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'   cfAus-es AND PREVEN‘HON om
swam Wm

Thais {or tho 0qu of M; S.
.MscHIGAN STATE COLLEGE
Rbberf Olaf Ringoen

195-1

0-169

Date

This is to certify that the

thesis entitled

Causes and Prevention of Engine Wear

presented by

Robert Olaf Ringoen

has been accepted towards fulfillment
of the requirements for

M.S. degree in Mechanical Engineering

,/

Major professor ‘ ~

llay 17 , 1951

 

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3135 A139 PSEVFIII‘I‘IOZ‘I CF ENGINE WEAR

(3
:p
(—4

A TEES IS

Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Hechanical Engineering

1951

THESIS

3.; P
\'
fl“.

ACKNOWLEDGEMENTS
The author is deeply indebted to Mr. Paul 3.
Lane of the Muskegon Piston Ring Company and Mr.
C. W. Ohly of the Thompson Products Company for
their co-operation in supplying pertinent informa-
. tion on various phases of the engine wear problem.
Sincere appreciation is also expressed for
the suggestions offered‘by Professor G. W. Hobbs
of Michigan State College in the preparation of

the manuscript.

‘V b ; ‘.;(
“afifltn 9‘)

I.
II.
III.
IV.
V.
VI.
VII.
VIII.

Table of Contents

Introduction

Pattern of Cylinder Wear
Classes of Cylinder Wear
Abrasion

Erosion

Corrosion

Conclusions

Bibliography

CAUSES AND PREVENTION OF ENGINE WEAR
I. Introduction

The subject of engine wear is a timely one in
view of the fact that present-day engines are being
designed to operate at higher speeds and greater
specific outputs than ever before. Such conditions
are bound to increase the wear to which the engine
is subjected, and make it imperative for the design-
er to consider all phases of the wear problem before
attempting to bring about its reduction.

Viewed broadly, the various factors which con-
tribute to cylinder wear are numerous and complex.
Often the various contributors to engine wear are
interrelated with one another. In addition, changes
in design and operating conditions to reduce one
type of wear may often result in increased wear in
some other form.

During the past fifteen to twenty years, a
considerable amount of original research has been
done on the wear problem. All phases of the ques-
tion have been dealt with, some in great detail.

It will be the purpose of this thesis to review some
of the results of this research with particular ref-
erence to recent literature on the subject. Also,

since most original research on the problem usually

deals with only one or several aspects or phases
of the question, it might be well to correlate and
combine the results of this work in such a way as
to show its relation to the entire picture.

Before proceeding with the subject at hand,
it might be advisable at this point to state that
this discussion will be confined to the wear problem
as it applies to the power assembly only-- consist-
ing of the cylinders, pistons, and rings. The rea-
son for this restriction is that most engine wear

is normally experienced here.

II. Pattern of Cylinder Wear

Before taking up the wear problem and all its
ramifications, it first might be well to discuss
the various characteristic patterns in which cyl-
inder wear is manifested.

Figure 1* shows a typical wear pattern for a
worn cylinder bore. It may be seen that maximum
wear occurs at the top of the stroke-- opposite
the top piston ring-- and tapers gradually through
a distance of two to three inches, reaching a min-
imum at mid-stroke; after which it remains relative-
ly constant until the end of the piston ring travel,
where it again increases slightly. Below the ring
travel the wear is almost negligible.

One of the foremost authorities on the wear

problem, Alex Taub,l

states that all cylinders of
the same engine do not wear uniformly; the rate of
wear varies from cylinder to cylinder. This vari-
ation in.wear is shown in Figure II.** In some
cases, however, greater bore wear is noted in the
intermediate cylinders which serves to illustrate

the complexity of the problem.

 

“lanarque, P. V., "Piston Ring and Cylinder
wear in Automobile Engines", En ineerin , December
22, 1944: Figure l, p. 498

**Taub, Alex, "Cylinder Bore wear and Corrosion“,
Autoggtive §2g_Aviation Industrie , March 1, 1944: p. 36

 

 

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Circumferential wear is not always uniform,
either. Tests conducted at the National Bureau of
Standards by C. S. Bruce and Jesse T. Duck2 indi-
cate that the point of minimum wear occurs, almost
without exception, on the side of the cylinder op-
posite the one receiving the thrust of the piston.
Maximum wear was found to usually occur on the sides
of the cylinder in line with the engine, except in
a few cases where the greatest wear was on the thrust
face of the cylinder. Sparrow and Scherger,3 of the
Studebaker Corporation, are of the opinion that the
piston rings, rather than the piston itself, are the
major contributors to bore wear. The mechanics of
the wear problem as related to the piston rings and

cylinder bore will be discussed at length later on
and will not be taken up in detail at this point.

III. Classes of Cylinder Wear

Among the authorities on the subject of cylin-
der wear, an almost complete agreement exists on
the theory that three distinct classes of wear do
occur. These types may be designated as follows:

1. Abrasion

2. Erosion or scuffing

3. Corrosion

As far as the relative importance of the classes
is concerned, considerable controversy still exists.
Max Roensch,4 now of the Ethyl Corporation, is of
the opinion that the factors as named in the order
above are in their approximate positions of impor-
tance. It should be borne in mind, however, that
variations in operating conditions, as well as chan-
ges in the design of the engine, air cleaner, piston
and ring equipment, and type of lubricating oil used,
may markedly affect the order of importance of these
factors.

Having now established the prime factors to
which engine wear may be attributed, it now becomes
necessary to discuss in detail the mechanics involved
in each of the three classifications and to show how
the various causes of wear are related to these main

classifications.

IV. Abrasion

Defigitiog g; Abrasion gag Sggggg§.g§_Abrag;ge Ngg;

Abrasion may be defined as wear which is brought
about by the scratching action of foreign particles
in the oil film between the two rubbing surfaces.

Abrasion is probably the most common form of
wear and can take place only under conditions of
boundary lubrication. Usually this type of wear
takes place in two distinct stages:

1. A wearing down or breaking off of the sur-
face peaks to produce sufficient area to
carry the load.

2. The scratching of at least one of the sur-
faces by hard particles.

The first stage may be taken as the cause of
high initial wear, as when two new surfaces are run
together. The second stage of wear usually results
from abrasives embedded in the metal.

The mechanism of abrasive wear alters in degree
with the hardness of the surfaces. In soft material,
or hard materials with soft spots, the abrasive is
embedded almost completely in the surfaces and a
lapping action takes place. With hard materials the
abrasive particles are embedded only sufficiently to

hold them in position and the rubbing surfaces acquire

a scored appearance.

Abrasives commonly encountered are metal par-
ticles, metal oxides, dust, carbon, and engine oil
sludge.

The most common sources of entry of abrasive
material, according to Roensch, are:

(1) Core sand, cast iron filings, chips and dirt
which may be left in the engine and not taken
out by the cleaning and washing process.

(2) Valve grinding compound or cylinder honing
residue which may be left in the engine due
to imprOper cleaning.

(3) Road dust which may enter the crankcase with
the ventilating air.

(4) Road dust which enters the engine through
the intake system.

While all the above sources of abrasive wear are
important, the last mentioned deserves the greatest
amount of attention, as it is the greatest single
source of cylinder and ring wear.

In abrasion, as well as in all other forms of
engine wear, cylinder and ring wear are interrelated;
thus any attempt to reduce cylinder wear will result
in a reduction of ring wear at the same time. Espec-
ially high rates of wear have been noted on the top

piston ring under abrasive conditions. This is un—

9
doubtedly due to the fact that the top ring serves
to pulverize the foreign material until the maximum
particle thickness is less than the oil film; thus
the abrasive action on the rings below is diminished.

Tests conducted by Sparrow and Scherger indicate
that abrasive wear has the same general characteris-
tics as encountered in normal operation (1.9., the
greatest rate of wear is at the tap of the bore) with
the exception that the entire wear pattern has been
multiplied or exaggerated under such conditions.

The question as to the quantity of dust entering
an engine depends on the efficiency of the filtration
system. Roensch states that the use of oil bath air
cleaners may remove 95 per cent of the air-borne dust,
but even the 5 per cent which remains may still cause
appreciable wear. He also points out that with cars
operated on paved roads without effective air clean-
ers the cylinder bore wear due to abrasion is increas-
ed (in summer driving) by 25 per cent. Am M. Brenneke,5
of the Perfect Circle CorpOration, states that abnor-
mal wear rates may be expected if the amount of dirt
admitted is in excess of 0.00025 grams per cubic foot
of air. This applies to air after it leaves the filter
and as it enters the engine, and refers to abrasive
particles of 5 microns or less. Particles larger than

this would have an even greater damaging effect.

10

Prevention o§:Abra§;Qg, In discussing the ways
and means of preventing abrasive wear it would be well
to consider the following factors, sinhe they have a
direct bearing on the extent to which abrasion can
occur:

1. The efficiency of the air cleaner in removing

air-borne dust.

