Automotive Innovation (2018) 1:95–113https://doi.org/10.1007/s42154-018-0020-1

Recent Progress in Automotive Gasoline Direct Injection EngineTechnology

Shijin Shuai1 · Xiao Ma1 · Yanfei Li1 · Yunliang Qi1 · Hongming Xu1

Received: 1 September 2017 / Accepted: 4 January 2018 / Published online: 21 June 2018© Society of Automotive Engineers of China (SAE-China) 2018

AbstractGasoline direct injection (GDI) engines are currently the dominant powertrains for passenger cars.With the implementation ofincreasingly stringent fuel consumption and emission regulations worldwide, GDI engines are facing challenges owing to highparticulate matter emissions and a tendency to knock, leading to a change in the research and design (R&D) issues comparedwith those in the twentieth century. This paper reviews the progress in research regarding GDI engine technologies over thepast 20years, focusing on combustion system configurations, and also highlights common issues in GDI R&D, including pre-ignition and deto-knock, soot formation and PM emissions, injector deposits and gasoline compression ignition (GCI). First,an overview of recent developments in the field as driven by regulations is provided, following which progress in injection andcombustion systems is examined. Third, the review addresses the occurrence and mechanism of deto-knock and considersmeans of suppressing this phenomenon. The fourth section discusses soot formation mechanisms and particulate matteremission characteristics of GDI engines and describes the application of gasoline particulate filter (GPF) after-treatment.The subsequent section summarizes studies regarding injector deposit formation, as well as pioneering research into GCIcombustion modes. Finally, a summary and future prospects for GDI engine technologies are provided.

Keywords Gasoline direction injection · Combustion system · Pre-ignition · Particulate matter · Injector deposit · Gasolinecompression ignition

AbbreviationsAI Artificial intelligenceBDC Bottom dead centerBMEP Brake mean effective pressureBSFC Brake specific fuel consumptionBTE Brake thermal efficiencyCAFC Corporate average fuel consumptionCGPF Coated gasoline particulate filterCTE Coefficient of thermal expansionCOV Coefficient of varianceCR Compression ratioDI Direct injectionDISI Direct injection spark ignitionDPF Diesel particulate filterEGR Exhaust gas recirculationEIVC Early intake valve closing

B Shijin Shuaisjshuai@tsinghua.edu.cn

1 State Key Laboratory of Automotive Safety and Energy,Tsinghua University, Beijing 100084, China

GCI Gasoline compression ignitionGDCI Gasoline direct compression ignitionGDI Gasoline direct injectionGPF Gasoline particulate filterHCCI Homogeneous charge compression ignitionITE Indicated thermal efficiencyLIVC Late intake valve closingLSPI Low-speed pre-ignitionMPCI Multiple premixed compression ignitionMPI Multiple point injectionNA Natural aspiratedNEDC New European driving cyclePFI Port fuel injectionPM Particulate matterPN Particle numberPPCI Partially premixed compression ignitionRCM Rapid compression machineRON Research octane numberSMD Sauter mean diameterSPCCI Spark-controlled compression ignitionTDC Top dead center

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THC Total hydrocarbonTWC Three way catalystVCR Variable compression ratioVVT Variable valve timing

1 Introduction

At present, automotive powertrains are undergoing diversi-fication and electrification in response to concerns regardingenergy supply security and air pollution. The production ofpurely electric vehicles has grown rapidly in recent years,especially in China. However, internal combustion enginevehicles, including hybrids, are still predicted to accountfor over 90% of the total light-duty vehicle powertrains in2030 [1]. Gasoline direct injection (GDI) engines can pro-vide benefits related to high thermal efficiency and potentialreductions ofHCs andNOx emissions andwill likely becomedominant among passenger cars [2]. The global proportionof GDI engines involving stoichiometric fuel mixtures isexpected to overtake that of port fuel injection (PFI) enginesby 2020 due to the implementation of increasingly stringentfuel consumption regulations [3].

In Europe, the major light-duty vehicle markets are mov-ing toward a CO2 emission target of 95g/km by 2020. Inthe USA, the average annual reductions in CO2 emissionsfrom 2017 to 2021 and 2022 to 2025 will be 3.5 and 5%,respectively. In China, the corporate average fuel consump-tion (CAFC) standards for passenger cars for 2020, 2025 and2030 are 5.0, 4.0 and 3.2L/100km respectively [4]. In addi-tion, the Euro 6 and China 6 emission regulations requiremuch lower particulate matter emissions, especially parti-cle number (PN) emissions [5]. Therefore, more advancedGDI engines have been developed to meet these new regula-tions, such as the Mazda SKYACTIV [6]. In Japan, industryand academia have initiated the Research Association ofAutomotive Internal Combustion Engines, with the goal ofimproving gasoline engine thermal efficiency to an unprece-dented level of 50% for GDI engines by 2020 [7]. In 2016,the US Department of Energy began the Co-Optimizationof Fuels and Engines (Co-Optima) project [8], which com-bines biofuel and combustion R&D and aims to increaseautomotive fuel efficiency to 50% (15% greater than the cur-rent level). Gasoline compression ignition (GCI) technologyhas also been intensively investigated in recent years. DelphiTechnologies [9] has developed a system known as gasolinedirect compression ignition (GDCI) and has launched a thirdgeneration GCI engine that has a thermal efficiency compa-rable to that of conventional diesel engines but with greatpotential for PM reduction.

To achieve enhanced thermal efficiency in newly devel-oped GDI engines, a high degree of exhaust gas recirculation(EGR) has been combined with increased tumble flow to

suppress knocking associated with turbo-charging and highcompression ratio (CR), thus reducing heat losses in asso-ciation with low temperature combustion. Over-expansioncycles resulting from variable valve timing (VVT) systems,which are especially applicable to dedicated hybrid gasolineengines, have also been widely applied. However, becauseGDI engines produce much more PM and PN emissions thanPFI engines, there are concerns regarding the particulatemat-ter emissions output of these engines.

It is evident that GDI engine technologies have sig-nificantly evolved and yet still have significant potentialto realize additional energy efficiency improvements andemission reductions. Thus, it is helpful to examine the devel-opment of GDI engines over the past 20years and to considerthe state of the art, following a prior review by Zhao et al. [2].

This paper first introduces the latest developments in com-bustion systems and fuelmixture formulation inmodernGDIengines inSect. 2. Subsequently, several important tropics arepresented in detail, including deto-knock in Sect. 3, soot andparticulate matter in Sect. 4, and injector deposits in Sect. 5.Section 6 describes the recent development of GCI engines.Finally, the outlook for the future development of next gen-eration GDI engines is delivered.

