The Catalytic Converter

While in Asia we got to talking about pollution, 2 stroke versus 4 stroke engines, and the catalytic converter. So I thought the first question to kick off the column would be a brief history and usage of the catalytic converter.

    All internal combustion engine operate on a 15 to 1
air fuel ration.  That is 15 pounds air to 1  pound
fuel.  That is a standard  as long as engine have been
around.  Some engines, called lean burn engines
(Chrysler), use a 17 to 1 air fuel ratio.  For this
topic we will use a simple one cylinder internal
combustion four cycle engine.  This means that the
engine has four cycles. Intake, compression, power and
exhaust.  Intake is when the fuel and air mixture is
introduced into the engine through the intake manifold
as the piston is traveling down.  Compression is when
the fuel and air mixture is being compressed as the
piston is traveling up.  Power is when the mixture is
ignited and the piston is being pushed down.  Exhaust
is when the exploded mixture is pushed out by the
piston being pushed up.  What makes the piston go up
and down is another story that can be answered in
another article.

    Early engines introduced fuel and air into the engine
by controlled suction of fuel through a device called
a carburetor.  Very crude in the beginning but became
so complicated that many could not be understood or
repaired by the average mechanic. A engine can run by
controlled dripping of fuel into the intake manifold.
That's what the carburetor is.  It gave the engine too
much fuel some times and then did not stop giving it
fuel in certain driving conditions such as going down
hill or slowing down.

    Because fuel was cheap and the environment was not a
concern, the carburetor was used.  When air quality
was becoming a issue, the basic fuel management system
was added, modified and compromised.  First we
completely closed all vents of the engine to the
atmosphere.  First by closing the road draft tube that
vented the engine fumes generated by combustion
chamber leakage past the piston rings and vapors from
hot oil.  Then the fuel vapor from the carburetor
float bowl and the gas tank was vented into the intake
manifold while the engine was running.  When the
engine was off the vapors were vented to a canister
filled with carbon to absorb it.  The canister was
also vented to the intake manifold when the motor was
running.

    Now we have a closed system and fuel was still cheap.
Then emissions standards became more strict and fuel
costs started to became more expensive.  Engine
management started controlling the carburetor and
ignition system with more controls to purge the
combustion chamber of unburned fuel by holding the
carburetor open for 3.5 seconds while decelerating and
changing the ignition timing to have more complete
combustion.  While doing this a new pollutant was
created which is called nitrous oxide and then a
device was added to stop this. This device added
exhaust gases back into the intake manifold to spoil
the combustion efficiency. Also a air pump was added
to some cars to pump air into the exhaust to achieve
burning of the unburned fuel that made it past the
efforts of all the controls.

    All this effort was to remove any unburned fuel from
making its way out the exhaust pipe.  Along came the
catalytic converter in 1986.  A device that looks like
a muffler on the exhaust pipe but located as near to
the engine as possible.  It was a metal container
containing a catalyst of platinum, palladium or
rhodium applied to a honeycomb of ceramic material.
One manufacture used little ceramic marbles coated
with the catalyst and the exhaust had to pas through
it. It operates at a high temperature and burns any
unburned hydrocarbons.  This material could not
tolerate the lead that was added to the fuel to
control premature detonation of the fuel air mixture.
Now the fuel was modified by taking out the lead and
we now have unleaded fuel.
    In the early days the catalytic converter was
considered a maintenance item and were sometimes
replaced at 25,000 miles and paid for by the
manufacturer.  They government mandated that emissions
related equipment must last 50,000 miles.  Many failed
and were routinely replaced. They were extremely
expensive and some still are.  Some of the common
problems that are still present today is contaminated
fuel, engines that are not maintained and overwork the
catalyst.  The honeycomb ceramic material can break
down and became loose.  This creates a rattle in the
converter canister and can clog the muffler.

    Today the modern engines have computer controlled
fuel management systems with electronic fuel injectors
to control the fuel delivery.   Engines run well and
produce clean tail pipe emissions.  Some cars have
more then one converter and many exhaust feedback
sensors controlling the fuel mixture and engine
timing.  It is said that the modern engine could not
be invented as a single project,  but is an evolution over many
years of solving one problem at a time.