Analysis of mixture formation of different diesel engines

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Medium-speed marine diesel  D42 Turbocharged truck diesel KamAZ Tractor diesel CMD with off-center injector Naturally aspirated diesel YaMZ 236

     The results of calculation of four diesels with different conditions of mixture formation and levels of boosting are presented below.
     On the basis of the analysis of obtained results it is possible to make the deduction about efficiency of a working process of engines and define the methods for their perfecting.
   As a result of calculation of a development of each fuel jet, the amount of fuel allocated in each of characteristic zones is evaluated:

-   in environment of free jet and in environment of a wall surface flow;
-    in a core of a free jet;
-   in a core of a wall surface flow;
-   in area of intersection of wall surface flows;
-   on a surface of the cylinder head;
-   on a surface of the cylinder wall.

   Inside one characteristic zone, there are approximately identical conditions of evaporation. The fuel evaporation speed of the various zones differ very strongly.
   The calculation method was carefully checked. The calculated data was compared to experimental ones.
   All presented images of the fuel jets are obtained by the "Fuel Jet Visualization" code.

   The low level of specific fuel consumption of a diesel can be obtained at a high speed of combustion when the small-sized drops are derived due to intensive injection and the large part of fuel is allocated in the front of free jet, in rare environment of a jet and in environment of wall surface flow. In these zones, there are the best conditions of evaporation and is no deficiency of oxygen.

   As small as possible of fuel should be allocated on a surface of  the cylinder head . The hit of fuel on the cylinder wall is intolerable.


Marine diesel D42

As an example of a volumetric mixture formation it is possible to present a working process of medium-speed marine diesel D42.

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Medium-speed marine diesel D42

 
D = 300mm
S = 380mm
rpm=750.
Pi=20.7 bar.

Nozzles: 8 0.5

All jets are identical

The allocation of fuel in the zones (at the end of injection)  allows to analyze the positive and negative factors of mixture formation.

Num. of jet 1 Sum

Positive factors:
      The intensive injection at maximum pressure 1300 bar allows to obtain relative small-sized drops d32=23 micron.
      Owing to intensive injection the majority of fuel of a free jet is allocated in environment of jet, the part of fuel allocated in a dense core of a free jet (with a poor condition of evaporation) does not exceed 5 % during all period of injection (See diagram, curve: Jet Core)
      The jets of fuel develop in conditions not confined strongly by walls. Therefore 84.3% of fuel of each jet are allocated in rare environment of the free jet and its wall surface flow, and also in a front of free jet. And only 13.5 % of fuel are allocated in a wall surface flow which develops on a hot surface of the cast iron piston.
      The evaporation constant of a wall surface flow on a hot surface is high enough. It ensures a high rate of evaporation (and combustion) in this zone.
The negative factors are not significant:
    
Intersection of wall surface flows is very small: only 3 % of fuel are allocated in these zones. As a result, the amount of fuel allocated in environment is reduced by 2 %, and the amount of fuel allocated in a core of a wall surface flow is reduced by 1 %. Negative consequences of intersection of wall surface flows is insignificant.
     On a cylinder wall surface (where is very poor conditions of evaporation) the fuel does  practically not hit  (0.41 %).

Environment of jet and of wall surface flow. 84.3% 82.2%
Free jet core. 1.73% 1.73%
Core of  wall surface flow of piston. 13.5% 12.5%
Zone of intersection of wall surface flows. 0.97% 3.0%
Surface of the cylinder head 0 0
Surface of the cylinder wall 0.41% 0.41%

   The resulting heat release is intensive enough: 95 % of fuel are burn off through 22 degrees after termination of injection.


Truck diesel KamAZ-7405

If the fuel jets develop in confined conditions of a transport engine piston bowl, the allocation of fuel has other character. The results of calculation of a mixture formation of the truck diesel KamAZ-7405 are presented below.

Truck diesel KamAZ-7405

D=120mm
S=120mm.
rpm=2200
Pi=12.1 bar.

Nozzles: 40.33

All jets are identical

The allocation of fuel in the zones  (at the end of injection) 

Num. of jet 1 Sum       The injection of average intensity at maximum pressure 650 bar allows to obtain the drops having diameter d32=25 micron.
      Owing to non-high level of injection intensity the amount of fuel in the dense core of a free jet (where are poor conditions of evaporation) is great. So in middle of injection the amount of fuel in a core reaches
20 % (See diagram: curve Jet Core
).
      The jets of fuel develop in conditions confined by walls. At the end of injection
74.5% of fuel of each jet are allocated in rare environment of free jet and in environment of its wall surface flow. And 22.2 % of fuel are allocated in a wall surface flow which develops on a hot surface of the steel piston.
      The constant of evaporation of a wall surface flow on a hot piston is high enough. It ensures a high rate of evaporation (and combustion) in this zone.
      The intersection between wall surface flows is absent.

