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Pressure fluctuations in a hydrogen supply system during hydrogen injection
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Pressure fluctuations in a hydrogen supply system during hydrogen injection Bai-gang Sun, Dong-sheng Zhang, Fu-shui Liu* School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
article info
abstract
Article history:
A test bench and simulation model are established to investigate the pressure fluctuations
Received 5 November 2012
in the port fuel-injected hydrogen system of an internal combustion engine. Analyses are
Received in revised form
conducted, with injection pressure varied from 0.2 to 0.45 MPa and injection pulse width
21 February 2013
varied from 2 to 10 ms. The test and simulation results show that more severe fluctuations
Accepted 24 February 2013
occur, fluctuation ranges are greater, and decay times are longer under higher injection
Available online 4 April 2013
pressures. Pressure decreases at the curve of pressure at the time during which the injector is opened. This time can be calculated by simulation. t0 decreases from 1.33 to 0.45 ms at an
Keywords:
increase in injection pressure of 0.2e0.45 MPa. Therefore, the hydrogen injection time of
Hydrogen
the internal combustion engine is accurately controlled by considering t0 under different
Hydrogen internal combustion en-
conditions. The results serve as guidance in the design of hydrogen rails in hydrogen in-
gine
ternal combustion engine.
Hydrogen supply system
Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All
Pressure fluctuation
1.
Introduction
Hydrogen has a faster combustion speed, lower ignition energy, shorter quenching distance, and cleaner exhaust products than does liquid fuel [1,2]. It is also suitable for use in internal combustion engines because it is more economical than hydrogen fuel cells [3,4]. Port fuel-injected (PFI) hydrogen internal combustion engines have been considered effective equipment because they present advantages such as uniform mixtures, easy organization of combustion, and development based on substantial insights derived from the traditional internal combustion engine industry [5e7]. Hydrogen has an extensive flammability range, and its mixture can be easily ignited within a hydrogeneair mixture volume ratio of 4%e75%. The torque of hydrogen internal combustion engines can therefore be controlled by adjusting the throttle position in light loads, in which mixture concentration is kept constant. When an increase in torque is
rights reserved.
required, the throttle position is fully initiated and mixture concentration increases through the injection of more hydrogen. Given that hydrogen has a low volumetric energy content, the injection volume of hydrogen in cylinders should be considerably higher than those of gasoline and diesel. Especially in diesel, the fluctuations of pressure upstream the nozzle holes are unavoidable because of the fast opening and closing of the needle valve during very short injection intervals and the propagation of pressure wave through narrow fuel passages [8]. Therefore, such a high volume of hydrogen will cause greater pressure fluctuations in pipes, especially under high speeds and high loads. The effects of hydrogen on the flow characteristics of pipe structures and the pipe pressure fluctuations caused by hydrogen leakage have been investigated by fewer researchers [9,10]. However, the hydrogen injection-induced pressure fluctuations in pipes remain unexplained. The flow characteristics of hydrogen injectors were studied by Sun [11],
* Corresponding author. Tel.: þ86 15011357196; fax: þ86 010 68945860. E-mail address: [email protected] (F.-s. Liu). 0360-3199/$ e see front matter Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
who found that pressure fluctuates upstream and downstream a pipe. The area where hydrogen injection pressure abruptly decreases is easily determined when the injector is switched on. The time required for the injector to be opened was calculated. To decrease the pressure fluctuations in the hydrogen rails of a PFI hydrogen internal combustion engine, Sun also investigated the design methods and configurations of hydrogen rails [12,13]. In the current study, numerous tests and calculations are carried out to examine the pressure fluctuations in the pipes of PFI hydrogen internal combustion engines.
computer (13). Hydrogen is injected into the container after the injector receives the opening signal, and the current and pressure signals are recorded by the analyzer. After the pressure signals are recorded, hydrogen is released through the gas release valve (9), and water is discharged by the water release valve (12) when too much water is in the water channel (11). The above-mentioned steps are repeated; the pressure fluctuations in the pipe and in the hydrogen container located before and after the injector can be easily determined by this test bench.
2.2.
2.
Materials and methods
2.1.
