Studying combustion and cyclic irregularity of diethyl ether as supplement fuel in diesel engine

7/26/2019 Studying combustion and cyclic irregularity of diethyl ether as supplement fuel in diesel engine

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Studying combustion and cyclic irregularity of diethyl ether as supplement fuel

in diesel engine

D.C. Rakopoulos, C.D. Rakopoulos , E.G. Giakoumis, A.M. Dimaratos

Internal Combustion Engines Laboratory, Department of Thermal Engineering, School of Mechanical Engineering, National Technical University of Athens (NTUA), Zografou Campus,

9 Heroon Polytechniou St., 15780 Athens, Greece

h i g h l i g h t s

" Experimental diesel engine fueled on 24% DEE supplement in diesel, at various loads.

" HRR diagrams delayed, pressures, temperatures, heat loss reduced, leaner operation.

" Stochastic techniques showed combustion stability with random cyclic irregularity.

" Moreover, no effect on cyclic irregularity of injection process or DEE/diesel blend.

a r t i c l e i n f o

Article history:

Received 13 December 2012

Received in revised form 6 January 2013

Accepted 7 January 2013

Available online 29 January 2013

Keywords:Diesel engine

Diethyl ether blend

Combustion

Cyclic irregularity

Heat release and stochastic analysis

a b s t r a c t

An experimental study is conducted to evaluate the effects of using diesel fuel blend with diethyl ether

(DEE) 24% by vol., a promising fuel that can be produced from biomass (bio-DEE), on the combustion

behavior of a standard, direct injection, Hydra diesel engine. Combustion chamber and fuel injection

pressure diagrams are obtained at four loads, using a high-speed, data acquisition and processing system.

A heat release analysis of the experimentally obtained cylinder pressure diagrams and plots of histories in

the combustion chamber of the gross heat release rate (HRR) and other related parameters, reveal some

interesting features of the combustion mechanism when using DEE blend. Cylinder pressures andtemperatures are reduced, HRR diagrams are delayed, and the engine runs overall a little leaner at

reduced heat losses, with the DEE blend compared to neat diesel fuel for all loads. Moreover, given the

shown low ignition quality of DEE/diesel fuel blend and reports for unstable engine operation at high

DEE blending ratios, the strength of cyclic (combustion variation) irregularity is examined as reflected

in the pressure indicator diagrams, by analyzing for the maximum pressure and rate as well as dynamic

injection timing and ignition delay, using stochastic analysis for averages, coefficients of variation, prob-

ability density functions, auto-correlations, and cross-correlation coefficients. The stochastic analysis

reveals the randomness of fluctuation phenomena observed in the engine, and the cross-correlation coef-

ficients showed that neither the injection process nor the DEE/diesel fuel blend had practical effect on

cyclic irregularity.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Stringent imposed emissions regulations have forced research-

ers to focus their interest on the domain of engine- or fuel-related

techniques [14]. Moreover, the ever increasing energy demands

in the energy generation and transport sectors, coupled with the

limited availability of fossil fuels and their detrimental environ-

mental effects, has guided research to seek alternative fuels for

gradually substituting conventional ones [57]. Among those,

bio-fuels have received increasing attention due to their attractive

features of being renewable in nature and reducing the net CO2emissions, and have been used in both conventional diesel and gas-

oline engines[812].

The share of bio-fuels in the automotive fuel market is expected

to grow rapidly in the next decade. In 2009, the new European reg-

ulation (Directive 2009/28/EC) introduced new targets for the

European Union member states (among those Greece), stating that

each state shall ensure that the share of energy from renewable

sources in all forms of transport in 2020 is at least 10% of the cor-

responding final energy consumption[13,14]. In the USA, the envi-

ronmental protection agency renewable fuel standard version 2

(EPA-RFS2) and the Californian low-carbon fuel standard are

driving the US market[15]. The most promising bio-fuels for fossil

0016-2361/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.01.012

Corresponding author. Tel.: +30 210 7723529; fax: +30 210 7723531.

E-mail address:[email protected](C.D. Rakopoulos).

Fuel 109 (2013) 325335

Contents lists available atSciVerse ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l
http://dx.doi.org/10.1016/j.fuel.2013.01.012mailto:[email protected]://dx.doi.org/10.1016/j.fuel.2013.01.012http://www.sciencedirect.com/science/journal/00162361http://www.elsevier.com/locate/fuelhttp://www.elsevier.com/locate/fuelhttp://www.sciencedirect.com/science/journal/00162361http://dx.doi.org/10.1016/j.fuel.2013.01.012mailto:[email protected]://dx.doi.org/10.1016/j.fuel.2013.01.012


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liquid fuels substitutes/supplements are: bio-alcohols and bio-

ethers primarily used for spark-ignition engines, and vegetable oils

[16], bio-diesels[17], bio-ethanol[1820]and bio-butanol[2124]

mixed in small proportions with diesel fuel for diesel engines.

Works originating from this laboratory studied the performance

and emissions behavior of the present single-cylinder, standard

diesel engine, fueled with blends of diesel fuel with the most

promising of those bio-fuels, such as: vegetable oils and bio-diesels

of various origins [13,25], ethanol[26], n-butanol[27], or diethyl

ether (DEE)[28], and with blends of cottonseed oil and its bio-die-

sel with eithern-butanol or DEE with no diesel fuel at all [29]. The

above investigations were extended on a six-cylinder, turbo-

charged, direct injection, Mercedes-Benz bus diesel engine used

by the Athens Urban Transport Organization, fueled with blends

of diesel fuel with vegetable oils and bio-diesels [30,31], ethanol

[32], orn-butanol[33].

