汽車正時鏈條之磨耗壽命分析

Abstract

正時系統(timing system)在一顆引擎中扮演協調各零組件的角色，一旦正時系統失效，除引擎無法正常運轉外，還會造成氣門被汽缸撞擊而損毀等嚴重問題。正時鏈條(timing chain)為正時系統中主要的傳動媒介，在運作時會因磨耗造成節距伸長，導致跳齒與脫鏈等失效情形發生，因此正時鏈條的使用壽命為各車廠所關心者。然而現有資料顯示，車廠在挑選鏈條時只能依靠特定環境下的實驗數據，並沒有考慮現實中汽車行駛的情況，對於鏈條的設計改善也無法直接反應在使用壽命上。有鑑於此，本研究應用彈性力學與Archard磨耗公式來得知負載下鏈條的磨耗量，再考量汽車於道路行駛時的各種參數，包括行車速度，加速度以及引擎轉速等，從引擎動力與這些參數的關係推導鏈條負載；最後透過可靠度工程概念，預測正時鏈條的壽命，並探討其更換的適當時機及鏈條參數對磨耗壽命的影響。本研究採用的正時鏈條係符合JIS與ANSI規格的標準鏈條，研究結果發現，該種鏈條在汽車行駛至85,000公里時可靠度仍高達0.949，但超過該里程數後可靠度便急速下降，引擎損壞的風險也驟然升高，因此建議在該時更換鏈條。此外，敏感度分析的結果顯示，影響汽車正時鏈條壽命最關鍵的兩因素為磨耗係數與鏈條節距，因此在設計改善時應優先考量此兩參數。

Timing system is the engine coordinate system of a vehicle. Once the timing system fails, it will not only cause engine to stop working, but also result in valve damage owing to piston impact. The timing chain is the major transmission medium of a timing system. Its wear will result in pitch elongation and chain slippage, and eventually cause failure of the chain. Therefore, life of the timing chain against wear has always been an important topic for vehicle manufacturers. However, from information collected, it seems that manufacturers do not take real driving conditions into account; and the design improvement has not been related to life of the timing chain either. In view of this, the present thesis employs elasticity theory and Archard’s wear function to describe the wear amount of a timing chain. Reliability theory is also adopted in the analysis when considering random parameters such as vehicle speed, vehicle acceleration and engine speed resulted from various kinds of drivers. From a derived equation relating these parameters to the engine power, the tension of timing chain is calculated. Accordingly, the life distribution of a timing chain measured in mileage is predicted. The proper mileage for replacement and design improvement of a timing chain are discussed as well. This thesis studies in detail a timing chain fulfilling standards of JIS and ANSI. The result indicates that after the timing chain is used for 85,000 km, its reliability is 0.949. Afterwards, the reliability drops rapidly and the risk of engine-damage increases tremendously. Therefore, the timing chain is suggested to be replaced at around this mileage. Moreover, from sensitivity study, the wear coefficient and the pitch are two most important factors that would affect the life of a timing chain. They should be considered seriously by manufacturers for design improvement.