Test equipment and steps
The layout of the test bench [14] is shown in Fig. 1. The pressure in the hydrogen bottle (1) is 12 MPa, which can be reduced by releasing the valve (2), thereby satisfying the working pressure required for injectors. A hydrogen rail (3) is set up before the injector (8) to maintain stable pressure. The injector operates when an opening signal is received, and hydrogen is injected into the calibration container (10) that measures hydrogen volume. Two Kistler transient-pressure sensors (4) are set up before and after the injector, and the pressures in the pipes located before the injector and in the container during injection are recorded using a Dewetron 5000 analyzer (6). First, the target injection pressure is set at 0.2e0.45 MPa at an interval of 0.05 MPa on the basis of the maximum working pressure of the injector. Under each pressure, the injection times and pulse width of the injector are set by using a
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Simulation model
To investigate the pressure fluctuations in the pipe of a PFI hydrogen internal combustion engine, a simulation model is established on the basis of the hydrogen state, continuity, and momentum equations. The hydrogen state equation, proposed by Sun [15], features high accuracy and is expressed as: PR ¼
TR 27=64 pffiffiffiffiffiffi ZC ðVR þ 0:13636Þ1=8 TR Z2C ðVR þ 0:13636Þ2
(1)
where PR is the ratio of pressure and critical pressure, TR denotes the ratio of absolute temperature and critical temperature, VR represents the ratio of volume and critical volume, and ZC is a constant that represents the compression factor of the critical isotherm of hydrogen. The continuity and momentum equations are as follows [16]: vr vr vV þV þr ¼0 vt vx vx
Fig. 1 e Layout of the test bench. (1. Hydrogen bottle; 2. release valve; 3. hydrogen rail; 4. transient pressure sensor; 5. electric control equipment; 6. data monitor and recorder system; 7. current caliper; 8. injector; 9. gas release valve; 10. hydrogen container; 11. water channel; 12. water release valve; and 13. computer.)
(2)
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r
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
vV vV vP þ rV þ ¼0 vt vx vx
(3)
where V is the flow speed of gas, P is the pressure, and r is the density of gas. The flow rates at the internal nodes of the injector are calculated using the following equation [16]: vdm vP r$ff $A$y2 $signðyÞ ¼ A$ vt vx 2$D
(4)
where y is the gas velocity, A denotes the cross-sectional area of the pipe, DP is the pressure drop, D represents the pipe diameter, r is the density, and ff is the friction factor. The process of hydrogen flow from the injector is assumed as orifice flow, and the mass flow equation is [16]: Pu _ ¼ A$Cq $Cm pffiffiffiffiffi ffi m Tu
(5)
where A is the area of the orifice, Cq is the flow coefficient calculated by Cq ¼ 0.72 þ 0.12 cos (p$Pd/Pu), Pu is the pressure of the orifice upstream, Pd denotes the pressure of the orifice downstream, Tu represents the temperature of the orifice downstream, and Cm is the parameter related to gas flow, whose value is constant when flow speed reaches the local velocity of sound. The flow speed can be calculated by Equation (6) [16], when this speed is slower than the local velocity of sound. ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffisffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g2 gþ1 2 Pd Pd g Cm ¼ ðg 1Þ Pu Pu
3.
Results and discussion
3.1.
Model testing and verification
The pressure fluctuations in the pipe upstream the injector and in the hydrogen container downstream the injector are shown in Fig. 2, which also presents the drive current. The hydrogen supply pressure is 0.4 MPa, and the injection pulse width is 8 ms. Pu rapidly decreases when the injector is switched on, and Pd increases during hydrogen injection. Given that liquid level gradually increases, the pressure in the hydrogen container also slowly rises. As the ratio of Pd and Pu is smaller than 0.527, the orifice of the injector is choked, which means that the pressure in the hydrogen container does not affect Pu. Additionally, the fluctuation of Pu mainly affected by the opening process of the injector, the lengths of the pipes, the damp though the pipes and the spread speed of pressure. Especially, the wave frequency of the pressure mainly depends on the length of the pipes and the spread speed of pressure. To verify the accuracy of the model, the simulation result of the pressure in the pipe upstream the injector has been compared with the test in Fig. 3. The simulation result is similar to the test result at an injection pressure of 0.45 MPa and an injection pulse width of 8 ms. The maximum error of the simulation result is within 0.3%, indicating that the model can be used to describe actual hydrogen injection systems.
3.2. (6)
Effects of opening and closing processes
The accurately measuring the movement of the armature is difficult to accomplish because of the limitations posed by the
Fig. 2 e Changes in Pu, Pd, and drive current over time.
