSFC with jatrophabiodiesel was more at compression ratio 14 and it was about 29%higher than diesel. In case of karanja biodiesel, the specific fuelconsumption was higher than jatropha biodiesel by about 11 to30% for different compression ratios at low loads. The authorspointed out that the differences in density and viscosity amongdifferent biodiesel and diesel were responsible for this kind ofbehavior.
At the end of this section, it can be said that except a few,almost all the studies showed an increase of BSFC in the range of5–15% for different biodiesels. For further illustration and com-parison experimental results from the work of Puhan et al. [75]and Sharon et al. [27] have been plotted in Fig. 5(a) and(b) respectively for mahua and used palm oil biodiesels.
It can be seen from both the figures that the specific fuelconsumption rates of both the biodiesels are higher than that ofdiesel over the entire load range. The maximum increase in BSFCnoted are not to be 20% and 14.55% with mahua and used palm oilbiodiesel respectively.
3.3. Effect on exhaust gas temperature
Exhaust gas temperature is an indicator of the heat release rateof the tested fuel during combustion period [86] and its effectiveutilization to produce power. It depends on the nature of com-bustion and the heat loss to the exhaust which again depends onthe fuel consumption rate. The higher fuel consumption rateresults higher amount of heat rejection, which causes higherexhaust gas temperature [41]. Generally, the fuel consumptionwith biodiesel is higher than that with diesel and also the com-bustion is improved due to the presence of excess oxygen in thefuel itself. These set the general trend of increased exhaust gastemperature with biodiesel fuels.
Godiganur et al. [38] experimented on a Cummins made sixcylinder turbocharged diesel engine using diesel, mahua oilmethyl ester and its blends with diesel as fuels. They observed anincrease in exhaust gas temperature with the increase in engineload. The exhaust gas temperature was found to increase with thepercentage increase of biodiesel in the blended fuel. The mean EGTof the engine with neat mahua oil methyl ester was noted to benearly 12% higher than that with diesel. The trend was due tomore heat loss with biodiesel as explained by Godiganur and co-researchers. Buyukkaya [87] experimentally investigated theeffects of rapeseed biodiesel and its blends with diesel on engineperformance, emission and combustion of a six cylinder turbo-charged diesel engine. They reported that with increase in bio-diesel share in the blended fuel the exhaust temperature increased
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and the maximum exhaust temperature with B100 rapeseed bio-diesel was found to be approximately 490 °C, whereas the corre-sponding value with diesel fuel was noted to be 475 °C running ata relatively high speed of 2100 rpm. Datta et al. [88] experimentedon a double cylinder, four stroke, diesel engine fueled with jatro-pha oil methyl ester and its blends with diesel. They observed anincrease in exhaust gas temperature with jatropha oil methyl esterand its blends due to the higher flame temperature of jatrophabiodiesel compared to that with diesel.
Puhan et al. [75] conducted an experimental study on a fourstroke, compression ignition, constant speed, vertical, watercooled, direct injection diesel engine using mahua oil methyl esteras fuel. It was reported by them that exhaust gas temperature ofmethyl ester of mahua oil was higher than that of mineral dieselapproximately by 3.7%. They identified the longer duration of afterburning period to be responsible for higher exhaust gas tem-perature. Behçet [86] observed an increment of exhaust gas tem-perature approximately by 12% compared to that of diesel whenfueled with anchovy fish biodiesel during experimentation on asingle cylinder direct injection diesel engine at a constant speed of3600 rpm. The possible reason of higher exhaust gas temperaturewith anchovy fish biodiesel might be the higher oxygen content ofit, which led to better combustion. This increased combustiontemperature as well as the exhaust temperature. Higher viscosityand density of biodiesel were also identified by the author asinfluencing parameters for increase of exhaust gas temperature. Inan experimental investigation, Banapurmath et al. [62] observedthat the exhaust gas temperature increased with marotti oilmethyl ester (biodiesel) compared to diesel. They explained theabove fact on the basis of the poor volatility, high viscosity and theslower combustion of marotti oil methyl ester.
Reverse trend was also observed by Kegl [89] while experimentingon a four stroke, six cylinder, in line, water cooled bus engine withrapeseed biodiesel. This type of behavior might be due to the lowheating value of biodiesel as reported in this study. Lin et al. [44]experimented on a single cylinder, four stroke, water cooled, directinjection diesel engine with various kinds of biodiesels prepared fromeight different oils namely, soybean, peanut, corn, sunflower, rapeseed,palm, palm kernel and waste fried oil using methanol as alcoholduring transesterification. They observed that the exhaust gas tem-peratures with those biodiesels were slightly lower than that withdiesel. It was reported by them that the lower energy content of thosebiodiesels reduced total heat release during combustion and hence theexhaust gas temperature reduced. Sureshkumara et al. [90] experi-mented on a single cylinder, four stroke, water cooled diesel engine ata constant speed of 1500 rpm with karanja oil methyl ester. Theyreported a lower exhaust gas temperature for karanja oil methyl ester
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compared to that of diesel. Oxygenated nature of the biodiesel whichled to more complete combustionwas held responsible for this kind ofvariation.
On the other hand, Raheman and Phadatare [41] reported thatthere was no such significant change in exhaust gas temperaturebetween karanja oil methyl ester and diesel due to nearly sameamount of heat loss at the exhaust.
To summarize the variation of exhaust gas temperature frombiodiesel fueled CI engines, some results related to exhaust gastemperature from two different experimental works carried out byPuhan et al. [75] and Datta et al. [88] are presented in Fig. 6(a) and(b) respectively and discussed.
Fig. 6(a) and (b) clearly show that the exhaust gas temperaturesusing mahua biodiesel and jatropha biodiesel as fuels are higherthan those with diesel under the same operating condition.
3.4. Summary of performance analysis
The performance related information under normal operatingconditions as observed by several researchers with different bio-diesels and their blends with diesel have been summarized andpresented in Table 4. The increase and decrease in different per-formance parameters as shown in the above mentioned table arewith respect to mineral diesel only.
It can be noted from the table that brake thermal efficiencydecreases in most of the cases by around 10% for neat biodiesel(B100). The decrease is marginal in case of B10 and B20 blends.The decrease is much more with straight vegetable oil. The BSFCincreases by more than 10% for neat biodiesel. The exhaust gastemperature is normally higher with biodiesel and its blends withdiesel. The maximum decrease in brake thermal efficiency is notedto be 27% with karanja biodiesel. On the other hand, the maximumincrease of brake specific fuel consumption is found to be 38.3%with neat mahua biodiesel. A 20% increase of exhaust gas tem-perature with neat jatopha biodiesel is reported to be highestamong all the studies reviewed in this work. The opposite trend,i.e., increase in brake thermal efficiency and decrease in BSFC aswell as exhaust gas temperature are also found in few cases.
Brake thermal efficiency of an engine depends upon theproperties of the fuel such as lower heating value (calorific value),viscosity and density. In case of neat biodiesel or diesel–biodieselblended fuels, calorific value of the fuel is less compared to that ofneat diesel and viscosity and density are higher. The decrease inlower heating value is mainly responsible for the higher BSFC incase of biodiesel. This higher fuel consumption and the improve-ment in combustion due to the oxygen enrichment with biodieselresult in higher exhaust gas temperatures. The analysis of all the
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experimental studies suggests that it will be beneficial and wise ifbiodiesel–diesel blends having 10–20% biodiesel are used withlittle sacrifice in the performance of the engine.
4. Effect of biodiesel on engine emissions
The increased use of fossil fuels in the form of diesel and petrol,particularly in the automobile sector and the continuously grow-ing emission of harmful pollutants from the tail pipe of engines arelargely responsible for several diseases and fast degradation of theglobal environment. The hydrocarbon emitted from the exhaust ofthe automobiles forms ground-level ozone which is the majorcomponent of smog. Ozone affects human beings causing lungdecease, eye irritation and respiratory problems. Hence, it is nowalmost mandatory for any fuel to be used as automobile fuel tomeet the stringent emission norms set by the different regulatingauthorities throughout the world. Keeping this in mind, theemissions from different biodiesel fueled CI engines have beencritically reviewed and summarized based on the diversifiedworks reported in the literature. The major pollutants from theengine exhaust that have been identified and considered for thisreview work are CO, CO2, NOx, hydrocarbon and smoke. The effectof biodiesel addition to diesel in different proportions on theabove said emissions from CI engines has been presented anddiscussed in the next few sections.
