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Renewable Energy 81 (2015) 113e122

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Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Combustion and emissions study on motorcycle engine fueled withbutanol-gasoline blend

Renhua Feng a, Jianqin Fu b, Jing Yang b, *, Yi Wang b, Yangtao Li b, Banglin Deng b,Jingping Liu b, Daming Zhang c

a Key Laboratory of Advanced Manufacture Technology for Automobile Parts, Ministry of Education, Chongqing University of Technology,400054 Chongqing, Chinab Research Center for Advanced Powertrain Technology, Hunan University, 410082 Changsha, Chinac Department of Industrial Technology, California State University, Fresno, CA 93740, USA

a r t i c l e i n f o

Article history:Received 28 October 2014Accepted 10 March 2015Available online 28 March 2015

Keywords:Butanol-gasoline blendCombustionEmissionsIgnition timingBlend ratioEngine load

* Corresponding author. Tel.: 86 13907481080.E-mail address: yangjing10@vip.sina.com (J. Yang)

http://dx.doi.org/10.1016/j.renene.2015.03.0250960-1481/ 2015 Published by Elsevier Ltd.

a b s t r a c t

In this paper, experimental studies were conducted on a single cylinder high speed spark ignition (SI)motorcycle engine under both full load and partial load at 6500 and 8500 rpm with pure gasoline, 30%and 35% volume butanol-gasoline blends. This study is trying to find out the influence on combustionheat release of high speed SI engine by variables including ignition timing, butanol blend ratio andengine load. The results show that butanol-gasoline blend provides higher knocking resistance byallowing advance ignition timing in SI engines, which leads to more efficient combustion. With butanolblend ratio increases, more complete combustion process will achieve with the optimum operatingparameters. With engine load increases, the rates of heat release become faster and ascend in peak valuefor both pure gasoline and butanol-gasoline blends. Furthermore, engine performance parameters suchas power, fuel economy and emissions have been compared and analyzed. The results also show thatengine power, torque, brake specific energy consumption, HC, CO and O2 emissions are better than thoseof pure gasoline at full load with 35% volume butanol addition, combined with ignition timing optimi-zation. But NOx and CO2 emissions are higher than those of the original level of pure gasoline.

2015 Published by Elsevier Ltd.

1. Introduction

In the 21st century, energy crisis and environmental protectionare two of the biggest challenges [1,2]. Due to the shortage of oilresources and the increasing oil price, it is very important to seekalternative fuels for internal combustion engine [3]. Biofuels can bemade from agricultural products [4]. Biofuels have been consideredas the alternative fuels in some ways [5]. At present, commonbiofuels include methanol, ethanol, butanol, biodiesel, biogas andbiohydrogen [6]. Butanol has been suggested as a future fuel bio-component [7]. Compared with conventional fuels, butanol hasmore excellent fuel properties and environment performance, suchas wildly production sources, more oxygen content and higher heatof evaporation [8]. As automotive fuel, butanol has more advan-tages compared to methanol and ethanol, including lower vola-tility, higher heating value, higher viscosity, less corrosive and

.

lower auto-ignition temperature [9]. Furthermore, butanol can beextracted from renewable resources, such as corn fiber [10], wheatstraw [11], distillers dry grains and solubles (DDGS) [12], cornstover [13], switchgrass, barley straw [14] and other plant materials[15]. Due to its superior physical and chemical properties, butanolhas become a very competitive biomass-based renewable fuel forinternal combustion engines to substitute or supplement gasoline[16,17]. Typical properties of gasoline, methanol, ethanol andbutanol [18e22] are shown in Table 1.

There are many researches in butanol utilizations on conven-tional spark ignition (SI) gasoline engines. Some of them focused onengine performance, fuel economy and emission characteristics.Alasfour [23e26] has conducted the pioneer work to study per-formance and emissions on a single cylinder engine with 30% vol-ume butanol gasoline blend. Williams et al. [27] studied the impactof butanol and other bio-components on thermal efficiency ofprototype and conventional engines. It was found that butanolblends offered measurable gains in thermal efficiency in line withthe relative octane lift over base gasoline. And butanol blends

Delta:1_fuelled Delta:1_given nameDelta:1_surnameDelta:1_given nameDelta:1_surnameDelta:1_given nameDelta:1_surnameDelta:1_given namemailto:yangjing10@vip.sina.comhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.renene.2015.03.025&domain=pdfwww.sciencedirect.com/science/journal/09601481http://www.elsevier.com/locate/renenehttp://dx.doi.org/10.1016/j.renene.2015.03.025http://dx.doi.org/10.1016/j.renene.2015.03.025http://dx.doi.org/10.1016/j.renene.2015.03.025

Table 1Typical properties of gasoline, methanol, ethanol and n-butanol.