2. Piston ring materials and design.

3. Cylinder-block hardness and alloy content.

4. Amount of abrasive material in the engine oil.

The efficiency of the carburetor air-cleaner is
of paramount importance, since if the dust-removal
capacity of the air cleaner is high, a major source
of abrasive wear has been eliminated. .Considerable
research is being currently conducted on the effect
of air filters on engine wear by the Fram Corporation
of Providence, Rhode Island. A progress report on
these activities was submitted by W. S. James and
B. G. Brown, both of the Pram Corporation, before the
S.A.E. Summer Meeting in June, 1950. In this report
dust tunnel tests are described in which the efficiency
of oil-bath and oil-wetted air cleaners are determined
by measurement of the engine parts before and after
the test, the difference in these values being an

index of the amount of wear which occurred during

l-l
operation. Those items which were measured included
increase in pistOn ring gaps, change in radial thick-
ness of the rings, changes in ring groove clearance,
changes in cylinder bore diameter and changes in bear—
ing clearances, thickness, and weight. A summary of
their findings includes the following:

1) Some oil—wetted types of cleaners are of no

I value in removing air-borne dust.

2) Oil-bath air cleaners will, in some cases,
reduce rings and bore wear to one-tenth of
that occurring in oil-wetted types.

3) An increase in air-cleaner efficiency of from
98 to 99 per cent may reduce engine wear by
one-half.

The effect of piston—ring design and material

on abrasion is worthy of attention. It has been
found that an.increase in piston-ring width results

in reduced abrasive ring wear. This is shown in
Figure III,‘ the ring wear being measured by the in-
crease in end clearance. From this relationship it
would seem that if abrasive wear resistance were the
only consideration, the widest possible rings would
be the best. Unfortunately, however, there are other
considerations to be taken into account which preclude
this possibility.

As regards piston-ring materials, tests by James

 

*Erenneke, A. M., "How Diesels Wear and What to Do
About It," S.A.E, Journal, April, 1950: Figure 4, p.36

 

 

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and Brown6 show that the use of chrome-plated top
rings will reduce the bore wear from_abrasion by 75
per cent over that for grey iron rings of the same
section thickness and unit pressure. In addition,
an increase in thickness of the chrome plate on the
top ring also contributes to increased ring life.

The use of chrome-plated cylinder liners in
heavy-duty engines is sometimes employed as a means
of minimizing abrasion. The subject of liners and
also the effect of cylinder block hardness and metal
structure on wear will be discussed at length later
on, and will not be taken up at this time.

The effect of abrasive materials in the engine
011 has been studied by c. G. Williams7 in his ex-
tensive research on cylinder wear. Tests were con-
ducted by him in which silica dust of 200-inch mesh
size was introduced at the carburetor air horn at a
rate of 8.5 grams per 100 hours. This rate is con-
siderably greater than would actually occur, even
under dusty road operation, so that the extremely
large increase in wear indicated would not likely
be realized in actual service. Figure IV* shows the
results of this test-- cylinder wear being plotted as
a function of oil dilution for both abrasive and non-

abrasive conditions. Dilution of the oil in these

 

*Williams, C. G., "Cylinder Wear and What to Do
About It", Automobile Enginee , July, 1933: p. 260

13
tests was accomplished by adding various percentages
of kerosene to the engine lubricating oil. Th in-
teresting feature to note is that, even in the pres-
ence of excessive amounts of abrasive, rapid wear
does not occur until dilution of the oil reached 80
per cent. bove this figure, however, a very rapid
increase in wear occurs which would lead to the con-
clusion that abrasives in the oil are harmful at high

amounts of oil dilution.

14
V. Erosion

Definition g§_Erosion and Sourggg g: Erosive
Eggg. Assuming that clean air is supplied to the
engine so that abrasive effects may be disregarded,
we may then focus our attention on a second source
of engine wear; namely, erosion or scuffing.

Erosion may be defined as wear produced by
metal-to-metal contact between the piston or rings
and the cylinder bore. The mechanics of erosion is
discussed by P. V. Lamarquea, who postulates that
under such conditions of wear a frictional form of
failure is induced due to the existence of boundary
lubrication conditions between the rubbing surfaces.
Local welding of the rubbing surfaces takes place,
accompanied by a sharp rise in temperature of the
welded areas, even when thesurrounding metal mass
is cold. This temperature rise takes place as a re-
sult of the liberation of energy following the shear-
ing of the metallic bridges. It is worthy of note
that only chemically clean surfaces are subject to
erosion; thus the presence of surface films on the
contacting surfaces will act as inhibitors toward
this form of wear.

Lamarque also states that the scuffing tenden-
cies of metals decrease with increases in surface

temperature. Thus, in the case of cast iron, in-

15
creasing the temperature from 300 to 450 degrees P.
will reduce the scuffing resistance by one-half.

Since erosive wear is brought about by metal-
to-metal contact of rubbing surfaces, the sources
of erosion would be those factors which contribute
to such a condition. These may be listed as follows:

1) Absence of lubricating 011 films between the

rubbing surfaces, due either to delay in es-
tablishing these films or dilution of the
film with fuel as in cold starting or warm-
up conditions.

2) Improper mixture ratios under both starting

- and fully warmed-up conditions.

3) Mechanical and thermal distortion of the

cylinder walls.

4) Piston-ring design, material, and surface

finish.

5) Piston design, material, and surface finish.

6) Undesirable metal structure of the cylinder

bore iron.

All of the above mentioned factors, except the
last one, contribute either directly or indirectly
to the establishment or destruction of the lubricating
oil film between the sliding surfaces of the piston
rings, or piston, and the cylinder wall.

It might be well at this point to discuss each

16
of these factors individually and in some detail,
as the minimization of erosive wear depends primarily
on the extent to which these conditions exist in en-
gine design and Operation.

Iub;;gation g§_the Rubbing Surfages and Egosive

Wear. The maintenance of an adequate lubricating oil

 

film is one of the prime requisites in the minimiza-
tion of scuffing. This depends chiefly on the follow-
ing factors:
1) The quantity of the oil supplied
2) The viscosity of the oil
3) The time required, as in cold starting, be-
fore adequate lubrication is established
4) The degree to which the lubricating oil has
been diluted by the fuel
Effect g§_g;; Quantity. The question as to the
effect of the quantity of oil supplied on bore wear
has been dealt with.by C. G. Williams. As a result
of tests conducted by him, he concludes that, with
cylinder temperatures ranging from 250 to 500 degrees
F. and under steady running conditions, a deficiency
in the amount of lubricating oil supplied to the cyl-
inder bores is unlikely to be a factor of practical
importance in regard to cylinder wear. At tempera-
tures below 194 degrees F., it is found that wear is

influenced to a marked extent by the quantity of

1?
lubricating oil supplied to the cylinder walls. A
discussion of the temperature effects on wear will
be presented in the section on corrosion, and will
not be taken up here.

The fact that the cuantity of oil supplied has
little effect on bore wear at normal Operating temp-
eratures does not mean that lubrication problems are
non-existent under these conditions, however. W. A.
Robotham9 points out that the wide speed range of
modern engines imposes severe problems in providing
adequate lubrication of the cylinder walls. This is
evident from referring to Figure V*, which shows the
relationship between engine speed and oil consumption.
This curve was obtained from dynamometer tests of a
1942 Chevrolet engine with 216.5 cubic inch displace-
ment, and operating at road load conditions. The
engine lubrication system was of the combined splash
and pressure type; ring equipment consisted of two
1/8 inch S.A.E. taper face compression rings and one
3/16 inch drilled channel 011 control ring. Pistons
were of the cast-iron slipper type.

‘Although the shape of this curve may vary some-
what among engines other than that tested, depending

on the lubrication system and ring equipment used,

 

*Piston Ripg Manual, Muskegon Piston Ring Company,
p. 50

 

 

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it is evident that if the speed range is increased
the amount of oil passing the rings at maximum speed
will be prohibitive unless the 011 control maintain-
ed by the rings is increased correspondingly. If
this is done, the Oil consumption under low speed
Operation may become microscopic, resulting in boun-
dary lubrication conditions and accelerated bore wear.

Effegt 9£_Q;I,Vi§cogity. The effect of oil vis-
cosity on erosion is discussed by RObotham, in which
‘he states that low-viscosity oil has been over-empha-
sized as a wear deterrent. Low-viscosity oils, he
adds, have a lower film strength which may contribute
to increased wear. He states that records taken from
cars in customers' hands fail to show any definite
wear reduction when the Oil is changed from S.A.E. 30
to S.A.E. 20. In cases where thin Oils are used, the
degree of ring control must be increased proportion-
ately to avoid excessive Oil consumption at maximum
speed, which might well counteract the advantages of
improved lubrication when starting. Robotham also
states that running-in experiments indicate that thin
oils are not as good as heavier ones under boundary
lubrication conditions.

Effect g§_2g;gy_;p_lubrication. The time requir-
ed to establish an oil film on the cylinder walls is

of paramount importance. Assuming that the engine is

19
started from a cold condition, the lubricating Oil
may be too viscous to be thrown freely onto the cyl-
inder walls during the first few minutes of opera-
tion. A certain amount of time must elapse before
the oil reaches the proper viscosity to insure ade-
quate lubrication of the cylinder walls. Obviously
the time required for the oil to attain this condi-
tion depends on the initial viscosity of the Oil,
the lower Viscosity oils showing to advantage in this
respect. Often this time interval is not reckoned
with in considering the lubrication requirements of
the cylinder assembly. The actual time for estab-
lishing an oil film on the cylinder walls will depend
to a large degree on the efficiency of the Oil con-
trol rings.

