Numerical and experimental study of water/oil emulsified fuel combustion

in a diesel engineq

Niko Sameca,*, Breda Kegla, Robert W. Dibbleb

aFaculty of Mechanical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor, SloveniabDepartment of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720-1740, USA

Received 1 February 2002; revised 30 April 2002; accepted 6 May 2002; available online 3 July 2002

Abstract

Numerical and experimental studies were made on some of the chemical and physical properties of water/oil emulsified fuel (W/OEF)

combustion characteristics. Numerical investigations of W/OEF combustion’s chemical kinetic aspects have been performed by simulation

of water/n-heptane mixture combustion, assuming a model of a homogenous reactor’s concentric shells. The injection and fuel spray

characteristics are analyzed numerically also in order to study indirectly the physical effects of water present in diesel fuel during the

combustion process. The experimental results of W/OEF combustion in the DI diesel engine are also presented and discussed. The results of

engine testing in a broad field of engine loads and speeds have shown a significant pollutant emission reduction with no worsening of specific

fuel consumption. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Combustion; Emission; Numerical modelling

1. Introduction

The development of internal combustion (IC) engines

has followed a dual strategy over the years:

† Improvement of engine performance and

† Reduction of pollutant emissions.

The incrassation of the fuel’s and lubricant’s quality is of

great importance in order to achieve the mentioned strategy.

Some of the chemical improvements in the fuels and

lubricants used in heavy-duty diesel engines consist of

numerous technologies in order to meet the demanded levels

of environmental compatibility (economy, emissions, noise)

and market requirements (reliability, lifetime, price, etc.).

Several conventional and unconventional techniques

have proved to be powerful tools when improving diesel

engine characteristics, especially NOx and soot emissions.

Among these approaches, the addition of water to diesel fuel

and its addition to gasoline as a modified fuel for spark-

ignited engines have a long history [1]. Several systems

have been invented and experimentally investigated. The

main physical effects of water on combustion processes

have also been studied theoretically [2–4]. Moreover, the

addition of water is considered to be one of the effective

approaches to the in-cylinder reduction of pollutant

formation whilst at the same time an economic way of

reducing NOx and particulate materials (PM).

In general, it has been concluded that the presence of

water vapor in reactants influences the physics and chemical

kinetics of combustion and has a beneficial effect on the

rate-of-heat release history and reduction of pollutants

emissions. During combustion vaporized water reduces the

flame temperature, changes the chemical composition of the

reactants, resulting in higher OH radical concentration

controlling the NO formation rate and soot oxidation, and

dilutes the reach zones in the combustion chamber.

Water may be added to the fuel in several ways:

† continuously into the air stream via a single point system

or periodically through intake valves via a multi-point

system [5,6],

† water is injected directly into the cylinder through a

separate nozzle, or is introduced to fuel within the injection

nozzle when fuel injection does not take place [7],

† by stratified fuel–water injection [10], or

† through the preparation of stabilized water/fuel

emulsion (W/OEF).

0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 01 6 -2 36 1 (0 2) 00 1 35 -7

Fuel 81 (2002) 2035–2044

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

* Corresponding author.

E-mail address: niko@samec.uni-mb.si (N. Samec).

A short features review of some different methods of

water addition and introduction is given in Ref. [8].

The effects of single and multi-points water addition

systems on the NOx and soot emissions of a vehicular

heavy-duty diesel engine have been investigated [5,9] and

studied previously in our laboratory. It may be concluded

based on these results, that both systems showed practically

the same beneficial influence on NO emission reduction, but

rather a poor effect on soot emission. Considering our

previous results and the results of several other investi-

gations performed recently [11,12] we have concluded that

more promising effects on NOx and soot reduction may be

expected when we turn our attention to W/OEF using the

same engine in our experiments.