4.1. Effect on CO emission
Carbon monoxide (CO) is produced by the incomplete com-bustion of carbon-containing substances. The carbon present inany fuel is converted to CO2 and CO (product of incomplete com-bustion) during burning of fuel in the presence of oxygen withinthe engine cylinder. It is obvious that the emissions of CO2 and COare interrelated i.e., if CO2 emission increases then CO emissiondecreases naturally. It is expected that CO emission will decreasewith the increasing biodiesel percentage in the biodiesel–mineraldiesel blends as biodiesel itself contains 11% oxygen in its mole-cules. Some of the previous experimental works on the effect onCO emission while using biodiesel and diesel–biodiesel blends asfuel for compression ignition engine have been presented andanalyzed in this section.
The effect of adding mahua oil methyl ester to diesel on theengine emission was studied by Godiganur et al. [38]. It wasobserved that CO emission decreased with diesel/biodiesel blendsdue to more complete oxidation of biodiesel than that of diesel.Some of the CO formed during combustion was further oxidized
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Table 4Performances of different biodiesel fueled engines compared to diesel at normal operating condition.
Name of researchers Biodiesel and it blends BTE BSFC EGT
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and converted to CO2 due to the presence of certain amount ofoxygen in biodiesel itself. It was also observed that CO emissioninitially decreased with load and later on increased sharply uptothe full load. Nabi et al. [45] experimented on a single cylinder,water cooled, naturally aspirated, four stroke, direct injectiondiesel engine using cotton seed oil biodiesel and reported that COemission with biodiesel was lower than that of diesel fuel. In caseof B30 blend of cotton seed oil, CO emission was reduced by 24% incomparison to neat diesel fuel. They also concluded that the oxi-dation process of CO to CO2 was enhanced due to the presence ofoxygen in biodiesel fuel.
Zou and Atkinson [91] carried out experiments on two vehicles,namely Toyota Helix Utility with oxidation catalytic converter andVolswagen Wolf without oxidation catalytic converter using canolabiodiesel. They used standard testing cycle of diesel engine for thisstudy. The Euro 2 drive cycle test was consisted of four urban
driving cycle test with a maximum speed of 50 km/h and one urbandriving cycle test with a maximum speed of 120 km/h. The sam-pling time for the emission test was 1200 seconds. They observed anarrow range of reduction (about 10%) of CO emission with bio-diesel as compared to that of diesel. During a comparative study byRakopoulos et al. [43] on a four stroke, direct injection (DI), Ricardo/Cussons ‘Hydra’ Diesel engine with methyl esters of cottonseed oil,soybean oil, sunflower oil, rapeseed oil and palm oil blended withdiesel (10% and 20% blend), it was observed that CO emission wasslightly reduced with the use of biodiesel blends in comparison tothat of the neat diesel fuel. The maximum reduction was noted withcotton seed biodiesel and it was approximately 14%.
It was observed by Çelikten et al. [46] that CO emission reducedwhile using rapeseed and soybean oil methyl esters as fuels in afour cylinder diesel engine. At higher injection pressure of 350 bar,it was also observed that CO emission decreased by 21% and 28%
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respectively for rapeseed and soybean biodiesel. Raheman andPhadatare [41] observed a huge reduction of CO emission whileusing karanja oil methyl ester and its blends with diesel instead ofmineral diesel. CO emission was found to be reduced by nearly 74%when neat karanja biodiesel was used instead of mineral diesel.Kegl [89] reported that more reduction in CO emission at tailpipeusing biodiesel was achieved by retarding the injection timing.Randazzo and Sodré [92] experimented on a station wagon havingturbo-charged, four cylinder diesel engine of compression ratio17.6:1. The test vehicle was also equipped with a platinum–rho-dium catalytic converter and exhaust gas recirculation valve toreduce pollutant emissions and NOx respectively. The resultsshowed that addition of soybean biodiesel to diesel had littleinfluence in lowering CO emission. But the addition of ethanolincreased the CO emission significantly. The possible reasonbehind this increment was the higher latent heat of vaporizationof ethanol, which led to a reduction of combustion temperatureand resulted in decrease of oxidation rate of CO into CO2.
Lin et al. [93] experimented on a Cummins direct injectionheavy duty diesel engine having compression ratio of 17.9:1 atconstant injection pressure of 250 bar and injection timing of 12.3°bTDC with waste cooking oil biodiesel. They reported decrease ofCO emission by 3.33% to 13.1% with the use of different blends (B5-B30) of the above said biodiesel and ultra low sulfur diesel. Yoonand Lee [94] experimented on a four cylinder, turbocharged dieselengine and employed both single fuel and dual fuel mode withbiogas and biodiesel. In both the conditions, the CO concentrationwas found to be lower with biodiesel due to 11% higher oxygencontent of biodiesel. This led to complete combustion resultinglower CO emission.
In an experimental study on a four cylinder, inline turbo-charged diesel engine with intercooler using karanja biodiesel andits blend with methanol as fuels, it was observed by Anand et al.[42] that CO emission was approximately 46.5% lower at higherload for biodiesel–methanol blend compared to neat biodiesel. Themore complete combustion due to the oxygen enrichment withmethanol blending was found to be the reason for this kind ofbehavior. Nabi et al. [50] experimented on a single cylinder fourstroke diesel engine with diesel and its blends with neem bio-diesel. They observed a 4% reduction of CO emission in case ofdiesel–neem biodiesel blends compared to that of conventionaldiesel fuel. In another experimental study by Datta et al. [88]observed that, in case of jatropha oil methyl ester and its blends,the CO emission was reduced by 24% compared to diesel. It wasalso reported by them that, due to more complete combustion, COwas decreased with biodiesel and its blends. Also, the presence ofoxygen in biodiesel and higher combustion temperature setfavorable conditions for oxidation of CO to CO2.
It was also observed by Puhan et al. [75,95] during experi-mental studies on Kirloskar made single cylinder, four strokediesel engine fueled with mahua oil alkyl esters that the higheroxygen content (about 10–12%) of mahua oil methyl and ethylesters enhanced the combustion process and thus oxidized the COinto CO2. It was reported that the difference in cetane number, andenergy content of biodiesel and petro-diesel were also responsiblefor this kind of behavior. The reduction in CO emission with mahuabiodiesel was reported to be approximately 67–79% compared tothat of diesel. Puhan and Nagarajan [96] experimented on thesame test rig and also observed the same trend for mahua oil ethylesters. Similar type of behavior was observed with cotton seed oilmethyl ester by Aydin and Bayindir [97] while using the biodieselon a single cylinder, direct injection, four stroke, water cooleddiesel engine. Gumus and Kasifoglu [98] experimented on aLombardini single cylinder diesel engine using apricot seed kerneloil methyl ester and its blends with diesel as fuels. A reduction ofCO emission was observed by them and they concluded that the
reduction in CO emission was mainly due to oxygen content ofbiodiesel which improved the combustion process. Behçet [86]also observed a reduction in CO emission while using wasteanchovy fish biodiesel and the maximum reduction was found tobe 31.2%. Sureshkumara et al. [90] also reported that karanja bio-diesel emitted lower amount of CO compared to diesel.
