Effect of external hot EGR dilution on combustion, performance and particulate emissions of a GDI engine

Energy Conversion and Management 142 (2017) 69–81

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Effect of external hot EGR dilution on combustion, performance and particulate emissions of a GDI engine Fangxi Xie, Wei Hong, Yan Su ⇑, Miaomiao Zhang, Beiping Jiang State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130025, China

a r t i c l e

i n f o

Article history: Received 11 December 2016 Received in revised form 3 March 2017 Accepted 12 March 2017

Keywords: GDI engine Hot EGR Combustion Performance Particle number emissions

a b s t r a c t In this paper, an experimental investigation about the influence of hot EGR addition on the engine combustion, performance and particulate number emission was conducted at a spark-ignition gasoline direct injection (GDI) engine. Meanwhile, the different effects between cooled and hot EGR addition methods were compared and the variations of fuel consumption and particle number emissions under six engine operating conditions with different speeds and loads were analyzed. The research result indicated that increasing hot EGR ratio properly with adjustment of ignition timing could effectively improve the relationship among brake-specific fuel consumption (BSFC), NOx and particle number emissions. When hot EGR ratio increased to 20%, not only BSFC but also the NOx and particle number emissions were reduced, which were about 7%, 87% and 36% respectively. Compared with cooled EGR, the flame development and propagation speeds were accelerated, and cycle-by-cycle combustion variation decreased with hot EGR. Meanwhile, using hot EGR made the engine realize a better relationship among fuel consumption, NOx and particle number emissions. The biggest improvements of BSFC, NOx and particle number emissions were obtained at low-medium speed and medium load engine conditions by hot EGR addition method. While engine speed increased and load decreased, the improvement of engine fuel consumption and emission reduced with hot EGR method. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Following with the sustainable growth in the automobiles industry, its emission and fuel-saving problems have become the focus of world attention. Gasoline direct injection (GDI) has been considered as an enormous potential technology to improve thermal efficiency, reduce harmful emissions and meet a good start-accelerate performance [1,2]. As a result, the number of GDI vehicles worldwide has increased significantly in recent years [3]. However, fuel consumption of commercial GDI engine is still worse compared with diesel engine in general, especially at mild and light load of engine. Furthermore, the problem of particulate emissions from gasoline engine has attracted more attention due to large adverse impacts of particulate on the human health in the recent years, especially for people living in cities [4,5]. In 2014, the particle number emissions for gasoline engine began to be regulated in the European VI emission standards. It is generally known that exhaust gas recycle (EGR) technique is an efficient method of suppressing engine NOx emission by decreasing in-cylinder combustion temperature and oxygen partial ⇑ Corresponding author. E-mail address: [email protected] (Y. Su). http://dx.doi.org/10.1016/j.enconman.2017.03.045 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

pressure [6–8]. EGR technique has been widely used on the compression ignition diesel engine. In recent years, using EGR technique in the spark-ignition engine has attracted a lot of scholars [9–11]. Compared with compression ignition diesel engine, the use of EGR technique in the spark-ignition engine can not only reduce the NOx emission but also suppress knock combustion and enhance engine thermal efficiency [12,13]. The Grandin [14], Duchaussoy [15] et al. reported that the cooled EGR was more effective to suppress knock under heavy load condition than air dilution. The Lattimore [16], Fontana [17] et al. found that the combustion center (CA50) and spark timing limited by knock under the high engine load could be advanced with cooled EGR addition. The research of Potteau [18] demonstrates that the rich mixture (k < 1) using under 5000r/min and 12.9 bar BMEP engine condition can be replaced by stoichiometric air-fuel mixture (k = 1) with 12.2% cooled EGR addition, which leads to a fuel economy improvement of 14%. Su et al. indicated that cooled EGR provided more brake thermal efficiency improvement than increasing geometric CR from 9.3 to 10.9, and the combined using of 18–25% cooled EGR and 10.9 CR led to 2.1–3.5% improvement in the brake thermal efficiency compared with the baseline (9.3 CR without EGR) [19]. Similar finds are also gained in literatures [20]. Meanwhile, Hoepk et al. indicated that the inhibitory effect of cooled EGR on knock

