  Cyclic-to-cycle variability, CCV, of intake-jet flow in an optical engine was
measured using particle image velocimetry (PIV), revealing the possibility of
two different flow patterns. A phase-dependent proper orthogonal decomposition
(POD) analysis showed that one or the other flow pattern would appear in the
average flow, sampled from test to test or sub-sampled within a single test;
each data set contained individual cycles showing one flow pattern or the
other. Three-dimensional velocity data from a large-eddy simulation (LES) of
the engine showed that the PIV plane was cutting through a region of high shear
between the intake jet and another large flow structure. Rotating the
measurement plane 10{\deg} revealed one or the other flow structure observed in
the PIV measurements. Thus, it was hypothesized that cycle-to-cycle variations
in the swirl ratio result in the two different flow patterns in the PIV plane.
Having an unambiguous metric to reveal large-scale flow CCV, causes for this
variability were examined within the possible sources present in the available
testing. In particular, variations in intake-port and cylinder pressure,
lateral valve oscillations, and engine RPM were examined as potential causes
for the cycle-to-cycle flow ariations using the phase-dependent POD
coefficients. No direct correlation was seen between the intake port pressure,
or the pressure drop across the intake valve, and the in-cylinder flow pattern.
A correlation was observed between dominant flow pattern and cycle-to-cycle
variations in intake valve horizontal position. RPM values and in-cylinder flow
patterns did not correlate directly. However, a shift in flow pattern was
observed between early and late cycles in a 2900-cycle test after an
approximately 5 rpm engine speed perturbation.