It was observed by Swaminathan and Sarangan [99] during anexperimental study on a Kirloskar made single cylinder, fourstroke, direct injection diesel engine, that the addition of diethy-lene glycol dimethyl ether to pongamia methyl ester (karanjabiodiesel) reduced CO emission. At full load condition, the reduc-tion was observed to be 44%. The additives provided extra oxygento oxidize CO. Amarnath and Prabhakaran [100] experimented ona Kirloskar made single cylinder, four stroke, water cooled dieselengine using karanja oil methyl ester and also observed a reduc-tion of CO emission. It was also pointed out by them that at highercompression ratio the CO emission further reduced due to bettercombustion. In addition, they tried to examine the effect ofinjection pressure on CO emission and observed a reduction of it athigher injection pressure. The availability of more surface area forcombustion of very fine droplets under this condition resulted toalmost complete combustion, which led to a less amount of CO attailpipe. Bayrakçeken [101] experimented on a single cylinder, fourstroke, air cooled diesel engine fueled with crude and refinedsoybean oil methyl ester. They observed that CO emissiondecreased with both crude and refined soybean oil methyl ester by11.98% and 6.96% respectively in comparison to that with mineraldiesel.
Venkata Subbaiah and Raja Gopal [102] experimented on asingle cylinder, four stroke, water cooled diesel engine with ricebran oil biodiesel and ethanol blending with mineral diesel. Theyreported that with rice bran oil biodiesel CO emission decreasedcompared to that with diesel by 25.8%. Small amount of ethanoladdition to biodiesel decreased the CO emission compared to thatof biodiesel, but when the percentage of ethanol in the blendsbecame more than 7.5%, higher amount of CO emission wasobserved than that with biodiesel. More complete combustionwith cotton seed oil methyl ester reduced the CO emission com-pared to that with diesel as observed by Aydin and Bayindir [103]during their study on a Rainbow-186, direct injection, singlecylinder diesel engine. The same trend was noted by Ulusoy et al.[104] and Sharma et al. [105] with sunflower oil methyl ester andneem biodiesel respectively. Ulusoy et al. [104] used a heavy dutyturbo diesel engine during the study whereas agri-genset enginewas used by Mathur et al. [105]. A significant reduction (29%) in COemission was observed by Aydin and İlkiliç [112] with rapeseedmethyl ester compared to that with diesel while experimenting ona similar type of test engine as used by Aydin and Bayindir [103].
İlkiliç [106] observed approximately 30% reduction in COemission from a Lombardini, single cylinder, air cooled dieselengine under different injection pressures when sunflower oilmethyl ester was used as fuel instead of diesel. Shirneshan [107]experimented on a four cylinder, water cooled, heavy duty dieselengine with waste frying oil methyl ester and reported that the COemission decreased with the addition of waste frying oil methylester (biodiesel) to diesel. In another study by Ranganathan andSampath [108] on a four stroke, single cylinder, air cooled, directinjection diesel engine with cotton seed oil biodiesel, it was foundthat CO emission was approximately 26% less than that with neatdiesel. The extra oxygen available in the biodiesel moleculeenhanced the combustion process and reduced the CO emission asmentioned by them. Rao et al. [114] also used cotton seed oilbiodiesel on a single cylinder, direct injection diesel engine andfound that CO emission decreased by 18% and 24% using B25 andB100 blend of used cotton seed oil biodiesel respectively.
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Fig. 7. Effect of different biodiesels on CO emission.
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On the other hand, some researchers found some oppositetrend of CO emission with the use of different biodiesels. Sahooet al. [40] experimented on a three cylinder water cooled tractorengine using jatopha, karanja and polanga based biodiesel forthree different engine speeds. They adopted the 8 mode test cycleof ISO 8178 type C1 for conforming the Euro 2 standard. Duringexperimentation, they observed that CO emission increased gra-dually with blending of higher concentration of biodiesel to dieselexcept karanja biodiesel. They indicated that the non-homogenousnature of the blended fuels and the higher latent heat of vapor-ization resulted poorer combustion and hence more CO formation.An air cooled, single cylinder, four stroke, DEUTZ F1L511, directinjection diesel engine was employed by Huzayyin et al. [109] forexperimentation with mixture of jojoba oil and gas oil as fuel. Theyobserved that with the increase in jojoba oil percentage in the fuelblend, CO emission increased. It was observed by Saleh [49] duringan experimental study on a two cylinder, four stroke, water cooleddiesel engine, that with the use of jojoba oil methyl ester as fuelincreased the CO emission compared to that of diesel due to thehigher viscosity of biodiesel which led to poorer atomization andpoorer distribution of air–fuel mixture.
Pugazhvadivu and Jeyachandran [110] observed an incrementin CO emission using waste frying oil (without preheating) as fuelon a Kirloskar made single cylinder diesel engine. The incrementwas attributed to the higher viscosity of waste frying oil, which ledto poorer combustion and formation of locally rich air–fuel mix-ture zone and thus, suitable condition for CO formation was cre-ated. Heavier molecular structure and higher viscosity of honge,jatropha and sesame oil methyl esters led to poor atomization andresulted to a higher CO emission for biodiesel compared to dieselas observed by Banapurmath et al. [76] during an experimentalstudy on a Kirloskar made single cylinder, variable compressionratio, computerized diesel engine.
Baiju et al. [39] conducted an experimental study on a singlecylinder, four stroke, naturally aspirated, constant speed com-pression ignition engine using blends of karanja oil methyl esterand ethyl ester with diesel. With karanja oil methyl ester, it wasobserved that at lower loads, CO emissions did not vary much forall fuels considered by them. However, at full load, higher COemissions were observed with B20 blend of karanja oil ethyl esterthan any other blends of methyl ester due to the enrichment ofoxygen which resulted in better combustion with methyl ester ofkaranja oil. Tsolakis et al. [111] conducted an experimental studyon a Lister-Petter TR1 naturally aspirated, air cooled, single cylin-der direct injection diesel engine. They also employed exhaust gasrecirculation technique by adding different percentages of exhaustgas to the intake air mainly to reduce NOx in the tailpipe. But, in
case of CO emission, the effect was found to be detrimental. Theyobserved that rapeseed oil methyl ester emitted less amount of COthan that of diesel in normal and 10% EGR conditions, but therewas practically no variation in CO emission between mineral dieseland biodiesel in the case of 20% EGR. The possible reason for lessCO emission in the first two cases was the oxygen enrichmentduring the time of combustion by means of oxygenated fuels(biodiesels). However, at 20% EGR, this effect was compensateddue to the dilution effect of the exhaust gas recirculation.
At the end of this section, the authors like to compare someresults on CO emission from CI engines using neat biodiesel andneat diesel as fuels from the previous works of Puhan et al. [75],Banapurmath et al. [76] and Sharon et al. [27]. It may be noted thatsimilar type of engine setup was used in the above said threestudies and the corresponding results on CO emissions have beenpresented in Fig. 7(a)–(c) respectively.
It is reflected from Fig. 7(a) and (c) that the emission of CO islower with mahua and used palm biodiesel than that with dieselat all loads conditions. The maximum reduction is found to be 30%and 52.9% for mahua and used palm biodiesel respectively. On theother hand, the emission of CO with jatropha biodiesel reported byBanapurmath et al. [76] is higher than that with diesel at all loadsas shown in Fig. 7(c). The maximum increment is found to be37.77% at full load condition. Poor atomization characteristics ofjatropha biodiesel due to its higher viscosity resulted in impropermixing of biodiesel with air and led to higher CO emission. Also,form the figures it is observed that, in general, the CO emissionincreases with load. At the end of the review of CO emissions frombiodiesel fueled engines, it is observed that CO emissions from CIengines decrease with biodiesel fuels in most of the cases. How-ever, a few studies show even the increase of CO emission to someextent with certain specified biodiesels under certain operatingconditions.
4.2. Effect on CO2 emission
It is well-known that complete combustion inside the com-bustion chamber helps in increasing CO2 (carbon dioxide) emis-sion rapidly. Although there is no possibility of occurring completecombustion, but it may be nearly complete combustion dependingupon the engine operating conditions and the fuel used for run-ning the engine. Most of the researches have measured CO2
emissions along with CO emission to find a correlation betweenthem. CO emission has been described just in the previous sectionand this section is devoted to CO2 emission. In most of the cases,the operating conditions and engine used to study the CO2
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emissions have already been described in the previous section ofCO emission.
