Dependence of diesel engine parameters on fuel jets direction
It is known, that an angle of inclination of nozzles
(the angle between the axis of the cylinder and axis of a jet) particularly for diesels
with volumetric mixture formation, should have optimum value at which best parameters of
the engine operation are reached.
Let g be a jets inclination angle defined as angle between the axis of the cylinder and axis of a jet.
The purpose of research: to calculate optimum value of g at which the fuel consumption is minimum.
At numerical experiment the angle g was varied from 55 to 85 degrees (number of
nozzles: 8, diameter: 0,5 mm). For each variant of a direction of the pump-injector
nozzles working process of a diesel was calculated (processes of mixture formation and
gas-exchange were calculated too). Two operation regimes were investigated: 1) rpm=750,
BMEP=17 bar; 2) rpm=600, BMEP=11 bar. The presented graphic materials are
corresponded with a regime of complete capacity.
The "Fuel Jet Visualization" code (build in DIESEL-2/4t) displays on the screen the moving image of development of fuel jets and their wall surface flows and also the diagrams of allocation of the sprayed fuel in characteristic zones. There are best conditions of evaporation in a zone of environment of free jet and its wall surface flow. There are worst-case conditions in a core of a free jet and on a surface of a cylinder wall.
|At large value of jet angle ( g =85°), about 20 % of fuel gets on the surface of the cylinder.|
|At small value of jet angle ( g =65°), the jets quickly reach the hot surface of the piston. The rising of pressure is increased.|
At small value of g the fuel quickly reaches a hot surface of the piston and
evaporates in an initial stage of injection (up to TDC). The increase of amount of evaporated fuel takes place because of an early beginning of formation of core of a wall surface flow with a good conditions for evaporation. The rising of pressure is increased.
Engine parameters at different values of jets inclination angle "g"
|g, deg.||Difference in fuel consumption, g / (kW h)||Share of fuel getting in to the environment of a jet*||Share of fuel getting on the cylinder wall||dP/df ,
|55||+5.6||50.3 %||0. %||5.6||rpm=750;
|65||-1.0||78.7 %||0.2 %||5.35|
|75||0.||84.3 %||0.46 %||4.8|
|85||+4.9||68.3 %||20.4 %||5.0|
|* At the end of injection.|
|55||+7.1||45.0 %||0. %||4.14||rpm=600;
|65||0||72.7 %||1.7 %||4.15|
|75||0.||76.4 %||1.9 %||4.08|
|85||+9.4||59.2 %||30.4 %||4.12|
At increase of angle g, the axis of jet comes nearer to surface of the cylinder head, duration and distance of free development of jets increase. At increase of a free jet length the amount of fuel in its dense axial core increases (in Fig. 1., curves "Jet Core"). The fuel in this zone evaporates very badly, therefore rate of burning in the first half of injection at increase of angle g decreases (in Fig. 2.)
However after collision of a jet with a wall the amount of fuel in environment is increased (see table) and reaches a maximum at g =75 deg., it results in increase of burning rate in the second half of injection (see in Fig. 2.). The specific fuel consumption is reduced.
However, this reduction occurs only until the appreciable part of fuel begins to get on the surface of the cylinder. From this moment the parameters of efficiency of process are sharply worsened. It is necessary to consider, the optimum angle of an inclination of jets to be equal to 75 degrees, at this angle the best fuel consumption is reached at the least value of dP/df. The same conclusions can be made from the experiments carried out by manufacturer: joint-stock company "KTZ".
Heat release curves at different values of jets inclination angle g
||At g =55
...65 a jets quickly reaches the piston, the wall surface layer is quickly formed and
evaporates. The value of dP/df grows.
At g =65 ...75 a jets forms an advanced environment with the best conditions of evaporation. The best profitability is reached at a small value of dP/df .
At g =85 a part of fuel gets on the surface of the cylinder.