4.Pistons Rings Rods

7/31/2019 4.Pistons Rings Rods

http://slidepdf.com/reader/full/4pistons-rings-rods 1/57

Pistons, Rings, and

Connecting Rods



Pistons

The piston's primary

responsibility is to take

thermal energy created

by the ignition of fuel

and air, and transform it

into linear motion. Linear motion acts on the

crankshaft journal and

becomes rotary motion.



A typical pistonillustrating thevarious partsand the names.



Piston Heads (Crown)

Domed

Flat Top

Recessed (valve reliefs)

Dished



A domed piston with valvereliefs or valve pockets.



A Flat Top piston



A Flat Top piston with valvereliefs or valve pockets.



A Dished piston with valvereliefs or valve pockets.



Piston Skirts

Moly Graphite



Piston Skirts

Finished

perpendicular

to the head

Scuffing

promotes

blow-by



A low-friction moly coating on the skirt ofthis piston helps prevent piston scuffingwhen the engine is cold.



Piston Composition @ Process

Iron – heavy machinery or “old school”




Aluminum - cast

Pour aluminum into a

mold Light-weight

economical

Some silicone added

General usage Brittle

Somewhat

unpredictable expansion

qualities




Hypereutectic

Cast aluminum

with a high siliconcontent

Light-weight

Higher performance

Less brittle

Predictable

expansion qualities




Aluminum - Forged Can be made lighter

weight (smaller) thancast because it’sstronger

Can withstand abuse

Newer designs have

predictableexpansion qualities

Silicon & Nickeladded

Greater piston to wallclearance



Notice the temperaturedifference between a forgedpiston and a cast piston.



Pistons are often cam-ground toproduce the elliptical shape whenthe piston is at room temperature.



Piston diameterbeing measured

using a micrometer.



Piston (wrist) Pins

High-Quality steel

Usually Hollow

Cross-sectional pistonpins.

Most piston pins are

hollow to reduceweight and have astraight bore.

Some pins use atapered bore to add

strength.



Piston pin is offset toward

the major thrust surface.

TECH

TIP



Engine rotation and rod angle during thepower stroke causes the engine to pressharder against one side of the cylinder,creating a major thrust surface.

In this clockwise-rotating engine, asviewed from the front of the engine, themajor thrust surface is on the left side.



Piston Pin Retaining Methods

Full Floating

Lock Rings

Circlips or snap ringshold full-floatingpiston pins in place.



Piston Pin Retaining Methods

Interference Fit

Pressed



Piston Rings

Compression

Oil – usually unidirectional

Ring expander



Compression Ring Composition

Pearlitic

Nodular Iron

Ductile Iron – flexible

Cast Iron

Chromium - .0004” - durable

Molybdenum – reduced scuffing

Chrome-moly



The preferred material for compression rings is a

low-alloyed, heat-treated nodular cast iron

(KV1/GOE 52). This material is characterized by

a high bending strength of min. 1300 MPa and a

high modulus of elasticity attributable to a

martensitic microstructure and spheruliticgraphite structure.



•In the 2nd groove, alloyed grey cast irons are used in aheat-treated condition.

•Besides having a high bending strength and modulus of

elasticity, an increased hardness of 320 to 470 HB is

produced in order to obtain the required wear resistance in

the uncoated condition.

•The demand for high wear strength is also met by the use

of a tempered, alloyed cast iron (GOE 44). This has the

benefit of a high bending strength of min. 800 MPa and

high modulus of elasticity.•The good wear resistance results from the combination of

a fine-pearlitic matrix structure and finely dispersed,

precipitated secondary carbides.



Unalloyed grey cast iron is used for 2-piece oil

rings in the 3rd groove. These ring materials (STD

/ GOE 12, GOE 13) are characterized by a fine-lamellar graphite structure in a pearlitic matrix and

have good conformability due to a relatively low

modulus of elasticity.



•Reduced width piston rings in gasoline engines to match

reductions in the overall height of pistons, and increasing

combustion pressures in diesel engines call for materials withincreased strength characteristics.

•These challenges are met by the use of high-chromium alloyed

steels and spring steels.

•The greater durability under increased stresses is demonstratedby the improved fatigue strength manifested as form stability in a

comparison of S/N curves for different piston ring materials

(spherulitic, heat-treated cast iron versus heat-treated 18%

chromium steel).



•The wear resistance derives from finely distributed

chromium carbides of the type M23C6 and M7C3 embedded

in the tempered martensite matrix.•For improved wear resistance these steels are mainly

used in a nitrided condition or with a peripheral coating.

•The steels mentioned are used chiefly as compression

ring materials for gasoline engines and truck diesel enginesas well as for the steel rails and expander-spacers of oil

control rings and for 2-piece profiled steel oil rings.



