Matching analysis on flow limiting valve of marine diesel engine common rail system

The flow limiting valve (FLV) designed for the high pressure common rail fuel system (CRS) was designed, the simulation model of the FLV and electronic controlled injector is established and the injector’s model was verified by experiment data. Through simulation, the maximum cycle injection quantity (CIQ) and close flow quantity (CFQ) of the CRS matched with the FLV are verified, and the influence of key FLV’s parameters on the matching performance was analysed. The maximum CIQ is 722mm3 and the CFQ is 1194.2mm3, which can satisfy the technical requirements of the CRS under overload conditions. The closing lift and orifice diameter of the FLV has a significant impact on the CFQ, which should be carefully designed according to the technical requirements, and the dimensional tolerance and processing accuracy should be guaranteed in the production and assembly process to ensure the consistency of the CFQ.


Introduction
High pressure common rail fuel system has been considered to be one of the key technologies to meet the current marine diesel engine emission regulations for it can flexibly adjust injection time, pressure, frequency and duration, improve spray quality and optimize combustion process significantly [1][2][3] .At present, CRS has been widely used in modern marine diesel engine to improve combustion efficiency, reduce harmful gas emissions and optimize overall performance [4,5] .It is necessary to configure flow limiting valves on the common rail pipe to realize the over flowing protection of high pressure circuit of the CRS, so as to meet the extremely high requirements of working stability and reliability of the CRS for marine generator set diesel engine [6][7][8] .
Meng J.M., Zhang J.Q. and An S.J. conducted simulation analysis of the FLV respectively and studied the influence of structural parameters on its performance [9,10] .Wang M. studied the match and calculation of the FLV and injector by using simplified physical model and calculation process [2] .Chen Y.X.analysed the influence of the orifice and spring stiffness on the close flow of the FLV through test [11]   .Jin J.S. realized the accurate diagnosis and recognition of 4 fault characteristics under 3 failure modes of the FLV through simulation and experiment [12] .However, when analysing the FLV's performance, the above studies mostly adopt simplified injector model or fail to consider the influence of injection process, which lead to the shortcomings in guiding the specific design and matching of the FLV.
In this paper, the matching analysis of the FLV to CRS of a 12V marine generator set diesel engine was carried out.A simulation model was established based on the parameters of FLV and the electronic controlled injector.Based on the simulation, the matching performance of the FLV to the CRS was verified, and the effects of the FLV's parameters on performance such as action response, close flow and injection rate were analysed.The research work of this paper can provide guidance for structural design, Overload cycle injection quantity 710mm 3 Close flow quantity of the FLV 1200mm 3 The structure of the FLV designed for the CRS is shown in Figure .2, it is composed of valve body, valve core, spring and stopper.The main parameters are shown in Table 2.The valve core can reset to its initial position if it hasn't closed the FLV's outlet by the end of injection, then the FLV will not affect the injection in that case.However, when there is a fault of excessive flow in the high pressure circuit, the valve core will move continually in the direction of the outlet until the outlet is closed.After that, the valve core will remain closed and cut off the fuel flow from the common rail pipe to the FLV's outlet, achieve over flowing protection and ensure the CRS continues to operate normally.

Simulation model of the FLV and injector
The Liebherr LI2 injector is shown in Figure.

Maximum cycle injection quantity (CIQ) and close flow quantity (CFQ)
The designed diesel engine requires the CRS can ensure no less than 110% of the maximum CIQ under overload conditions, that is, no less than 710mm 3 should be injected when the engine is overload operated at 200MPa pressure.).The injector will continue injecting fuel after the FLV has closed for there still has high pressure fuel between the FLV and the injector by this time, and injection will be stopped until the fuel pressure drops close to the cylinder pressure.Under this operating condition, the maximum fuel injection quantity of the injector, that is, the CFQ is 1194.2mm 3 , which can meet the requirements of the 1200mm 3 CFQ of the FLV.

Effect of closing lift
The VCL and IR curves are shown in Figure .8when closing lift increases form 6.5mm to 10.5mm by a step size of 1.0mm with injector continually opened.The curves show that the time required for the valve core to reach the closing position is longer with the increase of the closing lift.Which will delay the injection ending time, increase injection duration and resulting in an increase in CFQ.
The CFQ under the conditions of 6.5, 7.5, 8.5, 9.5 and 10.5mm closing lift are 997.0,1096.2, 1194.2, 1291.6 and 1387.9mm 3 , respectively, with a variation range of 7.0-10.0%with variation step.The CFQ is closest to the technical requirement when the closing lift is 8.5mm especially.Therefore, the closing lift has a significant effect on the CFQ, and which should be matched according to the requirement in design.And the closing lift also should be strictly controlled in production and assembly in order to ensure the consistency of the CFQ of FLV.The Figure.9 shows the VCL and IR under different valve core lift conditions with 3.56ms injection duration.The valve core can reach the maximum lift after the end of fuel injection and can be reset smoothly without affecting IR when the closing lift ranges from 7.5 to 10.5mm.However, the valve core will reach its closing lift before the end of the fuel injection when the closing lift is 6.5mm.And at this condition, the fuel injection quantity is 703.7mm 3 and the CFQ is 992.5mm 3 , which can't satisfy the technical requirements for CIQ and CFQ under the overload conditions.It means that under the premise of meeting the CFQ requirements, the change of closing lift has no significant effect on the fuel injection within the maximum injection duration range (32°CA), and the FLV can meet the technical requirements for CIQ of overload conditions.