P. V. Iamarque discusses results obtained in
motoring tests on a 6-cylinder, le-litre engine
which indicates to some degree the time element in—
volved. The engine- the bore, piston and piston
rings of which had been thoroughly cleaned and dried
before the test- was motored at 1000 r.p.m. with the
cylinder head removed and the time observed for an
oil film to form on one of the cylinders. The re-
sults are shown in Figure V1* for three oils of S.

A.E. grades 60, 50, and 10. In connection with these

 

*Iamarque, P. V., "Piston Ring and Cylinder Wear in

Autzggbile Engines," Engineering, December, 22, 1944:
p.

 

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20
tests, Lamarque states that in the case of the high-
er viscosity oils a large proportion of the time re-
quired to establish an oil film on the cylinder
walls is due to delay in oil discharge from the big
end of the connecting rod. Thus in the case of the
S.A.E. 60 oil, for instance, of the 200 seconds re-
quired to form an oil film at 14° F., 120 seconds
elapsed before any oil was thrown from the big end.
Additional tests were also conducted in which the
speed was reduced to 500 r.p.m., where it was found
that the time required to establish an oil film was
twice that required at 1,000 r.p.m. The delay in
this case was attributed not to the time required
for oil discharge at the big end bearing, but rath-
er the increased time required to spread the oil on
the cylinder walls.

Considerable variation in the time required for
oil films to form on different cylinders of the same
engine also occurs. Tests conducted on a 6—cylinder
engine, motored at 700 r.p.m. with the head removed
and bores wiped dry as before, showed that with an
S.A.E. 50 oil the time required for excess oil to
appear at the piston tops varies from 3 minutes to
over 30 minutes. Such a condition might help to ex-
plain the reason for non-uniform wear rates in dif-

ferent cylinders of the same engine.

21

Effect 9: 9;; Dilution. The question as to what
effect dilution of the oil has on scuffing has been
the subject of some investigation. C.G. Williams
undertook some original research on the problem in
which he conducted tests on a single-cylinder engine
operating at a speed of 1,600 r.p.m. and a brake
mean effective pressure of 59 lbs. per square inch.
The oil in the crankcase was diluted with various
percentages of kerosene, which resembles the lower
end of the gasoline volatility range most responsible
for crankcase dilution. The results are shown in
Figure IV. This curve indicates that up to 90 per -
cent dilution of the oil, little increase in wear
occurs provided normal Operating temperatures are
maintained. Tests were also run on kerosene alone,
and even then excessive wear was not observed; al-
though the viscosity of kerosene is l/26th that of
lubricating oil.

gig Eg§l_Mixtuzes -- Ratios éQQ.§ylindeg Egan,
Alex Taub10 states that under cold starting condi-
tions a 1/1 air fuel ratio is required to produce
an inflammable mixture. Obviously under these con-
ditions, when cylinder wall lubrication is Just be-
coming established, scuffing may easily occur due to
the washing of the lubricant off the cylinder walls

when excessive use of the choke is employed.

22

Under warming-up conditions Taub indicates an
8/1 or 9/1 mixture ratio is needed, while for fully
warmed-up, part-throttle operation any ratio up to
17.5 to l is desirable. During warm-up it has been
found that both cylinder and ring wear decrease as
the air-fuel ratio is decreased from 13/1 to 8/1.
Figure VII* shows the results of tests conducted on
the effect of mixture ratios on cylinder wear. The
tests were carried out on a single-cylinder engine
operating at 1,600 r.p.m. and a b.m.e.p. of 59 lbs.
per square inch. Examination of Figure VII shows
that as the mixture is progressively enrichened, the
wear on the top piston ring is reduced for low-mixture
temperatures. When the mixture temperature is in-
creased to 122° F., enrichening of the mixture ratio
has no effect on either ring or cylinder wear over a
range of air-fuel ratios from 10/1 to 14/1.

Under high temperature operation increases in
air-fuel ratio appear to produce the reverse effect.
Taub cites the case of an engine under test in which
the air-fuel ratio was increased from 12/1 to 14/1,
which resulted in a three to seven fold reduction in
cylinder bore wear. The fact that the engine was

definitely under-oiled might account for this exceed-

 

*Williams, C. G., "Cylinder Wear in Gasoline Engines,"
S.A.E. Jougngl (Transactions), May, 1936: p. 193

23
ingly large reduction; nevertheless, lean mixtures
do appear to have a pronounced effect in reducing
bore wear at high temperatures. Normally, it might
be thought that lean mixtures would promote oxidiz-
ing conditions within the cylinder which would affect
the bores adversely; thus it would seem that some
other reason must be given to account for this be-
havior. Two theories have been advanced by Taub
which might explain this phenomenon: l

l) Increases in the air-fuel ratio at high tem-
peratures will minimize dilution.

2) Leaner mixtures at high temperatures result
in lower explosion pressures which tend to
reduce the gas pressure behind the rings,
and consequently the pressure exerted by the
rings on the cylinder wall.

0f the two theories, the latter would seem to
be the more plausible; since dilution takes place
primarily under cold starting conditions.

In the case of warm-up operation a decrease in
the air~fuel ratio is believed to bring about a re-
duction in the formation of corrosive agents, par-
ticularly 002, thereby reducing bore wear. A dis-
cussion of corrosion and its effects on wear will
be withheld until later.

Since bore wear has been shown to be affected

24
by mixture ratio, it might well be that this is one
of the causes for non-uniform wear between different
cylinders of the same engine; inasmuch as the vari-
ous cylinders have different mixture ratios, depend-
ing on their particular location in the distribution
system.

Effect g§_Cylinder 2199K Distortion gg_§ggg,fl§gz,
Cylinder block distortion may be attributed to two
factors; namely,

1) Thermal distortion

2) Mechanical distortion
Both of these factors are primarily due to the pres-
ent practice of combining the cylinders in a monobloc
casting. Distortion of the cylinder barrel, pro-
duced from either of these two causes, may give rise
to high pressure areas which tend to break down the
oil film between the piston rings and cylinder wall,
resulting in scuffing of these rubbing surfaces.

Lamarque states that thermal distortion, more
than any other factor, is responsible for abnormal-
ities in the manner in which a cylinder wears. Im-
proper cooling of the cylinder walls is one of the
primary sources of thermal distortion. Care must be
taken to arrange for proper distribution of the cool-
ing water around each cylinder. The use of a water

distributing tube and full-length water Jackets will

25
go a long way toward preventing bore distortion from
uneven cooling and elimination of local hot spots.

The region near the top of the cylinder bore is
probably the most critical, and it has been found
that hot spots in this zone can rapidly reach a tem—
perature at which breakdown of lubrication may occur.
Recent research indicates that it is not unusual for
the temperature of a hot spot in a cylinder barrel
to increase from 330 to 500 degrees F. in 90 seconds,
after increasing the engine speed from 2,000 r.p.m.
to 3,500 r.p.m.- the temperature of the adjacent areas
of the barrel remaining substantially constant. This
non-uniform temperature condition would, of course,
readily promote thermal distortion of the cylinder
barrel.

Probably the chief source of mechanical dis-
tortibn of the cylinder barrel is at the Joint between
the head and block, although thermal distortion plays
a part here, too. Mechanical distortion at this
point is due to the bolting of the head to the block,
especially where inadequate support has been provid-
ed for the cylinder stud anchorages. Placement of
these anchorages on the Jacket wall, rather than the
cylinder wall, so that the pull of the stud produces
pure tensile stress on the material of the block may

help to prevent this difficulty. Increasing the

26
thickness of the cylinder wall in the section near-
est the top may also reduce distortion.

Piston Rigg.Desigg and Cylinder Weag, Max Roensch
states that prOper design of the piston rings is of
outstanding importance in the prevention of cylinder
wear by erosion. An enormous amount of research has
been and is being done by piston-ring manufacturers

in an attempt to bring about further improvements
in the design of piston rings which will contribute
to longer engine life.

In discussing piston-ring design and its effect
on cylinder wear it might be well to consider the
following factors:

1. Ring sticking and blow-by

2. Radial pressure

3. Ring width

4. 011 control

5. Ring materials and surface finishes

gggg_8ticking. While ring sticking is not ac-
tually tied up with piston-ring design, it is a fac-
tor which contributes to excessive scuffing of the
rings and bore. Ring sticking is caused by the ac-
cumulation of sufficient cementitious material in
the ring clearances to prevent movement of the ring
in its groove. In the intermediate stages, the ring

action becomes sluggish and results in increased oil

27
consumption. Restriction in the free movement of
the ring impairs its gas sealing pr0perties and gives
rise to an abnormal amount of blow-by passing from
the engine breather.

The chief source of ring sticking is the oxi-
dation and subsequent deposition of unstable products
of the lubricating oil in the ring clearances, scrap-
er ring oil holes, and rubbing surfaces of the pis-
ton. Two forms of ring sticking occur:

1) Low temperature ring sticking

2) High temperature ring sticking

Low temperature ring sticking, as its name im-
plies, occurs in cold weather operation, where idling

time constitutes a high prOportion of total operating
itime. Under such conditions carbonaceous material
derived from the products of combustion, together
with condensed water formed during the burning of the
fuel, acts to produce emulsification of the lubricat-
ing oil. The so-called sludge is then filtered out
into the ring clearances and scraper ring oil holes
where subsequent oxidation and decomposition occurs.
The only solution of the low temperature ring stick-
ing problem is to prevent the formation and accumu-
lation of moisture by rapid warm-up of the engine and
by providing adequate crankcase ventilation under

light-load conditions.

