Gasoline Engine Exhaust Gas Recirculation: A Review of Performance, Emissions, and Applications

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Gasoline engine exhaust gas recirculation – A review

Haiqiao Wei, Tianyu Zhu, Gequn Shu

^⇑

, Linlin Tan, Yuesen Wang

State Key Laboratory of Engines, Tianjin University, 92 Weijin Road, Tianjin 300072, China

a r t i c l e i n f o

Article history:

Received 25 November 2011

Received in revised form 24 March 2012 Accepted 6 May 2012

Available online 10 June 2012 Keywords:

Gasoline engine EGR

NOX

Stratiﬁed EGR Knock

a b s t r a c t

EGR technique, as one of effective measures to reduce NOXformation, was ﬁrstly adopted in diesel engines. But with the growing energy and environment problems, EGR has been commonly used also in gasoline engines together with other advanced techniques. Since there hasn’t been any comprehensive review on gasoline engine using exhaust gas recirculation, this paper is made by its motivation. Recircu- lating exhaust gas on gasoline engines is employed primarily to reduce throttling loss at part load range in order to reduce fuel consumption, and secondarily, to reduce NOXemission levels. In addition, EGR can replace fuel enrichment in gasoline engine to inhibit knock. The aim of this paper is to review the inﬂu- ence of EGR on the performance and emission of gasoline engine as well as to compare the application of EGR on GDI engines and on PFI engines. Furthermore, a detailed analysis of comparison between cooled EGR and hot EGR, the effect of EGR on knock suppression and the implementation of EGR on turbocharged gasoline engine are introduced. From the deep analysis, EGR can improve fuel economy, reduce NOX

emission and inhibit the tendency of engine towards knock. However, the maximum possible EGR rate is limited by high cyclic variations, misfire, the decrease of total efficiency and the increase of HC emission. This problem can be solved by stratified EGR system. The combination of EGR technique with supercharged direct injection engine is the trend of today’s gasoline engine.

Contents

1. Introduction . . . 534

2. EGR system . . . 535

3. EGR vs. NOX . . . 536

4. EGR in GDI engines and in PFI engines . . . 536

5. Stratified EGR . . . 537

6. Comparison between cooled EGR and hot EGR . . . 538

7. Knock suppression using cooled EGR . . . 538

8. Implementations of EGR for turbocharged gasoline engines . . . 541

9. Conclusion . . . 543

Acknowledgment . . . 543

References . . . 543

1. Introduction

In engine evolution, EGR technique was ﬁrstly adopted in diesel engines in order to limit thermal NOXformation rate by reducing combustion chamber temperature thanks to the dilution of fresh charge with a certain amount of exhaust gas recycled at engine intake. But with the growing issues of energy and environment, var-

ious countries in the world have developed more stringent regulations on vehicle emission and fuel economy, which drive gasoline engine towards downsizing direction[1–3]. Engines with turbo-charged spark ignition are becoming increasingly popular in the world market due to their compactness and high power density. However, due to the high power density of turbo-charged engine, knock combustion and high temperature of exhaust gas at high loads constitute problems[4]. The increasing temperature may also cause the problem of NO_Xemission increase. Accordingly, EGR is commonly used also in gasoline engines together with other advanced techniques[5,6].

http://dx.doi.org/10.1016/j.apenergy.2012.05.011

⇑Corresponding author. Tel./fax: +86 022 27891285.

E-mail address:[email protected](G. Shu).

Contents lists available atSciVerse ScienceDirect

Applied Energy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p e n e r g y

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Unlike diesel engines,kcan be actually used in gasoline engines to reduce NO_X emission effectively. Therefore, recirculating exhaust gas on gasoline engines is used primarily to reduce throttling loss at part load range, thus reduce fuel consumption, and secondarily, to reduce NOXemission levels[4]. In order to keep the same torque and power output after introducing EGR in gasoline engine, further opening of engine throttle is necessary to increase the trapped charge density, which can reduce pumping loss and to increase fuel economy compared with that when no EGR is used. On the other hand, gasoline engine works with a stoichiometric air–

fuel mixture in order to meet the need of TWC, so the O2concen- tration in the exhaust is very low, which will cause a corresponding lower NOXemission.

For a GDI engine that is designed to work with lean or even ultra-lean mixtures, it is unfortunately true that a conventional TWC cannot be used to remove NOX,therefore other techniques for in- cylinder NOX reduction or exhaust after-treatment must be employed. EGR is an effective measure. Compared with PFI engine, EGR rates as high as possible are required by GDI engine to reduce overall NOXemission levels. EGR is considered as a principle method to reduce NOXemission of GDI engine[7]. The application of EGR in GDI engine and in PFI engine is discussed in detail in this paper.

