The cowling surrounding a reciprocating engine is a sophisticated aerial garbage disposal. Its job is to throw away about $1.50 out of every $6 you spend on avgas.
Gasoline contains more energy per pound than TNT, but engines turn only about a quarter of that energy into useful power. What happens to the rest? It dribbles off as waste heat, to be absorbed by the atmosphere. Half blows out the exhaust pipe; some of that, however, can be harvested and put to good use by a turbocharger. The remaining quarter heats up the engine and its accessories. If the engine were uncooled it would become very hot indeed; gases get up to 3,000 degrees Fahrenheit and more in the combustion chamber. Aluminum begins to lose strength at around 500 degrees, however, and so it’s imperative to carry off the combustion heat to keep cylinder head temperatures at a safe level — preferably around 375 degrees. The job of the cowling is to dispose of that heat at the lowest possible cost in drag.
