rn

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xinjection. @DOI: 10.1115/1.1473157#

IntroductionPremixed compression ignition combustion, such as that ac-

complished by homogeneous charge compression ignition ~HCCI!~Ryan III et al. @1#! or by premixing at extremely early stages ofcompression stroke by direct-injection ~Takeda et al. @2#! has thepotential to achieve ultra-low exhaust emissions. However, al-though such combustion can sharply reduce NOx emissions, it isstill characterized by a number of lingering problems. For ex-ample, it is difficult to control the ignition timing and the operat-ing region is limited to lower engine loads. In addition, premixedcompression ignition combustion is characterized by high THCand CO concentrations and high fuel consumption. For this rea-son, a new combustion concept based on compression ignition ofa lean pre-mixture using multiple injection with a single injector~Yokota et al. @3#! and a two-stage split injection with multipleinjector and injection systems ~Hashizume et al. @4#! have beenproposed. The aim of both combustion systems is to obtain lowerNOx and particulate emissions by compression ignition of a leanpre-mixture derived from preliminary injection at partial load andby ordinary diesel combustion under lower concentration of oxy-gen by second-stage injection at higher engine load.

The combustion system proposed in the present study can beoutlined as follows. NOx concentration is reduced by combustionof a over lean-mixture formed by the first-stage injection. Most ofpre-mixture is combusted by second-stage injection, and thus the

amount of second-stage injection controls the engine load. Thiscombustion system is based on a hybrid of premixed compressionignition combustion and ordinary diesel combustion. The overlean pre-mixture is formed by the first-stage injection between theearly and middle stages of the compression stroke, and then aportion of that pre-mixture is combusted by compression ignition,and the remainder by second-stage injection. In this study, weinvestigate exhaust emissions, and particularly the emission ofHCHO derived from premixed combustion such as that in sparkignition engines, and we clarify the relationship between combus-tion and emission characteristics.Experimental Apparatus and Procedures

Test Engine and Operating Conditions. The experimentswere carried out on a single-cylinder, horizontal-type, four-stroke-

Contributed by the Internal Combustion Engine Division of THE AMERICAN SO-CIETY OF MECHANICAL ENGINEERS for publication in the ASME JOURNAL OFENGINEERING FOR GAS TURBINES AND POWER. Manuscript received by theICE Division, July 13, 2000; final revision received by the ASME Headquarters,December 5, 2001. Editor: D. N. Assanis. Fig. 1 Combustion chamber configuration

660 Vol. 124, JULY 2002 Copyright 2002 by ASME Transactions of the ASMEK. Yamanee-mail: yamane@mech.usp.ac.jp

Y. Shimamoto

Department of Mechanical Systems Engineering,The University of Shiga Prefecture,

2500 Hassaka, Hikone,Shiga 522-8533, Japan

CombustCharacteCompresMeans oEarly FueThe objective of this searly injection on thediesel engine. Engineengine and an injectiocontrolled high-pressucombustion and exhaua single-stage and eaneously measured theHCHO, CH3CHO, asingle injection and tfrom diesel combustiocharacteristic compontremely early injectiowith an increase in thof pre-mixture. At exthowever, the indicateddiesel combustion. Inincreased, so that ISFreducing HCHO, NOion and Emissionristics of Direct-Injectionsion Ignition Engines byf Two-Stage Split andl Injection

tudy was to experimentally clarify the effect of two-stage split andcombustion and emission characteristics of a direct-injection (DI)tests were carried out using a single-cylinder high-speed DI dieseln system, combining an ordinary jerk pump and an electronicallyre injection system, KD-3. In these experiments to compare thest emission characteristics with two-stage split and early injection,ly injection was tested. The FT-IR exhaust-gas analyzer simulta-exhaust emissions of 26 components. The results showed that

nd CH3COOH were emitted during the very early stage of bothwo-stage injection. These concentrations were higher than those

with ordinary fuel injection timings. These exhaust emissions areents of combustion by premixed compression ignition with ex-. In particular, the HCHO concentration in exhaust was reducedmaximum rate of heat release after cool flame due to pre-reaction

emely early injection, the NOx concentration was extremely low;specific fuel consumption (ISFC) was higher than that of ordinarythe case of two-stage injection, the degree of constant volume isC is improved. These results also demonstrated the possibility of, and smoke emissions by means of two-stage split and early

cycle DI diesel enginerated and water cooledthe test engine and injecas shown in Fig. 1 warpm and at a fixed coo

