Effect of Injection Rate and Split Injections on Diesel Engine

7/24/2019 Effect of Injection Rate and Split Injections on Diesel Engine

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February 28-March 3, 1994

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940668

Measurement of the Effect of Injection

Rate and Split Injections on Diesel

Engine Soot and NOx Emissions

D. A. Nehmer and R. D. ReitzUniversity of Wisconsin-Madison

Reprinted from: Diesel Combustion Processes and Emission Control(SP-1028)


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940668

Measurement of the Effect of Injection

Rate and Split Injections on Diesel

Engine Soot and NOx Emissions

D. A. Nehmer and R. D. ReitzUniversity of Wisconsin-Madison

ABSTRACT

This study was conducted to develop an understandingof how rate-shaped and split injections can affect the sootand NOx emissions of a heavy-duty diesel engine. Thetests were performed on a single cylinder version of theCaterpillar 3406 production engine, modified to accept anelectronically-controlled, high-pressure common-railinjection system that offers a very high degree of flexibilityin injection timing, split injections, and rate shaping of theinitial injection. The engine was instrumented forparticulate measurements with a full dilution tunnel, andCO, CO2 and NOx emission meters. Cylinder pressurewas used to study heat release rates, and the response tochanges in the injection scheme. The results show thatrate-shaped injection, when optimized for lowest BSFC,does not appreciably affect pressure rise or peak cylindergas pressures. Split injections, however, allowed peakpressures to be reduced by more than 45%, and have asignificant effect on the overall rate of pressure rise. The

emission measurements showed that split injections havea trend of reduced NOx as the quantity of fuel in the firstinjection is reduced, without particulate emissionsincreasing rapidly. Furthermore, it was determined thatsplit injection better utilizes the air charge and allowscombustion to continue later into the power stroke than fora single injection case, without increased levels of sootproduction. This indicates that pulsed injection mayprovide a means to reduce particulate emissions, andallow for reduced NOx from controlled pressure rise.

OF PRIMARY CONCERN FOR diesel engineemissions is the formation of nitrogen oxides (NOx) andparticulate or soot. Fuel initially injected into the engine

has a short ignition delay as it mixes with the hotcompressed air in the cylinder and reaches its ignitionpoint. It is generally believed that NOx is formed in localhigh temperature regions present in the cylinder duringinitial combustion, or the "premixed burn" (Plee et al.,1981). Because combustion in the premixed bum isapproximately stoichiometric, combustion gases will be attheir maximum temperature and near peak pressure whenthe piston is close to top-dead-center (TDC). With these

conditions present, and because the fuel-air charge is nohomogeneous, the excess oxygen and nitrogen combinat high rates to form NOx. NOx formation rapidly slowduring the later portion of the combustion process, anmost of the NOx which has formed "freezes" as it mixewith cooler cylinder gases and as the bulk cylindetemperature drops during the expansion stroke. Durinthe second phase of combustion, the "diffusion burn"particulates are formed (Plee et al., 1981). Air mixing witthe outer edges of the fuel jet sustains the diffusion burnFuel in the interior of the spray jet is subjected to higtemperatures and pressures but is starved for oxygenleading to soot production.

NOx and particulate emissions are now regulated bthe Environmental Protection Agency (EPA). Since 197the allowed emissions have been steadily decreased andare currently limited to 1/3 of the NOx and 1/4 of theparticulates allowed in the 1974 regulations. Furthereduction of engine emissions are necessary to meet th

1994 and 1998 standards. These levels, however, will bmuch more difficult to attain than the previous levelsMeeting new emission levels will not only require thincorporation of the many advanced techniques used tdate (such retarded injection timing and increasedinjection pressures) but also require significant researcon the effect of injection parameters such as optimumtiming, split injection, and rate shaping.

