Mahle GmbH (Ed.) Cylinder Components: Properties, applications, materials
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124 5 Crankcase and cylinder liners
5.5.4 Cylinders for four-stroke engines
New, more stringent exhaust regulations are forcing engine manufacturers to introduce the
four-stroke principle, even in small engines. Currently, various developments are underway
with the goal of providing designs that are high-revving, independent of orientation, and as
small and light as possible.
5.5.5 Bore coatings for light-alloy cylinders
Depending on the required service life and operational reliability, various running surface
coatings are available to the user of light-alloy cylinders. In any case, light-alloy cylinders
without or with thin bore coatings made of other metals are nearly equal in terms of heat dis-
sipation and weight as well as less expensive compared to cast iron cylinder liners.
Table 5.6 shows the ratings of light-alloy cylinders with various cylinder surface coatings in
comparison with gray cast iron cylinders. According to the relationship
W
o
o
T
T
Gray cast iron
Light metal
the temperatures at the base of the fin, near the combustion chamber floor, are compared
for gray cast iron and light-alloy cylinders.
Table 5.6: Ratings of various cylinder surface coatings, relative to gray cast iron cylinders
Cylinder Cylinder surface coating Rating of heat transfer T
Gray cast iron cylinder – 1
Light-alloy cylinder with
surface coating
NIKASIL
®
1.16 – 1.19
CROMAL
®
1.16 – 1.19
FE R R AL 1.16 – 1.19
Light-alloy cylinders without
reinforcement
– 1.17 – 1.2 0
The following coatings have been proven in day-to-day use:
NIKASIL
®
The running surface of light-alloy cylinders coated with NIKASIL
®
consists of a nickel disper-
sion, developed by MAHLE (Figure 5.28), which is about 0.05 mm thick in its finish-honed
state.
This galvanically applied nickel layer, with an even inclusion of hard silicon carbide particles
( < 2.5
μm edge length), results in a relatively “smooth” cylinder surface after honing.
5.5 Light-alloy cylinders 125
Figure 5.29 shows the profile diagram of a typical NIKASIL
®
layer. The bearing roughness
value TR has the magnitudes 0.4/96/1.2 (0.4 μm average roughness of the supporting sur-
face with 96% supporting rate; average depth of deep scoring 1.2 μm).
NIKASIL
®
, as a cylinder surface, leads to fast break-in and associated rapid and good seal-
ing of the piston rings. Together with the low frictional loss provided by NIKASIL
®
, optimal
properties are obtained with regard to engine load, quantity of blow-by, oil consumption, and
cylinder wear.
NIKASIL
®
cylinders are used in great numbers today, as standard, for air-cooled two-stroke
and four-stroke engines. NIKASIL
®
is employed worldwide in racing engines, even for water-
cooled four-stroke engines, with very great success. It has also been proven as a surface
coating on trochoids in rotary engines.
Figure 5.28:
Running surfaces with
NIKASIL
®
-coated cylinders,
unhoned and honed
Figure 5.29: Roughness profile of a NIKASIL
®
coating
126 5 Crankcase and cylinder liners
CROMAL
®
This coating is a galvanically applied hard chrome layer of about 0.06 to 0.08 mm in thick-
ness (Figure 5.30).
Wear of the chrome surface, and the piston rings that run against it, is significantly affected in
engine operation by the formation of the layer surface. This formation is intended to provide
as even a distribution and adhesion of the lubricating oil to the cylinder surface as possible.
Honing with diamond strips, which is mainly used today, generates a roughness R
a
of about
2.5 μm, which is then smoothed to create a plateau structure.
The roughness profile in Figure 5.31 shows a typical diamond-honed CROMAL
®
cylinder
surface with a bearing roughness TR of 1.2/77/4.5. Honing with diamond strips has largely
replaced the previously used process of porous chrome plating or linked chrome plating as
well as knurling. The correctly formed chrome surface provides a good counterpart for the
piston and the piston rings. The hardness and chemical resistance of the chrome layer lead to
low wear values for the cylinder and piston rings and to a long service life.