Firstly, the chemical–physical effects on NO and soot

formation have been numerically investigated and presented

in this paper by simulating W/OEF combustion with the

application of an engine combustion model with detailed

chemical kinetics and simplified mixing. Furthermore, both

the injection and W/OEF spray characteristics are analyzed

using numerical simulation. The effects of injection and

spray characteristics on emissions and other engine

characteristics at several operating regimes are discussed

for different fuel/water ratios on the basis of this analysis.

The overall effect of different amount of the water presence

(0, 10, and 15%) in a diesel fuel on NO, HC soot emissions

reduction has been experimentally performed by using a

four-cylinder air-cooled DI truck diesel engine.

2. Numerical investigations

Many experimental investigations of W/OEF combus-

tion in diesel engines have been made to show some of the

benefits of water addition to the diesel fuel on the reduction

of pollutant emission, specific fuel consumption, thermal

loading, and maximum combustion pressure [13–16]. They

could not offer, however, a more detailed explanation of

some of the specific physical and chemical effects of W/

OEF combustion processes. In this case, the corresponding

numerical investigations seem to be very helpful. Two

major groups of numerical investigations have been

performed: numerical simulation of W/OEF combustion

from the chemical kinetics point of view, and numerical

simulation of the injection processes for a better under-

standing of W/OEF atomization processes influenced by

vaporized water droplets.

2.1. Numerical simulation of W/OEF combustion

The numerical model of complete chemistry and

turbulent mixing is too vast. Usual simplifications are

moderately detailed fluid mechanics with simplified chemi-

cal kinetics. We here simplify the turbulent mixing and

retain detailed chemical kinetics. The model divides the

flow and reaction field inside the combustion chamber into a

series of concentric deformable shells that can be arbitrarily

convoluted by convection and turbulence; each shell is

modeled as a well-mixed reactor (WMR) [17,18]. The

thermo chemical data and detailed chemical kinetics for

each WMR are computed using the CHEMKIN package of

numerical codes.

Basic differential equations for a single WMR shown in

Fig. 1, can be derived from the conservation of mass,

species, and energy, respectively, [18]

dm

dt¼

XNi

i¼1

_mi 2 _me ð1Þ

djs

dt¼

1

m

XNi

i¼1

_miðjis 2 jsÞ þ_vsMs

rð2Þ

dT

dt¼

1

�cpm

XNi

i¼1

_mi

XNs

s¼1

ðhis 2 hsÞjis

" #2

XNs

s¼1

_vshsMs

r�cp

21

r�cp

dp

dtð3Þ

where m is the mass inside of reactor, _mi is the mass flow

rate into reactor at inlet i, _me is the total mass flow rate out of

the reactor, Ni is the number of inlets, js is the mass fraction

of species, jis is the mass fraction of species s at inlet i, Ns is

the number of chemical species, _vs is the production rate of

species s (calculated using CHEMKIN), Ms is the molecular

mass of species s, r is the mass density in the reactor, T is the

temperature, �cp is the mean specific heat at constant pressure

in the reactor, and p is the pressure.

In the multiple reactor case, as shown in Fig. 2, the

outlets of one reactor become the inlets of the adjacent

reactors. If the first reactor were ignited, hot gas would flow

into the second reactor, which would soon reach the ignition

temperature and ignite. This ignition process repeats itself

as the hot gas in the second reactor goes into the third

reactor and so forth. Since the fluid properties are the same

at all exits, the flow at all exits may be lumped together into

a single exit flow, even though there may be more than one

exit. When applying a series of various WMRs numbers of

different sizes, the typical combustion chamber under auto

Fig. 1. A WMR with multiple inlets and a single outlet.

Fig. 2. A series of WMRs.

N. Samec et al. / Fuel 81 (2002) 2035–20442036

ignition conditions can be modeled to show a conceptual

picture of a 12-reactor system in Fig. 3 considered in our

case.

The dp=dt term in Eq. (3) is modeled by assuming that all

reactors have the same pressure and noting that:

p ¼Rm

V

XNr

r¼1

mrTr

Mr

; ð4Þ

where Rm is the universal gas constant, V is the total volume

of all reactors, and Mr is the mean molecular weight of the

gas in the reactor.

