2012-087

CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 26,aNo. 1,a2013 ·1·

DOI: 10.3901/CJME.2013.01.***, available online at www.springerlink.com; www.cjmenet.com; www.cjmenet.com.cn

Simulation Research on the Effect of Cooled EGR, Supercharging andCompression Ratio onDownsized SI Engine Knock

SHU Gequn, PAN Jiaying, WEI Haiqiao*, and SHI Ning

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China

Received March 15, 2012; revised October 16, 2012; accepted November 14, 2012

Abstract: Knock in sparkignition(SI) engines severely limits engine performance and thermal efficiency. The researches on knock of downsized SI engine have mainly focused on structural design, performance optimization and advanced combustion modes, however there is little for simulation study on the effect of cooled exhaust gas recirculation(EGR) combined with downsizing technologies on SI engine performance. On the basis of mean pressure and oscillating pressure during combustion process, the effect of different levels of cooled EGR ratio, supercharging and compression ratio on engine dynamic and knock characteristic is researched with threedimensional KIVA3V program coupled with pressure wave equation. The cylinder pressure, combustion temperature, ignition delay timing, combustion duration, maximum mean pressure, and maximum oscillating pressure at different initial conditions are discussed and analyzed to investigate potential approaches to inhibiting engine knock while improving power output. The calculation results of the effect of just cooled EGR on knock characteristic show that appropriate levels of cooled EGR ratio can effectively suppress cylinder highfrequency pressure oscillations without obvious decrease in mean pressure. Analysis of the synergistic effect of cooled EGR, supercharging and compression ratio on knock characteristic indicates that under the condition of high supercharging and compression ratio, several times more cooled EGR ratio than that under the original condition is necessarily utilized to suppress knock occurrence effectively. The proposed method of synergistic effect of cooled EGR and downsizing technologies on knock characteristic, analyzed from the aspects of mean pressure and oscillating pressure, is an effective way to study downsized SI engine knock and provides knock inhibition approaches in practical engineering. Key words: exhaust gas recirculation, compression ratio, knock, pressure wave equation, pressure oscillation

1 Introduction ∗

At present, with the increasing issues of energy and environment all over the world, much attention has been paid to the more stringent emission regulations in light of Tokyo Agreement, which is characterized by taking CO2

emission as an important criterion of fuel consumption. Engine downsizing [1–4] refers to the way of reducing cylinder volume assisted with intake supercharger to maintain power output, or increasing engine power output without changing cylinder volume while improving engine efficiency. Engine downsizing is one of the most potential ways to improve fuel consumption and emission performance [5] . Depending on downsizing factors, a reduction in fuel consumption of up to 30% can be achieved in the New European Driving Cycle (NEDC) [6–10] . The advantages of downsized gasoline engines have been widely reported and proved being able to meet stringent emissions [11] . To meet normal power and torque output, engine

downsizing technology has to be assisted with high

* Corresponding author. Email: [email protected] This project is supported by National Natural Science Foundation of

China(Grant No. 51176138), and Tianjin Municipal Natural Science Foundation of China(Grant No. 12TJZDTJ28800) © Chinese Mechanical Engineering Society and SpringerVerlag Berlin Heidelberg 2012

supercharging and variable technologies [12–13] . However, knocking combustion is still the primary barriers for downsized SI engine to improve thermal efficiency. It is shown that the higher compression ratio and supercharging applied to SI engines can make knocking problem more serious [14] . Therefore, it is significant to investigate formation and inhibition mechanism of knock and propose new knocking suppression methods [15] . Because of the complexity of phenomena itself, the

formation mechanism of knock hasn’t been completely understood so far [16] . Autoignition theory demonstrates that knock is spontaneous combustion of end gas due to the inhomogeneity of high temperature and high pressure in end zone [17] . Detonation wave theory describes knock as a complex combustion of the unburned fuelair mixture ahead of the normal flame front by the action of a shock wave or true detonation wave [18] . According to flame propagation theory, knock is a result of accelerating flame propagation in homogeneous mixture [19] . Autoignition theory has been widely accepted due to its good explanation for various test results. However, all of these knocking theories find that when knock occurs, the velocity of flame propagation and heat release rate are considerably high, which is accompanied by unique highfrequency pressure oscillation in combustion chamber. Pressure oscillation, an acoustical parameter in nature, is

