Automotive mechanics (volume II)(Part 2, chapter15) emission controls

Emission controls 277 Motor vehicle pollution: sources 278 Evaporativecontrol system 278 Crankcase ventilation 281 Exhaust emissions 282 Catalytic converters 283 Engine management 284 Engine design 286 Carburettor engines: emission systems 288 Other emissionsystem devices 291 Emission standards 292 Servicing emission controls 293 Diagnosis guides 295 Technical terms 297 Review questions 297

Carburettor engines: emission systems

Other emission-system devices

Emissions from motor vehicles are considered to be a

major cause of air pollution For this reason, vehicle

manufacturers are required to control emissions so that

their vehicles meet specified standards.

To achieve these, engines have been fitted with

various devices to reduce emissions Electronic fuel

injection and engine management systems, in particular,

have made engines more fuel-efficient and also reduced

the number of emission devices that are needed.

The chemicals discharged from industrial processes

and from motor vehicles combine with other chemicals

in the air, and some can be affected by sunlight Under

certain conditions, the impurities in the air can be seen

as smog, which is a term that combines the words

smoke and fog However, air pollution does not have to

be seen to be present, and pollution can be a health

hazard whether or not it is visible.

This chapter deals with petrol engines, which are

subject to stringent pollution controls.

Motor vehicle pollution: sources

Polluting emissions from a motor vehicle with a petrol

engine originate from the following sources:

1 Fuel supply Petrol is a very volatile liquid and

requires very little heat to cause it to evaporate.

Liquid fuel turns to vapour and is lost to the

surrounding atmosphere because of heat from

the atmosphere or from the engine.

2 Engine crankcase Gases within the crankcase

increase as the engine wears and the piston and

rings allow more blowby Some unburnt fuel

(hydrocarbon, HC) also finds its way down the

cylinder walls.

3 Exhaust system Chemicals enter the engine in the

form of fuel and air Most of these are changed by

the process of combustion inside the engine and are

then discharged through the engine’s exhaust

system into the atmosphere.

■ Whenever an engine is operating, it is producing

some harmful chemicals that can cause pollution.

Emission controls

There are three general types of emission controls that

are designed to remove or reduce emissions from the

sources noted above.

1 Evaporative controls These are fitted to prevent

fuel vapour from the fuel system escaping into the

atmosphere.

2 Crankcase ventilation This directs vapour from the crankcase to the intake manifold, so that the crankcase is vented internally and not to the atmosphere.

3 Exhaust-emission controls These limit the noxious gases exhausted from the vehicle by providing more efficient combustion and also by treating the by-products of combustion.

Evaporative-control system

An evaporative control system for a petrol engine is shown in Figure 15.1 The functions of the parts are described in the following paragraphs.

Fuel tank assembly

A fuel tank has to be vented to allow for expansion and contraction of its contents (liquid fuel and fuel vapour) If the tank is vented directly to the atmosphere, fuel vapour, which is above the liquid fuel, would be forced out the vent during expansion During contraction, air would be drawn back into the tank.

To prevent fuel vapour from being discharged into the atmosphere where it would be harmful, it is vented into a charcoal canister where it is stored until it can be burnt by the engine.

■ The fuel tank has to breathe and it does this through the carbon canister.

Fuel tank vapour space

Space is provided in the fuel tank to store and dense the vapour rising from the surface of the fuel The space is determined by the position of the overfill- limiting pipe, which limits the level of the fuel during normal filling procedure Once the fuel reaches the lower end of the pipe, air cannot escape back up the filler neck, and it is difficult to add additional fuel This limits the fuel and provides the necessary vapour space.

con-Fuel tank cap

The tank cap in this system is fitted with a relief valve This opens when the pressure in the tank falls below atmospheric pressure, and allows air to enter the fuel tank (Figure 15.2).

vacuum-■ If a fuel tank cap is to be replaced, the new cap must be of the same design as the original.

Check valve

Fuel vapour is stored in the fuel tank in the space

above the fuel, but when pressure starts to build up, the

check valve opens to allow vapour to pass through to

the charcoal canister.

