Fuel 90 (2011) 3179–3192

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NOx and PM emissions reduction on an automotive HSDI Diesel enginewith water-in-diesel emulsion and EGR: An experimental study

Alain Maiboom ⇑, Xavier TauziaLaboratoire de Mécanique des Fluides, UMR CNRS 6598, Internal Combustion Engine Team, Ecole Centrale de Nantes, BP 92101, 44321 Nantes Cedex 3, France

a r t i c l e i n f o

Article history:Received 7 June 2010Received in revised form 9 June 2011Accepted 10 June 2011Available online 29 June 2011

Keywords:Automotive Diesel engineWater-in-diesel emulsionExhaust gas recirculationCombustionHeat release

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.06.014

⇑ Corresponding author. Tel.: +33 2 40 37 68 80; faE-mail address: alain.maiboom@ec-nantes.fr (A. M

a b s t r a c t

Automotive Diesel engines exhaust emissions must constantly be reduced to comply with more and morestringent regulations, all over the world. The introduction of water in the combustion chamber is alreadyused on some large marine diesel engines to cut down NOx emission.

In this paper an experimental study is conducted on a modern automotive 1.5 l HSDI Diesel enginewhile injecting a water-in-diesel emulsion (WDE) with a volumetric water-to-fuel ratio of 25.6%. Fourinjection strategies are considered with and without pilot injection, with two levels of injection pressure.First, the injection of WDE is compared to diesel-fuel in terms of combustion and NOx and PM emissionswithout using exhaust gas recirculation (EGR). Depending on the WDE fuelling rate and injection strategy(with or without a pilot injection before main injection), NOx emissions are most often reduced (of up to50%), and PM emission are most often decreased as well (the maximum relative reduction being 94%).The combustion is largely affected by the injection of WDE as compared with pure diesel-fuel, the mainobservations being an increased of the ignition delay and an improved mixing-process between the fueland the surrounding gases.

After that, the use of WDE in parallel with EGR (with various EGR rates) is tested with the aim atimproving the NOx–PM trade-off (reduction of NOx emission at a given PM emission level or reductionof PM emission at a given NOx emission level). The results show that this method is an effective wayfor NOx and PM emission reduction on an automotive Diesel engine.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In light of the current requirements as regards the reduction ofpollutant emissions of automotive Diesel engines such like EURO 6in Europe, manufacturers have to develop new in-cylinder strate-gies and/or aftertreatment devices [1]. With the upcoming pollu-tant regulations, NOx emission will become particularly criticalon automotive Diesel engines.

As regards the in-cylinder strategies aiming at reducing NOxemission, exhaust gas recirculation (EGR) into the engine intakeis the most used and studied technology. The decrease of NOxemission with EGR is the result of complex and sometimes oppo-site phenomena occurring during combustion [2–15]. The maineffect is the decrease of local temperatures in the combustionchamber, in particular those corresponding to zones where NO isproduced (on the lean side of the diffusion flame during fuel injec-tion [16] and in the combustion products after the end of injec-tion). The main drawback of EGR is the increase of PM emissionin the classical high temperature diesel combustion (HTC) andthe need to increase boost pressure at middle and high loads when

ll rights reserved.

x: +33 2 40 37 25 56.aiboom).

using EGR to maintain the air–fuel ratio (AFR) at a suitable level[5,6].

Another in-cylinder strategy to reduce local temperatures andconsequently the NO production rate is the injection of water(WI), either into the engine inlet [17–25], directly in the combus-tion chamber [26–33], or in emulsion with the fuel [18,19,21–23,34–56]. One advantage of WI as compared with EGR is the possiblereduction of NOx emission either at low loads and high loads with-out a substantial increase in PM emission.

The probably easiest way to inject water in the engine is inlet WI[17–25]. This technique has been used on some large marine Dieselengines [18] and various strategies to inject water in the inlet air arepresented in the literature. The main drawback of inlet WI is that awater mass of about 60–65% of the fuel is needed to achieve a 50%NOx reduction, and is very high (up to four times the amount of fuelmass) if trying to drastically reduce NOx emission [25].

Different strategies have been also proposed to inject waterdirectly into the combustion chamber, with the aim at reducingNOx emission while limiting the water quantity as compared withinlet WI. One advantage of direct WI as compared with water-in-diesel emulsion is the possibility to change the water-to-fuel ratio,while varying engine parameters (speed and load) or during enginewarm-up (cold start) [28–30].

Nomenclature

AFR air–fuel ratio (–)AFRst stoichiometric air–fuel ratio (–)AMF air mass flow (kg/h)FMF fuel mass flow (kg/h)MNO2 molar mass of NO2 (g/mol)NOx (ppm) nitrogen oxides concentration (ppm)NOx (g/h)

nitrogen oxides emission (g/h)P pressure (bar)PM particulate matter (g/h)Qexhaust exhaust gas flow (non-condensed) (m3/h)Qexhaust_dry exhaust gas flow (dry) (m3/h)Smoke (mg/m3) smoke emissions in mg/m3

Smoke (g/h) smoke emissions in g/hT temperature (�C)Vm molar volume (l/mol)XEGR EGR ratio (%)XCO2 CO2 concentration (percentage in volume, dry) (%)k excess air/fuel ratioqair air density (kg/m3)qburned-gas

burned gas density (kg/m3)

Subscripts1 ambient conditions2 post-compressor20 post-intercooler200 post-EGRair air

m relative to main injectionp relative to pilot injectionrail fuel rail

AbbreviationsATDC after top dead centreBMEP brake mean effective pressureBSFC brake specific fuel consumptionCA crank angle (deg)DOC Diesel Oxidising CatalystsEGR exhaust gas recirculationFSN filter smoke numberHP high pressureHSDI high speed direct injectionHTC high temperature combustionID ignition delayIGR internal gas residualIVC intake valve closingNEDC new European driving cyclePM particulate matterROHR rate of heat release (W)SOC start of combustion (CA deg)SOI start of injection (CA deg)TDC top dead centreUHC unburned hydrocarbonsVGT Variable Geometry TurbineWDE water-in-diesel emulsionWI water injection