It was observed by Godiganur et al. [38] and Nabi et al. [45] thatthe presence of excess oxygen in mahua biodiesel and cotton seedoil biodiesel respectively resulted better combustion, whichhelped to convert CO into CO2 and thus CO2 emission increased.Fontaras et al. [47] while experimenting on new European drivingcycle observed that during cold start, tailpipe CO2 emission froman automobile increased by 14% for B100 and 9% for B50 blend ofsoybean biodiesel. But, for Artemis driving cycles, it was noted thatB50 blend had no impact on CO2 emission and for B100 itincreased only slightly. Randazzo and Sodré [92] used soybeanbiodiesel upto maximum of 20% (B20) and observed that CO2
emission increased with the increase of biodiesel content in dieselbiodiesel blends. It was also reported by them that small percen-tages of ethanol addition to B20 blend reduced CO2 emissionsubstantially.
Huzayyin et al. [109] observed that with the increase in jojobaoil percentage in the blended fuel, CO2 emission increased. In anexperimental study on the emissions from dual fuel engine usingsoyabean oil methyl ester as one of the fuel, Yoon and Lee [94]observed more CO2 emission with biodiesel. They explained thatoxygen present in biodiesel allowed CO to oxidize into CO2. Theyalso commented that the life cycle CO2 emission was less withsoyabean biodiesel than that of diesel fuel as soyabean plantsabsorbed CO2 during harvesting through the process of photo-synthesis. Puhan et al. [75] and Puhan and Nagarajan [96] alsoobserved the same trend of increased CO2 emissions with methylester of mahua oil and ethyl ester of mahua oil respectively.Amarnath and Prabhakaran [100] observed that with the use ofkaranja biodiesel CO2 emission increased. They also concluded thathigher oxygen content of karanja biodiesel was responsible formore CO2 emission and less CO emission. Venkata Subbaiah andRaja Gopal [102] also reported a higher CO2 emission with ricebran oil biodiesel. They also noticed that when small amount ofethanol was added to biodiesel, a further increase of CO2 emissionwas observed because of the presence of oxygen in ethanolmolecules.
On the other hand, Gumus and Kasifoglu [98] observed thatapricot seed kernel oil methyl ester emitted lower amount of CO2
compared to that of diesel when used in CI engine. They concludedthat the scarcity of air in the mixture formation slowed down thecombustion process and thus decreased the formation of CO2. Thedecrease of CO2 emission with biodiesel fuels was also observed byBehçet [86]. Sureshkumara et al. [90] also reported a reduction inCO2 emission for karanja biodiesel. On the basis of the experi-mental study already mentioned in the previous section,
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Swaminathan and Sarangan [99] observed a reduction of carbondioxide emission in the range of 2–8% with the use of diethyleneglycol dimethyl additive with pongamia methyl ester. A reductionin CO2 by 16% was observed by Aydin and İlkiliç [112] with B20blend of rapeseed methyl ester compared to diesel. With sun-flower oil methyl ester, it was observed by İlkiliç [106] that sun-flower biodiesel produced less CO2 at all conditions. Lower CO2
emission was explained by him in the following manner. Biodieselis a low carbon fuel and has a lower elemental carbon to hydrogenratio than diesel fuel which leads to a lower CO2 emission. Shir-neshan [107] found that for waste frying oil methyl ester–dieselblended fuel, CO2 emission decreased compared to that with die-sel. However, at higher concentration of biodiesel in the blendedfuels, CO2 emission increased but still remained lower than thatwith neat diesel. Özcanli et al. [113] noted a decrease in CO2
emission with increase in castor biodiesel content in the blendedfuel and the most probable reason behind this was reported to bethe lower elemental carbon to hydrogen ratio in the biodieselinvestigated. Rao et al. [114] also reported a reduction of CO2
emission from CI engine with the use of cotton seed oil biodiesel.But, Zou and Atkinson [91] reported that CO2 emission remainedalmost same for both 100% canola biodiesel and 100% petroleumdiesel.
On the basis of the review and analysis of the reports availablein the literature on CO2 emissions from CI engines using differentkinds of biodiesel and diesel blends, two types of opposite trendshave been identified. Quite a large number of studies showedincrease of CO2 emissions due to the improved combustion uti-lizing the oxygen present in the biodiesel fuel itself. On the otherhand, substantial number of experimental investigations alsoreported reduction of carbon dioxide in the exhaust gas frombiodiesel fueled engines. The lower elemental carbon to hydrogenratio in the molecular structure of biodiesel can be identified to bethe possible reason to lower the CO2 emission. Accordingly, tworesults of opposite nature from the experimental works of Sharonet al. [27] and Gumus and Kasifoglu [98] are presented in Fig. 8(a) and (b) respectively for illustration.
It is quite evident from Fig. 8(a), that the CO2 emission of usedpalm biodiesel is higher than that with diesel at all loads and amaximum increase is noted to be 8.76%. However, the reverse typeof trend is observed in Fig. 8(b) with apricot seed kernel biodieseland the maximum reduction is noted to be 10.88%. Theoreticalinvestigation shows that the emission of CO2 from CI enginesshould increase with biodiesel addition to diesel fuel because ofimproved combustion due to the presence of oxygen in themolecular structure of biodiesel. However, the lower elementalcarbon to hydrogen ratio in biodiesel will tend to decrease the CO2
emissions. The final emission level of CO2 from CI engines using
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biodiesels as fuels will be decided on the relative influence ofthose two opposite effects and this can only be answered fromexperimental studies.
4.3. Effect on NOx emission
Mainly two oxides of nitrogen, namely, nitric oxide (NO) andnitrogen dioxide (NO2) are formed due to the oxidation of nitrogenpresent in the intake air during the combustion process. Theseoxides of nitrogen were found in the exhaust emissions and aretogether referred to as NOx. The amount of NOx formed, mostlydepends on the combustion temperature, the oxygen concentra-tion and residence time of the combustion product gases insidethe high temperature zone within the engine cylinder. If thecombustion temperature becomes more than 1200 °C, NOx isformed within the engine cylinder and it is emitted with theexhaust gas. Several studies on NOx formation in CI engine usingbiodiesel blended fuels have been briefly presented anddiscussed below.
Godiganur et al. [38] reported that the NOx emission increasedby 11.6% compared to diesel while using neat mahua oil methylester. According to them, the better combustion characteristics ofmahua biodiesel over neat petro-diesel increased the in-cylinderpressure and temperature and enhanced NOx formation. Baijuet al. [39] observed from their experimental investigation that NOx
emissions from the engine using karanja biodiesel/diesel blendsand neat karanja biodiesel were higher than that using diesel atpart load condition. However, NOx emission was found to behigher for diesel compared to biodiesel at loads close to themaximum load. They have also noted that biodiesel prepared fromkaranja oil using ethyl alcohol emitted more NOx than methyl esterof karanja oil under the same operating conditions. Nabi et al. [45]reported about 10% increase in NOx emission using biodiesel asfuel instead of neat mineral diesel. It was also suggested by themthat the reduction of NOx with biodiesel might be possible withthe proper adjustment of injection timing and introducing exhaustgas recirculation (EGR) technique.
Sahoo et al. [40] observed that the presence of oxygen moleculein polanga biodiesel caused an increase in combustion gas tem-perature which resulted in an increase in NOx emission. Celiktenet al. [46] observed no significant change in NOx emission at lowerengine speeds, but the emission was found to be more with bio-diesel compared to mineral diesel at higher engine speeds. Anincrease in NOx emissions with rapeseed and soybean oil methylesters were noted to be 12.7% and 20% respectively at an injectionpressure of 250 bar. It was also reported that NOx emissionincreased with the increase in injection pressure for all tested fuelsincluding neat diesel. The corresponding increase of NOx emissionwere reported to be 26%, 21% and 20% respectively for pure diesel,neat rapeseed and neat soyabean biodiesel when the injectionpressure was increased from 250 bar to 350 bar. A higher NOx
emission with rapeseed biodiesel was observed by Buyukkaya[87]. The increases in NOx emissions were found to be 6%, 9% and12% with B20, B70 and B100 blends of rapeseed biodiesel withdiesel respectively. However, the NOx emission decreased withincrease of speed for all the blended fuels. The increase in volu-metric efficiency of the engine, faster mixing of the fuel and airand shortened ignition delay at higher speeds were indicated to bethe possible reasons for this. Kegl [89] reported an increase in NOx
emission while using rapeseed biodiesel instead of mineral diesel.He also observed that by retarding the injection timing, NOx
emission with biodiesel could be brought down below the emis-sion level noted with mineral diesel. Soltic et al. [115] reportedthat NOx emissions decreased when GTL was used, but the use ofoxygenated fuels such as RME, neat soybean and rapeseed oilsincreased it to some extent. Yoon and Lee [94] reported the NOx
concentration increased with the addition of soyabean biodiesel tomineral diesel for all the test conditions due to relatively hightemperature prevailing in the flame zone and the maximumincrease was noted to be 15% at full load.