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Fig. 3 e Comparison of simulation and test results.
test conditions. Therefore, a simulation is performed to determine the effect of armature movement on pressure fluctuations. The armature can be seen as a mass block driven by several force, such as the electromagnetic force, the frictional force and the force generated from hydrogen pressure. Additionally, the force generated from hydrogen pressure can be calculated by the area of the armature and the hydrogen pressure. Fig. 4 shows that the opening area of the injector changes with armature movement; the pressure fluctuation at the orifice of the injector and the pressure sensor position are 0.45 MPa and 10 ms, respectively. The
pressure at the orifice of the injector decreases with the opening of the injector, after which the pressure at the pressure sensor position also decreases. The maximum amplitudes of the pressure fluctuations at the sensor position decrease and the time is delayed for the variations in the section area of the pipes. Then, the time needed for the injector to fully switch on can be determined by simulation. This time is defined as t0. Fig. 5 shows that t0 decreases with increasing injection pressure possibly because the force of hydrogen on the armature facilitates the opening process of the injector.
Fig. 4 e Changes in opening area, pressure of the injector input, and pressure sensor over time.
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Fig. 5 e Changes in t0 with supply pressure.
Thus, higher pressure results in shorter t0. However, the injection pulse width has little effect on t0 at a constant drive voltage. Fig. 6 indicates that at 0.45 MPa, the pressure fluctuations in the pipe upstream the injector at different pulse widths are nearly the same with each other before the closing process of the injector, which means that the pulse width has little effect on the opening process of the injector. However, hydrogen pressure will rapidly increase during the closing process of the injector because the hydrogen flow is cut off by armature movement.
3.3.
Effects of pulse width and injection pressure
Given that flow in the injector nonlinearly varies at a pulse width less than 2 ms [11], the pressure fluctuations in the pipe upstream the injector varies with injection pressure under different pulse widths (longer than 2 ms; Fig. 7). It can be seen that the rates of pressure decay under different pulse widths are nearly the same, and a long pulse width affects pressure fluctuations only for a short period. Fig. 8 shows the pressure fluctuations in the pipe upstream the injector at a pulse width of 10 ms and an injection pressure
Fig. 6 e Pressure fluctuations under different pulse widths at 0.45 MPa.
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Fig. 7 e Pressure decay with time under different pulse widths.
of 0.2e0.45 MPa. At higher injection pressures, greater pressure fluctuations and longer pressure times occur and remain stable. This result is attributed to the rapid increase in the injection volume of hydrogen with increasing injection pressure. However, Dp/p0 decreases slowly with the pressure increases, as seen in Fig. 9, where Dp is the absolute fluctuation magnitude and p0 is the baseline pressure. It seems that higher pressure may be better for accurate injection of hydrogen. However, longer time needed for the pressure recovering stably with higher pressure, which has a lasting effect on hydrogen pressure. Thus, at a sufficient hydrogen
flow, low injection pressure is better because it aids the control of injection accuracy.
3.4.
Effects of hydrogen rail diameter
Placing a rail before the injectors of PFI hydrogen internal combustion engines stabilizes the injection pressure of hydrogen, thereby improving engine performance. The diameter of the hydrogen rail is the most important factor that affects pressure fluctuations because the length of the rail cannot be easily changed given the fixed distances between
Fig. 8 e Pressure changes with time under different supply pressures.
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Fig. 9 e Variation of pressure fluctuation with hydrogen pressure.
intake ports. To reduce the costs incurred in designing a hydrogen rail, we simulate the rail design for a 2.0 L PFI hydrogen internal combustion engine. The model is established on the basis of the hydrogen supply system of the engine. The maximum speed of the engine is 5500 r/min and the maximum power is 60 kW, corresponding to a maximum hydrogen flow of 6 kg/h. Two hydrogen injectors are used per cylinder to supply hydrogen, and the pressure sensor is attached in the middle of the rail. This sensor is used to monitor pressure fluctuations. Fig. 10 shows that the variations of pressure fluctuations with rail diameter. Pressure fluctuations rapidly decrease when
diameter increases from 5 to 30 mm, and then remains constant at a diameter of 60 mm. In particular, when the diameter of the rail is larger than 50 mm, the variation of pressure fluctuations is smaller than 2%, indicating good rail performance. Therefore, the suitable hydrogen rail diameter is 50 mm. We fabricate a hydrogen rail and install it in the hydrogen internal combustion engine. The comparison between the optimal rail and the pre-rail for hydrogen pressure fluctuations in the pipe where the pressure sensor is installed is shown in Fig. 11. The optimal rail is also shown in Fig. 11. These fluctuations occur at an engine speed 5500 r/min, an injection pressure of 0.44 MPa,
Fig. 10 e Variations of pressure fluctuations with hydrogen rail diameter.
