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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.

In direct injection Diesel engines having a unit displacement less than 1 liter, the use of high compression ratios (19\20 instead of 16\17) is mandatory for solving cold smoke problems and reducing HC\NOx emissions. The increase of compression ratio however is associated to the increase of black smoke and specific fuel consumption. On the basis of systematic measurements of the main combustion quantities and 2-D computations of the air motion and spray development inside the cylinder, a re-entrant high compression ratio chamber was defined. Experimental results in terms of fuel consumption, black smoke and gaseous emissions are presented. The optimum squish-to-swirl ratio is discussed.

The main parameters affecting combustion of heavy-duty D.I. Diesel engines were studied by the utilization of a two-dimensional axisymmetric code taking into account air motion, spray formation and combustion. Model predictions are integrated with the analysis of the main combustion quantities coming from the recorded in-cylinder pressure, injection pressure and needle lift. The trend of the total air momentum and spray formation obtained by changing the swirl level, the combustion chamber shape, the in-cylinder air mass and the fuel jet distribution were compared with the trends of the smoke, fuel consumption, gaseous emissions and heat release rate. Experimental data and model predictions indicate that very different combustion chamber geometries can be adopted in medium and small bore engines. The ratio between the in-cylinder air mass and injected fuel momenta can be useful in defining the optimum air motion and spray formation of a given combustion system.