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combustion was contributed to reduce temperature of end mixture and combustion speed, and strengthen quenching reactions [21]. Li et al. revealed that replacement of fuel enrichment with EGR was a leading contributor for the improvement of fuel consumption at high engine load [22]. Due to low temperature and pressure in cylinder, although the spark-ignition engine operating under light and medium load conditions is almost free from knocking restrictions compared with high load, the EGR addition could also improve the engine fuel consumption by reducing pumping and hot losses to surroundings in theory [23]. But in fact, the fuel consumption improvement for engine light and medium load is usually weakened by cooled EGR addition due to the reduction of flame development and propagation speed and deterioration of combustion stability [24,25]. If the exhaust gas is re-circulated directly to the intake without cooling, the inlet charge temperature increases, which will be helpful to enhance combustion speed and improve thermal efficiency under the light and medium engine load compared with the cooled EGR dilution method to some extent. Xie’ s research at a portinjection and spark ignition methanol engine indicated that compared with cooled EGR, hot EGR could effectively improve engine combustion performance, and a better relationship between output torque and NOx emission was achieved [26]. Abd-Alla et al. also found that the engine thermal efficiency and NOx emission were improved with hot EGR, and at the same time the smoke reduced to almost zero under high natural gas fractions on a dual fuel engine [27]. In the literature [28,29], the lower unburned HC and CO emissions were gotten by hot exhaust gas. However, Safari et al. also presented that the indicated thermal efficiency of cooled EGR in a SI engine fueled with hydrogen was slightly higher than that of hot EGR strategy [30]. EGR also have an impartment influence on particulate matter emissions [31–34]. Alger et al. [35] found that particle mass and number emissions from a 2.4 l turbocharged PFI engine decreased obviously by high levels (up to 40%) with using cooled LP-EGR. Zhao’s research carried out on an air-guided GDI engine indicated that the nucleation mode particulate increased clearly, the accumulation mode particle number decreased gradually with the increase of EGR rate, and the typical size of nucleation mode particulate was in the range of 10–25 nm [36]. Gu et al. [37] demonstrated that the particle number emission under all blend rates of gasoline-n-butanol conditions could be reduced by adopting hot EGR dilution. Zhang et al. indicated that with EGR dilution the soot formation, even for fuel-rich areas in cylinder, decreased significantly at medium engine loads [38]. However, Lattimore’s research also showed that EGR addition could increase the accumulation mode particulate, but the nucleation mode particulate decreased [12]. Meanwhile, the literature [39] found that under engine speed of 2500r/min, the total particle number emission have decreased obviously at EGR ratio of 25%. Instead, under engine speed of 3500r/min the total particle number emission increased with the same EGR ratio. In conclusion, the influence of EGR on the PM emissions is complex, and it still needs to be investigated more deeply. EGR addition has great potential to improve engine fuel consumption and harmful emission simultaneously for GDI engine. However, the above studies mostly describe the influence of cooled EGR, there is little work related to the combustion and PM emissions characteristics of GDI engine under hot EGR addition condition. Therefore, this paper experimentally investigated the combustion and emission characteristics on a GDI engine under various hot EGR addition levels, especially for the number and size distribution of particulate emissions. Meanwhile, the different effects between cooled and hot EGR addition methods were compared and the variations of fuel consumption and particle number emissions under six engine operating conditions with different

speeds and loads were analyzed. This work will be useful in enhancing fuel economy and decreasing emissions of the gasoline direct injection engine.

2. Experimental setup and procedure 2.1. Experimental setup The experiment was conducted on a four-cylinder, four-stroke and water-cooled GDI engine. The specifications of the engine used in the experiment were given in Table 1. Meanwhile, the schematic of measuring and controlling system was shown in Fig. 1. The engine loads and speeds could be adjusted and controlled by an eddy current dynamometer with model of Nan-Feng160. The combustion pressure in engine cylinder was measured by an AVL quartz crystal pressure sensor with ZF42 model, and an AVL charge amplifier was employed to process and amplify pressure signal. The crankshaft angle was measured by a LF-72BM-C05E encoder with 0.5° resolution. The in-cylinder pressure and crankshaft angle were transmitted to an ONO SOKKI DS-9110 combustion analyzer, and the combustion characteristic parameters such as in-cylinder heat release rate, accumulated heat release, combustion temperature and so on were calculated. The intake and fuel flow were acquired by an air flow meter of SENSYCON Sensy flow P and fuel flow meter of ONO SOKKI DF-2420 respectively. In the test, the coolant was kept at 80 °C (±3 °C) by heat exchangers with closed-loop temperature control system. A self-developed electronic control unit (ECU) based on the 16-bit Freescale MCU was employed to adjust experimental engine’s fuel injection and ignition parameters online. Engine gas emissions, including THC, CO, NOx and CO2, were measured by an AVL DICOM 4000 gas analyzer. The sampling pipe was placed at upstream of the three-way catalytic converter. The particle size distribution and particle number concentration was measured by a particulate emission spectrometer of DMS 500, and its measure range of particle size was from 4.87 to 1000 nm with 38 measure stages of the particle size. The particulate sample was installed on the opposite side of AVL DICOM 4000 sampling point. Before the exhaust from sampling pipe entered into DMS 500, it was diluted by air (1st dilution ratio: air/exhaust = 5:1, 2nd dilution ratio: air/exhaust = 15:1) for holding a good signal to noise ratio. The air flow in the dilution system was managed by HEPA filtered compressed air. Exhaust sample passed through a heated line maintained at 150 °C in order to prevent the condensation of the water and hydrocarbons in exhaust. The data was collected at a sampling frequency rate of 10 Hz for 120 s, and then the average value was used. The main specifications of the measurement devices used in tests were presented in Table 2. Meanwhile, the error bars representing 95% confidence was given in the figures [40]. The EGR pipe was placed between exhaust manifold and inlet manifold after throttle, and an EGR valve was arranged within

Table 1 Specifications of the test engine. Engine parameters

Specifications

Engine type Combustion system Displacement Bore
stroke Compression ratio Intake valve opening Exhaust valve closing Maximum torque Maximum power

Spark-ignition, in line, 4-cylinder, 4-stroke Spray-Guided GDI 1.39 L 76.5 mm
75.6 mm 10:1 376.0°CA BTDC 144.0°CA ATDC 220 N m (1500–3500r/min) 96 kW (5000r/min)

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EGR Valve Water collector

Intake flow meter

Intake Cooler

Combustion Analyzer

Intake Cooler

Gas Analyzer

Charge Amplifier Pressure sensor

Lambda Meter Encoder Engine

EGR Cooler

Dynamometer

Particulate Pectrometer Fig. 1. Schematic of experimental setup.

