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Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine
Energy 36 (2011) 6948e6959
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Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine Seungmok Choia, Wonah Parka, Sangyul Leea, Kyoungdoug Mina, *, Hoimyung Choib a b
SNU Automotive Laboratory, School of Mechanical and Aerospace Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151e744, Republic of Korea Advanced Institutes of Convergence Technology, 864-1, Iui-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-270, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2011 Received in revised form 2 August 2011 Accepted 10 September 2011 Available online 1 November 2011
The effects of in-cylinder EGR stratification on combustion and emission characteristics are investigated in a single cylinder direct injection diesel engine. To achieve in-cylinder EGR stratification, external EGR rates of two intake ports are varied by supplying EGR asymmetrically using a separated intake runner. The EGR stratification pattern is improved using a 2-step bowl piston and an offset chamfer at the tangential intake port. When high EGR gas is supplied to the left (tangential) port, a high EGR region is formed at the central upper region of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and PM is reduced significantly. When high EGR gas is supplied to the right (helical) port, a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NOx is reduced without PM penalty. Stratified EGR potentially reduces NOx by maximum 45%, without penalties of performance and other emissions. A proper in-cylinder swirl with stratified EGR maximizes the effects and achieves simultaneous reduction of NOx by 7% and PM by 23%. Moreover, the robustness of stratified EGR is evaluated under various operating conditions and injection strategies. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Direct injection diesel engine Exhaust gas recirculation (EGR) Stratified EGR Swirl control valve (SCV) Nitric oxides (NOx) Particulate matters (PM)
1. Introduction High-speed direct injection (HSDI) diesel engines have advantages such as high efficiency, affordability, and excellent reliability compared to other internal combustion engines. Recent HSDI engines have achieved higher specific power and better fuel efficiency than previous engines by using high injection pressure provided by improved fuel injection technologies and advanced charge technologies, such as turbochargers and intercoolers. Recently, noise levels have also improved due to the application of common rail injection systems. However, HSDI diesel engines emit more nitrogen oxides (NOx), particulate matters (PM) and other pollutants than their counterparts. According to Euro VI, which will be enforced in 2014, NOx should be reduced by 55.6%, while PM must be maintained at the Euro V level [1]. EGR (exhaust gas recirculation) is one of the most effective ways of controlling the combustion and emissions of a diesel engine. EGR raises the specific heat capacity (thermal effect) and lowers the O2 concentration (dilution effect) of trapped gas under the same boost conditions. EGR lowers the flame temperature [2,3] and delays
* Corresponding author. Tel.: þ82 2 880 1661; fax: þ82 2 883 0179. E-mail address: [email protected] (K. Min). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.09.016
combustion (ignition delay, premixed combustion and diffusion combustion) [4]. As the EGR rate increases, NOx is reduced from the decrease in O2 concentration and burnt gas temperature. However, it prevents soot from oxidizing, and generally more PM is produced, which is the so-called ‘NOx-PM trade-off.’ To overcome the NOx-PM trade-off, new combustion concepts have been studied such as homogeneous charged compression ignition (HCCI) combustion [5], smokeless-rich diesel combustion [6], and modulated kinetics (MK) combustion [7]. In HCCI combustion, in-cylinder temperature, EGR rate and injection parameters are controlled precisely to allow well mixed prior to reaching its ignition temperature during compression. However, HCCI reaction in totally homogeneous mixture is so fast that it occurs knock-like rapid pressure rise and noise. To avoid it, stratification of temperature and mixture and fuel additives have been studied to control the reaction rate. Smokelessrich diesel combustion uses aggressive high EGR to initiate combustion at a temperature below the temperature needed to form PM. MK combustion concept reduces NOx and PM simultaneously using a high EGR rate to reduce the oxygen concentration and retarded injection to accomplish premixed combustion. While the new combustion concepts differ in many ways, one common characteristic among them is to lower the combustion temperature through a high EGR rate, usually more than 30e55%. With this high EGR, along with the lowered local burnt gas temperature, the local
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equivalence ratio is decreased by a prolonged ignition delay that exceeds the injection duration and allows for sufficient precombustion mixing [8]. Although these concepts are very attractive for both NOx and PM emissions, they are accompanied by a large increase in CO and HC emissions [9e11] and, in most cases, with an increase in fuel consumption [6,9,12]. In-cylinder stratification is a possible method for simultaneously reducing NOx and PM. In-cylinder stratification has mainly been studied in HCCI. Studies on in-cylinder thermal stratification [13e15], EGR stratification [16,17] and fuel stratification [13,18e20] have been performed to promote combustion phase control and suppress the rapid pressure rise and knocking associated with HCCI. Previous studies have shown that thermal and fuel stratification strongly affect combustion and extend the operating range at the high load by smoothing heat release. However, the effects of EGR stratification are not clearly understood yet. In HSDI diesel engines, where stratified fuel combustion occurs, thermal and EGR stratification have rarely been studied. Fuyuto et al. have suggested in-cylinder EGR stratification concept that reduced smoke while NOx was maintained at a medium speed/load level [21]. In this study, the potential for combustion and emission control by in-cylinder EGR stratification was evaluated in a single cylinder engine experiment. In-cylinder EGR stratification was improved by a 2-step piston and an offset chamfer in the tangential port, and the changes in EGR stratification patterns were predicted using a CFD flow simulation [22]. Both the 2-step piston and the offset chamfer were manufactured and adapted to a single cylinder engine, and the experiment was performed under various EGR supply methods for different in-cylinder stratification using the separated intake runner and swirl control valve (SCV). The robustness of the incylinder EGR stratification was verified under various speeds, loads, EGR rates, and injection strategies. 2. The concept of in-cylinder EGR stratification The motivation behind in-cylinder EGR stratification is increasing or decreasing the EGR concentration at the combustion site above or below the overall EGR rate. If the degree of stratification and distribution of EGR can be controlled so that combustion occurs at higher EGR concentrations, NOx is expected to be further reduced. In addition, PM can be maintained or reduced because of the same overall air to fuel ratio (AF) and the soot oxidation increment by the mixing with high-concentration O2 regions. The desired in-cylinder EGR stratification and spray targeting is shown in Fig. 1. Because combustion starts in the locally high EGR region, where the local O2 concentration is lower than average, the combustion processes are prolonged, and the combustion temperature decreases. As a result, further NOx reduction is
Fig. 1. Desired in-cylinder EGR stratification and spray targeting [22].
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Fig. 2. Mechanism of NOx and PM reduction by stratified EGR [22].
expected in the same overall EGR rate. Meanwhile, a longer ignition delay by locally high EGR increases the premixing of fuel and air, and soot formation is expected to decrease. A high-concentration O2 region is participating in combustion during the expansion stroke and promotes soot oxidation. Thus, PM emission is also expected to decrease (Fig. 2). Two main concerns in in-cylinder EGR stratification are which EGR stratification pattern is the most favorable and how the desired pattern can be obtained in a real engine. Regarding the favorable pattern, the degree of stratification should be maximized, and the higher EGR region should be symmetrically distributed so that the combustion region can be placed in the higher EGR region. A previous study on implementing EGR stratification showed that the gas through the tangential port gathers in the upper part of the cylinder, whereas the gas through the helical port moves down to the lower part of the cylinder. Thus, in-cylinder EGR can be stratified by an asymmetrical EGR supply through the intake ports [21]. The asymmetrical supply of external EGR, which supplies high EGR gas through the tangential port, is shown in Fig. 3. The intake port shape needs to be modified to control the in-cylinder gas flow direction for better stratified EGR pattern. However, an overall modification or an optimization of the intake port shape is difficult because other mechanical parts such as an injector, a cam and valve train, and oil and coolant passages should be newly designed to avoid an interference with a modified port. Thus, it means that a new engine head should be designed, which entails significant cost and effort. Instead, several minor changes were conducted in this study, among them a 2-step piston and an offset chamfer in the tangential port effectively controlled the in-cylinder EGR stratification pattern. A 2-step piston enhances the degree of in-cylinder EGR stratification [22]. Fig. 4 shows the CFD flow simulation result that compares the stratified EGR pattern of (a) a conventional piston and (b) a 2-step piston at near TDC. The EGR concentration of
Fig. 3. Asymmetrical external EGR supply for in-cylinder EGR stratification [22].
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Fig. 4. Enhancement of the degree of EGR stratification by a 2-step piston [22].
the high EGR region is increased in the 2-step piston by preventing a squish flow during the compression that mixes the upper gas of the higher EGR and the lower gas of the lower EGR. To improve the distribution of in-cylinder EGR stratification, an offset chamfer in the tangential port was proposed [22]. The base chamfer in the tangential port valve shifted 0.8 mm to the opposite direction of other intake valve as shown in the upper figure of Fig. 5. The stratified EGR pattern by CFD flow simulation in Fig. 5(a) and (b) showed that the offset chamfer improves the distribution of EGR stratification by shifting the biased high EGR region to the center of combustion chamber compared to the base chamfer. As a result, the stratified EGR pattern becomes more symmetric. The 2-step piston and the offset chamfered head were manufactured and adapted to a single cylinder engine to verify the effects experimentally.
Fig. 6. The intake runner for asymmetrical EGR supply to the intake ports.
3. Implementation of stratified EGR 3.1. Intake runner for stratified EGR To realize in-cylinder EGR stratification in the single cylinder engine experiment, a separate intake runner was designed to supply external EGR asymmetrically through the intake ports. As shown in Fig. 6, the runner is separated from the start, and each runner has an EGR inlet. Each EGR inlet is placed 220 mm upstream from the inlet of
Fig. 5. Improvement of the stratified EGR distribution by offset chamfer [22].
Fig. 7. EGR supply methods for stratified EGR.
