ILASS Americas, 25th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013

Effects of Spray Characteristics of Emulsified Diesel on Soot Emissions in a Constant Vol-

ume Chamber

Ming Huo1, Han Wu

2,1, Nan Zhou

3,1, Karthik Nithyanandan

1, Chia-fon F. Lee

*1.4

1Department of Mechanical Science and Engineering

University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

2School of Automobile, Chang’an University, Xi’an, Shaanxi, 710064, China

3State Key Laboratory of Automotive Simulation and Control,

Jilin University, Changchun 130025, China

4Center for Combustion Energy and State Key Laboratory of Automotive Safety and Energy

Tsinghua University, Beijing, 100086, China

Abstract

Emulsions of diesel and water are a potential solution to meet the increasingly stringent emission regulations as they

are able to simultaneously reduce both nitrogen oxide (NOx) emissions and particulate matter (PM) from diesel en-

gines. The PM reduction capability is often associated with the unique atomization characteristic known as micro-

explosion. In this work, emulsified diesel, with the amount of water ranging between 10% and 20% by volume, was

injected and combusted in an optical constant volume chamber. With controlled combustion of an acetylene mixture

prior to fuel injection, the chamber is able to provide high ambient temperature and pressure to mimic the real en-

gine operation. In this study, ambient temperatures ranging from 800K to 1200K were investigated. Mie scattering

images, at 15000 fps, were first taken to record the evolution of the spray using a Phantom 7.1 high speed CCD

camera coupled with a copper vapor laser as the light source. The spray images revealed longer liquid penetration

and wider cone angle at the beginning stage of the injection event for emulsified fuel, supporting the occurrence of

micro-explosion. The integrated broadband luminosity suggested that emulsified fuel coupled with low temperature

combustion may optimize the emission control strategy.

*Corresponding author: cflee@illinois.edu

ILASS Americas, 25th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013

Introduction

The simultaneous reduction of nitrogen oxides

(NOx) and particulate matter (PM) emissions without

after-treatment devices remain a challenge for compres-

sion ignition engines. Advanced combustion concepts

such as homogeneous charge compression ignition

(HCCI), reactivity controlled compression ignition

(RCCI), low temperature combustion (LTC) have been

developed in recent years to lower the NOx and PM

emissions. These technologies are generally associated

with engine modifications to some extent: highly ad-

vanced injection/port fuel injection for HCCI, heavy

EGR introduction for LTC, dual fuel utilization for

RCCI, etc. In this respect, water emulsified fuel re-

mains an attractive solution to reduce the NOx and PM

in exhaust gas simultaneously since it can be introduced

into the current fleet of diesel engines with no major

modifications on the engine. [1]

Numerous studies have been conducted to evaluate

the engine performance by replacing diesel with emul-

sified fuel. [2-10] Hironori et al. [2] investigated the

feasibility of using water-in-oil type emulsified fuels in

small DI diesel engines and reported that lower fuel

consumption and NOx emission can be achieved by the

control of injection timing for W/O emulsified fuels

with water content ratio less than 20%. Hountalas et al.

[3] showed comparative evaluation of EGR, intake wa-

ter injection and W/O emulsion as NOx reduction tech-

niques for a heavy duty diesel engine. Watanabe and

Tamura [4] introduced mono-dispersion emulsified fuel

produced by their own original way using multi-porous

glass and reported effectiveness of mono-dispersed

emulsified fuel on simultaneous reductions of NOx, PM

and fuel consumption under slow engine speed condi-

tion of 1000 RPM. Masatoshi et al. [5] studied the in-

fluence of fuel injection timing and water content on

diesel engine performance and concluded that influence

of the water is mainly attributed to the micro-explosion

feature of emulsified fuel because the water presented

in the combustion chamber by direct water injection

method has almost no influence on engine performance

except lowered NOx emission. The impact of cooling

loss reductions on engine performance with emulsified

fuel operation was investigated by Yasufumi [6] and it

was determined that the reduced cooling loss improved

indicated thermal efficiency. In comparison, optical

studies using laser diagnostic methods for the emulsi-

fied fuels are rarely found in the literature [7-10]. Raul

et al. [7] reported the spray and combustion characteris-

tics of water-in-diesel emulsions and micro-emulsions

in an optical chamber. Longer droplet penetration was

reported because of the lower volatility of the water.