2 GDI Combustion Systems

This section introduces the latest developments in combus-tion systems in modern GDI engines. Several importantcommon features of the recent GDI combustion systems arereviewed separately.

2.1 Development of GDI Combustion Systems

Among the three types of typical GDI combustion systems[10], the wall-guided configuration is the least used in recentengine production due to concerns regarding fuel impinge-ment and high THC and particle emissions [11]. However,the wall-guided configuration is still employed and is com-bined with the air-guided configuration to reduce costs insome systems [12]. Recent studies [13–15] have shown thatGDI combustion systems tend to consist of a combinationof air-guided and spray-guided configurations. Spray guid-ing is used as part of a stratified strategy incorporating lateinjections to eliminate impingement, while the in-cylinderair motion and turbulence are emphasized in the combustionsystem designs [16].

Figure 1 shows the typical configurations of side-mountedand central-mounted injectors. Other investigations [17–19]have shown that, compared with central-mounted injectors,side-mounted injectors exhibit better performance in termsof enhanced tumble motion, thermal load reduction, pre-ignition suppression and thermal efficiency improvement.

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Fig. 1 Typical GDIconfigurations (Courtesy to SAEInternational). a Side-mountedinjector [19], b central-mountedinjector [20]

Fig. 2 A GDI/PFI dual injection system [22] (Courtesy to SAE Inter-national)

However, it is more difficult for a side-mounted injectorto realize spray-guided operation due to the potential forimpingement. In contrast, central-mounted injectors are sit-uated closer to the spark plug, such that the spray can easilyapproach the spark area in the stratifiedmode. This simplifiesthe combustion chamber design and offers flexibility whenattempting to reduce PM/PN by avoiding wall-wetting. Inaddition, piezo injectors in GDI engines [20,21] are normallycentral-mounted to maximize the performance by usingmul-tiple injection in association with short intervals during fuelmixture preparation. The central-mounted design may alsobe the best option for future GDCI concepts [9].

Another important trend inGDI combustion systems is theuse of dual injection systems that combine direction injection(DI) and PFI, as shown in Fig. 2 First proposed by Ikoma etal. [22], such systems provide more flexibility in terms ofcontrol, particle reduction and knock suppression [23–26]and are currently used in GDI engines [14].

High thermal efficiency is the main goal when develop-ing combustion systems such as GDI engines. Increasing thecompression ratio (CR) and lowering heat losses are botheffective approaches to meeting this goal. Figure 3 plotscompression ratio values as functions of the maximum brakemean effective pressure (BMEP) based on data obtained fromgasoline engines in 2015 and 2016. The CR values of tur-bocharged GDI engines are primarily in the range from 9.5

Fig. 3 Relationship between the geometric compression ratio andmax-imum BMEP [33] (Courtesy to SAE International)

to 11.0 [19,27–30]. In the case of recently developed naturalaspirated (NA) GDI engines, the CR normally ranges from11.0 to 13.0 [16,31,32], while the highest CR reported inthe development stage is 15.0 [31]. The CR in a high boostdesign is limited by knocking; therefore, NA systems areable to take advantage of peak thermal efficiencies [31]. Toincrease the CR above 13.0, most such engines have to incor-porate variable compression ratio (VCR) [28] or VVT forover-expansion cycles (such as theAtkinson orMiller cycles)[29,31], and over-expansion cycles are discussed in Sect. 2.5.

Asthana et al. [34] described seven types of VCR con-figurations, the majority of which have challenges relatedto high-speed adaptability, reliability and cost. The Nissancorporation has reported a series of studies regarding VCRsystems [35–38] and announced a production-ready gasolineengine with a VCR turbo system in 2017 [28]. This systemuses a multilink rod crank and a servo motor to allow con-tinuous variation of the CR between 8.0 and 14.0. Both AVLand FEV [33,39] have adopted a different design conceptin which the rod length is varied in proof-of-concept GDIengines, while Honda [40] uses a dual piston system. It hasbeen reported that the VCR system can reduce brake specificfuel consumption (BSFC) by up to 8% [33,39].

A previous study has determined that a high tumble ratio isimportant in GDI engine downsizing [16], because it results

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Fig. 4 Spray structures of outward-opening, multi-hole [21] and slit nozzles [43] (Courtesy to SAE International). a Piezo-driven injector, bmulti-hole injector, c fan-shaped slit injector

Fig. 5 PM and PN emissions from different combustion strategies (Courtesy to SAE International). aResults fromRef. [50], b results fromRef. [47]

in faster fuel mixing and increased turbulence that improvethe burning velocity, reduce PM/PN and suppress knocking.Typically, the tumble ratios in recent combustion systemshave been in the range of 1.93 to 2.8 [27,41,42], althoughthere has been a report of a prototype engine with a ratio ofapproximately 3.4 [16].

2.2 Fuel Injection Systems

Figure 4 shows the structures of typical sprays generatedby multi-hole, outward-opening and slit nozzles. Currently,multi-hole nozzles are widely used due to the ability to con-trol jet orientation. However, Toyota uses slit nozzles togenerate a fan-shaped spray that exhibits wider dispersionand better atomization quality [43]. Piezo-driven injectorsshow much faster responses and higher control accuracythan solenoid-driven multi-hole injectors [44], although thehigh cost of piezo injectors has restricted their applications.In recent years, outward-opening piezo-driven nozzles havebeen utilized [45] in stratified charge models. These nozzles

can provide a relatively short spray penetration length thatreduces wall-wetting [46]. In addition, the flow velocity inthe vortex is lower than that in the spray mainstream, con-tributing to ignition stability.

2.3 Lean Burn Strategy

There has been continuous research regarding lean strati-fied strategies, although high engine-out NOx levels and lowexhaust temperatures remain as challenges associated withafter-treatment. Recent studies [47,48] have indicated that,comparedwith conventional stoichiometric gasoline engines,lean burn strategies can effectively reduceBSFCwhile reduc-ing the COV and knocking. Iida et al. [49] employed ahigh-energy ignition system and an increased tumble ratio(up to 2.5) to achieve super-lean burning (λ = 1.92) andobtained an indicated thermal efficiency (ITE) as high as46% in a single-cylinder engine.

However, lean burn GDI engines with stratified chargingalso tend to produce significant PM/PN emissions. Previous

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studies [47,50] have shown that the particle concentrationsgenerated by lean stratified combustion may be an order ofmagnitude higher than those produced by lean homogenousand stoichiometric combustion and that lean homogenousengines produce the least PN and soot (Fig. 5).

