PHOENIX — The Q50 Infiniti’s best-selling sedan gets a mid-cycle refresh for 2018. That means, rather than a complete redo, there’s tweaking to the front and rear fascia, modest changes to the interior as well as innovative technologies designed to support the driving experience. One would have to park the outgoing 2017 Q50 next to the refreshed 2018 to discern the difference, visually. Up front, there are new lower air intakes, a modestly reshaped front bumper, and a grille that’s a bit more dimensional.