Feasibility study of an oil-free turbocharger supported on gas foil bearings via on-road tests of a 2-liter class diesel vehicle

This paper presents the feasibility study of an oil-free turbocharger (TC) supported on gas foil bearings (GFBs) via on-road tests of a 2-liter class diesel vehicle. The oil-free TC is constructed using a hollow rotor with a radial turbine at one end and a compressor impeller at the other end, a center housing with journal and thrust GFBs, and turbine and compressor casings. The oil-free TC reuses parts of a commercial variable geometry turbocharger except for the rotor-bearing system. In a test rig driven by a diesel vehicle engine (EG), the rotordynamic performance of the oil-free TC is evaluated up to the rotor speed of 130 krpm, while being measured at the compressor end. The journal GFBs are modified to enhance the rotordynamic performance by inserting three metal shims between the bump-strip layers and bearing housing. The rotordynamic performance is also measured during on-road tests by replacing the original TC of the test diesel vehicle with the constructed oil-free TC. The journal GFBs have a relatively large bearing clearance and no metal shims to generate sub-synchronous motions at low TC and EG speeds. During normal vehicle driving, the TC rotor motions show steady rotordynamic operations. The oil-free TC rotates at 25 krpm ~ 50 krpm while the vehicle runs at 20 km/h ~ 30 km/h on the roads. Sub-synchronous rotor motions initiate with a frequency of ~ 100 Hz at the TC speed of ~ 37 krpm. The TC rotor motion also shows multiple EG-induced harmonics, as expected. Upon external shocks given by passing the vehicle on road-bumps, the sub-synchronous motions are excited only when the rotor rotates above the initiation speed of sub-synchronous motion. The excitation is non-destructive because the vehicle suspension absorbs most of the external shock. Incidentally, the external shocks appear to have no influence on the synchronous motion and engine-induced harmonics of the TC rotor.