Operational Life Testing of Flexible A/C Hoses for CO 2 - Systems

Transcription

1 Operational Life Testing of Flexible A/C Hoses for CO 2 - Systems B. Balmer, C. Burkhardt, Witzenmann GmbH (Germany) 2004 Alternate Refrigerant Systems Symposium June 29 - July 1, 2004; Scottsdale, Arizona

2 Introduction To decouple the relative movements between the enginefixed compressor and the body-fixed components of an A/C system, flexible connections are required. Source: BEHR When using R744 (CO 2 ) as an environmentally friendly alternative to R134a, these connections have to be permeation tight. 2

3 Introduction Currently proposed designs are based on a corrugated metal hose, functioning as a non-permeable layer. This hose can be either covered by a rubber/plastic hose with integrated braiding (a) or by a full metal braiding with or without intermediate layer (b). (a) (b) 3

4 Introduction In both variants, the key functions of the metal hose are to enable a gas-tight connection between body-fixed and engine-fixed components to decouple the relative movements of the A/C piping between engine and body To ensure a safe function of the A/C system over lifetime, the potential risks have to be identified and minimized during the development and validation process. 4

5 Failure modes A/C hoses based on metal hoses can exhibit different failure modes, resulting in a reduction of the operational life: Bursting Corrosion Wear Fatigue 5

6 Failure modes Bursting: The hose has to be designed with suitable safety factors. As the operating pressures are known, and bursting of the metal hose has no time-dependence, a simple standardized burst pressure test can be applied. Current specifications are met. Corrosion: Suitable materials have to be used. As the corrosion requirements of passenger cars are well known, standard test procedures are defined. A standardized corrosion test can be applied. Current specifications are met. 6

7 Failure modes Wear: The hose has to be designed so that wear between the components is prevented (by design). The wear of chosen components is well known by experience, suitable design measures can be applied. Current specifications are met. Fatigue: The hose has to be designed with suitable safety against material fatigue. Based on experience, this failure mode is prevailing for A/C hoses, the metal hose being the critical component. 7

8 Fatigue testing To prevent fatigue by design, it is necessary to know the operational load, being the actual distortions exerted on the metal hose during operation in the vehicle. The occuring distortions are multi-axial and lead to non-proportional loads in all 6 degrees of freedoms (DOF), i.e. 3 translations in x, y and z-axis and 3 respective rotations. The cause for the distortions are enginemovements due to load changes (e.g. starting forward or changing gear) or due to poor road excitations. 8

9 Fatigue testing Testing methods: To ensure the operational life of the hose, a suitable validation method has to be applied. Three different methods could be used: 1. Vehicle testing under real conditions (e.g. 150,000 miles) 2. Operational life test on a multiple axes test bench, correlating with the real vehicle conditions 3. Constant amplitude load test, correlating with the real vehicle conditions 9

10 Fatigue testing Testing methods: 1. Vehicle testing under real conditions (e.g. 150,000 miles) Easy setup, defined conditions, real load case ("100% reality") expensive, time-consuming, vehicle sometimes not (yet) available 2. Operational life test on a multiple axes test bench fast result; very close to vehicle testing, cheaper than 1. operational loads difficult to compile 10

11 Fatigue testing Testing method: 3. Constant amplitude load test Easy setup, fast and cheap test Correlation between vehicle testing and test on the rig very difficult Correlation based on todays information not possible Target for future validations Option 2 (Operational life test on a multiple axes test bench) currently the best choice! 11

12 Operational life testing 1. Measurement of the engine movements in 6 DOF The distortions of the A/C hose can be determined from the displacement at any point of the engine, assuming the engine being a rigid body. 12

13 Operational life testing The final validation drive test, usually carried out by the OEM, specifies the representative operational loads. This could be the basis for the design specification; with the resulting loads being unique for each vehicle (or even engine version) Typical driving maneuvres include: Starting forward/reverse normal and "kick down" Changing gear Poor road excitation Braking [...] 13

14 Operational life testing 2. Simulation of the measured distortions on a multiple axes test bench 14

15 Operational life testing 2. Simulation of the measured distortions on a multiple axes test bench The complete operational life distortion, as defined in the spec. book, can be applied onto the A/C hose to simulate the whole vehicle life. As the small displacements are irrelevant for the fatigue of metal hoses, a significant faster simulation is possible with an appropriate preparation of the load profile. A statistical assessment of the test results is necessary to obtain sufficient safety margins. 15