Vapour can pass through the check valve, but liquid will close the check valve and block off the vent line This prevents liquid fuel from reaching the charcoal canister.

figure 15.1 Evaporative emission-control system for an EFI engine HYUNDAI

plenum chamber

ECU

vapour space

figure 15.2 Fuel filler cap with a relief valve HYUNDAI

Charcoal canister

The charcoal canister, also referred to as a carbon

canister, contains granules of activated charcoal

(Figure 15.3) It has two pipe connections: one for the

fuel tank vent and one for the vapour pipe to the intake

manifold As well as these, there is also an

atmos-pheric vent to admit air.

valve that is controlled by the ECU When the engine

is running, the vapour pipe, connected to the intake manifold, purges the vapour from the canister and carries it into the intake manifold.

Purging will occur only when the engine is running

at reasonable speeds Purging does not occur at idle and slow speeds when it could upset the air–fuel mixture ratio.

■ Vehicles without EFI have a purge valve that is vacuum-controlled.

Fuel tank vents

Fuel tanks can have more than one vent pipe, depending on the shape and the location of the tank in the vehicle When there is more than one pipe, at least one pipe is above the level of the fuel, regardless of the position of the vehicle This not only vents the tank, but prevents fuel from syphoning through the vent pipe The pipes are connected at some point above the fuel tank to a common vent pipe.

In some systems, the pipes are carried to a small container above the fuel tank, called a condense tank

or a liquid-vapour separator This acts as a trap and prevents liquid fuel from reaching the charcoal canister A venting system of this type is illustrated in Figure 15.4 This is for a light-commercial vehicle.

Returnless fuel system

Traditional EFI fuel systems have a fuel return line from the engine compartment This carries excess fuel from the pressure regulator in the fuel rail back to the fuel tank The disadvantage of this arrangement is that the fuel being returned has been heated by being close

to the engine This raises the temperature of the fuel in

figure 15.3 Charcoal canister HOLDEN LTD

figure 15.4 Fuel tank with condense tank and venting system

The fuel tank can breathe through the canister.

Vapour enters and exits the canister as the fuel in the

tank expands and contracts The atmospheric vent in

the canister allows air to enter or leave the canister.

The canister shown has a solenoid-operated purge

the fuel tank and leads to unnecessarily high

evapora-tive emissions.

A returnless system does not have a fuel return line.

This is because the fuel pump, fuel filter and pressure

regulator are all housed inside the fuel tank as shown

in Figure 15.5 With this arrangement, excess fuel

from the regulator is returned to the tank without being

heated.

PCV valve operation

The PCV valve is located in the valve cover It has a metered orifice and a stepped and tapered plunger, which also acts as a valve These control the flow of air and crankcase gases into the intake manifold so that they will not upset engine operation.

The valve is operated by the difference in pressure between the intake manifold and the crankcase Its action is illustrated in Figure 15.7 as follows:

1 When the engine is not running, the plunger is held in the closed position by its spring (Figure 15.7(a)) This prevents crankcase gases from entering the intake manifold while the engine is being started.

2 At normal engine operation the balance between the manifold vacuum and the spring holds the plunger mid-way in the body allowing for normal flow (Figure 15.7(b)).

3 At idle speeds, when intake manifold vacuum is high, the plunger is drawn to the intake manifold end of the body (Figure 15.7(c)) This creates a small vacuum passage which results in a restricted flow.

4 During acceleration, and at higher load, the lower intake manifold vacuum allows the plunger to move towards the other end of the body giving maximum flow (Figure 15.7(d)).

figure 15.5 Arrangement of a returnless fuel system

TOYOTA

pulsation damper

valve cover

crankcase

blow-by gas fresh air

Crankcase ventilation

Positive crankcase ventilation (PCV) systems are used

to prevent pollution They supply clean air to the

crankcase and, at the same time, remove gases that are

harmful to both the engine and the atmosphere The

gases are recycled back to the combustion chambers

where the combustible elements are burnt and the

remainder discharged with the exhaust gases through

the exhaust system.

