Automotive mechanics (volume i)(part 2, chapter15) intake and exhaust systems

Intake and exhaust systems 231 Air cleaners 232 Carburettor air cleaners 232 EFI air cleaners 234 Diesel air cleaners 236 Aircleaner service 236 Engine manifolds 238 Intakesystem problems 240 Exhaust systems 240 Exhaustsystem service 243 Exhaustsystem problems 244 Technical terms 246 Review questions 246

Intake and exhaust systems

Chapter 15

Air cleaners

Carburettor air cleaners

EFI air cleaners

Diesel air cleaners

Air-cleaner service

Engine manifolds

Intake-system problems

Exhaust systems

Exhaust-system service

Exhaust-system problems

Technical terms

Review questions

The intake system is responsible for providing clean

air and carrying it into the cylinders of the engine The

system includes the air cleaner and the intake manifold

and other ducting and passages In petrol engines, the

air from the intake system carries the air–fuel mixture

into the cylinders In diesel engines, the intake system

carries only air.

The exhaust system carries the exhaust gases away

from the engine and reduces the exhaust noise It

consists of the exhaust manifold, exhaust pipes, and

one or more mufflers Most petrol engines also have a

catalytic converter to reduce the exhaust emissions.

■ This chapter covers normal intake and exhaust

systems Induction systems and turbochargers are

covered in Volume 2.

Air cleaners

An air cleaner has a filter element through which all

the air passes before it enters the engine The air

cleaner body also acts as a silencing chamber to muffle

the sound of the incoming air.

A considerable amount of air passes through the

intake system into the engine By mass, it is about

fifteen times the amount of fuel The air must be

filtered because, even under good conditions, dust and

grit are present in the air.

If unfiltered air is allowed to enter the engine, it

will act as an abrasive and cause premature wear to the

valve guides, piston rings, pistons and cylinder walls.

Types of air cleaners

There are a number of different types of air cleaners, all

of which contain some type of filtering element to

remove dust from the air Air cleaners for passenger

and light commercial vehicles have a dry filter element,

but air cleaners with wet elements have been used.

These have a filtering element that is wet with oil.

There are different arrangements of the filtering

systems for carburettor engines, EFI engines and diesel

engines.

Carburettor air cleaners

Most carburettor engines have a large circular air

cleaner like the one shown in Figure 15.1 This is

located on top of the carburettor Apart from its

filtering action, the filter of a carburettor engine can

act as a flame trap If there is an engine backfire, the

filter will contain the flame within the air cleaner.

Dry-element air cleaners

This type of air cleaner (Figure 15.2) contains a replaceable element, which is made from pleated paper

or cellulose fibre This is a very fine porous material It

is fine enough to filter the impurities from the air, but porous enough to allow the clean air to pass through with very little restriction.

The air cleaner body is made of pressed steel The top of the filter can be removed to access the replaceable filter element

figure 15.1 On a carburettor engine, the air cleaner is

usually mounted on top of the carburettor

figure 15.2 Dry-type carburettor air cleaner cut away to

show the filter element FORD

Hot-air controls in air cleaners

On carburettor engines, a shroud fitted around the exhaust manifold provides heated air, which can be taken into the air cleaner through a connecting pipe The air cleaner has a flap valve in its air inlet, which automatically controls the amount of heated air that enters the air cleaner The flap (or control valve)

in some air cleaners is operated by a thermostatic spring, in others, by a vacuum control.

The heated air provides better vaporisation of the

fuel under cold conditions It also reduces the amount

of unburnt hydrocarbons that are emitted through the

exhaust.

Thermostatic control

This arrangement is shown in Figure 15.3 When the

engine is first started the flap valve is fully open to

allow only heated air to the air cleaner As the engine

operating temperature increases, the thermostatically

controlled flap valve gradually closes off the hot air

passage and opens the normal air intake.

