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SAE TECHNICAL

PAPER SERIES 2003-01-0765

A Study On The Determination of the Amount

of Pilot Injection and Rich and Lean Boundaries

of the Pre-Mixed CNG/Air Mixture For a

CNG/Diesel Dual-Fuel Engine

Zhiqiang Lin and Wanhua SuState Key Lab of Engines,Tianjin University

Reprinted From: CI Engine Combustion Processes &Performance with Alternative Fuels

(SP-1737)

2003 SAE World CongressDetroit, Michigan

March 3-6, 2003

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2003-01-0765

A Study On the Determination ofthe Amount of Pilot Injectionand Rich and Lean Boundaries ofthe Pre-Mixed CNG/Air

Mixture for a CNG/Diesel Dual-Fuel Engine

Zhiqiang Lin and Wanhua SuState Key Lab of Engines, Tianjin University

Copyright 2003 SAE International

ABSTRACT

A sequential port injection, lean-burn, fully electronically-controlled compressed natural gas (CNG)/Diesel dual-

fuel engine has been developed based on a turbo-charged and inter-cooled direct injection (D.I.) dieselengine. During the optimization of engine overallperformance, the effects of pilot diesel and pre-mixedCNG/air mixture equivalence ratio on emissions (CO,HC, NOx, soot), knocking, misfire and fuel economy arestudied. The rich and lean boundaries of the pre-mixedCNG/air mixture versus engine load are also provided,considering the acceptable values of NOx and THCemissions, respectively. It is interesting to find that thereis a critical amount of pilot diesel for each load andspeed point, which proved to be the optimum amount ofpilot fuel. Any decrease in the amount of pilot diesel from

this optimum amount results in an increase of NOxemissions, because the pre-mixed CNG/air mixture mustbe made richer, otherwise THC emissions wouldincrease. However, the soot emissions remain almostunchanged at a very low level.

INTRODUCTION

CNG/diesel dual-fuel engines have many features incommon with spark-ignition, Otto-cycle engines,because the air and the primary fuel are pre-mixed in thecylinder before combustion. On the other hand, relyingon compression ignition of the pilot diesel, they also

share some characteristics with diesel engines, as wellas some unique advantages and drawbacks of their own[1,2,3]. It is shown in figure 1 that natural gas is injectedinto inlet ports of cylinders sequentially by electronic gasinjection valves during the inlet stroke, mixing with airinto homogeneous mixture quickly. Then some pilotdiesel, which would serve as millions of ignition sourcesites after their auto-ignition, is injected as the pistonapproaches the top of the compression stroke. Nearly allof the pilot diesel will mix with air to form ahomogeneous mixture while the amounts of pilot dieselare small, however, there will exist some diffusion comb-

Inlet portInlet manifoldDieselinjector

CNG common rail

CNG Electromagnetisminjector CNG leading pipe

Combustionchamber Diesel Spray

Fig.1 Illustration of the combustion process

in diesel/CNG dual-fuel engine

ustion if larger amount of pilot diesel is employed. It hasbeen indicated [4,5,6] that in diesel engine, localizedstoichiometric diffusion combustion produces high flametemperatures and high NOx emissions. In addition, highsoot emissions will be formed in diesel fuel rich zone.

In order to achieve effective, clean combustion, theproportion of diesel diffusion of CNG/diesel engine incombustion process should be reduced to a minimum

After analysis of a series of research [7,8,9] onimproving BSNOx vs. BSEC trade-off in dual-fueengines N.J. Beck et. al[10] drew a conclusion thaBSNOx levels can be reduced to 2g/hp-hr with anelectronic dual fuel system and to 1 g/hp-hr with micropilot. The primary reduction in NOx is attributed to theelimination of NOx emissions from the pilot oil. It seamsthat the methodology of Micro-pilot injection was the besin the reduction of NOx emissions. However, in this

study, it is found that there exists an optimum amount ofpilot diesel and accordingly an optimum air/CNGequivalence ratio for each engine speed and load pointsat which high efficiency and low emissions are obtainedThis paper presents our experimental results on theeffects of the amount of pilot diesel and the equivalenceratio of pre-mixed air/CNG mixture upon emissions (sooNOx, THC, CO), knock, misfire and fuel economy.