Figure 5.31: Roughness profile of a diamond-honed CROMAL
®
cylinder surface
Figure 5.30: Surfaces of CROMAL
®
-coated light-alloy cylinders, honed and unhoned
127
Glossary
1st piston ring
First piston ring on the combustion chamber side
Bearing clearance
Designed space between the friction partners for the oil film that supports
the bearings and distributes the pressure onto the journal
Beveled-edge
oil control ring
Top compression ring with outside edges chamfered
Cast-in liner
Cylinder liner cast into the engine block
Cavitation
Local material abrasion on the water-side cylinder wall due to implosions
of water vapor bubbles
Compression ring
see 1st piston ring
Conformability
Ability of a piston ring to conform to the cylinder surface
Conrod sweep
Space covered by the connecting rod during a revolution of the crankshaft
Cracking
Separation of the crank end by fracture
Cylinder liner
Liner installed in the engine block
Cylinder surface
Internal surface of the cylinder bore
Double-beveled
oil control ring
Oil control ring with two surfaces whose edges are equally chamfered
Dry liner
Cylinder liner that is cooled only indirectly by water, i.e., that does not have
any direct contact with the coolant
Expander ring
Spring-supported oil control ring
Fixed pin
connecting rod
Piston pin that is fixed in the small end bore by a shrink fit
Gap clearance
Spacing of the piston ring ends in the installed condition
I-ring
I-shaped oil control ring
Keystone ring
Piston ring with trapezoidal faces on one or both sides
L-ring
L-shaped piston ring whose vertical surface, oriented toward the piston
crown, contacts the cylinder surface
Napier ring
Special shape of a piston ring
Normal force
see side force
Oil control ring
Piston ring designed to remove oil from the cylinder surface in a defined
manner
Piston pin
Link between the piston and connecting rod
128 Glossary
Piston ring
Slotted, self-tensioning ring
Rectangular ring
Basic shape of 1st piston ring
Ring flutter
Occurrence of radial or axial vibrations of the piston ring
Ring gap
Ends of the open piston ring
Ring sticking
Sticking of the piston ring due to carbonization in the piston ring groove
Running face
see cylinder surface
Scraper ring
see oil control ring
Scuffing
Sign of local overheating between the piston ring and the cylinder surface
due to lack of oil
Side faces
Axial faces of the piston ring or piston ring grooves
Side force
Part of the force of combustion exerted on the piston that acts on the
cylinder via the piston skirt
Thrust bearing
Radial bearing with axial contact surfaces
Thrust washer
Axially acting bearing washer
Twist
Deformation of a piston ring chamfered only on one inner side
Wet liner
Cylinder liner washed with coolant from the outside
129
Index
1st piston ring 1, 11 f.
2nd piston ring 1, 9
3-S-ring 15
A
ALBOND® liner 112
aluminum alloys 62 ff., 100
aluminum liner 116
axial bearing simulation (ABAS) 60
B
barrel shape 12
bearing 49 ff.
–, load capacity 52
–, properties 52
–, seizure resistance 55
–, types 49 ff.
–, wear resistance 54
– clearance 56
– geometry 55 f.
– materials 61 ff.
beveled-edge oil control ring 7
boundary lubrication capability 3
bronze alloys 64 ff.
C
camshaft bushing 51
carbon steel 22
cast iron 18, 100
cast-in liner 112
cavitation 99
coil spring 13
coil spring loaded ring (SSF-ring) 13 f.
compression ring, see 1st piston ring
conformability 17
connecting rod 69 ff.
– –, component testing 88 ff.
– –, FE analysis 79 ff.
– –, pulsator test 87 f.
– –, stress 72
– – bearing 50
– – bolt 80
– – bolted joint 91 ff.
– – bushing 51
– – shank 76
– – small end 77
conrod sweep 72
contact pressure 2
contact surface 51
cooling 99
COSCAST
TM
method 107
CPS method 107
cracking (fracture splitting) 74
crank end 76 ff.
crankcase 95 ff.
–, materials 100 ff.
–, types 96 ff.
CROMAL
®
coating 126
cylinder liner 109 ff.
cylinder surface 1, 6, 109 ff.
D
double-beveled oil control ring 7
dry liner 114
E
eccentricity 57
F
fixed pin connecting rod 28
flange thickness 51
free spread 50
full-mold casting method 108
G
gap clearance 2, 16
gravity die casting 108
gray cast iron 20 ff., 116
I
I-shaped ring 14
K
keystone ring 5, 11
L
light-alloy cylinders 122 ff.
liner composite 112 f.
low-pressure die casting 108
L-shaped ring 11
lubrication, elasto-hydrodynamic 59
–, hydrodynamic 58
M
magnesium 106
main bearing 50
– – seat 98 f.
maximum oil film pressure (POFP) 56 ff.
130 Index
minimum oil film thickness (MOFT) 55 f., 59
molding sand 107
movement of journal 58
N
napier ring (NM-ring) 12
NIKASIL
®
coating 118, 124 f.
nitriding of running faces 24
nodular cast iron 21
O
oil control ring 1, 9, 13 ff.
oil groove 50 f.
oil hole 50 f.
ovality 18
overlaps 60
overlays 66 f.
P
parallel connecting rod 77
permanent mold casting 108
piston pin 27 ff.
– –, circlips 46 f.
– –, materials 42 ff.
– –, terms 40
– –, test bench 45
– –, types 33 f.
– – deformation 31 f., 38
– – strength 28 f.
piston ring 1 ff.
– –, design 16 ff.
– –, forces and stresses 4 ff.
– –, materials 18 ff.
– –, principles of operation 3
– –, purpose and function 1 ff.
– –, types 7 ff.
– – with internal bevel 10 f.
piston wear 33
pressure die casting 109
protective coatings 67
pulsator test 87 f.
R
radial pressure 3
rated power 56
rectangular ring 5, 9
– – with taper-faced running face 9
ring carrier piston 6
ring flutter 1
ring gap 10 f.
ring sticking 1
R-ring 10, 12
running face coatings 23
S
sand casting 106 f.
scuffing 1
seizure 55
semi-solid method 109
side face 3
side face coatings 23
side force 110 f.
slotted oil control ring 13
S-N curve 88 f.
squeeze casting 109
stainless steel 22
stepped connecting rod 78
surface protection 25
T
thrust bearing 51
thrust washer 51
torque 91
trapezoidal connecting rod 78
twist 10
U
U-flex-ring 15 f.
W
wet cylinder liner 114
white metal 65