Book Description

Abstract : Combustion plays a dominant role in power generation and transportation. In spark ignition (SI) engines, the combustion process is originated from an electrical discharge within the spark plug electrodes. One important physical parameter affecting the spark discharge process and subsequent flame kernel propagation is the in-cylinder crossflow motion. Increasing the crossflow velocity generates turbulence in the combustion chamber. This is attributed to the spark channel being elongated at higher crossflow velocities. A longer spark channel length contains a higher discharge voltage which can induce a new re-spark across the spark plug electrodes. Furthermore, a longer spark channel expands the spatial spark discharge volume, affecting the initial formation and propagation of the flame kernel. Understanding the flame evolution physics in the cylinder and the corresponding cyclic variability in the combustion process under turbulent flows are of utmost importance to increasing efficiency of advanced engine technologies. In particular, knowledge of the cycle-to-cycle variations in combustion could potentially improve engine efficiency and performance including fuel economy, driveability, and emissions. Therefore, the main goal of this research is to understand the effects of high-speed crossflows on the initiation and development of the spark discharge and cyclic flame kernel propagation using optical diagnostics. Ignition tests are conducted in an optically accessible constant-volume spray and combustion vessel under various high-speed crossflows, pressures, and spark plug orientations to quantify the spark discharge process including the spark discharge channel, discharge duration, and glow discharge energy. Results show that increasing high-speed crossflows shortens the discharge duration while the glow discharge energy increases. A correlation between the spark channel length and electrical measurements is provided. Furthermore, cyclic variability is studied in an optical SI engine with retarded ignition timing under stoichiometric conditions. A spark sweep and various in-cylinder tumble motions are performed to develop a fundamental understanding of the cyclic variability at different operating conditions. Here, optical diagnostics, in-cylinder pressure measurements, and ion signal waveforms are analyzed to quantify the cycle-to-cycle variations of candidate combustion metrics including indicated mean effective pressure (IMEP) and mass fraction burned (MFB). Results provide a set of correlations among in-cylinder pressure measurements, ion signal data, and flame front data obtained from high-speed combustion images. It is also found that the cyclic variability is amplified with retarding the spark timing.