NOx emissions remain an important issue in lean burnresearch, and it has been determined that very lean combus-tionmay suppressNOxgeneration due to the low combustiontemperature. Even so, high λ combustion has to addressthe problem of high combustion phasing sensibility andincreased COV values [48].

2.4 Cooled and Hot EGR

Engines using cooled EGR have the advantage of a highintake density, resulting in superior volumetric efficiency.This technique also decreases the combustion temperaturein the engine, resulting in NOx reduction and knock sup-pression, particularly at high load. In contrast, hot EGRincreases the intake temperature so that volumetric efficiencyis reduced, but results in low HC emissions at cold start andlow load conditions.

It has been reported [51] that, comparedwith cooled EGR,hot EGR shows pronounced COV tolerance and has a signif-icant effect on combustion duration and thermal efficiency.Studies [52,53] have shown that cooledEGRcan reduceNOxand CO by up to over 90 and 50%, respectively, and lowerBSFC values by more than 11%, although THC emissionsare significantly increased. The NOx emissions from a hotEGR can be four times higher than those from a cooled EGR,while theTHCemissions froma cooledEGRcan be 1.5 timeshigher than those from a hot EGR [54]. It is also evident thatlarger particles (>50nm) can be reduced by increasing thecooled EGR ratio. However, smaller particles (<10nm) arenot as sensitive to the ratio [53,55].

2.5 Over-Expansion Cycles

Over-expansion cycles are widely used and can be combinedwith VCR to achieve high BTE values in GDI combustionsystems, such as in the case of an early IVC (EIVC) known astheMiller cycle or a late IVC (LIVC) referred to as theAtkin-son cycle [33,56]. The EIVC mode results in stable engineperformance over a wide range of engine speeds, while theadvantage of LIVC in conjunction with BTE is only evidentat high speeds [57]. However, it has been demonstrated thatEIVC leads to considerable power loss at wide open throttle(WOT) conditions, especially at high speeds [58,59], suchthat a higher intake boost pressure is needed to compensate[60]. Previous studies [61,62] found that LIVC effectivelysuppresses knocking and reduces BSFC by up to 6.9% alongwith a significant increase in the low BSFC operating region[63,64]. In contrast, the EIVC mode only achieves a BSFC

reduction of 2.0% compared with the base cam [65]. It isworth noting that the ideal efficiency is higher in the caseof EIVC because of the lower compression work comparedwith LIVC [60,66]. However, detailed investigations havedetermined that EIVC has an adverse effect on in-cylindercharge motion and requires sophisticated optimization ofthe combustion system [33,67,68]. Therefore, LIVC is moreapplicable to GDI engines

3 Pre-ignition and Deto-Knock

This section addresses the occurrence and mechanism ofdeto-knock and considers means of suppressing this phe-nomenon. The effect of pre-ignition is discussed, and therequirements on fuels, lubricant oils and engine designs forsuppressing deto-knock are reviewed.

3.1 Characteristics of Deto-Knock

Downsizing, high boost and direct injection are all effec-tive approaches to enhancing the power density and fueleconomy of gasoline engines. However, increasing the boostratio tends to induce a new engine knock mode in conjunc-tion with a pressure rise more than one order of magnitudehigher than that associated with conventional knocking. Onestudy has shown a pressure exceeding 1000 bar during acycle, which is sufficient to damage the engine [69]. Thisnew knock mode has been termed unwanted pre-ignition byVW,Chao-ji knock [70] and deto-knock by Tsinghua in 2006and 2016, respectively, super-knock by Shell in 2009, megaknock by AVL in 2009 and low-speed pre-ignition (LSPI) bySWRI in 2001 [69]. In the event of deto-knock, the cylinderpressure rises before the normal spark timing, indicating thatpre-ignition occurs in the cylinder. After a period of smoothpressure rise, the pressure trace starts to oscillate, accom-panied by a very high amplitude. Although deto-knock istriggered by pre-ignition, in contrast to self-sustaining andrunaway surface pre-ignition [71], deto-knock occurs rarely,randomly and alternately with normal combustion, then dis-appears naturally. Deto-knock is normally rated using theoccurrence rate within a certain number of cycles. Typically,the frequency is lower than one in thousands [69], whichmakes it difficult to investigate the associated mechanism.

3.2 The Deto-KnockMechanism

3.2.1 Combustion Modes Associated with Deto-Knock

The deto-knock mechanism was first elucidated by Wang etal. in 2015 [72] using synchronous high-speed direct pho-tography and pressure measurements in a rapid compressionmachine (RCM). The results demonstrated that the mech-

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Fig. 6 A random pre-ignition induced deto-knock combustion process [72]. a An image showing pre-ignition, deflagration and detonation, b apressure trace with synchronous images

anism involves an initial hotspot-induced deflagration toproduce a secondary hotspot-induced detonation, as shownin Fig. 6. A detonation wave initiated in the near-wall end-gas area is clearly observed in this figure, generating a sharprise and discontinuity in the pressure trace. It should be notedthat the deflagration (pre-ignition) triggered by the first hotspot does not self-accelerate to detonation. Instead, it is thesecond hot spot located in the unburned end-gas that initiatesthe detonation.

3.2.2 Pre-ignition-Sources

Pre-ignition is an essential condition for deto-knock to occur.Lubricant oil contains long-chain alkanes that are prone toauto-ignition [73] and are assumed to act as the pre-ignitionsource. This has been verified by directly injecting a smallamount of oil into the cylinder [74]. The results showed thatthe injected oil droplets could induce pre-ignition and deto-knock just in that injection cycle. Dahnz et al. [71] postulatedthat the most probable manner in which oil droplets enterthe combustion chamber is the spray wetting and dilutionof the oil film on the liner wall, such that the diluted oilaccumulates in the piston crevices. Because gasoline dilutionreduces the viscosity and surface tensionof the oil, the dropletis likely to be released from the crevice because of the inertialforces during deceleration of the piston just before it reachesTDC. Currently, this explanation regarding the ingress of oilinto the combustion chamber is widely accepted [72,75]. Therelease of oil droplets from the piston crevice at the end ofthe compression stroke has also been observed by Kassai etal. [76]

Lauer et al. [77] confirmed that pre-ignition frequentlyleads to follow-up events. Their optical results suggested that

the first pre-ignition is triggered by oil/fuel droplets whilesubsequent events are initiated by hot particles. One plausi-ble scenario is that the first event causes pressure oscillationsthat detach deposits from the combustion chamber walls.These deposits persist and are heated during the followingcycle to serve as ignition sources. Similar findings have beenreported by other researchers [78,79]. Wang et al. [80] usedheated carbon particles as surrogates for detached depositsto initiate pre-ignition. The results indicated that hot solidparticles must be sufficiently large to initiate a pre-ignitionevent, since the incipient flame must reach a critical radiusbefore it becomes self-sustaining [81].