3180 A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192

The last method to inject water is the use of water-in-dieselemulsion or microemulsion [18,19,21–23,34–57]. Most engineexperiments and numerical studies using WDE technique show thatthe NOx reduction is accompanied with a large reduction of PM andsoot emissions. Main effects of WDE on PM emission are as follows:

� At a given fuel injection rate, the use of WDE leads to anincrease of the total injected mass, of which a consequence isan increase of the mixing rate between fuel and air, thus reduc-ing local fuel–air ratios and consequently PM production [51].� The vaporisation of water and the dilution effect of water lead

to a decrease of the temperatures within the core spray wheresoot is produced, thus reducing the soot production rate [35,51].� The increased presence of water within the combustion jet may

affect the chemical kinetic mechanisms of soot formation (inthe core spray) and soot oxidation (at the jet periphery)[35,51,56].� The presence of water in the emulsion has a tendency to

increase the ignition delay. Thus, the mass fraction of fuel thatburns under a premixed combustion is increased, which a directconsequence is the decrease of soot production rate[36,38,39,52,53].� It has been shown that the location of flame lift-off on diesel

fuel jet plays an important role in the soot formation process,by allowing fuel and air to mix upstream of the lift-off length(i.e. prior any combustion) [58–61]. Just downstream of thelift-off length, the partially premixed air–fuel mixture under-goes a premixed combustion that generates a significant localheat release and fuel-rich product gas that becomes the ‘fuel’for the diffusion flame at the jet periphery. The soot formationwas shown to be directly dependant of the equivalence fuel–air ratio at the lift-off length [58–61]. Local studies in an

optically-accessible engine show that that flame lift-off is sig-nificantly increased with WDE, leading to leaner mixtureswithin the jet during the diffusion stage of combustion [39].� The water droplets contained in the emulsion have a lower boil-

ing point than diesel-fuel. According to some researchers, thesudden and dramatic expansion of vaporising water (calledmicro-explosion) would enhance the mixing process betweenair and fuel [21,43–45,51,53] and lead to the combustion ofsmaller diesel droplets.� Polycyclic aromatic hydrocarbons (PAH) have been shown to

play an essential role as precursors for soot particles duringtheir formation and growth [51,61]. The combustion of WDEleads to lower PAH concentration within the core spray, thusreducing the soot production rate [51].� Finally, the vaporisation of water and the dilution effect of

water lead to a decrease of the temperatures at the jet peripherywhere soot is partially oxidised [61,67,68] that may reduce thesoot oxidation rate.

As regards unburned hydrocarbons (UHC) and carbon monoxide(CO) emissions, results are contradictory with some tests showingreduction [21,47] and other tests showing increase [22,34,35].

WDE is used on some large marine Diesel engines to cut downNOx emission [62–64] and some on-road tests have been carriedout on captive fleets of trucks and buses [65].

As mentioned above, the most efficient WI technologies to re-duce NOx emission are WDE or direct WI, because the water is in-jected directly into the combustion zone, allowing a large decreaseof combustion temperatures [18,23,51]. As a consequence, for a gi-ven quantity of injected water, the NOx reduction with directwater injection or WDE is around twice as high as with inlet water

Table 1Engine specifications.

Compression ratio 17:1Number of cylinders 4Number of valves per cylinder 2Combustion chamber Re-entrant bowl-in-pistonInjection system Common-rail solenoidMaximum injection pressure 1600 barNumber of injection holes 6

A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192 3181

injection [18,23]. As regards PM emission, the use of WDE seems toshow better results than other WI techniques.

Thus, the aim of this paper is to experimentally study the poten-tial of WDE in parallel with EGR as an in-cylinder strategy for bothNOx and PM emissions reduction on a modern automotive com-mon-rail DI Diesel engine for future emissions standards.

2. Material and methods

2.1. Description of the engine

The engine used for the experiment is a 1.5 l water-cooled HSDI4-cylinders diesel engine, with two valves per cylinder and whichconforms to Euro III standards. It is equipped with a common-railsolenoid injection system with a maximum injection pressure of1600 bar, re-entrant bowl-in-piston combustion chambers, a Vari-able Geometry Turbine (VGT) turbocharger, an intercooler, and aDiesel Oxidising Catalyst (DOC) (see Fig. 1). Engine specificationsare given in Table 1.

The engine is originally equipped with a high pressure (HP) un-cooled EGR loop. An EGR cooler has been added and an indepen-dent water circuit on the EGR-cooler was used in order to controlthe temperature of the recirculated gases, and thus the tempera-ture T200 of inlet gases after mixing with EGR. The mean EGR rateis defined as follows:

XEGR ð%Þ ¼ 100 � XCO2 inlet

XCO2 exhaustð1Þ

where XCO2_inlet and XCO2_exhaust are measured CO2 concentrations ininlet and exhaust manifolds respectively.

The original air/air intercooler was turned into a water/air inter-cooler allowing the air temperature T20 and/or T200 to be controlledseparately.

2.2. Water-in-diesel emulsion

Many experiments have been done to find a stable WDE (oversome days or weeks). Various concentrations of different types ofemulsifiers were tested in the WDE. The Hydrophilic LipophilicBalance (HLB) methodology was used to help finding a stableWDE. During the tests, the more stable emulsion was obtainedby using two emulsifiers, Span 80 (sorbitan monooleate) andTween 85 (polyoxyethylenesorbitan trioleate), the volumetric ra-tios being respectively 1.3% and 0.7% and the HLB number beingequal to 6.5. Also, various water quantities in the WDE have beentested on the test bench (with a volumetric water-to-fuel ratio ofup to 43%), in order to reduce NOx and PM emissions as much aspossible while limiting the negative impacts of water (high

Fig. 1. Engine configuration.

cycle-to-cycle dispersions and misfired cycles at very low loadconditions, large increase of CO and UHC emissions at low loadconditions or during engine warm-up, misfire problems at enginestart-up in particular in cold conditions). A volumetric water-to-fuel ratio of 25.6% was found to be a good compromise (thevolumetric fraction of water in the total WDE being thus equal to20%). The volumetric composition of the WDE tested in this studyis as follows:

� Diesel-fuel: 78%.� Water: 20%.� Span 80: 1.3%.� Tween 85: 0.7%.

