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The intention of the presented work was to develop a new simulation tool that fits into a CFD (computational fluid dynamics) workflow and provides information about the soot particle size distribution. Additionally it was necessary to improve and use state-of-the-art measurement techniques in order to be able to gain more knowledge about the behavior of the soot particles and to validate the achieved simulation results. The work has been done as a joint research financed by the European Community under FP5.

The paper describes the results of a parallel 1D thermo-fluid dynamic simulation and experimental investigation of a DI turbocharged Diesel engine. The attention has been focused on the overall engine performances (air flow, torque, power, fuel consumption) as well as on the emissions (NO and particulate) along the after-treatment system, which presents a particulate filter. The 1D research code GASDYN for the simulation of the whole engine system has been enhanced by the introduction of a multi-zone quasi-dimensional combustion model for direct injection Diesel engines. The effect of multiple injections is taken into account (pilot and main injection). The prediction of NO and soot has been carried out respectively by means of a super-extended Zeldovich mechanism and by the Hiroyasu kinetic approach.

The paper describes the development and validation of a quasi-dimensional multi-zone combustion model for Gasoline Direct Injection engines. The model has been embedded in the 1D thermo-fluid-dynamic code for the simulation of the whole engine system named GASDYN and developed by the authors [1, 2 and 3]. The GDI engine combustion model solves mass, energy and species equations using a 4th order Runge-Kutta integration method; the fuel spray is initially divided into a number of zones fixed regardless of the injected amount and the time step, considering the following break-up, droplet evaporation and air entrainment in each single zone. Experimental correlations have been used for the spray penetration and spatial information. Once the ignition begins it is assumed that the flame propagates spherically, evaluating its velocity by means of a fractal combustion approach and considering the local air-fuel ratio, which is the result of the spray evolution within the combustion chamber.

The DEXA Cluster consisted of three closely interlinked projects. In 2003 the DEXA Cluster concluded by demonstrating the successful development of critical technologies for Diesel exhaust particulate after-treatment, without adverse effects on NOx emissions and maintaining the fuel economy advantages of the Diesel engine well beyond the EURO IV (2000) emission standards horizon. In the present paper the most important results of the DEXA Cluster projects in the demonstration of advanced particulate control technologies, the development of a simulation toolkit for the design of diesel exhaust after-treatment systems and the development of novel particulate characterization methodologies, are presented. The motivation for the DEXA Cluster research was to increase the market competitiveness of diesel engine powertrains for passenger cars worldwide, and to accelerate the adoption of particulate control technology.

The paper describes the research work carried out on the thermo-fluid dynamic modeling of an S.I. engine coupled to the vehicle in order to predict the engine and tailpipe emissions during the ECE European driving cycle. The numerical code GASDYN has been extended to simulate the engine + vehicle operation during the first 90 seconds of the NEDC driving cycle, taking account of the engine and exhaust system warm-up after the cold start. The chemical composition of the engine exhaust gas is calculated by means of a thermodynamic multi-zone combustion model, augmented by kinetic emission sub-models for the prediction of pollutant emissions. A simple procedure has been implemented to model the vehicle dynamic behavior (one degree of freedom model). A closed-loop control strategy (proportional-derivative) has been introduced to determine the throttle opening angle, corresponding to the engine operating point when the vehicle is following the ECE cycle.

Multidimensional simulation of the soot formation process in a diesel engine is realised exploiting quantitative measurements of the soot volume fraction and diameter obtained by optical techniques. Broadband extinction and scattering measurements are performed on an optically accessible 4-stroke engine where a forced air motion allows a strong prevalence of the premixed stage of combustion with respect to the non-premixed one. Two semi-empirical models for soot formation are tested in the numerical simulation, which is performed using a customized version of the KIVA-3 code. The need of furnishing coherent values of the soot particles density and mean diameter to the one of the two models requiring this kind of information, is highlighted and demonstrated to be crucial in avoiding over-prediction of the soot concentration.

Polychromatic extinction and chemiluminescence techniques, from ultraviolet to visible, were applied in an optical diesel engine, in order to analyze the temporal and spatial evolution of a high pressure fuel jet interacting with a swirling air motion. A fully flexible Common Rail fuel injection system equipped with a single hole nozzle was used. The experiments were performed at fixed engine speed and air/fuel ratio for three injection strategies. The first one consisted of a main injection to compare with those operating at low pressure injection. The other ones were based on a pilot and main injections, typical of current direct injection diesel engines, with different dwell time. A detailed investigation of the mixture formation process inside the combustion chamber during the ignition delay time was performed. The liquid and vapor fuel distribution in the combustion chamber was obtained analyzing the polychromatic extinction spectra.

