Fuel 117 (2014) 8–14

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Fuel

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Applying Taguchi method to combustion characteristics and optimalfactors determination in diesel/biodiesel engines with port-injecting LPG

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.09.005

⇑ Corresponding author. Tel.: +886 6 2747018x223; fax: +886 6 2747019.E-mail address: z7708033@email.ncku.edu.tw (H.-W. Wu).

Zhan-Yi Wu, Horng-Wen Wu ⇑, Cheng-Han HungDepartment of System and Naval Mechatronic Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC

h i g h l i g h t s

� Diesel engine is optimized with diesel/biodiesel blend inducting LPG and cooled EGR.� Taguchi method finds optimal factors for high fuel consumption time, low NOX, smoke.� Coefficient of variation for peak pressure is lower than 1% not burning unstably.� We better brake specific fuel consuming, pressure, heat release at optimized engine.� The decrease rate is 52% for smoke and 31% for NOX at 60% engine load.

a r t i c l e i n f o

Article history:Received 20 March 2013Received in revised form 31 August 2013Accepted 2 September 2013Available online 13 September 2013

Keywords:Diesel/biodiesel mixtureLiquefied petroleum gasDiesel engineOptimal factors determinationTaguchi method

a b s t r a c t

This paper investigates combustion characteristics and decides the optimal operating factors by a Taguchimethod on a diesel engine using diesel/biodiesel mixture with liquefied petroleum gas (LPG) and cooledexhaust gas recirculation (EGR) inducted in the intake port. The optimal operating factors for acquiringthe largest fuel consumption time, the lowest smoke and NOX are decided for 1500 rpm and differentloads. In addition, this study compares the combustion characteristics (heat release rate and ignitiondelay) and emissions (NOX and smoke) between the optimum combination of factors and baseline dieselengine. The results display that predictions by Taguchi method are in fair consistence with the confirma-tion results, and this method decreases the number of experimental runs in this study. The best fuel con-sumption time, smoke, and NOX at each load is acquired at a combination of B10 (A1), 40% LPG (B3) and20% EGR ratio (C1). Moreover, the heat release rate for engine conditions is computed using a variablespecific heat ratio by the experimental in-cylinder pressure. Furthermore, the best combination decreasesboth smoke and NOX emissions. The decrease rate is 52% for smoke and 31% for NOX at 60% engine load.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Established in December 1997, the ‘‘Kyoto Protocol’’ regulatedthat all member countries emit amounts of six greenhouse gasesless in 2010 than in 1990. Studies on alternative fuel for fossil fuelsare a growing trend for more and more serious energy and envi-ronmental issues. Biodiesel was an alternate and sustainable fuelfor diesel engines. The use of biodiesel in diesel engines signifi-cantly reduced CO, HC, and particulate emissions [1]. On the otherhand, LPG is thought to be one of the promising alternative fuelsnot only as a substitute for petroleum but also as a means ofdecreasing NOX, smoke, and particulate matter [2]. Therefore, thestudy on diesel/biodiesel engines without/with port-injecting LPGhas drawn considerable attention.

Park et al. [3] showed that B20 fuel compared with B0, NOX

increased by 3.7%, CO and HC slightly declined; smoke decreased

up to 20%; and soot particles reduced by 24% on number and by16% amount after introducing the EGR. Sigar et al. [4] displayedthat smoke and NOX emissions reduced while CO and HC emissionsincreased for B20 biodiesel from yellow methyl ester with induct-ing LPG; however, the brake thermal efficiency of B20 biodieselfrom yellow methyl ester with inducting LPG was lower than thatof pure diesel under full load. Vijayabalan et al. [5] observed thatthe diesel and biodiesel as a primary fuel mixed with LPG had high-er HC and CO emissions than single-fuel system did but reducedNOX up to 70% and smoke up to 62%. Kapilan et al. [6] found thatan increase in LPG increased HC with diesel and mahua biodiesel(MOB) as basic fuel since this reduced diesel/biodiesel mixture todelay combustion and deterioration. Reddy et al. [7] showed thatpure diesel fuel system had a higher brake thermal efficiency atlow load but using biodiesel from mahua oil methyl ester andLPG at the intake had a higher brake thermal efficiency and lowersmoke and NOX at high load. Ramadhas et al. [8] reported thatcompared to diesel, biodiesel produced slightly higher NOX, andhigher cylinder temperature in the combustion process; while

Z.-Y. Wu et al. / Fuel 117 (2014) 8–14 9

EGR gas reduced combustion temperature and this combined effectslightly improved combustion efficiency.