A nozzle orifice diaexperiment. An injectioments to compare comordinary combustion chamber shape and nozzle orifice. Therefore,

h-

Table 1 Main specifications of a test engine and injectionsystems

Engine type

Bore x strokeDisplacementCompression ratioSwirl ratioCombustion chambeFuel injection systemInjection nozzlethe injection nozzle orifice is not an optimum one to attain bestefficiency and lowest exhaust emission at the two-stage split andearly injection.

Injection System and Method of Split Injection. To realizethe two-stage split and early injection ~SPEI!, two injection sys-tems connected with a check valve were used as shown in Fig. 2.The first-stage injection between the early and middle stages ofthe compression stroke was achieved by an electromagnetic con-trolled high-pressure injection system, KD-3, as shown Fig. 3.This injection system is applied oil-hammering in a convergentinjection pipeline, which was developed by one of the authors~Ikegami et al. @5#!. The injection timings of the first-stage injec-tion were changed by this system, and the injection quantity wasfixed. To change the engine load, the injection quantity at thesecond-stage injection was changed by means of a jerk-type pumpinstalled with an engine camshaft, and the injection timing wasfixed at TDC. To compare the combustion and emission charac-teristics with SPEI, a single-stage and early injection ~EI! wastested by using a KD-3 injection system.

Under engine-firing conditions, the injection fuel quantity wasmeasured by integration of an injection rate waveform which wasdetermined by applying a partial computer simulation technique.This method uses the measured pressure at the nozzle holder-inlet,the measured nozzle needle-lift, and the measured cylinder pres-sure as input signal data. The maximum injection pressure at the

Fig. 3 Schematic illustration of a highammering in a convergent pipeline, KD

Journal of Engineering for Gas Turbines and Powernozzle holder-inlet was controlled to 30 MPa to form a locallyrich-mixture for SPEI, and for EI it was elevated to 70 MPa torealize the combustion by compression ignition with premixedlean mixture.

Measurement of Exhaust Gas Emissions. Smoke densitywas measured by a Bosch-type smoke meter, and exhaust gasemissions of 26 components, including NO, NO2 , lower HCs, andunregulated components, i.e., HCHO, etc., as shown in Table 2,were simultaneously measured by an FT-IR exhaust gas analyzer~Horiba MEXA-4000FT!.

Experimental Results and DiscussionEffect of First-Stage Injection Timings on Combustion and

Emission Characteristics. Figure 4 shows the effect of first-stage injection timings, u j1 , on indicated specific fuel consump-tion, bi , indicated mean effective pressure, pi , degree of constantvolume, hglh , combustion efficiency, h i , NOx , smokeHC(C1 C7), HCHO, CH3CHO, and HCOOH for both the SPEIand EI at fixed engine speed of 1800 rpm and equivalence ratio off50.4. For SPEI, the amount of fuel injected at the first-stageinjection maintained at six times that of the amount injected at the

-pressure injection system using oil-3 Ikegami et al. 5

Fig. 2 Layout of second-stage injection system

JULY 2002, Vol. 124 661~Yanmar NFD170! that was naturally aspi-. Table 1 shows the main specifications oftion systems. A toroidal-type piston cavitys used. The engine was operated at 1800ling-water temperature of 80C.meter of 0.22 mm was employed in thisn nozzle orifice was fixed for all experi-

bustion and emission characteristics under

Direct-injection diesel engineSingle cylinder, water-cooled102 mm3105 mm0.857 liters17.8:12.4

r ToroidalKD-3 ~plunger dia.56.5 mm!PFR-1 ~plunger dia.59 mm!DLLA150P224

pressure of the second-stage injection, and thus that this smoke tion control is difficult.