A photography of the WDE has been obtained with a micro-scope (Fig. 2), showing that most water bubbles have a diameterlower than 40–50 lm, but with some slightly larger water bubbles.It must be underlined that the emulsion was not optimised interms of stability or bubbles diameter dispersions (such likeWDE commercialized by some manufacturers), but was found tobe good enough to undertake experimental studies on the testbench.

2.3. Evaluation of the mean gross rate of heat release (ROHR)

The gross ROHR is obtained for each operating condition thanksto a calculation procedure developed at the laboratory. The calcu-lation is classically based on the in-cylinder pressure. It is mea-sured with a Kistler 6055BB piezo electric pressure transducerand an encoder with a resolution of 0.36�CA. The cylinder pressureused is the mean value over 100 consecutive cycles, which wasfound to be enough for the mean value to be reliable. It must beunderlined that the mean ROHR gives no indication on eventual cy-cle to cycle dispersions. The gross ROHR was extracted from the netROHR by calculating the heat transfer to the combustion chamberwalls with Hohenberg’s model [66].

Since valve overlap is negligible on this engine, trapped mass offresh air can be estimated directly from air mass flow measured at

Fig. 2. Photography of the emulsion obtained with a microscope.

3182 A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192

the engine inlet. The mass of residual gas is calculated with perfectgas law applied at EVC, from volume, measured in-cylinder pres-sure and in-cylinder temperature estimated from exhaust gas tem-perature. The residual gas composition is deduced from exhaustgas analysis. When EGR is used, EGR mass flow is calculated fromCO2 concentrations measurements at the engine inlet and engineexhaust (that gives the ratio between EGR mass flow and totalair + EGR mass flow, the air mass flow being measured at the en-gine inlet). EGR composition is deduced from exhaust gas analysis.

2.4. Emissions measurement

NOx emission is measured with an ECO PHYSICS CLD 700EL gasanalyser, which uses the chemical luminescence detector (CLD)method. NOx emissions are converted from ppm to g/h:

NOx ðg=hÞ ¼MNO2 � NOx ðppmÞ � Qexhaust dry

103 � Vm

ð2Þ

where MNO2 = 46.005 g/mol and Vm = 22.41 l/mol at standard tem-perature and pressure.

PM emission at the exhaust is measured with an AVL 415Ssmoke-meter. The relation between filter smoke number (FSN)and smoke emissions in mg/m3 is given in the AVL 415S operatingmanual and is as follows:

Smoke ðmg=m3Þ ¼ 10:405

� 5:32 � FSN � expð0:3062 � FSNÞ ð3Þ

Smoke emissions in g/h are thus given by:

Smoke ðg=hÞ ¼ 10�3 � Smoke ðmg=m3Þ � Q exhaust ð4Þ

As the value of air excess is more than 1 for all tested operatingconditions, exhaust gases are composed of stoichiometric burnedgases and non-consumed air. Thus, the non-condensed exhaustgas flow Qexhaust is given by:

Q exhaust ¼ FMF � 1þ AFRst

qburned gasþ ðk� 1Þ � AFRst

qair

" #ð5Þ

where AFRst = 14.4 is the stoichiometric air–fuel ratio, qburned_gas =1.33 kg/m3 is the burned gas density of a stoichiometric mixture,qair = 1.293 kg/m3 is the air density, and k is the air excess.

Similarly, the dry exhaust gas flow Qexhaust_dry is given by:

Q exhaust dry ¼ FMF � ð1þ AFRstÞ � aqburned gas

þ ðk� 1Þ � AFRst

qair

" #ð6Þ

where a is the mass of dry exhaust gases in 1 kg non-condensed ex-haust gas (equal to 0.924 kg for diesel fuel).

The air excess is calculated with measured air mass flow AMFand fuel mass flow FMF:

k ¼ AMFAFRst � FMF

ð7Þ

Inlet and exhaust CO2 concentration are measured with a CAP-ELEC CAP3200 gas analyser and a SIEMENS ULTRAMAT 23 gas ana-lyser respectively, which use the non-dispersive infrared (NDIR)measurement technique. Each gas analyser is calibrated every 4 hof experiments with specific gas standards. If the necessary shiftis under 0.3%, then the experiments done since the previous cali-bration are validated.

2.5. Evaluation of fuel proportion injected during ignition delay

In order to interpret NOx and soot emissions it can be helpful toknow the proportion of fuel that burns during the premixed phaseand that which burns during the diffusion phase. In fact, it is rather

impossible to access directly this proportion. Thus, the proportion rof fuel injected during ignition delay (ID) was calculated for eachoperating condition tested. The injection rates were obtainedthanks to a simplified model of the injector (calibrated with mea-surements of instantaneous injection rates with diesel fuel madeon a standard injection test bench [69]) for each operating point,thus providing the instantaneous proportion of injected fuel ateach time step. The ignition delay is supposed to end when com-bustion acceleration (differential of ROHR, in W/s) reaches a criticalvalue, which was arbitrarily fixed at 2 � 108 W/s. It should be no-ticed that the air–fuel mixture formed during ID is generally rich[68], such that all the diesel-fuel injected during ID will not burnunder a premixed combustion mode. The ratio r can be thus con-sidered as an upper bound of the proportion of fuel that burns un-der a premixed combustion mode.

2.6. Error analysis

Table 2 sums up the measurement technique, calibrated range,accuracy and relative error of various instruments involved in theexperiment for various parameters. Errors in experiments can arisefrom instrument conditions, calibration, environment, observation,reading and test planning. The accuracy of the experiments has tobe validated with an error analysis. That was performed here usingthe differential method of propagating errors based on Taylor’ the-orem. It gives the maximum error u of a function f(x1, x2, . . . , xn) asfollows:

uðf ðx1; x2; . . . ; xnÞÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXðci � uðxiÞÞ2

qð8Þ

As a result, the maximum relative errors for XEGR, NOx (g/h), PM(g/h) are 1.4%, 1.5%, and 2.3% respectively.

