Mercedes D.III

The Mercedes D. III, or F1466 as it was known internally, was a six cylinder SOHC valvetrain 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 , but a series of changes improved this to in 1917, and 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 Compared to the Allied engines it faced, the D. III was generally outdated.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 aluminium 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 aluminium 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 each for a trio of adjacent pairs of cylinders. 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 into the cylinders via pipes on the left side of the engine, supplied from a twin barrel carburetor located just above the crankcase. Both the fuel and oil reservoirs were pressurized by an air compressor run off the crank. Daimler also used the pistons of the D. III to produce the reduction geared, eight cylinder Mercedes D. IV during this period, but it did not see widespread use. The lengthened crankshaft of the D. IV proved prone to breaking.

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 levels from to about , or by water cooled inline engines in the to range such as 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 higher rating under British HP numbers. Fuel consumption was 11.75 gallons 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. 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.