      On a surface of the head and the wall of the cylinder (where are very poor conditions of evaporation) fuel does not hit.
Environment of jet and of wall surface flow. 74.9% 74.9%
Free jet core. 2.9 % 2.9 %
Core of wall surface flow of piston. 22.2% 22.2%
Zone of intersection of wall surface flows. 0 0
Surface of the cylinder head 0 0
Surface of the cylinder wall 0 0

    

    In spite of the fact that the allocation of fuel in a combustion chamber of a high-speed diesel with a volumetric-film mixture formation is not so favorable then in the medium speed engine with a volumetric process*, the speed of heat release of a high-speed diesel remains enough high due to high level of air-fuel ratio a=2.15 (the medium speed diesel has a=1.75) and due to hot surface of steel piston and small-sized drops of fuel. 95 % of fuel in a combustion chamber of a diesel KamAZ 7405 burns down through 25 degrees after a termination of injection, almost as well as in a medium speed diesel. But this engine is much less boosted. If to try to increase its power, the duration of injection will increase, the wall surface flows will begin to hinder each other, and the speed of combustion will decrease. The deceleration of combustion owing to intersection of wall surface flows starts to happen then, when the part of fuel in these zones will exceed 10 %.

* (Volume Fuel Ratio [VFR] is the relation between amount of fuel allocated in volume of environment of jets and general amount of injected fuel. VFR of high-speed diesel is equal 74.9% and VFR of medium speed diesel is equal 82%.)


Tractor diesel CMD

 If the jets of fuel develop in the different conditions, the analysis should be carried out for each jet. The example of calculation of the tractor engine CMD are presented below.

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Numbers of jets
num_jets.gif (631 bytes)

Jet # 1 is represented on the sectional view.

Tractor diesel CMD

D = 120mm
S = 140mm
rpm=1800
Pi=10. bar.

Nozzles: 40.33

The allocation of fuel in the zones  (at the end of injection) %  

Num. of jet 1 2 3 4 Sum - At not intensive injection (maximum pressure not exceed 350 bar) the mean diameter of drops d32 is equal 30 micron. In these conditions, the jets have a developed dense core during of period of free development (See diagrams of fuel allocation, curves: Jet Core). Up to 20% of fuel are allocated in the dense core of lengthy jets #2 and #3 in middle of injection. This fuel very slowly evaporates. The constant of evaporation in the core of jet is less than in environment in 100 times. While the jets will not meet with a wall, in rare environment the amount of fuel is not great. And only after collision the environment intensively grows. As a result, the rate of evaporation in the first half of injection is small.
      The jets of fuel develop in different conditions. Two jets are boosted by a swirl, and two are braked. The part of fuel of one of jets is allocated in a clearance between the piston crown and the cylinder head and part of fuel settles on a surface of the cylinder head.
      The constant of evaporation of a wall surface flow on a surface of the aluminium piston is not high. For this reason, the rate of evaporation (and combustion) in a zone of the wall surface flow is not high too.
     
The wall surface flow are not intersected.
     
On a wall of the cylinder (where are very poor conditions of evaporation) the fuel does not hit.
Environment of jet and of wall surface flow. 17.0 19.6 21.0 17.1 74.7
Free jet core. 0.1 0.4 0.6 0.1 1.2
Core  of wall surface flow of piston. 7.8 5.0 3.4 7.57 23.8
Zone of intersection of wall surface flows. 0 0 0 0 0
Surface of the cylinder head 0.1 0 0 0.2 0.3
Surface of the cylinder wall 0 0 0 0 0


Allocation of fuel in the zones
Cmd_jet1.gif (3834 bytes)

The jet # 1 runs to a wall very fast. Due to low level of injection pressure (350 bar) the jet has a developed dense core which evaporates badly . The intensive formation of environment starts only after end of interaction of front of  jet with a wall. As a result: only 68% of a mass of this jet is allocated in volume of environment, and 31.6 % of fuel is allocated in wall surface flow. In this case, the small length of free jet is the positive factor, because only 7% of fuel are concentrated in slowly evaporating core of free jet at middle of injection. (Jet Core).

Allocation of fuel in the zones
Cmd_jet2.gif (3837 bytes)

The jet # 2 for a long time develops in volume, however because of low pressure of injection average 20 % of fuel are concentrated in a dense axial core of a free jet, in which fuel  does practically not evaporate (Jet Core). The intensive development of environment begins after impact of jet with a wall. At the end of injection, 78.4% of  fuel of this jet are allocated in environment, and 20 % are allocated on a wall.

Allocation of fuel in the zones
Cmd_jet3.gif (3935 bytes)

The jet # 3. Its development in volume is braked by swirl, and this jet runs to a wall after all. At free development for a long time 20 % of fuel does practically not evaporate in a dense axial core of free jet. (Jet Core) The front of jet sets slowly on a wall, and only after that the intensive development of environment begins. At the end of injection in the environment there is 84% of fuel, and on a wall it is 13.6 %.