2.7. Operating points

The study is conducted for five various operating conditions(load and speed). The engine speed, torque, pilot and main die-sel-fuel injections quantities, pilot and main start of injections(SOI) as well as approximate brake mean effective pressure (BMEP)are given in Table 3. Operating points 1–3 are operating pointssuch as those encountered in the European emissions test cycle(NEDC) for a classical vehicle (equipped with a moderated down-sized Diesel engine). The whole cycle is composed of four urbandriving cycles and one extra urban driving cycle.

Operating points 4 and 5 are operating points with higher loads,thus representative of operating points that could be encounteredin the extra urban driving cycle with a much downsized Diesel en-gine. Such operating points are particularly critical in terms of NOxand PM emissions in this test cycle. Actually, on the one hand, theuse of EGR is limited to a low level with the HP EGR loop at theseloads because an increase of EGR results in both a decrease of O2

concentration in the inlet and a decrease of boost pressure, thusreducing the in-cylinder O2 content. At part load operation, thiswould irremediably lead to a low value of the air excess, and thusto large amounts of PM, CO, and UHC emissions, as well as a largeincrease of brake specific fuel consummation (BSFC). On the otherhand, there is a clear tendency to further downsize automotiveDiesel engines, and thus there is a need to reduce pollutant emis-sions at higher loads. It is why operating points 4 and 5 have beenstudied in this paper.

Quantities of injected diesel fuel are held constant for eachoperating point. Thus, BMEP is little modified with various testedmodifications (use of WDE, EGR rate, inlet temperature). Initial val-ues of boost pressure for each operating point were held constantin all tests presented hereafter; when using HP EGR, the decreaseof boost pressure while opening the EGR valve is compensated

Table 2Relative measurement error.

Instrument Calibratedrange

Accuracy Relativeerror (%)

Inlet gas temperature (k-typethermocouple)

0–1000 �C ±1 �C ±0.75

Inlet gas pressure (2 barpiezoresistive relativepressure sensor HCS SensorTechnics)

0–2 bar ±5 mbar ±0.25

Air mass flow (hot wire air flowmeter)

0–800 mg/str ±4 mg/str 1

Fuel consumption (ROTRONICSDMC202)

0–40 kg/h ±40 g/h ±0.1

NOx (ECO PHYSICS CLD 700EL) 0–1000 ppm ±5 ppm 1Smoke (AVL 415S) 0–10 FSN ±0.1 FSN 2Inlet CO2 (CAPELEC CAP 3200) 0–20% ±0.15% 1.5Exhaust CO2 (SIEMENS

ULTRAMAT 23)0–20% ±0.1% 1

In-cylinder pressure (Kistler6055BB)

0–200 bar ±0.5 bar 1

Table 3Operating points.

Point 1 2 3 4 5

Engine speed (rpm) 1480 2035 1480 2065 1460Torque (Nm) 21.0 65.2 78.7 116.5 163.9BMEP (bar) 1.32 4.11 4.95 7.33 10.31Pilot quantity (mg diesel/stroke) 1.3 1.7 1.6 1.9 2.0Principal quantity (mg diesel/

stroke)5.3 12.1 14.8 21.1 28.8

Boost pressure P200 (mbar) 1040 1250 1230 1690 1630Pilot SOI (�CA ATDC) �27.0 �31.5 �32.6 �36.5 �40.0Main SOI (�CA ATDC) 3.2 3.1 3.5 1.7 �0.3

Table 4Rail pressures for various injection strategies.

Point 1 2 3 4 5

Diesel 405 655 520 880 900WDE – Prail 1 392 633 502 850 869WDE – Prail 2 490 1032 813 1400 1430

A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192 3183

by closing VGT vanes. Pilot and main SOI are maintained constantas well.

3. Influence of water-in-diesel emulsion without EGR

A first study has been carried out to investigate some of the ef-fects of WDE on combustion, NOx and PM emissions (withoutEGR). Actually, as described in the introduction, most of experi-mental studies dealing with the use of WDE presented in the liter-ature have been done on large Diesel engines. The aim of this firstsection is thus to present the effects of WDE on a recent automo-tive HSDI Diesel engine, equipped with a common-rail high pres-sure injection system.

Two injection strategies have been tested:

� The first strategy consists in injecting WDE with the same max-imum instantaneous mass injection rate through the injector aswith diesel fuel. This is achieved with a slight reduction of railpressure (because of the higher density of WDE as comparedwith diesel fuel, which a consequence is an increase of massflow rate through the injector if maintaining the same rail pres-sure). With this first strategy (‘‘WDE – Prail 1’’ on the followingfigures), to ensure that same fuel mass is injected per cylinderand per cycle, the injection duration has to be increased. Theinstantaneous injection rate of the fuel part in the WDE is thuslower than with diesel fuel being injected alone.� Second strategy consists in maintaining approximately the

same diesel fuel injection rate with WDE as compared with die-sel fuel alone. The WDE must thus be injected faster into thecombustion chamber to maintain the same instantaneous fuelintroduction. This is achieved by increasing rail pressure

(‘‘WDE – Prail 2’’ on the following figures). The injection dura-tion is kept approximately constant as compared with dieselfuel alone.

The calculation of the injection pressures Prail1 and Prail2 wasobtained thanks to the simplified model of the injector. Rail pres-sures Prail1 and Prail2 for each operating point are given in Table4. It can be noticed that the injection pressure Prail2 is much high-er than Prail1, especially for operating points 2–4.