A mixture of heated air and cool air (air at

atmospheric temperature) is provided until the engine

warms up With the engine warm, the flap valve will

have closed off the hot air passage and fully opened

the air intake partly open and air entering the air cleaner will be a At intermediate temperatures, the flap valve will be

mixture of cool air and hot air, being regulated by the thermosensor valve.

Wet-type air cleaners

There are two types of wet air cleaners: oil-wetted and oil bath.

Oil-wetted air cleaner

An oil-wetted air cleaner has an element made of metal strands packed closely together These are wet with oil The air moving through the air cleaner passes through the element where particles of dust are collected on the oily strands.

Oil-bath air cleaner

This type of air cleaner uses a similar element to an oil-wetted type but, in addition, has a bath of oil in the bottom of the cleaner Air entering the cleaner passes across the surface of the oil, which collects the particles of dust (Figure 15.5).

The air passing through the cleaner suddenly changes its direction as it reaches the oil The dust particles do not change direction, but continue on into

figure 15.3 Arrangement for heated air to a carburettor

air cleaner

1 air cleaner, 2 flap valve, 3 cool air, 4 thermostatic control,

5 hot air from around the exhaust manifold MAZDA

figure 15.4 Vacuum operated hot-air control on an air

cleaner MITSUBISHI

Vacuum control

A vacuum control unit is shown in Figure 15.4 This

has a diaphragm with a rod attached to a control valve

(flap valve) The chamber above the diaphragm is

connected by a tube to the intake manifold.

When the engine is started, intake manifold vacuum

(negative pressure) applied to the diaphragm opens the

control valve and allows hot air into the air cleaner.

A thermosensor valve, located inside the air

cleaner, senses changes in the air temperature When

this is less than 30°C, the thermosensor valve is open.

This allows vacuum to reach the diaphragm and open

the control valve to supply hot air.

When the air temperature rises to 45°C, the

thermo-sensor valve cuts off the vacuum The spring pushes

the diaphragm down and the control valve closes off

the bath of oil The dust is trapped in the oil and sinks

to the bottom of the container.

The air is further filtered through an oil-wet filter

element before entering the engine.

■ Oil-wetted or oil-bath air cleaners are only likely to

be found on older vehicles or in special applications.

EFI air cleaners

Multipoint electronic fuel injection (EFI) systems

usually have the air cleaner located at the side of the

engine and connected into the intake system by large

hoses and ducts.

Air cleaners used with EFI can be rectangular or

round Apart from their shape and location, the filter

elements are similar to air cleaners for carburettor

engines.

The EFI air cleaner in Figure 15.6 has a plastic

body The flat rectangular filter is made of cellulose

fibre and this is supported in the air-cleaner body and

enclosed by a cover The air cleaner is mounted beside

the engine It is located in the air-intake system, ahead

of the airflow sensor and throttle body.

Air enters through the air intake at the side of the

air cleaner body and passes upwards through the filter

element.

■ Engines with throttle-body EFI have the air filter mounted on top of the throttle body assembly and are similar to carburettor air cleaners.

EFI air-cleaner and ducts

Figure 15.7 shows the components of an EFI air-cleaner assembly with its hoses and ducting The air intake is made of moulded plastic material and located

at the front of the engine compartment The intake shown has an air chamber that can hold a volume of air This acts as a resonator to reduce the noise of the incoming air.

The system shown also has a supplementary resonator fitted after the air cleaner This helps to reduce air pulsation.

Figure 15.8 shows the air cleaner and air ducts for a different EFI system This carries air from the front of the vehicle to the throttle body on the intake manifold

of the engine.

Figure 15.8(a) shows the parts of the system The air cleaner is bolted to the body beside the engine There is a long duct between the air cleaner and the front of the car for air intake, and a flexible hose between the air cleaner and the throttle body.

Figure 15.8(b) shows the dismantled air cleaner assembly This has an air filter element and also a noise-reduction filter The airflow sensor for the EFI system is attached to the air cleaner cover When the cover is assembled to the body, the airflow sensor fits inside the filter element An electrical connector

on the sensor connects it to the electronic control unit.

