Chapter-3 Cylinder Heads, Cylinders & Liners
Chapter-3 Cylinder Heads, Cylinders & Liners
Chapter-3 Cylinder Heads, Cylinders & Liners
Chapter-3
Cylinder heads, Cylinders & liners
Most modern automotive engines have all of their cylinders and the greater part of their
crankcase poured in a single casting, so that cylinders and crankcase form a single unit. However,
cylinders and crankcase perform different functions.
Gaskets
Copper-Asbestos Gaskets.
Separate cylinder heads were rendered
practical by the introduction of the copper-
asbestos gasket. This consists of an asbestos
sheet cut or stamped to the required form,
which is armored with thin sheet copper.
There is a copper sheet on each side of the
asbestos sheet, and the two copper sheets lap
along the outer edges of the asbestos sheet, so
that the latter is completely encased. Copper
grommets are inserted in the waterway openings and sometimes also in the combustion-chamber
openings. In heavy duty engines the combustion-chamber grommet of the gasket may be reinforced
by a copper-wire loop or a copper washer. In these copper-asbestos gaskets the copper provides the
tenacity and the asbestos the compressibility needed in a packing. A gasket for a four-cylinder L-
head engine is shown in Fig.2.
Cylinder Material.
In the past automobile-engine cylinders have been generally cast of close-grained gray iron
approximating the following composition.
Percent
Silicon 1.9 to 2.2
Sulphur not over 0.12
Phosphorus not over 0.15
Manganese 0.6 to 0.9
Combined carbon 0.35 to 0.55
Total carbon 3.2 to 3.4
The SAE has standardized five grades of cast iron, of which four are recommended for
cylinder blocks and cylinder heads as follows: No. 111 for small cylinder blocks; No. 120 for
cylinder blocks generally. No.121 for truck and tractor-, and No. 122 for diesel engine cylinder
blocks. Pistons also are cast of these irons.
It was determined from tests conducted, that to obtain the better physical properties the total
carbon & silicon contents must be reduced and the phosphorus content held to a lower limit.
Among other points usually covered in specifications for cylinder castings arc the following:
Castings must be smooth, well cleaned and free from shrinkage cavities, cracks and holes, large
inclusions, chills, excess free carbides and any other defects detrimental to machinability,
appearance, or performance. They must finish to the size specified. When tensile tests are provided
for, the portion of the casting from which the test piece is to be machined is usually specified. .
The use of steel for cylinders has often been suggested, and for racing and aircraft engines,
cylinders are sometimes made from hollow steel forgings. Several American manufacturers use
cylinder castings of semi-steel, more properly called high-test cast iron. This material is made by
adding a certain percentage of scrap steel to the melt of cast iron, which results in a finer grain and
in somewhat better tensile properties.
To make it possible to successfully cast a multiple-cylinder block with thin walls, the iron
must pour well and have a "long life" (as the foundry men call it). These characteristics are
strengthened, by high phosphorus content, but, unfortunately, this element tends to make the iron
soft and less resistant to wear.
Nickel-Chromium irons.
Certain iron ore mined in Cuba contains small percentages of nickel and chromium, and the
metal made from this are, known as Mayari iron, is sometimes added to gray iron for cylinder
castings: Mayan iron therefore is a natural alloy. It is claimed that it is free from oxidation & has a
lower solidification point, and that the "longer life" of the iron improves the "feeding" of castings
when they are properly gated, in spite of low phosphorus content. Castings when sectioned -show
By B Dinesh Prabhu, Assistant Professor, P E S College of Engineering, Mandya, KARNATAKA 3
Theory and Design of Automotive Engines
sound metal even where there are heavy bosses and thick sections. Cylinder castings made of a
mixture containing 10 per cent of Mayari iron showed a tensile strength of 36,740 psi, according to
makers of the iron; a transverse strength of 4250 lb, and a Brinell hardness of 223-229. The same
iron is also used for cylinder heads and pistons. Results similar to those from Mayari iron are being
obtained by the addition of small quantities of nickel and chromium, and such alloy irons are now
used not only for cylinder blocks, but also for pistons, particularly for heavy duty, commercial-
vehicle engines.
The chief advantage of alloyed irons is that they possess greater hardness and wear
resistance, and that without being harder to machine. The machinability of grey iron is dependent
upon the absence of excess iron carbide of chilled or hard spots. Nickel acts to eliminate both, and
so to improve machinability. In many cases the alloyed iron, although having a Brinell hardness
from 30 to 40 points greater, is actually easier to machine than ordinary gray iron.
When nickel is used alone as an alloying element, the content usually ranges between 1.25
and 2.5%, whereas if it is used in combination with chromium, the nickel content ranges between
o.50 and 1.50 % and that of chromium between 0.25 and 0.50 % it is claimed that a combed content
of nickel and chromium of 1 per cent will give cast iron with a Brinell hardness of 207-217; of 2 per
cent, 223-235, and of 3 per cent, 241-255.
Chromium and nickel, however, are not the only alloying elements purposely added to
cylinder irons; others added to improve the fluidity of the molten iron, the resistance of the iron to
wear, its machinability, or both of the latter qualities, include, molybdenum, vanadium and titanium.
Removable Liners.
