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Transient measuring method for injection rate of each nozzle hole based on spray momentum flux
Fuel 125 (2014) 20–29
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Transient measuring method for injection rate of each nozzle hole based on spray momentum flux Fuqiang Luo ⇑, Huifeng Cui, Shaofeng Dong School of Automobile and Traffic Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China
h i g h l i g h t s A transient measuring method of injection rate of each nozzle hole was proposed. The differences in injection rates among nozzle holes were primarily analyzed. Non-uniform coefficient of cycle fuel injection quantities among nozzle holes was defined. The influence of the measurement procedure details was studied.
a r t i c l e
i n f o
Article history: Received 3 November 2013 Received in revised form 5 February 2014 Accepted 7 February 2014 Available online 19 February 2014 Keywords: Measuring method Injection rate Nozzle hole Multi-hole injection nozzle Spray momentum flux
a b s t r a c t For a diesel engine equipped with multi-hole injectors, its combustion process, pollutant formation and thermal load consistency of combustion chamber are directly influenced by the differences in injection rates among nozzle holes. However, there are few measuring methods and equipments suitable for the determination of injection rate of each nozzle hole. The aim of this paper is to evaluate a measuring method proposed based on the spray momentum measurement of each nozzle hole that could be used to determine its injection rate. For this purpose, a conventional injection system of pump-line-nozzle was utilized and a dedicated experimental rig was constructed. Under different operating conditions, the cycle fuel injection quantities of the measured injector and the transient injection rate of each nozzle hole were measured successively. Based on the experimental results, the reliability and stability of the proposed measuring method were validated, and the differences in injection rates among nozzle holes were analyzed. In order to further understand the measuring method proposed, the influence of the measurement procedure details such as the distance between the outlet and the target and the angle between the target and spray axis on the determination of the transient injection rate of each nozzle hole was experimentally studied. The experimental results show that when the distance between the outlet and the target is less than 12 mm and the angle between the target and spray axis is lower than 100°, the transient injection rate of each nozzle hole could be measured accurately using the measuring method proposed, and that with a higher injection pump speed or more cycle fuel supply quantity, the consistency of cycle fuel injection quantities among nozzle holes is improved gradually. The further increase of the distance or the angle will result in the reduction of the peak injection rate and cycle fuel injection quantity of the measured nozzle hole. Besides, the injection start, injection end, and the corresponding phase of peak injection rate of the measured nozzle hole will be delayed little by little with the further increment of the distance. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction For a diesel engine, in order to achieve an efficient combustion process with moderate emissions, the optimization of the injection process in terms of injection rate and spray characteristics is crucial [1,2]. In fact, the injection rate directly affects the ⇑ Corresponding author. Tel.: +86 13615280495; fax: +86 0511 88782845. E-mail address: [email protected] (F. Luo). http://dx.doi.org/10.1016/j.fuel.2014.02.011 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
evolution of the diesel spray, fuel air interaction, and the combustion process [3]. In other words, in a diesel engine, the injection rate has a direct influence on the combustion performance, the noise and pollutant emissions. Hence, the knowledge of the characteristics of injection rate could lead to a significant contribution to the design improvement and performance optimization of a diesel engine. To obtain reliable cycle fuel injection quantity and injection rate measurements, many methods and techniques have been
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F. Luo et al. / Fuel 125 (2014) 20–29
Nomenclature Ageo Cd Cn F L _ m _ M n Pi q qmax qmean qmin
geometrical outlet section discharge coefficient non-uniform coefficient of cycle fuel injection quantities among nozzle holes spray impact force the distance between the outlet and the target mass flow rate spray momentum flux injection pump speed injection pressure cycle fuel injection quantity of the measured nozzle hole maximal cycle fuel injection quantity among nozzle holes mean cycle fuel injection quantity among nozzle holes minimal cycle fuel injection quantity among nozzle holes
developed. The oldest as well as the most common methods are the Bosch measuring method [4–7] and the Zeuch’s measuring method [8–10]. With the charge measuring method, the determination of injection rate is based on the measured charge created by the frictions, of the fuel in the nozzle and the spray against the surface of the sensor, and by the Seebeck effect [11,12]. As reported in the literature, the Laser Doppler Anemometer could also be used to determine the injection rate by measuring the axial velocity, from which the actual volume flow rate signal could be deduced [13,14]. All of the above-mentioned measuring methods can give the accurate result of injection rate of single-hole nozzle. For the multi-hole nozzle, however, they can only give its total injection rate, and provide no information about the possible differences in injection rates among nozzle holes. As reported in the literature, the injection rate diversities among nozzle holes do exist due to the inaccuracies in workmanship and the differences in hydraulic conditions among nozzle holes [15,16], which will lead to the non-uniform spatial and temporal distributions of the fuel within the combustion chamber and the induced uneven thermal loads of the combustion chamber [15,17–19]. Nowadays the direct injection diesel engines are usually equipped with multi-hole injectors. It is easy to understand the necessity of studying the measuring method, by which the injection rate of each nozzle hole of a multi-hole injector could be determined. Nevertheless, only a small number of scholars have ever carried out the above related researches. Marcˇicˇ [15,19] developed a deformational measuring method, whose criterion of the fuel injected of each nozzle hole is expressed by the deformation of the membrane occurring due to the collision of the pressure wave in the measuring space against the membrane. Payri et al. [16] developed a hole to hole mass flow test rig, by which the fuel injected of each nozzle hole could be obtained from the corresponding siphon where the fuel–air mixture is carried to and the liquid fuel and the air are separated. On the other hand, spray momentum flux is a very important parameter, with which the effective velocity at the outlet, fuel density, and the effective diameter of nozzle hole could be brought together [18,20,21]. Meanwhile, some important parameters such as spray penetration, spray cone angle, and air entrainment depend largely on spray momentum flux [22]. For these reasons, several experimental techniques have been developed to measure the spray momentum [23–26]. The spray momentum can be used
Q Qcum Qmean t t0 u um V_
cycle fuel injection quantity of the measured injector cumulative fuel injection quantity mean cycle fuel injection quantity time t delay time, in seconds or in camshaft rotation angles real velocity mean outlet velocity injection rate in terms of volume flow rate
Greek symbols D relative error DP pressure drop q real density qf liquid fuel density qf,N,T liquid fuel density under normal atmosphere and fuel temperature of T
not only to evaluate internal flow characteristics of nozzle, spray outflow characteristics, and the spray evolution [24,27,28], but also to validate the models established and estimate the relevant model parameters [18,20,21,29]. The objective of this work is to evaluate a transient measuring method proposed based on the spray momentum flux, with which the possible differences in injection rates among nozzle holes of a multi-hole diesel injector can be obtained. In order to achieve the proposed objective, the following three aspects have been addressed. First of all, the relationship between the injection rate of a nozzle hole and its corresponding spray momentum flux needs to be established, which is the basis of the succeeding researches. Secondly, spray momentum test rig needs to be built, which is the key to the entire study. Besides, a lot of experiments also need to be conducted in order to validate the proposed measuring method and to analyze the discrepancies in injection rates among nozzle holes. This paper is composed of six sections. In Section 2, the transient measuring method of injection rate of each nozzle hole proposed based on spray momentum flux is introduced. In Section 3, the experimental facilities are described briefly. In the following Sections 4 and 5, the validation of the measuring method, the analyses of the discrepancies in injection rates among nozzle holes, as well as the discussion of the measuring method are presented. Finally, in Section 6, the most important conclusions of this work are drawn.
