Controlling DI Diesel Engine Emissions Using ...

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a single-cylinder version of the Caterpillar 3406 production heavy-duty truck .... and D7 whose dwells were 10, 8, and 6 crank angle degrees, respectively. The.
Controlling D.I. Diesel Engine Emissions Using Multiple Injections and EGR Rolf D. Reitz

Engine Research Center, University of Wisconsin 1500 Engineering Drive, Madison, WI 53706 [email protected]

Revision Submitted to Combustion Science and Technology June, 1998

Keywords: Diesel Engine, Multiple Injection, Split Fuel Injection, Emissions, EGR

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ABSTRACT

Experiments have been performed using a modern heavy-duty direct-injection diesel engine equipped with a common-rail, electronically-controlled high-pressure fuel injection system. The experimental results demonstrate the utility of using multiple or split fuel injections in combination with EGR for giving significant emissions reduction over the entire engine operating range. The mechanisms of emission reduction have been revealed by means of combustion visualization and CFD modeling. Soot is reduced due to the fact that the soot producing regions at the tip of the sprays are not replenished with fresh fuel when each injection is terminated and then restarted. The large reduction in soot possible with multiple injections allows the use of higher levels of EGR than is possible with single injections, so that NOx can be reduced significantly while soot levels are simultaneously kept low.

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INTRODUCTION

Engine manufacturers are faced with meeting increasingly stringent engine performance and emissions mandates.

In order to remain competitive in the global

marketplace the industry is adopting new fuel system and engine control strategies. The direct-injection (DI) diesel engine offers superior fuel consumption and is widely used in heavy-duty-transport applications. There is also presently much interest in improving the performance of the small-bore direct-injection automotive diesel engine. However, diesel engines are known to produce relatively high emissions of NOx and particulates. This paper presents an overview of recent research progress on the use of modern electronically-controlled fuel injection systems to control heavy-duty diesel engine emissions.

The paper reviews a collection of unique experimental and modeling

techniques that have been used and applied successfully to reveal information for controlling emission formation mechanisms in diesel engines by researchers at the Engine Research Center of the University of Wisconsin-Madison. Traditionally, diesel emission standards have been met by the industry with techniques such as increasing the fuel injection and boost pressures (e.g., Shundoh et al., 1991), using injection timing retard (Heywood, 1988) and, recently, through the use of electronically controlled injection systems (e.g., Glassey et al., 1993). In spite of these efforts, with proposed further reductions in emission levels, the viability of the diesel engine is being challenged. Extensive research continues today to find better methods to reduce both soot and NOx emissions from diesel engines. Conventional efforts to decrease NOx emissions in-cylinder have primarily centered on retarding the injection timing. However, the limits of improvement may have been

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reached in DI diesel engines (Murayama et al., 1989). Further retardation must be accomplished with the use of higher injection pressures, but this introduces concern for material strengths, increased parasitic losses, and increased fuel system cost.

Exhaust

Gas Recirculation (EGR) is widely used with turbo-charging for NOx control in high speed DI diesel engines for passenger cars in Europe (e.g., Ladommatos et al., 1996, Arcoumanis and Schindler, 1997). However, EGR has not been used widely for heavyduty engines due to the complexity of the required hardware (Baert et al., 1996). Multiple injections are a promising new strategy for emissions control that has been explored extensively at the Engine Research Center.

Both experimental and

computational studies on the effect of multiple injection on diesel engine performance and pollutant emissions have been made. Nehmer and Reitz (1994) used a single-cylinder version of a heavy-duty truck engine and varied the amount of fuel injected in the first injection pulse of a double injection scheme (from 10 to 75 percent of total amount of fuel).

It was observed that split injections could reduce NOx without a significant

increase in particulate emissions. This was attributed to the better utilization of the air charge and late combustion compared to the standard single injection case. Tow et al. (1994) extended the study to consider different length dwells between the injection pulses, and also considered triple injection schemes. The study showed that particulate emissions could be reduced by as much as a factor of three with only an insignificant increase in NOx with optimal injections.

In addition, for triple injection schemes both

NOx and particulate decreased compared to single injection schemes at all loads. Pierpont et al. (1995) further investigated the importance of fuel distribution and injection dwell on diesel engine emissions and also included the effects of EGR. As mentioned above, for diesel engines EGR has been shown to be an effective means to control NOx emissions.