28

High temperature ring sticking is caused chief—
ly by oxidation of the oil, mainly in the crankcase,
followed by decomposition of the oxidation products
at the high temperature points in the engine.

Ring sticking in compression-ignition engines
is also believed to be caused to some degree by the
fuel used. Aldehydes and unsaturated acids produced
by hydroxylation during combustion may resinify easily;
when they reach the ring grooves, they may result in
ring Sticking. Ring sticking in diesel engines is a
much more common complaint that in gasoline engines.

Blow-by. Blow-by, or the leakage of combustion
gases past the piston at high engine speeds, has be-
come an increasingly important problem. Under con-
ditions in which blow-by occurs, the piston rings
reach such a stage in the engine speed range where
their operation becomes unstable due to flutter or
chatter. At this point a rapid increase in blow-by
occurs (Figure VIII*) which results in rapid wear of
the rings and bore and, in extreme cases, breakage
of the rings. The point at which flutter of the rings
occurs depends upon the ring tension, the radial pres-
sure pattern, and the fit of the ring in the bore.
Thus it will be seen that control of blow-by depends to

a large degree on Judicious ring design.

 

*Taub, Alex, "Cylinder Bore Wear," Automobile Engin-
eeg, March, 1939: p. 85

 

 

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29

One of the most common sources of blow-by is
cylinder distortion, which was discussed in the pre-
ceeding section. In bores which have become distor-
ted the piston ring is unable to conform to the con-
tour of the cylinder barrel. At these points of non-
conformity, hot blow-by gases pass the rings, which
increases the temperature. A heavy oil film collects
at this distorted area, as the rings cannot make con-
tact with the cylinder walls with sufficient pres-
sure to reduce the oil film thickness. This excess
oil, in combination with the existing high temper-
atures, produces a layer of hard carbon on the cyl-
inder walls. The formation of this layer of carbon
prevents adequate lubrication of the cylinder wall
and prohibits the proper sealing action of the rings.
As‘a result, scuffing of the piston ring and cylin-
der walls occurs accompanied by further blow-by and
excessive oil consumption.

Obviously, in new engines, or in engines which
have new cylinders, blow-by may be somewhat of a
problem during the running-in period. That this is
true has become an established fact, and it has
been observed that the main destructive agency dur-
ing the running-in period has been the presence of
blow-by with its accompanying evils. In new cylin-

ders the piston rings must accommodate themselves

30
to changes in the shape of the bores due to the re-
lease of casting and machining stresses. Such con-
ditions promote bore distortion with resulting blow-
by and ring scuffing. Taub advocates the use of
pinned rings as a means of reducing scuffing caused
by blow-by. Such rings would reduce the time of
the running-in period and would promote better and
faster bedding-in, since the rings would then be
restricted as to rotational motion and would there-
fore conform to the contour of the bore more readily.

The question as to how much blow-by is permis-
sible is answered by Taub with the statement that
if blow-by is less than 1 cu. ft. per minute, no
scuffing will occur. If the blow-by is above 6 cu.
ft. per minute at 4,000 r.p.m. damage from scuffing
will result in two hours' running time. Excessive
amounts of blow-by also hasten the formation of
sludge in oilways and scraper ring grooves, which
may often lead to ring sticking and even piston seize
ure; however, a slight amount of blow-by in a new
engine is to be preferred over one in which piston-
ring wall pressures are so high that no blow-by ex-
ists.

Radial Pressure and Cylindeg Wear. The present

 

trend of modern engines toward high output and high

speeds have dictated changes in the amount of radial

31
pressure to be exerted by the rings, as well as the
manner in which this pressure is to be applied.

From 1920 to 1927 the common practice was to design
rings for a uniform radial pressure with a diametral
ring tension of from 7 to 9 pounds. From 1927 to
1932 the ring tension was increased to values rang-
ing from 9 to 11 pounds, and from 1932 to 1937 the
tension was increased to 16 to 18 pounds. At this
point it was found that increased temperatures,
pressures, and speeds made the control of blow-by a
dominant factor, and that further advances in piston-
ring design must be brought about by control of the
pressure pattern, as well as increased radial pres-
sure.

In Figure VIII it was seen that a break occur-
red in the blow-by curve at 3,400 r.p.m. Above this
speed blow-by becomes excessive, and at 3,900 r.p.m.
the blowbby is 4% cu. ft. per minute. By prOper de-
sign of the radial pressure pattern, the break in
the blow-by curve may be shifted to occur at higher
_engine speeds.

The requirements of a prOper pressure pattern
for a compression ring have been set forth.by R.R.
Teetor and H. Bramberry in a booklet published by
the Perfect Circle Piston Ring Company:

32
"Free piston ring shape is most important
in obtaining good performance and long life.
The most important portion of the rings to
form is the 180-degree are opposite the back
half; that portion having the Joint. The ring
must be shaped so as to exert maximum pressure
on the cylinder wall at the points and yet
have such a shape on either side of the Joint
as to exert sufficient pressure immediately
after installation to prevent blow—by. To
accomplish this result has required consider-
able study of the proportions of radial wall
thickness and free Joint opening to the diam-
eter. When these proportions are correct,
the ring will conform at once to the curve
of the cylinder wall with sufficient wall
pressure at all points."
Teetor has also stated that the useful life of
a ring having such a pressure pattern is governed
by the point pressure; when the point pressure reach-
es zero, the ring is worn out. An ideal pressure
pattern, advocated by Taub for rings of this type,
is shown in Figure IX? The high point pressure
typical of such a pattern is evident in this diagram.
Paul 8. Lane, in a private communication, also
supports the theory of high point pressure rings.
He states that inasmuch as the points or ends of
the ring are the weakest sections, Sufficient rig-
idity must be provided to prevent point flutter.
Also high point pressures should be provided to pre—
vent wear and fatigue from making the points "nega-
tive" as to circularity. If the points are provided
with sufficient "plus" circularity when new, the wear

occurring in service should cause the shoulders

 

* Taub, Alex, IICylinder Bore Wear", Automobile En:
gineer, March, 1939: p. 86

 

 

 

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33
(the region adjacent to the points) to make good

peripheral contact against the cylinder wall. It
is Lane's belief also that uniform contact of the
ring with the wall throughout the entire circum-
ference, or "light-tightness", is essential for
satisfactory ring performance. Examination of the
radial pressure pattern (Figure IX) shows lower
radial pressures at the shoulders, which might lead
one to wonder whether such a condition would provide
satisfactory light-tightness.

ging_flidthgggd_Cylindeg Egan, Another factor
which should be considered in connection with pis-
ton ring design is the effect of compression-ring
width on ring and cylinder bore wear.

It has been found that a decrease in the width
of the compression ring will result in a reduced
amount of scuffing. It will be remembered that the
reverse was true in the case of abrasive wear, where
narrower rings were found to have an adverse effect.
H. G. Braendel11 of the Perfect Circle Company at-
tributes this phenomenon partly to the fact that
narrower rings operate at face pressures which are
much less than would be the case for wide rings in-
stalled in cylinders of the same bore and subjected
to the same combustion pressure. Calculations made

by him show that face pressure ratios for two such

34
rings, both with the same radial wall thickness,
vary from 1.7 to 1 to 2.6 to 1 depending on whether
the frictional force between the ring and the groove
is acting with or against the back pressure of the
ring. Obviously the effect of a lower face pressure
would be to diminish the tendency for the ring to
scuff. This would be especially true at the t0p
portion of the stroke, since here the face pressures
would be at a maximum due to the high gas pressures
during combustion. Under such conditions the face
pressures might reach such a magnitude in a wide
ring that the lubricating 011 film might be squeez-
ed out at the relatively low sliding velocity during
this portion of the stroke, resulting in heavy wear.

Braendel also points out another reason which
might account for reduced wear on the part of nar-
row compression rings. He indicates that narrower
rings may have a greater degree of axial conforma-
bility to the cylinder wall. This would mean that
a narrower ring would conform to the bore at any
particular time with a greater proportion of its
area in contact with the wall than would a wider
ring. In.many cases, he believes, the actual con-
tact area may be the same for both.narrow and wide
rings, with instances occurring where the narrower

ring may have even a greater contact area.

35

The effect of ring width on blow-by is also im-
portant to note. Examination of Figure X‘E shows that
as the ring width is decreased, the break in the
blow-by curve is shifted farther to the right. This
would indicate that the sealing capacity of narrower
rings is superior to those of greater width.