The lowest carbon-chain ether, dimethyl ether (DME), CH3-OCH3, has been experimented as an ignition-improving additive

or replacement in diesel engines with success for lowering smoke

and nitrogen oxides emissions[34,35]. However, as DME is a gas-

eous fuel, its use in vehicles requires some engine fuel injectionsystem modifications [36], while the corresponding fuel delivery

infrastructure is not currently suitable for distributing large quan-

tities of gaseous fuels. Thus, a more appropriate fuel (ether) may be

diethyl ether (DEE), CH3CH2OCH2CH3, which is a fuel with similar

attractive properties to DME for use in diesel engines but in liquid

form (at ambient conditions). It can be produced from ethanol,

which is produced itself from biomass[26], via a dehydrating pro-

cess, thus being also a bio-fuel (bio-DEE).

DEE has several favorable properties for diesel engines [36],

including exceptional cetane number, reasonable energy density

for on-board storage, high oxygen content, low autoignition

temperature, broad flammability limits, and high miscibility with

diesel fuel. Bailey et al. [36]had reported a review of the subject

up to 1997 to identify the potential of DEE as a transportation fuel.Even up to date the testing of DEE in diesel engines performance-

and emissions-wise is limited to very few works [3741], which

were reviewed by the authors[28].

Thus, it is made obvious that a gap exists for the study of com-

bustion mechanism of this bio-fuel when fueling diesel engines,

with the relevant information being rare and incomplete, and with

some works reporting adverse behavior at higher DEE/diesel fuel

blend ratios or loads. Unlike works [37,39] that did not report

any engine stability problems though working up to high DEE/die-

sel fuel blends (30%) and loads, two works[40,41]reported unsta-

ble and heavy smoke engine operation with higher than15% DEE/

diesel fuel blends. In the light of the above and especially the al-

ways shown low ignition quality (higher ignition delay) behavior

of DEE/blends (despite the DEE high cetane number [36]) thatmay give rise to unstable operation[19], a pertinent investigation

is called for the detailed combustion mechanism and strength of its

cyclic irregularity (variability), by examining any cause and effect

relationships.

Therefore, this work reports the results of systematic experi-

mental investigation on a standard, experimental, four-stroke, sin-

gle-cylinder, Hydra, Ricardo/Cussons, naturally aspirated diesel

engine, which possesses high versatility and control over the vari-

ation of its operating parameters. It is a continuation of previous

work [28], where performance and emissions results were pre-

sented using various blends of diesel fuel with DEE, examining

the influence of varying the DEE/diesel fuel blending ratio (92/8,

84/16 and 76/24). The current work examines the influence of load,

the detailed combustion characteristics and the possible driving to

unstable engine operation, at various loads, for the highest blend-

ing ratio that is more likely prone to cyclic irregularity.

Two strong tools are used here for treating the experimentally

obtained cylinder pressure diagrams, viz. heat release analysis[42]

and stochastic techniques[43], which are reviewed briefly in later

sections. The stochastic techniques of auto- and cross-correlation

functions are powerful, objective, scientific tools for removing

the noise from signals and uncover any useful harmonics, thusdisclosing information on any cause and effect relationship, e.g.

here any instability due to fuel low ignition quality or erratic pump

operation.

Concluding this section, it is to be noted that DEE is an isomer of

butanol (the counterpart of ethanol), a very promising fuel for

which extensive research is carried out at present. It may then be

worth stating a brief comparison of the emission-wise behavior

for the same conditions and engine, fueled with the same percent-

ages (in diesel fuel) of either n-butanol, reported in[27], or DEE,

reported in[28], both by the present group. With increasing per-

centage of eithern-butanol or DEE in the blends, it was reported

[27,28] decrease of emitted smoke, nitrogen oxides and carbon

monoxide, and increase of unburned hydrocarbons, with no fuel

penalty. This is a noteworthy similar behavior of those isomerbio-fuels, showing a remarkable simultaneous decrease in both

emitted soot and nitrogen oxides.

2. Experimental engine test facilities, measuring apparatus and

procedure

Facilities to monitor and control engine variables such as speed,

load, water and lube oil temperatures, fuel and air flows, are in-

stalled on a fully automated test bed, single-cylinder, four-stroke,

water cooled, Ricardo/Cussons, Hydra, high-speed, experimental

standard engine. It has the ability to operate on the Otto (spark-

ignition) or direct injection (DI) diesel or indirect injection (IDI)

diesel, four-stroke principle. Here, it is used as a naturally aspi-rated, DI diesel engine having a re-entrant, bowl-in-piston

Nomenclature

cv specific heat capacity under constant volume(J/kg K)

h sampling time interval (s)he (sensible) specific enthalpy (J/kg)m cylinder charge mass (kg), or maximum lag number

N number of raw data valuesp pressure (Pa)Q heat (J)r lag numberR specific gas constant (J/kg K)bRr auto-correlation function of time record

bRxy cross-correlation function between time records x(t)andy(t)

t time (s)T absolute temperature (K)V cylinder volume (m3)

Greek symbolsH fuel lower calorific value (J/kg)q density (kg/m3)qxy sample cross-correlation coefficientu crank angle (deg)

326 D.C. Rakopoulos et al. / Fuel 109 (2013) 325335


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combustion chamber. It has a cylinder bore of 80.26 mm, a piston

stroke of 88.90 mm, a compression ratio of 19.8:1, and a speed

range of 10004500 rpm. The Bosch fuel injection pump has an

11 mm diameter plunger, and the Bosch injector nozzle has four

holes, 0.25 diameter each. The injector opening pressure is

250 bar, and the injection advance (at pump spill) can be varied

from 0to 40crank angle (CA). The engine is mounted on a fully

automated test bed and coupled to a McClure DC motoring dyna-mometer, equipped with a load cell for engine torque measure-

ments. Full details can be found in past publications by the

authors, e.g.[25,26].