16 Example: European Midsize Car The proposed approach for operational life testing was carried out on a European Midsize Car, known to have large engine movements (transverse engine, front wheel drive, soft pendulum engine mounting) Citroën Xsara Picasso, 1.8 l (petrol engine) 16

17 Example: European Midsize Car 1. Installation position: Assumption: The installation position of the R744 hose would be the same as with the current R134a A/C system 17

18 Example: European Midsize Car 2. Measured distortion of the A/C hose (flexible length 350mm), selected driving maneuvres: 20 displacements [mm], rotations [deg] x y z Rx Ry Rz -10 time [s] Forward start "kick down" 18 x y z Rx Ry Rz max 5,0 2,0 17,3 0,2 3,0 0,6 min -5,4-5,8-3,5-0,4-1,0-0,3

19 Example: European Midsize Car 2. Measured distortion of the A/C hose (flexible length 350mm); selected driving maneuvres: displacements [mm], rotations [deg] x y z Rx Ry Rz -15 time [s] Reverse start "kick down" 19 x y z Rx Ry Rz max 16,1 15,4 7,7 1,5 1,2 0,4 min -3,2-4,2-10,7-0,2-4,7-1,2

20 Example: European Midsize Car 2. Measured distortion of the A/C hose (flexible length 350mm); selected driving maneuvres: displacements [mm], rotations [deg] x y z Rx Ry Rz -15 time [s] Poor road, track "Europe" 20 x y z Rx Ry Rz max 5,7 9,0 14,1 0,7 2,1 0,6 min -5,3-6,5-9,1-0,4-1,5-0,8

21 Example: European Midsize Car 3. Measurement on the multiple axes test bench running order description of driving maneuvers 10 x 10 x 10 x 282 x m1 4 x m2 2 x m3 3 x m4 7 x m5 abbr. description no. of cycles m1 combined starting m2 combined starting sporty 4800 m3 poor road tracks 2050 m4 poor road tracks 3900 m5 town driving 7700 m6 combined maneuver m7 combined maneuver x m6 1 x m7 8 x m2 9 x m4 7 x m5 2 x m7 The samples were pressurized with 15 MPa (constant pressure), leak detection via pressure switch (<13,5 Mpa) 5 x m3 21

22 Example: European Midsize Car 3. Measurement on the multiple axes test bench Ex. 1 Ex. 2 Ex. 3 22

23 Example: European Midsize Car 4. Results: sample number of failure failure position operational life number load cycles mode percentage of spec crack moved end ("compressor side") 47% crack moved end ("compressor side") 68% crack moved end ("compressor side") 30% crack moved end ("compressor side") 23% crack moved end ("compressor side") 36% crack moved end ("compressor side") 30% average value 40% Based on the design specification for the examined car, the operational life target of the A/C hose is not met in the currently used installation position. 23

24 Example: European Midsize Car 4. Results: Operational loads are dependent on the engine movements, length of the hose and installation position. Assuming for example a slightly different installation position (~70 mm closer to the engine rock axis), the operational loads are reduced by 20%, leading to the following results: sample number of failure failure position operational life number load cycles mode percentage of spec crack moved end ("compressor side") 155% crack moved end ("compressor side") 79% crack moved end ("compressor side") 245% average value 160% An increase of the flexible length of ~50 mm would lead to the same result. 24

25 Summary For R744 A/C hoses based on the corrugated metal hose design, fatigue is the critical failure mode. Currently, operational life testing is the only reasonable test method on the component level. The feasability of the proposed approach has been shown in the example presented afore, including the measurement of the operational loads, the appropriate preparation of the load profile and the consequent testing on the multiple-axes testbench. As the operational loads are unique for each vehicle this approach is recommended by Witzenmann for component level validation of flexible R744 A/C hoses. 25

26 Acknowledgements Witzenmann would like to thank the PSA Group for the possibility to do the engine movement measurements on the PSA test track in La Ferté (France) and for the provision of the required geometrical data and design specifications. Thank you very much for your interest! 26