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

All the fuel burns instantaneously producing high energy release
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
Explosion in furnace
Fuel burns with less air

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Steam Boilers, Engines, Nozzles and Turbines
The cylinder dimensions of a compound engine may be designed on the basis of

All of these
Equal power developed in each cylinder for uniform turning moment
Equal temperature drop in each cylinder for economy of steam
Equal initial piston loads on all pistons for obtaining same size of piston rod, connecting rod etc. for all cylinders

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