This study was carried out at an engine speed of 1400 rpm, fuel injection pressure of 300 MPa, and with the boost pressure varying from 0.15 to 0.45 MPa with a change step of 0.1 MPa. The simulation was performed when the Db/T ratio varied from 3.4 to 10 at ε = 15.4, which corresponded to a change in Db from 67 to 100 mm.
During the simulation, the start of injection (SoI) with each speed mode needed to be optimized to achieve the lowest indicated fuel consumption (
gi) and the biggest indicated power (
Ni).
Figure 8 illustrates the result of the SoI optimal with a fuel injection pressure of 300 MPa and engine speed of 1400 rpm. From the results, the SoI optimal for this case was chosen as 5 degrees of crank angle BTDC. At this point, the
Ni obtained was the largest, and the
gi was the smallest.
3.1. Influence of Geometric Parameters of the Combustion Chamber and Boost Pressure on the Characteristics of the Injection Fuel Spray
Figure 9 and
Figure 10 demonstrate the characteristics of the development of the fuel spray as a function of time.
At the initial stage of the injection (up to 0.05 ms), the boost pressure and the
Db/T ratio had a weak effect on the injection process (
Figure 9 and
Figure 10). The oscillation of both the liquid and the vapor phases’ penetration and the spray tip velocity started after 0.1 (ms) time from the SoI. The length of penetration was limited by fuel evaporation and its movement in the gaseous state. As the boost pressure increased, the maximum values of the penetration length and the spray tip velocity decreased due to greater air resistance (
Figure 9). The boost pressure defines the pressure in the engine cylinder; therefore, the higher the boost pressure, the higher the spray deceleration. In this case, the number of fuel droplets at the periphery and on the shell of the spray increased. As a result, the width of the tip edge increased—in other words, the spray cone angle increased.
The values of the spray penetration and the spray cone angle changed significantly when the boost pressure increased from 0.15 to 0.25 MPa, while those values changed slightly with an increase in the boost pressure from 0.25 MPa to 0.35 MPa.
With an increase in
Db (
Db/T ratio), the spray parameters (liquid penetration) varied a little (
Figure 10). This behavior can be explained by the fact that the pressure in the combustion chamber and the fuel injection pressure do not depend on
Db/T (because the values of all the fuel injection pressure, the boost pressure, and the compression ratio ε are constant). As can be seen in
Figure 8, the spray cone angle increases, but both the fuel spray tip velocity and the penetration depth decrease simultaneously with the start of combustion (SoC), taking place earlier in the cycle owing to the higher air–fuel vapor mixing rate at the high boost pressure of 0.45 MPa and high temperature inside the cylinder. The
Db/T value affects the oscillation results of the spray penetration and the spray tip velocity. When increasing the combustion chamber diameter, their maximum value also grows, while their values decrease when the boost pressure decreases (
Figure 9).
In fact, the angle between the axis of the spray hole and the axis of the fuel injection nozzle (
φ) changes as the
Db/T ratio varies. This angle should be in accordance with the diameter of the combustion chamber to ensure that fuel is injected into the space in the combustion chamber. The amount of fuel injected should not be too close to the cylinder head or too much on the surface of the combustion chamber. To achieve that, it is necessary to optimize this angle with the maximum value of the indicated power. In the study, this angle was also optimized for each case of the diameter of the combustion chamber. It can be seen that the
φ angle changes according to the change in the diameter of the combustion chamber. Furthermore, the length of the nozzle holes increases too, while the wall thickness of the nozzle tip does not change (
Figure 11). This causes disturbances in the fuel flow and the spray parameters. The maximum value of the spray tip velocity in all cases (
Figure 9 and
Figure 10) exceeds the speed of sound.
3.2. Effect of the Geometric Parameters of the Combustion Chamber and the Boost Pressure on Combustion Characteristics
Figure 12 and
Figure 13 illustrate the dependencies of the excess air ratio α, the maximum pressure value
pmax, the maximum temperature value
Tmax, as well as the maximum pressure rise rate (
dp/dφ)
max on the value of
Db/
T ratio and the boost pressure.
The excess air ratio does not depend on the
Db/
T ratio, but it has a weak effect on
pmax (
Figure 12) at the
ε value unchanged condition.
