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Mitsubishi Heavy Industries, Ltd.Technical Review Vol.38 No.3 (Oct. 2001)

136

Increase of Thermal Efficiency and Reduction of NOx Emissions

in DI Diesel Engines

*1 General Machinery & Special Vehicles Headquarters*2 Nagasaki Research & Development Center, Technical H eadquarters

Akira Numata*1 Takuya Kumagai*1

Yoshinori Nagae*2 Shinnosuke Osafune*2

To increase thermal brake efficiency and reduce NOx emissions in a stationary diesel engine, we installed directinjection (DI) combustion consisting of a combustion chamber with a high compression ratio and fuel injection withhigh injection pressure in a single-cylinder diesel engine and tested it to determine its potential and basic properties.The high compression ratio was proven to have high thermal brake efficiency and low NOx emission at retarded fuelinjection timing. High injection pressure due to a small nozzle hole diameter restrained ini tial combustion, promoteddiffused combustion, and increased effective combustion efficiency. The high compression ratio and high injectionpressure with retarded fuel injection timing improved thermal brake efficiency and reduced NOx emission.

1. I ntroduction1. I ntroduction1. I ntroduction1. I ntroduction1. I ntroduction

The problems of global warming due to carbon di -oxide, and of environmental pollution including acid

deposition caused by nitrogen oxides, are now more

serious than ever, and minimization of harmful emis-

sions produced by combustion of fuels is increasingly

required. In addition, industr ies using cogeneration

system and stationary diesel engines for power gen-

eration need to take urgent measures.

Direct injection (DI ) diesel engines, which are now

the major type of stationary diesel engine, are excel-

lent in high thermal efficiency and are advantageous

in reducing CO2 emission, but still have problems in

reduction of nitrogen oxide (NOx) and smoke emis-

sions. With a view to reduction in NOx emission and

increase of the thermal efficiency of DI diesel engines,

thi s study was started with r esearch on a combus-

tion system with high compression ratio and high

fuel-injection pressure in order to determine the ba-

sic characteristics.

This report describes the performance of the above-

mentioned combustion system applied to a

single-cylinder engine, and discusses the results of

the combustion test.

2. Aim of technical studies2. Aim of technical studies2. Aim of technical studies2. Aim of technical studies2. Aim of technical studies

The brake thermal effi ciency e of diesel engine is

expressed by Equation (1)(1)

ethX glX combX (1-cool )X m (1)

where,

th: theoretical thermal efficiency

[=f (,K): function of compression

ratio and ratio of specific heat K]

gl: degree of constant volume

comb: combustion efficiency

cool: heat l oss rate

m: mechanical effi ciency.

To increase the thermal efficiency of DI diesel en-

gines, this study was carried out with the aim of

improving the rul ing factors in the above-mentionedbrake thermal efficiency, namely theoretical thermal

efficiency, degree of constant volume, combustion ef-

ficiency, heat loss rate and mechanical efficiency.

As shown in F igF igF igF igF ig..... 11111, higher compression ratio is

needed for increase of theoretical thermal efficiency,

shorter combustion period and control of ignition tim-

ing for improvement of degree of constant volume,

higher excess air ratio and higher fuel-air mixture

formation energy for increase of combustion effi-

ciency, lowering of heat transfer coefficient in gas by

control li ng swirl velocity for reduction of heat l oss,

and lower friction loss for increasing of mechanical

efficiency.

I n view of the above, it was planned in this study

to raise compression ratio, increase maximum com-

bustion pressure in the cylinder, shorten injection

period and control injection timing, increase fuel in-

jection energy by high-pressure injecti on, introduce

high-pressure ratio variable geometry (VG) turbo-

charger, reduce swir l ratio, reduce dead volume in the

combustion chamber and modify the cooling system

and piston rings.

F or reducti on of N Ox emission, modifi cati on ofengine is necessary to control combustion, that is to

keep the flame temperature lower and control oxy-

gen concentr at i on . F or reduct i on of f l ame

temperature, control of initial combustion and acti-

vation of diffusive combustion were attempted with

the aim of forming a constant pressure combustion

cycle, whi le charge air cooling and a New Water I n-

jecti on System (an inj ecti on syst em wi th singl e

nozzle through which fuel and water are alternately

sprayed)(2) are applied.

