Simulation of Emissions Reduction in DI Diesel Engine Using Multiple Injection A. Praptijanto, W.B. Santoso

The 14th International Symposium on Transport Phenomena, 6-10 July 2003, Bali, Indonesia

Simulation of Emissions Reduction in DI Diesel EngineUsing Multiple Injection

A. Praptijanto, W.B. Santoso

Internal Combustion Engine Laboratory, Indonesian Institute Of SciencesJl Sangkuriang (Komplek LIPI Gd20) Bandung 40135,Indonesia

ABSTRACTDiesel engines are known to produce relatively

high emissions of NOx and particulates. This research explains the influence of multiple injections in reducing DI Diesel Engine Emission. Different injection schemes (Schemes A and Schemes B) were considered, and cylinder pressure, heat release rate and soot and NOx emissions are simulation using CFD. The result shown that scheme A (75-8-25) produce the lowest emissions.

INTRODUCTIONEngine manufacturers are faced with increasingly

stringent engine performance and emission 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 research explains the influence of multiple injections in reducing DI Diesel Engine Emission. Engine experiments have shown that with high pressure multiple injection (two or more injection pulses per power cycles), the soot-NOx trade-off curves of diesel engine can be shifted closer to the origin than those with the conventional single-pulse injections, reducing both soot and NOx emissions significantly. In order to understand the mechanism of emission reduction, multidimensional computation was carried out for DI Diesel Engine with multiple injections.

MODEL FORMULATIONThree-dimensional CFD code was used to

perform all of the calculation in this research. The turbulence is modeled by standard k-. The spray is modeled by wave breakup model of Reitz [1], and was accounted for drop impingement onto the wall. The ignition model use was based on the multiple steps Shell ignition model [2]. The combustion model is a turbulent combustion model of Magnussen [3]. NOx is modeled with extended of Zeldovich the Nagle and Strickland-Constable oxidation model [4]. The models were applied to model a DI diesel

engine with the specifications and operating condition shown in Table 1.

Table 1. Engine Specifications and operating condition

Cylinder bore x stroke (mm) 92 x 96Displacement (cc) 638Compression ratio 17.7Swirl ratio 2.4Engine Speed (rpm) 1180Number of nozzle orifice x diameter (mm)

4 x 0.26

Fuel Diesel, C13H23

Fuel injected (m3/cycle) 5,052E-8Overall equivalence ratio 0.69Injection duration 26 deg CAStart of injection -6 deg. CA ATDC

The fuel was injected according to single injection rate diagram shown in figure 1.

Fig 1. Single injection rate diagram

For multiple injections, different injection schemes (Scheme A and Scheme B) were considered.

Fig 2. Scheme A Multiple Injection

The injection scheme A is shown schematically in Fig 2. In the nomenclature 75-8-25 (-6), the number in the brackets indicates the injection starting angle (ATDC). The total injection duration in scheme A is 34 deg CA.

25-8-75 (-6)

50-8-50 (-6)

75-8-25 (-6)

-8 0 8 16 24 deg CA

1.943E-9

1.943E-9

100

-8 0 8 16 24 deg CA


In scheme A injection velocity was maintained the same with that in single injection i.e. 1.943 x 10-9

m3/deg CA.

Fig 3. Scheme B Multiple Injection

Total injection duration in schemes B was the same with that in single injection. The injection velocity was about 2.808 x 10-9 m3/deg CA. The computational mesh at TDC is shown in Fig. 4.

Fig 4. Outline of the computational mesh at TDC

RESULT AND DICUSSIONSThe computed soot-NOx trade-off for various

cases is shown in figure 5. It is noticed that multiple injections are effective at reducing soot and NOx emission. For example, the soot emission of multiple injection in scheme A is reduced very significantly using the 75-8-25 (-6) injection, while NOx emission is almost the same. The 25-8-75 (-6) slightly reduces soot and NOx emission at the same time. In this study, scheme A multiple injection reduced both NOx and soot emissions. Scheme B results in reducing soot but on the other hand increasing NOx.

Fig 5. Computed soot-NOx trade-off of the designed injection schemes

NOx and soot emission mechanism of scheme B injection is similar to that of single injection with retarded injection timing. The temporal evolution of cylinder-averaged temperature is shown in Fig 6. It is evident that multiple injections delays some portion of the combustion and reduces the in-cylinder temperature which attributes to the NOx formation.

Fig. 6 In-cylinder temperature for scheme B multiple injection

NOx REDUCTION MECHANISME - Scheme A shows an interesting phenomenon since both NOx and soot emissions are reduced with the use of multiple injection. The cylinder-averaged NOx mass fraction history is shown in Fig 7. As can be seen, the NOx formation mass fraction of the 75-8-25(-6) case is quite similar to that of single injection case. The value is higher for a few crank angles after start of combustion, but due to injection pause, NOx mass fraction of this case is slightly lower for the remaining crank angle. The in-cylinder pressure and temperature of this case are very similar to those of single injection as can be seen in Fig 8. The equivalence ratio mass fraction that will result high NOx, i.e. 0.8 < < 1.1, for the two cases is very similar as well as shown in Fig 9.

The lowest NOx mass fraction is achieved when the 25-8-75(-6) is used. The in-cylinder temperature for this injection scheme is lower compared to the other injection scheme. Actually, the stoichiometric mixture of this scheme is higher than that of single injection. Due to lower in-cylinder temperature, the NOx mass fraction is lower because NOx formation is affected by temperature.