Figure 15.6 shows a PCV system for an in-line

engine Similar systems are used on both EFI and

carburettor engines Filtered air is drawn through the

valve cover and into the crankcase, where it mixes

with the crankcase gases caused by blowby.

The gases are drawn from the crankcase through

the positive-crankcase ventilation valve (PCV valve)

and into the intake manifold They are then burnt as

part of normal combustion.

Exhaust emissions

The emissions from the exhaust system account for the

bulk of the total emissions from a motor vehicle They

are more complex than evaporative emissions, and it is

necessary to consider their origin within the engine in

order to understand the methods of control.

Air–fuel mixture

The air–fuel mixture supplied to the engine consists of

four main chemical elements: oxygen (O 2 ), nitrogen

(N), hydrogen (H) and carbon (C) The oxygen and

nitrogen are obtained from the air, and the hydrogen

and carbon come from the fuel.

The fuel is mainly hydrocarbons (HC) which are

compounds of carbon and hydrogen When the air–fuel

mixture is ignited, combustion occurs and a number of

chemical reactions take place.

Emissions from combustion

If complete combustion could occur, the oxygen in the

air would combine with the hydrogen in the fuel to

form H 2 O (water) The oxygen in the air would also combine with the carbon in the fuel to form CO 2 (carbon dioxide), which is a relatively non-poisonous gas Emissions would then not present the problems that they do Ideal combustion does not occur within the engine; some hydrocarbons remain after combustion, and carbon monoxide (CO), a highly- poisonous gas, is produced instead of carbon dioxide.

■ It is because of the carbon monoxide from the exhaust that engines must be run in well-ventilated areas.

Chemicals in the exhaust

As the result of combustion, internal-combustion engines produce chemical emissions in the following form These will be present in the exhaust system.

1 Hydrocarbons (HC) – unburnt particles of fuel.

2 Carbon monoxide (CO) – a product of combustion.

3 Oxides of nitrogen (NO x ) – resulting from high temperatures in the combustion chambers.

4 Particulates – very small solids, such as particles of carbon and minute particles of metal.

Figure 15.8 illustrates how the main chemical elements that enter the engine combine within the combustion chamber and leave in a different form through the exhaust.

figure 15.7 PCV valve operation HOLDEN LTD

intake manifold end

valve cover end

engine not running valve closed passage closed

normal operation valve open passage open

idling or decelerating valve open

passage small

accelerating

or high load valve open passage large

figure 15.8 Chemical reactions of fuel and air in the

engine: four elements, H, C, N and O enter the engine – some leave in different forms

Methods of reducing exhaust emissions

Manufacturers have developed various ways of reducing exhaust emissions Some of these are inside the engine itself; others are external modifications or devices fitted to the engine.

The purpose of most of the devices is to improve combustion so that more of the fuel charge is burnt, resulting in less emission Some devices, such as catalytic convertors, do not directly improve

combustion but treat the exhaust gases to improve their

quality before they are released into the atmosphere.

Electronic fuel injection and engine-management

systems play a large part in reducing emissions They

have close control of the air–fuel ratio over the entire

operating range of the engine and this means that less

emissions are produced.

Catalytic converters

Figure 15.9 shows an exhaust system with a catalytic

converter and mufflers With V-type engines, two

catalytic converters are often used – one for each bank of

cylinders Catalytic converters are basically gas reactors,

which reduce the amount of harmful pollutants in the

exhaust gases as they pass through the exhaust system.

The construction of a catalytic converter can be

seen in Figure 15.10 It consists of a stainless steel

shell which encloses a ceramic honeycomb core The

core has a coating of alumina impregnated with noble

metals which act as catalysts to produce chemical

reactions in the exhaust gases These reactions result in

less harmful emissions.

A honeycomb core (approximately 160 cells per

cm 2 ) is used because it allows a free gas flow through the converter and also provides a large surface area of contact between the catalyst and the exhaust gas.

A large surface area is needed because reaction occurs

on the surface of the catalyst.