EXPERIMENT APPARATUS

A sequential port injection, lean burn, fully electronicallycontrolled CNG/Diesel dual-fuel engine was developed





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in our previous study [11], based on a turbo-charged andinter-cooled D.I. diesel engine. A schematic layout of theexperimental engine is shown in figure 2. The apparatusincludes several sensors, e.g., CNG temperature andpressure sensors, inlet air temperature and pressuresensors, crankshaft position sensor and rack locationfeedback sensor. The amounts of pilot fuel and theequivalence air/CNG ratio can be fairly controlled at all

load and speed points by the linear electromagnet andthe gas solenoid valve correspondingly. The enginespecifications are given in Table 1.

For emission measurement, NDIR was used for CO,HCLD was employed for NOx, and HFID was applied toTHC. A Bosch soot meter was used for soot emissions.

Table 1 specifications of the test engine

Bore (mm) 126

No. of cylinders 6

Displacement (litre) 9.726

Compression ratio 15

Charge type Turbocharger, inter cooling

Max. power 175kw at 2200 rpm

Max. torque 888Nm at 1400 rpm

EXPERIMENTAL RESULTS AND ANALYSISFOR THE CRITICAL AMOUNT OF PILOT DIESEL

EFFECTS OF PILOT DIESEL ON SOOT AND NOXEMISSIONS

Figure 3 shows the experimental relation of the air/CNG

equivalence ratio ( ) with NOx and soot and the

amounts of pilot diesel at 1400 RPM and the loads of50%, 75%, 100%, respectively. The relationship

between the pilot diesel and is interrelated andinterdependent. To obtain a certain engine power outpuwith a small amount of natural gas injection (namely a

larger air/CNG equivalence ratio ), a larger amount o

pilot diesel is required in order to have the same amountof heat release in the cylinders and vice versa. Howeveras the amount of pilot diesel is increased soot would be

1 electronical throttle 2 ECU 3 battery 4 CNG p ipe 5 CNG common rail 6 CNG temperature sensor 7 CNG pressure

sensor 8 craf t shaft sensor 9 inlet temperature sensor 10 inlet pressure sensor 11 supercharger 12 inlet mani fold 13

solenoid adapter 14 gas solenoid valve 15 engine body 16 diesel injector 17 cool water temperature sensor 18 lube

temperature sensor 19 lube pressure sensor 20 diesel pump 21 rack feedback sensor 22 l inear electromagnet 23 craft

shaft sensor 24 CNG pressure regulator 25 CNG filter 26 GNG cut off 27 CNG tank 28 CNG manual valve

Fig.2 Schematic layout of the experiment engine

Load=100%(diesel)

Load=75%(diesel)

Fig.3 Soot, NOx VS. the amount of pilot diesel

Load=50% (NOx)

Load=75% (NOx)

Load=100% (NOx)

Load=50% (soot)

Load=75% (soot)

Load=100% (soot)

Load=50%(diesel)

1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.0

0.1

0.2

0.3

4

5

0.6

0.7

0.8

0.9

0.

0.

S

oot(BSU)

10

20

30

40

50

60

70

200

400

0

0

1000

1200

diesel(mg/st)

60

80

N

Ox(ppm)





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produced in a diesel engine, when burning zone islocally fuel rich [6]. Under this experimental condition,the local pilot diesel-mixing zone is therefore the primesource of the soot production.

The experimental results in Fig.3 show that there is acritical amount of pilot diesel (CAPD) for each load andspeed point respectively. When the amount of pilotdiesel is less than this critical value, the soot emissionsare very low (0.1) and are not affected by changing theamount of pilot diesel and accordingly the air/CNGequivalence ratio. This is because nearly the entire pilotdiesel mixes with air into homogeneous mixture duringits ignition delay period, burning in a similar way to HCCIcombustion process. The relation between the CAPDand the load was determined from the experiments, asshown in figure 4. The higher the load, the shorter theignition delay, and the less the CAPD becomes. Whenthe amount of pilot diesel is greater than CAPD, theproportion of the diesel diffusion combustion is increased,resulting in higher soot emissions. Therefore, for theminimum soot emissions, the optimal amount of pilotdiesel (OAPD) should be less than CAPD. However, this

does not mean that the smaller amount of pilot diesel,the better the combustion process becomes.