Property Gasoline Methanol Ethanol n-Butanol

Chemical formula C4eC12 CH3OH C2H5OH C4H9OHMolecular weight (g/mol) 100e105 32 46 74Composition (C, H, O) (mass%) 86, 14, 0 37.5, 12.5, 50 52, 13, 35 65, 13.5, 21.5Lower heating value (MJ/kg) 42.9 20.1 26.8 33.1Density (kg/m3 at 20 C) 720e760 792 790 810Octane number (R M)/2 86e94 98.6 99.1 89Boiling temperature (C) 25e275 64.5 78.3 118Latent heat of vaporization (25 kJ/kg) 380e500 1178 904 716Auto-ignition temperature (C) 257 465 422 343Stoichiometric air/fuel ratio 14.7 6.4 9.0 11.2Laminar flame speed (cm/s)a,b 51 68 63 58.5Adiabatic flame temperature (K) 2370 1890 2310 2340

a T 325 K and P 100 kPa, at stoichiometric mixture.b The gasoline was represented by iso-octane here.

R. Feng et al. / Renewable Energy 81 (2015) 113e122114

allowed the greatest volume of crude derived gasoline displace-ment at fixed fuel oxygen content. Gu et al. [28] measured thespeeds of laminar flame of tert-butanol-air premixed mixtures andgot the results that laminar burning velocity of tert-butanol-airmixtures increase with the increase of initial temperature anddecrease with the increase of initial pressure. Gu et al. [22] alsoconducted an experiment on a port-fuel injection SI engine fueledwith blends of gasoline and n-butanol at different spark timingsand exhaust gas recycling (EGR) rates. It was found that advancingspark timing increases engine specific HC and NOx emissions andparticle number concentration while it decreases engine specificCO emissions. EGR can reduce engine specific NOx emissions andparticle number concentration simultaneously on SI engine fueledwith gasoline and n-butanol blends. These tendencies were similarto Ref. [29]. Venugopal and Ramesh [30] studied the effect of in-jection timing on a SI engine using dual injection of n-butanol andgasoline at the intake port. The results showed that with dual in-jection, n-butanol has to be used at higher throttle positions forgood performance and low emissions. Injection timing mainly in-fluences HC emission, and injection phasing has a small influenceon emissions. Mittal et al. [31] studied two different fuel blendscontaining 10% and 15% of butanol in Gasoline by volume, whichare tested on an engine dynamometer using the uncoated andceramic coated engines. The results strongly indicated that com-bination of ceramic coated engine and butanol gasoline blendedfuel has potential to improve engine performance. Besides theregulated emissions, Broustail et al. [32] studied the non-regulatedpollutants (methane, acetylene, ethylene, benzene, acetaldehydeand formaldehyde) on a single-cylinder port-fuel injection SI en-gine, and comparisons have been made between iso-octanebutanol blends and iso-octane ethanol blends. The resultsshowed that ethanol has a superior emission performance relativeto butanol and they are both better than iso-octane.

As the very important research factor and direction, the studiesabout engine combustion heat release of butanol-gasoline blendalso have been carried out. Dagaut and Togbe [33] examinedtheoretical and experimental analyses of combustionmechanism ofn-butanol-gasoline mixtures and studied oxidation mechanismusing a jet stirred reactor. They reported good agreement betweenexperimental results and the computations of detailed chemicalkinetic scheme for n-butanol-gasoline blend. Furthermore, com-bustion processes of SI engine fueled with butanol-gasoline blendor neat butanol were investigated. Yang et al. [34] and Deng et al.[35] achieved the combustion efficiency increasing by adjustingignition timing on a SI engine fueled with butanol-gasoline blend.Serras-Pereira et al. [36] studied combustion processes of SI engine

fueled with butanol-gasoline blend or neat butanol, and Tornatoreet al. [37] investigated the effect on the spark ignition combustionprocess of 40% of n-butanol blended in volume with 60% puregasoline through cycle resolved visualization applied in a singlecylinder SI engine working at low speed, medium boosting andwide open throttle. The results showed that laminar burning ve-locity is faster with butanol addition and similar results were foundin research of Ref. [38].