2. Theoretical background The internal flow of the nozzle hole is very complex in terms of the flow direction, flow velocity, as well as the cavitation phenomenon [23,24]. Under these complex flow conditions, the mass flow rate and the spray momentum flux at the outlet of nozzle hole can be defined as follows:
_ ¼ m
Z
u qdA
ð1Þ
u2 qdA
ð2Þ
Ageo
_ ¼ M
Z Ageo
Based on the mass conservation law, the above expressions can be simplified to:
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F. Luo et al. / Fuel 125 (2014) 20–29
_ ¼ qf um Ageo m
ð3Þ
_ MðtÞ ¼ Fðt þ t 0 Þ
_ ¼ q u2 Ageo M f m
ð4Þ
Consequently, the injection rate of a nozzle hole shown in Eq. (8) can be modified as follows:
With the use of spray momentum flux, a non-dimensional parameter, discharge coefficient, can be obtained [26]:
Cd ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ M
ð5Þ
2Ageo DP
According to the relationship [30] between the mean velocity at the outlet and the discharge coefficient shown in Eq. (6), the mean velocity at the outlet can be derived:
um ¼ C d
sffiffiffiffiffiffiffiffiffi 2 DP
Combining Eqs. (3), (4), and (7), the injection rate of a nozzle hole in terms of the volume flow rate can be evaluated by:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ Ageo M
ð8Þ
qf
It can be clearly seen from Eq. (8) that it is possible to obtain the transient injection rate of each nozzle hole of a multi-hole diesel injector with the determination of its spray momentum flux. In order to determine the injection rate of each nozzle hole, a special method as described by Payri et al. [24] has been used to test the transient spray momentum of each nozzle hole. Fig. 1 shows the schematic diagram of the spray momentum measuring principle. The transient spray momentum is measured by an indirect method in terms of the impact force exerted by the spray on a flat surface (named target). Within certain rang of the distance between the outlet and the target (named outlet-target distance), as long as the target perpendicular to spray axis is large enough to interact with the entire spray, the impact force measured by the sensor will be equal to the spray momentum at the outlet of nozzle hole or at any other axial position due to the conservation of momentum. As shown in Fig. 1, the impact force measured is delayed with respect to the spray departure from the outlet. The delay time in seconds or in camshaft rotation angles can be evaluated approximately by:
L 6nL t 0 ¼ qffiffiffiffiffiffiffiffiffiffi or t0 ¼ qffiffiffiffiffiffiffiffiffiffi 2DPðtÞ qf ðtÞ
q¼
Q¼ ð7Þ
2DPðtÞ qf ðtÞ
ð9Þ
The relationship between transient spray momentum and its corresponding impact force can be expressed by:
Fig. 1. Measuring principle of spray momentum flux.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Fðt þ t 0 Þ Ageo qf ðtÞ
ð11Þ
Integrating Eq. (11) over the entire injection duration, the cycle fuel injection quantity of the measured nozzle hole and hence that of the injector used can be obtained by:
ð6Þ
qf
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ M um ¼ qf Ageo
V_ ¼
_ VðtÞ ¼
ð10Þ
ffi Z sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Fðt þ t 0 Þ Ageo dt qf ðtÞ X
q
ð12Þ
ð13Þ
3. Experimental set-up In order to evaluate the proposed measuring method, a conventional injection system of pump-line-nozzle was utilized and a dedicated experimental rig was constructed. In the following, the fuel injection system as well as the experimental rig is first introduced briefly. Then, the determination of the fuel density, which directly affects the measuring accuracy of injection rate of each nozzle hole, is presented. 3.1. Fuel injection system The measurement of injection rate of each nozzle hole was performed with a five-hole diesel injector, as shown in Fig. 2. It is equipped with a mini-sac nozzle with a nozzle hole diameter of 0.2 mm and a fuel starting injection pressure of 22.5 MPa. The injector is fed directly from a inline pump. The injection pump is driven by a pump test-bed, of which injection pump speed, fuel temperature and measurement times of fuel injection can be set freely. 3.2. Spray momentum experimental rig Fig. 3 shows the schematic diagram of spray momentum experimental rig, on which the impact force exerted by some spray on corresponding target can be determined and so can the injection rate of corresponding nozzle hole. A calibrated piezoelectric force sensor was employed to detect the spray impact force, by means of a circular target screwed directly on the sensor head. A customized magnetic stand was equipped with a distance adjusting screw and an angle adjustment knob, which allow the target-sensor assembly to be moved for a
Fig. 2. Schematic diagram of injection nozzle.
F. Luo et al. / Fuel 125 (2014) 20–29
23
Fig. 3. Spray momentum experimental rig.
travel range from 0 mm to 40 mm in spray axis direction and to be rotated for an angle range from 90° to the hardware-limited maximum of 120° with respect to spray axis, respectively. The magnetic stand was inserted into an oil mist dispersal chamber where the spray was injected into, and was used for the positioning of the target-sensor assembly. In order to adjust the position of the target-sensor assembly timely, the surrounding walls of the oil mist dispersal chamber were transparent and removable. A clamp-on pressure sensor clamped on the upstream of mini-sac was used to measure the injection pressure. In order to make Fig. 3 more readable, only one target-sensor assembly is shown. 3.3. Determination of fuel density Commercially available diesel fuel (No. 0 diesel fuel) obtained from a petrol station is used in this study. As clearly shown in Eq. (11), the fuel densities under different fuel temperatures and injection pressures must be determined precisely in order to obtain the accurate result of injection rate of nozzle hole. Based on the fuel densities obtained under atmospheric pressure and different fuel temperatures [31], the ones used for the determination of injection rate of nozzle hole can be evaluated by:
qf ðtÞ ¼ qf ;N;T 1 þ
0:6 109 Pi ðtÞ
!
1 þ 1:7 109 Pi ðtÞ
ð14Þ
4. Experimental results As reported in the literature [1,23,24], the target impacted by the spray must be large enough to interact with the entire spray, and at the same time it must be as small as possible to provide adequate response characteristics during the transients mainly including both the opening and closing stages of the needle. Combining the experimental results of many installation tests of 5 target-sensor assemblies conducted on the spray momentum experimental rig, the existing study on the measurement of spray momentum flux, the difficulty and the reliability of non-overlap fixing of each target-sensor assembly perpendicular to corresponding spray axis, the outlet-target distance and the target diameter were both decided to 10 mm to conduct the subsequent experiments. In order to reduce the influence of cycle-to-cycle variation of injection process, all the test results presented in this section are the mean values of 100 injection events.
4.1. Validation of measuring method Under the operating condition with injection pump speed of 1200 r/min and cycle fuel injection quantity of 61.8 mm3/cyc, the measurements of injection rate of each nozzle hole were carried out. Fig. 4(a) and (b) shows the obtained time histories of spray impact force and injection rate of each nozzle hole, respectively. As shown in Fig. 4(a), there are some local differences in the curve profiles of spray impact force of each nozzle hole, but they all possess relatively consistent shapes resembling a hat [18,21]. In addition, the differences in the curve profiles of spray impact force of different nozzle holes during the opening and closing phases of the needle are well reproduced by the injection rate time-histories of corresponding nozzle holes shown in Fig. 4(b). Comparing spray impact force time-history of each nozzle hole during the main injection period with that of injection rate, it can be seen that the fluctuating range of the latter is much smaller than that of the former. Both of the above phenomena are mainly caused by the relationship between spray impact force and its corresponding injection rate shown in Eq. (11). Combining the measurement of injection rate of each nozzle hole with Eqs. (12) and (13), the total integral fuel injection quantity at end of the injection (named cumulative fuel injection quantity, Qcum), 59.53 mm3, is obtained, and is very close to the mean cycle fuel injection quantity of the measured injector obtained based on the pump test-bed (named mean cycle fuel injection quantity, Qmean), 61.8 mm3. Their relative error calculated based on Eq. (15) is only 3.67%. The experimental result indicates, to some extent, that the injection rate of each nozzle hole of a multi-hole diesel injector can be measured with a comparative accuracy using the measuring method proposed and the experimental rig constructed.
D¼
Q mean Q cum 100% Q mean
ð15Þ
In order to further validate the reliability and stability of the measuring method proposed, the mean cycle fuel injection quantity Qmean and the injection rate of each nozzle hole have been measured successively under different operating conditions. In more detail, the injection pump speed was set at 800 r/min, 1000 r/min, 1200 r/min and 1400 r/min, respectively, and the control rack position of injection pump was set at P50, P60, P70 and P80, respectively (Px is a control rack position of injection pump at which the mean cycle fuel injection quantity Qmean over 100 injection events is x mm3 under the injection pump speed of
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole1 Hole2 Hole3 Hole4 n=1200r/min Q mean=61.8mm3/cyc
Hole5
2.0 1.5 1.0 0.5 0.0
5
10
15
(b) 1.5 Injection rate [mm3/º CaA]
Spray impact force [N]
(a) 2.5
20
Hole5
1.2 0.9 0.6 0.3 0.0
25
Hole1 Hole2 Hole3 Hole4 n=1200r/min Q mean=61.8mm3/cyc
5
10
Cam rotation angle [º CaA]
15
20
25
Cam rotation angle [º CaA]
Fig. 4. Time histories of spray impact force and injection rate of each nozzle hole. (a) Spray impact force and (b) injection rate.
1000 r/min). Under each of the combinations, the mean cycle fuel injection quantity Qmean was measured on the pump test-bed first, and then the injection rate of each nozzle hole was determined on the experimental rig constructed. Fig. 5(a) and (b) shows the comparisons between the cumulative fuel injection quantity Qcum and the mean cycle fuel injection quantity Qmean, and the distribution of the relative errors under the above 16 different operating conditions, respectively. As expected, Qcum and Qmean are increased gradually with the increment of the injection pump speed or the cycle fuel supply quantity characterized by the control rack position of injection pump. The relative errors are not evenly distributed, but their values are not large and mainly between 2.2% and 4.6%. In general, the proposed measuring method and the constructed experimental rig have relatively higher precision and better testing stability. 4.2. Difference analysis of injection rates among nozzle holes Fig. 6 shows the time histories of injection rate of each nozzle hole under different injection pump speeds at P60, and Fig. 7 shows the same parameter as in Fig. 6 under different control rack positions at injection pump speed of 1000 r/min. From Figs. 6 and 7, it can be seen that under each of the operating conditions, the global shapes of injection rate time-histories of 5 nozzle holes are very similar. Meanwhile, it is also noticed that under the same operating condition, the injection start is delayed gradually, the injection end is advanced by degrees, and the fuel injection duration is decreased little by little with the increase of nozzle hole serial number (i.e. with the increase of the angle between nozzle hole axis and needle axis, named nozzle hole angle,
shown in Fig. 2). As reported in the literature [32–34], the above phenomena can be considered as a result of the comprehensive effect of the difference in pressure distribution of nozzle sac, the influence of the nozzle hole angle on entrance pressure loss of nozzle hole, and the complexity of the needle movement. Another interesting result is that under each of the operating conditions there is a spike existing respectively in the injection rate time-histories of Hole 1 and Hole 2 during the opening phase of the needle. This is mainly attributed to the unsteady flow caused by the needle opening process and the hydraulic hammering effect produced by the sudden stop of the needle prevented by the lift limiter [35]. The reason why there is no spike existing in the injection rate time-histories of Hole 3, Hole 4 and Hole 5 could be reasonably interpreted as that the delays of their injection starts suppress the influence of the unsteady flow and the hydraulic hammering effect. As shown in Figs. 6 and 7, there are no significant changes in injection start of each nozzle hole with the increment of injection pump speed or the cycle fuel supply quantity characterized by the control rack position of injection pump, but their injection ends are delayed gradually, and at the same time their corresponding fuel injection durations measured in camshaft rotation angle are increased little by little. With the increase of injection pump speed, the leakage of plunger and barrel assembly of injection pump is decreased gradually, meanwhile, the throttle effect of fuel return hole of the injection pump is enhanced little by little, which will result in a smaller falling rate of injection pressure after fuel return hole is opened by plunger and hence the later closing phase of the needle. The influence of the cycle fuel supply quantity is mainly attributed to the working principle of the injection pump used.