To provide for better mixing of fuel and air, highepressure injection systems have been incorporated intmodern diesel engines. This allows the use of smalleinjection holes for the same fuel delivery rate, resulting ismaller fuel droplets (Heywood, 1988). Faster fuel-a

mixing and better air utilization are then possible.. Higpressure injection also shortens the ignition delay so thafuel may be introduced later in the cycle, even after TDC(Lyn, 1968). Retreading the timing has the benefit olowering NOx emissions by reducing peak cylindetemperatures and pressures, thus reducing the rate oNOx formation. However, the engine thermodynamiefficiency is reduced and soot emissions are increased(Heywood, 1988).


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In standard injection systems the fuel injection rateincreases rapidly to the maximum, and the injectionduration is varied to control load. The end of injection issharp to minimize hydrocarbon and soot emissions whichresult from fuel entering the cylinder late in the cycle withpoor spray development (Obert, 1973). New concepts indiesel injection systems now have the ability to control therate at which fuel is delivered during the initial injection.Rate shaping is applied primarily at the beginning ofinjection, with a sharp cutoff used at the end of injection

due to the emission considerations (e.g., Nishizawa et al.,1987 and Needham et al., 1990).

Splitting the injection sequence into two events iscalled pilot or split injection. As early as 1937 pilotinjection was experimented with to reduce combustionnoise and allow the use of poor ignition quality fuels (lowcetane numbers). The pilot injection is used to shortenignition delay and to control rapid pressure rise (Augustinet al., 1991 and Schulte et al., 1989). Reducing ignitiondelay also reduces the quantity of premixed burn. Sincemost NOx is believed to be formed in the premixed burn,pilot and split injection are now being investigated as ameans to help control NOx emissions (e.g., Miyaki et al.,1991, Racine et al., 1991 and Shakal and Martin, 1990).Shundoh, et al., 1992 have reported that NOx can bereduced by 35%, and smoke by 60 to 80%, without apenalty in fuel economy if pilot injection is used inconjunction with high pressure injection.

The objective of this work is to help develop anunderstanding of the effect of rate-shaped and splitinjections on diesel engine performance. The approach isto use a modern, well instrumented research engineequipped with a flexible high pressure fuel injectionsystem. The results of this work are available to helpprovide guidelines for strategies to achieve simultaneousreductions of soot and NOx emissions in diesel engines.An additional objective is to provide experimental data forcomputer model development efforts (e.g., Patterson etal., 1994).

EXPERIMENTAL DETAILS

A single-cylinder research engine that is a version ofthe Caterpillar 3406 production engine was used in thestudy. The engine is capable of producing 54 kW at arated speed of 2100 RPM. Engine specifications pertinentto the present study are listed in Table 1.

Table 1 Engine Specifications

A high pressure, electronically controlled common-raiunit injector was mount(A in the engine (Racine, 1991).The injection system is capable of up to four injections perengine cycle. The injector allowed control over the needlerise and the initial rate of injection by means of a hardwarechange (changing a valve orifice) within the injector. Thevarious orifices allow for a reduced rate of injection duringinitial stages by controlling needle lift rates to produceinjections with rise rates of 2, 7 and 16 crank angledegrees to maximum open (at 1600 rev/min).

K type thermocouples were used for inlet air, exhaustgas, engine oil and coolant, and orifice air temperaturemeasurements. Inlet and exhaust surge tank pressuresand engine oil pressures were monitored. Engine intakeair is metered and flows through a heater to simulatetemperatures of turbo-charged inlet conditions. Tomaintain constant inlet pressures an inlet surge tank wasused. Back pressure was maintained in the exhaust surgetank to simulate turbocharger back pressure.

Cylinder pressure measurements were taken with aKistler Model 6061-A water cooled piezo-electrictransducer. The transducer passage diameter wasincreased, and its length decreased to give as large a

diameter-to-length ratio as possible to improve responsetimes. The transducer was also coated with RTV asrecommended by Brown (1967) to provide more stablereadings. An AVL, Model 8 QP 3000 high pressuretransducer was used to measure fuel line pressure. A BEoptical shaft encoder which provides a resolution of 1/2crank angle degree was used for engine crank shaftposition. A PEI data acquisition system was used torecord cylinder pressure, needle lift, and fuel line pressuredata.