When the value of the fuel injection in a cycle is unchanged (
Q = 60 mg), the excess air ratio α and the maximum pressure in the cylinder
pmax increase due to the significant rise in the boost pressure (
Figure 12b). The excess air ratio
α rises because the mass of the air entering the cylinder increases. Those variations are determined by the increase in the in-cylinder pressure at the end of the compression process. The
α and
pmax values grow by 2.84 and 2.1 times, respectively, when the boost pressure increases from 0.15 to 0.45 MPa (
Figure 12a).
The increase in the boost pressure has a positive effect on the mixing rate (the maximum value of the spray penetration decreased but the cone angles of the spray increased,
Figure 9); therefore, the maximum increment rates in the in-cylinder pressure (
dp/dφ)
max and
Tmax decrease as well (
Figure 13). Thus, at the value of
Db/T = 10, the boost pressure variation from 0.15 to 0.45 MPa leads to a decrease of 38% in (
dp/dφ)
max and of 9% in
Tmax. The decrease in the (
dp/dφ)
max value has a positive effect on the production of NO
x emissions and noise in the operating process of a diesel engine.
The growth in the boost pressure has a higher effect on the (dp/dφ)max than that on the Tmax value. As the boost pressure increases, especially above 0.35 MPa, its role in reducing the values of both (dp/dφ)max and Tmax decreases, especially significant in Tmax.
The highest values of (
dp/dφ)
max and
Tmax were achieved at the smallest diameter of the combustion chamber (
Figure 13) and the biggest depth value of the combustion chamber (
Db/T = 3.4), which is associated with more fuel striking against the combustion chamber wall until the autoignition occurs; however, the only exception for
Tmax is at
pin = 1.5 bar.
As the diameter of the combustion chamber grows, the (dp/dφ)max value reasonably or slightly decreases. At pin = 0.45 MPa, there is a gradual decrease in (dp/dφ)max with an increase in Db. Actually, the maximum pressure increment rate decreases by 13% when the Db/T ratio rises from 3.4 to 10. The (dp/dφ)max value also goes down significantly by 12.5% at pin = 0.25 MPa and within the variation range of the Db/T ratio from 3.4 to 5.6.
To explain the results presented in
Figure 13 in more detail, the characteristics of the RoHR (
Figure 14) and the in-cylinder temperature distribution (
Figure 15 and
Figure A1 in
Appendix B) at different diameters of the combustion chamber are displayed.
It is known that the fuel spray parameters are affected by the following factors. (1) The more that fuel injection pressure increases, the greater the fuel injection speed and the energy of the spray movement. In this study, the fuel injection pressure is kept constant; therefore, it does not affect the spray rate. (2) Increasing the boost pressure and compression ratio leads to deceleration of the spray (an increase in the width of the tip edge) and creates more fuel droplets; they are stalled on its periphery and the shell. The fuel injection pressure determines the energy of movement, while the boost pressure determines the energy of the spray deceleration. (3) Temperature of fuel flow in the cylinder and the boost pressure rise, which makes the fuel spray warm up and accelerates the ignition. (4) The bigger the diameter of the combustion chamber is, the longer the spray of the fuel, which requires more time for its warmup, ignition, and combustion in the volume [
39,
40].
Figure 13 shows that the spray tips move and quickly reach the wall of the combustion chamber. The smaller the diameter of the combustion chamber is, the greater the amount of the fuel portion which is injected spreads along the cylinder wall. This fuel warms up slowly (for the beginning of the heat release rate characteristics, see
Figure 14), evaporates from the hot wall surface, ignites, and burns near the parietal zone. The heat release process is delayed, which can be seen at the end of the rate of heat release.
When the boost pressure is 0.25 MPa, more fuel ignites. The fuel droplets warm up faster; therefore, they lose more energy when the spray moves. As a result, less fuel (compared to
pin = 0.15 MPa) falls on the cylinder wall, which results in more heat by the fuel spray from the burned fuel bring into the combustion chamber. Usually, the fuel spray absorbs more heat when it spends more time in the combustion chamber volume due to a higher total surface area of the spray fuel droplets directly exposed to the hot in-cylinder compressed air charge. However, the heat transfers from the gases to the fuel spray depend upon the injection pressure, combustion chamber design, pressure, temperature, and many variable parameters to be considered in each specific case. The fuel that falls on the cylinder wall evaporates and ignites quickly (a more rapid onset of RoHR,
Figure 14). The combustion duration is still quite long, although it is less than with the value
pin = 0.15 MPa.
