These engines are also commonly referred to as barrel engines as the cylinders are usually arranged around a center point and the axis of rotation is parallel to the cylinder centerline.
The usual method of converting the reciprocating motion of the piston is by either a wobble plate or a swash plate. Wobble plates are free to rotate about a “Z” shaped crankshaft throw while swash plates are usually a fixed plate set at an angle to the crankshafts center of rotation. One area of concern on cam drive engines is the shock load cause by the start of the expansion stroke that is applied to the cam drive face by a cam follower and the possibility of this roller or follower actually bouncing due to necessary clearances between the components.
There are variations of axial engines that include using single overhung crankshafts.
A sub class of axial engines are revolver engines as the cylinder rotates around a fixed point much like the bullet chambers of a revolver type pistol. This type of arrangement is commonly seen in hydraulic motors. This variant can also have the crankcase rotating in the opposite direction as the crankshaft to allow an extra stroke to be picked up for every crankshaft revolution.
A major advantage of this type of arrangement is that the line of force from the piston is always offset from the center of rotation but most of these mechanisms have a relatively high rotating and reciprocating mass.
Another advantage is decreased cross section surface area and concentrated mass center. This reduced frontal area can create cooling problems in the back cylinders as well as in the center.
Read more about axial, inline, opposed, radial, and rotary engine configurations and see 25 unique designs for each category in this volume , 125 in total.
Included in this class are engines that have an between cylinders less than 175 degrees but this is a grey area as to when a "V" type of cylinder block could actually change it's description from an inline cylinder arrangement to an opposed engine type.
Opposed piston engines usually have a common combustion chamber that is formed between the piston crowns and have multiple crankshafts while opposed cylinder engines usually have a single crankshaft located between the cylinders.
Have cylinders that radiate out from a single point. One major sub group has individual combustion chambers for each cylinder while the other type has a shared combustion chamber at the center of rotation.
Rotary engines can also be broken into 2 major groupings, ones with a cylinder block that rotates about a fixed point and the type that the rotor or piston revolves around a fixed point.