One of the mysteries of the English language finally explained.
1Combustion (in early use especially spontaneous combustion) occurring or supposed to occur within something; specifically the metabolic process by which heat is generated in the animal body.
2Combustion of petrol, oil, or other fuel within a closed space inside an engine, with the hot gases produced driving a piston or doing other work inside that space; frequently attributive, especially in "internal combustion engine".
Late 18th century; earliest use found in William Hutchinson (1732–1814), topographer. From internal + combustion.
internal combustion/ɪnˌtəːnl kəmˈbʌstʃ(ə)n/
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