Fabrice Foucher

Effect of Dilution by Nitrogen and/or Carbon Dioxide on Methane and Iso-Octane Air Flames

The impact of dilution on laminar burning speed of two different fuels (methane and isooctane) is studied. In the present study, three different diluents are used—nitrogen, carbon dioxide, and a mixture representative of exhaust gases issued from a stoichiometric combustion of methane. Experimental results and PREMIX computations of the CHEMKIN package, using two different kinetic schemes, are presented and compared with literature results, when available. Initial pressure and temperature conditions are respectively 0.1 MPa and 300 K. For both fuels, a larger decrease of the laminar burning speed is obtained for carbon dioxide dilution than for nitrogen dilution. This observation is directly linked to the increase in heat capacity of the dilution gas but also to the carbon dioxide dissociation, even if the heat capacity effect seems to be predominant.

Combustion and Emissions Characteristics of Valeric Biofuels in a Compression Ignition Engine

AbstractNew-generation biofuels are mainly produced from nonfood crops or waste. Although second-generation ethanol is one of the main options, valeric esters can also be produced from lignocellulose through levulinic acid. However, only few experimental results are available to characterize their combustion behavior. Using a traditional compression ignition (CI) engine converted to monocylinder operation, the engine performances and emissions of butyl and pentyl valerate (BV and PenV, respectively) were investigated. This paper analyses the experimental results for blends of 20%vol of esters in diesel fuel, taking diesel fuel as the reference fuel. The BV and PenV have a smaller cetane number and consequently the ignition delay of the blends is slightly longer. However, engine performances and emissions are not significantly modified by adding 20%vol of esters to diesel fuel. The BV and PenV then represent very good alternative biofuels for CI engines.

Flame wall interaction: effect of stretch

An experimental study of flame-wall interaction is presented. The flame is a laminar, flat methane-air one at three different equivalent ratio: 1, 0.8, 0.6. Temporal PIV system was used to acquire images sequences of the interaction. Evolution of velocity field and flame contour shows that stagnation points appear between the reactive zone and the wall. Even if the maximum velocity value is relatively low, fresh gases motion amplifies wrinkle perturbation of the flame front. To explore more precisely the curvature evolution and its repercussion on the stretch rate, the stretch rate was calculated by determining local strain rate and curvature stretch term. Evolution of the stretch rate versus the time shows that the quenching stretch rate can be reached and locally, the extinction of the flame can appear for the equivalent ratio near the stoichiometry. We show that the interaction between a flame and a wall is controlled and disturbed by the modification of the velocity field of the fresh gases trapped between the reactive zone and the wall.

Exhaust gas recirculation stratification to control diesel homogeneous charge compression ignition combustion

A single-cylinder diesel engine was used to investigate the potential of exhaust gas recirculation dilution stratification as a control technique for homogeneous charge compression ignition combustion with early direct injections. Experimental studies on both all-metal and optically accessible engines were performed to understand the processes involved when exhaust gas recirculation is introduced separately in the intake ports. Laser-induced fluorescence diagnostics were carried out in the optical engine in order to provide fuel and exhaust gas recirculation distributions. The results indicate that depending on the intake configuration, the exhaust gas recirculation stratification can be maintained until late timings corresponding to the combustion event, leading to decreased maxima of heat-release rates, as well as decreased combustion noise levels. This result suggests that exhaust gas recirculation stratification may be used as a control parameter for combustion speed and therefore may contribute to the extension of the homogeneous charge compression ignition operating range. However, although exhaust gas recirculation stratification appears to be an interesting new control technique for homogeneous charge compression ignition combustion, its effect on the combustion was shown to be very sensitive to parameters such as the intake system configuration or the exhaust gas recirculation composition, showing that industrial use of this control technique requires further understanding of the physical phenomena involved.

Evaluation of burning rates in the vicinity of the piston in a spark-ignition engine

The burning rate in the vicinity of the piston is estimated from the flamlet, formulation. The flame surface density Σ is measured by laser sheet tomography for three equivalence ratio methane/air flames (1.0.9, 0.8) in a spark-ignition transparent engine. Two imaging configurations are realized: five horizontal planes at different distances from the piston (0, 1, 2, 3, and 5 mm) and a vertical one at the center of the combustion chamber. To decrease the effect of the cycle-to-cycle variation on the average flame front thickness and on the determination of the mean progress variable, several methods are tested to extract the mean flame front contour. Two flame surface density methods are compared: one based on the Bray-Moss-Libby flamelet crossing density model and the other based on the burning rate formulation by Gouldin. It appears that the last one is better adapted for the determination of Σ for weakly wrinkled flames and a good compromise to estimate the burning rate locally in vicinity of the piston. All methods are also validated by applying to freely propagating spherical flames. Model adaptation is realized to include the evolution of the flamelet scalar orientation near the piston. Finally, the burning rate is determined as a function of the distance between the wall and the mean flame contour.