For measuring the cylinder pressure, a Kistler 6125B miniature

piezoelectric transducer is used, flush mounted to the cylinder

head and connected to a Kistler 5008 charge amplifier. Also, a Kis-

tler 4067A2000 piezoelectric transducer connected to a Kistler

4618A2 charge amplifier is fitted on the injector side of the pipe

linking the injection pump and injector, to provide the fuel

pressure signal. A Tektronix TDC (Top Dead Center) magnetic

pick-up marker is used for time reference. These output signals

are routed to the input of a Keithley DAS-1801ST A/D board in-

stalled on a Pentium III PC, which can acquire input data at a total

throughput rate of 312.5 ksamples/s from up to eight differential

analogue inputs, utilizing also dual-channel Direct Memory Access

operation. Control of this high-speed data acquisition system is

achieved by a developed computer code based on the TestPoint

control software.

The conventional diesel fuel was supplied by Aspropyrgos

Refineries of the Hellenic Petroleum SA, representing the typical,

Greek automotive, low sulfur (0.035%) diesel fuel (gas oil). The

diethyl ether (DEE) (otherwise called ethyl ether or more simply

ether) was purchased from local commercial representatives cer-

tified to a purity of 99.7% (analytical grade), and was blended with

the normal diesel fuel. Preliminary solubility evaluation tests with

blending ratios up to 30/70 proved that the mixing was excellent

with no phase separation for a period of days, thus requiring no

emulsifying agent. The properties of diesel fuel and DEE are shown

inTable 1. The density of the 24% DEE blend used was measured at0.810 kg/m3. It is true that addition of a low viscosity fuel (cf. val-

ues inTable 1) to diesel fuel, such as DEE or ethanol, can reduce

lubricity and create potential wear problems in sensitive fuel

pump designs[20]. Thus, reduction of lubricity is one of the rea-

sons for keeping low their percentage in the blends, apart from

the effect of reduced viscosity on spray.

In previous work [28], performance (brake specific fuel con-

sumption and thermal efficiency) and regulated emissions results

were reported at full load, for blends of diesel fuel with 8%, 16%

and 24% (by vol.) of DEE. Here, detailed combustion analysis and

stability results are presented for the highest 24% blend, denoted

hereafter and in the figures as DEE24-D. The engine is working at

the same speed of 2000 rpm and static (pump spill) injection tim-

ing of 29 CA before TDC, at various loads, viz. no-load, low load,medium load and high load, corresponding to brake mean effective

pressures (b.m.e.p.) of 0.00, 1.40, 2.57 and 5.37 bar, respectively.

Owing to the differences among the lower calorific values and oxy-

gen contents of the fuels, the comparison is effected at the same

b.m.e.p., i.e. load, and not injected fuel mass or airfuel ratio.

Combustion chamber (indicator) and injector pressure dia-

grams are obtained, where pressures are measured with accuracy

better than within 1% of full-scale output, while the accuracy of

the analogue input readings of the data acquisition system is with-

in 0.01%. These pressures are directly measured quantities (gener-

ic) possessing inherently the inaccuracy of the piezoelectric

transducers stated, which form the seeds for the computations

of the various heat release and stochastic analysis parameters.

The present test engine installation is a standard, versatile, exper-imental one with very accurate instruments and controls to keep

the same speed and load conditions, having also the capabilities

of keeping constant the temperatures (lube oil, cooling water,

etc.). Then, for experiments conducted in the same day, the repeat-

ability is expected to be very good for the various fuels tested.

3. Background of experimental data heat release analysis

In the study of combustion process in diesel engines, an impor-

tant means to analyze combustion characteristics is the calculation

and analysis of heat release rates (HRRs) according to actual

measurements of pressures in the combustion chamber [4244],

with a corresponding diagram of the fuel injection pressure assist-

ing towards this side. The experimental cylinder pressure (indica-

tor) diagrams are here directly processed in connection with the

pertinent application of the energy and state equations. The results

of the analysis for the HRR and other related parameters in the

combustion chamber reveal some interesting features, which aid

the interpretation of the combustion mechanism associated with

the use of DEE/diesel fuel blend in the diesel engine. Towards that

side assist also the widely differing physical and chemical proper-

ties of DEE against the normal diesel fuel, which forms the base-

line case.The method of processing the experimental cylinder pressure

diagrams and their analysis for heat release has been reported in

detail in previous publications, e.g.[26,44]. Thus, only a brief out-

line will be given below. A recording is made of the cylinder pres-

sure data for ten cycles in a contiguous file, with a sampling rate

corresponding to 0.5 CA. A signal from a magnetic pick-up, simul-

taneously recorded, indicates the position of the TDC in each cycle.