Another advantage in the reduction of ring

12 of the Per~

width is pointed out by Harold Myers,
fect Circle Company. He states that when narrow
rings are used the inertia loads on the ring groove
may be reduced, thus preventing the type of failure
as shown in Figure XI*#

9;; Control. The problem of oil control as it
affects cylinder wear is extremely complex and im-
plies a delicate balance between a number of design
factors. If an oil control ring can be designed to
exert a uniform pressure against the cylinder walls
throughout the length of the stroke, regardless of
the shape and changes of shape of the wall, a uni-
form oil film will be metered to the compression
rings and the tendency toward scuffing will be vir-
tually eliminated. With such a condition a corres:

pending decrease in oil consumption will then be ex-

 

‘1amarque, P. V., "Piston Ring and Cylinder wear",
En inee in , Dec. 22, 1944; Figure 8, p. 518

*‘Iiviyers, Harold, "Engine Wear", S,A.E. Jogmal,
August, 1951: p. 49 .

 

36
perienced, resulting in the paradoxical result that
engines which run relatively dry will, upon disas-
sembly, show exceptionally low wear. Fulfillment
of such conditions also indicates that minimum uni-
form oil metering has been attained and that the com-
pression rings have been provided with sufficient
lubrication at all times.

Proper 011 control implies considerably more
than keeping oil out of the combustion chamber.
Among the prime functions of a well designed oil
ring is to provide a circulation of the oil supplied
to the cylinder assembly. The capacity of the ring
to pass or circulate oil is important; obviously,
this is determined to some degree by the face width
of the ring itself. Thus oil rings with wider face
widths may have larger slots for oil drainage. For
example, in a 3/16-inch oil ring the slots may be as
wide as 0.093 inches; while in a 5/32-inch oil ring
the slot width is usually only 0.062 inches-- a re-
duction of almost one-third in oil-handling capacity.
In engines which are prone to sludging, varnish or
carbon formations, adoption of greater width oil rings
will provide increased ring life.

Adequate 011 control is essential if a copious
oil supply is to be maintained on the cylinder bores.

Improper design of the piston and rings, or disinte-

 

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37
gration of the rings due to blow-by, may contribute

to faulty oil control. Taub cites an example of
such conditions in an engine where the oil supply
is copious but an inadequate ring combination is
used, such as two low-tension compression rings and
two low-pressure oil rings. When the engine is new,
the two oil rings will hold back a considerable share
of oil and additional control will be maintained by
the compression rings. Later on, when the compres-
sion rings become subjected to blow-by, they will
soften and become covered with carbon, reducing their
efficiency all the more. The blow-by gases then af-
fect the upper oil ring, which causes it to lose
tension and become covered with carbon. The 011
control has now been reduced to a minimum with the
oil supply still remaining a maximum, with the re-
sult of trouble, even though no serious wear may be
indicated on the cylinder walls. Taub also points
out that a difference between scraper rings of 9
pounds and 13 pounds diametral tension may mean a
50 per cent increase in oil consumption with the low-
‘er tension rings.

Another factor, besides ring tension, which in-
fluences oil control is the magnitude of the radius
on the scraping edge of the compression and oil

rings. It has been shown that a radius of 0.010

38
inches on the skirt ring will result in a marked in-
crease in oil consumption. Obviously the sharpness
of this edge would govern the amount of oil stripped
from the cylinder walls by the ring. Changes in oil
viscosity would alter the sharpness or radius to be
maintained-- thinner oils requiring a smaller radius
to maintain the same degree of oil control.

ging_Materials gng Surface Finishes. The effect
of the material used in piston-ring manufacture should
not be underestimated. Even though attention to pro-
per design and proportions of the ring are important,
considerable thought must be given to the ring mater-
ials as well. Although grey iron is the universal
material for compression rings in automotive use,
some types of engines may require metallurgical char-
acteristics beyond the scope of this material. It is
obvious that, above all, the material must possess
good wearing properties; however, some irons which
posses this property do not necessarily make satis-
factory rings. Lubrication conditions may often over-
shadow variations in material specifications. Per-
haps one of the most important properties to be con-
sidered is that the ring should wear away with very
little tendency to accumulate the wear products on
the rubbing surfaces. A metal structure which, in

abrading, will break down into extremely small par-

39
ticles that will cause little disturbance to the
surfaces affected is essential. The "Piston Ring
Manual", published by the Muskegon Piston Ring Com-
pany, gives the following chemical analysis for auto-
motive type cast iron rings of 3/32 to 3/16 inch

section:
Silicon -- 2.60 - 3.00 per cent
Sulphur -- 0.08 max.

Phosphorus -- 0.40 - 0.70

Manganese -- 0.55 - 0.70

Carbon -- 3.50 - 3.75

Rockwell "B" -- 98 - 105 B

The use of steel oil rings has been a rather
recent innovation. These rings Operate under rel-
atively high unit pressures of 150 to 200 pounds
per square inch. This increased pressure restricts
greatly the amount of oil supplied to the upper ring
belt and may sometimes contribute to greater wear
of the cast iron compression rings. An advantage
of this type of ring is that it can restore oil
economy to engines having badly tapered bores with-
out the expense of reboring the cylinders. The
hardness of these steel rings ranges from 350 to 400
Brinnell, as compared with 240 to 260 Brinnell for
cast iron rings. This increased hardness has been
found to have no ill effects if the softer iron in
the cylinder walls is provided with normal lubrica-
tion. In fact, the high hardness of the steel used

in these rings reduces the tendency of the rings

40
toward becoming loaded with abrasive material which
may produce a lapping action on the cylinder walls,
causing increased wear.

The importance of surface finishes on piston
rings is receiving an increasing amount of attention.
Surface finishes may contribute toward reduced scuf-
fing during the running-in period only, or finishes
may be used which result in reduction of ring and
bore wear throughout the entire life of the ring.

0f the former class, the most common surface
treatments used are metallic plating, oxide coating,
phosphate coating, and acid etching. The material
most commonly used in metallic plating is tin or cad-
mium, which is plated to a depth of 0.0001 to 0.0003
inches. The rings may be either plated all over, or
on the outside diameter only. All-over plating has
an advantage in that it affords additional protection
from rusting. The oxide coating or Ferox treatment
consists in producing a coating of iron oxide by
treatment at 1,000 degrees F. in a gaseous oxidizing
agent. The phosphate coating is produced by immer-
sion of the ring in a water solution of phosphoric
acid saturated with iron and manganese phosphate at
210 degrees F. Both of these coatings are 0.00025 -
0.00030 inches thick. The acid etching treatment

(Graphitox) consists in impregnating the etched sur-

41
face with colloidal graphite which provides the
metal with excellent self-lubricating qualities.

A variation of the conventional Ferox treat—
ment is sometimes practiced in the manufacture of
piston rings for diesel engines. In order to facil-
itate the use of wide rings in these engines and
still reduce ring scuffing to a minimum, grooves are
cut in the ring faces to interrupt the continuity of
the metallic surfaces. The grooves are then filled
with a mixture of iron oxide (Ferox) bonded with
sodium silicate. Figure XII“ shows a cross section
of this type of ring. Brenneke states that the scuff
resistance of this type of ring in l/4-inch width
is comparable with that of 3/32-inch conventional
, rings.

In order to obtain a maximum degree of scuff
resistance of both rings and bore throughout the en-
tire life of the ring, it is becoming the practice
to employ chrome-plated top rings. The use of these
rings has been employed for some time in truck or
diesel engines and is now being extended to the pas-
senger car field. Present practice employs a plating
of hard rather than porous chrome, ranging from 0.004
to 0.007 inches in thickness and up to 0.008 inches

for heavy-duty equipment.

 

*Erenneke, A.M., "How Diesels Wear and What To Do
About It', S.A.E. Journgl, April, 1950: p. 37

42

Offhand it might be thought that chrome-plated
rings would contribute to increased wear of the cyl-
inder walls due to the high hardness of the chrome,
as compared with the relatively soft material in the
cylinder bore. The reverse appears to be the case,
however, and actually reduced wear of the cylinder
wall occurs when such rings are used. Braendel be-
lieves that two reasons for this peculiar behavior
exist. One is the lack of embedability of the sur-
face of the chrome-plated rings. Abrasive particles
are unable to become embedded in the ring surface
and then act as cutting edges on the cylinder wall.
Not only does this result in reduced cylinder wall
wear, Braendel states, but also the life of the cast
iron rings below the top chrome ring is increased
for the same reason.

A second reason for the reduced cylinder wear
experienced with chrome rings is the resistance of
this type of ring to temperature effects. The fact
that its melting point is higher than that of cast
iron reduces the tendency toward localized welding
of the rubbing surfaces when these are brought to
the melting temperature by insufficient lubrication.

Figures on wear reduction with chrome—plated
top rings are given by Brenneke13 as 80 per cent

over that experienced in the same installation using

 

 

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43
conventional ring equipment.

Piston Design gpd,Cylindeg flggg. Just in the
case of the piston rings, proper design of the pis-
ton will do much to alleviate cylinder bore wear.
Selection of the prOper materials, combined with
correct design procedure, is important. Also, since
piston design influences to a great degree the amount
of oil control maintained by the rings, it is neces-
sary to consider this factor if a satisfactory end
result is to be reached. Principles which should be
considered in proper piston design procedure are the
following:

1. Top land clearance

2. Skirt clearance

3. Ring side clearance

4. Location of the top ring groove

5. Size and position of the ring gaps

6. Degree of oil control to be maintained

7. Material and surface finish

Effegt 9; $92 ngg Clearance. Iemarque cites
results of tests in which the effect of top land
clearance on blow-by was studied. Results show that
the critical speed at which excessive blow-by occur-
red decreased as the top land clearance was reduced.
Thus, in the case of a top land clearance of 0.020

inches, the critical speed is 5,400 r.p.m.; while

44
with a clearance of 0.040 inches, the critical speed
increases to 5,900 r.p.m. A possible explanation
for this behavior might be due to the reduction in
gas pressure acting on the top face of the ring when
the top land clearance is reduced. This would allow
the ring to break away from the lower side of the
groove at a reduced engine speed, giving rise to a
rapid increase in blow-by.