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Fig. 11 e Changes in hydrogen pressure with crank angle.
and an injection pulse width of 6.8 ms. The variation of pressure fluctuations are within 1.34% with the optimal rail, better than the pre-rail (2.72%), confirming the favorable effects of the hydrogen rail on decreasing pressure fluctuations.
4.
Conclusion
1. A test bench is established to investigate the pressure fluctuations in hydrogen supply system. The time required from the injector to be fully opened decreases from 1.33 to 0.45 ms when supply pressure increases from 0.2 to 0.45 MPa. 2. Pressure fluctuation rapidly increases with increasing supply pressure, as well as with increasing pressure fluctuation amplitude and pressure decay time. Injection pulse width minimally affects fluctuation. 3. A low supply pressure exhibits more effectively decreases on pressure fluctuation at a sufficient hydrogen flow. 4. The variation of pressure fluctuation is within 1.34% at a hydrogen rail diameter greater than 50 mm, indicating that hydrogen rails can be effectively designed by simulation.
Acknowledgements This study is supported by National Natural Science Foundation of China (51276019).
references
[1] Mao ZQ. Moving towards hydrogen energy. Science & Technology Review 2007;23(2):34e8. [2] Verhelst S, Sierens R, Verstraeten S. A critical review of experimental research on hydrogen fueled SI engines. SAE Paper No. 2006-01-0430 2006.
[3] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Progress in Energy and Combustion Science 2009;35:490e527. [4] Delorme A, Rousseau A, Sharer P, Pagerit S, Wallner T. Evolution of hydrogen fueled vehicles compared to conventional vehicles from 2010 to 2045. SAE Paper No. 2009-01-1008 2009. [5] Kiesgen G, Klu¨ting M, Bock C. The new 12-cylinder hydrogen engine in the 7 series: the H2 ICE age has begun. SAE Paper No. 2006-01-0431 2006. [6] Szwabowski SJ, Hashemi S, Stockhausen WF, Natkin RJ, Reams L, Kabat DM, et al. Ford hydrogen engine powered P2000 vehicle. SAE Paper No. 2002-01-0243 2002. [7] Yang ZZ. Study on combustion and optimum control of hydrogen fueled engine. PhD thesis. Zhejiang University; 2001. [8] Wang X, Li K, Su WH. Experimental and numerical investigations on internal flow characteristics of diesel nozzle under real fuel injection conditions. Experimental Thermal and Fluid Science 2012;42:204e11. [9] Li L, Peng ZG, Wang XS. Analysis and research on flow-rate characteristics parameter of air tube (1). Chinese Hydraulics & Pneumatics 2004;04:5e7. [10] Mattei N, Schiavetti M, Carcassi MN. Experimental studies on wind influence on hydrogen release from low pressure pipes. International Journal of Hydrogen Energy 2012;36:2414e25. [11] Sun BG, Zhang DS, Liu FS. Investigation of the characteristics of hydrogen injector using experiment and simulation in hydrogen internal combustion engine. International Journal of Hydrogen Energy 2012;37:13118e24. [12] Sun BG, Zhang DS, Liu FS. A manufacturing method of rail for hydrogen internal combustion engines. CHN patent No. 201110403863.X. 2012 Apr 11. [13] Sun BG, Zhang DS, Liu FS. A pressure controlling rail for hydrogen internal combustion engines. CHN patent No. 201110403927.6. 2012 Jun 13. [14] Sun BG, Zhang DS, Liu FS. Equipment for calibrating the flow characteristics of hydrogen injectors. CHN patent No. 200910237444.6. 2010 May 12. [15] Sun BG, Zhang DS, Liu FS. A new equation of state for hydrogen gas. International Journal of Hydrogen Energy 2012;37:932e5. [16] Wang BG, Liu SY, Huang WG. Dynamics of gas flow. Beijing: Beijing Institute of Technology Press; 2005.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Pressure fluctuations in a hydrogen supply system during hydrogen injection Bai-gang Sun, Dong-sheng Zhang, Fu-shui Liu* School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
article info
abstract
Article history:
A test bench and simulation model are established to investigate the pressure fluctuations
Received 5 November 2012
in the port fuel-injected hydrogen system of an internal combustion engine. Analyses are
Received in revised form
conducted, with injection pressure varied from 0.2 to 0.45 MPa and injection pulse width
21 February 2013
varied from 2 to 10 ms. The test and simulation results show that more severe fluctuations
Accepted 24 February 2013
occur, fluctuation ranges are greater, and decay times are longer under higher injection
Available online 4 April 2013
pressures. Pressure decreases at the curve of pressure at the time during which the injector is opened. This time can be calculated by simulation. t0 decreases from 1.33 to 0.45 ms at an
Keywords:
increase in injection pressure of 0.2e0.45 MPa. Therefore, the hydrogen injection time of
Hydrogen
the internal combustion engine is accurately controlled by considering t0 under different
Hydrogen internal combustion en-
conditions. The results serve as guidance in the design of hydrogen rails in hydrogen in-
gine
ternal combustion engine.
