Steam Boilers, Engines, Nozzles and Turbines
The function of a piston in a steam engine is

None of these
To convert heat energy of the steam into mechanical work id) to exhaust steam from the cylinder at proper moment
To transfer motion from the piston to the cross-head
To guide motion of the piston rod and to prevent it from bending

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Steam Boilers, Engines, Nozzles and Turbines
The relative efficiency is defined as the

Product of thermal efficiency and Rankine efficiency
Ratio of thermal efficiency to Rankine efficiency
Ratio of brake power to the indicated power
Ratio of heat equivalent to indicated power to the energy supplied in steam

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