Numerical  research  of  possibility   to decrease a level of NOx emission in diesel engine

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  The program DIESEL - 2/4t allows to carry out numerical researches with the purpose to decrease a level of emission of harmful materials in engines with different conditions of a mixture formation
    The results of similar numerical research for four-stroke two-cylinder air-cooled tractor diesel (D/S = 105 / 120 mm) are presented below. Beforehand the program was customized on this engine. The verification of the calculated diesel parameters compared to experimental ones are presented in table.

Parameters

rpm=2000 rpm=1500
Experiment Calculation Experiment Calculation
Capacity, Pe, kW 22 21.6 18.2 18.2
Specific fuel consumption, SFC, g/kW h 235 238 233.5 234.4
Air flow rate, kg/s 0.0365 0.0358 0.0275 0.0266
Max. cylinder pressure, bar 76.7 77.6 88 88
Mean indicator pressure, bar 8.48 8.34 - 8.73
Mean pressure of pumping loop, bar 0.35 0.36 - 0.21
Mean temperature in the exhaust manifold, K 793 713 793 704
Level of smoke emission, Hartridge 17.5 17.2 32.5 30

    As can seen from the table, the program allows to obtain the high accuracy results.    

     The difference between calculated  and measured gas temperature in the exhaust manifolds caused by well known circumstance: the measuring by the thermocouple temperature of a pulsing flow  gives overestimated results. The error of this measurement increases when the relation between pulse pressure difference and average value of pressure increases.

The results of calculation of a level of NOx emission at different values of compression ratio were compared to results of corresponding experiences of the manufacturer.

     In the present research, the possibility of decrease of NOx emission was investigated at the expense of change of following parameters:
-  nozzles bore was decreased from 0.3 to 0.22 mm;
-  compression ratio;
-  injection lead;
-  piston bowl shape;
- angles of inclination of fuel jets.

  The effect of injection characteristic was not investigated, as the implementation of the radical increasing of injection pressure requires serious change of a diesel construction.

Results of calculation of mixture formation and combustion at the base configuration of diesel.

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Tractor diesel D-120

rpm=2000;
Pe=22 kW

Compression ratio:

CR =  16

Swirl number H =3

At decrease of nozzles diameter from 0.3 to 0.22 mm the characteristic of injection of nominal fuel equipment essentially varies. The maximum injection pressure is increased from 520 to 665 bar; the duration of injection is increased from 19.2 to 27.3 deg. of crank angle.

Injection velocity
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   These data were obtained by Ph. D. Grekhov which has carried out the hydrodynamic calculation with the help of the program INJECTION. Beforehand the program INJECTION was customized on a nominal configuration of the pump and injectors of the given diesel.


     Results of calculation of a working process of a diesel at base configuration but with a bore of nozzles of 0.22 mm (See tab. 1, config. #1,) have shown that there is an essential improvement of quality of an atomization of fuel. The effective diameter of drops d32 considerably reduces from 30 to 19 microns. However the prolonged injection is a reason of unfavorable allocation of fuel:
23.5 % of fuel is allocated in zones of intersection of wall surface flows. In these zones there is a low evaporation rate. (For matching: at nozzle diameter of 0.3 mm and injection timing of 19.2 deg.  1.7 % of fuel is allocated only in these zones).
- The amount of fuel in the rare environment of free jets and of their wall surface flows (where is the best conditions of evaporation) is reduced by 5 %.
- The amount of fuel in the not intersected parts of wall surface flows (where are also quite good conditions of evaporation) is 17.5 % (whereas at nozzles of 0.3 mm it reaches 37 %).

   Thus, at prolonged injection the jets have not enough space for a development and they hinder each other. The combustion process is protracted:
- The specific fuel consumption "SFC" is increased by 8 g / kW h;
- The emission of nitric oxides "NOx" is reduced by  20 %;
- The level of a smoke "Hartridge" is increased by 49 % (table 1).

Table 1

Configuration Base # 1
CR 16 16
Injection lead 16 16
SFC 239 247
NOx 3.4 2.7

Hartridge

17.2 25.6

    It is impossible to estimate satisfactory this result, because of  the decreasing of a level of NOx emission is accompanied by considerable increase of smoke emission and by increase of fuel consumption.    
    The change of compression ratio e and injection lead "Inj. lead" is a reserve for improvement of the engine parameters. The research of optimum combination of these parameters was made by scanning method.  The isolines of constant values of different parameters of engine are presented in Fig.1:
- "
NOx" is constant levels of nitric oxide emission, g/m3;
- "Pz"  is constant levels of max. cylinder pressure, bar;
- "SFC" is constant levels of specific fuel consumption, g/kW h.

     The smoke emission as a function of two variables: the compression ratio e and the injection lead, is presented in a Fig. 2.

   The configurations of a diesel are presented in table 2:
- Base  is a basic configuration: nozzles of 3 x 0.3;
- # 1  is a basic configuration: nozzles of 3 x 0.22;
- # 2  is a new configuration: the enlarged compression ratio, diminished injection lead, nozzles of 3 x 0.22.

Fig. 1. Isolines of NOx emission, fuel consumption  and max. cylinder pressure at different combination of compression ratio e and injection lead.

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Fig. 2. The smoke emission as a function of two variables: the compression ratio e  and the injection lead.d12_smk.gif (4742 bytes)

 

Table 2

Config. Basic # 1 # 2
CR 16 16 19.5
Inj. lead 16 16 11.5
SFC 239 247 242
NOx 3.4 2.7 1.8

Hartr

17.2 25.6 24.2

Reasons of choice of configuration #2:

- CR =19.5
 (if  CR >19.5 then cyl.head-piston clearance < 0.5 mm);

- Injection. lead = 11.5
 ( if Inj.lead < 11.5 there is increasing of fuel consumption and smoke emission;
if Inj.lead > 11.5 there is increasing of NOx emission).