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a common phenomenon in SI engines knocking cycles. For its generation, it’s considered that the rapid heat release of autoignition reaction results in local overpressure which then develops into pressure wave or shock wave propagating in the combustion chamber back and forth [20] . What’s more, knock intensity depends mainly on cylinder pressure oscillation and chemical heat release [21–23] . Therefore, some intrinsic correlation may exist between engine knock characteristics and pressure oscillation in combustion chamber. However, most current studies of pressure oscillation, paying more attention to experimental investigation, don’t give quantitative description and clear explanation for the relation between unsteady transients and pressure oscillation. In order to suppress engine knock, the currently common

ways are to delay ignition timing, take lower compression ratio or change intake components [24] . Since that the first two approaches can affect engine thermal efficiency obviously, researchers have shifted to the latter one in recent years. Lots of studies [25–29] have indicated that cooled exhaust gas recirculation(EGR) can suppress knock effectively without much loss of power output. PIROUZPANAH, et al [30] , investigated the effect of EGR on a dualfuel engine to improve thermal efficiency and emission of carbon monoxide at part load. The results show that the thermal effect and chemical effect of EGR play an important role in combustion process. KARIM, et al [31–33] , also considered that positive effects of EGR can be moderated by its diluting effect. In addition, turbocharged SI engine becomes popular in

the market due to its compactness and high power density. With regard to the effect of cooled EGR on knock performance in turbocharged SI engine, lots of literature has been published [34] . The effect of cooled EGR on a turbocharged sparkignited engine was investigated by GRANDIN, et al [35] , who investigated the potential for suppressing knock and reducing the exhaust gas temperature at high load conditions. The results indicate that the effect of cooled EGR on knock can be concluded as following aspects. The first one is that the dilution effect of EGR can extend combustion duration and reduce incylinder temperature, which is beneficial for inhibiting autoignition. However, the speed of flame propagation decreases with the utilization of cooled EGR, which is undesirable for knock inhibition. Although there is a basic understanding of inhibitory effect of EGR on knock, very few has researched the synergistic effect of cooled EGR, supercharging and compression ratio on SI engine knock, which is of great significance to optimize premixed combustion process. Combined with pressure wave equation of internal

combustion engine, the effect of cooled EGR, supercharging and compression ratio on SI engine performance, especially the knock characteristic, is discussed systematically. This paper is organized as follows. Firstly the effect of cooled EGR, EGR combined with

supercharging, and EGR combined with compression ratio on engine knock performance is investigated and analyzed, respectively. Then the synergistic effect of the cooled EGR, supercharging and compression ratio is discussed in depth. The results show that appropriate EGR level combined with downsizing technologies can suppress knock effectively while improving engine performance. Therefore, in this paper, the proposed research of synergistic effect of cooled EGR and downsizing technologies on suppressing cylinder pressure oscillations is an effective way to study downsized SI engine knock and reveal its inhibition mechanism in practical engineering.

2 Pressure Wave Equation of Internal Combustion Engine

In general, for the combustion process of internal combustion engines, two kinds of time scale can be utilized to describe the changing pressure history. Macrotime scale, equivalent to cycle period, can describe the macroscopic characteristics of cylinder pressure, and microtime scale, equivalent to combustion instantaneity, describes microscopic characteristics of incylinder pressure. Therefore, the total cylinder pressure can be decomposed into mean pressure and oscillating pressure by

= p p p′ + , (1)

where p—Total cylinder pressure, p—Mean pressure, p′—Oscillating pressure.

The macroscopic scale and microscopic scale of wave equations of internal combustion engine is theoretically deduced in Refs. [36-37]. The macroscopic pressure wave equation can be

represented by

( ) ( ) 2 0 = 1 c p Q A m

t t ρ γ ρε

∂ ∂ − + − − ∂ ∂ & & , (2)

where Q & —Heat release of combustion, γ —Adiabatic exponent, ρ —Gas density, 0 A —Laminar flow is 0, turbulent flow is 1,

ε —Turbulent energy term in model, m & —Increase in unit volume of quality, c —Speed of sound.