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intake air was used to avoid the local temperature variation under stratified EGR. The in-cylinder swirl effects on stratified EGR were also evaluated by increasing in-cylinder swirl under RS-EGR using the SCV placed in the helical port (RSþSCV). 4. Experimental description 4.1. Experimental apparatus Fig. 8. In-cylinder EGR stratification patterns according to the EGR supply method.
the intake port. To enhance mixing with air, each EGR inlet has two holes at the top and bottom. The SCV is installed downstream of the EGR inlet in the direction of the helical port. The length of runner upstream of the EGR inlet is 410 mm long, and a plenum with a volume of 200 cc is installed at each runner to prevent EGR backflow to the other port when EGR is supplied asymmetrically. A thermocouple, a pressure transducer and a gas sampling line are installed in each port as closely as possible to the head to measure temperature, pressure and gas composition at the port inlet. 3.2. EGR supply method Using the separated intake runner, external EGR can be supplied to either one of the two intake ports or to both intake ports. Fig. 7 shows the EGR supply methods for stratified EGR. In Uniform EGR, EGR is supplied to both intake ports at the same time so that the gas at each port has the same composition of air and EGR. In the leftport stratified EGR (LS-EGR), EGR is supplied to the tangential port so that the tangential port has a higher EGR rate, and only air is supplied through the helical port. In this case, in-cylinder EGR is stratified to have a higher EGR in the upper center region of the combustion chamber as shown in Fig. 8(a). In the right-port stratified EGR (RS-EGR), EGR is supplied through the helical port. Thus, the right-port has higher EGR rate and only air is supplied through the tangential port. In this case, in-cylinder EGR is stratified to have a higher EGR in the lower peripheral region of the combustion chamber as shown in Fig. 8(b). Fig. 9 shows CFD simulation results of spray and fuel mixture formation at just before the combustion start (3 CA ATDC) in speed of 1500 rpm, EGR rate of 30%, injection timing of 5 CA BTDC injection pressure of 750 bar and injection quantity of 15.0 mg. In this condition, ignitable local mixtures that has equivalence ratio higher than 0.8 are formed at the peripheral region of the combustion chamber, close to the surfaces of first and second bowls. In LS-EGR, locally low EGR regions which have up to about 5% lower than overall EGR are placed at the ignition position (Fig. 8(a)), and combustion is expected to start from the low EGR region. In RS-EGR, on the contrary, locally high EGR regions which have up to about 5% higher than overall EGR are formed at the ignition position (Fig. 8(b)). The ignition position in stratified EGR will stay in similar position as of Uniform EGR, because local equivalence ratio difference caused by 5% of local EGR rate change is only 0.036, and cooled EGR which has the same temperature with
A single cylinder HSDI diesel engine equipped with the 2-step piston and the offset chamfered head was used for a stratified EGR combustion experiment. This engine has a common rail system with a piezo injector that enables injection pressures up to 1600 bar. Detailed specifications of the engine are shown in Table 1. Engine speed was controlled by a 37 kW DC dynamometer, and the quality of the diesel was preserved by using a large-capacity fuel tank during the entire experimental period. The oil and coolant temperature were maintained at 80 C, and the fuel temperature was maintained at 40 C during the experiment. The fuel mass flow rate was measured using a mass burette type flow meter (ONO SOKKI, FX-203P). Air and EGR mass flow rates were measured using a thermal resistance flow rate sensor (MKP, TSM-150). Because this type of sensor is sensitive to the composition of the measured gases, the measured EGR mass flow rate was corrected using the AF and gas factors for each species provided by the manufacturer. The EGR rate of the intake gas was calculated using the mass air flow rate and the EGR rate, and it was verified with the EGR rate calculated from the CO2 fractions in the exhaust gas and the intake gas. The concentrations of NOx, THC, CO, CO2, and O2 in the exhaust gas were measured using an exhaust gas analyzer (HORIBA, MEXA-7100DEGR), and PM was measured using a smokemeter (AVL, 415S). The two intake port pressures were measured using an absolute pressure transducer (Kistler, 4045A5), and in-cylinder pressure was measured using a relative pressure transducer (Kistler, 6055Bsp). Signals from the pressure transducers were recorded at every 1 CA for 200 cycles for each case using a data acquisition system. An intake boosting and EGR control system was developed to simulate boosting, back pressure and EGR in a multi-cylinder turbocharged engine in the single cylinder engine and to control air and EGR flow rate accurately under various EGR supply methods. The configuration of the system is shown in Fig. 10. To keep the same air and EGR flow rates at any EGR supply methods, the flow rate was controlled using sonic orifices by making choke flow from the compressed air and exhaust gas. From the accurate control of the intake gas flow rates, the combustion conditions were well maintained during the engine operation. NOx fluctuation at the same operating condition was less than 1 ppm. 4.2. Test conditions The test conditions are listed in Table 2. Each experiment was performed at three speed-loads of 1500 rpme15.0 mg/str (BMEP 4 bar), 2,000 rpme18.2 mg/str (BMEP 5 bar) and 1750 rpme27.6 mg/ Table 1 Engine specifications.
Fig. 9. Distribution of vaporized fuel (mixture fraction (Z) and equivalence ratio (Phi)) at 3 CA ATDC (speed: 1500 rpm, injection timing: 5 CA BTDC, injection pressure: 750 bar, injection quantity: 15.0 mg).
Engine type Bore stroke (mm mm) Displacement (cc) Compression ratio Con. rod length (mm) No. of nozzle holes Spray angle Nozzle diameter (mm) HFR (cc/100 bar/30 s)
4-Stroke DI 83 92 497 15.5 145.8 7 153 0.128 380
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Fig. 10. Intake boosting and EGR control system.
str (BMEP 8 bar), which are representative points of the NEDC driving cycle for the vehicle with a commercial four cylinder engine with identical dimensions. Boost pressure, AF and injection pressure were chosen to have the similar values with those of the multi-cylinder turbocharged engine not to exceed realistic boundary. At 1500 rpme15.0 mg/str and 2000 rpme18.2 mg/str, two EGR rates were tested, one in a conventional EGR rate and another in a higher EGR rate. In the higher EGR rate cases, the boost pressure was also increased to maintain an AF, and boost pressures were limited to pressures compatible with a 2-stage turbocharger in a multi-cylinder engine. For each case, Uniform EGR, LS-EGR and RS-EGR supply methods were compared, and the effects of SCV in RS-EGR were tested by closing at 30 and 60 . At 1500 rpme15.0 mg/str, various EGR supply methods were tested under two injection pressures of a conventional injection pressure (750 bar) and a high injection pressure (1250 bar) to verify the robustness of stratified EGR. And double injection with pilot injection of which quantity has the similar amount with that of the commercial engine at 1500 rpme15.0 mg/str was applied to the various EGR supply methods to evaluate the compatibility of stratified EGR with pilot injection.
measured in-cylinder pressure through the heat release analysis [23]. The equation is shown in Eq. (1), which is derived from the energy conservation equation of open system by assuming no crevice flow and uniform cylinder contents.
dQht ¼ Ahc ðT Tw Þ dt
(2)
4.3. Heat release analysis
hc ¼ 3:26B0:2 P 0:8 T 0:55 w0:8
(3)
To investigate the combustion changes by EGR supply methods more clearly, apparent heat release rate was calculated from the
Average gas velocity (w) in Eq. (3) is determined as Eq. (4) and coefficients C1 and C2 were set to 2.28 and 0 for the compression period and 2.28 and 3.24 103 for the combustion and expansion periods respectively.
Table 2 Experimental cases.
Engine speed (rpm) Injection quantity (mg/str) AF EGR rate (%) EGR strategy SCV ( ) Intake pressure (bar) Number of injections Main injection timing ( CA) Pilot injection timing ( CA) Injection quantity (PiI:MI) Injection pressure (bar)
Case 1
Case 2
Case 3
Case 4
Case 5
1500 15.0 22.0 35 U/LS/RS 0, 30 1.06 1 BTDC 5 e e 750
1500 15.0 22.0 43.1 U/LS/RS 0, 30, 60 1.21 1, 2 BTDC 5 BTDC 38 i4.5:95.5 750, 1250
2000 18.2 19.6 30 U/LS/RS 0, 30 1.06 1 BTDC 5 e e 956
2000 18.2 19.6 41.9 U/LS/RS 0, 30 1.26 1 BTDC 5 e e 956
1750 27.6 17.9 39 U/LS/RS 0, 30 1.68 1 BTDC 5 e e 1100
g dQn dQch dQht dV 1 dP ¼ ¼ P V þ g 1 dt g 1 dt dt dt dt
(1)
dQn =dt is the apparent net heat release rate which is the difference between the apparent gross heat release rate (dQch =dt) and the heat transfer rate to the walls (dQht =dt). It can be calculated from cylinder volume (V), measured cylinder pressure (P) and the ratio of specific heats (g) at a certain time. g was set to 1.35 for the gas exchange and compression periods, and 1.30 for the combustion and expansion periods. To get the apparent gross heat release rate (dQch =dt), the heat transfer rate (dQht =dt) was calculated using the Eq. (2), where heat transfer coefficient was defined as well-known Woschni’s correlation [23] in Eq. (3).
