Question

Why efficiency of liquid fuels is lower than solid fuels?

Answer 1

They are more efficient as they're ready to burn, transport (they have more energy per m3) and most importantly have a higher fuel/energyconversion efficiency than solid ones. Also they carry much less impurities which results in lesser air pollution.

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Answer 2

Solids conduct heat better than liquids, which are in turn better conductors than gases. The particles in a solid are most tightly bond and their positions are more or less fixed relative to each other (Fig. 5a). The force between adjacent particles is strong, making heat transfer by collision the most efficient.