The microscopic pressure wave equation can be represented by

( ) ( )

2 2

2 2 2

0 2

1 1 1 = p p c c t a

Q A uu H

t

γ

ρ ε ρ

′ ∂ − ′ − ∇ ∂

∂ + + ∇ − ∇

∂

& r r , (3)

CHINESE JOURNAL OF MECHANICAL ENGINEERING ·3·

where ( ) 0 2 = 3 s H A k F g ρ σ ρ − ∇ + ∇ + +

r r r ,

κ —Energy dissipation term in k ε model, a —Dimensionless, related with pressure gradient

scaling (PGS) in KIVA, u—Fluid velocity, Fs—Momentum increment due to spray and other

causes, σ —Viscous force tensor, g —Acceleration due to gravity.

Finally, pressure wave equation of internal combustion engine can be obtained by integrating above formulas from Eq. (1) to Eq. (4):

( ) ( )

( ) ( )

( )

2 2

0 2 2 2

2 2 2 0

0 0 2

1 r 1

2 3

1 1 c s

s

p p p m Q A t c t a t

uu A k F g

p u A J A Q Q t c

γ ρ γ γ γ ρε

ρ ρ σ ρ

γγ σ ρε

∂ ∂ ∂ − ∇ = − − + − + ∂ ∂ ∂

+ ∇ ⋅ + ∇ −∇ ⋅ −∇ −∇ + − ∂ − ∇⋅ + − ∇ −∇⋅ + + + ∂

r & &

r r r r r r rr r r

r r r r r r r & & u . (4)

3 Experiment Subject and Simulation Model

Combined with wave equation of internal combustion engine, the synergistic effect of cooled EGR, supercharging and compression ratio on the knock characteristics of SI engine was researched based on previous work [38] . The experiment was performed on a modified, single cylinder twostroke gasoline engine, which was modified from a diesel engine. In order to obtain the expected knock level, the advanced ignition timing (–30 ºCA) and relatively low octane number of fuel (MON 53) were adopted in the test. In addition, a data acquisition system was established, and Kistler 6125 flushmount incylinder pressure sensor was installed at cylinder head, as shown in Fig.1. Related specifications of the engine are presented in Table 1.

Table 1. Technical specifications of SI engine chosen for calculation

Parameter Value

Engine type Single cylinder Bore L×stroke S/(mm×mm) 95.0×60.2 Compression ratio R 9.6

Intake temperature T1/ 60 Intake pressure p/MPa 0.12 Engine speed n/( r∙min –1 ) 900 Ignition Advance t/(ºCA) –30 EGR temperature T2/ 60

Fig. 1. Distribution of pressure sensor in combustion chamber

In order to validate the simulation model, the comparison of experimental data and calculated results of 900 r/min operating condition is analyzed, shown as Fig. 2. Compared with experimental data, the calculated results in this paper are proved to be within acceptable range. It is obvious that the calculated results agree well with the experimental pressure history and pressure oscillation trace. When the piston reaches approximately the top dead center(TDC), highfrequency oscillating pressure generating from end gas autoignition appears, and then deceases quickly in the later expansion stroke, and both the maximum of absolute incylinder pressure obtained from experiment and calculation are more than 10 MPa. Therefore, this computational model is able to utilized to investigate pressure oscillation characteristics in knocking cycles, and also the existing experimental data can be utilized effectively [39–40] .

2

25 20 15 10 5 0 5 10 15 5.0

2.5

0.0

2.5

5.0

Crank angle θ /°CA

Pressure oscillationp'/M

Pa Pressurep/M

Pa

0 2 4 6 8 10

(a) Experimental data


25 20 15 10 5 0 5 10 15 5.0

2.5

0.0

2.5

5.0

Pressure oscillation p'/M

Pa


Pressure p/M

Pa

0 2 4 6 8 10

（b）Calculated results

Fig. 2. Comparison of experimental data and calculated results

4 Results and Discussion

When autoignition occurs in combustion process, rapid release of chemical energy of the end gas will result in pressure wave or even shock wave spreading in the combustion chamber back and forth. Then chamber cavity is forced to resonate at its resonant frequency, emitting sharp metallic noises. By utilizing an appropriate level of EGR, the ignition delay timing and combustion duration can be extended, and the temperature of end zone is maintained relatively lower temperature which retards chemical reaction of end gas. As a result, the amplitude of pressure oscillations is reduced drastically. Ref. [41] mentioned that the change range of temperature caused by EGR could reach up to 1 100 K. Here, the EGR percentage is defined by [42]

1 e

1 2

=100 n

n n η ⋅

+ , (5)

where e η —Percentage of the EGR,

1 n —Moles of EGR or added gases,

2 n —Total moles of air and fuel charge.

