Effect of Injection Pressure on Local Temperature and Soot Emission Distribution of Flat-Wall Impinging Diesel Flame under Diesel Engine like-Condition

Abstract

Increasing heat transfer and heat transfer coefficient as the combined effect of impingement distance and injection pressure has been explored in the previous report. However, local temperature distribution was limited to the discussion. Clearly, investigation of near-wall temperature in the heat transfer analysis is absolutely necessary. This has a crucial effect on the local heat flux to understand the heat transfer phenomenon on the combustion chamber walls. The local temperature and KL factor were investigated by using a high-speed video camera and a two-color method by using a volume vessel with a fix-impingement wall. We found that the local temperature and KL factor distribution increase in low injection pressure. This result had a dominant effect on local heat transfer.

1. Introduction

Vehicle electrification is vigorously promoted to achieve net-zero CO₂ emissions by 2050, considering one of the significant contributors of global CO₂ emissions comes from road transport. However, even on the International Energy Agency (IEA’s) most aggressive scenario toward future renewable society, around 40% of vehicles sold in 2030 worldwide are still predicted to be powered by internal combustion engines [1]. Therefore, in order to practically minimize the long-term CO₂ emission, not only vehicle electrification but also thermal efficiency improvement of internal combustion engines is absolutely necessary.

As an effective remedy to improve the thermal efficiency of diesel engines, reduction of heat loss through the engine combustion chamber wall is known to have significant potential. Numerous researches have been done in this field, particularly on improving the thermal efficiency of diesel engines. It was conducted using a single-cylinder diesel engine [2, 3, 4, 5] as well as a wall insertion-type constant volume vessel (CVV) [6, 7, 8]. To enhance thermal efficiency in the design of future engines, complete knowledge of the heat loss pathway from combustion gas to cylinder wall is essential.

In order to elucidate the mechanism of the wall heat transfer during the diesel spray flame impingement, a series of parametric study of wall impinging diesel spray flame combining transient wall heat flux measurements and high-speed optical diagnostics was conducted detailed by authors [9, 10, 11]. The results indicate how various experimental conditions affect the spray/flame impingement behavior, with considerable heat loss resulting in some cases. Gas flame velocity, contact area, and temperature difference are important factors of affect substantial heat loss. Therefore, identifying local temperature distribution is most needed to clarify the temperature difference near the wall during the combustion period.

Regarding some experimental parameters study [12], combining higher injection pressure resulted in higher heat loss, which is naturally attributed to higher flame velocity impinging on the wall with increased heat transfer coefficient. Therefore, this study aims to investigate the local temperature distribution under different injection pressure conditions. Furthermore, it also attempts to investigate the soot emission (KL Factor) distribution to contribute to the realization of the carbon-neutral future society. We used the two-color method to observe the local temperature and soot emission distribution.

2. Experimental setup

2.1 High-pressure and high-temperature chamber vessel

The experimental research has been performed by the author [9] using high pressure and high-temperature chamber vessel with four side windows. The experimental setup was described in Figure 1. Two transparent quartz windows were placed for visualization purposes. The others were facing each other which are used for an injector and spray, respectively. Three K-type thermocouples were installed close to the wall with the gap of 5 mm. These thermocouples were placed among the nozzle tip and wall for injection time purposes. If the ambient temperature was reached of set-point temperature, the diesel fuel will be injected to the chamber. Temperature gradient of these thermocouple was 5K. The injection pulse was defined as injection rate measurement.

Figure 1.

High-speed video camera setup.

Figure 1 shows the high-speed camera setup and its arrangement. This high-speed video camera was installed to figure out the flame pattern at a frame rate of 20.000 fps (frames per second) with a resolution of 320 x 448 pixels. It indicates that each image has a resolution of 5 pixels per millimeter. Meanwhile, the two-color method was employed to investigate the relation between diesel flame and wall heat loss. Not only is the relation between diesel flame and wall heat loss, but also the distribution of soot and local temperature unable to observe by using the two-color method. The main principle of the two-color method is providing a number of assumptions by using different wavelengths of radiation intensity then the flame temperature and soot formation were found out. The soot concentration is defined from Hotel and Broughton’s model which is represent the KL factor. More explanations about KL Factor have already been mentioned in the previous work [9, 10].

The two-color technique was calibrated using a standard light illuminant. Nac Image Technology’s “Thermias” two-color pyrometry software was used to analyze the data. As a result, the flame temperature and KL factor were two-dimensional with line-of-sight information.