I n this study, the effects of compression rat io, fuel

injection pressure and fuel injection timing, which

determine the basic performance of the direct injec-

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22

20

18

16

0.5

0

16 14 12

: 16.0: 14.2

P

MP

a)

e/(

e

)STD

100(%)

NOx

/(NOx

)

100(%)

R

20

-20

4

0

-4

0

inj BTDCo

Combustion chamber Fuel injection nozzle

Fuel spray Piston

Engine speedNe=1 000 rpm, rated load

Symbol Compressionratio

Brakethermalefficiency

Referencedata

Referencedata

Fuel injection timing

Max

imu

m

combu

stion

pressu

reincylinder

Smoke(BOSCH)


138

of friction loss mean effective pressure.

As the compression ratio increases, theoretical

thermal efficiency increases, but friction loss in-

creases to lower mechanical efficiency m [=Pme/

(Pme+Pmf)]. However, in view of the fact that the prod-

uct of theoretical thermal eff iciency th and

mechanical efficiency m becomes greater, it is clear

that application of high compression ratio could im-

prove brake thermal efficiency.

F igF igF igF igF ig..... 33333 shows the effects of compression ratio and

fuel injection timing on engine performance and ex-

haust emissi ons. I n thi s figure, brak e thermal

efficiency and NOx emission are expressed in the val-

ues relative to the reference condition of compression

ratio=14.2 and fuel injection timing in j =BTDC15O. As fuel i njection ti ming is retarded, both N Ox

emission and maximum combustion pressure in the

cylinder decrease, but brake thermal efficiency de-

creases and smoke emission shows a slight tendency

to increase. However, the case of compression ratio

=16.0 shows a higher rate of reduction in NOx emis-

sion and maximum combustion pressure in the

cylinder and a lower rate of reduction in brake ther-

mal effi ciency as compared with the case of=14.2.

I t was veri fied from the above that a combined

appli cation of hi gh compression rat io and retarded

fuel injection timing is an effective means of increas-

ing thermal efficiency and reducing NOx emission.

3.2 Effect of nozzle hole diameter3.2 Effect of nozzle hole diameter3.2 Effect of nozzle hole diameter3.2 Effect of nozzle hole diameter3.2 Effect of nozzle hole diameter

The effects of the nozzle hole diameter on fuel in-

jection characteri stics were investigated using a fuel

injection pump tester. The test results are shown in

F igF igF igF igF ig..... 44444. When the nozzle hole diameter is decreased

gradually from 0.48 mm to 0.45 mm and 0.42 mm

under conditions of camshaft speed Nc=500 rpm

(equivalent to engine speed Ne=1 000 rpm), fuel i n-

jection quant ity qf=2 100 mm3/stroke (equivalent to

rated power) and injection through 10 nozzle holes,

the maximum injection pressure (at nozzle end) Pf

increases from 151 MPa to 157 MP a and 168 MP a,

but the fuel i njection peri od qf (camshaft angle) in-

creases from 11.2O to 12.0O and 13.0O, respectively.

F igF igF igF igF ig..... 55555 shows the effects of the nozzle hole diam-

eter on engine performance and exhaust emission

characteri stics. In thi s figure, brake thermal effi -

ciency and NOx emission are expressed in the values

relat ive to the reference condit ion of the nozzle hole

diameter d=0.48 and fuel injection timing i nj

=BT DC 15O

. When the nozzle hole diameter is de-creased (both injection pressure and injection period

increase) at fuel injection timinginj =BTDC 15O, the

maximum combustion pressure in the cylinder de-

creases, but NOx emission shows a tendency to

increase whi le brake thermal efficiency and smoke

emission remain almost the same. When the nozzle

hole diameter is decreased at fuel injection timing

inj =BTDC10O, NOx emission increases, but brake ther-

mal efficiency rises and smoke emission and

maximum combustion pressure in the cylinder show

a tendency to decrease. From the above evaluati on,

it is obvious that the effect of the reduction of the

nozzle hole diameter, that is to increase fuel injec-

F ig. 3Fig. 3F ig. 3Fig. 3Fig. 3

When the fuel injection timing is retarded at highcompression ratio, the rate of decrease of brake ther-mal efficiency is low, but high for NOx emission andmaximum combustion pressure in the cylinder.

F ig. 4F ig. 4F ig. 4F ig. 4F ig. 4

As the nozzle hole diameter decreases, maxi-mum fuel injection pressure rises but injectionperiod becomes longer.

