Engine Science Camshafts and Pistons

It’s also the portion of a cam on which the follower rides during times when the valve is seated. At the first increase of this base circle dimension, a given cam follower begins motion up the lobe “flank.” This ascension continues until the follower reaches the maximum amount of displacement up the flank (maximum lift), after which it continues down the “closing side” of the cam flank. When the follower once again reaches the base circle, the valve is seated and will remain so as long as the follower rides the base circle. It is the shape of the opening and closing flanks that determines rate of valve motion and, therefore, the rate at which the flow passage around a particular valve and seat can develop. Cams that have quick lift rates expose flow paths quickly, while those with slow rates offer

more flow restriction at the valve/ seat junction.
Duration is the measure of time a valve is off its seat and usually relates to degrees of crankshaft rotation. Since most cams rotate at one-half crankshaft speed (one cam revolution for every two turns of the crankshaft), it’s understandable that as much as 300 degrees of valve duration can exist and still have time for the compression and power strokes of the piston. It’s simply a matter of reference. The crankshaft controls piston movement, and because cranks normally drive the camshafts, valve timing figures are noted in degrees of crank rotation.
Lobe displacement angle and intake lobe centerline are often confused in discussions of cam basics. Displacement angle is the angular distance between the intake and exhaust lobes for a single cylinder of the engine. For example, if a cam is ground with a displacement angle of 110 degrees, this means there are 110 degrees of crankshaft rotation between the centerlines of a pair of intake and exhaust lobes (see illustration). If you’ll take a moment to study Figure C, it’s obvious that there is also an angular relationship between the opening point of the intake lobe and the closing point of the exhaust lobe. It is during this time that both intake and exhaust valves are unseated (intake opening, exhaust closing). This is called the overlap period, and it can be determined by numerically adding the value of the intake opening point (before top dead center piston position) to the exhaust closing after TDC piston position.

C. This is an illustration of “overlap” between intake and exhaust valves. Note the direction of cam rotation and you’ll see that as the exhaust valve is closing (downside on the exhaust lobe), the intake valve begins to open. From this point until the exhaust valve closes, both valves are off their respective seats. You might also note that the earlier an intake valve opens, the greater will be the cylinder pressure exposed to the intake manifold. This is reversion pressure, and it can lead to contamination of subsequent cylinder fillings. D. Here’s a roller follower (lifter) and its basic relationship to a camshaft lobe. Note that the amount of angular relationship between lifter axis and roller contact on the cam lobe is relatively small. This measurement, called pressure or contact angle, relates to the amount of side-thrust imparted to the lifter during times when the lifter is off the cam’s base circle. E. Flat-lifters such as conventionally found in solid or hydraulic follower designs usually cause greater pressure or contact angles than roller types. This increases the amount of side-load on the lifter, resulting in more force (or required engine power) to operate a given valvetrain at high rpm. It also means that valve lift rates are limited to cam/lifter materials and engine rpm.

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