Engine Science Camshafts and Pistons

A camshaft primer: the mystique of valve motion … as regulated by the shaft with the cams on it

To many of us, knowledge of an engine’s camshaft stops at lift and duration. Or perhaps it doesn’t even really go that far. So often, it seems, we tend to understand and select a particular cam on its merits as told to us by someone else, perhaps one whose engine combination is especially applicable to the cam he’s using, but not necessarily best suited to our specific engine.

The subject of cam function and design is a rather large one, so it is the intent of this month’s Series to deal with the ground-level basics of cams: What they do. How they do it. What terms are used to describe their parts. And what these terms mean. So that by the time you get around to picking your next cam, there’ll be a little more substance to your selection than, “Gimme that one ’cause I like its looks.” We know because we’ve been there. First of all, suppose we discuss what a camshaft is supposed to do in an internal combustion engine.
Air and fuel pass into an engine
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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
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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.
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It is at this point when the valve is being “dwelled” in its open position—no (or relatively little) upward or downward motion—that the line of action between cam lobe and lifter lies along the follower’s axis.
At all other times, there is a “pressure angle” (as shown in the illustrations) that tends to produce a side thrust motion in the lifter. This increases lifter drag or friction in its boss and should be avoided where possible by designing lobe profiles that produce the greatest amount of near lifter travel for a given amount of lift. And while this may seem momentarily deep, it is meant to point out that the relationships between lobe shape and lifter design (and type) are critical to best valve action and maintaining continuous contact between lobes and lifters, especially when lift rates and engine rpm are high. Valve springs can be depended upon to do just so much, and even these have limits of performance, as many a drag racer’s parts budget rejects. Of the various types of basic lobe shape, perhaps the parabolic (with constant acceleration and deceleration of follower motion), parabolic with constant velocity, and simple harmonic motion are the more common in automotive applications.
At least in theory, parabolic lifter motion has the least amount of follower acceleration for a specific lobe lift and engine speed. A standard deviation of this method includes periods of constant lifter velocity, in addition to parabolic motion, where it may be useful to have zero lifter acceleration and constant velocity along an opening or closing ramp. This is the second basic lifter motion method.
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Pistons and rings as they relate to an internal combustion engine