Table 2 Specifications of the measurement devices. Equipment

Model

Accuracy

Dynamometer

CW160

Pressure sensor Charge amplifier Crank angle encoder Air flow meter Fuel flow meter Exhaust gas analyzer

ZF42 FLEXIFEM PIEZO LF-72BM-C05E SENSYCON DF-2420 DICOM 4000

Torque: ±2 N m Speed: ±1 rpm ±0.3% ±0.6% ±0.5° ±0.5% ±0.2% THC: ±30 ppm CO: ±0.01% NO: ±20 ppm CO2: ±0.1%

the EGR pipe. The exhaust gas was recycled due to the pressure difference between exhaust and intake pipe. Meanwhile, the experiments were mainly undertaken at medium and low loads, which means the intake pressure was at relative low level. As a consequence, high EGR ratios, more than 20%, could be obtained just by such pressure difference.

the exhaust gas was introduced directly to the intake pipe without cooling, it was considered as hot EGR. Meanwhile, AVL DICOM 4000 gas analyzer was employed to determine the EGR ratio. The EGR ratio was computed by the CO2 concentration in exhaust and intake gas. The formula is as follows:

EGR rate ¼

/CO2 intake /CO2 exhaused

ð1Þ

In the test, 200 consecutive combustion cycles were collected and then used to measure heat release rate (dQ ), temperature dh TðhÞ, flame development, combustion duration and cycle-by-cycle variation coefficient of indicated mean effective pressure (COVIMEP). The corresponding calculations are shown as follows:

dQ r dV 1 dP dQ W ¼
P þ
V þ dh r  1 dh r  1 dh dh TðhÞ ¼

PðhÞVðhÞ WðhÞRðhÞ

rIMEP

ð2Þ

½41

ð3Þ


100

ð4Þ

2.2. Experimental methodology

COVIMEP ¼

When the influence of hot EGR ratio and ignition timing on the engine combustion and PM emission was investigated and the hot and cooled EGR were compared, the engine speed and fuel consumption were still remained at 2000 ± 2r/min and 3.1 ± 0.1 kg/h, the fuel injection timing was set at 260°CA BTDC (before top dead center on the compression stroke) and the injection pressure was set at 8.5 MPa. Meanwhile, the evaluation and comparison of the improvement of hot EGR addition for the engine performance and emission under different engine speeds and loads were conducted at six engine operating conditions, the key engine operating parameters were shown in Table 3. BMEP (brake specific effective pressure) under no EGR condition is also given in it. The excess air coefficient (k) under all EGR addition conditions was kept 1 ± 0.01 and monitored by an ETAS lambda meter. The lambda sensor was BOSCH LSU 4.9. Recycling exhaust which cooled to about 130 ± 5 °C was regarded as cooled EGR. While

where r (heat capacity ratio) is the ratio of specific heats, dVdh is change rate of in-cylinder volume with respect to crank angle, dPdh is change rate of in-cylinder pressure with respect to crank angle and dQ W dh is heat transfer rate. P is measured cylinder pressure (Pa). V is the volume (m3) of the cylinder. W is the mass (kg) of mixture in the cylinder. R is the gas constant (J/ (kg K)) of the in-cylinder mixture. The flame development is defined as the crank angle interval between spark-ignited start and combustion start. The combustion start is the crank angle at which five percent of total apparent heat release has reached. The combustion duration is defined as the crank angle between the combustion start and the combustion end [42]. The combustion end is defined as the crank angle at which 90% of the fuel mass is burned. The combustion center (CA50) is defined as the crank angle at which fifty percent of total heat release is reached [23].

IMEP

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Table 3 Key engine operating parameters. Mode

Speed (r/min)

Injection pulse width (ls)

Injection pressure (Mpa)

Injection timing (°CA BTDC)

BMEP (kPa)

1 2 3 4 5 6

2000 2000 1000 1500 2500 3000

1600 900 1600 1600 1600 1600

8.5 8.5 8.5 8.5 8.5 8.5

260 260 260 260 260 260

581 279 695 596 578 562

3. Results and discussion 3.1. Effect of hot EGR ratio and ignition timing on engine combustion and emission Fig. 2(a) gives the effect of hot EGR ratio on in-cylinder pressure, temperature and heat release rate curve at ignition timing of 20°CA BTDC. As shown in Fig. 2(a), with increasing of hot EGR ratio, the peak in-cylinder pressure and heat release rate decreased, and corresponding crank angle was delayed. Compared with hot EGR ratio of 0, the peak of in-cylinder pressure, temperature and heat release rate of hot EGR ratio of 20% were reduced by 1156 kPa and 12.9 kJ/°CA, and the corresponding crank angle was delayed 5.5 and 12°CA respectively. The addition of hot EGR was bad for the flame development and propagation, and the higher hot EGR ratio could result in the delay and slowdown of heat release in cylinder obviously. In the literature [43], the better engine dynamic performance could be obtained at combustion center of 8–9°CA ATDC, the delayed combustion process generally meant that the heat release quantity in the late combustion phase was increased,

