Catalytic
Converters TTC1.0
European emission control legislation dictates the
limits for toxic emissions from a vehicle’s
exhaust system. Only by post-engine exhaust gas treatment
can these limits be met. This function is carried
out by a catalytic converter.
What it does:
A catalytic converter can be thought of as a post-engine
exhaust filter that removes the harmful emissions
of an engine. Unlike a traditional filter it works
by utilising a combination of heat and precious metals
to promote a catalytic reaction which chemically converts
those harmful gases into naturally occurring ones,
so that no residues remain. Also unlike other filters
the catalytic converter is designed to last the life
of the vehicle to which it is fitted.
They are located in a position in the exhaust system
as close to the engine as possible so that they quickly
get to operating temperature. It outwardly resembles
a small exhaust silencer. They are a legal requirement
on all petrol engined cars made for the UK market
since 1993.
How does it work?
A honeycomb structure (either ceramic or metallic)
is treated with a wash-coat of precious metals (usually
platinum, palladium and rhodium) through which the
exhaust gasses flow. The surface of the honeycomb
material has a rough finish such that it allows the
maximum contact area available to the exhaust gasses.
Viewed from either end it resembles a handful of drinking
straws with hundreds of tiny flow channels that allow
the gasses to pass through.
Once the converter is up to its minimum operating
temperature of 350-400 degrees Celsius, a two or three
way (depending upon the converter type) process takes
place;
Unburnt Hydrocarbons are oxidised into water/steam
Carbon Monoxide is oxidised into Carbon Dioxide and
Nitrogen Oxides are converted into Nitrogen and Oxygen
Two way converters only carry out the first two processes
and are now rarely used.
Limitations:
Catalytic converters are an integral part of the
vehicle’s emission system. They are designed
to work within a very limited band of exhaust gas
concentration, too rich or too lean and the life of
the converter will be reduced. Only by accurate control
of the engine management system (EMS) can this specific
exhaust gas concentration be maintained. Failure of
any of the EMS components will have an adverse effect
on the performance and life expectancy of the converter
and this particularly includes the Lambda sensor.
The catalytic converter is designed to last the life
of the vehicle to which it was fitted. If it has failed,
there will always be a reason behind this failure.
Only by investigating and rectifying this reason can
you expect the new, replacement converter to perform
correctly and reliably for the foreseeable future.
Testing:
The most accurate method of testing a catalytic converter
is to measure the gasses entering it and then compare
the readings of the gasses exiting it. This directly
addresses the main problem with converter diagnosis
in that the converter itself can disguise a mild emissions
error.
A few vehicles have a pre-cat test point where the
exhaust gasses can be sampled before they enter the
converter but for most vehicles this is a luxury that
we have to do without. Absence of such a port makes
exhaust gas comparison tests impractical.
The Lambda sensor, if it is operating correctly,
can be used as a pre-cat exhaust gas concentration
tester, simply hook a voltmeter up to the output wire
and monitor the voltage. A high voltage will indicate
a rich mixture; check the vehicle data for actual
voltage outputs.
The operation of the converter can also be checked
by monitoring the temperature of the converter at
the entry and exit points. With a working converter
the exit temperature will always be higher than the
inlet temperature due to the catalytic reaction. The
actual temperatures are unimportant, just check for
a difference. Ensure that the vehicle is fully warmed
up before carrying out this test and use a non-contact
(infra-red) type thermometer. The temperature difference
is also, to some extent, unimportant. Later vehicles
or lean burn engines will have a much smaller temperature
increase than a vehicle running at a higher exhaust
gas concentration. This test will not highlight any
emission faults; merely confirm if the catalytic converter
is working.
Visual inspections can also be quite revealing;