When hot EGR is used in engine, the exhaust can be used to heat the intake, so combustion quality and thermal efﬁciency are improved. However, cooled EGR increases inlet density and thus increases volumetric efﬁciency of the engine. At the same time, the reduced temperature can further reduce NO_Xemission, but the cycle-by-cycle variations will be increased compared with hot EGR.

Cooled EGR is a key technology enabling downsized SI engines and providing a means to meet the market requirement of ‘‘doing more with less’’[8]. The comparison of the advantages and disadvantages between hot EGR and cooled EGR is given in this paper. As it is well known, the crucial point of gasoline engine behavior is the dramatic fall of energy conversion efﬁciency in part load operation.

So, much effort has been made to decrease pumping loss and to optimize engine thermodynamic efficiency. However, knock risks impose strong limitations to the performance and efficiency of gasoline engine. So, in full load operation, fuel enrichment was usually adopted to suppress knock. EGR can reduce the combustion pressure and inhibit knock as well, and additional benefit is fuel economy as well as the decrease of HC and CO emission.

Turbocharged gasoline engine is a typical representative of downsized engines. EGR technique combining with supercharged direct injection engine is the trend of today’s gasoline engine.

Using EGR in turbocharged gasoline engine may involve compati-

bility issue, thus two implementation methods are reviewed in this paper, and both the advantages and disadvantages of these methods are compared. Furthermore, the problem of turbo matching when introducing EGR is described brieﬂy.

2. EGR system

Cylinder charge dilution with exhaust gas can be classiﬁed into internal EGR and external EGR. With external EGR, exhaust gas is taken from the exhaust port and supplied into the inlet port. Inter- nal EGR is achieved by increasing NVO during exhaust stroke, which requires an improved cam that can rapidly switch cam pro- ﬁles to achieve any variable valve timing, otherwise it’s impossible to independently and effectively control EGR ratio. This greatly limits the application of internal EGR. As a result, external EGR has become widely used on today’s automobile engines. External EGR has a relatively low cost. It only needs to use dedicated EGR control valve, which can control EGR rate effectively under all work conditions of engine[9–12]. Only external EGR is discussed in this paper.

External EGR system consists of EGR pipe, EGR valve and EGR cooler (cooled EGR). Exhaust goes through EGR valve and EGR cooler, and then enter intake manifold. Constant coolant goes through EGR cooler. EGR valve can be adjusted to get various EGR rates.

Pipe material is stainless steel to avoid transfer of engine vibration to exhaust system and then to measuring instruments[9].

Nomenclature AFR air fuel ratio

BMEP break mean effective pressure CA crank angle

CA10 crank angle where 10% of MFB CA90 crank angle where 90% of MFB CO carbon monoxide

CO2 carbon dioxide COV coefﬁcient of variations EGR exhaust gas recirculation GDI gasoline direct injection HC hydrocarbon

H2O water

HPL high pressure loop

IMEP indicated mean effective pressure k air excess ratio

ISHC indicated speciﬁc hydrocarbons ISNOX indicated speciﬁc oxides of nitrogen LPL low pressure loop

MFB mass fraction burned N2 nitrogen

NOX oxides of nitrogen NVO negative valve overlap O2 oxygen

PFI port fuel injection SI spark ignition TDC top dead center

TWC three-way catalytic converter VGT variable geometry turbine WOT wide open throttle

Fig. 1.Centralized and decentralized EGR[7].

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For multi-cylinder engine, important parameters in designing EGR systems are good homogeneous EGR distribution to each cylinder and good dynamic response. The recirculated exhaust gas can be supplied to the engines either centralized or decentralized as schematically shown inFig. 1.

For centralized exhaust gas recirculation systems, EGR valve is far away from inlet valves, normally at the entrance to intake manifold, where collector provides a good mixture of exhaust gas and fresh air and thus optimal distribution to each cylinder. The decentralized EGR system directly supplies exhaust gas to inlet valves.

Two different applications are possible: one application is one valve for each cylinder, and the application is using a multiple valve. With the two applications, it is ensured that, with closed EGR valve, cross flow is impossible between intake manifold runners. The benefits of decentralized EGR system are optimized EGR distribution and good dynamic response. However, the location and orientation of EGR port and valve may influence in-cylinder EGR distribution significantly, and thus influence fuel stratification by producing an extra flow of recirculated exhaust gas within combustion chamber. This influence must be carefully evaluated when using this application.

In addition, the sealing of EGR valve or port and assembly constraints in the vicinity of cylinder cover is also important factors that constrain the application of this EGR strategy[13].

