Yasuo 1998 Engine Ignition Stratifies

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2055

Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2055–2068

A NEW ERA IN SPARK-IGNITION ENGINES FEATURING HIGH-PRESSUREDIRECT INJECTION

YASUO TAKAGI

Engine and Powertrain Research Laboratory Nissan Research Center Nissan Motor Co., Ltd.

Yokosuka 227, Japan

As a result of recent technological advances, direct-injection spark-ignition (S.I.) engines featuringhigh-pressure injection have been shown to be promising next-generation automobile engines, distinguished by their low energy consumption, high power output, and low emissions of substances that affect the envi-ronment. These engines can reduce fuel consumption by 20% compared with conventional S.I. engines,

as a result of realizing ultralean burn based on stratified charge mixture. They also have the potential toattain other performance improvements, including nearly a 10% improvement in power output whilesimultaneously reducing cold start unburned HC emissions by approximately 30%.

This article reviews characteristics of the combustion process, high-pressure spray used, and mixtureformation of direct injection S.I. engines currently developed and implemented in production passengercars, along with describing the attainment of improved fuel efficiency, higher power output, and lower HCemissions.

It can also be said that a new era is expected in S.I. engines because these performance improvementshave resulted from the challenge undertaken to overcome various technical limitations of conventional S.I.engines, such as pumping losses, knock, and intake port wall wetting, the resolution of which has been along-cherished wish of engine combustion researchers.

Introduction

Approximately 120 years have passed since the ap-pearance of the Otto engine, which was the arche-type of today’s spark-ignition (S.I.) engines. Duringthe intervening years, great strides have been madein S.I. engine technologies, leading to significantprogress in homogeneous charge S.I. engine tech-nologies and resulting in optimization of the enginedesign for performance, functionality, and reliability.As a result, these engines have found widespreadapplication, ranging from the motive power source

of transportation equipment, especiallyautomobiles,to the power source of household lawn mowers. Theglobal production of S.I. engines for automotive usealone now exceeds 30 million units annually.

Meanwhile, there have been rising demandsthroughout the world to protect the earth fromglobal warming and environmental degradation inorder to pass on a healthy planet to future genera-tions. Because S.I. engines are used in such largenumbers around the world, researchers and engi-neers involved with engine combustion have been

working vigorously to research and develop tech-nologies for further reduction in fuel consumption

so as to arrest global warming and for achieving evenlower emissions of substances that affect the envi-ronment.

Fortunately, it has been found that a combustionsystem based on direct injection, where finely at-omized fuel is injected directly into the combustionchamber, has the potential to overcome a difficultsituation by reducing fuel consumption, increasingpower output, and reducing unburned hydrocarbon(HC) emissions simultaneously. The attainment hasbeen contributed significantly by technological ad-

vances such as the “high-pressure” injection systemthat enabled the supply of finely atomized fuel to thecombustion chamber and computer control tech-nology that enabled selection of any injection tim-

ings to achieve some different type of combustionprocesses. Hereupon “high pressure” mentioned isranged between 5 and 12 MPa, which is much lowerthan that used in diesel engines.

In addition, it is important to note that in this pro-cess, several technical issues, which had been pre-

viously recognized as limitations on performanceim-provement but for which no concrete solutions hadbeen found, have been tackled and partly solved by the use of new combustion processes. These limi-tations include

1. fuel economy deterioration due to pumping loss

during part load operation.2. limitation on output improvement due to knock,

and



2056 INVITED PLENARY LECTURE

Fig. 1. Direct-injection S.I. engine used on Mercedes-Benz 300SL in 1954 [3].

3. increased HC emissions caused by wall wettingduring cold engine operation.

With the aim of resolving the problem of reducingpumping losses, charge dilution through the use of a lean mixture and exhaust gas recirculation (EGR)has been applied [1]. However, there have been lim-itations to the improvement of fuel efficiency be-cause the A/F ratio or the amount of EGR gas thatcan be applied has been limited. Stratified chargecombustion realized by the injection of fuel directly to the combustion chamber has mitigated the causeof pumping losses.