Then, the mean of the cylinder (indicator) and the fuel pressure

diagrams are obtained, while a light smoothing for the pressure

signals is applied that is based on performing a four-data points

weighted smoothing. This seems to offer reasonable compromise

between no-loss of valuable signal information and relatively

smooth values for the first derivative of pressure with respect to

crank angle.The measured pressure data processed for the heat release anal-

ysis concern the closed part of the thermodynamic cycle. A spatial

uniformity of pressure, temperature and composition in the com-

bustion chamber (single-zone model), at each instant of time or

during a crank angle step or instantaneous cylinder volume, is as-

sumed. By combining the first law of thermodynamics and the

perfect gas state equation in differential form for the cylinder gas

content, the net heat release ratedQn/du (with respect to crank an-

gle) is derived as[4547]:

dQndu

cvR

pdV

du V

dp

dupV

m

dm

du

p

dV

du he

dm

du 1

with the perfect gas equation of state pV mRT 2

Table 1

Properties of diesel fuel and diethyl ether (DEE).

Fuel properties Diesel fuel Diethyl ether

CH3CH2OCH2CH3

Density at 20C (kg/m3) 837 713

Cetane number 50 >125

Lower calorific value (MJ/kg) 43 33.9

Kinematic viscosity (mm2/s) 2.6 (at 40C) 0.23 (at 20C)

Bulk modulus of elasticity (bar) 16,000 13,000est.Boiling point (C) 180360 35

Latent heat of evaporation (kJ/kg) 250 355

Oxygen (% weight) 0 21.6

Stoichiometric air/fuel ratio 15.0 11.2

D.C. Rakopoulos et al. / Fuel 109 (2013) 325335 327


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Thus, the corresponding gross heat release ratedQg/du, which is the

energy released from the combustion of fuel is given by:

dQgdu

dQndu

dQwdu

3

TermdQw/du stands for the rate of heat transferred to the com-

bustion chamber walls, which is calculated by using the formula ofAnnand[48].

By knowing the fuel lower calorific value, the fuel burned mass

ratedmfb/duis computed as:

dmfbdu

1

H

dQgdu

4

If the differential equations are integrated[26]from the point of

inlet valve closing event up to any crank angle, one can obtain the

respective cumulative values in the chamber of Qg and mfb. The

specific internal energies (sensible part) of the components are

given [49] as fourth order polynomial expressions of T. Similar

expressions are then derived for the specific enthalpies, heat

capacities and their ratio, by applying the thermodynamic rela-

tions connecting these quantities for a perfect gas [46]. The mix-ture properties are then computed by knowing the prevailing gas

composition, as calculated by knowing the air and the fuel mass

burnedmfbup to the point in question[26,49]and the temperature

Tcalculated from Eq.(3).

4. Background of experimental data stochastic analysis

An internal combustion engine may display variations in the

cylinder pressure from one cycle to another, even under nominally

constant operating conditions[50]. Any deviation in the pressure

time development reduces the efficiency and reliability of the en-

gine, increases its noise and exhaust-gas emissions, and is one of

causes of power fluctuations[51]. Measurements and analysis of

cycle-by-cycle variations in spark-ignition engines have beenmade by many investigators[52,53]. However, it seems that corre-

sponding analyzes for diesel engines have not kept pace, though

randomness in the cylinder pressure was known to exist, probably

because of the lower strength cycle-by-cycle pressure variations

occurring in diesel engines.

A short literature review for this phenomenon in diesel engines

has been presented in [19], which deals with the engine in hand

with ethanol/diesel fuel blends. Wing [54] was the first to deal,

in depth, with this aspect of diesel engine operation. His experi-

mental study concerned a multi-cylinder, four-stroke, DI diesel

engine having a rotary distributor fuel injection pump, which

was suspect and proved to be the culprit of cyclic pressure varia-

tions (irregularity). Sczomak and Henein [55] in an extensive

experimental investigation on a CFR pre-chamber diesel engineoperating with various low-ignition quality fuels, correlated cyclic

pressure variations with ignition delay and dynamic injection tim-

ing, and pointed out that low cetane number fuels can cause cyclic

irregularity in diesel engines.

Following the heat release analysis above, the present work fo-

cuses on the study of cyclic combustion variations in the engine

running with DEE/diesel fuel blend at the same operating condi-

tions. The need for such a complementary study emanates from

the reporting in some works (stated in the Introduction) of diesel

engine unstable operation with DEE/diesel fuel blends, and more

generally motivated by the always reported behavior of those

blends presenting higher ignition delay than the neat diesel fuel

(cf. also next section), despite the much higher cetane number of

DEE [36]. Thus, by showing a low-ignition quality fuel behaviorthey need to be investigated in that respect according to the find-

ings of Sczomak and Henein[55]. The combustion cyclic variability

(irregularity) is tackled here in the way it is reflected in the pres-

sure indicator diagrams, by analyzing for the maximum pressure

and pressure rate, dynamic injection timing and ignition delay,

using stochastic analysis techniques.

For the stochastic analysis a recording is made of the cylinder

and fuel measured pressure data for 480 cycles in a contiguous file,

with a sampling rate corresponding to 0.5

CA. In contrast to thepreviously described HRR analysis, for the stochastic analysis the

480 pressure diagrams (cycles) are used separately (the mean is

meaningless here), again with light smoothing, since by definition

the parameters drawn from them will form the data record values

to be statistically processed. For assessing the errors involved with

the number of cycles chosen[43], the variations of the mean value

and the standard deviation of the maximum pressures and pres-

sure rates were plotted against the number of cycles, revealing that

a number of cycles greater than 400 form a safe limit.

