South Africa is on the point of following the global movement towards cleaner exhausts, and it is generally expected that by the beginning of 2008 all NEW cars will have to be fitted with catalytic converters.
Initial research showed that the problems posed by car exhaust emissions had to be tackled on two fronts. First, engines had to be designed to produce a smaller percentage of harmful products, and, second, a way had to be found to convert these substances into something less harmful before they left the exhaust pipe. Early attempts to build a successful converter failed, because even the most sophisticated carburettor cannot deliver a mixture that is anywhere near chemically correct, especially under various everyday driving conditions. For example, in a typical American six-cylinder engine fitted with a single carburettor, cylinders number two and five received a vaguely correct mixture, but cylinders number one and six ran slightly lean, while the two centre cylinders usually ran rich. Yet, a stoichiometric (chemically-correct, ie neither rich nor lean) mixture is necessary not only because it will result in minimal harmful products being produced inside the engine, but will also allow the catalytic converter to function efficiently.
Luckily, the initial clean air legislation coincided with the beginning of the computer age, so engineers were able to combine fuel injection with some form of electronic control to ensure that, under most conditions, the mixture was close to stoichiometric. Today, these control systems are extremely complicated, sometimes using more than 20 sensors to control the mixture strength and spark or fuel injection advance, to even cope with changing conditions in the cylinder between one power stroke and the next at any engine speed.
This explains why computer-controlled fuel injection systems have become the norm for the majority of petrol engines. Even diesel engines, which have had fuel injection since they were invented, are also now being equipped with computer-controlled systems. The result is that most modern exhausts are more than 95 per cent cleaner than the old systems used to be.
Combustion
Hydrocarbon fuels consist mainly of various combinations of hydrogen and carbon, as well as small quantities of sulphur, lead and phosphorous. The natural products of stoichiometric combustion are water (H2O) and carbon dioxide (CO2). Such a condition is almost impossible to achieve, so any engine produces a number of major pollutants. These are a family of hydrocarbons (HxCx), carbon monoxide (CO), various oxides of nitrogen (NOx), and particulates. Sulphur, lead and phosphor are minor pollutants.
In a modern petrol engine, the mixture is controlled to be close to stoichiometric during cruising, but has to be slightly rich during starting, idling, and full throttle operation. Diesel engine combustion is far more complicated. Seen as whole, the mixture is always very lean, running between more than 90 per cent excess air during idling, up to about 20 per cent excess air when demanding full power. However, a diesel engine does not have a single spherical flame front emanating from the spark, such as occurs in a petrol engine. Instead, each fuel droplet, originating from the injectors, is heated by the hot air resulting from the high compression ratio, and has to first evaporate and then find sufficient air to start its own little flame. In such rich conditions, the fuel breaks down into water (steam), carbon monoxide and solid carbon.
Luckily, further mixing due to turbulence above the pistons results in most of the carbon finding enough oxygen to form carbon dioxide, so that for most of the time very little solid carbon finds its way out of the exhaust. However, under full power the fuel/air ratio is richest, and the formation of carbon is maximised. If the fuel delivery is set correctly, this may result in a slight carbon haze in the exhaust, but when a diesel is over-fuelled, whether deliberately or because of engine wear, the smoke is likely to be very visible. This results in the formation of particulates, as we shall see.
Hydrocarbons
These substances are irritants and produce unpleasant smells, and some are carcinogenic. They exist as various combinations of carbon and hydrogen. Some of these molecules are present in the exhaust gases, because a percentage of the fuel remains unburnt, but the majority are the result of large fuel molecules cracking up during the combustion reaction, and not getting a chance to complete the combustion.
The production of hydrocarbons obviously depends a great deal on mixture strength. Rich mixtures imply that there is not enough oxygen to react with the fuel, resulting in the formation of large quantities of hydro-carbons. Such mixtures are unavoidable during start-up and full throttle acceleration, to prevent misfire. Interestingly enough, very lean mixtures also promote the formation of hydrocarbons because they usually lead to occasional misfiring.Very few, if any, combustion cycles are complete in even the most efficient engines, resulting in varying amounts of hydrocarbons being formed. This may be due to incomplete mixing of the fuel and air, or the flame cooling down or being extinguished when getting close to the cylinder walls, or as a result of the gases cooling down as the piston descends. A high percentage of exhaust gas remaining in the combustion chamber will also contribute to the formation of hydrocarbons. Dual spark plugs are often fitted to improve combustion and reduce the formation of hydrocarbons.
Other sources of hydrocarbons include unburnt fuel that fills the annular crevice between the pistons and the cylinder walls, mainly above the top rings, and especially in the ring gaps. Unburnt mixture can also escape, even before the spark has occurred, because of the valve overlap, ie the time that both intake and exhaust valves are slightly open. This is worst during idling, when the actual overlap time is greatest. Oil and combustion deposits also absorb small quantities of hydrocarbons during compression and combustion, when the pressures are high, and release them again during the exhaust stroke. The result is that worn engines, which have greater crevice volumes, more deposits, and more oil on the cylinder walls, naturally emit larger quantities of hydrocarbons. Diesel engines emit only about one-fifth the hydrocarbons of similar-sized petrol engines, because excess oxygen is always present to burn away the hydrocarbons.
Carbon monoxide
This odourless, colourless gas is harmful to living creatures because it stops the flow of blood when inhaled. It is formed when one carbon atom is joined to one oxygen atom (CO) instead of being joined to two oxygen atoms (CO2), which is carbon dioxide. This happens whenever the oxygen supply is not enough to convert all the carbon in the fuel into carbon dioxide.