Allocation of fuel in the zones
Cmd_jet4.gif (3813 bytes)

The jet # 4 is  deformed strongly by a swirl and is thrown on a wall. Its development is similar to the jet #1. But if the spot of the jet #1 is drawn out by a swirl as the lengthy ellipse, the spot of this jet is expanded in breadth. The swirl operates against inertial forces. Swirl squeezes out fuel deep into cameras and into head - piston clearance. About 1 % of fuel settles on a surface of the cylinder head (Cyl.Head).

     Amounts of fuel allocated in volume and on walls of the combustion chamber for a diesel CMD almost same as for the engine KamAZ. However, presence of large drops of fuel, low speed of evaporation, especially during the first half of injection, and also low speed of evaporation of a wall surface flow because of more cold aluminium piston, are the reasons that the resulting rate of heat release is low, despite of a high air-fuel ratio (a=2.0): 95 % of fuel burns down through 70 degrees after a termination of injection.


    The creation of swirl in a combustion chamber is an effective resource of increase of rate of evaporation and combustion. The swirl deforms not only free jets, but also wall surface flows generated by them. The spots of these flows are drawn out in a direction of a swirl and can intersected among themselves. If the square of intersection of wall surface flows becomes considerable, the part of fuel which goes from them into rare environment of jet is reduced. It is a reason of lowering of total rate of evaporation and heat release. Specific fuel consumption and level of smoke emission increase.

Naturally aspirated diesel YaMZ-236

     The results of calculation of a mixture formation of the naturally aspirated diesel YaMZ-236 are presented below. The level of a swirl intensity of this diesel is close to optimum. The optimum level of swirl intensity was obtained experimentally on a single-cylinder engine.

Jamz_pic.gif (8041 bytes)

Naturally aspirated diesel YaMZ-236

D = 130mm
S = 140mm
rpm=2100
Pi=9.15 bar

Nozzles: 40.32

The allocation of fuel in the zones  (at the end of injection)

Num. of jet 1 Sum       The injection of average intensity at maximum pressure 690 bar allows to obtain the drops having diameter d32=27 micron.
      The part of fuel in the dense core of a free jet (where are poor conditions of evaporation) does not exceed 10% during all period of injection (See diagram, curve: Jet Core).
      T
he jets of fuel develop in conditions confined by walls. The intensive formation of environment starts only after end of interaction of front of  jet with a wall. At the end of injection 65.8% of fuel of each jet are allocated in rare environment of free jet and in environment of its wall surface flow. And 22.2 % of fuel are allocated in a wall surface flow which develops on a not-hot surface of the aluminium piston.
    T
he jets generate extensive zones of wall surface flows, which are intersected among themselves: the average amount of fuel allocated in zones of intersection of wall surface flows reaches 13.4 %. As a result, the amount of fuel of rare environment of jets is reduced from 65.8%   to 57.5%, and the amount of fuel of simple wall surface flows is reduced from 28.08 %  to 23 %. It is a reason of reducing of evaporation and heat release rates. However high enough intensity of a tangential swirl neutralizes this negative effect. At further increase of a swirl intensity the rate of evaporation will be reduced more and more strongly and the specific fuel consumption will grow.
       Rather great amount of fuel settles on a surface of the cylinder head: 5.6%. However the head of the cylinder is an iron casting and it has rather hot surface. It is a reason that the rate of evaporation on this surface is not reduced strongly.
        On a wall of the cylinder (where are very poor conditions of evaporation) the fuel does not hit.
Environment of jet and of wall surface flow. 65.8% 57.5%
Free jet core. 0.55% 0.55%
Core of wall surface flow of piston. 28.07% 23.0%
Zone of intersection of wall surface flows. 5.07% 13.4%
Surface of the cylinder head 5.6% 5.6%
Surface of the cylinder wall 0 0

   Not favorable allocation of fuel in a combustion chamber of this diesel is a reason of a not high ratio of evaporation. The low rate of evaporation, especially during the first half of injection is a reason that the rate of heat release is gained not high: 95 % of fuel burns down through 45 degrees after a termination of injection.


     To achieve a good mixture formation, fast combustion and low level of fuel consumption one should directed each jet so that the maximum amount of fuel was allocated in zones with good conditions of evaporation. It is first of all, zone of environment of a jet and, to a lesser degree, core of a wall surface flow. It is necessary to avoid allocation of fuel in zones of intersection of wall surface flows, and also the settlement of fuel on a wall of the cylinder and on the head of the cylinder, especially, if the head is made of aluminium alloy and has low temperature.
     However for lowering a level of NOx emission it is necessary to realize other operations, which cause intersection of wall surface flows of adjacent jets, settlement of fuel on a surface of the cylinder head, slowing down of combustion and increasing of soot emission. The software  DIESEL - 2/4t and built-in "Fuel Jet Visualization" code allows to coordinate these opposite tendencies and to discover the favorable compromises.

 

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