Steam Boilers, Engines, Nozzles and Turbines
Spontaneous combustion is a phenomenon in which

Coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal
All the fuel burns instantaneously producing high energy release
Fuel burns with less air
Explosion in furnace

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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 guide motion of the piston rod and to prevent it from bending
To transfer motion from the piston to the cross-head

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