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Student Workbook

LV34 Engines (3)

kap all phase 2 & 3 6/11/03 11:35 am Page 3

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV34 ENGINES (3)

Contents

Page Page

Effects of fuel on valves and valve Management of the VVT - i system 28

Operation of valve rotators 9

Varying the valve timing 26

Page Page

Comparing the VTEC and VTEC–E 52 Components and Operation: 64

Continuous Variable Valve Timing

VANOS variable cam timing (BMW) 55

Valve timing gear operation 56

Operation of the double VANOS 60

Function 62

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

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)

Requirements of a valve

Valves

tight seal when closed

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

Valve guides

Shoulder type

Shoulder limits depth into cylinder head

Shoulder type

Shoulder limits depth into cylinder head

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

Shrink Fit Valve Seat Laser Clad Valve Seat

Valve seat 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

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 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

C

Rotator body Coil spring Plate spring Valve closed

Retainer

Valve spring

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

C

Rotator body Coil spring Plate spring Valve closed

Retainer

Valve spring

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

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

Twin camshaft four valve per cylinder engine

With camshafts removed Camshafts fitted

Piston

Cylinder block Water jacket Gasket

Valve guide Exhaust valve Valve spring

Valve lifter or cam follower

Adjusting shim Valve

Valve lifter or cam follower

Adjusting shim Valve

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

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

Operation of the scissor gear

Shown here is an in-line

four cylinder engine with

twin camshafts and four

The scissor gear

Shown here is an in-line

four cylinder engine with

twin camshafts and four

The scissor gear

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

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

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

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

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

Valve lash adjuster

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

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:

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

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:

Variable Valve Timing Systems

Valve timing advanced

Crankshaft Sprocket

Telescopic rod Camshaft

Telescopic rod extended to advance valve timing

Camshaft pulley

is pulled round

to new position Top dead centre

Valve timing advanced

Crankshaft Sprocket

Telescopic rod Camshaft

Telescopic rod extended to advance valve timing

Camshaft pulley

is pulled round

to new position Top dead centre

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

Conventional valve timing

Inlet valve timing advance

Exhaust valve timing advance

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

VVT-I controller

The controller consists of

a housing driven by a timing chain and the vane

is connected to the inlet camshaft

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

During normal driving

Timing is advanced re-burning a portion of exhaust gas, which minimises emissions

During full acceleration

Timing is retarded which delays the closing of the intake valve and this allows a greater amount of air/fuel mixture to be drawn in producing higher torque and

improved acceleration

During full power

Timing is advanced thus preventing intake lag associated with high rpm

Allows greater amounts of air/fuel mixture to be drawn in, improving output power

Varying the valve timing

No valve overlap

Focus attention here

No valve overlap

Focus attention here

No valve overlap - exhaust gas moving into the intake side is prevented by delaying inlet valve opening

Large valve overlap

Focus attention here

Large overlap

Focus attention here

Large overlap

As a result of increasing the valve overlap – inlet and exhaust valves are open at the same time, the inlet and exhaust gases mix and re-burning takes place

Intake valve closes quickly

Focus attention here

Closes quickly

Focus attention here

Closes quickly

By closing the inlet valve early, air/fuel mixture is prevented from being discharged

The actual inlet valve timing is fed-back to the ECU from the camshaft position sensor to control the target valve timing

Management of the VVT - i system

The engine ECU calculates the target-timing angle according to the travelling state and sensor inputs

Reason for the lock pin

Improved starting is achieved when the valve spring force rotates the inlet cam to the fully retarded position when the engine is stopped

The spring-loaded lock pin locks the vane and housing together After the engine starts the lock pin is released by engine oil pressure The lock pin prevents a

knocking noise due to lack of hydraulic pressure being applied to the controller immediately after the engine has started When the engine starts the lock pin is released by hydraulic pressure

Retard

Camshaft Timing Oil Control Valve Drive Signal

Camshaft Timing Oil Control Valve Drive Signal

Camshaft Timing Oil Control Valve Drive Signal

When the camshaft timing oil control valve is positioned as illustrated in the diagram, the resultant oil pressure is applied to the timing retard side vane chamber to rotate the camshaft in the retard direction

Hold

Camshaft Timing Oil Control Valve Drive Signal

Camshaft Timing Oil Control Valve Drive Signal

Camshaft Timing Oil Control Valve Drive Signal

After setting to the target timing, the valve timing is held by keeping the camshaft timing oil control valve in the neutral position unless the travelling state changes This adjusts the valve timing to the desired target position and prevents the engine oil from running out when it is unnecessary

Advance

When the camshaft oil control valve is positioned as shown by the ECU advance signal, oil pressure is applied to the timing advance side chamber to rotate the camshaft in the timing advance direction

Progress check 2

Answer the following questions:

1 List three main advantages of variable valve timing over conventional valve

operating systems:

2 Explain the meaning of valve overlap

3 Sketch a simple valve-timing diagram and indicate an advanced position for

the inlet valve

4 The Electronic Control Unit (ECU) requires information from various sensors

The ECU then calculates the target-timing angle according to the travelling state List three sensors that transmit the necessary information:

5 State the purpose of the lock pin on a VVT – i controller Describe briefly the

operation of the lock pin

Timing Oil Control Valve

The camshaft timing oil control valve selects the path to the VVT - i controller

according to the advance, retard or hold signal from the ECU The intake camshaft

is rotated by the VVT - i controller to advance, retard or hold the valve timing, these positions are governed by the oil pressure applied

Valve timing varied

Stabilised idling speed and improved fuel economy

Light load

Less overlap eliminates blowback into intake

Greater engine stability

Medium load

Overlap increases causing an internal EGR (exhaust gas re-circulation) which eliminates pumping losses

Improved fuel economy and emissions

Torque is improved

in the low

to medium engine speed range