V Tr w ¼ C1 Sp þ C2 d ðP Pm Þ Pr Vr
(4)
5. Results and discussion 5.1. Comparison of EGR supply methods The EGR supply method determines the in-cylinder EGR stratification pattern near TDC, and combustion and emission characteristics are influenced by the EGR stratification pattern. The
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changes of combustion and emissions by stratified EGR were analyzed experimentally, and experiments were performed at a speed of 1500 rpm, a fuel quantity of 15.0 mg/str, an EGR rate of 43.1% and single injection at 5 CA BTDC. Fig. 11 shows (a) measured cylinder pressures and (b) heat release rates calculated from the cylinder pressures in Uniform EGR, LS-EGR and RS-EGR. Combustion phase was delayed by the effect of stratified EGR in both LSEGR and RS-EGR. The ignition delay of the high temperature reaction prolonged in both LS-EGR and RS-EGR cases, however, the combustion duration was further prolonged by RS-EGR. The gross IMEP of all cases were similar, less than 0.5% variance, indicating that the stratified EGR had little effect on load and fuel consumption. Fig. 12 shows the experimental results of (a) NOx and PM variances and (b) CO and THC variances according to the EGR supply methods. Emissions are normalized to show rates of change easily. As compared to Uniform EGR, LS-EGR maintained NOx but substantially decreased PM by more than 30%. CO remained the same, and THC increased slightly. As shown in Fig. 8(a), when high EGR gas is supplied to the tangential port, a high EGR region is formed at the upper center of the combustion chamber, and as a consequence, combustion is initiated in the low EGR region. It is thought that LS-EGR decreases PM effectively due to a lower local EGR rate at the combustion site even for the same overall EGR rate with Uniform EGR. RS-EGR showed an opposite trend: NOx decreased by up to 7%, whereas PM remained unchanged. However, THC increased by 5%. This increase is because when high EGR gas is supplied to the right-port, a high EGR region is formed at the lower
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a
b
Fig. 12. (a) NOx and PM and (b) CO and THC changes by EGR supply methods (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
periphery of the combustion chamber as shown in Fig. 8(b), and consequently, combustion is initiated in the high EGR region. 5.2. Potential of stratified EGR
Fig. 11. (a) Measured cylinder pressures and (b) heat release rates in various EGR supply methods (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
An EGR swing test was performed to evaluate the maximum potential of stratified EGR at a speed of 1500 rpm, a fuel quantity of 15.0 mg/str, and EGR rates from 28% to 52%. Because LS-EGR showed great PM reduction in the same overall EGR rate without penalties of NOx and fuel consumption, an EGR swing test was performed in Uniform EGR and LS-EGR, and the results of NOx and PM were compared. During the test, boost pressure was maintained to 1.2 bar and emissions of NOx, PM and CO, and ISFC were measured. The experimental results are shown in Fig. 13. As the EGR rate increased, NOx dropped steadily from 250 ppm to 15 ppm, and CO increased from 550 ppm to 3600 ppm in both Uniform EGR and LS-EGR. PM showed a different trend according to the EGR rate. While the EGR rate increased from 28% to 44%, PM increased and NOx-PM trade-off occurred. However, PM decreased rapidly with an EGR rate of more than 44%, and NOx and PM decreased simultaneously as EGR increased. It is thought that low temperature combustion (LTC) occurred from the EGR rate at around 44%, and a rapid increase of CO and fuel consumption, which are the general
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Fig. 13. The potential of stratified EGR (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 28e52%).
characteristics of LTC, were also shown in this experiment. Fuel consumption was maintained during the conventional diesel combustion region (EGR 28 w 44%); however, it increased rapidly with an EGR increase in the LTC region (EGR 44% w). In any EGR rates between 28% and 52%, LS-EGR reduced PM at the same overall EGR without NOx, ISFC and CO penalties. PM was reduced on average by 25% in the conventional diesel combustion region (EGR 28 w 44%), and PM was reduced by an average of 10% in the LTC region (EGR 44% w). NOx can be further reduced by increasing the EGR rate in the conventional diesel combustion region while maintaining PM and fuel consumption. In the LTC region, CO and fuel consumption can be improved by decreasing the EGR rate and maintaining NOx and PM by LS-EGR. When the EGR rate increased approximately from 31% to 37%, PM of Uniform EGR increased approximately from 0.43 FSN to 0.55 FSN. However, PM of LS-EGR at EGR 37% is maintained at approximately 0.43 FSN, which is the same PM level of Uniform EGR at EGR 31% (along the Iso-PM line in Fig. 13). As the EGR rate increased by 6%, NOx decreased from 180 ppm to 100 ppm. Meanwhile, fuel consumption remains constant, and CO slightly increased. These results indicate that in the conventional diesel combustion region, NOx can be reduced by a maximum of 45% by stratified EGR (LS-EGR) without increasing PM and fuel consumption. In the LTC region, NOx is preserved at a very low level (less than 20 ppm); however, sudden increments of CO and fuel consumption are drawbacks. To overcome these drawbacks, EGR should be decreased as much as possible; however, decreasing EGR increases PM significantly. Because LS-EGR can decrease PM by about 10% at the same overall EGR, LS-EGR is expected to have the potential to decrease CO by about 10% and to improve fuel consumption by approximately 2% by decreasing
the EGR rate about 1% while maintaining the same PM with Uniform EGR as shown in Fig. 13. 5.3. The effect of in-cylinder swirl on stratified EGR To compensate for the drop of in-cylinder swirl near TDC due to the larger bowl diameter of the 2-step piston than that of conventional piston, the SCV was closed during the experiment. Generally, when the SCV is closed, in-cylinder swirl is enhanced due to the increase of flow through the tangential port, and NOx increases while PM decreases by the enhanced mixing of fuel and air and the fasten reaction rate. Moreover, CO and THC tend to decrease. The heat release rate of UþSCV30 (Uniform EGR with the SCV 30 closed) in Fig. 14(b) and NOx and PM in Fig. 16(a) and CO and THC in Fig. 16(b) of UþSCV30 follows this general trend. However, when the SCV was closed in the stratified EGR condition, the combustion and emission characteristics were somewhat different from those of the Uniform EGR condition, and some meaningful results were obtained. Fig. 15 shows the changes in NOx and PM by experiment according to the in-cylinder swirl with RS-EGR, at a speed of 1500 rpm and a fuel quantity of 15.0 mg/ str. In the base swirl condition, RS-EGR delayed the combustion processes (Fig. 14(b)) due to the locally high EGR in the combustion region and reduced NOx without PM penalty (Path 1). When the SCV was closed at 30 (RSþSCV30) for moderate swirl conditions with RS-EGR, PM additionally decreased due to the swirl effect without an increment of NOx (Path 2). As a result, NOx and PM were reduced by 7% and 23%, respectively, compared to Uniform EGR (Fig. 16(a)). The combustion processes of RSþSCV30 was faster than that of RS-EGR but still slower than that of Uniform EGR of the base swirl condition by the effect of EGR stratification, as shown in
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a
b
Fig. 14. The in-cylinder swirl and stratified EGR effects on (a) cylinder pressures and (b) heat release rates (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
Fig. 14(b). However, in RSþSCV60, which was high swirl conditions, both NOx and PM increased significantly (Path 3), and the combustion processes was faster than other conditions owing to the excessive swirl, as shown in Fig. 14(b). It is assumed that the NOx increment by the excessive swirl overwhelmed the NOx
Fig. 16. The in-cylinder swirl and stratified EGR effects on (a) NOx and PM and (b) CO and THC (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
reduction by stratified EGR, and PM was increased by spray-tospray interference due to spray deflection under excessive swirl conditions. The results indicate that in-cylinder swirl controls fuel spray and mixture formation and affects the effects of stratified EGR. Two solid arrows in Fig. 16(a) show the comparison of NOx reduction rates by stratified EGR under two different swirl conditions. When RSþSCV30 was compared with UþSCV30, NOx reduced significantly up to 12%, and PM increased slightly. The NOx reduction rate increased compared to the base swirl conditions of Uniform EGR and RS-EGR, and it can be concluded that NOx reduction by stratified EGR can be maximized under proper in-cylinder swirl conditions. As shown in CO and THC of Fig. 16(b), CO of RSþSCV30 was higher than that of UþSCV30 but still lower than that of Uniform EGR. The THC levels of RSþSCV30 and UþSCV30 were similar, reduced by about 8% compared to Uniform EGR. Compared to Uniform EGR, RSþSCV30 achieved NOx and PM simultaneous reduction without CO and THC penalty in the same overall EGR by balancing the effects of in-cylinder swirl and stratified EGR. 5.4. Error analysis of stratified EGR experiment
Fig. 15. NOx and PM changes as in-cylinder swirl increment in RS-EGR (speed: 1500 rpm, fuel quantity: 15.0 mg/str).
Experiments of eight repeated cases were performed at the same operation condition of speed 1500 rpm, fuel quantity 15.0 mg/str, and EGR 43.1%. Injection pressure and timing of a single shot were set to 750 bar and 5 CA BTDC respectively. Each case was
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a
b
c
d
e
Fig. 17. Experimental errors of (a) NOx, (b) PM, (c) CO, (d) THC, and (e) average variances of each emission from Uniform EGR under various EGR supply methods at the same condition (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
tested in different date, under various EGR supply methods. Fig. 17 shows the experimental errors of (a) NOx, (b) PM, (c) CO, and (d) THC normalized by average values of Uniform EGR of eight cases. Case numbers are shown in X-axis, and normalized emission values are plotted in Y-axis. The dotted line shows emission values of Uniform EGR, and those of other EGR supply methods of the same case are plotted in various dots. During the experiment intake and EGR flows were finely controlled and the emission fluctuations of the same case were less than 1%. However, the standard deviation between eight cases of Uniform EGR NOx is 0.070, which means average 7.0% error. In the same manner, PM, CO and THC have
average 28.4%, 10.0%, and 10.5% errors respectively. Such emission variations between cases might be occurred from daily difference of ambient conditions and also calibration errors in measurement equipment. The emission changes by stratified EGR can be regarded as experimental errors, because those are similar or less than the emission errors between cases. However, as shown in Fig. 17(a)e(d), the trends of emission changes by various EGR supply methods almost always similar for all of the eight cases. As compared to Uniform EGR, UþSCV30 (square dot) tested in cases of 1, 4, 6, and 8 increased NOx, and decreased PM, CO, and THC. LS-EGR
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(triangle dot) tested in seven cases except for 5, slightly increased NOx and decreased PM while maintaining CO and THC. RS-EGR (cross dot) tested in all cases decreased NOx while maintaining PM, but slightly increased CO and THC. And RSþSCV30 (asterisk dot) tested in cases of 1 and 5e8 decreased both NOx and PM without increasing CO and THC. The average variance of each emission from Uniform EGR under various EGR supply methods in Fig. 17(e) clearly shows these emission trends. Changes in emissions are plotted as a percentage of Uniform EGR, þ mean a higher value. It can be concluded that the emission changes under various EGR supply methods are caused by the in-cylinder EGR stratification, not by the experimental errors.
