Manipulating Frictional Performance of Wet Clutch Engagement through Material Properties and Operating Conditions
Round 1
Reviewer 1 Report
In this work the author investigates the effects of various factors on the tribological performance during wet clutch engagement. The work is purely computational, yet, sufficient results are reported and some important conclusions are drawn regarding the the torque transfer, methods to achieve a positive mu-V mode, and the effect of squeeze film. These results and conclusions have potential applications in wet clutch design and manufacturing. It seems that the two-dimensional modified Reynolds equation was solved. However it is unclear what technique has been used to solve the differential equations. Was a commercial code involved? I guess that the finite-difference method was employed instead of a commercial code. If so, the author should explain the details of the finite difference model including the step size, mesh density, numerical stability, convergence study, computational time, etc. On the other hand, since the work is purely computational, validation is a necessary step. Did the author validate the model against the existing solutions or experimental observation in some limiting cases? I understand that the author has made comparisons among the three different models: the base model, the modified base model and the wavy model. However, I would like to see how these models are compared with the results obtained from an entirely different approach by prior researchers. After all, there are a significant number of computational models for wet clutch research available in the literature. Briefly, the overall quality of the paper meets the requirements for publication, however the author should provide more details on the computational model, and the validation part should be strengthened as well.
Author Response
1) Abstract is slightly changed for the limitation of word count.
2) This manuscript is about computational study of torque transfer during the wet clutch engagement, so that it can make many parameters of the wet clutch design parameters possible to be compared. Otherwise, experimental work to verify each effects of wet clutch design parameters costs too much for building test machine and friction pad samples. Then it may not be possible to expect their detail effects. In this stage of the present manuscript, the main purpose of the study is to investigate the effects of the possible design parameters and to provide the design guidance of favorable performance of the wet clutch engagement based on the theoretical background of hydrodynamic lubrication mechanism. Overall the purpose of this work is to show that hydrodynamic pressure development is the main mechanism to manipulate equivalent friction coefficient in wet clutch engagement.
3) Pure computational work is performed by the lab-made code not by the commercial code. Commercial code cannot model the friction pad mechanism with the material properties of which the effects are numerically compared in detail. Numerical method for solving the modified Reynolds’ equation are added in the main text as below.
After Line 226 in the revised version,
The modified Reynolds’ equation (10) is solved by finite difference method regarding to the unknown value of hydrodynamic pressure . Finite element method [18,21] that has more freedom of degree on the mesh generation is also applicable but convergence is less stable than finite difference method. In order to get the value of , other values in this equation should be set to be known. Initial guess for the film thickness needs to be set at first and other values such as flow factors of surface roughness, permeability, geometrical gap and sliding velocities of both friction pad and steel plate are given to Eq. 10 at each time step appropriately according to the related modeling equations and values in the Table1 and 2. The mesh size over the friction pad area is about 481 in circumferential direction, 21 in radial direction and 31 in axial direction with equidistance mesh size. The convergence criteria for the hydrodynamic pressure is 0.05% at each time step with SOR numerical method. With the time step of 1.0ms that the convergence stability for solving Eq. (10) delicately depends on, the hydrodynamic reaction force is obtained by integrating the hydrodynamic pressure over the friction pad area. With the given film thickness at each time step, the asperity contact pressure, Eq. (9) is calculated and integrated over the friction pad area to get the mechanical contact load.
4) Validation of analytical work by experiments is always desirable. However, validation by the experiments to match with the results of analytical work is another task in terms of costs and research-efforts in particular for this case. To realize the verification of the numerical results in the present manuscript needs to build very accurate test apparatus better than SAE#2 machine. Build-up of similarly functioned test machine as well as preparing various samples of friction pads take more efforts and budge than it is expected.
Validation can be obtained by the comparison of other previous studies of computation and experiments, although they are compared with the one-dimensional analysis. Reference [1-3,5,7,9] experiment verifies general design parameters with flat friction pad but they compared the result with one-dimensional analytical method with flat friction pad. The present manuscript of two dimensional analysis can verify more detail about the effects of geometrical shape of friction pad with respect of wedge effect during the initial stage of clutch engagement that can provide positive characteristics.
The below part is added in the last part of section 5. Results and Comparisons. (After Line 518)
Many experimental works [1-3,5,7,9] that are compared with simple one-dimensional analysis of flat wet clutch engagement performance show good tendencies to our computational results of two-dimensional analysis. The present numerical results can provide more detail the lubrication mechanism in every area of wet clutch contact geometry.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The biggest concern is about the novelty/contribution of this study. Without clearly state and discuss it, the paper's quality is low. If a new or updated model was proposed, it should be compared with the existing model as well as the experimental data.
