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Energy 58 (2013) 185e191

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Energy

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GTL (Gas To Liquid) and RME (Rapeseed Methyl Ester) combustionanalysis in a transparent CI (compression ignition) engine by meansof IR (infrared) digital imaging

Ezio Mancaruso*, Luigi Sequino, Bianca Maria VagliecoIstituto Motori, CNR, Via G. Marconi, 4, 80125 Naples, Italy

a r t i c l e i n f o

Article history:Received 3 August 2012Received in revised form21 December 2012Accepted 24 December 2012Available online 16 January 2013

Keywords:BiofuelsCombustion analysisInfrared imagingTransparent engine

* Corresponding author. Tel.: þ39 081 7177187; faxE-mail address: e.mancaruso@im.cnr.it (E. Mancar

0360-5442/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.energy.2012.12.029

a b s t r a c t

In the present paper, (infrared) IR measurements were performed in order to study the behaviour ofbiofuels combustion in a transparent Euro 5 diesel engine operating in premixed mode. (Commercialdiesel fuel) REF, (Gas To Liquid) GTL and (Rapeseed Methyl Ester) RME biofuels have been used. Anelongated single-cylinder transparent engine equipped with the multi-cylinder head of commercialpassenger car and (common rail) CR injection systemwas used. A sapphire windowwas set in the bottomof the combustion chamber, and a sapphire ring was placed in the upper part of the cylinder. Mea-surements were carried out through both accesses by means of high-speed infrared digital imagingsystem. IR camera was able to detect the emitted light in the wavelength range 1.5e5 mm. Infraredimaging allowed acquiring larger amount of information than UV (ultraviolet) and visible cameras. Inparticular the IR camera was used for the characterization of injection and combustion process. Analysingthe IR images, it was possible to identify clearly the seven jets of vaporized fuel that react with air in thebowl. During the late combustion phase, the IR image showed a good capability to follow the hot burnedgas both in the bowl and above the piston. The IR camera has shown high sensibility permitting to followcarefully the soot oxidation process within the cylinder. The GTL shows an advance of about 8� crankangles in the evolution of combustion process with respect to the RME. On the contrary a longer chemicalactivity has been detected for the latter biofuel. Finally, the IR camera was revealed very useful tool tocharacterize the combustion process for long time allowing high quality of the results. Images of thereactions that happen in the combustion chamber and above the piston head were clearly acquired evenif the optical windows were obscured by the soot produced from the previous combustion cycles.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, one of the possible solutions to make cleaner andmore efficient the (internal combustion engine) ICE seems to be theuse of biofuels. The fast reduction of fossil fuel resources and theircontribution to environmental pollution from ICE, and theincreasing request for efficient and eco-friendly energy manage-ment have led to an increase in interest among researchers onstudy combustion characteristics of alternative fuels. Their blendsin a certain percentage can be used without modification of enginestructure. In particular, great attention is paid to the 1st and 2ndgeneration of biodiesel. The former is obtained from vegetable re-sources; it is commonly referred to as FAME (fatty-acid methylesters). Its performance is quite similar to those of diesel, in

: þ39 081 2396097.uso).

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particular, its main characteristic is the higher content of O2 withrespect to conventional fuels. On the other hand, moralesocialdebates are in place because its derivation from edible oil and in-terferences with the human food chain [1]. The 2nd generation ofbiodiesel, is produced by the FischereTropsch synthesis process,able to produce liquid fuels from the so-called syngas. It is usuallyindicated as xTL, where ’x’ denotes the specific source feedstockand TL (to Liquid) the conversion to liquid state. The input feedstockcan be either renewable Biomass (hence BTL) or fossil fuels, asnatural Gas (GTL) or Coal (CTL). Furthermore, the chemical origin ofthe xTL fuels provides them better combustion characteristic asattitude to autoignition and stability in the chemical compositionthan FAME, which is essentially driven by the synthesis processitself and not by the baseline feedstock [2,3].

Moreover, in the last decades, the development of high perfor-mance devices and their application in the research fields has pro-vided new techniques suitable for the monitoring of naturalphenomena. In the motorist area, the growing attention on these

Abbreviations

ATDC after top dead centreBTDC before top dead centreca crank anglesCR common railCV coefficient of variationECU electronic control unitEGR exhaust gas recirculationET energizing timingGTL Gas To LiquidIR infraredREF reference diesel fuelRME Rapeseed Methyl EsterROHR rate of heat releaseSOC start of combustionTDC top dead centreUV ultravioletVSA variable swirl actuator

Table 1Engine and injection system specifications.