Fig. 7. Temporal evolution of cylinder-averaged NOx

25-8-75 (-6)

50-8-50 (-6)

75-8-25(-6)

2.808E-9

-8 0 8 16 24 deg CA

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

0.00E+00 1.00E-02 2.00E-02 3.00E-02 4.00E-02

NOx mass fraction

So

ot

ma

ss

fra

cti

on

single (-6)

25-8-75(-6)B

50-8-50(-6)B

75-8-75(-6)B

75-8-25(-6)A

25-8-75(-6)A

50-8-50(-6)A


Fig 8. Comparison of cylinder pressure, temperature and heat release rates for single injection and double injection schemes A

Fig 9. Volume fraction of each equivalence ratio at 200 ATDC

Based on the above discussion, the NOx reduction mechanism of multiple injections is similar to that of retarding the injection timing. Combustion of the second injection is delayed by the injection pause. When the large portion of the fuel is injected in the first injection, the NOx mass fraction history of multiple injections is like that at single injection with the same injection timing. The effect of combustion of the second injected fuel does not influence the NOx formation significantly. As the amount of the fuel at the first injection reduced, the NOx formation

rate of the multiple injections becomes similar to that of single injection with retarded start of injection. In this case, combustion of the first injected fuel has important effect on NOx formation.

SOOT REDUCTION MECHANISM - As indicated in Fig. 5, both the two injection schemes can reduce soot emission significantly. Figure 10 shows the soot mass fraction history for injection scheme A. It is seen that multiple injection can suppress the soot production. The peak values of in-cylinder soot from the multiple injections are largely reduced due to the injection pause. For example, the soot emission of 75-8-25(-6) is similar to that of single injection for few crank angles. It cannot reach the peak value of that of single injection when the injection pauses. When the second injection starts, the soot production slightly increases and then gradually decreases.

Fig 10. Temporal evolution of cylinder-averaged soot

The reason is because in single injection combustion, the high momentum of the 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 multiple injection, however, the second-pulse-injected fuel enters into a relatively fuel-lean and high-temperature region, which left over from 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 multiple 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 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 formations.

0.00E+00

1.00E+06

2.00E+06

3.00E+06

4.00E+06

5.00E+06

6.00E+06

Pre

ssu

re

Pa

0.00E+00

5.00E+02

1.00E+03

1.50E+03

2.00E+03

Tem

p

K

Single

50_50

25_75

75_25

0.00E+00

2.00E+01

4.00E+01

6.00E+01

300 320 340 360 380 400 420 440

deg CA

RO

HR

kJ/

deg


Temperature @ 200 ATDC

Soot mass fraction @ 200 ATDC


Fig 11. Computed temperature (top) and soot mass fraction in the plane of the spray axis for single injection

This reasoning is supported by Fig 9, which shows the distribution of equivalence ratio in the cylinder at 200 CA ATDC. The amount of rich mixture ( > 2) is reduced significantly. This condition tends to lean out the mixture and the soot formation is therefore reduced.

The in-cylinder temperature and soot mass fraction is shown in Fig 11 and Fig 12. At 200 CA ATDC, the 75-8-25(-6) results in a slightly higher temperature, but the overall cylinder-averaged temperature is almost the same as can be seen in Fig 8. This leads to a slightly higher soot mass fraction shown in Fig 12 (middle). The second fuel injection is rapidly consumed and the soot cloud continues to oxidize. In single injection the spray plume is replenished continuously, a rich soot-producing region (( > 2) will accumulate. The soot mass fraction is reduced significantly for the remaining crank angle.

Temperature @ 200 ATDC



Fig 12. Computed temperature (top) and soot mass fraction in the plane of the spray axis for 75-8-25(-6) scheme A injection

CONCLUSIONSIn this research, computations were made to

investigate and to get the understanding of emission reduction in DI diesel engine. The results are summarized in the following:1. Two multiple injection schemes are implemented

to achieve diesel emission reduction. Of the two schemes, scheme A gives a better emission reduction since both NOx and soot are reduced.

2. NOx reduction mechanism of multiple injections is similar to that of retarding the injection timing. When the large portion of the fuel is injected in the first injection, the NOx mass fraction history of multiple injections is like that at single injection with the same injection timing. The effect of combustion of the second injected fuel does not influence the NOx formation significantly. As the amount of the fuel at the first injection reduced, the NOx formation rate of the multiple injections becomes similar to that of single injection with retarded start of injection. In this case, combustion of the first injected fuel has important effect on NOx formation.

3. In single injection combustion, the high momentum of the 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 multiple injection, however, the second-pulse-injected fuel enters into a relatively fuel-lean and high-temperature region, which left over from 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

REFERENCES1. Xin,J., Ricart,L and Reitz,R.D, Computer

Modeling of Diesel Spray Atomization and Combustion, Combust.Sci and Tech, 137, 171-194, 1998.

2. Halstead,M.P.,Kirsch,L.J. and Quinn,C.P., The Autoignition of Hydrocarbon Fuels at High Temperatures and Pressure-Fitting a Matematical Model, Combustion and Flame, 30, 45-60, 1977.

3. Magnussen, B.F. and Hjertager,B.H., On Mathematical Modeling of Turbulent Combustion with Special Emphasis on Soot Formation and Combustion, 16th Symposium(International) on Combustion, 719-729, 1976

4. Advanced Simulation Technology(AST), FireCFDv70b_p13 UserManual, AVL List GmbH, 1998

5. Han Z., Uludogan A., Hampson G.J., and Reitz R.D., Mechanism of Soot and NOx Emissions Reduction Using Multiple Injection in a Diesel Engine, SAE Technical Paper 960633, International, 1996

6. Reitz.R.D., Controlling D.I.Diesel Engine Emissions Using Multiple Injections and EGR, Combust.Sci and Tech,vol 138, 257-278,1998.

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Simulation of Emissions Reduction in DI Diesel Engine Using Multiple Injection A. Praptijanto, W.B. Santoso