There are several explanations for the low NOx 4

emissions with EGR; the most important factor is the reduced cylinder gas temperature. Also, the exhaust gases dilute the intake charge, thereby reducing the availability of oxygen. This lowers the adiabatic flame temperature in rich regions and inhibits NOx formation (Murayama et al., 1989). However, a number of undesirable effects are usually thought to be associated with the use of EGR. These include increased engine wear due to degradation of lubricant, and increased brake specific fuel consumption (BSFC). However, it is important to note that these effects are due to increased particulate emissions with the use of EGR. If a method could be found to reduce particulates, incylinder, the use of EGR would be more acceptable. Thus, with the NOx reducing benefits of EGR, and the soot reductions possible with multiple injections, it is has been of interest to investigate the combined effect of EGR and multiple injections as a possible route toward reduced emissions for future clean diesel engines. Pierpont et al. (1995) showed that significant particulate and NOx emissions reductions could be achieved simultaneously with the use of EGR and multiple injections at one high load operating condition of a heavy-duty diesel truck engine. This work was extended by Montgomery and Reitz (1996) to show that emissions could be reduced over the entire engine operating range. In addition, the study showed that significant fuel economy benefits were possible because the engine could be operated with more advanced injection timings, while still maintaining low particulate and NOx emissions. ENGINE EXPERIMENTS The experiments of Nehmer, Tow, Pierpont and Montgomery were conducted using a single-cylinder version of the Caterpillar 3406 production heavy-duty truck engine. Details of the engine geometry are summarized in Table 1. The engine layout is presented in Figure 1. A full dilution emission tunnel was used for measuring soot emissions (dry

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and SOF using Soxhlet extraction).

Gaseous emissions were monitored using a

chemiluminescent analyzer for NOx, NDIR for CO/CO2 and a heated Flame Ionization Detector for HC.

The apparent heat release rate was computed from the measured

cylinder pressure. The raw cylinder pressure data taken with a Kistler 6061a water cooled transducer was smoothed with a Fourier transform method. More details are given by Nehmer and Reitz (1994). Combustion visualization experiments have also been conducted using an AVL endoscope system (Kong et al. (1995a), Hampson and Reitz (1997), Ricart et al. (1997)). The optical arrangement is shown in Figure 2. The endoscope took the place of one of the two exhaust valves and thus the combustion chamber geometry was changed only minimally. The field of view allowed most of a single spray plume to be visualized. The endoscope was connected to an intensified camera consisting of an Electro-Optical Services Model 9006 gated intensifier and a CID (charge injection device) camera. Specially designed gate controller and frame grabber electronics were used to allow nanosecond level gate control and frame grabber triggering. A common-rail, electronically controlled high-pressure fuel injector with programmable injection rate shapes and timings was used Miyaki et al. (1991). Injection rate data was obtained with a Bosch injection-rate meter. The experiments considered both multiple and single injections, with and without EGR, over a wide range of engine operating conditions, as described by Tow (1994), Pierpont (1995) and Montgomery (1996). Fuel injection rates (injection velocities) from typical injection schemes used are shown in Figure 3. The terminology is that 50(8)50 means that in a double injection 50% of the fuel is injected, the injections ends and is followed by an 8 crank angle degree delay, and then the final 50% is injected. Similarly, for a triple or quadruple injection, numbers in parentheses indicate the dwell times in crank angles between the 6

injections. The nozzle tip featured 6 holes with hole diameters of 0.259 mm. The sprays were oriented with the spray axis at an angle of 27.5º to the head, as can be seen in Figure 2. Further details of the fuel system instrumentation are given by Tow et al. (1994) and Pierpont et al. (1995). Typical operating conditions for the experiments are summarized in Table 2.

COMPUTER MODELING The multi-dimensional KIVA code (Amsden et al., 1987) was used to model diesel engine combustion with multiple injections and EGR. Additional new models were introduced into the code to improve the accuracy of predictions, as described by Reitz and Rutland (1995).

Turbulence is modeled using an RNG k-ε model, modified to account for

compressibility effects, as described by Han and Reitz (1995). The 'blob' injection model (Reitz, 1987) is used in which the liquid is injected as particles with characteristic sizes equal to the effective injector nozzle exit diameter. In this model effects of the internal nozzle-passage flow and geometry on initial disturbances and the atomization process are incorporated into just one model constant (Han et al., 1996). The spray breakup model is based on a Kelvin-Helmholtz wave model (Reitz, 1987) and drop distortion and dynamically varying drop drag coefficients are considered, as described by Liu et al. (1993). The ignition delay is computed using a modified version of the Shell autoignition model in which eight generic species are used to represent the fuel, intermediate species and products, as described by Kong and Reitz (1993). A characteristic time model which considers both laminar chemical kinetic and turbulent mixing time scales is used for combustion (Kong et al., 1995b). Wall heat transfer is modeled with an improved law-of-the-wall model that is derived from the onedimensional energy equation and accounts for the increase of the turbulent Prandtl number

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in the boundary layer and for compressibility effects, as described by Han and Reitz (1997). NOx emissions are modeled using the extended Zel'dovich mechanism (Bowman, 1975). Soot formation is modeled using the Hiroyasu and Kadota (1976) model and soot oxidation is determined with the Nagle and Strickland-Constable (1962) model or a turbulence/mixing controlled oxidation model, modified to include kinetic limits (Hampson and Reitz, 1997). The accuracy of cylinder pressure and emission predictions is influenced by the residual gas contribution to the initial condition at intake valve closing (IVC) (Xin et al., 1997). The residual gas fraction depends on the intake and exhaust manifold pressures and the valve overlap, and was calculated using the method reported by Senecal et al. (1996), which is a modified version of a zero-dimensional emptying and filling model.

RESULTS EXPERIMENTS - Engine-out particulate and NOx measurements obtained using single injections at the baseline condition of 1600 rev/min and 75% load are summarized in Figure 4 (Tow et al., 1994).