When EGR is applied, engine intake consists of fresh air and recycled exhaust gas. EGR (%) usually represents the percentage of the recirculated exhaust gas. The percentage of exhaust gas recirculation is deﬁned as the percentage of recirculated exhaust in total intake mixture [14]. Where [mi=ma+mf+mEGR] is the mass of total intake mixture and [mEGR] is the mass of the EGR.

EGRð%Þ ¼ ðmEGR=mi100Þ ð1Þ The calculation of recirculation degree was deﬁned by relating the amount of CO2 in intake manifold with that in exhaust pipe, through Eq.(2). Where the CO2 concentrations used in the expression are such that [CO2]Ambientwas measured in the environment, [CO2]Intake gas was checked in the intake manifold and [CO2]Exhaust gaswas measured in the exhaust gases[15,16].

EGRð%Þ ¼ ½CO2_{Intake gas} ½CO2_Ambient

½CO2Exhaust gas ½CO2Ambient

ð2Þ

3. EGR vs. NOX

As it is well known, NOX is generated under the condition of high-temperature and oxygen –rich environment. The NOXemis- sion of internal combustion engine is mainly NO, but it will be oxi- dized into NO2quickly after entering air. The formation of NO is increased dramatically in exponential function with temperature increase. When temperature is below 1800 K, the rate of NO formation is very low, but it will reach a very high speed when the temperature reaches 2000 K. It can be generally considered that for every 100 K increase of temperature, NO formation rate will nearly doubles. The formation mechanism of NO is different from that of HC and CO. No is not the result of incomplete combustion of mixture. Its generation relates with the proliferation of mixed combustion, concentration distribution of ﬂame and heat transfer, thus reaction mechanism is very complex. Basically, nitrogen hardly ex- ists in gasoline. The formation of NOXoriginates from thermal reaction of N2and O2in the air in high temperature combustion. NO is formed inside combustion chamber in post-ﬂame combustion process in high temperature region. NO formation and decomposition inside combustion chamber can be described by extended Zeldob- ich Mechanism[15,17,18]. The principal reactions near stoichiometric fuel–air mixture governing NO formation from molecular nitrogen are:

N2þO()NOþN NþO2()NOþO NþOH()NOþH

ð3Þ

The initial rate controlling NO formation can be described with Eq.(4). In the expression, [NO] denotes molar concentration, and [O₂]_eand [N₂]_edenote equilibrium concentrations[19]. The sensi- tivity of NO formation rate to temperature and oxygen concentration is evident from this equation. Hence, in order to reduce NOX

formation inside combustion chamber, the temperature and the oxygen concentration in combustion chamber need to be reduced.

d½NO

dt ¼ 610¹⁶ T^0:5

!

exp 69;096 T

½O2^0:5_e ½N2_emol s=cm³ ð4Þ In gasoline engine, EGR is the most principal technique in reducing NO_X emission. The main constituents of EGR are N₂, H2O, O2and CO2. It is well known that CO2exerts three effects when being introduced into combustion process. They are:

(1)Thermal effect:Exhaust gas consists of gases of two-atom and three-atom, and the heat capacity of three-atom gas increases faster in combustion process. Due to the increase in heat capacity of the oxidizer, ﬂame temperature is reduced;

(2)Dilution effect, which results from the reduction of oxygen concentration in the main stream of the oxidizer and from the reduction of reactive species in combustion process, which, in turns, reduces their collision frequency;

(3)Chemical effect,since CO₂is an active species and thus partic- ipates chemically in the combustion process;

These three effects have been studied in the literature[20–22].

Engine tests have demonstrated that NOX is greatly suppressed when the O2 concentration in combustion chamber is reduced.

Fig. 2shows that the reduction in O₂concentration in the cylinder suppresses NOXemission. The ﬁgure shows the combustion in two GDI fuel sprays, one with and the other without EGR. In GDI engine, when using EGR, some O2in the cylinder is replaced with exhaust gas, and thus local O2concentration in the cylinder becomes lower.

With the local O2concentration reduced, a given amount of fuel will have to diffuse over a wider area before encountering sufﬁ- cient O2for a stoichiometric mixture to be formed. Now, for a given amount of fuel, this larger area contains not only stoichiometric mixture but also an additional quantity of CO2, H2O and N2. These additional gases absorb the energy released by the combustion, leading to lower ﬂame temperature and then lower NOXgeneration [9,23].

4. EGR in GDI engines and in PFI engines

PFI engines work under stoichiometric condition, and EGR is employed primarily to reduce throttling loss at part load range, thus to reduce fuel consumption, and secondarily, to reduce NOX

emission levels. In order to keep the same torque and power

Fig. 2.Increase in volume occupied by spray ﬂame with use of EGR.

