An experimental analysis on the evolution of the transient tip penetration in reacting Diesel sprays

Schlieren imaging has helped deeply characterize the behavior of Diesel spray when injected into an oxygen-free ambient. However, when considering the transient penetration of the reacting spray after autoignition, i.e. the Diesel flame, few studies have been found in literature. Differences among optical setups as well as among experimental conditions have not allowed clear conclusions to be drawn on this issue. Furthermore, soot radiation may have a strong effect on the image quality, which cannot be neglected.     

Combining Neural Networks and Genetic Algorithms to Predict and Reduce Diesel Engine Emissions

Diesel engines are fuel efficient which benefits the reduction of CO₂ released to the atmosphere compared with gasoline engines, but still result in negative environmental impact related to their emissions. As new degrees of freedom are created, due to advances in technology, the complicated processes of emission formation are difficult to assess. This paper studies the feasibility of using artificial neural networks (ANNs) in combination with genetic algorithms (GAs) to optimize the diesel engine settings. The objective of the optimization was to find settings that complied with the increasingly stringent emission regulations while also maintaining, or even reducing the fuel consumption. A large database of stationary engine tests, covering a wide range of experimental conditions was used for this analysis. The ANNs were used as a simulation tool, receiving as inputs the engine operating parameters, and producing as outputs the resulting emission levels and fuel consumption. The ANN outputs were then used to evaluate the objective function of the optimization process, which was performed with a GA approach. The combination of ANN and GA for the optimization of two different engine operating conditions was analyzed and important reductions in emissions and fuel consumption were reached, while also keeping the computational times low     

A Comprehensive Study of Particle Size Distributions with the Use of PostInjection Strategies in DI Diesel Engines

Control strategies such as variations in injection pressure and timing have been used by researchers to reduce in-cylinder exhaust emissions and meet legislation standards. Postinjection has been studied for several years and is now well known as an efficient strategy for reducing soot emission. Diesel gaseous and particle mass emissions have been progressively reduced over the last twenty years as a consequence of increasingly restrictive emission legislation and the application of aftertreatment devices. The main objective of this work is to better understand the effect of postinjection on particle size distribution in diesel exhaust. The approach uses a modern, well-instrumented research engine test cell equipped with a flexible high pressure fuel injection system. The results of this work provide guidelines for developing strategies to reduce particle size distribution in diesel engines. A major improvement in particle size distribution was found in the accumulation mode by using a close postinjection of a small quantity of fuel. For reduction in nucleation mode, a relationship was found with close postinjections of large quantities of fuel.     

Limitations on the use of the planar laser induced exciplex fluorescence technique in diesel sprays

The Planar Laser Induced Exciplex Fluorescence (PLIEF) technique is widely used to visualize and measure the fuel concentration fields in both liquid and vapor phases of DI Diesel sprays. However, the real limitations of the PLIEF technique in Diesel sprays and the accuracy of the results obtained are still a source of controversy. In this work, a complete methodology for maximum penetration and fuel concentration measurements in evaporating conditions in Diesel sprays has been developed and the reliability of the results obtained has been investigated. The methodology includes new procedures for measuring both liquid and vapor phases, adapting, when necessary, correlations available in the literature for calibration.

An experimental matrix of nine test points with different injection pressures and combustion chamber densities has been performed. A critical analysis of the different error sources for proper quantification is made. Results have shown that macroscopic features can be accurately determined using the PLIEF technique, but for fuel concentration measurements special considerations have to be taken into account, particularly in the regions where liquid and vapor coexist.     

The modification of the fuel injection rate in heavy-duty diesel engines. Part 1: Effects on engine performance and emissions

An experimental study has been performed on the effects of injection rate shaping on the combustion process and exhaust emissions of a direct-injection diesel engine. Boot-type injections were generated by means of a modified pump-line-nozzle system, which is able to modulate the instantaneous fuel injection rate. The interest of the study reported here was the evaluation of the effective changes produced in the injection rate at different engine operating conditions, when the engine rotating speed and the total fuel injected were changed. In addition, the influence of these new injection rates was quantified on the global engine performance and pollutant emissions. In particular, the focus was placed on producing “boot-like” injection rate shapes, with the main objective of reducing NO_x emissions.Results show how this system is capable of achieving boot-type injections at different boot pressures and boot durations. Also, even though the general trend of the system is to reduce NO_x and to increase soot and fuel consumption, emissions and performance trade-offs can be improved for some specific boot shapes. On the contrary, the modulation of the injection rate showed to be ineffective at medium engine load, since the increase in soot was greater than the relative decrease in NO_x.The analysis of the modifications produced by these strategies on the combustion process, and on the rate of heat release are the base of a second paper.     

