Study On Friction And Wear Behavior Of Valve Train Of BZ Engine

As one of the vital parts of engine, valve train plays a role in controlling air exchange quality and promoting combustion, and its performance has a direct influence on the dynamic performance, stability, economy and durability of the engine. The two main factors affecting the performance of valve train are friction and wear. Friction heat causes a large loss of energy, and wear is one of the main reasons leading to the failure of the valve train.In this work, the friction and wear characteristics of the critical friction pairs were thoroughly studied by taking the valve train of BZ engine as the object, which was based on the dynamic analysis and the bench test for the overall valve train. Moreover, influence of different Stellite alloy coatings on the friction and wear characteristics of the exhaust valve steel were investigated using a MMU-10 G high-temperature friction and wear tester at elevated temperatures.1. A multi-mass dynamic model was established for the valve train of BZ engine to analyze its dynamic performance on the basis of the calculation of boundary conditions and relevant parameters under calibration condition. The results showed that serious separation of the selected cam-tappet pairs appeared. There were many high peaks in the contact stress curve. The lubrication status consisted of elastic hydrodynamic lubrication, film lubrication and the mixed lubrication for the cam-tappet pairs. In addition, the seating force and velocit y were very high and the curve of the seating stress exhibited the characteristics of pulse signal and sine signal.2. A test bench was designed and built to study the friction and wear characteristics of the vital friction pairs of valve train in BZ engine, which ran in the condition close to the practical instance. The results showed that the most serious failure of the selected cams located at the peach tip arc region. The slender furrows generated during machining processing before the test were gradually removed, so the tested surfaces became smoother. The edge of the tappet was mainly damaged by the fatigue spalling of the phosphating layer, and there were many severe ring scratches in intermediate area, whereas the center area failed in the form of a severe abrasion. Besides, valves were damaged not only by the fatigue flake locating at the surface defect but also by abrasive wear caused by the valve’s rotation.3. Two kinds of Stellite alloy coatings were fabricated on the substrate of exhaust valve steel by plasma surfacing. Surface properties of the coatings were analyzed, including microstructure, hardness, phase structure, etc. Moreover, the friction and wear properties of the alloys were investigated at different temperatures compared with the substrate. The results indicated that serious adhesive wear appeared on the surface of the substrate which easily led to the failure of the friction pairs at elevated temperatures. Serious adhesive wear has been effectively avoided on the surface of the coatings. Further, the coating prepared by Stellite F alloy had more excellent wear resistance.