3.15.
Piston-rings
Piston rings (Fig. 3.90) are comprised of com-
pression rings, located towards the top of the
piston and oil-control (scraper) rings, located
below the compression rings. The function of the
compression rings is to seal the space between the
cylinder wall and the piston preventing the escape
of burning gases from the combustion chamber.
These rings help to obtain maximum power from
Fig. 3.90. Piston ring.
the combustion pressure by maintaining a seal
with the cylinder wall while keeping the friction at a minimum. The oil rings control the flow
of oil along the cylinder walls and keeps oil from getting into the combustion chamber. Both the
rings help to dissipate some of the piston heat of the cylinder wall.
3.15.1.
Piston-ring Nomenclature
Ring diameter is the diameter of the cylinder bore in which the ring operates. Radial
thickness is the shortest distance between the outer and inner circumferential faces of the ring.
Ring width is the distance between the top and bottom side faces of the ring. Side faces are
the flat parallel upper and lower faces of the ring which contact the sides of the ring groove.
Working face is the outer circumferential surface, which contacts the cylinder wall. Free joint
gap is the circumferential distance between the two open ends of the ring in the free state.
Fitted gap is the circumferential distance between the two open ends of the ring when it is
placed in its groove. Tangential load is the force applied tangentially between the two open
ends of the ring which is necessary to close the free joint gap to its fitted clearance. Cylinder-
wall pressure is the radial outward force per unit area of contact, assuming the pressure to
be equally distributed around the ring.
3.15.2.
Factors Affecting Ring Performance
(a) Radial Thickness.
The pressure exerted by a ring on the cylinder wall varies ap-
proximately as the cube of the ring’s radial thickness. This thickness is usually related to the
ring diameter, d, and should be at least rf/24.
(6) Width.
As increase in ring width increases the radial load on the cylinder wall, for a
given cylinder-wall pressure. Therefore, excessive width should be avoided. For a given tangen-
tial loading, narrow rings bed into the cylinder wall more quickly than wide ones, and also reduce
ring flutter and resultant blow-by of combustion products at high engine speeds.
(c) Free Joint Gap.
With a large free joint gap, the stress in assembling a ring over its
piston is less, while the stress in the fitted working piston is more. The situation reverses with
a small free joint gap. A nominal free gap is normally 3.5 times the radial thickness.
id) Radial Load and Pressure. The radial outward load exerted by the ring on the cylinder
wall depends upon a number of factors such as radial thickness, width, free joint gap, and the
strength or modulus of elasticity of the ring material. For a given radial load, a large contact
area reduces the wall pressure and increases the thickness of the oil-film formed between the
ring and the cylinder wall. Therefore, a narrow ring increases the wall pressure and reduces
the oil-film thickness. Also a large radial thickness considerably increases the radial load and
wall pressure and possibly squeezes the oil film right down to boundary-lubrication. The
dimensions of the ring cross-section, therefore, determine of gas-sealing and adequate lubrica-
tion of the upper cylinder walls.
3.15.3.
Compression-ring
The piston ring expands readily outwards when
fitted in its groove, without having any interference
between the ring side faces and the ring groove. In
its free state, the ring diameter is slightly larger
than the cylinder bore, therefore when it is fitted up
in the cylinder; it tries to open outwards so that a
pressure is applied on the cylinder wall. However,
gas pressure acting on the piston ring is mainly
responsible for the radial sealing force (Fig. 3.91).
During the upward compression stroke, the
compressed charge moves between the groove and
the ring side faces, passes behind the ring, presses
it against the cylinder wall, and in addition it pushes down the ring against the lower ring groove
of the piston. This gives an effective compressive seal without leakage, provided the surface
finish of the groove, ring, and cylinder wall is of required value. During the downward power
Fig. 3.91. Compression-ring action.
stroke a similar situation persists, but if the acceleration of the piston is greater than that of
the ring, the upper groove and ring side faces are held firmly together to form the seal.
3.15.4.
Types of Compression-rings
Most common types of compression-rings are as follows.
(a) Chromium-faced Plain or Inlaid.