This operating condition has been found to be a

representative operating condition for emissions testing (Montgomery and Reitz, 1996). The classic particulate vs. NOx tradeoff is seen as the injection timing was varied particulate increases and NOx decreases as the injection timing is retarded. The figure also shows the particulate vs. NOx emissions for double injections with a zero dwell after the first (pilot) quantity of the fuel is injected. The initial injected quantity is seen to have little effect on the overall particulate vs. NOx tradeoff; all injection schemes behave similarly.

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A considerable reduction in particulates is found when a longer dwell is used between the injections, as shown in Figure 5 (Tow et al., 1994).

A ratio of

approximately 50/50 fuel mass in the first to second injection was used in cases D5, D6, and D7 whose dwells were 10, 8, and 6 crank angle degrees, respectively.

The

difference in particulate level between D5 and D7 shows the effectiveness of a relatively long (10 degree CA) dwell which produces a combustion process in which the particulate emissions do not increase significantly with timing retard.

Even larger

reductions in particulate have been found with the use of triple and quadruple injections, as will be discussed below. Exhaust gas recirculation (EGR) is known to be effective at reducing NOx, but at high loads there is usually a large increase in particulate. As discussed above, multiple injections are effective at reducing particulate, thus it was of interest to see if it was possible to simultaneously reduce particulate and NOx with the combined use of EGR and multiple injections. In the experiments, EGR was obtained by connecting the intake and exhaust surge tanks (see Figure (1)) and by using an exhaust back pressure slightly above the intake pressure (e.g., see Table 2). This increases the amount of exhaust residual gas in the cylinder which reduces NOx emissions somewhat (Pierpont et al., 1995). When EGR is added, the intake air temperature is increased. Results have been obtained with and without intercooling since the increased intake air temperature tends to increase NOx. Figure 6 shows measured particulate-NOx tradeoff results for double and quadruple injections combined with EGR. As can be seen, with a quadruple injection and a 10% EGR rate, particulate and NOx are simultaneously reduced to as low as 0.1 and 1.5 g/bhphr, respectively. These extremely low emissions are in the range of future Federally mandated ULEV emission values.

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Multiple injections have been found to be very effective at reducing particulate emissions at high load, and multiple injections used in conjunction with EGR are effective at intermediate and light loads (Montgomery and Reitz, 1996). This is shown in Table 3 which compares overall cycle results for a six mode simulation of the FTP transient emissions test when the engine was operated with single injections and with optimized double injection/EGR combinations. For the baseline single injections without EGR, the total cycle particulate and NOx were 0.44 and 5.17 g/bhp-hr, respectively, and the cycle brake specific fuel consumption (BSFC) was 206 g/bhp-hr.

With the use of EGR,

multiple injections and reduced injection pressure at the idle condition (mode 1), the total cycle NOx was reduced to 3.7 g/bhp-hr, the particulate was reduced to 0.11 g/bhp-hr, and the BSFC was reduced to 184 g/bhp-hr. Thus, the cycle NOx, particulate and fuel consumption were reduced by 32%, 75% and 12%, respectively. It is worth noting that the low emission levels shown in Table 3 were obtained with the use of relatively low injection pressures (maximum rail pressure of 90 MPa), relatively low boost pressures, and a fuel injector nozzle with a relatively large exit orifice diameter and a high nozzle discharge coefficient. Also, no correction was applied to account for the increased friction of the single cylinder test engine. MODELING - Figures 7a and 7b show the measured and predicted cylinder pressure and rate of heat release for the baseline single injection case with start of injection (SOI) timing of 6 degrees BTDC (Chen et al., 1997). As can be seen, there is excellent agreement between the model and experimental results. The predicted engineout soot and NOx tradeoff is compared with the measured particulate NOx tradeoff in Figure 7c. The soot predictions with the mixing controlled soot oxidation model are better able to capture the trend of increased particulate at retarded injection timings than

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the kinetic NSC model, and the mixing model was thus used for the remainder of the study. KIVA model predictions of the effect of multiple injection on emissions have been conducted by Patterson et al. (1993), Han et al. (1996) and Chan et al. (1997). The modeling results also predict that significant particulate reductions are possible with the use of a multiple injections and EGR and are useful to provide explanations of the observed trends.

Figure 8 shows comparisons of predicted and measured in-cylinder

pressure, normalized heat release rate and the soot- NOx trade-off curve for 50(8)50 double injections with 0% and 10% EGR. As can be seen, the overall agreement in pressure-time history and rate of heat release match fairly well for both EGR cases. The peak pressure is slightly over predicted. This is due to the higher predicted combustion rate, as seen in the heat release figures.

