For a number of years, EGR has been used to assist in the reduction of the emissions of NOx. NOx is often referred to as Oxides or Nitrogen and these gases are formed during the combustion process if the combustion temperatures become excessive. In general terms, the higher the temperatures during combustion, the higher the levels of NOx, a figure of around 1,800 degrees centigrade is often quoted as a temperature above which excessive levels of NOx are formed. NOx is in fact a generally used term to cover the Oxides of Nitrogen that commonly exist in the exhaust gases. Nitrogen monoxide (NO) is a particular combination of Nitrogen and Oxygen, and because both gases are present in the atmosphere they are induced into the engine. High combustion pressures and temperatures accelerate the formation of Nitrogen Monoxide, which, once it has left the exhaust port, combines with oxygen to form Nitrogen Dioxide (NO2). NOx is a colourless gas, which can cause paralysis if it enters the bloodstream. NO2 can cause respiratory irritation and can damage the lungs. Additionally, Oxides of Nitrogen combined with Hydrocarbons can form smog. The reduction of NOx emissions in general, is therefore, high on the priority list. Legislation in the UK, USA, EEC, Japan and other countries has for some time forced vehicle manufacturers to fit additional equipment to aid the NOx reduction. The basic problem with regard to the internal combustion engine and NOx is that high combustion temperatures cause an increase in the speed of NOx formation and higher levels of NOx are produced. Also NOx levels increase when mixture strength (air/fuel ratio) is slightly lean (excess Oxygen), and these levels occur around the cruise to light load range. Any aspect of the engines design or operation that is liable to increase combustion temperatures, especially when the air/fuel ratio is slightly lean, is going to result in high NOx emissions. As an example, many modern engines run fairly high compression ratios which cause high combustion temperatures, also the advanced ignition timing usually used when the engine is running on the leaner mixture also causes high combustion temperatures. These conditions will only serve to increase the NOx emissions. The EGR system allows a percentage of the exhaust gas to be fed back into the intake system. Exhaust gas is non combustible (it may contain an extremely small amount of hydrocarbons which has no effect on the process) therefore, if a small amount of exhaust gas is mixed with the fresh air/fuel mixture, it occupies a portion of the mixture that would otherwise be combustible and less heat will be produced. Additionally, the recirculated exhaust gas absorbs some of the heat produced during combustion. In total the combustion temperatures are reduced thus reducing the formation of NOx.