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This paper presents a methodology for the 3D CFD simulation of the intake and compression processes of four stroke internal combustion engines.The main feature of this approach is to provide very accurate initial conditions by means of a cost-effective initialization step. Calculations are applied to a low stroke-to-bore SI engine, operated at full load and maximum engine speed. It is demonstrated that initial conditions for this kind of engines have an important influence on flow field development, particularly in terms of mean velocities close to the firing TDC. Simulation results are used to discuss the choice of a set of parameters for the flow field characterization of low stroke-to-bore engines, as well as to provide an insight into the flow patterns during the overlapping period.

The Department of Mechanical and Civil Engineering (DIMeC) of the University of Modena and Reggio is developing a new concept of small capacity HSDI 2-Stroke Diesel engine, featuring a specifically designed combustion system. The paper reviews the 2-Stroke engine design process, supported by CFD simulations, both 1D and multi-dimensional. A four stroke automobile Diesel engine is taken as a reference for a theoretical comparison in terms of brake performance at both full and partial load. This comparison shows the potential of the 2-Stroke, as an ultra-compact, efficient and clean engine.

The supercharging system investigated in this study is made up of a traditional turbocharger, coupled with a Roots-type positive displacement compressor. An electrically actuated clutch allows the compressor to be disengaged from the engine at high speed and under partial load steady operations (such as the ones occurring in a driving cycle). This concept of supercharging has been applied to the downsizing of a reference engine (a 2.5 litre, turbocharged, four cylinder, high speed DI Diesel engine), without penalization on the maximum brake power (110 kW) and transient response. For such a purpose, a “paper” engine has been theoretically characterized. The gross engine parameters have been optimised by means of 1-D numerical simulations, using a computational model previously validated against experiments. Performances of the reference and the downsized engine have been compared, considering both steady and transient operating conditions, full and partial load.

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC.

Engine knock is emerging as the main limiting factor for modern spark-ignition (SI) engines, facing increasing thermal loads and seeking demanding efficiency targets. To fulfill these requirements, the engine operating point must be moved as close as possible to the onset of abnormal combustion events. The turbulent regime characterizing in-cylinder flows and SI combustion leads to serious fluctuations between consecutive engine cycles. This forces the engine designer to further distance the target condition from its theoretical optimum, in order to prevent abnormal combustion to severely damage the engine components just because of few individual heavy-knocking cycles. A RANS-based model is presented in this study, which is able to predict not only the ensemble average knock occurrence but also a knock probability. This improves the knock tendency characterization, since the mean knock onset alone is a poorly meaningful indication in a stochastic event such as engine knock.

The simulation of combustion chemistry in internal combustion engines is challenging due to the need to include detailed reaction mechanisms to describe the engine physics. Computational times needed for coupling full chemistry to CFD simulations are still too computationally demanding, even when distributed computer systems are exploited. For these reasons the present paper proposes a time scale separation approach for the integration of the chemistry differential equations and applies it in an engine CFD code. The time scale separation is achieved through the estimation of a characteristic time for each of the species and the introduction of a sampling timestep, wherein the chemistry is subcycled during the overall integration. This allows explicit integration of the system to be carried out, and the step size is governed by tolerance requirements.

The paper reports the application of Proper Orthogonal Decomposition (POD) to LES calculations for the analysis of combustion and knock tendency in a highly downsized turbocharged GDI engine that is currently under production. In order to qualitatively match the cyclic variability of the combustion process, Large-Eddy Simulation (LES) of the closed-valve portion of the cycle is used with cycle-dependent initial conditions from a previous multi-cycle analysis [1, 2, 3]. Detailed chemical modelling of fuel's auto-ignition quality is considered through an ad-hoc implemented look-up table approach, as a trade-off between the need for a reasonable representation of the chemistry and that of limiting the computational cost of the LES simulations. Experimental tests were conducted operating the engine at knock-limited spark advance (KLSA) and the proposed knock model was previously validated for such engine setup [3].

The paper aims at defining a methodology for the prediction and understanding of knock tendency in internal combustion engine piston crevices by means of CFD simulations. The motivation for the analysis comes from a real design requirement which appeared during the development of a new high performance SI unit: it is in fact widely known that, in high performance engines (especially the turbocharged ones), the high values of pressure and temperature inside the combustion chamber during the engine cycle may cause knocking phenomena. “Standard” knock can be easily recognized by direct observation of the in-cylinder measured pressure trace; it is then possible to undertake proper actions and implement design and control improvements to prevent it with relatively standard 3D-CFD analyses.

The paper reports a numerical activity on the investigation of the spray evolution within the combustion chamber of an automotive DISI engine under low-temperature cranking operations. In view of the high injected fuel amount and the strongly reduced fuel vaporization at cold cranking, wall wetting becomes a critical issue. Under such conditions, fuel deposits around the spark plug region can affect the ignition process, and even prevent engine start-up. In fact, due to the low injection pressure at engine start-up, the fuel shows almost negligible atomization and breakup, and the spray structure at the swirl-type injector nozzle is characterized by a single column of liquid fuel, strongly limiting the subsequent vaporization and enhancing the fuel-wall interaction. In order to properly investigate and understand the many involved phenomena, experimental visualization of the full injection process by means of an optically accessible engine would be a very useful tool.