■ With EFI systems, it is essential that there are no air leaks All the air entering the engine is accurately measured by the airflow sensor An air leak at a connection after the sensor would upset the air–fuel mixture.

EFI intake system

Figure 15.9 shows the intake system for a four-cylinder engine The air is taken in through the air intake at the front of the engine It passes through the air cleaner, the airflow sensor and the throttle body to the plenum chamber This acts as a form of air reservoir Air is then carried through the intake manifold to the cylinders of the engine.

figure 15.6 Air cleaner for an EFI system

figure 15.7 Air cleaner and ducting for an EFI engine HYUNDAI

air intake hose

air intake

air cleaner body filter

cover

figure 15.8 Air intake system for an engine with EFI

Diesel air cleaners

Diesel engines often have more than one air cleaner

and these are often mounted away from the intake

manifold They have ducting which connects them to

the engine (Figure 15.10) Off-road vehicles are likely

to have a precleaner and a main cleaner.

The air intake of some off-road vehicles is mounted

externally and raised almost to the height of the

passenger cabin This keeps the air intake above the

dust created by the vehicle.

The cyclone filter is so named because of the swirling action of the air as it passes through the air cleaner To create this airflow, vanes are fitted inside the body of the air cleaner.

The air cleaner operates in the following way: Air entering the air cleaner passes over the angled vanes and these impart a rotary motion to the airflow This spins out the heavier particles of dust, which are collected in the bowl at the bottom of the cleaner The air then passes through a dry-filter element for further cleaning before passing on to the intake manifold.

■ The cyclone filter and a dry air filter are combined

in the filter shown, but cyclone filters are also made

as separate air cleaners.

Air-cleaner service

Filter elements should be removed for service at periods recommended by the manufacturer However,

if the vehicle is operating in unusually dusty conditions, the air cleaner must be serviced more frequently.

Servicing the element varies according to its type,

as indicated below.

■ The main purpose of the air cleaner is to keep damaging dust from entering the engine If the element becomes clogged, is damaged, or does not fit properly, it will not do its job and the engine will suffer.

figure 15.9 Air intake system and intake manifold for a

four-cylinder EFI engine TOYOTA

plenum chamber

throttle body

cover

air cleaner body

intake manifold

air intake

air ducting

figure 15.10 Air cleaner of the type used for diesel

engines

Cyclone-type air cleaners

A cyclone-type air cleaner is shown in Figure 15.11.

The cyclone filtering arrangement is not efficient

enough on its own, and so it is used as a precleaner for

a dry-type filter.

figure 15.11 Cyclone-type air cleaner

mounting bracket

body

vanes

baffle

dust bowl clamp bolt filter element

Dry-element cleaners

The element can be removed for cleaning and

examined against a light for punctures Paper-type

elements should not be washed in any type of cleaning

solution, as this will destroy the filtering effect.

There are two methods of cleaning:

1 The element can be carefully and lightly tapped on

a clean flat surface to dislodge the dust particles.

The element must be kept flat as this is being done.

Do not strike the element on its edge, or in such a

way that it will become distorted or damaged and

so fail to seal when reinstalled.

2 The element can be cleaned by blowing with air

(Figure 15.12).

With a flat filter, air is blown from the inside

to the outside, that is, in the opposite direction to

normal air flow.

With a round filter, air is carefully blown from the centre of the element outwards to dislodge dust

from the outer surface of the filter paper.

■ Care must be taken not to blow holes in the paper,

so the nozzle of the air gun must be held at least

100 mm away from the inside of the element.

a matching groove and tab Failure to match these during reassembly can distort the cover and cause air leaks.

Where the cover of the air cleaner is secured by a wing nut, it should not be overtightened because this can also cause distortion.

Oil-wetted and oil-bath cleaners

The element of an oil-wetted filter is serviced by immersing it in cleaning solvent and agitating it to remove the oil and dirt.