The overall prediction of the soot- NOx trade-off compares well with the measured values as seen in Figure 8c and 8f. However, as the amount of EGR increases the discrepancies increase. This was also observed for the single injection cases, as discussed earlier. The accurate predictive capability of the present models is seen in Figure 8. The computations capture the unusual trend of simultaneously decreasing NOx and soot that is also observed in the experiment with high EGR cases as the injection timing is retarded. Thereafter, as the injection timing is further retarded, a dramatic increase in soot is seen the soot 'catastrophe' (Hampson and Reitz, 1997). Figure 8 also indicates that there exists an optimum EGR-injection timing condition where low values of both NOx and soot emissions can be obtained. Figure 9 shows a comparison of the predicted and measured in-cylinder pressure and normalized heat release rate using a 48(6)36(6)16 triple injection for 0 and 10% EGR 11

cases (Chan et al., 1997). The soot-NOx curve is also shown.

Good agreement is

observed between the predicted and measured in-cylinder pressure and the rate of heat release. The overall nature of the heat release curves is similar to that of the double injection cases with high EGR. In the case of triple injection, the general trends of the soot-NOx trade-off curve are maintained throughout the injection timing range studied. Figure 9 also shows that the very low NOx values achieved with the use of triple injections and EGR are well predicted by the model. The above good agreements between computational and experimental results gives confidence in the use of modeling to help reveal the mechanisms of emissions reduction with multiple injections. MECHANISM OF EMISSION REDUCTION - The mechanism of emissions reduction with the use of multiple injections has been studied by Han et al. (1996) and is illustrated schematically in Figure 10. The computations show that the soot is formed and it accumulates in the tip region of the spray jet. This soot accumulation at the spray tip has also been observed experimentally by Dec and Espey (1996) and others in optically accessible DI diesel engines. In single injection combustion, the high momentum injected fuel penetrates to the fuel-rich, relatively low temperature region at the jet tip and continuously replenishes this rich region, producing soot. In a split-injection, however, the second-pulse-injected fuel enters into a relatively fuel-lean and high-temperature region which is left over from the combustion of the first pulse. Soot formation is therefore significantly reduced because the injected fuel is rapidly consumed by combustion before a rich soot-producing region can accumulate. In addition, the soot cloud of the first spray plume is not replenished with fresh fuel, but instead, continues to oxidize. As a result, the net production of soot in split-injection combustion can be reduced substantially, particularly if the dwell between the two injections is optimized long enough so that the soot formation region of the first injection is not replenished with

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fresh fuel, but short enough that the in-cylinder gas temperature environment seen by the second pulse remains high enough to prompt fast combustion, reducing soot formation. SOOT 'CATASTROPHE' - The measured and predicted soot-NOx tradeoff results in Figures 6, 8 and 9 show that a precipitous increase in soot occurs at very retarded injection timings - the soot 'catastrophe' (Hampson and Reitz, 1998). Since operation at retarded timings is of interest for NOx control it is important to investigate the reasons for the large increase in soot. Figure 11 shows endoscope images for a 48(6)36(6)16 triple injection at various crank angles at two start of injection timings, SOI +3 and +6 degrees ATDC. As can be seen in Figure 9, the +6 case has an order of magnitude more engineout soot than the +3 case. The reasons for this extreme sensitivity to injection timing are thus of interest.

In Figure 11 the injector is located at the top of each frame and

combustion details from one central spray and portions of two neighboring spray plumes can be seen in each image. The first visible combustion (i.e., ignition) is seen to occur near the injector location at 7 and 9 degrees after the start of injection for the +3 and +6 cases, respectively. By 12 degrees after the start of injection combustion appears to be similar in both cases. But the intensity of light emission is lower in the +6 degree case (the intensifier gate width was 100 µs versus 30 µs for the +3 case), indicating lower flame temperatures. Significant differences between the two cases are seen at 22 degrees after the start of injection. In particular, in the +6 case it appears from the dark regions in the center of the image that the second injection pulse did not ignite.

This poor combustion and the resulting

unburned fuel would thus explain the large increase in soot observed for the +6 case.

SUMMARY AND CONCLUSIONS

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An emissions and performance study has been conducted to explore the effects of exhaust gas recirculation (EGR) and multiple injections on DI diesel engine particulate, NOx and BSFC.

The experiments were conducted using a fully instrumented single cylinder

version of the Caterpillar 3406 heavy-duty truck engine.

The fuel system was an

electronically-controlled common-rail injection system which was capable of up to four independent injections per cycle at injection pressures up to 90 MPa. The results showed that significant NOx and particulate emissions reductions can be achieved, together with improved fuel economy, by using split injections combined with EGR. These benefits were found over the entire engine operating range. The engine was also instrumented with an endoscope system for optical access and combustion visualization experiments were performed. The combustion images showed that the large increases in soot observed at very retarded injection timings are due to poor combustion of the late injections in multiple injection schemes. A companion modeling study was also performed to help reveal the mechanisms of emissions reduction with the use of multiple injections. It was found that the soot is reduced due to the fact that split injections separate the soot forming regions spatially within the combustion chamber, and this promotes increased soot oxidation and reduced soot formation.

ACKNOWLEDGMENTS This work is the outcome of the efforts of numerous faculty, staff and students (many of whose names appear in the list of References) working at the Engine Research Center funded by the Army Research Office, DOE/NASA-Lewis, DOE/Sandia, Ford and other funding. Caterpillar also provided support for many staff and graduate students and

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supplied the engine and parts. A version of this paper was presented at the Internationale Konferenz “Common Rail Einspritzsysteme – Gegenwart und Zukunftspotential”, ETH Zurich, Nov., 1997.

