Lecture Notes - Internal Combustion Engine

INTERNAL COMBUSTION ENGINE

SQUISH AND TUMBLE:

As the piston approaches TDC, the gas mixture occupying the volume at the outer radius
of the cylinder is forced radially inward as this outer volume is reduced to near zero. This
radial inward motion of the gas mixture is called squish. It adds to other mass motions within
the cylinder to mix the air and fuel, and to quickly spread the flame front. Maximum squish
velocity usually occurs at about 10° BTDC. During combustion, the expansion stroke begins
and the volume of the combustion
chamber increases. As the piston moves away from TDC, the burning gases are propelled
radially outward to fill the now-increasing outer volume along the cylinder walls. This reverse
squish helps to spread the flame front during the latter part of combustion.

As the piston nears TDC, squish motion generates a secondary rotational flow called tumble.
This rotation occurs about a circumferential axis near the outer edge of the piston bowl as
shown in Fig. 5.5, (Willard W. Pulkrabek ).

Figure 5.5: Tumble action caused by squish as piston approaches TDC. Tumble
is a rotational motion about a circumferential axis near the edge of the clearance

EXHAUST BLOWDOWN

Exhaust blowdown occurs when the exhaust valve starts to open towards the end of the
power stroke, somewhere around 60° to 40° bBDC. At this time, pressure in the cylinder
is still at about 4-5 atmospheres and the temperature is upwards of 1000 K. Pressure in
the exhaust system is about one atmosphere, and when the valve is opened the resulting
pressure differential causes a rapid flow of exhaust gases from the cylinder, through the
valve, into the exhaust system (i.e., exhaust blowdown),

In an ideal air-standard Otto cycle or Diesel cycle, the exhaust valve opens at BDC and
blowdown occurs instantaneously at constant volume (process 4-5 in Fig. 5.6). This does not
happen in a real engine, where blowdown takes a finite length of time. So that the pressure
in the cylinder has been fully reduced by BDC when the exhaust stroke starts, the exhaust
valve starts to open somewhere around 60° to 40° bBDC. When this happens, the pressure
is quickly reduced, and what would have been additional useful work is lost during the last
part of the expansion stroke, (Willard W. Pulkrabek ).

44

INTERNAL COMBUSTION ENGINE

Figure 5.6: Air-standard Otto cycle in P-V coordinates showing exhaust gas after blowdown at
hypothetical state 7.

EXHAUST SYSTEM

EXHAUST SYSTEM MANIFOLD:
After leaving the cylinders by passing out of the exhaust valves, exhaust gases pass through
the exhaust manifold, a piping system that directs the flow into one or more exhaust pipes.
Exhaust manifolds are usually made of cast iron and are sometimes designed to have close
thermal contact with the intake manifold. This is to provide heating and vaporization in the
intake manifold.
From the exhaust manifold, the gases flow through an exhaust pipe to the emission control
system of the engine, which may consist of thermal and/or catalytic converters. One argument
says these should be as close to the engine as space allows minimizing heat losses. On
the other hand, this creates high temperature problems in the engine compartment. These
converters promote reduction of emissions in the exhaust gases by additional chemical
reaction. They are discussed in the next chapter, (Willard W. Pulkrabek ).
CATALYTIC CONVERTERS:
The most effective after treatment system for reducing engine emissions is the catalytic
converter found on most automobiles and other modern engines of medium or large size. HC
and CO can be oxidized to Hz 0 and COz in exhaust systems and thermal converters if the
temperature is held at 600°-700°C.
If certain catalysts are present, the temperature needed to sustain these oxidation processes
is reduced to 250°-300°C, making for a much more attractive system. A catalyst is a substance
that accelerates a chemical reaction by lowering the energy needed for it to proceed. The
catalyst is not consumed in the reaction and so functions indefinitely unless degraded by
heat, age, contaminants, or other factors.

45

INTERNAL COMBUSTION ENGINE

Catalytic converters are chambers mounted in the flow system through which the exhaust
gases flow. These chambers contain catalytic material, which promotes the oxidation of the
emissions contained in the exhaust flow.
Generally, catalytic converters are called three-way converters because they promote
the reduction of CO, HC, and NOx. Most consist of a stainless steel container mounted
somewhere along the exhaust pipe of the engine. In most converters, the ceramic is a single
honeycomb structure with many flow passages, (Willard W. Pulkrabek ).

