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A combined numerical and experimental investigation is carried out to analyze the cycle-to-cycle variations (CCV) in an optically accessible spark-ignition engine with port fuel injection. A stable and an unstable operating condition is considered. Well-established turbulence, combustion, and ignition models are employed in the large-eddy simulations (LES). High-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane are conducted in the experiments. A detailed comparison between LES and experiments is carried out, including the in-cylinder pressure, the flow fields, the spatial flame distribution, and the fields conditioned on fast and slow cycles. Good agreement is achieved for the variables considering all cycles; yet, some discrepancies are observed for the conditionally averaged quantities. A systematic quantitative correlation analysis between the selected influencing variables and the CCV is presented, in which the influencing variables are extracted from different length scales (r = 3 mm, 12 mm, and 43 mm) and the CCV are distinguished between the early flame kernel development and later flame propagation. Even though the most relevant influencing parameters are different for the two operating conditions, the location of the coherent vortex structure is found to be important for the CCV of both cases. [ABSTRACT FROM AUTHOR]

Cyclic variability is investigated in an optically accessible single-cylinder spark-ignition research engine by introducing artificial exhaust gas in controlled amounts to the homogenous air–fuel mixture before ignition. A skip-fire scheme ensures the absence of internal exhaust gas recirculation (EGR) and allows the engine to be fired continuously for acquisition of large statistics. Four operating conditions ranging from a stable 0% EGR case up to a highly unstable extreme EGR case are analyzed to examine the increasing effects of homogeneous EGR on the cycle performance. To that end, high-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane allow the determination of phenomena leading to various flame positions and sizes as well as faster and slower combustion cycles. Through extensive conditional statistical and multivariate correlation techniques, flames are found to be heavily influenced by large-scale velocity motion, especially with the presence of greater EGR which leads to lower flame speeds. The greater sensitivity of slower flames to variations in the velocity field manifests itself in an exponential increase in cyclic variability of the maximum in-cylinder pressure and causes misfire cycles where the flame is blown off or quenched at the cylinder roof. In the most extreme cycles at the highest EGR level, the state of the large-scale velocity structures at the time of ignition determines whether the flame propagates towards the center of the cylinder (and is blown off or quenched) or if the flame sustains growth by propagating within the lingering tumble vortex. [ABSTRACT FROM AUTHOR]