Moreover, the influence of WDE on combustion and pollutantemissions has been studied with both a single fuel injection, orwith a double-shot injection (one pilot injection before main injec-tion). One or two pilot injections before main injection are classi-cally used to decrease the ID of main injection such that thepremixed part of main combustion is lower, thus limiting the highin-cylinder pressure derivative and the corresponding combustionnoise. When using only one injection (without pilot injection), thefuel mass of pilot injection is added to the fuel mass injected dur-ing main injection, such that the total mass of fuel injected per cy-cle is constant for each injection strategy (with or without pilotinjection, diesel fuel alone, WDE with injection pressure Prail1,WDE with injection pressure Prail2).

Main results in terms of combustion and NOx–PM emissionswith these various injection strategies are presented hereafter.

3.1. Influence on combustion

3.1.1. Without pilot injectionBefore studying a classical pilot + main injection, it was found

interesting to study the influence of WDE on a single injection.Mean gross ROHR for operating points 4 and 5 are presented inFig. 3. The standard analysis of the rate of heat release [2,68] showsseveral effects. Without pilot injection, the ROHR diagram is com-posed of two phases: a ‘‘premixed peak’’ due to the fast combus-tion of some of the fuel injected during ID, followed by adiffusion part during which combustion is controlled by the mix-ing-process between diesel-fuel and the surrounding gases (seeRefs. [2,68] for more details about the ROHR analysis).

Table 5 gives the ID and the proportion of diesel-fuel injectedduring ID.

The very first effect of WDE is the increase of ID (Table 5 andFig. 3). This is in agreement with previous researches [21,35,36,52]. The ID increase is probably due to the evaporation of watercontained in the WDE that causes local temperatures in the fuel jetto decrease.

The ID increase is observed for both injection strategies, but thesecond strategy (that maintains the diesel-fuel introduction rateinto the combustion chamber owing to a higher injection pressure)results in a lower ID increase. For instance, for operating point 4,the ID increase with first WDE injection strategy is about 27%whereas it is about 4% for the second WDE injection strategy. Sametendencies are obtained for the others operating points. The lowerID increase with the second WDE injection strategy may be due toa higher WDE injection rate and better atomisation as comparedwith the first strategy, such that some fuel is mixed with air fasterand can ignite earlier.

Given experimental results, it seems that the first effect is pre-dominant, such that the second WDE injection strategy results in a

Fig. 3. ROHR comparison of diesel-fuel and WDE, without pilot injection.

Table 5Ignition delay and proportion of fuel injected during ignition delay (r).

Operating condition 4 4 4 5 5 5Diesel WDE WDE Diesel WDE WDE

Prail1 Prail2 Prail1 Prail2

ID (ms) 0.29 0.37 0.30 0.33 0.42 0.34ID increase – +27% +4% – +29% +5%r (%) 15 19 20 14 19 20

3184 A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192

large decrease of ID as compared with the first WDE injectionstrategy.

Another effect of WDE on combustion is the modification of thepremixed part of combustion. For operating points 4 and 5, the pre-mixed part of combustion (that results from the introduction ofdiesel-fuel during ID) is largely lower than the diffusion part ofcombustion. Actually, the proportion r of diesel-fuel injected dur-ing ID does not exceed 20% (see Table 5).

For these two operating points, the premixed part of combus-tion results in a ‘‘traditional’’ premixed peak on the ROHR diagram.Main factors that influence the magnitude of the premixed peakare as follows:

� The ID directly influences the quantity of fuel that is injectedduring ID, and thus that can mix with the surrounding air toburn in a premixed combustion mode [2,68].� In case of dilution of the diesel-fuel (by water in this study), for

a given quantity of injected mass during ID, the dilution of thefuel decreases the quantity of diesel-fuel injected during ID.� The intensity of the mixing rate between the injected fuel and

the surrounding air during ID in the combustion chamber alsoinfluences the premixed peak, a higher mixing rate resultingin a higher quantity of injected fuel that is mixed with air andthus that burns in a premixed combustion mode [2,68]. Withthe second WDE injection strategy, the increased injection pres-sure increases the mixing rate as well.� Moreover, a premixed combustion is kinetically-controlled,

such that higher temperatures in the air–fuel mixture formedduring ID will result in a higher premixed peak [2].� Finally, the gas composition (in case of EGR and/or IGR) clearly

influences the premixed peak as well. Actually, for a given mix-ing rate between injected fuel and surrounding gases during ID,the dilution of fresh air by burned gases – owing to the externalEGR loop or because of residual gases at intake valve closing(IVC) – decreases the mixing rate between fuel and fresh air.

In our case, for both WDE injection strategies, the first fouritems are affected. Only the dilution of the fresh air is unchangedsince EGR valve is closed and there is no reason for IGR to change.

With the first WDE injection strategy, the premixed peak onROHR diagram is increased as compared with diesel-fuel. The in-creased ID results in a slight increase of the ratio r, from 15% to19% for operating point 4 and from 14% to 19% for operating point5. Although the diesel-fuel introduction rate is reduced with firstWDE injection strategy as compared with diesel-fuel, there isslightly higher diesel-fuel injected during ID because of a longerID. The local temperatures in the premixed zone are certainly re-duced because of the water evaporation process, but this thermaleffect seems to have less influence than the increase of the propor-tion r such that the premixed peak on the ROHR is increased.

With the second WDE injection strategy, the ID increase is low-er as compared with the first WDE strategy, but the proportion r ofdiesel-fuel injected during ID is approximately constant as com-pared with first WDE injection strategy (r is equal to 20% against19% for both operating points 4 and 5). For these operating points,the ID decrease with Prail2 as compared with Prail1 compensatesfor the higher diesel-fuel injection rate with Prail2. Although thefuel proportion r is approximately the same with both WDE injec-tion strategies, the second strategy results in a largely higher pre-mixed peak on the ROHR diagram. This can be explained by ahigher mixing process between the WDE jet and the surroundinggases. Actually, since the mass introduction rate of water + die-sel-fuel is increased as compared with diesel-fuel alone, the airentrainment by the jet is increased, resulting in an increased airentrainment per unit of diesel-fuel in the WDE jet, in particularduring ID, resulting in a less rich air–fuel mixture formed duringID. Again, as mentioned previously, the improved mixing-processbetween surrounding gases and diesel-fuel is also a consequenceof the increased injection pressure with second WDE injectionstrategy; this effect would also occur if using only diesel-fuel withincreased injection pressure.