4.1 Effect of cooled EGR on mean pressure In this paper, the calculation of EGR is based on the

variation in CO2 concentration. The effect of different CO2

concentrations on mean pressure and temperature of combustion chamber is discussed and compared with original data without EGR utilization. From Fig. 3 and Fig. 4 we can see that, with the increase

of CO2 concentration, the ignition timing delays and incylinder temperature decreases obviously, especially the maximum of combustion temperature. Fig. 3 shows that with the increase of CO2 concentration, the maximum of mean pressure remains basically equal, which indicates that

appropriate EGR levels don’t result in significantly negative effect on combustion process. However, there is an obvious decrease in cylinder pressure when the CO2

concentration reaches 20%. The main reason is that the excessive CO2 dilutes the combustible mixture, which changes the airfuel ratio of the normal ignition and combustion to some extent.

30 20 10 0 10 20 30 0

2

4

6

8

10

Mean pressure p/M

Pa


Without EGR 2% CO 2 4% CO

2

6% CO 2 8% CO

2

Fig. 3. Mean pressure under different CO2 concentration condition

30 20 10 0 10 20 30

500

0


Without EGR 2% CO

2

4% CO 2 6% CO

2

8% CO 2

Temperature T/K

3 000

2 500

2 000

1 500

1 000

Fig. 4. Temperature under different CO2 concentration condition

The focus of this paper is to research the effect of cooled EGR on knocking combustion process, thus the temperature of end gas is discussed in detail. Fig. 4 shows that with the increase of CO2 concentration, the maximum of incylinder combustion temperature decreases more and more obviously. Compared with original temperature curve, the maximum of temperature, corresponding to 4% and 8% CO2 concentration cases, is decreased by 2.8% and 5.5%, respectively. The fact is due to that the comic molecule CO2

has larger specific heat capacity. Therefore, the dilution effect of cooled EGR not only reduces the maximum of combustion temperature, but also decreases flame propagation and prolongs combustion duration to some extent.


Fig. 5 is the schematic diagram of the combustion duration of different percentages of CO2 concentration. From the figure we can see that with the increase of CO2

concentration, the combustion duration is prolonged significantly. Although lower temperature in end gas zone can help avoid autoignition occurrence, the decrease in speed of flame propagation resulting from the utilization of cooled EGR has adverse effect on knock suppression. Therefore, it’s necessary to further discuss the effect of EGR on pressure oscillation.

0 2 4 6 8 10 0

10

20

30

40

50

Com

bustion duration

ω/

CO 2 percentage η /%

Fig. 5. Schematic diagram of the corresponding combustion duration under different CO2 concentrations condition

4.2 Effect of cooled EGR on oscillation pressure and knock intensity

It is known that there is a close relation between high frequency pressure oscillation and knock intensity. In this paper, knocking indicator, the maximum amplitude of pressure oscillations within certain window of crank angle, is utilized to define knock intensity [43] . Fig. 6 shows four oscillating pressure curves under the

condition of different CO2 concentrations. It is obvious that there is a significant increase in ignition delay timing and combustion duration with the increase of CO2

concentration, which is consistent with the variation of the mean pressure discussed above, shown as Figs. 6 (a) and (b). And meanwhile, the amplitude of pressure oscillation decreases significantly in general except for the case of 4% CO2 concentration. This is mainly due to the fact that the extension of combustion duration slows down the rate of chemical heat release, which weakens pressure intensity in combustion chamber to some extent. Therefore, compared with the four pressure oscillation curves, it can be concluded that the cooled EGR has a good inhibitory effect on SI engine knock intensity.

20 10 0 10 20 2

1

0

1

2


Pa

Crank angle θ / (°)

2% CO 2 4% CO

2

(a) Pressure oscillation of 2% and 4% CO2 concentrations

20 10 0 10 20 1.2

0.6

0.0

0.6

1.2

Crank angle θ /(°)


Pa 6% CO

2

8% CO 2

(b) Pressure oscillation of 6% and 8% CO2 concentrations

Fig. 6. Comparison of oscillating pressure under different CO2 concentration condition

Finally, engine power performance allowing for knock suppression is researched. Fig. 7 shows the maximum of outbreak pressure under different CO2 concentrations. It can be seen that with the increase of CO2 concentration, the maximum of mean pressure increases at first and then decrease, but the oscillating pressure goes downward in general. This demonstrates that appropriate levels of cooled EGR can inhibit knock intensity effectively without much loss of power output. Noting that the 4% CO2 concentration is an abnormal case that the maximum of mean outbreak pressure is the largest, however, the maximum of pressure oscillation is also considerable high, approximately 1.5 MPa. The reason is that the decrease in speed of flame propagation may affect autoignition of end gas to some extent.


