Fuel 99 (2012) 72–82

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Fuel

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Ignition timing sensitivities of oxygenated biofuels comparedto gasoline in a direct-injection SI engine

Ritchie Daniel a, Guohong Tian a,b, Hongming Xu a,c,⇑, Shijin Shuai c

a School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, UKb Sir Joseph Swan Centre, Newcastle University, Newcastle Upon Tyne NE1 7RU, UKc State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, China

a r t i c l e i n f o

Article history:Received 4 July 2011Received in revised form 24 January 2012Accepted 26 January 2012Available online 4 April 2012

Keywords:2,5-DimethylfuranDMFEthanolButanolMethanol

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.01.053

Abbreviations: aTDC, after top dead centre; bTDC,butanol; CAD, crank angle degrees; CFR, cooperativecarbon monoxide; CO2, carbon dioxide; COV, coefficiinjection spark-ignition; DMF, 2,5-dimethylfuran; ETIMEP, indicated mean effective pressure; KL-MBT, kntorque; LCV, lower calorific value; MBT, maximumfraction burned; MON, motor octane number; MToxides; PM, particulate matter; RON, research octanper minute; SI, spark-ignition; SR10, spark retard (10ULG, unleaded gasoline.⇑ Corresponding author.

E-mail address: h.m.xu@bham.ac.uk (H. Xu).

a b s t r a c t

Global concerns over atmospheric carbon dioxide (CO2) levels and the security of fossil fuel supply haveled to the development of biofuels; a potentially carbon-neutral and renewable fuel strategy. One newgasoline-alternative biofuel candidate is 2,5-dimethylfuran (DMF). In this paper, the potential of DMFis examined in a direct-injection spark-ignition (DISI) engine. Focus is given to the combustion perfor-mance and emissions sensitivity around the optimum spark timing, especially at 10 crank angle degreesretard (SR10). Such spark retard strategies are commonly used to reduce catalyst light-off times, albeit atthe cost of reduced engine performance and increased CO2. The results for DMF are compared to gasoline,ethanol, butanol and methanol so that its sensitivity can be positioned relatively. The overall order ofspark sensitivity at the highest load (8.5 bar IMEP) was: gasoline > butanol > DMF > ethanol > methanol.The four biofuels widen the spark window due to improved anti-knock qualities and sometimesincreased charge-cooling. This allows the increase of CO2 to be better minimized than with gasoline. Fur-thermore, DMF is the only biofuel to produce high exhaust gas temperatures, similar to gasoline andhelpful for fast catalyst light-off, whilst maintaining high combustion stabilities. This demonstrates thepotentially favorable characteristics of DMF to become an effective cold-start fuel.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

With ever-increasing concerns of fuel security and the problemof global warming, there is a greater need to pursue alternative en-ergy sources. Carbon-free fuels, which do not emit CO2 once con-sumed, are the long-term ideal in order to eradicate carbonemissions. However, biofuels offer a short- to mid-term solutionin reducing the dependence on mineral oil and life-cycle CO2

emissions.One particular biofuel candidate, which has benefitted from sig-

nificant technological breakthroughs in its manufacture is 2,5-

ll rights reserved.

before top dead centre; BUT,fuel research (engine); CO,

ent of variation; DISI, direct-H, ethanol; HC, hydrocarbon;ock-limited maximum brake

brake torque; MFB, massH, methanol; NOx, nitrogene number; RPM, revolutionsCAD); TDC, top dead centre;

dimethylfuran, otherwise known as DMF. In 2007, bioscientists atthe University of Wisconsin–Madison publicized the productionof high yields of DMF [1,2], whose techniques have since benefittedfrom further iterations by other institutions [3–7]. These develop-ments have attracted the attention from automotive researchers inthe potential to use DMF as an alternative energy carrier to gaso-line [8]. In comparison to ethanol, DMF has a higher energy density(approximately 40% higher) and is insoluble in water [1]. Currently,relatively few publications exist on DMF as a gasoline-alternativefuel. The first reported engine studies were conducted by theauthors’ group [9–11]. This added to the laboratory studies of thelaminar burning velocity [12–15], spray properties [16] and com-bustion intermediates of DMF [17]. Evidently, this publication con-tributes to a series of experiments led by the authors’ group toexplore the use of DMF as a fuel for automotive applications.

In spark-ignition (SI) engines, one of the main control parame-ters is the spark timing. It significantly affects the combustion pro-cess, which determines the fuel economy, torque output andemissions performance [18]. The spark timing is usually optimizedusing sophisticated mathematical approaches, including polyno-mial regression techniques [19,20], radial basis functions and neu-ral networks [21,22], and advanced design of experiment (DoE)methodologies [23–25]. Each technique requires extensive model

R. Daniel et al. / Fuel 99 (2012) 72–82 73

tuning in order to find the optimum, or minimum advance for besttorque (MBT) timing. Minimal advance or retard about this pointgives modest variation in power and fuel consumption but can leadto large changes in NOx and HC emissions. Therefore, in order tominimize emissions or counteract knock, it is common to employa spark retard or knock margin [18,26]. Clearly, when employingthese approaches, an alternative fuel which produces the largestreduction in emissions whilst achieving competitive performancehas great value in a practical application.

As mentioned, the onset of knock ultimately limits the maxi-mum allowable spark advance and prevents the use of the theoret-ical optimum (MBT) timing. The knocking tendency of a fueldepends on its physicochemical properties and is best representedby the research and motor octane numbers (RON and MON respec-tively). The Octane (Antiknock) Index, or OI is sometimes preferredas it combines the effects of RON and MON (OI = [RON + MON]/2)[18,27]. However, to date, there is no published octane number(RON or MON) for DMF using the CFR (cooperative fuel research)engine method. What is known, however, is that DMF has previ-ously been used as an octane enhancer with gasoline [28,29]. Nev-ertheless, in comparison to pure ethanol, this increased knocksuppression ability of DMF over gasoline has been shown to be lesssignificant [10].