 

 

 


Fig. 3. Heat release "x" and injection velocity "Uo" at basic configuration of diesel and
at #2 configuration.
 
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Conclusion: At decrease of nozzles diameter from 0.3 to 0.22 mm it is necessary to increase simultaneously the compression ratio from 16 to 19.5 and reduce the injection lead from 16  to 11.5 degrees of a crank angle.

In this case:
- the level of nitric oxides emission will be decreased from 3.4 to 1.8 g / m3 (- 47 %);
- the specific fuels consumption will be increased from 239 to 242 g / kW h           (+1.2 %);
- the level of smoke emission will be increased from 17.2 to 24.2 Hartridge deg.       (+ 41 %).

    To decrease a fuel consumption and smoke emission it is necessary to change the shape of a piston bowl and nozzles directions. The designing of the greater space for a fuel jets development will allow to increase the rate of heat release on a stage of afterburning.

    The further researches were carried out in a direction of perfecting of the piston bowl shape and optimization of fuel jets orientation. The change of a swirl intensity owing to change of piston bowl diameter and head - piston clearance was taken into account.

     Satisfactory results can be obtained by use of experimental piston bowl which presented in Fig. 4. In this bowl, the swirl number at TDC is equal 3.1 (almost as at the base configuration). The angles between axes of jets and axis of the cylinder are enlarged. External diameter of a piston bowl is enlarged. These measures allow to increase length of a free development of jets.  The angle of inclination of the generatrix of the piston bowl is selected so that a main development of a wall surface flow to route deep into bowl, instead of toward the piston crown and into head-piston clearance. Because of a shallow depth of the piston bowl the size of a head-piston clearance is enlarged up to 2.1 mm, that creates padding possibilities for increase of compression ratio and precludes the fuel getting from a wall surface flow on the cylinder head surface.
     The combination of measures: the experimental piston bowl, the atomizer of 3 x 0.22 mm with the enlarged a cone angle, the compression ratio of 19.5 and the injection lead of 11.5 degrees compounds configuration # 3.

     The angle of injection lead of 11.5 degrees is selected on the basis of data represented in Fig. 5.

    The comparison of diesel parameters obtained in the configuration # 3 and in base is listed in table 4.

Fig.4.  Results of calculation of mixture formation and combustion at the configuration # 3 of diesel. (The experimental piston bowl)
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The base piston bowl

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Table 4

Config. Base # 3
CR 16 19.5
Inj. lead 16 11.5
SFC 239 236
NOx 3.4 1.92

Hartr

17.2 17.9

Fig. 5.  The relation between the diesel parameters and injection lead at the configuration #3
  
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     The allocation of injected fuel in the experimental piston bowl is more favorable rather than in base:

Characteristic zone

Base Config. #3
Amount of fuel allocated in rare environment of free jets and of its wall surface flows (the best conditions of evaporation) 60% 69%
Amount of fuel allocated in core of wall surface flows (conditions of evaporation are worse than in environment) 37% 27.9%

    - The prolonged injection, the small delay of autoignition, the small amount of fuel which has evaporated during of delay period are the reasons of absence of the expressed first peak of heat release rate curve. The much smaller amount of fuel burns down by the volumetric mechanism at which there are high local temperature in volume of explosive combustion. The decrease of amount of fuel which is burned down at a high temperature is a reason of lowering NOx emission.

   - Both in the base piston bowl and in the experimental one the fuel does not hit in zones with poor conditions of evaporation (zone of a cylinder wall and zone of a cylinder head). Amount of fuel allocated in zones of intersection of wall surface flows is rather not significant for both piston bowls. It is 1.7... 2.5 %.

   - High quality of atomization of fuel (the mean diameter of drops is 19 microns) promotes the high rate of evaporation and combustion

Thus, the using of the experimental piston bowl in combination to other the said above measures makes possible the lowering  NOx emission without increase of smoke emission and even with a lowering fuel consumption.

Conclusion:  To reduce the emission of nitrogen oxides of the engine D-120 it is necessary to realize following measures:
-  To apply the experimental piston bowl (Fig. 4);
-  To reduce diameter of nozzles from 0.3 to 0.22 mm;
-  To increase the angle between axis of the cylinder and axes of nozzles;
-  To increase the compression ratio from 16 to 19.5;
-  To reduce the injection lead from 16 to 11.5 degrees of a crank angle.

In this case:
- the level of nitric oxides emission will be decreased from 3.4 to 1.92 g/m3  (-43.5%);
- the specific fuels consumption will be decreased from 239 to 236 g / kW h          (-1.2 %);
- the level of smoke emission will be increased from 17.2 to 17.9 Hartridge deg.      (+ 4 %).


   To find the paths of a further decrease of fuel consumption and of smoke emission it is necessary to carry out the extended numerical researches of effect of the piston bowl shape and direction of nozzles to provide to fuel jets more free space for their development and to increase the heat release rate on the stage of afterburning.

(The measures connected with the increasing of injection pressure, usage of supercharging and intercooling in the given example are not considered, though the program allows to make it. One of the most effective ways of reduction of smoke emission is the usage of supercharging which will allow to increase essentially an air fuel equivalence ratio)

Usage of the program DIESEL - 2/4t and built-in "Fuel Jet Visualization" code allows to coordinate frequently opposite tendencies caused by necessity of a decrease of fuel consumption and decrease of emission of harmful materials and to find the favorable compromises.

 

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