2.2 Test conditions

Combusting spray conditions were tested, which is containing the amount of air at high temperature to figure out the local temperature and KL factor under injection pressure. Uniform gas density is set to 16 kg/m³ with keeping the ambient pressure at 4.1 MPa. Measurement conditions on the small-size diesel engines were determined in actual operation to reach the optimum results. Temperature and pressure were adjusted to compression TDC (top dead center) in low load operation for self-ignite the fuel purposes. The impinging distance of 40 mm was decided between the nozzle tip and the wall. Meanwhile, the test conditions were performed by using nozzle with a hole diameter 0.133 mm at three types of injection pressure 80, 120, and 180 MPa. Experimental conditions in detail can be seen in Table 1.

Fuel	Diesel fuel
Ambient gas (MPa)	4.1
Ambient temperature (K)	873
Ambient density (kg/m^3)	16
Injector type	Piezo actuator
Number of nozzle holes	1
Injection quantity (mm^3)	5
Injection pressure (MPa)	80	120	180
Injection duration (ms)	1.4	1.2	0.9
Nozzle hole diameter (mm)	0.133
Impingement distance (mm)	40
Wall temperature (K)	460±10
Cooling method	Oil cooling

Table 1.

Experimental conditions.

3. Result and discussion

3.1 Combustion characteristics

In this section, the combustion characteristics of impinging flame at three types of injection pressure are discussed. At 40 mm of impingement distance, three types of injection pressure were employed i.e. 80, 120, and 180 MPa.

Figure 2 shows the impinging flame at three types of injection pressure. Regarding injection rate graphs in previous work [10], luminous flames occur just before the end of injection. Furthermore, comparing three different injection pressure, the luminous flame appears earlier with higher injection pressure. It means increasing injection pressure leads to shorter ignition delay due to generate higher premixing of fuel and air. The flame luminosity was figured out at 0.9 ms after start of injection (ASOI) only during injection pressure of 120 and 180 MPa. However, it was unseen at injection pressure 80 MPa at 0.9 ms ASOI when increasing the time to 1.2 ms ASOI, the flame luminosity appeared. During the time of 1.2 ms ASOI, most brightness of flame luminosity was captured at injection pressure of 180 MPa. In this case, the luminous flame is starting to decrease then disappear. Similarly, at timing of 1.8 ms ASOI, the injection pressure of 120 MPa shows luminous flame start to decrease. However, in this timing the luminous flame continuous to develop at injection pressure of 80 MPa. It evident from Figure 2. that injection duration was longer with lower injection pressure than combustion duration showed finish later.

Figure 2.

Impinging flame at three types of injection pressure. (a) Pinj = 80 MPa. (b). Pinj = 120 MPa. (c) Pinj = 180 MPa.

Figure 3(a–c) shows the temperature and KL factor distribution of impinging flame at different injection pressure. These figures were extracted from flame natural luminosity images by using a two-color method analysis. Brightness color in its distribution indicates temperature and KL factor. Flame luminosity was obtained to investigate the temperature and KL factor distribution, therefore both of them seem similar shapes.

Figure 3.

Flame temperature distribution (Top) and KL Factor (Bottom) at different injection pressures. (a) Dimp = 40mm Pinj = 80 MPa. (b) Dimp = 40mm Pinj = 120 MPa. (c) Dimp = 40mm Pinj = 180 MPa.

At high injection pressure, complicated distribution of temperature was spread to wider area. However, the phenomenon was contrarily at lower injection pressure. The temperature was contributed to heat transfer on the wall [9]. The lowest injection pressure in this study is 80 MPa where it has the highest temperature compared to other injection pressure variations. This is probably due to more flame natural luminosity was captured at lower injection pressure as shown in Figure 2.

Figure 3(a–c) describes the KL factor distribution at injection pressure variations. Based on liquid length data in evaporating conditions, the soot formation region was formed [10]. This liquid length existed before impingement during the injection period. A shorter period with higher injection pressure has been found through the soot formation that occurred around the center of the impingement wall as shown in Figure 3. Mixing the fuel and air better is an important decisive factor in the reduction of soot formation.

A similar trend of integrated flame luminosity, luminous flame area, mean temperature, and integrated KL factor was shown in Figure 4(a–d). It starts increasing and reaches the maximum point before decreasing at time variation in each injection pressure. The integrated luminosity is shown in Figure 4(a). The flame luminosity indicates soot combustion in case of insufficient oxygen by a rich mixture. Opposite relation was found between injection pressure and luminosity where higher injection pressure will have smaller and shorter luminosity. Increasing injection pressure means increasing the velocity. Therefore, air entertainment will improve spray atomization and premixing of fuel and air. Figure 4(b) shows the shorter flame area at higher injection as a result of premixing fuel and air.

Figure 4.

