The original D.III was introduced in 1914. 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. At the time, fighters were generally powered by lighter rotary engines of power output from to about , or by water-cooled inline engines in the to range like the earlier
Mercedes D.II. By 1916 new designs had grown in size and performance and the D.III became popular on new designs. By 1917 the D.III was being widely used in fighters, most notably on the famous
Albatros D.I. Production of this version was essentially wound down by May 1917, with only a handful continuing to be delivered until October. British HP ratings being slightly different (with 746 watts = 1 hp) to the German PS or
Pferdestärke standard of roughly 735 watts per PS horsepower, it is probable that this engine would have had a slightly lower rating under British HP numbers. Fuel consumption was per hour. Its compression ratio was 4.5:1. Development of the basic design led to the slightly modified
D.IIIa, which took over on the production lines in June 1917. The main change was to change the piston profile to have a flat head instead of the former concave one, thereby slightly increasing maximum compression to 4.64:1. The earlier "side-slot" rocker arm design had also been used for the Mercedes D.I and D.II engines' valvetrains. 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. arms. The newer arrangement was stated as being interchangeable as a set with the complete camshaft, rocker boxes, rocker arms and valve springs, with the D.III's earlier "side-slot" cam drive system design - the later system seems to have influenced both
Packard's and
Hall-Scott's engine designers to adopt a nearly identical "slotless" rocker-box overhead cam valvetrain design feature for the
Liberty L-12 Allied V-12 aviation engine. Confusingly, the "ü" was not an official part of the name. This leads to a number of problems in various references, which often confuse the IIIa with the IIIaü, listing the former as a engine. There are two D.IV engines — from the
IdFlieg aircraft powerplant class designations based on power output — one
the straight-eight based on the 140 mm bore D.III pistons; and the later, six-cylinder
Mercedes D.IVa with 160 mm bore cylinders, which was essentially unrelated. The D.III line of engines would find themselves eclipsed in performance by the
BMW IIIa of and then (British rated it at ) in 1918, however, the small number of BMW's produced ensured that the Mercedes D.III series would be the primary German fighter engine up to the last month or two of the war and it would still be seen in very large numbers even at the end. At the end of the war the D.IIIaü would still be the numerically predominant German fighter engine. 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. ==Variants==