By processing the fuel (injection) pressure diagram, the static

injection timing (at the injector) was determined at the crank angle

where this pressure rises above the almost constant residual in the

connecting pipe pressure value, after the (pump spill) injection

timing event. The dynamic injection timing was assumed to coin-

cide with the crank angle where the fuel pressure reaches the value

of the injector nozzle opening pressure, immediately following the

event of static injection timing [26]. The difference between dy-

namic injection timing and pump spill timing forms the injection

delay.

By processing the cylinder pressure diagram, the ignition timing

was located at the crank angleu where the first derivative of pres-

sure with respect to u changes slope, immediately following the

event of dynamic injection timing, going from a negative to a posi-

tive value and so presenting a local minimum. The ignition timing

was then determined either by using this condition, or by locating

the corresponding zeroing crank angle of the second derivative of

pressure with respect to u, assuming that this signal is smooth

enough. Note that with every differentiation of the pressure signal

the noise-to-signal ratio increases, while if over-smoothing is ap-plied this zero point might disappear as being ill conditioned.

The difference between the ignition and dynamic injection timing

forms the ignition delay. From the first and second derivatives of

cylinder pressure diagrams with respect to u, the crank angles of

maximum values of the first derivative of pressure and the pres-

sure itself can be computed, bearing also in mind that they imme-

diately follow the ignition timing and in that order.

The following statistical quantities are used for the analysis of

the N raw data values u i (i= 1,2,..., N) of a time record: averages,

standard deviations, and probability density functions, with the

Gaussian (or normal) probability density function with the same

mean value and standard deviation as that of the data record also

computed [56]. For computation of the auto- and cross-correla-

tions of the parameters involved, the mean value u has been sub-tracted from each value ui, i.e. the new time history record is

considered xt xt0 nh ui u i 1;2; . . . ;N where h is

the sampling time interval andn = 1,2,..., N.

The auto-correlation function is estimated by direct computa-

tion after any linear trend removal. For N data values xi(i= 1,2,..., N), from a transformed record x(t), the estimated auto-

correlation function at the time displacementrh is defined by the

formula[56,57]:

bRr 1N r

XNri1

xixir r 0;1;2; . . . ;m 5

where r is the lag number, m the maximum lag number, and b

Rrthe estimate of the true value Rr at lag r, corresponding to the



5/11

displacement rh. A normalized value for the auto-correlation

function is obtained by dividing bRrby bR0; where

bR0 bRx0 1N

XNi1

x2i x2 6

The cross-correlation between two time recordsx(t) andy(t) at

lag numbers r= 0,1,2, . . . , mis:

bRxy 1N r

XNri1

xiyir and bRyx 1

N r

XNri1

yixir 7

The maximum value ofrshould normally be[57]less than 10%

of N. The normalization of the cross-correlation function defines

the sample cross-correlation coefficient:

qxyrh bRxyffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffibRx0bRy0q

bRxyffiffiffiffiffiffiffiffiffiffix2y2

q 8

5. Discussion of the heat release analysis combustion results

All pressure diagrams in this section are mean-smooth, whichare then processed to produce the other related parameters. They

are presented below in the two fourfoldedFigs. 1 and 2.

Fig. 1a shows at the four loads considered the fuel (injection)

pressure against crank angle diagrams for the neat diesel fuel

and the DEE24-D blend. First it can be seen that with increasing en-

gine load the injection duration increases (as more fuel is injected)

-40 -20 0 20Degrees crank angle

0

200

400

600

800

Fuelpressure(bar)

Diesel, b.m.e.p.=5.37 bar

DEE24-D, b.m.e.p.=5.37 bar





Diesel, b.m.e.p.=0. barDEE24-D, b.m.e.p.=0. bar

(a)

-20 -10 0 10 20 30 40Degrees crank angle

0

20

40

60

80

100

Cy

linderpressure

(bar)







Diesel, b.m.e.p.=0. bar

DEE24-D, b.m.e.p.=0. bar

(b)

-10 0 10 20 30 40

Degrees crank angle

0

10

20

30

40

Gross

hea

tre

leasera

te(J/deg.)









(c)

-20 -10 0 10 20 30 40

Degrees crank angle

0

400

800

1200

1600

2000

Tempera

ture

(K)









(d)

Fig. 1. Fuel (injection) pressure (a), cylinder pressure (b), gross heat release rate (c), and cylinder temperature (d) against crank angle diagrams, at the four loads, for the neatdiesel fuel and the 24% diethyl ether blend cases.



6/11

for both fuels and the same holds true for the injection pressures.

Further, for each load considered, the DEE fuel pressure diagram is

distorted with respect to the corresponding neat diesel fuel one.

Specifically, its uprising leg acquires a lower gradient, which is

translated into a delay of the dynamic injection timing, and

furthermore its maximum value is slightly reduced and its final

falling leg delayed.

The different densities ql and bulk moduli of elasticity Kbm of

blends influence the whole injection process [58,59] following

the simplified analysis of Obert [60]. For a jerk pump when its

plunger begins to compress the fluid, a pressure wave is propa-

gated down the connecting pipe, at essentially the speed of sound

as= (Kbm/ql)1/2, reaching eventually the injector needle in order to

open it. Thus, depending on the values of these properties the

-20 0 20 40 60 80

Degrees crank angle

0

200

400

600

800

Cumu

lativegross

hea

tre

lease

(J)









(a)

-40 0 40 80

Degrees crank angle

0

0.2

0.4

0.6

Equ

iva

lencera

tio









(b)

-20 0 20 40

Degrees crank angle

1000

2000

3000

4000

5000

Hea

ttrans

fercoe

fficien

t(W/m2K)






DEE24, b.m.e.p.=1.40 bar


DEE24, b.m.e.p.=0. bar

(c)

-80 -40 0 40 80 120

Degrees crank angle

-100

0

100

200

300

Cumu

lative

hea

tloss(

J)






DEE24, b.m.e.p.=1.40 bar


DEE24, b.m.e.p.=0. bar

(d)

Fig. 2. Cumulative gross heat release (a), equivalence (fuelair) ratio (b), heat transfer coefficient (c), and cumulative heat loss (d) against crank angle diagrams, at the four

loads, for the neat diesel fuel and the 24% diethyl ether blend cases.