Hydrogen supply system
Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All
Pressure fluctuation
1.
Introduction
Hydrogen has a faster combustion speed, lower ignition energy, shorter quenching distance, and cleaner exhaust products than does liquid fuel [1,2]. It is also suitable for use in internal combustion engines because it is more economical than hydrogen fuel cells [3,4]. Port fuel-injected (PFI) hydrogen internal combustion engines have been considered effective equipment because they present advantages such as uniform mixtures, easy organization of combustion, and development based on substantial insights derived from the traditional internal combustion engine industry [5e7]. Hydrogen has an extensive flammability range, and its mixture can be easily ignited within a hydrogeneair mixture volume ratio of 4%e75%. The torque of hydrogen internal combustion engines can therefore be controlled by adjusting the throttle position in light loads, in which mixture concentration is kept constant. When an increase in torque is
rights reserved.
required, the throttle position is fully initiated and mixture concentration increases through the injection of more hydrogen. Given that hydrogen has a low volumetric energy content, the injection volume of hydrogen in cylinders should be considerably higher than those of gasoline and diesel. Especially in diesel, the fluctuations of pressure upstream the nozzle holes are unavoidable because of the fast opening and closing of the needle valve during very short injection intervals and the propagation of pressure wave through narrow fuel passages [8]. Therefore, such a high volume of hydrogen will cause greater pressure fluctuations in pipes, especially under high speeds and high loads. The effects of hydrogen on the flow characteristics of pipe structures and the pipe pressure fluctuations caused by hydrogen leakage have been investigated by fewer researchers [9,10]. However, the hydrogen injection-induced pressure fluctuations in pipes remain unexplained. The flow characteristics of hydrogen injectors were studied by Sun [11],
* Corresponding author. Tel.: þ86 15011357196; fax: þ86 010 68945860. E-mail address: [email protected] (F.-s. Liu). 0360-3199/$ e see front matter Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
who found that pressure fluctuates upstream and downstream a pipe. The area where hydrogen injection pressure abruptly decreases is easily determined when the injector is switched on. The time required for the injector to be opened was calculated. To decrease the pressure fluctuations in the hydrogen rails of a PFI hydrogen internal combustion engine, Sun also investigated the design methods and configurations of hydrogen rails [12,13]. In the current study, numerous tests and calculations are carried out to examine the pressure fluctuations in the pipes of PFI hydrogen internal combustion engines.
computer (13). Hydrogen is injected into the container after the injector receives the opening signal, and the current and pressure signals are recorded by the analyzer. After the pressure signals are recorded, hydrogen is released through the gas release valve (9), and water is discharged by the water release valve (12) when too much water is in the water channel (11). The above-mentioned steps are repeated; the pressure fluctuations in the pipe and in the hydrogen container located before and after the injector can be easily determined by this test bench.
2.2.
2.
Materials and methods
2.1.