Although the octane number can provide an insight into thesensitivity to spark variations, it does not take in account thecharge-cooling effect made possible with modern direct-injection(DI). Furthermore, for fuels which outperform iso-octane (100RON) the values can only be extrapolated. As such, the CFR engineoctane methods, developed in 1930, have received criticism fortheir relevance to the modern situation [27,30–32]. Therefore, inan effort to further establish the antiknock performance of DMF,the authors have proposed an alternative method, which is closerto modern reality. By analyzing the effect of spark timing sweepsat various loads and fixed engine speed, it is possible to determinethe spark sensitivity of each fuel, or the spark window for a givendecrease in load. Low sensitivity is ideal, as a wide spark windowprovides a greater opportunity to reduce the NOx and HCemissions.

Spark timing retard strategies are employed during cold-startsfor fast catalyst light-off (defined as the temperature to reach50% efficiency [18]), because the combustion phasing shifts to-wards the expansion stroke and raises the exhaust temperature.It is also used in turbocharged engines to allow a more rapid build-up of boost pressure, especially at low loads [33,34]. The extent ofraising the exhaust temperature for fast catalyst light-off is limitedby the reduced combustion stability and efficiency during spark re-tard. As the cold-start HC emissions can contribute to 80–90% ofthe total during the FTP test cycle, the need for fast catalystlight-off is paramount [35,36]. Although researchers have analyzedthe effect of HC emissions with spark retard [35,37], little is docu-mented about the reduction limitations due to the spark sensitivityof the fuel.

Through the assessment of the spark sensitivity of DMF, it ispossible to further hypothesize the octane rating. Therefore, theobjective for this investigation is twofold: (1) to position the knocksuppression ability of DMF amongst other alternative SI fuels and(2) to study the effect of spark retard on modern engine perfor-mance and emissions with such fuels. In an effort to achieve theseaims, the authors have examined the behavior around the opti-mum spark timing between 3.5 bar (low load) and 8.5 bar (highload) indicated mean effective pressure (IMEP) in 1 bar intervalsat a fixed engine speed of 1500 rpm. The performance of DMF isbenchmarked against gasoline and compared to ethanol, butanoland methanol. The experimental system is described in the nextsection and then the results are presented and discussed. A sum-mary of the conclusions is given at the end of the paper.

2. Experimental setup

2.1. Engine and instrumentation

The experiments were performed on a single-cylinder, 4-strokeSI research engine, as shown in Fig. 1. The 4-valve cylinder head in-cludes the Jaguar spray-guided direct-injection (DISI) technologyused in their V8 production engine (AJ133) [38]. It also includesvariable valve timing technology for both intake and exhaustvalves, which, for this study, was kept constant.

The engine was coupled to a DC dynamometer to maintain aconstant speed of 1500 rpm (±1 rpm), regardless of the torque out-put. The in-cylinder pressure was measured using a Kistler 6041Awater-cooled pressure transducer which was fitted to the side-wallof the cylinder head. The signal was then passed to a Kistler 5011charge amplifier and finally to a National Instruments data acqui-sition card. Samples were taken at 0.5 CAD intervals for 300 con-secutive cycles, so that an average could be taken. The crankshaftposition was measured using a digital shaft encoder mounted onthe crankshaft. Coolant and oil temperatures were controlled at85 �C and 95 �C (±3 �C) respectively using a Proportional IntegralDifferential (PID) controller. All temperatures were measured withK-type thermocouples.

The engine was controlled using software developed in-housewritten in the LabVIEW programming environment. High-speed,crank-angle-resolved and low-speed, time-resolved data was alsoacquired using LabVIEW. This was then analyzed using MATLABdeveloped code so that an analysis of the combustion performancecould be made.

2.2. Emissions and fuel measurement

The gaseous emissions were quantified using a Horiba MEXA-7100DEGR gas tower. Exhaust samples were taken 0.3 m down-stream of the exhaust valve and were pumped via a heated line(maintained at 191 �C) to the analyzer.

Particulate matter (PM) emissions were measured using a 3936Scanning Mobility Particle Sizer Spectrometer (SMPS) manufac-tured by TSI. This comprises of a 3080 Electrostatic Classifier, a3775 Condensation Particle Counter (CPC) and a 3081 DifferentialMobility Analyzer (DMA). PM samples were taken from the sameposition as the Horiba analyzer but measured asynchronously.A heated (150 �C) rotating disc diluter (Model 379020A, supplied byTSI) was used at a dilution ratio of 67:1. The SMPS measuredparticles from 7.23 to 294.3 nm in diameter and the sample andsheath flow rates were 1 and 10 l/min, respectively.

The fuel consumption was calculated using the volumetric airflow rate (measured by a positive displacement rotary flow meter)and the actual lambda value (Bosch heated LSU wideband lambdasensor and ETAS LA4 lambda meter). The LA4 lambda meter usesfuel-specific curves to interpret the actual air-fuel ratio (AFR) usingthe oxygen content in the exhaust. Before each test, the user inputsthe fuel’s hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C)ratios, as well as the stoichiometric AFR, so that the fuel composi-tion can be used to characterize the fuel curves.

2.3. Test fuels

The DMF used in this study was supplied by Shijiazhuang LidaChemical Co. Ltd., China at 99.8% purity. This was benchmarkedagainst commercial 97 RON gasoline and to ethanol, which wereboth supplied by Shell Global Solutions, UK. These three fuels arehereby referred to as the primary fuels used in the study. Bothmethanol and butanol constitute the secondary fuels and weresupplied by Fisher Scientific, UK with 99.5% and 99% purity,

Exhaust VVT

IntakeVVT

Exhaust Plenum

Chamber

Intake Plenum

ChamberIntakeFilter

Horiba MEXA -7100DEGR

Emissions Analyser (HC, CO, CO2, O2, NOx)

Scanning Mobility Particle Sizer

(SMPS)

High/Low Speed Data AcquisitionControl Tower

Lambda Meter

Air In

Exhaust

Oil/Water Cooler

Crank Angle Encoder

VAF MeterThrottleFuel

Accumulator

Compressed Nitrogen Cylinder

Pressure Gauge (150bar)

to Injector

Kistler Pressure Sensor

Fig. 1. Schematic of engine and instrumentation setup.

74 R. Daniel et al. / Fuel 99 (2012) 72–82

respectively. The high octane gasoline was chosen as this repre-sents the most favorable characteristics offered by the marketand provides a strong benchmark to the four biofuels. The fuelcharacteristics are shown in Table 1.