Differentiating Eq. (4) with respect to time, and

considering that combustion occurs at the constant volume

of each reactor gives an expression for dp=dt which can be

rearranged to give:

V

Rm

dp

dt¼

XNr

r¼1

mrTr

XNs

s¼1

1

Msr

djsr

dtþ

XNr

r¼1

mr

dTr

dt

XNs

s¼1

jsr

Msr

þXNr

r¼1

Tr

dmr

dt

XNs

s¼1

jsr

Msr

: ð5Þ

In the case of Nr reactors there is an energy Eq. (3) for

dTr=dt; which contains dp=dt: Eq. (5), which is the formula

for dp=dt also contains the dTr=dt of each of the reactors.

This forms a set of Nr of Eq. (3) in Nr unknowns that must

be solved simultaneously to give the dTr=dt terms.

2.2. Numerical simulation of W/OEF injection processes

To investigate some of the physical effects of water

present in fuel during combustion in a diesel engine, it is

important to consider major injection parameters, which

have a strong effect on the primary spray droplets size and

distribution over the combustion chamber. In the combus-

tion of W/OEF, however, the primary spray fuel droplets are

further divided as a result of explosive vaporization caused

by the rapid heating of water dispersed within the individual

fuel droplets. The internal water droplets undergo

spontaneous nucleation of steam bubbles at a temperature

well above the vaporization temperature depending on in-

cylinder pressure, causing a violent conversion of the water

droplet into steam. The vaporization, in turn, produces a

rapid expansion of the surrounding oil droplets, fragmenting

the oil into a vast number of smaller fuel droplets

representing very intensive micro explosions. The name

for this process is secondary atomization. The efficiency of

this secondary atomization can be indirectly investigated by

numerical simulation of the W/OEF injection processes by

calculating both the mean injection characteristics, and

primary spray droplets size. The droplets size of W/OEF is

one of the most important factors determining the

subsequent combustion characteristics.

The most important injection and spray characteristics

are calculated using our own mathematical model for the

numerical simulation of injection processes in an in-line

diesel fuel injection system [22,23,26]. In this mathematical

model, the flow of the fuel through the high-pressure (HP)

system is modeled as a one-dimensional flow. The HP

system consists of five parts: the in-barrel chamber, the

delivery valve chamber, the snubber valve chamber, the HP

tube, and the injector chamber. The response of this system

is governed by a system of 15 first-order ordinary

differential equations (equations of continuity and equations

of motion for the movable parts). In each HP part, the

instantaneous fuel properties and pressures are assumed to

be constant with the exception of the pressures in the HP

tube. The relationship between the response variables on

both sides of the HP tube is established by the equations of

pressure wave transport. The equation of pressure wave

transport is derived from equations of momentum and

continuity

›p

›xþ r

›w

›tþ rkw ¼ 0;

›w

›xþ

1

a2r

›p

›t¼ 0; ð6Þ

where w is the fluid velocity, r is the fluid density, k is the

flow resistance factor, p is the pressure, a is the velocity of

sound, and x is the coordinate along the high-pressure tube.

Only the injection rate history ð_qÞ and the needle lift ðhnÞ

from the set of all response variables are discussed in the

following. Additionally, the Sauter mean diameter ðd32Þ and

heat release ðQÞ histories are considered.

The injection rate can be expressed as

_q ¼ minAin

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2

rlpin 2 pal

sð7Þ

where the symbol ðminAinÞ denotes the effective nozzle flow

area, pin denotes the injection pressure and pa is the ambient

pressure.

The needle lift is calculated from the equation of motion

dhn

dt¼

0; Fn $ 0 and hn ¼ hmaxn

0; Fn # 0 and hn ¼ 0

vn; otherwise

8>><>>: ð8Þ

Fig. 3. Conceptual picture of typical locations of the reactors.