Effects of compression ratio on engineEffects of compression ratio on engineEffects of compression ratio on engineEffects of compression ratio on engineEffects of compression ratio on engineperformance and exhaust emissionsperformance and exhaust emissionsperformance and exhaust emissionsperformance and exhaust emissionsperformance and exhaust emissions

Effects of nozzle hole diameter onEffects of nozzle hole diameter onEffects of nozzle hole diameter onEffects of nozzle hole diameter onEffects of nozzle hole diameter onfuel injection characteristicsfuel injection characteristicsfuel injection characteristicsfuel injection characteristicsfuel injection characteristics

1

0-85 -80 75 70 65 60

200

150

100

50

0

13o

168MPa

Camshaft speedN

Fuel injection quantity (rated load)q

Injection nozzle

: 10-hole, 0.42 mm

: 10-hole, 0.45 mm

: 10-hole, 0.48 mm

Injectionpressu

reatnozz

leend

Pf

(MPa

Cam angle (o) N

eedlev

alvelift(mm)

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comb

1-

cool)

0.42 0.44 0.46 0.48

0.76

0.74

0.72

0.7

52

50

48

46

0.96

0.94

0.92

0.9

42

40

38

Engine speedNe=1 000 rpm

Rated load Compression ratio =15.0Injection timing BTDC10

o

Injection nozzle:

Nozzle holes 10Spray angle 160

o

Effectiv

ecombu

stion

efficiency

B

rakethermalefficiency

e

(%)

Combustionperiod

(o

)

Degreeofconstant

volume

Injection nozzle d (mm)

20

2000

1600

1200

800

15

10

5

0-30 0 30 60 90

4

3

2

1

0

: 10-0.42-160: 10-0.45-160: 10-0.48-160

Engine speed Ne=1 000 rpm

Rated load Compression ratio = 15.0

Injection timing BTDC10o

Injection nozzle

Cy

cle

temperaturein

cylinderT

(K)

Pressu

reincy

linder

P

(MPa)

Crank angle (ATDCo

)

Heatreleaserates

kJ/

o)

: BTDC10o

: BTDC15o

P

max

(MPa)

e

/

e)S

TD

100

(%)

NOx

/(NOx

)STD

100(%)

0

-4

-40

-20

20

22

20

18

16

0.5

0

0

-

80.42 0.44 0.46 0.48

Engine speedNe = 1 000 rpmRated load Compression ratio = 15.0

Injection nozzle:Sym-bol

Injectiontiming Nozzle holes 10

Spray angle 160o

Brak

ethermalefficiency

Nozzle hole diameter d (mm)

Reference data

Reference data

Maximu

m

combu

stion

pressu

reincy

linder

Smoke

(BOSCH)

R

Mi tsubishi Heavy Industries, Ltd.Technical Review Vol.38 No.3 (Oct. 2001)

139

tion pressure (although injection period becomes

longer), on the improvement of combustion charac-

teri stics become bigger in case of retarding the fuel

injection ti ming.

F igF igF igF igF ig..... 66666 shows the effects of the nozzle hole diam-

eter on heat release rates in case retarded fuel

injection t iming. As the nozzle hole diameter i s de-

creased, initial combustion is restrained and diffusive

combustion becomes active. Since the initial injection

pressure does not vary much with the difference in

nozzle hole diameter, as seen in F ig. 4, it is consid-

ered that the initial fuel injection quantity, namely

init ial injection rate, is restrained by use of a small er

diameter nozzle, allowing injection pressure to rise

in the later stage and the injection period to lengthen,with a resulting increase in the injection rate in the

later stage.

F igF igF igF igF ig..... 77777 shows the result of analysis of heat r elease

in the cylinder. The effective combustion efficiency

shown in this figure is as described below.

Assuming heat loss to be included in the total heat

release calculated from cylinder pressure, the ratio

of calorific value of fuel Humf(Hu: lower heating value

of fuel, mf: fuel mass) to total heat release QT is de-

fined as the effective combustion efficiency QT/(Humf)

(ratio of heating value actually received by the gas

inside combustion chamber), and the following Equa-

tion (3) should hold:

QT/(Humf) =combX (1-cool ) (3)

Hence, the effective combustion effi ciency in this

case is as shown in F ig. 7. When the nozzle hole di-

ameter is changed, compression ratio =15.0 is

constant and, accordingly, theoretical thermal effi-

ciency and mechanical efficiency are also constant

while, from Equation (1), brake thermal efficiency is

dependent on the degree of constant volume and ef-


Reduction of the nozzle hole diameter restrains ini tialcombustion and activates diffusive combustion.