2.4. Experimental procedure

The engine was considered warm once the coolant and lubricat-ing oil temperatures had stabilized at 85 �C and 95 �C, respectively.All tests were carried out at the stoichiometric AFR (k = 1) withfixed injection timing (280� bTDC), ambient air intake conditions(approximately 25 ± 2 �C) and constant valve timing (see Table2). The pressure data from 300 consecutive cycles was recordedfor each test using the in-house developed LabVIEW code.

When changing fuels, the high pressure fuelling system waspurged using nitrogen until the lines were considered clean. Once

Table 1Test fuel properties.

DMF Ethanol Methanol Butanol Gasoline

Chemical formula C6H8O C2H6O CH4O C4H10O C2–C14

H/C ratio 1.333 3 4 2.5 1.795O/C ratio 0.167 0.5 1 0.25 0Gravimetric oxygen

content (%)16.67 34.78 50 21.6 0

Density @ 20 �C (kg/m3)

889.7a 790.9a 792 811 744.6

Research octanenumber (RON)

n/a 107b 106b 98 96.8

Motor octane number(MON)

n/a 89b 92b 84 85.7

Octane Index, (K = 0.5) n/a 98 99 91 91.25Stoichiometric air fuel

ratio10.72 8.95 6.47 11.2 14.46

LHV (MJ/kg) 32.89a 26.9a 19.83a 32.71a 42.9LHV (MJ/L) 29.26a 21.3a 15.7a 26.5a 31.9LHV stoich. mix (MJ/

m3)3.49 3.29 3.16 3.33 3.4

Flash point (�C) 1 13 12 36 -40Heat of vaporization

(kJ/kg)332 840b 1103b 430 373

Initial boiling point (�C) 92 78.4 65 118 32.8

a Measured at the University of Birmingham.b Heywood [18].

the line was re-pressurized to 150 bar using the new fuel, the en-gine was run for several minutes. This removed any previous fuelfrom the injector tip and in any combustion chamber crevices be-fore the data was acquired. The ETAS LA4 lambda meter settingswere changed for each fuel using the stoichiometric AFR, O/C andH/C ratios in Table 2.

2.5. Spark advance

In this study, the MBT, or optimum ignition timing is defined asthe ignition timing to produce the maximum IMEP for a fixedthrottle position. If audible knock occurred, the MBT timing was re-tarded by 2 CAD, an arbitrarily safe margin, as advised by key en-gine researchers [18,26] and is then referred to as the knock-limited MBT timing (KL-MBT). For this work, the MBT/KL-MBT tim-ings were determined for each fuel from spark sweeps generatedbetween 3.5 bar and 8.5 bar IMEP, in 1 bar IMEP intervals and ata fixed engine speed of 1500 rpm. At each load, the spark timingwas advanced to find the knock limit or until a significant dropin performance or stability was seen (IMEP decrease >5% or COVof IMEP increase >3%). Retarding the timing further for emissionspreservation was not used, in order to eliminate subjectivity andbetter isolate the effect of spark sensitivity. Similarly, the sparktiming was retarded until the aforementioned drop in performancewas found. While performing each spark sweep, the fuel and airflow rates were kept constant for each fuel once the required loadand stoichiometric AFR was achieved at the anticipated MBT point(estimated from the spark sweep at the previous load). Firstly, thethrottle position was adjusted and then the fuel injection pulse

Table 2Engine specification.

Engine type 4-Stroke, 4-valveCombustion system Spray guided DISISwept volume 565.6 cm3

Bore � stroke 90 � 89 mmCompression ratio 11.5:1Engine speed 1500 rpmInjector Multi-hole nozzleFuel pressure and timing 150 bar, 280� bTDCIntake valve opening 16� bTDCExhaust valve closing 36� aTDC

37.5 32.5 27.5 22.5 17.5 12.5 7.5 2.57.8

7.9

8.0

8.1

8.2

8.3

8.4

8.5

8.6

8.7

8.81500rpm, λ = 1

IMEP

(bar

)

Spark Timing (°bTDC)

ETH DMF ULG

Fig. 3. Effect of spark timing at high load when using ethanol, DMF and gasoline.

R. Daniel et al. / Fuel 99 (2012) 72–82 75

width was adjusted finely (±1 ls) to find stoichiometry. Three re-peats were made with each fuel to produce an average.

Once the spark timing sweeps were analyzed, the engine wasrun again at each load using the chosen MBT/KL-MBT timingsand at a spark timing retard of 10 CAD. This allowed the engineperformance and emissions’ sensitivity to be analyzed under sub-stantial spark timing retard conditions. Once more, three sets oftests were carried out for repeatability. Each fuel was tested overthree consecutive days; however, the test order was varied eachday in order to minimize the effect of engine drift, as recom-mended by leading engine researchers [39]. Error bars have beenused where applicable in order to highlight such variations.

3. Results and discussion

3.1. Spark advance

The MBT/KL-MBT timings for each fuel are shown in Fig. 2. Ateach load, the MBT/KL-MBT timings are shown by the individualdata points which were observed experimentally. Polynomialtrend lines have then been applied to highlight the differencesand more clearly present the relationship with respect to load.

Throughout the entire load range, ethanol and methanol requirethe most advanced spark timing. At the highest load, the optimum(MBT) timing is 11 CAD more advanced than with gasoline and 5CAD more than with DMF. Until 6.5 bar IMEP, DMF and ethanolare separated by less than 1 CAD. Despite this, the maximum IMEPwhen using DMF is limited by audible knock and the theoreticalmaximum (MBT) cannot be achieved. Although DMF is believedto have a high octane number, the spark timing is relatively moreretarded than with ethanol, due to this onset of knock. However,the best spark timings for gasoline are clearly the most retarded,once again largely limited by knock.

When using 97 RON gasoline, a knock margin (2 CAD retard)was enforced as early as 4.5 bar IMEP, which restricted the abilityto find the theoretical optimum (MBT) timing. For DMF, however,this safety margin was not enforced until 5.5 bar IMEP and reaf-firms the knocking behavior discovered in earlier experimentalwork [9]. What is interesting is that the knock margin was also em-ployed for butanol at this same load, despite having a marginallylower OI than gasoline (see Table 1). This suggests the highercharge-cooling effect of butanol, which can be interpreted fromthe higher heat of vaporization, has a more pronounced effect onthe overall knock suppression, which is obviously not accountedfor in the CFR tests. Based on this trend, it is possible that the OIfor DMF is noticeably higher than 97 RON (86 MON) gasoline(OI = 91.25) as the heat of vaporization of DMF is lower. Clearly,this is hypothetical and no substitute for real CFR engine data.

