Research of Flow Field Simulation for Lubrication System and Effect Evaluation on a 7-Speed Dual Clutch Transmission Yinhui Lin, Zhihua Hu, Chaoqiang Xiong, Mengyan Zang, Yuan Jia, Yong Chen, Daguo Luo and Fuquan Zhao
Abstract The evaluation of lubricating effect of the transmission lubrication system is an important part in the transmission design. The Ansys-Fluent software is used in this paper. Firstly, the flow field of the single oil orbit of 7DCT transmission is simulated and the mass flow rate of each nozzle is taken as the input condition of the lubricating simulation and analysis in the whole transmission model. Then the whole transmission model is established to facilitate evaluating the lubricating effect of the oil catcher by setting the monitor surfaces at the positions of the oil catcher. The complex oil and gas phase flow field of transmission is simulated and analyzed so that the lubrication effects of three lubricating method are evaluated to achieve the purpose of evaluating the performances of the transmission lubrication system, including the forcing ejective oil lubrication, splash lubrication and oil passage lubrication. Keywords 7DCT
Transmission Lubrication CFD Effect evaluation
F2012-E03-033 Y. Lin (&) C. Xiong M. Zang South China University of Technology, Guangzhou, China e-mail:
[email protected] Z. Hu Y. Jia Y. Chen D. Luo F. Zhao Zhejiang Geely Automobile Research Institute Co Ltd, Hangzhou, China
SAE-China and FISITA (eds.), Proceedings of the FISITA 2012 World Automotive Congress, Lecture Notes in Electrical Engineering 195, DOI: 10.1007/978-3-642-33835-9_27, Ó Springer-Verlag Berlin Heidelberg 2013
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1 Introduction Effective transmission lubrication cannot only reduce the abrasion of contact surface of moving parts (shaft gears, gears, bearings, synchronizers, etc.), but also take away the impurities and heat of friction contact surface and act on cleaning and cooling, and also serve as the seal, antirust, damping and buffer. Thus it can ensure the normal operation of transmission and increase the service life of transmission. In the transmission lubrication system, the key problem is the lubrication of the gear. Due to the limitations of computing resources and other factors, the geardriven flow field simulation presently focuses on the internal flow field of twodimensional gear pump, such as Kris Riemslagh and etc., have calculated the flow characteristics of internal fluid of the rotary displacement pumps (cam pump and gear pump) by Lagrangian–Eulerian finite volume method [1]; John Vande Voorde etc., have calculated the internal flow field of rotary volumetric pump by the Fluent software [2]; Jiang Fan etc., have done the dynamic simulation of the internal flow field of the gear pump for diesel engine lubrication by the moving mesh method [3]. Considering aspects of transmission lubrication, Lemfeld etc., have done the related analysis of the two-phase flow model for the non-toothed gear under the conditions of different tilt angles and oil temperature [4]; Dong Chunfeng etc., have used the technique of dynamic mesh to carry on the flow field simulation for a pair of straight gears in gearbox [5]. Since the model of transmission lubrication system is a complex two-phase flow model of multi-pair driving bevel gears, its complexity is high and no analysis of such case has been presented in public at present. 7 Dual Clutch Transmission (7DCT) is a dual clutch automatic transmission including seven forward gears, dual input shafts and dual output shafts. Because of special layout structure for 7DCT transmission, the designers have used three lubrication methods including the oil bath lubrication, splash lubrication and pressure lubrication to achieve the purpose of lubrication for all moving contact parts of the transmission, and it can be expressed particularly as three ways including the forcing ejective oil lubrication, splash lubrication and oil passage lubrication, which have a direct impact on the performance of the transmission. Therefore the evaluation of the lubricating effect is an important part in the transmission design. The traditional evaluation method of lubrication is that the housings of major observation parts are wiped off and re-sealed using transparent materials and the experimental prototype is made to observe the lubricating effect of the key parts by running the bed experiment. Although the evaluation method of experimental prototype is intuitive and of high reliability, only the several local observation parts can be observed and the global understanding of the operating conditions of the lubrication system cannot be known, and this method can only be used in the late period of the transmission development. Thus it has an important engineering significance that one simulation analysis method is used to evaluate the lubricating
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effect of the transmission lubrication system at the time of the transmission design stage. The Ansys-Fluent software is used in this paper. Firstly, the flow field of the single oil orbit of 7DCT is simulated and the simulated result of the mass flow rate of each nozzle is taken as the input condition of the lubricating simulation and analysis in the whole transmission model, which is based on simulation confirmation and experimental results are basically the same. Then the whole transmission model is established to facilitate evaluating the lubricating effects of the concerned points by setting the monitor surfaces at the observing positions. Finally, the working condition of the first speed is simulated with the whole transmission model as the research object and the lubricant effects of three lubricating methods of 7DCT transmission have been studied particularly, including forcing lubrication, splash lubrication and oil passage lubrication.