7/11

dynamic injection timing is affected despite that the pump spill

timing is kept constant, as here for all fuel samples tested. The bulk

modulus of elasticity of DEE is not known, but is expected to bemuch lower than the diesel fuel one and near to the ethanol value

at around 13,000 bar[28,61]. Using the values ofql and Kbm from

Table 1,asis computed as 1382.6 m/s and 1350.3 m/s for the diesel

fuel and the DEE, respectively, showing indeed a relatively later

arrival of the pressure pulse at the injector needle for the DEE case.

Fig. 1b shows, at the four loads considered, the cylinder pres-

sure against crank angle diagrams for the neat diesel fuel and the

DEE24-D blend, focusing on their part around hot TDC. First it

can be seen that the pressures increase with load (with the

compression lines remaining the same), while the ignition delay

decreases with engine load for both fuels due to the increasing

gas temperatures with load. One can observe that for each load

considered, the DEE blend start of combustion occurs later (the

pressure rise due to combustion starts later) with respect to thecorresponding neat diesel fuel one, while its maximum pressure

falls and occurs later. The start of combustion is delayed as a

consequence of synergy of the lower dynamic injection timing

(cf.Fig. 1a) and increased ignition delay.It is worth explaining this behavior also in conjunction with

Fig. 1c, which shows the corresponding gross heat release rate

(HRR) diagrams. First it can be seen that the ignition delay

decreases with engine load for both fuels (since temperatures

increase), while the heat release rate values become higher. For

the higher loads, both parts of combustion, i.e. the premixed com-

bustion (the part under the first sharp peak) and the diffusion

combustion (the last part under the second rounded peak), are

apparent with the diffusion combustion diminishing with load

decrease. One can again observe that for each load considered,

the ignition delay for the DEE24-D blend is higher than the corre-

sponding one for the neat diesel fuel case. The increase of ignition

delay of DEE when blended with diesel fuel has also been reported

early in [35] and by later investigators despite its much highercetane number than diesel fuel, with possible explanations

0 2 4 6

b.m.e.p. (bar)

65

70

75

80

85

90

Meano

fmax

imump

ressure

(bar)

Mean for Diesel fuel

Mean for DEE 24%

COV for Diesel fuel

COV for DEE 24%

0

0.2

0.4

0.6

0.8

1

COVo

fmax

imumpressure

(%)

(a)

0 2 4 6

b.m.e.p. (bar)

1

2

3

4

5

Meano

fmax

imumpressurera

te(bar/

deg.)

Mean for Diesel fuel

Mean for DEE 24%

COV for Diesel fuel

COV for DEE 24%

0

2

4

6

8

10

COVo

fmax

imumpre

ssurera

te(%)

(b)

0 2 4 6

b.m.e.p. (bar)

8

8.4

8.8

9.2

9.6

10

Meanof

dynam

icinjec

tion

tim

ing

(deg.

bTDC)

Mean for Diesel fuelMean for DEE 24%

COV for Diesel fuel

COV for DEE 24%

1.6

1.8

2

2.2

2.4

2.6

COVo

fdynam

icinjec

tion

tim

ing

(%)

(c)

0 2 4 6

b.m.e.p. (bar)

4

4.4

4.8

5.2

5.6

6

Meanofignitiondelay(deg.)

Mean for Diesel fuelMean for DEE 24%

COV for Diesel fuel

COV for DEE 24%1.6

1.8

2

2.2

2.4

COVofignitiondelay(%)

(d)

Fig. 3. Cyclic variation, as a function of load, expressed as mean values and coefficients of variation (COV) of the maximum cylinder pressure (a), maximum rate of cylinder

pressure rise (b), dynamic injection timing (c), and ignition delay (d), for the neat diesel fuel and the 24% diethyl ether blend cases.



8/11

provided in[62,28], while the decreasing dynamic injection timing

and the higher latent heat of evaporation of DEE (seeTable 1) here

contribute also towards this side (injection into a lower tempera-

ture environment). Further, it is observed that the premixed

combustion (area under the first sharp peak) of the DEE blend

seems to decline against the corresponding neat diesel fuel case,

thus leading to lower pressures and temperatures during the initial

part of combustion process.

Fig. 1d shows the corresponding cylinder temperature dia-

grams. First it can be seen that there is a temperature increase with

engine load[45]for both fuels. One can observe that for each loadconsidered, with respect to the neat diesel fuel case, the tempera-

tures for the DEE24-D blend are lower up to around their maxi-

mum values and appear delayed (cf. previous paragraph for the

premixed part of combustion), while later on during expansion

they seem to recover and even slightly switch over the correspond-

ing diesel fuel ones. The latter is due to the delayed and prolonged

(last) part of diffusion combustion (area under the second

rounded peak in the HRR diagrams). It is reminded here that this

is a computed mixed temperature due to the inherent single-zone

assumptions of the heat release analysis followed.