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output after introducing EGR in gasoline engine, further opening of engine throttle is necessary to raise the trapped charge density.

This can reduce pumping loss and improvement of fuel economy compared with that when no EGR is used. The improvement in fuel consumption with increasing EGR is due to three factors: first, reduced pumping work, as EGR increases at constant brake load (fuel and air flows remain almost constant, hence, intake pressure increases); second, reduced loss of heat transferred to cylinder wall since burned gas temperature is decreased significantly; third, a reduction in the degree of dissociation in the high temperature burned gases, which allows more fuel’s chemical energy to be con- verted to sensible energy near TDC[24].

It is worth to mention that when PFI engine works at full load, the throttle has reached the WOT condition. In this case, the throttle cannot be opened more to increase intake density. Therefore, boosting intake pressure is necessary to gain the same level of torque and power output. If supercharge is not used to increase intake density, the power loss will increase with the increase of EGR ratio at full load[4]. Furthermore, in a conventional PFI engine, EGR trends to influence combustion stability and investigations have confirmed that both ignition delay and combustion period are extended with EGR due to the associated decrease in laminar flame speed. Consequently, it may be appropriate to use EGR in combination with other techniques[25,26].

In general, a larger NOXreduction can be realized with EGR in GDI engines than that in either PFI engines or diesel engines. For GDI engines, the available fuel–air mixing time is comparatively longer than that of diesel engines, and, as a result, EGR role is more effective and NOXemission can be further reduced. In a GDI engine, since there is no throttling effect, the engine works with lean or even ultra-lean mixtures and the in-cylinder AFR is higher, and the introduction of exhaust gas replaces part of fresh air directly.

For a given torque and power output, the amount of fuel that engine supplies must be constant. Therefore, the in-cylinder AFR decreases with the increase of exhaust gas, which can reduce NOX

emission effectively. Although in GDI engine, lean-NO_Xafter-treatment technologies can be used to reduce NOXemission, EGR was considered the only feasible way to reduce NOX[7]. In addition , EGR ratio as high as possible are required to keep GDI engine working under stoichiometric condition, so that TWC can work normally, and then we can use it to reduce HC and CO emission.Table 1shows the basic differences between GDI engine EGR system and PFI engine system for different applications.

The EGR application ranges of PFI engines and of GDI engines are different.Fig. 3shows the ranges of conventional PFI engines in comparison with the various working modes of GDI engines. In Fig. 3, in homogeneous mode of GDI engines, the use of recirculated exhaust gas corresponds to that of conventional PFI engines.

Due to the higher EGR tolerance of the engine, EGR rates are also higher in the case of homogeneous work. In stratiﬁed mode, EGR rates comparable to those of diesel engines are attainable and much higher than that of PFI engines. For a GDI engine using

charge stratiﬁcation, the mixture near spark gap is ideally either stoichiometric or slightly richer. Furthermore, mixture prepara- tion can be improved due to EGR heating effects. Robust combustion is therefore possible with a much higher level of EGR than that with homogeneous combustion. However, there is an associated compromise between a signiﬁcant NOX reduction and a simultaneous degradation in both HC emission and in fuel consumption due to the degradation of combustion quality [13].

Moreover, in a good EGR system, dynamic response is mandatory to avoid drivability problems in transition from one work mode to the other.

When EGR is used in GDI engine, providing appropriate amount of EGR has always been a design challenge, since a relative higher EGR mass flow rate must be metered and the flow must be distrib- uted uniformly to individual cylinders under a much lower pressure difference with that of traditional PFI engines[27]. The flow of so much EGR may require a moderate level of intake vacuum, which will cause great pumping loss, while GDI engines is supposed to reduce such loss substantially.

5. Stratiﬁed EGR

Whether in PFI engine or in GDI engine, homogeneous EGR system reduces laminar flame speed, which will cause reduce of burning speed, HC emission increase, cycle-by-cycle variations aggravation, and steady-state combustion being difficult to be achieved, and even causing fire [14,28]. High-diluted stratified EGR charge to separate air/fuel mixture in intake and compression strokes can be a good practice to generally overcome the above dif- ficulties. The stratified exhaust gas recirculation is characterized by separating EGR air and fresh air in combustion chamber. Due to a minimized exhaust gas concentration at spark plug region, flame propagation is improved compared with homogeneous EGR, EGR compatibility is increased. However, the flow structure in combustion chamber is extremely complex, so to realize complete separa- tion of air and exhaust gas is very difficult. Another challenge in this system is how to realize stratification in intake stroke and how to maintain the stratification in compression stroke prior ignition.