Engineers and researchers of S.I. engines havealso struggled endlessly with the problem of knock[2], even though it is now possible to use fuel withan octane number of 100 RON. Despite the adop-tion of such technology as combustion chamber de-sign with a shorter flame propagation distance, im-provement of the in-cylinder airflow and reductionof the mixture temperature, the compression ratiomost commonly used in S.I. engines today is limitedto about 10:1, which means there is still considerableroom for improvement. This issue has been resolvedby charge cooling achieved with direct injection of

fuel into the combustion chamber.Approximately 50% of the unburned HC emis-sions are emitted during the first 60 s after a coldengine start for vehicles complying with the low-emission vehicle (LEV) standards enforced in Cali-fornia. This is caused by the emission of unburnedHCs before the catalyst reaches its light-off tem-perature. In addition, engine-out HC emissions arehigher due to the supply of excess fuel at engine startowing to the fact that a large portion of the fuelinjected into the intake ports adheres to the port

walls and cannot follow the intake airflow in the ini-tial period after engine start. This wall wetting prob-

lem is eliminated by injecting fuel directly into thecombustion chamber.

Almost the same improvement trends have been

seen in the combustion process of direct-injectiondiesel engines by using higher pressure injectionthan that of previous systems, resulting in reducedexhaust emissions. Therefore, it can be said thathigh-pressure, computer-controlled fuel injection

has ushered in a new era of technology in both com-pression-ignition diesel engines and direct-injectionS.I. gasoline engines.

History of the Development of Direct-Injection S.I. Engines

Homogeneous Charge Direct-Injection Engines

An S.I. engine that injected fuel directly into thecombustion chambers was actually used in a fighterplane in Germany in the late 1930s. A famous ex-

ample is the inverted V-12 DB601A engine made by Daimler-Benz that powered one of Germany’s majorfighters, the Messerschnitt Me109, in 1937. It wouldappear that this injection system was adopted fromthe dual standpoints of the particular requirementsof aircraft engines and the absence of any alternativetechnology, given the technical levels of the day.Compared with the use of a carburetor, which wasthe mainstream fuel supply system of aircraft en-gines at that time, direct injection (DI) improvedaerial performance, especially at high altitude, by achieving good fuel distribution and eliminating theneed to install a throttle in the induction system, by allowing excellent drivability obtained with the ab-sence of a float and by removing any danger of icing.As an analogy with contemporary technology, the in-take port injection systems used in today’s automo-tive engines would seem to be sufficient. In theDB610A engine, fuel was injected in the intakestroke, and stratified charge combustion was not ac-complished, but the injection timing was selected tocontrol knock, which was mentioned in the previoussection as being one of the limitations on the im-provement of S.I. engine performance. That direct-injection technology was also subsequently incor-

porated in automotive engines for racing use, and itfound application in mass-production vehicles in1954 when it was adopted for the Mercedes Benz300SL sports coupe [3] (Fig. 1).

Initial Development of Direct-Injection, StratifiedCharge S.I. Engines

To the best of the author’s knowledge, the firstdirect-injection spark-ignition system to be appliedto a stratified charge engine was the Texaco Com-bustion Process (TCP) [4] presented in 1955 (Fig.2). Through stratified charge combustion achieved

by injecting the fuel in the compression stroke, theTCP engine operated at an air–fuel ratio of around50:1 attaining indicated thermal efficiency of 38%



A NEW ERA IN SPARK-IGNITION ENGINES 2057

Fig. 2. Combustion chamber geometry of the Texacocombustion process (TCP) direct-injection stratifiedcharge engine [4].

Fig. 3. Design of direct-injection stratified charge en-gines previously researched [11] (referred from Ref. [5];Texaco-TCP, Ref. [6]; White L-163-S, Ref. [7]; Ford-Proco,Ref. [8]; Mitsubishi-MCP, Ref. [9]; MAN-FM, Ref. [10];

GM-DISC).

with the technology available at that time. Becausethe system did not require a carburetor, it was alsoinsensitive to fuel properties and attracted great at-tention as a multifuel engine. Subsequently, many stratified charge engines were developed, as indi-cated in Fig. 3. As can be seen in the figure, nearly all of these engines shared the same basic concepts

with respect to the mixture formation, ignition, andcombustion processes. They were categorized as so-called late injection systems whereby the fuel was

injected in the latter half of the compression strokeand immediately ignited. The relative positions of the fuel injector and spark plug were arranged so as

to meet those requirements. By contrast, the MAN-FM combustion system was somewhat different inthat it adopted a stratified charge combustion modein which the fuel was injected toward the wall of abowl provided in the piston so as to assist evapora-

tion, and then it was ignited [5]. Because the injec-tion timing was somewhat early, it was categorizedas an early injection system. Among these engines,the TCP engine and Ford PROCO (ProgrammedCombustion) [6] engine were reportedly sold to lim-ited customers around 1980 for use on delivery ve-hicles for the purpose of reducing fuel consumption.However, in the end, they were never offered forsale to the general public.

