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The multidimensional simulation methods, today available for spray motion predictions, solve the spray equations including the mass, momentum and energy changes due to the interaction between the drops and the gas, considering also the collision and coalescence phenomena. As concerns break up, two models are the most commonly used: the TAB one, proposed by O'Rourke and Amsden and based on the Taylor analogy, and the WAVE model; developed by Reitz and Diwakar. Both models need the tuning of some empirical constants. Considering also that the mechanism, that controls atomisation, is not yet well understood, it seems that further calculations and experimental comparisons over a range of injection conditions may be useful to improve the prediction capability of these models. Therefore the present paper concerns a sensitivity analysis of the TAB and WAVE models to changes of the empirical constants.

Three dimensional computations of Diesel combustion were performed using a modified version of Kiva II code. The autoignition and combustion model were tuned on a set of experimental conditions, changing the engine design, the operating conditions and the fuel characteristics. The sensitivity of the model to the different test cases is acceptable and the experimental trends are well reproduced. In addition the peak of pressure and temperature computed by the code are quite close to the experimental values, as well as the pressure derivatives. Once tuned the combustion model constants, different but simple formulations for the soot formation and oxidation processes were implemented in the code and compared with the experimental measurements obtained both with fast sampling technique and two colors method. These formulations were found unable to give good prediction in a large range of engine operating conditions, even if the model tuning may be very good for each test point.

Cetane number, sulphur content and aromatic structure of Automotive Diesel Oil (ADO) were changed to assess their influence on emissions of light duty direct injection Diesel engine. The detailed chemical analysis of particulate soluble fraction allows to quantify the P.A.Hs emission. In addition also the aldehydes and volatile organic compounds were measured in the gaseous phase. The sulphur content of the fuel and its aromatic structure strongly influence particulate emission. The insoluble fraction of the particulate rises with an increase of the high sulphur content ADOs with about the same back end volatility. Unburned P.A.Hs control P.A.Hs emission at the part loads typical of normalized schedules for emission testing of light duty vehicles in Europe. Finally the level of emissions of benzene and 1-3 butadiene is comparable to the total P.A.Hs emission.

A modified version of KIVA II code was used to perform three-dimensional calculations of combustion in a DI diesel engine. Both an ignition delay submodel and a different formulation of the fuel reaction rate were implemented and tested. The experiments were carried out on a single cylinder D.I. diesel of 0.75 I displacement equipped with sensors to detect injection characteristics and indicated pressure. A fast acting sampling valve was also installed in the combustion chamber to allow the measurement of main pollutants during the combustion cycle, by an ensemble average technique. Computational and experimental results are compared and the discrepancies are discussed. Today the demand for light duty engines that produce less emission and consume less fuel is increasing. Thus, if limits on CO2 emissions are established, the direct injection diesel engine for light duty applications will become an attractive option.

The fluid-mechanic behaviour of straight-sided and re-entrant chamber geometries has been studied using laser doppler anemometry (LDA) technique. Measurements have been carried out during the compression stroke in a direct injection diesel engine, representative of medium size family, operating at 1000 rpm under motored conditions. The mean motion and turbulence intensity have been computed using a filtering procedure on the LDA data. Using the second version of KIVA code, the air flow field evolution during the same crank angle period has been also computed. To perform proper comparisons between measured and computed values of mean velocity and turbulence intensity, a careful choice of the initial conditions for computations has been performed. Reasonable agreement has been found between computed and measured mean swirl velocities for both combustion chamber geometries tested. On the contrary, the computed turbulence intensities underestimate those measured.

A methodological analysis of combustion parameters and pollutant emissions measuring procedures during transient operation of a D.I. T.C. light duty diesel engine was performed. Combustion process was characterized by ignition delay time, combustion pressure peak value and heat release law measurements during the transient ECE 15 schedule on a dynamic test bed with electronic simulation of inertia. The particulate emission was measured every 0.05 s by an I.R. optical method. In addition some correlations, based on pressure cycle and injection law evolution, were implemented in order to calculate instantaneous fuel delivery and transient NOx emission. Some activities were carried out in order to asses the limits of engine configurations ranking performed with steady state measurements of performances and emissions. Strong differences were detected between carbon emission during transient operations and the value obtained by interpolation from a steady state map.

The formation and oxidation of soot, light and heavy hydrocarbons, CO, CO2 and NOx in a D.I. diesel engine have been studied by means of direct fast sampling and chemical analysis of the combustion products collected during the combustion cycle. Particular attention has been paid to the histories of each fuel hydrocarbon class analyzing the chemical transformations that the paraffins, and monoaromatic and polyaromatic compounds, contained in a diesel fuel oil, undergo during the combustion cycle. This approach is able to give information on the origin of soot and heavy hydrocarbon emission from a diesel engine. The concentration of the heavy hydrocarbons decreases during the early stages of the combustion cycle and their profile corresponds roughly to the fuel disappearance rate because of the chemical similarity with the fuel compounds.

Comparisons are presented of computed and measured air flow fields in an open chamber diesel engine running at 1,000 and 2,000 rpm without combustion. Both Conchas spray and KIVA codes were tested. The effect of turbulence is represented using both K-ε and SGSD (Sub-grid Scale Differential) submodels. A Laser Doppler Velocimeter (LDV) was used to make velocity and turbulence measurements during the compression stroke. Reasonable agreement between numerical and experimental results for the engine examined was observed.

The behaviour of two combustion chambers, a toroidal and a turbulent one, has been compared. The engine performance in terms of imep and exhaust emissions were measured. Laser Doppler Anemometry technique was used to characterize the fluids dynamic aspect of combustion system. The axial asymmetry introduced in combustion chamber shape causes strong differences in the air flow field at the end of compression stroke. The tangential velocity profile is flattened to that obtained with toroidal chamber. Moreover the rms values of tangential velocity measured in turbulent combustion chamber are about three times higher than that measured in the toroidal chamber. At low engine speed the turbulent chamber allows to operate with low NOx levels without penalties of smoke emissions and fuel consumption as happens by using conventional toroidal chamber.

Two fuels having different aromatics content and different cetane numbers were tested in a direct injection diesel engine with thermally insulated pistons. Actually tests were carried out with a full aluminum piston, an aluminum piston modified to accept a stainless steel crown and a similar one coated with ceramic. Higher combustion noise and emissions were detected using the degraded fuel, having fixed the type of piston. Furthermore, the experiments showed that thermal barrier adoption has a positive effect on the combustion noise.

The paper describes some experiments carried out on two d.i. Diesel engines running with insulated pistons. Three different thermal barriers were tested; namely, a stainless steel cup, a Si3N4 cup and a stainless steel piston crown. The combustion process was characterized by heat release calculation and ignition delay measurements. The experiments showed that the indicated efficiency is not affected by thermal insulation adoption, Nox level increases while smoke level decreases consistently.