Christine Mounaïm-Rousselle

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.

Droplet Sizing by Mie Scattering Interferometry in a Spark Ignition Engine

A theoretical explanation is given of a technique based on Mie scattering interferometry (MSI), obtained by defocusing of the collecting optics, to size droplets. The originality of this study is the development of a droplet sizing method by planar laser light scattering for the case of a scattering angle range close to 90°. The feasibility of this method and its limitations are fully described. The dependence on intensity levels and refractive index variations can be neglected. After discussion of some practical details about particle size, imaging and camera constraints, the results obtained in the combustion chamber of a spark ignition (SI) engine, near the spark plug, prior to ignition and for different injection timings are described and discussed. It can be concluded that the implementation of the MSI method in this experimental set-up has been realized successfully to provide droplet distributions in an SI engine. To allow the easier use of the technique, image processing software will be developed in the Matlab environment.

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.

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.

Comparison of Combustion Characteristics of Magnesium and Aluminum Powders

ABSTRACT This work presents an experimental study of the combustion characteristics of micron-sized aluminum and magnesium powders under constant volume combustion experiments. Burning velocities were estimated from the measured pressure traces using both a simplified model for combustion on closed spherical bombs and a semi-empirical correlation for dust explosions, and compared to previous literature. Flame temperatures were measured by bi-color pyrometry and indicate that, for aluminum powders with a mean particle diameter smaller than 12 μm, the flame moves closer to the particle’s surface. However, emission spectra obtained during combustion indicate that vapor-phase oxidation exists for all studied powders. Analysis of the combustion products further supported the presence of a vapor-phase reaction. For aluminum, the residue is composed by partially crystallized nanometric spheres as fine as 200 nm. MgO was found in crystallized cubic structures of different sizes, the finest ones also about 200 nm.

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.

Effects of hydrogen addition under lean and diluted conditions on combustion characteristics and emissions in a spark-ignition engine

Experimental investigations on the effects of hydrogen addition to spark-ignition (SI) engines running under lean and diluted conditions are presented in this paper. Experiments were carried out in a mono-cylinder SI engine with different rates of nitrogen dilution (0 to 20 per cent by volume in the total mixture) and hydrogen/iso-octane blends (from 0 to 80 per cent by volume in the fuel). The study of the impact of hydrogen addition on combustion characteristics and emissions was performed for two different engine speeds and loads. The equivalence ratio, the rate of dilution, and the intake pressure were varied either separately or simultaneously in order to maintain a constant engine load at a fixed hydrogen fraction in the fuel. The lean and dilution operating limits were also determined for all the iso-octane/hydrogen/air/nitrogen mixtures investigated, and results show that these limits are extended only when the hydrogen percentage in the fuel is higher than 40 per cent by volume. At a fixed engine load, hydrocarbon (HC) and CO emissions decrease with an increase in the hydrogen fraction in the intake mixture, while NO x emissions are mainly affected by the equivalence ratio and by the amount of dilution. Pumping losses, combustion efficiency and indicated efficiency are also improved with the addition of hydrogen. High values of indicated engine efficiency with low values of HC, NO x , and CO emissions can be achieved by combining hydrogen addition with lean and/or diluted conditions.

Combustion Characteristics of p-Cymene Possibly Involved in Accelerating Forest Fires

A potential implication of volatile organic compounds (VOCs) emitted by vegetal species has been introduced in the literature to explain accelerating forest fires. The main purpose of this article is to determine the combustion characteristics of a VOC emitted by Rosmarinus officinalis shrubs, namely p-cymene. The emission of this compound is studied for the temperature range 353–475 K, and an emission peak is found at 448 K. Laminar burning speeds, Markstein lengths, and flame thicknesses are determined using outwardly propagating spherical flames in a combustion chamber at atmospheric pressure. The effects of equivalence ratio (0.8–1.4) and unburned gas temperature (358–453 K) are studied. A correlation is proposed to estimate laminar burning speeds as a function of equivalence ratio and temperature. Due to the lack of data concerning the combustion characteristics of p-cymene, our results are compared to experimental data of heavy molecules such as ethylbenzene, iso-octane, and α-pinene, and to computed data of JP-10 and n-decane. The computed laminar burning speeds of these last two molecules are obtained using the PREMIX code of the CHEMKIN package.

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.

Thermodiffusive Effect on the Flame Development in Lean Burn Spark Ignition Engine

In Spark Ignition engines, the heat release rate is not only piloted by the mixture reactivity but also by its sensitivity to stretch effects. Only few results can be found in the literature about flame stretch effect in SI engine configurations. For this study, three different fuels (Methane, Propane, Iso-octane) were studied, but at different air-fuel lean mixture conditions, to present almost equivalent laminar flame speeds and thermo-dynamical properties at ignition timing condition. Besides those mixtures present different Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography was then performed in an optical Spark Ignition (S.I.) engine. Using a high speed camera, flame propagation images were acquired through the piston. Thermodynamic analyses based on in-cylinder pressure traces were performed to estimate in-cylinder temperature and burnt mass fraction during the engine cycle. From the determination of flame areas, the global flame stretch and an equivalent propagation speed have been then defined and estimated. Results prove that mixtures have the same ranking in terms of flame stretch sensitivities as in the laminar regime. Probability density functions of flame curvature are centered on 0.05 mm−1 for the three mixtures whereas differences are observed on the global wrinkling. Finally the impact of the Lewis number on the Burned Mass Fraction curved was studied thus showing a linear relationship between crank angle corresponding to 5 and 10 % of burned mass fraction and the Lewis number.