However, only a few studies for SI engine combustion heatrelease analysis, performance, economy and emissions with thebutanol blend at the same time, especially for the very high speedgasoline engines. Furthermore, many studies just focused on con-ventional harmful exhaust emissions (CO, HC and NOx) when usebutanol as SI engine fuel. Although CO2 is a non-toxic gas, which isnot classified as an engine pollutant, it is one of the substancesresponsible for global temperature rises through the greenhouseeffect [39,40], and CO2 emission has not been usually taken intoaccount in many studies. Moreover, Oxygen (O2) emission canreflect some situations of other emissions but it was rarely con-cerned by investigators. Based on authors' previous studies[34,35,41,42], the objectives of this study will be find out commonprinciples about the influence on combustion heat release, perfor-mance, fuel economy and the exhaust emissions of SI engine forvariables such as ignition timing, engine load and butanol blendratio. In this study, experiments were conducted on a single cyl-inder high speed SI motorcycle engine for two operating modes offull load and partial load at 6500 and 8500 rpmwith pure gasoline,30% and 35% volume butanol-gasoline blends. Engine performance,fuel economy and exhaust (HC, CO, CO2, NOx and O2) emissionshave been tested and analyzed among pure gasoline, 30% and 35%volume butanol-gasoline blend.

2. Experimental setup

The engine used in this study is a single cylinder, four-stroke, 2-valve, air-cooling SI motorcycle engine with compression ratio of9.2. The specifications of this engine are listed in Table 2.

In this study, three fuels were tested, including pure commercial90# gasoline (PGS) which is used as the base fuel, two n-butanoland gasoline blends denoted as Bu30 and Bu35 (Bux means thevolume fraction of n-butanol in the blend is x). Table 1 showedmain properties of gasoline and butanol.

Experiments were conducted on a fully warmed engine. Enginewas tested on full load from 3000 to 8500 rpm with an interval of500 rpm. The partial loads were conducted at 6500 and 8500 rpm.Firstly, air/fuel ratio (AFR) was adjusted for adapting the 30% and

Table 2Engine specifications.

Items Content

Engine type 4 Strokes, 1 cylinder, spark ignitionCooling model Air coolingBore 56.5 mmStroke 49.5 mmCompression ratio 9.2:1Displacement 124.1 cm3

Connecting rod 104.5 mmMax power 6.99/8000 (kW/rpm)Max torque 9.25/6500 (N m/rpm)Min BSEC 13.44 [MJ/(kW h)]

R. Feng et al. / Renewable Energy 81 (2015) 113e122 115

35% volume butanol addition. Secondly, ignition timing was opti-mized to produce the maximum brake torque (MBT) [43]. All theoperation points' conditions are shown in Table 3. For butanol-gasoline blend fuels, all cases were tested twice. The first one wasperformed without modifying anything of the engine, and thesecond test was performed under the ignition timing optimization,denoted as OIT. Engine operating parameters such as intake,exhaust and inecylinder pressure, temperature, as well as engineperformance parameters such as torque, brake specific energyconsumption (BSEC) and exhaust emissions (HC, CO, CO2 and NOx),including O2 in exhaust were measured for each tested case. Thepicture and schematic diagram of experimental setup are shown inFig. 1.

Inecylinder pressure was measured by a pressure sensor (AVLZ121) which was fitted together with spark plug with a precision of0.001 bar. Heat release rate of fuel chemical energy was obtainedfrom AVL combustion analyzer. Engine emissions were alsomeasured by HORIBA MEXA-7100D analyzer. Engine AFR wasmeasured by analyzing the exhaust gas contents with a precision of0.1%. Due to lack of AFR control for the existing fuel system, fuelmass flow per cycle was kept at constant for all fuels in a givencondition. The stoichiometric AFR of gasoline is 14.7, 30% volumebutanol-gasoline blend is 13.64, and 13.47 for 35% volume butanol-gasoline blend, so the blend fuels always run at leaner fuel airmixtures relative to pure gasoline, as shown in Fig. 2. That meansthe engine was tested in the mode of equivalent absolute fuel airmixtures.

Table 3All the operation points' conditions.

Speed (rpm) Engine load Ignition timing (CA)

PGS Bu30 OIT3000 Full 20.5 253500 20.5 264000 20.5 274500 21 265000 21.5 285500 22 296000 22.5 286500 Full 23.5 28