Lin et al. [44] experimented on a diesel engine with biodieselprepared from various feedstocks such as soyabean, palm, sun-flower, rapeseed and waste fried oil and found an increase in NOx
emission compared to petroleum diesel due to the higher pressureand temperature attained during their combustion. The maximumincrease in NOx emission was noted to be 25.97% with rapeseedbiodiesel and minimum with palm kernel biodiesel (5.58%). It wasobserved by Saleh [49] that the NOx emission increased with thedecrease in engine speed as well as increase in biodiesel massfraction in the fuel. The maximum increase was found to be 16% at1600 rpm. He also employed exhaust gas recirculation techniqueand observed 50% reduction in NOx emission. The excess oxygencontent increased the NOx emission from the diesel engine withneem oil methyl ester–diesel blend compared to diesel, asobserved by Nabi et al. [50]. They also reported a substantialamount of reduction in NOx emission with the use of EGR tech-nique, particularly for B15 blend. In another study by Datta et al.[88], an increase of NOx by an amount of 24% was found whenjatropha biodiesel was used instead of mineral diesel. Gumus andKasifoglu [98] used apricot seed kernel oil methyl ester as fuel andnoted an increase in NOx emission by 10% with B100 blend com-pared to diesel. They commented that the higher oxygen content,the higher peak pressure, the higher combustion temperature andcombustion duration were responsible for higher NOx emissionwith biodiesel.
In case of anchovy fish biodiesel, Behçet [86] observed that theNOx emission of biodiesel was slightly higher compared to dieselbecause of higher oxygen content of biodiesel. With methyl esterof paradise oil, it was observed by Devan and Mahalakshmi [116]that the presence of oxygen in biodiesel molecules and a shorterignition delay for biodiesel advanced the start of combustion andled to higher NOx emission. The emissions were reported to behigher by 5% and 8%, respectively with B50 and B100 blends ofmethyl ester of paradise oil. It was observed by Swaminathan andSarangan [99] that the addition of oxygenative additive (diethy-lene glycol dimethyl ether) to karanja biodiesel reduced NOx
emission by 31%. Amarnath and Prabhakaran [100] reported thatthe more oxygen content of karanja biodiesel led to a higher NOx
emission in general. The oxygen of fuel reacted with nitrogen offresh air and formed nitric oxide and other oxides of nitrogen weremainly responsible for higher NOx emission. The higher the com-pression ratio and injection pressure, more oxygen was availableunder favorable condition which caused higher NOx emission.
It was reported by Venkata Subbaiah and Raja Gopal [102] thathigher oxygen content of rice bran oil biodiesel resulted in higherNOx emission in their experimental investigation. The increase wasfound to be 4%, but small quantity of ethanol addition to cotton-seed biodiesel reduced NOx emission by 9.4%. Aydin and İlkiliç[112] reported that the higher oxygen content of rapeseed bio-diesel and the resulting higher combustion temperature led tohigher amount of NOx formation compared to diesel. They foundan increase of NOx emission by 16.7% with B20 blend at mediumspeed of 2000 rpm and 11.8% with B100 at higher speed of3000 rpm. Shirneshan [107] found that NOx emission increasedwith the use of waste frying oil methyl ester (biodiesel) blendeddiesel fuels compared to neat diesel at all operating conditions.The maximum increase was found to be 11.66%. Özcanli et al. [113]reported comparatively a higher increase (44.68%) in NOx emissionwith castor biodiesel blended fuel compared to diesel. Accordingto them, better combustion utilizing the oxygen already present inthe fuel and enhanced combustion temperature as well as the in-cylinder temperature rise were responsible for higher NOx
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formation. Ranganathan and Sampath [108] and Rao et al. [114]used neat cotton seed oil biodiesel as fuels of CI engines and notedan increase of NOx emission by 11% and 10% compared to dieselrespectively.
On the other hand, Zou and Atkinson [91] observed a 10%reduction in NOx emission using canola biodiesel. Rakopoulos et al.[43] also noticed different trends in NOx emission during theirexperimental study with biodiesel produced from various feed-stocks. The NOx emissions were slightly reduced with the use ofbiodiesel or vegetable oil blends of various feedstocks compared tothat with neat diesel fuel. This reduction was higher with higherpercentage of biodiesel in the blend. An effective reduction of NOx
emission (around 26%) was reported by Raheman and Phadatare[41] in their work with neat karanja biodiesel. The B20 blend ofkaranja biodiesel and diesel showed the similar results with nar-row range of variation with respect to mineral diesel. The averagevalue of NOx emissions from the two test fuels containing soya-bean biodiesel (B50 and B100) during the New European andArtemis driving cycles showed opposite trends as reported byFontaras et al. [47]. In case of B50, a reduction of 2–3% in NOx
emission was noted, whereas it increased by 6–9% for neat soya-bean biodiesel. This was attributed to the fact that the thermal aswell as the fluid properties of the blended fuel determined theemission characteristics from the engine.
Huzayyin et al. [109] observed that NOx emission was reducedwhile using jojoba oil methyl ester–gas oil blends with respect to100% gas oil. The maximum reduction of NOx emission had beenobserved at the rated engine speed of 1500 rpm. Pugazhvadivuand Jeyachandran [110] observed a reduction in NOx emissioncompared to conventional diesel using normal and preheatedwaste frying oil as fuels. However, the reduction was found to beless with preheated oil. Puhan et al. [75,95] also observed reduc-tion in NOx emission while using methyl as well as ethyl esters ofmahua oil. The maximum reductions with respect to neat dieselwere found to be 9% and 27% respectively. In another experimentalstudy on the same engine, Puhan and Nagarajan [96] found anaverage reduction of NOx emission by 12% with mahua oil ethylester. Puhan and the co-workers opined that higher cetane num-ber of methyl and ethyl esters was the main factor for thisreduction. The lower ignition delay period, which in turn reducedthe peak temperature during premixed combustion reduced thepossibility of thermal NO formation. Banapurmath et al. [76] alsofound more NOx emission with diesel fuel compared to honge,jatropha and sesame biodiesels. The maximum reduction wasobserved with JOME and the minimum was with SOME. The cor-responding values were noted to be 10.18% and 7.4% respectively.Lower heat release rate of biodiesel and hence lower peak
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temperature during combustion compared to diesel were thoughtto be the probable reasons for this reduction. It was observed byBayrakçeken [101] that NOx emission decreased with the additionof both crude and refined soybean oil methyl ester by 20.5% and20.1% respectively compared to that of diesel fuel. Aydin andBayindir [103] and Sharma et al. [105] experimented with cottonseed and neem oil biodiesels and found a decrease in NOx emissioncompared to normal diesel. İlkiliç [106] observed 25% reduction inNOx emission from the CI engine for different injection pressuresof 150 bar, 200 bar and 250 bar when fueled with sunflower bio-diesel instead of diesel.