Integrated Flame luminosity, luminous flame, mean temperature, and KL factor under injection pressures. (a) Integrated flame luminosity. (b) Luminous flame area. (c) Mean temperature. (d) Integrated KL factor.

Figure 4(c) describes the mean temperature of three types of injection pressure where it has a lower temperature compared to the temperature at injection pressure of 80 MPa. The differences in temperature among them were approximately 100K. We can see that flame temperature distribution at three types of injection pressure were 1.2, 1.5, and 1.8 ms ASOI as shown in Figure 4(c). At the time 1.5 and 1.8 ms ASOI, the temperature distribution was almost uniform where the mean temperature will be high as shown in Figure 4(c). On other hand, the reduction of soot formation may affect premixed combustion at high injection pressure with a shorter injection duration.

Figure 4(d) shows the integrated KL factor which is consist of the soot formation. This formation was decreasing when the injection pressure increased. Meanwhile, in Figure 4(a–c), the integrated KL factor increases to peak value at maximum flame natural luminosity and flame area. As mentioned before, opposite relation between injection pressure and KL factor was found which is due to less air entertainment than spray atomization and fuel-air mixing decreasing. Next, it affects a high equivalent ratio. For soot production purposes, the correlation between high temperature and fuel-air mixing is interesting to discuss.

3.2 Local temperature and KL factor distribution

In this study, we evaluate the axial temperature distribution from the wall surface with position variations. Figure 5 shows the time 1.2 ms ASOI in three types of positions where it reaches the maximum at injection pressure 180 MPa. These temperature distributions shown in Figure 4 were obtained from Figure 3. On other hand, the increasing temperature has a reverse effect from some injection pressure. It means that 180 MPa of injection pressure contributed to the highest heat loss. In this case, the temperature gradient has an effect on the local heat flux and the heat transfer. In Figure 5(c) at position 3, the dominant temperature distribution at 180 MPa affect the wider area of flame distribution.

Figure 5.

Distribution of temperature from wall surface at 1.2 ms ASOI under injection pressures and positions. (a) Position1 at 1.2 ms ASOI. (b) Position2 at 1.2 ms ASOI. (c) Position3 at 1.2 ms ASOI.

One of the necessary parts of heat loss on the wall during flame impinging is the spatial distribution of local temperatures. Figure 6 shows the mean temperature near the wall at 0.8 mm for all variations. The result shows that the injection pressure of 180 MPa has the maximum temperature at all surface areas. It indicates that the flame temperature along the near wall is uniform. This made the local temperature gradient was higher.

Figure 6.

Temperature near wall at 0.8 mm from wall during injection pressure. (a) Position 1. (b) Position 2. (c) Position 3.

The spatial distribution of the local KL Factor near a wall at 0.8 mm is shown in Figure 7(a–c). The figure shows that the distribution of KL Factor varies at three areas namely Position1, Position2, and Position3 under different Injection Pressures. KL Factor decreases with increasing time after the start of injection at all injection pressures and Positions. It can be seen from Figure 7(a–c) that the higher injection has a shorter KL Factor period where it will have an impact on the total amount of KL factor. This means that the injection of 180 MPa can be considered to have contributed to the reduction of carbon neutrality for a future society.

Figure 7.

KL Factor near wall at 0.8 mm from wall under injection pressures. (a) Position 1. (b) Position 2. (c) Position 3.

4. Conclusion

The effects of injection pressure on local temperature and soot emission distribution were investigated in the flat-wall impinging diesel flame under diesel like-condition. General conclusions obtained from the study are summarized as follows:

1. Injection pressure of 80 MPa reached the most brightness in flame luminosity, which is attributed to the higher mean flame temperature and soot emission.

2. The axial temperature distribution from the wall surface was found higher local temperatures at 180 MPa compared with other pressures under 1.2 ms ASOI at all positions.

3. Injection pressure of 80 MPa has the maximum local temperature near a wall (0.8 mm) at all of the surface areas. This indicates that flame temperature is uniform along a near-wall which is lead large the local temperature difference between wall and flame.

4. Injection pressure of 180 MPa has a shorter KL Factor period which identifies a small amount of the total KL factor.

Acknowledgments

The authors thank to Prof. Keiya Nishida at Mechanical Power and Motor System Laboratory, University of Hiroshima for their support with measurement in this study and we gratefully acknowledge Prof. Tetsuya Aizawa-Meiji University for his valuable suggestions and discussions.

Appendices and nomenclature

P_inj

Injection Pressure

ASOI

After Start of Injection

TDC

Top Dead Center

Fps

Frame per Second

CVV

Constant Volume Vessel