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a
5.5. Robustness of stratified EGR The robustness of the stratified EGR effects was verified under various operating conditions (speed, load and EGR) and injection strategies (injection pressure and number). LS-EGR, RS-EGR and RSþSCV30 supply methods are tested at three speed-loads, two EGR rates, two injection pressures and two injection numbers. The variations in (a) NOx and (b) PM in the various operating conditions are shown in Fig. 18. The trends of the NOx and PM changes due to the different EGR supply methods were similar at any speed, load or EGR. In every condition, LS-EGR slightly increased or maintained NOx and reduced PM, RS-EGR reduced NOx and slightly increased or maintained PM and RSþSCV30 reduced NOx and PM simultaneously. However, the effectiveness of stratified EGR was influenced by the load and the EGR rate, and the effects were better at low loads and high EGR conditions. It is thought that under high-load conditions, the longer injection duration breaks the EGR stratification and combustion occurs in
b
Fig. 19. (a) NOx and (b) PM variations according to various injection strategies with EGR supply methods (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
a
b
Fig. 18. (a) NOx and (b) PM variations according to various speed-loads and EGR rates with EGR supply methods.
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both low and high EGR regions owing to the broadened combustion site. The higher the overall EGR rate, the greater the deviation in local EGR concentrations that can be achieved and the effectiveness of EGR stratification is increased. The variations in (a) NOx and (b) PM due to the various injection strategies are shown in Fig. 19. In the single injection condition, two injection pressures of 750 bar and 1250 bar were compared (SS750 bar and SS-1250 bar). With the same injection pressure, a single injection was compared with a double injection with 1 pilot injection of 4.5% of the total injection quantity. The trends in the NOx and PM changes were similar under any of the injection strategies. However, less NOx and PM reduction was found with a high injection pressure, and it is thought that the high speed and momentum of the high-pressure spray destroys the stratified EGR structure before combustion. The stratified EGR effect was retained in double injection, and the pilot injection does not affect the EGR stratification. 6. Conclusion In this study, the effects of in-cylinder EGR stratification on combustion and emission characteristics were examined in a single cylinder engine experiment. (1) The concept of stratified EGR was implemented in a single cylinder engine by the asymmetrical supply of external EGR. A 2-step piston and an offset chamfer at the tangential port were adapted to the single cylinder engine to enhance the degree of EGR stratification and to improve the distribution of EGR stratification. (2) The combustion and emission characteristics were changed according to the EGR supply method. When a high EGR gas is supplied to the tangential port (LS-EGR), a high EGR region is formed at the central upper area of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and NOx tends to increase slightly while PM is reduced by 30%. When a high EGR gas is supplied to the helical port (RS-EGR), a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NOx is reduced by 6 % while PM increases slightly. (3) The potential of stratified EGR was evaluated by an EGR swing test using LS-EGR, which has great PM reduction. NOx can potentially be reduced by 45% by increasing the EGR rate in the moderate EGR conventional diesel combustion region without penalties of PM and fuel consumption. In the heavy EGR LTC region, stratified EGR has potential of CO reduction by 10% and fuel consumption improvement by 2% by decreasing the EGR rate without PM penalty. (4) Proper in-cylinder swirl with RS-EGR maximized the effect of the stratified EGR and had positive effects on NOx, PM, CO and THC emissions, achieving simultaneous reductions in NOx (7%) and PM (23%). (5) The robustness of the effects of stratified EGR was verified under various operating conditions (such as speed, load and EGR rate) and various injection strategies (different injection pressures and numbers). Stratified EGR was less effective under a higher load, a lower EGR and under higher injection pressure conditions. Stratified EGR was implemented and verified in a production engine in this study. However, the ability to make large changes to the intake port shape to further optimize in-cylinder EGR distribution was limited. For this reason, only minor changes to the ports (such as offset chamfer) were possible, so the stratified EGR effect was thought to be limited. If the intake port shape can be fully
optimized for better in-cylinder EGR stratification, it is expected that stratified EGR will have more potential. Acknowledgements This work was sponsored by the Hyundai Motor Company and supported by the second stage of the Brain Korea 21 Project in 2010 and SNU-IAMD. References [1] Mori K, Matsuo S, Nakayama S, Shiino S, Kawatani T, Nakashima K, et al. Technology for environmental harmonization and future of the diesel engine. SAE Paper no.2009-01-0318; 2009. [2] Ladommatos N, Abdelhalim SM, Zhao H, Hu Z. The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissionsdpart 4: effects of carbon dioxide and water vapor. SAE Paper no.971660; 1997. [3] Ladommatos N, Abdelhalim SM, Zhao H, Hu Z. Effects of EGR on heat release in diesel combustion. SAE Paper no. 980184; 1998. [4] Maiboom A, Tauzia X, Hétet J-F. Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine. Energy 2008;33:22e34. [5] Ryan T. HCCIdupdate of progress 2005. 12th annual diesel engine emissions reduction (DEER) conference, Detroit, USA; 2006. [6] Akihama K, Takatori Y, Inagaki K, Sasaki S, Dean AM. Mechanism of the smokeless rich diesel combustion by reducing temperature. SAE paper no. 2001-01-0655; 2001. [7] Kimura S, Aoki O, Ogawa H, Muranaka S, Enomoto Y. New combustion concept for ultra-clean and high-efficiency small DI diesel engines. Trans SAE, J Engines 1999;108:2128e37 [SAE paper no. 1999-01-3681]. [8] Musculus MPB, Lachaux T, Pickett Lyle M, Idicheria CA. End-of-injection overmixing and unburned hydrocarbon emissions in low-temperaturecombustion diesel engines. SAE paper no. 2007-01-0907; 2007. [9] Beatrice C, Del Giacomo N, Bertoli C, Migliaccio M. Looking at new concepts for ultra-low emission diesel combustion system. ASME paper no. ICEF2002-486; 2002. [10] Johnson T. Diesel emission control technology in review. 11th annual diesel engine emissions reduction (DEER) conference, Chicago, USA; 2005. [11] Miles P. Sources and mitigation of CO and UHC emissions in low-temperature diesel combustion regimes: insights obtained via homogenous reactor modeling. 13th Annual diesel engine emissions reduction (DEER) conference, Detroit, USA; 2007. [12] Wagner RM, Green JB, Dam TQ, Edwards KD, Storey JM. Simultaneous low engine-out NOx and particulate matter with highly diluted diesel combustion. SAE paper no. 2003-01-0262; 2003. [13] Dec JE. Advantages of charge stratification in HCCI engines, ICE2007. 8th International conference on engines for automobiles, Capri, Napoli; 2007. [14] Sjöberg M, Dec JE, Babajimopoulos A, Assanis D. Comparing enhanced natural thermal stratification against retarded combustion phasing for smoothing of HCCI heat-release rates. SAE paper no. 2004-01-2994; 2004. [15] Sjöberg M, Dec JE. Effects of engine speed, fueling rate, and combustion phasing on the thermal stratification required to limit HCCI knocking intensity. SAE paper no. 2005-01-2125; 2005. [16] Mori S, Lang O, Pischinger S. Type analysis of EGR-strategies for controlled auto ignition (CAI) by using numerical simulations and optical measurements. SAE paper no. 2006-01-0630; 2006. [17] Thirouard B, Cherel J, Knop V. Investigation of mixture quality effect on CAI combustion. SAE paper no. 2005-01-0141; 2005. [18] Choi S, Lim J, Ki M, Min K, Choi H. Analysis of cyclic variation and the effect of fuel stratification on combustion stability in a port fuel injection (PFI) CAI engine. SAE paper no. 2009-01-0670; 2009. [19] Jung G, Sung Y, Choi B, Lim M. Effects of mixture stratification on HCCI combustion of DME in a rapid compression and expansion machine. Int J Automot Technol 2009;10:1e7. [20] Sjöberg M, Dec JE. Smoothing HCCI heat-release rates using partial fuel stratification with two-stage ignition fuels. SAE paper no. 2006-01-0629; 2006. [21] Fuyuto T, Nagata M, Hotta Y, Inagaki K, Nakakita K, Sakata I. In-cylinder stratification of external exhaust gas recirculation for controlling diesel combustion. Int J Engine Res 2010;11:1e15. [22] Choi S. The effects of in-cylinder EGR stratification on combustion and emission characteristics in a diesel engine. Ph.D. thesis, Seoul National University, Seoul, Korea; 2010. [23] Heywood JB. Internal combustion engine fundamentals. Singapore: McGrawHill; 2000.
