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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 light weight diesel engine family having high specific power output (up to 21 KW/1) has been developed for medium trucks (7/13 ton) starting from existing 6-cyl in-line N/A and T/C DI engines with a single displacement equal to 0.915 liters. To meet future U.S. federal and California emission standards changes in combustion chamber shape, swirl level, injection system and turbocarger matching have been introduced. Projections of fuel economy according to the new transient procedure proposed by EPA have been carried out. These have been compared with the projections of a conventional gasoline engine referred to the same vehicles. Finally the main relationships among the combustion parameters, fuel consumption and emissions have been explored.