Engine Science Camshafts and Pistons http://www.enginescience.maxa1.com Thu, 27 May 2010 19:50:59 +0000 http://wordpress.org/?v=2.9.2 en hourly 1 Engine’s Camshaft http://www.enginescience.maxa1.com/2010/04/15/hello-world/ http://www.enginescience.maxa1.com/2010/04/15/hello-world/#comments Thu, 15 Apr 2010 16:41:19 +0000 admin http://www.enginescience.maxa1.com/?p=1 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

and spent exhaust gas leaves it. And since the combustion process deals with very high cylinder pressures, intake and exhaust valves must seal and hold this pressure to provide usable power. Other than during these times, intake and exhaust valves must be opened and closed to allow the passage of air/fuel mixtures and exhaust gas into and out of the engine’s cylinders. Couldn’t get much simpler, right? But it is the timing of these openings and closings and the duration of these events that govern how a given camshaft affects power output.
On a common shaft there are typically located several noncircular lobes. As the shaft turns (make a good soap opera title, wouldn’t it?), each of these lobes can impart motion to a follower (lifter) that causes a rocker arm to open and close a particular valve. In pushrod engine design, there’s no direct contact between follower and rocker arm (a pushrod connects them). In overhead cam engine design, camshaft lobes impart direct motion to rocker arms.

But regardless of the specific method of valve operation, camshafts with lobes cause valves to (1) open, (2) close, (3) remain open, (4) be lifted a specific amount from the valve seat. Obviously, it is the design (shape) of each lobe and its position relative to other lobes on the shaft that affect valve motion and the effects of valve timing on engine performance.
Perhaps the most important aspect of camshaft design, selection and understanding is where in an engine’s rpm range are optimum volumetric efficiencies going to be required. Cams designed for low-rpm engine operation are not the same as those intended for higher rpm use. Discussion of differences among these types will follow. But to get us into that, let’s now spend a few minutes talking about some of the terminology relative to cam design.
Base circle is not the ring drawn on the ground to locate home plate. It’s the lobe circle (as shown in the illustrations) from which additional radiuses are referenced to cause valve motion.

A. Here you can see the relationships of base circle, lobe lift and valve duration. During times when a valve is in its closed position, follower (or lifter) travel is along the base circle surface of the lobe. From the time when a valve leaves its seat to when it is seated again, “off-seat” time (or valve duration) allows mixture/gas flow. Note that net lobe lift is measured from base circle radius to optimum lift, not from the point at which lift motion begins. B. The angular measurement (in crankshaft degrees) from the centerline of an intake valve lobe to the centerline of a corresponding exhaust valve lobe is called “lobe displacement angle.” Variations of this relationship govern the amount of “overlap” between intake and exhaust valves.


]]> http://www.enginescience.maxa1.com/2010/04/15/hello-world/feed/ 0 Maximum Lift http://www.enginescience.maxa1.com/2010/04/15/maximum-lift/ http://www.enginescience.maxa1.com/2010/04/15/maximum-lift/#comments Thu, 15 Apr 2010 19:21:25 +0000 admin http://www.enginescience.maxa1.com/2010/04/15/maximum-lift/ 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.

]]> http://www.enginescience.maxa1.com/2010/04/15/maximum-lift/feed/ 0 Position of the Camshaft http://www.enginescience.maxa1.com/2010/04/15/position-of-the-camshaft/ http://www.enginescience.maxa1.com/2010/04/15/position-of-the-camshaft/#comments Thu, 15 Apr 2010 19:32:48 +0000 admin http://www.enginescience.maxa1.com/?p=26 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

]]> http://www.enginescience.maxa1.com/2010/04/15/position-of-the-camshaft/feed/ 0 Amount of Lift http://www.enginescience.maxa1.com/2010/04/15/amount-of-lift/ http://www.enginescience.maxa1.com/2010/04/15/amount-of-lift/#comments Thu, 15 Apr 2010 19:42:03 +0000 admin http://www.enginescience.maxa1.com/?p=35 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.

