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Student Workbook

LV34
Engines (3)

LV34/SWB


Student Workbook for Technical Certificates in
Light Vehicle Maintenance and Repair

MODULE LV34
ENGINES (3)

Contents
Page

Page

3

Advanced Valve Systems:
Effects of fuel on valves and valve

seats
Requirements of a valve
Valve guides
Valve springs
Valve rotators
Operation of valve rotators
Camshafts
Twin camshaft four valve per
cylinder engine
Camshaft layout of a V6 engine
Fully assembled V6 engine
Operation of the scissor gear

10
11
12
13

Hydraulic Valve Lifters:
Operation valve closed
Operation valve open
Valve lash adjuster
Progress check 1

16
17
17
18
19

Variable Valve Timing Systems:
Conventional valve timing
Variable valve timing
During idling
During normal driving
During full acceleration
During full power
Varying the valve timing
Large valve overlap
Intake valve closes quickly

22
23
24
24
25
25
26
26
27
27

4
5
5
8
8
9
9

Operation of VVT - i
Management of the VVT - i system
Reason for the lock pin
Retard
Hold
Advance
Progress check 2

28
28
29
29
30
30
31

Timing Oil Control Valve:
Valve timing varied
Valve arrangement
Operation
Low and medium speed
High speed
Oil control valve
Progress check 3

33
34
36
36
37

38
38
41

VTEC System:
VTEC operation
Basic principles of VTEC operation
Main components of VTEC
Effect of change-over on torque
Hydraulically operated pin

43
44
44
46
47
47

Three Stages of VTEC Operation:
Stage 1
Stage 2
Stage 3

49
49
50
51

(Cont.)

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LV34: Engines (3) Issue 1


Page
Comparing the VTEC and VTEC–E
Main advantages
The VTEC-E
Progress check 4

52
52
53
54

Continuous Variable Valve Timing
Control:
VANOS variable cam timing (BMW)
Valve timing gear operation
Operation of the VANOS
Operation of the double VANOS
Function

55
55
56
58
60

62

Page
Components and Operation:
Comparing VTEC with Valvetronic
Variable valve timing (Ferrari)
The future
Progress check 5

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LV34: Engines (3) Issue 1

64
66
66
67
69


Advanced Valve Systems

Thomas Midgley, Jr.
To help to understand the need for improvement in materials that are used in valves
and valve seats, it is helpful to investigate briefly the history of the development of
fuel.
The story of leaded petrol goes back many years and involves Thomas Midgley, Jr.
(not usually ranked with the likes of Isaac Newton) who was born in 1889 and held a
PhD in engineering.

In 1916 he joined forces with Charles Kettering the inventor of coil ignition. Kettering
was having trouble with a farm engine that ran on kerosene and knocked very badly.
Midgley added iodine to the fuel and knocking was reduced. After six years he
found that tetra-ethyl lead worked beautifully.
In the 1920s tetra-ethyl lead was a wonderful invention. When the spark plug ignites
the fuel mixture a flame front travels through the combustion chamber. Tetra-ethyl
lead made the flame front travel more slowly and less turbulently. Lead virtually
eliminated knock, and overnight compression ratios jumped from 4:1 to 7:1,
therefore the modern high output engine was born. In today’s modern engines,
cylinder pressures reach forty times atmospheric pressure.

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LV34: Engines (3) Issue 1


Originally, his ideas were thought to be brilliant, only later did they turn out to be
disasters. Lead emitted from vehicle exhaust was affecting children all over the
world. Today, of course, we use unleaded petrol.
Thomas Midgley was the man who put lead in our petrol; at the time it was hailed as
a great advance but eighty years on we are still trying to deal with the consequences
of it.
Midgley also invented chlorofluorocarbons and we know the damage that has
occurred by using these wonderful chemicals. They are responsible for punching a
very large hole in the ozone layer which in 1992 was the widest and deepest ever
recorded. CFCs and the reduction of the ozone layer have increased ultra-violet
radiation levels, which are responsible for the increase in skin melanomas.
Paradoxically, in 1939 Thomas Midgley predicted the control of the ozone layer in
order to control the Earth’s climate.

In 1940, Thomas Midgley, Jr. was paralysed by polio. He built an assembly of
pulleys and ropes so he could move himself between his bed and his wheel chair. In
1944, he became entangled in the ropes and strangled to death in his own invention.

