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