After the accumulated and diluted oil is released fromthe piston crevice or the deposits are detached from the com-bustion chamber wall, considerable time is required for thesematerials to accumulate again. Therefore, pre-ignition occursvery occasionally and rarely.

3.3 Suppression of Pre-ignition and Deto-Knock

Because pre-ignition causes deto-knock, the most direct andeffective means of suppressing deto-knock is to preventpre-ignition. The main factors affecting pre-ignition are oilingress into the combustion chamber, deposit formation andmixture reactivity. Therefore, specific, practical approachesare related to improving oil properties, fuel properties, enginedesign factors and operating parameters.

3.3.1 Oil Properties

1. Base StocksIn general, less reactive base stocks, i.e., those withlong ignition delays, appear to have the lowest igni-

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tion tendency [82]. This is supported by the work ofTakeuchi et al. [83], who found that a tendency towardLSPI correlates well with the auto-ignition temperaturesof lubricants. Among the various physical and chemi-cal properties of base stocks, viscosity shows the highestcorrelation with pre-ignition events. Andrews et al. [84]found that engine oils formulated with the most vis-cous base stocks produced the highest frequency of LSPIevents. In addition to viscosity, volatility may also playa role in the occurrence of pre-ignition. Morikawa etal. [85] presented that oils with higher distillation tem-peratures led to more frequent pre-ignitions.

2. AdditivesAmong the metal-based oil additives, Ca is widely con-sidered to work as a pre-ignition promoter [86–93],independent of the type of Ca used in the detergent[92]. Ca-based additives act in almost direct proportionto the concentration of Ca contained in the formulation,although the effect of Ca on pre-ignition also depends onthe fuel type. Kassai et al. [86] observed that Ca additivesonly had an effect in premixed PRF/air mixtures, espe-cially with lower research octane number (RON) values,while no noticeable effect was observed with a premixedmethane/air mixture.

In contrast to Ca, Zn and Mo added to the oil inthe form of zinc dialkyldithiophosphate (ZnDTP) andmolybdenum dialkyldithiocarbamate (MoDTC), respec-tively, are regarded as pre-ignition inhibitors [83,86,88,89,91–94]. However, Hayakawa et al. [95] reported thatadding ZnDTP and MoDTC up to 800 and 980 ppm tothe fuel had no effect on fuel auto-ignition.

Other metals, such as Mg and Na, are reported notto show any remarkable effect on pre-ignition [87,90,92].However, in the presence of Ca, Na can also function as apre-ignition promoter and appears to be more active thanCa alone [92].

In addition to typical additives, wear metals canalso affect pre-ignition. Hirano et al. [88] found that theaddition of Fe and Cu compounds clearly modified thepre-ignition frequency, which was explained by the cat-alytic effect of these metals.

3.3.2 Fuel Properties

Fuel composition and properties have been shown to havea relatively large impact on the frequency of pre-ignitionoccurrence [75,96–99]. It is clearly evident and well estab-lished that aromatic content has a large positive correlationwith the pre-ignition frequency [96,99], although the associ-ated mechanism is not well understood. Mansfield et al. [99]speculated that poor vaporization and incomplete combus-tion of low-reactivity aromatic compounds could result in

the increase in deposits and soot formation, providing localignition sites.

Higher volatility was confirmed to be helpful in termsof reducing the pre-ignition frequency in previous research[94,100–102]. Chapman et al. [100] demonstrated that pre-ignition is well correlated with both T50 and T70 of the fuel.Fuels that had a low evaporation percentage within the dis-tillation range of 120–130 ◦C were associated with a highpre-ignition frequency. This temperature range is close tothe estimated range of the piston crevice metal temperature.Therefore, more fuel would be accumulated in the pistoncrevice in the case of fuels with lower evaporation percent-ages, leading to a higher pre-ignition frequency. A similarfindingwas reported byMayer et al. [102],who demonstratedthat the pre-ignition frequency exhibits a very good correla-tion with the final boiling point.

3.3.3 Engine Design Factors and Operational Parameters

Because pre-ignition is induced by the auto-ignition offuel-diluted oil droplets released from the piston crevices,reducing the spray wetting that forms oil films on the linerwall and limiting the accumulation of oil in the crevices canprevent pre-ignition.

Zahdeh et al. [97] found that piston wetting significantlyreduced the pre-ignition frequency compared with liner wet-ting. Furthermore, when piston wetting was combined with aflat top piston, no pre-ignitionwas observed at different injec-tion start and fuel pressure values while varying the sparktiming in conjunction with double injection. These resultssuggest that avoiding linerwetting is themost effectivemeansof preventing pre-ignition. Increasing the in-cylinder chargemotion can offset liner wetting because it reduces the spraypenetration depth and removes droplets near the liner [101].

Another approach to limiting spray wetting is to adjust theinjection timing and use split injection, which reduces thefuel-oil interactions by shortening the spray penetration [97,101,102]. Furthermore, optimizing the split injection timingand ratio for a double-injection strategy, Wang et al. [103]effectively reduced the pre-ignition frequency. In addition,the fuel consumption rate, exhaust temperature and emissionswere not deteriorated, as shown in Fig. 7.

Lowering the in-cylindermixture reactivity (i.e., by reduc-ing the pressure, temperature and composition in the cylin-der) and thus inhibiting auto-ignition is also an effectivemeans of reducing pre-ignition. Increasing the intake pres-surewill further increase the pre-ignition frequency [97,104],therefore boosting due to downsizing faces the problem ofpre-ignition and deto-knock. The effect of the intake temper-ature on pre-ignition is not as significant as that of pressure.An investigation by Zaccardi et al. [75] showed that, whenusing a low RON fuel with a minimal concentration of heavycomponents, the pre-ignition frequency increased along with

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Fig. 7 Deto-knock frequency,exhaust temperature and fuelconsumption values for differentinjection strategies [103]

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intake temperature. However, in the case of a high RON fuel,the pre-ignition frequency did not show an obvious correla-tion with the intake temperature and could even increase atlower temperatures. To summarize [75], a higher intake tem-perature may promote auto-ignition, but the same effect canalso be experimentally observed when lowering the intaketemperature. Low temperatures have a positive effect on igni-tion delay for pure gaseous mixtures, but can also promotethe formation of liquid films and rich gas mixtures, thus pro-ducing significant local decreases in ignition delay.

