DJA3032 CHAPTER 6

CHAPTER 6: PISTON AND PISTON RINGS

by

MOHD SAHRIL BIN MOHD FOUZI, Grad. IEM (G 27763)

DEPARTMENT OF MECHANICAL ENGINEERING

© MSF @ POLITEKNIK UNGKU OMAR

(DJA3032) INTERNAL COMBUSTION ENGINE

General Objective:

To understand the structure and various types of pistons and rings

Specific Objectives:

At the end of this unit you should be able to:

1. describe the application and types of pistons and rings

2. describe the application and types of piston pins.

3. draw and label types of piston rings and pins.

4. define and draw types of piston ring joints.

5. describe material selection for piston.

6. describe the usage and types of compression and oil rings.



Introduction

The piston and rod assembly are designed to transmit the power from

combustion to the crankshaft. There are several parts on this assembly.

Their main function is to contour for heat expansion.

Piston

Piston is a component found either in a cylinder with a 2-strokes engine or

4-strokes engine.

Piston heads are designed through casting (hypereutectic casting) and they are

flat and dome wedge.

The piston is slightly smaller than the cylinder bore. This will allow heat

expansion and lubrication.



Figure 6.1 : Structure Of Piston

In many pistons the pin is offset from the center of piston. Pistons must be

always installed in the right direction because of the offset pin and thrust faces.

Piston is directly involved in the explosion and compression cycle.

The following is the internal combustion cycle that involves piston:

i. piston is forced upward on the compression stroke.

ii. the cylinder pressure forces the piston against the cylinder wall.

iii. as the piston is driven down on the power stroke, the high cylinder

pressure drives the major thrust side of the piston against the cylinder

wall.



Material Selection for Piston

Pistons are commonly made by casting process which is hypereutectic casting

Figure 6.2: A Prototype Piston Made

By Casting Process

Figure 6.3: Sample Of Forged Piston

The main content in making a piston is aluminum strengthened with silicon.

The process that is involved in making piston is by pouring melted aluminum

into a mold that shapes the slug into a piston.

In contrast, forged pistons are formed, using a giant press that takes a block of

metal and pounds it into shape under thousands of tons of pressure.

The tooling needed to do this is much more expensive than the tooling used to

make a casting, and it wears out quicker. This makes forged pistons more costly

than castings.



Aluminum makes the piston lighter. However, some larger engines, especially

certain diesel engines, may use a cast iron piston. In this case the RPM would be

lower.

The lighter piston can operate more effectively in today’s gasoline engines,

which run in excess of 5,000 RPM.

Piston Expansion

When combustion occurs at the top of the piston, some of the heat is transmitted

down through the piston body. This causes the piston to expand.

If the expansion is too great, the piston might wear the cylinder to a point of

damage. To compensate for expansion, other pistons have a split skirt (Figure

6.4). When the piston expands, the slot closes rather than increases in size.



FIGURE 6.4: SPLIT SKIRT

The piston is ground in the shape of a cam or egg.

As the piston heats up during operation, it

becomes round.

The piston is designed so that maximum expansion

takes place on dimension B. Dimension A remains

about the same . (Figure 6.5 ).

FIGURE 6.5: CAM GROUND PISTON



Piston Head Shapes

FIGURE 6.6: DIFFERENT SHAPES OF PISTON HEAD DESIGN

The shape of the piston head varies according to the engine. Head shapes are

used to create turbulence and change compression ratios.

Generally, small, low-cost engines use the flat top. This head comes so close

to the valve on some engines that there is a recessed area in the piston for the

valve.

Another type of head is called the raised dome or pop-up head. This type is

used to increase the compression ratio.

The dished head can also be used to alter the compression ratio.



Piston Skirt

Since the 1970’s, it has become important to make the engine as small as possible

and yet still powerful. One way to do this is to keep the height of the piston and

connecting rod to a minimum.

This is done by shortening the connecting rod. To shorten the connecting rod, a

slipper – skirt is used.

Part of the piston skirt is removed so that the counterweights will not hit the

piston. This design means there can be a smaller distance between the center of the

crankshaft and the top of the piston.

FIGURE 6.7: SLIPPER SKIRT

The output power of the engine is not affected

because the bore and stroke still remain the

same ( Figure 6.7).



FIGURE 6.8: OIL GROOVE ON PISTON

The surface of the skirt is somewhat rough. Small

grooves are machined on the skirt so that

lubricating oil will be carried in the groove

(Figure 6.8).

This helps to lubricate the piston skirt as it moves

up and down the cylinder. If the engine overheats,

however, the oil will thin out and excessive piston

wear may occur .

