Engine Science Camshafts and Pistons

Let’s say a cam is ground with an intake event of 36 degrees opening before TDC and an exhaust closing event of 40 degrees after TDC. Overlap period would be 76 crankshaft degrees.
As the overlap period increases, there is less valve seated time available, resulting in higher engine rpm required to generate adequate cylinder pressure. For example, race cams usually have more overlap than those intended for stock applications. Think of it in terms of how long overlap periods allow usable cylinder pressure to be lost to atmosphere and the whole soggy mess gets a little clearer.
Intake lobe centerline has to do with the position of the camshaft as installed in an engine. It simply means that the centerline of a cam’s intake lobe (usually the No. 1 cylinder) is related to the position of the crankshaft (thus, piston position). Moving the camshaft ahead of this initial position is called “advancing” the cam and tends to help low-rpm power output. Moving the cam behind this initial position is called “retarding” the shaft and generally helps power at higher rpm. Functionally, advancing a cam increases low-rpm cylinder pressure, thus aiding fuel economy and throttle response. Retarding a cam increases the rpm point at which optimum volumetric efficiency is achieved, thereby raising the point of peak power. And whether the cam is of race design or a stocker, the same effects can be expected.

Okay. We’ve touched on lift, duration, overlap, displacement angle, lobe centerlines, advancing and retarding, and primary functions of a camshaft. Now suppose we work our way through the three basic types of camshaft lobe followers and see how each affects the performance of a particular lobe design.
All three of these types can be classified as radial followers. That is, each involves a follower that is held in some form of bushing (lifter or follower bore in the engine’s cylinder block) and actuates a valve based on radius changes in the lobe while the camshaft rotates (see illustration because this may not be a clear description). One, with a flat face, is typical of most “flat-tappet” design followers. The next, with a spherical face, is called a “convex” lifter and tends to provide increased rates of valve motion as compared to flat-faced followers. And the third, which incorporates a roller (or wheel) that follows lobe shape, is used primarily for exceptionally high rates of valve motion where lifter/lobe contact pressures can be minimized, particularly at high engine rpm.
Assume for a minute that we have an engine operating at 4000 rpm. At this speed (or any other), there is a specific amount of time in terms of crankshaft rotation in which to operate the intake and exhaust valves for a particular cylinder. To optimize the amount of intake and exhaust flow, it may be necessary to open, hold open, and delay closing of the valves to achieve maximum cylinder filling (volumetric efficiency). Since there is only a specific amount of time in which to do this, it may be good to have quick valve motion so that the valve/seat relationship offers the least amount of impedance to net flow. Such valve action tends to increase contact pressure (friction load or drag) between follower and lobe, suggesting the use of a roller design follower instead of a flat-faced one.
Such is the case with race-type camshaft lobes. And as a compromise, because the spherical lifter face imparts something of a “roller effect” to valve, the convex follower is frequently used in race cam design. This offers a degree of roller action without the need for a true roller tappet. Grand National NASCAR engines are a predominant user of this type of camshaft.
Also with respect to roller cam followers, the line of action (force) from the cam lobe to the follower cannot be along the follower axis, except when the follower is at or near maximum lift.

F. This is simple harmonic motion. We included such an illustration to show typical valve motion relative to camshaft rotation. The “slope” of the curve (angle relative to the horizontal axis) indicates how fast valve action is taking place. The more vertical the slope, the faster the valve action. Note that maximum valve movement takes place during the midpoints of the lift curve. The more vertical the slope, the faster the valve action. This is a basic lift curve. As mentioned in the story, variations of simple harmonic motion produce specific valve motion relative to specific engine requirements. G. This is the typical “convex” tappet design. Compare this type of lifter with the design shown in Figure E. This lifter design is one method of cheating roller-vs.-flat-tappet capabilities to use more radical profiles. Besides all this, cams designed for use with convex lifters really get the job done

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