Effect of Multiple Injection Strategies on Emission and Combustion

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The author is solely responsible for the content of the paper.SAE Customer Service: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-07902009-01-1354Effect of Multiple Injection Strategies on Emission and Combustion Characteristics in a Single Cylinder Direct-Injection Optical EngineJinwoo Lee, Jinwoog Jeon, Jungseo Park and Choongsik BaeKorea Advanced Institute of Science and Technology (KAIST)Copyright © 2009 SAE InternationalABSTRACTThe effect of multiple injections in a heavy-duty diesel engine was investigated by focusing on single-pilot injection and double-pilot injection strategies with a wide injection timing range, various injection quantity ratios, and various dwell times. Combustion characteristics were studied through flame visualization and heat release analyses as well as emissions tests.Single-pilot injection resulted in a dramatic reduction in nitrogen oxide and smoke emissions when the injection timing was advanced over 40° CA before the start of injection (BSOI) due to combustion with partially premixed charge compression ignition. A brown-colored flame area, which indicates a very fuel-rich mixture region, was rarely detected when more fuel was injected during single-pilot injection. However, hydrocarbon emission increased up to intolerable levels because fuel wetting on the cylinder wall increased. When double-pilot injection was applied, nitrogen oxide and smoke emissions were lowered by up to 73% and 84%, respectively, compared with single-injection. This result implies that double-pilot injection induced a better premixed mixture due to an improved turbulent effect between the two pilot injections. Furthermore, hydrocarbon emission was reduced by 50% compared with single-pilot injection because the spray tip penetration of double pilot injection was shortened, which resulted in a reduced amount of wall-wetted fuel. Images from the double-pilot injection experiment revealed that the diffusion burn portion was decreased and the luminosity intensity was the weakest among all the other experimental conditions.INTRODUCTIONDiesel engines generally offer better thermal efficiency and less emission of carbon dioxide (CO 2), a primary greenhouse gas, than gasoline engines [1-2]. Diesel engines are therefore considered to be attractive powertrain sources. However, the reduction of nitrogen oxide (NOx) and particulate matter (PM) emissions from diesel engines is essential because emission regulations are becoming increasingly stringent. These circumstances have forced many researchers to develop new techniques and combustion strategies.Among these novel techniques, a multiple injection strategy is known to be a useful method for reducing both NOx and smoke simultaneously by overcoming the NOx-smoke tradeoff [3-4]. A multiple injection strategy is an injection that is divided into two or more parts. Injection events before the main injection are referred to as pilot or split injections. Nehmer and Reitz [3] carried out a pilot injection strategy by changing the pilot injection quantity and timing with an engine speed of 1600 rpm and an equivalence ratio of 0.45, which is equivalent to a load of approximately 80%. They found that NOx could be reduced while maintaining the smoke level with a small pilot injection, while NOx increased and smoke was reduced as the pilot injection quantity increased. Moreover, they found that the dwell time between the pilot injection and the main injection did not have a significant effect on the emission characteristics. Tow et al. [5] reported that smoke and NOx could be reduced by 50% and 30%, respectively, when a two stage pilot injection system was contrasted with a single injection system under 25% (1.82 atm of intake pressure) and 75% (1.15 atm of intake pressure) loads of peak torque at an engine speed of 1600 rpm. Han etal. [4] carried out multidimensional computational analysis to determine the mechanism underlying the simultaneous reduction of NO and smoke when multiple injections were applied. They found that the NOx reduction mechanism is similar to that of a single injection with retarded injection timing. With respect to soot, soot formation is reduced after the injection pause between injection pulses due to the fact that the soot-producing rich regions at the spray tip are no longer replenished. Babu and Devaradgane [6] speculated that reducing NOx while maintaining the smoke level was possible because the oxidation of smoke occurred more vigorously after combustion when a split injection method was used. However, Park et al. [7] pointed out that pilot injection resulted in an