With a further increase in the boost pressure from 0.35 to 0.45 MPa, the more fuel warms up and ignites in volume, and the little fuel drops on the wall of the combustion chamber (
Figure 15). As a consequence, the combustion process passes faster. In the beginning, the RoHR is higher, and the combustion process is shorter (
Figure 14).
Thus, the boost pressure plays a role as a distributor to separate the amount of the combustion fuel in the spray (volumetric mixture formation process) and near the wall of the combustion chamber (near-wall mixture formation process). At the volumetric mixture formation condition, the amount of burned fuel at the start of the combustion process rises with increasing boost pressure, and the fuel concentration is higher near the wall of the combustion chamber at the end of the combustion process.
As the diameter of the combustion chamber grows, the length of the fuel spray also increases, which takes longer than time for warming up, ignition and burning. The less fuel that falls on the combustion chamber wall, the more fuel that is burned under the volumetric mixture formation conditions, especially at a lower boost pressure value of
pin = 0.15 MPa, compared with the RoHR curves of
Db = 67 and
Db = 100 mm (
Figure 13b). The reciprocal change of
Db and
T strongly affects both the shape and the movement of the burning cloud in the middle and at the end of the combustion process.
3.3. Economic and Technical Indicators
The indicated power increases with increases in the boost pressure for all cases of the
Db/T ratio, while the indicated specific fuel consumption (ISFC) decreases. The possible reason is that when the boost pressure increases, the maximum pressure of the cycle also goes up if the same quantity of fuel is injected, and, therefore, the work carried out in the cycle during the expansion process is greater under the condition that the displacement and the volume are unchanged (
Figure 16 and
Figure 17).
The ignition delay decreases (due to a rise in the ratio of the volumetric mixture formation process), while the value of the heat transfer into the wall of the combustion chamber grows with an increase in the Db/T ratio. Additionally, the combustion process is faster and produces more effective work by the expansion process. These two factors determine the best values of the indicated power and the ISFC, which depends mainly on the Db/T ratio (the presence of the optimal value of the Db/T ratio).
When the boost pressure increases, the heat release process begins earlier (
Figure 12), and more fuel gets burned in the combustion chamber volume. As a result, the optimal value of the
Db/T ratio increases, and the range of variation of the indicated power and ISFC becomes larger (
Figure 16 and
Figure 17). At the same time, the nature of the changes in the indicated power and the ISFC for the boost pressures of 0.35 and 0.45 MPa is different from those values obtained at the boost pressures of 0.15 and 0.25 MPa.
3.4. Emission Characteristics
Variation of toxic substances and soot emission in exhaust gases with a change in the boost pressure and the
Db/T ratio is illustrated in
Figure 18.
The reasons for the decrease in temperature-related NO
x emissions include the growth in the boost pressure and the decrease in the time of ignition delay. The boost pressure value rises, which leads to air mass growth; therefore, α goes up, and as a result,
Tmax (
Figure 13b) and NO
x (
Figure 16) decrease. At the high boost pressure, the geometry of the combustion chamber has a slight effect on the production of NO
x, while at the low boost pressure, the effect of the combustion chamber geometrical parameters on the variation of NO
x is significant (at
pin = 0.15 MPa). The NO
x production depends on the
Db/T ratio and reaches the minimum value of NO
x at a
Db/T ratio of about 5.0. The NO
x value decreases when the ratio
Db/T = (3.2 ÷ 4.5) and is almost unchanged with the ratio
Db/T = (4.5 ÷ 5.6) and continues to decrease when the ratio
Db/T = (7.8 ÷ 10). Variation of soot is opposite to the NO
x change. Except for
pin = 0.15 MPa, the NO
x value increases with the ratio
Db/T = (7.8 ÷ 10), and soot and CO are almost unchanged when the ratio
Db/T = (4.5 ÷ 10).
As the boost pressure goes up, more fuel concentrates in a smaller volume, and so the fuel density in the volume of the spray increases, which increases the resistance time of the spray. As a result, less air can be available to burn the injected fuel completely with a cleaner exhaust, with the cloud of burning fuel (
Figure 15) smaller at the end of combustion, especially when
Db = 90 mm. This, in turn, leads to an increase in CO, HC, and soot (
Figure 18).