Article reproduced with kind permission of Mike Fishwick

Although many of us regard may it as an irrelevance, the introduction of the catalytic converter may well be seen in historical terms as the most significant event in the history of the motorcar.

In the motor engineering world of the ‘seventies little thought had been given to a fundamental reduction of exhaust emissions until the US market required the catalytic converter and its associated unleaded fuel. This was then foisted upon the European automotive public as a result of our politicians’ wish to be seen to be doing something – anything – rather than waiting for the promised lean-burn technology, but it is here to stay for the foreseeable future. I have little doubt today’s children show us that freedom from automotive lead emissions is not related to intelligence, although at the time it was heralded as being responsible for the ills of a generation.

In spite of the fact that catalytic converters were available from BMW as an expensive option in the UK for several years, and are now standard equipment on all new models since 1990, few of us know much about them, as they are hidden within an apparently ordinary exhaust system.

Typical automotive converter, mounted close to the exhaust ports to cut warm-up time. Note the Lamda probe mounting bosses at each end.

The basic purpose of a catalytic converter is to reduce the levels of hydrocarbons, oxides of nitrogen, and carbon monoxide in the exhaust gas by means of a high-temperature chemical process, which converts most of them into carbon dioxide, nitrogen, and water. The necessary heat is initially provided by the exhaust gases, so starting the reaction which liberates more heat to raise the temperature still further, resulting in a faster reaction which produces still more heat, until maximum efficiency is achieved at about 600° Centigrade.

As produced in their original form, during the late nineteen-seventies, catalysts consisted of a ceramic honeycomb structure, coated with platinum and palladium. A typical converter has a total active surface area in the order of a quarter of million square feet!

These early catalytic converters soon gained a reputation for expensive fragility, as even a backfire caused by the associated weak mixture required in engines of the period could shatter the honeycomb. Driving through deep water could also damage the honeycomb, due to its sudden cooling effect, while being hit by a stone spelt the end for many converters. Attempts to strengthen the structure by increasing its thickness resulted in excessive thermal inertia, which increased the time taken for the converter to reach its working temperature. As with any chemical reaction, the conversion process operates better at high temperatures, and in short commuting use these old units were ineffective.

Subsequent development led to the converter being insulated and protected by an external shield, while the ceramic honeycomb was replaced by a thin metal structure of high mechanical strength and low thermal inertia. This design has increased reliability, and when coupled with the addition of Rhodium to the coating, improved conversion efficiency and reduced the warm-up period, making modern converters more effective under low speed and short distance conditions.

The low cross-section of the metal structure also reduced the exhaust restriction imposed by the converter, which had been responsible for a substantial loss of performance. Some manufacturers even developed electrically pre-heated converters to improve ‘cold’ performance. Several manufacturers, such as BMW, often fitted two parallel catalytic converters, such as on the 2.8 litre M52TU engine, so reducing the exhaust restriction still further.

In spite of these wonders of metallurgy, the converter remains at the mercy of the exhaust gas; the coatings can be ‘poisoned’ by the use of leaded fuel, or by an engine which is burning oil, particularly if the oil contains zinc or phosphorous-based additives. The use of such additives was popular as a boundary lubricant to provide protection for highly-loaded components such as cams and followers during cold starts, but their use was drastically reduced in oils from the ‘SJ’ approval classification onwards. These later oils are, in general, therefore not suitable for older types of engine.

Any unburnt fuel, such as may be admitted to the converter when unsuccessfully attempting to start the engine, can – after starting – raise converter temperature to a level where the honeycomb will melt, partially blocking the exhaust, and promoting other damage such as burnt exhaust valves. Such unused fuel is reduced in modern engines by a camshaft position sensor, which at low or starting speeds signals the injection system to supply fuel only to those cylinders about to fire.

The life of a modern catalytic converter can exceed the usual life of the vehicle, as was proven by the Mobil 1 endurance test, where (under ideal conditions on a test bed) a BMW E30 325i covered a million miles, during which the converter was replaced at 200,000 mile intervals.

Although many manufacturers, such as BMW, will ‘trade in’ old catalytic converters, few will sell them as separate items, preferring to only provide a complete all-welded system, but they are available from specialists such as Milltek and Piper.

The characteristics of the modern catalytic converter are therefore as efficient as the sum of its design and use of precious metals can permit, but its output quality is dependent upon that of the exhaust gas produced by the engine. Such a converter is known as an ‘Open Loop’ type, for reasons which will become obvious, and is now mainly used on diesel engines, which produce far smaller proportions of the same emissions as petrol engines.