A Bosch rate-of-injection meter was used to determinethe instantaneous rate of injection. The accuracy of therate-of-injection is a function of mass flow and was

measured within 1.0 % using a Flo-Tron flow meter.

A full dilution tunnel, shown in Fig. 1, was constructedfor the particulate measurements following EPAguidelines (EPA, 1990). The dilution tunnels primarypurpose is to cool exhaust gases by diluting them withfiltered air and to

4-Stroke,Simulated

Turbocharging

Bore 137.19 mm

Stroke 165.1mm

Connecting Rod Length 261.62

Displacement 2.44L

Compression Ratio 15.0:1

Intake Valve Timing opens 3 ATDC

closes 10 ABDC

Exhaust Valve Timing opens 19 BBDC

closes 7 BTDC

Figure 1. Dilution Tunnel


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uniformly mix them before a sample is drawn from thetuinnel. The full dilution tunnel allows for the use of largefilters (90 mm Pallflex) and large sample rates at lowfilter-face velocities. The use of a full dilution tunnel servesimprove test results over mini-dilution approaches byminimizing measurements errors associated with filterweighing accuracy and sample volume determination(MacDonald and Plee, 1980, and Hirakouchi et al., 1989).The filter method also allowed for the determination ofSoluble Organic Fraction (SOF) using Soxhlet extraction

as described by Tree, 1992.

The primary tunnel is composed of three parts: thediffuser, mixing, and sampling sections. Particulatesamples are drawn from the sampling section through a15.9 mm diameter 304 stainless steel tube which leads tothe secondary dilution tunnel and filter section. Tominimize temperature gradients in the tunnel and preventdeposition of soot on the cool metal surfaces(thermophoresis), insulation was placed on the tunnelwalls (Kittleson, 1991). Immediately at the end of themixing section is the filter section which is designed to usetwo filters in parallel with the secondary filter being used totrap any particulate which by-passes the primary filter.The temperature of the diluted products and filter paperwas monitored and maintained at 52 C since SOF loadinghas been shown to be dependent on filter temperature.Procedures used to collect and weigh the particulatefilters are described by Nehmer, 1993.

Gaseous emission samples were drawn from theexhaust surge tank with a single probe, designed to SAEJ177 specifications (SAE, 1992). The sample gases weredried using an ice bath and a Drierite desiccant dryer. Achemiluminiscent gas analyzer (Thermo EnvironmentalInstruments, Inc. model 10S) was used to monitorNO/NOx concentrations. The unit was equipped with athermoelectric cooler on the reaction chamber to stabilizelow range sensitivity, and accuracy is within 1% of fullscale. Horiba infrared gas analyzers were used to monitorCO and CO2 concentrations. The CO and CO2 analyzershave an nominal accuracy of 1.0% of full scale, while theCO2 meter has an improved accuracy of 0.5% of full scalefor the high sensitivity range which is needed to monitordilution ratios in the primary and secondary dilutiontunnels.

The pressure data was analyzed using a heat releaseanalysis based on the two-zone heat release modedeveloped by Borman and Krieger, 1966 and modified toincorporate the Woschni, 1967, correlation to predict heattransfer (Tree, 1992).

TEST CONDITIONS

The goal of this study was to quantify changes inemissions and engine performance caused by varying

injection Parameters. Therefore engine speed, inlettemperatures and pressures, exhaust back pressure, andequivalence ratio were held constant for testing, and thevalues are listed in Table 2. The values were chosen toreflect common loads placed on this engine (1600 rpmand a equivalence ratio of 0.45, or approximately 80%load) as well as the appropriate inlet temperature andpressure conditions (Brown, 1992).

