Mercedes D.III

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Mercedes D.IIIa mounted in a Fokker D.VII

The Mercedes D.III, or F1466 as it was known internally, was a 6-cylinder, liquid cooled inline aircraft engine 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. Compared to the Allied engines it faced, the D.III was generally outdated.

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 2/3rds 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. The ignition cables were protected in tubes running down either side of the cylinders. Fuel was fed the the cylinders via pipes on the left side of the engine (as viewed from the rear), supplied from a twin-barrel carburetor located just above the crankcase. Both the fuel and oil reservoirs were pressurized by an air pump run off the crank. 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.

The original D.III was introduced in 1914, but did not see widespread use until 1916 when the fighters grew to need that level of power; earlier designs were generally powered by engines of about 110 hp. By 1917 the D.III was being widely used, 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. Daimler also used the pistons of the D.III to produce the eight-cylinder 220 hp Mercedes D.IV during this period, but it did not see widespread use.

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.

One important change on the D.III series of engines, especially on engines after the original D.IIIs were produced, was a change in the external rocker arm drive setup. Originally, the square planform camshaft metal cases where the camshaft drove each rocker arm was situated directly over the top of each cylinder head, with the rocker arms emerging from each side of each case through a vertical slot in the case sides. Later, the rocker arm drive torque shafts were allowed to come through the front side of each case, with each rocker arm now fully exposed outside the case, with the cases themselves being offset rearwards atop each cylinder head.

A more "radical" upgrade was the 180 hp D.IIIaü, introduced in late 1917. This engine changed the pistons again, this time to a domed profile that further increased the maximum compression – ü for "über", overcompressed. Additionally, a new altitude-compensating carburetor was added, which improved performance at higher altitudes. To support operations at these altitudes, water from the radiator was used to heat the air intake and prevent icing in the carburetor. The aü model was widely used and designed into most fighter designs from late 1917 on. This included most of the entries in the First Fighter Competition at Adlershof in January 1918, notably the famed Fokker D.VII.

The final version was the 200 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 avü, accounting for most of the gains in power. It is unclear if any av's saw service use.

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.

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.