N. Samec et al. / Fuel 81 (2002) 2035–2044 2037

dvn

dt¼

0; Fn $ 0 and hn ¼ hmaxn

0; Fn # 0 and hn ¼ 0

Fn=min; otherwise

8>><>>: ð9Þ

where Fn ¼ ðAstn 2 Ase

n Þpin þ Asen psac 2 Fn

0 2 Cnhn; psac ¼

ðK2=ð1 þ K2ÞÞðpin 2 paÞ þ pa; K ¼ minAin=ðminAinÞmax;hmax

n is the maximal needle lift, Asen is the needle seat

cross-section area, Astn is the needle steam cross-section area,

Fn0 is the spring preloading force and Cn is the spring

stiffness.

The Sauter mean diameter was calculated from the well-

known Hiroyasu formula

d32 ðmmÞ ¼ 2:39 £ 103ðpinj 2 paÞ20:135r0:121

a q0:131 ð10Þ

where pinj; pa are the injection and ambient pressure,

respectively (Pa), ra is the air density (kg m23), and q is the

fuelling (m3 st21).

Furthermore, the heat release was calculated from the

equation of energy conservation

dQ 2 dQc þ hf dmf ¼ dðmcvTÞ þ p dV ð11Þ

where Q is the heat release or combustion energy, Qc is the

cooling loss, hf dmf is the apparent sensible enthalpy

derived from the injected fuel, mcvT is the sensible internal

energy of the gas in the cylinder and p dV is the expansion

work.

2.3. Numerical results and discussion

It is possible to make different calculations to find out

some chemical aspects of the water present in the fuel using

the presented simplified numerical model of W/OEF

combustion. A stoichiometric mixture of n-heptane (instead

of diesel fuel) has been applied for these calculations

considering a detailed n-heptane kinetic mechanism [19]

also including Zeldovich NOx formation reactions and the

corresponding soot model as follows:

C3H3 þ C3H3 ¼ A1

A1 ! 6CðSÞ þ 3H2

C4H2 ! 4CðSÞ þ H2

CðSÞ þ C2H2 ! 3CðSÞ þ H2

C2H2 ! 2CðSÞ þ H2

The first reactor (Fig. 3) involves the mixture of a given

amount of water and n-heptane, while other reactors involve

the corresponding amount of air with a temperature of

950 K assuring auto ignition.

When adding different amounts of water (0, 10, and 30%

by volume) to the fuel, the corresponding amount of fuel is

replaced by water resulting in lower combustion tempera-

ture as shown in Fig. 4. In a real system, the combustion

temperature is reduced additionally during the water

vaporization processes, but it is not considered in the

chemical mechanism used here. Lower combustion tem-

perature, however, directly influences thermal NO for-

mation during the reduction in chemical reaction rates for

Zeldovich mechanism reactions [24]:

O þ N2 !k1

NO þ N

k1 ¼ 1:8 £ 1012 expð2319 kJ mol21=RTÞ

ð12Þ

N þ O2 !k2

NO þ O

k2 ¼ 6:4 £ 109 expð226 kJ mol21=RTÞ

ð13Þ

N þ OH!k3

NO þ H k3 ¼ 3:0 £ 1013 ð14Þ

Only minor contribution to the NO formation may also be

expected by considering the following reactions

O þ N2 þ M Y N2O þ M; ð15Þ

N2O þ O Y 2NO; ð16Þ

which may become important at lower temperatures

because of their lower activation energy, especially for

reaction (16) with Ea ¼ 97 kJ mol21 [24]. Additionally, the

NO concentration may be reduced by reducing the O atoms

concentration (reactions (12), (15), and (16)) due to its

consumption at the OH radicals formation by the following

overall reactions

H2O þ O ! OH þ OH ð17Þ

H2O þ H ! OH þ H2 ð18Þ

as a consequence of the water present in the fuel. However,

Fig. 4. Temperature profile at the combustion of n-heptane/water emulsion

with 0, 10, and 20% of water.

Fig. 5. NO concentration during the combustion of n-heptane/water

emulsion of 0, 10, and 20% of water content.