Mass Production of Direct-Injection, StratifiedCharge S.I. Engines

Initially, Mitsubishi Motors [12] launched generalsales of their direct-injection stratified charge S.I.engines in 1996, followed by Toyota Motor in thesame year [13] and Nissan Motor [13] in 1997, com-mencing a new era in S.I. engines. The majorchanges of these engines from ones at an earlierstage shown in Fig. 3 include the installation of ahigh-pressure injector below the intake port and theprovision of a shallow bowl each uniquely shapedpiston crown (Fig. 4) in order to provide the re-quired performances. A shallow bowl piston thatdoes not increase the piston weight is used becauseat high speed these engine have to operate muchlike conventional passenger car engines. The loca-tion of the injector was found to be the best positionrelative to the spark plug for mixture preparationand transportation of the stratified mixture to thespark plug under the shallow bowl concept. The in-take air motion design differs in that the Mitsubishiengine uses reverse tumble while the Toyota andNissan engines use swirl air motion in the stratifiedcharge combustion mode. Yet, all three enginesadopt a similar combustion concept, in which an at-omized fuel spray is injected toward the bowl pro-

vided in the piston crown and airflow in the bowl

assists fuel evaporation and formation of a stratifiedcharge, which is then ignited to accomplish stratifiedcharge combustion. Therefore, it can be said thatthis process successfully adopts the concept of theMAN-FM combustion system that was developed inthe initial period.

The fact that a combustion concept from the initialdevelopment period has been implemented in pro-duction engines in recent years can undoubtedly beattributed to regulatory requirements and changedenergy circumstances that make such fuel-efficientengines necessary. Yet, in terms of technology, thisimplementation owes much to the quantum leap ad-

vances achieved in two peripheral areas as describedlater.

The first concerns the progress achieved in the




Fig. 4. Combustion chamber configuration and layout of peripheral components of mass production DI gasoline

engines launched from Mitsubishi in 1996 [16], Toyota in 1996 [13,20], and Nissan in 1997 [14].

fuel supply system. All the engines in the initial development period, including the TCP and MAN-FMengines, used a high-pressure injection system in

which the injection pressure was raised intermit-tently. That was the only system available at the time,and it was similar to the technology used for dieselengines. However, because the lubricity of gasolineis vastly lower than that of diesel fuel, it was notpossible to ensure high reliability with the technol-ogy that was available in those days. This problem

was resolved by the combined adoption of the com-mon rail system employing solenoid-driveninjectors,

which were considerably improved in applications toport injection S.I. engines, and a high-pressurepump achieving enhanced performance as a resultof advances made in materials and in machiningtechnologies. As a result, today’s injection systemsare capable of providing injection pressure up to 12MPa [13].

The second peripheral area that has contributedsubstantially to the implementation of this combus-tion concept is computer-controlled technology. Di-

rect-injection S.I. engines do not actually use thestratified charge combustion mode under all oper-ating conditions. Stratified charge combustion isused only under low- and light-load operating con-ditions where further reduction of fuel consumptionis required and is not used under high-load opera-tion because of smoke generation stemming fromthe presence of an excessively rich mixture in theflammable mixture region. Therefore, homogeneouscharge combustion, similar to that used in conven-tional S.I. engines, is employed at high-load oper-ating conditions to suppress the generation of smoke. In this mode, fuel is injected in the intake

stroke, making the injection timing greatly differentfrom that used for stratified charge combustion.Many other injection timings are also used to meet

the performance required by the engine or the ve-hicle under a variety of conditions. The technologiesthat have made such flexible injection timing possi-ble are the common rail fuel supply system and com-puter-control technology that allows the injectiontiming to be freely selected.

The other large contributions have also beenmade, in the author’s opinion, by visualization andsimulation techniques [15,21,25]. Experimental vi-sualization and computational simulation have pro-

vided the information that has enabled engine de-signers to implement the stratified chargecombustion concept in production engines. Theseadvances, combined with the application of precise,high-pressure, computer-controlled fuel injectionhave all occurred since the early work of the 1970s.The mass production of direct-injection S.I. enginesis the result of the combination of various techno-logical advances.