This is a single-piece ring tensioned by heat to form
so as to produce the desired pressure required for effective sealing when installed in the cylinder
(Fig. 3.92A). These rings also help oil control by influencing the thickness of the oil film. The
ring working face is provided with a hard chrome deposit to improve the operating life of both
the ring and the cylinder bore.
(b) Plain Inlaid.
These rings are used to retain a rectangular ring section (Fig. 3.92B). The
groove on the working face is filled by spraying of anti-friction material like chrome, molyb-
denum.
(c) Taper-faced.
A small amount of taper (1 to 1.5 degrees) is provided across the working
face of the ring which extends excellent bedding qualities, because of a high radial pressure on
the cylinder walls due to small area of contact. The line contact of the taper (Fig. 3.92C) gives
an effective scraping action on the down-stroke and hence it also controls the oil supply.
(d) Barrel-faced.
This curved face profile (Fig. 3.92D) of the ring maintains a constant face
radius. The degree of curvature is very critical for initial bedding. Occasionally these barrel
faces are copper plated to reduce excessive friction during bedding, which otherwise may cause
scuffing.
(e) Upward-run Groove.
The ring groove may be slightly inclined upwards at 5 degrees
to provide a bottom-edge line contact for rapid bedding as well as to have a good ring side support
without any strain. These rings (Fig. 3.92E), therefore, seal the gases at an early stage and also
maintain the oil control to a reasonably good extent.
(f) Wedge Section.
The problem of rings sticking in their grooves for some high-perfor-
mance and heavy-duty engines is occasionally minimized by tapering the side faces of the ring
(Fig. 3.92F) to an inclined angle of 15 degrees. The outward slope of the sides of the groove,
during flexing of the ring in its groove, reduces fretting and sticking between the side faces of
the rubbing pair. This relative movement between the ring and the groove is mainly attributed
by the change of piston tilt in the cylinder during its movement and the continuous radial
movement of the ring in its groove.
Fig. 3.92. Compression-rings.
A. Plain chromium faced- B. Plain inlaid face. C. Taper-faced- D. Barrel-faced.
E. Upward-run groove- F. Wedge section. G. Internally stepped. H. Ridge-dodger.
(g) Internally Stepped.
A small square section is removed from the top inside edge of the
ring (Fig. 3.92G) causing the ring to twist, which produces a dished effect when the ring is
assembled in its groove. As a result, this form a taper-periphery working face providing a
bottom-edge contact with the cylinder wall. This improves the ring bedding and provides
adequate degree of oil control on the down-stroke.
(A) Ridge-dodger.
These rings (Fig. 3.92H) clear the wear ridge formed at the top of the
cylinder bore when replacement rings are installed. Only the rings are replaced when the
cylinders with a relatively small wear loose compression or consume oil.
3.15.5.
Oil-control-ring (Oil-scraper-ring)
During crankshaft rotation, more oil than re-
quired for lubrication is splashed from the big-end
bearings on to the cylinder walls. The oil-control-ring
controls oil in the combustion-chamber zone of the
cylinder, and provides a film of oil over the whole
cylinder surface to lubricate the compression-rings,
the skirt, and the upper cylinder region. These rings
are installed with the bevelled working face pointing
towards the cylinder head (Fig. 3.93).
During the upward movement of the piston, the
lower side face of the ring presses hard against the
adjacent ring groove, and the bevelled working face of
the ring slides over the oil and also scraps a proportion
of the oil ahead of the itself. Excess oil accumulates in the clearance space formed between the
groove and its ring, until it overflows through a row of holes at the back of the groove to the
sump.
When the piston moves down its stroke, the upper side face bears hard against the top of
the ring groove. The sharp edge of the working face of the ring now scrapes the oil down the
bore so that surplus oil passes between the ring and its groove and out of the relief holes to the
sump. During this scraping process there is no build-up of oil pressure.
3.15.6.
Types of Oil-control-rings
There are several designs of oil-scraper rings to meet the various operating conditions and
some of them are described below.
(a) Beveled Scraper.
This is a single ring having a narrow bearing working face (Fig.