Figure 5.5
The surface of the ceramic passages contains small-embedded particles of catalytic material
that promote the oxidation reactions in the exhaust gas as it passes. Aluminum oxide (alumina)
is the base ceramic material used for most catalytic converters. Alumina can withstand the
high temperatures, it remains chemically neutral, it has very low thermal expansion, and
it does not thermally degrade with age. The catalyst materials most commonly used are
platinum, palladium, and rhodium.
TAILPIPE AND MUFFLER:
After exiting the catalytic converter, exhaust gases flow through a tailpipe that ducts the flow
away from the passenger compartment of the vehicle and vents it to the surroundings. This
is usually under and out the back (or side) of an automobile and often upward behind the cab
of large trucks.
Somewhere in the tailpipe section there is usually a larger flow chamber called the mummer.
This is a sound chamber designed to reduce the operating noise of the engine, most of which
is carried out with the exhaust flow. Mufflers use two general methods of sound reduction.
One method absorbs the energy of sound pulses by flow through a porous medium. Other
mufflers reduce sound by the cancellation of waves. Instead of fully dampening all engine
noise, some mufflers are designed to give a louder, sporty sound.

46

INTERNAL COMBUSTION ENGINE

6.0 PISTON AND PISTON 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.

47

INTERNAL COMBUSTION ENGINE
48

INTERNAL COMBUSTION ENGINE
49

INTERNAL COMBUSTION ENGINE
50

INTERNAL COMBUSTION ENGINE

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.
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.

Figure 6.1: Structure of Piston
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

Figure 6.3: Sample of Forged Piston

51

INTERNAL COMBUSTION ENGINE

The main content in making a piston is aluminium strengthened with silicon. The process that
is involved in making piston is by pouring melted aluminium into a mould 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 costlier than castings.
Aluminium 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

Figure 6.6: different shapes of piston head design

52

INTERNAL COMBUSTION ENGINE

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. The output power of the engine is not affected because the bore and
stroke still remain the same (Figure 6.7).

Figure 6.7: Slipper Skirt

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.

53

INTERNAL COMBUSTION ENGINE

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

RINGS

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 Figure 6.11 : Types of Ring Joint Control Ring

54

INTERNAL COMBUSTION ENGINE

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.

COMPRESISION RINGS

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 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 Chromeplate 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 RINGS

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.

55

INTERNAL COMBUSTION ENGINE

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.

Figure 6.13 : Different Shapes of Oil Rings
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

56

INTERNAL COMBUSTION ENGINE

7.0 VALVE AND COOLING VALVE

INTRODUCTION

In this chapter we are to discuss about valves in an engine. Valves can either
be in the block, or in the cylinders. Valve location is used to classify an internal
combustion engine.

TYPES OF VALVE LOCATION

Valves in the block (flat head), are also called L Head engine, because the cylinder
and combustion chamber are in the shape of an inverted ‘L’.
Engines with valves in the block have the intake valve on one side of the cylinder
and the exhaust valve on the other side, these are called T Head engines.
Valves in head (overhead valve), are also called I-Head engine because the
cylinder and combustion chamber are in the shape of an ‘I’.
One valve in head (usually intake) and one valve in block, are also called F head
engine; this is less common.

ARRANGEMENT OF VALVE

Figure 7.1 : Arrangement of Valve

57

INTERNAL COMBUSTION ENGINE

VALVE PARTS

Figure 7.2 : Parts of Valve

A valve has several parts and they are in the tip or rocker arm contact area, valve spring
retainer lock grooves, valve stem, fillet, face, margin and valve Head. The head of the valve
is the part that is inside the combustion chamber.
The valve can be designed with different head shapes Such as:
i. oval head
ii. Flat head
iii. Concave head
iv. Recessed head
The more metal on the head of valve, the more rigid the valve is. Less metal means the valve
will be able to conform to the seat more effectively. These valves are said to be elastic. Rigid
valve does not seat that well. Elastic valves seat better, but they may not last long.

58

INTERNAL COMBUSTION ENGINE

Figure 7.3 : Types of Head Valves

A - Flat head
B - Concave head
C - Oval head
D - Recessed head

VALVE MATERIALS

Intake and exhaust valve materials include:

i. Alloy steel valves with aluminized face and chrome stem
ii. Silchrome valve with aluminized face
iii. Austenitic steel with aluminized face and chrome stem
iv. SAE 21-2steel with nickel-plated face
v. Satellite
vi. Sodium-filled stems (for valve cooling).

VALVE PROTECTION

Corrosion tends to cause pitting on valve seat faces, as a result of which the hot gases start
to leak through, eventually burning a channel locally through which compression is lost.