Pearlitic Rings

Pearlite is a two-phased, lamellar (or layered) structurecomposed of alternating layers of alpha-ferrite (88 wt%) and

cementite (12%) that occurs in some steels and cast irons. The eutectoid composition of Austenite is approximately

0.8% carbon ; steel with less carbon content will contain acorresponding proportion of relatively pure ferrite crystallitesthat do not participate in the eutectoid reaction and cannot

transform into pearlite. The appearance of pearlite under the microscope resembles

mother of pearl (also a lamellar structure), from which ittakes its name.

http://en.wikipedia.org/wiki/Phase_(matter)

http://en.wikipedia.org/wiki/Lamellar_structure

http://en.wikipedia.org/wiki/Ferrite_(iron)

http://en.wikipedia.org/wiki/Cementite

http://en.wikipedia.org/wiki/Steel

http://en.wikipedia.org/wiki/Cast_iron

http://en.wikipedia.org/wiki/Carbon

http://en.wikipedia.org/wiki/Ferrite

http://en.wikipedia.org/wiki/Microscope

http://en.wikipedia.org/wiki/Nacre

http://en.wikipedia.org/wiki/Nacre

http://en.wikipedia.org/wiki/Microscope

http://en.wikipedia.org/wiki/Ferrite

http://en.wikipedia.org/wiki/Carbon


http://en.wikipedia.org/wiki/Steel

http://en.wikipedia.org/wiki/Cementite




http://en.wikipedia.org/wiki/Lamellar_structure






Nodular (ductile) Iron

Ductile iron, also called ductile cast iron,

spheroidal graphite iron, nodular cast iron, isa type of cast iron invented in 1943 by Keith

Millis.

While most varieties of cast iron are brittle,

ductile iron is much more flexible and elastic, dueto its nodular graphite inclusions.


http://en.wikipedia.org/wiki/Keith_Millis


http://en.wikipedia.org/wiki/Brittle

http://en.wikipedia.org/wiki/Flexible

http://en.wikipedia.org/wiki/Elastic

http://en.wikipedia.org/wiki/Elastic

http://en.wikipedia.org/wiki/Flexible

http://en.wikipedia.org/wiki/Brittle






Chromium rings

Chromium is a steely-gray, lustrous, hard

metal that takes a high polish and has a highmelting point.

It is odourless, tasteless, and malleable.

http://en.wikipedia.org/wiki/Lustre_(mineralogy)

http://en.wikipedia.org/wiki/Metal

http://en.wikipedia.org/wiki/Metal

http://en.wikipedia.org/wiki/Lustre_(mineralogy)



Chromium facing can be seen on the rightside of the sectional view of the piston ring.



Molybdenum Rings

Molybdenum (pronounced

/məˈlɪbdənəm)

It has the sixth-highest melting point

of any element, and for this reason

it is often used in high-strength steel

alloys.

Molybdenum was discovered in1778 by Carl Wilhelm Scheele and

first isolated in 1781 by Peter Jacob

Hjelm.

http://en.wikipedia.org/wiki/Wikipedia:IPA_for_English

http://en.wikipedia.org/wiki/Carl_Wilhelm_Scheele

http://en.wikipedia.org/wiki/Peter_Jacob_Hjelm




http://en.wikipedia.org/wiki/Carl_Wilhelm_Scheele

http://en.wikipedia.org/wiki/Wikipedia:IPA_for_English



Molybdenum facing can be seenon the right side of the sectionalview of the piston ring. TECH

TIP



This typical three-piece oil control ring uses ahump-type stainless steel spacer-expander.

The expander separates the two steel railsand presses them against the cylinder wall.



Ring Gaps

Ring gap must be checked prior to engine

assembly

Loose

Tight

Butt gap

Gapless?



The gapless ring overlaps, while theconventional ring design uses a gap.

Frequently

Asked Question



Gapless Ring



Combustion chamber pressure forcesthe ring against the cylinder wall andthe bottom of the ring groove.

These are the two sealing surfacesthat the top ring must be able to sealfor maximum engine power.



The piston rings musthave the specified sideand back clearance.

Fitting Piston Rings



The rectangular and the barrelface are the most commonly usedtop compression rings because

they provide the best seal.



The taper face ring provides good oil controlby scraping the cylinder wall.

If this design ring were accidentally installedupside down, the tapered face would pump oil

into the combustion chamber.



Torsional twist ringsprovide better compressionsealing and oil control than

regular taper face rings.



Cutting Rings



Connecting Rods

Some designs utilize spit holes or bleed

holes

Cast iron – general

duty

Forged steel –

stronger – generallyheavier



Connecting Rods

Cast aluminum

Forged aluminumLightweight

Stretch easily



Connecting Rods

Powdered metal Pre-balanced

Fractured parting line



Connecting Rods

H-Beam Design



A typical connecting rod and related engineparts. The connecting rod is probably themost highly stressed part in the engine.

Combustion forces try to compress it andwhen the piston stops at the top of the

cylinder, inertia forces try to pull it apart.



Some connecting rods have balancingbosses (pads) on each end of the rod.

Rod caps are unidirectional and mustbe reinstalled in the same rod position.



The rod bearing boresnormally stretch from topto bottom causing therod bearing to wear mostnear the parting line.



Rod Bore Measurement Use a bore gauge and rod fixture in vise

Check out-of-round

Rod caps must be torqued to mfg. specs.



Connecting Rod Installation

The

chamfered

side of the

bore will

always lead

towards the

crankshaftside on a v-

type motor.



A press used to removeand install connectingrods to the pistons.

Connecting rod

End

of

show



HighPerformance

Ti

Using a connecting rod bolt stretch gauge to

measure the amount the rod bolt stretches to

tighten the fastener to its ultimate strength.

Connecting rod clamp vise

Rod bolt stretch gauge

Connecting rod show

Documents

4.Pistons Rings Rods