In recent years, with the decrease in number of experimentalruns by orthogonal arrays and reliable quality control, the Taguchimethod has been used in the industry to decide factors which areessential for obtaining objectives. Using the Taguchi method opti-mizes qualitative characteristics through the setting of designparameters, and reduces the sensitivity of the system performanceto sources of variation of a single characteristic. However, theresearchers have not applied the Taguchi method to optimal anal-ysis until recent years. Wu and Ku [9] developed the optimalparameters estimation for proton exchange membrane fuel cells(PEMFCs) by this method. Nataraj et al. [10] confirmed that theTaguchi method was useful for predicting how different design fac-tors influenced the engine emissions. In addition, Saravanan et al.[11] explored how fuel injection timing, exhaust gas recirculation(EGR) ratio, and fuel injection pressure affected NOX from a dieselengine by Taguchi’s L9 orthogonal arrays. Ganapathy et al. [12]investigated the fitness of various kinds of biodiesel for optimizingengine performance by a thermodynamic model.

Much literature is available on the effect of diesel/biodieselmixture using LPG or EGR at a fixed flow rate on full-factorialexperiments but little attention was devoted to the combustioncharacteristics, emissions, and optimal factors decision of a dieselengine with diesel/biodiesel mixture using LPG and EGR inductedat the intake port. Regarding the emissions from the diesel engine,the smoke and NOX are major. The authors hence use orthogonalarray L9 (34) of Taguchi method to determine the optimum operat-ing condition of biodiesel/diesel mixture, premixed ratio of LPG bythe equal input energy law [13], and cooled EGR ratio for the larg-est fuel consumption time, the lowest smoke and NOX emissions.

2. Experiment facilities

Fig. 1 indicates a schematic view of the experimental systemsused in this paper. The engine employed in the experimentalsystems is a single cylinder, four-stroke, and water cooled diesel

17

15

2122

2

14

12

16

18

Fig. 1. Schematic view of experimental systems. 1. diesel engine, 2. fuel tank, 3. cylinder pNOX analyzer, 8. CO/CO2/HC analyzer, 9. smoke analyzer, 10. dynamometer, 11.crankevaporator, 16. flow controller, 17. LPG safety solenoid valve, 18. intake surge tank, 19.

engine (Kubota RK-125) having a displacement volume of624 cm3 with rated power of 9.2 kW. After the exhaust gas in theEGR system of the engine was cooled using the exhaust cooler, itwas introduced into the cylinder. When the appropriate EGR ratiowas obtained by EGR valve, the gas flow was controlled by the in-take air and EGR volume flow sensors. A pressure transducer (Kis-tler 6001) was connected with a charge amplifier (Kistler 5011B) toanalog/digital signal converter NI PCI-6259 for measuring the cyl-inder pressure. The crank angle (degree) signals were obtainedfrom a shaft encoder with an accuracy of ±0.05� and a resolutionof 1�. After NI PCI-6259 was quickly processed through a computer,the cylinder gas pressure-crank degree signal was acquired. LPGflow rate was measured by a thermal mass flow controller,GFC-37A-VAD with precision at ±1.5%. Smoke was measured byBOSCH EAM3.011, and NOX was taken by ACHO Physics CLD-60.The measurement uncertainty for pressure is ±0.5%, for LPG flowrate for ±1.5%, NOX ± 1.0%, and for smoke ±2.0% [14].

3. Combustion analysis descriptions and Taguchi technique

3.1. Combustion analysis descriptions

Using a concept of ‘equal input energy’ defines the premixed ra-tio of LPG by the following equation:

Premixed ratio ¼ ELPG

ðELPG þ Ef Þð1Þ

where ELPG is the mass flow rate of LPG (kg/min) multiplied by thelower heating value (MJ/kg) of LPG and Ef the mass flow rate of die-sel/biodiesel mixture multiplied by the lower heating value of die-sel. (ELPG + Ef) is maintained constant at the same operationcondition.