Nomenclature A: surface area of combustion chamber (m2) AF: air to fuel ratio (by mass) B: bore diameter (m) hc: convective heat transfer coefficient (W/m2 K)
S. Choi et al. / Energy 36 (2011) 6948e6959 LS-EGR: left-port stratified EGR NOx: nitrogen oxides P: instantaneous cylinder pressure (kPa) Pm: motored cylinder pressure (kPa) Pr: in-cylinder gas pressure at reference state (kPa) PM: particulate matters Qn: net heat release (J) Qch: gross heat release (J) Qht: heat transfer (J) RS-EGR: right-port stratified EGR RSþSCV: right-port stratified EGR with SCV Sp : mean piston speed (m/s) T: average temperature of gas in the cylinder (K) Tr: in-cylinder gas temperature at reference state (K) Tw: surface temperature of combustion chamber (K) THC: total hydrocarbon UþSCV: Uniform EGR with SCV V: instantaneous cylinder volume (m3) Vd: displacement volume (m3) Vr: in-cylinder gas volume at reference state (m3)
w: average gas velocity (m/s) Greek symbols
g: specific heat capacities ratio Abbreviations ATDC: after top dead center BMEP: break mean effective fuel consumption (bar) BTDC: before top dead center EGR: exhaust gas recirculation gIMEP: gross indicated mean effective pressure (kPa) HFR: hydraulic flow rate (cc/100 bar/30 s) HCCI: homogeneous charged compression ignition HSDI: high-speed direct injection ISFC: indicated specific fuel consumption (g/kWh) MK: modulated kinetics NEDC: new European driving cycle rpm: revolutions per minute SCV: swirl control valve CA: degrees of crank angle
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Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine Seungmok Choia, Wonah Parka, Sangyul Leea, Kyoungdoug Mina, *, Hoimyung Choib a b
SNU Automotive Laboratory, School of Mechanical and Aerospace Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151e744, Republic of Korea Advanced Institutes of Convergence Technology, 864-1, Iui-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-270, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2011 Received in revised form 2 August 2011 Accepted 10 September 2011 Available online 1 November 2011
The effects of in-cylinder EGR stratification on combustion and emission characteristics are investigated in a single cylinder direct injection diesel engine. To achieve in-cylinder EGR stratification, external EGR rates of two intake ports are varied by supplying EGR asymmetrically using a separated intake runner. The EGR stratification pattern is improved using a 2-step bowl piston and an offset chamfer at the tangential intake port. When high EGR gas is supplied to the left (tangential) port, a high EGR region is formed at the central upper region of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and PM is reduced significantly. When high EGR gas is supplied to the right (helical) port, a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NOx is reduced without PM penalty. Stratified EGR potentially reduces NOx by maximum 45%, without penalties of performance and other emissions. A proper in-cylinder swirl with stratified EGR maximizes the effects and achieves simultaneous reduction of NOx by 7% and PM by 23%. Moreover, the robustness of stratified EGR is evaluated under various operating conditions and injection strategies. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Direct injection diesel engine Exhaust gas recirculation (EGR) Stratified EGR Swirl control valve (SCV) Nitric oxides (NOx) Particulate matters (PM)
1. Introduction High-speed direct injection (HSDI) diesel engines have advantages such as high efficiency, affordability, and excellent reliability compared to other internal combustion engines. Recent HSDI engines have achieved higher specific power and better fuel efficiency than previous engines by using high injection pressure provided by improved fuel injection technologies and advanced charge technologies, such as turbochargers and intercoolers. Recently, noise levels have also improved due to the application of common rail injection systems. However, HSDI diesel engines emit more nitrogen oxides (NOx), particulate matters (PM) and other pollutants than their counterparts. According to Euro VI, which will be enforced in 2014, NOx should be reduced by 55.6%, while PM must be maintained at the Euro V level [1]. EGR (exhaust gas recirculation) is one of the most effective ways of controlling the combustion and emissions of a diesel engine. EGR raises the specific heat capacity (thermal effect) and lowers the O2 concentration (dilution effect) of trapped gas under the same boost conditions. EGR lowers the flame temperature [2,3] and delays
* Corresponding author. Tel.: þ82 2 880 1661; fax: þ82 2 883 0179. E-mail address: [email protected] (K. Min). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.09.016
combustion (ignition delay, premixed combustion and diffusion combustion) [4]. As the EGR rate increases, NOx is reduced from the decrease in O2 concentration and burnt gas temperature. However, it prevents soot from oxidizing, and generally more PM is produced, which is the so-called ‘NOx-PM trade-off.’ To overcome the NOx-PM trade-off, new combustion concepts have been studied such as homogeneous charged compression ignition (HCCI) combustion [5], smokeless-rich diesel combustion [6], and modulated kinetics (MK) combustion [7]. In HCCI combustion, in-cylinder temperature, EGR rate and injection parameters are controlled precisely to allow well mixed prior to reaching its ignition temperature during compression. However, HCCI reaction in totally homogeneous mixture is so fast that it occurs knock-like rapid pressure rise and noise. To avoid it, stratification of temperature and mixture and fuel additives have been studied to control the reaction rate. Smokelessrich diesel combustion uses aggressive high EGR to initiate combustion at a temperature below the temperature needed to form PM. MK combustion concept reduces NOx and PM simultaneously using a high EGR rate to reduce the oxygen concentration and retarded injection to accomplish premixed combustion. While the new combustion concepts differ in many ways, one common characteristic among them is to lower the combustion temperature through a high EGR rate, usually more than 30e55%. With this high EGR, along with the lowered local burnt gas temperature, the local
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equivalence ratio is decreased by a prolonged ignition delay that exceeds the injection duration and allows for sufficient precombustion mixing [8]. Although these concepts are very attractive for both NOx and PM emissions, they are accompanied by a large increase in CO and HC emissions [9e11] and, in most cases, with an increase in fuel consumption [6,9,12]. In-cylinder stratification is a possible method for simultaneously reducing NOx and PM. In-cylinder stratification has mainly been studied in HCCI. Studies on in-cylinder thermal stratification [13e15], EGR stratification [16,17] and fuel stratification [13,18e20] have been performed to promote combustion phase control and suppress the rapid pressure rise and knocking associated with HCCI. Previous studies have shown that thermal and fuel stratification strongly affect combustion and extend the operating range at the high load by smoothing heat release. However, the effects of EGR stratification are not clearly understood yet. In HSDI diesel engines, where stratified fuel combustion occurs, thermal and EGR stratification have rarely been studied. Fuyuto et al. have suggested in-cylinder EGR stratification concept that reduced smoke while NOx was maintained at a medium speed/load level [21]. In this study, the potential for combustion and emission control by in-cylinder EGR stratification was evaluated in a single cylinder engine experiment. In-cylinder EGR stratification was improved by a 2-step piston and an offset chamfer in the tangential port, and the changes in EGR stratification patterns were predicted using a CFD flow simulation [22]. Both the 2-step piston and the offset chamfer were manufactured and adapted to a single cylinder engine, and the experiment was performed under various EGR supply methods for different in-cylinder stratification using the separated intake runner and swirl control valve (SCV). The robustness of the incylinder EGR stratification was verified under various speeds, loads, EGR rates, and injection strategies. 2. The concept of in-cylinder EGR stratification The motivation behind in-cylinder EGR stratification is increasing or decreasing the EGR concentration at the combustion site above or below the overall EGR rate. If the degree of stratification and distribution of EGR can be controlled so that combustion occurs at higher EGR concentrations, NOx is expected to be further reduced. In addition, PM can be maintained or reduced because of the same overall air to fuel ratio (AF) and the soot oxidation increment by the mixing with high-concentration O2 regions. The desired in-cylinder EGR stratification and spray targeting is shown in Fig. 1. Because combustion starts in the locally high EGR region, where the local O2 concentration is lower than average, the combustion processes are prolonged, and the combustion temperature decreases. As a result, further NOx reduction is
Fig. 1. Desired in-cylinder EGR stratification and spray targeting [22].
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Fig. 2. Mechanism of NOx and PM reduction by stratified EGR [22].
expected in the same overall EGR rate. Meanwhile, a longer ignition delay by locally high EGR increases the premixing of fuel and air, and soot formation is expected to decrease. A high-concentration O2 region is participating in combustion during the expansion stroke and promotes soot oxidation. Thus, PM emission is also expected to decrease (Fig. 2). Two main concerns in in-cylinder EGR stratification are which EGR stratification pattern is the most favorable and how the desired pattern can be obtained in a real engine. Regarding the favorable pattern, the degree of stratification should be maximized, and the higher EGR region should be symmetrically distributed so that the combustion region can be placed in the higher EGR region. A previous study on implementing EGR stratification showed that the gas through the tangential port gathers in the upper part of the cylinder, whereas the gas through the helical port moves down to the lower part of the cylinder. Thus, in-cylinder EGR can be stratified by an asymmetrical EGR supply through the intake ports [21]. The asymmetrical supply of external EGR, which supplies high EGR gas through the tangential port, is shown in Fig. 3. The intake port shape needs to be modified to control the in-cylinder gas flow direction for better stratified EGR pattern. However, an overall modification or an optimization of the intake port shape is difficult because other mechanical parts such as an injector, a cam and valve train, and oil and coolant passages should be newly designed to avoid an interference with a modified port. Thus, it means that a new engine head should be designed, which entails significant cost and effort. Instead, several minor changes were conducted in this study, among them a 2-step piston and an offset chamfer in the tangential port effectively controlled the in-cylinder EGR stratification pattern. A 2-step piston enhances the degree of in-cylinder EGR stratification [22]. Fig. 4 shows the CFD flow simulation result that compares the stratified EGR pattern of (a) a conventional piston and (b) a 2-step piston at near TDC. The EGR concentration of
Fig. 3. Asymmetrical external EGR supply for in-cylinder EGR stratification [22].
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Fig. 4. Enhancement of the degree of EGR stratification by a 2-step piston [22].
the high EGR region is increased in the 2-step piston by preventing a squish flow during the compression that mixes the upper gas of the higher EGR and the lower gas of the lower EGR. To improve the distribution of in-cylinder EGR stratification, an offset chamfer in the tangential port was proposed [22]. The base chamfer in the tangential port valve shifted 0.8 mm to the opposite direction of other intake valve as shown in the upper figure of Fig. 5. The stratified EGR pattern by CFD flow simulation in Fig. 5(a) and (b) showed that the offset chamfer improves the distribution of EGR stratification by shifting the biased high EGR region to the center of combustion chamber compared to the base chamfer. As a result, the stratified EGR pattern becomes more symmetric. The 2-step piston and the offset chamfered head were manufactured and adapted to a single cylinder engine to verify the effects experimentally.
Fig. 6. The intake runner for asymmetrical EGR supply to the intake ports.
3. Implementation of stratified EGR 3.1. Intake runner for stratified EGR To realize in-cylinder EGR stratification in the single cylinder engine experiment, a separate intake runner was designed to supply external EGR asymmetrically through the intake ports. As shown in Fig. 6, the runner is separated from the start, and each runner has an EGR inlet. Each EGR inlet is placed 220 mm upstream from the inlet of
Fig. 5. Improvement of the stratified EGR distribution by offset chamfer [22].
Fig. 7. EGR supply methods for stratified EGR.
