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# 5.2 Technologies and standards

Over the last 50 years, several technological advances have significantly reduced the emissions from internal combustion engined vehicles.

## Reducing pollution from petrol engines

One of the most important developments in emission-control technology for petrol (and other spark ignition) engines has been the introduction of the three-way catalytic converter (see Box 3). 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.

## Box 3 Catalytic converters

Catalytic converters are an important type of ‘tailpipe’ or exhaust technology. Inside the catalytic converter, three catalysts are used, each to convert a different pollutant, hence the term ‘3-way’ catalytic converter. Platinum and palladium are used to help oxidise the unburned hydrocarbons and carbon monoxide to carbon dioxide, and rhodium is used to help convert the nitric oxides back to nitrogen and oxygen. The catalysts are applied to a support structure within the exhaust pipe with a high surface area through which the exhaust gases are made to flow.

A catalytic converter unit is usually protected in a steel 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 they have reached an initial ‘light-off’ temperature (between 150 °C and 300 °C), 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.

## Reducing pollution from diesel engines

There are a range of possible technologies:

- Diesel particulate filters (DPFs) and oxidation catalytic converters – also known as Continuously Regenerating Traps (CRTs). These use a filter to trap sooty particulates, a control system to monitor the soot level and to make sure that these particulates are ‘burned off’ from the filter later on, thus ‘regenerating’ the filter.
- Exhaust gas recirculation – NOx is only produced when the combustion temperature exceeds 1500 °C. A computerised engine management system can rapidly control the peak combustion temperature by recirculating some of the exhaust gas back into the engine air intake.
- Selective catalytic reduction (SCR) – this involves injecting ammonia (NH3) into the exhaust where it reacts with NOx emissions in the presence of a catalyst reducing the gases back to nitrogen. Vehicles using this technology have to top up with ‘diesel exhaust fluid’ (often known as ‘Adblue’), a mixture of urea (a compound of ammonia and oxygen) and water.

Particulate filters and oxidation catalysts are now fitted as standard to heavy-duty engines and are proven to reduce particulates by up to 90%.

As in the USA and Japan, legislation in Europe has been 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).

Note that these are measured in laboratory conditions, over a defined driving cycle. The tests represent a mix of urban and longer-distance inter-urban car journeys and can include long periods of running at a constant speed with little acceleration or deceleration.

Table 3 illustrates how the Euro standards have tightened since 1992.

## Table 3 European emissions limits for passenger cars (grams per km)

Standard | Year | Petrol | Diesel
--- | --- | --- | ---
CO | HC | NOx | HC+NOx | PM | CO | HC | NOx | HC+NOx | PM
Euro 1 | 1992 | 2.72 | – | – | 0.97 | – | 2.72 | – | – | 0.97 | 0.14
Euro 4 | 2005 | 1.0 | 0.10 | 0.08 | – | – | 0.50 | – | 0.25 | 0.30 | 0.025
Euro 6 | 2014 | 1.0 | 0.10 | 0.06 | – | 0.005 | 0.50 | – | 0.08 | 0.17 | 0.005

Source: data taken from DieselNet
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