Characteristics of Mixture Formation,

Combustion, and Spray in Current Direct-Injection S.I. Engines

Mixture Formation Process

The new generation direct-injection S.I. engineseschew the deep piston bowl that was used in earlierengines but which increased the piston weight, inorder to operate at high speeds similar to conven-tional passenger car engines as is compared in Figs.3 and 4. In the shallow bowl concept, a combinationof optimized air motion, piston bowl geometry, andspray is required to stabilize the stratified charge

combustion. As one of the concepts of cylinder airmotion, the use of horizontal swirl has been pro-posed [13–15]. In this concept, fuel is injected in the




Fig. 5. Behavior of evaporated fuel in the combustion

chamber with horizontal swirl and forward tumble air mo-tion as visualized by LIF [15].

Fig. 6. Flow field and distribution of evaporated fuel inthe combustion chamber in a DI gasoline engine obtainedby CFD-based simulation [15,21] (operating conditions:same as those in Fig. 5).

latter half of the compression stroke toward the pis-ton bowl, where evaporation and mixing with localair are promoted, resulting in the formation of astratified charge.

The series of photographs in Fig. 5 show the mix-ture formation process as visualized by laser-inducedfluorescence (LIF). To facilitate LIF measurement,

quartz windows are installed in a pentroof section of the combustion chamber of the test engine to visu-alize mixture formation in the combustion chamber.Although special care was taken to visualize the gas-eous phase fuel [15], the photographs in the figure

show both liquid and gaseous phases, as separationof the images obtained for each phase was not con-sidered. It is seen that evaporated fuel is transportedto the spark plug by way of piston bowl (not shownin the figure), where fuel is fully evaporated, fromaround a crank angle of 35 deg.BTDC. Because nofluorescence is observed around the evaporated fuelcloud, it is understood that stratified charge wasformed in the chamber. The transportation of evap-orated fuel is promoted by a vertically rotating mo-tion inside the bowl that was generated by the pres-ence of horizontal swirl to assist the upward motionof the mixture toward the spark plug. This vertical

flow is a resultant vector of swirling air motion andsquish motion generated by the piston crown ge-ometry, as shown in Fig. 6. The air-to-fuel ratioaround the spark plug electrode tip at the timing of spark measured by IR absorption method in this airmotion concept was about 5–10% richer than stoi-chiometry [26].

On the other hand, in the case of forward tumblemotion, which is the natural air motion generated inthe cylinder in conventional automobile enginesequipped with four-valve cylinder heads, the in-

jected spray is pushed directly upward to the sparkplug, as is seen between crank angles of 35 and 45

in the series of photographs on the left side of Fig.5. In this case, high cycle-to-cycle fluctuations occurbecause the injected spray contains large amounts of droplets owing to insufficient evaporation.

Another type of intake air motion used in the shal-low bowl concept is reverse tumble [12], which iscreated as a result of modifying the intake port ge-ometry from the horizontal orientation commonly used in conventional engines to an upright orienta-tion (left-hand side example in Fig. 4). Reverse tum-ble motion transports the evaporated fuel along withthe surface of the piston bowl to the spark plug

(Fig. 7).

Characteristics of High-Pressure Spray

The spray produced by the swirl injector in thenew-generation direct-injection S.I. engines is char-acterized as having a hollow cone with a fairly widespray cone angle of approximately 70–80 under at-mospheric pressure like the spray patterns shown inFig. 8 [12,15,27–29]. With a swirl injector, a widercone angle is preferred for better atomization andfor more uniform spatial distribution of the injectedfuel in the combustion chamber. This is especially

true under wide open throttle operation when sup-pression of smoke generation is one of the main is-sues under homogeneous mixture operation because




Fig. 7. Liquid and gaseous fuel behavior of thefuelspray in the cylinder in reverse tumble air motion achieved by upright intake port [16] (engine speed, 1000 rpm; injectionamount, 15 mm3 /St.; injection, 50 deg. BTDC).