3.94A) and is installed with the bevelled side towards the piston head (Fig. 3.93). Oil relief holes
are made at the back of the groove with most scraper rings for escaping of the accumulated
surplus oil to the inside of the piston, and then to the sump.
(6) Externally Stepped Scraper.
To reduce the number of compression rings, a stepped
scraper is used which acts both as a compression ring and as an oil-control scraper ring (Fig.
3.94B). The reduced width of the working face increases pressure against the cylinder walls.
This gives improved sealing and scraping without excessively stressing the ring.
(c) Stepped and Beveled Scraper.
These rings (Fig. 3.94C) operate in the same way as
both the stepped and the beveled scraper rings. These rings provide a more effective means of
regulating the upward movement of the oil and hence more suitable for the engine requiring
large amounts of oil to be splashed on to the cylinder walls.
Fig. 3.93. Oil-control-ring action.
Fig. 3.94. Oil-control rings.
A. Beveled scraper B. Externally stepped scraper
C. Stepped and beveled scraper D. Slotted scraper
E. Delayed-section double-groove scraper F. Intertia-flow scraper
G. Double-action stepped scraper H. Composite-rail scraper^
(d) Slotted Scraper.
These rings (Fig. 3.94D) have rectangular section with a recess or
groove round the centre of the working face. Between the rear of this central groove and the
back of the ring, elongated oil slots are also provided. The narrow ring lands of this scraper
provide a relatively high radial pressure against the cylinder walls and also establish a two-stage
scraping action.
(e) Delayed-action Double-groove Scraper.
During the initial stage of running the
engine, more oil may be required to lubricate the walls and piston assembly, but less oil is needed
as the cylinder walls is bedded. These slotted and grooved oil-control rings (Fig. 3.94E) use a
projected central land than the other two lands when new. As the ring settles down, the central
land wears away so that both of the outer lands control the scraping action.
(/) Inertia-flow Scraper.
With this design (Fig. 3.94F), oil flows freely past the top land of
the ring and is scraped by the bottom land during up stroke due to which oil moves through the
drain-holes in the ring to the back of the groove to escape out of the channels made in the piston
section. During down stroke, the top land scrapes oil away into the drain-holes. The degree of
oil control of this ring greatly depends on the inertia of the oil flow and not on the high radial
loadings.
(g) Double-action Stepped Scraper.
Most of the rings do not fully conform to the cylinder
walls due to ovality and heavy wear. To overcome this situation, twin scraper rings in a single
groove are incorporated. These two-piece rings (Fig. 3.94G) use narrow lands with stepped
segments to form oil slots. Each individual land is free to follow the contour of the bore at all
positions of the piston. A backing expander is sometimes incorporated to provide even more
efficient oil control through increased radial wall pressure.
(h) Composite-rail Scraper.
These multi-rail scrapers (Fig. 3.94H) are designed for use
in worn cylinder bores having ovality, taper and any other irregularities on the cylinder walls,
except deep scores which can hinder in controlling the oil supply. The ring assembly uses a
number of steel rails having radiused edges of hard chrome. A crimped spring expands the rails
against both sides of the groove. This does not permit the oil pumping between the ring and
groove which occurs with a conventional ring. Additionally an expander-ring spring installed
behind the rails pushes the rings radially outwards due to which they conform to the cylinder-
wall profile. The rounded rail edges provide a smooth wiping action against the cylinder wall
without causing bore wear.
3.15.7.
Piston-ring Materials and Methods of Manufacture
Piston rings are manufactured from high-quality cast iron containing about 3.4% carbon,
upto 0.5% chromium, and up to 0.25% molybdenum and nickel. Piston rings are manufactured
in various ways, but four of them are briefly described below.
(i) In the beginning, piston rings were manufactured from large sand-cast tubes or pots
from which individual rings are cut to the correct width and then split. This caused
residual compressive stresses in the ring due to hammering or penning round the
inside of the ring. More consistent homogeneous castings are obtained by centrifugal
casting of the pots, and in place of hammering the inside of the ring, a heat-forming
treatment is generally provided.