(a) EGR ratio

which would decrease the conversion efficiency from in-cylinder combustion energy to useful engine work. Fig. 2(b) gives the effect of injection timing on in-cylinder pressure, temperature and heat release rate curves at hot EGR ratio of 0. With advance of the injection timing, the peak of in-cylinder pressure and heat release rate increased, and corresponding crank angle was advanced. When the injection timing was 25°CA BTDC, the peak of in-cylinder pressure, temperature and heat release rate were highest in the test, and their corresponding crank angle was more close to compression top dead center. Compared with the injection timing of 5°CA BTDC, the peak of in-cylinder pressure, temperature and heat release rate of 25°CA BTDC increased by 1661 kPa and 7.8 kJ/°CA, and the corresponding crank angle was advanced 20.5 and 23°CA respectively. The ignition also had an obvious influence on in-cylinder combustion. The flame propagation could be accelerated and the combustion phase could be advanced by advancing ignition timing effectively. Combining the adjustment of ignition timing, the delay and slowdown problem of in-cylinder heat release under the higher hot EGR ratio was likely to be improved.

(b) Ignition timing

Fig. 2. Effect of hot EGR ratio and ignition timing on the in-cylinder pressure and heat release rate.

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Fig. 3. Effect of hot EGR ratio and ignition timing on the flame development and combustion duration.

Fig. 3 shows the effect of the hot EGR ratio and ignition timing on the flame development and combustion duration. For a given ignition timing, the flame development and the combustion duration were lengthened with increasing of hot EGR ratio, as the flame development and propagation slowed down. For example, when the hot EGR ratio increased from 0% to 20% at 25°CA BTDC, the flame development duration and the combustion duration were prolonged about 8°CA and 27.5°CA respectively. Meanwhile, the ignition timing also had an obvious effect on the flame development and combustion duration under hot EGR addition condition. Overall, as the ignition timing was advanced, the early flame growth rate decreased and the flame development duration prolonged, but the flame propagation speed was accelerated and the combustion duration shortened. Especially, when hot EGR ratio was higher, the influence of ignition timing advance on reducing combustion duration became more significant, and a faster combustion rate was also gotten. When hot EGR ratio was 25%, the combustion duration was about 30°CA at ignition timing of 50°CA BTDC, which reached roughly the same level as no EGR addition condition. Fig. 4 shows effect of hot EGR ratio and ignition timing on the combustion center (CA50) and variation coefficient of indicated mean effective pressure (COVIMEP). With hot EGR ratio increasing, the combustion center was delayed and COVIMEP was deteriorated obviously. When hot EGR ratio was enhanced to 20% at ignition timing of 25°CA, the combustion center (CA50) was delayed about 14.5°CA and the variation coefficient in indicated mean effective pressure was increased by 5.09% comparing with no EGR operation. Meanwhile, under the higher hot EGR addition level,

the combustion center and variation coefficient in indicated mean effective pressure could also be improved effectively by advancing ignition timing. When hot EGR ratio was 25%, the combustion center and variation coefficient of indicated mean effective pressure were about 7°CA ATDC (after top dead center on the compression stroke) and 2.315% at ignition timing of 50°CA BTDC. Combined with the ignition timing adjustment, hot EGR addition could reach a level of 25% EGR ratio without obvious combustion delay and deterioration. Fig. 5 shows the effect of hot EGR ratio and ignition timing on BSFC (brake-specific fuel consumption). Fig. 6 gives the corresponding minimum brake-specific fuel consumption (MBSFC) under different hot EGR ratios. The engine BSFC curves presented a trend that increased firstly and then decreased with the advancing ignition timing, and there existed a spark timing for a certain hot EGR rate which made engine reach maximum torque and MBSFC. Meanwhile, as increasing hot EGR, MBSFC spark timing needed to be advanced further because the development and propagation of flame were slowed as shown in Fig. 2. Without hot EGR addition, the MBSFC spark timing appeared at 20°CA BTDC. When hot EGR ratio increased to 25%, the MBSFC spark timing advanced to 45°CA BTDC. Under hot EGR addition condition, BSFC was decreased until reaching a hot EGR ratio of 20% with the MBSFC spark timing due to reduced heat transfer loss and pumping loss, and an increase in the specific heat ratio. When hot EGR ratio was 20%, the BSFC could be improved about 7% compared with no EGR addition condition. However, while hot EGR ratio was further increased to 25%, BSFC also started to increase, as the passive influences of EGR addition such as the extended combustion

Fig. 4. Effect of hot EGR ratio and ignition timing on combustion center (CA50) and variation coefficient of indicated mean effective pressure (COVIMEP).

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Fig. 5. Effect of hot EGR ratio and ignition timing on brake-specific fuel consumption (BSFC).

