A flat-4 engine has 4 cylinders organized in an opposite direction along a typical crankshaft. The pistons go away from each other at the same time. Examples are Honda CB750 and BMW K1200RS. Hub: the middle part of a wheel which contains bearings and it is where the spokes attach https://remingtonardpy.sharebyblog.com/34868035/5-simple-statements-about-engine-cycle-diagram-explained