Engine type 4-Stroke diesel single-cylinderBore 85 mmStroke 92 mmSwept volume 522 cm3

Combustion bowl 19.7 cm3

Vol. compression ratio 16.5:1Injection system Common railInjector type Solenoid drivenNumbers of holes 7Cone angle of fuel jet axis 148�

Hole diameter 0.141 mmRated flow at 100 bar 440 cm/30 s

E. Mancaruso et al. / Energy 58 (2013) 185e191186

methods is motivated by the need to achieve a more precisedescription of the processes occurring in the combustion chamber,in order to implement new optimized control methods assuringmore efficient and clean combustion systems. Optical diagnosticstrongly benefits from technological innovation; the microscopicand macroscopic analysis of the in-cylinder processes gives thepossibility to collect significant information. In particular, the op-portunity to inspect the phenomena in the (infrared) IR rangemakesit possible to investigate an area, outside the visible spectrum,where a lot of reactions take place. Each body with a temperaturehigher than 0 K emits energy, as an electromagnetic radiation, in thewhole spectral range fromtheultraviolet (UV) up to the infrared (IR).The visible range goes from 380 to 750 nm, so human eyes can’tdetect energy emitted at higher or lower wavelength. Infraredcameras candetect radiationwith awavelength longer than750nm,the infrared range goes from 750 nm to 1000 mmand it is divided innear-infrared (0.78e3mm),mid-infrared (3e50mm)and far-infrared(50e1000 mm) [4]. The use of infrared cameras in a diesel enginewith the aim to gather information about its functioning has manybenefits; themain challenge in this field is the definition of themostrepresentative flame signals and to derive the meaningful infor-mation required to diagnose the state of a flame. Fuel vapour is noteasily observed in the visible wavelength range but is well resolvedin the infrared region [5]. In the IR range it is also possible to capturethe radiation emitted by species of low-temperature reactions priorto running into rapid heat-releasing reactions [6]. Parker et al.monitored soot formation in the near-infrared for a diesel spray,observing that 9.4 mm was an appropriate wavelength for quanti-tative measurements of soot mass in the spray [7]. Moreover, fil-tering images from combustion chamber in the IR range alloweliminating the effects due to other substances; it is so possible tostudy better the stability of combustion [8]. Finally, more informa-tion about the combustion energy released is obtained for longertime than UV and visible imaging techniques. However, it isimportant to consider some limitations and shortfalls of currentinfrared technologies. In fact, for phenomenon as rapid as com-bustion process, only little image resolution is available for the highacquisition frequency needed. Moreover, some hot gases, such asoxygen and nitrogen, are mostly transparent in the infrared wave-lengths due to their low emissivity. So the temperature measure-ments will consider the radiation transmitted through these gasesrather than the direct radiation emitted by flames, causing diffi-culties in determination of temperatures [5e8].

This paper deals with the analysis of combustion process ina transparent Euro 5 diesel engine operating in premixed mode.The investigation of the phenomena occurring in the combustionchamber is made through IR digital imaging. A single-cylinderengine equipped with the head of a Euro 5 production engine hasbeen used. A multi injection strategy, consisting of a pilot anda main injection, has been performed with last generation highpressure (common rail) CR injection system. IR images have beenacquired from two different views: one from the bottom of thecylinder and the other from the side; image luminosity has beencomputed by using image processing techniques. The aim is toexplore the reactions that are not detectable using a visible de-tector. In particular, the engine, running at 1500 rpm, has been fedwith three different fuels: (commercial diesel fuel) REF, (Gas ToLiquid) GTL and (Rapeseed Methyl Ester) RME biofuels, in order toinvestigate how fuel properties influence combustion reactions.

2. Experimental apparatus and engine operating condition

A (single-cylinder) SC optical engine equipped with the com-bustion system architecture and injection system of a four-cylinder,16 valves, 1.9 L, Euro 5 engine has been used. Details and specifi-cations of the engine and the injection system are reported inTable 1. The elongated single-cylinder transparent engine had thestroke and bore of 92 mm and 85 mm, respectively, and the com-pression ratio is 16.5:1. The engine was equipped with a (commonrail) CR injection system managed by a fully opened (electroniccontrol unit) ECU. Bosch second-generation CR system injects fuelthrough a CRI2.2 injector, minisac type, with 7-hole nozzle, holediameter 0.141 mm.