N. Samec et al. / Fuel 81 (2002) 2035–20442038

assuming from the theoretical point of view that the N atoms

are in a quasi-steady state (fast reactions (13) and (14))

d½NO�=dt ¼ 0; the following NO formation expression is

obtained

d½NO�

dt¼ 2k1½O�½N2�; ð19Þ

indicating very high dependence on NO formation from the

O atoms concentration. Fig. 5 shows the calculated overall

time history of NO formation during the water/n-heptane

emulsion combustion.

The fuel’s chemical composition has been changed by

water addition to the fuel, resulting in higher OH radicals

production during combustion, reactions (17) and (18). At

the same time higher OH concentration drop is expected

because of their consumption during soot oxidation due to

the possible reaction

Cn þ OH ! CO þ Cn21 þ H

as shown in Fig. 6. It finally means less soot emission in the

combustion products. The OH radicals play an important

role in soot oxidation especially at higher temperatures,

because they become the leading oxidators of soot, whereas

oxygen in its molecular form is consumed by the flame [20].

It is also possible to produce higher OH concentration

resulting in higher soot emission abatement [21] by adding

some other additives to the fuel instead of water (i.e. H2O2).

Lower OH concentration seems to also have an additional

effect on NO formation during reaction (14). Fig. 7 shows

the overall history of soot emission in the case of emulsion

combustion prepared with 0, 10, and 20% (by volume) of

water and n-heptane. The overall numerical results of the

combustion simulation of fuel/water emulsions are collected

in Table 1.

The calculation of W/OEF injection characteristics has

been carried out under three engine-operating regimes at

injection timing of 13.58CA before TDC. For all three

different fuels: (i) neat diesel fuel, (ii) 10% W/OEF and (iii)

30% W/OEF, the engine torque was 575 N m and the power

was 102 kW at an operating regime of 1700 min21 and pe ¼

10 bar: The total quantity of diesel fuel is 129.6 mm3 st21 in

all three cases. Using 10% W/OEF and 30% W/OEF, the

quantity of diesel fuel during the first 0.5 ms of injection

process is smaller with respect to the neat diesel fuel by

about 7 and 23.5%, respectively, Fig. 8. In Fig. 9, the Sauter

mean diameter histories, using the Hiroyasu expression, are

presented for different amounts of water. The average

values of Sauter mean diameter are 29.29, 29.26, and

29.17 mm for all three fuels, respectively. Furthermore, the

in-cylinder pressure ðpcÞ and the rate-of-heat release time

histories ðdQ=dQÞ are presented in Fig. 10 for two different

fuels: neat diesel fuel (solid curve) and 10% W/OEF

(dashed curve).

The second tested engine-operating regime corresponds

to on engine torque of 451 N m and the power of 82.2 kW at

on operating regime of 1700 min21 and pe ¼ 8 bar: A

comparison of the influences of different fuels: (i) neat

diesel fuel, (ii) 10% W/OEF and (iii) 30% W/OEF is shown

in Figs. 11 and 12. The total quantity of diesel fuel is

102.3 mm3 st21 in all three cases. When considering the

10% W/OEF and 30% W/OEF, the quantity of diesel fuel

during the first 0.5 ms of the injection process is smaller

with respect to the neat diesel fuel by about 8.2 and 26.1%,

respectively, Fig. 11. The average values of Sauter mean

diameter are 28.96, 29.03, and 29.13 mm for all three fuels,

respectively. In this case, for neat diesel fuel (solid curve)

and 10% W/OEF (dashed curve) the in-cylinder pressure

and the rate-of-heat release time histories are presented in

Fig. 13.

The last tested engine-operating regime corresponds to

an engine torque of 299 N m and a power of 53.3 kW. A

comparison between the influences of the same three types

of fuels as in the previous cases is shown in Figs. 14 and 15.

The total quantity of diesel fuel is 71.05 mm3 st21 in all

Fig. 6. OH radicals time history during the combustion of n-heptane/water

emulsion with 0, 10, and 20% of water content.