3 4 5 6 7 8 90

5

10

15

20

25

30

35

401500rpm, λ = 1

Spar

k A

dvan

ce (°

bTD

C)

IMEP (bar)

ETH DMF ULG BUT MTH

Fig. 2. MBT/KL-MBT spark timings at various loads for ethanol, DMF, gasoline,butanol and methanol.

For gasoline, the octane rating is largely governed by the aro-matic content (fractions of benzene, toluene, etc.). However, forpure, oxygenated fuels another relationship prevails. Gautam andMartin have shown that the knock suppression capability of oxy-genated fuels can be related to the relative oxygen content [40].For DMF, a non-benzene ring aromatic, the oxygen content is lowerthan the other oxygenated compounds used in this work (see Table1). This could explain why the knocking tendency occurs at lowerloads when using DMF. For ethanol and methanol, fuels with rela-tively high oxygen content, no knock margin was required at anyload. In addition to their high oxygen content, these fuels also burnwith high velocity and produce a greater charge-cooling effect (seeTable 2), which helps to lower the combustion temperature anddiscourage end-gas pre-ignition.

3.2. Spark timing sensitivity

The variation of IMEP using DMF, ethanol and gasoline at thehighest load spark sweep (approximately 8.5 bar IMEP) is shownin Fig. 3 (the data for methanol and butanol has been omitted inorder to clearly present the methodology). At this load, there is aclear difference between the three fuels. Ethanol combustion,which is uninhibited by knock at this compression ratio, permitsa wide spark sweep and allows the IMEP to be analyzed either sideof the MBT timing (21� bTDC). DMF and gasoline on the other hand,are much more sensitive to the onset of knock and only the re-tarded timing from KL-MBT can be observed. With comparison,there appears to be a relationship between the MBT/KL-MBT loca-tion and rate of change of IMEP. It is evident that the more retardedthe MBT/KL-MBT spark timing is, the higher the rate of IMEP decaybecomes with spark retard. This rate of decay can be used as anindicator of the spark timing sensitivity for each fuel.

When normalizing the IMEP and spark timing data (by theirrespective MBT/KL-MBT values) from Fig. 3, these fuel effects be-come more obvious. This is shown in Fig. 4, using the term sparkretard, which represents the number of retarded CAD from MBT/KL-MBT. As the term suggests, a positive value represents retardedtiming from MBT/KL-MBT, whereas a negative value is advanced.This term has been previously used by Ayala et al. [41] to help de-velop their combustion retard parameter.

When using ethanol, the rate of decay is symmetrical about itsMBT and decreases at a lower rate than with DMF and gasoline.This is largely explained by the knock suppression superiority ofethanol. The data suggests that the initial rate of decay can also

-15 -10 -5 0 5 10 150.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01 1500rpm, λ = 1

IMEP

/IMEP

MB

T/K

L-M

BT

Spark Retard (θMBT/KL-MBT - θST, CAD)

ETH DMF ULG

Fig. 4. Effect of spark retard on normalized IMEP at high load when using ethanol,DMF and gasoline.

a

b

c

Fig. 5. Effect of spark retard on normalized IMEP when using (a) ethanol, (b) DMFand (c) gasoline.

76 R. Daniel et al. / Fuel 99 (2012) 72–82

indicate how far away the KL-MBT timing is from the theoreticalMBT timing (if unhindered by knock). For instance, the initial rateof decay using DMF is less than with gasoline, which suggests theKL-MBT timing for DMF is much closer to its theoretical MBT tim-ing. Within this range of IMEP decay, ethanol is the least sensitiveto spark timing variations. This is clearly shown in Fig. 4 at thearbitrary 5 CAD spark retard location, or SR5 (most retarded pointfor gasoline). Here, when using ethanol, there is a loss in IMEP ofapproximately 1% from MBT. However, when using DMF and gas-oline this loss increases to 2.5% and 7% respectively from theirKL-MBT timings. Evidently, it is gasoline which is the most sensi-tive to spark timing at this load, which is largely a function of itsrelatively low OI (see Table 1).

This normalization method is applied to the entire load range inFig. 5 (each fuel has been separated for clarity). There is a cleartrend in spark sensitivity with load, which is best shown with gas-oline (Fig. 5c). As the load increases, the rate of decay of IMEP fromthe MBT/KL-MBT point also increases. At the lowest load of 3.5 barIMEP, the spark sensitivity is also relatively low. For instance, atSR5, the loss in IMEP is <1%. However, with each 1 bar incrementin load, the loss in performance rapidly increases to a maximumof 7.2% at 8.5 bar initial IMEP. The increase in sensitivity with re-spect to load is also evident for DMF and ethanol but in an increas-ingly subtle manner. The spark sensitivity of ethanol is alsosymmetric either side of MBT due to the benefit of a higher octanenumber and greater charge-cooling effect (see Table 2).

For DMF, the spark timing sensitivity lies closer to that of etha-nol than to gasoline, which is shown by the lower spread betweenthe loads. However, it is difficult to examine this link more closelyby solely observing these graphs. Therefore, the arbitrary SR10location is used to help quantify the spark timing sensitivity whenanalyzing other key combustion performance and emissionsparameters. Although the SR5 location demonstrates clearly thespark sensitivity at the higher loads, the authors have chosen amore retarded timing of 10 CAD in order to emphasize the trendat the lower loads.

3.3. Combustion performance at SR10

In this section the spark sensitivity between the tested fuels isexamined in more detail in terms of combustion performancespecifically at SR10. As previously mentioned, these tests were per-formed once the spark sweeps were analyzed and the MBT/KL-MBT

timings were known. Error bars have been used where applicablefor the three primary fuels (ethanol, DMF and gasoline) but havebeen omitted for the secondary fuels (butanol and methanol) inorder to maintain clarity. Similarly, solid lines have been used toposition DMF between gasoline and ethanol, whereas dashed

3 4 5 6 7 8 90.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00 1500rpm, λ = 1

IMEP

SR10

/IMEP

MB

T/K

L-M

BT

IMEPMBT/KL-MBT (bar)

ETH DMF ULG BUT MTH

Fig. 6. Effect of SR10 on normalized IMEP with increasing engine load betweenethanol, DMF, gasoline, butanol and methanol.