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REFERENCES Amsden, A. A., Butler, T. D. and O'Rourke, P. J. "The KIVA-II Computer Program for Transient Multi-Dimensional Chemically Reactive Flows with Sprays," Society of Automotive Engineers Technical Paper 872072, 1987. Arcoumanis, C., and Schindler, K. P., “Mixture Formation and Combustion in the DI Diesel Engine,” SAE Paper 972681, 1997. Baert, R. S. G., Beckman, D. E., and Verbeek, R. P., “New EGR Technology Retains HD Diesel Economy with 21st Century Emissions,” SAE Paper 960848, 1996. Bowman, C.T., "Kinetics of Pollutant Formation and Destruction in Combustion," Prog. Energy Combust. Sci., Vol. 1, pp. 33-45, 1975. Chan, M., Das, S., and Reitz, R.D., "Modeling Multiple Injection and EGR Effects on D.I., Diesel Engine Emissions, Fall SAE Fuels and Lubricants Meeting, Tulsa, OK, 1997. Dec, J.E., and Espey, C., "Ignition and Early Soot Formation in a DI Diesel Engine Using Multiple 2-D Imaging Diagnostics," SAE Paper 950456, 1995. Glassey, S.F., Flinn, M.A., Camplin, F.A., "Development of the HEUI Fuel System Integration of Design, Simulation, Test and Manufacturing," SAE Paper 930271, 1993. Hampson, G. J., and Reitz, R.D., “Two-Color Imaging of In-Cylinder Soot Concentration and Temperature in a Heavy-Duty DI Diesel Engine with Comparison to Multidimensional Modeling for Single and Split Injections,” SAE Paper 980549, 1998.. 16

Han, Z. and Reitz, R.D., "Turbulence Modeling of Internal Combustion Engines Using RNG k-ε Models," Combust. Sci. and Tech. 106, 4-6, p. 267, 1995. Han, Z., Uludogan, A., Hampson, G., and Reitz, R.D., "Mechanisms of Soot and NOx Emission Reduction Using Multiple-Injection in a Diesel Engine," SAE Paper 960633, 1996. Han, Z. and Reitz, R.D., "A Temperature Wall Function Formulation for Variable Density Turbulent Flows with Application to Engine Convective Heat Transfer Modeling," International Journal of Heat and Mass Transfer, Vol. 40, No. 3, pp. 613-625, 1997. Heywood, J. B., “Internal Combustion Engine Fundamentals,” McGraw-Hill, Inc. New York, 1988. Hiroyasu, H, and Kadota, T., "Models for Combustion and Formation of Nitric Oxide and Soot in DI Diesel Engines," SAE Paper 760129, 1976. Kong, S.-C., and Reitz, R.D. "Multidimensional Modeling of Diesel Ignition and Combustion Using A Multistep Kinetics Models," Paper 93-ICE-22, ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 115, No. 4, pp. 781-789, 1993. Kong, S.-C., Ricart, L. M., and Reitz, R.D., "In-Cylinder Diesel Flame Imaging Compared with Numerical Computations," SAE Paper 950455, SAE Transactions, Vol. 104, Section 3, Journal of Engines, pp. 829-852, 1995a. Kong, S.-C., Han, Z., and Reitz, R.D., "The Development and Application of a Diesel Ignition and Combustion Model for Multidimensional Engine Simulations," SAE Paper 950278, SAE Transactions, Vol. 104, Section 3, Journal of Engines, pp. 502518, 1995b. 17

Ladommatos, N., Balian, R., Horrocks, R., and Cooper, L., “The Effect of Exhaust Gas Recirculation on Combustion and NOx Emissions inn a

High-Speed Direct-

Injection Diesel Engine,” SAE Paper 960840, 1996. Liu, A.B., Mather, D., and Reitz, R.D., "Effects of Drop Drag and Breakup on Fuel Sprays" SAE Paper 930072, SAE Transactions, Vol. 102, Section 3, Journal of Engines, pp. 63-95, 1993. Miyaki, M., Fujisawa, H., Masuda, A., and Yamoamoto, Y., "Development of New Electronically Controlled Fuel Injection System ECD-U2 for Diesel Engines," SAE Paper 910252, 1991. Montgomery, D.T., and Reitz, R.D., "Six-mode Cycle Evaluation of the Effect of EGR and Multiple Injections on Particulate and NOx Emissions from a D.I. Diesel Engine," SAE Paper 960316, 1996. Murayama, T., “Simultaneous Reduction of NOx and Smoke of Diesel Engines, without Sacrificing Thermal Efficiency,” JSME International Journal, Series B, 37, 1, 1989. Nagle, J., and Strickland-Constable, R.F., "Oxidation of Carbon between 1000-2000 C," Proc. of the Fifth Carbon Conf., Vol. 1, p. 154, 1962. Nehmer, D.A. and Reitz, R.D., "Measurement of the Effect of Injection Rate and Split Injections on Diesel Engine Soot and NOx Emissions," SAE Paper 940668, SAE Transactions, Vol. 103, Section 3, Journal of Engines, pp. 1030-1043,1994. Pierpont, D.A. , Montgomery, D. and Reitz, R.D. "Reduction of Diesel Soot and NOx emissions using EGR and Multiple Injections," SAE Paper 950217, 1995. Reitz, R.D. "Modeling Atomization Processes in High-Pressure Vaporizing Sprays," Atomisation and Spray Technology, 3, pp. 309-337, 1987. 18