In most engines the pistons hear directly on walls
forming part of the cylinder block, hut in some-and
particularly in engines with large cylinders-removable liners
are used. There are two types of these liners:
A "dry" liner is one which is in contact with metal of the
block over its whole length, or nearly its whole length, while
a "wet" liner is one which is supported by the block over
narrow belts only, and is surrounded by cooling water
between these belts.
In the United States "wet" liners came into use first,
especially in the engines of farm tractors and commercial
vehicles. Aside from the fact that any liner when worn or
damaged can be replaced at relatively low cost, the
construction offers the advantage that because of their
uniform wall thickness (being machined inside and lout) and
because they are very little affected by the tension of
cylinder-head studs, separate liners distort less in service than
the integral barrels of conventional cylinder blocks. Fig. 7 "Wet" cylinder liner with packing rings.
Chromium Plating.
Another method of reducing the rate of wear consists in chromium plating the bore. The
process differs radically from that of chromium plating for ornamental purposes. .It gives a "porous"
coating which holds oil, while the so called bright plating process gives a dense coating to which oil
will not adhere & which for this reason is readily is scored in service. From 200 to 500 times as
By B Dinesh Prabhu, Assistant Professor, P E S College of Engineering, Mandya, KARNATAKA 10
Theory and Design of Automotive Engines
much chromium as in conventional decorative plating is deposited per unit of area. If slightly too
much should be deposited, so that the bore is undersize by from 0.0005 to 0.001 in., the excess can
be removed by honing.Wear tests made on a plain gray-iron cylinder of 241 Brinell hardness and a
similar cylinder plated indicated that chromium plating reduces the rate of cylinder wear
approximately in the proportion of 7:1 and that the wear on the top piston ring is coincidentally
reduced about 4:1.
Such methods as nitriding and chromium plating of cylinder bores are applicable
particularly to bus and railcar gasoline engines and to Diesel engines, which have a much longer
service life than passenger-car engines. Cylinder bores in plain cast iron must be reconditioned
about every 50,000 miles, and with either a nitrided or chromium-plated bore, if reconditioning is
required at all, it will be required only after a much longer interval.
Length of Bore
In most modem engines of both the L-head and I-head type the combustion chamber is
formed in the cylinder head and at the end of the up-stroke the top of the piston is flush with the
finished top surface of the cylinder block. One reason for not making the piston overrun the end of
the bore is that that would bring the top ring beyond the upper end of the water jacket at the end of
the up-stroke, where it would not be so effectively cooled, in the ring groove. The lower end of the
piston generally is made to overrun the end of the bore slightly.
The total length of the finished bore evidently is equal to the length of stroke plus the length
of the piston minus any overrun of the piston at both ends, the overrun being considered negative
when the piston does not come quite to the end of the bore. To facilitate getting the piston rings into
the cylinder, the bore is chamfered at the end from which the piston is entered
Cylinder should be
- designed to withstand the high pr. & temp. conditions.
- be able to transfer the unused heat effectively so that metal temp. does not approach the
dangerous limit.
The Cylinder wall is subjected to gas pressure & the piston side thrust.
-Piston side thrust tends to bend the wall but the stress in the wall due to side thrust is very
small & can be neglected.
-The gas pressure Produces 2 types of stresses;
-longitudinal and circumferential, which act at right angle to each other & the net stress in
each direction is reduced. The longitudinal stress is usually small & can be neglected.
D2
p max
force 4
f l =longitudinal stress= =
area (
D 2O D 2 )
4
D=cylinder diameter,
DO= cylinder outside diameter,
p max =max. gas pr.
p max × D
f c =circumferential force=
2t
fc
Net f l = f l - , &
m
f
Net f c = fc - l ,
m
where 1 = poision’s ratio= 1
m 4
The thickness of the cylinder wall usually varies from 4.5mm to more than 25mm, depending upon
the cylinder size.
According to an empirical relation,
For liners of oil engines,
D
t near the top portion & through 20% of the stroke.
15
For dry liners,
The total thickness‘t’ is the thickness of the liner & that of the cylinder wall.
The thickness of the Dry liner is given as t ' =0.03D to 0.035D
The thickness of the inner walls of the automobile engine cylinders is usually given empirically as
t =0.045D+1.6mm
1 3
The thickness of Jacket wall is given as = to t , larger ratio for smaller cylinder
3 4
or =0.032D+1.6mm
The water space between the outer cylinder wall & inner jacket wall is =10mm
for a 75mm cylinder to about 75mm for a 750mm cylinder
or =0.08D+6.5mm
, D 2 × p max . = z × d c2 × f t Diameter
4 4
D 2 × p max . = z × d c2 × f t Outside
p max Diameter
dc = D ,
z × ft
where f t = allowable fibre stress, 35 to 70 N/mm2,
d c = core diameter
Low value of f t is taken since there is already high stress in the studs due to tightening of the nuts.
D D
The number of studs ' z ' may be taken as + 4 to + 4 , D in mm
100 50
Or the pitch of the bolts may be taken as 19 d to 28.5 d , where d is in mm.
3
In practice d generally varies from ( to 1) times the thickness of the flange.
4
In no case d should be < 16mm
Cylinder bore, mm 75 100 150 200 250 300 350 400 450 500
Reboring factor, mm 1.5 2.3 4.0 6.0 7.5 9.5 10.5 12.5 12.5 12.5
References:
1. High Combustion Engines – P M Heldt
2. M/C Design –Sharma & Agarwal