The paper describes the research work concerning the simulation of 1D unsteady reacting flows in s.i. engine pipe-systems, including pre-catalysts and main catalysts. The numerical model GASDYN has been developed to enable the concurrent prediction of the wave motion in the intake and exhaust ducts, the chemical composition of the gas discharged by the cylinder of a s.i. engine, the chemical and thermal behavior of catalytic converters. The effect of considering the transport of chemical species with reactions in gas phase (post-oxidation of unburned HC in the exhaust manifold) and in solid phase (conversion of pollutants in the catalyst) on the predicted wave motion is reported.

Nowadays 1D fluid dynamic models are widely used by engine designers, since they can give sufficiently accurate predictions in short times, allowing to support the optimization and development work of any prototype. According to the last requirements in terms of pollutant emission control, some enhancements have been introduced in the 1D code GASDYN, to improve its ability in predicting the composition of the exhaust gas discharged by the cylinders and the transport of the chemical species along the exhaust system. The main aspects of the methods adopted to model the combustion process and the related formation of pollutants are described in the paper. To account for the burnt gas stratification, two different approaches have been proposed, depending on the expected turbulence levels inside the combustion chamber. The reliability of the simulation of the pollutant formation process has been enhanced by the integration of the thermodynamic module with the Chemkin code.

Conventional methods to measure gas concentrations and, in particular, NO are typically based on sampling by valve, sample treatment and subsequent analysis. These methods suffer low spatial and temporal resolution. The introduction of high energy lasers in combination with fast detection systems allowed to detect the NO distribution inside optically accessible Diesel engines. In this paper, a high spatial and temporal resolution in-situ technique based on ultraviolet - visible absorption spectroscopy is proposed. The characterization of the combustion process by the detection of gaseous compounds from the start of combustion until the exhaust phase was performed. In particular, this technique allows the simultaneous detection of NO and OH absolute concentrations inside an optically accessible Diesel combustion chamber.

Non-intrusive diagnostic techniques based on broadband (190-550 nm) extinction and scattering spectroscopy were applied at undiluted exhaust Common- Rail (CR) diesel engine in real time. The influence of load and Exhaust Gas Recirculation (EGR) on soot mass concentration, size distribution of emitted particles and NO concentration was analyzed. NO concentration was evaluated by ""in-situ"" ultraviolet-visible absorption measurements and compared with those obtained by conventional analyzer. The extinction and scattering spectra were compared with those evaluated by the Lorenz-Mie model for spherical particles in order to retrieve the size, the number concentration of the emitted particles and particulate mass. The optical measurements showed that new generation diesel engines, in spite of a drastic reduction of the exhaust mass concentration, caused the emission in the atmosphere of high number concentration of carbonaceous nanoparticles.

Quantitative measurements of the soot volume fraction and diameter performed by spectroscopic techniques within the combustion chamber of a diesel engine are employed to aid multidimensional simulation of the soot formation and oxidation processes. By changing the start of fuel injection, two different operating conditions are considered, which are characterized by different relative importance of the premixed to the diffusive stage of the combustion process. Both the reduced models by Hiroyasu et al., and the one by Nagle and Strikland- Constable are employed within the numerical simulation. The reason of the peculiar over-prediction of soot concentration of the latter model is discussed and related to the need of furnishing coherent values of the soot particle density and mean diameter.

The calculation of the main pollutant emissions discharged into the atmosphere by means of numerical codes requires the development of integrated models, including either an accurate thermodynamic in-cylinder analysis and the simulation of reacting unsteady flows in the duct system. This paper describes the main features of the numerical model GASDYN developed by the authors, which in the last years has been enhanced in order to achieve this kind of objectives. A multi-zone approach has been adopted to predict the combustion process in s.i. engines, whereas the so called super-extended Zeldovich mechanism has been introduced to perform a more detailed description of all the chemical reactions involved in the NOx production process. The simulation of the reacting flows in the exhaust manifold has been completed by the introduction of further enhancements to predict the chemical behavior of gases inside the catalytic converters.

This paper describes some recent advances of the research work concerning the 1D fluid dynamic modeling of unsteady reacting flows in s.i. engine pipe-systems, including pre-catalysts and main catalysts. The numerical model GASDYN developed in previous work has been further enhanced to enable the simulation of the catalyst. The main chemical reactions occurring in the wash-coat have been accounted in the model, considering the mass transfer between gas and solid phase. The oxidation of CO, C3H6, C3H8, H2 and reduction of NO, the steam-reforming reactions of C3H6, C3H8, the water-gas shift reaction of CO have been considered. Moreover, an oxygen-storage sub-model has been introduced, to account for the behavior of Cerium oxides. A detailed thermal model of the converter takes into account the heat released by the exothermic reactions as a source term in the heat transfer equations. The influence of the insulating mat is accounted.