1. Line 72. The author mentioned “Wet clutch engagement involves three process….” The author shall discuss why two of them, i.e., asperity contacts and hydrodynamic lubrication, are important and studied in this paper. In addition, the author shall address the motivation and contribution of this study in the introduction section.
2. Lines 228-237. Is this paragraph the key contribution of this study? If so, the author shall summarize it in the introduction section. Also, will this model improve the wet clutch study, compared to the existing research works?
3. In section 5, the author compared based model, modified base model and wavy model. Is there experimental data can be compared to show the wavy model is the best?
Author Response
* Abstract is slightly changed for the limitation of word count.
* In the revised manuscript, references are rearranged and new references are added.
1) Line 72
It is mentioned that there are generally three divided regions, hydrodynamic, boundary lubrication[13,14] and mechanical contact in the introduction and references [2,3,15] (line 68,69) are re-quoted. And it is also mentioned that boundary lubrication is not fully considered (line 139) in this study, because it is beyond the scope of this work which should be described in detail with the respect of chemical reaction mechanism or experimental investigation. Therefore, the boundary friction coefficient is not considered too much and explained that some researches considered it as constant value in the main text. (line 277 [3,15] and line 327[2,6,15]) Instead, the scope of this work is focused on the boundary condition of lubricant slip behaviors on the porous friction pad, which is necessary to verify the hydrodynamic pressure development which is greatly engaged in the equivalent friction coefficient. References for the detail boundary lubrication mechanism are referred and added in the manuscript.
[8] Cho, J; Lee, Y;, Kim W.; Jang S. Wet single clutch engagement behaviors in the dual-clutch transmission system. Int. J. Automot. Technol., 2018, 19, 463−472.
[13] Eguchi, M.; Yamamoto, T.; Shear characteristics of a boundary film for a paper-based wet friction material: friction and real contact area measurement. Tribol. Int., 2005, 38, 327–335.
[14] Fei, J.; Li, H.; Huang, J.; Fu, Y.; Study on the friction and wear performance of carbon fabric/phenolic composites under oil lubricated conditions. Tribol. Int., 2012, 56, 30–37.
2) Lines 228-237
Revised in Introduction (line 85-91)
Numerical results shows that geometrical shape of waviness of friction pad generates strong hydrodynamic pressure development caused by wedge effect in addition to squeeze film effect with full two-dimensional analysis.
Added in Section 3 (line 248-253)
Many other studies investigated simple one-dimensional analysis[1-3] for fast computation of frictional torque transfer and compared to other experimental work.[2,3,6,7,9,10,19] Few two-dimensional studies[16-18] investigated the full hydrodynamic pressure developments that provide the favorable torque transfer performance. And also they did not point out the mechanism that enhancement of hydrodynamic pressure development at the initial stage of wet clutch engagement can provide positive m–V characteristics.
3) In section 5,
The work scope of this study is to verify how the hydrodynamic pressure development effects on the equivalent friction coefficient to have positive -V characteristics which is much related to the initial stage of wet clutch engagement. In order to accelerate the hydrodynamic pressure development, it needs to have additional wedge effects due to the geometrical shape of waviness. Flat friction pad cannot have wedge effect and does have only squeeze effect although other frictional material properties can have optimized performance. This study focuses on the hydrodynamic pressure development that happen most in the initial stage of wet clutch engagement and it makes it possible to have positive -V characteristics. To verify these behaviors with experiment efforts is another work to build test machine with accurate actuators and sensors, and various friction pad samples. The computational work will guide design method how to remove shudder phenomena while torque transferring with wet clutch mechanism.
The following is added for better understanding of this manuscript. (line 518-523)
Many experimental works [1-3,5,7,9] that are compared with simple one-dimensional analysis of flat wet clutch engagement performance show good tendencies to our computational results of two-dimensional analysis. The present numerical results can provide more detail the lubrication mechanism in every area of wet clutch contact geometry.
4) existing model as well as the experimental data issue
Most researches deal the analytical method for the hydrodynamic pressure computation of wet clutch engagement with one-dimensional domain, because it can provide computational results with fast and easy modes. However, if the computational domain has geometrical complexity, then the one-dimensional numerical method cannot provide meaningful design guidance. Experimental results [1-3,19] for the material parameters of wet clutch friction pad are obtained similarly to the intention of our work, but they did analyze the wet clutch contact mechanism only with one-dimensional method. Other researches that used two-dimensional method did not investigated the hydrodynamic pressure development of wedge effect that can possibly provide positive -V characteristics. This is the main difference of this study from others.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors properly addressed my comments.