nwould be reduced by the second-stage injection with high injec-tion pressure ~Yamaneet al. @6#!. The concentrations of gas com-ponents, HCHO, CH3CHO, and HCOOH for both of SPEI and EIwere remarkable high in the combustion. All three of these con-cetrations were clearly higher than the corresponding concentra-tions in ordinary diesel combustion. The pre-mixture formed byextremely early injection is oxidated at lower temperature, thenHCHO is formed. The mixture with the oxidation reaction reachesat cylinder wall layer and its reaction is quenched by wall layerand ambient air, because wall temperature and ambient air arelower than the mixture with oxidation reaction. As the result, ex-haust gas concentration of HCHO indicates high level.

Figure 5 shows the courses of cylinder pressure, p, net rate ofheat release, q , mean gas temperature, Tg , which is calculatedfrom measured cylinder pressure thermodynamically, and lift ofnozzle needle, hz , at different first-stage injection timings. In thisfigure it is shown that there are two peaks in the rate of heat

In the case of the latest injection timing of EI, an event of q bycool and hot-flame is exhibited at a little earlier crank angle thanthat at other injection timings. This will be because the injectedspray impinges at the top-land of piston and pre-mixture is formedmore quickly. However, in the present study the spray penetrationand piston cavity shape on the formation and ignition of pre-mixture are not clear, because of not changing spray nozzle ori-fices and piston cavity shapes.

The maximum of q in both peaks at SPEI is lower than that atEI. This is because the injection pressure for the first-stage injec-tion at SPEI is lower than that at EI. For this reason, the degree ofmixing of fuel and air is deteriorated and the rich mixture isformed locally. As a result, the amount of combustible mixturedecreases and the degree of pre-reaction is controlled. For SPEI,the rich mixture formed by the first-stage injection will be quicklyburnt in a diffusion combustion with the fuel injected at second-stage injection. As a result, hglh in the case of SPEI was im-proved. For this reason, it is seen that pi became higher and bibecame lower than in the case of EI.

Figure 6 indicates the effects of first-stage injection timings,u j l , on the concentration of exhaust gas emissions including un-regulated species measured by FT-IR. It is shown that the charac-teristic components in the exhaust gas species were HCHO,CH3CHO, CH4 , and C2H6 for both SPEI and EI. This indicatesthat the pre-mixture formed by the first-stage injection undergoes

Table 2 Gas components measured by FT-IR exhaust gasanalyzer

CO, CO2, NO, NO2, N2O, H2O, NH3, SO2, HCHO, CH3CHOCH3OH, CH3COCH3, (CH3)3COCH3, HCOOH,CH4, C2H4, C2H6, C3H6,C3H8, 1,3-C4H6, iso-C4H8,n-C4H10, iso-C4H10, C6H6, C7H8, NO1NO2

662 Vol. 124, JULY 2002 Transactions of the ASMEsecond-stage injection. h i was calculated as the ratio of theamount of heat release at the combustion end derived from mea-sured cylinder pressure to theoretical amount of heat release esti-mated from amount of injected fuel per one stroke, and hglh wascalculated as the ratio of the product of rate of heat release andOtto cycle thermal efficiency for small division in crank angle toan integrated amount of heat release derived from measured cyl-inder pressure.

We can see in this figure that concentration of NOx is a verylow level of less than 100 ppm for both of SPEI and EI at earlierinjection timing of 70BTDC. The smoke number ranges from 1to 2. It appears that this smoke resulted from the low injection