Fig. 7. Soot fraction time history during the combustion of n-heptane/water

emulsion with 0, 10, and 20% of water content.

Table 1

Reduction of NO and soot emission at the combustion of n-heptane/water

emulsion

Variable Emulsion with 10%

of H2O (%)

Emulsion with 20%

of H2O (%)

Temperature 10 15

NO emission 15 35

Soot emission 35 60

N. Samec et al. / Fuel 81 (2002) 2035–2044 2039

three cases. Taking into account the 10% W/OEF and 30%

W/OEF, the quantity of diesel fuel during the first 0.5 ms of

the injection process is smaller with respect to the neat

diesel fuel by about 5.4 and 25.2%, respectively. The

average values of the Sauter mean diameter are 28.28,

28.45, and 28.57 mm for all three fuels, respectively.

Finally, Fig. 16 shows the in-cylinder pressure and the

rate of the heat-release histories for two different fuels: neat

diesel fuel (solid curve) and 10% W/OEF (dashed curve).

The presented results correspond to the regime of n ¼

1700 min21 and pe ¼ 5 bar:

The gradient of heat release ðdQ=dQÞ during the

premixing phase of the diesel combustion is increased by

using W/OEF, Figs. 10, 13, and 16. The reason for this

seems to be in the fact, that the mixture is better prepared

due to prolonged ignition delay. In this way, a larger amount

of the mixture burns down in the kinetic phase of the

combustion process. It also has to be pointed out, that the

ignition locations of the emulsion fuels are different from

those of a pure diesel fuel [2]. Ignition occurs in the middle

of the combustion chamber with the diesel fuel, while it

occurs in the bottom region or at multiple points in the

middle simultaneously with W/OEF. The flame propagates

slowly from the ignition locations, so that it takes twice as

long or longer for the luminous flame to propagate over the

whole chamber using W/OEF. Some experimental investi-

gations [2] show that strong micro explosions of a group of

droplets can occur in the specific region of the luminous

flames near the spray tip. They affect the local shape and

brightness of the flames as small, dark, round regions due to

the explosion of superheated water in the droplets. The sizes

of the micro explosions range from barely identifiable small

ones to those with a diameter of a few millimeters. Micro

explosions of the emulsion fuels seem to enhance the

mixing of the fuel with the surrounding air for faster and

more efficient combustion also resulting in a higher heat

release gradient ðdQ=dQÞ at the beginning of the combus-

tion process.

When employing numerical simulation, it can observed

Fig. 8. Injection rate and needle lift histories. Engine-operating regime:

n ¼ 1700 min21; pe ¼ 10 bar:

Fig. 9. Sauter mean diameter histories. Engine-operating regime: n ¼

1700 min21; pe ¼ 10 bar:

Fig. 10. In-cylinder pressure and rate-of-heat release time histories. Engine-

operating regime: n ¼ 1700 min21; pe ¼ 10 bar:

Fig. 11. Injection rate and needle lift histories. Engine-operating regime:

n ¼ 1700 min21; pe ¼ 8 bar:

N. Samec et al. / Fuel 81 (2002) 2035–20442040

that the diesel fuel quantity during the first 0.5 ms of the

injection process decreases with higher water/diesel rates,

Figs. 8, 11, and 14, reducing the combustion temperature

and consequently NOx emission. It has also been verified by

previous numerical simulation of water/n-heptane emulsion

combustion and later experimental work.

W/OEF gives somewhat larger Sauter mean diameters,

Figs. 9, 12, and 15. In spite of this, the previously simulated

and measured soot emission values are lower at all engine-

operating regimes. This can be explained again by the

phenomenon of micro explosions, which probably leads to

better atomization, although, at the beginning of the

injection the emulsion droplets are rather large, which

seems to be normal, whilst the emulsion droplet involves a

given amount of smaller water droplets. Very small droplets

with a well-controlled size distribution are necessary in

order for secondary atomization to be more effective in a

combustion process. Insufficient energy will be produced if

the number of water droplets is to small, causing secondary

atomization. On the other hand, larger droplets reduce the

number of droplets for explosion and tend to produce less

violent explosions within the oil droplets because of

nucleation taking place at lower temperatures.

