The roadside and MOT gaseous emission tests used for vehicles in the UK correlate poorly with the type approval test. This is because the roadside and MOT tests use test conditions that do not subject the engine to external load. This project investigated the feasibility of using an improved type of dynamometer which can adequately and fairly assess exhaust emissions from motor vehicles and is relatively quick and cheap.
A prototype system was investigated using three cars. The dynamometer used free running chassis rollers and the vehicle brakes to apply speed and load to the engine, whilst emission concentrations were measured using a basic garage specification gas analyser. Data from the vehicles OBD diagnostic system was used in conjunction with the data from the gas analyser to measure mass emissions of Carbon Monoxide, Hydrocarbons and Nitric Oxide in relation to engine speed and load.
Although many of the measurements exhibited poor repeatability or were near the limit of resolution of the gas analyser, it was thought that the methodology used may be sufficient to identify excessive emissions caused by gross malfunctions of the engine, fuel system or after treatment system. It is thought that the poor repeatability may be due to variations in the catalyst temperature and the fuel air ratio used during the measurements.
Further work would be needed to clarify and verify these results. The best way of achieving this could be to purposely induce faults on a limited number of vehicles so the emissions would be similar to that expected from faulty vehicles and large enough to be measured accurately with a basic garage specification of gas analyser.

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