As regards the diffusive part of combustion, the use of WDE alsoproduces noticeable effects. For first WDE injection strategy, theROHR diagram shows a slower diffusive combustion speed as com-pared with diesel-fuel. This is due to a lower introduction rate ofdiesel-fuel with first WDE injection strategy. For the second WDEinjection strategy, while the diesel-fuel introduction rate is approx-imately identical as compared with diesel-fuel being injected alone,the combustion speed during diffusion combustion is largely in-creased, showing again that there is an enhanced mixing of sur-rounding gases per unit of injected diesel-fuel. One consequenceis a positive impact on PM emission as will be shown later.

3.1.2. With pilot injectionThe influence of WDE on the combustion with a ‘‘classical’’ dou-

ble-shot pilot + main injection is given in Fig. 4 for operating points2–5. Also given in Fig. 4 are the SOI for pilot and main injections.

A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192 3185

It can be noticed that the fuel injected during pilot injection hasa two-stage autoignition, with a ‘‘cool-flame’’ low-temperatureheat-release (LTHR) phase followed by a main high-temperatureheat-release (HTHR) phase (see annotations in Fig. 4a). Such a com-bustion mode is classically observed in compression ignition en-gines with fuel being injected sufficiently early in the cycle whenthe in-cylinder air temperature is too low for the fuel to ignite di-rectly, thus letting fuel enough time to mix with air. The air–fuelmixture ignites later in the compression stroke (ignition by com-pression) when in-cylinder temperature is higher and the corre-sponding combustion is kinetically controlled. Such combustionregimes are also involved in homogeneous charge compressionignition (HCCI) engines where all the fuel is injected very early inthe cycle (at the beginning of the compression stroke or in the in-take manifold).

For operating points 1–3, the use of WDE has a tendency to in-crease the ID of pilot combustion, for both WDE injection strate-gies, but with no remarkable difference on the ROHR diagrambetween the two WDE injection strategies. At higher loads (operat-ing points 4 and 5), the second WDE injection strategy leads to avery low ROHR for pilot combustion, showing that a large amountof fuel injected during pilot injection is not burning before maininjection. This is probably due to the fact that with the secondinjection strategy the increased injection pressure leads to a fasterfuel jet penetration in the combustion chamber such that the fuelcan reach the combustion chamber walls before ignition. In thatcase, the air–fuel mixture is cooled by the cold walls and will notburn before main injection. Another explanation for the lack ofcombustion for pilot injection with second WDE injection strategycould be the formation of mixtures that are too fuel-lean to ignitebecause of the fast mixing at high injection pressure.

Fig. 4. ROHR comparison of diesel-fu

As regards main combustion, when comparing Figs. 3 and 4,there is a little decrease of ID for main injection when using a pilotinjection, as traditionally observed. One consequence is a decreaseof the premixed peak on the ROHR diagram. The use of WDE hassame consequences on main combustion than in the case of a sin-gle injection (without pilot injection), described in details in previ-ous paragraph. Only for operating point 4, the fact that the fuelinjected during pilot injection does not burn in the second WDEinjection strategy leads to a higher ID of main combustion of sec-ond WDE injection strategy as compared with first WDE injectionstrategy, which is opposite to the tendencies observed with a sin-gle injection.

3.2. Influence on NOx and PM emissions

3.2.1. Without pilot injectionThe influence of WDE on NOx and PM emissions with a single

injection is given in Fig. 5 for operating points 4 and 5.For operating points 4 and 5, there is a noticeable decrease of

NOx emission with first WDE injection strategy as compared withdiesel-fuel (32% and 30% for operating points 4 and 5 respectively).This can be explained as follows:

� The evaporation of water causes local temperatures in the sprayto decrease, including those of zones where NO is produced (onthe lean side of the diffusion flame during injection and in thecombustion products after the end of injection), resulting in adecrease of NO production rate.� Moreover, for a given mass of burned fuel, the heat is released in

a higher mass of gas (because of the dilution of the fuel bywater). The dilution by water has thus a local thermal effect.

el and WDE, with pilot injection.

Fig. 5. Comparison of WDE and diesel-fuel on NOx and PM emissions, without pilot injection.

Fig. 6. Comparison of WDE and diesel-fuel on NOx and PM emissions, with pilot injection.

Fig. 7. Influence of EGR on ROHR with WDE.

3186 A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192

� Another effect is the combustion off-phasing by WDE(described in Section 3.1.1.) that shifts combustion further intothe expansion stroke and thus results in a global decrease of in-cylinder temperatures including local temperatures in the fueljet.� Finally, as described by some researchers, the presence of water

can form OH radicals which are involved in the formation ofthermal NO [2] and could thus have a chemical effect on NOformation.

With second WDE injection strategy, there is a large increase ofNOx emission as compared with first WDE injection strategy. Thisshould be mainly due to an increase of instantaneous ROHR thatmay result in higher local temperatures. The final result of secondWDE injection strategy as compared with diesel-fuel on NOx emis-sion depends on operating point: NOx emission increases for oper-ating point 4 and decreases for operating point 5 (Fig. 5a).

As regards PM emission, different tendencies are also observedbetween the two WDE injection strategies. For first WDE injectionstrategy, there is a very slight decrease of PM emission for oper-ating point 4 and a large relative increase for operating point 5(Fig. 5b). An increase of PM emission is contradictory with mostexperimental studies described in the literature. It shows thatthere are opposite phenomena on PM formation and oxidationprocesses occurring in the combustion chamber when injectingWDE. For operating point 5, the decrease of local temperatureat the jet periphery that decreases the soot oxidation rate seemsto have more impact on PM emission than all others effects thatdecrease PM emission (given in Section 1). With second WDEinjection strategy, there is a large decrease of PM emission ascompared with diesel-fuel (�90% and �28% for operating points4 and 5 respectively). This is most likely due to the increased

injection pressure that promotes the atomization of the fuel andthe mixing with surrounding air, and to the increase of the pre-mixed part of combustion. It must be noticed that the increasedinjection pressure would almost certainly reduce PM emissionswith diesel-fuel.

