Page 15
Next Page (16) Main Index Previous Page (14)

Other types of Diesel emissions - NOx

Diesels are by their nature lean-burn engines, and this can result in high NOx emissions due to the high pressures and excess of unburned oxygen. Earlier in the tech sections we discussed how Nitrogen, a normally unreactive gas, will Oxidise (combust) given high enough pressure and temperature. This creates NOx. The ECU can regulate the production of these gases in over-run and light throttle conditions by operating a throttle valve, a feature not formerly present on Diesel engines, to restrict the amount of air entering the combustion chamber, thus reducing the amount available for NOx formation.

Particulate filters

The remaining particulates still made by the Diesel engine are trapped in filters located in the exhaust system, called particulate filters. Obviously, given time these filters will clog up and require cleaning. This is done under the control of the ECU and is termed "filter regeneration".

Diesel exhaust because of its inherent thermal efficiency is not hot enough to oxidise the soot that has been collected on the filters, so the ECU opens the injector again right at the end of the combustion cycle, thus allowing unburned fuel to enter the exhaust system. This happens occasionally (approximately every 500-1000km) during steady cruising conditions, the fuel ignites in the hot exhaust and raises the temperature to about 650 degrees C, above the oxidation point of the soot. This completes the combustion within the filter, oxidising the soot particles and thus cleaning the filter. The soot particles are liberated from the filter, are carried out of the exhaust along with the normal exhaust gas flow, and discharged into the atmosphere.

Identifying different NOx pollutants

This applies to both Diesel emissions and direct-injection Petrol emissions. A quick reference guide to major NOx pollutants. The objective of a NOX catalyst is to reduce gases category yellow and red, turning them into the first (green).

Name Chemical Formula Description Harmful Pollutant Category
Nitrogen Gas N2 Completely harmless. Comprises almost 80% of the regular natural atmosphere. Not an oxide of nitrogen. Used where an inert gas is required eg. MIG welding Not at all  
Nitrous Oxide N2O Powerful oxidiser used for Nitrous Injection in high power cars, known as Laughing Gas when used as anaesthetic (surgical pain relief) Not really No  
Nitric Oxide NO Precursor to NO2 which then combines with O2 or HO2· (hydroperoxide) to make acid rain Yes  
Nitrogen Dioxide NO2 Brown coloured gas. Irritant. Inflames airways and causes dyspnea (loss of lung function), headaches, nausea Very  

Next Page (16) Main Index Previous Page (14)