Simple harmonic motion of a follower is especially suited to roller lifters, since maximum pressure angles will normally be smaller than with either types of parabolic motion. This also means that there will be less power required to rotate the camshaft, which is of particular benefit when either rpm or valve spring pressures are made high. Cams are frequently designed using combinations of these methods of follower control, and further examination of the theoretical aspects of each is beyond the scope of the Series. Just keep in mind that from the initiation of valve lift, up to a maximum, and down to the end of lift, we are dealing with an “elastic” system of energy in which there is damping, harmonics (especially since we are concerned with valve springs), and a general transfer of forces into and out of an entire valvetrain assembly. Aside from all this, the prevailing objective of the camshaft is to provide the proper amount of valve timing and lift to optimize cylinder filling in a specific range of engine rpm.
Such variables as total piston displacement, span of rpm, geometric relationship of crankshaft stroke and connecting rod lengths, cross-sectional size of intake and exhaust port passages, size of intake and exhaust valves, and compression ratio can each or all affect valve timing requirements. Thus, a particular lobe design and arrangement for one engine can perform totally differently in another. A cam that’s “big” in a 350-c.i.d. engine gets even “bigger” in an engine of smaller displacement. And one that’s “big” in a small engine becomes effectively “smaller” in a larger engine.
In many respects, we can return to a previous Series where it was suggested that atmospheric pressure “forces” air into an engine; and that the ability of atmospheric pressure to accomplish high levels of cylinder filling depends upon the difference in pressure between cylinder and atmospheric pressures; and that there is no such thing as vacuum, only the absence of atmospheric pressure.
Camshafts control valve motion. If an intake valve is opened too early (relative to some rpm), some amount of cylinder pressure will be lost into the induction system (reversion, if you will). If it opens too late, some time is lost in which to load the cylinder. Should an exhaust valve open too early, some effective “work pressure” will bleed to the atmosphere and power “will suffer. And if it closes too late, there’s a good chance of drawing some residual exhaust gas back into the cylinder (since the piston will have already begun the intake stroke).
So aside from the criticalness of designing suitable valve motion dynamics into a particular lobe, precise valve timing is required to optimize engine performance. It was the intent of this month’s Series to point out some of the basic terms and how they relate to specific camshaft functions. Should you decide that a more analytical or mathematical exploration of the subject would be to your liking, we suggest locating basic textbooks on the dynamics of mechanisms (found in most libraries).
Meanwhile, you’ll now be able to dazzle the guy behind your favorite parts counter with some fresh terms and knowledge. And when you’ve finished finding out what he knows about pressure angles and parabolic follower motion, you can still point to the cam you’ve chosen and say, “Gimme that one. I still like the way it looks.”

REVIEW QUESTIONS: True or False
1. A flat-nose lifter can normally bench-press more than 340 pounds.
2. Duration is measured in crankshaft degrees and relates to the amount of time required for either an intake or exhaust valve to reach maximum lift.
3. Lifter (or follower) motion off the lobe base circle is when valve lift usually begins.
4. Opening and closing “flank rates” have little to do with rate of lifter acceleration.
5. Camshafts typically rotate at twice crankshaft speed.
6. Camshaft lobe displacement angle and lobe centerline are terms that mean the same thing.
7. A camshaft ground with a displacement angle of 108 degrees means that there are 108 crank degrees between the intake lobe spacing of any two adjacent cylinders.
8. Valve overlap periods are normally long for street-type cams and short for race-type, high-rpm engines.
9. Advancing a camshaft (relative to piston position) tends to improve high-rpm power with little effect on low-rpm torque.
10. Convex valve lifters provide valve action similar to that of roller lifters.
11. Spherical followers and convex followers are the same thing.
12. When a valve (or follower) reaches the nose of a given lobe, it is said to be in its “dwell” position, where there is little or no upward or downward motion taking place.
13. Theoretically, parabolic follower motion provides the greatest amount of lifter acceleration, making it well-suited to race lobe designs.
14. Simple harmonic lifter motion is well-suited to flat-nose follower designs, especially where hydraulic lifters are used.
15. Connecting rod and crankshaft stroke relationships are about the only two primary engine variables not affected by camshaft design.