EGR can effectively reduce the occurrence of pre-ignitionand deto-knock because of combustion cooling. Amann etal. [105] and Zaccardi and Escudié [75] observed that evensmall amounts of cooled EGR (10%) could significantlyreduce the likelihood of pre-ignition and the intensity of deto-knock. It was also demonstrated that adding EGR allowsBMEP to be increased substantially while maintaining orreducing the level of pre-ignition risk. Increasing fuelingrates by 15% and BMEP by 17% was possible by operat-ing an engine with 10% EGR, while reducing pre-ignitionfrequency by roughly 80% and deto-knock intensity byapproximately 30% over the baseline, non-EGR condition[105].

Scavenging is another practical method for cooling com-bustion chamber walls. In boosted GDI engines, the scaveng-ing process can be controlled using VVT technology. Luoet al. [106] and Wang et al. [103] demonstrated the effective-ness of scavenging at high load in a turbochargedGDI engineusing appropriate control of VVT parameters. It is also evi-dent that deto-knock can bemitigated evenwhen pre-ignitionevents are not eliminated completely.

Varying the coolant temperature to suppress pre-ignitionhas a different or even opposite effect. Dahnz et al. [71] andZahdeh et al. [97] showed that the pre-ignition frequencydecreases with the increase in the coolant temperature. Onthe contrary, Amann et al. [96] reported that pre-ignition fre-

quency increases at higher coolant temperatures. The key tothe effect of coolant temperature on suppressing pre-ignitionis whether the coolant plays a dominant role in defining thein-cylinder temperature or in the evaporation of the fuel.

In summary, there are many factors affecting pre-ignitionand deto-knock. For different engines, the cause of pre-ignition may not be identical, so there may not be commonfactors and suppression measures. The key parameters musttherefore be considered on an engine-by-engine or evencylinder-by-cylinder basis [75].

4 Soot and Particulate Matter

This section reviews the studies on in-cylinder soot formationand particle emissions in the exhaust of GDI engines. Theapplication of gasoline particle filter (GPF) after-treatmentsystem is discussed.

4.1 In-Cylinder Soot Formation and Reduction

The chemical and physical processes involved with soot for-mation are complicated and quite constant regardless of theflame or fuel types [107]. These processes include the for-mation of soot precursors, such as C2H2 and polyaromaticshydrocarbons (PAHs), the nucleation or inception of par-ticles, the formation of large primary particles by surfacegrowth or coalescence, the formation of agglomerates, andsoot oxidation [108]. In GDI engines, soot generation stemslargely from three sources. (1) Pool fire because of sprayimpingement on the piston or wall [109], (2) imperfect mix-ing during stratified charge combustion [110,111], and 3)diffusion combustion of liquid fuel droplets near the injector[112]. Figure 8 shows the pool fire and diffusion combustionin a GDI engine. The main soot source in the homogenousmode is pool fire, while in the stratified mode the sources are

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Fig. 8 a Soot generated by poolfire [109], b diffusioncombustion [112] (Courtesy toSAE International)

Fig. 9 Effect of injection pressure on droplet size [113] (Courtesy toSAE International)

the locally rich mixture and pool fire. The soot generated bythe pool fire may persist until late in the cycle, while the oxi-dation reactants such as OH are consumed during the exhauststroke, leading to a slow oxidation rate.

There aremanymeasures that can reduce soot formation inthe cylinder. High-quality fuel and air mixing is regarded as afundamental strategy to reduce soot generation. To improvethe atomization quality, high injection pressures up to 35MPahave been utilized in modern GDI engines. Hoffmann etal. [113] showed that the Sauter mean diameter (SMD, D32)continually decreaseswith increases in the injection pressure,as shown in Fig. 9. However, there is also a report [114] indi-cating that the effect of reducing the SMD is much less whenthe injection pressure exceeds 20MPa. The injector designmay have different effects in this regard and more in-depthresearch is required.

4.2 Particulate Matter Emissions

The number concentration, mass concentration and massfraction of particles emitted from GDI engines all varyremarkably under the different conditions. In 2007, the JapanPetroleum Energy Center and other agencies measured theparticle number size distributions in different engines andvehicles with varying after-treatment technologies, and ana-lyzed the PN size distribution curves, as presented in Fig. 10

Fig. 10 PN size distributions of different engines and vehicles [115](Courtesy to SAE International)

[115]. It can be seen that the PN size distribution obtainedfrom direct injection spark ignition (DISI) gasoline vehiclesis unimodal with a peak particle size of approximately 85nm.Due to the fuel rich zone in lean burn DISI engines, the PNis approximately 10 times higher than that of stoichiometricDISI engines and much higher than that produced by multi-point injection (MPI) gasoline and diesel particulate filter(DPF) vehicles.

The authors studied the PN size distribution of particulatematter emitted from aGDI engine and a PFI engine operatingat 2000 r/min and at different loads (25, 50 and 75%) [116],with the results presented in Fig. 11. These data indicate thatthe PN size distribution of the GDI engine had a bimodaldistribution. In addition, the nuclei particle matter size wasgenerally less than 30 nmwhile the PN peak value was in therange of 10–20nm. The accumulated particle size was nor-mally between 30 and 110nm, with a PN peak value between60 and 90nm. The PN concentration of the PFI engine wasmuch less than that of the GDI engine, but the PN peak valuewas in the range of 125–132nm, larger than that of the GDIengine.

In 2007, Price et al. [117] studied the composition andmicroscopic morphology of particulates emitted from a GDIengine (see Fig. 12) using a fast particle mobility size spec-

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Fig. 11 Comparison of PN sizedistributions between GDI andPFI engines [116]

Fig. 12 Morphologies of different particulate matters [117] (Courtesy to SAE International). aMicrostructures of nuclei particles, bmicrostructuresof accumulated particles

trometer (DMS500) and transmission electron microscopy(TEM). It was found that the nuclei particle diameters werein the range of 30–50nm, and consisted of smaller, 5–10nmparticles. The accumulated particles were in the range from200 to 500nm and were composed of 30–80nm particles.

4.3 GPF After-treatment for PM Reduction

4.3.1 Layouts of GPF with TWC

Gasoline particulate filters (GPFs) and three way catalysts(TWCs) are two indispensable after-treatment technologiesfor GDI engines intending to meet Euro 6/China 6 emissionsregulations. A GPF can be located in a closed coupled orunderfloor position, as shown in Fig. 13 [118]. Bare GPFsare often placed underfloor, while coated GPFs are primar-ily located in the closed coupled position after the TWC. Inextreme cases, a traditional TWCmay be directly replaced bya coatedGPF, but this is rare at present. TheGPF position canaffect the particulate matter filtration efficiency, and a GPFpositioned underfloor will have a filtration efficiency about15% higher than that in a closed coupled position. This is dueto the lower volumetric gas flow rate caused by the reducedexhaust temperature [119].