Effect 9: Skigt Clearance. The effect of skirt
clearance on piston and ring wear is important. Even
in the presence of ample lubrication, skirt clearance
has a marked effect on top ring and cylinder wear;
increased skirt clearances inducing higher wear rates
as shown in Figure XIII*. Probably the reason for
this increase in wear with increased skirt clearance
is due to the destructive effect of piston slap.

The use of controlled-expansion pistons such as the
Auto-thermic and belted types, which maintain close
clearances over a wide range of operation, would serve
as an approach to this problem.

Effects g§_3igg,§;gg Clearance. Increases in
piston ring side clearance may be brought about by
mutual wear of the piston ring groove and ring. Such
wear is a common occurrence in the case of aluminum

pistons. According to Myers, most of the wear under

 

*Williams, C. 0., "Cylinder Wear in Gasoline Engines",
S.A.E. Journal, (Transactions), May, 1936: p. 194

45
such conditions occurs on the flat sides of the ring
which may eventually cause it to assume a "T" shaped
section as shown in Figure YIV*. It should be noted
that the cross of the "T" is not in the zone of max-
imum wear, being at the outer periphery of the ring
where it does not contact the ring grooves. If such
wear continues until the side clearance becomes ex-
cessive, pounding of the ring in the groove will
occur which may result in breakage of the ring.

In addition to causing groove pounding and
ring breakage, side clearance has some effect on
oil consumption. Tests, described by Lamarque, on
a single-cylinder gasoline engine fitted with two
compression rings and one oil ring indicate that at
an engine speed of 1,500 r.p.m. an increase in the
top ring side clearance from 0.004 to 0.012 inches
results in a 60 per cent increase in oil consump-
tion. When the engine speed was raised to 3,500
r.p.m. and the same increase in side clearance made,
the oil consumption increased to 120 per cent. In-
creasing the side clearance of both the top and sec-
ond compression ring to 0.012 inches brought a four-
fold increase in oil consumption at 2,500 r.p.m.

Effect of Location of the Top Ring Grgove.

“mun—I.“-

 

*Myers, Harold, "Engine Wear", S.A.E. Journal,
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46

Proper location of the top ring groove on the piston
has been found to minimize ring scuffing tendencies.
Brenneke advocates locating the groove at such a
point that when the piston is at the top of its stroke
the top ring should not travel beyond the end of the
water Jacket. t is obvious that such a practice
would avoid operation of the ring in excessively high
temperature zones which are especially conducive to
scuffing.

Provision of an adequate top land by locating
the top ring well down on the piston also serves to
protect the ring from the direct effects of combustion
as well as reducing its operating temperature. Ring
belts which operate at moderate temperatures have less
tendency toward ring sticking and poor sealing ability,
both sources of blow-by and accompanying rapid wear.
The top ring must also be adequately supported by a
strong second land if it is to have a low wear rate
and good sealing ability. Brenneke advocates 0.20
inches per inch of cylinder diameter for top land
width and 0.05 inches per inch of diameter for second

land W idth 0

.Effect of the Size and Position of the Ring gaps.
It has been found that the size and relative position
of the ring gaps on a piston have a marked effect on

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47
pressure of 15 pounds per square inch, increasing the
ring gap from 0.011 inches to 0.022 inches in an en-
gine with a bore diameter of 3.375 inches gave a two-
fold increase in oil consumption.

The effect of the position of the ring gaps on
oil consumption was observed when two rings, each hav-
ing 0.017-inch gaps, were installed on a piston, first
with the ring gaps on Opposite sides of the piston and
next with both gaps on the same side of the piston.
The oil consumption in the case of the former instal-
lation was slightly less than one-half of that exper-
ienced in the latter.

Effggt g; Piston Design gn_Qil,gontzol. As was
previously pointed out, piston design has a great ef-
fect on the amount of oil control to be furnished by
the rings. A piston which is not structurally rigid
may interfere with proper action of the rings, also
its shape must be such that it will assist in metering
the oil to the bores in order that a given combination
of piston rings may not have to do more than was orig-
inally contemplated in their design. A properly de-
signed piston should supply a new film of oil on the
cylinder walls at each stroke, the function of the rings
being to cut the 011 film to the final smoothness.

Taub points out two factors in piston design

which interfere with good oil control. These are:

48
l. Improper taper of the piston skirt
2. Improper amount of piston ovality

In many cases, due to manufacturing tolerances,

a reverse taper is provided on the piston skirt. Nor-
mally, the taper of the skirt should be such that the
clearance at the bottom is less than near the rings.
Owing to these tolerances the skirt may either be
0.0005 inches smaller at the bottom than near the
rings or it may be 0.001 inches larger at the bottom
than at the ring belt. The oil consumption with a re-
verse taper has been found to be almost double that
used with one which is correct.

In the case of ovality of the piston, tolerances
are allowed for the difference in dimensions between
the major and minor axes. According to Taub, the dif-
ference in oil consumption.between an ovality of 0.004
inches and one of 0.008 inches on one particular pis-
ton was 40 per cent in favor of the small ovality.

Figure XV* shows the design of a four-cycle die-
sel engine piston advocated by H. G. Eraendel. This
piston is noteworthy in that it illustrates how modi-
fications made in the present design can enhance oil
control. Changes made to accomplish this in this case
are:

1. The provision of a sharp edge at the bottom of

the skirt. This feature is said to provide as

 

*Eraendel, H. 0., "Three Piston Design Tips for Im—
proved 011 Control," S.A.E. Journal, April, 1950:
p. 40

49
much oil control as an additional oil ring.

2. The oil control ring groove is provided with

abundant drainage for both lands of the oil
control ring. This is accomplished by pro-
viding an undercut and drilling the drain—
back holes with their center lines coinciding
with the lower plane of the oil ring groove.

3. The addition of an undercut of approximately

25 per cent of the radial thickness of the
ring on the land between the oil control ring
and the lowest compression ring. This under—
cut serves as a reservoir space for the oil
scraped on the down stroke by the lowest com-
pression ring. On each subsequent up-stroke
this oil may then flow back to the inside of
the piston through the side clearance of the
oil ring.

Eigton flaterigl,agd_8urfacg Finish. As far as
the effect of piston material on cylinder wear is con-
cerned, there appears to be no great difference in th
amount of wear regardless of the material used. In
the case of aluminum pistons severe ring groove wear
has been found to occur, especially in heavy-duty,
high-speed engines. One remedy for this difficulty is
a bimetallic piston with a ferrous metal ring carrier

which is either cast or mechanically attached to the

50
aluminum piston. The increased hardness of ferrous
materials is often offset by the poorer thermal prop-
erties of this material. As a result, ring belt temp-
eratures may be much higher, causing a tendency to—
ward sticking.

Among the most common surface treatments applied
to pistons are tin plating and anodizing. Both treat-
ments facilitate run-in and lessen scuffing, espec-
ially during the warm-up period.

Under cold starting conditions the cooling water
surrounding the cylinder walls may prevent them from
warming up as quickly as the piston. As a result the
piston may become larger than the bore, causing "cold"
scuffing of the rubbing surfaces, especially when lube
rication is scanty. Anodizing prevents this by pro-
ducing a hard surface, and at the same time, one which
is porous enough to hold oil. Tin plating also pre-
vents this condition, the tin acting as a metallic
lubricant under the high shearing forces which ac-
company cold scuffing.

A rather new surface finish which has been ap-
plied to super-charged diesel engine pistons has been
developed by the Zollner Corporation. It consists in
hnurling the rubbing surface of the piston to provide
a series of tiny valleys or reservoirs for oil, which

materially improves the lubrication of the piston,

51
rings, and bore. Piston seizures, which previously
had been reported, have been prevented with his type
of finish.

Metal Stzucture and Surface Finish of tTe gzlr

inder Bore ggyRelated tg_§ea§ Resistance. In the
past, it has been the belief that bore hardness, rath-
er than metal structure, has been the main factor in
governing wear resistance in cylinder iron. Undoubt-
edly the reason for this belief has been.based on the
abrasive wear theory; for it is known that under pure-
ly abrasive conditions an increased hardness of the
cylinder bore will reduce wear. Actually a cylinder
iron of extremely high hardness, such as provided by
nitrided cast iron, is a distinct disadvantage from
the standpoint that such a material lacks the surface
bearing qualities of the more normal irons; also, such
an iron is more prone to scoring under conditions of
limited lubrication, especially when in combination
with equally hard surfaces.

The question of metal structure and its effect on
wear and scuffing has been dealt with in some detail
by Paul S. Lanel4. It is his belief that the wear
properties of cylinder iron are governed to a great
degree by the speed of cooling of the casting which is,
in turn, largely determined by the section thickness.