Due to this, coatings are applied for protection against corrosion. Materials that are used for
coatings include alloy containing:

i. nickel
ii. manganese
iii. cobalt
iv. chromium
v. silicon
vi. molybdenum
vii. tungsten
viii. titanium

However, these materials are costly. Due to that, a much economical coating system has
been developed. This coating system is called ‘Aldip process’ in which application of an
aluminium coating is used.

59

INTERNAL COMBUSTION ENGINE

The process is:
i. Aluminium material is sprayed in the form of a paste after valve seats have been
finished – machined and the stems rough grounds.
ii. The component, in a jig, are then dipped for a few seconds in a molten flux bath at
7600 C.
iii. Later it is removed from bath before surplus aluminium is blown off.
The outcome is a smooth permanently adhering coating of aluminium on an iron alloy underlay.
No further finishing is necessary.

VALVE SEAT

Valve seat is defined as a circular surface that is machined into the cylinder block or head.
There are two types of valve seats:
i. Integral
ii. Insert
The integral type is cast directly as part of the cylinder head; the insert type uses a metal ring
as the seat.
It is pressed into the cylinder block and ground to the correct angle. Insert valve seats can
be made from cast iron, hardened cast iron, hi-chrome steel, and satellite (very hard steel).
Valve seats are ground to a specific angle for correct operation. They are either 30° or 45°.
An interference angle is very common when grinding the valve.

Figure 7.4 : An Interference Angle
Figure 7.4 shows the valve is ground to 44° while the seat is ground to 45°. This interference
angle tends to cut through any deposits that have been formed on the seat.

VALVE ROTATORS

Valve rotators are used on certain engines. If valve is rotated a small amount each time they
are opened, valve life will be extended. This is especially true when using leaded fuel.

60

INTERNAL COMBUSTION ENGINE

Below are the effects of using valve rotators:
i. Minimize deposits of carbon on the stem of the valve,
ii. Keep the valve face and seat clean,
iii. Prevent valve burning caused by localized hot spots,
iv. Prevent valve edge distortion,
v. Help to maintain uniform valve head temperatures,
vi. Help to maintain even valve stem tip wear, and
vii. Help to improve lubrication on the valve stem.
There are several types of valve rotators. Most operate on the principle that as the valve spring
compressed, small balls inside the rotator roll up an inclined surface. This action causes the
valve rotates slightly as it is being compressed.

Figure 7.5 : Valve Rotator

Figure 7.5 : Rotorcap Valve Rotator Operation

VALVE TEMPERATURE AND COOLING

Exhaust valve temperature may reach approximately 704°C to 815°C. This means that they
are in fact running red hot. Good heat transfer, therefore, is essential.
It is important that valve can be fully seated when they are closed. The exhaust valve is
closed approximately two – third of the time while the engine is running.

Figure 7.6 : Zones of Heat

61

INTERNAL COMBUSTION ENGINE

It is during this time that a large part of the heat is transferred from the valve head to the seat.
The heat from the seat is transferred to the engine coolant. Figure 7.6 shows the zones of
heat and working temperature of the exhaust valve. The remaining heat transfer takes place
from the valve steam to the valve guide then to the engine coolant.

HOW SODIUM COOLED VALVES CAN MAINTAIN WORKING TEMPERATURE

Some exhaust valves also use a metallic sodium inside the stem. The sodium becomes a
liquid at operating temperature. Valve movement causes the liquid sodium transfers heat
from the head of the valve to the valve stem and then to the valve guide, which helps the
valve head to run cooler in Figure 7.7. Intake valve temperature is considerably lower than
exhaust valve temperature. In coming air-fuel gases cool the intake valve while the valve is
open. The valve seat has a great deal to do with good heat transfer.

Figure 7.7 : Sodium-Filled Stem Improves Cooling

OVERHEAD CAMSHAFT

Overhead camshaft (OHC) assembly procedures vary considerably between SOHC and
DOHC, split or full circle cam bearings. OHC may operate the valve directly through bucket-
type cam followers, through pedestal-mounted cam followers, or by rocker arms. On DOHC
engines with four valves per cylinder, there is one camshaft for intake valves and one for
exhaust valves. Now, most engines which have DOHC, operate without rocker arm, that means
the engine is less inertia from the valve linkage. The engine produces good performance.
Figure 7.8 shows DOHC.

Figure 7.8 : Double Over Head Cam with Four Valves per Cyclinder

62