3 N m 21 25.55 N m 22.5 277 N m 23 27.5

7000 Full 25.5 277500 29 308000 30.5 328500 Full 31.5 35.5

3 N m 28.5 30.55 N m 30 327 N m 31 33.5

3. Discussion of combustion heat release results

3.1. The impact of ignition timing

Ignition timing has a significant effect on SI engines [44]. It has aconsiderable influence on combustion characteristics, and there-fore, affects engine performance and combustion products.Generally speaking, too-advanced ignition timing causes cylinderpressure to increase substantially and rapidly before the end ofcompression stroke. This increases the work lost in compressionprocess, and therefore, decreases the net useful work. In contrast,too-delayed ignition timing results in a lower peak pressureoccurring very late in expansion process. This reduces the worktransfer from expanding gases to piston [45]. So there exists anoptimal choice for ignition timing. The optimum ignition timingproduces satisfactorily high cylinder pressure, with its peakoccurring just after top dead center. This ensures minimumcompression work and maximum work transfer during theexpansion stroke. Some properties of butanol are different fromthose of gasoline, such as boiling temperature, vapor pressure,latent heat of vaporisation, heating value, flame temperature,diffusion coefficient and octane number. So ignition timing shouldlikely also differ from those used with pure gasoline. Butanol pro-vides a higher knock resistance for allowing earlier ignition timing[40,46]. The optimized ignition timing under full load operation isshown in Fig. 3. It can be seen that the optimized ignition timing ofBu30 OIT and Bu35 OIT are advanced when compared to puregasoline at all speeds, which is coincided with the research ofRef. [47]. And it can also be seen in Fig. 3 that Bu35 achieved a moreadvanced ignition timing than Bu30 at most speeds, especially inthe range of 3500e7500 rpm which is commonly used in practicaldriving.

Figs. 4 and 5 show the rate of heat release (ROHR) andinecylinder pressure for PGS, Bu30 and B35 in different status atfull load at four specific speeds (low speed of 3000 rpm, middlespeed of 6000 rpm, high speeds of 7000 and 8500 rpm), respec-tively. At low speed, the profile of ROHR is narrow and high. But it iswide and low at high speed. That is because the ROHR is measuredby crank angle degree, and a greater number of crank angle degreewere covered as engine speed increased during the similar timeperiod magnitude of combustion processes. It is clearly seen, asexpected, that the peak ROHR is larger and occurred earlier as the

Air/fuel ratio ()Bu35 OIT PGS Bu30 Bu3524.5 0.79 0.85 0.9126 0.77 0.84 0.8827.5 0.81 0.87 0.9230 0.84 0.89 0.9531 0.84 0.9 0.9630 0.86 0.91 0.9631.5 0.87 0.92 0.9929.5 0.86 0.92 0.9726 0.92 0.98 1.0827.5 0.89 0.96 1.0529 0.87 0.94 1.0131 0.85 0.9 0.9430 0.85 0.89 0.9332 0.81 0.86 0.934 0.77 0.82 0.8531 0.85 0.89 0.9232.5 0.83 0.87 0.9033.5 0.80 0.84 0.87

Fig. 1. The picture (left) and schematic diagram (right) of experimental setup.

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Fig. 3. The ignition timing for PGS, Bu30 OIT and Bu35 OIT.

R. Feng et al. / Renewable Energy 81 (2015) 113e122116

ignition timing advancing. Furthermore, the difference among PGS,Bu30 and Bu35 is smaller at high speed. Similar characteristics wereobserved for inecylinder pressure (Fig. 5). Note that the ROHRs ofPGS are more oscillating than that of Bu30 and Bu35 (see Fig. 4),which indicated that butanol is helpful for stabilizing combustion

-30 -15 0 15 30 45 600.000.010.020.030.040.050.06

PGBuBuBuBu

)A

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RH

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Crank angle ( CA)

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(b)

Crank angle ( CA)

Fig. 4. The rate of heat release for PGS, Bu30 and Bu35 of (a):

process. This is coincided with the observation of Ref. [38], whereshowed that the coefficient of variance (COV) of net indicatedmeaneffective pressure (IMEP) for butanol is slighter than that for puregasoline.

The behavior of heat release at full load can be seen in Fig. 6. Theover-advanced ignition timing causes faster combustion and higherrate of heat release that depicted in Fig. 6a. The burning speeds ofBu30 and Bu35 are faster than that of PGS during the early flamedevelopment stage represented by 0e10% mass fraction burned(MFB) with or without OIT, as shown in Fig. 6b. That is because theearlier in the late stage of compression stroke, the lower temper-ature of mixture gas due to less compression work done by piston.And lower temperature of combustible mixture would result inlower reaction rate in pre-ignition phase [48]. This mainly explainsthe slower burning of Bu30 and Bu35 with OIT operation in earlyflame stage. In addition, higher heat of vaporization and lowerenergy density of butanol relative to pure gasoline further decreasecombustion temperature, this demonstrated the reason of Bu30and Bu35 without OIT were burned slower than PGS in all com-bustion development phases (Fig. 6bed). Nevertheless, those ef-fects on flame development brought by butanol can becompensated by advancing ignition timing, higher knocking resis-tance and faster laminar flame speed, leading to more advanced50% MFB location, shorter duration of 0e10% and 10e90% MFB, asshown in Fig. 6bed.