90
P50
P60
P70
P80
Qmean
P50
P60
P70
P80
Qcum
80 70 60 50 40
(b) Relative error [%]
Cycle fuel injection quantity [mm3/cyc]
(a)
800
1000
1200
Injection pump speed [r/min]
1400
5 4 3 2
P50 P60 P70 P80
1 0
800
1000
1200
1400
Injection pump speed [r/min]
Fig. 5. Validation results of the test method proposed. (a) Comparisons between Qcum and Qmean and (b) distribution of relative errors.
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
(b) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
(a) 1.8
5
10
15
20
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
5
10
15
20
Hole4
Hole5
0.9 0.6 0.3 5
10
15
20
25
Cam rotation angle [º CaA]
(d) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
1.8
Hole3
1.2
Cam rotation angle [º CaA]
(c)
Hole2
1.5
0.0
25
Hole1
25
Cam rotation angle [º CaA]
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
5
10
15
20
25
Cam rotation angle [º CaA]
Fig. 6. Instantaneous injection rates of each nozzle hole under different injection pump speeds at P60. (a) 800 r/min, P60, (b) 1000 r/min, P60, (c) 1200 r/min, P60 and (d) 1400 r/min, P60.
Under the above operating conditions, the corresponding cycle fuel injection quantities of each nozzle hole are shown in Fig. 8(a) and (b), respectively. It can be seen that the cycle fuel injection quantity of each nozzle hole is increased gradually with the increment of injection pump speed or the cycle fuel supply quantity characterized by the control rack position of injection pump. Besides, under the same operating condition, the cycle fuel injection quantity of a nozzle hole is decreased little by little with the increase of nozzle hole serial number (i.e. with the increase of the named nozzle hole angle shown in Fig. 2). These results are consistent with those shown in Figs. 6 and 7. In order to study the differences among diesel nozzle holes, an evaluating parameter named relative hole mass flow is defined by Payri et al. [16], which is calculated by dividing the cycle fuel injection quantity of the analyzed nozzle hole by the mean value of all the nozzle holes. Under the present testing conditions, the relative hole mass flows of each nozzle hole are shown in Fig. 9(a) and (b). As can be seen from the graphs, the relative hole mass flows of 5 nozzle holes have some difference under the same operating condition, but the relative hole mass flows of any nozzle hole obtained under different operating conditions are not evenly distributed and their variations are not significant. This is consistent with the experimental results obtained by Payri et al. [16]. In order to further understand the uniformity of injection rates among nozzle holes under different operating conditions, a new non-dimensional parameter, Cn, or non-uniform coefficient of cycle fuel injection quantities among nozzle holes is defined:
Cn ¼
qmax qmin 100% qmean
ð16Þ
It can be seen from Fig. 9(a) that the non-uniform coefficient Cn is gradually decreased with the increment of injection pump speed. The similar result is also shown by the experimental data obtained
by Marcˇicˇ for different operating conditions and multi-hole diesel injectors [15,19]. To some extent, this indicates that the uniformity of injection rates among nozzle holes is improved little by little with the increase of injection pump speed. As shown in Fig. 9(b), the non-uniform coefficient Cn is also decreased gradually with the increment of cycle fuel supply quantity characterized by the control rack position of injection pump. This is mainly because the cycle fuel injection quantity of each nozzle hole is increased significantly with the increase of cycle fuel supply quantity and so is the mean cycle fuel injection quantity of 5 nozzle holes (qmean), but the change in the difference between cycle fuel injection quantity of Hole 1 (qmax) and that of Hole 5 (qmin) is not distinct. The non-uniform coefficient is one of the evaluation parameters characterizing the injection consistency among nozzle holes indirectly, which characterizes the maximum difference among the relative hole mass flows of all the nozzle holes. The larger the non-uniform coefficient is, the worse the injection consistency among nozzle holes is. 5. Discussion of measuring method As reported in the literature [1,23], it is impossible for the gaseous phase of the spray (including the ambient air exchanging momentum with the spray and the fuel vapor) to impact entirely on the target, which will result in a loss of momentum flux. The influence of the above gaseous phase is always existed and cannot be eliminated completely by adjusting the measurement procedure details, but it can be diminished to some extent by advisably decreasing the outlet-target distance [1]. Assuming that the spray geometry is an axisymmetric body, since the entire spray is surrounded by ambient air, the force measured by the sensor is the same as the spray momentum flux at the
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
(b) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
(a) 1.8
5
10
15
20
Hole1
Hole1
Hole2
Hole3
Hole4
0.6 0.3 0.0
5
10
Hole5
0.9 0.6 0.3
10
15
15
20
25
Cam rotation angle [º CaA]
1.2
5
Hole5
0.9
25
1.5
0.0
Hole4
1.2
(d) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
1.8
Hole3
1.5
Cam rotation angle [º CaA]
(c)
Hole2
20
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
25
5
10
Cam rotation angle [º CaA]
15
20
25
Cam rotation angle [º CaA]
Fig. 7. Instantaneous injection rates of each nozzle hole under different control rack positions at injection pump speed of 1000r/min. (a) P50, 1000 r/min, (b) P60, 1000 r/min, (c) P70, 1000 r/min and (d) P80, 1000 r/min.
(b) 14
Hole1
Hole2
Hole3
Hole4
Hole5
Control rack position of P60 13 12 11 10 9
800
1000
1200
1400
Injection pump speed [r/min]
Cycle fuel injection quantity [mm3/cyc]
Cycle fuel injection quantity [mm3/cyc]
(a)
Hole1
Hole2
Hole3
Hole4
Hole5
Injection pump speed of 1000r/min
16 14 12 10 8
P50
P60
P70
P80
Control rack position of injection pump
Fig. 8. Cycle fuel injection quantities of each nozzle hole obtained under different operating conditions. (a) The values related to different injection pump speeds and (b) the values related to different control rack positions.
outlet of a nozzle hole due to the conservation of momentum [28]. As a matter of fact, however, spray geometry is not a normal axisymmetric body [36]. Besides, with the increase of the outlet-target distance, i.e. with the development of the spray, the spray geometry between the outlet and the target will become more and more irregular due to the air entrainment and the momentum exchange, which will inevitably affect the measurement of the transient spray momentum flux of each nozzle hole and hence the determination of corresponding injection rate. According to the spray momentum measuring principle shown in Fig. 1, one geometry condition that the target is normal to spray
axis must be satisfied in order to accurately measure the transient spray momentum flux of some nozzle hole, because otherwise there will be non-null residual velocity components in spray axis direction after spray-target impact, which will also affect the determination of spray momentum flux. Based on the above analysis, it can be seen that both the outlettarget distance and the angle between the target and spray axis (named target-axis angle) have a direct influence on the measurement of spray momentum flux of each nozzle hole and on the subsequent determination of corresponding injection rate. Consequently, it is necessary to study the influence of the above geometry
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole2
Hole3
Hole4
Hole5
24
Relative hole mass flow
Non-uniform coefficient
23
1.1
22
1.0
21 0.9
20
0.8 0.7
19
Control rack position of P60 800
1000
1200
18
1400
Injection pump speed [r/min] Hole1
Hole2 Hole3 Hole4 Non-uniform coefficient
Hole5
23
1.1 1.0
22
0.9 21 0.8
Injection pump speed of 1000r/min 0.7
P60
P50
P70
Non-uniform coefficient [%]
Relative hole mass flow
(b) 1.2
20
P80
Control rack position of injection pump Fig. 9. Relative hole mass flows of each nozzle hole and non-uniform coefficients obtained under different operating conditions. (a) The values related to different injection pump speeds and (b) the values related to different control rack positions.
parameters carefully in order to put forward some suggestions for the design and the improvement of the spray momentum experimental rig shown in Fig. 3.
5.1. Influence of the outlet-target distance Fig. 10 shows the time histories of injection rate of Hole 3 and its corresponding cycle fuel injection quantities concerning a 16 mm diameter target normal to spray axis, positioned at different outlet-target distances, i.e. L = 6 mm, 12 mm, 18 mm, 24 mm and 30 mm, under the operating condition with injection pump speed of 1000 r/min and control rack position of P60.