0 2 4 6 8 10 0.0 0.4 0.8 1.2 1.6 2.0


7.0

7.1

7.2

7.3

7.4 Maximum p' /MPa

Maximum p /M

Pa

Fig. 7. Maximum of cylinder pressure outbreak profile under different CO2 concentration condition

4.3 Cooled EGR combined with supercharging The maximum of outbreak pressure under different CO2

concentrations and different supercharging p0 is discussed in order to further analyze the effect of cooled EGR and supercharging on engine performance. The simulation results of maximum outbreak pressures under the condition of different CO2 concentrations combined with supercharging are shown in Fig. 8.

0 2 4 6 8 10 12 2

4

6

8

10

Max.mean pressure p/M

Pa

CO 2 percentageη /%

p 0 =0.132 MPa p

0 =0.172 MPa

p 0 =0.182 MPa p 0 =0.192 MPa

(a) Maximum value of mean pressure

0 2 4 6 8 10 12 0.0

0.5

1.0

1.5

2.0

Max.mean pressure p' /MPa


p 0 =0.132 MPa p

0 =0.172 MPa

p 0 =0.182 MPa p 0 =0.192 MPa

(b) Maximum value of oscillation pressure

Fig. 8. Maximum of outbreak pressures under the condition of different CO2 concentrations and supercharged pressure

Fig. 8(a) shows that with the increase of supercharging within a certain range, the mean pressure continuously increases, but the trend in growth slows down. This may due to the higher supercharging making more fuelair mixture participates in combustion reaction, in which more released heat is transformed into mechanic power. However, when the supercharged pressure increases to a certain level, the growth trend of cylinder pressure begins to decline. For certain supercharging, mean pressure remains basically equal in different CO2 concentrations, especially in the case of high supercharged pressure. Fig. 8 shows that when supercharged pressure reaches 0.192 MPa, the maximum of mean pressure maintains almost unchanged under different CO2 concentrations condition. The probable reason is that with increase in the intake density, the dilution effect of CO2 gas weakens drastically. Allowing for knocking characteristics, the oscillation

pressure is further investigated. Fig. 8(b) shows that, for certain supercharged pressure, the maximum of oscillating pressure decreases in general with increase of CO2

concentration. Noting that the oscillating pressure, corresponding to the range from 2% to 6% CO2

concentration, maybe locates in a transition region due to the fact that there isn’t clear variation law. It might have something to do with turbulence, flame propagation as well as autoignition in combustion chamber. For a certain CO2

concentration, oscillating pressure is significantly enhanced with the increase of supercharging due to the fact that under the condition of 0.192 MPa supercharged pressure and 0% CO2 concentration, the maximum amplitude of oscillating pressure goes up to 1.5 MPa. The probable reason is that for fixed compression ratio, the increase of the intake will lead to higher pressure and temperature of combustion chamber, which is prone to autoignition occurrence.


4.4 Cooled EGR combined with variable compression ratio

The maximum outbreak pressure under different CO2

concentrations and compression ratio is discussed to investigate the effect of cooled EGR and variable compression ratio technology on engine performance. The calculated results of the maximum outbreak pressure under the condition of different CO2 concentrations and compression ratios are shown as Fig. 9.

0 2 4 6 8 10 12 2

4

6

8

10

Maximum

of m

ean pressure p/M

Pa


R=8.6 R=9.6 R=10.6

(a) Maximum value of mean pressure

0 2 4 6 8 10 12 0.0

0.5

1.0

1.5

2.0

2.5

3.0


Maximum

of m

ean pressure p' /MPa

R=8.6 R=9.6 R=10.6

(b) Maximum value of oscillating pressure

Fig. 9. Maximum of outbreak pressure under different CO2 concentration and compression ratio condition