3.2. The impact of butanol blend ratio

As mentioned above, only 30% and 35% butanol blend ratios areconsidered due to the indistinguishable influence on engine per-formance below butanol blend ratio of 25%. Figs. 7 and 8 give thecombustion events at different speeds under full load operation. Asshown in Fig. 7, the 10e90% combustion durations of Bu35 is largerthan that of Bu30 when operated without OIT, but shorter than thatof Bu30 with OIT operations. From Fig. 8, the location of 50% MFB of

S3030+OIT3535+OIT

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0 5 10 15 20 25 30 35 40152025303540

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Fig. 7. The duration of 10e90% MFB at different speeds under full load operation of (a):without OIT, (b): with OIT. The 0% butanol blend ratio means the pure gasoline.

0 5 10 15 20 25 30 35 4005

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3000 rpm+OIT4000 rpm+OIT5000 rpm+OIT6000 rpm+OIT

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Fig. 8. The location of 50% MFB (reference to TDC, positive value means after TDC) atdifferent speeds with full load operation of (a): without OIT, (b): with OIT. The 0%butanol blend ratio means the pure gasoline.

R. Feng et al. / Renewable Energy 81 (2015) 113e122 117

Bu35 is more advanced than that of Bu30 without OIT, but almostthe same at the OIT operation. That is because butanol producesmore complete combustion due to the more oxygen content andleaner fuel air mixture (see Fig. 2), leading to more energy inputfrom fuel chemical reactions. Fig. 9 shows cylinder head tempera-ture in different operations. It can be seen that cylinder headtemperature of Bu35 is higher than that of Bu30 when operated atoriginal ignition timing, but opposite results were obtained withOIT operations. This is because there is more heat transfer tochamber wall from burned fuels with the combustion durationincrease. Thus, combustion conditions can be reflected by heattransfer between cylinder fuels and solid components in someways. From Fig. 9, as engine speed increases, cylinder head tem-perature will increase and the relative heat transfer loss (percent-age of total energy) will decrease [49]. It can also be seen that thedifferences of cylinder head surface temperature of Bu35 and Bu30at OIT operation are more close to each other than those at original

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PGSBu30Bu30+OITBu35Bu35+OIT

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HO

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Crank angle ( CA)-30 -15 0 15 30 45 60

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Fig. 11. The rates of heat release at different loads of 6500 rpm: (a) PGS, (b) Bu35 OIT.

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ignition timing. This can be explained when looking into Fig. 5,which shows that the curves between Bu35 and Bu30 almostoverlap when operated with OIT, but in the cases without OIT, itshows different statuses at different speeds. At low speed, Bu30have a more complete combustion, while it achieved a contraryresult at high speed, and they shook hands at medium speed. Thisindicates that, for some speeds, same ignition timing would causebig difference in combustion process for different fuels. In fact,when the engine fueled with butanol-blend fuels, it cannot obtainthe desired combustion process by using the original ignitiontiming. Moreover, different ignition timing is needed when usingdifferent butanol blend ratios [48]. It also indicates that cylinderhead temperature cannot reflect combustion condition absolutely.In addition, there is not a monotonicity or linear relationship for aspecific operating parameter as the speed changes for high speedengine due to its big span of speed operating range. Furthermore,we should note that the difference of 5% blend ratio in this studydid not bring a high identifiability in heat release. Obviously, theexperiments of higher butanol blend ratios are needed to extendthe investigated range, for searching more persuasive conclusions.

Crank angle (CA) Crank angle (CA)

Fig. 12. The rates of heat release at different loads of 8500 rpm: (a) PGS, (b)Bu35 OIT.

3.3. The impact of engine load

Engine load is the main effect factor on global efficiency, ratherthan engine speed or fuel type [50]. It can not only show enginecharacteristics, but also reflects engine actual working conditions.6500 and 8500 rpm were considered for analyzing the impacts ofengine load. The loads of 3 N m (low load), 5 N m (medium load)and 7 Nm (high load) were used, and only the 35% butanol-blendratio fuel was considered in engine load impact of this study.

