Mercedes D.III
| D.III | |
|---|---|
 |
|
| Mercedes D.IIIa mounted in a Fokker D.VII | |
| Type | Inline piston engine |
| Manufacturer | Mercedes |
| First run | 1914 |
| Developed from | Mercedes D.II |
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 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 Compared to the Allied engines it faced, the D.III was generally outdated.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 2/3 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. The ignition cables were protected in tubes running down either side of the cylinders. Fuel was fed into the cylinders via pipes on the port 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. 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. 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 lengthened crankshaft of the D.IV proved prone to breaking.
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