Fig. 8. Axial and radial cross-sec-tional shape and the effect of back

pressure of high-pressure hollowcone spray used in direct-injectionS.I. engines [15].

a large amount of fuel has to be injected to produce

high power output. However, this requirement is notcompatible with stratified charge combustion in

which a compact spray relative to the piston bowlsize has to be supplied in the compression stroke.Fortunately, the reduction of the penetration dis-tance and the cone angle that occurs in the presenceof high back pressure in the latter half of the com-pression stroke can be utilized to good advantage inthe stratified charge combustion mode to keep thefuel spray within the piston bowl. The changes inthe spray cone angle and the penetration distanceunder high back pressure are also seen in Fig. 8. Thereason for this is that the pressure difference be-

tween the cone interior and exterior is proportionalto the product of the density and square of the ve-locity inside the cone. Therefore, a higher back pres-sure with a higher air density increases this pressure

difference, causing the spray to be squeezed in- wardly. The spray particle size is on the order of 15–20 lm Sauter Mean Diameter (SMD) under injec-tion pressure in a range of 7–15 MPa.

A solid cone spray like that shown in Fig. 9 is also

used [13]. This spray has a fairly narrow cone angle,generated by higher injection pressure of 12 MPa,compared with that of the hollow cone mentionedearlier. As is seen in the figure, this spray has aninclined orientation from the center of the injectorto optimize the direction of injected fuel to the pis-ton bowl, which has a compound geometry previ-ously shown in the middle picture of Fig. 4. Thedroplet diameter is sensitive to the injection pres-sure up to about 13 MPa. The effect of the injectionpressure on fuel consumption improvement withthis spray is shown in Fig. 10. It is clear that forengine speeds of 1600 to 2400 rpm, fuel consump-

tion improves as the injection pressure increases;however, at 1200 and 800 rpm, the best efficiency isobtained around 8 MPa. At lower engine speeds, itis thought that the lower injection pressure producesa longer droplet traveling time that promotes evap-oration, at higher speeds, it is thought that the higherinjection pressure produces a shorter injection du-ration that results in a well stratified mixture. In theprocess of selecting the spray, the spray character-istics of a four-hole nozzle (Fig. 11) were also eval-uated [17]. However, a solid cone swirl injector wasfound to be preferable because it provides higheratomization, meets the lower injection pressure re-

quirement, and disperses the fuel better.A development of air-assisted injector is also re-

ported aiming the use for four-stroke automobile en-gines [30]. The spray characteristics created by this




Fig

. 9. Shape of solid cone spray taken under atmospheric pressureand 3 ms after injection start and ef-fect of injection pressure on dropletdiameter [13].

Fig. 10. Effects of injection pressure on fuel consump-tion improvement in solid cone spray [13].

Fig. 11. Spray of four-hole nozzle [17].

Fig. 12. Spray geometry and particle size distribution of air-assisted injector (denoted as low-pressure dual fluid).

injector is that much smaller droplet is generated with the assistance of pressurized air of up to 600kPa in spite of a lower fuel injection pressure of 720kPa than that generated by the high-pressure single-fluid injector as shown in Fig. 12.

Characteristics of Combustion Process

Two different combustion processes are used in anew generation of a direct-injection S.I. engine. Oneis that of homogeneous charge including stoichio-

metric and lean mixture that is the same as that of conventional homogeneous charge combustion be-cause fuel is injected in the intake process to form a

homogeneous mixture, as mentioned in the previoussection. The other is stratified charge combustion in

which combustion occurs under much richer air-to-fuel ratio than that of the supplied one called super-lean burn.

Figure 13 shows the spontaneous spectrum of flame luminescence obtained with a high-speed op-tical multichannel analyzer for stratified and uniformmixture combustion [16]. With early injection in

which a homogeneous mixture was formed, lumi-nescence radiation is attributed to OH and CHchemiluminescence as well as CO-O recombinationemission. Luminescence in the longer wavelengthregion is not observed. This is a typical characteristicof premixed lean or stoichiometric flames. With late

injection in which a stratified charge was formed, themajor component of the luminescence radiationconsists of a continuous solid emission from the soot




Fig. 13. Flame radiation andspectrum of the visible emissionfrom early and late direct injectionSI engine [16] (engine speed, 1500rpm; injection amount, 15 mm3 / stroke).

generated in the combustion chamber. This is a dis-tinctive characteristic of stratified charge combus-tion. However, this luminescence radiation was at-tenuated in a short time, as the soot was burned upbecause sufficient air had been entrained into thefuel before ignition.