Fig. 10 gives the combustion events of PGS, Bu35 and Bu35 OITin different status at 6500 and 8500 rpm. It can be seen that there isa similar trend for PGS, Bu35 and Bu35 OIT at different speeds.The more efficient combustion can also be achieved in partial loadsby advancing ignition timing with butanol additionwhich is similarto full load operation. It should be noted that the full load operationdid not follow the original trend, especially at 6500 rpm, mainlybecause of the knock limitations. More detailed heat release phe-nomena for Bu35 with OIT operation are depicted in Figs. 11 and 12,as can be seen that the curves for 8500 rpm are sharper near thepeak position probably due to the stronger turbulent intensity athigh speed. In addition, the difference between 7 Nm and 5 Nm aresmaller than those between 5 N m and 3 N m at both 6500 and8500 rpm.

4. Discussion of performance and emissions results

Engine performance, fuel economy and emissions at full loadbetween butanol-gasoline blend fuel and pure gasoline under

2 3 4 5 6 7 80

102030405060 PGS

Bu35Bu35+OIT

(b)

)A

C(

BFM

%09-01dna

%05 Engine torque (N.m)2 3 4 5 6 7 8

(a)

Engine torque (N.m)

Fig. 10. The combustion events versus engine torque in different status of (a):6500 rpm, (b): 8500 rpm. The top three lines are duration of 10e90% MFB and thebottom ones are location of 50% MFB (reference to TDC, positive value means afterTDC).

different operations and status have been compared, as shown inFig. 13.

Fig. 13a and b shows that engine power and torque of 30% and35% butanol-gasoline blend fuels without ignition timing optimi-zation are lower than those of pure gasoline at full load. But withignition timing optimization, engine power and torque ofBu30 OIT and Bu35 OIT are higher than those of pure gasolinefor all speeds. The variation tendency of engine power and torque atfull load are similar to that reported by Martin et al. [51] in 2012.That is because Bu30 and Bu35 without OIT are burned slower thanpure gasoline and the specific reasons have been analyzed in theimpact of ignition timing on heat release. Nevertheless, asmentioned before, with ignition timing optimization, butanol-gasoline blend fuel burns faster than pure gasoline. Moreover,Bu30 OIT and Bu35 OIT with higher knocking resistance, fasterlaminar flame speed, more advanced 50% MFB location, shorterduration of 0e10% and 10e90% MFB, leading high combustionefficiency.

Fuel consumption is an important indicator in engine evalua-tion. As the regular economy indicator, brake specific fuel con-sumption (BSFC) is not a very reliable parameter to compare fuelblends as calorific value and density of the blends follow a slightlydifferent trend [52]. In this paper, engine brake specific energyconsumption (BSEC) is used to compare volumetric consumption ofall test fuels for evaluating engine economy. It is described asmultiplication of BSFC and lower heating value (LHV) [18]. Thecomparisons of engine BSEC is shown in Fig. 13c. As shown inFig. 13c, engine BSEC of Bu30 and Bu35 are significantly reduced atfull load. And the higher butanol blend ratio, the lower BSEC. This isdue to higher density and lower heating value of butanol-gasolineblend compared to pure gasoline. On the other hand, Butanol-gasoline blend with leaner fuel mixture have higher combustionefficiency and higher fuel conversion efficiency [46]. It can also befound that BSEC of butanol-gasoline blend fuel is very sensitive toignition timing. That is because combustion efficiency improvedwith ignition timing optimization. In Fig. 13c, the BSEC ofBu35 OIT is the lowest.

Unburned Hydrocarbon (HC) emission from engine is mainlydue to completely unburned or only partially burned fuel. Low

2500350045005500650075008500234567

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(g)

2500350045005500650075008500

3000

5000

7000

9000

O2(p

pm)

Speed (r/min)

(h)

Fig. 13. Engine performance, fuel economy and emissions comparisons among pure gasoline, Bu30 and Bu35 (a): Power, (b): Torque, (c): BSEC, (d): HC (e): CO, (f): CO2, (g): NOx, (h):O2.

R. Feng et al. / Renewable Energy 81 (2015) 113e122 119

emission indicates that fuel and air is well mixed and that there issufficient oxygen to combust all fuels [53]. The amount of unburnedhydrocarbon depends on engine operating conditions and fuelproperties to some extent [31]. Fig. 13d shows the comparisons ofHC emissions for pure gasoline and butanol-gasoline blend fuel indifferent operations and status at full load. HC emitted by 30% and35% butanol-gasoline blend are much lower than that of puregasoline at all speeds. HC emission decreased with the increasing ofbutanol percentage. As shown in Fig. 13d, the higher butanol blendratio, the lower HC emission. HC emission decreasing can beexplained by butanol-gasoline blend fuel properties and combus-tion process. As mentioned above, 30% and 35% butanol-gasolineblend fuel burns more thoroughly than pure gasoline due to itshigher relative AFR and lower carbon content. The higher com-bustion efficiency of 30% and 35% butanol-gasoline blend causesthe reduction of HC emission. In Fig. 13d, it can also be found thatwith ignition timing optimization, HC emission of Bu30 OIT isalmost same with that of Bu30. Obviously, HC emissions ofBu35 OIT and Bu35 have the same trend. So we can draw con-clusions that HC emission is not sensitive to ignition timing butgreatly affected by properties of fuel to some extent.