Reitz, R.D., and Rutland, C.J., "Development and Testing of Diesel Engine CFD Models," Progress in Energy and Combustion Science, Vol. 21, pp. 173-196, 1995. Ricart, L.M., Xin, J., Bower, G.R., and Reitz, R.D., "In-Cylinder Measurement and Modeling of Liquid Fuel Spray Penetration in a Heavy-Duty Diesel Engine," SAE Paper 971591, 1997. Senecal, P. K., Xin, J., and Reitz, R. D., “Predictions of Residual Gas Fraction in IC Engines,” SAE Paper 962052, 1996. Shundoh, S., Kakegawa, T., Tsujimura, K., “The Effect of Injection Parameters and Swirl on Diesel Combustion with High Pressure Fuel Injection,” SAE Paper 910489, 1991. Tow, T., Pierpont, A. and Reitz, R.D. "Reducing Particulates and NOx Emissions by Using Multiple Injections in a Heavy Duty D.I. Diesel Engine," SAE Paper 940897, 1994. Xin, J., Montgomery, D. T., Han, Z., and Reitz, R. D., “Multidimensional Modeling of Combustion for a Six-Mode Emissions Test Cycle of a D. I. Diesel Engine,” Accepted for Journal of Engineering for Gas Turbines and Power, 1997.

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Table 1 Engine and Fuel System Instrumentation

Engine Details Cylinder Bore

137.2 mm

Stroke

165.1 mm

Compression ratio

14.5

Displacement

2.44 liters

Simulated turbocharge

to 4 atm.

Cooled EGR

0 -15% Fuel Injection System

Injectors

6 holes, 0.26 mm diameter

Common Rail Injection pressures

90 MPa

Single injection and up to 4 multiple injections

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Table 2 Six-mode engine test conditions (Montgomery and Reitz, 1996) Running Condition Speed (rev/min) Fuel Consumption Rate (kg/hr) Intake ( ° C) Temperature Intake Pressure (kPa) Baseline Exhaust Pressure (kPa) Exhaust Pressure at 6% EGR (kPa) Exhaust Pressure at 10% EGR (kPa) Exhaust Pressure at 20% EGR (kPa)

Mode 1

Mode 2

Mode 3

Mode 4

Mode 5

Mode 6

750

953

1074

1657

1668

1690

1600 rev/min 75% Load 1600

0.5

2.0

6.4

10.2

5.4

3.2

7.7

26

29 (40 at 20% EGR)

31

40

32

29

36

100

108

168

239

164

132

183

100

112

144

220

164

143

159

___

___ 181

286

197

___

___

___

___

___

___ 216 ___ 219

___

___

___

116 Test Conditions (nominal)

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___

___

___

Table 3 Results from 6-mode testing (Montgomery and Reitz, 1996)

90 MPa Single injections Mode

1

2

3

4

5

6

SOI

-8.7

-2.3

4.2

6

0

-3

NOx (g/hr)

14.0

134.2

195.1

145.1

104.7

104.6

Particulate (g/hr)

5.77

0.79

6.88

14.34

11.03

2.37

Power (kW)

0.3

7.6

27.0

38.4

21.5

11.3

0.0337

0.1052

0.1693

0.0888

0.0538

Fuel Consumed (kg/min) .0085

90 MPa double injections plus EGR (30MPa single injection at Mode 1 (idle)) Mode

1

2

3

4

5

6

SOI

-12

0.7

-4.8

-6

0

-3

EGR (%)

0

20

6

6

6

0

NOx (g/hr)

31.9

23.8

151.3

166.0

72.4

58.4

Particulate (g/hr)

0.56

0.68

2.86

6.58

2.80

2.70

Power (kW)

1.3

8.5

28.8

45.1

21.2

11.3

0.0345

0.1052

0.1040

0.0862

0.0528

Fuel Consumed (kg/min) .0092

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Mixing orifice

Baffle

T.C. Exhaust

Primary dilution air

Compressed air

Heater

T.C. EGR

HC NOx CO CO2

Filters

Exhaust

Intake

T.C.

Vacuum pump Volume meter T.C.

T.C.

Secondary dilution air

Exhaust

Cat 3401 Gate valve metering orifice

Fig. 1 Schematic diagram of Caterpillar engine and test cell layout.

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AVL Endoscope

Spray plumes

Endoscope view

Fig. 2 Schematic diagram of a one-sixth sector of the combustion chamber (top) and plan view of field of view of endoscope visualization imaging (bottom).