Generally, the PN emissions limit of 6× 1011 #/km in theEuro 6/China 6 regulations for GDI engines can be readilyattainedwith the application of a closed coupledGPF in asso-

Fig. 13 Possible layouts of a GPF and TWC [118] (Courtesy to Asso-ciation for Emission Control by Catalyst)

ciation with both the new European driving cycle (NEDC)and high dynamic aggressive driving cycles [120]. However,the tailpipe gas emissions may tend to increase over the life-time of the vehicle due to the aging effect of the close-coupledTWC. Applying a coating to a GPF installed in the under-floor position results in both additional catalytic activity toassist in the conversion of gases and improvements in the PNfiltration efficiency [121]. A study [122] with a coated GPF(CGPF) using two different GDI vehicles demonstrated thata TWC and CGPF after-treatment system improved tailpipe

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Fig. 14 PM filtrationmechanism (left-GPF;right-DPF) [119] (Courtesy toSAE International)

gas emissions, especially the NOx emissions. Even thoughCGPF has the same function as TWC and can reduce gaseousemissions such as THC, CO and NOx, a TWC is still indis-pensable [123] becauseCGPF leads to a significantly delayedlight-off during cold start operation compared with conven-tional TWC.

4.3.2 Differences Between GPF and DPF

The development of GPF technology is based on diesel par-ticulate filters (DPFs), which have been widely used since1990s in the USA. Although the structure and principle of aGPF is similar to that of a DPF, differences in the raw partic-ulate matter emissions and exhaust conditions between GDIand diesel engines result in different filtration and pressuredrop performances. GDI engines have lower engine-out PMemissions than diesel engines, and much less soot is accu-mulated in conjunctionwith higher exhaust temperatures andflow rates in the GPF, as seen in Fig. 14 [119]. This leads tolower particle filtration efficiency and a higher pressure dropin the GPF.

Because the exhaust temperature of aGDI engine is gener-ally much higher than that of diesel engines with dynamicalexhaust temperature heating and cooling, a GPF materialmust be chosen considering coefficient of thermal expansion(CTE) and thermal shock resistance performance. Currently,cordierite, silicon carbide (SiC) and aluminum titanate arethe most common materials used in ceramic wall flow DPFsfor heavy duty diesel engines [124]. Among these threeceramics, SiC has a higher CTE but inferior thermal shockperformance compared with cordierite, while aluminumtitanate may decompose at high temperatures. Therefore,cordierite is most frequently used as a substrate for GPFs.

4.3.3 PM Filtration and Regeneration in GPFs

Because of the high exhaust temperature in GDI engines,the passive regeneration of GPFs will likely occur duringdeceleration in conjunction with a fuel cut strategy, result-ing in an oxygen concentration of approximately 20% in theexhaust. Passive soot oxidation involves conditions underwhich soot is oxidized during normal engine operation with-out an active change in the engine control to create conditions

Fig. 15 Results of fuel cut experiments with a vehicle incorporatinga TWC-coated GPF in a close-coupled position [125] (Courtesy toSpringer)

more favorable for soot oxidation [125]. Figure 15 showsa typical passive regeneration of a GPF resulting from afuel cut during deceleration, as well as the consumption ofoxygen due to soot oxidation. Soot regeneration can occurat exhaust gas temperatures in the vicinity of 500 ◦C evenwith little available oxygen content in the exhaust [126,127].The GPF location has a minimal effect, and the particu-late matter regeneration can be expected during fuel cut atthe closed coupled and underfloor positions during NEDC[119]. Although the passive regeneration of GPFs can beaccomplished utilizing the fuel cut strategy, there is still someconcern as to whether active regeneration is needed becauseof the relatively low temperatures during the most commonmoderate city driving conditions [128]. Due to this issue,some GPF control strategies include the use of soot estima-

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tion algorithms, regeneration assistance and GPF protectionto meet the regeneration requirements [129]. The estimationof GPF soot load relies on a combination of open loop sootestimation, which is based on regressions involving enginecoolant temperatures and total fuel quantities, and pressuredrop-based soot estimation, which requires a pressure sen-sor in the GPF exhaust system. The regeneration assistanceapproach can involve a spark retard to increase the exhausttemperature, while GPF protection controls the oxygen flowto limit the soot burn rate.

5 Injector Deposits

A review by Xu et al. [130] in 2015 summarized the existingliteratures on GDI injector deposits, including the formationmechanism and factors affecting deposits, their effects onengine performance and emissions and approaches to miti-gating deposits. However, due to increasing concerns regard-ing PN emissions, many additional studies have been carriedout recently. Therefore, the main focus herein is on recentstudies regardingGDI deposits, including optical and numer-ical characterization of spray and mixture formation, engineperformance and emissions using deposited GDI injectors.

5.1 Regions and Characteristics of Injector Deposits

Typical terms of reference for injector deposits by scan-ning electron microscope (SEM) are shown in Fig. 16 [131],including the internal hole, external hole (counter bore), ball,seat and tip outer surface. Film-like depositswith a crystallinestructure are formed at the needle seat and ball. In addition,deposits have been observed in both internal and externalinjector holes, with deposits at the counter bore axially dis-tributed and tending to increase in density along one side ofthe hole. The deposits at the injector outer surface tend toexhibit a smooth topography, and deposits accumulate morereadily at the injector counter bore rather than the internalhole. Increasingly, the research focus is on the tip depositsdue to tip wetting because these are a key factor determiningwhether the latest emission regulations can be met.