2 CFD Theory Basis of a Transmission Lubrication System Compared with other automatic transmission oil, dual clutch automatic transmission oil (following lubricant oil for short) has better lubricating properties, which is used by 7DCT [6]. In the lubrication analysis, lubricating fluid is deemed usually as incompressible fluid, and the thermal expansion coefficient is ignored. Oil property standard is the property at 20 °C. Without considering heat conduction, convection and thermal radiation of the transmission lubrication system, the paper focuses on the problem of flow field calculation of the transmission lubrication system. In other words, energy conversion is ignored in this paper. Considering the disturbance of the transmission lubrication system, k-e turbulence model is used for lubrication analysis [3]. The region of flow field for lubrication system of the transmission will change over with time because of the rotating movement of gears. Therefore the technology of moving mesh is adopted in the simulation analysis.
2.1 Control Equations The continuity equation (mass conservation equation) and the equations of motion (momentum equation) are met firstly, when the liquidity problem of the transmission lubrication system is performed. Secondly, the rotating movement of the gears leads to the inner cyclone movement of the transmission, which makes the strain rate of the fluid higher and the degree of bending of flow lines bigger. The problems can be solved using the RNG k-e turbulence model [7]. The RNG k-e turbulence model is used for describing the fully developed turbulence model that has the high Re number. But the wall function method is used for dealing with the flow near wall region and low Re number flow [5].
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2.2 The Technology of Moving Grid The moving mesh can be used for simulating the flow field which changes shape with time because of boundary movement. The rotational movement of the gear can be achieved by compiling code. The grids change with the rotatable movement when gear is rotating, which leads to worse quality of grids and the more difficult computing convergence of the next step. Therefore the local re-mesh method is adopted by agglomerating the grids of poor quality and updating the moving grids of the mesh agglomerates, thus ensuring the quality of the grids and the success of computing process [5]. When the technology of moving mesh is used, the initial grids of good quality need to be defined and then grids can be automatically updated according to the boundary changes in each iteration step in the calculation process by setting the relevant parameters in order to achieve the purpose of the smooth calculation.
3 CFD Analysis of the Transmission Lubrication System 3.1 Simplification of Whole Model of Transmission Closed flow region should be established for the CFD analysis of the whole transmission model. Figure 1 shows that there are numerous parts in a transmission. It is difficult to form a closed chamber calculation because of complex geometry. So the geometry of transmission should be simplified and then surfaces of parts should be extracted. Finally, the closed flow region can be sutured successfully.
3.1.1 The Oil Orbit Model Figure 2 shows the solid model of oil orbit formed by extracting and suturing the inner surface. The oil orbit model consists of an inlet and seven nozzles. The shape of inlet and nozzle is all circular.
3.1.2 Dealing with the Jogged Gears The power transmission depends on the jogged contact of pairs of gears when the transmission is working. The oil film attaching to surface of gear is used for lubrication and cooling. The flow field of a pair of straight tooth gear can be simulated by increasing distance between two gears and adjusting tooth profile. Due to the limitations of computing resources, it is very difficult to simulate the process of bevel gear pairs in the transmission as shown in Fig. 1. Therefore, models of gear pairs are simplified by cutting tooth. As shown in Fig. 3, the tooth
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Fig. 1 Structure of transmission
Fig. 2 Solid model of oil orbit
Fig. 3 Simplification of jogged gear
of one gear in any pair is removed. In the whole transmission model, the drive gears on the input shaft should be jogged with several pairs of driven gears on the output shaft 1 and output shaft 2. The method of cutting teeth of drive gears above transmission oil surface and retaining teeth of driven gears is used to reduce the
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Fig. 4 Simplification of transmission housing
Fig. 5 Setting monitor surface
influence of splash lubricant effect of the oil liquid, which is caused by cutting teeth.
