Striving for reduced emissions
CAR|February 2022
Enthusiasts focus on tailpipe number, appearance and exhaust acoustics, often with little thought given to their critical role in meeting emission targets. We investigate the function of the exhaust gas treatment components to achieve reduced emissions
By Graham Eagle


The introduction of the positive crankcase ventilation (PCV) system in the early 1960s was the first move towards reducing pollution from production vehicles. Fumes containing unburnt hydrocarbons from the crankcase were routed to the intake manifold, instead of being released directly into the atmosphere where they contributed to the formation of photochemical smog.

By 1966, the first exhaust emission standards followed, and the US Clean Air Act of 1970 legislated that from 1975, all cars would have to reduce emissions of CO (carbon monoxide), HC (hydrocarbons) and NOx (nitrogen oxides) by 90%. By 1974, compliance was being achieved by engine detuning, which significantly reduced engine efficiency and increased fuel consumption. It was clear catalytic converters would be required if the industry was to meet the strict 1975 targets. This was not going to be possible with the existing leaded petrol available at that time, as lead residue coated the surface of the catalysts, effectively destroying them, so the introduction of unleaded fuels was fast-tracked to allow the introduction of catalytic converters. An added benefit was that vehicles were eliminated as a source of lead poisoning.

01 Cross-section of a ceramic monolith used in three-way catalytic converters. Exhaust gas flows longitudinally through the square section channels.


Early catalytic converters were known as two-way catalytic converters and were common fitment on most petrol engines until the 1980s. Also known as oxidation converters, two-way converters oxidise CO > CO, (carbon dioxide) and oxidise HC (unburnt or partially burnt fuel) > CO, and H,0 (water). More stringent emissions legislation effectively rendered them obsolete and they were superseded by three-way or oxidation-reduction converters. These are able to control the emission of NO (nitric oxide) and NO, (nitrogen dioxide) - covered by the common abbreviation of NOx-which are reduced to N, (nitrogen).

These three reactions in the catalytic converter occur most efficiently when the engine operates within a narrow band of air-fuel ratios near the stoichiometric point; this being the mass ratio of 14,7:1 for theoretically complete combustion, equating to 14,7 kg of air for one kilogram of fuel. If the engine runs too lean, which is ideal for optimum fuel consumption, the exhaust gases will contain excess oxygen that will limit the reduction of NOx. If operated too rich for optimised power, all available oxygen is consumed by the excess fuel, leaving only the oxygen stored within the catalyst available for the oxidation functions.

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