5.2 Effect of Injector Deposits on EnginePerformance

The positions where the deposits form will determine theimpact on the spray process. Deposits inside the GDI injectornozzle can modify the surface roughness of the internal holeand decrease the actual injector hole diameter, which in turnaffects the flow area and increases the length/diameter ratio.Song et al. [132] investigated the spray behavior of a six-hole GDI injector with internal deposits and found that theflow rate of the coked injector was decreased by about 10%

Fig. 16 Terms of reference for injector deposits [131] (Courtesy to SAEInternational)

due to the deposits. They further reported that the presenceof deposits could reduce the penetration length and increasethe spray cone angle. Jiang et al. [133] demonstrated thatdeposits were primarily located in the counter bore, whilevery few were formed inside the hole. A decrease in 2.21%in the fuel flow rate at a 150 bar injection pressure wasobserved. In addition, Jiang’swork found that the penetrationlength and mean droplet size of each jet were increased sig-nificantly, in contrast to Song’s observations. An increasedpenetration length was also observed by Henkei et al. [134].Wang et al. [69] indicated that deposits inside the counterbore could restrict air recirculation and entrainment, lead-ing to lower exiting turbulent kinetic energy of the sprayfrom a coked injector and producing a larger mean dropletsize. Wang et al. [135] studied the effect of injection strategyon the spray behavior of a fouled GDI injector. The resultsshowed that deposits led to a smaller cone angle and longerpenetration, as well as degraded atomization in conjunctionwith a single-injection strategy. With a split injection strat-egy, the potential for fuel-wall impingement was reducedbut the atomization quality was further degraded. How-ever, under strong flash boiling conditions, this reductionin atomization quality was nearly eliminated. It should benoted that the deposit location was not well described inRef. [135].

GDI deposits can also significantly affect engine perfor-mance and emissions. Joedicke et al. [136] performed anaccelerated deposit accumulation test at 5 bar BMEP and2000 rpm engine speed for 55h, and the results showed thatthe injector pulse width, fuel consumption rate and HC andCO emissions were increased by 23.5, 2.45, 20 and 93%,respectively. Results obtained byWen et al. [137] with a GDIvehicle having a mileage of 13,000km showed that particu-late mass and fuel consumption were increased by 376 and3.02%, respectively, comparedwith the values obtainedusingfresh injectors. In addition, gaseous emissions such as THC,non-methane hydrocarbon (NMHC) and NOx increased by17.1, 24.5 and 23.4%, respectively, and PN emissions couldbe increased by several orders of magnitude [133,134]. Thefuel stored in the fouled injector tip deposits during injection

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and then released later caused diffusive combustion eventsand thus increased PM and PN emissions [112].

5.3 Deposits Mitigation

According to the mechanism by which GDI deposits areformed, these deposits can be mitigated by several means.One approach involves using a fuel detergent. Zhang etal. [138] measured engine emissions before (base fuel) andafter using detergent (additivated fuel) and found that thedetergent greatly reduced the particle emissions, especiallyat high engine loads. Reducing the tip temperature is alsoeffective.However, lowering the injector tip temperaturemayhave a negative effect on the spray pattern and combustionstability [139]. Injector designs that lower the sac volumeor use valve covered orifices (VCOs) provide benefits interms of controlling the residual fuel [140,141]. In addition,a step hole or counter bore in the outlet side can decreasethe effects of deposit formation on the spray process [141].Increasing the fuel injection pressure has also been provento be an effective approach reducing deposit formation byincreasing the deposit removal rate [142]. The formation ofGDI injector deposits is dependent on the environment towhich the injectors are exposed, including tip temperatureand air motion. Thus, a typical spray-guided GDI systemwill show more significant injector tip wetting due to therelatively low air motion near the injector tip, in contrast toair-guidedGDI systems [2]. Additional optimization of com-bustion systems could therefore also be beneficial in termsof removing deposits.

6 Gasoline Compression Ignition

This section introduces the latest researches on gasoline com-pression ignition (GCI) mode with different octane numberfuels and the application of GCI modes in engines.

6.1 Motivation

GCI based on fuel direct injection, which is a promising com-bustion mode, has been widely investigated in recent yearsto improve thermal efficiencies. Gasoline readily forms suit-able pre-mixtures prior to ignition, which tends to reducesoot emissions. Gasoline homogeneous charge compressionignition (HCCI) has been considered to represent an idealcompression ignition mode since the 1970s [143]. How-ever, the difficulty in controlling the combustion phase andhigh combustion noise at high loads has limited the appli-cation of HCCI in actual engines [144]. In recent years,most studies have examined stratified charge compressionignition as a means of extending the engine load. Kalghatgiproposed a partially premixed compression ignition (PPCI)

Fig. 17 Injection strategies for HCCI, PPCI and MPCI

mode in 2006 [145]. In this process, an in-cylinder stratifiedcharge is used to reduce the pressure rise rate by inject-ing fuel near TDC [146]. Even so, the combustion noiseremains high at high loads, and a significant amount ofEGR is required to suppress NOx emissions. To mitigatethese shortcomings, Yang et al. proposed a multi-injectionstrategy to achieve a process based on “spray-combustion-spray-combustion” referred to as multiple premixed com-pression ignition (MPCI), as shown in Fig. 17 [147]. Ithas been demonstrated that MPCI involves both temporalmultistage combustion with different injection timings andspatial multi-zone combustion [148]. The first stage combus-tion involves premixed low temperature combustion, whilethe second is based on quasi-premixed high-temperaturecombustion. MPCI results in lower combustion tempera-tures and reduced pressure rise rates compared with PPCI,leading to lower NOx emissions and less dependency onEGR.

6.2 Low Octane Fuels for GCI

Kalghatgi [149] pointed out that the realization of premixedcombustion is quite difficult if the ignition delay of the fuel(e.g., conventional diesel) is short. In contrast, if the fuel has along ignition delay (e.g., conventional gasoline), combustionis difficult to initiate under certain operating conditions, suchas cold starts. Therefore, fuels with reactivity higher thanthat of gasoline but lower than that of diesel are desired. Theideal fuel for compression ignition appears to be a low octanenumber gasoline with a research octane number (RON) valuebetween 70 and 85 [149].

There are two viable means of obtaining low octane num-ber fuels. The first is to blend market gasoline and dieselfuels. These two fuels can be blended offline in the tankand are also referred to as wide distillation fuels [150] or“dieseline” [151,152]. Algunaibet et al. found that a blendof 20% (v/v) gasoline with diesel was sufficient to reducethe flash point below 40 ◦C and that diesel addition below50% (v/v) had a slight effect on the vapor pressure of gaso-line [153]. Therefore, gasoline can be safely blended withdiesel at lower ratios. The second approach involves the useof low octane number gasoline-like fuels, such as naphtha.Naphtha fuels normally have a 60–85 RON value and are

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Fig. 18 Combustion and injection systems in third generation GDCI engines [155] (Courtesy to SAE International)

mainly composed of straight-chain paraffins and naphthene,with a distillation range similar to that of gasoline (fromapproximately 20 to 200 ◦C) [154]. The use of naphtha alsosimplifies the oil refining process, leading to a less expensive,more environmental-friendly system.

6.3 Application of GCI

6.3.1 Delphi GDCI Engines

Delphi and partners have developed three generations ofmulti-cylinder gasoline direct injection compression igni-tion (GDCI) engines using US market gasoline since 2011(see Fig. 18) [155]. All three generations are operated inthe PPCI mode with a precisely optimized injection strategyusing RON91 E10 gasoline.