(a)
5.2. Influence of the target-axis angle Fig. 11 shows the time histories of injection rate of Hole 3 and its corresponding cycle fuel injection quantities concerning a 16 mm diameter target, positioned at an outlet-target distance of 10 mm with five different target-axis angles of 90°, 95°, 100°,
(b) 1.6
Injection rate [mm3/º CaA]
With outlet-target distance smaller than 12 mm, the time histories of injection rate fit well with each other in terms of the global shape, injection start, injection end, peak injection rate and the corresponding phase of peak injection rate. Furthermore, there are not any obvious changes in the corresponding cycle fuel injection quantity. With further increase of the outlet-target distance, however, the injection start, injection end, and the corresponding phase of peak injection rate are gradually delayed, meanwhile, the peak injection rate and the corresponding cycle fuel injection quantity are decreased little by little. More specifically, the injection start and the injection end are delayed by about 0.5° CaA measured at the outlet-target distance of 30 mm, the corresponding phase of peak injection rate is delayed by about 0.25° CaA, the peak injection rate is decreased by 6.37%, and the cycle fuel injection quantity is decreased by 5.96% with respect to those measured at the outlet-target distance of 12 mm. The above phenomena can be interpreted in terms of the respective contributions of the spray liquid phase and gaseous phase to momentum flux, and the measuring principle shown in Fig. 1. Within a certain range of outlet-target distance, the contribution of the spray liquid phase is dominant, which results in almost the same instantaneous spray momentum flux measured at different outlet-target distances [21] and hence approximately the same transient injection rate. With further increase of the outlet-target distance, the contribution of the spray liquid phase will be decreased remarkably in favor of the gaseous phase [1,21,23]. Meanwhile, the axial velocity of the spray will be decreased gradually and the spray geometry between the outlet and the target will become more and more irregular. In other words, the greater outlet-target distance will lead to a higher value of t0 obtained based on Eq. (9) and a bigger possibility that more gaseous phase cannot impact on the target. It hence gives rise to the delayed injection start, injection end, corresponding phase of peak injection rate, and the lower peak injection rate and cycle fuel injection quantity. Under the same operating condition, similar evaluations of Hole 1, Hole 2, Hole 4 and Hole 5 have been done at the above different outlet-target distances, respectively. Interestingly, almost identical results have been obtained. For brevity, they are not shown here.
6mm
12mm
18mm
24mm
30mm
1.2 0.8 0.4 0.0
8
10
12
14
16
18
Cam rotation angle [º CaA]
20
22
Cycle fuel injection quantity [mm3/cyc]
Hole1
Non-uniform coefficient [%]
(a) 1.2
13 12 11 10 9 8 7
5
10
15
20
25
30
Outlet-target distance [mm]
Fig. 10. Injection rate time histories of Hole 3 and its corresponding cycle fuel injection quantities measured at different outlet-target distances. (a) Injection rate time histories and (b) cycle fuel injection quantities.
28
F. Luo et al. / Fuel 125 (2014) 20–29
(b)
Injection rate [mm3/º CaA]
1.6
90°
95°
100°
105°
110°
1.2 0.8 0.4 0.0
8
10
12
14
16
18
20
22
Cam rotation angle [º CaA]
Cycle fuel injection quantity [mm3/cyc]
(a)
13 12 11 10 9 8 7
90
95
100
105
110
Target-axis angle [º]
Fig. 11. Injection rate time histories of Hole 3 and its corresponding cycle fuel injection quantities measured at different target-axis angles. (a) Injection rate time histories and (b) cycle fuel injection quantities.
105°, and 110°, at injection pump speed of 1000 r/min and control rack position of P60. The first interesting result shown in Fig. 11 is that the measured injection start, injection end, and the corresponding phase of peak injection rate are approximately the same under the above different target-axis angles. This is mainly because within a certain range of target-axis angle the flow patterns of the spray between the outlet and the target as well as the impact flow structures near the target have no significant changes for the same operating condition and outlet-target distance, and thus the values of t0 obtained based on Eq. (9) are almost the same. As shown in Fig. 11, while the target-axis angle is smaller than 100°, the time histories of injection rate measured at different target-axis angles agree well with each other, meanwhile, the change in the cycle fuel injection quantity is not significant. Specifically, for the target-axis angle of 100°, the cycle fuel injection quantity decreases by only 2.15% compared to that of 90° target-axis angle. With further increase of the target-axis angle, the change in the global shape of the injection rate time-history is very small, but the peak injection rate and the cycle fuel injection quantity decrease gradually. In more detail and compared to those of 90° target-axis angle, the peak injection rates measured at the target-axis angles of 105° and 110°, decrease by 4.41% and 6.97%, respectively, and the cycle fuel injection quantities decrease by 4.64% and 7.62%, respectively. The most feasible reason for these phenomena seems to be the difference, for the larger target-axis angles, in the rebound mechanism of fuel droplet colliding with the target. Generally, the obtained experimental data seem to indicate that the target-axis angle, especially the larger ones, has deep influence on the determination of injection rate of each nozzle hole. Hence, in order to obtain the injection rate of each nozzle hole precisely, the target-axis angle has to be kept below 100°.
6. Conclusions In this paper, a transient measuring method of injection rate of each nozzle hole of a multi-hole diesel injector is proposed. The method is based on the measurement of spray momentum flux, focusing on the impact force exerted by the spray on the target screwed directly on the force sensor head. Under different operating conditions, the mean cycle fuel injection quantities of the used injector were measured on the pump test-bed first, and then a dedicated experimental rig was constructed and employed to determine the injection rate of each nozzle hole. Combining the above experimental data, the reliability and stability of the measuring method was validated, and meanwhile, the differences in injection rates among nozzle holes were analyzed.
In order to make some suggestions for the design and the improvement of the constructed experimental rig, the influence of the distance between the outlet and the target and the angle between the target and spray axis on the determination of injection rate of each nozzle hole was experimentally studied. From this work, the major conclusions can be drawn as follows: Using the measuring method proposed, the injection rate of each nozzle hole can be tested accurately. With the distance between the outlet and the target of 10 mm and the angle between the target and spray axis of 90°, the relative errors of cycle fuel injection quantity of the measured injector obtained based on the measurements of the injection rate of each nozzle hole are less than 4.6% under different operating conditions. For the multi-hole diesel injector, the injection start of the nozzle hole with the larger angle between nozzle hole axis and needle axis will be delayed, the injection end will be advanced, and the cycle fuel injection quantity will be decreased. In order to further understand the uniformity of the injection rates among nozzle holes, a new non-dimensional parameter named non-uniform coefficient of cycle fuel injection quantities among nozzle holes is defined. For the injection system of pump-line-nozzle, the non-uniform coefficient is decreased gradually with the increase of the injection pump speed under the same control rack position or with the increase of cycle fuel injection quantity under the same injection pump speed. When the distance between the outlet and the target and the angle between the target and spray axis are less than 12 mm and 100°, respectively, the injection rate time-histories measured at different positions of the force sensor are close to one another for the same nozzle hole. But the further increase of the distance or the angle will result in the reduction of the peak injection rate and the cycle fuel injection quantity. Besides, the measured injection start, injection end, and the corresponding phase of peak injection rate will be delayed gradually with further increase of the distance.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51176068), Scientific Research Innovation Foundation for Graduate Students of Jiangsu Province (CXZZ12_0674) and a Project Funded by the Priority Academic Program Development of Jiangsu High Education Institutions.
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[18] Desantes JM, Payri R, Salvador FJ, Gil A. Development and validation of a theoretical model for diesel spray penetration. Fuel 2006;85:910–7. [19] Marcˇicˇ M. Measuring method for diesel multihole injection nozzles. Sens Actuators A 2003;107:152–8. [20] Desantes JM, Payri R, Garcia JM, Salvador FJ. A contribution to the understanding of isothermal diesel spray dynamics. Fuel 2007;86:1093–101. [21] Payri R, Ruiz S, Salvador FJ, Gimeno J. On the dependence of spray momentum flux in spray penetration: momentum flux packets penetration model. J Mech Sci Technol 2007;21:1100–11. [22] Rajaratnam N. Turbulent jets. Amsterdam: Elsevier; 1974. [23] Postrioti L, Mariani F, Battistoni M, Mariani A. Experimental and numerical evaluation of diesel spray momentum flux. SAE Paper 2009-01-2772; 2009. [24] Payri R, Garcı´a JM, Salvador FJ, Gimeno J. Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics. Fuel 2005;84:551–61. [25] Sangiah DK, Ganippa LC. Application of spray impingement technique for characterization of high pressure sprays from multi-hole diesel nozzles. Int J Therm Sci 2010;49:409–17. [26] Payri R, Salvador FJ, Gimeno J, Novella R. Flow regime effects on non-cavitating injection nozzles over spray behavior. Int J Heat Fluid Flow 2011;32:273–84. [27] Payri R, Salvador FJ, Gimeno J, Zapata LD. Diesel nozzle geometry influence on spray liquid-phase fuel penetration in evaporative conditions. Fuel 2008;87:1165–76. [28] Desantes JM, Payri R, Salvador FJ, Gimeno J. Measurements of Spray Momentum for the Study of Cavitation in Diesel Injection Nozzles. SAE Paper 2003-01-0703; 2003. [29] Salvador FJ, Ruiz S, Gimeno J, De la Morena J. Estimation of a suitable Schmidt number range in diesel sprays at high injection pressure. Int J Therm Sci 2011;50:1790–8. [30] Chen G. Study of fuel temperature effects on fuel injection, combustion, and emissions of direct-injection diesel engines. ASME Trans, J Eng Gas Turb Power 2009;131(2):022802. [31] Fuqiang Luo, Ziyu Wang, Liang Yu. Physicochemical properties and viscosity– temperature characteristic of jatropha curcas oil as fuel. Trans CASE 2010;5:227–31. [32] Miao XL, Zheng JB, Hong JH, Qiu Y, Xia SH, Ju YS. Spray characteristics of crossing hole in diesel nozzle. Trans CSICE 2012;5:435–9. [33] Wang QH, Zhang KW. CFD calculation of the two-phase flow inside K-factor injection nozzle. Des Manufacture Diesel Eng 2009;4:5–10. [34] Zheng JB, Miao XL, Hong JH, Wang XY, Fang ZH, Yang M. Flow analysis of SAC nozzle with CR and PLN system. Trans CSICE 2011;29:61–6. [35] Ganippa LC, Andersson S, Chomiak J. Transient measurements of discharge coefficients of diesel nozzles. SAE Paper 2000-01-2788; 2000. [36] Klein-Douwel RJH, Frijters PJM, Somers LMT, de Boer WA, Baert RSG. Macroscopic diesel fuel spray shadowgraphy using high speed digital imaging in a high pressure cell. Fuel 2007;86:1994–2007.
