Jean Arrègle

Development of a zero-dimensional Diesel combustion model. Part 1: Analysis of the quasi-steady diffusion combustion phase

Abstract The aim of this work is to identify and quantify the influence of injection parameters and running conditions on Diesel combustion. This theoretical–experimental analysis is the basis for the development of a zero-dimensional Diesel combustion model. The objective of this first part is to analyze the physical variables and processes that control the central phase of the quasi-steady Diesel diffusion combustion. For that purpose, a parameter as the apparent combustion time (ACT) characteristic of a diffusion combustion process has been used. This parameter allows to obtain explicit relations between, on the one hand, the injection rate law and in-cylinder conditions (air density, oxygen concentration…), and on the other hand, the rate of heat release. Results show a good correlation between the ACT and the instantaneous values of in-cylinder gas density, injection velocity, oxygen concentration and the nozzle diameter.

Diesel spray image segmentation with a likelihood ratio test

To characterize the macroscopic behavior of Diesel sprays and to validate and extend for current high-pressure injection systems the correlations existent in the literature, it is necessary to determine the spray geometry accurately, at least in terms of spray tip penetration and cone angle. These parameters are measured by analyzing Diesel spray images and are highly sensitive to the correct edge determination. An algorithm for segmentation of color images based on a likelihood ratio test is presented. This algorithm is compared with others available in the literature and has been validated, even for adverse experimental conditions. The experimental facilities, optical layouts, and image-processing algorithms are described.

Development of a zero-dimensional Diesel combustion model: Part 2: Analysis of the transient initial and final diffusion combustion phases

The objective of this work is the development of a zero-dimensional Diesel combustion model. After the development of a model based on the apparent combustion time for the central phase of the quasi-steady Diesel diffusion combustion [Appl. Therm. Eng. 23 (2003)], this second part describes the work of generalization of the model for the final and initial transient phases of the diffusion combustion. For this purpose, those transient phases are analyzed mainly on the basis of the results of CFD calculations for pulsed gas jets. The generalization for the final transient diffusion combustion phase is based on the analysis of the evolution of the turbulent effective viscosity dissipation at the end of the injection process. For the initial transient diffusion combustion phase the work is based on the analysis of the air/fuel mixture at the beginning of the injection process. The result is a zero-dimensional model capable of predicting the diffusion combustion in a Diesel engine. The validation of the proposed model has been performed on both a high speed Diesel engine and a heavy duty Diesel engine.

Segmentation of diesel spray images with log-likelihood ratio test algorithm for non-Gaussian distributions

A methodology for processing images of diesel sprays under different experimental situations is presented. The new approach has been developed for cases where the background does not follow a Gaussian distribution but a positive bias appears. In such cases, the lognormal and the gamma probability density functions have been considered for the background digital level distributions. Two different algorithms have been compared with the standard log-likelihood ratio test (LRT): a threshold defined from the cumulative probability density function of the background shows a sensitive improvement, but the best results are obtained with modified versions of the LRT algorithm adapted to non-Gaussian cases.

CHARACTERIZATION OF D.I. DIESEL SPRAYS IN HIGH DENSITY CONDITIONS

The characteristic parameters and the evolution of continuous Diesel sprays injected against a high density gas have been investigated using high speed photography and phase Doppler anemometry. The injector used for these tests was a two-spring one providing different injection conditions. Three test sections were analyzed at 10, 20 and 30 mm from the injector with several radial measurements for each one. The obtained results provided qualitative and quantitative information about the macroscopic evolution of the spray, but also about the drop velocity distribution and drop size evolution.

Evaluation of the Thermal NO formation mechanism under low-temperature diesel combustion conditions

Over the past two decades, the amount of exhaust gas pollutants emissions has been significantly reduced due to the severe emission legislation imposed in most countries worldwide. Initial strategies simply required the employment of simple after-treatment and engine control devices; however, as the restrictions become more stringent, these strategies are evolving in the development of different combustion modes, specially characterized by having low-temperature combustion characteristics. These new working conditions demand the need to check the suitability of the current NO predictive models that coexist nowadays under standard diesel combustion characteristics, paying closer attention to the Thermal mechanism. In order to do so, a common chemical-kinetic software was employed to simulate, for n-heptane and methane fuels, fixed local conditions (standard diesel and low-temperature combustion) described by constant pressure, relative mixture fraction, oxygen mass fraction and initial and final reaction temperature. The study reflects a common trend between all the studied cases, independently of the considered local conditions, making it applicable to more complex situations such as real NO formation processes in diesel sprays. This relationship was characterized by a fourth-degree polynomial equation capable of substantially improving the NO prediction by just using the Thermal NO predictive model.

On Board NOx Prediction in Diesel Engines: A Physical Approach

For pollutant emissions predictive physical modeling in diesel engines, three key points have to be taken into account:

Characterization of the pressure losses in a common rail diesel injector

A methodology to characterise the pressure losses in quasi-steady conditions (i.e. at full needle lift) of common rail diesel injectors was developed. The aim was to quantify the error when experimental results of nozzle internal flow are compared with computational fluid dynamics results, where pressure losses are usually neglected. The proposed methodology is based mainly on experimental tests that are complemented with some approximate calculations, based on the physics of the phenomenon, to take into account the effect of the needle deformation. The results obtained in the work lead to two important conclusions: on the one hand, that it is dangerous to extrapolate results relative to the injection (internal flow, spray atomization, spray penetration, etc.) and combustion processes from low permeability nozzles (e.g. single-hole nozzles) to high permeability nozzles (e.g. multi-hole nozzles), and, on the other hand, that the comparison of these results between experiments and computational fluid dynamics simulations should be carried out carefully, because the pressure losses in the injector can be high under certain conditions. Finally, people working on the study of the injection and/or combustion processes, through experiments or simulations, will find here some interesting information to better know the actual injection pressure to be used in their analysis and/or simulations.