4.2 Technologies and standards
During the last 30 years, as well as fuel formulations, several technological advances have significantly reduced the emissions from ICE vehicles. One of the most important developments in emission-control technology has been the introduction of the three-way catalytic converter for spark ignition engines (see Box 1). This technology was first used in the USA in the 1970s so that vehicles would conform to the US Clean Air Act, one of the first regulations that limited pollution from mobile (and stationary) sources. Since then, these catalyst systems have done much to improve air quality in the USA, Japan and Europe. More recently, diesel particulate filters (DPFs) have also been increasingly used to reduce emissions of particulates. These are a porous ceramic filter, normally with flow through walls, whereby the diesel exhaust gases flow through the filter trapping all of the particulates.
Box 1 Catalytic converters
Catalytic converters are an important type of 'end-of-pipe' technology that reduces emissions of CO, NOx and unburned HCs from the exhausts of petrol-engine vehicles (and are hence known as 'three-way' catalysts). Catalytic converters use a mixture of platinum, palladium and rhodium metals as their active components. In the presence of excess air, these catalysts promote chemical reactions that convert emissions to less harmful gases. The catalysts are applied to a high-surface-area support structure (within the exhaust pipe) through which the exhaust gases are made to flow.
A catalytic converter unit is protected in a steel or other metal canister located within the vehicle's exhaust pipe. Most systems have to meet stringent durability requirements, including working for 100 000 km or 5 years – whichever occurs first.
Catalytic converters do have some inherent drawbacks. They are relatively ineffective before the 'light-off' temperature is reached, which means that they are inactive during short trips. Also, they tend to slightly increase fuel consumption (and hence CO2 emissions). The precious metals in the converters can also be poisoned by certain fuel components such as lead and sulfur, which is why the use of catalysts has been dependent on the availability of lead-free and ultra-low sulfur fuels.
As in the USA and Japan, legislation in Europe continues to be tightened for vehicle emissions (see Table A.2) and has been highly successful in reducing some of the pollutants associated with road transport. Key European legislation for passenger cars has been the 'Euro' standards, introduced periodically from 1992; similar limits have been introduced for light commercial vehicles (vans) and heavy-duty vehicles (the latter specified in terms of grams per kWh of engine output).
Table 3 European emissions limits for passenger cars (grams per km)
Activity 3 (self-assessment)
Is it possible to be conclusive as to whether, in general, petrol or diesel cars produce the lowest amount of emissions?
This depends on which pollutant is chosen, but in general diesel now outperforms petrol on most pollutants and, as has already been noted, also has lower CO2 emissions per kilometre.