The Mercedes D.II was a six-cylinder,

SOHC An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion cha ...

valvetrain liquid-cooled inline

aircraft engine An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years ma ...

built by Daimler during the early stages of

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

. Producing about 110 to 120 hp, it was at the low-end of the power range of contemporary engines, and was generally outperformed by rotaries whose

power-to-weight ratio Power-to-weight ratio (PWR, also called specific power, or power-to-mass ratio) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measuremen ...

tended to be much better. It also had stiff competition from the

Ferdinand Porsche Ferdinand Porsche (3 September 1875 – 30 January 1951) was an Austrian-German automotive engineer and founder of the Porsche AG. He is best known for creating the first gasoline–electric hybrid vehicle (Lohner–Porsche), the Volksw ...

-designed 120 hp

Austro-Daimler 6 The Austro-Daimler 6 was a series of Austrian six-cylinder water-cooled inline SOHC aero engines first produced in 1910 by the Austro-Daimler company. Design and development The first Austro-Daimler six-cylinder engine was designed by Dr-Ing ...

. The D.II was produced only briefly as a result, but its design formed the basis for the later Mercedes D.III which saw widespread use throughout the war.

Design and development

The D.II was based on the Austro-Daimler to a large degree. Like the Austro-Daimler, it was built up from the

crankcase In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block. Two-stroke engines typically use a crankcase-compression design, resulting in the fuel ...

, which was milled from two pieces of cast aluminum bolted together at their midline. The cylinders were separately milled from steel and bolted to the top of the crankcase. Steel sleeves were fitted over the cylinders and welded on to form a cooling jacket. Much of this complexity is due to the differential rates of expansion of steel and aluminum, which precluded screwing the cylinders into the crankcase, and the alloys of the era meant that an aluminum cylinder was not possible. Both engines also used a scavenger pump to pump oil out of the crankcase to a separate cylinder, where a second high-pressure pump supplied oil to the engine. This arrangement allowed for a much smaller "sump" on the bottom of the crankcase, reducing the overall size of the engine, although in the case of the D.II it was not nearly as much as the Austro-Daimler. Where the D.II differed from the Austro-Daimler was largely in mechanical arrangement. For instance, the D.II featured a single

overhead cam An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion cha ...

, powered by a shaft leading up from the

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...

at the rear of the engine, whereas the Austro-Daimler had a more conventional valvetrain using pushrods driven from the crankcase. Another unique feature was the ability to shift the camshaft to a half-compression position for starting. The D.II used two carburetors located together on one side of the engine, feeding the cylinders through two manifolds; the Austro-Daimler separated its carburetors to locate them closer to the cylinders they fed. The D.II also used a unique cooling jacket design, with every two cylinders being covered by a single jacket. The D.II was introduced in 1914. It was fairly quickly replaced by the D.III, and ended production around 1916. The D.III was essentially a scaled-up D.II, although it abandoned the paired cooling jackets.

Applications

Aviatik B.II ''Note:'' Not to be confused with the Austro-Hungarian Aviatik B.II series 32, 32.7 and 34 which were different aircraft. The Aviatik B.II was a reconnaissance aircraft built in Germany during World War I. Design and development The (German) Av ...

* Aviatik C.I *

Albatros B.I The Albatros B.I, (post-war company designation L.1) was a German military reconnaissance aircraft designed in 1913 and which saw service during World War I. Design and development The B.I was a two-seat biplane of conventional configuration t ...

* Albatros B.II * DFW B.II *

Fokker D.I The Fokker D.I (company designation M.18) was a development of the D.II fighter. The D.I was also flown in Austro-Hungarian service as a fighter trainer aircraft under the designation B.III. Confusing the matter further, both the D.II and D.I ar ...

* Fokker V 6 * Friedrichshafen FF.29 *

Halberstadt D.II The Halberstadt D.II was a biplane fighter aircraft developed and manufactured by German aircraft company Halberstädter Flugzeugwerke. It was adopted by the ''Luftstreitkräfte'' (Imperial German Army Air Service) and served through the peri ...

Junkers J 1 The Junkers J 1, nicknamed the ''Blechesel'' ("Tin Donkey" or "Sheet Metal Donkey"), was an experimental monoplane aircraft developed by Junkers & Co. It was the world's first all-metal aircraft. Manufactured early on in the First World War, ...

''Blechesel'' * Zmaj Fizir FN

Engines on display

One surviving example of this engine had been on display at Old Rhinebeck Aerodrome-its crankshaft had sheared, and the engine is rumored to possibly have been brought to

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...

. Deutsche Museum in Munich has one example on display, this is a cutaway model that was created by Daimler Photograph cited in the Daimler Archives

Specifications (D.II)

References

Notes

Bibliography

*Gunston, Bill. ''World Encyclopedia of Aero Engines''. Cambridge, England. Patrick Stephens Limited, 1989. *''Jane's Fighting Aircraft of World War I''. London. Studio Editions Ltd, 1993.