Table 2 Test Conditions

To better determine the effects of changes in the fuelinjection schemes, an attempt was made to run all tests atmean best torque. Split injection, or rate shapingsignificantly increases the period over which fuel isinjected into the engine - over 50% longer for some testsconducted in this study - and if the injection timings werenot optimized for best BSFC, measurements of emissionsand performance would also include effects of retardedtiming resulting from the extended injection duration.

Injector settings were determined experimentally on a

Bosch rate-of-injection bench described by Bower, 1991The injector controller is adjusted for each injectionscheme by setting the solenoid initiation and duration forthe appropriate start of injection and fuel flow. Spacingbetween injections was set by monitoring needle lift andsetting the time between

Table 3 Injector Settings

Engine Speed 1600 RPM

Inlet Air Temperature 36 C

Inlet Air Pressure 184 kPa

Exhaust Back Pressure 159 kPaEquivalence Ratio 0.45

Fuel

1'st Inj.%

Fuel

2'nd Inj.%

Test Injection

SpacingCA

1'st Inj.

Durationmsec

Signal

SpacingCA

2'nd Inj.

Durationmsec

100 0 Base.6 - 1.930 - -100 0 Single.3 - 2.200 - -

100 0 Single.2 - 2.750 - -

10 90 10-90,3 3 0.324 7.6 1.700

10 90 10-90,8 8 0.324 13.6 1.700

25 75 25-75,3 3 0.600 10.3 1.430

25 75 25-75,8 8 0.600 15.7 1.410

50 50 50-50,3 3 1.040 14.7 0.940

50 50 50-50,8 8 1.040 20.0 0.940

75 25 75-25,3 3 1.500 19.3 0.500

75 25 75-25,8 8 1.500 24.2 0.500


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closing of the needle and its re-opening to the desired 3and 8 crank angle degree spacings (which corresponds to0.313 and 0.833 ms, respectively at 1600 rev/min). Theresulting injector settings and test conditions aredisplayed in Table 3. For all tests a common-rail pressure0 of 90 MPa was used.

RESULTS AND DISCUSSION

Results of the Bosch rate-of-injection meter tests for

single injection cases are presented in Figure 2. Shownfor each test case is needle lift and rate of fuel injected.The Base.6, or baseline data, utilizes the 0.6 min diametervalve orifice, which results in the fastest needle rise andthe shortest injection duration of all the tests. Single.3 andSingle.2 are

single injection cases with different diameter injector valveorifices to control needle lift and the initial rate of injectionThe Single.2 case utilizes a 0.20 min valve orifice and hasthe slowest lift and the Single.3 case uses a 0.30 minorifice for a faster needle rise.

Figure 3 presents needle lift and rate-of-injection datafor split injection cases having 3 crank angle degreespacing (dwell) and Fig. 4 presents data for an 8 crankangle degree spacing between injection events. All testswere performed with the 0.60 mm diameter orifice to attain

a fast needle rise for each injection. Anal~g the datarevealed two important details. First, that with 10% of thefuel in the initial injection, the needle only attains 70% oftotal lift in either the 3 or the 8 degree spacing tests(10-90,3 and 10-90,8). This may cause poor spraydevelopment and slow vaporization of the fuel in the initiainjection.

Secondly, it is noticed that the pressure waves that arecreated when the needle closes affect injection ratesduring the second injection event. Inspection of the 10-90and the 25-75 injection splits with 3 degree CA spacingshows that the rate of injection rises in the later half of thesecond injection event. A similar characteristic was notedwith the 8 degree CA spacing in the 10-90 and the 25-75

splits, but the high rate of injection is present in the firsthalf of the second event. It is the difference in dwell timesbetween injections which moves the high rate of injectionfrom the last half to the first portion of the injection eventThis corresponds with the arrival of the pressure wave inthe fuel line and it is the increased pressure that isresponsible for the increased rates of injection.