Test equipment and steps
The layout of the test bench [14] is shown in Fig. 1. The pressure in the hydrogen bottle (1) is 12 MPa, which can be reduced by releasing the valve (2), thereby satisfying the working pressure required for injectors. A hydrogen rail (3) is set up before the injector (8) to maintain stable pressure. The injector operates when an opening signal is received, and hydrogen is injected into the calibration container (10) that measures hydrogen volume. Two Kistler transient-pressure sensors (4) are set up before and after the injector, and the pressures in the pipes located before the injector and in the container during injection are recorded using a Dewetron 5000 analyzer (6). First, the target injection pressure is set at 0.2e0.45 MPa at an interval of 0.05 MPa on the basis of the maximum working pressure of the injector. Under each pressure, the injection times and pulse width of the injector are set by using a
11157
Simulation model
To investigate the pressure fluctuations in the pipe of a PFI hydrogen internal combustion engine, a simulation model is established on the basis of the hydrogen state, continuity, and momentum equations. The hydrogen state equation, proposed by Sun [15], features high accuracy and is expressed as: PR ¼
TR 27=64 pffiffiffiffiffiffi ZC ðVR þ 0:13636Þ1=8 TR Z2C ðVR þ 0:13636Þ2
(1)
where PR is the ratio of pressure and critical pressure, TR denotes the ratio of absolute temperature and critical temperature, VR represents the ratio of volume and critical volume, and ZC is a constant that represents the compression factor of the critical isotherm of hydrogen. The continuity and momentum equations are as follows [16]: vr vr vV þV þr ¼0 vt vx vx
Fig. 1 e Layout of the test bench. (1. Hydrogen bottle; 2. release valve; 3. hydrogen rail; 4. transient pressure sensor; 5. electric control equipment; 6. data monitor and recorder system; 7. current caliper; 8. injector; 9. gas release valve; 10. hydrogen container; 11. water channel; 12. water release valve; and 13. computer.)
(2)
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r
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
vV vV vP þ rV þ ¼0 vt vx vx
(3)
where V is the flow speed of gas, P is the pressure, and r is the density of gas. The flow rates at the internal nodes of the injector are calculated using the following equation [16]: vdm vP r$ff $A$y2 $signðyÞ ¼ A$ vt vx 2$D
(4)
where y is the gas velocity, A denotes the cross-sectional area of the pipe, DP is the pressure drop, D represents the pipe diameter, r is the density, and ff is the friction factor. The process of hydrogen flow from the injector is assumed as orifice flow, and the mass flow equation is [16]: Pu _ ¼ A$Cq $Cm pffiffiffiffiffi ffi m Tu
(5)
where A is the area of the orifice, Cq is the flow coefficient calculated by Cq ¼ 0.72 þ 0.12 cos (p$Pd/Pu), Pu is the pressure of the orifice upstream, Pd denotes the pressure of the orifice downstream, Tu represents the temperature of the orifice downstream, and Cm is the parameter related to gas flow, whose value is constant when flow speed reaches the local velocity of sound. The flow speed can be calculated by Equation (6) [16], when this speed is slower than the local velocity of sound. ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffisffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g2 gþ1 2 Pd Pd g Cm ¼ ðg 1Þ Pu Pu
3.
Results and discussion
3.1.
Model testing and verification
The pressure fluctuations in the pipe upstream the injector and in the hydrogen container downstream the injector are shown in Fig. 2, which also presents the drive current. The hydrogen supply pressure is 0.4 MPa, and the injection pulse width is 8 ms. Pu rapidly decreases when the injector is switched on, and Pd increases during hydrogen injection. Given that liquid level gradually increases, the pressure in the hydrogen container also slowly rises. As the ratio of Pd and Pu is smaller than 0.527, the orifice of the injector is choked, which means that the pressure in the hydrogen container does not affect Pu. Additionally, the fluctuation of Pu mainly affected by the opening process of the injector, the lengths of the pipes, the damp though the pipes and the spread speed of pressure. Especially, the wave frequency of the pressure mainly depends on the length of the pipes and the spread speed of pressure. To verify the accuracy of the model, the simulation result of the pressure in the pipe upstream the injector has been compared with the test in Fig. 3. The simulation result is similar to the test result at an injection pressure of 0.45 MPa and an injection pulse width of 8 ms. The maximum error of the simulation result is within 0.3%, indicating that the model can be used to describe actual hydrogen injection systems.
3.2. (6)
Effects of opening and closing processes
The accurately measuring the movement of the armature is difficult to accomplish because of the limitations posed by the
Fig. 2 e Changes in Pu, Pd, and drive current over time.
