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Energy 68 (2014) 495e509

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Energy

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Effect of compressed natural gas dual fuel operation with diesel andPongamia pinnata methyl ester (PPME) as pilot fuels on performanceand emission characteristics of a CI (compression ignition) engine

Abhishek Paul a,*, Raj Sekhar Panua a, Durbadal Debroy a, Probir Kumar Bose b

aDepartment of Mechanical Engineering, National Institute of Technology, Agartala 799055, Indiab Jadavpur University, Kolkata 700032, India

a r t i c l e i n f o

Article history:Received 5 October 2013Received in revised form26 February 2014Accepted 6 March 2014Available online 31 March 2014

Keywords:DieseleCNG combinationPPMEeCNG combinationBSFC diesel equivalentNOx reductionPongamia pinnata methyl esterPerformanceeemission tradeoff

* Corresponding author.E-mail addresses: v1.abhishek@gmail.com,

(A. Paul).

http://dx.doi.org/10.1016/j.energy.2014.03.0260360-5442/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The experimental work presents a comparative study of performance and emission using Pongamiapinnata methyl ester (PPME) and Diesel as pilot fuel in a CI (compression ignition) engine with com-pressed natural gas (CNG) as the primary fuel. The results show that PPMEeCNG dual fuel operation ismore effective than DieseleCNG dual fuel operation in improving the performance and emission char-acteristics of the engine. CNG is found to share higher quantity of input energy with PPME pilot operationthan pilot Diesel operation. Low amount of CNG injection also increases the brake thermal efficiency ofthe engine. PPMEeCNG operations with low amount of CNG injections are also more instrumental inreducing CO (carbon monoxide) emission and smoke opacity than DieseleCNG operations. NOx emissionfrom the engine is found to increase a bit for PPMEeCNG operations in comparison to DieseleCNGoperation. PPMEeCNG operation is also more effective in reducing hydrocarbon emission than DieseleCNG operations. The study also shows that CNG injected at 10� ATDC (after top dead center) for aduration of about 4500 ms with PPME as pilot fuel can produce better performance and emission sig-natures than DieseleCNG operation. The tradeoff study also consolidates the fact that PPMEeCNG dualfuel operation is instrumental in resolving the high performanceelow emission paradox.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Compression ignition engines take a very important role inmodern transportation and power generation sector owing to theirhigher thermal efficiency, excellent fuel economy and low emis-sions of unburned hydrocarbon and carbon monoxide (CO). How-ever, these engines greatly affect the environmental aspectsbecause of their high NOx and particulate matter emissions [1].Moreover, the ever-quenching petroleum reserves and stringentemission norms followed worldwide nowadays has put seriousquestion on usability of Diesel engine in future. Consequently,much of the research is now concentrated toward finding analternative fuel source with better performanceeemissioncharacteristics.

www.abhishek16@gmail.com

Natural gas is a promising fuel for the engine, whose propertiesare distinctly different from the properties of any liquid fuel. CNG(compressed natural gas) has ignition energy of 0.21 mJ (at 8.5% byvolume in air) [2,3] that allows ignition of very lean mixtures. CNGis readily suitable for spark ignition engines because of its highOctane number [4e6]. However, due to its low cetane number andhigh self-ignition temperature of about 540 �C [3], it is not possibleto achieve ignition of CNG by compression alone in existing CI(compression ignition) engines. Hence, some source of ignition hasto be created inside the combustion chamber to ensure ignition.This issuewas encounteredwhen attempts weremade to burn highoctane fuels like CNG in CI engine under ‘Dual-Fuel’ mode. In dual-fuel mode, the primary high octane fuel is ignited by the combus-tion of a small amount of high cetane pilot fuel. Over the years, anumber of researchers have investigated the combustion of CNG ina CI engine with Diesel as the pilot fuel. Hallquist et al. [7] foundthat buses running on CNG emitted more fine particulates but lessquantity of particulates compared to the Diesel-fueled buses. Kar-abektas et al. [8], Serrano and Bertrand [9] found that dual fuelyields higher CO and HC (hydrocarbon) emissions at all loads along

Nomenclature

CNG compressed natural gasBSEC brake specific energy consumptionBSFC brake specific fuel consumptionhbth brake thermal efficiencyTDC top dead centerATDC after top dead centerDI direct injectionms micro secondDAQ data acquisitionTSU total sampling uncertaintyEGR exhaust gas recirculation

PPME Pongamia pinnata methyl esterLNG liquefied natural gasLHV lower heating valueNOx oxides of nitrogenNO nitrous oxideCO carbon monoxideCO2 carbon dioxideHC hydrocarbonTHC total hydrocarbonUBHC unburned hydrocarbon20%DCNG-0 20% load condition, diesel without CNGx%DCNG-y x% load condition, diesel with CNG strategy y, where,