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intake air was used to avoid the local temperature variation under stratified EGR. The in-cylinder swirl effects on stratified EGR were also evaluated by increasing in-cylinder swirl under RS-EGR using the SCV placed in the helical port (RSþSCV). 4. Experimental description 4.1. Experimental apparatus Fig. 8. In-cylinder EGR stratification patterns according to the EGR supply method.
the intake port. To enhance mixing with air, each EGR inlet has two holes at the top and bottom. The SCV is installed downstream of the EGR inlet in the direction of the helical port. The length of runner upstream of the EGR inlet is 410 mm long, and a plenum with a volume of 200 cc is installed at each runner to prevent EGR backflow to the other port when EGR is supplied asymmetrically. A thermocouple, a pressure transducer and a gas sampling line are installed in each port as closely as possible to the head to measure temperature, pressure and gas composition at the port inlet. 3.2. EGR supply method Using the separated intake runner, external EGR can be supplied to either one of the two intake ports or to both intake ports. Fig. 7 shows the EGR supply methods for stratified EGR. In Uniform EGR, EGR is supplied to both intake ports at the same time so that the gas at each port has the same composition of air and EGR. In the leftport stratified EGR (LS-EGR), EGR is supplied to the tangential port so that the tangential port has a higher EGR rate, and only air is supplied through the helical port. In this case, in-cylinder EGR is stratified to have a higher EGR in the upper center region of the combustion chamber as shown in Fig. 8(a). In the right-port stratified EGR (RS-EGR), EGR is supplied through the helical port. Thus, the right-port has higher EGR rate and only air is supplied through the tangential port. In this case, in-cylinder EGR is stratified to have a higher EGR in the lower peripheral region of the combustion chamber as shown in Fig. 8(b). Fig. 9 shows CFD simulation results of spray and fuel mixture formation at just before the combustion start (3 CA ATDC) in speed of 1500 rpm, EGR rate of 30%, injection timing of 5 CA BTDC injection pressure of 750 bar and injection quantity of 15.0 mg. In this condition, ignitable local mixtures that has equivalence ratio higher than 0.8 are formed at the peripheral region of the combustion chamber, close to the surfaces of first and second bowls. In LS-EGR, locally low EGR regions which have up to about 5% lower than overall EGR are placed at the ignition position (Fig. 8(a)), and combustion is expected to start from the low EGR region. In RS-EGR, on the contrary, locally high EGR regions which have up to about 5% higher than overall EGR are formed at the ignition position (Fig. 8(b)). The ignition position in stratified EGR will stay in similar position as of Uniform EGR, because local equivalence ratio difference caused by 5% of local EGR rate change is only 0.036, and cooled EGR which has the same temperature with
A single cylinder HSDI diesel engine equipped with the 2-step piston and the offset chamfered head was used for a stratified EGR combustion experiment. This engine has a common rail system with a piezo injector that enables injection pressures up to 1600 bar. Detailed specifications of the engine are shown in Table 1. Engine speed was controlled by a 37 kW DC dynamometer, and the quality of the diesel was preserved by using a large-capacity fuel tank during the entire experimental period. The oil and coolant temperature were maintained at 80 C, and the fuel temperature was maintained at 40 C during the experiment. The fuel mass flow rate was measured using a mass burette type flow meter (ONO SOKKI, FX-203P). Air and EGR mass flow rates were measured using a thermal resistance flow rate sensor (MKP, TSM-150). Because this type of sensor is sensitive to the composition of the measured gases, the measured EGR mass flow rate was corrected using the AF and gas factors for each species provided by the manufacturer. The EGR rate of the intake gas was calculated using the mass air flow rate and the EGR rate, and it was verified with the EGR rate calculated from the CO2 fractions in the exhaust gas and the intake gas. The concentrations of NOx, THC, CO, CO2, and O2 in the exhaust gas were measured using an exhaust gas analyzer (HORIBA, MEXA-7100DEGR), and PM was measured using a smokemeter (AVL, 415S). The two intake port pressures were measured using an absolute pressure transducer (Kistler, 4045A5), and in-cylinder pressure was measured using a relative pressure transducer (Kistler, 6055Bsp). Signals from the pressure transducers were recorded at every 1 CA for 200 cycles for each case using a data acquisition system. An intake boosting and EGR control system was developed to simulate boosting, back pressure and EGR in a multi-cylinder turbocharged engine in the single cylinder engine and to control air and EGR flow rate accurately under various EGR supply methods. The configuration of the system is shown in Fig. 10. To keep the same air and EGR flow rates at any EGR supply methods, the flow rate was controlled using sonic orifices by making choke flow from the compressed air and exhaust gas. From the accurate control of the intake gas flow rates, the combustion conditions were well maintained during the engine operation. NOx fluctuation at the same operating condition was less than 1 ppm. 4.2. Test conditions The test conditions are listed in Table 2. Each experiment was performed at three speed-loads of 1500 rpme15.0 mg/str (BMEP 4 bar), 2,000 rpme18.2 mg/str (BMEP 5 bar) and 1750 rpme27.6 mg/ Table 1 Engine specifications.
Fig. 9. Distribution of vaporized fuel (mixture fraction (Z) and equivalence ratio (Phi)) at 3 CA ATDC (speed: 1500 rpm, injection timing: 5 CA BTDC, injection pressure: 750 bar, injection quantity: 15.0 mg).
Engine type Bore stroke (mm mm) Displacement (cc) Compression ratio Con. rod length (mm) No. of nozzle holes Spray angle Nozzle diameter (mm) HFR (cc/100 bar/30 s)
4-Stroke DI 83 92 497 15.5 145.8 7 153 0.128 380
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Fig. 10. Intake boosting and EGR control system.
str (BMEP 8 bar), which are representative points of the NEDC driving cycle for the vehicle with a commercial four cylinder engine with identical dimensions. Boost pressure, AF and injection pressure were chosen to have the similar values with those of the multi-cylinder turbocharged engine not to exceed realistic boundary. At 1500 rpme15.0 mg/str and 2000 rpme18.2 mg/str, two EGR rates were tested, one in a conventional EGR rate and another in a higher EGR rate. In the higher EGR rate cases, the boost pressure was also increased to maintain an AF, and boost pressures were limited to pressures compatible with a 2-stage turbocharger in a multi-cylinder engine. For each case, Uniform EGR, LS-EGR and RS-EGR supply methods were compared, and the effects of SCV in RS-EGR were tested by closing at 30 and 60 . At 1500 rpme15.0 mg/str, various EGR supply methods were tested under two injection pressures of a conventional injection pressure (750 bar) and a high injection pressure (1250 bar) to verify the robustness of stratified EGR. And double injection with pilot injection of which quantity has the similar amount with that of the commercial engine at 1500 rpme15.0 mg/str was applied to the various EGR supply methods to evaluate the compatibility of stratified EGR with pilot injection.
measured in-cylinder pressure through the heat release analysis [23]. The equation is shown in Eq. (1), which is derived from the energy conservation equation of open system by assuming no crevice flow and uniform cylinder contents.
dQht ¼ Ahc ðT Tw Þ dt
(2)
4.3. Heat release analysis
hc ¼ 3:26B0:2 P 0:8 T 0:55 w0:8
(3)
To investigate the combustion changes by EGR supply methods more clearly, apparent heat release rate was calculated from the
Average gas velocity (w) in Eq. (3) is determined as Eq. (4) and coefficients C1 and C2 were set to 2.28 and 0 for the compression period and 2.28 and 3.24 103 for the combustion and expansion periods respectively.
Table 2 Experimental cases.
Engine speed (rpm) Injection quantity (mg/str) AF EGR rate (%) EGR strategy SCV ( ) Intake pressure (bar) Number of injections Main injection timing ( CA) Pilot injection timing ( CA) Injection quantity (PiI:MI) Injection pressure (bar)
Case 1
Case 2
Case 3
Case 4
Case 5
1500 15.0 22.0 35 U/LS/RS 0, 30 1.06 1 BTDC 5 e e 750
1500 15.0 22.0 43.1 U/LS/RS 0, 30, 60 1.21 1, 2 BTDC 5 BTDC 38 i4.5:95.5 750, 1250
2000 18.2 19.6 30 U/LS/RS 0, 30 1.06 1 BTDC 5 e e 956
2000 18.2 19.6 41.9 U/LS/RS 0, 30 1.26 1 BTDC 5 e e 956
1750 27.6 17.9 39 U/LS/RS 0, 30 1.68 1 BTDC 5 e e 1100
g dQn dQch dQht dV 1 dP ¼ ¼ P V þ g 1 dt g 1 dt dt dt dt
(1)
dQn =dt is the apparent net heat release rate which is the difference between the apparent gross heat release rate (dQch =dt) and the heat transfer rate to the walls (dQht =dt). It can be calculated from cylinder volume (V), measured cylinder pressure (P) and the ratio of specific heats (g) at a certain time. g was set to 1.35 for the gas exchange and compression periods, and 1.30 for the combustion and expansion periods. To get the apparent gross heat release rate (dQch =dt), the heat transfer rate (dQht =dt) was calculated using the Eq. (2), where heat transfer coefficient was defined as well-known Woschni’s correlation [23] in Eq. (3).
