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Modelling small and large scale fluctuations of fuel distribution is of high interest for stratified direct injection spark ignition (DISI) engines. Homogeneous combustion models need to be extended or replaced in order to account for these fluctuations. They are presently neglected in most engine simulations. Effects of mean fuel/air equivalence ratio gradient have been recently included in previous homogeneous mixture approaches. To account for local fluctuations of mixture composition, the new model ECFM-Z has been developed on the basis of recent Direct Numerical Simulation results and Coherent Flame Surface modelling. The model has been implemented in a CFD code (KMB) The influence of mixture fraction is integrated in the Extended Coherent Flame Surface combustion model. The model is based on a conditional approach. Unburnt hydrocarbons produced by lean flame local extinctions are taken into account.

The purpose of the European « SPACE LIGHT » (Whole SPACE combustion for LIGHT duty diesel vehicles) 3-year project launched in 2001 is to research and develop an innovative Homogeneous internal mixture Charged Compression Ignition (HCCI) for passenger cars diesel engine where the combustion process can take place simultaneously in the whole SPACE of the combustion chamber while providing almost no NOx and particulates emissions. This paper presents the whole project with the main R&D tasks necessary to comply with the industrial and technical objectives of the project. The research approach adopted is briefly described. It is then followed by a detailed description of the most recent progress achieved during the tasks recently undertaken. The methodology adopted starts from the research study of the in-cylinder combustion specifications necessary to achieve HCCI combustion from experimental single cylinder engines testing in premixed charged conditions.

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.

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.

In order to determine the area where knock occurs in a single cylinder engine, an acoustic methodology needs a minimum of four simultaneous pressure measurements in the combustion chamber. A specific cylinder head gasket integrating 12 pressure sensors has been developed and tested. The gasket is based on a bonded multilayer technology including high temperature piezoelectric cells, metallic and insulating sheets and printed circuit films. The total thickness is close to 1.25 mm (1/20 inch) and allows a straight forward substitution of the original gasket without modification. The sensors have large frequency bandwidth (typically 3-100 kHz) and withstand severe conditions (heat, combustion, pressure, vibrations, static pre-stress, electromagnetic fields and shocks). Signal processing adaptation of the dedicated exploitation software has brought good success for the single cylinder prototype, which remains operational after 100 hours of extreme conditions running (high knock).

In the whole engine development process, 0D/1D simulation has become a powerful tool, from conception to final calibration. Within the context of control strategy design, a turbocharged spark ignition (SI) engine with variable camshaft timing has been modelled on the AMESim platform. This paper presents the different models and the methodology used to design, calibrate and validate the simulator. The validated engine model is then used for engine control purposes related to downsizing concept. Indeed, the presented control strategy acts on the in-cylinder trapped mass, the in-cylinder burnt gas fraction and the air scavenging from the intake to the exhaust. Consequently, it permits to reduce not only the fuel consumption and pollutant emissions but also to improve the transient response of the turbocharger

In the context of increasingly stringent pollution norms, reduced engine emissions are a great challenge for compressed ignition engines. After-treatment solutions are expensive and very complex to implement, while the NOx/PM trade-off is difficult to optimise for conventional Diesel engines. Therefore, in-cylinder pollutant production limitation by the HPC combustion mode (Highly Premixed Combustion) - including Homogeneous Charge Compression Ignition (HCCI) - represents one of the most promising ways for new generation of CI engine. For this combustion technology, control based on torque estimation is crucial: the objectives are to accurately control the cylinder-individual fuel injected mass and to adapt the fuel injection parameters to the in-cylinder conditions (fresh air and burned gas masses and temperature).

An experimental investigation into the structure and flame propagation characteristics of stratified and homogeneous combustion has been performed in an optically-accessible, direct-injection spark ignition (DISI) engine using OH planar laser-induced fluorescence (PLIF) imaging. Homogeneous and stratified operation was achieved by employing either early or late injection timing strategies during the intake or compression stroke respectively. Planar LIF OH images obtained revealed that for stratified operation, the 3D structure of the combustion zone is highly inhomogeneous and is predominantly due to high fuel concentration gradients which are formed as a result of local fuel mixture stratification. The images reveal a combustion structure which suggests that the flame propagation pathway is ultimately determined by the presence of these local fuel mixture inhomogeneities.

Three dimensional CFD tools are commonly used to simulate spray injection and combustion in DI Diesel engines. However typical computations are strongly mesh dependent. By now it is not possible to enhance grid resolution since it would violate the underlying assumptions for the Lagrangian liquid phase description. Besides, a full Eulerian approach with an adapted mesh is not practical at the moment mainly because of prohibitive computer requirements. Based on the Lagrangian-Eulerian approach, new approaches have been developed: the Coupled Lagrangian-Eulerian (CLE) model for the two-way coupling between the spray and the air flow and a new combustion model (CFM3Z) which allows a description of the fuel-oxidizer sub-grid mixing. The previously introduced CLE model consists in retaining vapor and momentum along parcel trajectories as long as the mesh is insufficient to resolve the steep gradients created by the spray.

The influence of piston geometry on the in-cylinder mixture distribution and combustion process in an optically-accessible, direct injection HCCI Diesel engine has been investigated. A new, purpose-designed piston which allows optical access directly into the combustion chamber bowl permitted the application of a number of optical diagnostic techniques. Firstly, laser-induced exciplex fluorescence (LIEF) has been applied in order to characterize the fuel spray and vapor development within the piston bowl. Subsequently a detailed study of the auto-ignition and two-stage Diesel HCCI combustion process has been conducted by a combination of direct chemiluminescence imaging, laser-induced fluorescence (LIF) of the intermediate species formaldehyde (CH2O) which is present during the cool flame and LIF of the OH radical later present in the reaction and burned gas zones at higher temperature.

In the context of modern engine control, one important variable is the individual Air Fuel Ratio (AFR) which is a good representation of the produced torque. It results from various inputs such as injected quantities, boost pressure, and the exhaust gas recirculation (EGR) rate. Further, for forthcoming HCCI engines and regeneration filters (Particulate filters, DeNOx), even slight AFR unbalance between the cylinders can have dramatic consequences and induce important noise, possible stall and higher emissions. Classically, in Spark Ignition engine, overall AFR is directly controlled with the injection system. In this approach, all cylinders share the same closed-loop input signal based on the single λ-sensor (normalized Fuel-Air Ratio measurement, it can be rewritten with AFR as they have the same injection set-point.

The purpose of the ELEVATE (European Low Emission V4 Automotive Two-stroke Engine) industrial research project is to develop a small, compact, light weight, high torque and highly efficient clean gasoline 2-stroke engine of 120 kW which could industrially replace the relatively big existing automotive spark ignition or diesel 4-stroke engine used in the top of the mid size or in the large size vehicles, including the minivan vehicles used for multi people and family transportation. This new gasoline direct injection engine concept is based on the combined implementation on a 4-stroke bottom end of several 2-stroke engine innovative technologies such as the IAPAC compressed air assisted direct fuel injection, the CAI (Controlled Auto-Ignition) combustion process, the D2SC (Dual Delivery Screw SuperCharger) for both low pressure engine scavenging and higher pressure IAPAC air assisted DI and the ETV (Exhaust charge Trapping Valve).

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.