The following model is for heat release rate [15] which de-scribes the combustion events and stages.

dQdh¼ 1

c� 1cp

dVdhþ V

dpdh

� �� pV

ðc� 1Þ2dcdh

ð2Þ

1

3

10

11

19

13

20

7

9

8

6

54

LPG

ressure sensor, 4. signal amplifier, 5. analog /digital signal converter, 6. computer, 7.angle detector, 12. external jacket water cooler, 13. bump, 14. LPG tank, 15. LPGexhaust cooler, 20. carbon absorber, 21. EGR valve, 22. EGR flow sensor.

Table 1ANOVA results for fuel consumption time at 30% different loads.

Factor SS DOF MS F F0.1 SS’ Contribution (%)

ANOVA results for fuel consumption time at 30% loadA 0.25 2 0.13 1.93 3.46 0.12 –B 2.74 2 1.37 20.81 3.46 2.61 83.20C 0.07 2 0.03 0.49 3.46 �0.07 –e 0.08 2 0.04 0.57 3.46 �0.06 –Error 0.40 6 0.07 – – 0.53 16.80Total 3.14 8 3.14 100

ANOVA results for fuel consumption time at 60% loadA 0.38 2 0.19 20.90 4.32 0.37 6.47B 5.23 2 2.62 284.58 4.32 5.22 92.23C 0.01 2 0.00 0.49 4.32 –0.01 –e 0.03 2 0.01 1.51 4.32 0.01 –Error 0.04 4 0.01 – – 0.07 1.30Total 5.65 8 5.65 100

ANOVA results for fuel consumption time at 90% loadA 0.35 2 0.17 85.97 9 0.34 6.26B 4.99 2 2.49 1242.95 9 4.98 91.50C 0.11 2 0.05 27.37 9 0.11 1.94e 0.00 2 0.00 1.00 9 – –Error 0.00 2 0.00 – – 0.02 0.30Total 5.45 8 5.45 100

10 Z.-Y. Wu et al. / Fuel 117 (2014) 8–14

where dQ/dh denotes the heat release rate (J/degree), p cylinderpressure (Pa), V the volume of the cylinder (m3), and h the crank an-gle in degrees. c represents the specific heat ratio calculated by thecontents and temperature of cylinder gases versus crank angle [16],and dc/dh is hence obtained. EGR is one of techniques to decreaseNOX from internal combustion engines. The EGR (%) [17] is calcu-lated by

EGR ð%Þ ¼ V 0EGR=ðV0air þ V 0f þ V 0EGRÞ ð3Þ

where V 0EGR is the volume of exhaust gas from the engine per cycle;V 0air and V 0f are the volumes of the intake air and of the fuel per cycle(m3/cycle).

During the steady engine, fuel consumption time is measuredwhen the fuel is consumed in 10 cm3 operation, and the ignitiondelay is obtained by a heat-release analysis using pressure versuscrank angle data [18]. BSFC (brake specific fuel consumption) is de-fined as the fuel mass flow rate divided by the brake output powerin term of the following equation:

BSFCðg=kW hÞ ¼_mf

BPð4Þ

where _mf is mass rate (g/h) of the diesel/biodiesel mixture, and BPthe brake power (kW) of the engine.

BTE (brake thermal efficiency) denotes fuel conversion effi-ciency calculated by the following equation [18]:

BTE ¼ BPð _mf � hf Þ þ ð _mLPG � hLPGÞ

ð5Þ

where hf denotes the lower heating value of diesel/biodiesel fuel,and hLPG the lower heating value of LPG. If cyclic combustion varia-tion is greater, then the engine will operate unstably and will burnincompletely to influence combustion characteristics of engine. Thecyclic combustion variation in this study is represented by coeffi-cient of variations (COV) of peak pressure, and this COV is calcu-lated using 40 of cycle number [19].

