Numerical research of possibility to decrease a level of NOx emission in diesel engine
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.
|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:
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.
Tractor diesel D-120
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.||
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).
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
- 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.
Fig. 2. The smoke emission as a function of two variables: the compression ratio e and the injection lead.
Reasons of choice of configuration #2:
- CR =19.5
- Injection. lead = 11.5
Fig. 3. Heat release "x" and injection velocity "Uo" at basic configuration of diesel and
at #2 configuration.
|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.
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
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)
Fig. 5. The relation between the diesel parameters and
injection lead at the configuration #3
The allocation of injected fuel in the experimental piston bowl is more favorable rather than in base:
|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%|
- 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.
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 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)