A system for stratifying recycled exhaust gas (EGR) to substantially increase dilution tolerance has been applied to a port injected four-valve gasoline engine. This system, known as Combustion Control through Vortex Stratification (CCVS), has shown greatly improved fuel consumption at a stoichiometric air/fuel ratio. Both burnrate (10-90% burn angle) and HC emissions are almost completely insensitive to EGR up to best economy EGR rate. Cycle to cycle combustion variation is also excellent with a coefficient of variation of IMEP of less than 2% at best economy EGR rate. This paper describes a research programme aimed at gaining a better understanding of the in-cylinder processes in this combustion system. Results are presented from the application of a number of diagnostic techniques: CFD modelling of the flow field and residual concentration during the intake and compression strokes Dynamic visualisation of the flow field during the intake stroke using a sophisticated water analogy rig Study of flame propagation using a 16 channel ionisation probe head gasket Study of crank angle resolved HC signatures in each exhaust port using a fast response FID Use of these diagnostic techniques has led to an improved understanding of the flow and mixing processes during the intake and compression strokes. Both CFD and the flow visualisation rig have identified an unsteady vortex motion during the intake stroke, which causes mixing of EGR and fresh charge. CFD has shown that the flow processes during the compression stroke leading to stratification are more complex than the simple tumble stratification theory of CCVS operation. The dynamic flow rig has also shown the ability to differentiate between two CCVS cylinder heads which showed the same tumble ratio on a steady flow rig, but different combustion characteristics. The ionisation gasket has shown that flame propagation and detonation characteristics at full load are similar to those of a conventional four-valve combustion system. The exhaust port HC signatures have provided evidence to support the theory that the mechanism of HC emission control in the CCVS combustion system is a reduction in the amount of HC trapped in the crevices and oil layer on the EGR side of the cylinder.