The dot lines in figure 3 show the relation of NOx

emissions versus air/CNG equivalence ratio . Figure 5shows the burnt zone temperature calculated from P-Vdata using two-zone model. It always shows that lean

burn benefits the reduction of NOx emission for < 2

When is greater than 2, however, the proportion odiesel diffusion combustion increases with the increasingamount of pilot diesel. Continuing increase in theair/CNG equivalence ratio would yield less effect on the

NOx emissions and the burning zone temperature. Asexpected, too much pilot diesel with too lean air/CNGmixture leads to serious diffusion combustion of diesespray and hence the increase in NOx emission, as isshown in Fig.3 for the condition of 75% load.

In the case of the amount of pilot diesel less than CAPDthe engine maintains low soot emissions. However, aricher CNG/air mixture results in a higher NOx emissionIf the air/CNG equivalence ratio is increased properlythe amount of pilot diesel has to be increasedaccordingly for a constant load. When the amount ofpilot diesel is increased up to the CAPD, the engineworks at a fairly lean condition without diffusion

combustion, resulting in low emissions for both soot andNOx.

Hence a conclusion can be drawn that too smalamounts of pilot diesel or too rich CNG/air mixtures arenot the best matching in regard to NOx and sootemissions. On the other hand, there exists an optimaamount of pilot diesel (OAPD) for each work point, whichis usually the CAPD as shown in Fig.4. The conclusiondrawn from this study is actually a challenge to someprevious research, which calls for the micro-injection opilot diesel for the reduction of NOx emissions [7,8,910]

EFFECT OF PILOT DIESEL AND AIR/CNGEQUIVALENCE RATIO ON CO AND HC EMISSIONS

Figure 6 indicates the experimental results of COemissions VS. and the amount of pilot diesel at 1400

Fig.4 Amount of diesel that can be formed

into homogeneous mixture

load %50 75 100

26

27

28

29

30

31

32

33

Pilotdiesel

mg/st

Load=100%(diesel)

Load=75%(diesel)

Fig.5 Burnt zone temperature VS. and

amount of pilot diesel

Fig.6 CO VS. and the amount of pilot diesel

diesel(mg/st)

CO(ppm)

2

Load=50%(diesel)

1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

1000

000

3000

4000

10

20

30

40

50

60

70

Load=75% (CO)

Load=50% (CO)

Load=25% (CO)Load=100% (CO)





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M

isfirezone

OAPDof100%

load

OAPDof50%

load

OAPDof75%

load

Amount of pilot diesel mg/st

Fig.7 HC VS. the amount of pilot diesel

rpm and various loads. It is shown that CO emissionsare very high if the CNG/air mixture is rich ( 2. This isbecause CO is the product of incomplete combustion,which is caused either by low temperature in combustionzone due to too lean CNG/air mixture or by lack ofoxygen in combustion zone due to inhomogeneouscondition of mixture. However, the increase in pilotdiesel would increase the temperature in combustion

chamber and reduce CO emissions at lean conditions ofCNG/air mixture ( > 2.5).

The relationship among HC, air/CNG equivalence ratio and the amount of pilot diesel is shown by isolines of HCemission in Fig. 7. The experimental results show thatfor a given amount of pilot diesel, HC emissions increasewith an increase in the air/CNG equivalence ratio. Onthe other hand, for a given equivalence air/CNG ratio, asthe amount of pilot diesel is increased, HC emissions aredecreased. If the CNG mixture is in the range of 1.4 < < 2 with the amount of pilot diesel equal to OAPD, theHC emissions can be as low as 8 g/kw.h. However,

when the amount of pilot diesel is reduced from OAPDand the air/CNG equivalence ratio is increased, the HCemissions are increased. After the HC isoline of 10g/kwh,the rate of increase of HC emission increases. As shownin Fig. 7 the isoline is squeezed closer together in theshaded part, which is the area with very small amount ofpilot diesel and the rather high air/CNG equivalence ratio.Under such a situation, the HC emissions are very high.It is due to the incomplete combustion arising from thevery low combustion temperature and very low burningrate, which is the misfire zone.