3 4 5 6 7 8 90.81

0.83

0.85

0.87

0.89

0.91

0.93

0.95

0.97

0.99

1.01 1500rpm, λ = 1

IndE

ff SR10

/IndE

ff MB

T/K

L-M

BT

IMEPMBT/KL-MBT (bar)

ETH DMF ULG BUT MTH

Fig. 7. Effect of SR10 on normalized indicated efficiency with increasing engineload between ethanol, DMF, gasoline, butanol and methanol.

R. Daniel et al. / Fuel 99 (2012) 72–82 77

lines are used for the secondary fuels. Firstly, the effect of sparksensitivity on IMEP is quantified in Fig. 6 at SR10 for each load witheach fuel.

As surmised from Fig. 5, the loss of IMEP at SR10 with increasinginitial load, quantifiably decreases from gasoline, to DMF and final-ly to ethanol. This is more clearly shown in Fig. 6. When fuelledwith ethanol, the loss of IMEP is always less than 7% (60.5 bar)across the entire load range, suggesting that the exact MBT timingfor ethanol is less critical than the other two primary fuels. ForDMF, the decay of IMEP is much closer to ethanol than it is for gas-oline. Up to 6.5 bar IMEP, the SR10 performance is almost identicalto that seen with ethanol. Above this load, the sensitivity increasesand ethanol outperforms DMF. In comparison to gasoline, this lossis less significant. At 8.5 bar IMEP, the decay of IMEP when usingDMF is only 0.76 bar, or 9%. However, for gasoline this loss in-creases to 1.54 bar (18.3%). Evidently, gasoline is much more sen-sitive to spark retard in terms of IMEP, than both ethanol andDMF, which is largely a function of its relatively low OI (see Table1).

The performance of butanol and methanol reside below that ofDMF and above ethanol, respectively. When using methanol, thefuel which exhibits the greatest knock resistance (OI = 99, see Ta-ble 1), the IMEP decay is less than 4% at all loads and is consistentlysuperior to ethanol. Although the difference in OI between ethanoland methanol is marginal, the greater charge-cooling effect ofmethanol plays a key role in further knock suppression. This is alsotrue for butanol, despite a similar OI to gasoline; due to the greaterheat of vaporization (see Table 1), the spark sensitivity is far supe-rior. This observation helps us to explain the performance of DMFand hypothesize its OI. It is possible that DMF produces a relativelyhigh OI because its lower heat of vaporization would not be takeninto account in the CFR engine tests. In reality, the low charge-cooling effect when using DMF would counterbalance the benefitof the increased OI to suppress knock. Therefore, as observed byother researchers, it is important to consider the charge-cooling ef-fect and not only the OI, when selecting a fuel to improve knocksuppression [42].

The indicated efficiency is a measure of the fuel conversion effi-ciency and compares the total work done to the theoretical energyavailable from the fuel supplied. The experimental study revealsthe reduced effect on indicated efficiency at SR10 when using eth-anol and DMF compared to gasoline due to their lower spark sen-sitivity. Fig. 7 shows the loss of indicated efficiency for the three

fuels at SR10, which demonstrates a similar trend between thefuels seen in Fig. 6. Once again, the low decay in indicated effi-ciency of ethanol (and methanol) reiterates the low sensitivity tospark timing retard. At 8.5 bar IMEP, when using gasoline, the nor-malized indicated efficiency drops by 18%, almost double the lossexperienced with DMF (10%) and a factor of 3.6 more than withethanol (5%). The low sensitive fuels benefit from an earlier opti-mum, where the effect of spark retard has less of an impact. Nev-ertheless, there is a clear difference in spark sensitivity betweenethanol and methanol, despite a similar MBT timing. This couldbe explained by the faster burning rate of methanol, which enablesthe energy from the air-fuel mixture to be more fully utilized ear-lier in the expansion stroke. Clearly, the varying degree of effi-ciency losses due to spark retard between the fuels will also havea detrimental impact on the fuel consumption rate.

The effect of spark sensitivity on the combustion stability andexhaust temperature is shown in Fig. 8. Both graphs use absoluteand not normalized units, in order to show the negative and posi-tive effects of spark retard, respectively. Fig. 8a highlights theadvantage of the oxygenated fuels on combustion stability overgasoline. Although the effect of spark sensitivity for all fuels de-creases with load (with the exception of butanol), the instabilityof gasoline remains the highest. This is due to reduced combustiondurations resulting from more readily available oxygen moleculesand more advanced spark timing (at SR10). In general, it is metha-nol that offers the highest stability (lowest COV of IMEP) throughspark retard. For gasoline and DMF, the MBT/KL-MBT timing ismore retarded and closer to top dead centre (TDC). At this pointthe in-cylinder turbulence is slightly reduced which subsequentlycompromises the burn rate [26]. For ethanol, the MBT timing ismore advanced, so the combustion at SR10 occurs during higherturbulence intensity, which enhances the burn rate. Despite this,during the mid-loads (4.5–7.5 bar IMEP), DMF offers slightly im-proved combustion stability over ethanol. This is possibly due tothe offset of improved fuel droplet vaporization because of thelow heat of vaporization of DMF (see Table 1). In fact, DMF isknown to produce smaller fuel droplets than ethanol at 150 barinjection pressure and with increasing distance from the injectornozzle [16]. Furthermore, the lower charge-cooling effect of DMFresults in higher initial combustion temperatures. This also helpsto promote mixture homogenization prior to ignition because themarginally elevated temperatures (compared to ethanol) improvethe rate of molecular diffusion of the fuel vapor within the air.

3 4 5 6 7 8 91.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0 1500rpm, λ = 1

CO

V of

IMEP

(%) SR

10

IMEPMBT/KL-MBT (bar) IMEPMBT/KL-MBT (bar)

ETH DMF ULG BUT MTH

3 4 5 6 7 8 9500

550

600

650

700

750

800 1500rpm, λ = 1

Exha

ust T

empe

ratu

re (°

C) SR

10 ETH

DMF ULG BUT MTH

a b

Fig. 8. Effect of SR10 on (a) coefficient of variation of IMEP and (b) exhaust gas temperature with increasing engine load between ethanol, DMF, gasoline, butanol andmethanol.