The presence of “glowing spots” was observed in the

flames of the emulsified fuel which was attributed to

micro explosion.

In general, the following effects on engine perfor-

mance are expected with the application of emulsified

fuel: the adiabatic flame temperature is lowered with

the presence of water due to water evaporation heat ,

thus NOx emission decreases. Carbon monoxide usual-

ly declines due to more H2O available for the water gas

shift reaction. Soot is also reduced, explained by a

number of reasons: micro explosion of emulsified fuel

enhances atomization and evaporation of the spray in-

jected in the chamber, providing better fuel-air mixing;

the ignition delay is usually extended providing more

mixing time thus shift the combustion mode towards

premixed combustion; the OH release from water dis-

sociation also helps oxidizing the soot. [1]

In spite of the intensive studies on droplet micro-

explosion [11-16], the micro-explosion phenomena are

not expected to be visually observed under high pres-

sure conditions like engine combustion. The challenge

results from the fact that the water nuclei formation and

bubble growth in an emulsified fuel droplet actually

takes time before they can “break out” the entrapping

fuel. This time scale is believed to be longer than the

primary breakup time scale and comparable to the sec-

ondary breakup time scale. Therefore in theory, the

micro-explosion is expected to take place downstream

of the spray jet, where fine droplets may already exist

due to the combined effect of a number of spray physics

such as secondary breakup, evaporation, coalescence,

wall impingement etc. As a consequence, it is extreme-

ly difficult to identify the droplets produced by micro-

explosion.

The primary motivation of this study is to acquire a

more comprehensive understanding of the spray and

combustion characteristics of emulsified diesel under

different ambient temperature conditions using experi-

mental methods and to evaluate the impact of spray

features on soot emission. Emulsified diesel with the

amount of water ranged between 10% and 20% by vol-

ume was injected and combusted in an optical constant

volume chamber. High speeding imaging of both MIE

scattering and broadband luminosity were acquired to

illustrate time resolved data. On the other hand, because

of the current limitations in our understanding of the

spray and jets of the emulsified fuel, the modeling of

the emulsified fuel spray is still very limited. CFD

spray sub-models are often calibrated with measure-

ments of spray penetration and spreading angle in a

well-defined ambient environment. In this respective,

this study also serves as the CFD model calibration in

the future work.

Experiment Setup and Procedure

Apparatus

A constant volume chamber with a bore of 110 mm

and a height of 65 mm is used in this study. The cham-

ber can imitate the spray and combustion process of a

diesel engine, allowing a maximum operating pressure

of 18 MPa. The chamber has an open end on the top

with a fused silica window installed opposite to the

injector allowing optical access (See Fig 1.). The win-

dow, sealed with an energized spring seal, has dimen-

sion of 130 mm in diameter and 60 mm in thickness,

with a high UV transmittance down to 190 nm. A Cat-

erpillar hydraulic-actuated electronic-controlled unit

injector (HEUI) is mounted at the bottom of the cham-

ber. The injector is VCO type, with orifice diameter of

0.145 mm. The injection pressure and duration was kept

at 700 bars and 3.5 ms respectively through the tests.

The cylinder wall is heated to 380 K before the experi-

ment to mimic the wall temperature of a diesel engine

as well as to prevent water condensation on the optical

windows. A quartz pressure transducer (Model: Kistler

6121), embedded in the chamber wall in conjunction

with a dual mode differential charge amplifier, is re-

sponsible for recording the in-cylinder pressure. The

apparent heat release rate can be calculated from this

using the first law of thermodynamics [17]. The high

temperature and pressure environment is created by

burning a mixture of acetylene, air, and N2 using spark

ignition before the injection is triggered. The details of

the experiment procedure can be found in [18].