x ¼ 20, 40, 60, 80, 100% of load; y ¼ 1, 2, 3, 4, 5

A. Paul et al. / Energy 68 (2014) 495e509496

with lower NO emissions except for high loads. Liu et al. [10] foundthat DieseleCNG dual fuel mode reduces NOx and PM (particulatematter) emissions, although PM emission increasedwith increasingpilot fuel quantity. Cheenkachorn et al. [11] concluded that amaximum of 77.90% natural gas at 1300 rpm could be used in dualfuel engine operation. Yoshimoto et al. [12] studied the effect ofcetane number of the pilot fuel in dual fuel mode and found thatfuels with cetane number higher than 45, there is a strong negativeimpact on IMEP and brake thermal efficiency. Works of Tomita et al.[13] and Selim [14] separately came to the same conclusion that theuse of moderate EGR (exhaust gas recirculation) was very effectiveto reduce NOx emission and increase the thermal efficiency underDieselenatural gas dual fuel mode. Gong et al. [15] found that about25e35% percent increase in power density could be achievedwithout adversely affecting the NOx emissions in an HCCI (homo-geneous charge compression ignition) engine using DieseleCNGdual fuel operation. In two separate works, Ahmad et al. [16] andIshiyama et al. [17] found that low substitution ratios of CNGimproved the performance of the engine. Cordiner et al. [18] foundthat for high degrees of CNG substitution, significant improvementin PM emission could be achieved. All these previous researchersagreed on the same point that the dual fuel mode is reasonablysuccessful in reducing NOx and soot emission because of higherDiesel fuel supplement ratio [19,20]. Further, CNG readily formshomogeneous mixture with air, which can be burned easily over awide flammability range (5e16%) [21,22]. This improves the pre-mixed combustion phase and aids in reducing NOx and sootemissions compared to Diesel. Again, CNG generally burns quickerdue to significantly high laminar flame speed of CNGeair mixture(about 0.374 m/s for stoichiometric levels at 293 K and 1 atm) [23],which minimizes the combustion duration. This also results inincomplete combustion and increases hydrocarbon and CO emis-sions [24,25].

Since the properties of the igniter pilot fuel greatly influence thecombustion characteristics of a CI engine under dual-fuel mode, asignificant amount of work can be done by using different types ofpilot fuels along with the gaseous fuels. Although Diesel is used asthe primary pilot fuel in dual-fuel study, many other alternative andsustainable fuel sources have been investigated. One such alter-native fuel source is biodiesel. Biodiesel is alkyl ester of fatty acid,produced by trans-esterification of renewable resources such asvegetable oil, animal fat, waste cooking oil etc. Biodiesels are quiteidentical to Diesel in terms of their properties and their effect onperformance and emission characteristics of the engine. Over theyears, different biodiesels have been used as pilot fuels. Namasi-vayam et al. [26] used Rapeseed methyl ester as the pilot fuel for anatural gas fueled CI engine. This study observed significant in-crease in thermal efficiency and NOx emission. Side by side, a

distinct decrease in CO and HC emission was also witnessed. Selimet al. [27] observed improved performance, reduced combustionnoise, extended knocking limits and reduced cyclic variability ofcombustion using Jojoba methyl ester as pilot fuel for LNG (lique-fied natural gas) dual fuel operation. Geo et al. [28] used rubberseed methyl ester with Hydrogen under dual fuel strategy andobserved significant improvement in brake thermal efficiency andsmoke emission. Banapurmath and Tewari [29] used Honge methylester as pilot fuel with produced gas dual fuel operation and theyalso observed decrease in brake thermal efficiency and NOx emis-sion with increase in HC and CO emission. Korakianitis et al. [30]experimented with hydrogen and natural gas dual fuel operationusing rapeseed methyl ester as the pilot fuel. They found that NOxincreases with hydrogen use, whereas HC emission increases withnatural gas use. Namasivayam et al. [31] in a separate study withRapeseed methyl esterenatural gas dual fuel operation observed atendency of lower emissions of smoke and oxides of nitrogen, buthigher emission of carbon monoxide and unburned hydrocarbons.Yoon and Lee [1] studied the combustion and exhaust emissioncharacteristics of biogasebiodiesel dual-fuel combustion. Theyfound lower NOx emission with superior performance in reductionof soot emission with the mentioned fuel combinations. Ryu [32]observed significant reduction in NOx emission with biodieseleCNG dual fuel operation. One common feature of biodiesel, whichhas been observed from these studies, that the calorific values ofthese biodiesels are generally lower than Diesel, which increasesthe fuel consumption of the engine [33,34]. However due to itsoxygen content, biodiesels allows widespread oxidation of fuel,resulting in more complete combustion. This also reduces the for-mation of CO, HC, and soot particles [35e38]. Pongamia pinnatamethyl ester (PPME) is one such biodiesel, which is produced fromthe oil of P. pinnata seed by transesterification process. It is a non-conventional and bio based alternative to Diesel. It has higher Ce-tane number than the biodiesels mentioned earlier [39], whichmakes it more suitable as a pilot fuel for CNG dual fuel operation.PPME contains 11% dissolved oxygen [39] in its chemical compo-sition, which enables better oxidation of the fuel and compensatesfor its relatively lower calorific value. This inbuilt oxygen is bene-ficial in dual-fuel combustion as it can partially compensate for theloss in intake air due to gaseous fuel injection. Due to its high ce-tane number, PPME significantly reduces the ignition delay [40]. Itis also instrumental in reducing soot and hydrocarbon emissions[39].

The present study aims to add in this field by introducing a highcetane pilot fuel (PPME) for CNG dual fuel operation. The study ofliterature shows that most of the work in DieseleCNG dual fueloperation is done with low concentration of CNG and there is a lackof study on the effect of high CNG injection into the engine. The

Table 2Properties of CNG.

Properties

Density (kg/m3) 0.72Flammability limits (volume % in air) 4.3e15Flammability limits (Ø) 0.4e1.6Auto ignition temperature in air (�C) 723Quenching distance (mm) 2.1Stoichiometric fuel/air mass ratio 0.069Stoichiometric volume fraction % 9.48Calorific value (kJ/kg) 45,765

A. Paul et al. / Energy 68 (2014) 495e509 497

present experimental work thus provides a comparative studybetween Diesel and PPME as pilot fuel for CNG dual fuel operation.