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Injection Velocity (cm/s/deg)

8 104 single double triple

7 104 6 104 5 104 4 104 3 104 2 104 1 104 0 0

5

10

15 20 Crank Angle (deg)

25

30

35

Fig. 3 Injection velocity profiles obtained from the measured mass flow rate for: (a) single; (b) double 50(8)50; (c) triple 48(6)36(6)16 injections

25

0.2



0.18





Particulate (g/bhp-hr)

0.16 0.14

✧ ●



[S1] Single .6



[D1] 13-(0)-87



[D2] 16-(0)-84



[D3] 21-(0)-79



[D4] 32-(0)-68

[S1] [D1] [D2]



0.12



0.1

[D3]

✳ ✧

0.08

[D4]

★ ✦





0.06

★✧ ✳ ✦





★ ✳





0.04



0.02 3

4

5

6

7

8

9

NOx (g/bhp-hr)

Fig. 4 Particulate vs. NOx tradeoff curves for double injections (1600 rev/min, 75% load). Start-of-injection timings varied from -14 to –2 degrees ATDC. Inset plots show measured injection rate profiles.

26

0.2 0.18

Particulate (g/bhp-hr)

0.16 0.14 ●



[S1] Single .6



[D5] 48-(10)-52



[D6] 55-(8)-45 .2



[D7] 46-(6)-54



[D8] 75-(10)-25

[S1] [D5] [D6]

0.12

[D7] 0.1

❑ ■





[D8]

0.08 ● ▼

✖ ✖

0.06



✖ ■

0.04



✖ ■



● ▼ ■ ✖ ■ ■ ✖





0.02 3

4

5

6 7 NOx (g/bhp-hr)

8

9

Fig. 5 Particulate vs. NOx tradeoff curves for double injections (1600 rev/min, 75% load). Start-of-injection timings varied from -12 to 1 degrees ATDC. Inset plots show measured injection rate profiles.

27

0.7 J

0.6 0.5

B

Base

J

50(0.98)50 Double, 6% Cold EGR

H

41(0.50)26(0.50)21(0.80)12 Quad, 6% Cold EGR

F

41(0.50)26(0.50)21(0.80)12 Quad, 10% Cold EGR

F B

0.4 B J

0.3

H

0.2

B

J F F

0.1

FH

F J

H

J H

B

H

0 0

1

2

3

4

5

6

NOx (g/bhp-hr)

NOx vs. particulate tradeoffs for baseline, double injections and quadruple injections with 6% and 10% EGR (1600 rev/min, 75%load)

Fig. 6 Effect of single (base), double and quadruple injections and EGR on particulate and NOx emissions

28

12

Pressure (MPa)

10 8

Single, 0% EGR, SOI = -6 Deg Measured NSC Model Mixing Controlled

6 4 2 Single, 0% EGR

0 -100

-50

0 Crank Angle (deg)

50

100

Normalized Heat Release per Degree

(a.)

0.06 Single, 0% EGR, SOI = -6 Deg 0.05

Measured NSC Model Mixing Controlled

0.04 0.03 0.02 0.01 0 -0.01 -10

0

10

20 30 40 Crank Angle (deg)

50

60

70

(b.)

5 Single, 0% EGR single soot NSC Model Mixing Controlled

Soot (g/kgfuel)

4 (0)

3

2 (0)

1

(-3)

(0)

(-3)

(-6)

(-3)

(-9)(-9)

(-6) (-6)

(-9)

0 10

15

20

25 30 NOx (g/kgfuel)

35

40

(c.)

Fig. 7 Comparison of a.) predicted cylinder pressure and b.) heat release rates for single injection at -6 deg. ATDC, and c.) predicted and measured soot-NOx tradeoff obtained with kinetic NSC and mixing controlled soot oxidation models.

29

10 Double, 0% EGR, S.O.I. -6 ATDC

Measured Predicted

Pressure (MPa)

8

6

4

2

0 -100

0

-50

50

100

Crank Angle (deg)

Normalized Heat Release per Degree

(i.)

0.06 Double, 0% EGR, S.O.I. -6 ATDC

0.05

Measured Predicted

0.04 0.03 0.02 0.01 0 -0.01 -10

0

10

20 30 40 Crank Angle (deg)

50

60

70

(ii.)

4

(3)

Double, 0% EGR

3.5

Measured Predicted

Soot (g/kgfuel)

3 2.5 2 1.5 (3)

1 (0)

0.5

(0)

(-3)

(-6)

0

10

20 NOx (g/kgfuel)

(-9) (-9)

(-6)

0

30

40

(iii.)

Fig. 8a Comparison of predicted (dash lines) and measured (solid lines) for the double injection cases with 0% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve

30

10 Double, 10% EGR, S.O.I. -6 ATDC

Measured Predicted

Pressure (MPa)

8

6

4

2

0 -100

-50

0 Crank Angle (deg)

50

100

Normalized Heat Release per Degree

(a.)

0.06 Double, 10% EGR, S.O.I. -6 ATDC

0.05

Measured Predicted

0.04 0.03 0.02 0.01 0

-0.01 -10

0

10

20 30 40 Crank Angle (deg)

50

60

70

(b.)