It is known that higher compression ratio can improve thermal efficiency of internal combustion engine significantly. However, the knocking tendency also increases obviously due to the increase of combustion temperature, especially in the end gas zone. Fig. 9 (a) shows that for a certain level of CO2 concentration, the maximum of mean pressure is significantly increased in higher compression ratio case, but the effect weakens continually. The reason is that the increase in temperature of combustion chamber improves the combustion process and the velocity of flame propagation. Fig. 9 (b) shows that

the maximum of oscillating pressure is greatly increased with the improvement of compression ratio. The maximum of oscillating pressure, corresponding to compression ratio 10.6 and CO2 concentration from 0% to 4%, is beyond 2.0 MPa, which indicates that the increase of compression ratio can lead to knock easily. However, with the increase of CO2

concentration, the oscillating pressure at high compression ratio decreases significantly. Compared with the maximum of pressure oscillation under the condition of compression ratio 10.6 and 2% CO2 concentration, the oscillation pressure in the case of 10.6 compression ratio and 8% CO2

concentration falls by about 46%. However, the maximum of pressure oscillation under the condition of compression ratio 10.6, and 8% CO2 concentration is equal to that of compression ratio 9.6 and 2% CO2 concentration. This illustrates that engine knock can be suppressed effectively by the utilization of cooled EGR.

4.5 Cooled EGR combined with supercharging and compression ratio From the above analysis we know that power output and

engine knock intensity can be increased by both compression ratio and supercharging. Supercharged technology increases the cylinder pressure by improving density of the intake, while compression ratio affects combustion process by changing the volume of combustion chamber directly. For the improvement of combustion process, variable compression ratio technology is more significant than supercharging. Therefore, it is necessary to further discuss the synergistic effect of cooled EGR, supercharging and compression ratio on the engine performance and knock characteristic based on the preceding analysis. Fig. 10 is the curve of maximum outbreak cylinder

pressure under the condition of CO2 concentrations from 0% to 12%, compression ratio10.6 and initial pressure 0.192 MPa. The figure shows that the variation of mean pressure and pressure oscillation is similar to that of the previous ones. However, the maximum of cylinder pressure is improved by the higher initial pressure and compression ratio significantly. The mean pressure goes up to 8.19 MPa, and the maximum amplitude of oscillating pressure reaches 3.2 MPa. Although engine performance is improved remarkably, a higher level of cooled EGR should be utilized in order to suppress engine knock intensity effectively. As is shown in Fig. 10, when using 8% CO2

concentration, the maximum amplitude of pressure oscillation is reduced to 1.7 MPa, but the maximum mean outbreak pressure can reach about 8.2 MPa. Unfortunately, the effect of cooled EGR on combustion emissions aren’t discussed here, which may be investigated in future research.


0 2 4 6 8 10 12 1.2

1.8

2.4

3.0

3.6

Maximum p' /MPa

Maximum p

/MPa

7.6

7.8

8.0

8.2

8.4


Fig. 10. Maximum of outbreak pressure under the condition of different CO2 concentrations and R=10.6, p0=0.192 MPa

Fig. 11 and Fig. 12 are the curves of mean pressure and oscillating pressure in the 8% CO2 concentration and 0% EGR case, respectively. In this condition, the compression ratio and initial pressure is set as 10.6 and 0.192 MPa, respectively. With the increase of CO2 concentration, ignition timing and combustion duration are extended and combustion temperature is reduced obviously, which is shown as Fig. 13 and Fig. 14. However, in order to achieve higher power output and effective knock suppression under the condition of high initial pressure and high compression ratio, several times more cooled EGR than that under the original condition is necessarily utilized in order to suppress knock effectively. is necessary. It can be seen from Fig. 12, the maximum of pressure oscillation in 8% CO2

concentration case decreases to approximate 1.6 MPa which is lower 1.0 MPa than that without EGR case,.

20 10 0 10 20 30 2

4

6

8

10


Mean pressure p

/MPa

p 0 =0.192 MPa, R=10.6,

no EGR p 0 =0.192 MPa, R=10.6,

8% CO 2

Fig. 11. Comparison of mean pressure curves

30 20 10 0 10 20 30 2

1

0

1

2

3


p 0 =0.192 MPa, R=10.6,

no EGR p 0 =0.192 MPa, R=10.6,

8% CO 2

Oscillation pressure p'/M

Pa

Fig. 12. Comparison of oscillation pressure curves

25 20 15 10 5 0 5 10 15 20 0.0

0.2

0.4

0.6

0.8

1.0


p 0 =0.192 MPa, R=10.6,

no EGR p 0 =0.192 MPa, R=10.6,

8% CO 2

Heat release

χ /%

Fig. 13. Comparison of cumulative heat release rate

30 20 10 0 10 20 30 0

500

1 000

1 500

2 000

2 500

p 0 =0.192 MPa,R=10.6,

no EGR p 0 =0.192 MPa,R=10.6,

8% CO 2


Temperature T /K

3 000

Fig. 14. Comparison of cylinder temperature under 8% CO2 and without EGR condition