3.2. Taguchi technique

The results of the Taguchi experiments are analyzed in a stan-dard series of phases. First, the main effects are evaluated andthe influences of the factors are determined in qualitative terms.The optimum condition and performance at the optimum condi-tion are also determined from the factorial effects. In the nextphase, the analysis of variance (ANOVA) is performed on the exper-imental data to identify the relative influence of the factors in dis-crete terms. The term variation is indicated by severalmathematical descriptions as follows:

Total sum of square : SST ¼Xn

i¼1

ðy2i � CFÞ ð6Þ

Where CF is correlation factor

CF ¼Xn

i¼1

y1

!2

=n ð7Þ

The total and factor sums of squares are the basic calculationsneeded for ANOVA. Four other quantities calculated as part of theANOVA table information are all derived from the original sums ofsquares. For the factor A, they are as follows:

Mean variance : MSA ¼SSA

fAð8Þ

F-ratio : FA ¼MSA

Veð9Þ

Pure sum of squares SS0A ¼ SSA � ðVe � fAÞ ð10Þ

Percent contribution : qA ¼SS0ASST

ð11Þ

where SSA is sum of square, fA is the degrees of freedom (DOF) of fac-tor A, and Ve is the variance for the error term (obtained by calculat-ing error sum of squares and dividing by error degrees of freedom).

The signal-to-noise (S/N) ratio in Taguchi methods [20] is toevaluate the variation of performance of an output characteristic.The fuel consumption time in the experiments belongs to the lar-ger-the-better quality characteristics in the following equation:

S=N ratio ¼ �10 log1n

Xn

i¼1

1y2

i

!ð12Þ

Smoke and NOX emissions in the experiments belong to the smaller-the-better quality characteristics in the following equation:

S=N ratio ¼ �10 log1n

Xn

i¼1

y2i

!ð13Þ

where n represents the test number and yi the quality characteris-tic’s value evaluated by a trial. Finally, using the parametric levelsof the highest S/N ratio in the orthogonal array finds out the opti-mum combination of setting.

4. Results and discussion

The L9 orthogonal array in this study is an unsaturated designand the authors choose (A) diesel/biodiesel mixture, (B) premixedratio of LPG, and (C) the percentage of EGR as the three factorsfor the engine. Furthermore, the error is chosen as the fourth factorin the L9 orthogonal array. Three levels of A are B10, B20, and B30,and three levels for B are 20%, 30%, and 40%, and three levels for Care 20%, 30%, and 40%. The optimal operating factors are thendetermined for the largest fuel consumption time, the lowestsmoke and NOX emissions under different loads.

4.1. Results of Taguchi process

ANOVA was performed using Eqs. (8)–(11) to identify the mostsignificant factors and to quantify their effects on fuel consumptiontime, NOX, and smoke. The ANOVA results are shown in Tables 1–3for response at various loads while the negative/little contribution

Table 2ANOVA results for NOX at different loads.

Factor SS DOF MS F F0.1 SS’ Contribution(%)

ANOVA results for NOx at 30% loadA 0.63 2 0.32 0.54 4.32 �0.55 –B 18.65 2 9.32 15.73 4.32 17.46 24.46C 50.36 2 25.18 42.49 4.32 49.18 68.90e 1.74 2 0.87 1.46 4.32 0.55 –Error 2.37 4 0.59 – – 4.74 6.64Total 71.38 8 71.38 100

ANOVA results for NOx at 60% loadA 12.68 2 6.34 20.52 3.46 1206 12.27B 10.32 2 5.16 16.70 3.4 9.70 9.87C 73.43 2 36.72 118.88 3.46 72.82 74.09e 1.85 2 0.93 3.00 3.46 – –Error 1.85 2 0.31 – – 3.71 3.77Total 98.28 8 98.28 100

ANOVA results for NOx at 90% loadA 53.02 2 26.51 12.49 4.32 48.78 27.24B 4.97 2 2.48 1.17 4.32 0.72 –C 117.56 2 58.78 27.70 4.32 113.31 63.28e 3.52 2 1.76 0.83 4.32 – –Error 8.49 4 2.12 – – 16.98 9.48Total 179.06 8 179.06 100.40

Table 3ANOVA results for Smoke at 90% at different loads.

