Nitrogen oxide is a term used to describe a number of nitrogen based pollutants, but it is nitrogen dioxide that is of most concern because it affects people's health.
Two different types of technology are being evaluated as part of these trials - Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR). Both of these systems work in combination with a particulate filter.
Exhaust Gas Recirculation
EGR technology works by taking some of the exhaust gases that have passed through a particulate filter and recirculating them back into the engine.
The gases are cooled before they are fed back into the engine and this helps to reduce the amount of NOx produced. Initial tests have shown that this technology can reduce NOx emissions by about 45 per cent.
There are currently 10 double decker buses on trial with this technology. These are operated by Travel London.
Selective Catalytic Reduction
Once exhaust gases have passed through a particulate filter, they enter an SCR unit. This is where a liquid solution of urea, also known as AdBlue, is added in order to reduce the NOx. The SCR consists of two chambers: one is used to mix the urea with the exhaust gas while the second contains a catalyst specifically designed to help speed up the chemical reaction to reduce the NOx.
Over the London Bus test cycle initial results have shown that SCR can reduce NOx emissions by around 55-65 per cent.
There are currently 14 single decker buses on trial with SCR technology. These are operated by FirstGroup and London United.
- How the oxidation catalyst works (Flash movie SWF 19KB)
- How the filter works (Flash movie SWF 19KB)
- How the SCR section works (Flash movie SWF 45KB)
Performance of the technologies has been assessed over the course of a year and will continue until summer 2006.
Tests will also be carried out in a laboratory to assess what emissions benefits these technologies can deliver.
As test results are collated and analysed, the technologies are being modified and enhanced with the aim of producing reliable, durable systems that can then be rolled out across the fleet.
Once the trials are completed and we are satisfied with the performance of the systems, we will implement a retrofit programme from 2006/07.
Find out more information about the various technologies and partners by visiting these websites:
STT Emtec is the supplier of the EGR systems
Eminox supplies the SCR systems
Johnson Matthey supplies technology and catalysts for the NOx abatement systems