The observed above increase of delay of the fuel pressure and

heat release rate diagrams (and consequent fall in cylinder pres-

sures and temperatures) with the use of DEE in the diesel fuel

blend, points to the influence on the combustion and emissionsformation processes[59,61]. This is effected through a later and

slower spray development with possible impingement on the com-

bustion chamber walls [58], apart from any possible poor fuel

injection (and so atomization) due to vapor locks because of the

high volatility of DEE as mentioned in [40,41].

Fig. 2a shows the corresponding cumulative gross heat release

diagrams. One can observe that for each load considered, the

cumulative gross heat release curve for the DEE24-D blend lies,

at the beginning, a little lower than the corresponding one for

the neat diesel fuel case and catches up later on into the expansion

stoke, thus revealing the slower rate of combustion as also ex-

plained with reference to Fig. 1c above. Then, the corresponding

final (almost) equal cumulative gross heat release values are trans-

lated into the same brake thermal efficiency, given the constantengine speed and load. Fig. 2b shows the corresponding fuelair

equivalence ratio (i.e. the actual fuelair ratio divided by its stoi-

chiometric value) diagrams. One can observe that for each load

considered, the fuelair equivalence ratio curve for the DEE24-D

blend lies a little lower than the corresponding one for the neat

diesel fuel case. This proves that the engine runs overall a little

leaner with the DEE24-D blend, at least at the beginning, for the

same engine load and speed conditions, noting that the calculation

of fuelair equivalence ratio was made by considering all the fuel-

bound oxygen.

Fig. 2c shows the corresponding gas side heat transfer coeffi-

cient (from the cylinder gas to the combustion chamber walls) dia-grams. One can observe that these diagrams follow in shape closely

the corresponding ones of (cylinder) temperatures (cf.Fig. 1d). This

is explained as the gas side heat transfer coefficients are computed

from the relevant formula of Annand[48], which is an increasing

monotonic function of gas temperatureT. It can be easily proved

by assuming, for example, variation laws[45]of gas thermal con-

ductivitykgas= T0.75, and dynamic viscosity lgas= T

0.62. Fig. 2d

shows the corresponding cumulative heat loss (to the combustion

chamber walls) diagrams. One can observe that for each load con-

sidered, the cumulative heat loss curve for the DEE24-D blend lies

a little lower than the corresponding one for the neat diesel fuel

case. This is due to the lower cylinder temperatures and heat trans-

fer coefficients encountered with the DEE blend case (cf. Figs.1d

and2c), as the cumulative heat loss is effectively the integral, overthe cycle, of the product of these two quantities.

6. Discussion of the stochastic analysis results of combustion

parameters

In the figures to follow, results are presented at all four loads

considered, and for the neat diesel fuel and the blend of 24% (by

vol.) diethyl ether (DEE) in diesel fuel. From the large amount of

data collected at each operating condition, only representative

sample plots are presented owing to imposed conservation of

space. Preliminary tests to determine the extent of cyclic variation

in combustion over the load range examined, used both the maxi-

mum cylinder pressure and the maximum cylinder pressure rate asmeasures of the cyclic variation (the effect). Their variations are

0 10 20 30 40 50

Cycle difference

-0.4

0

0.4

0.8

1.2

Au

to-corre

lation

func

tion

(norm.)

MAX. PRESSURE RATEHigh load

Diesel fuel

DEE 24%

0 10 20 30 40 50

Cycle difference

-0.4

0

0.4

0.8

1.2

Auto-correlationfunc

tion(norm.)

IGNITION DELAYHigh load

Diesel fuel

DEE 24%

(a) (b)

Fig. 4. Normalized auto-correlation functions of the maximum rate of pressure rise (a), and ignition delay (b), at the high engine load, for the neat diesel fuel and the 24%

diethyl ether blend cases.



9/11

distinct and obviously have a close reference to the combustion

process itself but, in any case, a rather strong degree of correlation

exists between those as will be shown in lastFig. 5. The dynamic

injection timing was chosen[54]as potential cause of any influ-

ence of the injection process on the cyclic variation, while the

ignition delay was chosen as corresponding potential cause of

any influence of the fuel[55].

Fig. 3a and b presents the cyclic variation of the maximum

cylinder pressure and the maximum rate of cylinder pressure rise,respectively, expressed as mean values and coefficients of variation

(COV), i.e. standard deviation divided by the mean value, as a func-

tion of the engine b.m.e.p. (load) for the cases of the neat diesel fuel

and the 24% addition of DEE in the blend.Fig. 3c and d presents the

corresponding cyclic variations of the dynamic injection timing

and the ignition delay, respectively. The observed variation (mean

values) with either the load or the addition of DEE in the blend has

already been discussed with reference toFig. 1ac. FromFig. 3ad,

one can conclude, by observing the coefficients of variation (COV)

values, that the addition of DEE in the blend, at least for up to 24%

DEE, does not practically affect the cyclic variability (irregularity)

with respect to the neat diesel fuel case, which in any case is

already small.

The probability density functions of the experimental maxi-mum cylinder pressure, pressure rate, dynamic injection timing

and ignition delay, for the neat diesel fuel and the 24% addition

of DEE in the blend cases, followed quite closely the correspond-

ing Gaussian ones (computed) having the same mean value and

standard deviation. They showed a slightly different skewness

(in the range 0.1 to +0.1) and kurtosis (in the range0.2 to

0.6) against the corresponding values of zero for the Gaussian

ones. Hence, the error of the analysis will be insignificant if a

normal distribution is assumed for the purpose of determining

the statistical nature of the above four parameters, as has already

been tacitly assumed in previous Fig. 3. This implies that the

cause of the fluctuations of these parameters is rather random

(stochastic) and does not depend on its value of any other cycle,

i.e. on any residual effects of previous combustions taken place inthe cylinder[43,54].