3.1.3 Simplification of Transmission Housing A closed chamber is formed by extracting and suturing the housing of inner surface after plenty of geometry cleanup of the transmission, as shown in Fig. 4. There are five oil catchers in the housing. Monitor surfaces are set at the position of the oil catchers for directly observing whether the oil enters these oil catchers and are obtaining the mass flow rate of the oil, as shown in Fig. 5. The number of monitor surfaces is the same as that of oil catchers shown in Fig. 4. In the CFD computing, the monitor surface is set to interior boundary condition, thus flow fluid can pass the monitor surface freely so that the effect of collecting oil will not be affected.
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Fig. 6 Model of oil orbit at two-phase flow
Fig. 7 Simulation and experimentation result of mass flow rate of each nozzle
3.2 The CFD Analysis of Transmission Lubrication System 3.2.1 The CFD Analysis of Single Oil Orbit The CFD analysis of the ejective oil condition of single oil orbit has been firstly carried on and compared with the related experiment result to validate the feasibility of the simulation method. The simulation result of mass flow rate of each nozzle of the oil orbit is taken as the input condition of the whole transmission model. Since the working condition of oil orbit is the complex two-phase flow of oil and gas in transmission, a rectangular chamber is built in the lateral of nozzles of oil orbit as shown in Fig. 6. Oil is inside and air is outside the oil orbit in the initial condition.
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Fig. 8 Experiment of oil orbit
Fig. 9 The whole model of 7DCT transmission
Unstructured tetrahedral grids of oil orbit shown in Fig. 6 is meshed using Gambit software. Transient simulation of oil orbit is computed with the VOF model of the Fluent software. The density and the dynamic viscosity of the oil are 840 kg/m3 and 0.0324 Pa s at 20 °C respectively; the corresponding parameters of air are 1.225 kg/m3 and 1.789 9 10-5 Pa s. The design value of the volume flow of the oil orbit inlet is 4L/min. The simulation result of the mass flow rate of each nozzle, as shown in Fig. 7, is similar to the experimentation result shown in Fig. 8, which indicates the validity of analysis result of the mass flow rate of each nozzle.
3.2.2 CFD Analysis of the Whole Model of Transmission The closed chamber of the transmission inner, shown in Fig. 9, is meshed with unstructured tetrahedral grids, and the initial number of grid is 2.6 million. Simulation conditions are as follows: working temperature 20 °C, the direction of
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gravity negative z-axis, the rotational speed of input shaft 1,000 r/min and the working condition first speed. The mass flow rate of each nozzle obtained by the single oil orbit simulation is set as the mass inlet condition and the rotation direction of output shaft 1 is set as Fig. 9, which is corresponding to other working parts. The model of VOF, RNG k-e and the technology of moving mesh are used for simulating and computing [7]. Although the model has been simplified farthest by the method of cutting tooth, the present number of computing grids has been almost 6 million because of the complex of the structure of the research object and the flow field, and the use of the technology of moving mesh, which is a big challenge to the computing resources.
4 The Effect Evaluation of Transmission Lubrication System 4.1 The Forcing Ejective Oil Lubrication An oil orbit spout assembly is used in the forcing ejective oil lubrication, in which the lubricant oil orbit supplies the oil to the inlet through the filter and cooler under a certain pressure and then sprays on the gears, shift synchronizer set and other parts of the output shaft 1 by the nozzle to realize the lubrication and cooling through the oil orbit nozzle. The objective of the forcing lubrication is to confirm whether the oil ejected from the oil orbit can reach the lubricant point and is affected by the splash oil. Figure 10 shows the transmission oil liquid volume distribution observed from the front side of the model when the physical time is 0.0234 s and the ejected oil of each nozzle can be clearly observed. From the figure, the oil thrown by the gears of the output shaft 2 and final drive gear cannot interrupt the oil ejected from the oil orbit nozzle. Figure 11 shows the condition at the same physical time that the oil ejected from the nozzle 2 and nozzle 4 rotates with the gears. It indicates that the ejecting oil which reaches the gears can overcome the centrifugal force generated by the gears rotation and rotates to the jogged position together with the gears. The above result shows that oil ejected by the oil orbit can successfully reach the corresponding gears and rotate with the gears to the jogged place, which can lubricate the jogged gears and is not affected by the splash oil. It proves the rationality of the forcing lubrication system including the oil orbit.
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Fig. 10 Volume distribution contour plot of transmission oil at the front side
Fig. 11 Volume distribution contour plot of transmission oil on the left side
Fig. 12 Splash oil liquid distribution at first speed
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Fig. 13 Oil liquid distribution around the final drive gear at first speed
Fig. 14 Splash condition of the final drive gear at seventh speed
4.2 The Splash Lubrication The splash lubrication, as an automatic lubrication that is formed from which the rotation gears immersed in the oil sump of a sealed case, can bring the lubricant oil from the oil sump into the friction pair.