(ii) After individually casting the rings, they are machined and split, and then ‘heat
formed’. In this process the split ring is opened on to a spacer and is heated until all
the induced stresses created during ring opening are removed. After cooling, the ring
retains its stretched shape so that it exerts an outward force when fitted in the bore.
(Hi) For mass production, rings are cast directly to the normal-free-gap ring profile using
pattern plates milled to the calculated shape for one bore diameter. Therefore a whole
series of pattern plates is required. Once machined, split and gapped, these rings
become a truly circular shape producing a uniform loading around the cylinder wall.
(iv) Also rings are cast to a truly circular shape and then ‘cam-turned’ to obtain free-state
working-face shape. The rings are then split and gapped to exact size.
Phosphate-coated Rings.
To reduce the scoring and tearing of the surfaces while in use,
the piston rings are generally provided with a porous phosphate coating. This surface coating
protects the ring faces against very high local surface temperatures. To form these coating, the
ring is immersed in a bath of phosphoric acid and manganese. An etched layer of iron manganese
phosphate is obtained all over the surface of the ring, which minimizes the rubbing friction. This
porous surface also acts as an oil reservoir which persists even after the coating has worn away.
After treatment, the rings are immersed in a hot bath of oil which is readily absorbed into the
surface.
Chromium-faced Rings.
The chromium-plated compression rings provide not only in-
creased ring life but has also considerably reduced bore wear. Usually working faces of the first
compression ring is chromium plated as these are subjected to the highest working temperatures
and the corrosive products of combustion. For heavy-duty operations, additional rings are
chromium-plated and also on both side faces of the ring. The chrome face reduces the amount
of abrasive particles becoming embedded on the ring surfaces causing a very little ring-face
wear. Also, a very minute quantities of metallic chromium are transferred from the ring face to
the cylinder walls during running. This supports in protecting the relatively soft cast-iron
cylinder-wall surfaces from abrasive wear and corrosive attack.
3.15.8.
Piston and Piston-ring Working Clearance
Three types of piston-ring clearance in the piston grooves and in the cylinder are described
below.
Piston-ring Side Clearance.
The piston rings must be provided with groove side
clearance, which is the gap between the ring and land side faces. Insufficient clearance does not
allow required expansion of the land to wedge the rings in their grooves, thereby stopping the
flexing and rotary movement required during running. Even the gas-sealing properties of the
ring may be hampered causing blow-by and consequently leading to the destruction of the oil
film followed by overheating and seizure. But, a loose fit of the ring in its groove develops the
ring flutter, which causes the ring to hammer against the groove faces, producing rapid groove
wear. Excessive side clearance in extreme case may also create a pumping action of the oil into
the combustion zone.
Typical minimum ring side clearance for pistons having diameter between 60 mm and 120
mm are :
Compression ring has 0.050 mm for petrol engine and 0.060 mm for diesel engine.
Oil-control ring has 0.040 mm for petrol engine and 0.040 mm for diesel engine.
Piston-ring Joint Butting Clearance.
A clearance must be provided at the piston-ring
joint to accommodate the expansion from cold condition to hot working temperatures. With
insufficient clearance the ring ends butt, expanding the ring against the cylinder walls with a
high outward pressure so that the oil film is sheared away causing semi-dry friction and
overheating. Finally the brittle rings have a tendency to buckle until they eventually fracture.
On the other hand, rings with large gaps may cause loss of compression, with the consequent
blow-by and over heating effects.
Typical minimum ring joint clearances are :
Water-cooled four stroke engines have 0.03 mm per cm diameter.
Air-cooled four stroke engines have 0.04 mm per cm diameter.
Piston-skirt-to-bore Clearance.
The clearance between the skirt and the cylinder wall
is required to avoid piston slap when the engine is cold and the skirt seizing under heavy driving
conditions.
Typical minimum skirt clearances are :
Aluminium solid skirt piston has 0.010 mm per cm diameter.
Aluminium solid skirt with thermal slot piston has 0.008 mm per cm diameter.
Aluminium split-skirt piston has 0.005 mm per cm diameter.