This paper compares different engine solutions for the FIM MotoGP World Championship. Starting from the general guidelines given in a previous paper [2], in this study the specific features of each engine architecture (3 and 4 in line, V4, V5 and V6) are considered. 1-D engine simulations, based on a previously validated model, are extensively used to optimize each solution, as well as to provide a comparison among the engines in terms of dynamometer performances. Some issues concerning engine balance, engine overall dimensions, intake and exhaust system lay-out are discussed. Finally, the influence of the engine on the bike acceleration is calculated by means of a simple simulation at the Mugello track. The comparison has shown slight differences among the proposed configurations. Globally, the V engines, with four and five cylinders, have resulted to be the best solutions.

The paper reports the application of a look-up table approach within a LES combustion modelling framework for the prediction of knock limit in a highly downsized turbocharged DISI engine. During experimental investigations at the engine test bed, high cycle-to-cycle variability was detected even for relatively stable peak power / full load operations of the engine, where knock onset severely limited the overall engine performance. In order to overcome the excessive computational cost of a direct chemical solution within a LES framework, the use of look-up tables for auto-ignition modelling perfectly fits with the strict mesh requirements of a LES simulation, with an acceptable approximation of the actual chemical kinetics. The model here presented is a totally stand-alone tool for autoignition analysis integrated with look-up table reading from detailed chemical kinetic schemes for gasoline.

The paper critically discusses Large-Eddy Simulation (LES) potential to investigate cycle-to-cycle variability (CCV) in internal combustion engines. Particularly, the full load/peak power engine speed operation of a high-performance turbocharged GDI unit, for which ample cycle-to-cycle fluctuations were observed during experimental investigations at the engine test bed, is analyzed through a multi-cycle approach covering 25 subsequent engine cycles. In order to assess the applicability of LES within the research and development industrial practice, a modeling framework with a limited impact on the computational cost of the simulations is set up, with particular reference to the extent of the computational domain, the computational grid size, the choice of boundary conditions and numerical sub-models [1, 2, 3].

The paper reports an activity aiming at characterizing cycle-to-cycle variability (CCV) of the spark-ignition (SI) process in a high performance engine. The numerical simulation of spark-ignition and of early flame kernel evolution are major challenges, mainly due to the time scales of the spark discharge process and to the reduced spatial scales of flame kernel. Typical mesh resolutions are insufficient to resolve the process and a dedicated treatment has to be provided at a subgrid level if the ignition process is to be properly modelled. The focus of this work is on the recent ISSIM-LES (Imposed Stretch Spark-Ignition Model) ignition model, which is based on an extension of the flame surface density (FSD) transport equation for a dedicated flame kernel treatment at subgrid scales. The FSD equation is solved immediately after spark discharge.

In this paper a detailed analysis focused on lumped parameters numerical modeling of a variable displacement vane pump for high speed internal combustion engine lubrication is presented and discussed. This particular volumetric unit is characterized by very extreme performance, both in terms of rotational speed, delivery pressure and displacement variation. First of all, a comprehensive description of the simulation environment properly tailored for the numerical modeling of the vane pump operation is introduced and all its geometric, kinematic and fluid-dynamic characteristics are described in depth. Then, the results coming from an exhaustive experimental campaign have been compared with simulations, finding a general good accordance that demonstrates the reliability of this numerical approach.

A new generation of highly downsized SI engines with specific power output around or above 150 HP/liter is emerging in the sport car market sector. Technologies such as high-boosting, direct injection and downsizing are adopted to increase power density and reduce fuel consumption. To counterbalance the increased risks of pre-ignition, knock or mega-knock, currently made turbocharged SI engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter induce an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A possible solution to increase knock resistance is investigated in the paper by means of 3D-CFD analyses: water/methanol emulsion is port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins.

The design of 4-stroke engines, complying with the new Motorcycle Road Racing World Championship regulations is discussed. Similarity rules and non dimensional parameters from a database on racing engines are used to define some general guidelines. More specific information about friction losses and combustion is derived from experiments, carried out on a 3-cylinder MotoGP prototype engine. These experiments provided the input needed to set up and validate a base model for 1D thermo-fluid-dynamic calculations. Engine simulation is employed for optimizing several design parameters. A comparison between the proposed methodology and a few design criteria presented in literature is made. Finally, the brake performances of some optimized engines are predicted.

The paper is focused on the influence of the eddy viscosity turbulence models (EVM) in CFD three-dimensional simulations of steady turbulent engine intake flows in order to assess their reliability in predicting the discharge coefficient. Results have been analyzed by means of the comparison with experimental measurements at the steady flow bench. High Reynolds linear and non-linear and RNG k-ε models have been used for simulation, revealing the strong influence of both the constitutive relation and the ε-equation formulation on the obtained results, while limits in the applicability of more sophisticated near-wall approaches are briefly discussed in the paper. Due to the extreme complexity of typical ICE flows and geometries, the analysis of the behavior of EVM turbulence models has been subsequently applied to a test-case available in literature, i.e. a high-Reynolds compressible flow over a inclined backward facing step (BFS).

The paper presents the development of a novel approach to the solution of detailed chemistry in internal combustion engine simulations, which relies on the analytical computation of the ordinary differential equations (ODE) system Jacobian matrix in sparse form. Arbitrary reaction behaviors in either Arrhenius, third-body or fall-off formulations can be considered, and thermodynamic gas-phase mixture properties are evaluated according to the well-established 7-coefficient JANAF polynomial form. The current work presents a full validation of the new chemistry solver when coupled to the KIVA-4 code, through modeling of a single cylinder Caterpillar 3401 heavy-duty engine, running in two-stage combustion mode.