Effects of large-scale turbulence on cyclic variability in spark-ignition engine

► Experiments are conducted in a Ported Optical Engine with large skip-firing ratio. ► Turbulence statistics is distinguished for fast, average and slow combustion events. ► A clear link is shown between the variability of turbulence and rate of combustion. ► Faster combustion corresponds to higher values of root-mean-square velocity. ► Faster combustion corresponds to higher root-mean-square rather than mean vorticity. An investigation is carried out into the connection between the various characteristics of the velocity field and the burning rate in a spark-ignition engine. The experiments are performed on the Leeds University Ported Optical Engine with a large skip-firing ratio; the engine allows a full optical access to the combustion chamber. Velocity fields are obtained using a Particle Image Velocimetry (PIV) and a systematic distinct averaging is performed with an account of the cyclic variability of the burning rate, that is the properties of the turbulent velocity fields are derived separately for the fast, middle and slow cycles using the peak pressure as a proxy measure for the burning rate. Even though the velocity fields are nearly homogeneous in the mean, they reveal very significant intermittency where the regions of intense fluctuations have an extent more than half the clearance height. It is shown that the variations of the burning rate are correlated with the fluctuations in root-mean-square fields of the velocity magnitude, vorticity and shear strain rate. Faster combustion is induced by the fields with larger root-mean-square values. No discernible correlation of the burning rate with the average vorticity or shear strain fields has been detected. The integral scales of the large-scale motion derived from the PIV do not show any systematic variation between fast and slow cycles.