Search Results

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 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.

Optimized valve timing, according to engine load, may lead to significant improvements in pumping losses and internal EGR generation. Thus, VVT technology constitutes an effective way to reduce both fuel consumption and pollutant emissions. In this paper, the behavior of a small displacement, 2 valve, Spark-ignition engine, with variable valve timing, has been numerically and experimentally analyzed. The use of VVT allows obtaining combined internal EGR and Reverse Miller Cycle effects so to achieve a significant dethrottling at part load operation. High EGR rates require high turbulence intensity in order to accelerate the combustion rate. The engine performs an accurate combustion chamber design and a tangential intake port able to generate optimized swirl motion, according to the engine speed and load, during both the exhaust gas re-aspiration and the intake stroke. Engine performances at different cam phaser positions have been calculated by means of a 3-D computer code.

A small spark-ignition engine, in wide spread commercial usage since numerous years, is at present under study with the aim of improving its performance, in terms of a reduction of both fuel consumption and pollutant emissions. In previous papers, the influence of piston geometry [1] and intake system [2] on the combustion process has been evaluated by means of a 3-D computational model. In this paper, a more extensive analysis of the parameters affecting the combustion rate, hence thermal efficiency, pollutant formation and engine stability, has been carried out. In particular, at ELASIS Research Center, three prototypes featuring different combustion chambers have been realized and analyzed to the aim of assessing the influence of the squish area percentage on the flame front propagating in a quiescent charge. Furthermore, the AVL FIRE computer code has been utilized in order to simulate the engine behavior at full load operation.

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.

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.

Variable Valve Timing (VVT) technology is more and more adopted in modern spark-ignition engines for the optimization of torque delivery. Furthermore, a proper choice of valve timing could reduce the typical pumping losses of these engines thus improving fuel economy at part load. VVT mainly influences gas exchange processes, then the engine volumetric efficiency; in some circumstances, variations of valve timing could modify the charge composition and therefore the flame development and propagation. In this paper, the combustion process of a small displacement, 2 valve, spark-ignition engine, with variable valve timing, has been numerically and experimentally analyzed. The use of VVT allows obtaining combined internal EGR and Reverse Miller Cycle effects so to achieve a significant dethrottling at part load operation. A 3-D computer code has been utilized in order to calculate the details of the flow field within the cylinder and the combustion rate at different valve points.

During recent years several VVT devices have been developed, in order to improve either peak power and low end torque, or part load fuel consumption of SI engines. This paper describes an experimental activity, concerning the integration of a continuously variable cam phaser (CVCP), together with an intake port deactivation device, on a small 4 cylinder 16V engine. The target was to achieve significantly lower fuel consumption under normal driving conditions, compared to a standard MPFI application. A single hydraulic cam phaser is used to shift both the intake and the exhaust cams to retarded positions, at constant overlap. Thus, high EGR rates in the combustion chamber and late intake valve closure (“reverse Miller cycle”) are combined, in order to reduce pumping losses at part load.