increase in PM under engine operating conditions of 800 rpm and an idling load. This group reported that the increase in PM may have been a result of the higher temperature in the cylinder at the time of the main injection reducing the lift-off length and decreasing the charge air entrainment into the jet. Kastner et al. [8] investigated the effects of various pilot injection schemes, injection timing, and injection quantity on performance and emission characteristics using a piezo-actuated injector. They found that NOx and HC simultaneously increased as the dwell time between the pilot injection and the main injection approached zero. This is because ignition delay is lengthened by lowering the mixture temperature due to the latent heat of the main injection, which results in longer spray tip penetration, a locally lean mixture, and incomplete combustion, all of which cause HC emission.E hleskog et al. [9] carried out experiments where they split both pilot injection and main injection over time. They reported that PM and CO emissions decreased, while NOx increased when the main injection was split into two injections. The reduction of NOx with increased PM, CO, and HC was also reported because combustion duration was prolonged and CA 50 timing was retarded when the main injection was divided into three and four injections. Okude et al. [10] investigated the effects of different injection quantities and timings on pilot injection. They reported that the reduction of emission by pilot injection itself is an important factor that should be considered to further reduce emission with a pilot injection strategy. They proposed advancing pilot injection timing, increasing the pilot injection quantity and splitting the pilot injection. Several studies have investigated spray characteristics to understand the fuel-air mixing process with a multiple injection strategy [11-12]. Su et al. [11] reported that the main injection spray has a faster spray tip penetration speed than the pilot injection spray due to reduced friction between the spray and ambient air because the main injection spray enters into the flux induced by the pilot injection. Yuyin et al.[12] found that a split injection strategy enhanced the fuel-air mixing rate because high turbulent kinetic energy, which is called “the turbulent effect”, is generated when the pilot injection timing and quantity are controlled properly.Based on previous studies that have investigated and characterized multiple injection strategies, the timing of multiple injection, the quantity of fuel injected, and the dwell time between each injection are the main parameters to take into account when attempting to reduce the emission of NOx and PM. Nevertheless, there is still room for further studies of multiple injection strategies, owing to the very high degree of freedom of fuel injection schedules offered by the common-rail system. Furthermore, few physical analyses using an optical technique have been used to investigate multiple injection strategies.In this paper, we report the results of our investigation that focused on both single-pilot and double-pilot injection strategies with a wide range of injection times, various injection timing ratios, and various dwell times. Combustion characteristics were studied through flame visualization and heat release analyses as well as emissions tests.Table 1 Engine specifications.EngineSingle-cylinder, direct injection,four-valves, optical diesel engine Bore x Stroke 128 x 142 mmDisplacement 1817.6cm3Compression ratio 17 :1Fuel injection typeCommon rail injection system(up to 1800 bar)Injector8 holes, HFR 860 cc/30s,injection angle 146°Table 2 Engine operating conditions.Engine speed [rpm] 1200Injection pressure [MPa] 30, 140Total injection quantity[mg/stroke]60Single pilot injection quantity[mg/stroke]10, 30, 50 % of totalinjection quantity Double pilot injection quantity[mg/stroke]15, 25 % of totalinjection quantity foreach pilot injection Main injection timing[°CA ATDC]-28 ~ 4Single pilot injection timing[° CA BSOI]10 ~ 80Double pilot injection timing[° CA 1BSOI/2BSOI]10 ~ 80ComputerFigure 1 Schematic diagram of the experimental set-up.EXPERIMENTAL APPARATUSRESEARCH ENGINEThe test engine was a single-cylinder direct injection compression ignition (DICI) engine equipped with a common-rail injection system. The engine specifications are listed in Table 1. The schematic diagram of the experimental setup is shown in Fig. 1. The speed of the engine was controlled constantly by a DC dynamometer (90 kW). Fuel injection parameters including injection pressure, injection quantity, and injection timing were controlled by a programmable injector driver (Zenobalti Co., IDU 5000B). A common-rail