Fig. 7Fig. 7Fig. 7Fig. 7Fig. 7

Reduction of the nozzle hole diameter lengthens thecombustion period and lowers the degree of constantvolume, but increases effective combustion efficiency.


I ncrease of inj ection pressure with decreasednozzle hole diameter has a favorable effect onengine performance and exhaust emissions whenthe fuel injection timing is retarded.

E ffects of nozzle hole diameter onE ffects of nozzle hole diameter onE ffects of nozzle hole diameter onE ffects of nozzle hole diameter onE ffects of nozzle hole diameter onengine performance and exhaustengine performance and exhaustengine performance and exhaustengine performance and exhaustengine performance and exhaustemissionsemissionsemissionsemissionsemissions

E ffects of nozzl e hole diameter onE ffects of nozzle hole diameter onE ffects of nozzl e hole diameter onE ffects of nozzle hole diameter onE ffects of nozzl e hole diameter oncombustion characteristicscombustion characteristicscombustion characteristicscombustion characteristicscombustion characteristics

dQ/d

Effects of nozzle hole diameter on heatEffects of nozzle hole diameter on heatEffects of nozzle hole diameter on heatEffects of nozzle hole diameter on heatEffects of nozzle hole diameter on heatrelease ratesrelease ratesrelease ratesrelease ratesrelease rates

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140

fective combusti on efficiencycombX (1-cool). For this

reason, these rul ing factors of thermal efficiency are

shown in F ig. 7. As the nozzle hole diameter decreases,

combustion period becomes longer and the degree of

constant volume decreases, but effective combustion

efficiency increases, contri buting to the improvement

of brake thermal efficiency.

4. Conclusion4. Conclusion4. Conclusion4. Conclusion4. Conclusion

To increase thermal efficiency and reduce N Ox

emission in stationary diesel engines, this study was

conducted to research the direct-injection combustion

system using a single-cylinder engine and determine

its basic characteristics. The foll owing results were

obtained:

(1) Combinations of high compression ratio, high-pres-

sure fuel injection and retarded fuel injection

timing were found to be an effective means of in-

creasing thermal efficiency and reducing NOxemission in this combustion system.

(2) Application of high compression ratio increases the

product of theoretical thermal efficiency and me-

chanical efficiency, and has the potential to

increase brake thermal efficiency. Al so, when the

fuel injection timing is r etarded for combustion at

a high compression ratio, the rate of reduction is

low in brake thermal efficiency but high i n N Ox

emission and maximum combustion pressure in the

cylinder.

(3) Reduction of the nozzle hole diameter increases

the maximum injection pressure and extends the

injection period. Reduction of the nozzle hole di-

ameter is a fairly effective means of obtaining

improvements in the combustion performance

when retarding fuel injection timing, such as in-

crease of brake thermal efficiency and reduction

of smoke emission and maximum combustion pres-

sure in cylinder. I t also restrains ini tial combustion

and activates diffusive combustion, lengthens the

combustion period and lowers the degree of con-

stant volume while effectively increasing

combustion efficiency.

AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements

The study reported here was conducted as part of

a technical development project carried out by the Pe-

troleum Energy Center (PEC) and subsidized by the

Min istr y of Economy, Trade and Industry of J apan.

Mitsubishi Heavy I ndustries, Ltd. wishes to express

its deep appreciation to Land Engine ManufacturersAssociation (LEMA) and many other parties for their

valuable guidance and cooperation in the accomplish-

ment of this study.

ReferencesReferencesReferencesReferencesReferences

(1) Harada, T. et al., New Combustion System with Centered-

P rechamber for L ow-NOx Di esel Engine (1st Report ,

Influence of F undamental Design Parameters), Trans. J SME

(1996) No. 598B, p. 2534. (in J apanese)

(2) Harada, T. et al., New Combustion System with Centered-

P rechamber for L ow-NOx Di esel E ngine (3rd Report,

Application of Strati fied Fuel-Water I njection System),.

Trans. J SME (1996), No. 603B, p. 4000. (in J apanese)

(3) M il li ngton, B. W. et al., SA E Paper, N o. 680590 (1968)

pp.1-7

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