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Fig. 15 Left view of the lubricant oil splash condition
Figure 12 shows the condition of the splash lubrication of the transmission lubricant system at the first speed at the physical time of 0.6 s. The oil liquid splashed by the driven gear of the output shaft 2 and immersed in the oil sump has already reached the top of the output shaft 1. But because of the low rotation speed, the oil liquid splashed by the final drive gear leans to the right due to the influence of the bevel gear and gravity, and thus it makes against the supply of oil to the left oil catcher 1. According to Fig. 13, the gear lateral has already broken away from the oil liquid which only leaves a little of oil in the gear root after the gear rotates to the point A and there is not any oil liquid thrown from the highest point B of the gear. So the splash lubrication effect of the transmission under the first speed condition is poor because only little oil liquid can be splashed by the final drive gear. In order to further verify the splash lubricant effect of the final drive gear, the seventh speed condition has been simulated and the result is shown in Fig. 14 that the final drive gear is enwrapped completely by the oil liquid and can realize the oil supply of the oil catcher 1.
4.3 The Oil Passage Lubrication The oil retainer and oil catcher are designed under the transmission housing. The oil retainer can bring the oil liquid splashed by gears into the oil catcher, which will flow into the hollow drive shaft and realize the lubrication of needle bearing between each gear and shaft. This paper sets the monitor surface at the oil passage inlet of the oil catcher to confirm the mass flow rate of the lubricant oil of each oil passage. Figure 15 shows the left view of the lubricant oil splash condition at the time of 0.6 s. Although there is no lubricant oil splash into the oil catcher 1 and oil catcher 2, the oil liquid seems to enter the oil catcher 3, oil catcher 4 and oil catcher 5. But this does not happen in truth. Figure 16 shows the relationship between the lubricant oil flux entering each monitor surface of the oil catcher and the physical time, which indicates that the oil only has entered the oil catcher 5 and the mass
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Fig. 16 Time course of the mass flow rate of entering each monitor surface
Fig. 17 Front view of the oil passage lubrication
flow rate of the inlet oil increases with the time, but the oil has not entered other oil catchers. The result illuminates that the transmission lubricant system has not reached the dynamic stability far away and needs the further calculation. The condition that the oil catcher 3 and oil catcher 4 have not captured oil also can be affirmed by the front view of the lubricant oil splash condition as shown in Fig. 17. There are truly several oil liquid assembled around the oil catcher 5, but there are still some distances between the splash oil and the wall of the oil catcher although the splash oil has reached the height of the oil catcher 3 and oil catcher 4. Therefore there is basically no oil entering the monitor surface of the oil catcher 3 and oil catcher 4. Obviously, what can be avoided includes not only making the wrong estimation result because of the different observation angle, but also quantificationally evaluating the oil collecting condition of each oil catcher set to the monitor surface
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near each oil catcher. Due to the limitation of the computing model size and the computing resource, the physical time of the simulation computing is only 0.6 s which cannot surely judge whether the oil can enter each oil catcher and the mass flow rate of the inlet oil, so the effect of the oil passage lubrication cannot be evaluated. But what can be sure is that the transmission splash oil will enter the dynamic stability state with the physical time increase and the mass flow rate of the lubricant oil which enters each oil catcher through each monitor surface, which will also become more stable. The evaluation standard of the stable state is that the mass flow rate entering each oil catcher is no longer changing.
5 Conclusion With the software Ansys-Fluent, the paper firstly simulates the flow field of the single oil orbit of 7DCT transmission and takes the simulation result of the mass flow rate of each nozzle as the input conditions which are based on confirming simulation and experimental results are basically the same. Then the lubricant simulation model of the transmission has been established and the monitor surfaces have been set up at the positions of the oil catcher for the evaluation of the oil passage. Finally the working condition of the first speed is simulated with the whole transmission model as the research object and the lubricant effects of three lubricating methods of 7DCT transmission have been studied particularly, including the forcing lubrication, splash lubrication and oil orbit lubrication. Although the dynamic stability state of the splash lubricant oil has not been obtained because of the restriction of computing resources, the simulation method presented in the paper will undoubtedly have important significance for simulation evaluation of the lubricant properties of the transmission.
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