Carbon monoxide will burn in the presence of oxygen, so its presence in the exhaust will mean that fuel is being wasted. This explains why the CO content in the exhaust is a measure of how rich the mixture is, but even a very lean mixture will produce a small amount of CO because of poor mixing. Diesel engines produce far less carbon monoxide than petrol engines, again because of the excess oxygen.
Oxides of nitrogen
The exhaust contains mainly nitrogen oxide (NO), but a small amount of nitrogen dioxide (NO2) is also present, as well as traces of other nitrogen-oxygen compounds. These are lumped together as NOx. These compounds form ozone, which is one of the major components of photochemical smog.
NOx forms mainly at temperatures in excess of 2 250 degrees C, when excess oxygen is present. These conditions can be found when the mixture is slightly lean of stoichiometric. Modern petrol and diesel engines use mainly exhaust gas recirculation (EGR) to reduce the formation of NOx by reducing combustion temperatures. This seems paradoxical, because the exhaust gases are very hot, but they have already taken part in a combustion process and will not take part a second time. Combustion chamber shapes that reduce combustion time also help because the relevant chemical reactions need a certain time for completion. Indirect injection diesel engines produce NOx more readily than direct injection engines, because the former maintain higher combustion temperatures.
Particulates
Up to this point, diesel engines appear to be cleaner than petrol engines, but this is too good to be true, and the fly in the ointment is particulates. These are solid carbon soot particles generated mostly in the fuel-rich zone around each fuel droplet, but up to about 25 per cent comes from vaporised lubricating oil components that take part in the combustion process. In addition, the normally high diesel compression ratio implies a high expansion ratio, so diesel exhausts usually run a lot cooler than petrol exhausts, especially at part-throttle. This causes all sorts of components from the fuel and oil to condense on the soot particles, making these particles a lot nastier for humans and other life forms than if they were pure carbon.
Finally, EGR has been successful in reducing NOx, but too much will increase the formation of particulates and hydrocarbons. A longer combustion time will reduce the formation of particulates but increase the formation of NOx, showing that designing and controlling a modern diesel engine is very much like walking a tight-rope, hence the swing towards electronic control.
Petrol engine catalytic converters
The modern converter oxidises hydrocarbons and carbon monoxide to water and carbon dioxide. In addition, NOx is reduced to less harmful nitrogen compounds, as well as water and carbon dioxide. Unfortunately, these reactions only take place at high temperatures, so a cold engine is still very dirty, in spite of a having a converter. By contrast, when its temperature rises to above 400 degrees C, the converter will remove 99 per cent of carbon monoxide, 95 per cent of NOx, and more than 95 per cent of hydrocarbons, provided the mixture is close to stoichiometric.
The converter is usually a stainless steel container mounted in the exhaust system as close to the engine as possible to keep the temperature high. Inside, there is a porous ceramic structure, or a loose granular ceramic, with many flow passages to accommodate the airflow. Catalytic materials, consisting of platinum, palladium or rhodium, are embedded in the passages, so that gases can flow past them. A catalyst is any material that speeds up a chemical process by lowering the energy needed, but without taking part in the process.
From the definition, it’s obvious that a catalyst should last a very long time. In practice, this is not so, because it gets degraded by excess heat, dirt and poisoning of the active material. Under ideal conditions it should last for 200 000 km, but there are many ways to reduce its life. These include having a badly-tuned engine that will promote excessive lean or rich conditions, bad starting, misfiring, push-starting an engine, or testing for a spark by removing one of the plug leads. All of these will generate excessive heat inside the converter, leading to degradation of the active material. In addition, lead and sulphur from the fuel, as well as zinc, phosphorous, antimony, calcium and magnesium from the oil, will coat the catalyst and reduce its efficiency.
Diesel engine catalytic converters
Modern diesel engines use a converter that is similar to the petrol version, but they also need a particulate filter, and there are a number of different designs. The most common type uses a ceramic honeycomb monolith, like an emissions catalyst, but with larger passages and having alternate passage ends blocked. This forces the gases to flow through the channels, allowing the particles to deposit themselves on the walls. Other designs use sintered metal plates, foamed metal structures and fibre mats. Modern units are able to remove 99 per cent of the solid matter, but the non-solid portion ends up in the exhaust stream, so that the overall particulate efficiency is just over 90 per cent.
A particle filter will get blocked, so it has to be cleaned continuously, or regenerated from time to time. The continuous, or passive, system uses a catalyst at the front of the filter to generate nitrogen dioxide from the nitrogen oxide in the exhaust, and uses this gas to reduce the carbon to carbon dioxide while releasing the nitrogen oxide again. Active systems use oxygen to burn the carbon away, but the temperature inside the converter has to be raised to above 600 degrees C. This is done by injecting diesel fuel into the exhaust system, either directly, or post injection (into the combustion chamber), whenever a sensor tells the control unit that the filter needs it.
Mercedes-Benz recently released details of its ” Bluetec” diesel engine technology. This combines an oxidising catalyst, a particulate filter, and a storage catalytic converter that injects ammonia into the gas, converting the nitrogen oxides into harmless nitrogen. The ammonia is obtained from a substance that Mercedes calls AdBlue, stored in a separate tank that is refilled during a dealer service.
Conclusion
The compulsory fitting of converters is a heavy price to pay for cleaner exhausts. Not only will cars that do not have converters at present cost at least R10 000 more, but there’s no point in making converters compulsory unless it becomes mandatory to test these units from time to time. We’ve seen that it’s easy to render a converter useless, so these tests will become a heavy financial burden for most motorists. At current prices, a new converter costs between R12 000 and R20 000, or even more in some cases. In addition, converter theft is a growth industry in many overseas countries, because the units contain valuable metals. For many local motorists, the only solution may be to keep their pre-2008 cars for as long as possible. . .