Compared with the first two generations, the third gener-ation of engines increases the piston stroke from 85.2 to 105mm, thus achieving a long stroke-bore ratio (S/B = 1.28)to increase TDC clearance. In addition, the use of compres-sion ratios from 15 to 16 improves the part-load combustion.The injector is central-mounted and the injection pressurecan be greater than 350 bar. The piston bowl geometry iscarefully matched with the spray characteristics to ensure awet-less injection process. While the intake valve train liftis fixed, the exhaust valve train has variable lift and tim-ing to control exhaust rebreathing, which is beneficial forpart loads. In addition, a thermal management system andan air system including a variable nozzle turbocharger, asupercharger and a two-path cooled EGR system have beencarefully designed. Based on these systems and the reductionin engine-out NOx and soot emissions by incorporating anafter-treatment system, a minimum BSFC of 205g/kWh wasachieved at aBMEPof 10 bar. This engine thus exhibits betterfuel economy than the first two generations and approachesthat of diesel engines, as shown in Fig. 19.

6.3.2 Mazda SKYACTIV-X-Engine

In 2017,Mazda launched the SKYACTIV-X engine featuringspark-controlled compression ignition (SPCCI), thus com-bining the benefits of spark and compression ignition. Themain SPCCI concept is presented in Fig. 20. In this system,

Fig. 19 BSFC variations in GDCI engines [155] (Courtesy to SAEInternational)

the geometric compression ratio is set such that the air-fuelmixture is at the ignition boundary when the piston is atTDC. The spark plug works under all conditions to create anexpanding fireball upon spark ignition that increases the localpressure and temperature to initiate compression ignition.Hence, by controlling the spark ignition timing, the com-pression ignition process can be tuned. Based on seamlessswitching between spark and compression ignition, a HCCIcombustion mode could be very widely applied.

This system allows a double-injection strategy in conjunc-tion with a high-pressure injection system operating at 500bar and an injector mounted at the center of the combustionchamber. A lean mixture for compression ignition is initiallygenerated inside the cylinder during the intake stroke, afterwhich a sufficiently rich air-fuel mixture forms around thespark plug for spark ignition. A strong swirling motion isalso produced in the cylinder to enhance fuel vaporization.The piston crown has a volcano-shaped bowl to tumble therich mixture near the spark plug. Based on the low exhaustenergy, a supercharger with a clutched belt drive is used,which is capable of switching between natural aspirated andboosted operation. The EGR can be cooled using a coolantheat exchanger and the application of VVT enables overlapat the end of the exhaust stroke to scavenge hot gases fromthe cylinder.When necessary, theMiller cycle can be appliedby closing the intake valves after BDC.

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Fig. 20 Combustion concept and fuel consumption reduction for the Mazda SCCI [157] (Courtesy to Mazda)

The current prototype engine can reach 133kW and230N·m at 6000 r/min using regular gasoline. The fuel econ-omy is improved by approximately 20% compared withSKYACTIV-G, the new Mazda gasoline engine, and evenequals or exceeds that of Mazda’s latest diesel engine, theSKYACTIV-D under some conditions [156].

7 Summary and Outlook

Over the past 20years, GDI engines have been improved tobecome highly efficient and to offer clean combustion, as aresult of high tumble flow design, cooled EGR, high com-pression ratios, over-expansion cycles with VVT, boosting,high-pressure injection and other design features. Improv-ing the fuel/oil properties and optimizing the combustionsystem have been found to represent the most effectiveapproaches to suppressing deto-knock, and in-cylinder sootformation has been primarily attributed to fuel impinge-ment and pool fire phenomena. Research has shown thathigh injection pressures and optimized mixture formationcan greatly reduce soot formation in the cylinder, and thatthe PM/PN levels in GDI engine exhaust are much higherthan those generated by PFI engines, particularly during coldstarts. GPF combined with TWC is evidently essential tomeeting upcoming emissions regulations. Deposits on GDIinjectors have been confirmed to have a significant impacton PM/PN emissions, and fuel quality plays an importantrole in the formation of deposits. Reducing the injector tiptemperature, using proper additives, increasing the injectionpressure and optimizing the injector design have all beenfound to be effective in reducing deposits. Thus, comprehen-sive co-optimization involving fuel reforming, combustionsystem and after-treatment design has the potential to furtherincrease thermal efficiency and decrease emissions.

It is clear that hybrid systems will represent the future oflight-duty power trains as fuel economy regulations becomemore and more stringent and that GDI engines with stoi-chiometric combustion will be widely employed in hybridpassenger vehicles. The BTE values of hybrid GDI enginescan be further enhanced by over 40% based on adopting

a simpler structure that also lowers cost due to its narrowoperational region and reduced power output compared withconventional GDI engines.

GCI with lean burn and high CR values offers signifi-cant advantages with regard to fuel economy. It is believedthat, in the case of future engines with peak BTE values inthe range of 45–50%, the diluted lean burn GCI combustionstrategy will be a viable option. Difficulties related to thecontrol of GCI engines are eliminated in GCI engine hybridsincorporating electric motors. The after-treatment for GCIengines is evidently quite complicated and may require anintegrated system involving an oxidation catalyst, selectivecatalytic reduction of NOx or a lean NOx trap and GPF tomeet future emissions regulations.

In future, connected vehicles powered by GDI engineswill be smart, resulting in a 10 to 20% reduction in fuel con-sumption and much fewer harmful emissions under actualdriving conditions [158]. Research is underway to developartificial intelligence (AI)-based engine calibration and con-trol [159,160]. This development will move forward in threesteps: (1) from manual calibration to AI-based calibration,(2) from map-based to model-based prediction control, and(3) from model-predictive control to model-free machinelearning-based control. This development trendwill progressin concert with the advent of V2X (vehicle to everything)technology involving big data and cloud computing.

Acknowledgements The authors acknowledge theChinaNationalNat-ural Science Foundation Project “Formation and Evolution of PM fromGDI Engines: From Primary Particles to Secondary Aerosols” (GrantNo. 51636003), and the National Key R&D Plan Project “IntegrationTechnology of PM Capture and Clean Emissions for GDI Vehicles”(Grant No. 2017YFC02110004). The authors also wish to thank theircolleagues Prof. Jianxin Wang and Prof. Zhi Wang as well as Ph.D.students Shuai Liang, Zhou Zhang, Wenbin Zhang, Zexian Guo andHengjie Guo for their contributions to this review.

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