Figure 1. Schematic of the test chamber

Diagnostics

High speed imaging for both spray and combus-

tion studies was carried out using a non-intensified high

speed digital camera (Phantom V7.1) located above the

optical chamber. The camera is coupled with a Nikkor

UV lens with 105 mm focal length. For the spray stud-

ies, the light source is supplied by a copper vapor laser

(Oxford Lasers LS20-50) which can be externally con-

trolled to run up to a maximum frequency of 50 kHz

with pulse duration of 25 ns. The high-speed camera

and the copper-vapor laser were synchronized to 15,037

frames per second with an exposure time of 3 µs to

produce time resolved measurement at a spatial resolu-

tion of 512×256 pixels. The peak wavelength of the

copper vapor laser beam is at 510 nm, thus a 510 nm

narrow band pass filter (10 nm FWHM) is fitted in front

of the lens to block the signals of other wavelengths

emitted from the burning spray. The liquid spray is cap-

tured because of the difference in refractive index be-

tween the fuel and ambient gas. The continuous-wave

laser beam was expanded to completely illuminate the

liquid spray. This “volume-illumination” method, rather

than a laser sheet, was utilized to ensure that all drop-

lets spreading from the nozzle were illuminated to iden-

tify the maximum axial and radial distances of any liq-

uid-phase fuel. The input beam was directed at a slight

angle to avoid interference with the camera.

The broadband natural flame luminosity imaging

setup was very similar to the spray studies except that

both the laser beam as well as the band-pass filter was

removed. Two different camera configurations were

used as shown in Fig.2. In the first configuration (the

same configuration as in MIE scattering, thus exactly

the same field of view), the camera resolution is

512×216 with a speed of 15037 fps. A relative larger

camera aperture size of f/22 was chosen such that

stronger signal at lift-off region can be captured; in the

second configuration, the resolution is set to 640×480

with speed of 8082 fps. The minimum aperture size of

f/32 on the lens was used to ensure no pixel saturation

occurred downstream of the image. Quantitative analy-

sis such as spatial integrated broadband luminosity as

well as soot lift-off length will be based on this configu-

ration.

Figure 2. Broadband luminosity images a)

512×216 resolution, 15037 fps, camera aperture f/22, b)

640×480 resolution, 8082 fps, camera aperture f/32, the

images are from two different spray events

Test Fuel

Ultra low sulfur diesel with minimum cetane index

of 40, 90% distillation point between 293 oC to 332

oC

and viscosity around 3 cst was used as a base fuel in

emulsified diesel in current study. Span 80 and Tween

80 were used as surfactants to create O/W/O type emul-

sion. The detailed fuel preparation procedure can be

found in [9]. Two kinds of emulsified diesel with water

content 10% and 20% by volume were tested. The pure

diesel and two water emulsified diesel will be referred

to as D100, W10 and W20 in the rest of the text. All the

prepared fuels were found to be stable for at least two

weeks before separation was observed.

Image processing

The image processing procedure for the continuous

liquid penetration length and cone angle will be detailed

in the following section. The raw images obtained from

each complete injection sequence were first corrected

by the first three images of the respective sequence

which was taken right before the fuel injection. The

histogram equalization was then performed to enhance

the contrast of each image and minimize the effect of

the illumination intensity variation due to the ambient

temperature difference and light degradation from case

to case. It is also found that this procedure eliminates

the bulk noise of the background which later makes

easier the determination of both the liquid penetration

and cone angle. The liquid penetration length can be

defined as the distance between the injector tip and the

first pixel above a preset threshold along the jet center-

line. It is also worthwhile to mention that the algorithm

searches the aforementioned pixel from the spray tip,

thus it is referred as “continuous” liquid penetration. As

a matter of fact, droplets and ligaments were also ob-

served at the tip of the spray. At this point, we have not

fully developed an algorithm that can completely define

the droplets and ligaments boundary. As a simple way

to account for these breakup events, the penetration is

artificially decided by 3×3 pixel arrays whose values

are all above the threshold instead of just “one” pixel

such that the possibility of detecting false penetration

tip can be minimized. Once the liquid penetration was

determined, the cone angle can be measured by finding

the farthest 3×3 pixel array above the same preset

threshold perpendicular to the jet centerline (at 2/3 of

the penetration length) in a similar fashion as the liquid

penetration determination. All the quantitative analyses

were averaged over at least five shots for a statistical

base.

