Root Causes of Cycle-to-Cycle Combustion Variations in Spark Ignited Engines

This document is the PhD thesis of Philipp Schiffmann in the University of Michigan, Ann Arbor, USAStricter governmental emission regulations, climate change concerns, and consumer demands for high fuel efficiency push the development of advanced cleaner and more efficient combustion strategies. Many strategies that rely on spark ignition are limited in their peak efficiencies by excessive cycle-to-cycle combustion variations (CCV). In this study, various laser-based and passive optical techniques are used to measure flow fields, spark discharge and other factors that impact early flame growth from which CCV originate. Bulk flow motion, as one contributing factor to CCV, is characterized in an optical engine under motored and fired conditions. In the fired cases, the flow velocities are higher during the gas exchange period but lower at the time of ignition, due to higher charge viscosities, caused by higher gas temperatures. Ten different fuel-air mixtures are strategically chosen to isolate the effects of laminar flame speed, thermo-diffusive mixture properties and change of stoichiometrically deficient species on the mechanisms that are responsible for cycle-to-cycle variability. Single value decomposition methods are found to be inefficient in identifying flow structures that are related to combustion variability. Physical flow parameters such as velocity magnitude and shear strength around time of ignition are identified to affect combustion variability. The relative impact of these parameters on energy output and combustion phasing are quantified for all mixtures and show some weak dependence on Markstein number and laminar flame speed. In a more fundamental fan-stirred combustion vessel experiments, variability effects of flame-flow interactions on CCV are isolated and thermo-diffusive effects are shown to impact combustion variability. Unstable negative Markstein number mixtures tend to exhibit higher combustion variability when interacting with gradients in the flow field around the time of ignition. High shear strength at the point of ignition causes an increased flame wrinkling, increasing the surface area,leading to faster combustion. This is an important finding because the common Lewis number equals 1 assumption in CFD simulations might lead to an under-prediction of CCV in low turbulence cases for negative Markstein number mixtures.The bulk of the work was supported by funding through the General Motors Research and Development through the Engine Systems Division in the General Motors - University of Michigan Collaborative Research Laboratory. Some funding was received from the Université d'Orléans to enable a research visit to Orléans to conduct combustion vessel experiments.https://deepblue.lib.umich.edu/bitstream/2027.42/137636/1/Schiffmann_Philipp_PhD_2016.pdfDescription of Schiffmann_Philipp_PhD_2016.pdf : PhD thesis (Philipp Schiffmann