Abstract
An experimental study was conducted to investigate the effect of the structure of the induction flow on the characteristics of early flames in a lean-stratified and lean-homogeneous charge combustion of compressed natural gas (CNG) fuel in a direct injection (DI) engine at different engine speeds. The engine speed was varied at 1500 rpm, 1800 rpm and 2100 rpm, and the ignition timing was set at a 38.5° crank angle (CA) after top dead center (TDC) for all conditions. The engine was operated in a partial-load mode and a homogeneous air/fuel charge was achieved by injecting the fuel early (before the intake valve closure), while late injection during the compression stroke was used to produce a stratified charge. Different induction flow structures were obtained by adjusting the swirl control valves (SCV). Using an endoscopic intensified CCD (ICCD) camera, flame images were captured and analyzed. Code was developed to analyze the level of distortion of the flame and its wrinkledness, displacement and position relative to the spark center, as well as the flame growth rate. The results showed a higher flame growth rate with the flame kernel in the homogeneous charge, compared to the stratified combustion case. In the stratified charge combustion scenario, the 10° SCV closure (medium-tumble) resulted in a higher early flame growth rate, whereas a homogeneous charge combustion (characterized by strong swirl) resulted in the highest rate of flame growth.
Highlights
Modern induction systems in spark-ignition (SI) engines are designed to produce controlled air intake in order to generate the appropriate turbulence inside the cylinder [1]
Distortion of the early flame for the different engine speeds and induction flows is shown in Generally, the results showed that the level of flame distortion increased with the increase in the engine speed in all cases of induction for both the stratified and homogeneous charge combustions
Flame images were taken by an endoscopic intensified CCD (ICCD) camera, and quantitative information was retrieved from the images using elliptic Fourier analysis (EFA) and image-processing algorithms
Summary
Modern induction systems in spark-ignition (SI) engines are designed to produce controlled air intake in order to generate the appropriate turbulence inside the cylinder [1]. The nature of the flow at the time of ignition determines the flame kernel development characteristics, and eventually, the combustion performance of the engine [4]. The flame kernel development period is the time over which the initial flame kernel propagates from the spark gap and begins to interact fully with the turbulent flow field [5,6]. This portion of the Energies 2017, 10, 964; doi:10.3390/en10070964 www.mdpi.com/journal/energies
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