An external air compressor was used to supply pressurizedintake air in order to obtain the same in-cylinder conditions of thereal multi-cylinder engine. The intake air, before reaching theintake manifold, was filtered, dehumidified, and preheated. More-over, a (variable swirl actuator) VSA systemwas employed in orderto manage the air swirl motion in the intake manifold.

Finally, the presence of a pressure valves in the exhaust pipepermitted the recirculation of the right amount of burned gasesthrough the cooled (exhaust gas recirculation) system EGR. A Hall-effect sensor was applied to the injector current line in order todetect the drive injector signal. Moreover, the in-cylinder pressure,in motored and fired conditions, was monitored by a piezoelectricpressure transducer set in the glow plug seat of the engine head.The in-cylinder pressure and the drive injector current were dig-italized and recorded at 0.2� crank angles (ca) increments andensemble-averaged over 150 consecutive combustion cycles.

(Commercial diesel engine) REF, first-generation biofuel(Rapeseed Methyl Ester) RME and second-generation biofuel (GasTo Liquid) GTL have been used. RME is a biofuel from vegetablesources obtained from seeds of rape. In Table 2 their propertieshave been briefly summarized.

Table 2Fuel properties.

Density @ 15 �C[kg/m3]

Viscosity @ 40 �C[mm2/s]

Cetanenumber

Lower heatingvalue [MJ/kg]

REF 840 3.14 51.8 43.11GTL 777 2.56 73.9 43.53RME 883 3.26 52.3 37.35

Fig. 1. Optical setup.

E. Mancaruso et al. / Energy 58 (2013) 185e191 187

The engine operating condition analysed is representative of the(new European driving cycle) NEDC. It corresponded to enginespeed of 1500 rpm, and low load of 2 bar of (break mean effectivepressure) BMEP, with (exhaust gas recirculation) EGR of 57%. Thehigh EGR level allows realizing a strong premixed combustion. Bothinjection and engine parameters for all tested fuels have beenreported in Table 3. The injection strategy consisted of two in-jections per cycle, pilot and main, performed with injection pres-sure of 615 bar. It can be noted that the (Energizing Time) ET of themain injection is longer for RME fuel. It in fact has a lower heatingvalue smaller than other fuels tested.

Fig. 1 shows the engine lay-out and optical apparatus. The op-tical engine utilizes a conventionally extended piston with a pistoncrown sapphire window. In order to provide a full view of thecombustion bowl a flat window was fitted in the piston head anda fixed 45� visibleeIR mirror was set inside the extended piston.Moreover, a sapphire ring was placed on the top of the cylinder; itprovided a view of the in-cylinder volume above the piston headeven if it is influenced by the piston movement. IR imaging wasperformed using a fast camera (320 � 256 pixels) able to detectlight in the range 1.5e5 mm. The IR camera had a sensor made of(Indium Antimonide) InSb. It was equipped with a 70 mm objec-tive, F/1:2.3. The resolution of camera was 2 pixels per mm at2.25 kHz and 9 pixels per mm at 650 Hz. IR images were acquired at4� ca step in the same engine cycle. The high sensitivity IR cameradid not require a light source for the spray imaging. Images wereacquired with an exposure time of 111 ms, corresponding to 1� ca at1500 rpm. Finally, the IR camera can move on a rail in order toacquire separately the images from the bottom of the bowl or fromthe side of the cylinder. The synchronization of the camerawith theengine was made by a delay unit connected to the engine shaftencoder.

3. Results and discussion

The engine operating condition reported in Table 3 for severalpure fuels was widely investigated in previous paper by means ofdigital imaging in the visible and UV wavelength range [9]. The twoinjections performed (pilot and main) were well discernible on thedrive injector current signals. Moreover, the in-cylinder pressuregave macroscopic information on the combustion evolution of thealternative fuels with respect to the REF. In particular, the (start ofcombustion) SOC was identified analysing the rate of heat releasetrace and it corresponded to the point where the energy releasedbegins to exceed the energy lost due to the fuel evaporating pro-cess. The (rate of heat release) ROHR was computed from theensemble-averaged pressure using the first law of thermodynamicsand the perfect gas model [10]. At the start of main combustion,

Table 3Injection strategies.