Fig. 6. Minimum brake-specific fuel consumption (MBSFC) with different hot EGR ratios.

duration and the raise of compression work with highly advanced spark timing (Fig. 3) becoming dominant at high hot EGR addition level. Fig. 7(a) shows the effect of hot EGR ratio and ignition timing on the NOx emission. Fig. 7(b) gives the relationship between BSFC and NOx emission. While the ignition timing advanced, NOx emission significantly increased. When the ignition timing was delayed from 25°CA BTDC to 5°CA BTDC under no EGR addition operation, NOx emission decreased from 3103 to 1393 ppm. The ignition timing had an important influence on NOx emission and a delayed

ignition timing could effectively reduce NOx emission. Meanwhile, hot EGR also played a remarkably role on NOx emission, and increasing hot EGR ratio could reduce NOx emission obviously. Under ignition timing of 25°CA BTDC, the NOx emission decreased from 3103 ppm (EGR rate of 0%) to 89 ppm (EGR rate of 25%). So the hot EGR may also effectively suppress NOx emission. According to NOx formation theory developed by Zeldovich, the in-cylinder NOx formation was affected by oxygen concentration, combustion temperature and response time. When the ignition timing was retarded, the in-cylinder combustion process was delayed and heat release speed reduced as shown in Fig. 2, which could cut down the combustion peak temperature and reduce the duration of high temperature. For hot EGR addition, the peak of combustion temperature and oxygen concentration in the cylinder may reduce effectively. Meanwhile, in Fig. 7(b) there could also find that with increase of hot EGR ratio, the relationship curve between BSFC and NOx emission moved to the left bottom corner at hot EGR ratio of 0–20%. Under same NOx emission level, better fuel consumption could be obtained with optimized ignition timing under higher hot EGR ratio. At hot EGR ratio of 20% with ignition timing of 40°CA BTDC, the engine gained not only fuel saving about 7% but also 87% reduction in NOx emission. Meanwhile, when hot EGR ratio increased to 25%, there was still a great potential for further reduction of NOx emission. Fig. 8(a) shows the effect of the ignition timing on the particle number-size distribution under no EGR addition condition. In addition, the corresponding total particle number and particle number of two modes - nucleation and accumulation - are given in Fig. 8 (b). The nucleation mode was composed of smaller size particulate on the left-hand side (0–30 nm) and the accumulation mode was composed of larger size particulate on the right-hand side (30– 500 nm) [16] in Fig. 8(a). According to the literature [19], the nucleation mode was associated with volatile particulate, which was mainly attributed to condensation of hydrocarbon in exhaust. The accumulation mode particulate stemmed from solid core (soot) agglomerates formed at rich-fuel area in cylinder. The ignition timing showed a significant influence on particle number emissions under this study condition. With the spark timing delayed, the particle number-size distribution curve showed a decreasing tendency, including total particle number, nucleation mode particle number and accumulation mode particle number. When the ignition timing was delayed from 25°CA BTDC to 5°CA BTDC, they reduced by 48.4%, 48.3% and 48.1% respectively. The decrease of particulate emissions with delayed spark timing could be ascribed to the reduction of the peak combustion temperature and the rise of exhaust temperature during engine expansion and exhaust strokes. Retarding ignition timing could make the in-cylinder combustion process delayed (as shown in Fig. 2) and

Fig. 7. Effect of hot EGR ratio and ignition timing on NOx emission and relationship between brake-specific fuel consumption and NOx emission.

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Fig. 8. Effect of the ignition timing on particle emissions at EGR ratio of 0.

the peak of combustion temperature reduce, and then the H-abstraction reaction at high temperature combustion in cylinder was reduced, and the formation of carbonaceous particulate was suppressed. Retarding spark timing could also enhance gas temperature during the expansion and exhaust processes, which was conducive to the particulate oxidation and the decrease of hydrocarbon emission in exhaust. Meanwhile, when the ignition timing was retarded, the time of fuel-air mixing increased, and local richfuel phenomenon in cylinder was weakened. Fig. 9(a) shows the effect of hot EGR ratio on the particle number-size distribution at ignition timing of 20°CA BTDC. The corresponding total particle number, and particle number of two modes - nucleation and accumulation- were also shown in Fig. 9 (b). Overall, with hot EGR increased, the total particle number and accumulation particle number decreased obviously. As hot EGR ratio increased from 0% to 20%, the total particle number and accumulation particle number reduced by 70.36% and 84.69% respectively. It was distinguished from particulate emissions on diesel engines with diffusion combustion, which gave an increase in accumulation particle number with addition of EGR [44,45]. In diesel engine, the addition of EGR reduced in-cylinder oxygen concentration, the local rich-fuel phenomenon in cylinder was more serious, and the carbonaceous particulate formation increased. In GDI gasoline engine, the mixture was premixed and quasihomogeneous, the cleavage and dehydrogenation reaction at high temperature combustion was a key cause of primary carbonaceous particulate formation [37]. When the hot EGR was added to GDI engine, the in-cylinder heat release speed was lowered, combustion process was retarded, and the heat capacity of the in-cylinder mixture increased, which make the engine combustion temperature decrease, and then the H-abstraction reaction at high temperature combustion in cylinder reduced. Meanwhile, hot EGR