Fig. 4 Effect of first-stage injection timings, u jl on indicated specdegree of constant volume, hglh , combustion efficiency, h i , coHCOOHrelease. The first peak results from the cool-flame caused by thepre-reaction of pre-mixture and corresponding generation of form-aldehyde, i.e., HCHO ~Yanagihara @7#!. The second peak of qrepresents a hot flame of pre-mixture combustion. This peak at theretarded injection timings is higher than that at the advanced in-jection timings. At SPEI, it drops before TDC and links with thatby diffusion combustion by a second-stage injected fuel. It lasts toexpansion stroke after TDC. Thus the indicated mean effectivepressure, pi , goes down at the late injection timing. In the case ofthe late injection timings that the peak of hot-flame shows highlevel in both EI and SPEI, unstable ignition occurs and combus-

ific fuel consumption, bi , indicated mean effective pressure, pi ,centrations of NOx , smoke, HCC1C7, HCHO, CH3CHO, and

a lower temperature oxidbons, simultaneously, anding zone; wall layer.

In generally, the chemthe species of ignition,chemical reaction referreterized by lower temperHCHO results in a quickflame at high pressure. T

CHO is emitted in thelecule of HCHO do notuse the cool-flame withh lower temperature, andrbons.

bustion and Emissionect of equivalence ratio,haust emissions at SPEI

Journal of Engineerin 2002, Vol. 124 663ation reaction with cracking of hydrocar-these species are stored in the quench-

ical species of HCHO, which is one ofis emitted and stored by an oxidationd to as a cool-flame reaction and charac-ature and sufficient reaction time. Theoxidation reaction referred to as a hot-

hen, the HCHO is consumed. In Fig. 4 it

is found that a higher concentration of Hexhaust. This result suggests that all moalways transit to hot-flame. This is becaHCHO is quenched at the wall region witHCHO is emitted with unburned hydroca

Effect of Equivalence Ratio on ComCharacteristics. Figure 7 shows the efff, on combustion characteristics and ex

Fig. 5 Courses of cylinder pressure, p, rate of heat release, q, mean gas tem-perature, Tg , and lift of nozzle needle, hz , at different first-stage injection tim-ings ne1800 rpm, f0.4

g for Gas Turbines and Power JULY

Fig. 8, rate of heat release, q , before the start of the second-stageinjection is slightly changed. This is due to the pressure fluctua-and ordinary diesel combustion. Equivalence ratio, f, waschanged by the amount of fuel injected at the second-stage injec-tion of SPEI, and the first-stage injection timing, u j l , was fixed at82 deg BTDC. The amount of first-stage injected fuel to that ofsecond-stage was the same with the case of f50.4 in Fig. 4which a stable combustion was obtained. The ordinary dieselcombustion was realized by using a KD-3 injection system. In thiscase, an injection timing was fixed at 6 deg BTDC. The bestthermal efficiency was indicated in this engine speed and thisinjection timing by employed injection and combustion system inexperiments. For this reason we fixed the injection timing to 6 degBTDC. Figure 8 shows the courses of p, q , Tg and hz at differentequivalence ratios for SPEI, and for KD-3 the courses of p, q ,Tghz and pressure in the nozzle inlet, pN , are shown in Fig. 9. In

Fig. 6 Concentrations of exhaust gas emissions including un-regulated species analyzed by FT-IR for different first-stage in-jection timings, u j1 ne1800 rpm,f0.4

664 Vol. 124, JULY 2002tion of first-stage injection. This fluctuation little affected combus-tion and emission characteristics in this experiment.

From Fig. 7, it is found that the engine performance bi , pi ,hglh by SPEI is worse than those by the ordinary diesel combus-tion. However, concentration of NOx is especially lower than thatof ordinary diesel combustion, which is below 100 ppm. Andsmoke density is lower than that of the ordinary diesel combustionunder the range of lower equivalence ratio. Under higher equiva-lence ratio, smoke density in the case of ordinary diesel combus-tion reaches high in spite of high-pressure injection of 70 MPa.This may be due to the secondary injection as shown in thecourses of needle lift hz in Fig. 9. There is a tendency for HCHO,CH3CHO, and HCOOH to be reduced with an increase in equiva-lence ratio. This result suggests that HCHO with high concentra-tion is consumed and decomposed by diffusion combustion of fuelinjected at the second-stage injection.