Effect of orifice diameter
When the orifice diameter is 0.8, 1.0 and 1.2mm, the VCL and IR curves with the injector kept opened are shown in Figure .10.The change of the orifice diameter has no obvious effect on the IR.However, the throttling effect is weakened with the increase of the orifice diameter, which will reduce the pressure difference of both sides of the orifice and decrease the lift of the valve core.Therefore, the rising rate of the valve lift curve decreases with the increase of the orifice diameter, and the time to reach the closing lift is delayed, which also leads to the falling section of the IR curve moves backward and increases the injection duration.
Figure .11shows the VCL and IR curves with different orifice diameters at 3.56ms injection duration.The variation of the orifice diameter has little effect on the IR and will not change the overload CIQ.The CFQ is 1136.0,1194.2 and 1270.6mm 3 when the orifice diameter is 0.8, 1.0, 1.2mm, respectively, and the variation range is about 5% with variation step.
It means that the orifice diameter has obvious effect on the CFQ, and it should be matched carefully according technical requirements.At the same time, the processing quality of the orifice should be strictly controlled to ensure the consistency of the flow.

Effect of spring stiffness
When the spring stiffness is 1.0, 1.5 and 2.0N/mm, the VCL and IR curves with the injector continually opened are shown in Figure .12.The increase of spring preload of the FLV will enlarge the resistance of the valve core to lift up, slow down the rise rate of the valve core.Therefore, with the increase of spring stiffness, the rising rate of the VCL decreases and the time to reach the closing lift is delayed.While the IR is not affected by the variation of spring stiffness, it's falling section merely moves backward a little after the FLV has been closed.

Conclusion
(1) A simulation model was established based on the structural parameters of the FLV and injector, and the accuracy of the model was verified by the test results of the injector.
(2) The simulation results show that the maximum CIQ is 722mm 3 and the CFQ is 1194.2mm 3 under overload conditions, which can satisfy the technical requirements of the marine generator set diesel engines for the high pressure common rail fuel system.
(3) The variation of closing lift and orifice diameter has a significant effect on the performance of the FLV.The change of CFQ caused by the variation of closing lift with 1.0mm step size is 7-10%, while caused by orifice diameter with 0.2mm step size is about 5%.The two parameters should be matched carefully, and the tolerance of closing lift and the processing quality of orifice should be strictly controlled during production and assembly as well to ensure the consistency of the CFQ.(4) Within a limited range of change, the variation of spring stiffness and the clearance between the valve core and valve body has no obvious effect on the performance of the FLV.

Figure 1 .
Figure 1.Structure of the common rail pipe assembly.

3 .
The simulation model, as shown in Figure.4,was established based on the parameters of the FLV and LI2 injector.The accuracy of the injector model has been verified by the experiment data of the LI2 injector, as shown in Figure.5.Comparison curves show that the injector model can accurately predict fuel injection quantity under rated working pressure of 220MPa with high calculation accuracy.The rated speed of 1500r/min and the rated injection pressure of 220MPa are used in the following analysis if there is no special description.

Figure 5 .
Figure 5.Comparison curves of fuel injection quantity at 220MPa rated pressure.
Figure.6 shows the curves of valve core lift (VCL) and injection rate (IR) with 3.56ms (32°CA-crank angle) injection duration.Under this operating condition, the maximum VCL is 7.48mm, which has not yet reached the closing lift (8.5mm), and the CIQ is 722mm 3 , which can meet the requirements of no less than 710mm 3 as well.

Figure 7 .
Figure 7. VCL, IR and injection quantity when injector continually opened.The Figure.7 shows the VCL, IR and injection quantity curves with the injector continually opened.It shows that the VCL increases gradually with the injector's continuous injection until it reaches the closing lift(8.5mm).The injector will continue injecting fuel after the FLV has closed for there still has high pressure fuel between the FLV and the injector by this time, and injection will be stopped until the

Figure 8 .
Figure 8. VCL and IR with different closing lifts when injector continually opened.

Figure 9 .
Figure 9. VCL and IR with different closing lifts at 3.56ms injection duration.

Figure 10 .
Figure 10.VCL and IR with different orifice diameters when injector continually opened.

Figure 11 .
Figure 11.VCL and IR with different orifice diameters at 3.56ms injection duration.

Figure 12 .
Figure 12.VCL and IR with different spring stiffness when injector continually opened.3.6Effect of clearance VCL and IR curves are shown in Figure.13 with the clearance between the valve core and valve body is 0.010, 0.015, 0.020mm respectively when the injector is kept open.The curves show that within a limited range of variation, clearance has no obvious effect on VCL and IR.

Figure 13 .
Figure 13.VCL and IR with different clearance when injector continually opened.

Table 1 .
Main parameters of the diesel engine and the CRS.

Table 2 .
Main parameters of the FLV.