Potential for Performance Improvements withDirect-Injection S.I. Engines

Substantial Reduction in Fuel Consumption With the aforementioned specifications men-

tioned in the previous section, this direct-injectionengine achieves stable combustion of a lean stratifiedcharge combustion even at an air–fuel ratio as leanas 40:1, as indicated in Fig. 14. As a result, fuel con-sumption is reduced by 20% compared with that of conventional port injection engines operated at astoichiometric air–fuel ratio. In a lean burn S.I. en-gine, factors to contribute to fuel consumption im-provement are analyzed and reported to be a reduc-tion of pumping losses caused by charge dilution,

heat losses to the cylinder walls caused by reducedburned gas temperature, and increased transfer of burned energy to working gas in the cylinder causedby modified gas property [31]. In the direct-injectionstratified charge engine, the major reason for thisimprovement is the reduction of pumping loss, asindicated by the reduced intake depression seen inthe figure. Because much of the mixture is burnednear the stoichiometric air–fuel ratio, emission levelof nitrogen oxides (NO

x) are relatively higher than

for the conventional mixture burning. However, theapplication of EGR reduces NO

xemissions to the

target level, as seen in the figure, to meet the current

emission standard enforced in Japan, within the al-lowable engine stability limit defined by coefficientof variation of indicated mean effective pressure pi

(COV-pi). Unburned HC emissions are somewhathigher than the levels for conventional homoge-neous charge combustion, which, among other rea-sons, is due to quenching of the flame when a leanmixture exceeds the flame propagation limit, as hasbeen noted by many researchers before [7]. How-ever, the level is within the range where it can bereduced by improved oxidation capability of a cata-lyst, for which remarkable advances have beenachieved in recent years.

The combustion process realized with this strati-fied charge makes it possible to control engine out-put by regulating only the fuel injection amount,

without throttling the intake air at low loads.

Effect on Improved Power Output

As mentioned earlier, direct-injection engines op-erate in the homogeneous charge mode when highpower output is demanded. In this case, becausefinely atomized fuel is supplied directly to the com-bustion chamber, latent heat of fuel evaporation

works to cool the intake air for improved charging

efficiency. As shown in Fig. 15, it has the effect of improving charging efficiency by 3% in the low rpmrange. Simultaneously, the lower intake air tempera-ture helps to suppress knock, with the result that theignition timing can be advanced by approximately 2deg.CA. Together with the improved charging effi-ciency, this has the effect of increasing brake torqueby more than 6% under the operating conditionsnoted in the figure. An improvement in engine out-put of between 6 and 9% can be obtained compared

with conventional port injection engines in all enginespeed ranges.

These results indicate that direct injection also

plays a role in reducing knock, which was noted ear-lier as one of the unfortunate characteristics of ho-mogeneous charge S.I. engines.




Fig. 14. Reduction in fuel consumption by stratifiedcharge combustion [15] (A/F and G/F denote the ratio of air and air plus EGR gas to the fuel, respectively).

Fig. 15. Effect of increased charging efficiency and sup-pression of knock on improvement in torque output [15].

Fig. 16. Comparison of fuel be-havior in intake process under cold

engine conditions visualized by LIF[15] (sapphire cylinder optical en-gine is used under firing operation).

Reduction of Cold-Start HC Emissions

In direct-injection engines, finely atomized fuelcan be supplied directly to the combustion chambereven under cold-engine operation, making it possi-ble to avoid wall wetting at cold starts, which wasmentioned earlier as one of the limitations of im-proving performance of S.I. engines. A comparisonof the photographs visualized by LIF in Fig. 16clearly indicates that fuel behavior in direct-injectionengines is vastly different from that observed in con-

ventional port injection engines. This difference infuel behavior makes it possible to suppress the un-burned HC emission peak that occurs after engine

cold start due to wall wetting in conventional port

injection engines (Fig. 17). This results from the factthat direct-injection engines can be started readily

without any increase in the fuel supply as is also il-lustrated in Fig. 17. The reduction in cold-start HCemissions shown in Fig. 17 corresponds to 30% of the total quantity emitted for vehicles complying

with low-emission vehicles (LEV) standards en-forced in California.

Reduction of cold-start HC emissions has alsobeen achieved by applying high-pressure direct in-

jection to overcome the limitations of conventional

S.I. engines.

Potential for Eliminating Other Limitations

At the outset, this review emphasized the effect of direct-injection S.I. engines on overcoming variouslimitations that have held back performance im-provements in conventional S.I. engines. Some of these limitations have been mitigated or partly re-solved, as discussed in the previous sections. How-ever, this engine still faces other limitations on per-formance improvement:

1. limitations due to fixed valve events,




Fig. 17. Reduction of cold-start HC emissions underLA4 vehicle driving mode [15].