V Tr w ¼ C1 Sp þ C2 d ðP Pm Þ Pr Vr
(4)
5. Results and discussion 5.1. Comparison of EGR supply methods The EGR supply method determines the in-cylinder EGR stratification pattern near TDC, and combustion and emission characteristics are influenced by the EGR stratification pattern. The
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changes of combustion and emissions by stratified EGR were analyzed experimentally, and experiments were performed at a speed of 1500 rpm, a fuel quantity of 15.0 mg/str, an EGR rate of 43.1% and single injection at 5 CA BTDC. Fig. 11 shows (a) measured cylinder pressures and (b) heat release rates calculated from the cylinder pressures in Uniform EGR, LS-EGR and RS-EGR. Combustion phase was delayed by the effect of stratified EGR in both LSEGR and RS-EGR. The ignition delay of the high temperature reaction prolonged in both LS-EGR and RS-EGR cases, however, the combustion duration was further prolonged by RS-EGR. The gross IMEP of all cases were similar, less than 0.5% variance, indicating that the stratified EGR had little effect on load and fuel consumption. Fig. 12 shows the experimental results of (a) NOx and PM variances and (b) CO and THC variances according to the EGR supply methods. Emissions are normalized to show rates of change easily. As compared to Uniform EGR, LS-EGR maintained NOx but substantially decreased PM by more than 30%. CO remained the same, and THC increased slightly. As shown in Fig. 8(a), when high EGR gas is supplied to the tangential port, a high EGR region is formed at the upper center of the combustion chamber, and as a consequence, combustion is initiated in the low EGR region. It is thought that LS-EGR decreases PM effectively due to a lower local EGR rate at the combustion site even for the same overall EGR rate with Uniform EGR. RS-EGR showed an opposite trend: NOx decreased by up to 7%, whereas PM remained unchanged. However, THC increased by 5%. This increase is because when high EGR gas is supplied to the right-port, a high EGR region is formed at the lower
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Fig. 12. (a) NOx and PM and (b) CO and THC changes by EGR supply methods (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
periphery of the combustion chamber as shown in Fig. 8(b), and consequently, combustion is initiated in the high EGR region. 5.2. Potential of stratified EGR
Fig. 11. (a) Measured cylinder pressures and (b) heat release rates in various EGR supply methods (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
An EGR swing test was performed to evaluate the maximum potential of stratified EGR at a speed of 1500 rpm, a fuel quantity of 15.0 mg/str, and EGR rates from 28% to 52%. Because LS-EGR showed great PM reduction in the same overall EGR rate without penalties of NOx and fuel consumption, an EGR swing test was performed in Uniform EGR and LS-EGR, and the results of NOx and PM were compared. During the test, boost pressure was maintained to 1.2 bar and emissions of NOx, PM and CO, and ISFC were measured. The experimental results are shown in Fig. 13. As the EGR rate increased, NOx dropped steadily from 250 ppm to 15 ppm, and CO increased from 550 ppm to 3600 ppm in both Uniform EGR and LS-EGR. PM showed a different trend according to the EGR rate. While the EGR rate increased from 28% to 44%, PM increased and NOx-PM trade-off occurred. However, PM decreased rapidly with an EGR rate of more than 44%, and NOx and PM decreased simultaneously as EGR increased. It is thought that low temperature combustion (LTC) occurred from the EGR rate at around 44%, and a rapid increase of CO and fuel consumption, which are the general
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Fig. 13. The potential of stratified EGR (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 28e52%).
characteristics of LTC, were also shown in this experiment. Fuel consumption was maintained during the conventional diesel combustion region (EGR 28 w 44%); however, it increased rapidly with an EGR increase in the LTC region (EGR 44% w). In any EGR rates between 28% and 52%, LS-EGR reduced PM at the same overall EGR without NOx, ISFC and CO penalties. PM was reduced on average by 25% in the conventional diesel combustion region (EGR 28 w 44%), and PM was reduced by an average of 10% in the LTC region (EGR 44% w). NOx can be further reduced by increasing the EGR rate in the conventional diesel combustion region while maintaining PM and fuel consumption. In the LTC region, CO and fuel consumption can be improved by decreasing the EGR rate and maintaining NOx and PM by LS-EGR. When the EGR rate increased approximately from 31% to 37%, PM of Uniform EGR increased approximately from 0.43 FSN to 0.55 FSN. However, PM of LS-EGR at EGR 37% is maintained at approximately 0.43 FSN, which is the same PM level of Uniform EGR at EGR 31% (along the Iso-PM line in Fig. 13). As the EGR rate increased by 6%, NOx decreased from 180 ppm to 100 ppm. Meanwhile, fuel consumption remains constant, and CO slightly increased. These results indicate that in the conventional diesel combustion region, NOx can be reduced by a maximum of 45% by stratified EGR (LS-EGR) without increasing PM and fuel consumption. In the LTC region, NOx is preserved at a very low level (less than 20 ppm); however, sudden increments of CO and fuel consumption are drawbacks. To overcome these drawbacks, EGR should be decreased as much as possible; however, decreasing EGR increases PM significantly. Because LS-EGR can decrease PM by about 10% at the same overall EGR, LS-EGR is expected to have the potential to decrease CO by about 10% and to improve fuel consumption by approximately 2% by decreasing
the EGR rate about 1% while maintaining the same PM with Uniform EGR as shown in Fig. 13. 5.3. The effect of in-cylinder swirl on stratified EGR To compensate for the drop of in-cylinder swirl near TDC due to the larger bowl diameter of the 2-step piston than that of conventional piston, the SCV was closed during the experiment. Generally, when the SCV is closed, in-cylinder swirl is enhanced due to the increase of flow through the tangential port, and NOx increases while PM decreases by the enhanced mixing of fuel and air and the fasten reaction rate. Moreover, CO and THC tend to decrease. The heat release rate of UþSCV30 (Uniform EGR with the SCV 30 closed) in Fig. 14(b) and NOx and PM in Fig. 16(a) and CO and THC in Fig. 16(b) of UþSCV30 follows this general trend. However, when the SCV was closed in the stratified EGR condition, the combustion and emission characteristics were somewhat different from those of the Uniform EGR condition, and some meaningful results were obtained. Fig. 15 shows the changes in NOx and PM by experiment according to the in-cylinder swirl with RS-EGR, at a speed of 1500 rpm and a fuel quantity of 15.0 mg/ str. In the base swirl condition, RS-EGR delayed the combustion processes (Fig. 14(b)) due to the locally high EGR in the combustion region and reduced NOx without PM penalty (Path 1). When the SCV was closed at 30 (RSþSCV30) for moderate swirl conditions with RS-EGR, PM additionally decreased due to the swirl effect without an increment of NOx (Path 2). As a result, NOx and PM were reduced by 7% and 23%, respectively, compared to Uniform EGR (Fig. 16(a)). The combustion processes of RSþSCV30 was faster than that of RS-EGR but still slower than that of Uniform EGR of the base swirl condition by the effect of EGR stratification, as shown in
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Fig. 14. The in-cylinder swirl and stratified EGR effects on (a) cylinder pressures and (b) heat release rates (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
Fig. 14(b). However, in RSþSCV60, which was high swirl conditions, both NOx and PM increased significantly (Path 3), and the combustion processes was faster than other conditions owing to the excessive swirl, as shown in Fig. 14(b). It is assumed that the NOx increment by the excessive swirl overwhelmed the NOx
Fig. 16. The in-cylinder swirl and stratified EGR effects on (a) NOx and PM and (b) CO and THC (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
reduction by stratified EGR, and PM was increased by spray-tospray interference due to spray deflection under excessive swirl conditions. The results indicate that in-cylinder swirl controls fuel spray and mixture formation and affects the effects of stratified EGR. Two solid arrows in Fig. 16(a) show the comparison of NOx reduction rates by stratified EGR under two different swirl conditions. When RSþSCV30 was compared with UþSCV30, NOx reduced significantly up to 12%, and PM increased slightly. The NOx reduction rate increased compared to the base swirl conditions of Uniform EGR and RS-EGR, and it can be concluded that NOx reduction by stratified EGR can be maximized under proper in-cylinder swirl conditions. As shown in CO and THC of Fig. 16(b), CO of RSþSCV30 was higher than that of UþSCV30 but still lower than that of Uniform EGR. The THC levels of RSþSCV30 and UþSCV30 were similar, reduced by about 8% compared to Uniform EGR. Compared to Uniform EGR, RSþSCV30 achieved NOx and PM simultaneous reduction without CO and THC penalty in the same overall EGR by balancing the effects of in-cylinder swirl and stratified EGR. 5.4. Error analysis of stratified EGR experiment
Fig. 15. NOx and PM changes as in-cylinder swirl increment in RS-EGR (speed: 1500 rpm, fuel quantity: 15.0 mg/str).
Experiments of eight repeated cases were performed at the same operation condition of speed 1500 rpm, fuel quantity 15.0 mg/str, and EGR 43.1%. Injection pressure and timing of a single shot were set to 750 bar and 5 CA BTDC respectively. Each case was
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a
b
c
d
e
Fig. 17. Experimental errors of (a) NOx, (b) PM, (c) CO, (d) THC, and (e) average variances of each emission from Uniform EGR under various EGR supply methods at the same condition (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
tested in different date, under various EGR supply methods. Fig. 17 shows the experimental errors of (a) NOx, (b) PM, (c) CO, and (d) THC normalized by average values of Uniform EGR of eight cases. Case numbers are shown in X-axis, and normalized emission values are plotted in Y-axis. The dotted line shows emission values of Uniform EGR, and those of other EGR supply methods of the same case are plotted in various dots. During the experiment intake and EGR flows were finely controlled and the emission fluctuations of the same case were less than 1%. However, the standard deviation between eight cases of Uniform EGR NOx is 0.070, which means average 7.0% error. In the same manner, PM, CO and THC have
average 28.4%, 10.0%, and 10.5% errors respectively. Such emission variations between cases might be occurred from daily difference of ambient conditions and also calibration errors in measurement equipment. The emission changes by stratified EGR can be regarded as experimental errors, because those are similar or less than the emission errors between cases. However, as shown in Fig. 17(a)e(d), the trends of emission changes by various EGR supply methods almost always similar for all of the eight cases. As compared to Uniform EGR, UþSCV30 (square dot) tested in cases of 1, 4, 6, and 8 increased NOx, and decreased PM, CO, and THC. LS-EGR
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(triangle dot) tested in seven cases except for 5, slightly increased NOx and decreased PM while maintaining CO and THC. RS-EGR (cross dot) tested in all cases decreased NOx while maintaining PM, but slightly increased CO and THC. And RSþSCV30 (asterisk dot) tested in cases of 1 and 5e8 decreased both NOx and PM without increasing CO and THC. The average variance of each emission from Uniform EGR under various EGR supply methods in Fig. 17(e) clearly shows these emission trends. Changes in emissions are plotted as a percentage of Uniform EGR, þ mean a higher value. It can be concluded that the emission changes under various EGR supply methods are caused by the in-cylinder EGR stratification, not by the experimental errors.
