TRL was commissioned by the Department for Transport to review the approach used in the National Atmospheric Emissions Inventory (NAEI) for estimating emissions from road vehicles, and to propose new methodologies. This Report summarises the models which are available for estimating ‘cold-start’ emissions. Exhaust emissions of regulated pollutants are higher during the cold-start phase than when the engine, lubricants, coolants, exhaust catalyst, drive train and tyres are at their design temperatures. This is due to a combination of incomplete fuel combustion in the engine, catalyst inefficiency, and increased friction in the engine and drive train and at the tyre-road interface. A wide range of factors affect cold-start emissions, and the Report includes a brief summary of a number of studies dealing with the subject. The method used in the NAEI for estimating cold-start emissions is summarised, and potential limitations are highlighted. The Report also identifies a number of potential sources of data which might be used to update and refine the modelling approach in the NAEI. The main conclusion of the Report is that before any more detailed modelling of cold-start emissions is attempted the current NAEI model (which is based upon the COPERT II model) should be updated to reflect the COPERT 4 methodology. However, new methodologies are scheduled to be available in 2009, and when these are published they should be considered for inclusion in the NAEI.

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