Research work conducted by Lane and others indicates

52
that cylinder wall thickness is a critical factor in
controlling the cooling rate of the metal in zones
at, or adjacent to, the bore. Failure to consider
this relationship between wall thickness and cooling
rate may result in the establishment of micro struc-
tures in the iron which may produce abnormally high
wear rates and consequently reduced engine bore life.

From the point of view of obtaining a desirable
metal structure a case exists for the use of cylinder
liners, either wet or dry, because of the more uniform
cross-section and the greater simplicity of the cast-
ing involved.

In regard to the surface finish of cylinder bores
it has been found that, in addition to the actual
smoothness of the surface, the metal structure left
by the finishing process is of some importance.

Iamarque points out that cutting and abrasive
methods of finishing cylinder bores result in a rel-
atively fragile, crystalline surface structure which
is not particularly conducive to the establishment
of a stable oil film. 'During the running-in period
this crystalline structure is transformed into an
amorphous layer which greatly improves the load car-
rying capacity and reduces the rate of wear as long
as adequate lubrication of the rubbing surfaces is

maintained. In addition, running-in tends to remove

53
any loose material remaining from the finishing pro-
cess. A good deal of the uncertainty of the running-
in process might be avoided if, in the first place,
the cylinder bore could be finished to a state ap-
proaching the run-in condition.

One of the most important factors in cylinder
surface finishes is that the finish produced must be
of a sufficient degree of roughness to promote quick
seating and wearing-in of the rubbing surfaces, as
well as maintain sufficient oil retention properties.
Braendel advocates a scratch-honed cylinder with a
cross-hatched pattern having a roughness of 15-30
micro-inches as one which will give the best results.

Treatment of the cylinder-bore walls by a chem-
ical sulphidizing process has been recently employed
as a surface finish. Advantages claimed for this pro-
cess include reduced scuffing during the running-in
process as well as improved oil retention properties.

A new deveIOpment in surface finishes for cylin-
der bores, which shows considerable promise, is the
Bramberry Characterized Liner shown in Figure XVI.“
A "characterized" pattern is cut into a previously
machined, ground and honed liner sleeve. The pattern

consists of two sets of 30-degree helical grooves,

cut in opposite directions, with each groove having

a "V" section and depth of 0.00# inches. This forms

 

*Ohly, C. W.,"?rogress Report on Bramberry Liner,"
Thompson Products Company, January 18, 1950 ,

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54

a completely interconnected network of grooves and
leaves an arrangement of diamond-shaped plateaus
whose total area represents approximately 60 per cent
of the bore surface. .After a careful cleaning, the
grooves are filled with a carbonaceous, plastic base
compound. Following this, the entire bore surface
is sprayed with a very fine graphitic solution which
leaves a thin film of graphite on the wear surface.

Results of field and laboratory tests on such
liners have been submitted by Mr. C. W. 0hly15 of
the Thompson Products Company. Tests show that these
Bramberry sleeves have an average wear rate of be-
tween 0.0003-0.0005 inches per 10,000 miles in con-
ventional cylinders of the same engine. One advan-
tage of such liners lies in the fact that since the
diamond heads are isolated from one another, a hot-
spot develOping in one location does not spread. In
addition, the carbonaceous material has high oil re-
tention properties which, when combined with the for-
ces of capillary action and surface tension, spreads
a thin film of oil over each diamond head. This en-
ables continuous lubrication of the cylinder wall to
be maintained, probably accounting for the extremely
low wear rates with this type of liner.

Reduction in blow-by values of from 30-35 per

cent have also been reported with these liners.

55

Chrome plating of the cylinder wall surfaces
is being employed to some degree, although its use
is more or less restricted to diesel engines and ap-
plication outside the automotive field. The results
obtained are similar to those experienced with chrome
plated top rings. Chrome plating is especially ef-
fective under abrasive wear conditions and often ex-
cells nitrided liners in wear resistance. Abrasive}
wear with chrome plated cylinder wall surfaces is
often reduced in the ratio of 15/1 to 40/1 over that
of alloyed cast iron bores.

Present practice in the chrome plating of cyl-
inder bores is to produce a porous, rather than hard,
chrome surface in order to provide better 011 reten-
tion properties. In some instances only the top third
of the cylinder bore is subjected to the chrome plat-
ing treatment. Under such conditions the reduction
in wear of the top portion'by chrome plating also ap-
pears to reduce the rate of wear of the remainder of
the bore. This is manifested by no sudden increase
in diameter immediately below the chrome-plated por-
tion, even after considerable mileage. The reason for
such behavior might be explained by the fact that chrome
plating the top portion of the bore reduces the amount
of material worn off this portion, such material acting
as an abrasive agent, which acts to increase the amount

of wear during the lower portion of the stroke.

56

VI. Corrosion

Definition g§_Corrosion and Source§_g£,Corro—

 

giyg_ߤgg, As its name implies, corrosion is the de-
struction of the metal surface of the cylinder walls
by oxidation or chemical reaction with the products
of combustion. It is difficult to differentiate be-
tween corrosion and erosion as the two phenomena of-
ten go hand-in-hand. Most authorities are inclined
to agree that the greatest effects of corrosion are
manifest during low temperature or low-speed, light-
load operation. Under such conditions the condensa-
tion of water vapor on the cylinder walls may produce
wear by either one of two ways: by washing off the
011 film, thus giving rise to wear by abrasion or by
corrosion. It is often difficult to determine just
how much wear may be attributed to corrosion alone for
this reason. Surfaces attacked by corrosion often
present a pitted and pockmarhed appearance, as shown
in Zone "A", Figure XVII*. It would appear evident
from this sketch that corrosion is taking place only
in this zone. Actually, a closer examination would
indicate that the entire area designated by "B" has
also been affected, and that in Zone "c" the products
of corrosion have been scrubbed off, leaving a bright

surface.

 

*Taub, Alex,:"5ylinder Bore Wear and Corrosion,"
Automotive and Aviation Indust ies, March 1, 1944:
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57

Knowing that corrosion is caused by acid attack

on metal surfaces, it might be well to discuss those

acids which are formed during the combustion process

and their effects on cylinder wear. Williams, in his

corrosion hypothesis, believes that the following

acids are contributors to corrosion:

l.

2.

3.

Organic acids, such as formic (CH202) and
acetic (02H402) acid which are intermediate
products in the combustion process. Williams
states that formic acid, when added in pro-
portions of 0.5 per cent to the water of com-
bustion, will double the rate of corrosive
wear.

Sulphuric acid formed from the sulphur in the
fuel.

Nitric acid. At high temperatures some of the
nitrogen and oxygen in the atmosphere combine
to form nitric oxide (NO) which can be further
oxidized to nitrogen peroxide; addition of
water to the peroxide will subsequently form
nitric acid. A condition necessary for the
production of nitric oxide is rapid cooling.
Such a condition usually exists within the
combustion chamber during low temperature
operation.

Carbon dioxide. Solutions of carbon dioxide

58
in water are slightly acidic and therefore
possess corrosive properties.

Knowing that corrosive wear is favored both by
the condensation of water vapor and by the acid-
chemical reactions occurring during the combustion
process, it would be well to examine those factors
which might contribute to such conditions. These
contributors might be listed as follows:

1) low cylinder wall and operating temperatures.

2) Inadequate lubrication of the cylinder walls.

3) Chemical and physical characteristics of the

fuel.

4) Piston ring and cylinder wall materials.

Efgegt gflggg Cylinder fla;l_ang,0pezating Eggpr
eratures gg_Co:rosiye Eggg, In view of the fact that
corrosive wear depends to a large degree on the amount
of moisture condensed on the cylinder walls during th
combustion process, it seems logical that reduced cyl-
inder wall and operating temperatures would contribute
to a marked increase in corrosion by allowing a great-
er amount of condensation to take place.

Research on the corrosion problem was undertaken
by Williams in an attempt to determine the relation
between cylinder wall temperatures and the rate of
corrosive wear. As a result of a large number of tests

carried out on several types of engines, he found that

59
it was possible to construct a curve showing the re-
lation between the rate of wear and cylinder temper-
atures under steady running conditions. Examination
of Figure XVIII* indicates that between the range
of 212 to 510 degrees F. the rate of wear is almost
independent of the temperature; while below a temp-
erature of 194 degrees F. there is a rapid increase
in the rate of wear.

The reason for the increase in corrosive wear
at temperatures below 194 degrees F. is associated
with the condensation of moisture from the products
of combustion. In the initial state this water is
in the form of highly superheated steam within the
main body of the combustion gases. Contact with a
sufficiently cold surface, however, is capable of re-
ducing the temperature locally to below the dew point,
thus effecting condensation.

The effect of cylinder wall temperature on cor-
rosion has also been pointed out by Taub. Tests are
described by him in which engines operating under
favorable conditions, including a minimum number of
cold starts, with alcohol and water as cooling media
and thermostats set to operate at 145° F., showed
wear rates as l to 3 with alcohol and water, respective-
ly. The cylinder wall temperature was found to be 30

degrees cooler with water than with alcohol.

 

*Williams, C. G., "Cylinder Wear in Gasoline Engines",
S.A.E. Journal (Transactions), may, 1936: p. 192.

