Page 543 - Cam Design Handbook

THB16 9/19/03 8:04 PM Page 531

AUTOMOTIVE CAMSHAFT DYNAMICS 531

16.2 INTRODUCTION

In this chapter we present the latest techniques necessary to design a cam-follower valve
gear system for a new automobile internal combustion engine or to replace a cam in an
existing engine. Included in this broad subject are automotive engines for public trans-
portation, racing cars, motorcycles, and diesel engines. The presentation will largely utilize
computer software which is listed in App. D.
Curve information in Chaps. 2, 3, and 4 and the material lubrication and manufactur-
ing information of Chaps. 9 and 10 are particularly pertinent. Also, the reader should refer
to Chapters 11, 12, and 13 which present the dynamic study of compliant high-speed cam
systems applied to automotive internal-combustion valve-control systems. An excellent
reference for the subject of automotive camshaft design is Hubbard (2000). Valuable infor-
mation was shared with Crane (2000) and Dour (2002).
Automotive cam valve-gear systems are of two types:

• Open-track cam system
• Closed-track cam system (desmodromic)
The open-track cam follower, which is most popular, has a spring to maintain con-
straint of the mechanism. The closed-track follower is held on the cam by virtue of its
mechanism design. This type of follower system is largely applied to motorcycles.
The following types of cam-driven automotive valve installations are shown:
• Direct acting on valve, L-head, overhead cam (OHC)
• Push-rod rocker arm overhead valve train with flat or roller contact (OHV)
• Cam-on-rocker arm (CORA) valve train with radius or roller follower (OHC)

Most automotive engines made in the United States before World War II were flat-head
(L-head) designs, Fig. 16.2. The L-head design has a short stiff-valve follower train
which lessened potential dynamic problems, such as follower jump. After 1950, most
United States manufacturers switched to overhead valve designs (OHV) which became
the United States standard.
The OHV designs, Fig. 16.3, used an underhead camshaft which drove a long, com-
pliant pushrod to actuate the overhead valves located above the pistons. At high speeds
the long, flexible follower trains of these pushrod designs caused significant dynamic prob-
lems with the valve trains.
Postwar European and Japanese engine designers adopted the overhead camshaft
design, Fig. 16.4. The camshaft is located in the head above the valves. It provides a
short, light, and stiff follower train similar to the old-fashioned flathead engines.
This system allowed higher speeds of engine performance since the follower train
had a higher natural frequency. Accordingly, United States manufacturers are currently
shifting from pushrod overhead valve designs to overhead camshaft designs in the new
engines.

16.3 CAMSHAFT MATERIALS

In this section we will discuss the camshaft materials that are currently used. These mate-
rials are applied in either the sliding or rolling action of the follower; see Chaps. 9 and
10. The performance and life of mating materials is dependent on the compatibility of the
materials, the lubricants selected (additives), the engine speed, the newness of the cam-