Fuel Rpm SOI pilot[� ca]

ET pilot[ms]

SOI main[� ca]

ET main[ms]

Prail[bar]

EGR[%]

VSA[%]

REF 1500 �16 290 �6 545 615 46 66GTL 1500 �16 290 �6 545 615 46 66RME 1500 �16 290 �6 587 615 46 66

a fast rate due to the exothermic reactions of combustion wasobserved. Two well resolvable peaks were discernible on the ROHRcurve for REF fuel. SOC of pilot andmain injections occurred at 8� cabefore top dead centre (BTDC) and 1� ca BTDC, respectively [9].Moreover, a comparison between all tested fuels was made withrespect to the curve of in-cylinder pressure, the ROHR and thecurrent signals. It was noted that the in-cylinder combustion peakpressure was around 50 bar for all the fuels; in particular, GTLreached the highest value (51 bar) and in advance with respect tothe REF and RME. This is due to its high cetane number that allowsfaster chemical reactions in the combustion chamber. On the con-trary the lower peak of pressure was detected for RME (49.5 bar).The pilot injection ignited at 8� ca BTDC for all fuels, after thisphase, the curves of heat release raised. In particular, it was foundthat GTL shows the same start of pilot combustion of REF but it hadthe highest rate of heat release peak due to the pilot combustion.While the RME fuel showed later SOC and the lowest pilot com-bustion peak. These features influenced the ignition delay time ofthe subsequent main injection and its combustion evolution.Regarding the main combustion, the GTL fuel had the fastest ROHRbehaviour; on the contrary, the RME fuel had the lowest andretarded peak this is ascribed to its smaller heating value. Also itscombustion duration is longer, because the injected fuel mass isbigger.

In this paper, in order to focus the analysis on the behaviour ofbiofuels in the IR range, a set of images of combustion from thebottom view, has been reported in Fig. 2. They are for GTL and RMEat several crank angle degrees after top dead centre (ATDC) andthey are normalized with respect to their maximum intensity atfixed crank angle. In the images, the red areas (in web version)denote maximum energy, as indicated in the colour bar. It can benoted that the IR camera detects clearly the seven jets of vaporizedfuel before the starting of main injection at 8� ca BTDC and beforethe start of main injection. However, as reported in the previouspaper [11], the IR images show better the seven burning jet thana camera with high performance in the visible wavelength range.The latter detected only some bright spots near the nozzle tip.Moreover, in the IR images it seems that the vaporized jets are notstrongly affected by the in-cylinder air motion. It is so possible toidentify the non-homogeneous distribution of reactants in thebowl, a fundamental factor that influences the evolution of theprocess. The flames due to the pilot injectionwere recorded at 4� caBTDC. At (top dead centre) TDC, the seven atomized jets of maininjection are burning and the energy released by the flames is

Fig. 2. Combustion images from the bottom for GTL and RME fuels.

REFRME

E. Mancaruso et al. / Energy 58 (2013) 185e191188

detected by the IR camera. At 8� ca ATDC, the combustion flamemoved towards the bowl wall and consumed the fuel along the jetdirection. At 20� ca ATDC, the IR emission is still intense, while thevisible light is very weak [11]. Another peculiarity is that IR cameracan follow the reactions that take place during the late combustion.In particular, in the IR wavelength residual flame and hot burnedgas distributed in the bowl and above the piston head emittedenergy and impressed the IR detector. The combustion activity wasrecorded up to 60� ca ATDC. This can help to better understand themotion of the hot gas and air into the cylinder and their evolutionduring the soot reduction. The energy released by the hot burnedgases was detected up to 40� and 60� ca ATDC for RME and GTL,respectively where the heat release is already finished [10].

Moreover, it is interesting to analyse the results detectedthrough the sapphire ring from the lateral view. In Fig. 3, images ofcombustion from the side have been reported for REF, GTL and RME.Images refer to the late combustion from 18� ca ATDC. A cloud ofhot burned gases lies above the piston head, it can be explainedconsidering that the oxygen stored in the crevice, when the pistonwas at TDC, now is mixingwith the hot gas and continues to oxidizethe unburned species [12] in the cylinder volume during theexpansion stroke. The images reported in Fig. 3 show the com-bustion reactions taking place outside the bowl, on the top of thepiston surface for all the fuels investigated. As the expansion strokegoes on, the piston goes down and a higher percentage of the ringwindow is discovered. It must be remembered that it is not possibleto have images across the TDC because the piston blocks thevisibility.