Engine Performances and Emissions of Second-Generation Biofuels in Spark Ignition Engines: The Case of Methyl and Ethyl Valerates

As an alternative to second generation ethanol, valeric esters can be produced from lignocellulose through levulinic acid. While some data on these fuels are available, only few experiments have been performed to analyze their combustion characteristics under engine conditions. Using a traditional spark ignition engine converted to mono-cylinder operation, we have investigated the engine performances and emissions of methyl and ethyl valerates. This paper compares the experimental results for pure valeric esters and for blends of 20% of esters in PRF95, with PRF95 as the reference fuel. The esters propagate faster than PRF95 which requires a slight change of ignition timing to optimise the work output. However, both the performances and the emissions are not significantly changed compared to the reference. Accordingly, methyl and ethyl valerate represent very good alternatives as biofuels for SI engines. Future studies will focus on testing these esters in real application engines and performing endurance tests.

Towards HCCI Control by Ozone Seeding

Nowadays, the main objectives in the automobile engine field are to reduce fuel consumption and pollutant emissions. HCCI engines can be a good solution to meet pollutant emission requirements and to achieve high combustion efficiency. However, before an HCCI engine is used as a conventional engine, several problems must be overcome, in particular control of the progression of combustion. Many studies have been conducted into possible control methods. A new strategy consists in using oxidizing chemical species such as ozone to seed the intake of a HCCI engine. As increasingly smaller ozonizers are now being designed, this kind of device could be integrated on a vehicle and on a HCCI engine, in order to control combustion phasing and promote the future use of this engine as a conventional engine. In the present study, experiments on a HCCI engine fuelled with iso-octane were carried out with ozone seeding in the intake. Results showed that when assisted by the addition of ozone, combustion can be enhanced and moved forward. Consequently, the use of oxidizing chemical species can be a means to control HCCI combustion. Depending on the inlet temperature, the control of combustion phasing may be more or less easy due to sensitivity to the ozone concentration. The present results also show the existence of a cool flame in the case of iso-octane combustion, indicating that ozone seeding can also be used in order to study iso-octane cool flame in a HCCI engine.

Separation between coherent and turbulent fluctuations: what can we learn from the empirical mode decomposition?

The performances of a new data processing technique, namely the empirical mode decomposition, are evaluated on a fully developed turbulent velocity signal perturbed by a numerical forcing which mimics a long-period flapping. First, we introduce a “resemblance” criterion to discriminate between the polluted and the unpolluted modes extracted from the perturbed velocity signal by means of the empirical mode decomposition algorithm. A rejection procedure, playing, somehow, the role of a high-pass filter, is then designed in order to infer the original velocity signal from the perturbed one. The quality of this recovering procedure is extensively evaluated in the case of a single tone perturbation (sine wave) by varying both the amplitude and the frequency of the perturbation. An excellent agreement between the recovered and the reference velocity signals is found, even though some discrepancies are observed when the perturbation frequency overlaps the frequency range corresponding to the energy-containing eddies as emphasized by both the energy spectrum and the structure functions. Finally, our recovering procedure is successfully performed on a non-stationary perturbation (linear chirp) covering a broad range of frequencies.

ESTIMATE MEASUREMENT OF SOOT DIAMETER AND VOLUME FRACTION INSIDE THE BOWL OF A DIRECT-INJECTION-COMPRESSION-IGNITION ENGINE: EFFECT OF THE EXHAUST GAS RECIRCULATION

Abstract An original application of the Laser-Induced-Incandescence (LII) technique was set up to quantify soot particles inside the combustion chamber of an optically accessible Direct-Injection Diesel engine. Planar soot concentration and local particle diameter were measured for several Exhaust Gas Recirculation (EGR) rates. The impact of the injection timing on the soot evolution for the highest EGR rate was also studied. Based on the analysis of LII images it is shown that the planar distribution of soot becomes more and more uniform across the combustion chamber and globally the soot maximum more important with the EGR rates increase. High EGR rates, combined with a retarded start of injection may lead to lower soot production inside the combustion chamber. Comparison between exhaust and in-cylinder soot concentration highlights the effect of post-combustion oxidation on the particle-emissions amount.

Flame Speeds of α-Pinene/Air and Limonene/Air Mixtures Involved in Accelerating Forest Fires

Several researchers have reported that under certain conditions, forest fires with normal behavior suddenly start to propagate at unusual and very high rate of spread. Over the last decades, these accelerating forest fires were responsible for many fatalities in Europe. A thermochemical approach, based on the ignition of a volatile organic compounds (VOCs) cloud, has been proposed previously to explain this phenomenon. Indeed, some vegetal species emit volatile substances when they are heated. A typical Mediterranean plant, Rosmarinus officinalis, emits 14 components, mainly α-pinene and limonene. The acceleration of the rate of spread can be the consequence of the ignition of these emitted gases. The determination of α-pinene/air and limonene/air premixed flame speeds is essential to take into account this approach in forest fire modeling. It is the main purpose of this article. The spherical expanding flames methodology coupled with a nonlinear model was used to determine the unstretched premixed flame speeds of the two major compounds involved in accelerating forest fires. Experiments were performed in a spherical vessel at atmospheric pressure. The results for different equivalence ratios and unburned gas temperatures are given for the first time. The unstretched premixed flame speeds are discussed relatively to rates of spread of three real accidents.