Unit 9 · Web viewSelf-assessment evaluates your understanding of each unit. Question 9.1 Describe the combustion cycle that involves piston Question 9.2 Draw and label types of piston

PISTON AND PISTON RING J3182 / UNIT9 /

Unit 9Title: Piston and Piston Ring

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.

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Input This section introduces the subject matter that you are going to learn.9.0 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. In this unit we will discuss

piston and parts of assembly in an engine.

9.1 Piston

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

or 4-strokes engine. It plays a very important role in the combustion process /

cycle. 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. A structure model of a

piston is shown in Figure 9.1:

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Figure 9.1 : 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.

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9.1.1 Material Selection for Piston

Pistons are commonly made by casting process which is

hypereutectic casting (Figure 9.2 and 9.2.1). 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.

Figure 9.2: A Prototype Piston Made By Casting Process

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Figure 9.2.1: Sample Of Forged Piston

9.1.2. 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 9.3 ). When the piston expands, the

slot closes rather than increases in size.

Figure 9.3: Split Skirt

The T slot, is also used on older engine. In this case the T slot tends to

hold back the transfer of heat from the head to the skirt. Some pistons

use steel rings, which are cast directly into the pistons. These steel rings

will not expand as much as the aluminum. The steel rings have a

tendency to control or minimize expansion. Cam ground piston is also

used to control the expansion.

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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 9.4 ).

Figure 9.4: Cam Ground Piston

9.1.3 Piston Head Shapes

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.

Figure 9.5, illustrates different types of piston head designs.

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Figure 9.5: Different Shapes Of Piston Head Design

9.1.4 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. The output power of the engine is not affected because the bore

and stroke still remain the same ( Figure 9.6).

Figure 9.6: Slipper Skirt

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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 9.7). 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.

Figure 9.7: Oil Groove on Piston

9.1.5 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 9-8). 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

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iv. set screw type piston

Figure 9.8: Types Of Piston Pins

Activity 9AThis section tests your understanding of the subject matter. You are to answer the following questions.9.1 What is the purpose of piston in the internal combustion engine?

9.2 Describe how to make the piston and what material is used?

9.3 Piston pins on some engines are offset to the

(a) right side

(b) left side

(c) major thrust side

(d) minor thrust side

9.4 Based on the figures below, name the types of the piston head shapes

9

.


1._________________________________________

2._________________________________________

3._________________________________________

4._________________________________________

9.5 List down three types of piston pins

i.________________________________________________

ii._______________________________________________

iii._______________________________________________

9.6 Explain the purpose of slipper skirt at the piston.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

9.7 What is the purpose of an oil small groove on the piston skirt?

_______________________________________________________________

_______________________________________________________________

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.

Input This section introduces the subject matter that you are going to learn.

9.2 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 9.9.

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Figure 9.9: Ring Location & Ring

Figure 9.9 : compression ring & oil control ring

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. There are three types of ring joints (Figure 9.10).

i) 450 angle joint

ii) butt joint

iii) step joint

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Figure 9.10: Types Of Ring Joint

9.2.1 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 9.2 ….)

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

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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 (Figure 9.11). The rings also wear

off, increasing ring side clearances even further. If excessive, ring

breakage can occur.

Figure 9.11: Taper-Face And Chrome-Plate Top Compression Ring Action

9.2.2 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.

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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:

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

Figure 9.12 shows four types of oil rings which are used in internal combustion

engine.

Figure 9.12 : Different Shapes Of Oil Rings

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Activity 9BThis section tests your understanding of the subject matter. You are to answer the following questions.9.8 The purpose of piston rings is to control

(a) combustion pressures

(b) cylinder wall lubrication

(c ) oil consumption

(d) all of the above

9.9 List down four types of oil rings

i. __________________________

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ii. __________________________

iii. __________________________

iv. __________________________

9.10 During engine operation piston rings are subjected to

(a) constantly changing direction and speed

(b) heat and friction

(c ) dynamic pressure


9.11 During engine operation oil ring are subjected to

(a) scrape oil from the cylinder walls.

(b) stop any oil from entering the combustion chamber.

(c) lubricate the walls to prevent excessive wear.


9.12 Based on the figure below, name the types of piston head shapes

1._________________________________________

2._________________________________________

3._________________________________________

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Self –AssessmentSelf-assessment evaluates your understanding of each unit.

Question 9.1

Describe the combustion cycle that involves piston

Question 9.2

Draw and label types of piston ring joints

Question 9.3

Define the scraping process for an oil ring

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Question 9.4

Draw and name three (3) types of piston pins

Question 9.5

Describe how to make the piston and what material is used

Question 9.6

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

Question 9.7

Draw the right position of piston rings at the piston.

Question 9.8

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

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Documents

Unit 9 · Web viewSelf-assessment evaluates your understanding of each unit. Question 9.1 Describe the combustion cycle that involves piston Question 9.2 Draw and label types of piston