The baseline (Single.6) injection scheme was run at 5different injection timings ranging from 5 degrees to 15degrees BTDC to develop a reference NOx, Particulatetrade-off curve. The engine was also run for each injectionscheme listed m Table 3 while advancing timing until thedynamometer load appeared to be maximized indicatingthat minimum BSFC had been reached. The measured

engine performance for each injection scheme, orinjection timing, is presented in Table 4.

Cylinder pressure data from the Baseline singleinjection timing tests are presented in Fig. 5a. The cylinderpressure trend of increased peak pressure with timingbeing advanced from 5 degrees BTDC to 15 degreesBTDC is as expected. The 5 degree BTDC timing isinteresting because it is actually late enough to show peakmotored pressure, and expansion, before increasing fromcombustion. An injection timing of 11 degrees BTDCproduced the lowest BSFC and was therefore used as acomparison to all other data.

Single injection cylinder pressure data for different

rate-shaped injections is presented in Fig. 5b. In thisfigure the advanced timing of the. Single.2 test (20degrees BTDC) is apparent by the early ignition and sharppressure rise at around 6 BTDC. Both the baseline case(11 degree BTDC injection) and the Single.3 (14 degreeBTDC injection) configuration show premixed burn andrapid pressure rise starting at about 2 degrees BTDC. Thepeak pressure is 9.64 MPa at 12.5 degrees ATDC for boththe baseline and Single.3 configurations, while theSingle.2 test reaches a peak pressure of 9.67 MPa at 11.5degrees ATDC, a result of the earlier injection timing. Thisdata shows that the effect of a slow

Figure 2. Needle Lift and Injection Rate, Single Injections.


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Figure 3. Needle Lift and Injection Rate, 3 Split Injection

Figure 4. Needle Lift and Injection Rate, 8 Split Injection


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Table 4Engine Performance

rise in rate of injection can be compensated for by

advancing the injection timing.Figures 6a and 6b present the heat release data for

the single injection runs. The rate of heat release isplotted as a fraction of the total energy released versusengine crank angle position. As with the pressure data,clear trends are visible in the heat release data of Fig. 6aas the injection timing is changed. The heat releasecurves arc curves for each condition, and are just shiftedby the changes in injection timing.

Figure 6b presents the heat release data for the threesingle injection runs with different rate-shapes. TheSingle.2 curve shows the premixed bum occurring beforeeither the Single.3 or Base.6 case and reflects the earlyinjection timing used. The maximum rate of heat releaseduring the diffusion burn is lower, and peaks earlier, thanfor the other two test cases as a result of the slow needlerise and early injection.

Differences in the heat release curves for the Single.3and Base.6 case are very small and do not reveal that theSingle.3 case was run with more advanced timing than theBase.6 case. This is primarily because the initial needle lift(first 15%) of the Single.3 configuration is much slowerthan the

Base.6 case, but the rise in injection rates arc nearly

identical after that point. Therefore, after the delayaccompanying the initial needle rise both cases aresubjected to similar rates of fuel injection and the resultingheat release curves are nearly identical. The Single.3case does show ignition taking place approximately 0.5crank angle degree earlier, indicating that fuel is injectedduring the initial needle rise, but only a very small quantityand the initial heat release begins more gradually.

Pressure data for the split injection cases with 3 crankangle degree spacing between injections is presented inFig. 7. The lowest pressure and highest SFC is associatedwith the 10-90,3 injection scheme. As the percent of fuein the first injection is increased, peak pressures increase

BSFC decreases, and the engine performanceapproaches that of a single injection with the 75-25,3scheme.