Different possibilities of exhaust gas stratification are shown in Fig. 4, including radial stratification, lateral stratification and axial stratification. Lateral EGR stratification, consisting of air/fuel mixture at the side of cylinder intake and of only EGR at the exhaust side, may separate them well. However, this method requires strong tumble intensity to maintain vertical flow momentum.

When the piston moves upwards to TDC, the tumble flow is easily destroyed by turbulent and/or squeeze flow. Axial EGR stratification divides the cylinder into top air zone and bottom EGR zone.

The injected fuel can penetrate into the pure air zone, mix with the air, and then be ignited and burned at this zone. In this method, a vertical flow, such as tumble flow or squeeze flow, may make the

Table 1

The basic differences between EGR systems for the different applications.

EGR System for GDI Engines EGR System for PFI Engines

Target First, reduction in nitrogen oxides First, reduction in fuel consumption

Second, reduction in fuel consumption Second, reduction in nitrogen oxides

Max. EGR rate 50%, Stratiﬁed mode

25%, Homogeneous mode

25%

Max. exhaust temperature in the operating range 450°C

(650°C, homogeneous mode)

650°C

EGR cooling Under discussion Required

Other requirements Good dynamics

Good resolution capability Good distribution

Reduction in power losses at high EGR ratios

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gases easily mix. As a contrast, radial EGR stratification seems to be the most appropriate flow structure for compression stroke. Since the central air–fuel cylinder and the outer EGR tubular cylinder are concentric with the engine cylinder, as the piston moves upwards in compression stroke, the two cylinders will be compressed in axial direction. Particularly, if both EGR and air are swirling in the same direction, the conservation of angular momentum may make the two zones readjust their interface location to reach new force balance. Radial stratification can sustain much longer time towards the end of compression stroke[29–31]. In summary, radial stratification is the most appropriate method for getting stratification of intake and exhaust.

6. Comparison between cooled EGR and hot EGR

The operation that exhaust gas is recycled to intake directly is called hot EGR, and the operation that EGR after cooling is applied to recycled exhaust is called cooled EGR[15]. The engine using hot EGR can use the high temperature exhaust to heat the intake, promote combustion and thus improve the thermal efﬁciency. While cooled EGR increases intake density, thereby increases volumetric efﬁciency of engine. At the same time, the decreased temperature can further reduce NOXemission, but HC emission and cycle-by- cycle variations are increased compared with that of hot EGR.

The comparison of the characteristics between cooled EGR and hot EGR is given inTable 2.

Fig. 5a shows COVIMEPas a function of cooled and hot EGR in percentage. The maximum EGR tolerance is deﬁned as 10% COVIMEP. As

can be seen fromFig. 5a, increasing cooled EGR fraction will increase COV_IMEP. This might be because of non-uniform mixture composition consisting of recirculated EGR, air and residual gases.

Better mixing is expected when only hot EGR is used, so the COVIMEP

is lowered than that of cooled EGR.

Fig. 5b and c shows 0–10% and 0–90% MFB durations.Fig. 5b shows the data of 0–10% MFB duration, indicating the process of flame initiation in the mixture. One can see that, with increasing EGR, EGR replaces air, the MFB duration becomes longer and the mixture becomes more difficult to be ignited.Fig. 5c shows the increased temperature caused by hot EGR enhances flame propagation, while cooled EGR does not affect the overall burn duration.

Fig. 5d and e shows HC and CO emissions as function of cooled EGR and hot EGR in percentage. HC emission map shows, the fraction of cooled EGR and hot EGR has great effect on HC emission.

Generally, increasing cooled fraction increases HC emission, while increasing hot fraction reduces the emission. This effect is mainly attributable to the decrease of combustion stability with increased cooled EGR. The heating effect of hot EGR improves the combustion temperature and then gains improvements in fuel–air mixing, therefore HC emission will be reduced. With further increasing EGR ratio, HC emission began to deteriorate, as the introduction of EGR narrows ﬂammability limits.Fig. 5e shows, NOXemission decreases signiﬁcantly with increasing percentages of cooled EGR as a result of lower combustion temperature presented in this case.

While increasing percentage of hot EGR tends to leading to slight increase in NOXemission.

Fig. 5f shows relative efficiency as a function of cooled and hot EGR in percentage. The efficiency decreases with increased cooled EGR. This can be attributed to the changes in combustion temperature, and reduced temperature leads to a reduction in combustion speed and then in thermal efficiency.