2. Pilot fuels and their properties

The main liquid fuels used in this experimental study are high-speed Diesel and P. pinnata methyl ester (PPME). The PPME is anontoxic, renewable and biodegradable source of fuel and it is amixture of mono-alkyl ester of different chain length and saturatedfatty acids [41,42]. The PPME (Commercially known as Biodiesel) isproduced by trans-esterification of Karanja (P. pinnata) seed oil andthe whole process is carried in the IC engine laboratory of NITAgartala. Trans-esterification is an equilibrium reaction, that re-quires three moles of alcohol equivalently and the reaction requiresexcess alcohol for completion. The product of this reaction is anester and the by-product is glycerin. The raw oil is collected fromKaranja seeds and the fatty acid in the P. pinnata oil is esterified byits reaction with Methyl Alcohol and Potassium Hydroxide. Thereaction is shown in Eq (1). The CNG used in this study is collectedfrom a local CNG outlet of TNGCL (Tripura Natural Gas CorporationLtd.). The properties of the Diesel and PPME are shown in Table 1.Table 2 shows the properties of CNG.

(1)

3. Instrumentation and methodology

3.1. Experimental apparatus

3.1.1. Experimental setupThe experiments are conducted on a 5.2 HP 4 stoke CI engine as

detailed in Table 3 and conforming to Indian standard IS 11170-1985. The engine is coupled to an eddy current dynamometer(Make:Saj test plant Pvt. Ltd, Model-AG10) for load measurement.The engine is also synchronized to a crank angle sensor (Make-Kubler-Germany, Model 8.3700.1321.0360) for measuring the en-gine rpm. The crank angle sensor is calibrated in terms of 1� in-terval. Two piezoelectric pressure transducers (Make KISTLER, Type6056A31U20) are used to sense the in-cylinder pressure and theinjection pressure of the liquid pilot fuel. All the instrumentsattached to the engine are interfaced to the computer through aLabView� (National Instrument) based centralized data acquisitionsystem (DAQ), synchronized to the engine rotation onto a GUIbased Post processing software ‘Engine Soft�’ (Developer: Apex In-novations Pvt. Ltd.). The DAQ is preprogrammed to acquire in cyl-inder and fuel pressure data at 1� crank angle. The data stream issmoothen over 100 consecutive cycles to compensate any cyclicvariation for a particular case of engine operation. The specific fuel

Table 1Properties of diesel and PPME.

Property Diesel PPME (biodiesel)

Density (kg/m3) 820 886Kinematic viscosity (cSt) 2.51 8.68Calorific value (kJ/kg) 42,650 35,866Flash point (�C) 52 217Fire point (�C) 64 220Cetane index 46 55.48

consumption of the liquid fuels is measured by utilizing a fuelburette of 12.4 mm diameter and the fuel consumption is measuredfor a time interval of 60 s. The airflow into the engine is measuredfrom sensedmanometric depression in the air box integrated to theairflow circuit. Special care is taken to keep the speed of the engineconstant (�10 rpm) during data acquisition for each case of engineoperation at different load and different CNG induction strategies.The whole experimentation is done at an ambient temperature of28e29 �C and at a relative humidity of 70%. The complete enginecircuit is shown in Fig. 1.

3.1.2. CNG injection circuit developmentConsidering the risks involved in working with gaseous fuels,

sufficient care is taken to incorporate safety measures in CNG in-duction circuit. This is done by incorporating a number of safetydevices, gas flow control and gas flow measurement devices into aspecially developed intake manifold. The CNG from the main cyl-inder is first routed to a secondary cylinder, which not only worksas a buffer tank, but also provides security against any flash back.The pressure of the CNG is reduced from 196 bar to 150 bar in thesecondary cylinder by means of a pressure reduction valve (Make-CONCOA, USA, model-405 2021). A pressure gauge (Make-OMEGA,Model-PGC-25L-600) is connected to the CNG flow line after thesecondary cylinder for gas pressure measurement. A pressureregulator/reducer (Make-CONCOA, USA, Model-3123322-01-B04)is connected after the pressure gauge to manually reduce the gaspressure to the required working pressure of 1.2 bar of a solenoidgas injector (Make-DYMCO Corp, Model-i1000). A gas flow meter(Make-CLESSE, Type-G1.6) is connected to the flow line after thepressure regulator/reducer to measure the CNG flow into the intakemanifold of the engine. The specifications of the solenoid injectorused are given in Table 4 [43]. The injector is mounted at a distanceof 1.5D (where D is the manifold outer diameter) from the engineintake manifold to ensure homogeneous mixture of inducted CNGto the incoming air. The controlling of the solenoid is done by acontrol panel, which is synchronized to the rotation of the enginevia a crank angle encoder to enable a precise induction of CNG atany desired crank angle and for any desired induction duration. Thecomplete CNG injection circuit has been shown in Fig. 2.

Table 3Specifications of the engine.

Engine type Kirloskar, model tv-1, 4 stroke water cooled, vcr engineBore and stroke 87.5 mm and 110 mmMax. power 5.2 kW (@1500 rpm)Cr range 17.5Swept volume 661 ccCombustion system Direct injectionFuel injection pressure 205 barDynamometer Eddy current type (Make: SAJ test plant pvt. Ltd)Crank angle sensor Model 8.3700.1321.0360, Make KUBLERPressure transducer Piezoelectric type (Make: KISTLER), Model-6056a31u20

Fig. 1. Schematic diagram of complete experimental circuit.

A. Paul et al. / Energy 68 (2014) 495e509498

3.1.3. Emission analysis instrumentsAn AVL Digas 444 5-gas analyzer and an AVL 437opacimeter are

used to analyze the exhaust of the engine. The 5-gas analyzer isused to measure the emissions of CO, CO2, and O2 in terms of vol-ume percentage. The emission of UBHC and NOx is measured interms of ppm (Vol.). The opacimeter is used to measure the per-centage opacity of the exhaust smoke. The analyzers are interfacedthrough their respective RS 232C serial communication bus to anemission data acquisition platform that recorded the emission datafor 120 s at an interval of 20 s. CO, CO2, and UBHC emissions aremeasured on the basis of Non-Dispersive-Infrared (NDIR) detectionprinciple, while NOx and O2 were measured by means of precali-brated electrochemical sensors.