could enhance the gas temperature in the engine intake and compression strokes and the fuel evaporation and fuel-air mixing improved, which also contributed to the reduction of carbonaceous particulate formation. In Fig. 9(b), it can also find that the addition of hot EGR had a significant effect on mode particle number. As hot EGR ratio changed from 0% to 5%, the nucleation mode particle number increased slightly, which was in common with the research results described in literature [36]. However, while hot EGR ratio further increased from 5%, the different tendency appeared, the nucleation mode particle number reduced. It might be because when the hot EGR was added properly, the HC emission decreased in the research as shown in Fig. 10 due to the prolonged combustion duration (as shown in Fig. 3), raised in-cylinder temperature during engine expansion and exhaust strokes and improved fuel-air mixing, so the nucleation particulate decreased. But the paradoxical phenomenon still existed, when the hot EGR ratio were 15% and 20%, although HC emission increased (as shown in Fig. 10), the nucleation mode particle number still kept on decreasing. In the study of SI Peng-kun et al. about effects of fuel physical and chemical properties on low temperature combustion, it found that under LTC conditions with high EGR rate, the component of HC emission was changed obviously, where methane was the main composition of THC, which reached about 60% [46], so the change of HC composition might be an important factor for further decrease of nucleation particle number under hot EGR ratios of 15% and 20%. Meanwhile, in the research of Myung [47], it found that the primary carbonaceous particle size was about 10–80 nm, which means the primary carbonaceous particulate also could be a sources of nucleation mode particulate. When the addition ratio was higher, the combustion temperature reduced obviously, the formation of primary carbonaceous particle may be suppressed.

Fig. 9. Effect of hot EGR ratio on particulate emissions at ignition timing of 20°CA BTDC.

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reduction with a relatively good fuel economy. Combined with the above research, applying hot EGR addition method on GDI engine will improve engine fuel consumption, NOx and particle number emissions.

3.2. Effect of cooled EGR vs. hot EGR on combustion and emission performance

Fig. 10. Effect of hot EGR ratio on HC emission at ignition timing of 20°CA BTDC.

Through above discussion, the particle number emission could be effectively decreased with hot EGR addition. But, under high hot EGR addition level the ignition timing usually needed to be advanced to avoid combustion deterioration and improve engine fuel consumption, which may lead to increase in particulate emissions. To fully explore the effect of hot EGR on particle number emission, Fig. 11(a) gives further effect of hot EGR and ignition timing on the total particle number emission. And Fig. 11(b) shows the relationship between the brake thermal efficiency and total particle number emission under different hot EGR addition levels. As shown in Fig. 11(a), there was no doubt that the hot EGR was an efficient technology to reduce total particle number emission. Adopting the same ignition timing, total particle number could be reduced obviously with the increasing of hot EGR ratio. Meanwhile, even if the advanced timing was used, the higher hot EGR ratio still obtained a relatively low total particle number emission level. In Fig. 11(b), it also found BSFC and particle number emission were similar to the relationship curve between BSFC and NOx emission, with hot EGR ratio increased from 0% to 20%, the relationship curve between BSFC and total particle number emission moved to the left bottom corner. Under the same particle number emission level, the higher hot EGR ratio could obtain better fuel consumption. The total particle number emission of the hot EGR ratio of 20% with MBT ignition timing (40°CA BTDC) decreased about 36% than no EGR dilution condition with MBSFC ignition timing (25°CA BTDC), while the fuel consumption improved by 7%. Meanwhile, when the hot EGR ratio increased from 20% to 25%, although the lowest BSFC increased from 274 g/kW h to 281 g/kW h, the total particle number still had a great potentiality for further

Fig. 12 demonstrates the change of intake and EGR temperature under EGR ratios of 10%, 15% and 20% for hot and cooled EGR methods with ignition timing of 20°CA BTDC. In the test, while EGR ratio was 5%, the EGR temperature could only reach about 120°C due to itself heat loss of recycle exhaust system. When EGR ratio was 25%, the engine was difficult to maintain stability for cooled EGR method, so EGR ratios of 10%, 15% and 20% had been researched. In Fig. 12, with the increasing of EGR ratio, the EGR temperature continued to rise for the hot EGR method. When EGR ratio increases from 10% to 20%, EGR temperature was increased from 203 °C to 324 °C. Meanwhile, with the increasing of EGR ratio, the intake temperature increased because of the exhaust introducing by either hot or cooled EGR method. This effect was more significant with hot EGR addition compared with cooled EGR. Under EGR ratios of 10% and 20%, the intake temperature of hot EGR method raised by 6.1 °C and 45.3 °C than that of cooled EGR method respectively. Fig. 13 shows the flame development duration, combustion duration, combustion center and coefficient of variation of indicated mean effective pressure under EGR ratios of 10%, 15% and 20% for hot and cooled EGR methods with ignition timing of 20°CA BTDC. The effect of cooled EGR method on flame development and combustion duration was more pronounced. Under the same EGR ratio, cooled EGR shown a longer flame development duration and combustion duration. For example, when the EGR was 20%, the flame development duration and combustion duration of cooled EGR method were prolonged about 2.5°CA and 18°CA than that of hot EGR method. Meanwhile, as shown in Fig. 13(b), with the EGR ratio increased, the combustion center (CA50) was delayed and the variation coefficient of indicated mean effective pressure was increased. Compared with the cooled EGR method, the increasing of combustion center (CA50) and COVIMEP were more obvious. When EGR ratio increased from 10% to 20%, the combustion center (CA50) and COVIMEP of hot EGR method increased 8.5°CA and 157.8%, while they increased 17°CA and 289.1% for cooled EGR method. Overall, compared with the cooled EGR, the in-cylinder flame development and propagation speed had been improved to a certain extent for hot EGR addition.

