Gray Marine 6-71 Diesel Engine

The 6-71 Gray Marine Diesel Engine is a marinized version of the General Motors Detroit Diesel 6-71 engine produced by the Gray Marine Motor Company. It was used in landing craft during World War II and is used today in private boats and training facilities.

A 6-71 Gray Marine training engine aboard the Training Ship Golden Bear

Engine characteristics

The 6-71 is an inline six cylinder diesel engine.[1] The 71 refers to the displacement in cubic inches of each cylinder.[1] The firing order of the engine is 1-5-3-6-2-4.[1] The engine's compression ratio is 18.7:1 with a 4.250 inch bore and a 5.00 inch stroke.[1] The engine weighs 2,185 lb (991 kg) and is 54 inches long, 29 inches wide and 41 inches tall.[1] At 2,100 revolutions per minute the engine is capable of producing 230 horse power (172 kilowatts).[1] V-type versions of the 71 series were developed in 1957.

The 6-71 is a two-stroke cycle engine.[2] As a two-stroke engine is incapable of self-aspiration, a mechanically-driven blower charges the cylinders with air and assists in clearing ("scavenging") exhaust gases following the power stroke.[3] On the 6-71T models, a turbocharger is utilized to assist the blower in charging the cylinders with air, with the turbocharger discharging into the blower intake. Unlike the turbocharger of a four-stroke engine, the primary purpose of the 6-71T's turbo is to reduce the blower's parasitic loading at higher engine power levels, which improves fuel economy.[2]

Fuel is provided by unit injectors, one per cylinder, each injector being cycled by the camshaft that also cycles the exhaust valves.[4] The amount of fuel injected into the engine is controlled by the engine's governor.[1] The engine cooling is via liquid in a water jacket. In a boat, cool external water is pumped into the engine and passed through a heat exchanger mounted on or nearby in a "double loop" configuration.[4]

Side cutaway view of the Gray Marine 6-71

Development

The Gray Marine 6-71 Marine Diesel was developed from the General Motors 6-71 diesel engine that was in use for on-road applications, farm machinery, and other off-road applications. Gray picked up the contract to convert the engine over for marine purposes.[5]

Use

The 6-71 was used for Higgins LCVP, a landing craft used in larger numbers during World War II.[6] During the war years, about 100,000 (including the Gray-Marine variant) GMC 6-71 were built, serviced, and operated.[3]

References
  1. Barrington Diesel club. "Detroit Diesel 6-71 engine specs at Barrington Diesel Club". Archived from the original on 8 March 2012. Retrieved 11 April 2012.
  2. Tugboat Enthusiasts Society of the Americas. "DETROIT DIESEL INLINE 71 SERIES". Retrieved 11 April 2012.
  3. Caprio, Dennis. ""Jimmy" Rules: Detroit Diesel 6-71". Yachting Magazine. Retrieved 18 April 2012.
  4. G.M. Grey Marine Model 64 HN9. Seattle, Washington: Hatch and Kirk INC.
  5. Homfield, Max. "A Brief History of Gray Marine Engines". Gas Engine Magazine. Retrieved 18 April 2012.
  6. Grayson, Stan (1999). Engines Afloat. Devereux Books. ISBN 0-9640070-7-X.
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