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Transient measuring method for injection rate of each nozzle hole based on spray momentum flux Fuqiang Luo ⇑, Huifeng Cui, Shaofeng Dong School of Automobile and Traffic Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China
h i g h l i g h t s A transient measuring method of injection rate of each nozzle hole was proposed. The differences in injection rates among nozzle holes were primarily analyzed. Non-uniform coefficient of cycle fuel injection quantities among nozzle holes was defined. The influence of the measurement procedure details was studied.
a r t i c l e
i n f o
Article history: Received 3 November 2013 Received in revised form 5 February 2014 Accepted 7 February 2014 Available online 19 February 2014 Keywords: Measuring method Injection rate Nozzle hole Multi-hole injection nozzle Spray momentum flux
a b s t r a c t For a diesel engine equipped with multi-hole injectors, its combustion process, pollutant formation and thermal load consistency of combustion chamber are directly influenced by the differences in injection rates among nozzle holes. However, there are few measuring methods and equipments suitable for the determination of injection rate of each nozzle hole. The aim of this paper is to evaluate a measuring method proposed based on the spray momentum measurement of each nozzle hole that could be used to determine its injection rate. For this purpose, a conventional injection system of pump-line-nozzle was utilized and a dedicated experimental rig was constructed. Under different operating conditions, the cycle fuel injection quantities of the measured injector and the transient injection rate of each nozzle hole were measured successively. Based on the experimental results, the reliability and stability of the proposed measuring method were validated, and the differences in injection rates among nozzle holes were analyzed. In order to further understand the measuring method proposed, the influence of the measurement procedure details such as the distance between the outlet and the target and the angle between the target and spray axis on the determination of the transient injection rate of each nozzle hole was experimentally studied. The experimental results show that when the distance between the outlet and the target is less than 12 mm and the angle between the target and spray axis is lower than 100°, the transient injection rate of each nozzle hole could be measured accurately using the measuring method proposed, and that with a higher injection pump speed or more cycle fuel supply quantity, the consistency of cycle fuel injection quantities among nozzle holes is improved gradually. The further increase of the distance or the angle will result in the reduction of the peak injection rate and cycle fuel injection quantity of the measured nozzle hole. Besides, the injection start, injection end, and the corresponding phase of peak injection rate of the measured nozzle hole will be delayed little by little with the further increment of the distance. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction For a diesel engine, in order to achieve an efficient combustion process with moderate emissions, the optimization of the injection process in terms of injection rate and spray characteristics is crucial [1,2]. In fact, the injection rate directly affects the ⇑ Corresponding author. Tel.: +86 13615280495; fax: +86 0511 88782845. E-mail address: [email protected] (F. Luo). http://dx.doi.org/10.1016/j.fuel.2014.02.011 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
evolution of the diesel spray, fuel air interaction, and the combustion process [3]. In other words, in a diesel engine, the injection rate has a direct influence on the combustion performance, the noise and pollutant emissions. Hence, the knowledge of the characteristics of injection rate could lead to a significant contribution to the design improvement and performance optimization of a diesel engine. To obtain reliable cycle fuel injection quantity and injection rate measurements, many methods and techniques have been
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F. Luo et al. / Fuel 125 (2014) 20–29
Nomenclature Ageo Cd Cn F L _ m _ M n Pi q qmax qmean qmin
geometrical outlet section discharge coefficient non-uniform coefficient of cycle fuel injection quantities among nozzle holes spray impact force the distance between the outlet and the target mass flow rate spray momentum flux injection pump speed injection pressure cycle fuel injection quantity of the measured nozzle hole maximal cycle fuel injection quantity among nozzle holes mean cycle fuel injection quantity among nozzle holes minimal cycle fuel injection quantity among nozzle holes
developed. The oldest as well as the most common methods are the Bosch measuring method [4–7] and the Zeuch’s measuring method [8–10]. With the charge measuring method, the determination of injection rate is based on the measured charge created by the frictions, of the fuel in the nozzle and the spray against the surface of the sensor, and by the Seebeck effect [11,12]. As reported in the literature, the Laser Doppler Anemometer could also be used to determine the injection rate by measuring the axial velocity, from which the actual volume flow rate signal could be deduced [13,14]. All of the above-mentioned measuring methods can give the accurate result of injection rate of single-hole nozzle. For the multi-hole nozzle, however, they can only give its total injection rate, and provide no information about the possible differences in injection rates among nozzle holes. As reported in the literature, the injection rate diversities among nozzle holes do exist due to the inaccuracies in workmanship and the differences in hydraulic conditions among nozzle holes [15,16], which will lead to the non-uniform spatial and temporal distributions of the fuel within the combustion chamber and the induced uneven thermal loads of the combustion chamber [15,17–19]. Nowadays the direct injection diesel engines are usually equipped with multi-hole injectors. It is easy to understand the necessity of studying the measuring method, by which the injection rate of each nozzle hole of a multi-hole injector could be determined. Nevertheless, only a small number of scholars have ever carried out the above related researches. Marcˇicˇ [15,19] developed a deformational measuring method, whose criterion of the fuel injected of each nozzle hole is expressed by the deformation of the membrane occurring due to the collision of the pressure wave in the measuring space against the membrane. Payri et al. [16] developed a hole to hole mass flow test rig, by which the fuel injected of each nozzle hole could be obtained from the corresponding siphon where the fuel–air mixture is carried to and the liquid fuel and the air are separated. On the other hand, spray momentum flux is a very important parameter, with which the effective velocity at the outlet, fuel density, and the effective diameter of nozzle hole could be brought together [18,20,21]. Meanwhile, some important parameters such as spray penetration, spray cone angle, and air entrainment depend largely on spray momentum flux [22]. For these reasons, several experimental techniques have been developed to measure the spray momentum [23–26]. The spray momentum can be used
Q Qcum Qmean t t0 u um V_
cycle fuel injection quantity of the measured injector cumulative fuel injection quantity mean cycle fuel injection quantity time t delay time, in seconds or in camshaft rotation angles real velocity mean outlet velocity injection rate in terms of volume flow rate
Greek symbols D relative error DP pressure drop q real density qf liquid fuel density qf,N,T liquid fuel density under normal atmosphere and fuel temperature of T
not only to evaluate internal flow characteristics of nozzle, spray outflow characteristics, and the spray evolution [24,27,28], but also to validate the models established and estimate the relevant model parameters [18,20,21,29]. The objective of this work is to evaluate a transient measuring method proposed based on the spray momentum flux, with which the possible differences in injection rates among nozzle holes of a multi-hole diesel injector can be obtained. In order to achieve the proposed objective, the following three aspects have been addressed. First of all, the relationship between the injection rate of a nozzle hole and its corresponding spray momentum flux needs to be established, which is the basis of the succeeding researches. Secondly, spray momentum test rig needs to be built, which is the key to the entire study. Besides, a lot of experiments also need to be conducted in order to validate the proposed measuring method and to analyze the discrepancies in injection rates among nozzle holes. This paper is composed of six sections. In Section 2, the transient measuring method of injection rate of each nozzle hole proposed based on spray momentum flux is introduced. In Section 3, the experimental facilities are described briefly. In the following Sections 4 and 5, the validation of the measuring method, the analyses of the discrepancies in injection rates among nozzle holes, as well as the discussion of the measuring method are presented. Finally, in Section 6, the most important conclusions of this work are drawn.