■ A catalyst is a substance that promotes a chemical reaction but remains unchanged by the reaction.

Two-way and three-way converters

Catalytic converters are referred to as two-way or three-way converters.

Two-way (oxidising) converters use platinum and palladium as catalysts and convert HC and CO to H 2 O and CO 2 H 2 O is water, and CO 2 is a gas (carbon dioxide) that is not harmful to health Two-way catalytic converters do not react with oxides of nitrogen (NO x ), and so the engine still requires an exhaust gas recirculation (EGR) system to control NO x emissions.

■ See later section ‘Exhaust-gas recirculation (EGR)’.

figure 15.9 Exhaust system with a catalytic converter FORD

A three-way converter has an additional catalyst of

rhodium which reduces the NO x in the exhaust gases to

nitrogen (N 2 ) and oxygen (O 2 ).

Exhaust manifold catalytic converter

The catalytic converter shown in Figure 15.11 is

contained in a housing attached directly on to the

exhaust manifold This type of converter can be used

on its own, usually with smaller engines It can also be

used with a second catalytic converter further along the

exhaust system.

The sensors shown are part of a closed-loop system

for monitoring the exhaust gases Signals from the

sensors go to the ECU and are used for the engine management system.

Chemical conversion path

The diagram in Figure 15.12 shows the basic chemical conversions that take place in an engine and its exhaust system These start from the point where the air and fuel are drawn into the intake manifold This is followed by combustion and subsequent treatment processes to the stage where the end products are expelled into the atmosphere.

Engine management

Engine management provides a greater and more precise control of the systems and components that operate in the vehicle Development has generally been directed towards vehicle performance and economy, but there is emphasis on the control of emissions to comply with strict anti-pollution regulations.

The engine management system controls the air–fuel mixture and the ignition to achieve as complete combustion as is possible within the combustion chamber Modern vehicles utilise a wide array of sensors and ECU-controlled actuators to achieve a high level of control of the combustion process.

exhaust gas in

hydrocarbon carbon monoxide oxides of nitrogen

stainless steel shell

carbon dioxide nitrogen water vapour ceramic monolith

coated with platinum rhodium and palladium

exhaust gas out

figure 15.11 Three-way catalytic converters attached to

the exhaust manifold TOYOTA

exhaust manifold

air/fuel ratio sensors

three-way converters

heated oxygen sensors

Spark advance is needed to obtain the maximum

benefit from the fuel charge However, there are

some conditions, such as idle and slower speeds or

when the engine is cold, where a slightly retarded

spark will reduce emissions The ECU controls the

spark to provide the best ignition settings for all

speeds and conditions.

Electronic ignition also helps reduce emissions

because it can produce a spark that can jump the large

spark plug gap required to ignite the leaner fuel

mixtures that are used in more emission-compliant EFI

systems.

Electronic fuel injection (EFI)

Engines with electronic fuel injection and

engine-management systems need fewer emission-control

devices than carburettor engines.

The main reason for this is that the ECU is able to

continuously adjust the injectors so that they provide

an accurate quantity of fuel to suit the various

conditions of engine speed and load encountered by

the vehicle This gives more efficient combustion and

hence reduces exhaust emissions.

With a closed-loop system, the oxygen sensor in the

exhaust (Figure 15.13) provides a continuous

feed-back It signals the amount of the oxygen in the

exhaust gas to the ECU This enables combustion to be

monitored at all times.

Based on the information it receives from the

oxygen sensor, the ECU will vary the pulse width of

the signals that are sent to the injectors If the signal

from the oxygen sensor indicates that the mixture is

too lean, the pulse width to the injectors is increased If

the signal indicates that the mixture is too rich, the

pulse width is reduced This method of control is

shown as a diagram in Figure 15.14.

figure 15.12 Chemical conversion path

air

air induction

manifold

3-way catalytic converter

tail pipe

petrol injection

air injection HC

exhaust manifold

HO2S

figure 15.14 Closed loop process for adjusting the air–fuel

ratio FORD

closed loop air/fuel

reduce pulse width

increase pulse width ideal ratio

ideal ratio

oxygen sensor signals rich

oxygen sensor signals lean

Engine design

Engine-management systems, with electronic ignition

and electronic fuel injection, play a large part in

emission control The following are some other engine

features that also assist in the control of exhaust

emissions.