Fig. 18. Combustion strategy used in the direct-injec-tion S.I. engine system [13].

2. dependence of engine performance on fuel prop-erties,

3. cylinder-to-cylinder variations,4. cycle-to-cycle variations, and5. heat losses associated with flame impingement on

wall.

Experimental results relating to these issues havenot always been clear or reported in the literatureso far. However, combining a system for variable

valve timing and lift, such as a phase shifter and acam profile selector [22], with direct injection could

expand the range of the variable system. For in-stance, because the intake air does not contain any fuel, the benefit of late intake valve closing or the

delivery of air by late exhaust valve closing could besuccessfully utilized. As for the strong dependenceon fuel properties, development work on direct-in-

jection engines originally started from the conceptof a multifuel engine, that is, low sensitivity to fuel

properties, so further research work can be expectedto shed some light on this issue. It is also expectedthat innovative work will be undertaken to tackle theissues of reducing cylinder-to-cylinder and cycle-to-cycle variations and heat loss.

Outgrowth of Direct Injection

Increased Controllability of Mixture Formation

Because the new generation of direct-injectionS.I. engines incorporates a fully electronically con-

trolled fuel injection system, a combination of com-bustion processes is being used under different op-erating conditions. As mentioned earlier, stratifiedcharge combustion and homogeneous charge com-bustion are used under low-load operating condi-tions and high-load conditions, respectively. In ad-dition, as shown in Fig. 18, these engine systems usea variety of combustion process derived from differ-ent mixtures, namely, lean homogeneous mixturecombustion and two-stage mixture combustion, un-der transitional conditions to and from stratified andhomogeneous charge combustion modes [13]. Leanhomogeneous charge combustion is used to enhanceimprovement of fuel consumption.

Two-stage injection, in which the fuel is injectedfirst in the intake stroke and then again in the com-pression stroke, is employed in order to minimizetorque discontinuities in the transition betweenstratified charge combustion and homogeneouscharge combustion by extending the range of strat-ified charge combustion, as shown in Fig. 19 [13].This effect is achieved by suppressing smoke gen-eration under middle range operating conditions.

Another two-stage combustion used for the quick warming up of the under floor catalyst is proposed

[23]. In this combustion, a supplementary injectionis performed during the later stage of the expansionstroke, while the engine is operated lean, adoptingthe late injection mode under cold conditions. As alarge amount of air exists in the burned gas and thegas temperature is sufficiently high, supplementary supplied fuel is ignited to increase the exhaust gastemperature resulted in reducing light-off tempera-ture of the catalyst up to 200 s.

Improvement in Catalyst Technology

To comply with much more stringent emission

regulations set to be enforced in the United States,EU, Japan, and other countries, it will be necessary to use a lean-NO

xcatalyst system that reduces both




Fig. 19. Effect of two-stage injection on smoke sup-pression under 1200 rpm [13].

Fig. 20. Mechanism showing NO x

storage and reduction[13].

Fig. 21. Durability of lean-NO x

catalyst [23].

HC and NO x

emissions simultaneously even at leanair–fuel ratios in place of the conventional three-way catalyst system.

To meet this requirement, two kinds of lean-NO x

catalyst have been proposed—the NO x

storage cat-

alyst and the selective reduction catalyst [23]. As itsname implies, the NO x

storage catalyst stores NO x

during lean operation, and the stored NO x

are re-duced to nitrogen during a short stoichiometric orrich excursion period, as illustrated in Fig. 20 [13].Selective reduction catalysts make use of the directreduction reaction of NO

xby hydrocarbons.

As has been reported [23] and shown in Fig. 21,an NO

xstorage catalyst shows higher reduction ef-

ficiency than a selective catalyst under fresh catalystconditions. However, the NO

xstorage catalyst is sus-

ceptible to sulfur poisoning and shows a greater lossin activity than the selective catalyst over a short

driving mileage where gasoline containing 200 ppmsulfur is used as the fuel, as shown in Fig. 21. There-fore, improvement of the reduction performance of the selective type catalyst and resolution of the sulfurpoisoning problem of NO

xstorage catalyst by sulfur

trapping [24] or sulfur regeneration of the poisonedcatalyst are still high priority issues in catalyst tech-nology development.