system allows for the variation of pressure (up to 1600 bar), timing, and number of injections, and is capable of five injections (e.g., two pilot, one main, and two post injections) per cycle. A rotary encoder (Autonics, 3600 pulses/resolution) mounted on camshaft was used to control fuel injection timing. In-cylinder pressure was recorded with a piezoelectric pressure transducer (KISTLE R, 6043 Asp). The cylinder pressure was recorded at every 0.2° crank angle (CA). The cylinder pressures of 100 engine cycles were recorded and the average of these was used to calculate the heat release rate.VISUALIZATIONThis research engine can also be optically accessed to visualize the combustion process. An elongated piston was applied to enable the mounting of a 45° mirror beneath the piston quartz window. A high speed digital video camera (Vision Research Inc., Phantom V.7.0), which can record in color, was used to take images ofFigure 2 Injection profile for multiple injections.the natural luminosity produced by combustion. This high speed imaging system can record up to 10,000 frames per second, so images could be taken every 0.72° CA at an engine speed of 1200 rpm. The exposure duration was 10 ȝs to obtain clear images. No filters to detect specific wavelengths of visible light were used. EXHAUST GAS MEASUREMENTGaseous emissions were analyzed with an exhaust gas analyzer (HORIBA ME XA 1500D) that contains a nondispersive infrared absorption (NDIR) analyzer to measure carbon monoxide (CO) and carbon dioxide (CO 2), a chemiluminescence detector (CLD) to measure NOx, and a flame ionization detector (FID) to measure hydrocarbons. A smoke meter (AVL, 415S) was used tomeasure smoke emission. Part of the exhaust gas flow is sampled by means of probe in the exhaust line and drawn through a filter paper. The resultant blackening of the filter paper is measured by a reflectometer and represents a measure of the soot content in the exhaust gas. A filter smoke number (FSN) of 1 corresponds to a concentration of 17.84 mg/m3 at the sampling point. The resolution of this device is 0.001 FSN and 0.01 mg/m3.ENGINE OPERATING CONDITIONSThe engine was operated at 1200 rpm under fired conditions. The coolant temperature was set to 80°C. No exhaust gas recirculation was used during the investigation. The various engine operating conditions are described in Table 2. The total injection quantity applied was 60 mg/stroke to represent a middle-load condition. The fuel injection schedule included a single-pilot and double-pilot injection. The single-pilot injection timing was defined as the crank angle degree between single-pilot injection and main injection before the start of the main injection (BSOI). For the case of double-pilot injection, the 1st pilot injection timing was defined as the crank angle degree between the 1st pilot injection timing and the 2nd pilot injection timing (1BSOI) and the 2nd pilot injection timing and the main injection timing (2BSOI). A more specific injection profile is shown in Fig. 2. Single-pilot injection timing was varied from 10 to 80° CA BSOI with the single pilot injection quantity ranging between 10 and 50% of the total injection quantity. For double-pilot injection, the quantity of fuel in each of the two pilot injections was set to either 15 or 25% of the total injection quantity, while each of the pilot injection timings was varied from 10 to 80°CA BSOI.RESULTS AND DISCUSSIONE xperimental testing was performed to investigate the effects of low injection pressure & medium load and high injection pressure & medium load on emissions.RESULTS AT LOW INJECTION PRESSURE (P inj. = 30 MPa) & MEDIUM LOAD (Q inj.: 60 mg/stroke)The effects of varying the injection timing and quantity of single pilot injectionsFigure 3 presents the NOx emission as a function of the timing and injection quantity of single-pilot injections at an injection pressure of 30 MPa and a total injection quantity of 60 mg/stroke. The main injection timing was fixed at -20° ATDC for both single injection and single-pilot injection. NOx increased as more fuel was injected during single-pilot injection to 20° CA BSOI of single-pilot injection timing. This is conceivably because the presence of a large amount of near-stoichiometric mixture in the fuel spray resulting from insufficient lean mixing due to too short a period between fuel injection and the start of combustion by single-pilot injection itself causes quick combustion, which in turn results in increased NOx emission [10]. E mission of NOx tended to decrease from 20 to 40° CA BSOI of single-pilot injection timing. This phenomenon can be explained by the common mechanism by which pilot injection reduces NOx emission; pilot injection