In the last years, there has been an increasing concern on the emission of ultrafine particles in the atmosphere. A detailed study of formation and oxidation of these particles in the environment of the diesel cylinder presents many experimental difficulties due to the high temperatures, pressures and extremely reactive intermediate species. In this paper, in order to follow the different phases of diesel combustion process, high temporal and spatial resolution optical techniques were applied in the optically accessible chamber of diesel engine, at 2000 rpm and A/F=80:1 and 60:1. Simultaneous extinction, scattering and flame chemiluminescence measurements from UV to visible were carried out, in order to study the diesel combustion process from the droplet ignition to the formation of soot, through the growth of its precursors.

Ignition delay in diesel engine combustion comprehends both a chemical and a physical amount, the first depending on fuel composition and charge temperature and pressure, the last resulting of time needed for the fuel to atomize, vaporize and mix with air. Control of this parameter, which is mandatory to weight the relative amount of premixed to diffusive stage of the hydrocarbon combustion, is here considered. Experimental measurements of flame intensity spectra obtained by in situ measurements on an optically accessible test device show the presence of peaks corresponding to radicals as OH and CH appearing at the pressure start of combustion. Since OH radicals result from chain branching reactions, a numerical simulation is performed based on a reduced kinetic scheme which allows to measure the branching agent concentration, and whose approximate nature is adequate to the proportion chemical aspects contribute to the overall delay.

This paper deals with some recent advances in the field of 1D fluid dynamic modeling of unsteady reacting flows in complex s.i. engine pipe-systems, involving a catalytic converter. In particular, a numerical simulation code has been developed to allow the simulation of chemical reactions occurring in the catalyst, in order to predict the chemical specie concentration in the exhaust gas from the cylinder to the tailpipe outlet, passing through the catalytic converter. The composition of the exhaust gas, discharged by the cylinder and then flowing towards the converter, is calculated by means of a thermodynamic two-zone combustion model, including emission sub-models. The catalytic converter can be simulated by means of a 1D fluid dynamic and chemical approach, considering the laminar flow in each tiny channel of the substrate.

The paper describes the 1-D fluid dynamic modeling of unsteady flows with chemical specie tracking in the ducts of a four-cylinder s.i. automotive engine, to predict the composition of the exhaust gas reaching the catalyst inlet. A comprehensive simulation model, based on classical and innovative numerical techniques for the solution of the governing equations, has been developed. The non-traditional shock-capturing CE-SE (Conservation Element-Solution Element) method has been extended to deal with the propagation of chemical species. A comparison of the MacCormack method plus FCT or TVD algorithms with the CE-SE method has pointed out the superiority of the latter scheme in the propagation of contact discontinuities. A realistic composition of the exhaust products in the cylinder, evaluated by a two-zone combustion model including emission sub-models, has been imposed at the opening of the exhaust valve, considering the effect of short-circuit of air during valve overlap.

The influence of fuel composition on mixture formation and first stage of combustion, occurring in a small high swirl combustion chamber of an IDI Diesel engine, was analyzed from measurements of spectral extinction and flame emissivity. Measurements were carried out in an optically accessible combustion chamber in which an air swirling flow is forced from the main chamber through a tangential passage. A conventional injection system was used to inject Tetradecane, N-heptane and Diesel fuel. The distribution of liquid and vapor and the interaction of the jet with air swirl were detected by UV-visible extinction measurements. The autoignition phase was characterized by UV-visible chemiluminescence measurements. For all fuels examined, it was observed that initially the liquid fuel penetrates almost linearly with time until reaching a maximum characteristic length, slightly dependent on the fuel.

The characterization of a turbulent diesel spray combustion process has been carried out in a divided chamber diesel system with optical accesses. Laser Doppler Anemometry, spectral extinction and flame intensity measurements have been performed from U.V., to visible from the start of injection to the end of combustion, at fixed air/fuel ratio and different engine speeds. Spatial distribution of fuel and vapor as well as the ignition location and soot distribution have been derived in order to study the mechanism of the air-fuel interaction and the combustion process. The analysis of results has shown that the high swirling motion transports the fuel towards the left part of the chamber and breaks up the jet into small droplets of different sizes and accelerates the fuel vaporization. Then, chemical and physical overlapped phases were observed during the ignition delay, contributing both to autoignition.