cat in your future

There's a Cat in Your Future.

There's a Cat in Your Future.

Whenever someone mentions the word cat, we think of the four-legged creatures. This cat in cars has an important environmental function.

We are referring to a catalytic converter (to give it its full name), fitted somewhere in the path of a vehicle's exhaust gases for the purpose of converting the harmful compounds formed during combustion into less harmful gases.

Rumours circulating in the motor industry suggest that an agreement has been reached among all South African car manufacturers and importers (perhaps with some gentle persuasion from government) that all new cars will be fitted with "cats" fairly soon.

At present several of the entry-level models don't have them. Such a step will, sadly, spell the end of the road for the few carburettor-equipped cars still being produced, because a catalytic converter requires fuel injection and electronic engine management in order to work properly.

How does a cat perform its clean-up act, and are the untreated exhaust emissions really so harmful that we must put up with the extra complication (and cost) to have them cleaned up? We consider these questions today, and next week we shall look at how to care for your cat.

The main offenders in the cocktail of exhaust gases pumping out of the combustion chambers of a petrol engine are:

l Nitrogen oxide - often denoted by NOx to indicate the mixture of NO1 and NO2. These play a significant role in the formation of smog which adversely affects people, especially those with lung conditions such as asthma.

They also react with oxygen in the air to form ozone which is an irritant. And when dissolved in atmospheric moisture, they form nitric acid, a component of acid rain.

l Carbon monoxide (CO) - this is a colourless, odourless poisonous gas.

lUnburnt hydrocarbons (HC for short) - essentially petrol vapour that was not fully burnt in the combustion process; a major contributor to urban smog, as well as being poisonous.

To reduce the three main regulated emissions listed above, "three-way" catalytic converters are used on most modern cars.

They do their job in two stages: the first stage takes care of the NOx emissions. Here the exhaust gases are led through a ceramic honeycomb structure coated with the catalyst which at this stage is a mixture of platinum and rhodium. (A catalyst is a substance that accelerates a reaction without itself taking part in it.)

As the NOx molecules come in contact with the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen atoms to form molecules of oxygen gas. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst to form molecules of harmless nitrogen gas.

Thereafter the exhaust gas, now purged of NOx and slightly oxygen-rich, is fed to the second stage of the converter, which consists of another ceramic honeycomb coated with catalyst, this time a mixture of platinum and palladium. Here CO and HC emissions are dealt with. The catalyst helps CO to combine with oxygen in the exhaust gas to form CO2 - the same gas that we exhale when breathing - and likewise the unburnt hydrocarbons are fully burnt by combining with oxygen.

Both stages only work well when the air: fuel ratio in the engine's cylinders is in a very narrow band around the ideal ratio of 14,7:1. To keep it there, oxygen sensors (also called lambda sensors) are used in the exhaust path upstream of the cat. These communicate with the on-board computer which continuously regulates the amount of fuel injected. The first stage in the converter prefers slightly oxygen-poor exhaust gas (fuel mixture slightly rich), while the second stage prefers slightly oxygen-rich gas. It has been found that by providing a constantly varying mixture which have rapid, infinitesimal oscillations around the ideal ratio, very high conversion efficiencies can be achieved.

Diesel engines have their own set of emission problems, basically stemming from the fact that diesel exhaust gas is always oxygen-rich.

Thus the second stage of the process described above would work well on diesel engines, but not the first stage; they have problems getting rid of NOx . They also emit soot, the fine carbon particles in the black diesel exhaust fumes which we sometimes see, and that has to be removed by a separate soot filter.