Figure 8 displays the results from the split injectiontests with 8 crank angle (CA) degree spacing betweeninjections. As with the 3 CA degree spacing, a largerpercentage of fuel in the first injection, gives a highercylinder pressure. As with the 75-25,3 the 3 CA degreespacing of a 75-25 split performed much like a singleinjection. The results show

Run

#

Inj. Timing

BTDC

Power

kW

BMEP

kPa

BTDC

g/kw-hr

Eqv. Ratio

Base.6 5 35.2 1080 210 0.485

Base.6 8 36.3 1110 199 0.459

Base.6 11 37.5 1150 187 0.436

Base.6 13 38.7 1180 189 0.461

Base.6 15 39.2 1200 189 0.468

Single.3 14 38.3 1170 192 0.461

Single.2 20 38.0 1160 194 0.46010-90,3 12 39.2 1200 203 0.501

10-90,8 16 37.9 1160 206 0.494

25-75,3 14 38.0 1160 190 0.451

25-75,8 is 37.6 1150 193 0.453

50-50,3 12 38.8 1190 187 0.455

50-50,8 13 37.6 1150 192 0.454

75-25,3 12 40.6 1240 186 0.473

75-25,8 12 38.0 1160 192 0.455

Figure 5a. Baseline Cylinder Pressure Data Figure 5b. Single Injection Cylinder Pressure Data


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that, with more fuel in the first injection, peak pressuresoccur at later crank angles. This is expected, since later

injection timings are required to attain the lowest BSFCand to keep cylinder pressures within acceptable limits asa greater quantity of fuel is injected in the first injection.The 10-90, 3 case in Fig. 7, which has an almost flatpressure trace, is an exception. In this case, analysis ofthe BSFC data also suggested that the injection timingmay not have been quite optimized.

Heat release data for the 3 degree CA split injectioncases are shown in Fig. 9. The premixed burn spike isapparent for all cases but there is very little diffusion burnprior to the second injection for the 10-90,3 and 25-75,3cases. The premixed burn for the 25-75,3 case starts 2CA degrees before that of the 10-90,3 case, and shows

the earlier start of injection employed. Both the 50-50,3and 75-25,3 tests show a diffusion burn taking place afterthe premixed burn. The 50-50,3 case shows a period ofrelatively constant heat release between 0 and 6 degreesATDC before rising again as fuel from the second injectionevent begins to burn. The 75-25,3 case shows two peaksin the diffusion burn, with a drop in heat release beingcaused by the split in the injection. Due to the largequantity of fuel in the first injection, the rate of energyrelease is almost that of the peak rate achieved during thesecond injection and 67% higher than the maximum

energy release associated with the first injection of the 50-50,3 case.

Heat release curves for the 8 split injection test cases

are presented in Fig. 10. Both the 10-90,8 and 25-75,8cases are very similar to the 3 degree CA spacing tests,with the larger dwell time between injections resulting in alater diffusion burn. Again there is a very low rate ofenergy release after the premixed burn due to the smallquantity of fuel present in the first injection. The premixedburn for the 10-90,8 case does begin 1 degree earlier thanfor the 25-75,8 case, reflecting the more advanced timingused. The 50-50,8 and 75-25,8 test cases are also similarto the 3 degree spacing runs. As expected the larger dweltime between injections results in a greater separationbetween peaks in the diffusion burns, and a more notabledrop in energy release between injection events. The

greatest difference between the 3 and 8 degree CAspacing is in the 75-25 case with an 8 degree spacingbecause the greatest energy release is now in the firstdiffusion burn peak.

Engine emissions measured during the experimentaruns are shown in Table 5. Figure 11 is a plot of particulateversus NOx for all test points. From this graph it appearsthat all the split injection and rate-shaped (single injection)data points he approximately on the particulate, NOxtrade-off curve developed for the Base.6 case by varyingthe injection

Figure 6a. Normalized Heat Release Rate vs Injection

Timing, Baseline

Figure 7. Pressure Data, 3 Split Injection

Figure 6b. Normalized Heat Release Rate, Single

Injections

Figure 8. Pressure Data, 8 Split Injection


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Figure 9. Normalized Heat Release Rate Data, 3 Split Injection

Figure 10. Normalized Heat Release Rate Data, 8 Split Injection


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Table 5Engine Emissions

timing. However, closer evaluation of the graph suggeststhat 25-75 and even the 50-50 cases may allow areduction in NOx without as great an increase inparticulate as the Base.6 trade-off curve.