4 Double, 10% EGR

3.5

(3)

Measured Predicted

Soot (g/kgfuel)

3 2.5 (-6)

(3)

2

(-3) (-6)

1.5 (0)

1

(-3)

0.5 (0)

0 0

10

20

30

40

NOx (g/kgfuel)

(c.)

Fig. 8b Comparison of predicted (dash lines) and measured (solid lines) for the double injection cases with 10% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve

31

10 Triple, 0% EGR, S.O.I. -3 ATDC

Measured Predicted

Pressure (MPa)

8

6

4

2

0 -100

-50

0 Crank Angle (deg)

50

100

(i)

Normalized Heat Release per Degree

0.06 Triple, 0% EGR, S.O.I. -3 ATDC

0.05

Measured Predicted

0.04 0.03 0.02 0.01 0 -0.01 -10

0

10

20

30

40

50

60

70

Crank Angle (deg)

(ii)

2 (6)

(3)

Triple, 0% EGR

1.75

Measured Predicted

Soot (g/kgfuel)

1.5 1.25 1 0.75 (-6)

0.5

(3)

0.25

(0)

(-3) (0)

(-6)

(-3)

0 5

10

15

20

25

30

35

40

NOx (g/kgfuel)

(iii)

Fig. 9a Comparison of predicted (dash lines) and measured (solid lines) for the triple injection case with 0% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve

10 Triple, 10% EGR, S.O.I. -3 ATDC

Measured Predicted

Pressure (MPa)

8

6

4

2

0 -100

-50

0 Crank Angle (deg)

50

100

(i)

Normalized Heat Release per Degree

0.06 Triple, 10% EGR, S.O.I. -3 ATDC

0.05

Measured Predicted

0.04 0.03 0.02 0.01 0 -0.01 -10

0

10

20 30 40 Crank Angle (deg)

50

60

70

(ii)

2 Triple, 10% EGR

(-3)

1.75

Measured Predicted

Soot (g/kgfuel)

1.5 1.25

(0) (6)

1

(-3)

(3)

0.75

(3)

(0)

0.5 0.25 0 0

5

10

15

20

25

30

35

40

NOx (g/kgfuel)

(iii)

Fig. 9b Comparison of predicted (dash lines) and measured (solid lines) for the triple injection case with 10% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve

Incoming spray

Air entrainment

Low-temperature fuel-rich region Continuous replenishment of fuel-rich region

Soot formation

(a.)

Second-pulse spray

High-temperature fuel-lean region

Non-replenished soot cloud

Reduced soot formation

(b.)

Fig. 10 Schematic diagram showing soot-reduction mechanisms of split injections. a.) single injection, b.) split injection.

+3 100 µs

30 µs

20 µs

20 µs

7 12 0

9

22

28

20

40 ASOI

+6 100 µs

100 µs

30 µs

20 µs

Fig. 11 Endoscope images of 48(6)36(6)16 triple injections with start of injection 3 deg. ATDC (top) and +6 deg. ATDC (bottom). Injector is located at the top of each image.

FIGURE CAPTIONS Fig. 1 Schematic diagram of Caterpillar engine and test cell layout. Fig. 2 Schematic diagram of a one-sixth sector of the combustion chamber (top) and plan view of field of view of endoscope visualization imaging (bottom). Fig. 3 Injection velocity profiles obtained from the measured mass flow rate for: (a) single; (b) double 50(8)50; (c) triple 48(6)36(6)16 injections Fig. 4 Particulate vs. NOx tradeoff curves for double injections (1600 rev/min, 75% load). Start-of-injection timings varied from –14 to –2 degrees ATDC. Inset plots show measured injection rate profiles. Fig. 5 Particulate vs. NOx tradeoff curves for double injections (1600 rev/min, 75% load). Start-of-injection timings varied from -12 to 1 degrees ATDC. Inset plots show measured injection rate profiles. Fig. 6 Effect of single (base), double and quadruple injections and EGR on particulate and NOx emissions Fig. 7 Comparison of a.) predicted cylinder pressure and b.) heat release rates for single injection at -6 deg. ATDC, and c.) predicted and measured soot-NOx tradeoff obtained with kinetic NSC and mixing controlled soot oxidation models. Fig. 8a Comparison of predicted (dash lines) and measured (solid lines) for the double injection cases with 0% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve

Fig. 8b Comparison of predicted (dash lines) and measured (solid lines) for the double injection cases with 10% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve Fig. 9a Comparison of predicted (dash lines) and measured (solid lines) for the triple injection case with 10% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve Fig. 9b Comparison of predicted (dash lines) and measured (solid lines) for the triple injection case with 10% EGR, (i) In-cylinder pressure for SOI=-6, (ii) Rate of heat release rate at SOI=-6 and, (iii) Soot-NOx trade-off curve Fig. 10 Schematic diagram showing soot-reduction mechanisms of split injections. a.) single injection, b.) split injection.

Fig. 11 Endoscope images of 48(6)36(6)16 triple injections with start of injection 3 deg. ATDC (top) and +6 deg. ATDC (bottom). Injector is located at the top of each image.