]]> http://www.enginescience.maxa1.com/2010/04/15/amount-of-lift/feed/ 8 Pistons http://www.enginescience.maxa1.com/2010/04/15/pistons/ http://www.enginescience.maxa1.com/2010/04/15/pistons/#comments Thu, 15 Apr 2010 19:57:27 +0000 admin http://www.enginescience.maxa1.com/2010/04/15/pistons/ 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.

Such an approach has also been used in engines built for fuel economy whereby the top compression ring has been located very near the piston’s top, thus reducing so-called “crevasse volume” or the air space standing between the cylinder wall and piston just above the top compression ring. There’s a photo showing one experimental example of crevasse volume reduction for purposes of improved fuel economy and lowered exhaust emissions. You might know of this approach as a Dykes ring design, but it’s different in that the top ring is shaped like a laid-over letter U: with the open end of the U facing the piston. Just a little food for thought.
Compression rings help hold combustion compression; thus the name. Oil rings are designed to help lubricate cylinder walls while preventing the passage of oil into the combustion chamber. Oil in this area can encourage detonation, reduce fuel economy, and generally diminish overall combustion efficiency.

Piston domes (or cavities) are used to establish specific mechanical compression ratios, and piston skirts are intended to provide support within a cylinder while aiding lubrication to remainder of the piston. The barrel shape of a piston skirt assists piston wear (and engine silencing) during times when a piston is “cocked” as a function of connecting rod angle and load. Piston skirt “cam” is a feature affecting the amount of skirt material in contact with the cylinder wall. As shown in the illustrations, the greater the cam angle, the greater will be the amount of thrust loading absorbed by a given piston’s skirt. And the more a piston can be designed to absorb thrust loadings, the less force will be transmitted into the cylinder walls. This reduces friction, which decreases power loss, which increases power and fuel economy, which . . . lived in the house that Jack built. Other design features are included in the illustrations, along with explanations.

A. Here you can see basic piston terminology, including major and minor thrust locations. Of particular note is the fact that piston pin offset can be used to counteract combustion pressures on the piston, resulting in the reduction of piston-to-wall frictional losses. Connecting rod length is a consideration in determining the amount of pin axis offset. For example, the longer the rod the less the tendency for a piston to become “cocked” in the cylinder, thus affecting the amount of offset that can be used. Piston skirt “barrel” (dotted line) helps maintain skirt clearance and aids wear life. B. Piston skirt “cam” dimension relates to the amount of skirt material in contact with cylinder wall (shaded area). As this contact area is increased, thrust loads are distributed over a greater amount of piston skirt material.

]]> http://www.enginescience.maxa1.com/2010/04/15/pistons/feed/ 0 And Rings or Fries http://www.enginescience.maxa1.com/2010/04/15/and-rings/ http://www.enginescience.maxa1.com/2010/04/15/and-rings/#comments Thu, 15 Apr 2010 20:06:48 +0000 admin http://www.enginescience.maxa1.com/?p=54 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

and combustion volume. And the greater the combustion surface-to-volume, the lower the temperature of oil above a piston during combustion.
All this nice theory may seem of little value right about here, but it relates directly to how combustion-efficient an engine actually is and how long it can run before pistons and rings require replacement. And that relates to something called dollars.
Basic piston design can be categorized as follows: closed type, open type, and closed slotless type (which form the basis of many high-performance piston designs). The closed-type design incorporates a machine-turned (or sawed) slot located in the oil ring groove, separating the skirt area from the area in which all ring grooves are cut (the so-called “ring belt”). Such a cut allows good heat transfer from the ring belt to the piston skirt and aids operational bore-to-skirt clearance.
The open-type design of piston incorporates a slot located just below the oil ring. This gap separates the ring belt from the piston’s skirt. It is placed and designed to reduce piston skirt distortion and help control piston-to-bore clearance. It also helps free the oil ring from distortion caused by piston skirt loading. All this aids oil control and improves the retention of cylinder pressure, which is both power and fuel economy.
The closed slotless design relates to the high-performance type of piston. Here there is no separation slot between the ring belt (piston head) and skirt. This method provides very good resistance to high cylinder pressure and works well with high horsepower and rpm levels.