After draining, and blowing with air, the element is re-oiled with engine oil It is then allowed to stand so that surplus oil will drain off before it is reinstalled The filter element of an oil-bath air cleaner is removed and serviced in a similar way to an oil-wetted type The oil in the cleaner body is discarded and the body is washed to remove dust deposits It is then filled to the level mark with engine oil.

■ Avoid overfilling, as this can have a restricting effect on the air flow and cause loss of engine power.

Cyclone cleaner service

This type of air cleaner is serviced by removing the dust bowl from the bottom and cleaning out the dust The inside of the body can be wiped out with a damp cloth When operating in dusty conditions, the bowl should be removed and cleaned frequently to prevent it from becoming overloaded If this occurs, the filter element of the main filter will be doing all the filtering and could become clogged.

If the cyclone air cleaner also has a filter element, this can be removed and cleaned in the same way as other dry-type elements.

EFI air-cleaner service

With EFI filters, clips are used to secure the cover to the body Releasing the clips enables the cover to be separated from the body The filter element can then

be removed.

The element is cleaned in the same manner as other dry-type elements If electrical connections have to be disconnected to access the filter, this should be done carefully.

The ducts and hoses should be checked to make sure that the connections are tight and that the joints do not have air leaks.

figure 15.12 The air-cleaner element can be blown lightly

to remove dust MAZDA

outside

inside

Renewing the filter element

While the filter element can be cleaned, the pores of

the filtering material will gradually become blocked.

This will restrict air flow and so manufacturers specify

that the filter element should be renewed after a certain

period For example, every 30 000 km of operation, or

more often if the vehicle is used in dusty conditions.

When reassembling the element to the air cleaner,

make sure that it fits and seals correctly Sealing is

important to prevent unfiltered air from entering the

system.

On some carburettor air cleaners, the body and the

cover are located in relation to each other by means of

Engine manifolds

There are basic differences between the manifolds

used on carburettor engines, EFI engines and diesel

engines This applies particularly to intake manifolds.

Some of their features are:

1 Carburettor engines have intake manifolds that

carry a mixture of air and fuel into the engine.

2 EFI engines with throttle-body injection have

intake manifolds that are similar to carburettor

manifolds They also carry a mixture of fuel and air

into the engine.

3 EFI engines with multipoint injection have intake

manifolds that carry air Injectors in the manifold

spray fuel into the intake ports of the cylinder

head.

4 Diesel engines have manifolds that carry air only.

The fuel is injected directly into the cylinder.

Carburettor engine manifolds

Figure 15.13 shows a typical cylinder head and its intake and exhaust manifolds This is for a four-cylinder carburettor engine It is a crossflow four-cylinder head, with the intake manifold on one side and the exhaust manifold on the other.

The intake manifold has a mounting for the carburettor and a flange which bolts onto the cylinder head It has four branches that carry the air–fuel mixture from the carburettor to the cylinders The manifold is made from aluminium alloy for reduced weight and good heat transfer.

Most carburettor intake manifolds are heated to improve the vaporisation of the air–fuel mixture when the engine is cold The manifold shown has a water-jacket under the carburettor mounting This is supplied with a flow of engine coolant.

figure 15.13 Cylinder head and manifolds for a carburettor engine HOLDEN LTD

EFI engine intake manifolds

An inlet manifold for an EFI engine is shown in

Figure 15.14 This is made of aluminium alloy It has

long branches and a plenum chamber The plenum

chamber provides a surge chamber which reduces

intake air resistance.

The branches of the manifold are designed to be of

equal length The long branches create an

inertia-charging effect that improves intake efficiency.

V-type engine intake manifolds

V-type engines have the intake manifold located in the

valley between the two cylinder heads Branches of

the manifold go to the intake ports at each side.

