Like most inlines of the era, it used a large aluminium crankcase as the main structural component, with separate cylinders made from steel bolted onto it.

Jackets for cooling water covered the top 2/3 of the cylinder, feeding a radiator via connections at the back of the engine.

The D.III featured a rather prominent overhead cam operating the single intake and exhaust valves, powered by a shaft running up from the crankshaft at the rear of the engine.

Ignition was provided by two sets of spark plugs, one located on either side of the cylinders, each powered by a separate magneto for redundancy.

Fuel was fed into the cylinders via pipes on the left side of the engine, supplied from a twin-barrel carburetor located just above the crankcase.

While it saw widespread use in early examples of the C-series of two-seat general-purpose biplanes, the D.III was too large for contemporary fighter designs and did not see use in that role.

This engine changed the pistons again, this time to a domed profile that further increased the maximum compression – the ü was for "über", meaning "overcompressed".

To support operations at these altitudes, water from the radiator was used to heat the air intake and prevent icing in the carburetor.

The av used slightly longer pistons made of aluminium (possibly a first for a production engine), increasing the compression yet again, while at the same time allowing them to move faster due to the reduced weight.

In later versions of the D.III engines, the boxes were relocated rearward on the tubular camshaft housing, and the now easily sealable cylindrical rocker arm shafts protruded forwards through the front surfaces of the boxes, operating the now fully exposed rocker arms with the exposed shaft ends, as shown at right.

As a result, the Fokker D.VII's (those not equipped with BMW IIIa's) and the Pfalz D.XII's would be engine-limited in performance (as opposed to "airframe-limited") and yet would still be formidable adversaries to their Allied counterparts.

The D.IIIaü was considered the optimum engine for the Roland D.VI, Pfalz D.IIIa, and Albatros D.Va fighters, whose airframes were of an earlier, "all-wood" generation in design.