3.2. CNG injection strategy development

The CNG injecting strategy followed in this experimental work isformulated to achieve a controlled stream of CNG into the intakemanifold. In order to study the effect of different amount of CNG onperformance and emission of the engine, it is required to increasethe induction of CNG into the cylinder gradually. In absence of a gasflow controller, this is done by injecting CNG for some calculatedinjection durations. These injection durations are formulated onthe basis of degree of crank angle rotation of the engine. The valve

Table 4Specifications of test engine.

Properties Specifications

Make DYMCO CorpModel GISM-i100Operating temperature range 40e120 �CApproval pressure 1.2 bar � 0.05 barOperating pressure 0.2e4.2 barMax pressure 4.5 barOpening time 3.0 msClosing time 1.0 msOperating flow rate (L/min, 1.2 bar) 44

timing diagram as shown in Fig. 3 shows that the intake valveopens 4.5� before TDC (top dead center) and closes 35.5� after BDC(bottom dead center), which produces a maximum injectionduration of 220� (4.5� þ 180� þ 35.5�). Again, the exhaust valveclosed at 4.5� after TDC. Hence a valve overlap of 9� (4.5�þ4.5�) isproduced, which further reduced the effective induction period to211� (220�e9�). This available induction period of 211� is dividedinto 5 parts. Hence, CNG is injected for a crank rotation of 42� forthe 1st strategy, 84� for the 2nd strategy, 126� for 3rd strategy, 168�

for 4th strategy and 210� for 5th strategy. As per the programmingof the CNG injector control system, the variable input parametersare degree of start of injection (in � of crank angle) and injectionduration in terms of ms. Hence, the CNG injection durations areconverted from degree of crank angle to ms by using Eq (2). Thewhole calculation is shown in Supplementary material 1. As per thecalculations, the CNG injection durations are shown in Table 5.

Injection Duration ðmSÞ ¼ 60� q� 106

N � 360(2)

where

q ¼ Degree of crank rotation for a Specific injection strategy:N ¼ rpm for the same strategy:

3.3. Experimental methodology

The engine is first tested with pure Diesel, which provides abaseline data set for comparing different DieseleCNG and PPMEeCNG combinations. The engine test starts at 20% load and subse-quently is taken to full load condition through load steps of 20%increment. Once all the baseline data are collected, CNG is inductedas per the 5 different strategies as discussed in Section 3.2 at eachload stepping. Once the engine is tested with Diesel as pilot fuelalong with all the CNG strategies for the whole load spectrum, thecomplete liquid fueling system is cleaned properly to avoid any

Fig. 2. CNG induction circuit.

A. Paul et al. / Energy 68 (2014) 495e509 499

contamination of the PPME. In the second stage of experimenta-tion, the whole process is repeated using PPME as the pilot fuelalong with 5 CNG strategies for all load conditions. Data collectionis initiated only when CNG flow to the intake manifold is founduniform and engine speed is not varying more than 10 rpm. Specialcare is taken to keep a constant water flow rate into the engine andcalorimeter. To avoid any cyclic variation that may occur duringdata acquisition, all the data from the engine is averaged for 80cycles. Again, each set of data is taken 6 times and averaged toincrease the authenticity of readings. The emission analyzers areintroduced on to the exhaust pipe after the engine has gained asteady state condition.

4. Uncertainty analysis

Measurement of a physical quantity cannot be entirely accurate.Errors and uncertainties in the experiments may occur due to se-lection of instrument, working condition, calibration, environment,observation and method of conducting the test [44,45]. The devi-ation of the true value from the measured value of the quantity canbe calculated by means of an uncertainty analysis. Hence, it isnecessary to study the degree of uncertainty of the measured datain order to authenticate the repeatability of the experimentation.The combined uncertainty analysis for the performance parametershas been carried out on the basis of the root mean square method,where the total uncertainty U of a quantity Q has been calculated byusing Eq. (3) [46], depending on the independent variables x1, x2,.,xn (i.e., Q ¼ f[x1, x2,., xn]) having individual errors Dx1, Dx2,., Dxn.

Table 5CNG injection strategies.

CNG injectionstrategy

Injection duration(� of crank angle)

Injectionduration (ms)

Strategy-1 42 4500Strategy-2 84 9000Strategy-3 126 13,500Strategy-4 168 18,500Strategy-5 210 23,000

The percentage of uncertainty of the performance parameters isshown in Table 6.

DU ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�vUvX1

DX1

�2

þ�vUvX2

DX2

�2

þ/þ�vUvXn

DXn

�2s

(3)

The accuracy of emission parameters is calculated on the basis ofthe average value of six consecutive observations over a samplingspan of 120 s. The level of accuracy of the measuring instruments,i.e. AVL DiGAS 444 five-gas analyzer and AVL 437 Smoke meter is

Fig. 3. Valve timing diagram of the engine.

Table 6Total percentage of uncertainty of computed performance parameters.

Computed performance parameter Measured variables Instrument involved inmeasurement

% Uncertainty ofmeasuringinstrument [46]

Calculation Total % uncertaintyof computedparameters

BP (brake power) Load, RPM Load sensor 0.2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið0:2Þ2 þ ð0:1Þ2 þ ð0:1Þ2

q1.02

Load indicator 0.1Speed measuring unit 1.0

BSFC (brake specific fuel consumption) SFC (liquid fuel) Fuel measuring unit 0.065ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið0:065Þ2 þ ð1:5Þ2 þ ð1:02Þ2

q1.81

Fuel flow transmitter 1.5BP As for BP measurement 1.02

BSFCEquivalent (diesel equivalentbrake specific fuel consumption)

SFC (liquid fuel) As for SFC measurement 1.81ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1:81ð Þ2 þ 1:02ð Þ2 þ 0:25ð Þ2

q2.09

BP As for BP measurement 1.02CNG flow CNG mass flow meter 0.25

A. Paul et al. / Energy 68 (2014) 495e509500

given in Table 7 [46]. The total sampling uncertainty (TSU) of eachobservation set is computed as per Eq. (3) for each of the pollutant.In order to increase the trustworthiness of the error analysis, anadditional index of standard deviation has been computed for eachengine operating condition. The total sampling uncertainty (TSU)and the standard deviation of the emission parameters are shownin Table 8.