In another study, Aydin and Bayindir [97] experimented withdifferent blends of cottonseed biodiesel and mineral diesel. Theyobserved maximum NOx emission with the use of B5 blends atmedium engine speed (around 1750 rpm). Tsolakis et al. [111] usedexhaust gas recirculation technique to reduce NOx emission sig-nificantly from a rapeseed oil methyl ester fueled compressionignition engine and the emission level became even less than thatfrom ultra low sulfur diesel fueled engine. The addition of ethanolto soybean biodiesel–mineral diesel blended fuels reduced the NOx
emission as observed by Randazzo and Sodré [92]. Banapurmathet al. [48] found that NOx emissions were lower for producer gas–diesel dual fuel operation compared to producer gas–honge oil orits methyl ester dual fuel operation due to availability of higheroxygen in honge oil or its methyl ester molecular structure. It wasclearly observed that producer gas did not change the basic natureof NOx emissions when added to diesel, vegetable oil orbiodiesel fuels.
The analysis of the different studies mentioned above showsthat NOx emissions are more with biodiesel in most of the cases.However, some studies are also found where reductions in NOx
emission are reported under all load conditions or at certain loadrange. For ready reference, some results regarding NOx emissionfrom the previous works of Godiganur et al. [38] and Puhan et al.[75] are presented here in graphical form in Fig. 9(a) and(b) respectively.
In both the studies, biodiesel prepared frommahua oil has beenused as fuel. From Fig. 9(a), it can be observed that the maximumrise of NOx emission with biodiesel as fuel is noted to be 11.6%higher than that with petro-diesel. On the other hand, 4% reduc-tion in NOx can be noted in Fig. 9(b) for the same biodiesel.
4.4. Effect on HC emission
The emission of unburned hydrocarbon (HC) from enginesdepends on the compositions and combustion characteristics ofthe fuels used. If combustion is improved and shifts towardscompleteness, then HC emission decreases and vice versa. Since
0 1 2 3 40
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biodiesel contains some amount of oxygen within its own struc-ture, it is expected that HC emission will decrease with the use ofbiodiesel–diesel blended fuels and also with neat biodiesel. It mayalso be noted that a few studies reported in the literature showincrease of HC emission to some extent with the use of biodieselblended fuels under certain operating conditions. Some of theimportant studies reported in the literature regarding HC emis-sions from biodiesel fueled CI engines are described here alongwith the findings from those experimental investigations.
Godiganur et al. [38] observed a 32% reduction in HC emissionlevel with blends of methyl ester of mahua oil and diesel withrespect to neat diesel. Nabi et al. [45] reported that PM emissionwith cotton seed oil biodiesel was lower than that with diesel fuel.With B20 blend of cotton seed biodiesel, the PM emission wasreduced by 24%. A narrow range of reduction (about 10%) of HCemission was observed with canola biodiesel by Zou and Atkinson[91]. Sahoo et al. [40] mentioned that the excess oxygen helped toreduce the emissions of HC and PM by improving combustion ofkaranja, jatropha and polanga based biodiesel blends. The max-imum reductions of HC were reported to be 20.64%, 20.73% and6.75% with neat karanja, jatropha and polanga biodiesels respec-tively. The maximum reduction of PM in the exhaust was found tobe in the range of 40% for all the three tested fuels. Kegl [89]reported that with the use of rapeseed biodiesel, HC emissionreduced by 25% compared to that with mineral diesel wheninjection pump timing was retarded. Fontaras et al. [47] observedthat with soyabean biodiesel, HC emission increased (by 31% forB50 blend and 58% for B100) for New European driving cycle, butan average decrease of HC by 20% was noted for Artemis drivingcycle. Tsolakis et al. [111] noted that the use of neat rapeseed oilmethyl ester as fuel instead of ultra low sulfur diesel resulted to areduction of HC emission by nearly 50% and it increased slightlywhen EGR technique was used.
Randazzo and Sodré [92] experimentally investigated the effectof biodiesel and ethanol addition to diesel on HC and otheremissions. They observed a slight reduction of HC emission withsoybean biodiesel–mineral diesel blend. However, with the addi-tion of ethanol to the blended fuel, the HC emission was found tobe more. Due to presence of higher amount of organic condensatesand volatile particles, the soot formation and PM emission weremore in case of RME, neat soybean and rapeseed oil as observed bySoltic et al. [115]. In another experimental study with wastecooking oil biodiesel by Lin et al. [93], it was observed that the useof biodiesel instead of ultra low sulfur diesel reduced the PM andHC emissions. The use of different blends of ultra low sulfur dieseland waste cooking oil biodiesel as fuels decreased PM emission by5.29–8.32% and HC by 10.5–36.0%. Lin et al. [44] observed thatwhen fueled with various vegetable oil methyl esters, THC emis-sions were low in case of VOME due to lower carbon and hydrogencontent compared to petroleum diesel. The maximum and mini-mum HC reductions of 33.14% and 22.47% were obtained withPKOME and PNOME respectively.
Puhan and his co-workers [75,95,96] used different kinds ofmahua biodiesel and noted that the emission of HC was too lowfor MOME and MOEE compared to diesel. The maximum reduc-tions with methyl ester and ethyl ester of mahua oil were found tobe 60% and 63% respectively. It was mainly due to presence ofoxygen in the fuel, which enhanced the combustion process toreduce exhaust emissions. Gumus and Kasifoglu [98], Sur-eshkumara et al. [90] and Devan and Mahalakshmi [116] experi-mented with biodiesels prepared from apricot kernel seed, karanjaand paradise oil respectively and reductions in HC emission wereobserved in all the cases. Gumus and Kasifoglu [98] found HCreductions of 18.66% with B100 and 2.66% with B5 blend of apricotseed kernel oil methyl ester compared to neat diesel. Sur-eshkumara et al. [90] found almost zero HC emission except B20
blend of paradise biodiesel where small amount of HC was notedat no load and full load conditions. The higher cetane number ofparadise biodiesel and the inherent oxygen in the biodieselmolecules were identified as the factors for better combustion andhence less HC emission. Swaminathan and Sarangan [99] observeda reduction in HC emission in the range of 20 to 38% with theaddition of diethylene glycol dimethyl ether as oxygenative addi-tive to karanja biodiesel. The reduction of HC was supposed to bemainly due to the additive which acted as a catalyst and reducedunburned HC. Amarnath and Prabhakaran [100] observed thatwith the increase in karanja biodiesel percentage in the blendedfuel from 20% to 100%, HC emission was reduced by 50%. With theincrease in compression ratio and injection pressure, the HCemission was found to be less due to the better combustion.Venkata Subbaiah and Raja Gopal [102] reported that HC emissiondecreased with rice bran oil biodiesel compared to diesel by 54%.
Ulusoy et al. [104] and Sharma et al. [105] observed 11.1% and10.3% reductions in HC emission with sunflower oil methyl esterand neem biodiesel respectively compared to diesel. However,Ulusoy et al. [104] pointed out that life cycle HC emission waslarger with biodiesel by 35% than diesel and most of the HC isproduced during agricultural processes. Shirneshan [107] foundthat for waste frying oil methyl ester–diesel blended fuel, HCemission became less than that of diesel and the reductionincreased with the increase of biodiesel percentage in the blendedfuel. The lower volatility of biodiesel can be a contributor to thedifference between the HC emission of diesel and waste frying oilmethyl ester–diesel blended fuel. Ranganathan and Sampath [108]and Rao et al. [114] observed that cotton seed oil biodiesel pro-duced lesser amount HC emission (reduction by 33% and 36%) dueto better combustion of biodiesel and promoted the oxidationprocess in the fuel rich zones utilizing the oxygen present (nearly10–11%) in the fuel itself.
Banapurmath et al. [48] observed that the HC emission washigher for producer gas–diesel dual fuel operations compared toproducer gas–HOME operations. In case of karanja biodiesel, theHC emissions were slightly higher for biodiesel–methanol blendscompared to neat biodiesel at lower load conditions as observedby Anand et al. [42].
On the other hand, with jojoba methyl ester, it was observed bySaleh [49] that the HC emission was higher due to the increase inthe amount of fuel per stroke which led to improper mixing of fuel.On the basis of experimental investigations, Banapurmath et al. [76]observed an increase in HC emission while using honge, jatrophaand sesame oil methyl esters as fuels. The increments in HC emis-sion were reported to be 60.49%, 65.43% and 48.14% respectively.This was attributed to heavier molecular structure and higherviscosity of the biodiesels which led to poor atomization.