 

 

 

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60

Brenneke is also in agreement on the effect of
cylinder wall temperature on corrosion. It is his
belief that a low coolant temperature is the most im-
portant contributor to corrosive wear. He maintains
that Jacket temperatures below 120° F. should be a-
voided if high rates of corrosive wear are to be
prevented.

Since evidence leads to show that cylinder wall
temperature plays an important part in the low-temp-
erature corrosion problem, it would be well to con-
sider this factor in its relation to cold-starting
procedure. The common theory in the past has been
to operate under very light load and low speed until
the proper coolant temperature has been reached.

From the standpoint of the corrosion theory, this
procedure is the worst possible, as such operation
does not contribute to high Jacket water temperature.
The present recommendation is that the engine should
be brought up to speed and the load applied as quick-
ly as possible.

lubrication ang_lt§_Effect QQ_Coer§ion. It is
obvious that lubrication of the cylinder walls plays
an important part in the reduction of corrosion. Ex-
perience definitely indicates that if an oil film can
be maintained on the cylinder walls the effect of the

corrosive acids will be minimized. Conditions which

61
tend to destroy this film, the most prominent of
which is blow-by, need to be carefully controlled
through proper design of the piston rings.

Even the establishment of an adequate oil film
is not complete insurance against corrosion. Williams
points out that measurable amounts of corrosion have
taken place on oil-covered metal plates after ten
minutes' exposure to an atmosphere of air and condens-
ing steam.

The effect of the quantity of oil supplied on
corrosive wear is also worthy of note. It will be
remembered that under normal operating conditions,
the rate of wear was very largely independent of oil
consumption. Below temperatures of 1940 F., however,
the wear is influenced to a marked extent by the quan-
tity of lubricating oil supplied to the cylinder walls.
Tests conducted by Williams show that by an eight-fold
increase in oil consumption it is possible to reduce
cylinder wear at 1130 F. from 0.0013 to 0.0005 inches
per thousand miles.

The properties of the lubricating oil itself may
also affect the amount to which corrosion may or may
not take place.' Experiments conducted on the effect
of lubricating oil composition indicate that a high
degree of oiliness is necessary to combat corrosive

effects.

62

Egg; Characteristics gag Corrosion. In the
preceding pages, a discussion was presented on the
various acids formed during the combustion process.
The extent to which these acids are formed depends
Sprimarily on the chemical composition of the fuel
used.

Sulfur is one of the most important contribu-
tors to corrosion. Just how sulfur causes wear is
‘not definitely known, but it is believed that most
of the sulfur in the fuel is converted to sulfur tri-
oxide. This, in combination with the water formed
during combustion, produces the highly corrosive sul-
furic acid. Wear has been shown to be proportional
to the sulfur content with amounts of sulfur in ex-
cess of one per cent causing marked increases in cyl-
inder wear. An increase in sulfur content from 0.2
per cent to 1.0 per cent has been reported to give a
two to six-fold increase in ring and cylinder wear.
Since the sulfur content of diesel fuels is normally
higher than for gasoline, more trouble from this source
might be expected in diesel engines.

Sulfur content is not the only factor which may
contribute to increased wear in diesel fuels. Vola-
tility and viscosity of the fuel are also important
in that they affect the combustion process; thus,

fuels of low volatility and high viscosity may cause

63
poor combustion resulting in a dirty engine with
consequent increased wear. Tests show that improper
combustion of fuels containing no sulfur may cause
more wear than a fuel having two per cent sulfur and
burned under favorable conditions.

Considerable research has been done in recent
years on the effect of substitute fuels on corrosive
wear in gasoline engines. Cyclic wear tests conduc-
ted by the National Bureau of Standards using a fuel
composed of 95 per cent alcohol and 5 per cent water
have been described by Eruce, Duck and Pierce. These
tests were conducted on five 1942—mode1 engines oper-
ating 24 hours a day on a test cycle which included
a 20-minute operating time and a 10-minute shut-down
time each half hour. During the 20-minute running
time the engine was operated at conditions equivalent
to a 40 m.p.h. road speed and also at an idling speed
of 500 r.p.m. During the shut-down period cold water
was forced through the cooling system and oil pan in
order to simulate normal starting conditions at the
beginning of the operating period. Wear measurements
made during the test showed that the wear with alcohol
fuels was approximately one-half that of gasoline in
tests made under comparable conditions. The reason
for this increase in wear is believed to be due to the

difference in the corrosive characteristics of the two

64
fuels under conditions of low temperature, inter-
mittent operation.

Effegt 9;; P ston m m C‘rlinc‘ler all m-
iglg 9n Corrosion. Obviously corrosive effects may
be minimized by providing materials which are resis-
tant to acid attack. The use of chrome-plated rings
and cylinder liners is of value in this respect; al—
though some reports indicate that high sulfur fuels
even produce harmful effects on these materials. In
cases where porous chrome plating is used, as on cyl-
inder liners, absorption of corrosive materials into
the pores may accelerate the rate of wear, thus re-
ducing the life proportionately. Under conditions
of high temperatures cracking has been observed on the
surface layers of chrome plate. Such cracks might
permit the penetration of corrosive fluids and thus
introduce considerable corrosive wear.

Considerable attention has been given to the use
of austenitic cylinder and ring materials in England.
Cast iron having 14 to 15 per cent nickel, 6 to 7
Der cent cooper and 2 to 4 per cent chrome is austen-
itic and, as such, has increased resistance to cor-
rosion. In fact, Williams points out, the resistance
of this material to corrosion from certain dilute
acids is several hundred times greater than that of

ordinary cast iron. Austenitic iron, however, does

65
not appear to have any advantage in the presence of
dilute nitric acid. Williams also reports that tests
on austenitic piston rings show thvt the top piston
ring wear with such material is reduced in the ratio
of 2 to 1 over that of conventional rings, and that
a marked reduction in cylinder wear is also realized.
xIn view of the fact that austenitic materials have
a lower abrasion resistance than normal cast iron,
this reduction in wear is undoubtedlv due to the in-
creased corrosion resistance of this material.

Several disadvantages in the use of austenitic
materials are its high cost and poor machinability.
One way of getting around these undesirable factors
is the use of short austenitic sleeves about 1% to
'2 inches long which are shrunk into the top portion
of a cast-iron.bore, the zone of greatest corrosive
attack. The cylinder is then bored to size through-

out its entire length.

66

VII. Conclusions

From the discussion presented, it is evident
that the wear problem, as a whole, is governed by
two primary factors: operating conditions and design
characteristics. The control of wear ty providing
favorable operating conditions alone is difficult,
if not impossible. Only by taking into considera-
tion the detailed design features discussed will it
be possible to build a successful power assembly of
piston, rings and cylinders which will not only pro-
vide extremely low wear rates but also provide good
performance over a long period of operating life,
even under heavy-duty operation.

Application of these design principles in a
number of cases may be, at best, a compromise be-
tween the desired effects to be gained and the in-
creased first cost. Present trends toward high spe—
cific output will eventually make the adeption of
such design features as chrome plated piston rings,
wear-resistant liners and other means of minimizing
wear mandatory, if satisfactory service and long

engine life is to be realized.

67
BIBLIOGRAPHY

Taub, Alex, "Cylinder Eore Wear and Corrosinn",
Automotive and Aviation Industries, March 1,
1944: pp. 36-38

Bruce, C. S., J. T. Duck and A. R. Pierce,
"Effects of Substitute Fuels on Automotive En-
gines", g;§; Bureau 9; Standards Journal of
Research, August, 1948: pp. l35~149

Sparrow, S. W. and T. A. Scherger, "Cylinder
Wear, Where and 3.y", S.A.E. Journal {Transag-
tions), April, 1936: pp. 117-125

Roensch, Max M., "Observations on Cylinder Bore
Wear", filagfii.issznal.iizassasiisaal. March.
1937: pp. 89-96 '

Erenneke, A. M., "How Diesels Wear and What to
Do About It", S.A.E. Jopngl, April, 1950: pp.
34-38

James, W. S. and B. G. Brown, "Engine Wear as
Affected by Air and Oil Cleaners," S.A.E. Summer
Meetina Pa er, June, 1950

Williams, C. G., "Cylinder Wear", Automobile

En ineer, July, 1933: pp. 259-264

Lamarque, P. V., "Piston Ring and Cylinder Wear
in Automobile Engines", En ineerin , December 22,

1944: pp. 495-500; 517-519

68
EZELIOGRAPHY (Continued)

9. Robotham, W. A., "Some Problems of Cylinder Bore

Wear", Engineeriné, May 9, 1947: pp. 392-396

10. Taub, Alex, "Cylinder Bore Wear", Automobile
Engineer, July, 1933: pp. 259-264

11. Braendel, H. G., "Design Features Affecting Wear",
S.A.E. Summer Heating Paper, June 6, 1950

12. Myers, Harold, "Engine Wear", S.A.E, Journal,
August, 1950: pp. 46-49 ~

13. Brenneke, A. M., "Chrome Plated Piston Rings,
Design and Application", §.A.E. Paper, October
3, 1949 '

14. Lane, P. 5., "Wear and Scuffing of Cylinder Bore
iron", Metal_P coress, March, 1941: pp. 315-149

15. Only, C.W., "Progress Report on Pramberry Liners",

Thompson Products 00., January 18, 1950

 

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