2. Theoretical background The internal flow of the nozzle hole is very complex in terms of the flow direction, flow velocity, as well as the cavitation phenomenon [23,24]. Under these complex flow conditions, the mass flow rate and the spray momentum flux at the outlet of nozzle hole can be defined as follows:
_ ¼ m
Z
u qdA
ð1Þ
u2 qdA
ð2Þ
Ageo
_ ¼ M
Z Ageo
Based on the mass conservation law, the above expressions can be simplified to:
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F. Luo et al. / Fuel 125 (2014) 20–29
_ ¼ qf um Ageo m
ð3Þ
_ MðtÞ ¼ Fðt þ t 0 Þ
_ ¼ q u2 Ageo M f m
ð4Þ
Consequently, the injection rate of a nozzle hole shown in Eq. (8) can be modified as follows:
With the use of spray momentum flux, a non-dimensional parameter, discharge coefficient, can be obtained [26]:
Cd ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ M
ð5Þ
2Ageo DP
According to the relationship [30] between the mean velocity at the outlet and the discharge coefficient shown in Eq. (6), the mean velocity at the outlet can be derived:
um ¼ C d
sffiffiffiffiffiffiffiffiffi 2 DP
Combining Eqs. (3), (4), and (7), the injection rate of a nozzle hole in terms of the volume flow rate can be evaluated by:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ Ageo M
ð8Þ
qf
It can be clearly seen from Eq. (8) that it is possible to obtain the transient injection rate of each nozzle hole of a multi-hole diesel injector with the determination of its spray momentum flux. In order to determine the injection rate of each nozzle hole, a special method as described by Payri et al. [24] has been used to test the transient spray momentum of each nozzle hole. Fig. 1 shows the schematic diagram of the spray momentum measuring principle. The transient spray momentum is measured by an indirect method in terms of the impact force exerted by the spray on a flat surface (named target). Within certain rang of the distance between the outlet and the target (named outlet-target distance), as long as the target perpendicular to spray axis is large enough to interact with the entire spray, the impact force measured by the sensor will be equal to the spray momentum at the outlet of nozzle hole or at any other axial position due to the conservation of momentum. As shown in Fig. 1, the impact force measured is delayed with respect to the spray departure from the outlet. The delay time in seconds or in camshaft rotation angles can be evaluated approximately by:
L 6nL t 0 ¼ qffiffiffiffiffiffiffiffiffiffi or t0 ¼ qffiffiffiffiffiffiffiffiffiffi 2DPðtÞ qf ðtÞ
q¼
Q¼ ð7Þ
2DPðtÞ qf ðtÞ
ð9Þ
The relationship between transient spray momentum and its corresponding impact force can be expressed by:
Fig. 1. Measuring principle of spray momentum flux.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Fðt þ t 0 Þ Ageo qf ðtÞ
ð11Þ
Integrating Eq. (11) over the entire injection duration, the cycle fuel injection quantity of the measured nozzle hole and hence that of the injector used can be obtained by:
ð6Þ
qf
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ M um ¼ qf Ageo
V_ ¼
_ VðtÞ ¼
ð10Þ
ffi Z sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Fðt þ t 0 Þ Ageo dt qf ðtÞ X
q
ð12Þ
ð13Þ
3. Experimental set-up In order to evaluate the proposed measuring method, a conventional injection system of pump-line-nozzle was utilized and a dedicated experimental rig was constructed. In the following, the fuel injection system as well as the experimental rig is first introduced briefly. Then, the determination of the fuel density, which directly affects the measuring accuracy of injection rate of each nozzle hole, is presented. 3.1. Fuel injection system The measurement of injection rate of each nozzle hole was performed with a five-hole diesel injector, as shown in Fig. 2. It is equipped with a mini-sac nozzle with a nozzle hole diameter of 0.2 mm and a fuel starting injection pressure of 22.5 MPa. The injector is fed directly from a inline pump. The injection pump is driven by a pump test-bed, of which injection pump speed, fuel temperature and measurement times of fuel injection can be set freely. 3.2. Spray momentum experimental rig Fig. 3 shows the schematic diagram of spray momentum experimental rig, on which the impact force exerted by some spray on corresponding target can be determined and so can the injection rate of corresponding nozzle hole. A calibrated piezoelectric force sensor was employed to detect the spray impact force, by means of a circular target screwed directly on the sensor head. A customized magnetic stand was equipped with a distance adjusting screw and an angle adjustment knob, which allow the target-sensor assembly to be moved for a
Fig. 2. Schematic diagram of injection nozzle.
F. Luo et al. / Fuel 125 (2014) 20–29
23
Fig. 3. Spray momentum experimental rig.
travel range from 0 mm to 40 mm in spray axis direction and to be rotated for an angle range from 90° to the hardware-limited maximum of 120° with respect to spray axis, respectively. The magnetic stand was inserted into an oil mist dispersal chamber where the spray was injected into, and was used for the positioning of the target-sensor assembly. In order to adjust the position of the target-sensor assembly timely, the surrounding walls of the oil mist dispersal chamber were transparent and removable. A clamp-on pressure sensor clamped on the upstream of mini-sac was used to measure the injection pressure. In order to make Fig. 3 more readable, only one target-sensor assembly is shown. 3.3. Determination of fuel density Commercially available diesel fuel (No. 0 diesel fuel) obtained from a petrol station is used in this study. As clearly shown in Eq. (11), the fuel densities under different fuel temperatures and injection pressures must be determined precisely in order to obtain the accurate result of injection rate of nozzle hole. Based on the fuel densities obtained under atmospheric pressure and different fuel temperatures [31], the ones used for the determination of injection rate of nozzle hole can be evaluated by:
qf ðtÞ ¼ qf ;N;T 1 þ
0:6 109 Pi ðtÞ
!
1 þ 1:7 109 Pi ðtÞ
ð14Þ
4. Experimental results As reported in the literature [1,23,24], the target impacted by the spray must be large enough to interact with the entire spray, and at the same time it must be as small as possible to provide adequate response characteristics during the transients mainly including both the opening and closing stages of the needle. Combining the experimental results of many installation tests of 5 target-sensor assemblies conducted on the spray momentum experimental rig, the existing study on the measurement of spray momentum flux, the difficulty and the reliability of non-overlap fixing of each target-sensor assembly perpendicular to corresponding spray axis, the outlet-target distance and the target diameter were both decided to 10 mm to conduct the subsequent experiments. In order to reduce the influence of cycle-to-cycle variation of injection process, all the test results presented in this section are the mean values of 100 injection events.
4.1. Validation of measuring method Under the operating condition with injection pump speed of 1200 r/min and cycle fuel injection quantity of 61.8 mm3/cyc, the measurements of injection rate of each nozzle hole were carried out. Fig. 4(a) and (b) shows the obtained time histories of spray impact force and injection rate of each nozzle hole, respectively. As shown in Fig. 4(a), there are some local differences in the curve profiles of spray impact force of each nozzle hole, but they all possess relatively consistent shapes resembling a hat [18,21]. In addition, the differences in the curve profiles of spray impact force of different nozzle holes during the opening and closing phases of the needle are well reproduced by the injection rate time-histories of corresponding nozzle holes shown in Fig. 4(b). Comparing spray impact force time-history of each nozzle hole during the main injection period with that of injection rate, it can be seen that the fluctuating range of the latter is much smaller than that of the former. Both of the above phenomena are mainly caused by the relationship between spray impact force and its corresponding injection rate shown in Eq. (11). Combining the measurement of injection rate of each nozzle hole with Eqs. (12) and (13), the total integral fuel injection quantity at end of the injection (named cumulative fuel injection quantity, Qcum), 59.53 mm3, is obtained, and is very close to the mean cycle fuel injection quantity of the measured injector obtained based on the pump test-bed (named mean cycle fuel injection quantity, Qmean), 61.8 mm3. Their relative error calculated based on Eq. (15) is only 3.67%. The experimental result indicates, to some extent, that the injection rate of each nozzle hole of a multi-hole diesel injector can be measured with a comparative accuracy using the measuring method proposed and the experimental rig constructed.
D¼
Q mean Q cum 100% Q mean
ð15Þ
In order to further validate the reliability and stability of the measuring method proposed, the mean cycle fuel injection quantity Qmean and the injection rate of each nozzle hole have been measured successively under different operating conditions. In more detail, the injection pump speed was set at 800 r/min, 1000 r/min, 1200 r/min and 1400 r/min, respectively, and the control rack position of injection pump was set at P50, P60, P70 and P80, respectively (Px is a control rack position of injection pump at which the mean cycle fuel injection quantity Qmean over 100 injection events is x mm3 under the injection pump speed of
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole1 Hole2 Hole3 Hole4 n=1200r/min Q mean=61.8mm3/cyc
Hole5
2.0 1.5 1.0 0.5 0.0
5
10
15
(b) 1.5 Injection rate [mm3/º CaA]
Spray impact force [N]
(a) 2.5
20
Hole5
1.2 0.9 0.6 0.3 0.0
25
Hole1 Hole2 Hole3 Hole4 n=1200r/min Q mean=61.8mm3/cyc
5
10
Cam rotation angle [º CaA]
15
20
25
Cam rotation angle [º CaA]
Fig. 4. Time histories of spray impact force and injection rate of each nozzle hole. (a) Spray impact force and (b) injection rate.
