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Rotordynamic Performance of Shimmed Gas Foil Bearings for Oil-Free Turbochargers

[+] Author and Article Information
Kyuho Sim, Lee Yong-Bok

Energy Mechanics Center,  Korea Institute Of Science And Technology, 39-1 Hawolgok-Dong, Songbuk-Gu, Seoul, 136-791, Korea

Tae Ho Kim1

Energy Mechanics Center,  Korea Institute Of Science And Technology, 39-1 Hawolgok-Dong, Songbuk-Gu, Seoul, 136-791, Koreathk@kist.re.kr

Jangwon Lee2

Power Train NVH Team, Hyundai and Kia Corporate Research and Development, Division 772-1, Jangduk-Dong, Hwaseong-Si, Gyeonggi-Do, 445-706, Korea

1

Corresponding author.

2

Conducted work as a research assistant at the Korea Institute of Science and Technology, Seoul, Korea.

J. Tribol 134(3), 031102 (Jun 25, 2012) (11 pages) doi:10.1115/1.4005892 History: Received November 09, 2011; Revised January 13, 2012; Published June 25, 2012; Online June 25, 2012

Oil-free turbochargers (TCs) will increase the power and efficiency of internal combustion engines, both sparking ignition and compression ignition, without engine oil lubricant feeding or scheduled maintenance. Using gas foil bearings (GFBs) in passenger vehicle TCs enables compact, lightweight, oil-free systems, along with accurate shaft motion. This paper presents extensive test measurements on GFBs for oil-free TCs, including static load-deflection measurements of test GFBs, rotordynamic performance measurements of a compressed air driven oil-free TC unit supported on test GFBs, and bench test measurements of the oil-free TC driven by a passenger vehicle diesel engine. Two configurations of GFBs, one original and the other modified with three shims, are subjected to a series of experimental tests. For the shimmed GFB, three metal shims are inserted under the bump-strip layers, in contact with the bearing housing. The installation of shims creates mechanical preloads that enhance a hydrodynamic wedge in the assembly radial clearance to generate more film pressure. Simple static load-deflection tests estimate the assembly radial clearance of the shimmed GFB, which is smaller than that of the original GFB. Model predictions agree well with test data. The discrepancy between the model predictions and test data is attributed to fabrication inaccuracy in the top foil and bump strip layers. Test GFBs are installed into a TC test rig driven by compressed air for rotordynamic performance measurements. The test TC rotor, 335 g in weight and 117 mm long, is coated with a commercially available, wear-resistant solid lubricant, Amorphous M, to prevent severe wear during start-up and shutdown in the absence of an air film. A pair of optical proximity probes positioned orthogonally at the compressor end record lateral rotor motions. Rotordynamic test results show that the shimmed GFB significantly diminishes the large amplitude of subsynchronous rotor motions arising in the unmodified GFB. Predicted synchronous rotor amplitudes and rigid body mode natural frequencies agree reasonably well with recorded test data. Finally, the oil-free TC is installed into a passenger vehicle diesel engine test bench. The TC rotor speed is controlled by the vehicle engine. Speed-up tests show dominant synchronous motion (1X) of the rotor. Whirl frequencies of the relatively small subsynchronous motions are associated with the rigid body natural mode of the TC rotor-GFB system as well as (forced) excitation from the four-cylinder diesel engine. The bench test measurements demonstrate a significant reduction in the amplitude of subsynchronous motions for the shimmed GFB, thus verifying the preliminary test results in the TC test rig driven by compressed air.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Configuration for automotive TC rotor [6]

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Figure 2

Oil-free TC rotor, journal GFB (turbine side), and thrust GFB

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Figure 3

Locations of three shims relative to top foil spot-weld in test bearings. Turbine side GFB.

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Figure 4

Photo of GFB structure load-deflection test setup and schematic view of test procedure for GFB loading-unloading tests

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Figure 5

GFB displacement versus static load recorded during two consecutive loading-unloading tests. Test data compared to model predictions for (a) original and (b) shimmed GFBs (turbine side). Hysteresis loops highlighted.

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Figure 6

GFB structural stiffness versus displacement recorded during two consecutive loading-unloading tests. Test data compared to model predictions for (a) original and (b) shimmed GFBs (turbine side).

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Figure 7

Photo of oil-free TC test rig driven by compressed air

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Figure 8

Waterfall plot of rotor speed-up and speed-down response to 82,000 rpm (1.36 kHz) measured at compressor end. Original GFB.

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Figure 9

Waterfall plot of rotor speed-up and speed-down response to 86,000 rpm (1.43 kHz) measured at compressor end. Shimmed GFB.

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Figure 10

Amplitude of synchronous and subsynchronous rotor motions versus speed, recorded during rotor speed-down tests for (a) original and (b) shimmed GFBs. Amplitude A  =  (X2  +  Y2 )0.5 where X, Y are vertical and horizontal amplitudes, respectively. Measurements and predictions from compressor end.

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Figure 11

Amplitude of subsynchronous rotor motions versus speed and subsynchronous frequency versus speed recorded during speed-down tests for (a) original and (b) shimmed GFBs. Measurements from compressor end.

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Figure 12

Rotor orbital motions at top speeds of (a) 82,000 rpm for original GFB and (b) 86,000 rpm for shimmed GFB. DC-offset subtraction. Measurements from compressor end.

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Figure 13

Photo of passenger vehicle diesel engine test bench for oil-free TC

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Figure 14

Waterfall plot of rotor speed-up and speed-down response to 90,000 rpm (1.5 kHz) measured at compressor end during diesel engine bench test. Vertical direction.

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Figure 15

(a) Filtered whirl frequency and (b) amplitude of subsynchronous motions versus rotor speed for original and shimmed GFBs. Measurements recorded during speed-up tests in the vertical direction.

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Figure 16

TC rotor speed versus engine speed for original and shimmed GFBs, recorded during speed-up tests in diesel engine test bench

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Figure 17

(a) GFB displacement versus static load recorded during two consecutive loading-unloading tests for (b) three configurations of GFBs: Original GFB, GFB with a single shim of 360° arc length, and GFB with three shims. Hysteresis loops denoted. Estimated nominal radial clearance of cnom  =  70 μm for original GFB.

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Figure 18

Rotordynamic model of oil-free TC rotor supported on GFBs. Rotor mass: 0.336 kg. Rotor length: 116.65 mm.

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Figure 19

Predicted stiffness and damping coefficients versus rotor speed for original and shimmed GFBs. Turbine and compressor side GFBs.

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Figure 20

Predicted natural frequency versus rotor speed for (a) original and (b) shimmed GFBs. Note both vertical and horizontal axes in log scale.

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