At present, despite of the advanced combustion systems and emission control systems, cooled EGR is still the most effective measure to reduce NOXemission[33]. When using EGR, the intake composition changes in a large extent. Since the exhaust temperature reduces the intake density, the exhaust gas must be cooled to maintain a high volumetric efﬁciency to avoid the increase of heat loss. Cooled EGR is a key technology in the trend of downsized SI engines, and it provides a means to meet the market requirement of doing more with less[8]. Despite of cooling process, high temperature exhaust will increase the intake temperature at high load.

Therefore, volumetric efﬁciency is reduced.

7. Knock suppression using cooled EGR

In engine revolution, EGR technique was ﬁrstly adopted in diesel engines in order to limit thermal NOX formation rate by reducing combustion chamber temperature thanks to the dilution of fresh charge with a certain amount of exhaust gases recycled at engine intake. Some studies also suggest that appropriate EGR ratio Fig. 3.EGR ranges within the characteristic map[7].

Fig. 4.EGR-stratiﬁcation modes.

Table 2

A comparison of the characteristics between cooled EGR and hot EGR.

Cooled EGR Hot EGR

Characteristics Lower NOXvalues Better knock suppression Complex structure Higher cost

Lower combustion duration Lower HC values Simple structure

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can reduce combustion noise by lower the pressure rise rate and the pressure high frequency oscillation magnitude [34,35]. But with the growing energy and environmental issues, various countries in the world have developed more stringent regulations on vehicle emission and fuel economy to drive gasoline engine to be developed towards downsizing direction. Turbo-charged spark ignition engines are becoming increasingly popular in the world market due to their compactness and high power density. How- ever, due to the high power density of turbo-charged engine, knock combustion and high exhaust gas temperatures constitute problems at high loads [19]. As a result, knock control is becoming increasingly important.

In the past century, many scholars conducted studies on knock [36–39]. Knocking combustion is generally accepted as the effect of

auto-ignition of the portion of cylinder charge where propagating turbulent flame hasn’t reached. This view had been proved by experimental evidence. The pressure/temperature of end-gas end is controlled by the displacement of piston and the expansion of combusted gases. With advanced facing of the combustion, end- gas temperature increases, and finally, the reaction rate in end- gas reaches a level at which spontaneous ignition occurs. Another view is based on the interpretation of flame propagation theory. It’s saying that knock is the result of flame acceleration in homogeneous mixture. When flame speed is higher than sound speed, knock occurs[40]. When knock occurs, pressure and heat transfer increase sharply, which will cause serious damage to engine. Nor- mally, the number of knocking cycles is on-line determined using band-pass filtered cylinder pressure signal. For each cycle, the Fig. 5.Comparison between hot EGR and cold EGR (the maximum EGR tolerance is indicated by the highlighted 10% COVIMEPcontour. All data is for lean conditions with constant fuel flow and constant diluents (air + EGR) flow)[32].

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amplitude of the signal from oscillating cylinder pressure is compared with a reference voltage. If the amplitude exceeds reference voltage, the cycle indicates knocking [41]. The paper presented here does not care about the causes of knock, but only about knock suppression methods.

Several knock suppression methods have been studied in re- lated research. These methods can be divided into three categories [42,43]. The ﬁrst is to decrease effective compression ratio or delay ignition timing to limit cylinder pressure. The second is to inject excessive fuel in the mixture to decrease AFR. The third, an alterna- tive and efﬁcient solution, emerging as a promising technology to limit knocking, is through dilution, such as cooled EGR.

To reduce exhaust temperature and to limit knock, the most commonly used method is excessive fuel to decrease combustion temperature. Under the condition of fuel enrichment, HC and CO emissions will increase, and TWC can only work under stoichiometric condition, which causes problems of fuel economy and emission. However, using EGR as a diluents allows the use of an overall stoichiometric composition of the charge, thus insures a good conversion of all emissions. Therefore, using cooled EGR instead of excessive fuel to inhibit knock and to reduce emission is an effective measure in gasoline engine[41–43].

InFig. 6a, maximum BMEP, as a function of the number of the amount of EGR and ignition angle, is presented. For these tests maximum BMEP was limited by knocking combustion and exhaust temperature. The knock detection was set to detect cycles with amplitudes of the oscillating cylinder pressure above 250 kPa, and the exhaust temperature was limited to 960°C[39]. We can see clearly that after increasing EGR ratio from 7% to 13%, ignition angle and maximum BMEP are increasing. But with further advanced ignition, BMEP will not increase, which is limited by knocking combustion. The increase of EGR ratio will reduce this limitation. Because an increase of EGR increases the mass in the cylinder, and therefore lowers gas temperature, and subsequently, more fuel can be combusted before reaching temperature limit.