3. Experimental approach and set-up

A four-cylinder air-cooled DI truck diesel engine has

been employed for the experimental investigations of the

overall effects of water present in diesel fuel. This engine

with a displacement volume of 7118 cm3 offers a maximal

power of 150 kW and a maximal torque of 315 N m.

Practical W/OEF preparation by adding a very small amount

of special emulsifier (Span 85), requires a very precise

mixing methodology in order to obtain a stable emulsion

with no separation of constituents during a long time period,

which is acceptable for the practical application of W/OEF

in all fields of land and marine use.

The load-speed test map is presented in Fig. 17 and all

the data have been taken at each point during the stabilized

engine-operating regime. Three series of experiments have

been carried out in each series using either pure diesel fuel,

10% W/OEF or 15% W/OEF at the same engine-operating

regime as in the case of the numerical simulation of the W/

OEF injection processes. The engine test was carried out on

a completely computer controlled Zollner B-350AC test

bench, where all important data (temperatures, pressures,

fuel, and air consumption) determining engine-operating

regime were measured and stored for further processing and

computing engine performance data (volumetric efficiency,

air–fuel equivalence ratio, etc.). The concentrations of

some combustion products in the engine exhaust gases were

measured with indicated standard instrumentation: NO,

NO2 (Chem.-lum. Analyzer), CO(NDIR), THC(FID),

Fig. 12. Sauter mean diameter histories. Engine-operating regime: n ¼

1700 min21; pe ¼ 8 bar:

Fig. 13. In-cylinder pressure and rate-of-heat release time histories. Engine-

operating regime: n ¼ 1700 min21; pe ¼ 8 bar:

Fig. 14. Injection rate and needle lift histories. Engine-operating regime:

n ¼ 1400 min21; pe ¼ 5 bar:

Fig. 15. Sauter mean diameter histories. Engine-operating regime: n ¼

1400 min21; pe ¼ 5 bar:

N. Samec et al. / Fuel 81 (2002) 2035–2044 2041

O2(ZrO hard electrolyte) and soot (AVL soot analyzer,

Bosch unit). The indicative pressure (KISTLER) time

history was in-line monitored, stored and processed at

each regime representing the base for heat release history

computation, already presented in the section of numerical

investigations.

3.1. Experimental results and discussion

A comparison of the exhaust emission data (NOx, HC,

soot) and specific fuel consumption (be) for three different

fuels (i) neat diesel fuel (D2), (ii) 10% W/OEF, and (iii)

15% W/OEF and three different engine-operating regimes

are presented in Figs. 18–20, considering the D2 pollutant

emission as 100%. The relative emission reduction of

component ‘i’ in the exhaust gases, at the same engine-

operating conditions, is expressed as follows:

DEirel½%� ¼

½E�i;D2 2 ½E�i;W=OEF

½E�i;D2

£ 100 ð20Þ

Comparing the results of this W/OEF investigation with the

results of our previous experiments, when water was added

to the fuel separately via single and multi-point injection [5,

25], or with the results of other authors when other modes of

water addition were applied [7,8,10], it may be concluded

that water addition via W/OEF is the most appropriate

approach to decreasing NOx, soot and THC emissions in a

diesel engine exhaust, with no worsening of fuel consump-

tion as can also be seen from Figs. 18–20. It is also the

cheapest way; no changes are needed in the engine or its

ancillaries.

The effect of W/F ratio was so unclearly expressed in our

experimental results, probably because of smaller differ-

ences of water fraction in the emulsion. If the W/F ratio is

increased, however, the trend of pollutant emissions

reduction (reported elsewhere, i.e. [8,11]) has a positive

gradient of reduction as it can also be estimated by results

from our numerical experiment.