Influence of nozzle geometry on ignition and combustion for high-speed direct injection diesel engines under cold start conditions

Starting at low temperatures (below 0 °C) is an important issue for current and near future diesel engine technology. Low ambient temperature causes long cranking periods or complete misfiring in small diesel engines and, as a consequence, an increased amount of pollutant emissions. This paper is devoted to study the influence of nozzle geometry on ignition and combustion progression under glow-plug aided cold start conditions. This study has been carried out in an optically accessible engine adapted to reproduce in-cylinder conditions corresponding to those of a real engine during start at low ambient temperature. The cold start problem can be divided in two parts in which nozzle geometry has influence: ignition and main combustion progress. Ignition probability decreases if fuel injection velocity is increased or if the amount of injected mass per orifice is reduced, which is induced by nozzles with smaller hole diameter or higher orifice number, respectively. Combustion rates increase when using nozzles which induce a higher momentum, improving mixture conditions. For these reasons, the solution under these conditions necessarily involves a trade-off between ignition and combustion progress.     

Investigation of the Influence of Post-Injection on Diesel Exhaust Aerosol Particle Size Distributions

The effect of double-pulse fuel split injection on the exhaust aerosol particle size distribution emitted by a state-of-the-art Heavy Duty Diesel engine was experimentally investigated. The influence of post-injection fuel quantity and dwell was evaluated at four steady-state engine conditions by analyzing the changes in the accumulation mode particle number, mean diameter and geometric standard deviation, with respect to the baseline case of single injection. Generally, rising post-injection fuel amount was found to increase accumulation mode particle number and mean diameter of the size distributions. Particle number reduction efficiency resulted dependent on the operating conditions and post-injection parameters scheme. Reductions in the particle number around 50% with respect to the baseline single-pulse injection case were observed for 1500 rpm partial load cases. For 1800 rpm 75% load conditions the reduction was lower and was achieved only when the fuel amount was below 20% of the total fuel injected. An increase in the particle number was produced when post-injection was applied at low speed and high load conditions, due to the low accumulation mode particle number emitted at this operation mode. An optimum post-injection fuel amount for particle number reduction was only seen for the 1500 rpm 75% load operation conditions. The results are indicative that further research is necessary in order to explore the existence of optimum post-injection schemes for particle number reduction at the other operating conditions studied.

Geometric standard deviation from post-injection results was higher than that corresponding to the single injection baseline cases, while an increase in the σ _g value was observed for larger dwells in some operating conditions.     

Microprocessor-Based Control of Engine Valve Timing

One of the most interesting ways of improving the lowspeed performance of spark-ignition engines is the variation of the valve timing in order to reduce mechanical losses.     

A 1D model for the description of mixing-controlled reacting diesel sprays

The paper reports an investigation on the transient evolution of diesel flames in terms of fuel-air mixing, spray penetration and combustion rate. A one-dimensional (1D) spray model, which was previously validated for inert diesel sprays, is extended to reacting conditions. The main assumptions of the model are the mixing-controlled hypothesis and the validity of self-similarity for conservative properties. Validation is achieved by comparing model predictions with both CFD gas jet simulations and experimental diesel spray measurements. The 1D model provides valuable insight into the evolution of the flow within the spray (momentum and mass fluxes, tip penetration, etc.) when shifting from inert to reacting conditions. Results show that the transient diesel flame evolution is mainly governed by two combustion-induced effects, namely the reduction in local density and the increase in flame radial width.     

Computational study on the deposition of ultrafine particles from Diesel exhaust aerosol

Short-term in vitro assays are arising in order to determine the toxicity of native Diesel aerosol particles, due to its association with adverse health effects. Estimation of the real quantity and characteristics of the particles deposition on the cells cultures in these assays becomes necessary to establish correlations between the particle deposition conditions and the biological indicators of toxicity of the exposed cells.

On that basis, it is the aim of this paper to analyze the deposition of sub-micron particles in an air/liquid interface cells exposure system by means of a CFD (computational fluid dynamics) code. Some comparisons with experimental and modelled results are shown to probe the validity of the method and a detailed analysis on the deposition of particles and efficiency of exposure-cells systems, with perpendicular contact, is carried out.

Furthermore, a parametric study on the effect of particle diameter, particle density and flow inlet velocity revealed the importance of such parameters in the efficiency and pattern of particle deposition.