During a comparative study by Rakopoulos et al. [43] usingbiodiesels prepared from different origins such as cotton seed,sunflower, rapeseed and soyabean, it was noted that there was notsuch mentionable variation of unburned hydrocarbon (HC) emis-sion with respect to neat diesel. Buyukkaya [87] observed thatreduction in unburned HC emission was negligible for all theblends of rapeseed biodiesel with respect to mineral diesel.
The nature of HC emission from biodiesel fueled CI engines cannow be summarized by mentioning that it decreases with thebiodiesels with a few exceptions also. This fact has been illustratedby supplementing the results from the experimental investigationsof Puhan et al. [75], Banapurmath et al. [76] and Sharon et al. [27]in Fig. 10(a)–(c) respectively.
It is quite evident from Fig. 10(a) that the HC emission of neatmahua biodiesel is lower than that of diesel and the maximumreduction is observed to be 35%. On the other hand, the resultswith biodiesel produced from jatropha oil as plotted in Fig. 10(b) show the reverse trend. HC emission increases with the use of
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)
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80
HC
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)
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Diesel Mahua Biodiesel
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30
40
50
60
70
80
90
100
HC
(ppm
)Brake Power (kW)
Diesel Jatropha Biodiesel
Fig. 10. Effect of different biodiesels on HC emission.
0 1 2 3 40.0
0.5
1.0
1.5
2.0
2.5
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3.5
4.0
Sm
oke
Den
sity
(BS
N)
Brake Power (kW)
Diesel Mahua Biodiesel
0 1 2 3 4 5 60
10
20
30
40
50
60
70
80
90
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oke
Opa
city
(HS
U)
Brake Power (kW)
Diesel Jatropha Biodiesel
0 1 2 3 4 5 610
20
30
40
50
60
70
Sm
oke
Den
sity
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U)
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Fig. 11. Effect of different biodiesels on smoke emission.
A. Datta, B.K. Mandal / Renewable and Sustainable Energy Reviews 57 (2016) 799–821816
neat jatropha biodiesel as fuel. The maximum increment isobserved to be 65.43%. The reason behind this kind of behaviormay be the improper mixing of fuel and air which leads to anincomplete combustion resulting higher HC emission. Fig. 10(c) isbasically showing a maximum reduction of 38.09% in HC emissionwith used palm biodiesel. However, a slight increase of HC emis-sion at low load is also observed. The increased gas temperatureand higher cetane number of biodiesel are the possible factorsresponsible for the decrease in HC emission.
4.5. Effect on smoke emission
Smoke is formed due to the incomplete combustion of the fuel.As discussed earlier, biodiesel contains some amount of oxygen inits molecule which enhances the combustion process. As a con-sequence, the formation and the emission of smoke are likely to bereduced when neat biodiesel or its blends with mineral diesel areused as CI engine fuels.
Baiju et al. [39] observed that the presence of excess oxygencontent in karanja oil methyl ester led to better combustion andresulted in less smoke formation for all load conditions of theengine. The reduction was found to be nearly 52% compared todiesel. They also reported that biodiesel produced using methanolemitted less amount of smoke than biodiesel produced usingethanol from the same karanja oil. Nabi et al. [45] reported a 14%reduction in smoke emission using B10 blend of cotton seed bio-diesel. Sahoo et al. [40] observed a significant reduction of smokeemission with jatropha, karanja and polanga based biodiesels as
fuels. Rakopoulos et al. [43] reported that smoke density wasconsiderably lowered with the use of biodiesels obtained fromvarious feedstocks compared to that noted with neat diesel fuel.They found the maximum reduction of smoke with B20 blend ofcotton seed oil biodiesel among the tested biodiesels.
A considerable amount of reduction in smoke level wasobserved by Çelikten et al. [46] using rapeseed and soyabeanbiodiesels. It was also observed that smoke level was furtherreduced with the increase in injection pressure from 250 to350 bar. Raheman and Phadatare [41] observed an effectivereduction of smoke density when various blends of karanja oilmethyl ester and diesel were used instead of neat diesel. Buyuk-kaya [87] recorded the smoke opacity using rapeseed biodiesel/diesel blends and diesel and reported a maximum of 60% reduc-tion with neat biodiesel compared to that with neat diesel. Tso-lakis et al. [111] observed that the use of rapeseed oil methyl esteras fuel resulted in a reduction of smoke and the reason behind thatwas explained to be the oxygenated nature of the biodiesel fuel. Itwas also reported by them that the smoke was slightly (10%)increased when injection timing was retarded. However, smokewas found to be increased when EGR technique was employed.
In case of karanja biodiesel, exhaust smoke emission was sig-nificantly lower (96.4%) for biodiesel–methanol blend compared toneat biodiesel for all the load condition, as reported by Anand et al.[42]. Lin et al. [44] observed that when fueled with variousvegetable oil methyl esters; the smoke emission from the enginewas reduced compared to petroleum diesel, due to the uniformair–fuel mixing and the extra oxygen content in vegetable oil
Table 5Emissions of different biodiesel fueled engine compared to diesel at normal operating condition.
Name of researchers Biodiesel and its blends CO CO2 NOx HC Smoke
Nabi et al. [45] Cotton seed oil; B10, B20 and B30 decrease 24% for B30 increase increase 10% for B30 – decrease 14% for B10Çelikten et al. [46] rapeseed and soybean oil; B100 decrease 21% and 28% – increase 21% and 20% – decrease 122% for rapeseed
biodieselFontaras et al. [47] Soybean oil; B50 and B100 – increase 9% and
Aydin and Bayindir [103] Cotton seed oil; B5, B20 and B50 decrease – – increase
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atta,B.K.M
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SustainableEnergy
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(2016)799
–821817
Table
5(con
tinu
ed)
Nam
eof
researchers
Biod
ieselan
ditsblen
dsCO
CO2
NOx
HC
Smok
e
increa
seforB5,
decreaseforB20
and
B50
Ulusoyet
al.[10
4]Su
nflow
eroil;B10
0decrease
––
decrease11.1%
decrease
Sharmaet
al.[10
5]Nee
moil;B20
decrease
–decrease
decrease10
.3%
decrease
İlkiliç
[106
]Su
nflow
eroil;B10
0decrease30
%decrease
decrease20
%–
–
Shirneshan
[107
]W
aste
fryingoil;B20
,B40
,B60
,B80
and
B10
0decrease
decrease
increa
sedecrease
–
Ran
ganathan
andSa
mpath
[108
]Cottonseed
oil;B20
,B40
andB10
0decrease26
%–
increa
se11
%decrease33
%decrease34
.5%
Huzayy
inet
al.[10
9]Jojoba
oil;B20
,B40
andB60
increa
seincrea
sedecrease
–-
Puga
zhva
divuan
dJeya
-ch
andran[110
]W
aste
fryingoil;B10
0increa
sesign
ificantly
–decrease44
%–
increa
sesign
ificantly
Tsolak
iset
al.[111]
Rap
esee
doil;B20
,B50
,B10
0decrease
–increa
sedecrease
decrease
Ayd
inan
dİlkiliç
[112
]Rap
esee
doil;B20
,B10
0decrease29
%decrease16
%increa
se–
–
Özcan
liet
al.[113]
CastorBea
noil;B5,
B10
,B25
,B50
andB10
0decrease
decrease
increa
se44.68
%–
–
Rao
etal.[114]
Cottonseed
oil;B25
,B50
,B75
andB10
0decrease24
%decrease
increa
se10
%forB10
0decrease36
%–
Soltic
etal.[115]
Rap
esee
doil;B10
0decrease
–increa
sedecrease
–
Dev
anan
dMah
alak
shmi
[116
]Pa
radiseoil;B20
,B40
,B50
andB10
0–
–increa
se5%
and8%
for
B50
andB10
0decrease22
%an
d27
%forB50
and
B10
0decrease33
.5%an
d39
.4%forB50
andB10
0
A. Datta, B.K. Mandal / Renewable and Sustainable Energy Reviews 57 (2016) 799–821818
methyl ester which resulted in a superior combustion. Significantreductions of 72.73% and 59.09% were observed with palm kerneloil methyl ester and palm oil methyl ester respectively. Nabi et al.[50] observed that compared to conventional diesel fuel, thesmoke emission was reduced by 4% only using B15 blend of NOME.
