Q I understand that a well-known car ferry was at
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one time powered by semi-diesel engines. Could you explain the principle of the semi-diesel?
AThe engines you mention must have
been produced a great many years ago but the question is, in fact, of current interest because it is possible that a modern form of semi-diesel will be produced in the near future. A low-compression diesel with a compression ratio as low as 9 to 1 equipped with a turbocharger giving a high-boost pressure offers a number of advantages over a conventional high-compression diesel, including a very much increased output with no increase in mechanical or thermal loading. But the unit is difficult to start unless it is fitted with variable-compression-ratio pistons, which are heavy and complicated. And low-speed running is erratic.
A well-known research engineer has suggested that using sparking plugs to ignite the fuel when the engine is being started and run under light load would be preferable to employing variable-compression-ratio pistons. No details are currently available of older forms of system applied to marine semidiesels.