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The paper presents a combined experimental and numerical investigation of a small unit displacement two-stroke SI engine operated with gasoline and Natural Gas (CNG). A detailed multi-cycle 3D-CFD analysis of the scavenging process is at first performed in order to accurately characterize the engine behavior in terms of scavenging patterns and efficiency. Detailed CFD analyses are used to accurately model the complex set of physical and chemical processes and to properly estimate the fluid-dynamic behavior of the engine, where boundary conditions are provided by a in-house developed 1D model of the whole engine. It is in fact widely recognized that for two-stroke crankcase scavenged, carbureted engines the scavenging patterns (fuel short-circuiting, residual gas distribution, pointwise lambda field, etc.) plays a fundamental role on both of engine performance and tailpipe emissions.

The paper presents a 1D-3D numerical model to simulate the scavenging and combustion processes in a small-size spark-ignition two-stroke engine. The engine is crankcase scavenged and can be operated with both gasoline and Natural Gas (NG). The analysis is performed with a modified version of the KIVA3V code, coupled to an in-house developed 1D model. A time-step based, two-way coupled procedure is fully described and validated against a reference test. Then, a 1D-3D simulation of the whole two-stroke engine is carried out in different operating conditions, for both gasoline and NG fuelling. Results are compared with experimental data including instantaneous pressure signals in the crankcase, in the cylinder and in the exhaust pipe. The procedure allows to characterize the scavenging process and quantify the fresh mixture short-circuiting, as well as to analyze the development of the NG combustion process for a diluted mixture, typically occurring in a two-stroke engine.

CFD techniques are more and more utilized in the development of new solutions for performance improvement of internal combustion engines. Three dimensional models, in general, are able to provide detailed and sound information on engine phenomena, but often they are time consuming and hard to be implemented. On the other hand, one-dimensional models can reproduce the entire engine cycle with acceptable computational times; however they need semi-empirical correlations in order to model the flow field details and the burning speed within each cylinder. In this paper, an example of hierarchical structure of 3-D and 1-D models has been proposed. The main performances of a small turbocharged spark-ignition engine have been calculated. Variable-speed and full load operating points have been analyzed. The 3-D model provided the details of the in-cylinder flow field and turbulent indices.

The paper describes preliminary results of a research activity finalized to the development of a new scavenging concept for the reduction of the HC emitted by a small-size two-stroke carbureted crankcase-scavenged SI engine. Further developments of a well-established model (1Dime code) are presented, with particular emphasis on combustion and scavenging processes simulation. The rate of heat release is computed through a two-zone model, based on a “fractal” representation of the turbulent flame front. A CAD procedure evaluating, at each crank-angle and flame radius, the intersections between the flame surface and the actual combustion chamber walls, has been developed. Scavenging is modeled through an original two-zone approach which accounts for mixing and short-circuiting processes. The latter are directly related to the in-cylinder turbulent flow regime, inlet and exhaust flow velocities, and engine speed.

The paper reports the research activity related to the development of a twin-spark SI engine equipped with a variable valve timing (VVT) device. Improvements on the fuel consumption at part load are expected when an high internal exhaust gas recirculation (internal EGR) level is realized with a proper phasing of the VVT device. The twin-spark solution is implemented to improve the burning speed at low load, and to increase the EGR tolerance levels. Both experimental and theoretical analyses are carried out to investigate the real advantages of the proposed engine architecture. In particular an original quasi-dimensional model for the simulation of the burning process in a twin-spark engine is presented. The model is mainly utilized to find the proper combination of VVT device position (and hence EGR level) and spark advance for different engine operating conditions. A comparison with the single-spark solution is also provided.

