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From september 1988 a fleet test started in Italy on urban buses equipped with a single ceramic monolith filter plus a by-pass and Diesel fuel burners for regeneration. The operation of the particulate trap, during the normal bus running in the city, has been assessed in three different ways: 1. Continuous control, by means of a data acquisition system purposely designed, of the main parameters affecting regeneration (i.e. time between consecutive regenerations, temperature in three selected zones in the filter during regeneration,filter loading); 2. Periodic control of smoke opacity of the bus measured, with and without the filter, under free acceleration and when bus is accelerating from 50 to 65 km/h on proving ground; 3. Particulate collection efficiency of the trap by using a dilution tunnel with the engine operated at the test bench. Information carried out from the fleet tests and additional information deriving from laboratory results were used for the system improvement.

A 2.0 liter displacement gasoline fueled car, with closed-loop control and 3-way catalyst, was adapted to operate on CNG (Compressed Natural Gas). A system allowing the detection and measurement of aldehydes, ketones, polycyclic aromatic hydrocarbons (PAH) and speciated organic materials was set up. Running on different fuels, such as CNG, gasoline and gasoline/oxygenates blends, tests were carried out according to US (FTP '75) and European (ECE+EUDC) procedures with and without the original 3-way catalytic converter. Apart from the absence of evaporative emissions because of the required closed delivery system, test results pointed out that CNG use yields substantial air toxics benefits in terms of regulated and, above all, unregulated emissions.