3 4 5 6 7 8 90.50

0.55

0.60

0.65

0.70

0.75 1500rpm, λ = 1

Pmax

SR10

/Pm

axM

BT/

KL-

MB

T

IMEPMBT/KL-MBT (bar)

ETH DMF ULG BUT MTH

3 4 5 6 7 8 90.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30 1500rpm, λ = 1

SPK

-MFB

5 SR10

/SPK

-MFB

5 MB

T/K

L-M

BT

IMEPMBT/KL-MBT (bar)

ETH DMF ULG BUT MTH

a

b

Fig. 9. Effect of spark retard on In-cylinder pressure and MFB at the highest engineload between gasoline and DMF.

78 R. Daniel et al. / Fuel 99 (2012) 72–82

Similarly to the combustion instability, the exhaust tempera-ture should not exceed a component protection limit. However, itshould be high enough to improve cold-start performance (for cat-alyst light-off) and enable rapid boost pressure build-up throughspark retard. Although these tests have been performed in a warmcondition, the trends can help us to understand the impact on acold engine. The high exhaust temperatures with load when usingDMF in Fig. 8b, demonstrates its suitability to potentially meetcold-start (fast catalyst light-off) and boosting design require-ments, whilst maintaining high combustion stability. At the lowestload, the exhaust temperature at SR10 matches that of gasoline(the most favorable fuel to meet the aforementioned demands)and, with increasing load, remains close to gasoline and the highestbetween all oxygenated fuels. Methanol, on the other hand, despiteoffering high combustion stability with low spark sensitivity interms of IMEP, produces the lowest exhaust temperature and dem-onstrates its unsuitability as a cold-start fuel.

These differences in performance and efficiency can be moreclearly explained when analyzing the in-cylinder pressure data,in particular, the maximum pressure (Fig. 9a) and resulting heatrelease data. In this instance, the initial combustion duration (de-fined as the CAD from ignition to 5% mass fraction burned (MFB))has been selected, in order to best highlight the detrimental impactat SR10 (Fig. 9b). For each combustion (and emissions) parameter,the absolute values when using ethanol, DMF and gasoline, havebeen compared in a previous publication by the authors [10]. Thisexamines the absolute behavior at fuel-specific MBT timings andretarded gasoline KL-MBT timings. Therefore, it is the aim of thepresent work to present the relative decay in combustion (andemissions) from these optimum conditions, in order to examinethe robustness of each fuel. In general, the effect of retarded igni-tion timing with load is a dramatic reduction in the maximumin-cylinder pressure and increase in the change in normalized ini-tial combustion duration (see Fig. 9). At low load, the decay in max-imum in-cylinder pressure is similar between fuels; the range at3.5 bar IMEP is <2%. However, as the load increases, the differencesbecome self-evident, whereby gasoline exhibits the greatestchanges at SR10. At 8.5 bar IMEP, the ignition timing at SR10 forgasoline is TDC. This delays the combustion phasing towards theexpansion stroke and produces a 48% reduction in maximum in-cylinder pressure. Amongst the oxygenated fuels, butanol andDMF follow similar reductions in maximum in-cylinder pressuremainly due to their similar knock suppression abilities. Up to5.5 bar IMEP, ethanol also behaves similarly, but is less affected

at higher loads. Methanol, which has the greatest OI, is least af-fected at SR10.

R. Daniel et al. / Fuel 99 (2012) 72–82 79

Clearly, this change in pressure impacts the initial combustionduration. At low loads for gasoline (64.5 bar IMEP) and DMF(66.5 bar IMEP), and almost all loads for ethanol, butanol andmethanol, the initial combustion duration actually reduces atSR10. This is due to greater in-cylinder pressures as the point ofignition approaches TDC. However, because gasoline requires themost retarded MBT/KL-MBT timings, this benefit is rapidly lostabove 5.5 bar IMEP. With methanol, however, the most retardedignition timing at SR10 is 11�bTDC (at 8.5 bar IMEP). Therefore,combustion originates later in the compression stroke when thepiston is closer to TDC and the in-cylinder pressure is higher (com-pared to MBT). For DMF, the initial combustion duration is more af-fected than the other oxygenated biofuels at 8.5 bar IMEP, but thiseffect is still less than with gasoline. Here, the increase in initialcombustion duration when using DMF is 6.7% (0.96 CAD), whereasfor gasoline the effect is much worse (27% increase, or 3.93 CAD).

3.4. Gaseous emissions at SR10

The sensitivity of the regulated indicated specific emissions andcarbon dioxide (CO2) to variations in spark timing is shown to be ascritical as the performance criteria. This section analyses the im-pact of spark retard at SR10 on the emissions, as well as on partic-ulate matter (PM), with fuel and load.

Fig. 10a shows the decrease in indicated specific nitrous oxide(isNOx) emissions at SR10. The formation of NOx is strongly relatedto the combustion temperature [26]; as the ignition is retarded, the

3 4 5 6 7 8 90.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75 1500rpm, λ = 1

isN

Ox SR

10/is

NO

x MB

T/K

L-M

BT

IMEPMBT/KL-MBT (bar)

IMEPMBT/KL-MBT (bar)

ETH BUT DMF MTH ULG

3 4 5 6 7 8 90.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0 1500rpm, λ = 1

isC

OSR

10/is

CO

MB

T/K

L-M

BT

ETH DMF ULG BUT MTH

a

c

Fig. 10. Effect of SR10 on normalized indicated specific (a) NOx (b) HCs (c) CO and (d) CO2

peak in-cylinder pressure and temperature lowers and the isNOx isreduced. This effect is shown for every fuel across the entire loadrange at SR10. Between the primary fuels at SR10, the isNOx reduc-tion using ethanol is the most effective, closely followed by gaso-line (especially P7.5 bar IMEP) and finally DMF. At 5.5 bar IMEP,the isNOx emissions for ethanol, gasoline and DMF reduce by62%, 53% and 44%, respectively at SR10. Therefore, not only doesspark retard have a minimal impact on efficiency when using eth-anol, it also produces the greatest benefits in isNOx reductions. Forgasoline, these benefits are outweighed by the severe performanceand efficiency losses, especially at higher loads.