With the application of increasingly stringent emission control legislation during the ‘eighties, most car manufacturers adopted electronic fuel injection systems, so setting the stage for the next development of the catalyst. This took the form of a probe – generally known as a ‘Lambda’ probe – which was developed by Bosch during the early ‘seventies. The sensing element consists of a zirconia-ceramic cylinder, coated with platinum, which senses the level of oxygen in the exhaust gas. A weak mixture, for example, will contain a surplus of oxygen, while a rich mixture will have very little.

The probe was inserted into the exhaust system ahead of the converter, where it was able to sample the quality of the basic exhaust gas, and send an appropriate signal to the injection system. The probe signal is operative from idling speed and is able to modify the exhaust quality to ensure optimum low speed performance as the condition of components such as spark plugs and injectors deteriorate. When the engine is operating at high load – from about two-thirds of full throttle – the probe signal is ignored to protect the engine, the injection system enriching the mixture to suitable levels.

The probe output, of course, is a feedback signal, and in monitoring the exhaust output it is used to modify the injection period, and also in many cases the ignition timing, to control the basic exhaust quality. This basic exhaust – probe signal – injection control modified exhaust cycle is referred to a closed control loop, and therefore this type of converter is known as a ‘Closed Loop’ device, as is fitted to virtually all modern cars.

Detail of modern spiral-wound converter

American regulations now require a second Lambda sensor to be positioned behind the catalytic converter to monitor its performance, as a healthy converter will reduce the level of oxygen in its exhaust gas. Remember that the converter uses excess oxygen to change carbon monoxide into carbon dioxide, hydrocarbons into water, and oxides of nitrogen into nitrogen – a failing converter will have an excessive level of oxygen in its output.

In normal conditions 30,000 to 50,000 miles is regarded as an average life for a probe before soot accumulation reduces it effectiveness, while the self-cleaning heated probes introduced in 1998 can last as long as 100,000 miles. During one of the UK’s periodic petrol shortages , many drivers filled up with leaded fuel, which can be guaranteed to destroy both catalytic converters and lambda probes, while a recent petrol contamination problem showed that a probe can be ‘poisoned’ by even a small level of silica content. For this reason joints in fuel systems must not contain silicone rubber, while the use of silicone grease to fit ‘O’ rings etc is also proscribed.

It is interesting to see that this technology is also used to monitor oxygen levels in the mixed gases used by deep divers and anaesthetists, where rapid and accurate control has life-dependant considerations.

In a world where emission requirements are being progressively tightened, it is ironic to consider that when the currently projected exhaust emission regulations come into action, engines will be required to produce exhaust gases which may be purer than the ambient atmosphere in which they operate!

Silencer with uncontrolled converter units for MAN 450 bhp diesel truck engine

It is now over twenty years since the German government gave notice of their intention to withdraw Type Approval from all vehicles not fitted with catalysts, should atmospheric conditions deteriorate sufficiently, with similar legislation being prepared in France, and no doubt also in the UK . . . just in case it is useful.

Initial targets will no doubt be the many motorcycle engines which are at least as large as those in a small car, and whose fuel consumption is appreciably higher. When we also consider that most motorcycle use takes place in urban areas, it is inevitable that the level of untreated exhaust emissions produced by them will come to the notice of governments. As with cars, the cost of fitting the uncontrolled catalytic converters which will become mandatory will mean that many will be scrapped.

Other targets will doubtless be the ‘Historic’ pre-1970 vehicles, such as those currently granted freedom from Vehicle Excise Duty, and which are currently on the road at the pleasure of Her Majesty’s Government. It would only take the stroke of a pen for this dispensation to end, as the right of such vehicles to pay VED no longer exists – a chilling thought.

Such legislation would remove almost every older vehicle from our roads, so making them into museum pieces, only able to be used for transit to designated old vehicle events. The government, of course, probably imagine that sales of new cars would boom, so generating additional VAT. In order to remain fully road-legal it may be possible for some vehicles to be fitted with a catalytic converter, but as any well-worn oil-burning engine would wreck its catalyst, this would not be a practical proposition in many cases.

This will no doubt be seen as an attractive vote-catching bonus for politicians, but its true significance will not be lost on enthusiasts who take a pride in the regular use of an older car.

A more insidious effect of catalytic converters is the effect of their less benign by-products. We have all experienced the ‘rotten eggs’ odour of hydrogen sulphate when travelling behind various cars, usually those of far eastern origin. This, however, may not be the only unpleasant side-effect.

The fumes given off by any process involving the use of hot platinum are known to produce symptoms which are one of the many covered by the loose description ‘Industrial asthma.’

Could it, perhaps, be mere chance that the period since catalytic converters became mandatory has coincided with a dramatic rise in the incidence of asthma, particularly amongst children, who, it was claimed, were most at risk from the effect of exhaust fumes?
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