reduces the amount of premixed combustion that occurs, thereby reducing thesmoke emission at a low injection pressure & medium load conditionISFC at a low injection pressure & medium load conditionstuck to the cylinder wall as a result of the advanced single-pilot injection. The effect of the timing and quantity of single-pilot injection on smoke emission is shown in Fig. 4. When single-pilot injection timing was retarded over 20° CA BSOI, more smoke was emitted than in the single injection case. This is likely because fuel that was not used in premixed combustion during pilot injection was used for diffusion combustion. However, smoke emission decreased as the single-pilot injection quantity increased over 20° CA BSOI. Two possible explanations for this result are provided below. Firstly, fuel-air mixing that takes place during the compression stroke is likely to reduce hot and locally rich zones where soot formation normally occurs. Thus premixed combustion results in low smoke emission [14-15]. Another possible reason is that the amount of fuel injected during the main injection decreases when an increasing quantity of fuel is injected during single-pilot in combustion. Combustion as a result of main injection commenced earlier with a larger amount of single-pilot injection, because fuel injected during the main injection evaporated more easily due to a higher ambient temperature. However, combustion by main injection with a higher pilot combustion rate (50% single-pilot injection case) resulted in too much premixed burn portion while the 30% single-pilot injection case showed the most moderate overall heat-release rate. This result explains why the 50% single-pilot injection case had the highest NOx emission and the 30% single-pilot injection case had the lowest NOx emission at 30° CA BSOI. It is clear from Fig. 7 that the diffusion burn phase of the main combustion decreased as more fuel was injected during single-pilot injection. This result confirms our hypothesis as to why smoke emission was reduced when a greater quantity of fuel was injected during single-pilot injection. From Fig. 8, it is apparent that the combustion characteristics of pilot injection were similar,320330340350360370380390400501001502002500102030405060708090Crank Angle [0]I n -c y l i n d e r p r e s s u r e [b a r ]Single Injection: Q inj : 60 mg/st Pilot injection: Q inj : 6 mg/st Pilot injection: Q inj : 18 mg/st Pilot injection: Q inj : 30 mg/stEngine speed = 1200 rpmP inj = 300 barMain Injection Timing: - 20 0CA ATDCPilot Injection Timing: 30 0CA BSOIH e a t R e l e a s e R a t e [J /d e g ]Figure 7 Combustion characteristics according to varioussingle pilot injection quantities at a single pilot injection timing of 30° CA BSOI and a low injection pressure & medium load condition320330340350360370380390400501001502002500102030405060708090H e a t R e l e a s e R a t e [J /d e g ]Crank Angle [0]I n -c y l i n d e rp r e s s u r e [b a r ]Engine speed = 1200 rpmP inj = 300 barMain Injection Timing: - 20 0CA ATDC Q Total Inj.: 60 mg/st Q Pilot Inj.: 18 mg/stSingle InjectionPilot Injection Timing: 10 0CA BSOIPilot Injection Timing: 30 0CA BSOIPilot Injection Timing: 80 0CA BSOIFigure 8 Combustion characteristics according to various single pilot injection timings at a single pilot injection quantity of 18 mg/stroke and a low injection pressure & medium load conditionwhereas the combustion characteristics of the main injection had different heat release shapes according to the single-pilot injection timing. The most advanced single-pilot injection case (80° CA BSOI) showed the lowest level of NOx emission even though this case had a medium-sized premixed burn. Combustion phasing by combustion of the main injection with the most advanced single-pilot injection timing was not advanced, while other cases (10 and 30° CA BSOI of single-pilot injection) showed a relatively advanced main injection combustion phasing. It is possible that the most advanced single-pilot injection case resulted in a low ambient temperature due to PCCI combustion, which caused relatively retarded combustion phasing compared to the retarded single-pilot injection cases. Differences in the combustion phasing of the main injection combustion might be the reason behind thedifferent NOx emission characteristics, because earliercombustion results in greater NOx emission [1]. This observation suggests that partial PCCI combustion is a very useful method to reduce NOx.Images of the single-pilot injection are shown in Fig. 9for various single-pilot injection quantities at a low injection and medium load condition. Luminous flames are seen only in heterogeneous combustion regions of the main injection, as explained in the