A Design Approach of Asymmetrical Cam Profile and Its Effect on Performance of High-Speed Automotive Engine 2004-01-0610
The valve train is an important system in automotive engine. It can assure valves open and close at the right time by controlling cam profile. The adequate duration of valve opening, suitable velocity and acceleration of valve closing can be assured by the cam profile. The design of the valve train directly influences engine performance, exhaust emission, reliability, vibration and noise. In particular, the high-speed automotive engine requires valve train not only operating smoothly, reliably but also having a good performance. So it increases the challenge of valve train design. Typically, the designers choose the symmetrical cam profile. The symmetrical valve cam is suitable to low and medium speed engines, but with the rising of engine speed, valve closing velocity and acceleration increase, vibration and noise coming from valve loading become intense, so abrasion of valve will be accelerated and durability of the valve train will be deteriorated. This paper focuses on the valve train design problems that designers encounter in designing cam profile of automotive engine with increasing speed, and suggests using an asymmetrical N-harmonic cam profile to decrease the loading velocity (In this paper, loading velocity is defined as the valve velocity when valve contacts with valve seat) and acceleration. In the paper an asymmetrical N-harmonic mathematics model of cam profile is constructed, and the software on design of cam profile is compiled. Using the program, the author designs an asymmetrical N-harmonic cam profile of a high-speed diesel engine. The calculation results of dynamics and engine experiment demonstrate that, compared to a symmetrical cam-profile, the new asymmetric N-harmonic cam profile shows an improved dynamics performance, emission level and fuel economy.