Carbonmonoxide (CO) is a toxic gas and must be controlled. It isan intermediate product formed during combustion of hydrocar-bon fuels. Some reasons for the formation of CO include incompletecombustion and poor air fuel management [53,54]. And the pres-ence of oxygen plays a major role in CO emissions in SI engine [31].Fig. 13e shows CO emissions of pure gasoline and butanol-gasolineblend under different status and operations at full load. It can beseen that CO emissions of 30% and 35% butanol-gasoline blend aremuch lower than that of pure gasoline at all engine speeds. COemission decreased with the increasing of butanol percentage. This

result is in accordance with the studies which have been reportedby many studies [22,32,34,38]. The reasons for that are the samewith that of HC emission. So we can also draw the conclusions thatCO emission is greatly affected by the properties of fuel in someways [55]. In Fig. 13e, it can also be found that with ignition timingoptimization, CO emission of Bu30 OIT is lower than that of Bu30.But CO emission of Bu35OIT is almost the samewith that of Bu35.The possible reasonmay be that Bu30 didn't combust completely atthat AFR (Fig. 2) during the test together with original ignitiontiming, and advanced ignition timing can help to reduce CO emis-sion for the 30% butanol-gasoline blend. But Bu35 at that AFR(Fig. 2) is much leaner than that of Bu30. At that situation, COemission is not sensitive to ignition timing.

Carbon dioxide (CO2) occurs naturally in the atmosphere and is anormal product of combustion. CO2 is considered as a majorgreenhouse gas. It is an important component in global warmingand should be controlled at the acceptable levels [56]. The idealcombustion of HC fuels under the stoichiometric air fuel ratio orslightly lean mixture conditions should produce only CO2 andwater [57]. CO2 emission is directly related to the efficiency of en-gine combustion process [42]. In Refs. [58], the author mentionedthat the higher efficiency of combustion process, the lower CO2emissions. But in this paper, we found opposite behaviors. That is, ifcombustion more complete, more CO2 would emit. As shown inFig. 13e and f, CO2 emission has an opposite behavior whencompared with CO emission [59]. CO2 emissions of 30% and 35%butanol-gasoline blend fuel are much higher than that of puregasoline at full load. CO2 emission increased with the increasing ofbutanol percentage. It is also found that CO2 emissions ofBu30 OIT and Bu35 OIT are higher than that without ignitiontiming optimization. That's because the advancing ignition timing

R. Feng et al. / Renewable Energy 81 (2015) 113e122120

will create a higher cylinder temperature and this augmentedchemical reaction speed. Fig. 13f also shows that the maximum CO2emissions of pure gasoline and butanol-gasoline blend fuel appearat around 6500 rpm, which produces the maximum torque fromthe test engine, as shown in Table 2. This is because, one, the AFR isrelatively high at 6500 rpm (see Fig. 2), which means high oxygencontent for fuel oxidation; Two, the temperature is also relativelyhigh at around 6500 rpm (see Fig. 9), which can produce higherconversion from CO to CO2 [60].

Nitrogen oxides (NOx) formation is dependent on combustiontemperature, residence time, oxygen concentration and the effec-tive volume of combustion zone as stated in Zeldovich mechanism[61e65]. There are many factors influence NOx formation,including engine load, temperature, combustion chamber content,combustion chamber homogeneity and mixture density [31]. Dur-ing combustion process, NOx forms in both flame-front and post-flame [66]. As shown in Fig. 13g, NOx emission of Bu30 is lightlyhigher than that of PGS. NOx emissions of Bu30 OIT and Bu35 aremuch higher than that of PGS at all speeds. NOx emission ofBu35 OIT is the highest. There are some possible reasons. First,the combustion gas temperature with butanol-gasoline blend ishigher than that of pure gasoline, which is reflected by measuredcylinder head surface temperature as seen in Fig. 9. Second,butanol-gasoline blend contains oxygen and has a lower stoichio-metric AFR, which is shown in Table 1. While this engine wascalibrated for pure gasoline and at themeantime it was not possibleto adjust the AFR. The absolute AFR for all tested fuels were kept atthe same value for a given condition. So the combustion was rela-tively leaner and more oxygen was available to produce NOx forbutanol-gasoline blend fuel. Therefore, with the increasing ofbutanol blend ratio, NOx emission increased. And advancing igni-tion timing causes NOx emission increased. This is because theadvancing ignition timing would increase cylinder temperatureduring combustion process, which has been explained before.