Therefore, it can further widen ignition advance angle.

Fig. 6b shows cylinder pressure traces limited by knock as a function of EGR ratio. In the ﬁgure, all ignition angles are the angle before top dead center. The peak pressure tolerance of the engine is slightly increased with EGR increases. The possible reason is that the dilution and thermal effect of EGR reduce combustion temperature and inhibit knocking combustion, therefore allow a greater cylinder pressure.

With high load, the major role of EGR is to replace fuel enrichment to inhibit knock. When EGR ratio is 10% with akof 0.9, the

Fig. 6.Maximum BMEP and cylinder pressure as a function of EGR and the ignition angle (engine speed of 4000 rpm, EGR temperature of 120 ± 4°C, charge air temperature of 40 ± 2°C)[42].

Fig. 7.Comparison of combustion characteristics for 11% fuel enrichment and 10%

EGR (engine speed of 4000 rpm, boost pressure of 90 kPa, spark ignition timing of 6.6 CAD BTDC)[42].

Fig. 8.Engine emissions before catalyst from various EGR rates (engine speed of 4000 rpm, stoichiometric fuel–air mixtures)[42].

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engine has BMEP values of 1645 KPa and 1673 KPa respectively (calculated fromFig. 7). That is to say, if knock doesn’t occur, using high EGR ratio can get the same level of power output compared with that of fuel enrichment. And using EGR can ensure stoichiometric combustion, so this will greatly improve fuel economy and lower emission.

Fig. 8shows CO, HC and NOXemissions before catalyst at various EGR rates. The values from fuel enrichment case are in the upper part of the ﬁgure. The major effect using EGR instead of fuel enrichment is that CO is signiﬁcantly decreased from over 4% with fuel enrichment to a more reasonable value of 0.8% with a stoichiometric mixture. The negative aspect is that NO_Xis increased with a stoichiometric mixture. But with the increase of EGR rate, NOX

decreases gradually.

Fig. 9shows intensity of knocking cycles at 5000 rpm and under full load conditions. The fuel rich cases show consistently high average values of knock intensity, while the EGR cases display quite a number of high knock intensity, but average knock intensity is low. The possible explanation is that, the greater instability of the cycles has a rapid initial combustion phase, leading to higher cylinder pressure and consequently to higher end-gas temperature.

When knock limit is set to 50 KPa, the average knock intensity of the knocking cycles in the two cases are the same, but in the fuel rich cases, 87% of the cycles are knocking, while only 49% exhibits knock with EGR. Consequently, using EGR to replace fuel enrichment achieves relatively good results.

In the full load cases with a rich mixture, fuel consumption is very high. Naturally the consumption is reduced when fuel enrichment is replaced by EGR. This is shown inFig. 10, where fuel consumption of a case with 13% EGR is compared with a case with k= 0.9. With EGR, the fuel consumption at a comparable knocking intensity reduces by more than 10%.

8. Implementations of EGR for turbocharged gasoline engines The implementation of EGR is simple for naturally aspirated gasoline engines, since exhaust tailpipe backpressure is normally higher than intake pressure. With the development of downsized gasoline engine, turbocharged gasoline engine is becoming increasingly popular in the market. The implementation of EGR is, therefore, more difﬁcult, which leads to a coexistent problem in engines. Two different schemas are shown inFig. 11 [44].

The first structure is LPL EGR, as shown inFig. 11a. Exhaust gas goes into turbine at first, and then into compressor together with fresh air. Tailpipe pressureP₄is larger than compressor inlet pres- sureP1, so a positive difference between turbine outlet and compressor inlet is generally available. In the structure with low pressure, pressure difference in EGR circuits is very low, therefore a high permeability for the EGR valve and EGR cooler is required to ensure sufficient EGR flow. Furthermore, EGR cooler must have high efficiency to ensure the efficiency of compressor, otherwise it will cause intake air overheat and reduce volumetric efficiency.

However, conventional compressors and inter-coolers are not designed to ensure the temperature and fouling of exhaust, which limits the application of LPL EGR.

LPL EGR is divided into two structures: clean EGR and dirty EGR, as shown inFig. 12a and b. The impact of the two structures on compressor is quite different. When using clean EGR, exhaust goes into TWC and then goes through compressor together with fresh air. The ceramic particles from TWC may corrode compressor wheel, therefore additional coating is required to protect the wheel. When using dirty EGR, a major difference is the presence of sticky hydrocarbons, which might result in additional necessity of component protection. On the other hand, dirty EGR will also bring some additional beneﬁt[45]:

Improved rate and efﬁciency of combustion due to re-burn of hydrocarbons, H2 and CO in the cylinder, and improved fuel economy.