Some pistons have an impregnated silicon surface on the skirt of the piston.

Impregnated silicon (silicon particles placed into the external finished on the

piston) helps to reduce friction between the skirt and the cylinder wall.



Piston Pins

FIGURE 6.9: TYPES OF PISTON PINS

Piston pins are used to connect the piston to the connecting rod. These pins are

made from hard steel alloy and have a finely polished surface.

Most piston pins are hollow, to reduce weight. Piston pins are passed fit and

clamped to the connecting rod, or full floating.

In the full floating design the pins are free to turn in both the piston and

connecting rod ( Figure 6.9).

Piston pins are usually offset toward the major trust side from 15 to 22 mm, to

reduce piston slap as the piston moves through TDC from the compression to

the power stroke.



Clearance between the pin and piston may be as little as 0.0125 mm. There are

four types of piston pins:

i. full floating

ii. oscillating in bushed piston

iii. oscillating in piston and

iv. set screw type piston

Ring

Piston ring provides a dynamic seal between the piston and the cylinder wall. Its

purpose is to prevent combustion pressures from entering the crankcase and

crankcase oil from entering the combustion chamber.

This also controls the degree of cylinder wall lubrication. Types of piston rings

which include compression rings and oil control rings are shown in Figure 6.10.



FIGURE 6.10 : COMPRESSION RING & OIL

CONTROL RING

FIGURE 6.11: TYPES OF RING JOINT

Most automobile engines have two compression rings at the top of piston and an

oil control ring is just below the compression rings. Chrome-faced cast iron

compression rings are commonly used in automobile engine.

Rings and gaps are required to allow ring expansion without the ring ends butting

and causing damage to the cylinder.



Compression Ring

Compression rings are made of cast iron. This material is very brittle and can

break easily if it is bent. However, the brittle material can wear off easily.

Certain heavy duty engines and some diesel engines use ductile iron as piston

ring material. This material is stronger and resists breaking, but the cost of these

rings is higher.

Some high quality piston rings have a fused outside layer of chromium or

molybdenum. This is to reduce wear on the rings and cylinder walls; and also to

prevent the rings from breaking when they expand.

Counter bores and chamfers on compression rings assist the rings to slide over

the oil on the cylinder walls during upward movement of the piston and scrape

the oil of the cylinder walls on downward movement. Tapered-face and barrel-

face ring designs are also used for this purpose



FIGURE 6.12: TAPER-FACE AND CHROME-PLATE TOP COMPRESSION RING ACTION

Expanders used behind specially designed compression ring increase ring

pressure against the cylinder wall for increased sealing ability. Ring without

expansion rely on ring tension alone for static pressure against the cylinder

wall.

Piston rings are subjected to dynamic pressures, friction, heat, constant change of

direction and speed, and inertia. Since there is some side clearance between the

ring and the land, the piston ring moves up and down in the ring groove on the

different strokes of the engine.

Due to ring pressure against the cylinder wall and the inertia of the piston rings;

the rings tend to stay behind when the piston changes direction. This causes the

rings to move up and down in the groove and eventually causes ring groove wear.



The rings also wear off, increasing ring side clearances even further. If excessive,

ring breakage can occur.

Oil ring

All oil control rings are designed to scrape the oil off the wall on the down

stroke. Oil ring are made to:

i) scrape oil from the cylinder walls.

ii) to stop any oil from entering the combustion chamber.

iii) to lubricate the walls to prevent excessive wear.

After being scraped off the cylinder walls, the oil passes through the center of the

ring. It then flows through holes on the piston and back to the crankcase.

The scrapping process helps to remove carbon particles that are in the ring area.

The oil flow also helps to seal the piston.



Oil ring comes with an expander. The expander is used to push the ring out

against the cylinder walls. There are four types of oil rings:

FIGURE 6.13 : DIFFERENT SHAPES OF OIL RINGS

i. slotted cast iron oil control ring.

ii. slotted cast iron oil control ring with abutment type expender.

iii. circumferential steel oil control ring (3 pieces).

iv. multi-piece steel oil control ring



Question 1

Describe the combustion cycle that involves piston.

Question 2

Draw and label types of piston ring joints.

Question 3

Define the scraping process for an oil ring.

Question 4

Draw and name three (3) types of piston pins.

Question 5

Describe how to make the piston and what material is used.

Question 6

Describe the functions of the piston ring in the internal combustion engine.

Question 7

Draw the right position of piston rings at the piston.

Question 8

What are the materials needed to make a compression ring and an outside layer itself?

SELF-ASSESSMENT



DJA3032 CHAPTER 6

Automotive

Transcript of DJA3032 CHAPTER 6