The burning of air and fuel inside an engine creates both heat and pressure. It is the job of pistons and piston rings to contain this pressure so that the maximum amount of work is transmitted to the engine’s crankshaft and vehicle drivetrain. There is also a measure of oil control that must be provided by an engine’s oil rings, rounding out the piston ring requirement. Just how all this is accomplished, in addition to what constitutes basic piston design, is the subject of this month’s Series. First, let’s define some basic terms.
Spend a few minutes studying the illustrations. These will introduce you to the fundamental terminology of pistons and rings. Note that cylinder pressure can be used to improve ring-to-wall seal. This is a typical method in the building of race or high-performance street engines where it is beneficial to have low piston ring tension (against the cylinder wall) yet maintain good combustion pressure ring seal.
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Let’s now get into some of the functional considerations in the understanding of both pistons and rings.
There are five basic design features of a piston. It should (1) reduce operating friction, (2) transfer maximum heat to the cylinder walls, (3) be of low net weight, (4) provide necessary piston pin support (to cylinder pressure loading), and (5) prevent oil passage into the combustion chamber (inasmuch as piston design can accomplish this).
The consideration of how well a piston lubricates a cylinder wall should include the fact that its oil is partially “carbonized.” This is the result of oil clinging to the cylinder wall above the piston during combustion, resulting in exposure to the burning air/fuel mixtures. This condition also affects piston ring design and material selection. As mentioned in a previous Shop Series, there is a relationship between combustion surface
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Because this design incorporates 360-degree support of the ring belt area, there is excellent transfer of heat and retardation of piston ring groove deflection. This particular feature reduces the criticalness of machining tolerance requirements and makes for a less expensive piece to produce, unless material costs are higher than for a comparable design.
The closed-type design is probably the most popular among original equipment manufacturers. It is simple to design and has most of the better (and fewer of the poorer) qualities of the other two basic types. Piston-to-wall clearances can be on the order of 0.0015-0.0025-inch (diametrical), which is far less (and much quieter) than that expected from a true high-performance piston. And they’re usually less expensive. Piston rings and ring-land designs are regulated by the intended applications, frequently standarized by the SAE (Society of Automotive Engineers). This is especially true of ring design with ring-land configurations often determined by the experience of a given piston ring manufacturer.
Lower piston skirt area directly influences net piston noise, piston life and stability in the cylinder bore. Actually, this is the area of a piston that extends from the centerline of the piston pin to the lowest extremity of the piston. By comparison, the upper skirt area is fixed by the compression height minus the combined height of all rings and ring-land thicknesses.
Two “thrust” areas exist on any piston. The minor thrust side is opposite piston pin offset (per illustration). Here a piston must be capable of absorbing pin offset loads, in addition to those generated when a piston
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Piston ring breakage can result from a variety of conditions. Excessive side clearance, detonation, ring-end deflection leading to the cocking of rings in their respective grooves, and ring sticking are all sources of ring failure. Insufficient end gap can also lead to ring failure. Such a problem can result in damaged or broken rings and ring lands, excessive oil passage into the combustion chamber, fouled spark plugs and a sour engine. High-temperature creep in piston rings (especially top and second rings) can lead to toss of ring tension and failure to provide adequate gas seal. Another problem associated with incorrect end gap is called “butting,” as evidenced by bright worn spots at the ends of a ring. This can be corrected by increasing the amount of end gap for the size of cylinder in which rings are used. As top rings are moved closer to the top of a given piston, end gaps should be correspondingly increased to compensate for higher combustion temperatures and attendant ring expansion.
So much for all the nice theory. As this Series has developed during the last year, increased concern for over-the-highway engine efficiency and fuel economy has become more prominent. With this in mind, we offer the following with respect to piston/ring selection and fitting for these types of engines.

First, as engine rpm decrease, more piston ring tension (against cylinder walls) can be incorporated without concern for high-rpm power loss. If net engine rpm is going to be less than 4000, you can begin working toward better cylinder pressure seal without fear of high-rpm power loss from friction between cylinder walls and rings.
Moving the top compression ring up on the piston will reduce crevasse volume, decrease the amount of exhaust gas left in the combustion space and improve fuel economy. Frictional losses between pistons and cylinder walls are also critical to low-rpm fuel economy, since power lost to friction increases the amount of fuel required to provide everyday engine operation.
What you want to keep in mind is that the burning of air and fuel creates both heat and pressure. Unless your engine’s pistons and rings are capable of converting this form of work into usable and fuel-efficient power, you might not be able to afford the next

REVIEW QUESTIONS: True or False
1. Low-tension piston rings are beneficial for over-the-highway fuel economy.
2. Top rings operate at much lower temperatures than oil rings.
3. Crevasse volume is the space between the cylinder wall, piston and upper surface of the top compression ring.
4. “Barrel shape” relates to the dimension of a piston’s skirt as it contacts an engine’s cylinder wall.
5. Piston pin offset has nothing to do with “piston rattle.”
6. Oil clinging to cylinder walls subjected to an engine’s combustion process does not affect piston ring function.
7. An open-type piston design incorporates a slot located just above the oil ring.
8. Closed slotless piston designs are not related to high-performance pistons.
9. The most popular piston design of original equipment manufacturers is the closed-type design.
10. Major and minor piston thrust areas have little to do with piston noise and wear.
11. A piston ring’s end-gap dimension is not affected by ring temperature.
12. Free graphite is not usually found in cast-iron piston rings, regardless of ring location on a piston.
13. Cast-iron oil rings are typical in contemporary engines.
14. Molybdenum disulphide is a substance so hard that it is seldom used in piston rings.
15. Detonation cannot affect piston ring efficiency.
16. Piston ring location has nothing to do with net engine performance.
17. Bill Jenkins doesn’t read HOT ROD’s Shop Series.
18. Most pistons weigh too much.

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