Effects of fuel on valves and valve seats
As leaded petrol burns, the tetra-ethyl lead turns into a tan-coloured layer of lead
oxide, which covers valves and the combustion chamber. The valves hit their seats
hard several thousand times a minute, the lead oxide acts as a cushion therefore
protecting the valves and seats. Lead oxide has lubrication properties, which
reduces wear on the valve guides.
If an old leaded engine is run on unleaded fuel, damage occurs, but only if the
engine is fitted with ‘soft’ metallurgy, and only in high temperature areas such as the
exhaust valves, guides and seats.
On a ‘soft’ cast iron valve seat at high temperatures, iron oxides form and these
oxides flake off and actually embed themselves in the soft face of the exhaust valve.
These tiny particles work like very small grinding wheels as the valve operates,
grinding away the seat taking up the valve clearance preventing the valve from
closing properly. They cause edges of the valve to burn since heat cannot be
conducted away through the seat (burnt valve).

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LV34: Engines (3) Issue 1


Requirements of a valve

Valves
• must ensure a gas

tight seal when closed

Valves
Valve operating
mechanism

• offer as little
opposition to the flow
of gas possible
• have a reliable
operating mechanism
• operate with
minimum wear under
extreme conditions,
long life
• be accurately timed
to open and close to
provide maximum
engine efficiency
(power)

The use of unleaded fuel demands more robust valve components as follows:

Valve guides
Shoulder limits
depth into cylinder
head
This distance
is important
Why?

The valve guide in this case is a
machined hole in the cylinder
head

Common plain type

Integral type

Shoulder type

Silicon-aluminium bronze valve guides are used (good heat conducting properties),
usually in engines with double overhead cam (DOHC). Silicon-aluminium bronze is
fairly soft therefore it wears rapidly. In DOHC engines the valve moves up and down
with virtually no side force on the valve stem.

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In engines fitted with tappets, the end of the tappet moves in an arc, therefore it
forces the valve into the wall of the valve guide, in this case steel or cast iron valve
guides are used.
Note: Steel and cast iron valve guides may be used because they are less
expensive.

Poppet Valve
Valves are faced with hard stellite. Alloys that have been developed to satisfy

conditions during the operation of valves (they are subject to approximately 650°C
during the exhaust stroke) are varying amounts of manganese, silicon, nickel and
chromium. A new material used is molybdenum and titanium, which makes them
highly resistant to heat and also reduces the valve weight by approximately 20%.
Heat is passed from the seat directly to the cylinder head and along the stem
through the guide to the cylinder head.
In some extreme operating conditions the valve stem is sometimes made hollow and
filled with sodium. Sodium is a soft metal with a low melting point of 98°C. In its
molten state it splashes up and down the valve stem, therefore assisting the transfer
of heat from the head of the valve.
Some valves are coated with aluminium to improve heat transfer from the valve to
the engine block. These valves cannot be re-surfaced (ground) in the normal way
due to the surface coating being thin. Seventy five percent of exhaust valve heat is
dissipated through the valve seat area.
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Valve seat
Valve seat

Shrink Fit Valve Seat

Laser Clad Valve Seat

Valve seats are stellite or hard chromium (60% alloy valve seat inserts). An
alternative is to use a laser clad valve seat, which is a highly wear resistant alloy,
and is welded onto the cylinder head and subsequently machine cut to form the seat

angle. With this system the seat can be made thinner, the result is that the valve
seat diameter can be made larger and the cooling effect around the valve seat is
improved.

The valve seat is shaped like a cone and is normally at an angle of 45 degrees
although manufacturers use alternative angles as shown above.
The valve seat is in the shape of a cone to conform to the shape of the valve. The
valve seat contact width is generally 1.2 to 1.8 mm.
Excessive valve seat contact width is likely to cause carbon intrusion between the
valve face and seat, although the cooling effect will be high. If it is too narrow, gas
tightness will improve but the cooling effect will decrease.

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LV34: Engines (3) Issue 1


Valve springs

Valve springs are used to close
the valves quickly, most
engines have just one spring
per valve but some use two
springs per valve.

To prevent valve surging
(bounce) when the engine is
running at high speed uneven
pitch springs or double springs

are used.
The wider
pitch is
always
installed at
the top of the
valve

Valve surging causes abnormal noise generation from the engine when the engine is
operated at high speed, it can also cause interference between the piston and the
valve, which may lead to damage of both parts.

Valve rotators

Valve
keepers

Valve

A

Rotator body
Coil spring
Spring plate

B
C

Retainer

Valve spring
Rotator body
Valve closed

Some engines have valve rotators
fitted rather than conventional
valve retainers. The purpose of the
rotator is to prevent improper
seating of the valve cause by lead
compounds when leaded fuel is
used or carbon sticking to the valve
surface

Coil spring
Plate spring

Valve open

Valve closed

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LV34: Engines (3) Issue 1


Operation of valve rotators
When the valve is opened, the valve spring is compressed therefore the tension on it
becomes greater. This causes the outside circumference of the plate spring to flex
upward slightly, causing the coil spring to flatten even more, the rotator body then

turns. At this time point A slides, but points B and C do not slide.
When the valve closes, the spring extends and the tension in it weakens. The spring
returns to its original condition, this causes slipping to occur at points B and C but no
slipping occurs at point A. Therefore the rotator body remains in the same position
as when the valve is open.