Fig. 4a and b shows sample normalized auto-correlation func-

tions of the maximum rate of pressure rise and the ignition delay,

respectively, for the cases of the neat diesel fuel and the 24% addi-

tion of DEE in the blend, at the high engine load (b.m.e.p. = 5.37 -

bar). The auto-correlation function for the other engine loads and

the other parameters were similar, not exceeding the critical value

(0.20) at the 1% significance level. From observation of the auto-

correlation values, it is concluded that there is no correlationbetween the fluctuations of different cycles, thus confirming the

same conclusion as of the sample probability density functions dis-

cussed above.

For examining the influence of the injection process (potential

cause) and the kind of fuel used via its cetane number (another

potential cause) on the cyclic pressure variation, a cross-

correlation analysis was carried out. This computed the degree

of correlation between the dynamic injection timing and the

maximum rate of pressure rise, between the dynamic injection

timing and the ignition delay, and between the ignition delay

and the maximum rate of pressure rise. Also, the degree of

correlation between the maximum cylinder pressure and the

maximum rate of pressure rise is presented only for reference.

The reason is that the values of the maximum rate of pressure

rise were selected as the measure of cyclic variation (the effect)

in the combustion chamber.

Thus,Fig. 5 presents all these correlation coefficients (Eq. (8)

withr= 0) for the cases of the neat diesel fuel and the 24% addition

of DEE in the blend, as a function of load. It can be observed that

there is a minimal to slight correlation of these parameters (abso-

lute values much less than 0.5), with the exception of the expected

rather strong (positive) correlation between the maximum cylin-

der pressure and the maximum cylinder rate of pressure rise[43]

that seems to be decreasing with load.

All the results of the above analysis indicate clearly that neither

the injection process (through the dynamic injection timing), nor

the kind of DEE/diesel fuel blend used (through the shown low

ignition quality) have any practical effect on the above cyclic vari-

ations (irregularity). Therefore, there is no unstable operation ofthe engine at least for up to 24% addition of DEE. These findings

are in accord with works [37,39]that did not report any stability

problems though working up to high DEE blending ratios (30%)

and loads, thus not encountering the findings of the two works

[40,41], reporting unstable and heavy smoke engine operation

with higher than 15% (up to 25%) of DEE in its blends with diesel

fuel. The latter researchers (working on essentially the same en-

gine) attributed this behavior to erratic combustion, possibly due

to phase separation of the blends that resulted in cavitations

(vapor locks because of the high volatility of DEE) in the fuel line

and injector nozzle, thus leading eventually to poor fuel injection

(large droplets) in the combustion chamber. It is noticed that their

injection system was already operating in (or over) the limit for the

neat diesel fuel with high smoking at the high load points, and thusdeteriorating its performance when a different fuel (DEE blends)

was tried.

7. Conclusions

An extended experimental study is conducted to evaluate and

compare the use of DEE, a promising bio-fuel, as supplement to

the conventional diesel fuel in a high-speed, direct injection diesel

engine, operating at four loads.

A heat release analysis of the experimentally obtained pressure

diagrams revealed that with the use of DEE blend against neat die-

sel fuel, at all loads, the fuel injection pressure diagrams are de-

layed (with the uprising leg inclined), dynamic injection timingdecreased, ignition delay increased, maximum cylinder pressures

0 2 4 6

b.m.e.p. (bar)

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Corre

lationcoe

fficien

ts

Diesel, pr. - pr. rate

DEE24-D, pr. - pr. rate

Diesel, dyn. inj. - pr. rate

DEE24-D, dyn. inj. - pr. rate

Diesel, dyn. inj. - ign. del.

DEE24-D, dyn. inj. - ign. del.

Diesel, ign. del. - pr. rate

DEE24-D, ign. del. - pr. rate

Fig. 5. Correlation coefficients between dynamic injection timing (dyn. inj.) and

maximum rate of pressure rise (pr. rate), dynamic injection timing (dyn. inj.) andignition delay (ign. del.), ignition delay (ign. del.) and maximum rate of pressure rise

(pr. rate), and maximum cylinder pressure (pr.) and maximum rate of pressure rise

(pr. rate), as a function of load, for the neat diesel fuel and the 24% diethyl ether

blend cases.



10/11

and temperatures decreased, while the engine runs overall a little

leaner with reduced heat losses.

The acquired data were statistically analyzed and shown in this

paper for the maximum pressure and its maximum rate of pressure

rise, the dynamic injection timing, and the ignition delay. The cy-

cle-by-cycle variation was expressed as the mean and coefficient

of variation of these parameters. The analysis of probability density

and auto-correlation functions of the various parameters, revealedthe randomness (stochastic nature) of fluctuation phenomena

observed in the engine. Cross-correlation coefficients showed

clearly that neither the injection process (through the dynamic

injection timing) nor the DEE/diesel fuel blend used (through the

cetane number) have any practical effect on the above cyclic

variations (irregularity). Thus, there is no unstable operation of

the engine at least for up to 24% addition of DEE in its blend with

diesel fuel.

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Documents

Studying combustion and cyclic irregularity of diethyl ether as supplement fuel in diesel engine