5. Result and discussion

5.1. Performance study

In this study, brake thermal efficiency, BSFC (brake specific fuelconsumption) Diesel equivalent and CNG energy share have beenanalyzed for determining the influence of DieseleCNG and PPMEeCNG combinations on the performance of the engine.

5.1.1. Brake thermal efficiencyBrake thermal efficiency (hbth) defines the combustion quality of

the engine. Fig. 4 shows the variation of hbth for different DieseleCNG and PPMEeCNG dual fuel combinations at the tested loadconditions and Fig. 5 shows the percentage change in hbth fordifferent DieseleCNG and PPMEeCNG combinations in comparisonwith plain Diesel operation. It can be seen from the figures thatCNGeDiesel dual fuel operation reduces the brake thermal effi-ciency of the engine. PPMEeCNG operation with CNG injection formore than 18,500 ms is also found to reduce the hbth of the engine.This reduction in hbth is due to the deficiency of oxygen caused byindirect injection of CNG via intake manifold. Deficiency of oxygencauses incomplete combustion and subsequent decrease in con-verting the input fuel energy. Similar decrease in hbth is alsoobserved by Maji et al. [47]. It can also be seen that, low amount of

Table 7Accuracy of emission measuring instruments.

Instrument Measuring range Accuracy

AVL DiGAS 444 five gas analyzerCarbon monoxide (CO) 0e10% vol <0.6% vol: �0.03% vol >0.6%

vol: �5%Carbon dioxide (CO2) 0e20% vol <10% vol: �0.5% vol >10%

vol: �5% volHydrocarbon (HC) 0e20,000 ppm vol <200 ppm vol: �10 ppm

vol >200 ppm vol: �5%NOx 0e5000 ppm vol <500 ppm vol: �50 ppm

vol �500 ppm vol: �10%Oxygen (O2) 0e22% vol <2% vol: �0.1% vol �2%

vol: �5% vol

AVL 437 smoke meterSmoke opacity 0e100% 1%

CNG (CNG strategy 1, 2 and 3) with pilot PPME, increases the hbth ofthe engine at almost all load conditions. This is due to the oxygenrich content of PPME that supplies oxygen upon thermal decom-position and helps in better combustion. However, this inbuilt ox-ygen of PPME is not sufficient to compensate the lack of intake airduring higher CNG injection strategies. As a result, the hbth reduceswith higher amount of CNG injections with pilot PPME.

5.1.2. BSFC diesel equivalentThe BSFC diesel equivalent is a comparative parameter that

compares the quantity of a certain fuel sample with respect toDiesel for producing the same amount of power. Therefore, it bringsdifferent types of fuels to a common platform where they can beproperly compared according to their consumption rate. Fig. 6shows the variation of BSFC Diesel equivalent of the engine fordifferent DieseleCNG and PPMEeCNG combinations and Fig. 7shows the percentage of increase or decrease in BSFC Dieselequivalents with respect to plain Diesel operations. It is visible fromthe figures that, CNG reduces the consumption of liquid fuel forboth Diesel and PPME operations. This is due to the fact that theCNG has a comparatively higher calorific value than Diesel andPPME. As a result, for producing same amount energy less amountof pilot fuel is required when CNG is acting as the primary fuel.Hence, CNG provides the main stream of inlet energy for DieseleCNG and PPMEeCNG dual fuel operation. It can also be noticed thatthe fuel consumption is significantly reduced in case of PPMEeCNGcombinations. This is an indication that PPMEeCNG dual fueloperation is more fruitful than DieseleCNG operation.

5.1.3. CNG energy shareIn this present study, the CNG induction strategies are designed

in such a way that they provide a window to simultaneously in-crease the amount of CNG inducted into the engine. Again, sinceCNG is the primary fuel hence, bulk of the energy is supposed to besupplied by the CNG itself. Fig. 8 shows the amount of energyprovided by CNG for both Diesel and PPME pilot injections. It can beseen from the figure that increasing CNG content systematicallyshares the major portion of the input energy for both Diesel as wellas PPME pilot injections. However, it can also be seen that CNG

Table 8Average % TSU and std. deviation in observation sampling of all measured emissions.

Sampled emission Average TSU (%) Average std. deviation

CO 0.3707 0.000182HC 1.3384 0.389761CO2 0.5925 0.041385NOx 0.8193 0.487766Opacity 0.3088 0.004533

Fig. 4. Variation in hbth with load for different DieseleCNG and PPMEeCNG combination.

Fig. 5. Percentage change in hbth with load for different DieseleCNG and PPMEeCNG combination with respect to diesel.

Fig. 6. Variation in BSFC diesel equivalent with load for different DieseleCNG and PPMEeCNG combination.

A. Paul et al. / Energy 68 (2014) 495e509 501

Fig. 7. Percentage change in BSFC diesel equivalent with load for different DieseleCNG and PPMEeCNG combination with respect to diesel.

Fig. 8. Variation in CNG energy share with load for different DieseleCNG and PPMEeCNG combination.

Fig. 9. Variation in CO emission with load for different DieseleCNG and PPMEeCNG combination.