The experimental results obtained above on engineemission characteristics of NOx, soot, CO, HC show thatoo small amount of pilot diesel (or too high CNG filling inat a given load) is not the optimal selection. Theamounts of pilot diesel should be CAPD for each engineworking point. On the contrary, HC emissions and fueconsumption will become worsened if CNG mixturebecomes too lean, even at the condition of CAPD for theamounts of pilot diesel. However, as shown in figure 4by using as much as CAPD for the amounts of pilot

diesel, the engine can work under leaner conditions

(1.4 < < 2) at various loads with lower NOx, HC, COand soot emissions. Hence, the CAPD is referred to asthe optimal amount of pilot diesel (OAPD).

RICH AND LEAN BOUNDARIES OF THE PRE-MIXED AIR/CNG MIXTURE

LEAN BOUNDARY OF AIR/CNG EQUIVALENCERATIO

For CNG/diesel dual-fuel engine, there is a certain tradeoff relationship among the emissions of NOx, soot, HCand CO. It has been mentioned above that lean burn

and less diesel diffusion combustion is recognized as anapproach to improve NOx emissions. However, there wilbe high HC and CO emissions and even misfire at theconditions of large air/CNG ratio and too little pilot diesel

As shown in figure 7, the engines working range musbe located far away from the misfire area, inconsideration of the reductions in HC and CO emissionand fuel efficiency. Therefore, there should exist a lean

boundary of air/CNG equivalence ratio as engine loadis varied. The lean boundary is obtained as shown infigure 8, which is controlled by HC emissions. In thisstudy it is determined as the HC emissions is less than





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10g/kw.h for different loads. It is found that at reducedloads the lean boundary moves towards larger air/CNGequivalence ratio, and lower values of the air/CNGequivalence ratio as the engine load is increased. It isrelated to the factors that the OAPD increases as theengine load is decreased.

RICH BOUNDARY OF AIR/CNG EQUIVALENCERATIO

According to Daugas theory [12], auto-ignitiontemperature of natural gas (Tai) depends on its pressure

(Pm) and . Equation 1 shows the relationship betweenTai and Pm. When Pm is higher over 20bar, the effect of

Pm on Tai is not significant, while plays a key role.Equation 2 shows Daugas [12] relationship between Tai

and (0.8 < < 2.2), regressed from the experimentaldata.

Based on the data from a large number of experiments,the authors have developed a two-zone combustionmodel incorporating equations (1) and (2) and calculatedthe temperature in the unburned zone at different loads

and of the engine. Figure 9 shows the calculated

results of the temperature of the unburned zone versus

and crank angle . The knock boundary is obtained bycomparing the calculated knock temperature Tai with thecalculated temperature Tu of the unburned zone. If Tu isequal to Tai, the corresponding air/CNG equivalence

ratio is then taken as the points on the knock boundaryline.

The shadow area, where knock occurs, is called theknock boundary (KB), and shown in the left side of figure10. When the engine load is increased, the knockboundary of air/CNG equivalence ratio (KB) is alsoincreased, because higher compression temperaturenear TDC results in higher temperature in unbu- rnedzone. The engine should work far away from the knockboundary for the safe operation of the engine. In practice,the engine has to operate at even leaner conditions for

lower emissions of NOx. Therefore, it is not that KBrepresents the rich boundary (RB), but the rich boundarydetermined by NOx emissions is the real rich boundaryof air/CNG equivalence ratio. Obviously the richboundary is ultimately determined by the emissionlegislation. Fig.10 shows the optimal results for the richboundaries of the air/CNG equivalence ratio with variousemission legislations. The rich boundary is also the fasburning boundary for high thermal efficiency andacceptable Nox, HC and CO emissions.

2.4 2.6 2.8 3.0 3.2 3.4

4

6

8

10

12

Lean boundary

BMEP(bar)

Load=100%

Load=75%

CONCLUSION

In consideration of low emissions and high economythere exists an optimal amount of pilot diesel (OAPD) foeach work point in CNG/diesel dual-fuel engine. At theconditions of OAPD the injected pilot diesel mixes withair into a rather homogeneous mixture during its ignition

delay period, so that the pilot diesel burns with thefeature of homogeneous charge compression ignitionresulting in negligible contribution to the engine soot andNOx emissions. The OAPD does not mean a minimumamount of pilot diesel. Too small amount of pilot diese

leads to rich burning and higher NOx emissions. Theconclusion drawn from this study represents a challengeto some previous research, which calls for microinjection of pilot diesel for the reduction of NOxemissions. OAPD is related to engine loads. The higherthe load, the smaller the OAPD becomes. It has beenfound that lean burn and less diesel diffusion combustionare recognized as an approach to improve NOx