1000 r/min). Under each of the combinations, the mean cycle fuel injection quantity Qmean was measured on the pump test-bed first, and then the injection rate of each nozzle hole was determined on the experimental rig constructed. Fig. 5(a) and (b) shows the comparisons between the cumulative fuel injection quantity Qcum and the mean cycle fuel injection quantity Qmean, and the distribution of the relative errors under the above 16 different operating conditions, respectively. As expected, Qcum and Qmean are increased gradually with the increment of the injection pump speed or the cycle fuel supply quantity characterized by the control rack position of injection pump. The relative errors are not evenly distributed, but their values are not large and mainly between 2.2% and 4.6%. In general, the proposed measuring method and the constructed experimental rig have relatively higher precision and better testing stability. 4.2. Difference analysis of injection rates among nozzle holes Fig. 6 shows the time histories of injection rate of each nozzle hole under different injection pump speeds at P60, and Fig. 7 shows the same parameter as in Fig. 6 under different control rack positions at injection pump speed of 1000 r/min. From Figs. 6 and 7, it can be seen that under each of the operating conditions, the global shapes of injection rate time-histories of 5 nozzle holes are very similar. Meanwhile, it is also noticed that under the same operating condition, the injection start is delayed gradually, the injection end is advanced by degrees, and the fuel injection duration is decreased little by little with the increase of nozzle hole serial number (i.e. with the increase of the angle between nozzle hole axis and needle axis, named nozzle hole angle,
shown in Fig. 2). As reported in the literature [32–34], the above phenomena can be considered as a result of the comprehensive effect of the difference in pressure distribution of nozzle sac, the influence of the nozzle hole angle on entrance pressure loss of nozzle hole, and the complexity of the needle movement. Another interesting result is that under each of the operating conditions there is a spike existing respectively in the injection rate time-histories of Hole 1 and Hole 2 during the opening phase of the needle. This is mainly attributed to the unsteady flow caused by the needle opening process and the hydraulic hammering effect produced by the sudden stop of the needle prevented by the lift limiter [35]. The reason why there is no spike existing in the injection rate time-histories of Hole 3, Hole 4 and Hole 5 could be reasonably interpreted as that the delays of their injection starts suppress the influence of the unsteady flow and the hydraulic hammering effect. As shown in Figs. 6 and 7, there are no significant changes in injection start of each nozzle hole with the increment of injection pump speed or the cycle fuel supply quantity characterized by the control rack position of injection pump, but their injection ends are delayed gradually, and at the same time their corresponding fuel injection durations measured in camshaft rotation angle are increased little by little. With the increase of injection pump speed, the leakage of plunger and barrel assembly of injection pump is decreased gradually, meanwhile, the throttle effect of fuel return hole of the injection pump is enhanced little by little, which will result in a smaller falling rate of injection pressure after fuel return hole is opened by plunger and hence the later closing phase of the needle. The influence of the cycle fuel supply quantity is mainly attributed to the working principle of the injection pump used.
90
P50
P60
P70
P80
Qmean
P50
P60
P70
P80
Qcum
80 70 60 50 40
(b) Relative error [%]
Cycle fuel injection quantity [mm3/cyc]
(a)
800
1000
1200
Injection pump speed [r/min]
1400
5 4 3 2
P50 P60 P70 P80
1 0
800
1000
1200
1400
Injection pump speed [r/min]
Fig. 5. Validation results of the test method proposed. (a) Comparisons between Qcum and Qmean and (b) distribution of relative errors.
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
(b) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
(a) 1.8
5
10
15
20
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
5
10
15
20
Hole4
Hole5
0.9 0.6 0.3 5
10
15
20
25
Cam rotation angle [º CaA]
(d) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
1.8
Hole3
1.2
Cam rotation angle [º CaA]
(c)
Hole2
1.5
0.0
25
Hole1
25
Cam rotation angle [º CaA]
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
5
10
15
20
25
Cam rotation angle [º CaA]
Fig. 6. Instantaneous injection rates of each nozzle hole under different injection pump speeds at P60. (a) 800 r/min, P60, (b) 1000 r/min, P60, (c) 1200 r/min, P60 and (d) 1400 r/min, P60.
Under the above operating conditions, the corresponding cycle fuel injection quantities of each nozzle hole are shown in Fig. 8(a) and (b), respectively. It can be seen that the cycle fuel injection quantity of each nozzle hole is increased gradually with the increment of injection pump speed or the cycle fuel supply quantity characterized by the control rack position of injection pump. Besides, under the same operating condition, the cycle fuel injection quantity of a nozzle hole is decreased little by little with the increase of nozzle hole serial number (i.e. with the increase of the named nozzle hole angle shown in Fig. 2). These results are consistent with those shown in Figs. 6 and 7. In order to study the differences among diesel nozzle holes, an evaluating parameter named relative hole mass flow is defined by Payri et al. [16], which is calculated by dividing the cycle fuel injection quantity of the analyzed nozzle hole by the mean value of all the nozzle holes. Under the present testing conditions, the relative hole mass flows of each nozzle hole are shown in Fig. 9(a) and (b). As can be seen from the graphs, the relative hole mass flows of 5 nozzle holes have some difference under the same operating condition, but the relative hole mass flows of any nozzle hole obtained under different operating conditions are not evenly distributed and their variations are not significant. This is consistent with the experimental results obtained by Payri et al. [16]. In order to further understand the uniformity of injection rates among nozzle holes under different operating conditions, a new non-dimensional parameter, Cn, or non-uniform coefficient of cycle fuel injection quantities among nozzle holes is defined:
Cn ¼
qmax qmin 100% qmean
ð16Þ
It can be seen from Fig. 9(a) that the non-uniform coefficient Cn is gradually decreased with the increment of injection pump speed. The similar result is also shown by the experimental data obtained
by Marcˇicˇ for different operating conditions and multi-hole diesel injectors [15,19]. To some extent, this indicates that the uniformity of injection rates among nozzle holes is improved little by little with the increase of injection pump speed. As shown in Fig. 9(b), the non-uniform coefficient Cn is also decreased gradually with the increment of cycle fuel supply quantity characterized by the control rack position of injection pump. This is mainly because the cycle fuel injection quantity of each nozzle hole is increased significantly with the increase of cycle fuel supply quantity and so is the mean cycle fuel injection quantity of 5 nozzle holes (qmean), but the change in the difference between cycle fuel injection quantity of Hole 1 (qmax) and that of Hole 5 (qmin) is not distinct. The non-uniform coefficient is one of the evaluation parameters characterizing the injection consistency among nozzle holes indirectly, which characterizes the maximum difference among the relative hole mass flows of all the nozzle holes. The larger the non-uniform coefficient is, the worse the injection consistency among nozzle holes is. 5. Discussion of measuring method As reported in the literature [1,23], it is impossible for the gaseous phase of the spray (including the ambient air exchanging momentum with the spray and the fuel vapor) to impact entirely on the target, which will result in a loss of momentum flux. The influence of the above gaseous phase is always existed and cannot be eliminated completely by adjusting the measurement procedure details, but it can be diminished to some extent by advisably decreasing the outlet-target distance [1]. Assuming that the spray geometry is an axisymmetric body, since the entire spray is surrounded by ambient air, the force measured by the sensor is the same as the spray momentum flux at the
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
(b) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
(a) 1.8
5
10
15
20
Hole1
Hole1
Hole2
Hole3
Hole4
0.6 0.3 0.0
5
10
Hole5
0.9 0.6 0.3
10
15
15
20
25
Cam rotation angle [º CaA]
1.2
5
Hole5
0.9
25
1.5
0.0
Hole4
1.2
(d) 1.8 Injection rate [mm3/º CaA]
Injection rate [mm3/º CaA]
1.8
Hole3
1.5
Cam rotation angle [º CaA]
(c)
Hole2
20
Hole1
Hole2
Hole3
Hole4
Hole5
1.5 1.2 0.9 0.6 0.3 0.0
25
5
10
Cam rotation angle [º CaA]
15
20
25
Cam rotation angle [º CaA]
Fig. 7. Instantaneous injection rates of each nozzle hole under different control rack positions at injection pump speed of 1000r/min. (a) P50, 1000 r/min, (b) P60, 1000 r/min, (c) P70, 1000 r/min and (d) P80, 1000 r/min.