A. Paul et al. / Energy 68 (2014) 495e509502

A. Paul et al. / Energy 68 (2014) 495e509 503

shares more energy with pilot PPME operation than pilot Dieseloperation. For any given CNG strategy, pilot injection of PPMEproduces higher CNG energy share than Diesel. This is a clearindication that dual fuel operation with CNG is more fruitful withpilot PPME.

5.2. Emission study

In this study, emission of CO, NOx, Unburned hydrocarbon andsmoke opacity has been analyzed to assess the emission signatureof the tested fuel combinations.

5.2.1. CO emissionCarbon monoxide (CO) is formed in fuel rich mixture in the

flammability region, where the fueleair equivalence ratio is higherthan the rich flammability limit of the charge. As a result, this re-gion does not contain sufficient amount of oxygen that leads toincomplete combustion of the fuel. Hence, the quantity of COemission is an indicator of the quality of combustion taking placeinside the cylinder. Here, Fig. 9 shows the CO emission from theengine for the tested fuel for the defined load conditions and Fig. 10shows the percentage of increase or decrease in CO emission for thetested fuel combination with respect to plain Diesel operation. TheCO emission for the DieseleCNG and PPMEeCNG dual fuel opera-tions is found to decrease with increasing load. This is because, theeffective pressure inside the cylinder gradually increases withincreasing load. This causes an increase in cylinder temperature.The increased cylinder temperature aids in further oxidation ofcarbon and carbon monoxides to produce carbon dioxide, thusreducing CO formation in the process. Again, it can be seen thatPPMEeCNG operations produces lower CO emissions than theirDieseleCNG counterparts did. This can be attributed to the higheroxygen content of the PPME that promotes better combustion [60].It is also observed that CO emissions increase with increasing CNGcontent irrespective of the pilot fuel. This is due to replacement ofintake air by CNG, which reduces the possibility of completeoxidation of carbon molecules and produces Carbon monoxide.Yusaf et al. [48] also observed similar CO emission for DieseleCNGcombinations.

5.2.2. NOx emissionThe main cause for the increase of NOx emission is high com-

bustion temperature and equivalent ratio [49,50]. The NOx emis-sion from the engine for DieseleCNG and PPMEeCNG

Fig. 10. Percentage change in CO emission with load for different D

combinations is shown in Fig. 11. Fig. 12 shows the percentage ofincrease or decrease of NOx emission for the tested fuel combina-tions with respect to Diesel. It can be seen from the graphs that, theplain PPME operation produces highest NOx emission. The inherentoxygen content of PPME enables a more complete combustion ofthe charge and thus produces an increased in-cylinder tempera-ture. This elevated in cylinder temperature increases thermal NOxformation [59,60]. In addition, due to the higher cetane number ofPPME, the ignition delay becomes shorter, which consequentlycauses a faster combustion of air/fuel mixture, generating a fasterheat release at the beginning of the combustion process. This alsocauses rapid temperature rise, which helps in the formation of NOx[51]. It can also be seen that CNG reduces NOx formation, especiallywith Diesel as pilot fuel. This decrease in NOx emission is primarilydue to cooling effect caused by injection of cold CNG into the cyl-inder. This leads to decrease in peak temperature and hencedecrease in NOx emission [47]. Similar decrease in NOx emission isalso seen with cooling effect of ethanol [56]. Further, Diesel has alower cetane number than PPME and it causes longer ignition delayfor Diesel. This reduces the combustion rate, generating less incylinder temperature and subsequently lower NOx emission.Moreover, injection of cold CNG into the air produces a cooling ofthe intake air and subsequent cooling of the combustion chamber.This also reduces in NOx emissions [52]. In case of PPMEeCNG dualfuel operations, the oxygen released by PPME partially fills up thedeficiency of oxygen in intake air and helps in better combustion ofcharge. As a result, higher in-cylinder temperature is produced andconsequently, NOx emission increases. However, with higher CNGinjection strategies, the oxygen released from PPME cannotcompensate the lack of intake air and causes incomplete combus-tion and lower in cylinder temperature. As a result, with increasingCNG strategies the emission of NOx decreases with PPMEeCNGcombination.

5.2.3. Unburned hydrocarbon emissionUnburned hydrocarbon emission is the result of incomplete

combustion. The hydrocarbon emission from the tested fuel com-binations is shown in Fig. 13. Fig. 14 shows the percentage of in-crease or decrease of unburned hydrocarbon emissions for thetested fuel combinations in comparison to base Diesel operation. Itcan be seen from the figures that hydrocarbon emission increaseswith increasing amount of CNG content inducted into the enginefor both Diesel and PPME pilot operations. This trend is veryprominently visible with pilot Diesel operations. The increase in

ieseleCNG and PPMEeCNG combination with respect to diesel.

Fig. 11. Variation in NOx emission with load for different DieseleCNG and PPMEeCNG combination.

Fig. 12. Percentage change in NOx emission with load for different DieseleCNG and PPMEeCNG combination with respect to diesel.

Fig. 13. Variation in UBHC emission with load for different DieseleCNG and PPMEeCNG combination.

A. Paul et al. / Energy 68 (2014) 495e509504

hydrocarbon emissionwith increasing CNG content is an indicationthat the injection strategies defined in Section 3.2 might beinjecting excess amount of CNG into the engine. With higher in-jection durations, major portion of this CNG is exhausted without

significantly burning inside the combustion chamber [57]. How-ever, plain PPME and PPMEeCNG operation with low CNG con-centration showed appreciable decrease in unburned hydrocarbonemission. PPME has higher cetane number than Diesel fuel, which

Fig. 14. Percentage change in UBHC emission with load for different DieseleCNG and PPMEeCNG combination with respect to diesel.

