CENTRO DE TECNOLOGIA DA MOBILIDADE

Numerical Investigation of the Effects of In-Cylinder Flow and Multiple-Injection Strategy on Overall Spray Behavior, Evaporation, and Mixture Formation

Deborah Domingos da Rocha, Leonardo Mayer Reis, Fábio de Castro Radicchi, Paulo Henrique Terenzi Seixas, and Ramon Molina Valle

Centro de Tecnologia da Mobilidade – UFMG

Abstract

Using computational fluid dynamics (CFD), the effects of in-cylinder flow and injection timing were analyzed for two conditions of ethanol direct injection. The first condition consists in a unique injection during air admission, with a duration of 1.4ms, while the second condition aims to analyze the effect of splitting this injection in two injections of 0.7ms: one during air admission and another during compression. It was possible to study the spray behavior and differences between the conditions investigated, visualizing and tracking the droplets. Furthermore, it was possible to evaluate ethanol mass fraction, evaporation, mixture formation characteristics and influence of tumble and swirl on spray development during the engine cycle. Considering the cases of study, it was possible to verify the escape of fuel through admission port and poor mixture formation when the injection is made during the air admission, although fuel evaporation rate is better in this case than using two injections. It was also verified that multiple-injection can decrease the formation of fuel films, and that it is a good strategy to attain a good mixture formation and efficient combustion.

Introduction

The study of spray injection systems represents a current and promising possibility of increasing thermal efficiency, reducing specific fuel consumption and achieving the acceptable gases emission levels for internal combustion engines. An important aspect of this investigation is the analysis of injection strategy, which takes into consideration, among other factors, its moment of occurrence, duration and quantity of fuel injected. This paper evaluates different injection strategies for ethanol fuel, taking advantage of techniques as computational fluid dynamics (CFD) to analyze and compare them.

An important characteristic of the injector is its positioning, which may be at the admission port (PFI – port fuel injection) or inside cylinder (DI – direct injection). Both sprays produced are essential parts for the air-fuel mixture formation in Otto cycle engines. One of the greatest advantages using direct injection (DI) is the flexibility of the start and splitting injection in a larger range of crank angles, which allows its operation with homogeneous or stratified charge of air-fuel. According to Çelic (2010), when the engine operates with stratified charge, three combustion systems may be used to transport and produce an adequate mixture close to the spark plug at the combustion time. These systems are sketched in Figure 1.

Figure 1. Combustion systems for charge stratification, ÇELIK & OZDALYAN, 2010.

Alkidas and Tahry (2003) explains that DI engines with spark ignition may work with stratified charge during low loads and speed operation, while homogeneous charge at higher loads and speeds, resulting in a configuration that allows the highest fuel economy benefit potential.

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