Camshafts

On single and double overhead valve engines the crankshaft drives the camshaft via
a belt, gear or chain system.
Double overhead cams are used on engines with four valves per cylinder, inline
engines have two camshafts and ‘V’ engines have four. Having more valves per
cylinder increases the flow of gas, therefore increasing the power output of the
engine.
Camshafts are made from steel, either forged or cast, and then machined, case
hardening is used on the cam lobes, while cast shafts are usually hardened by
chilling during casting. More compact and lighter camshafts are made from high
carbon, high chromium alloy and are then tempered to withstand increased pressure
between the cam and valve operating mechanism of high lift high-pressure cam
lobes. Camshafts are supported in plain bearings but sometimes roller bearings are
used.

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Twin camshaft four valve per cylinder engine

With camshafts removed

Valve
keepers

Camshafts fitted

Spark plug

Adjusting shim

Intake valve

Valve lifter or
cam follower

Oil seal

Valve spring
Exhaust valve
Valve guide

Intake

Exhaust

Valve seat
Gasket

Combustion

chamber

Water jacket
Cylinder block

Piston

The diagram above shows a cross section of a twin overhead camshaft engine
showing the main component parts.

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LV34: Engines (3) Issue 1


Camshaft layout of a V6 engine
Each cylinder has two intake valves and two exhaust valves, the valves are directly
opened and closed by four camshafts.
The intake camshafts are driven by a timing belt and the exhaust camshafts are
driven through gears on the intake camshafts.

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LV34: Engines (3) Issue 1


Fully assembled V6 engine

The intake camshaft drives the exhaust camshaft through a scissor (sub-gear) gear
mechanism, which allows for the valves to be placed at a narrower angle.

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LV34: Engines (3) Issue 1


Operation of the scissor gear

The scissor gear

Shown here is an in-line
four cylinder engine with
twin camshafts and four

The exhaust camshafts are driven by the gears on the inlet camshafts. The scissor
gear mechanism is used on the exhaust camshaft to control backlash and therefore
reduce gear noise.
To prevent the tooth surfaces from seizing when in mesh they are designed to have
backlash (clearance between the teeth in mesh).

Backlash creates noise especially when driving the valve gear, due to fluctuations in
torque on the camshaft.

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LV34: Engines (3) Issue 1

Scissor Gear Mechanism
The scissor gear is a means of preventing this noise occurring. The scissor gear
mechanism uses a sub gear with the same number of teeth as the drive gear and is
attached to the gear on the driven side.

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LV34: Engines (3) Issue 1


Through the reactive force of the scissors spring, these two gears act to pinch the
drive gear, reducing backlash to zero and eliminating gear noise.
The teeth on the driven and sub-gears are thus always engaged with the teeth of the
driven gear so that the gear train is free of backlash.
The camshaft driven gear is a press fit onto the camshaft and is provided with a pin,
which holds one end of the scissor spring. The sub-gear is secured to the inlet
camshaft by a snap ring and a wave washer; both gears have the same number of
teeth on them, the pin on the sub-gear holds the other end of the scissor spring.
The scissor spring is located between the camshaft driven gear and the sub-gear, its
ends, are held by the pins, the camshaft driven gear transmits torque in the direction
of rotation to the sub-gear via the scissor spring.

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LV34: Engines (3) Issue 1

Hydraulic Valve Lifters

Another way of reducing valve noise is to fit hydraulic valve lifters (cam followers),
usually a high-grade alloyed cast iron is used on the bottom surface of the valve
lifters. The lifters may be slightly rounded to aid lifter rotation by the cams. The
purpose of hydraulic valve lifters is to maintain zero clearance at all times therefore
removing the need to adjust the valve clearances.
As engine temperature varies the valve clearance also varies, maintaining the
correct clearance at all engine temperatures is impossible with normal conventional
tappets. The variation in valve clearance results in an unwanted change in valve
timing, which affects the power output of the engine.
The oil pump provides oil to the plunger in the valve lifter by way of the oil passage.

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LV34: Engines (3) Issue 1


Operation valve closed
The spring pushes the plunger upwards ensuring that the valve clearance is kept at
zero. The pressurised oil from the engine lubrication system oil pump pushes the
check ball against the check ball spring and it flows into the working chamber of the
valve lifter.