A. Paul et al. / Energy 68 (2014) 495e509 505

facilitates an easier ignition and more complete combustion. Thiscauses less unburned hydrocarbon emission [51]. Again, it can beseen that unburned hydrocarbon emission reduces with increasingload for all fuel combinations. With increasing the load, the meaneffective pressure rises, producing a higher exhaust temperaturewhich causes the reduction of unburned hydrocarbon [47].

5.2.4. Smoke opacityOpacity of the emitted smoke primarily depends on the amount

of soot particles present in it. Higher the amount of soot, higher willbe the opacity of the gas. Fig. 15 shows the opacity of the exhaustgas for different fuel combination and Fig. 16 shows the percentageof increase or decrease of smoke opacity for different fuel combi-nations when compared with Diesel operation. It can be seen fromthe figures that, plain PPME operation produces the lowest smokeopacity among all the fuel combinations tested here. Similardecrease is also observed by Saravanan et al. [58]. It can also be seenthat, CNG reduces smoke opacity for both the pilot Diesel and pilotPPME operations. Since the H/C ratio in CNG is higher, so it containssmaller hydrocarbon structure as compared to Diesel. Hence, sootformation is less with CNG, which leads to a reduction in smokeopacity [47]. For PPMEeCNG combination, the reduction of smokeopacity is even higher, especially with CNG strategy-1. The lowestsmoke opacity of all the fuel combinations is observed from CNGstrategy-2 with PPME pilot operation. This combination produces

Fig. 15. Variation in smoke opacity with load for diff

an average reduction of 71.63%. The smoke generation takes placein fuel-rich areas of the combustion chamber, especially in the fuel-spray core (liquid phase) of the pulverized jet. Since CNG is portinjected, it gets sufficient time to produce a homogenous mixtureinside the cylinder that produces better combustion. Thus, it re-duces the formation of fuel rich zones and reduces soot formation.This causes lower smoke opacity. Again, the oxygen content of thePPME ensures sufficient oxidant for pyrolysis of carbon particles.This causes a decrease in formation of solid particles [53] resultingin lower smoke opacity.

5.3. Tradeoff study

NOx and soot are considered to be most detrimental emissionfrom an automobile engine as the former is responsible for severalrespiratory disorders with the round level smog formation [54,55]and the later one is a key constitute to global warming. Again, dueto depleting fuel reserves, fuel consumption of the engine is alsodesired to be kept low. Therefore, a tradeoff study, which combinesNOx, smoke opacity and fuel consumption, provides a platform forsimultaneously comparing the major performance and emissionparameters and provides a scope of optimal study. Figs. 17e21show the tradeoff analysis of all the fuel combinations studied inthis work on the basis of NOx emission, smoke opacity and BSFCDiesel equivalent at the tested load conditions.

erent DieseleCNG and PPMEeCNG combination.

Fig. 16. Percentage change in smoke opacity with load for different DieseleCNG and PPMEeCNG combination with respect to diesel.

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Observing the tradeoff graph at 20% load condition in Fig. 17 fordifferent DieseleCNG and PPMEeCNG combinations, it can be seenthat, CNG significantly reduces the equivalent BSFC for bothDieseleCNG and PPMEeCNG combinations, as it draws the tradeofffrom the highest fuel consumption zone A to the lowest fuel con-sumption zone B. It can also be seen that DieseleCNG operationreduces the NOx emission with increase in smoke opacity. CNGstrategy 4 with Diesel as pilot fuel (shown as 20%DCNG-4) shows54.55% reduction in NOx emission, 28.38% reduction in smokeopacity with 67.78% lower equivalent BSFC among all fuel combi-nations at 20% load condition. Again it is also evident from thegraph that PPME pushes the tradeoff to a high NOx emission zone(zone C) with a visible reduction in equivalent BSFC and smokeemission. PPMEeCNG operation not only reduces the NOx emissionbut also the equivalent BSFC. However, PPMEeCNG with higherCNG strategies shows a marginal increase in smoke opacity (zonesCeE) as compared to low CNG strategies with same pilot fuel (zonesC and D).

Fig. 18 shows the tradeoff at 40% load condition. It can be seenfrom the graph that CNG with Diesel as pilot fuel, pulls the tradeoffzone closer to the origin with significant reduction in equivalentBSFC and smoke opacity (From zone F to G). However, a marginalincrease in NOx emission with CNG strategy-5 (shown as 40%DCNG-5) is also witnessed. PPME on the other hand, pushes the

Fig. 17. Tradeoff between NOx emissioneSmoke opacity and BSFC diesel equivalent at20% load condition.

tradeoff zone far away from the origin to a zone of very high NOx

emission and high equivalent BSFC with reduced smoke opacity(Zone-H). At this point, there is an increase of 380.95% in NOxemission and 10.20% in equivalent BSFC with 69.78% decrease insmoke opacity. Again, it can be seen that PPMEeCNG dual fueloperation reduces the NOx emission and equivalent BSFC andbrings down the tradeoff zone from H to I. However, the NOxemission at these CNG strategies is still higher than Diesel andDieseleCNG combinations. At 40% load condition, the PPMEeCNGcombination is found to be beneficial in terms of lower equivalentBSFC with almost similar smoke opacity signatures with respect tothe plain Diesel operation.

The tradeoff between NOx emission, Smoke opacity and equiv-alent BSFC at 60% load for the tested fuel combinations is shown inFig. 19. As it can be seen from the graph that CNGeDiesel dual-fuelstrategies reduces the equivalent BSFC and pulls the tradeoff zonetoward origin. The DieseleCNG strategies also produces small in-crease in NOx emission with marginal decrease in smoke opacity.The lowest NOx emission (with 35.59% decrease) and smokeopacity (with 10.34% decrease) for DieseleCNG operation is seenwith CNG strategy-1 (shown as 60% DCNG-1). PPME however, asseen in previous load conditions, pushes the tradeoff zone to a veryhigh NOx emission and high equivalent BSFC region (zone K). It isalso observed that PPMEeCNG dual fuel strategies again

Fig. 18. Tradeoff between NOx emissioneSmoke opacity and BSFC diesel equivalent at40% load condition.