Factor SS DOF MS F F0.1 SS’ Contribution (%)

ANOVA results for Smoke at 30% loadA 1.21 2 0.60 75.88 9 1.19 15.65B 1.71 2 0.86 107.77 9 1.70 22.32C 4.67 2 2.34 293.69 9 4.66 61.19e 0.02 2 0.01 1.00 9 – –Error 0.02 2 0.01 – – 0.064 0.84Total 7.61 8 7.614 100

ANOVA results for Smoke at 60% loadA 7.29 2 3.65 0.99 3.46 �0.10 –B 61.90 2 30.95 8.38 3.46 54.51 64.84C 9.39 2 4.70 1.27 3.46 2.00 –e 5.48 2 2.74 0.74 3.46 �1.91 –Error 22.17 6 3.70 – – 29.56 35.16Total 84.07 8 84.07 100

ANOVA results for Smoke at 90% loadA 0.95 2 0.47 0.57 3.46 �0.72 –B 30.53 2 15.27 18.34 3.46 28.87 81.25C 3.42 2 1.71 2.05 3.46 1.76 –e 0.63 2 0.31 0.38 3.46 – –Error 4.99 6 0.83 – – 6.66 18.75Total 35.52 8 35.52 100

Table 4S/N ratios and overall optimal factors for different loads.

Factors Levels S/N ratio for Fuelconsumption time

S/N ratiofor NOX

S/N ratiofor smoke

Overalloptimum

(a) at 30% loadA 1 – – �8.94 A1

2 – – �9.673 – – �9.76

B 1 36.16 �44.31 �10.05 B3

2 36.71 �43.33 �9.303 37.50 �40.88 �9.01

C 1 – �44.91 �8.71 C3

2 – �44.08 �9.233 – �39.53 �10.43

(b) at 60% loadA 1 33.56 15.48 – A1

2 33.28 15.39 –3 33.06 15.23 –

B 1 32.37 �49.69 �46.26 B3

2 33.29 �48.13 �49.513 34.24 �47.08 �52.06

C 1 – 16.36 – C1

2 – 15.26 –3 – 14.47 –

(c) at 90% loadA 1 31.06 11.54 – A2

2 30.81 12.24 –3 30.58 11.76 –

B 1 29.90 – �33.97 B3

2 30.83 – �31.643 31.72 – �29.46

C 1 30.94 13.62 – C1

2 30.84 12.45 –3 30.67 9.47 –

Z.-Y. Wu et al. / Fuel 117 (2014) 8–14 11

is not listed because the factor has no obvious variance in the test.The contribution level for the premixed ratio of LPG to the fuel con-sumption time is from 83.2% (at 30% load) to 91.50% (at 90% load).This is since some diesel and biodiesel mixture is replaced by light-er LPG. At any load operation, the contribution for premixed ratioof LPG to fuel consumption time is the highest and biodiesel vol-ume fraction contributes secondly. At different loads, the contribu-tion of EGR percentage to NOX is higher than that of other factors toNOX since increasing EGR percentage decreases gas temperatureand thus reduces formation of NOX. At 30% load all the three factorshave significant effects on smoke. However, when the load is at90%, premixed ratio of LPG factor effect is significantly higher at81.25% while the contribution level of biodiesel volume fractionand EGR percentage to smoke is smaller. This is because at lowload, temperatures in cylinder does not enough make LPG burnfully and inducting EGR also reduces air entering cylinder to in-crease fuel air ratio. In addition, the viscosity of biodiesel is higherthan that of diesel to have larger droplet size and to bad mix with

air leading to locally rich mixture and increasing smoke emission.At high load, an increase in cylinder temperature helps LPG of lowcarbon content gasify and burn so as to reduce smoke.

Table 4 consequently shows the optimum factors setting deter-mined for each response at various loads. The optimized combina-tion is decided using the highest S/N ratio and much morecontribution. At 30% load, control factor A is more significant onsmoke than on other objectives; in addition, the highest S/N ratiofor smoke is A1. The S/N ratio for fuel consumption time, smokeand NOX are considered, and the highest S/N ratio is B3. As for fac-tor C, the contribution is higher to NOX than to other objectives;and C3 is thus confirmed to be the optimal factor. After the sameprocesses are applied at the other two loads (60% and 90% loads),the best combination at 60% load is B10 (A1), 40% LPG (B3) and20% EGR ratio (C1), and at 90% load is B20(A2), 40% LPG(B3) and20% EGR(C1). A1B3C1 is then chosen as the best combination whenthe three combinations are integrated under various loads. In addi-tion, the predicted best combinations of Taguchi method must beverified by the confirmation experiments [20]. For example, the90% confidence interval for NOX is equal to ±1.95 at 30% load.The optimum NOX is therefore predicted at –42.72 ± 1.95 dB. Toverify the predicted optimum NOX, this paper conducted a confir-matory experiment by running several replications at the optimalsettings of the process parameters determined from the analysis.The S/N ratio of the NOX at 30% load from the confirmation exper-iments is –42.43 dB, which falls within the predicted 90% confi-dence level. In the same way, it can be found that otheroptimum objectives also fall within the predicted 90% confidencelevel. The predictions from Taguchi method hence fairly agreedwith the confirmation results.