Operation valve open
As the camshaft rotates and the cam pushes on the lifter body, the oil pressure in
the working chamber raises the check ball which closes the oil passage. The lifter
body is pushed up, along with the plunger as the camshaft rotates and the engine

valve is opened by means of the valve operating mechanism.
As the lifter is pushed against the valve, a small amount of oil in the working
chamber escapes through the clearance between the body and plunger. The cam
continues to rotate and as it does so the engine valve closes and the oil again
pushes against the check ball and re-enters the working chamber, therefore
maintaining the engine valve clearance at zero.
During engine warm-up the engine valve expands and this decreases the volume of
oil in the working chamber to take up the play between the engine valve and the
cam. This occurs over a period of time and is compensated for by a small loss of oil
in the working chamber. This occurs every time the engine valve is operated,
consequently zero valve clearance is maintained, irrespective of engine
temperature.
A slight loss of oil from the lifter occurs when the engine has been unused for some
considerable time, therefore it is usual to hear a rattle from the valve operating
mechanism for a short time after the engine is started. If the noise persists, then an
investigation as to the cause must be carried out which may lead to a new lifter
being fitted.

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Valve lash adjuster

Shown here is a different arrangement for taking up the clearance between the
engine valve and the camshaft.

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Progress check 1
Answer the following questions:

1.

State three requirements of an engine valve:

2.

What major change made it necessary to improve the durability and strength
of engine valves and valve seats?

3.

What material contributes to making engine valves approximately 20%
lighter?

4.

State the purpose of using sodium in engine valves:

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LV34: Engines (3) Issue 1

5.

What, is meant by a laser clad engine valve seat? Describe the differences
when compared to a shrink fit valve seat.

6.

What methods are used to prevent engine valve surging (bounce)?

7.

State the purpose of rotating engine valves. Describe briefly how the rotating
device operates.

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LV34: Engines (3) Issue 1


8.

Discuss the main reason for using a scissor gear (sub-gear) to drive
camshafts. With the aid of sketches explain the operation of the scissor gear.

9.

Hydraulic tappets (valve lifters) are used because they:

10.

The purpose of the check ball in a hydraulic tappet (valve lifter) is to:

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LV34: Engines (3) Issue 1


Variable Valve Timing Systems
Valve timing
advanced

Top dead centre
Camshaft

Camshaft pulley
is pulled round
to new position

Telescopic
rod

Telescopic rod
extended to
advance valve
timing

Crankshaft
Sprocket

The dynamics of airflow through an engine combustion chamber change
dramatically over an engine range of 2000 to 6000 rpm. Using a standard valve
drive arrangement is a compromise which allows the engine to start, run and provide
strong acceleration with good cruising speeds, but engines are rarely ever in the
‘sweet zone,’ which results in wasted fuel, reduced performance and excess exhaust
emissions.
Inertia forces apply when trying to get air to move, it is hard to get moving and once
moving is hard to stop. It is well understood that the intake valve opens before the
piston reaches the top of the cylinder and closes after the piston reaches the bottom.
The exhaust valve begins to open as the piston reaches the bottom of the cylinder
and begins to close after the piston reaches the top.
As engine speed increases air will gain inertia force and even when the piston
reaches the bottom of the cylinder air will continue to flow in. Thus to obtain as
much air as possible without causing inefficiencies from these inertia forces, the best
solution would be to have the valve timing change as engine speed changes.
Variable valve timing has been developed to increase engine performance, improve
fuel economy and reduce exhaust emissions throughout all the engine’s operating
range.
The effect on fuel economy, power output and exhaust gas emissions is
considerable.

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LV34: Engines (3) Issue 1

Conventional valve timing
Inlet valve
timing advance

Exhaust
valve timing
advance

One of the main factors influencing engine performance is the amount of valve
overlap. The duration of valve overlap determines the amount of exhaust gas left in
the cylinder when the exhaust valve closes.
At higher engine speeds a longer inlet valve-opening period would increase the
power developed, but this will cause an increase in valve overlap and at idle would
greatly increase hydrocarbon emissions.
To overcome these and other problems variable valve timing is used.

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LV34: Engines (3) Issue 1


VVT-I controller

The controller consists of
a housing driven by a
timing chain and the vane
is connected to the inlet
camshaft

A typical variable valve-timing layout is shown here.

Variable valve timing
VVT-i, VVC and VTEC are all acronyms, which embrace a range of engine design
enhancements.
There are two basic methods of valve timing, cam-changing and cam-phasing.
Cam-changing (VTEC) provides different cam profiles allowing earlier opening of the
inlet valves and later closing including greater valve lift at high engine speeds. Cam
phasing is described below.

During idling

Timing is retarded which prevents exhaust gas intermixing with the intake air/fuel
mixture.
It provides stable combustion and engine idle speed can be lowered, which
improves fuel economy.
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