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The current work is based on the computational design optimisation for a two-stroke, uniflow scavenging free piston engine prototype for use in series hybrid vehicles to reach high efficiency levels combined with low emissions. Here the performance goals have been attempted via Homogeneous Charge Compression Ignition (HCCI) Diesel combustion with increased levels of exhaust gas recirculation (EGR) and direct injection. For optimal mixture preparation and composition, a highly tuned gas exchange system is required and also adapted injection parameters are needed to induce heat release at around top dead center (TDC). A computational methodology has been developed based on iterations between zero-, one-dimensional and simplified Computational Fluid Dynamics (CFD) simulations to define the operating conditions and overall geometrical parameters which give the best engine performance.

The short time available for injection and mixing in high-speed engines requires an accurate modeling of the fuel related processes to obtain a valuable in-cylinder charge description, and then a good combustion performance prediction. An advanced version of the KMB code of IFP has been used to compute a racing engine. It includes a fitted on experiments spray model, a comprehensive wall-film model, the AKTIM ignition and ECFM combustion models. A major difficulty was the necessity to compute numerous cycles before reaching a cycle-independent solution. A procedure has been defined to minimize calculation time. Another difficulty was the high concentration of liquid in some zones, which requested a careful meshing. Effects such as the influence of the strong acoustic waves on the spray dynamic, the wall wetting effects on the engine time response, injector position on fuel distribution in the cylinder, charge homogeneity on the combustion process have been investigated.

A methodology has been developed to determine, for every cycle on which significant knock is detected, the area in which self-ignition occurs. This methodology is based on the exploitation by a dedicated algorithm of a minimum of 4 simultaneous combustion chamber pressure measurements. The algorithm has been first tested on the results of engine knocking simulation, then applied with success on a single-cylinder engine equipped with classical pressure transducers and with an instrumented cylinder head gasket developed for this application. The results obtained with these two kinds of transducers on several engine configurations and tunings are similar. If the timing and intensity of knock events depend on all engine parameters, its location is especially sensitive to such design parameters as fluid motion into the combustion chamber and spark plug position.

The present research focuses on the understanding and improved prediction of knock at full load in a four-cylinder passenger car spark-ignition (SI) engine using computational fluid dynamics (CFD) methodology. The emphasis is on the possibility of controlling the knock limit via optimised engine cooling mechanisms. To date, CFD simulations of the combustion chamber and cooling circuit are performed separately, while chamber wall temperatures are derived from either experiments or experience. This, however, entails the risk of employing inadequate boundary and hence in-cylinder conditions for a combustion and knock simulation. CFD simulations are performed for all four combustion chambers and metal components, including the cooling circuit. Both types of simulations are thermally coupled via the conditions on the chamber walls.

Currently, the development of higher specific output and higher efficiency S.I. engines requires better control and knowledge of knock mechanisms. As it is not easily possible to instrument an engine to determine the beginning of fuel auto-ignition, knock modeling by means of 3D CFD simulation, can be a powerful tool to understand and try to avoid this phenomenon [1, 2, 3]. The objectives of the work described in this paper are to develop and validate a simple model of auto-ignition. This model, developed at IFP, is implemented in the 3D CFD code KMB [4, 5]. It is based on an AnB model [6, 7] which creates a ‘precursor’ species transported with the flow in the combustion chamber. When its concentration reaches a limiting value, the auto-ignition phenomenon occurs.

Gasoline Direct Injection (GDI) is today more regarded as a suitable technology for relatively high displacement engines. The literature shows that the R&D effort on GDI engines is generally made for bores larger than 80 mm. But because GDI appears to be the most relevant way to improve fuel efficiency of S.I. engines, it should also be considered for small bore engines (bore below 75 mm). Nevertheless, locating an injector in already congested cylinder heads, with ultra lean stratified combustion capability while maintaining high engine specific power and proper cylinder head cooling is a real challenge. For such an engine, IFP “narrow spacing” proposal is a 3-valve per cylinder layout or NSDI-3 concept, with a spark-plug-close-to-the-injector design and a suitable piston to confine the fuel spray within the vicinity of the ignition location. This paper describes stage by stage the prototype engine realization using this novel concept.

Multidimensional computations are carried out to aid in the development of a direct injection Diesel engine. Intake, compression, injection and combustion processes are calculated for a turbo-charged direct injection Diesel engine with a single intake valve. The effects of engine speed and engine load, as well as the influence of exhaust gas recirculation are compared to experimental measurements. The influence of piston bowl shape is investigated. Three dimensional calculations are performed using a mesh built from the complete CAD definition of the engine, intake port, cylinder and piston bowl. The injection characteristics are found to be of primary importance in the control of the combustion process. At a given injection set, piston bowl shape can be optimized for fluid dynamic and combustion.