The change in combustion characteristics and emissions withthe variation for process parameter level is graphically representedby S/N response curves and the highest S/N ratio is the optimumfactors combination for individual objective at each load. The S/N

35

36

37

38

A1 A2 A3 B1 B2 B3 C1 C2 C3

S/N

(dB

)

-11

-10

-9

-8

A1 A2 A3 B1 B2 B3 C1 C2 C3

S/N

(dB

)

-46

-44

-42

-40

-38

A1 A2 A3 B1 B2 B3 C1 C2 C3

S/N

(dB

)

Fuel consumption at 30% load

Smoke at 30% load

NOx at 30% load

Fig. 2. S/N responses for fuel consumption time, smoke, and NOX at 30% engineload.

0

20

40

60

80

100

30 60 90

Load (%)

Fue

l con

sum

ptio

n ti

me

(s /1

0cm

³)

Baseline diesel engine

Optimized diesel engine

(a)

0

10

20

30

40

50

60

30 60 90

Load (%)

Smok

e (%

)

0

200

400

600

800

30 60 90

Load (%)

NO

X(p

pm)

Baseline diesel engineOptimized diesel engine

Baseline diesel engine

Optimized diesel engine

(b)

(c)

Fig. 3. Comparison of (a) fuel consumption time (b) smoke and (c) NOX betweenbaseline diesel engine and optimized engine.

12 Z.-Y. Wu et al. / Fuel 117 (2014) 8–14

responses for fuel consumption time, smoke, and NOX are dis-played in Fig. 2 at 30% load. The optimum factors combinationfor the largest fuel consumption time in Fig. 2 is at B10 (A1), 40%LPG (B3) and 20% EGR ratio (C1) at 30% load. The optimum combi-nation for the lowest smoke is at B10 (A1), 40% LPG (B3), and 40%EGR ratio (C1). The optimum combination for the lowest NOX isat B10 (A1), 30% LPG (B3) and 40% EGR ratio (C3) at 30% load. TheS/N responses for fuel consumption time, smoke, and NOX at 60%and 90% loads will not be shown in figures for considering the lim-ited total pages. However, after in the same way, this study also ob-tains that at 60% load, the optimum combination for the largestfuel consumption time is A1B3C1, for the lowest smoke A1B3C1,and for the lowest NOX A1B3C3; at 90% load, the optimum combina-tion for the largest fuel consumption time also from is A1B3C1, forthe lowest smoke A2B3C1, and for the lowest NOX A1B3C3.

4.2. Variation of combustion characteristics and emissions foroptimized engine and baseline diesel engine

Fig. 3(a) shows the difference in fuel consumption time be-tween baseline diesel engine and optimized engine; and fuel con-sumption time is larger with optimized engine. Fig. 3(b) indicatesthat smoke for the optimized engine is less than that for the base-line diesel engine under varied loads. At low load, there is almostno difference in smoke between optimized engine and baselinediesel engine, and the difference is increasing all the time as theload increases. The similar result was found in Ref. [5]. At 60% load,the reduction value for the smoke reaches about 52% since the car-bon content of LPG is lower than that of diesel fuel and is moreprone to oxidation, and the flame spreads rapidly. In Fig. 3(c),NOX is also reduced with the optimized engine under varied loads.At 60% load, the reduction rate is 31% and it is 35% at 90% load. Thehigher the EGR is, the lower the NOX becomes; this is because theEGR causes worse combustion and thus lower gas temperature.Moreover, when the premixed ratio of LPG is higher, the amount

