Component testing data

7.17.3 Component testing data

reduces combustion noise and, on most modern Figure 7.47 is a printout from a CD, showing

vehicles, reduces harmful pollutants in the what data is available for testing a specific

exhaust stream. The main parts of the system component.

are the exhaust manifold, the silencer or muffler,

Figure 7.46 Engine management circuit diagram

Engine systems 145

the pipes connecting them, and a catalytic- allow movement because the engine is also rubber converter.

mounted, and they also reduce vibration noise. Most exhaust systems are made from mild

An exhaust manifold links the engine exhaust steel, but some are made from stainless steel

ports to the down pipe and main system. It also which lasts much longer. The system is suspended

reduces combustion noise and transfers heat down- under the vehicle on rubber mountings. These

stream to allow the continued burning of hydro- carbons and carbon monoxide. The manifold is connected to the down pipe, which in turn can be connected to the catalytic converter. Most exhaust manifolds are made from cast iron, as this has the necessary strength and heat transfer properties.

The silencer’s main function is to reduce engine noise to an acceptable level. Engine noise is a mixed up collection of its firing frequencies (the number of times per second each cylinder fires). These range from about 100 to 400 Hz (cycles/ sec). A silencer reduces noise in two main ways:

● interior chambers using baffles, which are tuned to set up cancelling vibrations;

● absorptive surfaces function like sound- deadening wall and ceiling panels to absorb

noise. When the exhaust gases finally leave the

exhaust system, their temperature, pressure and noise have been reduced considerably. The over- all length of an exhaust system including the silencers can affect the smooth flow of gases. For this reason do not alter the length or change the layout of an exhaust system. Figure 7.48 shows a

Figure 7.47 Testing data

silencer/muffler box.

Figure 7.48 Exhaust noise reduction methods

146 Advanced automotive fault diagnosis

7.18.2 Catalytic converters

The core has traditionally been made from ceramic of magnesium aluminium silicate. Due

Stringent regulations in most parts of the world to the several thousand very small channels, this have made the use of a catalytic converter neces-

provides a large surface area. It is coated with sary. The three-way catalyst (TWC) is used to

a wash coat of aluminium oxide, which again great effect by most manufacturers. It is in effect

increases its effective surface area by about sev-

a very simple device; it looks similar to a stand- eral thousand times. ‘Noble’ metals are used for ard exhaust silencer box. Note that in order for

the catalysts. Platinum helps to burn off the the ‘cat’ to operate correctly, the engine must be

hydrocarbons (HC) and carbon monoxide (CO), always well tuned. This is to ensure that the right

and rhodium helps in the reduction of nitrogen ‘ingredients’ are available for the catalyst to

oxides (NO x ). The whole three-way catalytic con- perform its function. A catalytic converter works

verter only contains about three to four grams of by converting the dangerous exhaust gases into

these precious metals. Some converters now use gases which are non-toxic.

metal cores (substrates). Figure 7.49 shows the

xide

ogen N 2 nitr

carbon dio

CO 2 H 2 O water

xides nitr ogen o NO xide x

CO carbon mono ydr ocarbon HC h

During the catalytic conversion become carbon dioxide of exhaust pollutants

and water (steam) through into environmentally compatible

oxidation whereas nitrogen components carbon monoxide

is produced from nitrogen and hydrocarbon

oxides through reduction.

Lambda sensor controlled mixture

Since the NO x emissions behave contrarily to

Precious metal

the CO+HC values the exhaust gas purification cannot start from the respective minimum. The smallest common denominator lies at the stoi- chiometric airfuel ratio of 14.7:1, the lambda window. An oxygen sensor in the exhaust flow,

Washcoat

the lambda sensor, controls the optimum mix-

NO x

HC

ture via control electronics, thus ensuring this

CO

optimum in all engine load cases.

Ceramic carrier

Top: the structure of Below: the active catalytic

Conversion λ-window

a ceramic monolith

layer in the exhaust gas flow

10 Since pollutants behave very differently in the

0.925 0.95 0.975 1.0 1.025 1.05 1.075 exhaust flow the common rich

Stoichiometric

lean

optimum has to be determined

for conversion: this is lambda = 1.

Figure 7.49 Catalytic converter operation

Engine systems 147

operation of a catalytic converter and how the modern low styled bonnets. Because it can be lambda window is important.

cleaned and fresh oil added, an oil bath air cleaner The engine can damage a catalytic converter

might still be used for vehicles operating in dusty in one of two ways:

conditions.

Air temperature control is used to help the firstly by the use of leaded fuel which can vehicle conform to emission control regulations cause lead compounds to be deposited on the and for good driveability when the engine is cold. active surfaces;

Good vaporisation of the fuel is the key. An auto- secondly by engine misfire which can cause matic control is often fitted to make sure that the the catalytic converter to overheat due to air intake temperature is always correct. The air burning fuel inside the unit. cleaner has two intake pipes, one for cold air and

Some manufacturers use a system on some vehicles the other for hot air from the exhaust manifold or where a sensor checks the output of the ignition

hot box. The proportion of hot and cold air is con- HT system and, if the spark is not present, will

trolled by a flap, which is moved by a diaphragm not allow fuel to be injected. Misfire detection is

acted on by low pressure from the inlet manifold. also part of current on-board diagnostic (OBD)

The flap rests in the hot air pick up position. legislation in some countries and future legisla-

A thermo-valve in the air stream senses the tion in others.

temperature of the air going into the engine. When a temperature of about 25°C is reached, the