(b) 14
Hole1
Hole2
Hole3
Hole4
Hole5
Control rack position of P60 13 12 11 10 9
800
1000
1200
1400
Injection pump speed [r/min]
Cycle fuel injection quantity [mm3/cyc]
Cycle fuel injection quantity [mm3/cyc]
(a)
Hole1
Hole2
Hole3
Hole4
Hole5
Injection pump speed of 1000r/min
16 14 12 10 8
P50
P60
P70
P80
Control rack position of injection pump
Fig. 8. Cycle fuel injection quantities of each nozzle hole obtained under different operating conditions. (a) The values related to different injection pump speeds and (b) the values related to different control rack positions.
outlet of a nozzle hole due to the conservation of momentum [28]. As a matter of fact, however, spray geometry is not a normal axisymmetric body [36]. Besides, with the increase of the outlet-target distance, i.e. with the development of the spray, the spray geometry between the outlet and the target will become more and more irregular due to the air entrainment and the momentum exchange, which will inevitably affect the measurement of the transient spray momentum flux of each nozzle hole and hence the determination of corresponding injection rate. According to the spray momentum measuring principle shown in Fig. 1, one geometry condition that the target is normal to spray
axis must be satisfied in order to accurately measure the transient spray momentum flux of some nozzle hole, because otherwise there will be non-null residual velocity components in spray axis direction after spray-target impact, which will also affect the determination of spray momentum flux. Based on the above analysis, it can be seen that both the outlettarget distance and the angle between the target and spray axis (named target-axis angle) have a direct influence on the measurement of spray momentum flux of each nozzle hole and on the subsequent determination of corresponding injection rate. Consequently, it is necessary to study the influence of the above geometry
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F. Luo et al. / Fuel 125 (2014) 20–29
Hole2
Hole3
Hole4
Hole5
24
Relative hole mass flow
Non-uniform coefficient
23
1.1
22
1.0
21 0.9
20
0.8 0.7
19
Control rack position of P60 800
1000
1200
18
1400
Injection pump speed [r/min] Hole1
Hole2 Hole3 Hole4 Non-uniform coefficient
Hole5
23
1.1 1.0
22
0.9 21 0.8
Injection pump speed of 1000r/min 0.7
P60
P50
P70
Non-uniform coefficient [%]
Relative hole mass flow
(b) 1.2
20
P80
Control rack position of injection pump Fig. 9. Relative hole mass flows of each nozzle hole and non-uniform coefficients obtained under different operating conditions. (a) The values related to different injection pump speeds and (b) the values related to different control rack positions.
parameters carefully in order to put forward some suggestions for the design and the improvement of the spray momentum experimental rig shown in Fig. 3.
5.1. Influence of the outlet-target distance Fig. 10 shows the time histories of injection rate of Hole 3 and its corresponding cycle fuel injection quantities concerning a 16 mm diameter target normal to spray axis, positioned at different outlet-target distances, i.e. L = 6 mm, 12 mm, 18 mm, 24 mm and 30 mm, under the operating condition with injection pump speed of 1000 r/min and control rack position of P60.
(a)
5.2. Influence of the target-axis angle Fig. 11 shows the time histories of injection rate of Hole 3 and its corresponding cycle fuel injection quantities concerning a 16 mm diameter target, positioned at an outlet-target distance of 10 mm with five different target-axis angles of 90°, 95°, 100°,
(b) 1.6
Injection rate [mm3/º CaA]
With outlet-target distance smaller than 12 mm, the time histories of injection rate fit well with each other in terms of the global shape, injection start, injection end, peak injection rate and the corresponding phase of peak injection rate. Furthermore, there are not any obvious changes in the corresponding cycle fuel injection quantity. With further increase of the outlet-target distance, however, the injection start, injection end, and the corresponding phase of peak injection rate are gradually delayed, meanwhile, the peak injection rate and the corresponding cycle fuel injection quantity are decreased little by little. More specifically, the injection start and the injection end are delayed by about 0.5° CaA measured at the outlet-target distance of 30 mm, the corresponding phase of peak injection rate is delayed by about 0.25° CaA, the peak injection rate is decreased by 6.37%, and the cycle fuel injection quantity is decreased by 5.96% with respect to those measured at the outlet-target distance of 12 mm. The above phenomena can be interpreted in terms of the respective contributions of the spray liquid phase and gaseous phase to momentum flux, and the measuring principle shown in Fig. 1. Within a certain range of outlet-target distance, the contribution of the spray liquid phase is dominant, which results in almost the same instantaneous spray momentum flux measured at different outlet-target distances [21] and hence approximately the same transient injection rate. With further increase of the outlet-target distance, the contribution of the spray liquid phase will be decreased remarkably in favor of the gaseous phase [1,21,23]. Meanwhile, the axial velocity of the spray will be decreased gradually and the spray geometry between the outlet and the target will become more and more irregular. In other words, the greater outlet-target distance will lead to a higher value of t0 obtained based on Eq. (9) and a bigger possibility that more gaseous phase cannot impact on the target. It hence gives rise to the delayed injection start, injection end, corresponding phase of peak injection rate, and the lower peak injection rate and cycle fuel injection quantity. Under the same operating condition, similar evaluations of Hole 1, Hole 2, Hole 4 and Hole 5 have been done at the above different outlet-target distances, respectively. Interestingly, almost identical results have been obtained. For brevity, they are not shown here.
6mm
12mm
18mm
24mm
30mm
1.2 0.8 0.4 0.0
8
10
12
14
16
18
Cam rotation angle [º CaA]
20
22
Cycle fuel injection quantity [mm3/cyc]
Hole1
Non-uniform coefficient [%]
(a) 1.2
13 12 11 10 9 8 7
5
10
15
20
25
30
Outlet-target distance [mm]
Fig. 10. Injection rate time histories of Hole 3 and its corresponding cycle fuel injection quantities measured at different outlet-target distances. (a) Injection rate time histories and (b) cycle fuel injection quantities.
28
F. Luo et al. / Fuel 125 (2014) 20–29
(b)
Injection rate [mm3/º CaA]
1.6
90°
95°
100°
105°
110°
1.2 0.8 0.4 0.0
8
10
12
14
16
18
20
22
Cam rotation angle [º CaA]
Cycle fuel injection quantity [mm3/cyc]
(a)
13 12 11 10 9 8 7
90
95
100
105
110
Target-axis angle [º]
Fig. 11. Injection rate time histories of Hole 3 and its corresponding cycle fuel injection quantities measured at different target-axis angles. (a) Injection rate time histories and (b) cycle fuel injection quantities.
105°, and 110°, at injection pump speed of 1000 r/min and control rack position of P60. The first interesting result shown in Fig. 11 is that the measured injection start, injection end, and the corresponding phase of peak injection rate are approximately the same under the above different target-axis angles. This is mainly because within a certain range of target-axis angle the flow patterns of the spray between the outlet and the target as well as the impact flow structures near the target have no significant changes for the same operating condition and outlet-target distance, and thus the values of t0 obtained based on Eq. (9) are almost the same. As shown in Fig. 11, while the target-axis angle is smaller than 100°, the time histories of injection rate measured at different target-axis angles agree well with each other, meanwhile, the change in the cycle fuel injection quantity is not significant. Specifically, for the target-axis angle of 100°, the cycle fuel injection quantity decreases by only 2.15% compared to that of 90° target-axis angle. With further increase of the target-axis angle, the change in the global shape of the injection rate time-history is very small, but the peak injection rate and the cycle fuel injection quantity decrease gradually. In more detail and compared to those of 90° target-axis angle, the peak injection rates measured at the target-axis angles of 105° and 110°, decrease by 4.41% and 6.97%, respectively, and the cycle fuel injection quantities decrease by 4.64% and 7.62%, respectively. The most feasible reason for these phenomena seems to be the difference, for the larger target-axis angles, in the rebound mechanism of fuel droplet colliding with the target. Generally, the obtained experimental data seem to indicate that the target-axis angle, especially the larger ones, has deep influence on the determination of injection rate of each nozzle hole. Hence, in order to obtain the injection rate of each nozzle hole precisely, the target-axis angle has to be kept below 100°.
6. Conclusions In this paper, a transient measuring method of injection rate of each nozzle hole of a multi-hole diesel injector is proposed. The method is based on the measurement of spray momentum flux, focusing on the impact force exerted by the spray on the target screwed directly on the force sensor head. Under different operating conditions, the mean cycle fuel injection quantities of the used injector were measured on the pump test-bed first, and then a dedicated experimental rig was constructed and employed to determine the injection rate of each nozzle hole. Combining the above experimental data, the reliability and stability of the measuring method was validated, and meanwhile, the differences in injection rates among nozzle holes were analyzed.
In order to make some suggestions for the design and the improvement of the constructed experimental rig, the influence of the distance between the outlet and the target and the angle between the target and spray axis on the determination of injection rate of each nozzle hole was experimentally studied. From this work, the major conclusions can be drawn as follows: Using the measuring method proposed, the injection rate of each nozzle hole can be tested accurately. With the distance between the outlet and the target of 10 mm and the angle between the target and spray axis of 90°, the relative errors of cycle fuel injection quantity of the measured injector obtained based on the measurements of the injection rate of each nozzle hole are less than 4.6% under different operating conditions. For the multi-hole diesel injector, the injection start of the nozzle hole with the larger angle between nozzle hole axis and needle axis will be delayed, the injection end will be advanced, and the cycle fuel injection quantity will be decreased. In order to further understand the uniformity of the injection rates among nozzle holes, a new non-dimensional parameter named non-uniform coefficient of cycle fuel injection quantities among nozzle holes is defined. For the injection system of pump-line-nozzle, the non-uniform coefficient is decreased gradually with the increase of the injection pump speed under the same control rack position or with the increase of cycle fuel injection quantity under the same injection pump speed. When the distance between the outlet and the target and the angle between the target and spray axis are less than 12 mm and 100°, respectively, the injection rate time-histories measured at different positions of the force sensor are close to one another for the same nozzle hole. But the further increase of the distance or the angle will result in the reduction of the peak injection rate and the cycle fuel injection quantity. Besides, the measured injection start, injection end, and the corresponding phase of peak injection rate will be delayed gradually with further increase of the distance.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51176068), Scientific Research Innovation Foundation for Graduate Students of Jiangsu Province (CXZZ12_0674) and a Project Funded by the Priority Academic Program Development of Jiangsu High Education Institutions.
F. Luo et al. / Fuel 125 (2014) 20–29
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