Optical diagnostics were used in two different single-cylinder research engines to investigate cycle-to-cycle variations (CCV). A range of physical characteristics, including the flow field, mixture formation, and flame propagation, were measured in two dimensions and subsequently combined with pressure-based analysis. Fuel was primarily injected through direct-injection (DI). Depending on the specific aspects of the study reference cases with port-fuel injection (PFI) were also measured. By varying parameters like injection timing, injection pressure, and ignition location the impact of the resulting changes in flow field and fuel concentration at ignition on CCV was investigated.

To determine the in-cylinder flow field, particle image velocimetry (PIV) was performed. Laser-induced fluorescence (LIF) of a tracer (anisole or di-fluorobenzene) evaporated into the fuel allowed examining mixture formation at selected crank angles during the compression stroke. The natural chemiluminescence of the flame was used to image flame growth and movement. Subsequently, the acquired data was analyzed and correlated in conditional averages, correlation maps, and independent component analysis (ICA) – the latter being further developed here specifically for analysis of engine CCV.

In the case of a homogeneous air-isooctane mixture delivered via PFI, it was observed that in slower cycles, the flame tends to move closer to the intake side of the spark plug. With inhomogeneous mixtures provided by DI the flame preferentially burns towards the exhaust side. Flames in fast cycles, on the other hand, increasingly burn centrally in the combustion chamber for both types of mixtures. Particularly with PFI, flow field analysis showed different movements of the vortex centers for fast and slow combustion, which were further investigated through ICA. The results of this analysis reveal various flow patterns that correlate with the combustion speed.

Correlation maps of the flow field and flame growth showed regions that correlate with the combustion process, often deviating from the location of the tumble vortex center. With DI, correlations between the equivalence ratio and combustion speed showed a significant impact of the mixture gradient on combustion behavior.

In the second part of this work, iso-octane was replaced with hydrogen (H₂). In motored operation, the variability of mixture formation was investigated. As expected, retarding injection timing and reducing injection pressure increases mixture inhomogeneity in some regions of the combustion chamber. A subsequent analysis with fired operation demonstrated that later injection timings reduce combustion CCV. This suggests that in terms of CCV, the benefits of increased turbulence associated with later injections outweigh the drawbacks of mixture inhomogeneity in the given experimental conditions.