Fig. 3. Combustion images from the side for REF, GTL and RME fuels.

Making a comparison between the three fuels, it can be notedthat an intense cloud of burned gases on the top of the piston ispresent for all. In particular, at 18� ca ATDC the images are not ableto put in evidence relevant differences between the fuels. After, thegases exiting from the bowl, they are clearly discernible in theimages at 30� ca ATDC, until they fill the entire available volume asthe piston goes down (42� ca ATDC).

In order to evaluate the variation of IR intensity during theevolution of the combustion process, images have been post-processed; in particular, the integral luminosity of the images ateach crank angle has been calculated. The methodology applied forthe computation has been faithfully described in Ref. [11]. In Fig. 4,the IR luminosities detected through the piston window (bottomview) have been reported for all the investigated fuels. It is the in-tegrated value of the pixel intensity reported in Fig. 2 for each crankangle. In order tomake a comparisonwith the physical and chemicalprocesses that happens in the engine during the combustion wemust to take in mind that the IR images were recorded with 4� castep. For this reason, it is not possible to detect exactly the start ofcombustion of the several fuels. However the IR evolution cancharacterize the energy released during the first phase of combus-tion. Fromthe analysis of the intensities emitted through the bottomwindow, it can be noted that the IR luminous emissions start toincrease from 12� ca BTDC for GTL, before the SOC of pilot injection

-20 0 20 40 60 80

Crank angle [°]

GTL

0.2

0.4

0.6

0.8

1

0

No

rm

alize

d IR

lu

min

os

ity

(b

otto

m) [a

.u

.]

Fig. 4. Normalized IR integrated luminosity measured from the bottom view for REF,RME, and GTL fuels.

0 20 40 60 80 100

Crank angle [°]

REFRMEGTL

0.2

0.4

0.6

0.8

1

0

No

rm

alized

IR

lu

min

os

ity

(s

id

e) [a

.u

.]

Fig. 5. Normalized IR integrated luminosity from the side view for REF, RME, and GTLfuels.

E. Mancaruso et al. / Energy 58 (2013) 185e191 189

detected from ROHR curve. It is the earliest detected IR signal.However, we must consider that the next frame analysed with thisprocedure was at 8� ca BTDC, when the SOC of pilot was detected.Probably, the IR camera is also able to catch the energy during theevaporation and mixing formation phases. Moreover, this is due tothe cold combustion that occurs before the luminous combustion, itreleases a small quantity of energy that doesn’t influence the ROHRcomputation. Then, the intensities increase slowly up to TDC. Afterthis crank angle the main combustion occurs into the bowl andproduces strong light emission in a broad wavelength range. Thepeak of the curves is at about 9� ca ATDC; it occurs 5� ca after thepeak of the ROHR curve. This happens because in the cylinder can bedetected the maximum energy only after the energy release hasreached itsmaximumrate, that is thepeakof ROHRcurve. In fact, theIR camera acquires the energy emitted in the cylinder during a cer-tain period, the exposure time, as integral of the instantaneousvalues. At 9� ca ATDC, the highest contribution to the total release ofenergy has already been given. Finally, it can be noted that during

Crank angle [°]

0.0x10

4.0x10

8.0x10

1.2x10

1.6x10

Infra

red

lu

min

osity (b

otto

m) [a

. u

.]

REF

Cran

-60 -40 -20 0 20 40 60 80 -60 -40 -20 0

Fig. 6. Dispersion of IR integrated luminosity fro

the late combustion, after 60� ca ATDC, the IR emission hasn’treached the zero value yet, sign that chemical reactions are still inprogress. In the IR range it has been possible to investigate thecombustion for a total duration longer than 90� ca ATDC.

Analysing the two biofuels tested some interesting consid-eration can be made. For GTL, first IR emission anticipates the REF,due to the high value of its cetane number. Moreover, it keepshigher values for the entire rising phase, until it reaches a peakvalue quite similar to that of REF. Finally, the reduction of the IRemission is similar to the values of REF. The analysis of the RMEbehaviour showed a delay in the activation of this fuel with respectto others. In fact, even if it reaches its peak of IR emission almost atthe same crank angle of REF, its intensity is very lowwith respect toother fuels. This may be due to a delay in the chemical reaction offuel in the bowl, because of a slower mix with the air due to itshigher density and low cetane number. On the other hand, the first-generation biofuel has shown a delay also in the reduction of the IRemission. Its values are higher than REF and GTL up to 30� ca ATDC.This means that the chemical activity is still in progress during thelate combustion phase. This is also due to the longer main injectiontime.