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Fig. 3 e Comparison of simulation and test results.
test conditions. Therefore, a simulation is performed to determine the effect of armature movement on pressure fluctuations. The armature can be seen as a mass block driven by several force, such as the electromagnetic force, the frictional force and the force generated from hydrogen pressure. Additionally, the force generated from hydrogen pressure can be calculated by the area of the armature and the hydrogen pressure. Fig. 4 shows that the opening area of the injector changes with armature movement; the pressure fluctuation at the orifice of the injector and the pressure sensor position are 0.45 MPa and 10 ms, respectively. The
pressure at the orifice of the injector decreases with the opening of the injector, after which the pressure at the pressure sensor position also decreases. The maximum amplitudes of the pressure fluctuations at the sensor position decrease and the time is delayed for the variations in the section area of the pipes. Then, the time needed for the injector to fully switch on can be determined by simulation. This time is defined as t0. Fig. 5 shows that t0 decreases with increasing injection pressure possibly because the force of hydrogen on the armature facilitates the opening process of the injector.
Fig. 4 e Changes in opening area, pressure of the injector input, and pressure sensor over time.
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Fig. 5 e Changes in t0 with supply pressure.
Thus, higher pressure results in shorter t0. However, the injection pulse width has little effect on t0 at a constant drive voltage. Fig. 6 indicates that at 0.45 MPa, the pressure fluctuations in the pipe upstream the injector at different pulse widths are nearly the same with each other before the closing process of the injector, which means that the pulse width has little effect on the opening process of the injector. However, hydrogen pressure will rapidly increase during the closing process of the injector because the hydrogen flow is cut off by armature movement.
3.3.
Effects of pulse width and injection pressure
Given that flow in the injector nonlinearly varies at a pulse width less than 2 ms [11], the pressure fluctuations in the pipe upstream the injector varies with injection pressure under different pulse widths (longer than 2 ms; Fig. 7). It can be seen that the rates of pressure decay under different pulse widths are nearly the same, and a long pulse width affects pressure fluctuations only for a short period. Fig. 8 shows the pressure fluctuations in the pipe upstream the injector at a pulse width of 10 ms and an injection pressure
Fig. 6 e Pressure fluctuations under different pulse widths at 0.45 MPa.
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Fig. 7 e Pressure decay with time under different pulse widths.
of 0.2e0.45 MPa. At higher injection pressures, greater pressure fluctuations and longer pressure times occur and remain stable. This result is attributed to the rapid increase in the injection volume of hydrogen with increasing injection pressure. However, Dp/p0 decreases slowly with the pressure increases, as seen in Fig. 9, where Dp is the absolute fluctuation magnitude and p0 is the baseline pressure. It seems that higher pressure may be better for accurate injection of hydrogen. However, longer time needed for the pressure recovering stably with higher pressure, which has a lasting effect on hydrogen pressure. Thus, at a sufficient hydrogen
flow, low injection pressure is better because it aids the control of injection accuracy.
3.4.
Effects of hydrogen rail diameter
Placing a rail before the injectors of PFI hydrogen internal combustion engines stabilizes the injection pressure of hydrogen, thereby improving engine performance. The diameter of the hydrogen rail is the most important factor that affects pressure fluctuations because the length of the rail cannot be easily changed given the fixed distances between
Fig. 8 e Pressure changes with time under different supply pressures.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
Fig. 9 e Variation of pressure fluctuation with hydrogen pressure.
intake ports. To reduce the costs incurred in designing a hydrogen rail, we simulate the rail design for a 2.0 L PFI hydrogen internal combustion engine. The model is established on the basis of the hydrogen supply system of the engine. The maximum speed of the engine is 5500 r/min and the maximum power is 60 kW, corresponding to a maximum hydrogen flow of 6 kg/h. Two hydrogen injectors are used per cylinder to supply hydrogen, and the pressure sensor is attached in the middle of the rail. This sensor is used to monitor pressure fluctuations. Fig. 10 shows that the variations of pressure fluctuations with rail diameter. Pressure fluctuations rapidly decrease when
diameter increases from 5 to 30 mm, and then remains constant at a diameter of 60 mm. In particular, when the diameter of the rail is larger than 50 mm, the variation of pressure fluctuations is smaller than 2%, indicating good rail performance. Therefore, the suitable hydrogen rail diameter is 50 mm. We fabricate a hydrogen rail and install it in the hydrogen internal combustion engine. The comparison between the optimal rail and the pre-rail for hydrogen pressure fluctuations in the pipe where the pressure sensor is installed is shown in Fig. 11. The optimal rail is also shown in Fig. 11. These fluctuations occur at an engine speed 5500 r/min, an injection pressure of 0.44 MPa,
Fig. 10 e Variations of pressure fluctuations with hydrogen rail diameter.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 1 5 6 e1 1 1 6 3
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Fig. 11 e Changes in hydrogen pressure with crank angle.