Therefore the results suggest that split injection may

provide some benefit in terms of reduced emissions.However, the fact that results were found to be sensitiveto injection timing makes it difficult to draw definitiveconclusions. A timing trade-off curve therefore needs tobe run for each split injection case to determine how muchthese points lie below the Base.6 curve, as has been donerecently, for example, by Tow et al., 1994.

Evaluation of the end of combustion (which is definedarbitrarily as the time when 95% of the fuel has burned)

versus Particulate emissions revealed a very interestingresult as shown in Fig. 12 for both the baseline data andall of the injection schemes tested in the study. In all testsrate-shaped or split-injection particulate emissions arebelow the baseline curve. Thus, even though combustionwas continued longer into the expansion stroke, lessparticulates were produced. This is contrary to what wouldbe expected, since late combustion is usually thought tolead to high particulate levels (Plee et al., 1981). Thissuggests that the changes in injection schemes effectssoot production, and/or oxidation, and new split-injectionschemes may allow reductions in NOx by retarding timingwithout the normal increase in soot emissions. Thisconclusion has indeed been borne out by recentmeasurements by Tow et al., 1994. The only common

Run#

Inj. TimingBTDC

Particulateg/bhp-hr

SOF% of Part.

NOxg/bhp-hr

COg/bhp-hr

CO2%Dry

Base.6 5 0.209 4.2 2.99 - 7.01

Base.6 8 0.137 9.7 3.76 - 7.05

Base.6 11 0.104 11.3 3.90 - 6.84

Base.6 13 0.083 13.3 4.58 - 7.01

Base.6 15 0.075 15.2 5.20 - 7.01

Single.3 14 0.103 9.4 4.09 0.73 7.01

Single.2 20 0.117 12.0 4.15 0.78 6.97

10-90,3 12 0.163 10.7 3.39 1.29 7.49

10-90,8 16 0.184 7.3 3.19 1.46 7.31

25-75,3 14 0.095 14.0 3.73 0.73 7.09

25-75,8 15 0.084 14.7 3.91 0.65 7.08

50-50,3 12 0.100 13.6 4.32 0.58 7.10

50-50,8 13 0.083 14.6 4.13 0.46 7.06

75-25,3 12 0.076 17.7 4.82 0.50 7.38

75-25,8 12 0.074 21.3 4.65 0.41 6.92

Figure 11. NOx vs Total Particulate Data


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point among all of the present injection schemes is that

the fuel is delivered over a greater number of crank angles(a slower overall rate) compared to the baseline singleinaction case.

Evaluation of the small first quantity injected 10-90tests suggest that these cases performed much like theBase.6 at late injection timings. This follows from the heatrelease data, which showed the premixed burn consumingmost of the fuel, and then a long delay time followed bythe diffusion burn during the second injection, with burningcontinuing late into the expansion stroke. This is typical oflate injection and high particulate levels. More advancedinjection timings need to be tested to determine if theparticulate levels can be reduced without significantlyincreasing NOx emissions.

A statistical analysis was performed on the presentsplit injection data. The modeling showed that while acorrelation between particulate and NOx exists, it isdependent on the quantity of fuel in the first and secondinjection. Good correlation to a reduction in NOx, andincrease in particulate, results as the quantity of fuel in thefirst injection is reduced. This trend is very similar to thatof retarding injection timing. A correlation betweeninjection spacing and emission levels could not be found.This indicates that a small spacing of the injection eventshas little effect on emission levels. However the data in

Fig. 12 suggests that a larger spacing in split injectioncases allows for longer burning into the expansion strokefor a given particulate level.

The work of Golding, 1992, indicates that significantreductions in soot are possible by enhancing mixingduring the expansion stroke. The results in Fig. 12 alsoappear to suggest that split injections serve to enhancemixing. This indicates that multiple injections (more than 2split injections) may be beneficial, as confirmed by therecent results of Tow et al., 1994.