With methyl ester of paradise oil it was observed by Devan andMahalakshmi [116] that the smoke emission was less with bio-diesel. A 39.4% reduction in smoke emission for neat MEPS (B100)and 33.5% reduction for MEPS 50 blend were recorded. The highercombustion temperature and the longer combustion duration withmore diffusive combustion of biodiesel were attributed to thiskind of behavior. With the addition of diethylene glycol dimethylether as an additive to pongamia methyl ester (karanja biodiesel),smoke density reduced to 55% at part load and 13% at full loadcondition, as observed by Swaminathan and Sarangan [99]. Ulusoyet al. [104] reported that the smoke emission of sunflower oilmethyl ester was lower than that with diesel fuel because ofcomplete combustion with biodiesel. Ranganathan and Sampath[108] noted that cotton seed oil biodiesel produced less amount ofsmoke emission (34.5% reduction) than diesel. According to them,better combustion of biodiesel due to oxygen present in the fuelitself promoted the oxidation process in the fuel rich zones andthereby smoke was reduced.
On a contrary, Banapurmath et al. [76] observed more smokeemission with honge, jatropha and sesame oil methyl esters incomparison to that with neat petro-diesel. The values of percen-tage increase in smoke compared to neat mineral diesel werereported to be 16.18%, 26.41% and 32.07% respectively. Accordingto them, the heavier molecular structure and higher viscosity ofbiodiesel caused poor atomization and that led to higher smokeformation for biodiesel compared to diesel. Amarnath and Prab-hakaran [100] observed an increase in smoke opacity with the useof karanja biodiesel. The increase was noted to be 33% when bio-diesel share in the blended fuel was increased from 20 to 100%.The reason as described by them was the higher viscosity andlower volatility of karanja biodiesel which led to a difficulty whileatomizing the fuel in the combustion chamber and resulted in anincomplete combustion. However, at higher compression ratio thecombustion process was complete enough in reducing the smokeby 45%. Higher injection pressure also improved atomizationwhich led to a better combustion and resulted in less amount ofsmoke. The reductions in smoke at the exhaust were found to beby 57.9% and 53.8% for diesel and karanja biodiesel respectively forincrease of injection pressure from 150 to 250 bar. Pugazhvadivuand Jeyachandran [110] observed higher smoke emission with thewaste frying oil (both non-preheated and pre-heated) compared todiesel due to poor volatility and higher viscosity of WFO’s con-stituents. However, the emission was found to be slightly lower incase of preheated oil. Aydin and Bayindir [103] observed that thesmoke opacity was higher at higher loads and it was not muchaffected with the use of biodiesel.
Smoke emission is generally expressed in terms of Bosch smokenumber (BSN) and Hartrige smoke unit (HSU). The review of theexperimental reports of several researchers presented in thissection clearly shows that the smoke emission is reduced with theuse of biodiesel/diesel blends and neat biodiesel. However, a fewstudies are also there which show a reverse trend, i.e., the increaseof smoke with the use of biodiesel. Some results of the experi-mental works from the literature related to smoke emission fromdiesel engines have been shown in Fig. 11(a)–(c).
The first two figures are based on the works of Puhan et al. [75]and Sharon et al. [27] and these show reduction in smoke. Themaximum reduction is observed to be 11% and 19% with mahuaand used palm biodiesel respectively. The possible reason asexplained by many researchers is the better combustion due to thepresence of oxygen in the biodiesel or biodiesel blended diesel
A. Datta, B.K. Mandal / Renewable and Sustainable Energy Reviews 57 (2016) 799–821 819
fuels. Fig. 11(c) is prepared on the basis of the experimental workof Banapurmath et al. [76] and it shows an increase in smokeemission with jatropha biodiesel. The maximum increase is notedto be at medium load range and it is nearly 26%. It is due to thehigher viscosity and heavier molecular structure of biodiesel,which results a poorer combustion and leads to an excess smokeas explained by them.
4.6. Summary of emission analysis
At the end of this section, it can be said that the inherentoxygen in biodiesel molecules plays a key role in the formation ofdifferent pollutants. The availability of extra oxygen enhanced thecombustion when biodiesel or its blends with mineral diesel areused as fuels. As a result, the emissions of CO, HC and smokeformation are reduced significantly, but the maximum tempera-ture during combustion increases. The enhanced combustion withbiodiesel and its blends with diesel increases the amount of CO2
and the increased combustion temperature increases the amountof NOx in the exhaust compared to that with neat diesel.
The emission parameters investigated by several researcherswith various types of biodiesel and diesel–biodiesel blends havebeen summarized and listed in Table 5. The increase and decreaseof different pollutants in percentages have been shown withrespect to mineral diesel under normal operating conditions only.
The table shows that the maximum reductions in CO, HC andsmoke emissions are by 73%, 63% and 72.73% with neat karanjabiodiesel, neat mahua biodiesel and waste fried biodiesel respec-tively. However, few studies show reverse trends. An increase ofHC emission by even 65.43% with neat jatropha biodiesel andincrease of smoke emission by 33% with neat karanja biodiesel arealso reported. The maximum increments in NOx and CO2 emissionare 44.68% and 20% with castor bean biodiesel and rice bran bio-diesel respectively. In some cases, opposite trends such as 27%reduction in NOx emission with neat mahua oil ethyl ester and 16%reduction in CO2 emission with rapeseed biodiesel have also beenobserved.
The authors feel that the condition of the engine, purity of thefuel, proper method of biodiesel production and the experimentaluncertainties should be ensured to obtain the maximum advan-tage of biodiesel fuel. The negative effects of biodiesel as CI enginefuel can be nullified to some extent using different pre-treatmentsof the fuel such as preheating and addition of ethanol andmethanol as supplementary fuels. Exhaust gas recirculation tech-nique may also be employed to reduce the amount of NOx in theexhaust.
5. Conclusion
On going through the different studies minutely, conducted bydifferent researchers on the various aspects of biodiesel as CI enginefuel, the following conclusions have been drawn. Biodiesel offers widerange of benefits such as renewability, biodegradability, nontoxicnature, reduction in import oil bills and less pollutant emissions ingeneral. The properties of biodiesel are dependent upon the fatty acidstructure of the raw feedstocks and to some extent on the productionprocess. Biodiesel is mainly produced by a chemical reaction calledtransesterification using various renewable feedstocks such as vege-table oils (both edible and non-edible), algal oils and animal fats.
Brake thermal efficiency with biodiesel is found to be less thanthat with diesel by around 2% on an average in most of theinvestigations reported in the literature. The brake specific fuelconsumption rate on the other hand increases with biodieselapproximately by 13%. It has been noted that the exhaust gastemperature with biodiesel is 10% higher than that with diesel. The
opposite trend is also observed and reported by a few researchersunder certain engine operating conditions. The higher cetanenumber and the intrinsic oxygen of biodiesel enhance the com-bustion process which leads to reductions in HC, CO and smokeformation by 20%, 30% and 50% respectively on an average. Theoxygen content of biodiesel is responsible for the higher NOx
emission and CO2 emission. General trends observed with bio-diesel fuel are the increase in emissions of NOx and CO2 approxi-mately by 12% and 14% respectively.
The blended fuels reduce the harmful pollutant emissionsexcept NOx and CO2 significantly at the cost of a bit sacrifice in theenergetic performances of the engine. However, the use of posttreatment technique of exhaust gas can be employed to reduceNOx emission. Also, the life cycle CO2 emission from biodiesel fuelis less than that of diesel as the basic feedstocks consume CO2
during its cultivation. So, in the decade of growing energy crisisand environmental degradation, a certain percentage of biodieselshould be blended with mineral diesel in all sectors those areusing diesel engine as their primary energy producing device.
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