In Fig. 5, the normalized integrated IR luminosity from the sideview has been reported for all investigated fuels. Images representthe luminous emissions above the piston head during the expan-sion strokes. In the first phase (before 40� ca ATDC), it is due to theflames that burn outside the bowl. In fact, the air stored in thecrevice, when the piston was at TDC, nowmoves toward the centreof the cylinder, when the piston goes down, mixes with theremaining vaporised hydrocarbons and ignites [12]. In the secondphase (after 40� ca ATDC), images regarding the hot gases thatmove on the head of the piston while it goes down.

From the computation of the IR emission it can be noted, asexplained above, that no data are available across the TDC due tothe presence of the piston that covers the area of interest. More-over, at about 10� ca ATDC it is possible to see the first luminousemission above the piston head. Despite of the first instants of theexpansion stroke, where the view available is very thin and thedetection of the emission is highly subjected by errors, the REF, GTLand RME have the same emission intensity at 20� ca ATDC. Afterthis crank angle, the behaviour is very different; it rises with vari-ous slopes, REF is the one that shows first high intensity, followedby RME and GTL. This configuration is evident also after the peakvalue, when the emission drops. For this reason, the position of themaximum values is at 30�, 45�, and 55� ca ATDC for REF, RME, andGTL, respectively. Moreover, the REF peak unless some uncertainty

k angle [°]

RME

20 40 60 80 -60 -40 -20 0 20 40 60 80

Crank angle [°]

GTL

m the bottom for REF, RME and GTL fuels.

Crank angle [a]

0.0x10

10

2.0x10

3.0x10

4.0x10

5.0x10

In

fra

re

d lu

min

os

ity

(s

id

e) [a

. u

.]

REF

Crank angle [°]

RME

0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140

Crank angle [°]

GTL

Fig. 7. Dispersion of IR integrated luminosity from the side for REF, RME and GTL fuels.

E. Mancaruso et al. / Energy 58 (2013) 185e191190

remains constant up to 50� ca ATDC. These behaviours can beexplained considering also the curves of IR emission from thebottom. For example, REF emission decreases and shows a knee at30� ca ATDC, corresponding to the peak from the side view, thismeans that the combustion is moving toward the volume whichhas become available on the top of piston. Moreover, the fuel hasnot burned completely yet, so it needs about 20� ca for the com-bustion. Similarly, RME and GTL reach their maximum intensitiesfrom the side, when the emission from the bottom view extin-guishes. In particular, it is retarded with respect to REF and burns inthe bowl volume for a longer period.

In order to evaluate the variation of the IR intensity during theevolution of the combustion process, the post-processed imageshave been analysed for a large number of combustion cycles. Asample of 5 cycles chosen among all the repetitions has beenextracted for all the tested fuels and both views. In Figs. 6 and 7, thedispersion of experimental data has been shown for all the testedfuels, from the bottom and side view, respectively. A good agree-ment between measurements is detectable. The repeatability of IRintegral energy curves is consistent with the typical variation of thecombustion cycles in a compression ignition engine.

Moreover, a quantitative analysis of the data dispersion has beencarried out through the evaluation of the (coefficient of variation)CV for all the measurements performed during the test. It is definedas the ratio between the standard deviation (s) and the mean value(m) of the several tests performed (Eq. (1)). One hundred repetitionsof combustion sequences have been analysed.

CV ¼ s=m (1)

Table 4Coefficient of variation (CV) analysis for REF, RME and GTL, from bottom and sideviews, respectively.

CV REF RME GTL

Range From �20� ca to 40� caView BottomMean 0.18 0.22 0.25Max 0.74 0.58 0.84(�ca) of Max �2 �7 �2@ 4� ca 0.08 0.31 0.1

Range From 0� ca to 100� caView SideMean 0.12 0.12 0.16Max 0.62 0.24 0.78(�ca) of Max 17 13.6 13.6@ 40� ca 0.04 0.08 0.06