Reduced pumping loss as a result of improvement of pressure drop across the turbine.

Improved EGR cooling due to lower enthalpy pre-catalyst (based on exothermic reactions in the catalyst).

In order to ensure compressor durability, it is necessary to con- duct more in-depth theoretical and experimental studies to resist corrosion in compressor.

The one that has wide applications is the second structure, as in Fig. 11b. In this structure, exhaust gas is from the upstream of turbine to the downstream of compressor or of intercooler. However, such HPL EGR must ensure that turbine upstream pressure is suf- ﬁciently higher than boost pressure. That is to say,P3>P2, otherwise we cannot acquire enough exhaust gas being introduced into intake air. Especially under low-speed and high-load condition, exhaust backpressure is lower than intake pressure, as shown inFig. 13. Therefore, in this case, exhaust cannot go into intake naturally, thus remedies must be made by either increasing turbine upstream pressureP3or reducing boost pressureP2. The ﬁrst method is to intake throttling or exhaust throttling that installs a throttle valve in the downstream of compressor or upstream of turbine.

Another widely used method is to use VGT to provide desired EGR Fig. 9.Knock intensity of knocking cycles (engine speed of 5000 rpm, full load

condition, 8°CA BTDC spark ignition timing ofk= 0.88 and 20°CA BTDC spark ignition timing of 13% EGR)[43].

Fig. 10.Comparison of fuel consumption at high load (IMEP1650 kPa, knock intensity 62 kPa at ignition timing 20°CA BTDC in case of EGR and 81 kPa at ignition timing 8°CA BTDC in case of fuel enrichment)[43].

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driving pressure without substantially sacriﬁcing the performance of turbocharged engine. The shrinking of the ﬂow passage of turbine nozzles will increase turbine upstream pressure P3and reduce boost pressureP2.

When using EGR in turbocharged engine, the matching of tur- bocharger relies on selected EGR structure.Fig. 14shows the compressor map without EGR, HPL EGR and LPL EGR. For each structure, EGR variation rate has a different impact on the evolution of initially set point in compressor map[41]:

In high pressure conﬁguration, increasing EGR rate increases only pressure ratio. This can induce surge line problems at low speed and high load.

In low pressure conﬁguration, increasing EGR rate increases simultaneously pressure ratio and air mass ﬂow. This can induce over-speed problem at high speed and high load.

Fig. 11.Low pressure loop EGR and high pressure loop EGR.

Fig. 12.Clean vs. dirty EGR circuits.

Fig. 13.Inlet and exhaust pressure vs. speed of the engine[41].

Fig. 14.Compressor MAP analysis[41].

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In summary, it is possible to naturally recirculate exhaust into intake in LPL EGR, but high temperature exhaust might corrode compressor and inter-cooler. LPL EGR is divided into two structures: clean EGR and dirty EGR. The impact of the two structures on compressor is quite different. The pressure difference over HPL EGR circuit is very low. Therefore, it is necessary to use VGT or throttle valve to generate a driving force to ensure sufficient EGR flow. In HPL EGR configuration, EGR gases are taken before turbine, thus before after treatment. EGR gases contain HC and CO, which could be a source of corrosion in EGR circuit. Further- more, using EGR in turbocharged gasoline engine must consider turbo matching problem.

9. Conclusion

(1) EGR technique can reduce fuel consumption of gasoline engine and meet more stringent emission regulations in the future together with other advanced techniques.

(2) The engine using hot EGR can use exhaust to heat intake, promote combustion, and thus improve thermal efﬁciency.

While cooled EGR increases intake density, thereby increases volumetric efﬁciency of engine. At the same time, decreased temperature can further reduce NOXemission, but HC emission and cycle-by-cycle variations are increased compared with that of hot EGR.

(3) Knock is a main problem to improve BMEP of gasoline engine. Commonly used method to inhibit knock is fuel enrichment. Using cooled EGR instead of excessive fuel to inhibit knock and to reduce emission is an effective measure in gasoline engine.

(4) With the development of downsized gasoline engine, turbocharged gasoline engine is becoming increasingly popular in the market, and the implementation of EGR is, therefore, more difﬁcult, which lead to a coexistence problem in engines. Furthermore, using EGR in turbocharged gasoline engine must consider turbo matching problem.

Acknowledgment

Authors wish to express their sincere appreciation to the ﬁnan- cial support of the National Natural Science Found of China NSFC (No. 51176138) and the key project of Tianjin’s application infra- structure with cutting-edge technology research program (Natural Science Foundation 12TJZDTJ2800).

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