experimental apparatus section. Generally, white, yellow-white, yellow, and orange-red colors in the diffusion flame represent carbon particle burn-up regions, while a brown color indicates soot clouds from very fuel-rich mixture regions [1]. The diffusion flame was detected earlier due to an increased ambient temperature when more fuel was injected during single-pilot injection, which implies a decreased main injection ignition delay, resulting in the production of smoke at an earlier CA. However, in the case of the 50% single-pilot injection, flame luminosity was much stronger than the other three cases, showing larger regions of white color, but the diffusion burn duration was much shorter. This phenomenon may indicate that smoke produced earlier was oxidized more in a shorter period, which resulted in a lower smoke level at the tail-pipe. However, the single injection case was characterized mainly by a brown color region, which implies the presence of a fuel-rich zone at the squish region for longer. The diffusion burn period was also longer than in the other cases, which implies more production of smoke in the single injection case. This optical investigation again confirms the results from emission measurements that less smoke was emitted in the 50% single-pilot injection case. Furthermore, this observation supports the reasons we proposed to explain the observed smoke emission characteristics. Effect of the double pilot injection strategyFigures 10 and 11 show the NOx emission when a double-pilot injection strategy for the 15%:15% and 25%:25% case was applied. NOx emission tended to decrease as pilot injection timing advanced for both the 1st pilot injection and the 2nd pilot injection. However, NOx emission exhibited an anomalous characteristic at the 40° CA 1BSOI of the 1st pilot injection timing: a sudden increase of NOx emission. The reason for this phenomenon is not clear, and requires further investigation. However, one possible explanation is the interaction between rail-pressure fluctuation and injection timing, which influences the fuel quantity that is actually delivered [19]. The double-pilot injection strategy resulted in a greater reduction of NOx emission compared to the single-pilot injection case, because a more homogeneous charge mixture is formed due to improved turbulent effects when single-pilot injection is split into double-pilot injection [12]. The degree of reduction in NOx increased when more fuel was split into double-pilot injection, most likely because more fuelconditionparticipated in PCCI combustion. Moreover, it is likely that the 2nd pilot injection timing has a more dominant influence than the 1st pilot injection timing on NOx emission. Smoke emission was further reduced with advanced 1st and 2nd pilot injection timings, as shown in Figs. 12 and 13, in comparison to single-pilot injection. This is because of the presence of a reduced fuel-rich zone as a result of the formation of a more homogeneous charge [21]. Further reduction of smoke was observed when the amount of fuel injected during the double-pilot injection increased due to a decrease in the diffusion burn phase. The 2nd pilot injection timing was found to exert more influence on smoke emission than the 1st pilot injection timing, similar to what was observed for the NOx emission results. Figure 14 NOx emission at a low injection pressure & medium load conditionpresents ISFC for double-pilot injections. ISFC under all double-pilot injection conditions was decreased compared to ISFC under single injection conditions. ISFC decreased in value as the injection timing of each pilot injection advanced. This result may be due to incomplete combustion in the area close to the walls because of poor evaporation conditions caused by fuel impinging onto the cylinder walls or the formation of too lean a mixture. Furthermore, advanced combustion phasing of the main injection due to combustion from the pilot injection could explain the observed ISFC result. As shown for the single-pilot injection case, the general pattern observed for ISFC is similar to the pattern observed for HC emissions. Figures 15 and 16 show thesmoke emission at a low injection pressure & medium load conditionHC emissions for the double-injection strategy. HC emissions increased as the 1st pilot injection and 2nd pilot injection timing advanced, which is similar to what was seen for the single-pilot injection case. However, HC emission for the 15%:15% double-pilot injection case was half that of the single-pilot injection case. This is because spray tip penetration by was shortened due to the double-pilot injection, resulting in a reduced amount of wall-wetted fuel. However, double-pilot injection with