( )208.171433300 ++= ai

T

ConstPnR

ET

m

ai+=

ln

2

1

Fig.8 Lean boundary VS. BMEP Load=25%

Load=50%

Fig.9 Unburned zone temperature VS. and

the amount of pilot diesel

Fig.10 Different rich boundary

Knock

High NOxemission

Rich boundaryfor euro II indexin Nox emission

Knock boundary Rich boundary foreuro III index inNox emission





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emissions. However, there will be high HC and COemissions and even misfire at the conditions of largeair/CNG ratio and too little pilot diesel. The engine mustwork far away from the misfire area, in consideration ofthe reduction of HC and CO emissions and fuelefficiency. The lean boundary of air/CNG equivalence

ratio is determined in this study, according to theincreasing rate of unburned HC emissions.

The engine should work far away from the knockboundary from the viewpoint of safety of engineoperation. In practice, the engine has to operate at evenleaner conditions for lower emissions of NOx. The richboundary (RB) of air/CNG equivalence ratio isdetermined by the NOx emissions. Therefore the richboundary is ultimately determined by the emissionlegislation. The rich boundary is also the fast burningboundary, at which the highest thermal efficiency can beobtained with acceptable NOx emissions.

ACKNOWLEDGMENTS

This research is supported by:

1. The key project of NSF of China titled as New

concept of combustion processes for vehicle

engines.Grant Number: 59936130

2. The-state key project of fundamental research plan

titled as new generation of engine combustion

principle and approach to application of alternative

fuelsGrant Number: 2001CB209202

REFERENCES

1. C.S.Weaver and S.H.Turner Dual-fuel Natural

Gas/Diesel Engines: Technology, Performance, and

Emissions SAE 940548(1994)

2. Karim G.A. and Zhigang Liu A Predictive Model

for Knock in Dual-fuel Engines. SAE 921550(1992)

3. Z Liu and G.A. Karim The Ignition Delay Period

in Dual-fuel Engines. SAE 950466(1995)

4. Otto Uyehara, Factors-Effects the formation of NOx

in Diesel Engines, SAE 910732(1991)

5. D.W. Stewart, T.W. Ryan III, A.C. Matheaus, NOx

Control in Heavy-Duty Diesel Engines What is the

Limit? SAE paper 980174(1998)6. John E. Dec, A Conceptual Model of D.I.Diesel

Combustion on Laser Sheet Imaging , SAE

970873(1997)

7. Hupperich. P, and Durnholz, M., Time

Controlled Pilot Injection for Stationary and Heavy

Duty Gas Engines. SAE 971713(1997)

8. Gebert,K. Beck, N.J., Barkhimer, R.L., Wong, H.C.

and Wells, A.D., "Development of Pilot Fuel Injection

System for CNG Engines", SAE 961100(1996)

9. Gebert,K. Beck, N.J., Barkhimer, R.L., Wong, H.C.

"Strategies to Improve Combustion and Emission

Characteristics of Dual-Fuel Pilot Ignited Natura

Gas Engines", SAE971712(1997)

10. Beck, N. J., R. L. Barkhimer, W. P. Johnson, H.-C

Wong, and K. Gebert. Evolution of Heavy Duty

Natural Gas Engines Stoichiometric Carbureted

and Spark Ignited to Lean Burn Fuel Injected and

Micro Pilot. SAE 972665(1997)

11. Wanhua Su Zhiqiang.Lin, Wang Yang Xie HuiWANG Jiang Pei Yi-qiang Fei Xiang-yang Liu

Wen-sheng Development of a Sequential Por

Injection, Fully Electronically-Controlled Gas/Diesel/-

Dual-fuel Engine Transactions of CSICE, Vol.19

No.2( 2001.5)

12. Daugas, C. Bastenholf, D. Cumbustion of Future

Residual-fuels and New Fuels in 4-Stroke Medium

Speed engines , 82-DGP-13, ASME (1982)

CONTACT

Dr. Zhiqiang Lin

Associate ProfessorState Key Lab of EnginesTianjin UniversityTianjin, [email protected]

Prof. Wanhua SuProfessorState Key Lab of EnginesTianjin UniversityTianjin, [email protected]



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a study on the determination of the amount of pilot injection and rich and lean boundaries of the...

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