Effect of High-Pressure Injection on DI DieselEngine Performance

Because of their lower fuel consumption, DI die-sel engines are increasingly being used even in smallautomobile engine applications in place of indirect-injection (IDI) diesel engines that have an auxiliary chamber into which the fuel is injected. A great dealof research has been done on technologies for re-ducing emissions of NO

x, soot, and other substances

affecting environment from DI diesel engines, be-cause reduction of emissions has been one technicalissue of concern for these engines. One approachbeing pursued toward this end is to use high-pres-sure injection similar to the technology applied toS.I. engines.

Recently, a new combustion concept called mod-ulated kinetics concept (MK concept) combustion, which simultaneously reduces both NO

xand PM

emissions, as shown on the left side of Fig. 22, hasbeen developed by Nissan Motor. It is planned toimplement this concept in a wide range of produc-tion engines [18,19]. The MK combustion conceptis based on a more homogeneous charge combustionprocess than that of conventional DI diesels. This isaccomplished by optimizing the combustion cham-ber geometry, strengthening in-cylinder air motion,applying cooled EGR gas, and also by retarding theinjection timing, all of which contribute to stable

combustion while increasing the degree of premixedcombustion of the injected fuel. Figure 23 comparesMK combustion and ordinary diesel combustion in




Fig. 22. Effect of MK combustion concept on reducing NO x and particulate emissions of direct-injectiondiesel enginesand effect of high-pressure injection on extending operating conditions [18,19].

Fig. 23. High-speed photographs and heat release toshow characteristics of MK combustion process [18].

Fig. 24. High-pressure spray used in MK combustionconcept direct-injection diesel engine visualized 1.0 ms af-ter injection start.

terms of the heat release rate and combustion pro-cess as visualized with high-speed cinematography.It can be seen that in the MK concept, most of theinjected fuel burned with premixed combustion be-cause none of the bright luminescence that is typicalof diffusion flame is not observed in the figure. High-pressure injection is used in order to apply MK com-bustion even at higher engine loads, where it is al-

ready known that it would be hard to use this newcombustion process at a conventional injection pres-sure of, say, 30 kPa. It has been common knowledge

so far that the use of a high-pressure spray in a DIdiesel engine reduces emissions of soot and partic-ulate matter (PM), but on the negative side, NO

x

emissions increase, as indicated in Fig. 22. In con-trast, by combining the MK concept with high-pres-

sure injection using the spray geometry shown inFig. 24, simultaneous reduction of NO x

and PMemissions can be achieved, something that has notbeen possible hitherto with a high-pressure fuelspray in a conventional combustion system at highengine loads, as shown on the right side of Fig. 22.It can be concluded that high-pressure injection canbe expected to reduce the fuel consumption and ex-haust emissions of diesel engines.

Conclusion

It was shown that direct-injection S.I. engines, which have been developed at a rapid pace in recent years, have potential to attain major performance




improvements simultaneously, including a reductionin fuel consumption of around 20%, improved out-put of from 6 to 10%, and a substantial reduction incold-start unburned HC emissions. These improve-ments result from the adoption of a new combustion

system, featuring direct injection of finely atomizedfuel into the combustion chamber, which is sup-ported by a number of new technologies such ashigh-pressure injection, computer-control technol-ogy, a new lean-NO

xcatalyst system, and diagnostic

and simulation techniques regarding combustion. Itcan also be said that these improvements areachieved by overcoming various limitations previ-ously thought to be hopelessly unresolvable in S.I.engines. As further improvements are expected by using direct injection at high-injection pressures, itcan be said that a new era in S.I. engines is antici-pated, especially in order to improve performance

regarding the exhaust emissions of substances thataffect the environment and contribute to global

warming.The same tendency is seen in the application of a

high-pressure injection to DI diesel engines in orderto achieve a further reduction of emissions such asNO

xand particulate that are major issues for this

type of engine. It is hoped that high-pressure fuelinjection technology will be a new approach to fur-ther improving the performance of both S.I. enginesand DI diesel engines.

Acknowledgments

The author would especially like to thank Dr. H. Andoof Mitsubishi Motor Corp. and Mr. D. Sawada of ToyotaMotor Corp., who kindly provided some of the originaldata. Thanks are also due the research engineers of theNissan Research Center for providing excellent data suit-able for this article, especially Mr. S. Kimura, who fur-nished data on the MK concept DI diesel engine, and Mr.T. Fujii, who provided information concerning the history of direct-injection S.I. engine development.

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