In an Adaptive Cycle Engine (ACE), thermodynamics favors combustion starting while the compressed, premixed air and fuel are still flowing into the cylinder through the transfer valve. Since the flow velocity is typically high and is predicted to reach sonic conditions by the time the transfer valve closes, the flame might be subjected to extensive stretch, thus leading to aerodynamic quenching. It is also unclear whether a single spark, or even a succession of sparks, will be sufficient to achieve complete combustion. Given that the first ACE prototype is still being built, this issue is addressed by numerical simulation using the G-equation model, which accounts for the effect of flame stretching, over a 3D domain representing a flat-piston ACE cylinder, both with inward- and outward-opening valves. A k-epsilon turbulence model was used for the highly turbulent flow field. It was found that the flame would suffer local blow-off under most operating conditions, but the blow-off is never complete so that the regions affected are later re-ignited by the remaining parts of the flame, and combustion is completed eventually. The interplay of blow-off and re-ignition causes a delay in the overall combustion process, which has been quantified into a modified set of parameters for a Wiebe equation model, applicable to ACEs with premixed charge via port injection, and other engines where combustion might begin before the valves are fully closed.