Combustion chamber design

In the combustion chamber, burning of the air–fuel

mixture starts at the spark plug and moves across the

chamber, consuming the charge as it goes Most of

the particles of fuel are completely burnt, but some are

not The design of the combustion chamber can

influence the amount of fuel that remains unburnt This

can be seen by comparing a wedge-shaped chamber

with a hemispherical chamber These designs are

shown in Figure 15.15.

Wedge-shaped combustion chamber

In a wedge-shaped combustion chamber burning starts

at the spark plug at the side of the combustion chamber

and moves across the chamber A feature of this type

of chamber is the quench area which is a relatively cool section This reduces the temperature of the burning charge and so prevents detonation.

However, the quench area is a disadvantage as far

as emissions are concerned, because the cooler area can allow particles of unburnt fuel to collect on its surface These are then exhausted as hydrocarbon emissions.

Hemispherical combustion chambers

A hemispherical combustion chamber has the spark plug located centrally so that burning can occur in all directions There are no quench areas and there is less surface area The ratio of the surface area of the combustion chamber to its volume is kept low for emission control purposes.

Hemispherical combustion chambers are designed

to promote swirl during the intake stroke (Figure 15.16) This is done so that the fuel will mix thoroughly and remain atomised The location of the valve ports and the shape of the piston head also have a bearing on combustion chamber design.

■ Many cylinder heads are designed with some form

of hemispherical-type combustion chamber.

figure 15.15 Combustion chamber designs

figure 15.16 Combustion chamber designed to provide

is designed to maintain uniform temperatures that are high enough to minimise quench areas.

Exhaust-gas recirculation (EGR)

With EFI, the levels of hydrocarbon and carbon monoxide are greatly reduced Oxides of nitrogen are

produced by high combustion temperatures and so are

present in an EFI engine Oxides of nitrogen can be

greatly reduced, however, by employing some form of

exhaust-gas recirculation (EGR) system.

With an exhaust-gas recirculation system, some of

the exhaust gas is circulated back through the engine,

but this is not to burn out any hydrocarbons that

remain in the exhaust gas The purpose is to add a

combustible gas to the air–fuel charge Any

non-combustible gas would do, but the exhaust gas is

readily available for use.

The reason for adding a gas is to absorb heat The

exhaust gas, at a lower temperature than combustion

temperature, absorbs some of the heat of combustion

and so reduces the temperature during the engine

cycle This in turn reduces the oxides of nitrogen

(NO x ) emissions that are produced as the result of high

temperatures.

EGR valve

An EGR system is shown in Figure 15.17 Its

opera-tion depends on the engine load, engine speed,

engine-coolant temperature, and radiator-engine-coolant temperature.

The engine ECU has inputs from a number of sources

and an output to the EGR solenoid of the vacuum

switching valve (VSV).

The parts of the system function as follows:

1 EGR valve Recirculates a portion of the exhaust

6 Throttle sensor Signals the throttle-valve opening

to the ECU, so that exhaust gas does not recirculate

at idle or at full throttle.

7 Coolant temperature sensor Signals the coolant temperature to the ECU.

engine-8 Thermo switch Sends an off signal to the control unit when the radiator coolant is cold, and an on signal when it is warm.

Valve overlap

During combustion, some unburnt hydrocarbons can

be trapped in the cylinder on quench areas and in crevices These are released during the exhaust stroke

as emissions, but are among the last of the exhaust gases to leave the cylinder.

The valve overlap has been changed on some engines to reduce the scavenging effect of the intake charge This allows the unburnt hydrocarbons to remain in the engine where they can be burnt instead

of being exhausted as hydrocarbon emissions.

figure 15.17 Exhaust-gas recirculation system for an EFI engine MAZDA

VSV (solenoid valve)

ECU

coolant temperature sensor EGR valve