Though Oxygen (O2) is not harmful, yet its content in engineexhaust gas can reflects some situations of other emissions. Oxygenin exhaust is the result of excessive air (leaning out) in the AFR [67].Fig. 13h shows the variation of oxygen (O2) emission with enginespeeds for different operations and status at full load. The O2 con-tent of Bu35 OIT is the highest, PGS of that is the lowest, andothers are in the middle. There are many factors, such as air fuelratio, butanol blend ration, fuel properties, ignition timing andother operation parameters. In this study, as shown in Fig. 9, Bu30and Bu35 have the higher cylinder head temperature than that ofpure gasoline, which will produce more O2 concentration at hightemperature combustion zone in cylinder [66]. Furthermore, theleaner mixture of Bu30 and Bu35 can accommodate more O2 fromthis dissociation process than that of pure gasoline which in-troduces more complete combustion of cylinder charge duringafter-combustion period. The excess O2 will be removed as exhaustgas during exhaust stroke [68]. In addition, the chemically boundoxygen in butanol-gasoline fuel provided another excess source ofoxygen component, which added to excess inlet air in reactantmixture. Hence, burning butanol-gasoline blend fuel producedmore residual oxygen emissions than pure gasoline fuel. And withignition timing optimization, the combustion will be improved andthereby releasing more O2.

Through the experimental results and analysis of engine per-formance, fuel economy, and emissions at full load, we found thatwith 35% volume butanol addition, combined with using the op-timum ignition timing, the tested engine has the best integrativeperformances. Compared to pure gasoline, Good results have beenachieved from Bu35 OIT in torque (increased by 2.1% on average),engine BSEC (decreased by 15.8% on average), HC emission(decreased by 22.4% on average), CO emission (decreased by 48.1%

on average) and O2 emission (increased by 63.4% on average). Onthe other hand, NOx and CO2 emissions (increased by 165.3%and12.2% on average respectively) are worse than those of puregasoline. However, it should be noted that the absolute value ofNOx emission is very low relative to CO emission.

5. Conclusions

Combustion heat release, performance, fuel economy andemissions of a single cylinder high speed SI motorcycle engine havebeen studied by experiment. Based on this study, the followingconclusions can be drawn:

1). Combustion heat release analysis results show that butanol-gasoline blend provides higher knocking resistance forallowing advancing ignition timing in SI engines, leading tomore complete combustion. With butanol blend ratioincreasing, higher oxygen content and anti-knock ability areuseful to improve combustion efficiency. Engine load is moreinfluencing on combustion heat release than fuel type.

2). Engine power, torque, BSEC, CO, HC, NOx and CO2 emissionshave been compared and analyzed among pure gasoline, 30%and 35% volume butanol-gasoline blend. Good results havebeen achieved in engine power, torque, BSEC, HC, CO, and O2emissions with 35% volume butanol addition, combinedwithusing the ignition timing optimization. But NOx and CO2emissions are higher than those of original level of puregasoline.

3). Engine power, torque, and NOx emission depend more onthe operating parameters, which directly affect enginecombustion process. HC, CO, CO2 and O2 emissions are highlyrelated to fuel properties. And engine economy is affected byboth fuel properties and engine operating parameters tosome extent. Nevertheless, this conclusion is limited to thepresent tested engine and it needs to be further confirmed bymore studies.

Acknowledgments

The work is supported by the National Natural Science Foun-dation of China (Program No. 51175164) and the Major State BasicResearch Development Program of China (973 Program,2011CB707201). The authors would like to thank Hunan University,China and California State University, Fresno for the support to thefirst author as visiting scholar. The authors appreciate the anony-mous reviewers and the editor for carefully reading this paper andsuggesting many helpful comments in improving the originalmanuscript.

Nomenclature

AFR air/fuel ratioBSFC brake specific fuel consumptionBSEC break specific energy consumptionCA crankshaft angleCOV coefficient of varianceCO carbon monoxideCO2 carbon dioxideDDGS distillers dry grains and solublesEGR exhaust gas recyclingHC hydrocarbonIMEP indicated mean effective pressureLHV lower heat valueMBT maximum brake torqueMFB mass fraction burned

R. Feng et al. / Renewable Energy 81 (2015) 113e122 121

NO nitric oxideNOx nitrogen oxidesOIT optimize ignition timingO2 oxygenPGS pure gasolineROHR rate of heat releaseSI spark ignitionTDC top dead center

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