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The Detroit Diesel Series 71 is a two-stroke diesel engine series, available in both inline and V configurations, with the inline models including one, two, three, four and six cylinders, and the V-types including six, eight, 12, 16 and 24 cylinders. The two largest V units used multiple cylinder heads per bank to keep the head size and weight to manageable proportions, the V-16 using four heads from the four-cylinder inline model and the V-24 using four heads from the inline six-cylinder model. This feature also assisted in keeping down the overall cost of these large engines by maintaining parts commonality with the smaller models.
The inline six-cylinder 71 series engine was introduced as the initial flagship product of the Detroit Diesel Engine Division of General Motors in 1938. The V-type first appeared in 1957. The 71 in the model series designation refers to the displacement per cylinder in cubic inches (actually 70.93 cu in / 1,162.4 cc). Bore and stroke is the same to all units, at 4.25 x 5.0 inches (107.95 x 127 mm).
All Series 71 engines use uniflow scavenging, where a gear-driven Roots type blower mounted to the exterior of the engine provides intake air through cored passages in the engine block and ports in the cylinder walls at slightly greater than atmospheric pressure. The engine exhausts through pushrod-operated poppet valves in the cylinder head(s), with either two or four valves per cylinder. Unit injection is employed, one injector per cylinder, with no high fuel pressure outside of the injector body. The injectors are cycled from the same camshaft responsible for opening the exhaust valves.
As a two-stroke diesel engine that does not use crankcase aspiration cannot naturally aspirate (draw in) intake air, a blower is necessary to provide sufficient air to scavenge exhaust gasses from the cylinders and to supply air for combustion. Later high-performance versions were available with turbochargers, and turbochargers with intercooling, the turbochargers discharging into the Roots blower intake. However, all 2 cycle Detroit Diesel engines are considered naturally aspirated unless they have the addition of a turbocharger (T series engine).
The 71 Series went off of the market in the summer of 1995, and the Four Stroke Detroit Diesel Engine was introduced as a replacement.
The most popular incarnations of the series 71 engine as used for highway vehicle applications included the inline 6-71, the V-block 6V-71 (both widely used in transit buses) and the 8V-71. In addition to motorcoach propulsion, both inline and V types have found extensive usage in trucks, fire apparatus, motor homes, construction and industrial machinery, a few farm tractors, and military vehicles and equipment.
The 71 series is very popular in marine applications, not only as a propulsion engine in small craft (Gray Marine 6-71) but as auxiliary power to drive generators, winches and other heavy shipboard machinery.
The inline 6-71 engine, in all of its variations, was also available as a 'pancake engine' (here variably called either 6L-71 or 6N-71) for horizontal (underfloor) mount applications, such as on larger Crown and Gillig school buses and articulated puller transit buses (such as the Crown-Ikarus 286).
Over the years, the 71 series has enjoyed a reputation for dependability and ease of maintenance. Due to their ubiquity and operating characteristics, inline models acquired a variety of nicknames from those who used and serviced them. Most common were "Screaming Jimmy" (Jimmy being trucker slang for GMC trucks) or "Rocky Mountain Humming Bird," which terms referred to the engine's sound at full throttle. Other nicknames include "Green Leaker" and "Driptroit Diesel," referring to the powerplants' factory color and propensity for leaking oil on everything in or near the engine compartment. The V12 has been called the "Buzzin Dozen" due to the higher RPM needed for it to produce power and what sound it makes when the engine brake is on.
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The inline 6-71 was adapted to British requirements as the power plant for Canadian (and later British) built Valentine tanks where it was known as the GMC 6004, orders being placed in late September 1940.
The 6046 Diesel was a twin engine setup used by US and British tanks & tank destroyers.
In the Soviet Union, there had been acquired in 1939 the license to produce the 71 Series engines at the Yaroslavl automobile factory (YaAZ). Throughout World War II, the 4-71 engine both in locally assembled form (built by Lend-Lease provided American industrial equipment) and from USA-supplied kits had been used for Ya-12 light artillery tractors and trucks. After 1945, the 4-71 engine entered production in a slightly modified configuration to suit the conditions of the Soviet Union branded "YaMZ-204". After 1947 the factory used a copy of the 6-71 engine branded "YaMZ-206" in the YaAZ 200 / 210 / 214 series of heavy trucks built from 1947 to 1960. Production was transferred to KrAZ in Kremenchuk, Ukraine in 1959, where newer versions of the YaMZ-206 stood in production until the appearance of the four-stroke V8-engined KrAZ-255 in 1967.
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