At lower equivalence ratio, the concentration of unburned hy-drocarbon HC (C1 C7) shows a high level, and bi increases re-markably. This is because that amount of fuel injected at secondstage is less. For this reason, most of the pre-mixture formed byfirst-stage injection does not burn and remains.

From Fig. 7 it is seen that degree of constant volume hglhdecreases with an increase in equivalence ratio f in the case ofSPEI. This is because the combustion period is elongated to latercrank angle with an increase in amount of fuel injected at second-stage as shown in Fig. 8.

Figure 10 indicates a relationship between HCHO concentra-tion in exhaust and the maximum rate of heat release, qmax . It isclear that HCHO concentration is greatly reduced with an increasein qmax . This shows that decomposition of HCHO strongly de-pends on the appearance of hot-flame linked diffusion flame. Andit is found that the slope falls quickly, as shown in Fig. 10. Thissuggests that the oxidation rate of HCHO in the transition fromcool-flame to hot-flame is very high.

At a higher maximum rate of heat release, that is, at qmax ofover 50J/deg, HCHO concentration increases. In this case, thefirst-stage injection timing was retarded at EI. This result indicatesthat the amount of HCHO quenched and stored at the cylinderwall increases with the retardation of injection timings, and due tothe combustion without diffusion flame for EI the amount ofHCHO decomposition is limited. Hence, it is important to reducethe maximum rate of heat release due to hot-flame in order toreduce the concentration of HCHO in the exhaust.

Relationship Between NOx , Smoke, and Indicated SpecificFuel Consumption ISFC. This section shows a present viewabout the possibility of SPEI to reduce emission and fuel con-sumption by comparing ordinary diesel combustion in a limitedengine operating range of present experiments.

Figure 11 shows the relationship between smoke, NOx , andISFC at the first-stage injection timing changes and fixed thesecond-stage injection timing, TDC for SPEI and EI, and the in-jection timing changes for ordinary diesel combustion. It can beseen that the relationships between smoke and NOx , ISFC, andNOx in the case of retarding injection timing for the SPEI and EIis on the opposite relation of that in the ordinary diesel combus-tion. In the case of the SPEI and EI, and smoke density reduceswith a decrease in NOx concentration with extremely advancingfirst-stage injection timing. This is because the pre-mixture overlean-limit is formed and is quenched at the region of cylinderwall. As the first-stage injection timing retards, smoke and NOxconcentrations increase. This is considered that a portion of thatlean pre-mixture becomes hot flame, then this flame heats thequenched mixture reached at the wall region. For this reason, sootis formed at the wall region and is emitted. But NO concentrationdoes not reach a high level because of a lean combustion withouta rapid pressure rise.

Transactions of the ASME

At the later injection timing of both EI and SPEI there is atendency to smoke reduction. This will be because the amount oflean flammable pre-mixture grows, most of their mixture quickly

comes high. But due to the excess air in the mixture, smoke den-sity does not becomes high.

In the case of the ordinary diesel combustion, smoke and NOx

Journal o 4 665Fig. 8 Courses of p, q, Tg , and hz for different equivalence ratio f

f Engineering for Gas Turbines and Power JULY 2002, Vol. 12burns, and the rapid pressure rise with a fluctuation in a tail of theheat release rate curve occurs as shown in Fig. 5. For this reason,combustion temperature rapidly goes up and its burnt gas is keptat higher temperature. Especially at EI, NOx concentration be-concentrations are reduced at extremely retarded injection timing.This is because the ignition delay becomes long and a largeramount of pre-mixture is formed. Due to lower gas temperatureby expansion stroke, smoke and NOx are reduced.Fig. 7 Effect of equivalence ratios, f, on engine performance and exhaust gas emissions at SPEI andordinary diesel combustion obtained by KD-3 injection system

higher pressure. Therefore, SPEI has the possibility to improveISFC at a late injection timing of first-stage injection. However, to

of hot-flame isFig. 10 Relatiomum of rate of h

e

666 Vol. 124 of the ASMEcontrol the timing and intensity of the occurringindispensable.nship between HCHO from exhaust and maxi-

eat release qmax

Fig. 11 Relationship between smoke density, indicated specific fuel consumption ISFC,and NOx concentration obtained from experimental results by first-stage injection timingchanges for SPEI and EI, and injection timing changes for ordinary diesel combustion