Fig. 11. Effect of hot EGR ratio and ignition timing on total particulate number emission and relationship between brake-specific fuel consumption and total particle number.

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Fig. 12. Comparisons of intake and EGR temperature between cooled and hot EGR addition methods.

Especially when the EGR ratio was higher, the combustion improvement was more significant. Fig. 14 shows the comparisons of BSFC and NOx emission between cooled and hot EGR addition under EGR ratios of 10%, 15% and 20% with ignition timing of 20°CA BTDC. In Fig. 14(a), although the brake-specific fuel consumption was increased and the engine fuel economy was deteriorated with the increase of EGR ratio, BSFC of cooled EGR method was increased obviously than that of hot EGR method. In the same EGR ratio, BSFC of cooled EGR was higher. This is mainly because the intake temperature of hot EGR raised more than that of cooled EGR, which made engine combustion improve, and the engine flame development duration and combustion duration were shortened, the combustion center was advanced and the cycle-by-cycle combustion variation was decreased (as shown in Fig. 13). Meanwhile, as the engine combustion process had been delayed and the in-cylinder gas temperature was reduced for cooled EGR method, the NOx emission of cooled EGR method was also obviously decreased than that of hot EGR method in the same EGR ratio as shown in Fig. 14(b). When EGR ratio was 20%, NOx emission of cooled EGR was further decreased about 36 ppm than that of hot EGR method. Fig. 15 shows the total particle number and the relationship between BSFC and total particle number emission for hot and cooled EGR additions under EGR ratios of 10%, 15% and 20% with ignition timing of 20°CA BTDC. In Fig. 15(a), with the increasing of EGR ratio, the total particle number emission of hot and cooled EGR methods decreased. In addition, the total particle number of cooled EGR was reduced significantly. Compared with the hot

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EGR addition, the total particle number of cooled EGR addition decreased by 15.76% and 11.84% respectively, under EGR ratios of 15% and 20%. Combined with the contrast curve of in-cylinder temperature for hot and cooled EGR additions at EGR ratio of 20% as shown in Fig. 16, the peak combustion temperature of hot EGR method was higher than that of cooled EGR method, while its temperature during engine expansion and exhaust strokes were lower. The higher combustion temperature could increase the carbonaceous particulate formation, and the lower temperature during engine expansion and exhaust strokes could suppress the later particulate oxidation rate, which leaded to the increasing of total particle number for hot EGR method compared with cooled EGR method. However, it also needed to be heeded that the relationship between BSFC and total particle number emission was significantly better than that of cooled EGR method as shown in Fig. 15 (b). Under the same BSFC level, hot EGR method shown relatively lower total particle number emission. This was because that compared with the cooled EGR method, the income of the improvement of fuel consumption for hot EGR emission was higher than deterioration level of particle number emission. 3.3. Effect of hot EGR dilution on performance and emission under different engine operating conditions Fig. 17 shows BSFC under six engine operating conditions and different EGR ratios with MBSFC ignition timing. Meanwhile, the change of MBSFC under different operating conditions are given in Fig. 18. The corresponding no EGR addition condition with MBSFC ignition timing was used as the baseline. With the increasing of hot EGR ratio, BSFC curves for the six engine operating conditions decreased at first and then increased. There was an optimized EGR ratio to obtain the lowest BSFC and good fuel economy. Under this condition, the optimized EGR ratio for obtaining minimum brake-specific fuel consumption was regarded as MBSFC EGR ratio. Compared with the medium (mode1) and low (mode 2) engine load, the optimized EGR ratio for obtaining the lowest BSFC had some differences. The optimized EGR ratios of mode 1 and mode 2 were 20% and 5% respectively. BSFC was improved about 7.1% and 5.7% than the corresponding no EGR addition condition. For engine medium load (mode 1), the tolerance of hot EGR was higher, and the more fuel saving was realized. This was because that in-cylinder combustion conditions, such as temperature, pressure and so on, was worse at low load than high load, the EGR addition was easier to produce combustion delay and instability. As for mode 2, when the hot EGR ratio just increased to 10%, the in-cylinder combustion deteriorated obviously, and BSFC increased sharply. Meanwhile, by BSFC curve of mode1 (2000r/min), mode 3 (1000r/min), mode 4 (1500r/min), mode 5 (2500r/min) and mode 6

Fig. 13. Comparisons of combustion parameters between hot and cooled EGR addition methods.

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Fig. 14. Comparisons of brake-specific fuel consumption and NOx emission between cooled and hot EGR addition methods.

Fig. 15. Total particle number and relationship between brake-specific fuel consumption and total particle number emission for hot and cooled EGR methods.

Fig. 16. In-cylinder temperature of hot and cooled EGR methods at 20% EGR ratio.

Fig. 17. Brake-specific fuel consumption under six engine operating conditions and different EGR ratios with MBSFC ignition timing.