Particulate emissions were analyzed for soluble

organic fractions (SOF) and the results are presented as apercent of total particulate emissions in Table 5. SOF liebetween 4.2 and 21.3 % (see Table 5) of the totaparticulate, and these levels are representative of thosefrom modern low emission engines. Moreover, it isimportant to note that the actual SOF emissions werebetween 0.009 and 0.017 g/bhp-hr for all tests, regardlessof the actual particulate concentrations. This consistencyin emission levels indicates that the engine is in goodcondition, with little lubricating oil being entrained into theexhaust.

SUMMARY AND CONCLUSIONS

The goals of this study were to develop anunderstanding of the effect of rate-shaped and splitinjections on diesel engine performance and emissionsFor this purpose, a well characterized research enginetest facility has been constructed. The engine test data isalso available to help the development of engine models(e.g., Patterson et al., 1994).

It was found that a single injection with a fast rate ofinjection produced the lowest particulate emissions, andthe lowest BSFC of all tests. Evaluating the split injectiontests, it was noted that the quantity of fuel in the firstinjection has a significant effect on the rate of in-cylinderpressure rise. The percent of fuel in the first injection was

also found to be correlated with engine emissions. Asmore fuel was injected in the first injection, Nox emissionsincreased, while particulate emissions decreasedHowever, there was no apparent correlation between thespacing (dwell) between the injections and engineemissions for the tests conducted in this study, whichconsidered 3 and 8 crank angle degree spacings.

Split injection also) appears to affect theNOx-particulate trade-off. By enhancing mixing, andimproving air utilization, NOx emissions for the 25%.75%and 50%-50%

Figure 12. Total Particulate vs End of Combustion


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injection cases were reduced without the increases in sootsuggested by the trade-off curve obtained for singleinjections. The split injections also allowed combustion tocontinue later into the expansion stroke without anaccompanying increase in particulate emissions. Inaddition, the data suggests that an optimum quantity offuel in the first injection exists, and it lies somewherebetween 10-90 and 25-75.

For all split injection tests it was determined that a 3

CA degree spacing produced lower BSFC than an 8 CAdegree spacing, as expected since efficiency improveswith shorter combustion periods closer to TDC.

Rate-shaped injection did appear to have a smallpenaltythe NOx-particulate trade-off curve. However, withslower injection rates it was possible for combustion tocontinue later into the cycle with lower soot levels thanthose for the fast-injection-rate tests. Rate-shapedinjection also had an increased BSFC over the baselinefast-injection-rate case, as would be expected becauseinjection of the fuel is extended over a longer duration.

The 25-75, 50-50, and 75-25 split injection results doindicate that NOx can be reduced with only a minimal

increase in particulate emissions by means of splitinjection. For the 10-90 split injection scheme most of thefuel in the first injection is consumed in the premixed burn,and the second injection sustains the diffusion burn. Therelatively long dwell time between the two injections,however, makes the diffusion burn take place late in thecycle and the emissions respond similar to those of asingle late injection. Finally, it was noted that both rate-shaped and split-injection schemes allowed forcombustion to continue later in the expansion stroke,while producing less soot than the baseline, fast-rate,single-injection scheme. This suggests that particulateemissions can be reduced with optimized injection

characteristics which enhance mixing and differ from thetraditional single injection strategy.

ACKNOWLEDGMENTS

This work was supported by the Caterpillar Engine Co.and by NASA-Lewis under grant NAG 3-1087. Additionalfacilities and support were provided by the ArmyResearch Office contract DAAL03-86-K-0174. Thecontributions of Bill Brown and the ERC staff and studentsare greatly appreciated. The authors particularly thankTryg Tow for his help with the experiments and forproviding his recent emissions measurements forcomparison, and thank Bob Bair who helped with thedesign of the dilution tunnel, and Joe Shakal who

performed a statistical analysis of the data.

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Effect of Injection Rate and Split Injections on Diesel Engine