Computer simulation of mixture formation and combustion in diesel engine.

Brief exposition of the basic principles of simulation method.

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     Mixture formation and combustion in diesel engines are simulated by Prof. Razleytsev's method, in further modified by Dr. Kuleshov.
     At simulation, the assumption is made that the
heat release process consists of  four main phases. They differ by physical and chemical peculiarities and factors limiting the rate of the process.
a) Induction period. b) Period of initial flare. c) Period of controlled combustion on a stage of fuel injection after flare. d) Period of diffusion burning after the ending of fuel injection.

Distribution of fuel in diesel jet.

The diagram of a diesel jet.
Notations:
1 - Rare environment of a jet.
2 - Dense axial core of free jet.
3 - Dense forward front.
4 - Rare environment of a wall surface flow.
5 - Dense core of a wall surface flow on a piston bowl surface.
6 - Forward front of a wall surface flow.
7 - Axial conical core of a wall surface flow.

   The development of a free jet consists of two main phases. a) Initial phase of pulsing development. b) Basic phase of cumulative development.  Amount of fuel getting in the characteristic zones with different conditions of evaporation and burning are calculated during movement of  jets. In a number of these zones, except listed above, zones on the piston crown, on the surface of the cylinder wall and on the head of the cylinder are included.

   The trajectories of free jets and movement of wall surface flows   formed by jets are calculated in view of influence of tangential air swirl and angle of a clash of a jet with a wall. The intensity of an air swirl is set by swirl ratio H (or swirl number H). H is a relation between swirl angular velocity ws (in the combustion chamber at the end of compression) and crank rotation velocity wr

Example: Calculation of development of fuel jets in the combustion chamber of tractor diesel CMD

Result of fuel jet development simulation
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Allocation of fuel in the zones  (jet #1)
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Film-gramme of wall surface flows  development

Notations:
Environ. - Fuel allocated in rare environments of the free jet and its wall surface flow.
Jet.Core - Fuel allocated in dense core of the free jet.
Pst.Wall - Fuel allocated in the wall surface flow.
Cyl.Head - Fuel settled on cylinder head surface.
Cyl.Wall - Fuel settled on cylinder wall surface.

   The given example presents the comparison between the result of calculation of fuel jets and  wall surface flows  movement with  the experimental film-gramme of wall surface flows development  in the combustion chamber of a tractor diesel CMD (rpm=1800, BMEP=7.7 Bar). The experimental data are obtained of PO GSKBD (Ukraina). The intensity of air swirl in the combustion chamber at the TDC corresponds to swirl ratio H=3.15. On the diagrams it is shown how much fuel in each moment of time has got to the characteristic zones.

Evaporation of sprayed fuel in volume and on the walls of a combustion chamber.

   During injection of fuel and development of fuel jets the rate of combustion is limited mainly by the rate of evaporation of fuel. In the free diesel jet there exist zones of intensive heat exchange and evaporations of sprayed fuel. It is forward front and rare environment of a jet. In a high-speed and dense axial flow core the warming is low  and evaporation of drops are insignificant.
   At clash of a jet with a wall, the evaporation rate of fuel accumulated in forward front  is reduced  sharply  to a minimum at the moment of end of stacking of front on a wall. It is caused by the lower (in comparison with gas) temperature of a wall, reduction of blow of drops, condensation of drops and gas mixture on a wall, merge and interfusion of vanguard drops with more cold drops flying up to a wall. After stacking of front on a wall the biphase mixture begins to be distributed on a wall outside the limits of a cone of a jet. The evaporation rate of fuel in a wall surface zone is increased, though remains smaller than in the volume of the chamber. When the fuel is distributed on the surface of the piston a part of fuel can penetrate into a clearance between the piston crown and a head of the cylinder. Fuel  get on the head and walls of the cylinder.
   The evaporation rate of fuel arriving in each zone of intensive heat exchange is equal to a sum of evaporation rates of separate drops. The evaporation of each drop before and after ignition of fuel is simulated by the Sreznevsky's equation.
   The fuel equipment of boosted diesels provides rather uniform similar atomizing of fuel, especially on the basic phase of injection. Therefore, the calculation of evaporation of fuel can be carried out on a base of  an average drop diameter d32.
    Constants of evaporation of fuel in various zones  are determined with the purpose of calculation of evaporation rate. The estimation of constants is made by known equation  in which are entered:
- Nusselt's criterion for process of diffusion;
- Factor of a diffusion for fuel vapors;
- Pressure of saturated steams;
- Density of liquid fuel;
- Characteristic pressure and temperatures including temperatures of walls.

Combustion of sprayed fuel.

   Upon termination of ignition lag there occurs explosive distribution of a flame on an activated mixture in an environment of  jets. The value of the first maximum of heat release rate curve depends on the following factors:
- the amount of fuel which evaporates in the period of self-ignition delay;
- degree of vapor activation;
- the relation between evaporation rate of fuel during flare and mass of injected fuel;
- quality of fuel atomizing and distribution;
- time of evaporation;
- physical, chemical, thermodynamic and gasdynamic characteristics of a fuel mixture.

    After initial flare and combustion of fuel vapours have formed in the period of delay of self-ignition  the  heat release rate is determined, in general, by the rates of  evaporation and burning out of the products of incomplete combustion in the volume of the cylinder. The latter depends on the average concentration of unused oxygen in volume.

   In the period of diffusion burning, after the termination of injection and termination of development of jets, there occurs the decrease of combustion rate. It is connected with reduction of weight of unburned fuel and with a limiting role of process of a diffusion in this period. Flame resolves into a set of the centers around the local congestion of fuel in core of jet. If the significant part of fuel is allocated on a wall of a piston bowl, especially on the surfaces near to the head of the cylinder, in the interval of 15-30 degrees of crank angle after TDC, on the curves of heat release rate one more small peak is observed. It is connected with the indignation and destruction of quasi-laminar wall surface layer at sharp increment of a gas pole above the appropriate surface.

Example: Calculation of heat release rate in a tractor diesel CMD.

Comparison of calculated and experimental curves of heat release rate dx/df

rpm=1800,
BMEP=7.7 Bar.

     The submitted technique allows to carry out the calculation of combustion in engines both with the volumetric and with the film mixing processes.

Example: Calculation of mixture formation and combustion  in the medium-speed marine diesel engine at full load.

D42_pic.gif (26423 bytes) Notations:
Environ. - Fuel allocated in rare environments of the free jet and its wall surface flow.
Jet.Core - Fuel allocated in dense core of the free jet.
Pst.Wall - Fuel allocated in the wall surface flow.
Cyl.Head - Fuel settled on cylinder head surface.
Cyl.Wall - Fuel settled on cylinder wall surface.

Example: calculation of mixture formation and combustion  in the high-speed automobile diesel engine.

Visualization of mixture formation
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Jet #1 (short)
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Notations:
Environ. - Fuel allocated in rare environments of the free jet and its wall surface flow.
Jet.Core - Fuel allocated in dense core of the free jet.
Pst.Wall - Fuel allocated in the wall surface flow.
Cyl.Head - Fuel settled on cylinder head surface.
Cyl.Wall - Fuel settled on cylinder wall surface.
     Verification of the calculated data compared to experimental ones for a diesel of the truck KamAZ is represented in figures of other page (all calculations are carried out at strictly identical values of adjusting coefficients).

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