Summarizing our experimental emission data by aver-

aging them over the whole field of engine-operating regimes

(Fig. 17), the reduction of pollutant emissions is given in

Table 2 and offers general information of pollutant

abatement by using W/OEF in diesel engine. It has to be

emphasized again that the overall emission abatement

results are the consequence of the mutual chemical and

physical effects treated separately in our numerical

investigations. Comparing the experimental and numerical

results of the NOx and soot emission reduction (Tables 1 and

2), one can find out that the chemical effect seems to be

more important at NOx emission reduction and physical

effects seem to be dominant at soot emission reduction.

Moreover, on the base of our experimental results and

results obtained by simulation of W/OEF injection pro-

cesses, it may be concluded that higher soot emission

reduction is achieved at higher engine loadings (Figs.

18–20) when the fuel droplet size is bigger containing more

of the smaller water droplets assuring, effective micro

explosions. In the case of lower loadings, the injected rate of

W/OEF is lower and, consequently, the fuel droplet size is

smaller which now contain less water droplets. Higher

injection pressures, however, assure better fuel atomization

resulting in smaller fuel droplets. Therefore, a negligible

effect of secondary atomization at the modern high-pressure

diesel injection may be expected, especially in the case of

common rail injection systems because of its constant high

level injection pressure.

Fig. 16. In-cylinder pressure and rate-of-heat release time histories. Engine-

operating regime: n ¼ 1700 min21; pe ¼ 5 bar:

Fig. 17. The load speed test map.

Fig. 18. The relative reductions of exhaust emissions and specific fuel

consumption be. Engine-operating regime pe ¼ 10 bar; n ¼ 1700 rpm:

Fig. 19. The relative reductions of exhaust emissions and specific fuel

consumption be. Engine-operating regime pe ¼ 8 bar; n ¼ 1700 rpm:

N. Samec et al. / Fuel 81 (2002) 2035–20442042

4. Conclusions

The following conclusions can be made based on the

results of the numerical and experimental investigations.

† The strong influence of chemical kinetics on ignition

reactions and on the rate of pollutant formation is

discussed, emphasizing the role of O and OH radicals,

taking into account our results obtained by numerical

simulation of water/n-heptane mixture combustion,

employing an engine combustion model with detailed

chemical kinetics and simplified mixing.

† With numerical analysis of the injection and fuel spray

characteristics, as well as the processes in the cylinder, it

is possible to investigate some of the physical effects of

water present in diesel fuel during combustion processes

and their benefits to the harmful emissions reduction. The

physical impact of water in fuel on the external diesel

combustion characteristic has been evaluated by analyz-

ing in-cylinder pressure, and the rate-of-heat release time

histories. In relation to the net diesel fuel combustion, the

ignition delay became longer by about 10% and the

gradient of rate-of-heat release during premixed burning

increased up to 26%, when W/OEF was used as fuel

under the same engine-operating conditions.

† Testing the vehicular DI diesel engine under several

loads and speeds, using 10% and 15% W/OEF, NOx

concentration in exhaust gas was reduced by nearly 20%

and concentration of soot (Bosch unit) by up to 50%,

with practically no worsening of specific fuel consump-

tion. When comparing the numerical and experimental

results, it becomes clear that chemical kinetics play an

important role, especially in the NOx formation whilst

some physical effects (secondary atomization due to

micro explosions) seem to be more important at soot

emission reduction. At a higher injection pressure,

however, the effect of secondary atomization is expected

to be reduced because of too small fuel droplets

containing insufficient amounts of smaller water

droplets.

† W/OEF can be successfully used to reduce heavy-duty

diesel engine exhaust pollutant emissions, especially

NOx and soot. It is supposed, that this unconventional

technique to reduce NOx and soot emissions in diesel

exhaust is a quite suitable technique to be employed on

vehicular diesel engines for special purposes working

primarily in urban area or on stationary engines, when

they have to satisfy ultra low emission standards.

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