, JULY 2002 TransactionsFig. 9 Courses of p, q, Tg , hz and the pressurlence ratio, f, by using a KD-3 injection systemThere is a tradeoff relationship between ISFC and NOx forSPEI, EI and ordinary diesel combustion. At ordinary diesel com-bustion as the injection timing retards NOx decreases and ISFCincreases. At SPEI and EI, as the first-stage injection timing re-tards, NOx increases and ISFC decreases. At a late injection tim-ing of first-stage injection, the level of ISFC at SPEI is lower thanthat at EI, and that curve approaches a curve at ordinary dieselcombustion. But this ISFC level is approximately 50% higher thanthat of the ordinary diesel combustion. This is due to a long com-bustion duration. This problem will be improved by reduction ofcombustion duration by means of the second-stage injection with

in nozzle holder-inlet, pN , for different equiva-

ConclusionThis study experimentally investigated the effects of second-

stage split and early injection ~SPEI! on the combustion and emis-sion characteristics of a DI diesel engine. The results showed thatHCHO, CH3CHO, and CH3COOH were emitted during the veryearly stage of both single injection ~EI! and second-stage injection~SPEI!. In particular, the HCHO concentration in exhaust wasreduced with an increase in the maximum rate of heat release aftercool-flame due to pre-reaction of pre-mixture. The NOx concen-tration of EI and SPEI was extremely low at extremely advancedtiming of the first-stage; however, the indicated specific fuel con-sumption ~ISFC! was higher than that of ordinary diesel combus-tion. SPEI have a possibility to approach ISFC to that of ordinarydiesel combustion at a retarded timing of first-stage injection. AlsoSPEI can extend the engine operating region; changing engineloads, which is one of the problems in the premixed compressionignition combustion. However, at higher engine load, a high in-jection pressure of over 100 MPa, for the second-stage injection,will be needed to greater reduce the smoke density and the fuelconsumption.

References@1# Ryan, T. W. III, and Callahan. T. J., 1996, Homogeneous Charge Compres-

sion Ignition of Diesel Fuel, SAE Technical Paper 961160.@2# Takeda, Y., Nakagome, K., and Niimura, K., 1996, Emission Characteristics

of Premixed Lean Diesel Combustion With Extremely Early Staged Fuel In-jection, SAE Technical Paper 961163.

@3# Yokota, H., Kudo, Y., Nakajima, H., Kakegawa, T., and Suzuki, T., 1997, ANew Concept for Low Emission Diesel Combustion, SAE Technical Paper970891.

@4# Hashizume, T., Akagawa, H., and Tsujimura, K., 1997, Emission ReductionUsing Multiple Stage Diesel Injection, Proc. of The 14th Internal CombustionEngine Symposium, SAE of Japan, Tokyo, pp. 259264 ~in Japanese!.

@5# Ikegami, M., Yamane, K., Neichi, T., and Zhu, Q. M., 1993, A High-PressureDiesel Fuel Injection System Using Spool Acceleration and Oil Hammering,SAE Trans., 102, pp. 743754.

@6# Yamane, K., and Shimamoto, Y., 1999, Prediction of NO and Soot FromDiesel Engines With Fuel Injection Rate Shaping Using Multidimensional En-gine Simulation, Proc. of The 15th Internal Combustion Engine Symposium(International), SAE of Japan, Tokyo, pp. 489495.

@7# Yanagihara, H., Sato, Y., and Mizuta, J., 1996, A Study of DI Diesel Com-bustion Under Uniform-Higher Dispersed Mixture Formation, Proc. of The13th Internal Combustion Engine Symposium, JSME, Tokyo, pp. 365371 ~inJapanese!.Journal of Engineering for Gas Turbines and Power JULY 2002, Vol. 124 667