It must be underlined that PM emission is very low in each case,because of the high air–fuel ratio since no EGR is used. It will beseen in Section 4.2 that when EGR is used, a decrease of PM emis-sion with WDE was always observed.

3.2.2. With pilot injectionThe influence of WDE on NOx and PM emissions with a ‘‘classi-

cal’’ double-shot pilot + main injection is given in Fig. 6 for operat-ing points 1–5.

A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192 3187

For operating points 1–4, same tendencies are observed thanwithout pilot injection, from a qualitative point of view:

Fig. 8. Influence of EGR on in-cylinder pressure with WDE.

Fig. 9. Comparison of WDE and diesel-fuel

� The first WDE injection strategy results in a large decrease ofNOx emission, whereas second WDE injection strategy resultsin either a little decrease or increase of NOx emission as com-pared with diesel-fuel.� With first WDE injection strategy there is a decrease of PM

emission (that is particularly drastic for operating points 1–3)except for operating point 5, as it is the case without pilot injec-tion. The second strategy results in a PM emission reduction forall operating points, which is drastic for operating points 1–4 (ofup to 94%). Again, there are opposite phenomena occurring inthe combustion chamber as regards PM formation and oxida-tion processes that can result in opposite tendencies on the finalPM level at the exhaust; in most cases there is a decrease of PMemission. The combustion analyse has shown than with bothWDE injection strategies, there is an increase of the air entrain-ment per unit of diesel-fuel mass; this effect results in adecrease of PM emission.

on NOx emission for various EGR rates.

Fig. 10. Comparison of WDE and diesel-fuel on PM emission for various EGR rates.

3188 A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192

4. Influence of water-in-diesel emulsion with EGR

As described in previous section, the use of WDE results in mostcases in a large decrease of PM emission and a noticeable decreaseof NOx emission. The aim of this section is to study the use of ex-haust gas recirculation in parallel with WDE to further decreaseNOx emission while maintaining PM emission at a low level. Ithas been shown than second WDE injection strategy results in alarge increase of instantaneous ROHR for main combustion andcan have negative impact on pilot combustion, as well as on NOxemission. As a matter of fact, the first WDE injection strategy hasbeen used here, with a double injection pilot + main.

4.1. Influence on combustion

The influence of EGR in parallel with WDE on combustion andin-cylinder pressure is given in Figs. 7 and 8 respectively, for oper-

ating point 2. Same tendencies have been observed for other oper-ating points.

It is shown that the use of EGR has a slight impact on ROHR forboth pilot and main combustion. First, for pilot combustion, thereis a decrease of ID while increasing EGR rate (Fig. 7a). This is con-tradictory with most experimental studies on the influence of EGRon combustion. In fact, the increase of EGR rate has two oppositeconsequences on ID:

� The dilution by EGR results in an increase of ID [4–6,8].� The increase of EGR rate results in an increased inlet tempera-

ture T200, which a consequence is an increase of the temperatureat pilot SOI. Since pilot combustion ID is mainly kinetically con-trolled, this tends to decrease ID. The EGR cooler used here isnot effective enough to maintain inlet temperature T200 at alow level while increasing EGR rate, such that the increase ofinlet temperature has a predominant effect on ID in theses tests.

Fig. 11. NOx–PM trade-offs comparison with diesel-fuel and WDE.

A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192 3189

As regards main combustion, there is a slight increase of ID thatresults in a slight increase of the premixed part of combustion. Thismay be accompanied with an increase of ROHR peak (as for in-stance for operating point 2 while increasing EGR rate of up to8.2%: see Fig. 7a). For main combustion, the dilution effect ofEGR has a predominant influence on ID over the temperature in-crease at main SOI. The slight off-phasing effect of EGR on maincombustion results in a decrease of in-cylinder pressure duringmain combustion. It has thus a negative impact on cycle efficiencyand on specific diesel-fuel consumption. A readjustment of maininjection by advancing main SOI is thus needed if trying to main-tain approximately the same in-cylinder pressure evolution. Froma qualitative point of view, main tendencies observed while vary-ing EGR rate are not impacted by the use of WDE instead of die-sel-fuel alone.

4.2. Influence on NOx and PM emissions

4.2.1. NOx emissionNOx emission while increasing EGR rate with diesel-fuel or

WDE is given in Fig. 9.Whether with diesel-fuel or WDE, there is a decrease of NOx

emission while increasing EGR rate. The decrease of NOx emissionwith EGR has been largely described and documented in the liter-ature. The main effect of EGR is the decrease of combustiontemperatures, in particular those of zones where NO is produced[4–13].

For each operating point, there is a decrease of NOx emissionwith WDE for a given EGR rate, from 14% of up to 75%. At low loadconditions (operating points 1 and 2), very low NOx emission canbe achieved with EGR and WDE. The decrease of NOx emission at

3190 A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192

higher load conditions is less interesting, in particular for operatingpoint 5 (Fig. 9e). These tests conducted at a given water-to-fuel ra-tio show that the variation in NOx emission is not only a functionof the water content in the WDE, as depicted in some studies onlarge Diesel engines, but strongly depends on the load, and proba-bly on some other engine parameters.

4.2.2. PM emissionPM emission while increasing EGR rate with diesel-fuel or WDE

is given in Fig. 10.With diesel-fuel, while increasing EGR rate, there is a large in-

crease of PM emission, as traditionally observed in the classicalHTC mode [4–6,11–14]. Main explanations for the increase of PMemission with EGR are the decrease of local temperature in zoneswhere PM is partially oxidised (at the jet periphery) and the reduc-tion of in-cylinder oxygen quantity that decrease PM oxidation rateas well [61,67,68]. The higher the load, the higher is PM emissionlevel for a given EGR rate, because of a reduced oxygen quantityin the combustion chamber.

