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The low-speed engine is characterized by rated speeds in the range of 80–120 revolutions per minute. ... Whereas medium-speed engines are widely employed ashore, the low-speed engine is almost exclusively a marine engine that is designed to match efficient propeller speeds without recourse to a speed-reducing.
Medium-speed diesel engines. A driving force in marine propulsion. Rolls-Royce is a leading developer and producer of medium-speed diesel and gas propulsion engines and generator sets.
The diesel engine is high compression, self-ignition engine. Fuel is ignited by the heat of high compression. HSD is normally used as a fuel in medium and high speed compression ignition engines (operating above 750 rpm) in commercial vehicles, stationary diesel engines, locomotives and pumps etc.
The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn which enables heat dissipation by the excess air. A small efficiency loss is also avoided compared to two-stroke non-direct-injection gasoline engines since unburnt fuel is not present at valve overlap and therefore no fuel goes directly from the intake/injection to the exhaust. Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can have a thermal efficiency that exceeds 50%.[1][2]
Diesel engines are manufactured in two-stroke and four-stroke versions. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships. Use in locomotives, trucks, heavy equipment and electricity generation plants followed later. In the 1930s, they slowly began to be used in a fewautomobiles. Since the 1970s, the use of diesel engines in larger on-road and off-road vehicles in the US increased.
I agree with Mr . Basim Natour
Thanks for invitation -
They are the engines are designed for the purpose of benefit from it according to certain standards, according to it's needed - the differences between them are in the number of engine cycles !
Short answer !!!
The term medium refers to diesel engines that operate with in the approximate speed range to revolutions per minute. High speed is usually rpm and above, while slow speed diesel engines have to rpm. Each has their various advantages and disadvantages for various duties on board ship. The slow speed two stroke cycle diesel is used for main propulsion units since it can be directly coupled to the propeller and shafting. It provides high power, can burn low grade fuels and has high thermal efficiency. The cylinders and crankcase are isolated, which reduces contamination and permits the use of specialized lubricating oil in cylinders and bearings. The use of two stroke cycle usually means there are no inlet and exhaust valves. This reduces maintenance and simplifies engine construction. The medium speed diesel engines are generally to operate on the four stroke cycle and are trunk piston construction. These egines provide a better power to weight ratio and power to size ratio and there is also a lower initial cost for equivalent power. The higher speed how ever requires the use of gearbox and flexible coupling for main propulsion use. These engine may be constructed in either “In-line” or “vee configuration” (shape). Vee types engines are constructed with two banks of cylinders arranged at an angle using a common crankcase and bed plate with considerable saving in size and weight. The most common method for connecting two pistons to each throw of the shaft is the side arrangement with two bottom end bearings.
High speed engines are having rated speed of 1200 rpm and above ,
Medium engines have a rated speed of 400 rpm or more but less than 1200 rpm
Low speed engines have a rated speed of less than 400 rpm.
Slow-Speed Engines means diesel engines having a rated speed of less than 400 rpm.
Medium Speed Engines diesel engines having a rated speed of 400 rpm or more; but, approximately less than 1200 rpm.
High-Speed Engines means diesel engines having a rated speed of approximately 1200 rpm or more.