more fuel (25%:25%) resulted in greater HC emission, because of increased fuel wetting on the cylinder wall. Figures 17 and 18 are shown to compare the combustion characteristics for various 1st and 2nd pilot injection timings with a double-pilot injection strategy. The starting point of combustion for the various double-pilot injections was almost the same, while the most retarded 1st pilot injection timing case showed the conditionhighest peak heat release rate (Fig. 17). The ignition delay of the main injection decreased as the 1st pilot injection timing was retarded. The peak heat release rate was reduced and the heat release rise-rate was mitigated as the 1st pilot injection timing advanced. Combustion with the most advanced 1st pilot injection timing case occurred in leaner mixture conditions. This observation is consistent with our previous observation that NOx emission decreased as the 1st pilot injection timing advanced. Changing the timing of the 2nd pilot injection did not have a significant effect on the heat release rate, as shown in Fig. 18. However, the heat release rate of the main injection was affected by the 2nd pilot injection timing, which manifested in a longer ignition delay, lower peak heat release rate and a more alleviated increasing rate of heat release rate in case of more advanced 2nd pilot injection timing. These resultsHC emissions at a low injection pressure & medium load condition3203303403503603703803904000501001502002503000102030405060708090Crank Angle [0]Single Injection1st Pilot Injection: 10 0CA 1BSOI1st Pilot Injection: 40 0CA 1BSOI1st Pilot Injection: 80 0CA 1BSOIEngine speed = 1200 rpm, P inj = 300 bar Q total inj.: 60 mg/st Q 1st inj., Q 2nd inj.: 15 mg/stMain Injection Timing: - 20 0CA ATDC2nd Pilot Injection Timing: 30 0CA BSOIH e a t R e l e a s e R a t e [J /d e g ]I n -c y l i n d e r p r e s s u r e [b a r ]Figure 17 Comparison of combustion characteristics forvarious 1st pilot injection timings at a 2ndpilot injection timing of 30° CA BSOI and a low injection pressure & medium load conditionprobably occurred because combustion in the advanced 2nd pilot injection timing case provided lower a ambient temperature due to the PCCI combustion that occurs when the fuel for the main combustion was injected. In general, the double-pilot injection strategy was approved as a promising method over the single injection and single-pilot injection methods to reduce NOx and smoke emissions simultaneously by selecting appropriate double-pilot injection timing.Figure 19 shows direct imaging of double-pilot injection with various 2nd pilot injection timing. A luminous flame was detected when the 2nd pilot injection timing was retarded. This case also showed a stronger luminosity intensity, while the diffusion burn phase lasted longer.H e a t R e l e a s e R a t e [J /d e g ]40° CA BSOI and a low injection pressure & medium load conditionIn contrast, the luminosity intensity of the most advanced 2nd pilot injection case was weak. At the same time, the series of visible diffusion flames ended at the earliest crank angle compared with the images of the other experimental conditions. A possible explanation for this result is that the main injection with a more retarded 2nd pilot injection resulted in a higher temperature combustion than for the more advanced 2nd pilot injection case. This observation implies that partial PCCI combustion by advancement of the 2nd pilot injection timing contributed to a relatively lean and homogeneous combustion, even during the main injection.RE SULTS AT A HIGH INJE CTION PRE SSURE (P inj. = 140 MPa) & MEDIUM LOAD (Q inj.: 60 mg/stroke)The smoke emission results for single-pilot injection under a high injection pressure & medium load condition are not presented here because no meaningful patterns were observed. Furthermore, the smoke level ranged from 0.07 to 0.05 FSN for these tests. It is believed that a high injection pressure results in rapid mixing and a leaner mixture in the time domain, which in turn enhances the rate of fuel/air mixing and subsequently suppresses the formation of smoke [21]. The HC emission pattern was similar to that observed under low injection pressure. NOx emission as a function of the single-pilot injection timing and quantity at an injection pressure of 140 MPa and a total injection quantity of 60 mg/stroke is shown in Fig. 20. The general trend of NOx emission is similar to that seen for the low injection pressure case. However, more advanced single-pilot injection timing was required to achieve low NOx emission as the quantity of fuel in the single-pilot injection increased. NOx emission from the double-pilot injection case was slightly less than that emitted by the。