The effect of spark retard on the indicated specific hydrocarbons(isHC) is shown in Fig. 10b. Although it is believed that FID analyz-ers could have a reduced sensitivity to oxygenated fuels [43,44],the results in Fig. 10b show a trend in the remaining total hydro-carbon emissions. Future work will include a detailed hydrocarbonemissions investigation for more accurate quantification usingFourier Transform Infrared Spectroscopy (FTIR). Nevertheless, theresults in Fig. 10b provide a good starting point. Here, as the igni-tion timing is retarded away from the MBT/KL-MBT location, theisHC production decreases. This is due to the increased time forthe mixing of induced air and injected fuel, which generates a morehomogenous mixture. This method is very effective in reducingisHC for the primary fuels (less so for ethanol when above5.5 bar IMEP due to compromised mixture quality). The resultsfor DMF consistently show more competitive reductions than withethanol across the entire load range. For instance, at 8.5 bar IMEP,

IMEPMBT/KL-MBT (bar)

IMEPMBT/KL-MBT (bar)

3 4 5 6 7 8 90.78

0.81

0.84

0.87

0.90

0.93

0.96

0.99

1.02

1.05 1500rpm, λ = 1

isH

CSR

10/is

HC

MB

T/K

L-M

BT

ETH BUT DMF MTH ULG

3 4 5 6 7 8 90.99

1.02

1.05

1.08

1.11

1.14

1.17

1.20

1.23 1500rpm, λ = 1

isC

O2 SR

10/is

CO

2 MB

T/K

L-M

BT

ETH DMF ULG BUT MTH

b

d

with increasing engine load between ethanol, DMF, gasoline, butanol and methanol.

80 R. Daniel et al. / Fuel 99 (2012) 72–82

the isHC reduction is 17.1% compared to 5.6% with ethanol. Previ-ous testing also showed how DMF produces lower isHC emissionsthan gasoline because of the oxygen contained in the fuel [9,10].This positive impact of spark retard on isHC emissions is coupledby the lower loss in IMEP when using DMF, than with gasoline.For butanol and methanol, the reduction is less impressive, despiteshowing large reductions in isNOx under the same conditions. Thissuggests the unburned HCs for the secondary fuels are alreadyhighly oxidized and the use of spark retard is an ineffective wayto further minimize their emissions.

When using oxygenated fuels, the indicated specific carbonmonoxide (isCO) emissions (see Fig. 10c) general decrease withignition retard (except for some instances above 7.5 bar IMEP).However, with gasoline, the isCO emissions dramatically increasefrom 5.5 bar IMEP. Although a lower combustion temperaturehelps to reduce NOx, the effect is detrimental to CO, especially athigh loads. As the spark is retarded at higher loads, combustion oc-curs very late in the expansion stroke for gasoline, which reducesthe temperature and pressure. These sub-optimal conditions resultin pockets of localized oxygen-deprived mixtures which generatehigher CO emissions as a result of incomplete combustion. At6.5 bar IMEP, the gasoline isCO emission increases by 22%, whereasno increase is observed with the oxygenated fuels. This rapidlyclimbs to 76% for gasoline, as the load is increased to 8.5 bar IMEP.Nevertheless, when using ethanol there are always isCO reductionsat SR10, regardless of the load, and until 7.5 bar IMEP when usingDMF. Once more, the large emissions reductions with spark retard

10 100101

102

103

104

105

106

Nucleation Mode

AccumulationMode

ETH MBT ETH SR10

ST = 21°bTDCIMEP = 8.5bar

dN/d

LogD

p (#

/cm

3 )

dN/d

LogD

p (#

/cm

3 )

Particle Diamater (nm)

1500rpm, λ = 1

10101

102

103

104

105

106

Nucleation Mode

Particle D

MBT

MBT

c

a

Fig. 11. PM size distributions at high load using (a) ethanol, (b) DMF and (c)

when using biofuels is attractive when their performance decay isso low.

Although CO2 is a non-toxic gas, which is not classified as an en-gine pollutant, it is one of the substances responsible for globaltemperature rises through the greenhouse effect. Therefore, a con-sideration of the indicated specific CO2 (isCO2) emissions withspark retard is made between the fuels in Fig. 10d. Unlike withthe previous emissions (except for some instances with isCO),the isCO2 emissions increase at SR10 and with increasing engineload. This emissions penalty is due to the increase in fuel consump-tion and reduction in indicated efficiency, as shown in Fig. 7. Infact, the inverse of the CO2 emissions almost equals the trend inindicated efficiency at SR10. As discovered with indicated effi-ciency, spark retard with gasoline results in the highest changein isCO2 emissions, while ethanol produces the least and DMF pro-duces only slightly more than with ethanol. At 8.5 bar IMEP, theisCO2 increase with gasoline is 19.4%, whereas with ethanol andDMF it is 5.6% and 10.7%, respectively. However, the low spark sen-sitivity of methanol results in the lowest change in isCO2 emissionsamongst all five fuels (3.2% at 8.5 bar IMEP). In summary, the in-crease in isCO2 is a function of the spark sensitivity of each fuel.

3.5. PM emissions at SR10

In addition to the CO2 emissions, the monitoring of PM numberemissions from gasoline engines is set to be enforced. Currently,PM number emissions do not form part of the emissions legisla-

dN/d

LogD

p (#

/cm

3 )

10 100101

102

103

104

105

106

Nucleation Mode

AccumulationMode

DMF KL-MBT DMF SR10

ST = 16°bTDCIMEP = 8.5bar

Particle Diamater (nm)

1500rpm, λ = 1

100

AccumulationMode

ULG KL-MBT ULG SR10

ST = 10°bTDCIMEP = 8.5bar

iamater (nm)

1500rpm, λ = 1

KL-MBT

KL-MBT

KL-MBT

KL-MBT

b

gasoline to compare the effect at MBT/KL-MBT and SR10 spark timings.