Due to the fact that there is some close relation between knock and cavity resonance of combustion chamber, it is necessary to analyze frequency characteristics of oscillating pressure. Fig. 15 is the FFT (Fast Fourier Transform) frequency spectrum curve of oscillating pressure. Fig. 15 shows that there are two resonant peaks, approximately 5.8


KHz and 11 KHz, respectively, in which the vibration energy is mainly distributed during engine combustion. The 5.8 kHz frequency is the natural vibration of the firstorder transverse mode of cylinder gas and 11 kHz is the second order transverse mode. Compared with the case without EGR, not only the amplitude corresponding to 8% CO2

concentration is lower, but also the whole spectrum curve shifts to the high frequency side. This may due to that the lower combustion temperature resulting from cooled EGR affects sound speed in combustion chamber. However, the effect of combustion chamber geometry on cavity resonance characteristic isn’t discussed in this paper, which needs to be investigated in future study.

0 5 10 15 20 25 0.0

0.1

0.2

0.3

0.4 p 0 =0.192 MPa, R=10.6,

no EGR p 0 =1.92 MPa,R=10.6,

8% CO 2

Frequency f /KHz

Amplitude m

/MPa

Fig. 15. FFT spectrum curve from cylinder oscillating pressure

5 Conclusions

(1) On basis of mean pressure and oscillating pressure, the coupling model of KIVA3V combustion model and pressure wave equation is established to research the synergistic effect of cooled EGR and downsizing technologies on knock characteristic. The calculation results show that this method is an effective way to study knock characteristic of downsized SI engine. (2) With the increased of CO2 concentration within a

certain range, ignition delay timing and combustion duration are extended correspondingly, and combustion temperature decreases obviously, which is beneficial for inhibition of autoignition occurrence. And meanwhile, the suppression of highfrequency pressure oscillations without obvious decrease of mean pressure indicates that cooled EGR can be utilized to suppress knock effectively. (3) The results of synergistic effect of different levels of

cooled EGR, supercharging and compression ratio on engine performance indicates that both supercharging and compression ratio can improve engine power output, but the latter is more obvious. Under the condition of high supercharged pressure and compression ratio, several times more cooled EGR than that under the original condition is

necessarily utilized in order to suppress knock. (4) For relation between knock characteristic and cavity

resonance of combustion chamber, the Fast Fourier Transform analysis of oscillating pressure shows that frequency spectrum shifts to low frequency region as a whole due to the fact that lower combustion temperature resulting from the utilization of cooled EGR affects sound speed in combustion chamber.

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Biographical notes SHU Gequn, born in 1964, is currently a professor and a director at State Key Laboratory of Engines, Tianjin University, China. He received his bachelor degree in internal combustion engines from Zhejiang University, China and both his master and PhD degrees from Department of Thermal Energy Engineering, Tianjin University, China. He has been working in Tianjin University, China. His research interests include engine noise and vibration and modern design technique, etc. Tel: +862227409558; Email: [email protected]

PAN Jiaying, born in 1987, is currently a PhD candidate at State Key Laboratory of Engines, Tianjin University, China. He received his bachelor degree in thermal energy and power engineering from Shandong University of Science and Technology, China and his master degrees from Internal Combustion Engines, Tianjin University, China. His research interests include engine combustion, SI engine knock, etc. Email: [email protected]

WEI Haiqiao, born in 1974, is currently an associate professor at State Key Laboratory of Engines, Tianjin University, China. He received his bachelor degree in North China Electric Power University, China. He received his PhD degree from Department of Mechanical Engineering, Tianjin University, China. His research interests include combustion processes and engine noise and vibration, SI engine downsizing. Tel: +862227891285; Email: [email protected]

SHI Ning, born in 1988, is currently a master candidate at State Key Laboratory of Engines, Tianjin University, China. He received his bachelor degree from Nanjing Agricultural University, China. His research interests include engine combustion, SI engine knock, etc. Email: [email protected]

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2012-087