Fig. 19. Tradeoff between NOx emissioneSmoke opacity and BSFC diesel equivalent at60% load condition.

Fig. 21. Tradeoff between NOx emissioneSmoke opacity and BSFC diesel equivalent atfull load condition.

A. Paul et al. / Energy 68 (2014) 495e509 507

bring down the tradeoff zone to a much lower NOx emission andlower equivalent BSFC signatures (zone L). Low CNG strategies viz.CNG strategy 1 and 2 are not very instrumental in reducing NOx andequivalent BSFC. But the higher CNG strategies viz. CNG strategy-3,4 and 5 significantly reduces NOx emission and equivalent BSFCwith minor increase in smoke opacity (as shown by zone L).

It can be seen in Fig. 20 that, at 80% load condition, CNG withpilot Diesel operation, significantly reduces the NOx emission andequivalent BSFC and brings down the tradeoff zone from M to N.DieseleCNG operations with low amount of CNG (CNG strategy-1and 2) are also found to reduce the smoke opacity and increaseequivalent BSFC. High CNG strategies (CNG strategy-3, 4 and 5)shows marginally higher smoke opacity with significant reductionin equivalent BSFC. Among all DieseleCNG combinations, CNGstrategy-5 shows one of the better tradeoff potential with 88.71%reduction in NOx emission and 58.13% reduction in equivalent BSFC.However, an increase of 27.81% in smoke opacity is also observedwith the said fuel combination. It can also be seen that the plainPPME operation pushes the tradeoff zone to very high NOx andequivalent BSFC dominated area (zone O). At this point, an increaseof 369% in NOx emission can be detected when compared to Diesel.

Fig. 20. Tradeoff between NOx emissioneSmoke opacity and BSFC diesel equivalent at80% load condition.

PPMEeCNG dual fuel operations, especially high CNG strategies arefound to pull down the tradeoff zone to a much lower NOx andlower equivalent BSFC signatures. High CNG concentration withPPME pilot operation (CNG strategy-5) is found to reduce equiva-lent BSFC and NOx emission but with some increase in smokeopacity.

Fig. 21 shows the tradeoff graph at full load condition. Observingthe graph it is again evident that DieseleCNG dual fuel strategiesare definitely beneficial in reducing equivalent BSFC. It can be seenthat there is a proportionate increase in smoke opacity withincreasing CNG content. Similar to all previous load conditions,PPME here also shows very high equivalent BSFC and NOx emission(zone S). But PPMEeCNG operation, especially with higher CNGstrategies, again produces a proportionate reduction in equivalentBSFC and NOx emission. However, the smoke opacity increases withit (zone T). The emission signatures of PPME with low CNG in-jections are found to be very high as compared to plain Dieseloperation.

6. Conclusion

The study conducted here provides a systematic comparisonbetween DieseleCNG and PPMEeCNG dual fuel operations. Forconducting such experimental work, the test engine is marginallymodified by attaching a CNG injection system at the intake mani-fold. The CNG quantity is gradually increased to find the optimumquantity that would give better performance and emission char-acteristics. Further, two liquid fuels, Diesel and PPME are tested aspilot fuel to find the effect of pilot fuel variation on the performanceand emission of the engine using CNG as the primary fuel. Thestudy also draws a comparison between plain Diesel and plainPPME operation. The vital findings of this study are summarized asfollows,

1. DieseleCNG strategies reduce the brake thermal efficiency ofthe engine. Increase in CNG causes further reduction in hbth ofthe engine. Increasing CNG quantity also produces a simulta-neous decrease in BSFC Diesel equivalent, as major portion ofthe intake energy is provided by CNG itself. Along with that,CNG also shows a commendable decrease in emissions of NOxand smoke opacity. However, there is an increase in CO andunburned hydrocarbon emission.

A. Paul et al. / Energy 68 (2014) 495e509508

2. PPME shows an increase in brake thermal efficiency with almostsame fuel consumption as compared to plain Diesel operation.CNG is found to respond far better in case of pilot PPME run andproduced higher energy share than Diesel at the same condi-tions. It also produces significant reductions in smoke opacity,CO emission and unburned hydrocarbon emission. However,due to better quality of combustion, PPME produces very highNOx emission.

3. PPMEeCNG operations produce the best results among all thetested fuel combinations, both in terms of performance andemission criterion. It is observed that low amount of CNG withpilot PPME not only improves the brake thermal efficiency of theengine, but also produces commendable decrease in fuel con-sumption. In terms of emission reduction, increasing CNG con-tent produces a consistent decrease in CO, NOx, and HC emissionwith noticeably clearer smoke emission.

4. The tradeoff study conducted in the course of this experimentalwork effectively shows the potential of CNG in reducing the fuelconsumption and NOx emissionwith acceptable values of smokeopacity. It also confirms the fact that PPMEeCNG dual fueloperation is a better alternative than DieseleCNG operation, asit improves the performance of the engine with reduction insignificant number of emission constituents.

This experimental work thus reveals the potential of CNG withpilot PPME operation in meeting the paradoxical objective of op-timum performanceeemission criteria without any major modifi-cation of modern engine setup. Further the study also shows thatCNG injected at 10� ATDC (after top dead center) for a duration ofabout 4500 ms with PPME as pilot fuel can produce better perfor-mance and emission signatures than DieseleCNG dual fueloperation.

Appendix A. Supplementary material

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.energy.2014.03.026.

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