ETH DMF ULG0

1

2

3

4

5

6

7

8IMEPMBT/KL-MBT = 8.5bar

1500rpm, λ = 1

Tota

l Con

cent

ratio

n (#

/cm

3 x10

4 )

MBT/KL-MBT SR10

ETH DMF ULG25

28

31

34

37

40

43

46IMEPMBT/KL-MBT = 8.5bar

1500rpm, λ = 1

Mea

n D

iam

eter

(nm

)

MBT/KL-MBT SR10

a bFig. 12. (a) PM total concentrations and (b) mean particle diameters at high load using ethanol, DMF and gasoline to compare the effect at MBT/KL-MBT and SR10 sparktimings.

R. Daniel et al. / Fuel 99 (2012) 72–82 81

tions for gasoline spark-ignition engines in Europe or the US. How-ever, control of these emissions is expected to commence in Euro-pean regulation in 2014 [45]. This will require not only themonitoring of particulate mass, but also the particulate numberfor all light-duty vehicles. Therefore, an understanding of theseemissions will become much more important, especially whenusing biofuels. In this section, the PM emissions between the threeprimary fuels only are studied at MBT and SR10 at the highest tar-get load (8.5 bar IMEP). The PM size distributions are shown inFig. 11. Typically, the PM size distribution is bimodal and consistsof a nucleation and an accumulation mode. The former constitutesliquid particles, whereas the latter constitutes solid carbonaceousspecies. Although the separation between these two modes is ill-defined [46], in this study, a particle diameter of 50 nm has beenapplied to separate the nucleation (<50 nm) and accumulationmodes (>50 nm) as used in previous publications by the authors[9,10].

The separation between the nucleation and accumulationmodes is shown clearly by the inflection in size distributionsaround 50 nm for all fuels in Fig. 11. Clearly, the nucleation modeis the dominant mode for all three fuels. The total concentration ofthis mode is higher when using the two biofuels, compared to gas-oline but the accumulation mode is much smaller, a similar resultfound by other authors [47]. The PM emissions variation withspark retard appears to be the most sensitive when using ethanol,whereas with DMF, it is the least. At SR10, the peak number con-centration using ethanol is 359,614 particles/cm3, with a particlediameter of 38.5 nm, which is 46% and 33% more than at MBT,respectively. However, with DMF, the increase in particle concen-tration and diameter is less than half of this (20% and 15%, respec-tively). This trend might be a function of the in-cylindertemperature, whereby its change using ethanol at SR10, is greaterthan that with DMF (surmised from the NOx emissions in Fig. 10a).Although this helps to reduce the isNOx emissions, it conversely af-fects the PM nucleation mode with little effect on the accumulationmode. Overall, spark retard at SR10 largely affects the nucleationmode and not the accumulation mode distribution.

Fig. 12 shows the total PM concentration and mean particlediameter for the three fuels at the highest initial load (8.5 barIMEP). At SR10, the total PM concentration and particle diameterincreases in almost every case. As shown with the size distribu-tions in Fig. 11, the change in total concentration when usingDMF is the lowest, albeit at a greater absolute value. For instance,from KL-MBT timing to SR10, the total PM concentration with DMFincreases by 1429 particles/cm3 (2.1%), whereas with ethanol thisis 12,620 particles/cm3 (26.6%). However, the two biofuels have

larger total concentrations compared to gasoline, which is mainlydue to the dominant nucleation mode. Nevertheless, in terms ofmean particle diameter, the two biofuels produce a lower meanparticle diameter than with gasoline. Ethanol, which shows thegreatest sensitivity to spark retard, has a low mean diameter atMBT timing of 29.6 nm, whereas for gasoline this is 42 nm. How-ever, for ethanol this rapidly increases to 38.7 nm at SR10 high-lighting its sensitivity, whereas there is minimal change withgasoline.

4. Conclusions

This study compares the spark sensitivity of three primaryfuels: DMF (2,5-dimethylfuran), commercial gasoline and ethanol,with two secondary fuels: butanol and methanol. The experimen-tal engine tests were performed on a single cylinder DISI enginefrom 3.5 bar to 8.5 bar IMEP in 1 bar IMEP increments and at afixed engine speed of 1500 rpm. The engine was first tested usingeach fuel under various spark sweeps and then an arbitrary 10CAD spark timing retard was chosen, denoted SR10. Based on theseexperiments, the following conclusions can be drawn:

1. All five fuels have different spark sensitivities with respect toengine load. In terms of IMEP and indicated efficiency, the orderof increasing sensitivity is: methanol > ethanol > DMF >butanol > gasoline.

2. When selecting a fuel to improve knock suppression, it isimportant to consider the charge-cooling effect and not onlythe OI.

3. At SR10, DMF produces high combustion stability andhigh exhaust temperature. However, gasoline and ethanolsuffer from either low stability or exhaust temperature,respectively.

4. Both isNOx and isHC decrease for all fuels at SR10. The isCOemissions are largely reduced for all oxygenated fuels, but notso for gasoline, which increased to a maximum of 76%.

5. The trend in isCO2 is inversely proportional to that seen withindicated efficiency. Once more, gasoline is the most sensitiveto spark retard and DMF performs similarly to ethanol.

6. The PM emissions increase at SR10 (largely nucleation modeparticles) for ethanol, DMF and gasoline at the highest, wherebyethanol is the most sensitive.

7. The widened spark window when using oxygenated fuels canhelp to improve calibration flexibility by increasing the rangein which to maximum the reduction of emissions.

82 R. Daniel et al. / Fuel 99 (2012) 72–82

In summary, these experiments highlight the benefit of biofuelsover commercial gasoline, in terms of spark sensitivity. Gasoline ishindered by the onset of knock, which requires more accurate con-trol of the spark timing. However, it has been shown that this isless critical with certain biofuels, as large NOx and HC emissionsbenefits are achieved with little performance degradation.

Acknowledgments

The present work is part of a 3-year research project sponsoredby the Engineering and Physical Sciences Research Council (EPSRC)under the grant EP/F061692/1. The authors would like to acknowl-edge the support from Jaguar Cars Ltd., Shell Global Solutions andvarious research assistants and technicians. The authors are alsograteful for the financial support from the European RegionalDevelopment Fund (EUDF) and Advantage West Midlands(AWM). Finally, the authors would like to acknowledge the supportfrom their international collaborators at Tsinghua University,China.

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