Results and Discussion

Spray characteristics

The sequence of spray images without histogram

correction for W10 and D100 at ambient temperature of

1000 K are shown in Fig.3. Although we focused on

only one plume from the injector, all six plumes were

actually more or less presented in the image. Since both

sets of images were taken from an individual injection

event, plume-to-plume variation was expected. The

images were “reversed” for better presentation of the

downstream portion of the spray jet, which means that

the “black” region is the area that was actually illumi-

nated while the “white” region is actually dark in the

original raw image. By comparing the spray images of

the two fuels, there are apparently some similarities and

differences. In the first shot of the spray image, W10

saw a larger spreading cone angle and “fattened” spray

pattern whereas the spray jet from D100 is narrow and

sharp which may indicate the impact of micro-

explosion. Subsequently, as the jet progresses along the

axial direction, spray tip thickening was observed for

both fuels till around 1.63 ms. Due to the tip thickening,

a sharp gradient existed between the spray and the

background. After 1.7 ms, the abrupt change at the

spray tip with chunk of ligaments detached from the

main jet body can be observed indicating that vigorous

droplet breakup was taking place. The pixel gradient

between the spray and background became less appar-

ent. A blurred area was observed after the droplets and

ligaments region which grew and stretched into the

downstream region; the continuous part of the liquid jet

however did not penetrate further. The signal captured

in the blurred area may come from a number of sources.

The pure white area is most likely caused by the soot

emitted at the flame front extinguishing the laser light,

which can be confirmed from the broadband luminosity

images. The area with coarse texture may be contribut-

ed by the strong soot incandescence that was not com-

pletely blocked by the narrow bandpass filter; the com-

bustion waves left by the premixed burn could also ac-

count for some signals. These blurry areas might also

be attributed to the Rayleigh scattering since the down-

stream region was also illuminated by the laser beam.

Experiments are currently being carried out in the au-

thor’s lab to inject the fuel into inert environment so

that the spray would not burn. This would help to con-

firm if the signal at the downstream region is mainly

from unfiltered flame luminosity or gas scattering.

Figure 4. Shot-to-shot variation on continuous liquid

penetration length

Using the image post processing method, the con-

tinuous liquid penetration length can be acquired. Fig.4

shows data averaged over five individual runs. Shot-to-

shot variation can be clearly seen and the uncertainty

varies with time. During the initial stage of the injec-

tion, a “bump” can be seen on the penetration length

curve and the variation is minimal among individual

runs; once it reaches a peak value, the penetration

length starts to decrease and reaches a quasi-steady

state before the injection event is terminated. It is with-

in this duration that much higher variations were de-

tected. As visualized in Fig.3, the gradient between the

tip of the spray and background became less apparent

due to the vigorous breakup events, which posed a chal-

lenge on the precise determination of the penetration

length. Among all the cases conducted, shot-to-shot

variation is typically around 10% during this period.

Figure 5. Continuous liquid penetration at different

ambient temperature for W10 and D100

Fig.5. illustrates the continuous penetration curve

for W10 and D100 at different ambient temperatures.

The temperature impact on the continuous penetration

length is very obvious as higher temperature yields

lower penetration. It is also of great interest to see that

the “bump” becomes much more remarkable at lower

ambient temperature while at the ambient temperature

of 1200K; the penetration length curve almost remains

a plateau. This feature is closely associated with the

ignition delay at the given condition. Lower ambient

temperature yields much longer ignition delay and

therefore longer penetration, which has been discussed

in a number of studies. [19-20]. Once the jet was ignit-

ed and the diffusion flame is formed, the temperature in

the reaction zone ahead of the spray tip rises quickly

and as a result, the spray evaporation speeds up and the

droplets breakup becomes much more violent, which

essentially swallows back the continuous liquid jet

body. In comparison, the ignition delay was much

shorter at high ambient temperature so that the diffusion

flame quickly formed and developed upon the start of

injection; therefore the penetration length was limited

to a quasi-steady state value throughout the entire injec-

tion event.

Regarding the water impact, the emulsified fuel

generally presented longer penetration at the “bump”

area on the curves due to the lower volatility of the wa-

ter. It is seen that the lower the ambient temperature,

the larger the discrepancy between the two fuels. At

low temperature of 800 K in particular, significant

longer penetration length is observed for W10 indicat-

ing that the impact of water addition is much more pro-

nounced at low ambient temperature environment. It is

also noted that after the initial bump on the curve, the

trend of longer penetration with emulsified fuel no

longer remains as short penetrations were observed for

W10 especially at 800K. One possible explanation is

that the atomization of the emulsified fuel was en-

hanced due to micro-explosion making the continuous

liquid part shorter.

