Emissions intake manifold drawing the vapour

7.9 Emissions intake manifold drawing the vapour

through the charcoal canister. This must be controlled by the manage-

7.9.1 Introduction

ment system, however, as even a 1% concentration of fuel vapour would

The following table lists the four main exhaust

shift the lambda value by 20%. This is

emissions which are hazardous to health, together

done by using a ‘purge valve’, which

with a short description of each.

under some conditions is closed (full load and idle for example) and can be progressively opened under

Substance Description other conditions. The system moni- tors the effect by use of the lambda

Carbon monoxide This gas is very dangerous even in sensor signal (CO)

low concentrations. It has no smell or

Hydrocarbons become concentrated taste and is colourless. When inhaled

Crankcase fumes

in the crankcase mostly due to it combines in the body with the red

(blow by)

pressure blowing past the piston blood cells preventing them from

rings. These gases must be conducted carrying oxygen. If absorbed by the

back into the combustion process. body it can be fatal in a very short time

This is usually via the air intake Nitrogen oxides

Oxides of nitrogen are colourless and system. This is described as positive (NO x )

odourless when they leave the engine crankcase ventilation but as soon as they reach the atmosphere and mix with more oxygen, nitrogen oxides are formed. They are

7.9.2 Exhaust gas recirculation

reddish brown and have an acrid and pungent smell. These gasses damage

(EGR)

the body’s respiratory system when

This technique is used primarily to reduce peak

inhaled. When combined with water vapour nitric acid can be formed which

combustion temperatures and hence the produc-

is very damaging to the windpipe and

tion of nitrogen oxides (NO x ). EGR can be either

lungs. Nitrogen oxides are also a

internal due to valve overlap, or external via a sim-

contributing factor to acid rain

ple arrangement of pipes and a valve (Figure 7.22

Hydrocarbons (HC) A number of different hydrocarbons

shows an example) connecting the exhaust mani-

are emitted from an engine and are

fold back to the inlet manifold. A proportion of

part or unburnt fuel. When they mix

exhaust gas is simply returned to the inlet side of

with the atmosphere they can help to form smog. It is also believed that

the engine.

hydrocarbons may be carcinogenic

This process is controlled electronically as

Particulate matter This heading in the main covers lead

determined by a ROM in the ECU. This ensures

(PM) and carbon. Lead was traditionally

that driveability is not effected and also that the

added to petrol to slow its burning

rate of EGR is controlled. If the rate is too high,

rate to reduce detonation. It is

then the production of hydrocarbons increases.

detrimental to health and is thought to cause brain damage especially in

One drawback of EGR systems is that they

children. Lead will eventually be

can become restricted by exhaust residue over a

phased out as all new engines now

period of time thus changing the actual percent-

run on unleaded fuel. Particles of

age of recirculation. However, valves are now

soot or carbon are more of a problem

available that reduce this particular problem.

on diesel fuelled vehicles and these now have limits set by legislation

The following table describes two further sources of emissions from a vehicle.

Source Comments Fuel evaporation

Fuel evaporation causes hydrocarbons from the tank and

to be produced. The effect is greater system

as temperature increases. A charcoal canister is the preferred method for reducing this problem. The fuel tank is usually run at a pressure just under atmospheric by a connection to the

Figure 7.22 EGR valve

Engine systems 121

7.9.3 Catalytic converters

the delay in the catalyst reaching this temperature. This is known as catalyst light off time. Various

Stringent regulations in most parts of the world methods have been used to reduce this time as sig- have made the use of a catalytic converter almost

nificant emissions are produced before light off indispensable. The three-way catalyst (TWC) is

occurs. Electrical heating is one solution, as is a used to great effect by most manufacturers. It is a

form of burner which involves lighting fuel inside very simple device and looks similar to a stand-

the converter. Another possibility is positioning ard exhaust box. Note that in order to operate

the converter as part of the exhaust manifold and correctly, however, the engine must be run at or

down pipe assembly. This greatly reduces light off very near to stoichiometry. This is to ensure that

time but gas flow problems, vibration and exces- the right ‘ingredients’ are available for the cata-

sive temperature variations can be problems that lyst to perform its function.

reduce the potential life of the unit. Figure 7.23 shows some new metallic sub-

Catalytic converters can be damaged in two strates for use inside a catalytic converter. There

ways. The first is by the use of leaded fuel which are many types of hydrocarbons but the example

causes lead compounds to be deposited on the illustrates the main reaction. Note that the reac-

active surfaces thus reducing effective area. The tions rely on some CO being produced by the

second is engine misfire which can cause the cat- engine in order to reduce the NO x . This is one of

alytic converter to overheat due to burning inside the reasons that manufacturers have been forced

the unit. BMW, for example, use a system on some to run the engine at stoichiometry. The legisla-

vehicles where a sensor monitors output of the tion has tended to stifle the development of lean

ignition HT system and will not allow fuel to be burn techniques. The fine details of the emission

injected if the spark is not present. regulations can in fact have a very marked effect

For a catalytic converter to operate at its opti- on the type of reduction techniques used. The

mum conversion rate to oxidise CO and HC whilst main reactions in the ‘cat’ are as follows:

reducing NOx, a narrow band within 0.5% of ● 2CO ⫹ O 2 → 2CO 2 lambda value 1 is essential. Lambda sensors in

● 2C 2 H 6 ⫹ 2CO → 4CO 2 ⫹ 6H 2 O

use at present tend to operate within about 3% of ● 2NO ⫹ 2CO → N 2 ⫹ 2CO 2 the lambda mean value. When a catalytic con- verter is in prime condition this is not a problem

Noble metals are used for the catalysts; platinum due to storage capacity within the converter for promotes the oxidation of HC and CO, and

CO and O 2 . Damaged converters, however, cannot rhodium helps the reduction of NO x . The whole

store sufficient quantity of these gases and hence three-way catalytic converter only contains about

become less efficient. The damage as suggested three to four grams of the precious metals.

earlier in this section can be due to overheating The ideal operating temperature range is from

or by ‘poisoning’ due to lead or even silicon. If about 400 to 800°C. A serious problem to counter is

the control can be kept within 0.5% of lambda

Figure 7.23 Catalytic converter metallic substrates

122 Advanced automotive fault diagnosis the converter will continue to be effective even if

7.10.2 Emissions fault diagnosis

damaged to some extent. Sensors are becoming

table

available which can work to this tolerance. A second sensor fitted after the converter can be

Symptom

Possible cause

used to ensure ideal operation.

EGR valve sticking

Buildup of carbon Electrical fault

7.10 Diagnostics – Rich mixture

High CO and high HC

Blocked air filter

emissions Damaged catalytic converter

Engine management system fault

Low CO and high HC

Misfire