of diesel and biodiesel mixture injected into the cylinder is lower.The more the LPG in cylinders is, the more heat is absorbed, so thegas temperature is lower. Since the impact of LPG is greater athigher load, the difference in NOX between optimized engine andbaseline diesel engine is also a larger. The same trend was observedin Ref. [5]. The optimized engine has higher heat release rate thanthe baseline diesel engine at various loads in Fig. 4. At 30% load, thedifference of heat release is not obvious because in-cylinder gastemperature is not sufficient to enable full gasification and com-bustion of LPG owing to small fuel amount supplied. At 60% and90% load, heat release rate is higher at optimal factor combinationsthan at diesel fuel since in-cylinder gas temperature also increasedto make LPG get better combustion. In addition, the higher the LPGhas higher heat release rate at higher load. The ignition delay inFig. 5 is larger for the optimized engine than for the baseline dieselengine under varied loads. At 30% load, ignition delay for the base-line diesel engine is 0.66 ms and it is 0.74 ms for the optimized en-gine. This is because atomization of fuel to mix with air becomesworse by adding biodiesel owing to a higher viscosity. The sametrend was seen in Ref. [6]. Furthermore, EGR is also an importantfactor for the increase of ignition delay due to lack of oxygen sinceLPG has a high octane rating and increasing LPG reduces cetanenumber. The COV of Pmax values at all operating conditions are low-er than 1% (from 0.44% to 0.78%) in Fig. 6. As a result, added LPGand EGR here do not cause unstable combustion [18]. BSFC inFig. 7(a) for the optimized engine decreases by 23.3% and more

Fig. 4. Variation of heat release rate with engine load for optimized engine andbaseline diesel engine.

0.50

0.55

0.60

0.65

0.70

0.75

0.80

30 60 90

Load (%)

Igni

tion

del

ay (

ms) Baseline diesel engine

Optimized diesel engine

Fig. 5. Variation of ignition delay with engine load for optimized engine andbaseline diesel engine.

0.0

0.2

0.4

0.6

0.8

1.0

30 60 90

Load (%)

CO

V o

f P

max

(%

)

Baseline diesel engineOptimized diesel engine

Fig. 6. Effect of engine load on COV of Pmax for optimized engine and baseline dieselengine.

150

200

250

300

350

30 60 90

Load (%)

BSF

C (

g/kW

·h)

Baseline diesel engineOptimized diesel engine

(a)

20

25

30

35

30 60 90

Load (%)

Bra

ke t

herm

al e

ffic

ienc

y (%

)

Baseline diesel engineOptimized diesel engine

(b)

Fig. 7. Variation of (a) BSFC and (b) BTE with engine load for optimized engine andbaseline diesel engine.

Z.-Y. Wu et al. / Fuel 117 (2014) 8–14 13

at different loads. At 90% load, the decrease rate is 29%; this is sinceLPG has higher heating value than biodiesel/diesel mixture soinducting LPG in the cylinder replaces more amount of biodiesel/diesel fuel. The optimized engine has higher BTE than the baselinediesel engine in Fig. 7(b). However, BTE of optimized engine justincreases up to 4.7%. This is because biodiesel has higher oxygen

content than diesel does, which causes to burn better. Moreover,the LPG replaces equal input energy from biodiesel/diesel mixture.BTE therefore slightly changes in this way.

5. Conclusions

This study has determined the optimal operating factors of thelargest fuel consumption time, the lowest smoke and NOX withTaguchi method for a diesel engine with diesel/biodiesel mixtureusing LPG and cooled EGR at the intake port. The predictions ofTaguchi method are verified to consist with the confirmation re-sults at a confidence interval of 90% in this study. The combustioncharacteristics and emissions of the optimized engine have alsobeen compared with those of baseline diesel engine. The best fuelconsumption time, smoke, and NOX at each load is acquired for acombination of B10 (A1), 40% LPG (B3), and 20% EGR ratio (C1). Thiscombination improves combustion characteristics. At variousloads, fuel consumption time, cylinder pressure, heat release rate,and ignition delay are higher at the optimized engine than at thebaseline diesel engine. In addition, the best combination does notcause the unstable combustion of the engine. The optimized enginecan also decrease smoke and NOX emissions. The decrease rate is52% for smoke and 31% for NOX at 60% engine load.

Acknowledgement

The authors are indebted to the National Science Council of Tai-wan, ROC, for its grant from NSC 98-2622-E-006 -028-CC3.

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