Combustion Characteristics

The apparent heat release rate calculated based on

in-cylinder pressure is shown in Fig. 6. More premixed

burn indicated by the first peak on the heat release

curve is observed with the decrease of the temperature,

which is expected as longer ignition delay at low ambi-

ent temperature provided more time for air-fuel mixing.

Longer ignition delay is also generally expected with

the emulsified fuel mainly due to the evaporation heat

of water. At 800K, the W20 exhibited much retarded

ignition timing compared with W10 and D100 indicat-

ing the impact of water on ignition delay only becomes

apparent beyond a certain mixing percentage. In con-

trast, little difference was perceivable at a higher ambi-

ent temperature of 1000 K. Since the injection duration

was kept the same, the total amount of fuel injected into

the chamber is a constant, leading to lower energy input

for emulsified diesel. Therefore, the total heat release

decreased with the addition of water.

Figure 6. Apparent Heat Release Rate at different am-

bient temperature with injection pressure of 700 bar

The spectral sensitivity of the high speed camera is

around 400 nm to 700 nm, thus although termed

“broadband”, the signal captured should be within this

spectral range. The flame luminosity consists of two

parts; chemiluminescence and soot incandescence. The

latter is much stronger than the former one, thus it is

reasonable to claim that the soot luminosity can be well

represented by the broadband luminosity. The sequence

of the broadband luminosity images for W10 and W20

are illustrated in Fig. 7. Larger aperture size f/22 was

used in this configuration so that slight image saturation

was yielded downstream of the flame. The line-of-sight

flame front and flame liftoff can actually be visualized

under this configuration. In the conceptual diesel com-

bustion model raised by Dec [21], the central region

just downstream of the lift-off is where fuel rich mix-

ture reacts and the premixed flame is hypothesized.

This is also the primary region where soot precursors

started to form. A relatively darker region compared

with the bright soot incandescence was indeed observed

at this location of the lift-off which supports this theory.

Space integrated broadband luminosity at different

temperature for W10 and D100 derived from the picture

in the second configuration are illustrated in Fig 8. In

this configuration the overexposure was by no means

avoided and the entire flame, including flame wall in-

teraction, was included in the field of view. It can be

seen that very limited reduction of luminosity resulted

from the water addition at temperature above 900 K. At

low ambient temperature of 800 K however, the lumi-

nosity is decreased by up to 50% by W10. As explained

above, the broadband luminosity can be regarded as a

good representative of the actual soot emission; the

results indicate that the advantage of using emulsified

fuel to suppress soot formation can be maximized if

coupled with a low temperature combustion strategy.

The reason for this feature can be mainly explained by

the different spray characteristics of W10 at 800 as par-

ticular long initial continuous liquid penetration was

observed which greatly enhanced the air fuel mixing.

Figure 8. Space integrated broadband luminosity at

different temperature for W10 and D100

Conclusion

In this work, the spray and combustion characteris-

tics of emulsified diesel were investigated in a constant

volume chamber. A number of conclusions can be

reached based on the study. At low ambient tempera-

ture, emulsified fuel showed significantly longer liquid

penetration at the beginning stage of the injection indi-

cating that the impact of water is more pronounced at

low ambient temperatures. This finding is also support-

ed by the space integrated broadband luminosity, which

saw the most reduction only at low ambient temperature.

It is thus suggested that emulsified fuel coupled with a

low temperature combustion may optimize the emission

control strategy. The beginning stage of the liquid pen-

etration also saw a larger spreading cone angle and “fat-

tened” spray pattern for emulsified diesel which could

be attributed to micro-explosion. Regarding the ignition

delay, the retarded ignition only becomes apparent at

low ambient temperature and with a water content of 20%

by volume.

Acknowledgements

This material is based upon work supported by the

Nation-al Science Foundation under Grant No.

CBET‐1236786. Any opinions, findings, and conclu-

sions or recommendations ex-pressed in this publication

are those of the author(s) and do not necessarily reflect

the views of the National Science Foundation. The au-

thors are grateful for the support from NSF.

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Figure 3. Sequence of liquid spray evolution at ambient temperature of 1000 K with injection pressure 700bar a)

W10, b) D100

Figure 7. Sequence of broadband natural flame evolution at ambient temperature of 1000 K with injection pressure

700 bar a) W10, b) D100