Internal Combustion Engines
Volatility of diesel fuel oil is

Minimum temperature to which oil is heated in order to give off inflammable vapours in sufficient quantity to ignite momentarily when brought in contact with a flame
It catches fire without external aid
Indicated by 90% distillation temperature, i.e., when 90% of sample oil has distilled off
Temperature at which it solidifies or congeals

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Internal Combustion Engines
Compression ratio of I.C. Engines is

Volume displaced by piston per stroke and clearance volume in cylinder
Swept volume/cylinder volume
Ratio of pressure after compression and before compression
The ratio of volumes of air in cylinder before compression stroke and after compression stroke

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