Mercedes D.III / F1466
The Mercedes D.III, or F1466 as it was known internally, was a six-cylinder, liquid cooled inline aircraft engine, first run in 1914, built by Daimler and used on a wide variety of German aircraft during World War I. The initial versions were introduced in 1914 at 160 hp, but a series of changes improved this to 170 hp in 1917, and 180 by mid 1918. These later models were used on almost all late-war German fighters, and its only real competition, the BMW III, was available only in very limited numbers.
The D.III was based on the same pattern as the earlier Mercedes D.II, suitably scaled up for higher power settings. Like most inlines of the era, it used a large aluminum crankcase as the main structural component, with separate cylinders made from steel bolted onto it. The technology for screwing a threaded cylinder of steel into an aluminum crankcase did not exist at that time. Jackets for cooling water covered the top 2/3 of the cylinder, feeding a radiator via connections at the back of the engine.
The only obvious design change from the earlier D.II was to use separate cooling jackets for each cylinder, whereas the D.II used one jacket for every two cylinders. Daimler also used the pistons of the D.III to produce the reduction geared, eight-cylinder 220 hp Mercedes D.IV during this period, but it did not see widespread use.
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 did not see use in fighters until 1916 when the fighters grew to need that level of power; earlier designs were generally powered by lighter rotary engines of about 110 hp or by water cooled inline engines in the 100 to 120hp range such as the earlier Mercedes D.II. 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 to the German PS or (Pferd Starke) standard, it is probable that this engine would have had a slightly higher rating under British HP numbers.
Development of the basic design led to the slightly modified 170 hp 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. Other changes were mainly in design details, notably a redesigned crankcase and new carburetor. Many of the accessories were also redesigned or moved around on the engine. This model was produced only briefly, for use on the Albatros D.III but there are indications that possibly some early Albatros (Alb.) made Fokker D.VII's were also equipped but probably had the engines upgraded or replaced as quickly as possible. This engine has been referred to in postwar British analysis as generating 180 hp.
A final version attempting to keep the D.III block competitive was the 200 hp (200-217 hp) D.IIIav (or avü), introduced mid-October 1918. The av used slightly longer pistons made of aluminum (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. The maximum allowable RPM increased from 1,400 in the earlier models to 1,600 in the av, accounting for most of the gains in power. It is unclear if any av's saw service use. The increased use of Benzol in German aviation fuel may have helped this final upgrade of power, it's higher octane rating being better suited for the higher compression ratio.
All of the D.III series were generally very similar to other models, with the exception of the piston profile and carburetor details. It appears that upgrades were available for many of the engines, certainly for the III to IIIa, and IIIa to IIIaü. It would seem unlikely that early III's would ever make it to the IIIaü standard, as they would almost certainly have been worn out in service before then. A more obvious change concerned the layout of the rocker arms that operated the valves. Early models had square cases positioned directly over the cylinders with the rocker arms exiting through vertical slots cut into the sides of the boxes. In later versions of the engines, the boxes were moved rearward and the 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. The newer arrangement can be seen in the image above (compare with the image of the D.II) and were stated as being interchangeable as a set with the complete camshaft, rocker boxes, rocker arms and valve springs, with the D.III's earlier cam drive system design.
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 180 hp engine. It should also be noted that there are two D.IV engines, one the eight-cylinder based on the D.III pistons, and the later six-cylinder D.IVa which was essentially unrelated.
The D.III line of engines would find themselves eclipsed in performance by the BMW IIIa of 185 and then 200 hp (British rated it at 230HP) 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.