The paper reports the research activity related to the development of a High Exhaust Gas Recirculation (EGR) Spark-Ignition (SI), 8 valve engine equipped with a variable valve timing (VVT) device. The latter imposes an equal phase displacement on both intake and exhaust camshafts (dual dependent cam phaser). Both experimental and theoretical analyses are carried out to characterize the performance of this engine architecture, and particularly to analyze the combustion process arising at low load and high EGR conditions. To this aim, a quasi-dimensional model for the simulation of the burning process is included as an external user-defined routine in a commercial 1D simulation code (GT-Power®). The whole model is validated at both wide open throttle (WOT) conditions and part-load, and then it is mainly utilized to find, by means of a parametric analysis, the lowest fuel consumption at low load.

Recently, a tendency is consolidating to produce low displacement turbocharged spark-ignition engines. This design philosophy, known as “engine downsizing”, allows to reduce mechanical and pumping losses at low load as a consequence of the higher operating Brake Mean Effective Pressure (BMEP). The presence of the turbocharger allows to restore the maximum power output of the larger displacement engine. Additional advantages are a higher low-speed torque and hence a better drivability and fun-to-drive. Of course, at high loads, the spark-advance must be carefully controlled to avoid the knock occurrence and this determines a substantial penalization of the fuel consumption. The knowledge of the knock-limited spark timing is hence a key point in order to reduce the fuel consumption drop at high loads.

The paper reports the research activity related to the development of a “downsized” turbocharged Spark-Ignition (SI) engine. Both experimental and theoretical analyses are carried out to characterize the performance of this engine architecture, and particularly to analyze the matching conditions with the turbocharger and the combustion process at wide-open-throttle conditions. To this aim, a quasi-dimensional model for the simulation of the burning process is included as an external user-defined routine in a commercial 1D simulation code (GT-Power®). The rate of heat release is computed through a two-zone model, based on a “fractal” representation of the turbulent flame front. A turbulence sub-model is included and it is properly tuned with respect to turbulence results computed by a 3D CFD code. A CAD procedure evaluating, at each crank-angle and flame radius, the intersections between the flame surface and the actual combustion chamber walls, is also presented.

A theoretical-experimental analysis of a VVT engine and a methodology for the definition of its optimal control is presented. The analyses are based on the employment of a very accurate 1D simulation model of the engine, developed by the authors. The code is validated by comparison with experimental data collected on a traditionally fixed- and a variable-valve timing engine as well. The model is then linked to an efficient optimization procedure, which is able to select - for each assigned operating condition - the most appropriate values of control parameters (spark advance, intake/exhaust valve opening/closing, and valve lift), with the objective of pursuing part-load BSFC improvements. Various VVT or VVA arrangements are analyzed and compared.

An experimentally derived map of the engine volumetric efficiency is usually employed to control the A/F ratio in a SI-ICE. In the case of a variable valve timing (VVT) engine, a different efficiency map must be considered at each camshaft position, as a consequence of the influence on the air flow exerted by the actual position of the intake/exhaust camshaft. In this paper, an attempt is reported to theoretically derive a correlation of the volumetric efficiency as a function of engine speed, manifold absolute pressure, and camshaft position. The correlation is not based on experimental data but on the results of a one-dimensional simulation model (1Dime code) developed at DIME. An extensive validation of the 1D model is preliminary reported in the first part of the paper. The procedure is developed with reference to a four-cylinder, SI engine, equipped with a phased intake and exhaust VVT device.

An experimental and numerical work has been performed to characterize the performance of a medium-size spark-ignition engine and the related gas-dynamic noise emitted at the intake mouth. The noise attenuation of the main component of the intake system, namely the air flow box, has been experimentally measured and compared to the numerical results obtained using a tri-dimensional code. Then, the 3D-CFD code has been used to improve the noise attenuation of the above component through the introduction of a Helmholtz and a column resonator along the inflow pipe. Both the base and the modified air box have been coupled to the engine, installed inside a vehicle. An experimental analysis has been carried out to measure the engine performance and the gasdynamic noise at the intake. Some comparisons have been then reported with the numerical results derived from a one-dimensional analysis of the whole engine.