and an injection pulse width of 6.8 ms. The variation of pressure fluctuations are within 1.34% with the optimal rail, better than the pre-rail (2.72%), confirming the favorable effects of the hydrogen rail on decreasing pressure fluctuations.
4.
Conclusion
1. A test bench is established to investigate the pressure fluctuations in hydrogen supply system. The time required from the injector to be fully opened decreases from 1.33 to 0.45 ms when supply pressure increases from 0.2 to 0.45 MPa. 2. Pressure fluctuation rapidly increases with increasing supply pressure, as well as with increasing pressure fluctuation amplitude and pressure decay time. Injection pulse width minimally affects fluctuation. 3. A low supply pressure exhibits more effectively decreases on pressure fluctuation at a sufficient hydrogen flow. 4. The variation of pressure fluctuation is within 1.34% at a hydrogen rail diameter greater than 50 mm, indicating that hydrogen rails can be effectively designed by simulation.
Acknowledgements This study is supported by National Natural Science Foundation of China (51276019).
references
[1] Mao ZQ. Moving towards hydrogen energy. Science & Technology Review 2007;23(2):34e8. [2] Verhelst S, Sierens R, Verstraeten S. A critical review of experimental research on hydrogen fueled SI engines. SAE Paper No. 2006-01-0430 2006.
[3] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Progress in Energy and Combustion Science 2009;35:490e527. [4] Delorme A, Rousseau A, Sharer P, Pagerit S, Wallner T. Evolution of hydrogen fueled vehicles compared to conventional vehicles from 2010 to 2045. SAE Paper No. 2009-01-1008 2009. [5] Kiesgen G, Klu¨ting M, Bock C. The new 12-cylinder hydrogen engine in the 7 series: the H2 ICE age has begun. SAE Paper No. 2006-01-0431 2006. [6] Szwabowski SJ, Hashemi S, Stockhausen WF, Natkin RJ, Reams L, Kabat DM, et al. Ford hydrogen engine powered P2000 vehicle. SAE Paper No. 2002-01-0243 2002. [7] Yang ZZ. Study on combustion and optimum control of hydrogen fueled engine. PhD thesis. Zhejiang University; 2001. [8] Wang X, Li K, Su WH. Experimental and numerical investigations on internal flow characteristics of diesel nozzle under real fuel injection conditions. Experimental Thermal and Fluid Science 2012;42:204e11. [9] Li L, Peng ZG, Wang XS. Analysis and research on flow-rate characteristics parameter of air tube (1). Chinese Hydraulics & Pneumatics 2004;04:5e7. [10] Mattei N, Schiavetti M, Carcassi MN. Experimental studies on wind influence on hydrogen release from low pressure pipes. International Journal of Hydrogen Energy 2012;36:2414e25. [11] Sun BG, Zhang DS, Liu FS. Investigation of the characteristics of hydrogen injector using experiment and simulation in hydrogen internal combustion engine. International Journal of Hydrogen Energy 2012;37:13118e24. [12] Sun BG, Zhang DS, Liu FS. A manufacturing method of rail for hydrogen internal combustion engines. CHN patent No. 201110403863.X. 2012 Apr 11. [13] Sun BG, Zhang DS, Liu FS. A pressure controlling rail for hydrogen internal combustion engines. CHN patent No. 201110403927.6. 2012 Jun 13. [14] Sun BG, Zhang DS, Liu FS. Equipment for calibrating the flow characteristics of hydrogen injectors. CHN patent No. 200910237444.6. 2010 May 12. [15] Sun BG, Zhang DS, Liu FS. A new equation of state for hydrogen gas. International Journal of Hydrogen Energy 2012;37:932e5. [16] Wang BG, Liu SY, Huang WG. Dynamics of gas flow. Beijing: Beijing Institute of Technology Press; 2005.