Figure 15.15 shows the parts of an intake manifold

assembly for a V-6 engine The lower intake manifold

fits between the cylinder heads and the upper intake

manifold is bolted to it The cover is then bolted to the

upper intake manifold to form the plenum chamber (air

chamber).

Seals are used between the parts so that the

assembly is airtight The injectors fit into holes in

the lower intake manifold They are attached to the

fuel rails, which supply them with fuel.

Variable intake manifold

With EFI engines, the branches of an intake manifold

(also called runners) are designed to have a particular

length and diameter to suit the engine The design is

something of a compromise, because the requirements

of an intake manifold at high engine speeds are

different to those at low speeds.

At low engine speeds, the branches of the intake

manifold need to be of small diameter so that the

velocity of air is maintained At high engine speeds, the branches need to be larger in diameter so that the air flow is not restricted.

There is also the pulsation of the air in the manifold

to be considered A ram air effect can be created in the manifold if it is tuned to the right length The ram effect originates with piston and valve action, which produces pulsations in the manifold The pulsations can be accentuated by the design of the manifold and used to produce the ram effect.

Dual branches or runners

To provide for the different requirements at high and low engine speeds, intake manifolds can be designed with dual branches as shown in Figure 15.16 The

figure 15.14 Intake manifold for an EFI engine – arrows

show the air flow HYUNDAI

figure 15.15 Parts of an intake manifold assembly for a

V-type engine – the fuel rails and injectors are also shown HOLDEN LTD

cover

plenum chamber

upper intake manifold

fuel rails and injectors

seal

lower intake manifold air inlet

diagrams show a cross section of one branch There are

two paths that the air can take, a long path and a short

path These are controlled by throttle-type valves.

At low speeds, the valve plate is closed as shown in

Figure 15.16(a) and the air is directed around the

longer and narrower path to the engine.

At high speeds, the valve plate is opened as shown in

Figure 15.16(b) Most of the air then passes through the

valve and takes the shorter and wider path to the engine.

The valves are located between the plenum

chamber and the manifold branches (Figure 15.17).

There is a valve plate for each branch and these are

mounted on a common shaft that is rotated by a

vacuum control unit This, in turn, is controlled by the

engine’s power control module.

Intake-system problems

Some problems for EFI and carburettor engine

air-intake systems and the likely effects are:

1 Restricted filter A blocked filter element in the air

cleaner could restrict air flow and cause loss of

engine power, particularly at higher speeds.

2 Air leaks, EFI system An air leak after the air filter

and ahead of the throttle body will admit unfiltered

air into the system.

An air leak after the throttle body will upset the fuel mixture, because the air will be additional to that measured by the airflow sensor.

3 Air leaks, carburettor system An air leak between the air cleaner and the carburettor will allow unfiltered air into the system.

Air leaking into the system between the carburettor and the intake manifold will weaken the air–fuel mixture.

Exhaust systems

Exhaust manifolds are usually made of cast iron, which is able to resist the high exhaust temperatures Exhaust manifolds can also be fabricated from stainless steel, which is lighter than cast iron.

The exhaust manifold in Figure 15.13 is made of cast iron and has four branches, one for each exhaust port The branches carry the exhaust gases from the exhaust ports, and join together to form the exhaust flange The manifold flange provides a connection for the exhaust pipe.

The exhaust manifold is covered by a shroud This shields other parts of the engine assembly from the heat that radiates from the manifold On carburettor engines, it also provides a ‘stove’ from which the air cleaner can receive heated air.

Parts of exhaust systems

An exhaust system for a V-type engine is shown in Figure 15.18 With a V-type engine, there is an exhaust manifold on each side of the engine, so dual com-ponents are used There are two catalytic converters

figure 15.16 Intake manifold with dual branches (runners)

(a) air flow at low engine speeds (b) air flow at

high engine speeds FORD

intake port

valve open

throttle body cylinder head

throttle body

plenum

valve closed

(a)

(b)

figure 15.17 Lower part of an intake manifold with dual

branches and control valves FORD

control valves

lever

valve plate