Emission Control Strategies (Automobile)

17.4.

Emission Control Strategies

The concern to limit the amount of toxic gases produced by automobiles first arose in the USA during the 1960s due to the deteriorating quality of air in many large cities. The problem was a specifically acute in Southern California where a high traffic density and a sunny climate led to the formation of photochemical smog.
The first US emission limits came into force by the 1968 Clean Air Act. As per this act vehicles sold in 1970 should have emission levels some two-thirds less than the uncontrolled 1967 levels (Fig. 17.3). Sub­sequently, Senator Muskie gained the approval of Congress in 1970 for further steep reductions in automobile emissions. The Muskie Laws required that by 1975-76 all vehicles should have emission levels of about 10 percent of the 1970 levels. To enact this law the US Environmental Protection Agency (EPA) was established to test and Type Approve vehicles.
To achieve this daunting task in a short time, the US automobile industries immediately considered all possible technologies and came to the conclusion that only exhaust emission control using catalytic con­verters would be available in time. Initially, oxidation catalysts were used in the exhaust system to limit HC and CO emissions. Then exhaust gas re-circulation (EGR) was added to reduce NOx emissions. Sub­sequently, as stringent NOx limits were set in the late 1970s, the three-way catalytic converter was developed to reduce the emissions of all three pollutants. By the end 1980s practically the entire US petrol engined cars were fitted with this emission-control system and it has since become a standard system throughout most of the developed countries.
European emission control legislation is about a decade behind the US, enabling European car makers to learn from the experiences in the US and to use more advanced electronic control techniques. Many European manufacturers, including Rover, Volkswagen and Peugeot-Citroen, developed emission control systems based on lean-burn engine technology so that low emission levels were achieved with greater fuel economy using simpler catalytic converters. Sub­sequently, due to political pressure from some European governments stringent emission control legislation was imposed so that the use of US-style catalytic converters become necessary (Table 17.1), and since January 1993 all new cars have been fitted with such systems.

Table 17.1 European emission regulations for petrol engines.

Fig. 17.3. Reduction in exhaust emission since 1967 (USA).
Although these vehicles produce less pollutants than those without a catalytic converter they still generate enormous quantities of toxic gases. It has been estimated that over a ten year period a medium size catalyst-equipped car covering 12800 km per year at an average fuel consumption of 12.5 km per litre produces approximately,
• 44 tonnes of carbon dioxide,
• 5 kg of sulphur dioxide,
• 47 kg of nitrogen dioxide,
• 325 kg of carbon monoxide, and
• 36 kg of unburned hydrocarbons.
This amount of exhaust gas emission gives rise to about 1 billion cubic metres of polluted air. It is one of the reasons for many car makers (especially in Japan) to continue working on lean-burn engines, and it is likely that future legislation to limit CO2 emission may enhance interest in lean-burn technology.