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In order to contribute to the solution of controlling the auto-ignition in a Homogeneous Charge Compression Ignition (HCCI) engine, parameters linked to External Gas Recirculation (EGR) seem to be of particular interest. Experiments performed with EGR present some difficulties in interpreting results using only the diluting and thermal aspect of EGR. Lately, the chemical aspect of EGR is taken more into consideration, because this aspect causes a complex interaction with the dilution and thermal aspects of EGR. This paper studies the influence of EGR on the auto-ignition process and particularly the chemical aspect of EGR.The diluents present in EGR are simulated by N 2 and CO 2 , with dilution factors going from 0 to 46 vol%. For the chemically active species that could be present in EGR, the species CO, The fuels used for the auto-ignition are n-heptane and PRF40. It appeared that CO, in the investigated domain, did not influence the ignition delays, while NO had two different effects.At concentrations up until 45 ppm, NO advanced the ignition delays for the PRF40 and at higher concentrations, the ignition delayed. The influence of NO on the auto-ignition of nheptane seemed to be insignificant, probably due to the higher burn rate of n-heptane. CH 2 O seemed to delay the ignition. The results suggested that especially the formation of OH radicals or their consumption by the chemical additives determine how the reactivity of the auto-ignition changed.

A snapshot proper orthogonal decomposition (POD) is performed from 2D time-resolved PIV measurements obtained in the tumble plane of a Spark Ignition engine flow. Based on this filtering approach, the in-cylinder flow field is decomposed into a mean part, a coherent part and a turbulent incoherent part. The analysis of the one-point statistical moments of orders 3 and 4 (skewness and flatness coefficients) as well as the analysis of the probability density function of the velocity field show that the POD extracts an incoherent random velocity field having Gaussian distribution properties. These small amplitude background fluctuations are also homogeneous. We then demonstrate the ability of the POD procedure, based on an energy criterion, in separating the coherent part and random Gaussian fluctuations. Based on this POD flow partitioning, providing a new triple decomposition of the instantaneous turbulent flow, the flow engine cycle-to-cycle variations associated with each velocity field contribution are then accessed. Thus, one of the first full field quantitative analyses of the cyclic variabilities associated with each part of the in-cylinder flow field is provided from the analysis of the corresponding POD temporal coefficients.

A Laser Extinction Method has been set up to provide two-dimensional soot volume fraction field time history at a tunable frequency up to 70 Hz inside an axis-symmetric diffusion flame experiencing slow unsteady phenomena preserving the symmetry. The use of a continuous wave laser as the light source enables this repetition rate, which is an incremental advance in the laser extinction technique. The technique is shown to allow a fine description of the soot volume fraction field in a flickering flame exhibiting a 12.6 Hz flickering phenomenon. Within this range of repetition rate, the technique and its subsequent post-processing require neither any method for time-domain reconstruction nor any correction for energy intrusion. Possibly complemented by such a reconstruction method, the technique should support further soot volume fraction database in oscillating flames that exhibit characteristic times relevant to the current efforts in the validation of soot processes modeling.

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The role of different combustion modes that govern the combustion propagation inside a rapid compression machine is discussed. Aiming at the control of the compression generated turbulence, the careful design of the UPMC-RCM is described. A methodology is then proposed to investigate the influence of the residual post-compression turbulence level on the visible combustion propagation process. Through the fuel type, the main parameter varied is the ratio of the ignition delay time to the characteristic decay time for the post-compression turbulence. A highspeed camera images the visible combustion. Particle Image Velocimetry provides two-dimensional velocity fields of the post-compression flow prior to ignition. The residual turbulence level is shown to influence the combustion propagation phenomenology, highlighting the coexistence of both volumetric and frontlike combustion modes for the shorter aforementioned ratios. This conclusion could contribute to an explanation of the experimental discrepancies among the ignition delays measured in different RCMs as the residual turbulence level is highly set-up dependent.

This paper aims to investigate cycle-to-cycle variations of non-reacting flow inside a motored singlecylinder transparent engine in order to judge the insertion amplitude of a control device able to displace linearly inside the inlet pipe. Three positions corresponding to three insertion amplitudes are implemented to modify the main aerodynamic properties from one cycle to the next. Numerous particle image velocimetry (PIV) two-dimensional velocity fields following cycle database are posttreated to discriminate specific contributions of the fluctuating flow. We performed a multiple snapshot proper orthogonal decomposition (POD) in the tumble plane of a pent roof SI engine. The analytical process consists of a triple decomposition for each instantaneous velocity field into three distinctive parts named mean part, coherent part and turbulent part. The 3rd-and 4th-centered statistical moments of the proper orthogonal decomposition (POD)-filtered velocity field as well as the probability density function of the PIV realizations proved that the POD extracts different behaviors of the flow. Especially, the cyclic variability is assumed to be contained essentially in the coherent part. Thus, the cycle-to-cycle variations of the engine flows might be provided from the corresponding POD temporal coefficients. It has been shown that the in-cylinder aerodynamic dispersions can be adapted and monitored by controlling the insertion depth of the control instrument inside the inlet pipe.

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