C. Additional compression ring pressure against the cylinder wall can be accomplished by the drilling of small (usually 0.025-inch or thereabouts) holes down through the tops of pistons. Such holes are located to intersect top compression rings in the “back clearance” area (behind the ring) so that ring-to-wall pressure is increased. This improves cylinder pressure seal and aids net engine output. Although usually associated with race-type engines, this particular piston modification can also improve an engine’s fuel economy by virtue of increased cylinder pressure and more net work on pistons. D. Cylinder pressure paths typically follow down onto the top compression ring (forcing ring movement against the cylinder wall) and then onto the second compression ring (for similar pressure movement). It is this type of pressure/ring relationship that leads to low-tension rings (pressure against the cylinder wall) that maintain good pressure seal during an engine’s power stroke. And in most cases, reduced ring pressure (other than during the power stroke) against the cylinder walls increases net power output. E. These are typical compression and oil scraper ring designs. Varations on these types have been developed, but the basic concept of each remains unchanged. And while we are not able to show all variations of piston rings currently in use, these represent the basic types.

]]> http://www.enginescience.maxa1.com/2010/04/15/and-rings/feed/ 0 Groove Ring Man http://www.enginescience.maxa1.com/2010/04/15/groove-ring-man/ http://www.enginescience.maxa1.com/2010/04/15/groove-ring-man/#comments Thu, 15 Apr 2010 20:15:24 +0000 admin http://www.enginescience.maxa1.com/?p=63 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

cycles from upward to downward motion within the cylinder.
The major thrust side is involved in piston “noise” control. If you’ve ever heard the audible results of “piston slap,” you know the sound we’re discussing. This is the result of the major thrust side of a piston hammering its face against a cylinder wall. And as major thrust side frictional loads are increased, some net power is lost. It’s a careful balance among piston pin offset, pin location relative to compression height, and how fast the engine is running.
In terms of piston ring terminology, we should probably mention two basic areas: (1) end gap and (2) side clearance. End gaps are provided to allow installation of a ring and compensate for thermal expansion of ring material in the presence of combustion heat. End gaps that are excessive allow cylinder pressure to be lost past the ring, and gaps too tight cause premature ring wear against cylinder walls and a general lack of correct piston ring performance. Side clearance allows for vertical ring expansion, in addition to allowing combustion gas pressure to provide ring pressure against the cylinder walls (see illustrations for specific methods by which this is accomplished).
Piston rings are normally made of fine-grained cast iron, although some versions using stainless steel have been successfully used in racing engines. Such rings are typically top and second compression rings, the preference being the top ring. Free graphite found in fine-grained cast iron is one reason this material is commonly used. This is beneficial during certain conditions of ring and cylinder wall lubrication. The insertion of molybdenum disulphide (MOS,) or “moly” in compression rings can improve ring wear (such moly has lubricity properties comparable to graphite). This material also allows for increased oil ring control, since less engine oil is required to maintain proper compression ring seal and life.
Top rings, because of proximity to combustion heat and remoteness from the cooling effects of crank-case oil, tend to run hotter than rings located lower on a piston. This condition makes adequate top ring service life difficult, so it is common to have these rings plated with tin or a porous chromium material that is applied to the face (friction) of the ring. Of these two plating materials, chromium seems to be superior (especially when used in conjunction with hard-surface cylinder walls), since oil becomes trapped in the chrome surface irregularities, resulting in improved lubrication and parts life.

f. This is an example of crevasse volume reduction by the design of a top compression ring located near the piston’s top. Such a ring is U-shaped, with the open end of the U facing toward the piston. A loose fit on the piston assures ring-to-wall pressure during times when cylinder pressure is high (power stroke). End gaps of such rings are critical, since combustion heat is nearer and material expansion is greater, resulting in an increase in gap dimension requirement. The same is true for compression rings not of this particular design. Increased combustion heat means more expansion of the top rings. End gaps should increase accordingly.

]]> http://www.enginescience.maxa1.com/2010/04/15/groove-ring-man/feed/ 0 Fouled Spark Plugs http://www.enginescience.maxa1.com/2010/04/15/72/ http://www.enginescience.maxa1.com/2010/04/15/72/#comments Thu, 15 Apr 2010 20:25:30 +0000 admin http://www.enginescience.maxa1.com/?p=72 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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