Due to the presence of the mean value at the denominator,when this is close to zero, the CV approaches to infinity and itsvalue is no longer significant. For this reason, a confidence intervalhas been set for the computation of the CV. Time interval in termsof crank angles has been chosen the region considering the startand the end of combustion process for both views. In particular, forthe images from the bottomview the confidence interval goes from20� ca BTDC up to 40� ca ATDC, and for the images from the sideview it goes from TDC (0� ca) up to 100� ca ATDC. In Table 4, resultsof the CV calculation have been reported. It can be noted that thevalues of CV are around less than 0.25 and 0.16 for data acquiredfrom bottom and side view, respectively. They are the highest dis-persion values and they are referred to GTL fuel. Also the maximumvalue of the CV has been measured and it gives information aboutthe critical phases of the combustion process. The high dispersionhas still been noted for the GTL fuel. Moreover, higher dispersionhas been detected at the start of increasing rate for all the testsduring the early combustion phase (i.e. 2� ca BTDC for REF and RME,and 7� ca BTDC for GTL, respectively, for the bottom view). In fact,the amount of energy detectable is strictly linked to the progress ofchemical reactions occurring in the combustion chamber. More-over, for the images from the side view, in the early crank anglesduring the expansion stroke, the available view is very thin and thedetection of the flame emission is very difficult. Finally, the CV ofthe peaks of integral IR emission curves reported in Figs. 6 and 7 hasbeen calculated. Although the peaks did not occur in the same lo-cations for all the repetitions performed at fixed fuel, low values ofCV emerge because of the CV is normalized with respect to themean value. The RME shows the highest CV, 0.31 for the bottomview. This is due to the temporal resolution of the acquisition of twoconsecutive frames. In fact, the RME peak occurs at 9� ca ATDCwithan uncertainties 4� ca (Fig. 6).

A good repeatability of themeasurements has been observed forall the experimental data. The analysis of the CV has revealeda small dispersion along the whole combustion process. Significantvariations of measured values are present at the start of energyrelease due to the autoignition fuel characteristics.

4. Summary and conclusions

In the present paper IR digital imaging has been carried out tostudy the combustion process of alternative diesel fuels. Two bio-fuels, GTL and RME, have been tested, and compared with com-mercial diesel, REF. They fuelled a transparent single-cylinderdiesel engine equipped with the latest generation Euro 5 engine

E. Mancaruso et al. / Energy 58 (2013) 185e191 191

head. Images have been recorded via two optical accesses: one inthe head of the piston and another along the cylinder line. Theintegrated values of energy released in the IR wavelength rangehave been calculated as function of the crank angle and analysed.Infrared imaging has allowed acquiring a large amount of infor-mation. It allows distinguishing chemical and/or physical activity ofthe injection and combustion process in advance or with moredetail with respect to typical visible images.

GTL starts burning before the other fuels due to its higher cetanenumber. This means that the GTL has less time to premix it with theair in the bowl and thus realize more soot during the combustion.This is also in good agreement with the results of high PM (par-ticulate matter) emission and in-cylinder OH rates detected inprevious papers. Moreover, the energy released on the top of thepiston is the slowest than the other fuels, while it decreases quicklyas the REF fuel. This is in good agreement with the fastest rate bothfor the formation and the oxidation of the soot. Probably, the GTLcompletes its combustion in the bowl and the unburned fuel andburned hot gases moves out the bowl slowly producing retarded IRenergy detection. The GTL combustion behaviour entirely offset thebenefit of fuel lack of aromatics.

RME showed the most retarded start of combustion due to itsLHV (lower heating value). Very low energy during the pilot com-bustion with respect to the other fuels has been detected. More-over, the longest time for the autoignition, due to its lower cetanenumber, and the high oxygen content improved the RME mixingprocess providing a low soot combustion process. RME had thelargest IR emission due to the highest energizing time of the maininjection. Chemical activity is in progress up to 30� ca ATDC. Finally,the analysis of reactions occurring outside the piston bowl, shownRME flames have a propensity to migrate toward the volumewhichhas become available on the top of the piston during the expansionstroke slower than REF and faster than GTL.

The introduction of infrared technologies in the study of com-bustion engine functioning has revealed a good way to investigatethe influence of alternative fuel in the combustion process

especially when the visible imaging is not able to catch useful in-formation. In particular, during the late combustion phase, the IRimage showed a good capability to follow the hot burned gasesboth in the bowl and above the piston. Finally, the IR digital imagingof combustion process has revealed a tool with high potential.

Acknowledgements

The authors thank Mr. Carlo Rossi and Mr. Bruno Sgammato fortheir precious help.

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