(3000r/min), it could be found that the optimized EGR ratio for obtaining the lowest BSFC was basically the same, which was 20%. For mode3 (1000r/min), it was difficult to add more than 20% hot EGR into engine due to system limit. When the engine speed was in the range of 1000-2500r/min, the improvement of fuel consumption was also roughly at same level, which was about 6.8%-8%. However, when the engine speed increased to 3000r/min (mode 6), the optimized EGR ratio for obtaining the lowest BSFC decreased, which was only 15%. Meanwhile, the improvement of hot EGR on BSFC also decreased. Compared with the no

EGR addition condition, BSFC was only reduced about 4.0% for mode 6. Fig. 19 shows NOx and total particle number emission under MBSFC EGR ratio and no EGR under six engine operating conditions. Fig. 20 gives the corresponding change rate of NOx and particle number emissions. The hot EGR addition method with MBFC EGR ratio could efficiently reduce the NOx and particle number emission compared with the condition without hot EGR addition at the same time. By comparison, the decrease of NOx and particle number also had some differences from the six engine operation

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Fig. 18. Change rate of minimum brake-specific fuel consumption (MBSFC) under different operating conditions.

conditions. For the mode 1 (2000r/min), mode 3 (1000r/min), mode 4 (1500r/min) and mode 5 (2500r/min) the decrease degree of NOx was relatively high, which reached about 80%. When the engine worked under the mode 2 and mode 6 with MBSFC ignition timing and EGR ratio, the decrease of NOx was relatively low, which was only about 64% and 71%. This was mainly because at mode 2 and mode 6 MBSFC EGR ratio was only 5% and 15% respectively, which were lower than mode1 (2000r/min), mode 3 (1000r/min), mode 4 (1500r/min) and mode5 (2500r/min) with MBSFC hot EGR ratio of 20%. For the engine operating at medium

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load and low to medium speed, hot EGR addition could make NOx emission improve more effectively. But, when the engine worked at lower load and higher speed, the improvement of NOx emission by hot EGR addition was reduced. Meanwhile, when the engine worked at mode 1 (2000r/min), mode 4 (1500r/min) and mode 5 (2500r/min) with MBSFC hot EGR ratio, the particle number emission reduced obviously than no EGR addition condition, which was about 30–40%. However, when the engine operation conditions were mode2, mode3 and mode6, although particle number emission could also be decreased by hot EGR addition, the decrease was relatively weak, which was only 17.1%, 10.4% and 15.0%. Compared with mode1, the MBSFC hot EGR ratio of mode2 and mode3 was lower, and the decrease of the particle number emission was suppressed. Although, the mode3 also had provided higher MBSFC hot EGR ratio, the engine speed was lower and in-cylinder gas flow was weakened, and fuel-air mixing was relatively bad. And then, in Fig. 10(a) the total particulate emissions was higher than any other modes. The addition of hot EGR could make the local rich-fuel phenomenon in cylinder more serious, and the improvement of hot EGR on total particle number emission was restricted. For mode 6, the MBSFC hot EGR ratio was relatively low; on the other hand, the time of fuel-air mixing was shortened at the high engine speed condition, and the addition of hot EGR also could increase the local rich-fuel phenomenon in cylinder, the decrease level of total particle number was reduced. Overall, the optimal improvement of BSFC, NOx and particle number emission could be obtained in low- medium speed and medium-load engine conditions by hot EGR addition method. While the engine speed was higher or lower, and the load was lower, the improvement of BSFC, NOx and particle number emission were weakened.

Fig. 19. NOx and total particle number emissions under MBSFC EGR ratio and no EGR.

Fig. 20. Change rate of NOx and total particle number emissions.

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4. Conclusion In this paper, the influence of hot EGR addition on the engine combustion and particle number emission characteristics was investigated experimentally on a spark-ignition GDI engine. The engine combustion and emission characteristics of cooled and hot EGR addition methods were compared and estimated. Meanwhile, the variations of BSFC, NOx and PM emissions under six engine operating conditions were summarized with different speeds and loads. The main conclusions are as follows: (1) While hot EGR ratio increases, the flame development and propagation duration are extended, retarded combustion and cycle-by-cycle variation are increased. However, combined with the ignition timing advancing, the combustion duration, cycle-by-cycle variation and combustion center can also reach the same level as no EGR dilution condition under high hot EGR ratio of 25%. (2) Properly increasing hot EGR ratio can improve the relationship among brake thermal efficiency, NOx and particle number. When hot EGR ratio increases to 20% under research condition, fuel saving reaches 7%, NOx decreases 36% and particle number reduces 87%. (3) Compared with cooled EGR, the flame development and propagation speed are accelerated, and the cycle-by-cycle combustion variation is weakened with hot EGR. Meanwhile, using hot EGR can realize a better relationship among brake thermal efficiency, NOx and particle number emissions. (4) The biggest improvement of BSFC, NOx and particle number emissions can be obtained under low-speed and mediumload engine conditions by hot EGR addition method. While engine speed rises and the load decreases, this improvement will be weakened.

Acknowledgements This work was supported financially by the National Natural Science Foundation of China (grant numbers 51276080), Jilin Province Natural Science Foundation (grant numbers 20170101137JC and 20170101132JC), Science and Technology Research Projects of Jilin Province education department for 13th Five Year Plan (grant numbers [2016]423).

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