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5.5. Robustness of stratified EGR The robustness of the stratified EGR effects was verified under various operating conditions (speed, load and EGR) and injection strategies (injection pressure and number). LS-EGR, RS-EGR and RSþSCV30 supply methods are tested at three speed-loads, two EGR rates, two injection pressures and two injection numbers. The variations in (a) NOx and (b) PM in the various operating conditions are shown in Fig. 18. The trends of the NOx and PM changes due to the different EGR supply methods were similar at any speed, load or EGR. In every condition, LS-EGR slightly increased or maintained NOx and reduced PM, RS-EGR reduced NOx and slightly increased or maintained PM and RSþSCV30 reduced NOx and PM simultaneously. However, the effectiveness of stratified EGR was influenced by the load and the EGR rate, and the effects were better at low loads and high EGR conditions. It is thought that under high-load conditions, the longer injection duration breaks the EGR stratification and combustion occurs in
b
Fig. 19. (a) NOx and (b) PM variations according to various injection strategies with EGR supply methods (speed: 1500 rpm, fuel quantity: 15.0 mg/str, EGR: 43.1%).
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Fig. 18. (a) NOx and (b) PM variations according to various speed-loads and EGR rates with EGR supply methods.
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both low and high EGR regions owing to the broadened combustion site. The higher the overall EGR rate, the greater the deviation in local EGR concentrations that can be achieved and the effectiveness of EGR stratification is increased. The variations in (a) NOx and (b) PM due to the various injection strategies are shown in Fig. 19. In the single injection condition, two injection pressures of 750 bar and 1250 bar were compared (SS750 bar and SS-1250 bar). With the same injection pressure, a single injection was compared with a double injection with 1 pilot injection of 4.5% of the total injection quantity. The trends in the NOx and PM changes were similar under any of the injection strategies. However, less NOx and PM reduction was found with a high injection pressure, and it is thought that the high speed and momentum of the high-pressure spray destroys the stratified EGR structure before combustion. The stratified EGR effect was retained in double injection, and the pilot injection does not affect the EGR stratification. 6. Conclusion In this study, the effects of in-cylinder EGR stratification on combustion and emission characteristics were examined in a single cylinder engine experiment. (1) The concept of stratified EGR was implemented in a single cylinder engine by the asymmetrical supply of external EGR. A 2-step piston and an offset chamfer at the tangential port were adapted to the single cylinder engine to enhance the degree of EGR stratification and to improve the distribution of EGR stratification. (2) The combustion and emission characteristics were changed according to the EGR supply method. When a high EGR gas is supplied to the tangential port (LS-EGR), a high EGR region is formed at the central upper area of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and NOx tends to increase slightly while PM is reduced by 30%. When a high EGR gas is supplied to the helical port (RS-EGR), a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NOx is reduced by 6 % while PM increases slightly. (3) The potential of stratified EGR was evaluated by an EGR swing test using LS-EGR, which has great PM reduction. NOx can potentially be reduced by 45% by increasing the EGR rate in the moderate EGR conventional diesel combustion region without penalties of PM and fuel consumption. In the heavy EGR LTC region, stratified EGR has potential of CO reduction by 10% and fuel consumption improvement by 2% by decreasing the EGR rate without PM penalty. (4) Proper in-cylinder swirl with RS-EGR maximized the effect of the stratified EGR and had positive effects on NOx, PM, CO and THC emissions, achieving simultaneous reductions in NOx (7%) and PM (23%). (5) The robustness of the effects of stratified EGR was verified under various operating conditions (such as speed, load and EGR rate) and various injection strategies (different injection pressures and numbers). Stratified EGR was less effective under a higher load, a lower EGR and under higher injection pressure conditions. Stratified EGR was implemented and verified in a production engine in this study. However, the ability to make large changes to the intake port shape to further optimize in-cylinder EGR distribution was limited. For this reason, only minor changes to the ports (such as offset chamfer) were possible, so the stratified EGR effect was thought to be limited. If the intake port shape can be fully
optimized for better in-cylinder EGR stratification, it is expected that stratified EGR will have more potential. Acknowledgements This work was sponsored by the Hyundai Motor Company and supported by the second stage of the Brain Korea 21 Project in 2010 and SNU-IAMD. References [1] Mori K, Matsuo S, Nakayama S, Shiino S, Kawatani T, Nakashima K, et al. Technology for environmental harmonization and future of the diesel engine. SAE Paper no.2009-01-0318; 2009. [2] Ladommatos N, Abdelhalim SM, Zhao H, Hu Z. The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissionsdpart 4: effects of carbon dioxide and water vapor. SAE Paper no.971660; 1997. [3] Ladommatos N, Abdelhalim SM, Zhao H, Hu Z. Effects of EGR on heat release in diesel combustion. SAE Paper no. 980184; 1998. [4] Maiboom A, Tauzia X, Hétet J-F. Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine. Energy 2008;33:22e34. [5] Ryan T. HCCIdupdate of progress 2005. 12th annual diesel engine emissions reduction (DEER) conference, Detroit, USA; 2006. [6] Akihama K, Takatori Y, Inagaki K, Sasaki S, Dean AM. Mechanism of the smokeless rich diesel combustion by reducing temperature. SAE paper no. 2001-01-0655; 2001. [7] Kimura S, Aoki O, Ogawa H, Muranaka S, Enomoto Y. New combustion concept for ultra-clean and high-efficiency small DI diesel engines. Trans SAE, J Engines 1999;108:2128e37 [SAE paper no. 1999-01-3681]. [8] Musculus MPB, Lachaux T, Pickett Lyle M, Idicheria CA. End-of-injection overmixing and unburned hydrocarbon emissions in low-temperaturecombustion diesel engines. SAE paper no. 2007-01-0907; 2007. [9] Beatrice C, Del Giacomo N, Bertoli C, Migliaccio M. Looking at new concepts for ultra-low emission diesel combustion system. ASME paper no. ICEF2002-486; 2002. [10] Johnson T. Diesel emission control technology in review. 11th annual diesel engine emissions reduction (DEER) conference, Chicago, USA; 2005. [11] Miles P. Sources and mitigation of CO and UHC emissions in low-temperature diesel combustion regimes: insights obtained via homogenous reactor modeling. 13th Annual diesel engine emissions reduction (DEER) conference, Detroit, USA; 2007. [12] Wagner RM, Green JB, Dam TQ, Edwards KD, Storey JM. Simultaneous low engine-out NOx and particulate matter with highly diluted diesel combustion. SAE paper no. 2003-01-0262; 2003. [13] Dec JE. Advantages of charge stratification in HCCI engines, ICE2007. 8th International conference on engines for automobiles, Capri, Napoli; 2007. [14] Sjöberg M, Dec JE, Babajimopoulos A, Assanis D. Comparing enhanced natural thermal stratification against retarded combustion phasing for smoothing of HCCI heat-release rates. SAE paper no. 2004-01-2994; 2004. [15] Sjöberg M, Dec JE. Effects of engine speed, fueling rate, and combustion phasing on the thermal stratification required to limit HCCI knocking intensity. SAE paper no. 2005-01-2125; 2005. [16] Mori S, Lang O, Pischinger S. Type analysis of EGR-strategies for controlled auto ignition (CAI) by using numerical simulations and optical measurements. SAE paper no. 2006-01-0630; 2006. [17] Thirouard B, Cherel J, Knop V. Investigation of mixture quality effect on CAI combustion. SAE paper no. 2005-01-0141; 2005. [18] Choi S, Lim J, Ki M, Min K, Choi H. Analysis of cyclic variation and the effect of fuel stratification on combustion stability in a port fuel injection (PFI) CAI engine. SAE paper no. 2009-01-0670; 2009. [19] Jung G, Sung Y, Choi B, Lim M. Effects of mixture stratification on HCCI combustion of DME in a rapid compression and expansion machine. Int J Automot Technol 2009;10:1e7. [20] Sjöberg M, Dec JE. Smoothing HCCI heat-release rates using partial fuel stratification with two-stage ignition fuels. SAE paper no. 2006-01-0629; 2006. [21] Fuyuto T, Nagata M, Hotta Y, Inagaki K, Nakakita K, Sakata I. In-cylinder stratification of external exhaust gas recirculation for controlling diesel combustion. Int J Engine Res 2010;11:1e15. [22] Choi S. The effects of in-cylinder EGR stratification on combustion and emission characteristics in a diesel engine. Ph.D. thesis, Seoul National University, Seoul, Korea; 2010. [23] Heywood JB. Internal combustion engine fundamentals. Singapore: McGrawHill; 2000.
Nomenclature A: surface area of combustion chamber (m2) AF: air to fuel ratio (by mass) B: bore diameter (m) hc: convective heat transfer coefficient (W/m2 K)
S. Choi et al. / Energy 36 (2011) 6948e6959 LS-EGR: left-port stratified EGR NOx: nitrogen oxides P: instantaneous cylinder pressure (kPa) Pm: motored cylinder pressure (kPa) Pr: in-cylinder gas pressure at reference state (kPa) PM: particulate matters Qn: net heat release (J) Qch: gross heat release (J) Qht: heat transfer (J) RS-EGR: right-port stratified EGR RSþSCV: right-port stratified EGR with SCV Sp : mean piston speed (m/s) T: average temperature of gas in the cylinder (K) Tr: in-cylinder gas temperature at reference state (K) Tw: surface temperature of combustion chamber (K) THC: total hydrocarbon UþSCV: Uniform EGR with SCV V: instantaneous cylinder volume (m3) Vd: displacement volume (m3) Vr: in-cylinder gas volume at reference state (m3)
w: average gas velocity (m/s) Greek symbols
g: specific heat capacities ratio Abbreviations ATDC: after top dead center BMEP: break mean effective fuel consumption (bar) BTDC: before top dead center EGR: exhaust gas recirculation gIMEP: gross indicated mean effective pressure (kPa) HFR: hydraulic flow rate (cc/100 bar/30 s) HCCI: homogeneous charged compression ignition HSDI: high-speed direct injection ISFC: indicated specific fuel consumption (g/kWh) MK: modulated kinetics NEDC: new European driving cycle rpm: revolutions per minute SCV: swirl control valve CA: degrees of crank angle
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