When using WDE, there is a remarkable decrease of PM emis-sion (of up to 90% in these tests) as compared with diesel-fuel, inparticular at high EGR rates. Only for operating point 5 the de-crease of PM emission with WDE is moderate (around 30–45%).Main explanations for the decrease of PM emission with WDE havebeen listed in Section 1. The separation of these effects would bevery interesting but is very hard to perform, in particular on anon-optically accessible engine. As described earlier, these testsshow an evident increase of air entrainment per unit of mass ofdiesel-fuel, of which a consequence is a decrease of PM emission.

4.2.3. NOx–PM trade-offThe influence of WDE on the NOx–PM trade-off while varying

EGR rate is finally depicted in Fig. 11.Since there is a decrease of both NOx and PM emissions with

WDE, the NOx–PM trade-off is largely improved with WDE as com-pared with diesel-fuel (decrease of NOx emission level for a givenPM emission level, or decrease of PM emission level for a given NOxemission level). The use of WDE in parallel with EGR is thus aninteresting in-cylinder method for both NOx and PM emissions.The NOx emission decrease is mainly due to EGR, while the useof WDE allows PM emission to be maintained at a low level whileincreasing EGR rate and thus compensates for the negative effect ofEGR on PM emission observed with pure diesel-fuel. The improve-ment of NOx–PM trade-off is less interesting at higher load condi-tions (operating point 5).

4.3. Influence on brake specific diesel-fuel consumption (BSFC)

Brake specific diesel-fuel consumptions with pure diesel-fueland WDE while varying EGR rate are given in Fig. 11. In all cases,there is an increase of BSFC while increasing EGR rate, whetherwith diesel-fuel or WDE. The increase of BSFC with increasedEGR rate while maintaining injection parameters (pilot and mainSOI, pilot and main quantities and injection pressure) is mainlydue to the off-phasing effect of EGR on combustion that resultsin lower in-cylinder pressures during the compression stroke.Some tests (not presented here) have been done to try to main-tain the combustion phasing while increasing EGR rate byadvancing the injections events and have shown that BSFC is keptat the same level than without EGR. Only for high EGR rates thereis an increased BSFC that cannot be compensated by advancingthe injections, because of a reduction of the combustionefficiency.

As regards the difference between diesel-fuel and WDE, oppo-site tendencies are observed between the different operatingpoints tested here. There is a slight decrease of BSFC with WDE

for operating points 3 and 5, a slight increase for operating points2 and 4, and a large increase for operating point 1, at a given EGRrate. Opposite phenomena can explain this variation between thedifferent operating points and have been also observed in someprevious studies [22,57]:

� Since injection parameters are kept constant, in particular theSOI and the injection pressure, WDE has a tendency to delaythe combustion process, thus lowering cylinder pressure(described in Section 4.1). This off-phasing effect of WDE hasthus a negative effect on BSFC.� On the other hand, the cooling of in-cylinder content due to

evaporation tends to diminish the temperature differencebetween the gases and cylinder wall, thus reducing the heattransfer. This, in turn, may lead to an increase of thermal effi-ciency and a slight reduction in BSFC. An increase of thermalefficiency has been observed for instance by Ghojel et al. [47].

Since the off-phasing effect of WDE on combustion is verymarked for operating point 1 (low load condition), this effect ispredominant and results in a large increase of BSFC for operatingpoint 1. For the other operating points, the variation of BSFC islow (around 2–3%).

5. Conclusion

An experimental study on the use of water-in-diesel emulsionhas been conducted on a common-rail high injection pressureautomotive Diesel engine, with a volumetric water-to-fuel ratioof 25.6%. Main conclusions are as follows:

� The effect of WDE on combustion and emissions depends oninjection strategy and operating conditions (engine speed andload).� Most often, WDE increases ID (of up to 29%), since water evap-

oration lowers in-cylinder temperature. Consequently, pre-mixed part of combustion increases whereas diffusioncombustion rate can be slightly decreased when injection totalmass flow rate is kept constant (in that case, fuel mass flow ratedecreases when WDE is used and injection duration increases).When injection pressure is increased with WDE, so that fuelmass flow rate is approximately the same with WDE as withdiesel fuel, diffusion combustion rate increases due to airentrainment enhancement (water addition increasing spraymomentum).� With the lower injection pressure NOx emission is always

reduced when using WDE, compared to pure diesel fuel (theNOx emission relative reduction can vary from 30% to 50%). Thiscan be explained by flame temperature reduction caused bywater evaporation, thermal dilution with water and combustionoff-phasing. When injection pressure is increased, NOx emissioncan even increase (of up to 24%) with WDE compared to purediesel, due to faster combustion.� The use of WDE usually reduces PM emission compared to pure

diesel fuel. This is especially true at low load conditions (themaximum observed PM relative reduction being 94%) and wheninjection pressure is increased (this result is also classical withpure diesel fuel). However, PM emission sometimes increaseswhen WDE is used at higher loads. Indeed WDE has several con-trary effects on both PM production and oxidation.� Finally, when used in combination with EGR, WDE allows

reducing both NOx and PM emissions, the relative reductionbeing approximately the same whatever the EGR rate. Thus,the traditional NOx-PM trade-off is largely improved and verylow emission level can be achieved.

A. Maiboom, X. Tauzia / Fuel 90 (2011) 3179–3192 3191

By the way, even if the concept seems promising, several issuesshould be addressed before any industrial use:

� Cold-start may be impossible with WDE, so that injection sys-tem should be purged before engine stop in order to use purediesel fuel for engine start.� There might some reliability trouble with some injection com-

ponents, in particular high pressure pumps that might needsome specific modifications.� Long term emulsion stability could be problematic if a WDE dis-

tribution network would be developed. A possible alternative isan on-board emulsion fabrication, which could allow water/fuelratio to vary.

Acknowledgments

The authors would like to thank Mr. Sébastien Trébuchère, Mr.Anthony Pelletier, Ms. Carole Querel, and Ms. Sheddia Didorally,students at the Ecole Centrale de Nantes, for their valuable partic-ipation to this project, and Dominique Tarlet and his colleagues forintroducing us to the fabrication of WDE.

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